SCHOOL OF ADVANCED STUDIES

Doctorate course in Chemical Sciences

PhD thesis

Cesium and rubidium salts of Keggin-type

heteropolyacids as stable meso-microporous matrix

for anode catalyst for H2/O2 Proton Exchange

Membrane Fuel Cell, Direct Methanol Fuel Cell and

Direct Ethanol Fuel Cell

Cycle XXI

Scientific-sector CHIM/01

PhD Candidate Tutor

Artur śurowski Professor Roberto Marassi

Professor Paweł J. Kulesza

2005/2006 – 2007/2008

1

Table of contents Overview of the dissertation ……………………………………………………………4

Introduction

1. Fuel cell……………………………………………………..………………………7

1.1 History………………………...…………………………………………7

1.2 Basic principles, types and efficiency………………………………...…8

2. Anode electrocatalysis in PEMFC…………………………………………..…...…14

2.1 Hydrogen oxidation…………………………………………………….14

2.2 Methanol oxidation……………………………………………………..16

2.3 Ethanol oxidation…………………………………………………….....20

3. Heteropoly compounds……………………………………….............…….............24

3.1 Structure of heteropoly compounds (heteropolyacids)…………………24

3.2 Properties of heteropolyacids…………………………………………..27

3.3 Salts of the heteropoly compounds…………………………………..…30

4. Experimental techniques……..………………………………..………………..34

4.1 Cyclic voltammetry……………………………………………….…….34

4.1.1 Rotating disk voltammetry…………………………………………37

4.2 Chronoamperometry……………………………………………………39

4.3 Staircase voltammetry………………………………………………….40

4.4 Electrochemical impedance spectroscopy……………………………...41

4.5 Transmission electron microscopy ……………...……………………..43

4.6 Scanning electron microscopy ……..…………………………………..45

4.7 Infrared spectroscopy………………………………………………….46

Experimental part

5. Chemical reagents and measuring equipment…………………..………..………...48

6. Preparation and characterization of Keggin-type matrix………………..………….51

6.1 Preparation of Keggin-type heteropolyacid salts…………..…………...51

6.2 IR characterization of Keggin-type matrix……………………………..51

6.3 SEM characterization of Keggin-type matrix…………………………..54

6.4 Cyclic voltammetry characterization of Keggin-type matrix…………..57

6.5 Conclusions…………………………………………………………….61

7. Hydrogen oxidation reaction (HOR) on the catalytic layers

containing Cs2.5PW12 matrix…………………………………………………....62

2

7.1 Preparation of the catalytic layers…………………………….……..62

7.1.1 Mixing method……………………………………………………..62

7.1.2 Electrochemical method……………………………………………63

7.2 Electrochemical measurements on the Pt/C modified with

Cs2.5PW12 system prepared by mixing method……………………….65

7.2.1 Cyclic voltammetry (CV) and rotating disc voltammetry

(RDE) measurements……………………………………………….65

7.2.2 Test of the Cs2.5H0.5PW12O40-containing anode’s

catalyst in working single PEM fuel cell…………………………...74

7.3 Electrochemical measurements on the Pt/C modified with

Cs2.5H0.5PW12O40 system prepared by electrochemical method..…….78

7.3.1 Cyclic voltammetry (CV) study….……………………….………..78

7.3.2 CO electrooxidation on the catalytic layer containing

Cs2.5H0.5PW12O40 as a matrix……………………………………….80

7.3.3 HRTEM characterization…………………………………………..81

7.3.4 Rotating disc voltammetry (RDE) measurements…………………83

7.4 Comparison of catalytic layers containing Cs2.5PW12

matrix prepared by mixing and electrochemical methods……………88

8. Methanol oxidation reaction (MOR) on the catalytic layers

containing Keggin-type heteropolyacid salts as a matrix……………..…….……...90

8.1 Preparation of the catalytic layers……………………………………...90

8.2 Electrochemical measurements at the Pt40%/C

modified with Cs2.5-HPAs matrix………………………………...……91

8.2.1 Cyclic voltammetry (CV) study……………………………………...91

8.2.2 Staircase voltammetry (SV) measurements…………………………..95

8.2.3 Chronoamperometry (CA) measurements……………………………97

8.2.4 Electrochemical impedance spectroscopy

for methanol electrooxidation ………………………………………..99

8.2.5 CO stripping voltammetry study at Pt/C-Cs2.5HPAs…….………….105

8.2.6 Electrochemical stability of investigated materials

containing Cs2.5-HPAs matrix………………………………………107

3

8.3 Electrochemical measurements at system

containing Rb2.5-HPAs matrix…………………………….………...108

8.3.1 Cyclic voltammetry (CV) measurements…………………………...108

8.3.2 Staircase voltammetry (SV) measurements…………………………112

8.3.3 Chronoamperometry (CA) measurements…………………………..114

8.3.4 Electrochemical impedance spectroscopy measurements…………..116

8.3.5 CO stripping voltammetry study at Pt/C

modified with Rb2.5-HPAs………...………………………………...118

8.3.6 Electrochemical stability of investigated materials

containing Rb2.5-HPAs……………………………………………...120

8.4 Summary and conclusion……………………………………………..122

9. Ethanol oxidation reaction (EOR) on the Pt40%/Vulcan XC-72 carbon

modified with Cs2.5-HPAs………………………………………………………...127

9.1 Cyclic voltammetry (CV) measurements……………………….…….127

9.2 Staircase voltammetry (SV) measurements…………………………..130

9.3 Chronoamperometry (CA) measurements……………………………132

9.4 Electrochemical impedance spectroscopy

for ethanol electrooxidation…………………………………………...134

9.5 Electrochemical stability of investigated materials

containing Cs2.5-HPAs………………………………………………...136

Conclusions…………………………………………………………………………...138

Bibliography…………………………………………………………………………..141

4

Overview of the dissertation

In various applications, fuel cells are widely recognized as very attractive

devices to obtain directly electric energy from the electrochemical combustion of

chemical products. Low temperature fuel cells, which generally utilize proton

electrolyte membranes, seem to be also of utility for a large of power applications.

However, the final choice of the fuel is still difficult and depends greatly on the field of

application. Utilization of pure hydrogen or hydrogen-rich gases, rather than alcohols,

as fuels in Polymer Electrolyte Membrane Fuel Cell (PEMFC) leads to higher electric

efficiencies. Due to the problems related to the hydrogen storage, the hydrogen-oxygen

PEMFC is the best choice for stationary applications. Direct alcohols (methanol and

ethanol) fuel cells based on solid polymer electrolytes are widely proposed for portable

and mobile applications due to relatively low prices of the methanol and ethanol fuels

and easiness of storage. The major impediment in the development of fuel cells

operating on hydrogen, methanol, and ethanol is deactivation of the anode

electrocatalyst by trace level of CO. Thus, the most active Pt electrocatalyst for the

oxidation of hydrogen and alcohols (e.g. methanol, and ethanol) is deactivated by strong

adsorption of carbon monoxide, and leads to decreased performance of fuel cell.

Therefore, a new inexpensive, stable electrocatalyst must be developed, which is

tolerant to high levels of CO (particularly, for direct alcohol fuel cell) or that could

preferably utilize CO.

There are currently two state-of-the-art methods which increase CO tolerance of

the fuel cell anode. One method is to use a Pt-alloy catalyst (e. g. PtRu, PtSn, PtMo,

PtW).[1-6] The other method is to use the zeolite as matrix, and limit the preferential

formation of CO clusters on platinum by the steric constraints imposed by the zeolites

framework, followed by facile oxidation to CO2 by interaction with the surface or

bridged hydroxyls of the zeolites. Furthermore, the porous nature of the zeolite support

material provides relatively improved gas permeability and minimizes the disadvantage

associated with restricted gas diffusion in the electrode. The ideal support would also an

enhanced electrochemically active surface area by dispersion of the metal catalyst.[7, 8]

Keggin-type heteropolyacids show appreciable acid catalytic properties[9-14] that

are of practical importance[15] as illustrated in numerous recent reviews.[13, 15-18] The fact

that heteropolyanions undergo spontaneous adsorption (from aqueous solution) on

5

various substrates[32,33] provides a simple tool for modification of electrode surfaces[19-

22] Among other important issues related to electrocatalysis are their ability to undergo

fast reversible multi-electron transfers and super-acid properties resulting in the

increased availability and mobility of protons at the electrocatalytic interfaces.

Consequently, polyacids were considered for fuel cell research.[23-29] With respect to the

oxygen reduction, the adsorbed heteropolyacid (particularly H3PW12O40) nanostructures

do not seem to block access of reactant molecules[21] to catalytic Pt. Further, their

presence at the interface shifts formation of the inhibiting Pt-oxo (PtOH or PtO) species

towards more positive potentials thus increasing the potential range where catalytic

metallic (Pt0) sites exist. It is reasonable to expect that, by analogy to the activating role

of WO3 and partially reduced hydrogen tungsten oxide bronzes during elect oxidation of

methanol, the related heteropolyblue tungstates should also enhance reactivity of Pt

during oxidation of organic fuels.[26, 30] Stanis and co-workers[27] reported that the

addition of adsorbed HPAs can improve the performance of Pt anodes in a fuel cell

under CO-poisoned conditions. Also Farell et al. postulated that HPAs can act as co-

catalysts with platinum for methanol electrooxidation.[28]

Among limitations in practical applications of heteropolyacids is their very good

solubility in aqueous solutions including acids as well as their ability to undergo

desorption during long-term operation. Thus it is necessary to stabilize heteropolyacid

layers at electrocatalytic interfaces without loosing their activating properties. An

interesting alternative arises from the possibility of formation of acidic salts of

heteropolytungstates or molybdates by partial exchange of protons with large cations

(such as Cs+, Rb+, NH4+ or K+) in the parent heteropolyacid. Consequently, a water-

soluble polyacid of low surface area (<5 m2g-1) is transferred into a water-insoluble acid

salt precipitate characterized by the surface area exceeding 100 m2g-1.[31, 32] Contrary to

zeolites exchanged with alkali metals[14, 33], heteropolyacid salts remain strong acidity.

The resulting materials have occurred to be efficient solid shape-selective acid catalysts

for a variety of organic liquid-phase reactions[18] that include hydrogenations and

oxidations. The pore size and acidity of heteropolyacid salt can be controlled by the

cation content. For example, when the Cs content, x, is initially increased in CsxH3-

xPW12O40 from 0 to 2, the number of surface protons decreases but, later, it significantly

increases when x changes from 2 to 3 to show the highest surface acidity at x = 2.5.[14]

In the present work, we consider the salts of Keggin-type heteropolyacids

containing 2.5 moles of Cs+ and Rb+ cations in 1 mole of the heteropoly salt. The

6

system with such Cs or Rb-content is micro-mesoporous, and it is characterized by very

good stability and, while being insoluble in water, it exhibits high acidity (proton

availability and mobility).

The aim of this work is to study the applicability of the Cs and Rb salts of

Keggin type heteropolyacids as a stable meso-microporous matrix for anode catalyst for

H2/O2 Proton Exchange Membrane Fuel Cell, Direct Methanol Fuel Cell and Direct

Ethanol Fuel Cell.

The experimental part is divided into four chapters (from 6 to 9). In the Chapter

6, we present characterization of cesium, rubidium and ammonium salts of Keggin

types heteropolyacids by infrared spectroscopy (IR), scanning electron microscopy

(SEM), and cyclic voltammetry (CV) measurements.

In Chapter 7, we illustrate incorporation and activation of Pt centers in the

conductive high surface-area zeolite-type robust, Cs2.5H0.5PW12O40 matrices through

application of mixing and electrochemical methods. To evaluate electrocatalytic activity

towards the hydrogen oxidation of the investigated electrode material, we have

performed the diagnostic rotating disc electrode voltammetric measurements. In a case

of the catalytic layer prepared by corrosion of Pt counter electrode (electrochemical

method), CO-stripping and HRTEM measurements have been performed to comment

the electrochemically active area of catalyst, dimensions of Pt particles and platinum

loading.

In Chapter 8, we report on the performance of electrocatalysts (prepared by

mixing method) towards methanol oxidation in acidic media. The Cs and Rb salts of

H3PW12O40, H3PMo12O40, H4SiW12O40, H4SiMo12O40 were used as zeolite matrix for

Pt40%/Vulcan XC-72 carbon nanoparticles (Pt/C). The techniques of cyclic

voltammetry, staircase voltammetry, chronoamperometry, electrochemical impedance

spectroscopy, CO stripping voltammetry were applied to compare the Pt-base

electrocatalyst activity and stability to methanol oxidation.

In Chapter 9, the system composed of Pt40%/Vulcan XC-72 carbon modified

with Cs2.5-HPAs matrix (prepared by mixing method) has been examined with respect

to ethanol electrooxidation by several different electrochemical techniques. Comparison

has been made to commercial Pt/C.

7

1. Fuel cell

1.1 History

The principle of the fuel cell was discovered by the German scientist, who in

1839 published paper about current production in reaction between hydrogen and

oxygen. Based on this work, in 1839, British physicist and lawyer, Sir William R.

Grove develops the first working fuel cell. By connecting a hydrogen anode and an

oxygen cathode, he produced an electric current with the experimental set-up shown in

Fig. 1.

Fig.1. Four-cell version of Grove’s gas battery.[34]

He also, in 1844/1845, presented the first fuel cell power generator which consisted 10

cells connected in series and was supplied with hydrogen from corrosion of zinc in acid.

Fig. 2. Grove’s fuel cell power generator.[34]

In 1905 Wilhelm Ostwald and Walter H. Nerst presented a general theory of the

fuel cells. Due to easily accessible and large amounts of oil and the invention of the

8

combustion engine by Carl Friedrich Benz and Gottlieb Daimler fuel cells were

forgotten until the middle of the 20th. Because of US Apollo space programme in the

1960’s fuel cells exhibited their first renaissance. Gemini 5 was the first space shuttle

using a polymer membrane fuel cell instead of battery. In 1969 alkaline fuel cells were

used in the Apollo missions and supplied the electric power when the USA landed on

the moon.

The first oil crisis in 1973 led to the second renaissance of the fuel cells. At this

time interest for large power plants based on high-temperature fuel cells considerably

increased. In 1970 professor Karl Kordesch (early fuel cell pioneer) form University of

Graz, Austria, and his co-workers developed an alkaline fuel cell motorbike and a car.

The focus on increasing pollution level over the last two to three decades has

forced world society to search for cleaner energy technology, and thus fuel cells have

experienced an exponential increase in attention.

1.2 Basic principles, types and efficiency

A fuel cell is electrochemical device that continuously converts chemical energy

into electric energy (and some heat) for as long as fuel oxidant is supplied.

In the exchange membrane fuel cell (PEMFC), hydrogen (fuel) is oxidised to

protons and electrons at the anode. Protons migrate through the membrane electrolyte to

the cathode. As the membrane is an electric insulator, electrons are forced to flow in an

external electric circuit. At the cathode, oxygen (oxidant) reacts with protons to produce

water, which is the only waste product from a hydrogen-operated PEMFC. A schematic

representation of a fuel cell with the reactant/product gases and the ion conduction flow

directions through the cell is shown in Figure 3.

Anode reaction: H2 →2H+ + 2e- (1)

Cathode reaction: ½O2 + 2H+ + 2e- →H2O (2)

Total cell reaction: H2 + ½O2 →H2O (3)

9

Fig.3. Schematic of an individual fuel cell.[35]

From hydrogen and oxygen we obtain water, heat and power. There are other fuels,

electrolytes and charge-transferring ions for the other fuel cell types - but the principle

is the same.

When an external resistance, commonly referred to as a “load”, is applied to the cell,

non-equilibrium exists and a net current flows through the load. The net rate of an

electrochemical reaction is proportional to the current density which is defined as the

current of the electrochemical system divided by the active area devices. The cell

voltage becomes smaller as the net reaction rate increase because of irreversible

losses.[36] A representative polarization curve for hydrogen-oxygen PEMFC is illustrates

in Fig. 4. The voltage-current density relationship for a given fuel cell (geometry,

catalyst/electrode characteristic, and electrolyte/membrane properties) and operating

conditions (concentration, flow rate, pressure, temperature) is a function of the kinetic,

ohmic, and mass transfer resistance. Fuel crossover and internal current losses result

from the flow of fuel and electric current in the electrolyte. The electrolyte should only

transport ions, however a certain fuel and electron flow will always occur. Although the

fuel loss and internal currents are small, they are the main reason for the real open

circuit voltage (OCV) being lower than the theoretical one (Erev).

10

Fig. 4. A typical performance curve for a solid polymer fuel cell showing the relative effects of

electrode activation (kinetic losses), ohmic resistance and mass transport losses.[37]

Activation losses are caused by the slowness of the reactions taking place on the

electrode surface. The voltage decreases somewhat due to the electrochemical reaction

kinetics. The ohmic losses result from resistance to the flow of ions in the electrolyte

and electrons through the cell hardware and various interconnections. The

corresponding voltage drop is essentially proportional to current density, hence the term

"ohmic losses". Mass transport losses result from the decrease in reactant concentration

at the surface of the electrodes as fuel is used. At maximum (limiting) current, the

concentration at the catalyst surface is practically zero, as the reactants are consumed as

soon as they are supplied to the surface. In the thesis the investigation was principally

concerned with the activation losses of low temperature fuel cells.

Fuel cells can be classified according to their operating temperature, electrolyte

and the corresponding conductive ions (Tab. 1).

11

Alkaline Fuel Cell (AFC)

Proton Exchange

Membrane Fuel Cell (PEMFC)

Phosphoric Acid Fuel Cell

(PAFC)

Molten Carbonate Fuel Fell (MCFC)

Solid Oxide Fuel Cell (SOFC)

Operating temperature

70-220°C Up to 120°C 130-220°C 600-800°C 700-1000°C

Electrolyte Potassium hydroxide

(KOH)

Polymer membrane

Concentrated phosphoric

acid

Melted Li/K carbonate

Solid oxide ceramic

Fuels Pure hydrogen

Hydrogen (+reformate),

methanol, ethanol

Hydrogen, natural gas

Hydrogen, natural gas

Hydrogen, natural gas

Power range

realised

Up to 12 kW Up to 250 kW Up to 1 MW Up to 2 MW Up to 10 MW

Application Space, sub-marine

Portable, mobile, APU,

CHP

Small power plants, APU,

CHP

Power plants Power plants,

APU, CHP APU - Auxiliary Power Unit, CHP - Combined Heat and Power

Table. 1. Typical properties of the different fuel cell types.[37]

To the low-temperature group of fuel cells we can include Polymer Electrolyte

Membrane Fuel Cells (PEMFC), Direct Methanol Fuel Cells (DMFC), Direct Ethanol

Fuel Cells, Phosphoric Acid Fuel Cells (PAFC) and Alkaline Fuel Cells (AFC). Solid

Oxide Fuel Cells (SOFC) and Molten Carbonate Fuel Cells (MCFC) are high-

temperature fuel cells.

The most important disadvantage of fuel cell at the present time is the same for all

types – the cost. However, the are varied advantages[38]:

Efficiency. Fuel cell is generally more efficient than combustion engines

whether piston or turbine based. They are small system which can be just as

efficient as large ones. This is very important in the case of the small local

power generating system needed foe combined heat and power system.

Fuel cells are not limited due to the Carnot theorem therefore, they are more

efficient in extracting energy from a fuel than conventional power plant. Waste

heat from some cells can also be harnessed, boosting system efficiency still

further. Typical efficiency of different fuel cells is placed below, in Table 2.

12

Type Efficiency

Solid Oxide 45 – 65%

Molten Carbonate 50%

Phosphoric Acid 40%

Alkaline 50 – 60%

Direct Methanol 40%

Proton Exchange

Membrane (PEM)

40%

Table. 2. Typical efficiency of fuel cells.[37]

Simplicity. The essential of a fuel cell are very simple, with few if any moving

parts. This can lead to highly reliable and long-lasting system.

Low emissions. The by-product of the main fuel cell reaction, when hydrogen is

the fuel, is pure water, which means a fuel cell can be essentially ‘zero

emission’. This is their main advantage when use in vehicles, as there is a

requirement to reduce vehicle emissions, and even eliminated them within cities.

1 2000 U.S. EPA Average Annual Emission for Passenger Cars and Light Trucks 2 Calculations from Desert Research Institute 1mile = 1.6 km

Table. 3. Systems emissions.[39]

Engine Type

Water Vapor

g / mile

CO2 g /mile

CO g /mile

NOx g /mile

Hydro Carbons g /mile

Gasoline ICE Passenger Car 1

176.90

415.49

20.9

1.39

2.80

Gasoline ICE Light Truck 1

N/a

521.63

27.7

1.81

3.51

Methanol FC 2

113.40

68.04

0.016

0.0025

0.0034

Hydrogen FC 2

113.40

0.00

0.00

0.00

0.00

13

Silence. Fuel cell are very quiet, even those with extensive extra fuel processing

equipment. This is very important in both portable power applications and for

local power generation in combined heat and power schemes.

The advantages of fuel cell impact particularly strongly on combined heat and

power system (for both large and small scald applications), and on mobile power

systems, especially for vehicles and electronic equipments such as portable computers,

mobile telephones, and military communications equipment. These areas are the major

fields in which fuel cells are being used. A key point is the wide range of applications of

a fuel cell power, from systems of a few watts up to megawatts.

Table. 4. Chart to summarized the application of fuel cells of different types and different

applications.[38]

Among the applicable low-temperature acid-type systems, polymer electrolyte

membrane fuel cells (PEMFCs), direct methanol fuel cells (DMFCs) and direct ethanol

fuel cells (DEFCs) are probably the most promising devices, and they are subjects of

interest in many laboratories worldwide. While in the case of PEMFCs, hydrogen is

utilized; the methanol or ethanol fuel is oxidized in the anodic compartment of DMFCs

or DEFCs, respectively. To improve power densities of PEMFCs, DMFCs, and DEFs

there is a need to develop new electrocatalyst to inhibit the poisoning and significantly

increase the rate of electrooxidation. This study has been concerned with the catalysts of

the three low temperature fuel cells, the PEMFC, the DMFC and the DEFC.

14

2. Anode electrocatalysis in PEMFC

2.1 Hydrogen oxidation

The hydrogen oxidation reaction (HOR) is very important reaction which, serve

as a model for electrocatalytic reaction, and has significant practical utility in some

kinds of electrochemical sensors and fuel cells.[40-57] In acid media the catalysis of the

reaction is usually promoted by the platinum catalyst dispersed as fine particles on high

surface area carbon supports.[44,58,59] One important class of such devices are those

assembled using polymer electrolyte (PE) and polymer impregnated gas diffusion

electrodes, most commonly employing Nafion® (E.I. DuPoint, U.S.A.) membranes and

ionomers. The electrochemical oxidation of dihydrogen:

H2 → 2H+ + 2e- (E0 = 0.00 V vs. RHE) (4)

on noble metal surfaces such as Pt and Pd is very facile.[44] Other metals also show high

activity for H2 electrooxidation, but in acidic electrolytes, noble metals show the

greatest stability towards corrosion or passivation.

The accepted mechanism of hydrogen electro-oxidation on Pt in acidic

electrolytes is formed by a primary chemical:

H2 + 2Pt → 2Pt-Hads (Tafel reaction) (5)

and / or electrochemical:

H2 + Pt → Pt-Hads + H+ + e- (Heyrovsky reaction) (6)

adsorption steps, followed by a discharge path of the adsorbed hydrogen atom given by

MH → M + H+ + e- (Volmer reaction) (7)

For a given electrode material and the electrolyte, the rates of reactions (5) and (6) may

be quite different, and the mechanism may be formed preponderantly by the

Tafel/Volmer steps or alternatively by the Heyrovsky-Volmer steps. On Pt electrode, a

Tafel-Volmer mechanism, with dissociative adsorption of dihydrogen (Tafel reaction)

being the rate determining step (rds), has been proposed for this reaction[34,44,45,51] in

acidic electrolytes.

15

While pure H2 is the ideal choice of fuel for the PEMFC, economical sources of

pure H2 are not readily available. Therefore, currently the most practical source of H2 is

the catalytic processing of hydrocarbons. Hydrogen produced by the stream reforming

or partial oxidation of hydrocarbon fuels (gasoline, diesel, methane, alcohols) contains

impurities such as CO (1 - 3%) and CO2 (19 – 25%). It is well known that CO binds

strongly Pt sites and reduces the number its catalytic centers available for H2 adsorption

and oxidation. Although the electrochemical oxidation of CO:

CO + H2O → CO2 + 2H+ + 2e- (E0 = -0.1 V vs. RHE) (8)

is thermodynamically favorable, in practice a large overpotential is required on pure Pt

surfaces before oxidation occurs. For example, on dispersed Pt catalysts, the onset of

CO oxidation is no observed until 0.5 V at 80 0C. Therefore, in the potential region

where anodes need to operate (i.e. 0 - 0.1 V), CO is an inert adsorbate. The degree of

CO poisoning of Pt catalysts is very dependent on both temperature and CO

concentration.[34]

CO2 poisoning on pure Pt catalysts is modest when compared to the effect of

CO, especially when the large differences in relative concentrations in reformate are

considered. The poisoning effect comes from two possible mechanisms:

1) H2 + CO2 → H2O + CO (9)

2) CO2 + 2Pt-Hads → Pt-CO + H2O + Pt (10)

Both are forms of the “reverse water-gas shift” reaction with (9) being the

familiar gas-phase reaction, and (10) the electrochemical equivalent. In both cases, the

product is CO, which has the same effect as fuel stream CO.

The most elegant way to overcome anode poisoning is through the development

of CO- and CO2- tolerant electrocatalysts. Much effort has been spent modifying Pt

with others metals to improve CO tolerance. Niedrach et al.[60], in early 1960s, found

that the addition of Ru, Rh and Ir to Pt in the form of unsupported mixed metal powders

(blacks) get substantial tolerance, compared to Pt black alone, to the presence of CO in

a fuel stream at 85 0C in 2.5 M H2SO4 electrolyte. Moreover, they found that mixing Pt

blacks with metal oxides, such as CoMoO4, MoO2 and WO3, also improved CO

tolerance over Pt alone.[60] In particular, the oxide-containing electrodes showed

16

remarkable performances with pure CO as a fuel, with reported activities approaching

those with pure H2 even at low potentials.[34]

In the recent years, a number of workers have reported the superior CO and CO2

tolerance of carbon-supported PtRu catalysts when compared to Pt-only catalysts.[1,61-67]

2.2 Methanol oxidation

Introduction

The electro- oxidation of methanol is very important from practical point of

view and has been studied for more than three decades.[1,28,61-67,68-97] Methanol is used as

a liquid fuel in Direct Methanol Fuel Cell (DMFC). Among the different possible

alcohols, methanol is the most promising organic fuel because its use as a fuel has

several advantages in comparison to hydrogen: high solubility in aqueous electrolytes,

liquid fuel available at low cost, easily handled, transported and stored, high theoretical

density of energy (6 kWh/kg) comparable to that of gasoline (10-11 kWh/kg).[68,69,97]

Fig. 5. Sketch of a DMFC illustrating proton, water and methanol permeation

across the PEM and related characterization methods.[98]

The sketch of the DMFC is shown in Fig. 5.[98] The DMFC consist of an anode

at which methanol is electro-oxidized to CO2 and a cathode where oxygen, generally

from air, is reduced to form water. Both electrodes, usually formulated with platinum or

Pt-based catalysts, are separated by a proton-conducting electrolyte. The direct

17

methanol oxidation involves the transfer of six electrons to the electrode for complete

oxidation to carbon dioxide. In an acidic medium, this reaction can be written as

follows:

Anode reaction: CH3OH + H2O → CO2 + 6H+ + 6e- (11)

Cathode reaction: 3/2 O2 + 6H+ + 6e- → 3 H2O (12)

Overall reaction: CH3OH + 3/2 O2 → CO2 + 2 H2O (13)

The free energy associated with the overall reaction at 25 oC and 1 atm and the

electromotive force are[77,99]: ∆G = -686 kJ mol-1CH3OH; ∆E = 1.18 V vs. SHE

Methanol oxidation reaction

Iwastia[84] reported that the thermodynamic potential for methanol oxidation to

CO2, is very close to the equilibrium potential for hydrogen (E0=0.02 V), Eq. 11.

However, compared with hydrogen oxidation, this reaction is by several orders of

magnitude slower and requires high loading Pt-based electrocatalyst (≈ 2 mg cm-2).[77]

In fact, the ideal anodic reaction is not completely reached as methanol is mainly

decomposed into CO, which can further be oxidized into CO2. The formations of CO

and CO2 are assumed occurring according to the dual path mechanism in the oxidation,

one leading to CO (12) and another to CO2 (11)[100]:

CH3OH → CO+ 4H+ + 4e- (14)

Other CO-like species are also formed during the adsorption phase, COHads, HCOads,

HCOOads, and the principle by-products are formaldehyde and formic acid.[77,93, ,101,102]

The generally accepted mechanism of methanol electro-oxidation pathway is considered

as follows[83,85,103]:

Pt + CH3OH → Pt(CH3OH)ads (15)

Pt(CH3OH)ads → Pt(CH4-nO) + n H+ + n e- (16)

M + H2O → M-OH + H+ + e- (17)

Pt-CO + M-OH → CO2 + H+ + e- (18)

Pt-CO + H2O → CO2 + 2 H+ + 2 e- (19)

18

where, M is a metal, e.g. Pt, Ru, W. The complexity of the methanol oxidation reaction

on involving different path of reaction is shown in Fig. 6.

Fig. 6. Detailed reaction mechanism of the oxidation of methanol on a platinum electrode.[97]

Since the complete oxidation of methanol to CO2 involves the transfer of 6 electrons to

the electrode, the overall reaction mechanism (Eqs. 15 – 19) involves several steps

including dehydrogenation, chemisorption of methanolic residues, rearrangement of

adsorbed residues, chemisorption of oxygenated species (preferentially on the alloying

element) and surface reaction between CO and OH to give CO2. There is a reasonable

consensus regarding the general mechanisms at different potentials on Pt surfaces. It is

known that at low potentials, less than 0.4 V vs. RHE, the rate determining step for Pt

and PtRu electrode is the methanol dissociative adsorption expressed by Eqs. (15) and

(16).[82,84] The most important potential regime for a fuel cell is ca. 0.4 - 0.45 V vs. RHE

and the consensus agreement is that at this potential the rate-determining process is the

oxidation of CO.[34,82]

Clean Pt is a good catalyst for methanol oxidation and initially show very high

activity for methanol oxidation, but these very rapidly decay in current on the formation

of strongly bound intermediates.[62,77,104-106] These intermediates (mainly COads) are only

removed on going to high overpotentials where they are oxidized. At potentials below

0.45 V, the surface of Pt becomes poisoned with a near-monolayer coverage of CO, and

19

further adsorption of water or methanol cannot occur. Hence, the methanol oxidation

rate drops to an insignificant level.[34]

The development of advanced Pt based catalysts has focused on the addition of a

secondary component (e.g., Ru, Sn, W) that is able to provide an adsorption site capable

of forming OHads species at low potentials adjacent to poisoned Pt sites. The adsorption

site is also less effective at adsorbing methanol itself. The OHads is then able to react

with the bound CO to produce CO2 and free sites for further methanol adsorption.

Superior catalytic activities have been reported for Pt based alloys, such as Pt-Ru, Pt-

Mo, Pt-Sn, Pt-Os, Pt-Ru-Os.[61-66,106-108] This enhancement effect has been explained by

models such as the “bifunctional mechanism”[64,109,110] and/or the “electronic

effect”[65,111,112], which indicates a promotional effect of the alloyed metal on Pt. For

“promoters” such as Ru, stable methanol oxidation currents occur at significantly lower

potentials (<0.25 V) to Pt[34], indicating the Ru is capable of the formation of OHads

without itself being poisoned by CO.[62,84]

Ru + H2O → (OH)ads + H+ + e- (20)

(CO)ads + (OH)ads → CO2 + H+ + e- (21)

Until now the most successful results have been achieved through the alloying of Sn

and Ru with Pt. It has been shown that these alloys give rise electrocatalysis which

strongly promote the oxidation of both methanol and CO. The promotion of CO

oxidation reaction on Pt-Sn catalysts appears to be mainly due to a modification of the

electronic environment around Pt-sites.[113-115] It becomes clear that Pt-Ru catalysts are

more effective for methanol oxidation since the reaction desires the electrocatalyst to be

operated in a potential regime where labile-bonded oxygen should be present on the

surface. In this situation, the supply of active oxygen to the surface is of paramount

importance since this apparently would facilitate the oxidation of adsorbed methanolic

residues to CO2. The presence of strongly-bonded oxygen species on Sn-sites in the Pt-

Sn system limits the oxidation of methanol to CO2.[113-115]

Out of all tested catalysts, for the practical applications in DMFC the most active

and stable catalysts are these based on platinum ruthenium alloys.[1,61,63,64,66,67,112]

20

2.3 Ethanol oxidation

Introduction

Direct ethanol fuel cells (DEFCs) have spurred more and more in the recent years

due to intrinsic advantages such as its low toxicity, renewability, and its easy production

in great quantity by the fermentation from sugar-containing raw materials.[97,116]

Furthermore, the high theoretical mass energy density (about 8.00 kWh/kg)[4,97]

provides it with a potential candidate fuel for polymer electrolyte membrane fuel cells.

Therefore, ethanol electro-oxidation reaction is extensively studied by numerous of

researchers.[2-6,79,91,92,97,116-139]

Fig. 7. Schematic principle of a direct ethanol/oxygen fuel cell (DEFC).[117]

The parts of which a direct ethanol fuel cell (DEFC) constitutes and its working

principle are shown in Fig. 7. At the anode (negative pole of the cell) the electro-

oxidation of ethanol aqueous solutions takes place as follows[97,117,118]:

CH3CH2OH + 3 H2O → 2 CO2 + 12 H+ + 12 e- (22)

E10 = 0.085 V vs. SHE

whereas the cathode (positive pole) undergoes the electro-reduction of oxygen, i.e.:

O2 + 4 H+ + 4 e- → 2 H2O (23)

E20 = 1.229 V vs. SHE

21

where Ei0 are the standard electrode potentials versus the standard hydrogen electrode

(SHE). This corresponds to the overall combustion reaction of ethanol in oxygen:

CH3CH2OH + O2 → 2 CO2 + 3 H2O (24)

This gives a standard electromotive force (Eemf) equal to 1.145 V.

The complete oxidation of ethanol involves 12 electrons per methanol molecule

(Eq. 22). and the cleavage of the C-C bond. In view of the direct electro-oxidation of

ethanol in the fuel cells, the materials that could facilitate ethanol complete oxidation

and shift the onset oxidation potentials to lower values are the most interest.[119]

Ethanol oxidation reaction

The complete oxidation of ethanol to CO2 is the central challenge in the

electrocatalysis of this alcohol. In the recent years, several spectroscopic and

electrochemical studies have been taken to evaluate the mechanism of ethanol electro-

oxidation on Pt-based catalysts.[2,79,92,97,120-122] A generally accepted reaction sequence

comprises of the following steps (where M is an active site)[2]:

M + CH3–CH2OH → M–CHOH–CH3 + H+ + e- (25)

M–CHOH–CH3 → M–CHO–CH3 + H+ + e- (26)

M–CHO–CH3 → M–CO–CH3 H+ + e- (27)

M–CO–CH3 + M → M–CO + M–CH3 (28)

M + H2O → M–OH + H+ + e- (29)

M–CO + M–OH → 2M + CO2 + H+ + e- (30)

Ethanol can also react with adsorbed hydroxyl species directly to produce acetate via a

four-electron oxidation pathway[2,122]:

C2H5OH + M–OH → M–CH3COO + 4H+ + 4e- (31)

An efficient electrocatalyst should facilitate each of the processes of dehydrogenation

(Eqs. (25)–(27)), C–C bond cleavage (Eq. (28)) and COads oxidation (Eq. (30)) for the

complete conversion of ethanol to CO2 to take place. In addition, water activation (Eq.

(29)) at low electrode potential is important for the subsequent COads oxidation step.

22

Platinum is recognized to be the most active material for ethanol oxidation, however, it

should be noted that the self-inhibition happens in the case of Pt alone, especially in the

steady state operation mode. Furthermore, in order to increase the fuel utilization and

the fuel cell efficiency, it is crucial to break C-C bond and provoke its complete

oxidation into carbon dioxide. Therefore, in order to improve the electrocatalytic

activity of platinum to ethanol electro-oxidation, platinum was often modified by a

second or a third additive like PtRu[2,3], PtRh[123], PtMo[124], PtSn[4,5], PtW[6], PtRu-

MeOx (Me = W, Mo, V ect.).[91,125-130] Fig. 8 presents the effect of different additives to

Pt’s activity to ethanol electrooxidation in a single DEFC. As one can distinguish that

all the additives can promote more or less the platinum’s electrocatalytic activity

towards ethanol oxidation.[119]

Fig. 8. The effect of the different additives on the Pt’s activity toward ethanol electrooxidation

in single direct ethanol PEMFCs under the same operation conditions. Tcell = 90 0C. Anode:

PtM/C, 1.3 mg Pt/cm2, Cethanol = 1.0 mol/L, flow rate: 1.0 mL/min. Cathode: Pt/C (20%,

Johnson Matthey Corp.), 1.0 mg/ cm2, PO2 = 2.0 atm. Electrolyte: Nafion1®-115 membrane.[119]

From the attempts of Lamy et al.[131] to identify the suitable electrocatalysts for ethanol

oxidation, it was concluded that Sn can lead to encouraging results especially at lower

potential values, three times increase in maximum power density. They also found that

when the Sn atomic ratio in PtSn electrocatalyst is in the range of 10–20%, it shows

desirable results. Moreover, they found that the addition of tin to Pt promotes the

oxidation of ethanol to acetic acid at lower potentials and proposed the mechanism of

ethanol electrooxidation over Pt and PtSn catalysts.[119] Both the bifunctional

mechanism and ligand effect have been proposed to be involved in the ethanol

electrooxidation over PtSn catalyst.[79]

23

At the end of this chapter we have to note that even if PtSn/C catalysts exhibit

higher electrocatalytic activity to ethanol oxidation, the majority of the oxidation

products are still the species containing C–C bond, which will have an obviously

negative effect on the fuel cell performance.[4] It is crucial and necessary to develop a

novel catalyst or add a third element to modify the PtSn/C and PtRu/C to present higher

specific activity of dehydrogenation, C–O and C–C bond cleavage during the ethanol

oxidation process.

24

3. Heteropoly compounds

Transition metal polyoxometalates (POMs) are well-defined clusters with an

enormous variation in size, metal-oxygen framework topology, composition, and

functions. The preparation of POMs is based on the programmed self-assembly of metal

oxide building blocks, which result in discrete, structurally uniform, nanoscopic

clusters.[140] Although the first polyoxometalates [PMo12O403-] was reported in the 1826

by Berzelius[16] they have wide drawn attention since 1933 when it was possible to

characterized a believing variety of POM structure and the first X-ray crystal structure

analysis of the Keggin anion, [PW12O403-], appeared in the literature.[141] Most of the

chemical and physical properties of POMs have been growing interest for variety

applications, particularly in medicine, homogeneous and heterogeneous catalysis.[18,142]

There are two generic families of polyoxometallates[19]:

(1) the isopoly compounds (isopolyanions or isopolyoxometalates) contain only d0

metal cations and oxide anions

(2) the heteropoly compounds (called heteropolyanions, heteropolyoxometalates, or

heteropolyacids, when contain in the structure H+, H3O+, H5O2

+) contain one or

more p-, d- or f-block “heteroatom” in addition the other ions.

As heteropolyanions are more numerous and their structural and electronic properties

are easier to modify synthetically than those of the isopolyanions, the former is a field

of increasing importance (particularly, in acid catalysis).

3.1 Structure of heteropoly compounds (heteropolyacids)

Heteropolyacids (HPAs) are complex proton acids that incorporate

polyoxometallates anions (heteropolyanions) having metal-oxygen octahedral as basic

structure units.[18] Their general formula may be presents as [XxMmOy]q- (x < m) for

heteropoly anions. M is usually Mo or W and to a lesser extent V, Nb, or Ta. The

heteroatom X, can be one of 64 elements that belong to a various groups of the periodic

table except the noble gases.[143]

25

Heteropolyacids can be classified into four different groups according to their

molecular architectures[144,145]:

(1) Keggin (e. g. H3[PW12O40]),

(2) Wells-Dawson (e. g. H7[P2Mo17VO62]),

(3) Finke-Droege (e. g. Na16[Cu4(H2O)2(P2W15O56)2]) and

(4) Pope-Jeannin-Preyssler (e. g. (NH4)14[NaP5W30O110]).

Among the various structural classes, the heteropolyacids comprised of the

Keggin structure (represented by the formula Xn+M12O4n-8), and in particular the

compounds containing molybdenum and tungsten, are the most often studied due to

their acids and redox properties, stability at elevated temperatures, availability and

relative ease of synthesise what is the most important in catalysis.[16] In 1864 Marignac

observed two isomeric forms of the H4SiMo12O40, now know as the α- and β-

isomers.[146] In 1933 Keggin reported an the structure the α-isomer of [PW12O40]3-.[147]

The α-Keggin anions are know with a wide range of heteroatoms (for example, X =

Al(III), Si(IV), P(V), Fe(III), Co(III), Cu (I) and Cu (II) for M=Mo and W. β-isomers

are much less common and the structure of β-[SiW12O40]4-, was first determinated by

crystallography in 1973 by Yamamura and Sasaki.[146]

Figure 9 illustrates two views of the α-Keggin structure. The Keggin anions has

a diameter of ca. 1.2 nm and is composed of a central tetrahedron XO4 surrounded by

12 ege- and corner–sharing metal-oxygen octahedra MO6. The octahedra are arranged in

four M3O13 groups, one of which is shown in Fig. 1A. Each group is formed by three

octahedra sharing edges and having a common oxygen atom which is also shared with

the central tetrahedron XO4 to give molecular symmetry. The metal atoms M occupy the

centres of distorted octahedra with one terminal M-Ot bond. There are four types of

atoms in the Keggin anion: 12 terminal M=Ot, twelve edge-bridging angular M-Oc-M

shared by the octahedra within a M3O13 group, 12 corner-bridging quasi-linear M-Oe-M

connecting two different M3O13 groups, and 4 internal X-Op-M.[148] The β-isomer

structure may be considered as derived from the α-structure by rotation of one M3O13

group by 60o about its threefold axis.

26

Op

X

A B

Fig. 9. The α-Keggin (primary) structure shown (A) as a combination of [M3O13] groups and

(B) as individual bonds showing the distorted octahedral geometry around each metal.[28,146]

The examples of the Keggin type heteropolyacids:

H3PW12O40 (PW12) 12-phosphotungstic acid

H3PMo12O40 (PMo12) 12-phosphomolybdic acid

H4SiW12O40 (SiW12) 12-silicotungstic acid

H4SiMo12O40 (SiMo12) 12-silicomolybdic acid

Generally, heteropolycompound (heteropoly acids and their salts) form ionic

crystals composed of heteropolyanions (primary structure), countercanions (H+, H3O+,

H5O2+), hydratation water, and other molecules (Fig. 10). This three – dimensional

arrangement is the secondary structure (pseudoliquid).[149] A relatively stable form of

hydrated HPA contains six water molecules per Keggin unit (KU). The hexahydrate has

a body-canter cubic structure with Keggin units at the lattice points and H5O2+ bridges

along the face. Each of 12 terminal oxygen atoms of the Keggin unit is bond to

hydrogen atom of an H5O2+ bridge.

The secondary structure of HPAs depends on the amount of hydration water and

heteropolyanion and can change from flexible to solid. Fro example 12-

tungstophosphoric acid (H3PW12O40) exhibits different packing arrangements as the

hydration water is lost.[148] This water can be easily removed on heating, whereby the

acid strength is increased due to the dehydration of protons. This a reversible process

accompanied by chancing the volume of crystal cell. Unlike the rigid network structure

of zeolites, in HPA crystal the Keggin anions are quiet mobile. Not only water but also

27

a variety of polar organic molecules can enter and leave HPA crystal. Such structural

flexibility is important when using HPA as a heterogeneous catalyst.

Fig. 10. Schematic diagram illustrating the structure of the hexahydrate H3PW12O40·6H2O as

two interpenetrating simple cubic structures. The H+ of an H5O2+ species coordinated to the

body centered KU is located at the midpoint of an edge of the conventional cubic cell. The KUs

are shown in polyhedral representation.[150]

The tertiary structure is the structure of heteropolyacids as assembled. The size

of particles, pore structure, surface area, and distribution of protons in the particles are

the elements of the tertiary structure.[14]

3.2 Properties of heteropolyacids

Heteropolyacids (HPAs), also know as polyoxometalates (POMs), are early

transition metal oxygen anion clusters that exhibit a wide range of molecular size,

compositions and architectures.[16,151]

A most attractive attribute of heteropoly compounds is the size dependence of their

physicochemical properties. Most notable is the size-dependent tendency of the metal-

oxygen framework to accommodate excess electrons.[140,152] Heteropoly compounds are

commonly strong oxidizing agents, which are readily reversibly reduced by addition of

various specific numbers of electrons depending upon the pH and the potential

employed. The ability of heteropoly compounds to accept electrons under alteration of

the optical properties can be use for the construction of functional electrooptical

materials.[153,154] The reduction products, which typically retain the general structures of

their oxidized parents, are characteristically deep blue in colour and comprise a vary

large group of complexes known as the “poly blue”, or “heteropoly blue” if the

28

framework includes heteroatoms.[155] Heteropoly blues corresponds to class II systems

in the Robin and Day classification of mixed-valence compounds. The added (“blue”)

electrons are delocalized over certain atoms or regions of the structure. The electron

delocalization is viewed as operating through two mechanisms[156]:

a thermally activated electron hopping processes from one addendum (e. g. Mo

or W) atom to the next,

a ground-state delocalization presumably involving π-bonding through bridging

oxygen atoms from reduced metal atom to its neighbours.

The heteropoly blue provide important potentialities as specialized reducing agent, with

a wide range of controllable reduction potentials. Heteropoly blue are use as

electroreduction catalyst. By fixing the potential, one fixes the heteropoly blue species

involved and thus controls the number of electrons per reduction event.[147]

At the lattice energies of heteropoly compounds are low, and so are the salvation

energies of heteropoly anions, the solubility of heteropoly compounds largely depends

on the salvation energy of the cation. Heteropolyacids (HPAs) are extremely soluble in

water and oxygen-containing organic solvents such as lower alcohols, ethers, ketones,

ect. On the other hand, they are insoluble in nanpolar solvents such as benzene.[148] The

Keggin anions may be consider a nanoscale, isolated particle within an aqueous matrix,

which allows classification of Keggin type heteropolyacids as a strongly acidic particle

hydrate.[149,157] In aqueous solution H3PW12O40 (PW12), H4SiW12O40 (SiW12) and

H3PMo12O40 (PMo12) are strong fully dissociated acids, and there are stronger then the

usual mineral acids such as H2SO4, HCl, HNO3, ect. (Table 5).[18]

Table 5. Dissociation Constants of Heteropolyacids in Acetone at 25oC.[18]

The acid strength of crystalline heteropolyacids decrease in the series PW12 > SiW12 >

PMo12 > SiMo12 which is the same to that in solution (Table 5). The tungsten acids are

markedly stronger than molybdenum ones. Although, the effect of the central atom is no

29

as great as that of the addenda atoms, phosphorus-based heteropolyacids are slightly

more acidic than silicon based heteropolyacids.

A strong polarization of the outer electrons of the surface oxygen towards the

Mo6+ ions gives a strong Brønsted acidity to all surface oxygen, particularly the terminal

oxygen (Ot). As a result, the equilibrium reaction is shifted strongly the right to create a

high concentration of protons in hydrogen-bonded aqueous matrix with which the

anions are located (Equation 32).[149]

(32)

However, when the HPA has been fully dehydrated, the location of the protons is not as

easily defined. The terminal (M=Ot) and bridging (M-O-M) atoms of oxygen are the

most probable positions for the protons. The calculation were proved that the most

energetically favourable site for the acid proton is a bridging oxygen atom.[158]

The heteropolyacids are widely use as acid, redox, and bifunctional catalysts in

homogenous and heterogeneous system because of their high solubility in polar solvents

and fairly high thermal stability in the solid sate.[14,15,18] The thermal stability of

hydrogen forms of heteropolyacids change as follows: H3PW12O40 > H3PMo12O40 >

H4SiMo12O40 > H4SiW12O40.

The ability of heteropolyanions to undergo spontaneous adsorption from

aqueous solution on various electrode substrates[159,160] provides a simple tool for the

modification of the surface.[19-23] Further, their ability to exhibit fast reversible multi-

electron transfers and super-acid properties resulting in the increased availability and

mobility of protons at the electrocatalytic interface. Heteropolyacids have been known

as good matrix for traces of platinum reactive towards reduction of oxygen[19-21].

Because of interaction between heteropolyanions and Pt surface by mainly corner

oxygen (from heteropolyanions) only a few percent of interfacial reactive platinum

atoms would be blocked to the access of oxygen molecules.[21] Another issue concerns

ability of the adsorbed heteropolyanions (particularly H3PW12O40) to shift of

voltammetric peaks referring to the formation of Pt-oxo (PtOH or PtO) species towards

more positive potential and increase of the potential range where platinum is not

30

covered with platinum oxide adsorption and activation of oxygen molecules during

electrocatalysis may be facilitate.[21] Heteropolyanions due to their exceptionally high

solid state protonic conductivity were considering for fuel cell research.[23-25,27,28,161]

Stanis and co – workers have presented that the addition of adsorbed heteropolyacids

can improved the performance of Pt anodes in a fuel cell under CO – poisoned

conditions.[27]

Though, heteropolyacids have been identified to be good candidates as materials

for catalytic applications, the insolubility in water and increasing the surface area of

heteropoly compound are necessary. For this purpose, the mine approach has been

followed in the literature: the direct preparation of acid porous and water-insoluble salts.

3.3 Salts of the heteropoly compounds

Keggin type heteropolyacid salts, described by formula M1xHy-xM

2M312O40,

where M1 is Cs+, Rb+, M2 is P or Si, M3 is W or Mo, x is 2.5 and y is 3 or 4 if M2 is P

or Si, respectively, are produced by partially exchanging protons in the parent

heteropolyacids. They are efficient solid acid catalyst for a variety of organic reaction,

particularly for liquid-phase reaction.[14,18,162]

Salts of heteropoly compounds can be classified into two groups[163]:

(1) group A, the small cation group (like Na+, Cu2+), which posses:

low surface area (1 – 15 m2 g-1),

high solubility in water,

absorption capability of polar or basic molecules in the solid bulk,

(2) group B, the large cations group (like Cs+, Rb+), which is:

with high surface area (50 – 200 m2 g-1),

insoluble in water,

unable to adsorb molecules.

Partial exchange of heteropolyacids with large cations, such as Cs+, Rb+, NH4+, K+, ect.

change a water-soluble acid with low surface area (<5 m2g-1) into a water-insoluble acid

salt precipitates with surface area exceeding 100 m2g-1.[31,32] Further, strong acidity

remains on acids salts what is opposite to alkali-exchanged zeolites.[14,33]

31

The crystal structure of

Cs salts are the same as

the cubic of

H3PW12O40·6H202, with

cations at the sites of H+

(H2O)2 sites, called

second structure, which

corresponds to the

micocrystallites.

Aggregates of the

microcrystallites are

called the tertiary

structures corresponding. [14, 164]

Fig. 11. Hierarchical structure of Keggin

type heteropolyacids salts.[164,165]

Okuhara has presented the change of the surface area with the extent of Cs

substitution for proton in H3PW12O40 (Fig. 12).[162] The pore size of heteropolyacids

salts can be precisely controlled by the cation content. The surface are decrease when

the Cs content, x in CsxH3-xPW12O40 increase from 0 to 2, and then the surface are

increase when x change from 2 (1m2 g-1) to 3 (156 m2 g-1). The author has estimated the

acid amount on the surface (called surface acidity) from the surface area and the formal

concentration of proton attached to the polyanion. Figure 12 illustrates that the surface

acidity decreases at first with the content of Cs, but sharply increases when x exceeds 2.

The maximum appeared at x = 2.5. The Cs2.5H0.5PW12O40 and H3PW12O40 have similar

acid strength.[162,166]

H+ (H2O)n, Cs2+, Cu2+, K+, Mg2+, ect.

Primay structure

Secondary structure

Tertiary structure

W3O13

P

32

Fig. 12. Changes in the surface area and surface acidity of CsxH3−xPW12O40 as a function of the

Cs content.[162]

Okuhara and co – workers have estimated from surface area the particles size of

the heteropoly aids and its salts.[160] The particles size were 2000 Å for H3PW12O40 and

5000-10000 Å for Cs1 (x = 1) and Cs2 (x = 2), respectively. Conversely, particle sizes of

Cs2.5 and Cs3 were 60-70 Å. Morphologies of the CsxH3−xPW12O40 were examined using

scanning electron microscopy (Fig. 13). It was apparent that the surface of Cs2 is

smooth (Fig. 13A). The same observations were made for Cs1. On the contrary, Cs2.5 is

composed of fine particles with the size about 100 Å and the surface are rough (Fig.

13B). As well as the primary spherical particles, pores between particles can be seen for

Cs2.5. The authors obtained the similar SEM image for Cs3 to that of Cs2.5.

Fig. 13. SEM images of (A) Cs2HPW12O40 and (B) Cs2.5H0.5PW12O40.

[160]

It is well know that the pore-width of zeolites can be controlled by the kind of

cation and the pore-width increased as the cation size decreased (Cs > Rb > K).[167] The

salts of Rb and K gave change in the surface area (Fig. 14) similar to that of Cs salts

33

(Fig. 12).[162] A marked increase of the surface area when the Rb content increases was

observed at x = 1.8, while it was detected at 2.1 for the Cs salts. In the case of K salts,

the change of the surface are with the increase of the content of K was rather loose.

Fig. 14. Surface areas of RbxH3−xPW12O40 (A) and KxH3−xPW12O40 (B) as a function of cation

content.[162]

Heteropoly salts are frequently more stable than the parent acid. The relative

stabilities, however, depended on the counteraction.[148] The thermal stability change

generally in the order of Ba2+, Co2+ < Cu2+, Ni2+ < H+, Cd2+ < Ca2+, Mn2+ < Mg2+ <

La3+,Ce3+ < NH4+ < K+, Tl+, Cs+.[14] The acidic cesium salt Cs2.5H0.5PW12O40 is more

stable than H3PW12O40. No decomposition of the salt was observed at 500 0C.

Due to the presence of meso-micropore in the Keggin type heteropolyacids salts

(particularly, where x = 2.5), it is possible to introduce the platinum

nanoparticles.[162,164] The size and dispersion of nanoparticles of Pt can be control by

quantity of platinum in the structure of heteropolyacids salt. Okuhatra and Nakato

reported that the presence of Pt 0.5 wt% in the Cs2.5H0.5PW12O40 did not influence the

pore width of these Cs salts, and the size of Pt was probably less than 10 Å.

The Keggin type heteropolyacids salts (particularly, when cation content is equal

2.5, e.g. Cs2.5H0.5PW12O40) are evidently a vary promising materials as matrix for

catalytic centres (for example Pt nanoparticles) for fuel cell research.

34

4. Experimental techniques

4.1 Cyclic voltammetry

Cyclic voltammerty (CV) is often the first experimental performed in an

electroanalytical study. In particular, it offers a rapid location of redox potentials of the

electroactive species, and convenient evaluation of the effect of media upon the redox

process.[168]

Cycling voltammetry consists of scanning linearly the potential of a stationary

working electrode immersed in a quiescent solution (Fig. 15) and measuring the

resulting current.[168-170]

Fig. 15. Potential – time excitation signal in cyclic voltammetric experiment.[168]

In CV a constant-surface electrode (platinum, gold, glassy carbon, hanging

mercury drop electrode) are used as working electrodes.

During the change of the potential (when the oxidized form, O) from E1 to E2 (in

negative direction), a cathodic current begins to increase, until a peak is reached (Fig.

16). After traversing the potential region in which the reduction process take place, of

the potential sweep is reversed. During the reverse scan, R molecules (generated in the

forward half cycle, and accumulate near the surface) are deoxidised back to O and the

anodic peak results.[168]

35

Fig. 16. Typical cyclic voltammogram for a reversible O + ne- ↔ R redox process.[168]

In cyclic voltammetry for the reversible systems the position of the peaks on the

potential axis (Ep) is related to the formal potential of the redox process. The formal

potential for a reversible couple is centered between the anodic peak potentials (Ep,a)

and the cathodic peak potentials (Ep,c)[168]:

2

EEE p,cp,ao +

= (33)

The separation between the peak potentials (for reversible couple) is given by

Vn

0.059EE∆E p,cp,ap =−= (34)

Thus, the peak separation ca be used to determinate the numbers (n) of electrons

transferred, as a criterion for a Nernstian behaviour. Accordingly, a fast one-electron

process exhibits a ∆Ep of about 59 mV. Both the cathodic and anodic peak potentials are

independent of the scan rate. For irreversible process, the individual peaks are reduced

in size, widely separated and depended of the scan rate.

36

The peak current (ip) for a reversible electrode process is presented by the

Randles-Sevcik equation[168,169]:

ip = 2.72 x 105n3/2D1/2AV1/2c0 (35)

It follows from Eq. 35 that the peak current depends on the concentration of the

depolarized in the bulk of the solution (c0), the diffusion coefficient of the substance

being reduced or oxidized (D), the area of the electrode surface (A), and the number of

electrons taking parts in the elementary electrode process (n). Furthermore, the current

increase with the increasing polarisation rate (V). The liner dependence of the peak

current on the concentration of the reacting substance makes this method useful in

quantitative analysis. The peak dependent on the square root of the scan rate, and the

liner dependents means that the value of current is control by diffusion of electroactive

substance to the electrode surface.

Different behaviour is observed when the reagent or product of an electrode

reaction is adsorbed strongly or weakly on the electrode. The separation between the

peak potentials is smaller than expected for solution phase process. The ideal Nernstian

behaviour of surface-confined nonreacting species is manifested by symmetrical cyclic

voltammetric peaks (∆Ep = 0), and a peak half-width of 90.6/n mV (Fig. 17).

Fig. 17. Ideal cyclic voltammetric behaviour for a surface layer on an electrode. The surface

coverage, Γ, can be obtained from the area under the peak.[168]

37

The peak current is directly proportional the surface coverage (Γ) and potential scan

rate[168]:

RT

AFni p 4

22 νΓ= (36)

The surface coverage, Γ, can be calculate from the peak area (i.e., the quantity of charge

consumed during the reduction or adsorption of the adsorbed layer):

Q = nFAΓ (37)

This can be used for calculation the area occupied by the adsorbed molecule and hence

to predict its orientation of the surface.

In practices, the ideal behaviour is approached for a relatively slow scan rate, and for

adsorb layer that show non intermolecular interaction and fast electron transfer.[168]

The cyclic voltammetry method was used in the thesis for initial electrochemical

studies of Keggin type heteropolyacids salts and examination of proposed catalysts

towards oxidation of hydrogen, methanol and ethanol.

4.1.1 Rotating disk voltammetry

The rotating disk electrode (RDE) is vertically mounted in the shaft of the

synchronous controllable-speed motor and rotated with the constant angular velocity (ω)

about an axis perpendicular to the plan disc surface (Fig. 18).[168]

Fig. 18. Rotating disc electrode.[168]

38

The primary advantages gained by the utilization of electrochemical techniques based

on rotated electrodes is the precise, quantitative control of mass transport to the

electrode through forced convection included by electrode rotation.[171] This is possible

because the motion of a RDE drags a layer of fluid near the disc surface along with it as

rotates. At the same time the liquid layers is subjected to centrifugal forces that cause to

move rapidly away from the rational axis of the electrode describing an S-shape path as

it does so. As a consequence of fluid motion parallel to the disk surface, new liquid is

drawn to the disc along the path that is parallel to the rotational axis of the electrode.

According to the simple Nernst diffusion layer concept, a thin layer of stagnant solution

is present at the electrode surface, within which the concentration of the electroactive

species that is undergoing oxidation or reduction varies linearly from its value in the

bulk solution to a new value at the electrode surface. The current, i, is given

approximately by the expression[171]:

δ)( ccnFAD

io −= (38)

where A is electrode area, δ is the thickness of diffusion layer, F is the Faraday

constant, D is the diffusion coefficient, n is the number of electrons taking parts in the

elementary electrode process, co and c are the concentration of the electroactive species

in the bulk solution and at the electrode surface, respectively. For RDE in a solution

with the kinematic viscosity, ν, Levich presented that the diffusion layer thickness is

dependent on the inverse square root of the angular velocity ω of the rotating electrode:

δ = 1.61D1/3ν1/6ω-1/2 (39)

The limiting current il (for a reversible system) is thus proportional to the square root of

the angular velocity, as described by Levich equation[169]:

i l = 0.62nFAD2/3ω1/2ν-1/6co (40)

An increase in ω from 400 to 2500rpm thus results in a twofold increase of the signal. A

deviation from linearity of a plot of il vs. ω-1/2 suggests some kinetic limitations.

39

Since 1960 the rotating disc technique has been employed in the study of the

kinetics of electrode process, as well as of that of chemical reactions taking place at the

electrode surface.[169]

In the thesis rotating disc voltammetry was applied to study the kinetics of

hydrogen oxidation reaction (Chapter 7.2.1 and 7.3.4).

4.2 Chronoamperometry

Chronoamperometry[168] involves stepping the potential of the working electrode

from a value at which no faradic reaction occurs to a potential at which the surface area

concentration of the electroactive species is effectively zero (Fig. 19a). A stationary

working electrode and unstirred solution are used. The resulting current – time

dependence is monitored. As mass transport under these conditions is solely by

diffusion, the current – time curve reflects the change in the concentration gradient in

the vicinity of the surface. This involves a gradual expansion of the diffusion layer

associated with the depletion of the reactants, and hence decreased slope of the

concentration profile as time progress (Fig. 19b).

The current corresponding to the transformation of Ox in to Red, for linear

diffusion of reactants to the electrode, change with the time according to the Cottrell

equation[169]:

2/12/1

02/1

t

AcnFDi OxOxl π

= (41)

where, iL is the limiting current, i.e. the maximum current that can be obtained under the

given conditions. Its value depends on the bulk depolarized concentration (C0ox), the

diffusion coefficient (Dox), the electrolysis time (t), and the electrode surface area (A). F

is the Faraday constant, and n is the number of electrons exchanged between one ion or

molecule of a reactant an electrode. According to equation 41 the current of

chronoamperometric electrocatalysis tends to zero when the time tends to infinity. This

is due to the progressive decrease of the reactant concentration in the region close to the

surface, and change the current with time corresponds to the curve illustrates in Fig.

19c.

40

Fig. 19. Chronoamperometric experiments: (a) potential time waveform, (b) change of

concentration profiles with time, (c) the resulting current – time response.[168]

In the thesis the chronoamperometry method was used to evaluate the reactivity

of our electrocatalytic system for the methanol (Chapter 8.2.3, 8.3.3) and ethanol

(Chapter 9.3) oxidation.

4.3 Staircase voltammetry

Staircase voltammetry[168,172] has been proposed as a useful tool for rejecting

background charging current. The potential – time waveform involves successive

potential step (∆E) of c.a. 10 mV height and about 50 ms duration (tp) (Fig. 20). The

current is sampled at the end of each step, where the charging current has decayed to a

negligible value. This method of polarization of the electrode enable the double layer to

be charged, making it possible to discriminate capacitive current component, if the

current is measured at a sufficiently long time after application of the pulse. Such a

method of polarisation and current sampling should give current vs. potential curves

that are similar to those recorded in linear scan voltammetry.[169]

41

Fig. 20. Potential – time waveform used in staircase voltammetry.[168]

In the thesis staircase voltammetry method was applied to get insight into the

system’s reactivity towards oxidation of methanol and ethanol (Chapter 8.2.2, 8.3.2,

9.2).

4.4 Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy is an experimental technique that

involves imposing a small sinusoidal (AC) voltage or current signal of knows amplitude

and frequency - the perturbation – to an electrochemical cell and monitoring the AC

amplitude and phase response of the cell. The AC perturbation is typically applied over

a wide range of frequencies, from 10 kHz or greater to less than 1 Hz, hence the name

impedance spectroscopy. The ratio and phase-relation of the AC voltage and current

signal response is the complex impedance, Z (jω). The results of an impedance

spectroscopy experiment is a rich data set from which many properties of the

electrochemical cell may be extracted via application of physically – responsible

equivalent circuit models.[36,173] Properties of the electrochemical system commonly

evaluated using impedance spectroscopy include ohmic (bulk) resistance, electrode

properties such as charge transfer resistance and double layer capacitance, and transport

(diffusion) effects.

An electrochemical impedance experiments is based on monitoring the AC

response of an electrochemical cell that results from imposing a small AC signal (Fig.

21). The impedance is the ratio of the AC voltage and current output.

42

Fig. 21. Schematic of an electrochemical impedance spectroscopy.[36]

A sinusoidal current signal of amplitude IAC (amps) and frequency ω

(radiations/seconds) can be defined:

I(ω) = IAC·sin (ω·t) (42)

where t is time (s). The output AC voltage signal from the electrochemical cell can be

defined:

V (ω) = VAC·sin[(ω·t)-Ө] (43)

where VAC is the amplitude for the output voltage signal (volts) and Ө is the phase

angle (radians). The phase angle is the difference in the phase of a sinusoidal voltage

and current signals. In the case of AC signal, the “resistance” of circuit of the

electrochemical device, which is not purely resistive, will be function of the frequency

of oscillation of the input signal. Ohm’s Law for the AC case is expressed:

Z(jω) = V(jω) / I(jω) (44)

where Z (jω) is the complex impedance (Ω) and j is imaginary operator,

j ≡ √-1 (45)

Equation (44) indicates that impedance is a complex value. That is, it can take on both

real and imaginary components. Note that the imaginary component of the impedance is

a real measurable quantity: “j” is for bookkeeping purposes and allows description of

the out-of-phase component of the impedance. The complex relationship of impedance

is implicit so Z(jω) is normally written as Z(ω). Although one can think of impedance as

“resistance” to current, it is more general than that because it takes into account the

phase difference between voltage and current. Equation (44) also indicated that

43

impedance depends on the frequency at which it is measured. Z can change as the

frequency of the AC signal changes.

Frequency in cycles per second, f (Hz = 1/s), is obtained through the relation:

ω = 2π·f (46)

Equation (44) can be written in complex notation:

Z = Z´ + Z´´ (47)

where,

Z´ = Re(Z) = |Z|cosӨ real (in-phase) component of impedance (48)

Z´´ = Im(Z) = |Z|sinӨ imaginary (out-of-phase) component of impedance (49)

22 )()( ZZZ ′′+′= magnitude of impedance (50)

and,

Ө = tan-1 (Z´´/Z´) (51)

In the impedance spectroscopy experiment, the frequency of the AC perturbation

is swept over a range, from ~ 10 kHz to less than 1 Hz and the impedance is evaluated

as a function of frequency to evaluate the properties of the electrochemical system

under investigation. Usually the real (Z´) and imaginary (Z´´) parts of the impedance are

plotted as a function of frequency.

In the thesis the AC-Impedance was used to analyzed reaction mechanism of

methanol and ethanol electrooxidation on Pt40%/Vulcan XC-72 carbon modified with

Keggin type heteropolyacids salts (Chapter 8.2.4, 8.3.4, and 9.4).

4.5 Transmission electron microscopy

The fathers of electron microscopy were Knoll and Ruska (1931), and the first

commercial TEM was built in 1939 by Siemens. Since then, the theory and the

instrumentation have developed and modern TEM’s have become a fundamental tool

for material science.[174]

44

A TEM microscope is composed by an optical column, operated under high

vacuum that enclose[175]:

Illumination system. It takes the electrons from

the gun and transfers them to the specimen

giving either a broad beam or a focused beam. In

the ray-diagram, the parts above the specimen

belong to illumination system.

The objective lens and stage. This combination

is the heart of TEM.

The TEM imaging system. Physically, it

includes the intermediate lens and projector lens.

Fig. 22. Schamatic of a Transmition Electron Microscope.[175]

The diffraction pattern and image are formed at the back focus plane and image plane of

the objective lens. If we take the back focus plane as the objective plane of the

intermediate lens and projector lens, we will obtain the diffraction pattern on the screen.

It is said that the TEM works in diffraction mode. If we take the image plane of the

objective lens as the objective plane of the intermediate lens and projector lens, we will

form image on the screen. It is the image mode.

TEM is applied to analysis of electrochemical power sources, since novel

electrode materials for electrochemical energy storage devices are currently synthesized

with constantly smaller dimensions, down to nanosize level. This is very important in

catalysts used for fuel cells, since smaller particles have higher specific surface and

catalytic activity. Control of morphology, granulometry and microstructure at

nanometric level is, thus, very important in predicting or explaining the performances

obtained by the different electrodes or preparation methodologies.

In the thesis TEM was used to investigate size of Pt nanoparticles obtained by

corrosion of platinum counter electrode (Chapter 7.3.3).

45

4.6 Scanning electron microscopy

In Scanning Electron Microscopy (SEM), a high energy (up to 50 keV), very

thin electron beam is finely focused over a sample and swept in a raster across the

surface. The electron beam/sample interactions cause the emission of different signals

that are collected by specific detectors and converted into an image of the sampled area

and viewed or recorded on a cathode ray tube (CRT). The generated signals include

secondary electrons, backscattered and Auger electrons, photons of various energies and

characteristic X-rays. The signals of greatest interest for surface topography are

secondary and backscattered electrons.[174] The basic components of the SEM

microscope are presented in Fig. 23.

Fig. 23. Schematic of operation a typical Scanning Electron Microscope.[174]

The SEM permits the observation of materials in macro and submicron ranges. When

SEM is used in conjunction with EDS (Energy Dispersive X-ray Spectrometer) the

analyst can perform an elemental analysis on microscopic sections of the material or

contaminants that may be present.

46

In the thesis the scanning electron microscopy was used to investigate

morphology of the Keggin types heteropolyacids salts (Chapter 6.3).

4.7 Infrared spectroscopy

Infrared spectroscopy (IR) involves examination of the twisting bending,

rotating and vibrational motions of atoms in a molecule.[170]

Molecules contain bonds of specific spatial orientation energy. These bonds are

seldom completely rigid, and when is supplied, they may band, distort or stretch. A vary

approximate model compares the vibration to that of a harmonic oscillator, such as an

ideal spring. If the spring has a force constant, k, and masses mA and mB at the end, than

the theoretical vibration frequency ν is given by[176]:

ν = (1/2π)√(k/µ) (52)

where µ = mA · mB / (mA + mB) is called the reduced mass.

Each type of molecular vibration is characterized by a vibrational quantum

number, υ. For a simple stretching vibration, there is a series of levels whose energy is

given by approximately by:

E = hν0·(υ + ½) (53)

This means there is a set of levels spaced energy by hν0 or in wavenumber by ν 0. The

selection rule for an ideal harmonic oscillator allows transition where ∆υ = ± 1, giving a

single, fundamental vibrational absorption peak.

However, when the bonds are stretched they weaken, so better model takes this

into account, and the molecules are treated as anharmonic oscillators. Thus, where high

energies are involved, larger energy transition may occur, where ∆υ = +2, +3, ect.

giving the first overtone at wavenumber approximately double that the fundamental,

and so on.

The electric field of incident radiation interacts with the molecular dipole. When

the frequency of the radiation (~1013 Hz) resonates with a molecular vibration,

absorption can occur, particularly if excitation of that vibration has an effect on the

47

molecular dipole moment. The energy changes involved in exciting vibrational modes

in this way correspond to the infrared spectral region. A full infrared spectrum consists

of bands (group frequencies), assignable to particular moieties (e.g. -CH2-, -CH3, C=O),

in characteristic frequency regions that are relatively independent of the other groups in

the molecule. Since infrared spectroscopy probes molecular vibrations that involve

changes in the dipole moment, the vibrations of polar molecular bonds generally

correspond to strong infrared bands.

Most IR spectra are recorded by measuring the absorption of the incident

radiation as a function of wavelength (wavenumber) in the range of 2.5 - 25 µm (4000 -

400cm-1) by solid, liquid or gaseous samples. Both qualitative and quantitative

information can be obtained from vibrational spectra. The use of IR spectrometry for

qualitative measurements is extensive and wide ranging, and for this purpose

transmission spectra are conventionally recorded as function of wavenumber. In order

to make quantitative measurements, it is necessary to convert the transmittance readings

to absorbance, A, the relation between the two being[176]:

A = log (100 / T%) (54)

In the thesis infrared spectroscopy was used as an established tool for the

structural characterization of heteropolyacids salts (Chapter 6.2).

48

5. Chemical reagents and measuring equipment

A) Chemical reagents using in the measurements:

Phosphotungstic acid hydrate, H3PW12O40·nH2O (PW12), Sigma-Aldrich

Phosphomolybdic acid hydrate, H3PMo12O40 nH2O (PMo12), Sigma-Aldrich

Tungstosilicic acid hydrate, H4SiW12O40·nH2O (SiW12), Sigma-Aldrich

Silicomolybdic acid hydrate, H4SiMo12O40·nH2O (SiMo12), Sigma-Aldrich

Cesium nitrate, CsNO3, Aldrich

Rubidium nitrate, RbNO3, Aldrich

Ammonium Chloride, NH4Cl, Carlo Ebra

Potassium chloride, KCl, Aldrich

Sulphuric acid 99,999%, H2SO4, Aldrich

Methanol 99, 9%, CH3OH, J. T. Baker

Ethanol 99, 9%, C2H5OH, J. T. Baker

Nafion solution 5% wt, Ion Power, Inc

Pt10% on Vulcan XC-72 (Pt/C), E-tek

Pt40% on Vulcan XC-72 (Pt/C), E-tek

Vulcan XC-72, E-tek

High purity argon, carbon monoxide, hydrogen, oxygen gas

Ultra-pure water (Millipore Milli-Q)

B) Equipments used during performed experiments:

IR spectra in the range 1800 to 500 cm-1 were recorded with a Perkin

Elmer System 2000 FT-IR instrument.

The morphology of platinum particles was monitored using a JEM-

2100F electron microscope (TEM) operating at 200 kV.

The morphology of heteropolyacid salts was monitored using a JOEL

Model JSM-5400 scanning electron microscope (SEM).

Electrochemical measurements were carried out by CHI 660B

electrochemical working station (CH Instruments Inc.).

49

RDE voltammetric measurements were done using a variable speed

rotator, Pine Instruments, USA.

For the electrochemical impedance studies, the data were collected by

CHI 660B with a frequency sweep of 0.05 Hz to 100 KHz and ac sine

wave amplitude of 5 mV.

The PEMFC test stand consists of fuel cell hardware, backpressure

regulators, water traps, a humidifier and mass flow controller. The fuel

cell hardware (Fuel Cell Technologies Inc.) is a single cell with area 4.84

cm2 and single-serpentine flow fields.

C) Electrodes applied during the measurements:

Glassy carbon electrode (as working electrode), GC (area 0.071cm2), CH

Instruments, Inc.

Rotating disc (glassy carbon) electrode (as working electrode), RDE

(area 0.1256cm2), Pine Researcher Instrumentation.

A saturated calomel electrode (as references electrode), SCE, Amel

Electrochemistry

Platinum flag (as counter electrode, area ca. 1cm2)

Nafion Membrane 115 (as solid electrolyte in PEM cell), Ion Power, Inc.

Low-temperature ELAT® GDL microporous layer on woven web, tin

configuration (as diffusion layer in electrode for fuel cell), E-tek

D) Experimental conditions:

The measurements were carried out in a three-electrode cell with a

platinum flag as a counterelectrode and a saturated calomel electrode

serving as the reference electrode, placed in a separated compartment

and connected to the main cell by a Luggin capillary.

CO stripping voltammetry study: prior to electrochemical measurements

0.5 mol dm-3 sulphuric acid solution was purged with argon for 30

minutes. Subsequently, five consecutive CV (scan rate 20 mV s-1) were

performed in the potential range 0.025 – 1.125 V vs. RHE. For CO

50

stripping measurements, pure CO was bubbled into the electrolyte for 10

minutes and then its adsorption on the electrode was driven under

potential control at 0.1 V vs. RHE for 4 minutes. The electrolyte was

purged for 35 minutes with argon, keeping electrode potential at open

circuit potential (OCP) to eliminate CO reversibly adsorbed on the

surface. Five cyclic voltammetry (scan rate 20 mV s-1) were recorded

from 0.025 to 1.125 V versus RHE. The first anodic sweep from 0.025 to

1.125 V vs. RHE was performed to electro-oxidize the irreversibly

adsorbed CO and the subsequently voltammetries in order to verify the

completeness of the CO oxidation.

At the beginning of experiments, each catalytic layer was cycled

continuously (san rate, 50 mV s-1) through the potential region from 0 to

1.05 V vs. RHE until a steady state voltammogram was obtained

Before experiment working electrode was polished (on a cloth) with

Al 2O3 water suspension, particle size 5 - 0.05 µm.

All the electrochemical experiments were carried out at 22 ±2 0C (except

PEMFC measurements).

All potentials in these studies were reported here versus RHE.

51

6. Preparation and characterization of Keggin-type matrix

This thesis is focused on applying salts of Keggin-type heteropolyacids as a

material for the new stable, water-insoluble matrix for the films containing Pt

nanoparticles. From several possible salts of Keggin-type heteropolyacids those

containing 2.5 moles of Cs+, Rb+ and NH4+ cations in 1 mole of the heteropolyacids salt

have been chosen. Advantages of these salts are the highest surface acidity, high ionic

conductivity, water insolubility and high surface area characterized by the presence of

meso and micro pores.

6.1 Preparation of Keggin-type heteropolyacid salts

Keggin-type heteropolyacids salts were prepared by adding stoichiometric

amounts of 0.25 mol dm-3 aqueous solution of CsNO3, RbNO3 or NH4Cl, to the desired

volume of 0.1 mol dm-3 aqueous solution of desired heteropolyacid[177-180], using a

molar ratio M/Y = 2.5, (where M = Cs+, Rb+ or NH4+ and Y = P or Si). The suspension

was then stirred for 24 h. The precipitates was washed four times with ultra pure water,

separated from the liquid phase by centrifugation and freeze dried (40 - 75 0C).

6.2 IR characterization of Keggin-type matrix

IR spectra of H3PW12O40 (HPW), Cs2.5H0.5PW12O40 (Cs2.5PW) and

Rb2.5H0.5PW12O40 (Rb2.5PW) are shown in Fig. 24A. Four bands at 700-1100 cm-1

region corresponding to Keggin unit structural vibrations[181,182] are observed for all

samples what suggesting that the framework of the primary Keggin structure remained

unaltered. The origin of the Keggin anion vibration bands is as follows: at 1077 cm-1 is

from the νas (P-Oa) vibration; 971 cm-1 is due to the terminal νas (W=Od) vibration; at

883 and 756 cm-1 should be assigned to νas (W-Ob-W) and νas (W-Oc-W), respectively.

Weaker absorptions appearing at 592 and 523 cm-1 are due to bending vibrations of the

type δ (Oa-P-Oa) and νs (W-O-W), respectively.[183] The absorption at c.a. 1697 cm-1 is

indicative of the presence of the protonated water clusters, probably proton-type H5O2+,

and it is assigned to δ (H2O) vibration.[178,184] The proton substitution with Cs+ or Rb+

ion to form M2.5H0.5PW12O40 (M = Cs+, Rb+) causes a decrease of the intensity of the δ

52

(H2O) peak at 1697 cm-1. Absorption band detected at 1620 cm-1 for cesium and

rubidium salt of 12-phosphotungstic acid can be attributed to the presence of neutral

water.[178]

Fig. 24B shows the IR spectra of 12-phosphomolybdic acid, H3PMo12O40

(HPMo) as well as its cesium and rubidium salts. There is no apparent difference

between the three spectra. All showed bands at 1060, 961, 875, 760 cm-1 that are in

agreement with those reported in the literature.[22,185-187] A shift in the frequency in the

oxygen Mo-Ob-Mo stretching mode from 875 cm-1 (for HPMo) to 865 cm-1 (for

Cs2.5H0.5PMo12O40 or Rb2.5H0.5PMo12O40) is probably related to the organization and in

particular to the presence of positively charged cation in the material. The absorption at

1610 cm-1 is related to the presence of neutral water.[178]

The IR spectra recorded for Fig. 24C shows the 12-silicotungstic acid,

H3SiW12O40 (HSiW) and its cesium and rubidium salts, while Fig. 24D shows the IR

spectra of 12-silicomolybdic acid, H3SiMo12O40 (HSiMo) and their cesium and

rubidium salts. The band around 1700 cm-1 (Fig. 24C) for HSiW is due to oxonium ions

(H3O+) or more likely to dioxonium ions (H5O2

+).[188,189] As in the case of HPW, proton

substitution with Cs+ or Rb+ causes a decrease of the intensity of the peak at 1700 cm-1.

The absorption band at c.a. 1624 cm-1 is indicative of the presence of “neutral” water.

From the spectrum of HSiW and their cesium and rubidium salts in the 700-1100 cm-1

region, we can obtain information on the bonds between Keggin units.[190] The bands at

ca. 730 and 871 cm-1 were assigned to the stretching of tungsten-oxygen-tungsten

chains, the former to W-Oc-W, and the latter to W-Ob-W. The Oc oxygen atom is

common for two [WO6] octahedra in [W3O10] subunits, joined by Ob atoms. The band at

ca. 905 cm-1 was assigned to Si-Oa stretching (there are four Oa atoms connected to the

central Si atom). The absorption at 970 cm-1 is related to the W=Od bond. The

assignments of the bond at ca. 1020 cm-1 is not known.

In the spectra of HSiMo (Fig. 24D) and its cesium and rubidium salts, we can

see four bands at 700-1100 cm-1 region corresponding to Keggin unit structural

vibrations. This means that, also in this case, the structure of the salts still retains the

basic framework of the Keggin structure.

53

6 008 001 00 01 2001 4001 6001 800

5 23

52 3

883

W a venu m be r / cm -1

H3P W

12O

40

10 77

97 18 83

75 6

1 697

9 757 60

59 2

59 2

Tra

nsm

ittan

ce

C s2 .5

H0.5

P W12

O40

10 77

52 3

592

R b2.5

H0.5

P W12

O40

A

16 20

1 62 0

1 07 7

97 5

88 3

76 0

60080010001200140016001800

961 591760

W avenumber / cm -1

H3PMo

12O

40

1610

1060

961

875

760590

1060

961

865

1610

591

760

Tra

nsm

ittan

ce

Cs2.5

H0.5

PMo12

O40

1060865

1610B

Rb2.5

H0.5

PMo12

O40

60080010001200140016001800

1021

910

871

W avenum ber / cm -1

H4S iW

12O

40

1700

727

871905

970

970

910735

T

rans

mitt

ance

C s2.5

H1.5

S iW12

O40

735

1020

1020

1624

970871

C

R b2.5

H1.5

SiW12

O40

1624

60080010001200140016001800

900851

740

W avenum ber / cm -1

H4SiM o

12O

40

740

851901

9641610

1612

959

900851

740

T

rans

mitt

ance

C s2.5

H1.5

S iMo12

O40

1614

959

D

R b2.5

H1.5

SiM o12

O40

Fig. 24. IR spectra of the: A) H3PW12O40 and their cesium and rubidium salts (Cs2.5H0.5PW12O40,

Rb2.5H0.5PW12O40), B) H3PMo12O40 and their cesium and rubidium salts (Cs2.5H0.5PMo12O40,

Rb2.5H0.5PMo12O40), C) H4SiW12O40 and their cesium and rubidium salts (Cs2.5H1.5SiW12O40,

Rb2.5H1.5SiW12O40) and D) H4SiMo12O40 and their cesium and rubidium salts

(Cs2.5H1.5SiMo12O40, Rb2.5H1.5SiMo12O40) recorded at ambient temperature.

54

We can conclude from the results that the primary Keggin structure remains

unaltered even when the protons form parental heteropolyacid are substituted by the

cesium or rubidium cations. The peak at ca. 1700 cm-1 in the HPW and HSiW IR

spectra is due to the presence of dioxonium ions (H5O2+) and their relative intensity

decay is observed upon proton substitution by Cs+ or Rb+ cation. Such pattern is

observed here because the hexahydrate structures isomorphous with H3PW12O40·6H2O

also exist in other dodecaheteropolyacids, like tungstosilicic acids in which only three

of four protons are forming dioxonium ions. Thus their proper formula is

(H5O2+)3(HSiW12O40

3-) as described in the literature.[190]

6.3 SEM characterization of Keggin-type matrix

Scanning electron micrographs (SEM) of pure 12-phosphotungstic acid, 12-

phosphomolybdic acid, 12-silicotungstic acid, 12-silicomolybdic acid and their cesium

and rubidium salts are shown in Fig. 25, Fig. 26, Fig. 27 and Fig. 28, respectively. The

SEM image of pure phosphotungstic acid (HPW) reveals the presence of a (Fig. 25A)

mixture of small (micro size) crystals together with few larger crystals.[191,192] The SEM

micrograph of Cs2.5H0.5PW12O40 (Fig. 25B) shows that the bulk salt consists of

agglomerates of smaller nanocrystals. The nanosize spherical aggregates of bulk

Rb2.5H0.5PW12O40 (Fig. 25C) appear to be bigger than those of Cs2.5H0.5PW12O40.

Fig. 26A shows the SEM micrograph of the pure phosphomolybdic acid (HPMo)

in which arrays of uniformly small crystals are observed. The SEM micrograph of pure

HPMo suggests a more uniform crystal texture as compared to that of pure HPW. The

presence of Keggin structure is not so prominent in HPMo as compared to HPW

because molybdenum (Mo) atom is of smaller size, Cs2.5H0.5PMo12O40, when compared

to tungsten (W). Both salts consist of aggregates composed of small spherical

nanocrystals grains that appear to be smaller for Cs2.5PMo12 than for the analogous

rubidium salt.

55

Fig. 25. SEM micrographs of

A) H3PW12O40 (3000x magnification),

B) Cs2.5H0.5PW12O40 (8000x magnification),

C) Rb2.5H0.5PW12O40 (8000x magnification).

Fig. 26. SEM micrographs of

A) H3PMo12O40 (3000x magnification),

B) Cs2.5H0.5PMo12O40 (8000x magnification),

C) Rb2.5H0.5PMo12O40 (8000x magnification).

56

The SEM image of pure silicotungstic acid (Fig. 27A) and silicomolybdic acid

(Fig. 28A) shows sizeable agglomerates containing mixture of micro size crystals. In

contrast, the cesium and rubidium salts of HSiW and HSiMo are composed of fine

Fig. 27. SEM micrographs of

A) H4SiW12O40 (3000x magnification),

B) Cs2.5H1.5SiW12O40 (8000x magnification),

C) Rb2.5H1.5SiW12O40 (8000x magnification).

Fig. 28. SEM micrographs of

A) H4SiMo12O40 (3000x magnification),

B) Cs2.5H1.5SiMo12O40 (8000x magnification),

C) Rb2.5H1.5SiMo12O40 (8000x magnification).

57

particles. Smaller particles are obtained for Rb+ than for Cs+ salts of HSiMo and HSiW

(Fig. 27, 28), what is opposite to cesium and rubidium salts of HPW and HPMo (Fig.

25, 26).

In conclusion, we have to note that the surface state and microstructure of Cs+

and Rb+ salts are in strict contrast with heteropolyacids. The relatively smallest particles

were obtained for cesium salt of tungsten heteropolyacid (Fig. 25B).

6.4 Cyclic voltammetry characterization of Keggin-type matrix

In order to prepare inks for cyclic voltammetric characterization, 0.02 g of

cesium, rubidium and ammonium salts of 12-phosphotungstic acid, (HPW), 12-

phosphomolybdic acid, (HPMo), 12-silicotungstic acid, (HSiW) or 12-silicomolybdic

acid, (HSiMo) (described by the general formula 40312

2xy

1x OMMHM − , where M1 is Cs+,

Rb+ or NH4+ and M2 is P or Si, M3 is W or Mo and x is 2.5 and y is 3 or 4 if M3 is P or

Si, respectively) was mixed with Nafion (5% alcoholic solution) and ethanol (>99.9%)

as a solvent and left for 12 hours on the magnetic stirrer. Next 3 µl of the suspension

was dropped on the glassy carbon electrode surface (GC) and left for 30 minutes to dry.

Cyclic voltammetric response of investigated heteropolyacid salts deposits on

glassy carbon electrode recorded in argon saturated 0.5 mol dm-3 H2SO4 (scan rate: 50

mV s-1) are presented in Fig. 29.

Fig. 29a shows typical cyclic voltammogram for Keggin-type 12-

phosphotungstic acid. The two couples of peaks at potentials -27 mV and 240 mV

correspond to two redox reactions and can be described in terms of two consecutive

reversible one-electron processes[20,21,193]:

PW12VIO40

3- + ne- + nH+ ↔ HnPWnVW12-n

VIO403- (55)

where n is equal to 1 or 2. Voltammograms recorded for cesium (Fig. 29b), rubidium

(Fig. 29c) and ammonium (Fig. 29d) salt of tungsten heteropolyacid are slightly

different from those obtained for H3PW12O40. In the cyclic voltammogram of

Cs2.5H0.5PW12O40 and Rb2.5H0.5PW12O40 instead of two couples of peaks only one peak

at circa 68 mV can be seen in the potential range from –0.2 to 0.85 V. Thus the peaks

characteristic of salts are shifted towards lower potentials in comparison to the original

tungsten heteropolyacid and that only the first reduction can be observed in the potential

58

window examined. A different behavior is observed for ammonium salt of HPW,

((NH4)2.5H0.5PW12O40). In the Fig.29d we see two couples of peaks at 23 mV and 235

mV. This voltammogram is similar to that of pure HPW and may indicate that the salt is

partially soluble in aqueous solution of H2SO4 thus releasing the acidic anion.

Cyclic voltammogram of 12-phosphomolybdic acid (HPMo) is presented in

the Fig. 29e. In this case we can see three couples of peaks at 280, 445 and 640 mV.

These peaks correspond to the three consecutive two-electron processes that can be

described by the following reaction[21,22,187]:

PMo12VIO40

3- + ne- + nH+ ↔ HnPMonVW12-n

VIO403 (56)

where n is equal to 2, 4 or 6. Fig. 29f-h illustrates the cesium (Cs2.5PMo12), rubidium

(Rb2.5PMo12) and ammonium ((NH4)2.5PMo12) salt of HPMo. In all these

voltammograms we observe three couples of peaks like in parental 12-

phosphomolybdic acid, but only for Cs2.5PMo12 the peaks are shifted towards less

positive potentials values (from 640 to 570 mV, from 445 to 400 mV and from 280 to

210 mV). The cyclic voltammogram of (NH4)2.5H0.5PMo12O40 is very similar to that

obtained for unmodified H3PMo12O40.

A typical cyclic voltammogram for Keggin-type silica heteropolyacid,

H4SiW12O40 (HSiW) is shown in Fig. 29i. The two couples of peaks that are present in

the cyclic voltammogram at potentials -170 mV and 42 mV correspond to two redox

reaction and can be described in terms of two consecutive reversible one-electron

processes[20,193]:

SiW12VIO40

4- + ne- + nH+ ↔ HnSiWnVW12-n

VIO404- (57)

where n is equal to 1 or 2. In Fig. 29j we can see that for Cs2.5SiW peaks are shifted

towards less positive potential values (from 42 to –25 mV and from –170 to –220 mV).

59

-0.6

-0.3

0.0

0.3

0.3 0.4 0.5 0.6 0.7 0.8-0.04

-0.02

0.00

0.02

0.2 0.4 0.6 0.8-0.4

-0.2

0.0

0.2

j / m

A c

m-2

j / m

A c

m-2

e

g

f

E / V vs. RHE

E / V vs. RHE

m

n

o

-0.04

0.00

0.04

-0.2

0.0

0.2

-0.1

0.0

0.1

-0.2 0.0 0.2 0.4 0.6 0.8

-0.2

0.0

0.2

j / m

A c

m-2

a

b

c

d

E / V vs. RHE

-0.10

-0.05

0.00

0.05

-0.4

-0.2

0.0

0.2

-0.4

-0.2

0.0

0.2

-0.2 0.0 0.2 0.4 0.6 0.8-0.10

-0.05

0.00

0.05

i

j

k

l

E / V vs. RHE

-0.6

-0.3

0.0

0.3

0.6

-0.06

-0.03

0.00

0.03

-0.2

-0.1

0.0

0.1

0.2 0.4 0.6 0.8-0.6

-0.3

0.0

0.3

0.6

e

f

g

h

E / V vs. RHE

Fig.29. Cyclic voltammetric response of

(a) H3PW12O40, (b) Cs2.5H0.5PW12O40,

(c) Rb2.5H0.5PW12O40, (d) (NH4)2.5H0.5PW12O40,

(e) H3PMo12O40, (f) Cs2.5H0.5PMo12O40,

(g) Rb2.5H0.5PMo12O40, (h) (NH4)2.5H0.5PMo12O40,

(i) H4SiW12O40, (j) Cs2.5H1.5SiW12O40,

(k) Rb2.5H1.5SiW12O40, (l) (NH4)4SiW12O40,

(m) H4SiMo12O40, (n) Rb2.5H1.5SiMo12O40,

(o) Cs2.5H0.5SiMo12O40

deposits on glassy carbon electrode.

Electrolyte: argon saturated 0.5 mol dm-3 H2SO4.

Scan rate: 50 mV s-1.

60

This behavior is not observed for rubidium (Rb2.5SiW12) and ammonium

((NH4)4SiW12) salts of 12-silicotungstic acid. Fig. 29l shows, instead of cyclic

voltammogram of (NH4)2.5H1.5SiW12O40 ((NH4)2.5SiW12), voltammetric response of

(NH4)4SiW12O40. Having in mind the fact that ammonium salt of 12-silicotungstic acid

containing 2.5 moles of NH4+ is completely soluble in water, it cannot be used as a

matrix material.

Fig. 29m shows a typical cyclic voltammogram of Keggin-type 12-

silicomolybdic acid. The three couples of peaks at potentials 270 mV, 450 mV and 550

mV correspond to three consecutive redox processes each involving two-electron[20,194]:

SiMo12VIO40

4- + ne- + nH+ ↔ HnSiMonVW12-n

VIO404- (58)

where n is equal to 2, 4 or 6. The cyclic voltammetric responses of Rb2.5H1.5SiMo12O40

(Rb2.5SiMo12) and Cs2.5H1.5SiMo12O40 (Cs2.5SiMo12) are presented in Fig.29n and

Fig.29, respectively. Three peaks located at potential similar to those of HSiMo are seen

for Rb2.5SiMo. In the potential range studied, only two couples of peaks can be

observed for Cs2.5SiMo12. The cyclic voltammetric response of ammonium salt of

HSiMo is not presented because (NH4)2.5H1.5SiMo12O40 is completely soluble in water.

Full substitution of all the protons in the 12-silicomolybdic acid to produce

(NH4)4SiW12O40 also results into a completely soluble product.

Some of the voltammograms recorded for investigated materials deposited on the glassy

carbon electrodes shows similar behavior to those voltammograms obtained for their

analogues in the solution. It is due to partial solubility in solution.

It can be concluded, judging from CV measurements, that the most promising

materials for the anode matrix in fuel cells are cesium salts of Keggin-type

heteropolyacids (HPA). It seems that rubidium salt of HPA could also be of practical

importance. Ammonium salts of Keggin-type heteropolyacids are certainly less

attractive because they are more soluble in comparison to cesium and rubidium salts.

61

6.5 CONCLUSIONS

1) The results obtained using Infrared Spectroscopy technique (IR) demonstrate

that the primary Keggin structure remains unaltered when the protons existing in

parental heteropolyacid (HPW, HPMo, HSiW, HSiMo) are substituted with cesium or

rubidium cations (Cs2.5PW12, Rb2.5PW12, Cs2.5PMo12, Rb2.5PMo12, Cs2.5SiW12,

Rb2.5SiW12, Cs2.5SiMo12, Rb2.5SiMo12). The peak at c.a. 1700 cm-1 in the HPW and

HSiW IR spectra due to dioxonium ions (H5O2+) decreases in intensity with increasing

the number of protons substituted by Cs+ and Rb+ cation.

2) Images obtained using Scanning Electron Microscope (SEM) clearly show

that cesium and rubidium substituted Keggin-type heteropolyacids posses higher surface

area than theirs parental heteropolyacids. Consequently formation of small spherical

crystallites where proton in HPA is substituted by cesium or rubidium cations is

feasible.

The smallest nanoparticles were obtained when protons in Keggin-type 12-

phosphotungstic acid were substituted by cesium cations (to produce Cs2.5H0.5PW12O40).

3) The voltammograms indicate that the most promising material for the anode

matrix in fuel cells are salt of Keggin-type heteropolyacids containing cesium and

rubidium cations.

4) Results obtained here show that, relatively high solubility in the water,

ammonium salts of HPAs are less attractive than their cesium or rubidium salt

analogues. This observation is of practical importance when it comes to preparation of

catalytic layers for fuel cells.

5) On the basis of our results described in this chapter, the salts of Keggin-type

heteropolyacids containing 2.5 moles of cesium and rubidium cations in 1 mole of the

heteropolyacids salts (Cs2.5PW12, Rb2.5PW12, Cs2.5PMo12, Rb2.5PMo12, Cs2.5SiW12,

Rb2.5SiW12, Cs2.5SiMo12, Rb2.5SiMo12) have been chosen for future research.

62

7. Hydrogen oxidation reaction (HOR)

on the catalytic layers containing Cs2.5PW12 matrix

This chapter is devoted to the development of a catalyst consisting of Pt

nanoparticles supported on insoluble salts of Keggin-type heteropolyacids to be used for

hydrogen oxidation reaction (HOR). Two methods of preparation of catalytic layers are

described through mixing and the electrochemical method. The best systems in each

preparation method contained a new type of matrix as demonstrated and compared to

unmodified commercial carbon supported platinum catalyst using rotating disc electrode

(RDE) voltammetry. High resolution transmission electron microscopy (HRTEM) has

been used to study the dispersion platinum nanoparticles deposited on the matrix by

using electrochemical method. CO stripping voltammetry has also been applied to

estimate the active surface of Pt existing in the catalytic layer (prepared by

electrochemical method).

7.1. Preparation of the catalytic layers

7.1.1. Mixing method

The steps followed for the preparation of composite catalytic layers by mixing is

shown schematically in Fig. 30. A suspension in ethanol of Pt10%/Vulcan XC-72

carbon and heteropolyacid salt (Cs2.5H0.5PW12O40) was mixed with know amount of

Nafion (5% alcoholic solution) and stirred in a close vial for 12 h. The mass ratio

between Pt10%/Vulcan XC-72 : heteropolyacids salts : Nafion was 1:2:1.1,

respectively. A portion of the resulting ink was then dropped using micropipette on to

the surface of glassy carbon electrode (RDE electrode) in such way to obtain Pt loading

equal 30 µg cm-2. The resulting catalytic layer was left to dry for 30 min. at room

temperature. For comparison, the heteropolyacid salt-free ink of Pt10%/Vulcan XC-72

carbon and Nafion was also prepared.

63

Fig. 30. Schematic of the preparation of a composite catalytic layer by mixing.

7.1.2. Electrochemical method

To prepare the matrix for electrochemical plating the Cs2.5H0.5PW12O40 was

mixed with Vulcan XC-72 carbon, Nafion solution (5% of aliphatic alcohols) and

ethanol. After 12 hours of mixing (under magnetic stirring) desired amount of the

suspension was dropped on the glassy carbon rotating disc electrode (RDE) surface and

left for 30 minutes to dry. The best results were obtained when the catalytic ink was

prepared by mixing Cs2.5H0.5PW12O40 with Vulcan XC-72 carbon and Nafion in the

following proportion 2:1:1.1.

A standard three-electrode cell (described previously in the chapter 5) was used

to prepare films and to perform other electrochemical measurements. A platinum flag

served as a counter electrode; a saturated calomel electrode (SCE), placed in a separated

compartment and connected to the main cell through a Luggin capillary, was used as

reference electrode.

Preparation of catalytic layer Pt-Cs2.5H0.5PW12O40 with Vulcan XC-72 carbon

and Nafion was accomplished by cycling (2500 cycles) the electrode between -0.05 V

Ethanol

Cs2.5H0.5PW12O

Pt on Vulcan

Nafion

Glassy carbon Nafion

Mixing for 24h

Pt 10%/C – Cs2.5PW12

Pt 10%/C –

Cs2.5PW12 with

Nafion

64

to 1.05 V vs. RHE at scan rate 50 mV s-1 in a solution of 0.5 M of H2SO4 containing 5

10-3 M KCl. During cycling the counter electrode can reach very anodic potentials

causing partial oxidation of the Pt flag. The Pt ions may eventually deposit on the

working electrode. The presence of ions Cl- in the solution helps the process, and

decrease of the standard oxidation potential Pt/Pt(II), E0 = 1.2 V vs. RHE, by formation

of complexes[195]:

PtCl62- + 2e = PtCl4

2- + 2Cl- 0.68 V vs. RHE (59)

PtCl42- + 2e = Pt + 4Cl- 0.73 V vs. RHE (60)

After deposition of platinum within the matrix, the electrode was washed with water

and subjected to cycling in 0.5 M H2SO4 in potential range from 0 V to 1.05 V (vs.

RHE) to remove Cl- from the catalytic film. The steps followed for the electrode

preparation are shown schematically in Fig. 31.

Fig. 31. Scheme of the preparation catalytic layer by electrochemical method.

Matrix (Cs2.5H0.5PW12O40)

Counter electrode

Pt flag

Anodic dissolution of

Pt counter electrode

Working

Electrode (GC)

65

7.2 Electrochemical measurements on the Pt/C modified with

Cs2.5PW12 system prepared by mixing method

7.2.1 Cyclic voltammetry (CV)

and rotating disc voltammetry (RDE) measurements

Fig. 32 shows the cyclic voltammetric responses (recorded in argon-saturated

0.5 M H2SO4 ) of a glassy carbon electrode modified with Nafion-treated (---)

unmodified Pt10%/Vulcan XC-72 carbon (Pt10%/C) and (—) Pt10%/Vulcan XC-72

carbon modified with cesium salt of 12-phosphotungstic acid (Cs2.5H0.5PW12O40). The

platinum loading in both cases was 30 µg cm-2.

0.0 0.2 0.4 0.6 0.8 1.0-6

-4

-2

0

2

4

j / m

A c

m-2

E / V vs. RHE Fig. 32. Cyclic voltammetric response of Nafion-containing films (deposited on rotating disc

glassy carbon electrode) of (—) Pt10%/Vulcan XC-72 carbon modified with Cs2.5H0.5PW12O40,

and (---) unmodified Pt10%/Vulcan XC-72 carbon. Electrolyte: argon saturated 0.5 M H2SO4.

Scan rate, 50 mV s-1.

It is apparent from Fig. 32 that reduction of Cs2.5H0.5PW12O40 (in the range of potentials

from 0 to 0.3 V) tends to overlap with the so-called hydrogen adsorption peaks that

typically exist on clean bare platinum. The electroactivity of Cs2.5H0.5PW12O40 in the

hydrogen adsorption/desorption region of Pt (at potential lower than 0.35 V) is clearly

evident in Fig. 32. Further, following CV response for Cs2.5PW12 with Pt/C, the

66

voltammetric peak (at potential higher than 0.7 V) referring to the formation of Pt-oxo

(PtO or PtOH) species is shifted towards more positive potentials. Increase of the

potential range, where platinum is not covered with platinum oxide, may facilitate

adsorption and activation of hydrogen molecules during electrocatalysis.[21]

The hydrogen electrooxidation reaction on Pt in acid solution is one of the fastest

known electrochemical reactions. To evaluate electrocatalytic activity towards the

hydrogen oxidation, we have performed the diagnostic rotating disc electrode (RDE)

voltammetric measurements using a glassy carbon disc electrode modified with Nafion-

treated (Fig. 33A) Pt10%/Vulcan XC–72 carbon modified with Cs2.5H0.5PW12O40 in

comparison to (Fig. 33B) unmodified Pt10%/Vulcan XC–72 carbon. The RDE

measurements (Fig. 33) for the hydrogen oxidation have been performed at different

rotation rates (ranging from 900 to 2500 rpm). The anodic potential was limited to 100

mV, because the diffusion limiting current for both catalytic layer reach a constant

value at ca. 70 mV, and the coupled hydrogen adsorption-electron transfer steps of the

reaction are only observed at potentials below 50 mV. The examination procedure was

the same for both systems.

It becomes obvious that current density obtained for catalytic layer modified

with cesium salt of tungsten heteropolyacid (Cs2.5H0.5PW12O40) is higher (Fig. 33A),

when compared to the corresponding current density of the bare commercial

Pt10%/Vulcan XC-72 carbon nanoparticles (Fig. 33B). The above results may imply

that modification of Pt10%/Vulcan XC-72 carbon with Cs2.5H0.5PW12O40 matrix results

in increasing dispersion of catalytic centers (higher exposing of Pt particles) on which

proceed hydrogen oxidation reaction (HOR). It can be also explained by the different

diffusion speed into the catalytic layer, what was described in the literature.[46]

67

0.00 0.03 0.06 0.09 0.12 0.150.0

0.5

1.0

1.5

2.0

2.5

3.0

B

E / V vs. RHE

j / m

A c

m-2

2500

900

ω / rpm

Fig. 33. RDE voltammograms recorded in the hydrogen saturated 0.5 M H2SO4 solution at

different rotation rates (900, 1100, 1300, 1500, 1700, 1900, 2100, 2300, 2500 rpm) using a

glassy carbon disc electrode covered with Nafion-treated (Fig. 33A) Pt10%/Vulcan XC–72

carbon modified with Cs2.5H0.5PW12O40 and (Fig. 33B) unmodified Pt10%/Vulcan XC–72

carbon. Scan rate, 10 mV s-1. The loading of Pt 30 µg cm-2.

The above effects become clearer when we compare them at certain rotation rate

for modified and unmodified systems (Fig. 34). Moreover, based on the literature

data[27,28] we can expect that HPAs can act as redox mediators for the electrochemical

0.00 0.03 0.06 0.09 0.12 0.150.0

0.5

1.0

1.5

2.0

2.5

3.0

j / m

A c

m-2

E / V vs. RHE

A

2500

900

ω / rpm

68

oxidation of CO. This could be advantageous knowing that Pt can easily poison with

CO.

Fig. 34. Comparison of RDE voltammograms recorded in the hydrogen saturated 0.5 mol dm-3

H2SO4 solution using rotating disc glassy carbon electrode covered with films of Nafion-

containing inks of (red line) Pt10%/Vulcan XC-72 carbon modified with Cs2.5H0.5PW12O40, and

(black line) unmodified Pt10%/Vulcan XC-72 carbon Rotation rate, ω = 2500 rpm. Scan rate,

10 mV s-1.

Fig. 33 and Fig. 34 confirm that modification of commercial Pt10%/Vulcan XC-

72 by Cs2.5H0.5PW12O40 matrix increases dispersion of platinum centers which are in

contact with hydrogen.

The dependence of the RDE limiting currents versus the square root of rotation

rates (Fig. 35) shows linearity behavior, i.e. ideal behavior characteristic of a system

limited solely by convective diffusion of hydrogen in solution.

0.00 0.03 0.06 0.09 0.12 0.150.0

0.5

1.0

1.5

2.0

2.5

3.0

LPt = 30 µg/cm2

LPt = 30 µg/cm2

10%Pt/Vulcan and Nafion

Cs2.5H0.5PW12O40 with 10%Pt/Vulcan and Nafion

E / V vs. RHE

j / m

A c

m-2

69

Fig. 35. Levich plots j vs. ω1/2 prepared using the data of Fig. 33 for (red symbols) Cs2.5PW12-

modified GC-supported Pt10%/Vulcan XC-72 carbon nanoparticles. Currents were measured

at 0.1 V. For comparison the same plot is provided for (black symbols) GC-supported

Pt10%/Vulcan XC-72 carbon nanoparticles of the same loading (30 µg cm-2).

Assuming laminar flow and the mass transport rate, the diffusion limited current

density is mathematically described by the Levich equation, as a function of the

rotational frequency of the RDE, ω (in radians per seconds)[196,197]:

ίd = 0.62nFD2/3 ν

-1/6 c0ω1/2 = Bc0ω

1/2 (61)

where, D is the diffusivity of hydrogen in 0.5 M H2SO4 (D298 K = 3.7 x 10-5 cm2/s,

estimated from the product of H2 diffusivity at infinite dilution and the ratio of the

dynamic viscosities of the electrolyte and pure water), n is the number of electrons in

the H2 oxidation reaction (i.e., n = 2), ν is the kinematics viscosity of the electrolyte

(ν298 K = 1.07 x 10-2 cm2/s), c0 is the solubility of H2 in 0.5 M H2SO4 (c298 K = 7.14 x 10-3

M). In 0.5 M H2SO4, the calculated value of Levich constant, B, and the solubility, c0,

i.e., Bc0, at 298 K is equal to 6.54 x 10-2 (mA cm-2)rpm-1/2.

It have to be point out that in the presence of Nafion® film, mass-transport limitations

through the film lowered the limiting current at each rotation speed. That’s why for the

0 10 20 30 40 500.0

0.5

1.0

1.5

2.0

2.5

3.0

ω1/2 / (rpm)1/2

j / m

A c

m-2

Pt10%/C (X

C-72) +

Nafion

Pt10%/C (X

C-72) +

Cs2.5H 0.5

PW 12O 40

+ Nafion

70

Nafion®-coated RDE we have additional term including hydrogen diffusion into the

layer[46]:

i film = nFADfcf / δf (62) where, Df is the diffusion coefficient of H2 in the recast Nafion® (cm2 s-1), A is a

geometric area of an electrode area (cm2), cf is a solubility of H2 in recast Nafion® (mol

cm-3), δf is a nafion film thickness.

The thickness of the Nafion® film on the investigated catalytic layers was equal

0.23 µm. Maruyama and coworkers[46] reported absence of the mass transfer effect when

the thickness of Nafion® film varied in the range 1 to 13 µm. This means that in our

case the influence of the i film is negligible and this element can be omission.

According to the Eq. 61, a plot of the current density at constant potential versus ω1/2

(Fig. 35) ought to result in a straight line with the slope defined by Bc0. The regression,

made for ours new type of system, yielded a slope of Bc0 = 6.07 · 10-2 (mA cm-2) rpm-1/2

which is in fairly good agreement with above calculated value (6.54 x 10-2 (mA cm-2)

rpm-1/2) (-8%) considering the uncertainties associated with the evaluation of the

diffusivity of hydrogen in the electrolyte. Incidentally, the above values apply to the

entire potential range between 0.1 and 0.3 V. For Nafion-containing Pt10%/Vulcan XC-

72 carbon (Cs2.5PW12-free system showed as a comparison) the regression yielded a

slope of Bc0 = 6 · 10-2 (mA cm-2) rpm-1/2.

Fig. 36, presents mass transport corrected Tafel diagrams for the investigated

catalytic layers. Mass transport correction for rotation disc electrode is described as

follows[54]:

ik = id i / id – i (63)

where, i is the experimentally obtained current, id refers to the measured diffusion-

limited current, and ik the mass-transport free kinetic current.. The RDE polarization

data analyzed here allow obtaining information on the kinetics of the HOR. On

polycrystalline platinum, oxidation of hydrogen involving two electrons in acidic media

can proceed according to two different pathways:

by chemical adsorption step (Tafel-Volmer mechanism)

H2 + 2M → 2MH (Tafel reaction) (5)

71

or electrochemical adsorption step (Heyrovsky-Volmer mechanism)

H2 + M → MH + H+ + e- (Heyrovsky reaction) (6)

followed by the hydrogen atom discharge path given by

MH ↔ M + H+ + e- (Volmer reaction) (7)

The rates of the reactions (5) and (6) can change depending on the electrode material

and electrolyte.[44,51] Taking attention that in our case influence of the i film is negligible

the equations have been derived for analysis of the RDE voltammograms for the cases

of reversible or irreversible electrochemical reaction. The kinetic equations

are[45,49,52,198]:

−−=L

L

j

jj

nF

RTEE log

303.2'01 (64)

−⋅−=

jj

jj

nF

RTEE

L

Llog303.2'0

2 α (65)

where, for the reversible (Eq. 64) and irreversible (Eq. 65) cases, respectively. In the

equations E10’ and E2

0’ are current independent constants, j is the current density, jL is

the diffusion limiting current density and α is the change transfer coefficient.

72

-1.0

-0.5

0.0

0.00 0.01 0.02 0.03 0.04

0

1

-1.0

-0.5

0.0

0.00 0.01 0.02 0.03 0.04-1

0

1

lo

g[(j

L-j)

/jL]

900 rpm 1700 rpm 2100 rpm

a

b

lo

g[(

j Lx

j)/(j

L -

j)]

E / V vs. RHE

900 rpm 1700 rpm 2100 rpm

c

log

[(j L

-j)/j

L]

900 rpm 1700 rpm 2100 rpm

d

E / V vs. NHE

log

[(j L

x j)/

(jL -

j)]

900 rpm 1700 rpm 2100 rpm

Fig. 36. Mass transport corrected Tafel diagrams [E ~ log(jL – j / jL) and E ~ log(jL · j / jL - j)]

for HOR on catalytic layer prepared from Nafion-containing Cs2.5H0.5PW12O40 with Pt10% on

Vulcan XC-72carbon, (a) equation (64) and (b) equation (65) and as a comparison catalytic

layer made from Nafion-containing Pt10% on Vulcan XC-72 carbon, (c) equation (64) and

(d)equation (65) in 0.5 M H2SO4. Rotation rates: () 900 rpm, (ο) 1700, (∆) 2500 rpm.

Data taken from Fig. 33.

If either condition is satisfied, according to equations (64) and (65), the plots

log(jL – j / jL) vs. E should be linear with slopes independent of the rotation rates and

equal to nF/2.303 RT or αnF/2.303 RT.[169,197]

As we can see in Fig. 36, for all the catalytic layers the plots are linear only if a

reversible kinetics is assumed. The reverse values of the slopes (the Tafel slope E ~

log(jL · j / jL - j)), presented in Table 6, are very close to the theoretical value obtained

for platinum 29 mV dec-1 at temperature 293 K. However, there is no agreement with

the prediction coming from equation (65). In this case we should obtain slope equal 58

mV dec-1 from mass transport corrected Tafel diagram. For potentials below 0.01 V,

there is no linear relationship for the plot E ~ log(jL · j / jL - j), however, if we consider

slope values for E > 0.01 V, we obtain values on the level 28 - 31 mV dec-1 what is

more then two times lower in comparison to the predicted value of 58 mV dec-1. From

these results it may be concluded that the HOR takes places via the Tafel-Volmer

mechanism with the atom-atom recombination step (Tafel) as a rate determining step

73

(rds) at both layers. On this type of electrode, in strong acidic media, a Tafel-Volmer

mechanism, with Tafel as the rds, has also been proposed by Mello and co-workers.[44]

Exchange currents (I0) was calculated from the slope of the linear polarization response

and corrected for diffusion by using the following equation[52]:

+=

∆∆

LIInF

RT

I

E 11

0

(66)

where: n = 2 and IL is the limiting current. Values obtained from this equation are

presented in Table 6.

Composition of catalytic layer

breversible

(mV dec-1)

birreversible (mV dec-1)

Theoretical breversible

(mV dec-1)

Theoretical birreversible

(mV dec-1)

I0

(mA)

Pt10% on Vulcan XC-72 carbon

Cs2.5H0.5PW12O40 with Pt10% on Vulcan XC-72

carbon

Babić at all[52]. Pt20% on Vulcan

XC-27 carbon

32

32

32

28

31

35

29 (Tafel-Volmer) 118 (Heyrovsky-

Volmer)

58 (Tafel-Volmer,

Heyrovsky-Volmer)

0.63

0.83

0.55

Table 6. Theoretical and experimental Tafel parameters, mechanism and corresponding values

of exchange currents for HOR in acid medium at 25 0C for the catalytic layer made from:

Nafion-containing Cs2.5H0.5PW12O40 with Pt10%/Vulcan XC-72 carbon and for Cs2.5H0.5PW12O40

– free system (as a comparison).

In conclusion, we should note that, in both systems, hydrogen oxidation reaction

follows the Tafel-Volmer mechanism with the atom-atom recombination step (Tafel) as

rds. Calculated values of Bc0 for investigated materials are close to the theoretical value

(6.54 x 10-2 mA cm-2 rpm-1/2). Nevertheless the value of diffusion limiting current for

ours system is 12% higher than that for unmodified one. Moreover exchange current

calculated for Cs2.5PW12-containing catalytic layer is 32% higher than that for the

system containing unmodified commercial available Pt/Vulcan XC-72 carbon

nanoparticles. The value of I0 obtained for unmodified system (Pt/C) is in a good

agreement with that reported by Babić at all.[52] All these issues clearly show that our

74

catalytic layer containing Pt10%/Vulcan XC-72 carbon together with a matrix obtained

from cesium salt of 12-phosphotungstic acid (Cs2.5H0.2PW12O40) possesses many

advantages in comparison with Cs2.5PW12-free system.

7.2.2 Test of the Cs2.5H0.5PW12O40-containing anode catalyst

in working single PEM fuel cell

The mixing method was used to prepare inks to be utilized as anode catalyst in

a working fuel cell. Cs2.5H0.2PW12O40, Pt10%/Vulcan XC-72 and Vulcan XC-72 were

mixed in 99% ethanol and stirred in a close container for 24 hours. 200 µl of Nafion

solution were then added to the homogeneous suspension and stirring was continued for

12 hours. The relative amounts of the different component are listed in Table 7.

Name of the component Weight in grams Cs2.5H0.2PW12O40 0.0073 Pt10% on Vulcan XC-72 carbon 0.0059 Vulcan XC-72 carbon 0.0067 Nafion 0.0087

Table 7. Amounts of components used for preparation anode for PMEFC.

The ink was then spread homogeneously onto the surface of a commercial gas

diffusion layer (low-temperature ELAT® GDL microporous layer on woven web, tin

configuration), purchased from E-TEK, (5x2.5 cm) previously weighted using a brush.

The resulting GDL with catalytic layer (CL) was left to dry in an oven for 30 min. at 80 0C and weighted again to determine the amount of catalytic mixture in terms of Pt

content per cm2. Pt loading was 43 µg cm-2. The same procedure was used to prepare a

Cs2.5H0.2PW12O40-free catalytic layer using commercial Pt10%/Vulcan XC-72 carbon

from E-TEK. The Pt loading for this electrode was 50 µg cm-2. 2.2 x 2.2 cm portions of

the two layers were then cut to obtain the electrode to be used for preparing a MEA.

Membrane electrode assemblies (MEA) are built by hot pressing two of the

above electrodes at the opposite sides of a Nafion membrane (thick, 127 µm). Nafion®

Membrane 115 purchased from Ion Power Inc., (dimension) 7.5x7.5 cm, was used as a

proton conducting membrane.

Two types of MEA have been built and tested: MEA1 was prepared using the

composite electrode at one side and the commercial catalyst on the other side; MEA2, to

75

be used for comparison, had the commercial catalyst on both sides. The preparation

conditions are listed in Table 8 in the case of MEA1.

Anode electrode Cathode electrode

Membrane Temperature / 0C

Pressure / bar

Time / min.

Cs2.5H0.2PW12O40

Pt10%/C Vulcan XC-72

Pt10%/C Vulcan XC-72

Nafion® 115

125 15 3

Table 8. Components and conditions for the hot pressing MEA preparation. The MEA have been tested using a 5 cm2 single cell, from Fuel Cell Technologies,

operated equipped with single serpentine flow channel as gas feeds, showed in Fig. 37.

The cells were operated using pure H2 and O2.

Fig. 37. Picture of the (A) single PEM fuel cell and (B) one side of PEMFC with bipolar

plate and single serpentine flow channel

Fig. 38 and Fig. 39 show the polarization curves and the power curves for both

MEA under the conditions specified in Table. 9.

A

gas intlet gas outlet

O2 part

H2 part

B

bipolar plate

serpentine flow channel

76

Fig. 38. Current-voltage PEMFC’s single cell performances of (red symbols) Cs2.5H0.5PW12O40-

containing and (black symbols) Cs2.5H0.5PW12O40-free anode: 70 0C of cell temperature, 70 0C

for both humidifiers, 3 bars of operating pressure.

0 200 400 600 8000

100

200

300

400

H2 -- L

Pt = 50 µg/cm2 Pt // Pt L

Pt = 50 µg/cm2 -- O

2

H2 -- L

Pt = 43 µg/cm2 Pt + Cs

2.5H

0.5PW

12O

40 // Pt L

Pt = 50 µg/cm2 -- O

2

Current density / mA cm-2

Pow

er D

ensi

ty /

mW

cm

-2

Fig. 39. PEMFC’s single cell performances of (red symbols) Cs2.5H0.5PW12O40-containing and

(black symbols) Cs2.5H0.5PW12O40-free anode: 70 0C of cell temperature, 70 0C for both

humidifiers, 3 bars of operating pressure.

0 200 400 600 8000,0

0,2

0,4

0,6

0,8

1,0

MEA2

Current density / mA cm-2

Cel

l vol

tage

/ V

H2 -- L

Pt = 50 µg/cm2 Pt // Pt L

Pt = 50 µg/cm2 -- O

2

H2 -- L

Pt = 43 µg/cm2 Pt + Cs

2.5H

0.5PW

12O

40 // Pt L

Pt = 50 µg/cm2 -- O

2

MEA1

77

Catalysts and their loading (µg cm-2)

Feeding conditions TCell (0C)

TAnode (0C)

TCathode (0C)

Electrolyte

Anode Cathode Anode Cathode Cs2.5H0.2PW12O40

Pt10%/C Vulcan XC-72 43 µg cm-2

Pt10%/C Vulcan XC-72 50 µg cm-2

H2, 50 ccm 3 bars

O2, 50 ccm 3 bars

70 70 70 Nafion® 115

Pt10%/C Vulcan XC-72 50 µg cm-2

Pt10%/C Vulcan XC-72 50 µg cm-2

H2, 50 ccm 3 bars

O2, 50 ccm 3 bars

70 70 70 Nafion® 115

Table 9. Composition of electrodes, their loadings and operation conditions of single working

PEMFC.

The polarization curves were obtained by applying 50 mA steps and recording

the cell potential after 60 sec.

As it may be seen better performances are obtained (either in terms of potential

and power) when using MEA1. In this type of fuel cells the performances are limited by

the cathode where oxygen reduction (a very sluggish electrochemical reaction) occurs.

As the cathode catalyst is the same in both cases, the better performances of MEA1

have to be ascribed to the anode side that contains a catalyst layer modified with

Cs2.5H0.5PW12O40.

Our results suggest that modification of commercial Pt10%/Vulcan XC-72

carbon with cesium salt of 12-phosphotungstic acid (Cs2.5H0.2PW12O40) leads to better

utilization of catalytic centers as apparent from Fig. 38 and Fig. 39. Similar results were

obtained by Stanis et al. where Pt/C was modified with H3PW12O40 heteropolyacid was

used as an anode in PEM fuel cell operating on pure hydrogen as a fuel.[27]

In conclusion, we should also remark that results obtained for our new system by

using single working PEM fuel cell are in agreement with previous results received by

using electrochemical methods, particularly cyclic voltammetry (CV) and rotating disc

voltammetry (RDE) methods.

78

7.3 Electrochemical measurements on the Pt/C modified with

Cs2.5H0.5PW12O40 system prepared by electrochemical method

7.3.1 Cyclic voltammetry study

Corrosion of Pt was accomplished by applying sufficiently positive potential to

platinum flag counter electrode in 0.5 mol dm-3 H2SO4 containing ions Cl- (as was

described previously in chapter 7.1.2) on composite catalytic layers and layers prepared

using the commercial catalyst alone. During negative potential scans applied to the

working electrode electrodeposition take places onto catalytic layers existing on the

surface of glassy carbon RDE electrode. The voltammograms before and after

electrochemical deposition were recorded in the 0.5 mol dm-3 H2SO4 without Cl- (Fig.

40 and 41). Before recording voltammetric curve of the catalytic layer after

electrodeposition of platinum, the modified electrode was washed with water and

subjected to cycling in 0.5 M H2SO4 in potential range from 0 V to 1.05 V (vs. RHE) to

remove Cl- from the catalytic film.

0.0 0.2 0.4 0.6 0.8 1.0-8

-6

-4

-2

0

2

4

0.0 0.2 0.4 0.6 0.8 1.0

-0.4

-0.2

0.0

0.2

0.4

after Pt corrosion before Pt corrosion

j / m

A c

m-2

E / V vs. RHE

Carbon paper as counter electrode

after cycling beafore cycling

j / m

A c

m-2

E / V vs. RHE

Pt as counter electrode

Fig. 40. Cyclic voltammetric responses of Cs2.5H0.5PW12O40 with Vulcan before (dash line) and

after (solid line) Pt corrosion from platinum counter electrode; Inset: Cs2.5PW12O40 with Vulcan

before (dash line) and after (solid line) cycling where carbon paper was used as counter

electrode; Electrolyte: argon saturated 0.5 M H2SO4. Scan rate, 50 mV s-1.

79

Fig. 40 shows the final cyclic voltammetric response of the composite catalytic

layer before and after Pt continuous cycling together with the initial cyclic

voltammograms. The final cyclic voltammogram shows the classic signature of

hydrogen adsorption/desorption on Pt.

The insert in Fig. 40 shows the cyclic voltammograms obtained under the same

conditions with the same electrode when carbon paper instead of Pt was used as counter

electrode. Apart from a slight increase of the currents in the hydrogen region, due to the

reduction of the cesium salts (chapter 6.4), the shape of the curve is practically

unaltered after prolonged cycling. The obvious conclusion that may be drawn is that the

high current increase when the counter electrode is Pt is due to deposition of Pt on the

composite electrode because of corrosion of the counter electrode.

For a comparison the same procedure has been applied to Nafion-containing

catalytic layer made from Vulcan XC-72 carbon (Cs2.5PW12-free system). Fig. 41 shows

the cyclic voltammetric response of Vulcan before and after Pt corrosion. The results

demonstrate that the procedure works also on Vulcan alone.

0.0 0.2 0.4 0.6 0.8 1.0

-10

-8

-6

-4

-2

0

2

4

6

Vulcan before Pt corrosion Vulcan after Pt corrosion

j / m

A c

m-2

E / V vs. RHE

Fig. 41. Cyclic voltammetric responses of Vulcan before (dash line) and after (solid line)

Pt corrosion from platinum counter electrode; Electrolyte: argon saturated 0.5 M H2SO4; Scan

rate, 50 mV s-1.

Again the development of the cyclic voltammetric curves with increasing number of

scan reveals the progressive grow of the characteristic hydrogen adsorption/desorption

waves close to 0 V that indicate presence of Pt on the electrode.

80

In order to obtain further evidences, beside the signature of hydrogen

adsorption/desorption, of the effectiveness of the proposed method to dope the

composite electrode (and Vulcan) with Pt nanoparticles by corrosion of a Pt counter

electrode and to have an idea of the electrochemically active area, dimensions and Pt

loading that may be obtained, alternative methods to detect Pt have been used. These

include both CO stripping and TEM. The electrochemically active area at Pt electrodes

is usually measured using the hydrogen adsorption/desorption peaks. In this case this

method can not be applied because the Cs salts is electro-active in the same potential

region and, hence, the integrated areas under the peaks contain the contributions due to

the reduction of the compound itself.

7.3.2 CO electrooxidation on catalytic layer

containing Cs2.5H0.5PW12O40 as a matrix

It is well known in the literature that CO adsorbs on Pt poisoning it and

cancelling any catalytic activity for hydrogen oxidation. Adsorbed CO can be stripped

from the Pt surface at potentials in the range 0.8 - 1 V vs. RHE. The charge

corresponding to one monolayer of adsorbed CO is equal to 0.484 mC cm-2.[199-201]

Hence, by measuring the charge under the stripping peak, after subtraction of the

background, the electrochemically active area can be obtained using the formula:

EAS = QCO / 0.484 (67)

where QCO is the measured charge.

Fig. 42 shows several voltammograms obtained at 20 mV/s using the composite

electrode under different conditions. The black continuous line is relative to the Pt

containing composite electrode as prepared, while the dashed black line shows the

voltammogram obtained after CO adsorption. The red line corresponds to the

voltammogram of the composite electrode before Pt deposition and the dashed blue line

to that obtained with the same electrode after CO adsorption. The last two

voltammograms demonstrate that there is no absorption of CO before Pt corrosion and,

hence, that no errors are introduced due to CO adsorption in the salt.

81

The peak relative to hydrogen desorption present in the pristine electrode after

activation in the hydrogen region is completely absent after CO absorption. This

testifies that the entire Pt surface is completely blocked by CO. The stripping of CO at

about 0.95 V is very well defined and sharp. The computed charge under the peak is

equal to 0.2207 mC. Using Eq. 67 this translates into an electrochemically active area of

0.456 cm2.

0.0 0.2 0.4 0.6 0.8 1.0 1.2-1

0

1

2

3

j / m

A c

m-2

E / V vs. RHE

Cs2.5

PW12

-containing system after Pt corrosion

CO-adsorption on Cs2.5

PW12

- containing system after Pt corrosion

Cs2.5

PW12

-containing Pt-free system

CO-adsorption on Cs2.5

PW12

- containing Pt-free system

CO

Fig. 42. Base voltammetry (—) and CO-stripping (---) on a Nafion-containing Cs2.5H0.5PW12O40

with Vulcan XC-72 carbon and corroded platinum and for a comparison base voltammetry (—)

and CO-stripping (---) at a Pt-free Nafion-containing Cs2.5H0.5PW12O40 with Vulcan XC-72

carbon electrode. Electrolyte, argon saturated 0.5 mol dm-3 H2SO4. Scan rate, 20 mV s-1.

7.3.3 HRTEM characterization

Fig. 43a and b show HRTEM images of Pt nanoparticles obtained using powders

from the composite and the pure Vulcan electrodes after electrochemical Pt deposition.

The images show that the nanoparticles of electrodeposited Pt on catalytic layer

containing cesium salt of tungsten heteropolyacid are largely spherical in shape with a

diameter of about 2 nm (Fig. 43a), while for the catalytic layer containing only Vulcan

XC-72 carbon the platinum particles have diameter about 4 nm (Fig. 43b).

A likely explanation of the different particle size may be that in the case of the

Cs2.5H0.5PW12O40 containing composite Pt deposition occurs inside the nanochannels of

82

the porous tertiary structure of the salts that posses characteristic diameters while in the

case of pure Vulcan the particles grow on the surface where are more prone to grow.

(a) (b)

Fig. 43. HRTEM image of nafion-containing catalytic layer made from: (a) Cs2.5H0.5PW12O40

with Vulcan XC-72 carbon and corroded Pt and (b) Vulcan XC-72 carbon with corroded Pt for

a comparison.

In the case of the composite catalytic layer where the surface area is known one

may attempt to compute the Pt loading by assuming uniform diameters and the absence

of bigger aggregates average. Knowing dimensions of Pt nanoparticles and assuming

theirs spherical shape we are able to calculate platinum loading into the catalytic layer

by using the following equation[52,202]:

S = (6 x 103) / ρd (68)

where S is specific surface area (m2 g-1), d is the mean particle size in nm (from

HRTEM) and ρ is the density of Pt metal (21.4 g cm-3). Assuming that all nanoparticles

have dimension ca. 2 nm as well as the absence of aggregated nanoparticles, we obtain

the surface area for one nanoparticle equal to 140.18 m2 g-1. Knowing EAS,

electrochemically active area (0.456 cm2) received from CO stripping measurements

(assuming 100% coverage) and S for one particle we are able to calculate loading of

platinum into the catalytic layer by applying following equation:

83

LPt = (EAS/ S) / A (69)

where, A is the area of the RDE electrode (A = 0.1256 cm2). According to this equation

the calculated Pt loading was 2.6 µg cm-2. This approximate calculation was made to

comment on the order of magnitude of platinum. The amounts of Pt are indeed varying

very low what means that we are able to decrease quantity of this material without

decreasing the systems performance towards hydrogen oxidation reaction (HOR).

7.3.4 Rotating disc voltammetry measurements

The kinetic data of the HOR occurring at the catalytic layers prepared by

electrochemical method has been done by applying the same type of analysis (using the

same procedure) as for the systems prepared by mixing method (chapter 7.2.1).

Fig. 44 presents the hydrogen oxidation polarization curves recorded at several

rotation speeds (1300 rpm ≤ ω ≤ 2500 rpm) in 0.5 mol dm-3 H2SO4 on the following

catalytic layers: (A) Cs2.5H0.5PW12O40 with Vulcan XC-72 carbon, Nafion and corroded

platinum and (B) Vulcan XC-72 carbon with Nafion and corroded platinum (for

comparison). The anodic potential limit was to 100 mV because the limiting currents

reach a constant value at ca. 80 mV in the case of both catalytic layers. Also the coupled

hydrogen adsorption-electron transfer steps of the reaction are only observed at

potentials below 50 mV. Current density obtained for catalytic layer containing

Cs2.5H0.5PW12O40 matrix is higher than that obtained for catalytic layer containing only

Vulcan XC-72 carbon.

84

0.00 0.02 0.04 0.06 0.08 0.10 0.120.0

0.5

1.0

1.5

2.0

2.5

A

j / m

A c

m-2

E / V vs. RHE

2500

1300

ω / rpm

0.00 0.02 0.04 0.06 0.08 0.10 0.120.0

0.5

1.0

1.5

2.0

2.5

B

j / m

A c

m-2

E / V vs. RHE

2500

1300

ω / rpm

Fig. 44. Hydrogen oxidation RDE voltammograms for several rotation speeds obtained in

0.5 M H2SO4 under H2 atmosphere on Nafion-containing catalytic layer made from (A)

Cs2.5H0.5PW12O40 with Vulcan XC-72 Carbon and corroded platinum and (B) Vulcan XC-72

Carbon with corroded platinum as a comparison, at different speed rotation, 1300 rpm ≤ ω ≤

2500 rpm. Scan rate, 2 mV s-1.

85

Comparison of hydrogen oxidation RDE curves at 2500 rpm recorded for the

system containing Cs2.5PW12 matrix with corroded platinum and this unmodified one

with corroded platinum is illustrated in Fig. 45. Higher current density obtained for the

modified material could be explained by formation of smaller Pt nanoparticles not only

on the surfaces but also possibly inside the channels in micro-meso porous structure of

the matrix.

Fig. 45. Comparison of hydrogen oxidation RDE voltammograms at Nafion-coated catalytic

layers made from: (red line) Cs2.5H0.5PW12O40 with Vulcan XC-72 Carbon and corroded

platinum and (black line) Vulcan XC-72 Carbon with corroded platinum in 0.5 M H2SO4 under

H2 atmosphere. Rotation rate, ω = 2500 rpm; scan rate, 2 mV s-1.

The dependence of the respective RDE limiting currents versus the square root

of rotation rates (Fig. 46) shows linearity behavior. Assuming laminar flow, the mass

transport rate and the limiting diffusional current density can be described

mathematically by the Levich equation, as a function of the rotational frequency of the

RDE, ω (in radians per seconds) (Equation 61).

0.00 0.02 0.04 0.06 0.08 0.10 0.120.0

0.5

1.0

1.5

2.0

2.5

3.0

j /

mA

cm

-2

E / V vs. RHE

Vulcan XC-72 carbon after corrosion of Pt flag

Cs2.5

H0.5

PW12O

40 + Vulcan XC-72 carbon after corrosion of Pt flag

86

Fig. 46. Levich plots j vs. ω1/2 prepared using the data of Fig.44 for (red symbols)

Cs2.5H0.5PW12O40 with Vulcan XC-72 carbon and corroded platinum. Currents were measured

at 0.1 V. For comparison the same plot is provided for (black symbols) unmodified GC-

supported Vulcan XC-72 carbon with corroded platinum.

The Bc0 calculated from the slope of Levich plot (Fig. 46) is equal 5.1 · 10-2 (mA cm-2)

rpm-1/2 and is about 20% lower than the theoretical value of Bc0 in 0.5 mol dm-3 H2SO4,

at 25 0C (6.54 · 10-2 (mA cm-2) rpm-1/2).

In order to overcome limitation of the technique employed, steady-state

experiments were also carried out. They are complementary to the RDE results. Steady

state data were used to prepare Tafel plots for the hydrogen oxidation reaction occurring

at the investigated electrode material and for Nafion-containing Cs2.5PW12-free system

as a reference. The Tafel diagrams for Nafion-containing catalytic layer made from

Cs2.5PW12 matrix with Vulcan XC-72 carbon and corroded platinum are presented in

Fig. 47. As a comparison, the same graphs were made for unmodified system.[44,45,50,51]

Like it was described previously, on polycrystalline platinum two electron HOR in

acidic media can proceed by chemical adsorption step (Tafel) or electrochemical

adsorption step (Heyrovsky), followed by the adsorbed hydrogen atom discharge step

(Volmer), as described in Eqs. (5) – (7).

0 10 20 30 40 500.0

0.5

1.0

1.5

2.0

2.5

Cs 2.5H 0.5

PW 12O 40

+ Vulc

an X

C-72 ca

rbon a

fter c

orros

ion of

Pt flag

Vulcan

XC-72 ca

rbon a

fter c

orrosi

on of

Pt flag

j / m

A c

m-2

ω1/2 / (rpm)1/2

87

-1.6

-0.8

0.0

0.00 0.02 0.040.00 0.02 0.04

-0.8

0.0

0.8

(c)

lo

g[(j L-j)

/j L]

(a)

(d)

log[

(j Lx j)/

(j L - j)

]

E / V vs. RHE

(b)

Fig. 47. Mass transport corrected Tafel diagrams [E ~ log(jL – j / jL) and E ~ log(jL · j / jL - j)]

for HOR on catalytic layer prepared from Nafion-containing Cs2.5H0.5PW12O40 with Vulcan XC-

72 carbon and corroded platinum, (a) equation (64) and (b) equation (65) and as a comparison

catalytic layer made from Nafion-containing Vulcan XC-72 carbon with corroded platinum, (c)

equation (64) and (d) equation (65) in 0.5 M H2SO4. Rotation rates: () 1500 rpm,

(ο) 2500 rpm. Data taken from Fig. 44.

The values of the Tafel slopes for reversible reaction, presented in Table 10, are

very close to theoretical value obtained for platinum 29 mV dec-1 at temperature 295 K.

Based on the experimental data reversible nature of the electrochemical hydrogen

oxidation reaction at both investigated catalytic layers is electrochemically reversible

seems experimentally supported. The HOR on investigated systems takes places via the

Tafel-Volmer mechanism with the atom-atom recombination step (Tafel) as a rate

determining step (rds).[44] Exchange currents (I0) was calculated from the slope of the

linear polarization response and corrected for diffusion by using previously showed and

described equation (66). Values obtained from this equation are presented in Table 10.

In conclusion, we should admit that the results obtained for catalytic layers

prepared by electrochemical method studied using CV and RDE method, clearly shows

that modification of the system by using insoluble cesium salt of 12-phosphotungstic

acid matrix provide to better performance towards hydrogen oxidation reaction (HOR).

88

The exchange current (I0) calculated for Cs2.5PW12-containig material is almost twice of

the value obtained for unmodified one.

Composition of catalytic layer

breversible

(mV dec-1)

birreversible (mV dec-1)

Theoretical breversible

(mV dec-1)

Theoretical birreversible

(mV dec-1)

I0

(mA)

Cs2.5H0.5PW12O40 with Vulcan XC-72 carbon and corroded Pt Vulcan XC-72 carbon with corroded Pt

31

31

28

28

29 (Tafel-Volmer)

118 (Heyrovsky-Volmer)

58 (Tafel-Volmer,

Heyrovsky-Volmer)

0.98

0.5

Table 10. Theoretical and experimental Tafel parameters, mechanism and corresponding values

of exchange currents for HOR in acid medium at 22 0C for the Nafion-containing catalytic layer

made from: Cs2.5H0.5PW12O40 with Vulcan XC-72 carbon and corroded Pt and for

Cs2.5H0.5PW12O40 – free system (as a comparison).

7.4 Comparison of catalytic layers containing Cs2.5PW12

matrix prepared by mixing and electrochemical methods

Table 11 presents summary of results obtained for Cs2.5H0.5PW12O40-containing

systems prepared by mixing method and by electrochemical method.

Composition of catalytic layer

breversible

(mV dec-1)

Theoretical breversible

(mV dec-1)

I0

(mA)

Pt10% on Vulcan XC-72 carbon Cs2.5H0.5PW12O40 with Pt10% on Vulcan XC-72 carbon Cs2.5H0.5PW12O40 with Vulcan XC-72 carbon and corroded Pt

32

32

31

29 (Tafel-Volmer)

118

(Heyrovsky-Volmer)

0.63

0.83

0.98

Table 11. Summary table containing mechanistic parameters and corresponding values of

exchange currents for HOR in acid medium at 22 0C for the Nafion-containing catalytic layer

made from: Cs2.5H0.5PW12O40 with 10% Pt/C, Cs2.5H0.5PW12O40 with Vulcan XC-72 carbon and

corroded Pt and 10% Pt/C (for comparison).

89

The mechanism of the hydrogen oxidation reaction is the same for all

investigated electrode materials.

The value of exchange currents (I0) calculated for the system containing

Cs2.5PW12-matrix is higher than that obtained for Cs2.5PW12-free catalyst. Furthermore,

the catalyst prepared by electrochemical deposition appears to be more active, towards

hydrogen oxidation than that prepared by mixing method.

Catalytic layer, prepared by mixing method, containing Cs2.5H0.5PW12O40 matrix

was tested as an anode in single working PEM fuel cell operating on pure hydrogen as a

fuel. Advantage of this method is its simplicity. Results obtained from this

measurements confirm that the modification of the commercial available Pt/C by using

Cs2.5H0.5PW12O40 matrix provide to better performance of the system towards HOR

what is in agreement with RDE measurements.

It is important to underscore that results described in this chapter being the

part of the Italian and international patent.[203,204]

90

8. Methanol oxidation reaction (MOR) on the catalytic layers

containing Keggin-type heteropolyacid salts as a matrix

Methanol oxidation reaction is a very important reaction from a practical point

of view. This small organic molecule has attracted considerable attention due to the

development of direct liquid fuel cells that require highly reactive fuels with high

energy density. However, the formation of strongly adsorbed intermediates species such

as (CO)ads on the Pt catalyst, which is the most active metal for methanol oxidation,

results in high oxidation overpotentials, usually far from the thermodynamic limit.

Therefore this chapter is devoted to optimize of Pt-based electrocatalysts, by applying

new type of matrices with high porosity, strong acidic properties and high ionic

conductivity.

New systems have been characterized with respect to their electrochemical

properties ( voltammetry, CO stripping) and their electrocatalytic activity for methanol

oxidation. The stability of catalytic properties will be also discussed.

The chapter is divided into two parts related to the cation present into the

Keggin-type heteropolyacid salts (Cs+ or Rb+) used as a matrix for commercial

Pt/Vulcan XC-72 carbon. Methanol oxidation reaction on the unmodified catalytic layer

(Nafion-containing Pt/Vulcan XC-72 carbon) is shown for a comparison.

8.1 Preparation of the catalytic layers

The electrocatalyst layer was prepared by mixing via the following procedure.

Calculated amounts of commercial Pt40%/Vulcan XC-72 carbon and of cesium or

rubidium salt of Keggin-type heteropolyacid matrix were mixed in 1:1 ethanol - water

mixture (proportion 1:1) and stirred for 24 h. A measured volume of Nafion (5%

alcoholic solution) was then added and the stirring continued for additional 12 h to

obtain a homogenous dispersion. The mass ratio between Pt40%/Vulcan XC-72 :

heteropolyacids salts : pure Nafion solution was 1:1.5:0.3, respectively. The resulting

ink was dropped with a micropipette the surface of a glassy carbon disk electrode (

0.071 cm2 surface area) . The thin film catalytic layer was dried at room temperature

for 20 minutes. For comparison, heteropolyacid salts-free ink of Pt40%/Vulcan XC-72

carbon and Nafion was also prepared. The platinum loading in each system is equal to

100 µg cm-2.

91

8.2 Electrochemical measurements

at the Pt40%/C modified with Cs2.5-HPAs matrix

8.2.1 Cyclic voltammetry (CV) study

Fig. 48 shows the cyclic voltammetric responses (recorded in argon-saturated

0.5 mol dm-3 H2SO4) of the glassy carbon electrode modified with Nafion-treated pure

Pt40%/Vulcan XC-72 carbon (Pt40%/C) and Pt40%/Vulcan XC-72 carbon modified

with matrix made from cesium salts of Keggin-type heteropolyacids: Cs2.5H0.5PW12O40,

Cs2.5H0.5PMo12O40, Cs2.5H1.5SiW12O40 and Cs2.5H1.5SiMo12O40. The platinum loading in

all investigated electrocatalytic films was equal to 100 µg cm-2. The voltammetric peaks

appearing at about 0.46 V (Fig. 48, light-blue line) and 0.43 V (Fig. 48, green line)

should be attributed to the reduction of Cs2.5H1.5SiMo12O40 and Cs2.5H0.5PMo12O40,

respectively, while Cs2.5H1.5SiW12O40 (Fig. 48, dark-blue line) is electroactive at

potential lower ca. 0.1 V. Cs2.5H0.5PW12O40 becomes electroactive (Fig. 48, red line) at

the more negative potentials and its reduction overlaps with the so-called hydrogen

adsorption peaks that typically exist on clean bare platinum at potentials lower than 0.35

V.

The presence of cesium salts of Keggin-type heteropolyacids (particularly Cs2.5PMo12

and Cs2.5SiMo12) causes a shift of the voltammetric peaks (at potential higher than 0.8

V) relative to the formation of Pt-oxo (PtO or PtOH) species towards more positive

potentials.

92

0.0 0.2 0.4 0.6 0.8 1.0-0.9

-0.6

-0.3

0.0

0.3

0.6

j / m

A c

m-2

E / V vs. RHE

Fig. 48. Cyclic voltammetric responses of Nafion-containing films (deposited on glassy carbon

electrode) of (—) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PW12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PMo12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon. Electrolyte: argon

saturated 0.5 mol dm-3 H2SO4. Scan rate, 10 mV s-1. Temperature, 240C.

To investigate the influence of the matrix in the catalytic behavior towards

methanol oxidation reaction (MOR), cyclic voltammetric measurements (Fig. 49) were

carried out on four kinds of systems (Pt40%/C with Cs2.5PW12, Pt40%/C with

Cs2.5PMo12, Pt40%/C with Cs2.5SiW12 and Pt40%/C with Cs2.5SiMo12) and on

commercial unmodified electrocatalyst (Pt40%/C) as a reference. Because the slow

kinetics, methanol oxidation reaction occurs at high oxidation overpotentials, far from

the thermodynamic limit (E0 = 0.02 V).[84]

In Fig. 49, for all investigated materials we observe two characteristic

irreversible current peaks (A and B) during the electrooxidation of methanol. The peak

obtained in the forward scan (peak A) at around 0.85 V is typically attributed to the

methanol electrooxidation and the backward peak (peak B) at ca. 0.73 V is known to be

due to the oxidation reaction on the Pt of residual intermediate species such as CH2OH,

CH2O,HCOOH and CO.[205,206]

93

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

25

30

35B

E / V vs. RHE

j / m

A c

m-2

A

Fig. 49. Cyclic voltammetric responses of Nafion-containing films (deposited on glassy carbon

electrode) of (—) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PW12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PMo12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon. Electrolyte: argon

saturated 0.5 mol dm-3 CH3OH + 0.5 mol dm-3 H2SO4. Scan rate, 10 mV s-1. Temperature, 240C.

Moreover, Fig. 49 clearly show that modification of commercial Pt40%/C by

cesium HPA salts matrix increases noticeably the electrocatalytic activity of the system

for MOR. The highest current densities were obtained for the catalytic layers containing

Cs2.5H0.5PMo12O40 and Cs2.5H1.5SiW12O40 as a matrix.

It is also important to note that methanol oxidation reaction starts at more negative

potentials for all modified systems in comparison to unmodified catalytic layer

containing commercial Pt40%/C. This indicates that the presence of the matrix can

increase the kinetics of methanol oxidation by lowering the onset potential for this

reaction.

The reaction mechanism was investigated on the basis of the Tafel plot analyses.

Fig. 50 shows the Tafel plots for the methanol oxidation obtained from the cyclic

voltammograms at the scan rate 5 mV s-1 for the systems modified by cesium salts of

Keggin-type heteropolyacids and, as a comparison, for the unmodified catalytic layer.

94

0,3 0,4 0,5 0,6 0,7

-3

-2

-1

0

1

region 2

93 mV dec-1

95 mV dec-1

106 mV dec-1

95 mV dec-1

88 mV dec-1lo

g j /

mA

cm

-2

E / V vs. RHE

region 1

Fig.50.Tafel plots of methanol oxidation at the Nafion-containing (--) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H0.5PW12O40, (--) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H0.5PMo12O40, (--) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiW12O40, (-♦-)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiMo12O40 and (--) unmodified

Pt40%/Vulcan XC-72 carbon. Temperature, 240C.

The reverse of the slopes (RT/nF) of the Tafel lines (1 / b, expressed in mV dec-

1) change at ca. 0.45 V, indicating the presence of two different reaction mechanisms.

The Tafel slope in region 1 is strongly dependent on the concentration of methanol as

well as the reaction time[85] and this does not allow to draw conclusions on the kinetic

equation in region 1. Since the Tafel slope in region 2 is unchanged with the

concentration of methanol and reaction time a kinetic parameters at a given reaction

time can be determined from this region.[85]

The main slope, in the region 2, for unmodified 40% Pt/C is 88 mV decade-1.

The Tafel slopes for the catalytic layers containing Cs2.5H0.5PW12O40,

Cs2.5H0.5PMo12O40, Cs2.5H1.5SiW12O40 and Cs2.5H1.5SiMo12O40 matrix were estimated, in

the same manner as for the pure Pt40%/Vulcan XC-72 carbon system, to be 93, 95, 106

and 95 mV decade-1, respectively. Those results are in good agreement with those

reported by Inada and co-workers.[85] They obtained Tafel slope of the methanol

oxidation at the Pt electrode to be 96 ± 10 mV decade-1 in the potential range of 0.55 –

0.7 V vs. RHE at room temperature[85], which well agrees with the result of Fig. 50.

Thus, it is postulated that the number of electrons involved in the electrode reaction n is

95

ca. 1 (n ≈ 1) at 240C. The methanol electrooxidation pathway is considered as

follows[85]:

CH3OH ↔ (CHnO)ads + (4 – n) H+ + (4 – n) e- (70)

H2O ↔ (OH)ads + H+ + e- (71)

(CHnO)ads + (OH)ads → CO2 + n (H)ads (72)

(H)ads → H+ + e- (73)

where methanol is oxidized via the strongly adsorbed species (CHnO)ads. If (CHnO)ads is

(COH)ads, step (72) is written as

(COH)ads + (OH)ads → (CO)ads + H2O (74)

Reaction described in equation (70) (methanol dissociative adsorption) occurs around

0.1-0.3 V vs. RHE at Pt[84,85] and it is not include to the rate-determining process (Tafel

slop is observed at 0.45 – 0.66 V). Therefore, the rate determining step is step showed

in Eq. (71) or (72); although Eq. (71) is reported to be rate determining process only by

several workers.[83,84,207]

8.2.2. Staircase voltammetry (SV) measurements

For a better insight of the system reactivity towards methanol oxidation reaction

staircase voltammetry has been used. Fig. 51 shows dependencies of staircase

voltammetric responses (step period of 50 s recorded every 25 mV) of methanol

oxidation on catalytic layers containing Nafion treated Pt40%/Vulcan XC-72 carbon

and cesium salts of Keggin-type heteropolyacids (Cs2.5PW12, Cs2.5PMo12, Cs2.5SiW12,

Cs2.5SiMo12) as a matrix and on commercial unmodified electrocatalyst (Pt40%/Vulcan

XC-72 carbon) for a comparison. For all materials containing Keggin-type cesium salts

as a matrix a significant increase of electrocatalytic currents was observed. This can be

rationalized in terms of the relative ability, potential mutual interactions, and the

existence of sufficient numbers of Pt centers for efficient oxidation of methanol.

Presence of Cs2.5H0.5PMo12O40 and Cs2.5H1.5SiW12O40 salts in the system results in some

increase of methanol electrocatalytic currents (compare curves for unmodified and

modified systems in Fig. 51).

96

0.4 0.6 0.8 1.0

0

5

10

15

20

25

j /

mA

cm

-2

E / V vs. RHE

Fig. 51. Staircase voltammetric current densities for the methanol (0.5 mol dm-3) oxidation

recorded every 25 mV (between 0.25 and 1.07 V) following application of 50-s potential steps at

the Nafion-containing layer of (--)Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H0.5PW12O40, (--) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PMo12O40, (--)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiW12O40, (-♦-) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H1.5SiMo12O40 and (--) unmodified Pt40%/Vulcan XC-72 carbon.

Electrolyte: argon saturated 0.5 mol dm-3 H2SO4..Temperature, 240C.

Fig. 52 presents dependencies of staircase voltammetric responses (step period

of 50 s recorded every 25 mV) of methanol oxidation on the same layers as in Fig. 51

but in a shorter potential range (from 0.43 to 0.65 V). An important issue of the data of

Fig. 52 is that the methanol oxidation currents densities tend to appear at less positive

potentials than on bare Pt. The best performances are shown by a catalytic layer

containing Cs2.5H0.5PMo12O40 salt as a matrix. The methanol oxidation reaction starts at

potential c.a. 20 mV less positive than for the unmodified system containing Nafion-

treated Pt40%/Vulcan XC-72 carbon. This enhancement effect and also the increased

current densities, may originate from the fact that PMo12 (in Cs2.5PMo12), added to

platinum, increase the CO tolerance of Pt-based system.

97

0.45 0.50 0.55 0.60 0.65

0

2

4

6

8

j /

mA

cm

-2

E / V vs. RHE

Fig. 52. Staircase voltammetric current densities for the methanol (0.5 mol dm-3) oxidation

recorded every 25 mV (between 0.43 and 0.65 V) following application of 50-s potential steps at

the Nafion-containing layer of (—)Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H0.5PW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PMo12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon.

Electrolyte: argon saturated 0.5 mol dm-3 H2SO4.. Temperature, 240C.

8.2.3 Chronoamperometry (CA) measurements

To further evaluate the reactivity of our electrocatalytic systems towards the

methanol oxidation, current-time measurements at different constant potentials (0.47 or

0.52 V) were performed (Fig. 53). As expected, the largest currents densities are

observed at the more positive applied potential (Fig. 53B). As observed in Fig. 53, when

investigated materials are polarized at constant potential (0.47 V or 0.52 V) in methanol

solutions, the current density decays continuously indicating a pronounced loss in

activity. The current density reaches almost stationary state after 800 seconds (Fig.

53A) and 600 seconds (Fig. 53B). The factor causing the decay of current density is

apparently, a blockage of the surface by some organic residue, which is slowly formed

and can only be oxidized at high anodic potentials.[84] The impregnation of cesium salts

98

of Keggin-type heteropolyacids matrix with commercial Pt40%/Vulcan XC-72 carbon

results in an increase of the methanol electrooxidation current densities.

0 200 400 600 800 10000.0

0.1

0.2

0.3 A

t / s

j / m

A c

m-2

0 200 400 600 800 10000.0

0.2

0.4

0.6

j /

mA

cm

-2

t / s

B

Fig. 53. Chronoamperometric curves recorded for the methanol oxidation at the Nafion-

containing layer of (—)Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PW12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PMo12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon upon application of (A)

0.47 V and (B) 0.52 V. Electrolyte: argon saturated 0.5 mol dm-3 H2SO4.. Temperature, 240C.

99

At both studied constant potentials, catalytic layers containing Cs2.5H0.5PW12O40 and

Cs2.5H1.5SiMo12O40 shows only small difference in current densities in comparison to

unmodified platinum (at 0.47V current decreased to the same level with that of pure Pt).

The highest activity in both potentials (0.47 V and 0.52 V) is displayed by the system

containing Cs2.5H0.5PMo12O40 salts as a matrix. We have to state that we not have

produced a practically more active catalyst here. The system’s activity here is due to the

higher number of the Pt active site which the methanol molecules reached to CO2 per

second, per surface site, but not to the mass of platinum used.

8.2.4 Electrochemical impedance spectroscopy

for methanol electrooxidation

In order to further compare the activity of methanol electrooxidation on a

different electrochemically polarized catalysts and to investigate the reaction

mechanism, electrochemical impedance spectroscopy (EIS) was carried out at different

potentials. Fig. 54 shows Nyquist plots of methanol electrooxidation for different

electrochemically polarized catalysts modified by cesium salts of Keggin-type

heteropolyacid matrix (Pt40%/C-Cs2.5HPA) and for unmodified Pt40%/Vulcan XC-72

carbon catalyst as a reference, at potentials of 0.50, 0.65 and 0.75 V.

The EIS results indicate that the methanol electrooxidation on Nafion-containing

Pt40%/Vulcan XC-72 carbon with cesium salts of HPAs (Cs2.5PW12, Cs2.5PMo12,

Cs2.5SiW12, Cs2.5SiMo12) matrix at various potentials shows different impedance

behaviors. In order to analyze reaction mechanism of methanol electrooxidation on

Pt/C-Cs2.5HPAs catalysts, a simple two-step model for methanol electrooxidation can be

assumed[209]:

−+ ++→ eHCOOHCH adsI 441

3 (75)

−+ ++→+ eHCOOHCO Iads 2222

2 (76)

I1 is the rate leading to the adsorbed surface intermediates COads. I2 is the rate for

oxidation of COads. IF is the Faradaic current and stand for net rate of charge transfer. In

this assuming, we just consider only one intermediate COads in methanol

electrooxidation.

100

)1(11 COmckI θ−= (77)

OHCOkI θθ22 = , 21 III F += (78)

where θCO and θOH are the fractional surface coverage of COads and OH, respectively.

For simplifying the analysis, the variation of θOH is assumed to have little effect on

impedance behavior of methanol electrooxidation, defined as

)()( 2121 IIKIIq

F

dt

d

CO

−=−==Θ θ (79)

where qco is the quantity of charge needed to complete a full coverage of CO on the

electrocatalyst.[209]

According to the kinetic theory derived by Harrington and Conway[218] and Cao[219] for

reactions involving intermediate adsorbate, in the electrode process of methanol

electrooxidation, the Faradaic current depends on the electrode potential E and one

other state variable θCO varying with E and affecting the Faradaic current.[209]

So the Faradaic admittance of methanol electrooxidation is[209]

ωja

B

RY

ctF +

+= 1 (80)

where Rct = (∂E/∂IF)SS is charge transfer resistance of the electrode reaction and is the

only circuit element that has a simple physical meaning describing how fast the rate of

charge transfer during methanol electrooxidation changes with changing electrode

potential when the surface coverage of the intermediate is held constant. The subscript

“ss” denotes steady state. Rct also can be defined as Rct = limω→0 ReZf where ReZf

is the real component in complex plots (Nyquist plots, Fig. 54), ω the circular frequency

ω = 2πf. So Rct can be obtained directly from Nyquist plots.[209]

In Eq. 80, a = −(∂Θ/∂θ)SS > 0 a is the always positive for a stable steady process

and with a dimension of s−1, defined as B = mb where m = ∂IF/∂θ, b = (∂Θ/∂E)SS =

dθ/dE, and B with dimensions of Ω−1 cm−2 s−1.

The impedance Nyquist plots can be classified according to the sign of B value.

When B > 0, Eq. 80 can be rewritten as follows[209]:

LjRRRY

ctBj

Ba

ctF ωω +

+=+

+=0

1111 (81)

101

The dimension of R0 is Ω cm2 and of L is cm2. Thus there will be an inductive

component involved in Faradaic impedance. The impedance Nyquist plot will be a

capacitive arc in the high frequency range and an inductive arc in the low frequency

range.

When B < 0, in this case:

ωja

B

RY

ctF +

−= 1 (82)

The Faradaic impedance can be rewritten as

aa

act

ct

ctct

FF CRj

RR

jBRa

BRR

YZ

ωω ++=

+−+==

1

1 2

(83)

with Ra = Rct 2|B|/a − Rct|B|, Ca = 1/Rct2|B|.

The dimension of Ra is Ω cm2 and of Ca, F cm−2. In this group there are still two cases,

which can be classified.[209]

When a − Rct|B| > 0, then Ra > 0, two capacitive arcs will be displayed on the

first quadrant of Nyquist plot. When a − Rct|B| < 0, then in this case Ra < 0, and the

capacitive arc in low frequency range will enter into the second quadrant of Nyquist

plot.[209]

According to above analysis, for methanol electrooxidation on Pt/C-Cs2.5HPAs

catalysts, the impedance parameters from Eqs. 77 and 78 can be deduced and are as

follows[209]:

[ ] [ ]COOHmmSS

kckckRT

FK

E

IIK

Eb θθα

)()(

21121 +−=

∂−∂=

∂Θ∂= (84)

mOHSS

F ckkIII

m 1221 )( −=

∂−∂=

∂∂= θ

θθ (85)

1 When methanol electrooxidation at low potential range (0.5 V), assumes

reaction (1), methanol dehydrogenation is rate-determining step, then k1 « k2. So, when

k2θOH > k1cm, According to Eqs. 84 and 85, thus b < 0, and m > 0, namely B < 0.

From Eq. 83, Nyquist plots of EIS at 0.4V (Fig. 54A) should show capacitive

behaviors. Moreover, in this case, a − Rct|B| > 0, so two overlapped capacitive

semicircles should exist in Nyquist plot and signify a reaction with one adsorbed

intermediate.[209]

102

Fig. 54. Nyquist plots

recorded in Ar saturated

0.5 mol dm-3 CH3OH +

0.5 mol dm-3 H2SO4

aqueous solution for

Nafion-containing

(--) Pt40%/Vulcan XC-72

carbon modified with

Cs2.5H0.5PW12O40,

(--) Pt40%/Vulcan XC-72

carbon modified with

Cs2.5H0.5PMo12O40,

(--) Pt40%/Vulcan XC-72

carbon modified with

Cs2.5H1.5SiW12O40,

(-♦-) Pt40%/Vulcan XC-72

carbon modified with

Cs2.5H1.5SiMo12O40 and

(--) unmodified Pt40%/Vulcan

XC-72 carbon catalysts at

different potentials: (a) 0.50 V;

(b) 0.65 V; (c) 0.75 V.

The frequency range is from

0.05Hz to 100kHz.

Each electrode contained 100 µg

cm-2 of the platinum.

0 1000 2000 3000 4000 5000

0

1000

2000

3000

4000

5000

6000

-Z

imag

/ oh

m

Zreal

/ ohm

A

0 200 400 600 800 1000-400

-300

-200

-100

0

100

200

300

400

500 B

-Z

imag

/ oh

m

Zreal

/ ohm

-500 -400 -300 -200 -100 0 100

-400

-300

-200

-100

0

100

200

300

-Zim

ag /

ohm

Zreal

/ ohm

C

103

The analyses from EIS indicate that, at low potential region , reaction (I1) might be rate-

determining step, and means the oxidation of COads with the OHads is fast.[208,209]

2. When methanol electrooxidation at intermediate potential range (0.65 V), with

an increase of potential, the rate of reaction (75) is increasing, but is not enough to

exceed the rate of reaction (76) obviously. So in this case the rate-determining step of

methanol electrooxidation is in transition region[209,210]. Thus maybe has following

relationship[209]:

k2 θOH θCO + k1 cm θCO > k1 cm > k2 θOH

From Eqs. 84 and 85, b < 0, and m < 0 can be deduced and then B = mb > 0.

According to Eq. 81 an inductive arc in low frequency range will be exhibited in

Nyquist plot. The inductive behavior from theoretical analysis is also observed in our

experiments shown in Fig. 54B. In general, the condition of occurrence of an inductive

behavior in Nyquist plot is mb > 0. This means, if the variation of the electrode potential

causes a variation of the Faradaic current density not only through its effect on the

strength of the electric field in the double layer but also through its effect on another

variable X and the both effects act in the same direction, an inductive component will be

involved in the Faradaic impedance.[219] According to impedance parameters, b and m,

inductive behavior in methanol electrooxidation reveals the COads coverage decreases

with increasing potential (b < 0), and the decreasing COads lead to an increase of

Faradaic current (m < 0). A reasonable explanation is with potential increasing, the large

amounts of OHads are formed on Cs2.5HPAs (particularly tungsten containing HPAs)

sites and react with COads and decrease its coverage. Meanwhile, the decreasing surface

coverage of COads will contribute to adsorption of methanol on Pt and enhance the

Faradaic current. So at the intermediate potential range, with an increase of potential

from 0.5 to 0.65 V, the transition from capacitive behavior to inductive behavior

indicates that the rate-determining step maybe is changing.[209,217] The similar inductive

behavior in EIS at about 0.65V was also observed by Wang and co-workers.[217]

3. When methanol electrooxidation at high potential range (0.75 V), in this case,

reaction (76) can be assumed as rate-determining step.[209,217]

Thus, k1 » k2 and then k2θOH < k1cm.

According to Eqs. 84 and 85, b > 0 and m < 0 can be deduced, then B < 0.

Moreover, in this case a − Rct |B| < 0. The capacitive arc at low frequency in Nyquist

plot will flips to the second quadrant with the real component of the impedance

104

becoming negative. The theoretical analyses agree well with the experimental results

shown in Fig. 54C. This means that resistance Ra becomes negative resulting from

passivation of electrode surface.[221] Melnick et al[220] indicated that the passivation of

the Pt electrode during methanol electrooxidation is probably due to the reversible

formation of oxide species. Meanwhile, due to reaction (76) is rate-determining step, the

oxidation of COads with OH is very slow, so the passivation at higher potentials can be

explained by the formation of a large amount of COads and OH on surface of Pt/C-

Cs2.5HPAs catalyst. Therefore, adsorption of methanol on Pt sites is inhibited due to an

increase of coverage of COads and OH on Pt sites and the electrooxidation rate is almost

no obvious increase.[209] From the EIS analyses, reaction (76) as rate–determining step

at high potential range can well explain experimental results.[209,217]

Impedance spectroscopy of the electrooxidation of methanol on

electrochemically polarized Pt/C-Cs2.5HPAs catalysts indicates that cathodic

polarization leads to an enhancement for methanol oxidation. The reaction proceeds

with one adsorbed intermediate of appreciable surface concentration and lifetime. The

different impedance behaviors in three different potential regions reveal that the

mechanism and rate-determining step in methanol electrooxidation vary with potentials.

At low potential range (0.50 V), methanol dehydrogenation is rate-determining step

while at high potential range (0.75 V), the oxidation and removal of COads became rate-

determining step. Meanwhile, at intermediate potential region(0.65 V), the rate-

determining step in methanol electrooxidation is maybe in transition range.[209,217]

In spite of the different impedance behaviors at different potentials, the

diameters of the primary semicircle of the Nafion-containing Pt40%/C with Cs2.5PMo12

and Pt40%/C with Cs2.5SiW12 catalysts are always smaller that of the Nafion-containing

Pt40%/C with Cs2.5PW12, Pt40%/C with Cs2.5SiMo12 and unmodified Pt40%/C systems.

This behavior means that the lowers charge transfer resistances for the electrode

reaction can be obtained on the Pt40%/Vulcan XC-72 carbon modified by

Cs2.5H0.5PMo12O40 or Cs2.5H1.5SiW12O40 matrix.

105

8.2.5 CO stripping voltammetry study

at Pt/C-Cs2.5HPAs

Fig. 55 shows cyclic voltammograms obtained with catalytic layers modified by

cesium salts of Keggin-type heteropolyacids (Cs2.5PW12, Cs2.5PMo12, Cs2.5SiW12 and

Cs2.5SiMo12) and with unmodified system from pure Pt40%/Vulcan XC-72 carbon with

a CO adsorbed ad-layer. An insert is put into the figure to emphasize the difference

between the potential of CO oxidation in the first anodic cycle for all investigated

systems. It is clear that the first anodic cycle is different from the second anodic cycle in

that a peak between 0.632 and 0.89 V exist in the first anodic cycle in each

voltammogram. The peak between 0.632 and 0.89 V is the oxidation of adsorbed CO.

For all the layers CO-adsorbed on the Pt surfaces is oxidized already in the first anodic

cycle, which is confirmed by the absence of peak in the second anodic cycle at

potentials between 0.632 and 0.89 V. Moreover, for the CO oxidation peaks, important

information such as onset and peak potential for CO oxidation is summarized in Table

12.

Catalyst Onset potential (V)

Peak potential (V)

Pt/C Pt/C-Cs2.5PW12 Pt/C-Cs2.5PMo12 Pt/C-Cs2.5SiW12 Pt/C-Cs2.5SiMo12

0.743 0.665 0.692 0.665 0.632

0.813 0.792 0.817 0.784 0.733

Table 12. Onset and peak potentials for oxidation of adsorbed CO with the four modified

systems and for pure Pt/C as a reference.

Among the Pt/C, Pt/C-Cs2.5PW12, Pt/C-Cs2.5PMo12, Pt/C-Cs2.5SiW12 and Pt/C-

Cs2.5SiMo12 catalysts, the Pt/C-Cs2.5SiMo12 system has the lowest onset and peak

potentials for CO oxidation, while the Pt/C has the highest ones. CO starts to be

oxidized with the Pt/C-Cs2.5SiMo12 electrode at a potential of 0.632 V which is 111 mV

lower than with the Pt/C electrode. The addition of others cesium salts of Keggin-type

heteropolyacids (Cs2.5PW12, Cs2.5PMo12, Cs2.5SiW12) also results in shifting of the onset

potential of CO oxidation in to a lower values (Table 12). These results suggest that the

HPA cesium salts behave as a redox mediator for the CO oxidation reaction with Pt

present in the system. The proposed mechanism proceeds via the following reaction[27]:

106

HPA + CO + H2O → CO2 + H2HPA (86)

H2HPA → HPA + 2H+ + 2e- (87)

This mechanism is considered particularly for the molybdenum containing

heteropolyacids, in which oxidation potential is shifted to the more positive values of

potentials in comparison to the tungsten containing heteropolyacids.[19,21]

0,6 0,7 0,80

2

4

0,0 0,2 0,4 0,6 0,8 1,0 1,2

-1

0

1

2

3

4

5

j /

mA

cm

-2

E / V vs. RHE

j / m

A c

m-2

E / V vs. RHE

Fig. 55. Cyclic voltammograms in the potential range 0.025 to 1.125 V vs. RHE on the Nafion-

containing layer of (—)Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PW12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PMo12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon with (first anodic cycle)

and without (second anodic cycle) a CO adsorbed ad-layer. Electrolyte, argon saturated 0.5

mol dm-3 H2SO4. Scan rate, 20 mV s-1. Temperature 24 0C.

The catalytic enhancement of the layer with cesium salt of tungsten containing

heteropolyacids (Cs2.5PW12, Cs2.5SiW12) as a matrix may be due to the synergistic effect

between Pt and PW12 (or SiW12). Assuming that this enhancement effect may originate

from the fact that tungsten containing heteropolycompound is analogous to the parent

tungsten oxide[74,211], tungsten units may provide additional -OH groups or radicals

107

capable of facilitating oxidation of passivating intermediates (COads)[26,74,222] on Pt. This

reaction can be described as follows[222]:

CO + OH → CO2 + H+ + e- (88) Pt WO3

Alternatively, introduction of PW12 (or SiW12) may induce morphological

differences and lead to better Pt catalyst utilization.[74]

8.2.6 Electrochemical stability of investigated materials

containing Cs2.5-HPAs matrix

Electrochemical stability, that is a very important factor in the application to

practical fuel cells, was tested using long-term CV (Fig. 56) for methanol oxidation

reaction under argon flow. Current densities where obtained from the forward peak of

the last cycle (9th cycle) of methanol oxidation recorded in argon saturated 0.5 mol dm-3

H2SO4 containing 0.5 mol dm-3 CH3OH solution at 50 mV s-1. The gap between

measurements was 20 min. As Fig. 56 shows, the electrochemical stability of the system

containing Cs2.5H1.5SiW12O40 and Cs2.5H1.5SiMo12O40 as a matrix are fairly good,

indicating that the catalytic layers are stable in these conditions.

Moreover, also Cs2.5H0.5PW12O40 and Cs2.5H0.5PMo12O40 –containing systems

are quite stable during long-term stability test. It is important to note that for all

modified systems the current densities of MOR are higher than for unmodified catalytic

material containing Nafion treated Pt40%/Vulcan XC-72 carbon. The reduction of

currents densities during long-term stability test may result not only from accumulation

of poisonous species (such as COads) on the surface of Pt particles but also because of

methanol consumption during the successive scans and change of the surface structure

of the Pt particles. Nevertheless, the best results (the highest current densities) during

long-term stability test were obtained for the catalytic layers containing

Cs2.5H1.5SiW12O40 and Cs2.5H0.5PMo12O40 salts as a matrix.

108

0 200 400 600 8000

5

10

15

20

25

30

j p / m

A c

m-2

t / min

Fig. 56. Long-term stability test of the Nafion-containing (--) Pt40%/Vulcan XC-72 carbon

modified with Cs2.5H0.5PW12O40, (--) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H0.5PMo12O40, (--) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiW12O40, (-♦-)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiMo12O40 and (--) unmodified

Pt40%/Vulcan XC-72 carbon electrodes in argon saturated 0.5 mol dm-3 H2SO4 containing 0.5

mol dm-3 CH3OH solution at 50 mV s-1. Temperature, 240C. Same Pt loadings mounted in all

cases (LPt = 100 µg cm-2).

8.3 Electrochemical measurements

at system containing Rb2.5-HPAs matrix

At the beginning of this subsection we have to mark that the interest of the

rubidium salts of Keggin-type heteropolyacids were much lower than theirs cesium

equivalents. Nevertheless they seem to be very attractive in point of view theirs future

application as a matrix in anode materials for the alcohol fuel cells.

8.3.1. Cyclic voltammetry (CV) measurements

Fig. 57 illustrates cyclic voltammetric responses of a glassy carbon electrode

modified with Nafion-treated pure Pt40%/Vulcan XC-72 carbon (Pt40%/C) and

Pt40%/Vulcan XC-72 carbon modified with a matrix made from rubidium salts of

109

Keggin-type heteropolyacids (Rb2.5H0.5PW12O40, Rb2.5H0.5PMo12O40, Rb2.5H1.5SiW12O40

and Rb2.5H1.5SiMo12O40) recorded in argon-saturated 0.5 mol dm-3 H2SO4 at room

temperature. The platinum loading in all investigated materials was equal to 100µgcm-2.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-0.9

-0.6

-0.3

0.0

0.3

0.6

E / V vs. RHE

j / m

A c

m-2

Fig. 57. Cyclic voltammetric response of Nafion-containing films (deposited on glassy carbon

electrode) of (—) Pt40%/Vulcan XC-72 carbon modified with Rb2.5H0.5PW12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Rb2.5H0.5PMo12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Rb2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with

Rb2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon. Electrolyte: argon

saturated 0.5 mol dm-3 H2SO4. Scan rate, 10 mV s-1. Temperature, 240C.

The most positive potential of anodic peak (0.46 V, Fig. 57 light-blue) is due to

the reduction of Rb2.5H1.5SiMo12O40 salt. The voltammetric peak appearing at about

0.43 V (Fig. 57, green line) should be attributed to the reduction of Rb2.5H0.5PMo12O40

salt, while Rb2.5H1.5SiW12O40 (Fig. 57, dark-blue line) and Rb2.5H0.5PW12O40 (Fig. 57,

red line) salts becomes electroactive at a less positive potential of ca. 0.35 V. The

presence of rubidium salts of Keggin-type heteropolyacids (particularly Rb2.5PMo12)

leads to shift of the voltammetric peaks referring to the formation of Pt-oxo (PtO or

PtOH) species toward more positive potentials. Cyclic voltammetric responses of the

110

systems containing rubidium salts of HPAs matrix are very similar to these obtained for

their cesium equivalents.

To investigate the influence of the matrix in the catalytic layer on the behavior

towards methanol oxidation reaction (MOR) cyclic voltammetric measurements (Fig.

58) were carried out with four kinds of systems containing Nafion treated

Pt40%/Vulcan XC-72 carbon modified by rubidium salts of Keggin-type HPAs

(Rb2.5PW12, Rb2.5PMo12, Rb2.5SiW12 and Rb2.5SiMo12) matrix and a commercial

unmodified electrocatalyst (Pt40%/C) as a reference.

In the Fig. 58 for all investigated materials we observe two characteristic

irreversible peaks (A and B) during the methanol electrooxidation. Forward scan peak

A, at potential ca. 0.85 V, is the oxidation current density that involves the formation of

intermediates.[205,206] As it was mention previously, it is certain that (CO)ads is a poison

to platinum catalyst for further oxidation of methanol. The backward peak B at ca. 0.73

V, correspond to the oxidation of the intermediates created during forward cycle.

0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40B

A

j /

mA

cm

-2

E / V vs. RHE

Fig. 58. Cyclic voltammetric response of Nafion-containing films (deposited on glassy carbon

electrode) of (—) Pt40%/Vulcan XC-72 carbon modified with Rb2.5H0.5PW12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Rb2.5H0.5PMo12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Rb2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with

Rb2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon. Electrolyte: argon

saturated 0.5 mol dm-3 CH3OH + 0.5 mol dm-3 H2SO4. Scan rate, 10 mV s-1. Temperature, 240C.

111

Moreover, Fig. 58 clearly shows that modification of commercial Pt40%/C by rubidium

HPA salts matrix increases noticeably electrocatalytic activity of the system for MOR,

what can be attributed to the better Pt catalyst utilization. The highest current density

was obtained for the catalytic layer containing Rb2.5H1.5SiW12O40 as a matrix.

Nevertheless the current densities obtained for materials containing Rb2.5H0.5PMo12O40

and Rb2.5H1.5SiMo12O40 are only 7% lower from this obtained from the layer containing

Rb2.5SiW12. Furthermore the onset potential of methanol oxidation on all modified

catalytic layers is shifted to less positive values of potential in comparison to

unmodified catalytic layer containing commercial Pt40%/C. This indicates that addition

of the new type of matrix can facilitate oxidation of methanol and produce passivating

intermediates during this process.

The reaction mechanism was investigated on the basis of the Tafel plot analyses.

The Tafel plots for the methanol oxidation obtained by cyclic voltammograms at the

scan rate 5 mV s-1 at the systems modified by rubidium salts of Keggin-type

heteropolyacids and for the unmodified catalytic layer made from Pt40%/Vulcan XC-72

carbon are showed in Fig. 59.

The main slope, in the region 2 (1/b, expressed in mV dec.-1), for unmodified

Pt40%/C is 88 mV decade-1. The other Tafel slopes for the catalytic layers containing

Rb2.5H0.5PW12O40, Rb2.5H0.5PMo12O40, Rb2.5H1.5SiW12O40 and Rb2.5H1.5SiMo12O40

matrix were estimated, in the same manner as for the pure Pt40%/Vulcan XC-72 carbon

systems, to be 106, 102, 106 and 113 mV decade-1, respectively. These results are in

good agreement with those reported by Inada and co-workers.[85] Small deviations are

observed for the system containing Nafion treated Pt40%/C with Rb2.5H1.5SiMo12O40

matrix.

For the slope in the shown range, it is postulated that the number of electrons

involved in the electrode reaction n is c.a. 1 (n ≈ 1) at 240C[82] and that the rate

determining process is the step in which CO from Pt surfaces is oxidized by the –OH

groups to CO2.[85] In spite of this Eq. (71) showed previously is reported to be rate

determining process only by several workers.[83,84,207]

112

0,3 0,4 0,5 0,6 0,7-3

-2

-1

0

1

region 2

113 mV dec-1106 mV dec-1

106 mV dec-1

102 mV dec-1

log

j / m

A c

m-2

E / V vs. RHE

88 mV dec-1region 1

Fig. 59.Tafel plots of methanol oxidation at the Nafion-containing (--) Pt40%/Vulcan XC-72

carbon modified with Rb2.5H0.5PW12O40, (--) Pt40%/Vulcan XC-72 carbon modified with

Rb2.5H0.5PMo12O40, (--) Pt40%/Vulcan XC-72 carbon modified with Rb2.5H1.5SiW12O40, (-♦-)

Pt40%/Vulcan XC-72 carbon modified with Rb2.5H1.5SiMo12O40 and (--) unmodified

Pt40%/Vulcan XC-72 carbon. Temperature, 240C.

8.3.2 Staircase voltammetry (SV) measurements

Reactivity of the Pt/C modified with Rb2.5-HPAs towards methanol oxidation

was also examined by using staircase voltammetry method. Fig. 60 present

dependencies of staircase voltammetric responses (step period of 50 s recorded every 25

mV) of methanol oxidation on catalytic layers containing Nafion treated Pt40%/Vulcan

XC-72 carbon and rubidium salts of Keggin-type heteropolyacids (Rb2.5PW12,

Rb2.5PMo12, Rb2.5SiW12, Rb2.5SiMo12) matrix and unmodified electrocatalyst

(Pt40%/Vulcan XC-72 carbon) as a reference.

For all materials containing Keggin-type rubidium salts matrix a significant

increase of electrocatalytic currents was observed. This can be rationalized in terms of

the relative ability, potential mutual interactions, and the existence of sufficient numbers

of Pt centers for efficient oxidation of methanol. The highest current densities for

113

methanol electrooxidation were obtained for catalytic layers containing rubidium salts

of 12-silicotungstic and 12-silicomolybdic acid as a matrix.

0.4 0.6 0.8 1.00

5

10

15

20

25

E / V vs. RHE

j / m

A c

m-2

Fig. 60. Staircase voltammetric current densities for the methanol (0.5 mol dm-3) oxidation

recorded every 25 mV (between 0.25 and 1.07 V) following application of 50-s potential steps at

the Nafion-containing layer of (--)Pt40%/Vulcan XC-72 carbon modified with

Rb2.5H0.5PW12O40, (--) Pt40%/Vulcan XC-72 carbon modified with Rb2.5H0.5PMo12O40, (--)

Pt40%/Vulcan XC-72 carbon modified with Rb2.5H1.5SiW12O40, (-♦-) Pt40%/Vulcan XC-72

carbon modified with Rb2.5H1.5SiMo12O40 and (--) unmodified Pt40%/Vulcan XC-72 carbon.

Electrolyte: argon saturated 0.5 mol dm-3 H2SO4..Temperature, 240C.

Fig. 61 present dependencies of staircase voltammetric responses of methanol

oxidation on the same layers as in Fig. 60 but in a shorter potential range (from 0.43 to

0.65 V). An important issue of the data of Fig.60 is that the methanol oxidation currents

densities tend to appear at less positive potentials than on bare Pt, the same as for Cs+

salts of HPAs. The best performance shown catalytic layer containing

Rb2.5H1.5SiW12O40 salt as a matrix. The onset potential of methanol oxidation is ca. 25

mV less positive than for the unmodified system containing Nafion-treated

Pt40%/Vulcan XC-72 carbon. However, it is important to underline that the onset

potentials for the catalytic layers modified by the residue rubidium salts of 12-

phosphotungstic acid, 12-phosphomolybdic acid and 12-silicomolybdic acid are also

shifted to the less positive values of potential. This enhancement effect, as well as the

114

increasing of current densities, may originate from the fact that rubidium salts of

Keggin-type heteropolyacids matrix added to platinum increase the CO tolerance of Pt-

based system.

0.45 0.50 0.55 0.60 0.65

0

2

4

6

8

E / V vs. RHE

j / m

A c

m-2

Fig. 61. Staircase voltammetric current densities for the methanol (0.5 mol dm-3) oxidation

recorded every 25 mV (between 0.43 and 0.65 V) following application of 50-s potential steps at

the Nafion-containing layer of (—)Pt40%/Vulcan XC-72 carbon modified with

Rb2.5H0.5PW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with Rb2.5H0.5PMo12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Rb2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Rb2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon.

Electrolyte: argon saturated 0.5 mol dm-3 H2SO4.. Temperature, 240C.

8.3.3 Chronoamperometry (CA) measurements

To further evaluate the reactivity of our electrocatalytic systems for the methanol

oxidation, current-time measurements at different constant potentials (0.47 or 0.52 V)

were performed (Fig. 62). As observed in Fig. 62, when investigated materials are

polarized at constant potential (0.47 V or 0.52 V) in methanol solutions, the current

density decays continuously indicating a pronounced loss in activity. The current

density reaches almost stationary state after 600 seconds (Fig. 62A) and 400 seconds

(Fig. 62B). As it was mentioned before the factor causing the decay of current density is

115

apparently, a blockage of the surface by some organic residue, which is slowly formed

during methanol oxidation process.[84] In Fig. 62B, for the Rb2.5SiW12 containing

catalytic layer the initial current decay is greater than for the others catalysts. This

behavior can be addressed to the greater extent of the surface coverage with partially

oxidized intermediates, than for other layers.

0 200 400 600 800 10000.0

0.1

0.2

0.3

t / s

j / m

A c

m-2

A

0 200 400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0

B

t / s

j / m

A c

m-2

Fig. 62. Chronoamperometric curves recorded for the methanol oxidation at the Nafion-

containing layer of (—)Pt40%/Vulcan XC-72 carbon modified with Rb2.5H0.5PW12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Rb2.5H0.5PMo12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Rb2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with

Rb2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon upon application of (A)

0.47 V and (B) 0.52 V. Electrolyte: argon saturated 0.5 mol dm-3 H2SO4.. Temperature, 240C.

116

All catalytic layers modified with rubidium salts of Keggin-type heteropolyacids

matrix exhibit higher methanol electrooxidation current densities (at constant potential

0.47 or 0.52 V) during all experiment time in comparison to unmodified system

(Pt40%/C). The highest activities in both constant potentials are displayed by systems

containing Rb2.5H0.5PMo12O40, Rb2.5H1.5SiW12O40 and Rb2.5H1.5SiMo12O40 salts as a

matrix. This can be explained by the increasing Pt active surface area, which can be

reached by methanol molecules and increment the CO tolerance of Pt-based system.

8.3.4 Electrochemical impedance spectroscopy measurements

In order to further compare the activity of the catalysts containing rubidium salts

of HPA`s matrix for methanol oxidation, electrochemical impedance spectroscopy (EIS)

were carried out at different potentials in 0.5 mol dm-3 H2SO4 and CH3OH aqueous

solutions. The Nyquist plots for the corresponding Pt40%/C-Rb2.5HPAs systems and for

unmodified one, as a reference, recorded at 0.50, 0.65 and 0.75 V are showed in Fig. 63.

It is clearly seen that EIS of the methanol electrooxidation on investigated electrode

materials at various potentials shows different impedance behavior. The polarization

resistance can be measured from diameter of the primary semicircle. As it was in the

case of Cs2.5-HPAs modified catalytic layers, at low potentials range of methanol

electrooxidation Pt/C-Rb2.5HPA electrode interface is dominated by adsorption and

electrical double layer at low potential range.[208] The Nyquist plots of electrochemical

impedance spectroscopy should show capacitive behaviors, which could be seen in Fig.

63A. At intermediate potentials range (Fig. 63B) the rate-determining step of MOR is in

transition region. The inductive behaviors observed in our experiments shown in Fig.

63B are in agreement with the literature data.[209,210]

When EIS are performed at high potential range (Fig. 63C), the oxidation and removal

of COads become rate-determining step. The capacitive arc at low frequency in Nyquist

plot reverses to the second and third quadrants, with the real component of the

impedance becoming negative[209], which is shown in Fig. 63C.

117

Fig. 63. Nyquist plots

recorded in Ar saturated

0.5 mol dm-3 CH3OH +

0.5 mol dm-3 H2SO4

aqueous solution for

Nafion-containing

(--) Pt40%/Vulcan XC-72

carbon modified with

Rb2.5H0.5PW12O40,

(--) Pt40%/Vulcan XC-72

carbon modified with

Rb2.5H0.5PMo12O40,

(--) Pt40%/Vulcan XC-72

carbon modified with

Rb2.5H1.5SiW12O40,

(-♦-) Pt40%/Vulcan XC-72

carbon modified with

Rb2.5H1.5SiMo12O40 and

(--) unmodified Pt40%/Vulcan

XC-72 carbon catalysts at

different potentials: (a) 0.50 V;

(b) 0.65 V; (c) 0.75 V.

The frequency range is from

0.05Hz to 100kHz.

Each electrode contained 100 µg

cm-2 of the platinum.

0 1000 2000 3000 4000 5000

0

1000

2000

3000

4000

-Zim

ag /

ohm

Zreal

/ ohm

A

0 200 400 600 800 1000-400

-200

0

200

400

-Z

imag

/ oh

m

Zreal

/ ohm

B

-400 -300 -200 -100 0-400

-300

-200

-100

0

100

200

-Zim

ag /

ohm

Zreal

/ ohm

C

118

The electrochemical impedance spectroscopy of methanol electrooxidation

presented in Fig. 63 shows that the mechanism and rate-determining step vary with

potentials. At low, intermediate and high potentials range the rate-determining steps of

methanol electrooxidation are methanol dehydrogenation, transition range and oxidation

and removal of COads, respectively.

Moreover the diameters of the primary semicircle of the Nafion-containing

Pt40%/C with Rb2.5SiW12 and Pt40%/C with Rb2.5SiMo12 catalysts are always smaller

than that of the Nafion-containing Pt40%/C with Rb2.5PW12, Pt40%/C with Rb2.5PMo12

and the unmodified Pt40%/C systems in all electrochemical impedance spectroscopy

(EIS) measurements, performed at different potentials

This behavior can be explained by the lower charge transfer resistance of the

electrode reaction on the Pt40%/Vulcan XC-72 carbon modified by Rb2.5SiW12 and

Rb2.5SiMo12 matrix.

8.3.5 CO stripping voltammetry study

at Pt/C modified with Rb2.5-HPAs

The cyclic voltammograms obtained on Nafion containing catalytic layers

modified by rubidium salts of Keggin-type heteropolyacids (Rb2.5PW12, Rb2.5PMo12,

Rb2.5SiW12 and Rb2.5SiMo12) and on unmodified system (Pt40%/Vulcan XC-72 carbon),

as a comparison, with a CO adsorbed ad-layer are shown in Fig. 64. An insert is put into

the figure to emphasize the difference between the potential of CO oxidation in the first

anodic cycle for all investigated systems. For all the layers, CO-adsorbed on the Pt

surfaces, is oxidized already in the first anodic cycles, as confirmed by the absence of

peak in the second anodic cycles at potentials between 0.63 and 1 V. Moreover, for the

CO oxidation peaks in the first anodic cycles onset and peak potentials for CO oxidation

are summarized in Table 13.

119

0.0 0.2 0.4 0.6 0.8 1.0 1.2-2

-1

0

1

2

3

4

5

0.6 0.8

0

2

4

E / V vs. RHE

j / m

A c

m-2

j / m

A c

m-2

E / V vs. RHE

Fig. 64. Cyclic voltammograms in the potential range 0.025 to 1.125 V vs. RHE on the Nafion-

containing layer of (—)Pt40%/Vulcan XC-72 carbon modified with Rb2.5H0.5PW12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Rb2.5H0.5PMo12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Rb2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with

Rb2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon with (first anodic cycle)

and without (second anodic cycle) a CO adsorbed ad-layer. Electrolyte, argon saturated 0.5

mol dm-3 H2SO4. Scan rate, 20 mV s-1. Temperature 24 0C.

Catalyst Onset potential

(V)

Peak potential

(V)

Pt/C

Pt/C-Rb2.5PW12

Pt/C-Rb2.5PMo12

Pt/C-Rb2.5SiW12

Pt/C-Rb2.5SiMo12

0.743

0.679

0.656

0.656

0.632

0.813

0.831

0.790

0.780

0.750

Table 13. Onset and peak potentials for oxidation of adsorbed CO with the four modified

systems and for pure Pt/C as a reference.

From the Table 13 we can easily calculate that the catalytic enhancement toward CO

oxidation is on the order of 64 – 110 mV in comparison to the unmodified

Pt40%/Vulcan XC-72 carbon electrode. The lowest onset potential of oxidation of CO-

120

adsorbed on the Pt surfaces was recorded for the system containing Rb2.5SiMo12 matrix.

This shift to less positive values of potential is in well agreement with the onset

potential obtained for catalytic layer containing Cs2.5SiMo12 salt as a matrix. These

results suggest that the salts containing SiMo12 heteropolycompound can provide the

best protection of the Pt surface against poisoning by CO, moreover they can help

oxidize strongly adsorbed CO, e.g. by weakening the Pt-CO bond. For the others

catalytic layers modified by rubidium salts of Keggin-type heteropolyacids matrix

(Rb2.5PW12, Rb2.5PMo12, Rb2.5SiW12) we also obtain lower value of the onset potential

for CO oxidation (Table 13). This shift of potential on the modified catalyst could be

attributed to the presence of oxygenated species on Rb2.5-HPAs at lower potentials

compared to platinum. According to the bifunctional mechanism[212], these oxygenating

species allow the oxidation of CO into CO2 at lower potentials.

These results, obtained in this subchapter, suggest that the presence of POMs

anions (attend in the rubidium salts of Keggin-type heteropolyacids) might facilitate the

electrooxidation of intermediate species such as the CO that adsorbed on the Pt catalyst

surfaces[27], leading to suppression of the poisoning effect on Pt catalysts by CO or CO-

like intermediates. The catalytic enhancement of the layer containing Rb2.5SiW12 matrix

may be due to the synergistic effect between Pt and SiW12. This behavior can be

explained by assuming that tungsten units may provide additional -OH groups or

radicals capable of facilitating oxidation of passivating intermediates (COads) on Pt (like

in the parent WO3).[74,211] Tungsten containing salts may also induce morphological

differences and lead to better Pt catalyst utilization.

8.3.6 Electrochemical stability of investigated materials

containing Rb2.5-HPAs matrix

The long-term stability of Nafion treated Pt40%/Vulcan XC-72 carbon modified

by Rb2.5-HPAs matrix systems in 0.5 mol dm-3 H2SO4 containing 0.5 mol dm-3 CH3OH

aqueous solution has been investigated by cyclic voltammetry and the corresponding

results are shown in Fig. 65. Long-term stability test for unmodified system (Pt40%/C)

is presented as a reference.

121

0 200 400 600 8000

5

10

15

20

25

30

j p / m

A c

m-2

t / min

Fig. 65. Long-term stability test of the Nafion-containing (--)Pt40%/Vulcan XC-72 carbon

modified with Rb2.5H0.5PW12O40, (--) Pt40%/Vulcan XC-72 carbon modified with

Rb2.5H0.5PMo12O40, (--) Pt40%/Vulcan XC-72 carbon modified with Rb2.5H1.5SiW12O40, (-♦-)

Pt40%/Vulcan XC-72 carbon modified with Rb2.5H1.5SiMo12O40 and (--) unmodified

Pt40%/Vulcan XC-72 carbon electrodes in argon saturated 0.5 mol dm-3 H2SO4 containing 0.5

mol dm-3 CH3OH solution at 50 mV s-1. Temperature, 240C. Same Pt loadings mounted in all

cases (LPt = 100 µg cm-2).

The current densities were read from the forward anodic peak of the last cycle (9th

cycle). The gap between measurements was 20 min. The most stable performances

during long-term test are shown by catalytic layers containing Rb2.5H0.5PW12O40 and

Rb2.5H0.5PMo12O40, in which drop of the current densities was equal 23% and 27%

respectively. However, the best results (the highest current densities) during long-term

stability tests were obtained for the catalytic layers containing Rb2.5H1.5SiW12O40 and

Rb2.5H1.5SiMo12O40 salts as a matrix. We should also mention that, during all stability

tests, the current densities of methanol electrooxidation on modified systems were

higher that that for Nafion treated Pt40%/Vulcan XC-72 carbon electrode. The

reduction of currents densities during long-term stability tests may result not only from

accumulation of poisonous specious (such as COads) on the surface of Pt particles but

also because of methanol consumption during the successive scans and change of the

surface structure of the Pt particles.

122

8.4. Summary and conclusions

1) The values of peak current densities of methanol electrooxidation obtained from

cyclic voltammetry measurements, as a function of electrode composition are presented in

Fig. 66. Our result clearly shows that the modification of commercial Pt catalyst by Cs+

or Rb+ salts of HPAs increase electrocatalytic activity of the system towards methanol

electrooxidation. Based on this results we can conclude that the presence of Cs2.5-HPAs

or Rb2.5-HPAs matrix into the catalytic layer may facilitate the electrooxidation of

intermediate species such as the CO that is adsorbed on the Pt catalyst surfaces leading

to suppression of the poisoning effect on Pt catalysts by CO or CO-like intermediates.

0 4 8 12 16

0

10

20

30

Pt40%

/C-R

b 2.5SiM

o 12

Pt40%

/C-R

b 2.5SiW 12

Pt40%

/C-R

b 2.5PM

o 12

Pt40%

/C-R

b 2.5PW 12

Pt40%

/C-C

s 2.5PW 12

Pt40%

/C-C

s 2.5PM

o 12

Pt40%

/C-C

s 2.5SiW 12

Pt40%

/C-C

s 2.5SiM

o 12

Pt40%

/C

j p /

mA

cm

-2

Fig. 66. Peak current densities obtained from forward scan of methanol electrooxidation on the

investigated materials recorded in argon saturated 0.5 mol dm-3 CH3OH + 0.5 mol dm-3 H2SO4.

Data taken from Fig.49 and 58.

2) The kinetic data of the methanol electrooxidation on investigated catalytic materials

obtained from the Tafel plot analyses shows that, in all cases, the rate determining step

is the step in which CO from Pt surfaces is oxidized by the –OH groups to CO2.

123

Catalyst Tafel slope

b / mV dec-1

Catalyst Tafel slope

b / mV dec-1

Pt/C 88 Pt/C + Rb2.5PW12 106

Pt/C + Cs2.5PW12 93 Pt/C + Rb2.5PMo12 102

Pt/C + Cs2.5PMo12 95 Pt/C + Rb2.5SiW12 106

Pt/C + Cs2.5SiW12 106 Pt/C + Rb2.5SiMo12 113

Pt/C + Cs2.5SiMo12 95

Table 14. Tafel slope values for all investigated catalytic materials.

3) The results received from staircase voltammetry (Fig. 67), which were used to better

insight into the systems reactivity, clearly shows that the onset potential of the methanol

oxidation reaction is lower for almost all modified materials. Only in the case of

Cs2.5PW12 and Cs2.5SiMo12 modified layers onset potential is not change in comparison

to the Pt/C electrode. These results are very important in a point of view of theirs

practical application as an anode materials for the fuel cells.

0 4 8 12 16

0,50

0,52

0,54

0,56

Eon

set /

V v

s. R

HE

Pt40%

/C-R

b 2.5SiM

o 12

Pt40%

/C-R

b 2.5SiW 12

Pt40%

/C-R

b 2.5PM

o 12

Pt40%

/C-R

b 2.5PW 12

Pt40%

/C-C

s 2.5PW 12

Pt40%

/C-C

s 2.5PM

o 12

Pt40%

/C-C

s 2.5SiW 12

Pt40%

/C-C

s 2.5SiM

o 12

Pt40%

/C

Fig. 67. Onset potential of methanol electrooxidation on the investigated materials recorded in

argon saturated 0.5 mol dm-3 CH3OH + 0.5 mol dm-3 H2SO4. Data taken from Fig. 52 and 61.

Based on the results obtained by Samant et al.[7] we can conclude that the

superior oxidation kinetics in presence of zeolites matrix could be due to the preferential

formation of CO clusters on platinum that are limited by the steric constraints imposed

124

by the zeolites framework, followed by facile oxidation to CO2 by interaction with the

surface or bridged hydroxyls of the zeolites species.

4) Chronoamperometry tests on catalytic layers (Fig. 68) confirm earlier obtained

results that modification of commercial Pt/C with ours zeolite matrix increase catalytic

activity of the system towards methanol oxidation. The best performance after 1000

seconds of polarization at constant potentials (0.47 and 0.52 V) is obtained with the

system modified with Rb2.5SiW12 matrix. Nevertheless, the layers containing

Cs2.5PMo12, Cs2.5SiW12, Rb2.5PW12, Rb2.5PmO12 and Rb2.5SiMo12 salts also exhibit

higher current densities than the unmodified Pt/C electrode at both studied constant

potentials.

0 ,0 0

0 ,0 2

0 ,0 4

0 ,0 6

0 4 8 12 16

0 ,0 0

0 ,1 5

0 ,3 0

A

B

j / m

A c

m-2

Pt40%

/C-R

b 2.5SiM

o 12

Pt40%

/C-R

b 2.5SiW 12

Pt40%

/C-R

b 2.5PM

o 12

Pt40%

/C-R

b 2.5PW 12

Pt40%

/C-C

s 2.5PW 12

Pt40%

/C-C

s 2.5PM

o 12

Pt40%

/C-C

s 2.5SiW 12

Pt40%

/C-C

s 2.5SiM

o 12

Pt40%

/C

Fig. 68. Current densities of methanol electrooxidation as a function of electrode composition.

The data were obtained from chronoamperometric curves (Fig. 53 and 62) after 1000 seconds

of electrodes polarization at (A) 0.47 V and (B) 0.52 V in argon saturated 0.5 mol dm-3 CH3OH

+ 0.5 mol dm-3 H2SO4. T = 24 0C

125

5) The CO stripping voltammetry is a very important method, because CO is one of the

main poisoning intermediate products during methanol electrooxidation. Therefore the

studies on increasing resistance of the system towards this strongly adsorbed

intermediate species were made. The results presented in Fig. 69, clearly shows that the

onset potential of the CO oxidation is shifted to the lower value of potentials for the

systems modified with cesium and rubidium zeolite matrix.

Our results confirm that the system modification increase tolerance towards CO. The

PMo12 or SiMo12 containing materials behave as a redox mediator for the CO oxidation

reaction with Pt present in the system, while the PW12 and SiW12 materials may provide

additional hydroxide groups which facilitate oxidation of passivating intermediates

(COads) on Pt.

0 4 8 12 16

0,55

0,60

0,65

0,70

0,75

E

onse

t / V

vs.

RH

E

Pt40%

/C-R

b 2.5SiM

o 12

Pt40%

/C-R

b 2.5SiW 12

Pt40%

/C-R

b 2.5PM

o 12

Pt40%

/C-R

b 2.5PW 12

Pt40%

/C-C

s 2.5PW 12

Pt40%

/C-C

s 2.5PM

o 12

Pt40%

/C-C

s 2.5SiW 12

Pt40%

/C-C

s 2.5SiM

o 12

Pt40%

/C

Fig. 69. Onset potential of methanol electrooxidation on the investigated materials recorded in

argon saturated 0.5 mol dm-3 CH3OH + 0.5 mol dm-3 H2SO4. Data taken from Fig.55 and 64.

6) The results of the cyclic voltammetry long-term stability test on investigated

electrode materials are placed in Fig. 70. The peak current densities obtained at the

forward scan were read at 600 minutes of the test.

126

0 4 8 12 16

5

10

15

20

j p /

mA

cm

-2

Pt40%

/C-R

b 2.5SiM

o 12

Pt40%

/C-R

b 2.5SiW 12

Pt40%

/C-R

b 2.5PM

o 12

Pt40%

/C-R

b 2.5PW 12

Pt40%

/C-C

s 2.5PW 12

Pt40%

/C-C

s 2.5PM

o 12

Pt40%

/C-C

s 2.5SiW 12

Pt40%

/C-C

s 2.5SiM

o 12

Pt40%

/C

Fig. 70. Peak current densities obtained at 600 minute of long-term stability test of methanol

electrooxidation on the investigated materials recorded in argon saturated 0.5 mol dm-3 CH3OH

+ 0.5 mol dm-3 H2SO4. Data taken from Fig. 56 and 65.

For all the modified catalytic layers the values of the peak current densities of methanol

electrooxidation were higher in comparison to the platinum electrode. The highest value

of jp was obtained for the Rb2.5SiMo12 containing material. However, the most stable

during long-term cyclic voltammetry stability test appears to be the catalytic layers

modified with Rb2.5PW12 and Rb2.5PMo12 matrix.

127

9. Ethanol oxidation reaction (EOR) on

the Pt40%/Vulcan XC-72 carbon

modified with Cs2.5-HPAs

Among the small organic molecules, ethanol, being a renewable biofuel, is a

promising candidate for a fuel cell. Compared with methanol, ethanol has many merits:

less toxic, better stability, lower permeability across proton exchange membrane.

Ethanol is safer than other hydrocarbons, and the energy density of ethanol is higher

than that of methanol (1325.31 and 702.32 kJ mol-1 for ethanol and methanol,

respectively). Nevertheless, the process of the oxidation of ethanol is more difficult than

that of methanol with is necessity to break the C-C bond to obtain its complete

oxidation. However, the formation of strongly adsorb intermediates species such as

(CO)ads on the Pt catalyst, which is usually considered as the best single metal to adsorb

organic molecules and break intermolecular bond, reflect in high oxidation

overpotentials. Therefore the aim of the present chapter is to investigate the influence of

the modification of Pt nanoparticles for the electrooxidation of ethanol.

Studies of the electrooxidation of ethanol in acidic media on the new systems

prepared by mixing method were carried out using different electrochemical techniques

such as cyclic voltammetry, staircase voltammetry, chronoamperometry, A.C.

Impedance and Tafel plots. The stability of catalytic properties will be also discussed.

Cyclic voltammograms of the systems modified by Keggin-type heteropolyacid

salts as a matrix recorded in argon saturated 0.5 mol dm-3 H2SO4 as well as CO

stripping of these layers will be not presented because of the great similarity to the

voltammograms obtained previously in chapter 8.2.1 and 8.2.5, respectively.

9.1 Cyclic voltammetry (CV) measurements

In order to compare the electrocatalytic capabilities of all four electrodes studied

(and Pt40%/C electrode as a reference), all the steady cyclic voltammograms of the

electrooxidation of ethanol in argon saturated 0.5 mol dm-3 C2H5OH in 0.5 mol dm-3

H2SO4 aqueous solution are presented in Fig. 71. In the forward scan, ethanol oxidation

produced a prominent symmetric peak (A) around 0.89 V. In the reverse scan, an anodic

peak current density (B) is detected at around 0.79 V. This peak is due to the oxidation

128

of all adsorbed carbonaceous species (e.g. Pt-OCH2CH3, Pt-CHOH-CH3, (Pt)2=COH-

CH3, Pt-COCH3 and Pt-C≡O).

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

25 B

j /

mA

cm

-2

E / V vs. RHE

A

Fig. 71. Cyclic voltammetric response of Nafion-containing films (deposited on glassy carbon

electrode) of (—) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PW12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PMo12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon. Electrolyte: argon

saturated 0.5 mol dm-3 C2H5OH + 0.5 mol dm-3 H2SO4. Scan rate, 10 mV s-1. Temperature,

240C.

It is noticeable in Fig. 71 that the Nafion-treated Pt40%/Vulcan XC-72 carbon

modified with Cs2.5H0.5PMo12O40 matrix shows the highest positive scan peak current

density for ethanol electro-oxidation among the catalysts investigated here. It is quite

apparent from the figure that backward anodic current density peak (jb) is greater for an

electrode where forward anodic current density peak (jf) is grater. Moreover, the ratio of

the forward anodic peak current density (jf) to the reverse anodic peak current density

(jb), jf / jb, can be used to describe the catalyst tolerance to carbonaceous species

accumulation. These results as well as peak currents densities and corresponding peak

potentials of the forward and backward scan are listed in Table 15. Low jf / jb ratio

indicates poor oxidation of ethanol to carbon dioxide during the anodic scan, and

extensive accumulation of carbonaceous residues on the catalyst surface. High jf / jb

ratio shows the converse case.

129

Catalysts

Forward peak

potential /

V vs. RHE

j f /

mA cm-2

Backward peak

potential / V vs.

RHE

j b /

mA cm-2

j f/j b

Pt40%/C

Pt40%/C-Cs2.5H0.5PW12O40

Pt40%/C-Cs2.5H0.5PMo12O40

Pt40%/C-Cs2.5H1.5SiW12O40

Pt40%/C-Cs2.5H1.5SiMo12O40

0.894

0.897

0.897

0.898

0.898

16.60

21.62

25.43

20.30

19.31

0.796

0.800

0.804

0.802

0.798

14.77

18.44

22.09

19.14

17.00

1.12

1.17

1.15

1.06

1.14

Table 15. CV results of the investigated catalysts (24 0C).

It is clearly shown that the layers containing Cs2.5H0.5PW12O40,

Cs2.5H0.5PMo12O40 and Cs2.5H1.5SiMo12O40 salts as a matrix have higher ability to

oxidize ethanol to CO2 (particularly Pt-(CO)ads to CO2 ) in comparison to the platinum

electrode (higher value of jf / jb).

0.4 0.5 0.6 0.7

-1.0

-0.5

0.0

0.5

1.0

162 mV dec-1

E / V vs. RHE

log

j / m

A c

m-2

Fig. 72. Tafel plots of ethanol oxidation at the Nafion-containing (--) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H0.5PW12O40, (--) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H0.5PMo12O40, (--) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiW12O40, (-♦-)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiMo12O40 and (--) unmodified

Pt40%/Vulcan XC-72 carbon. Temperature, 240C.

130

Moreover, Fig. 71 clearly show that modification of commercial Pt40%/C by cesium

HPA salts matrix increases noticeably electrocatalytic activity of the system for EOR

(higher current densities were obtained for all modified catalytic layers). This enhanced

activity could be attributed to the increased surface area of the catalytic sites created by

high dispersion of Pt.

A very convenient way of comparing the performance of different electrode materials

for an ethanol electro-oxidation process is the use of steady-state polarization curves

and the corresponding Tafel plots. This innovative procedure allows a clear and

pictorial view of two important parameters, namely the starting potential and the current

density value for the systems under investigation. Thus, Fig. 72 shows the Tafel plots

carried out in the potentiostatic mode for ethanol oxidation process at the Nafion-

containing (red symbols) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PW12O40,

(green symbols) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PMo12O40, (dark

blue) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiW12O40, (light blue)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiMo12O40 and (black symbols)

unmodified Pt40%/Vulcan XC-72 carbon as a reference. For the oxidation of ethanol on

the investigated Pt - Cs-HPAs composite electrodes and for the unmodified Pt/C system

the b (slope) value is ca. 162 mV dec-1. However, the existing mechanism[124] is

extremely complex and not allow for the establishment of the rite determining step (rds)

for theoretical calculations of the Tafel slope value (b). It is clear that the current

density related to the active surface area (at constant value of potential) increases with

platinum dispersion by shifting the Tafel plot cathodically: e.g. at 0.5 V the current

density for modified layers (particularly for the systems containing Cs2.5PW12 and

Cs2.5SiW12 matrix) is much higher that this for pure system (Pt40%/C). Nevertheless,

the same value of b shows that there is no change in the reaction mechanism,

particularly the rate determining step. These results suggest a weaker poisoning for the

electrodes modified by Cs2.5-HPAs matrix.

9.2 Staircase voltammetry (SV) measurements

Fig. 73 present dependencies of staircase voltammetric responses (step period of

50 s recorded every 25 mV) of ethanol oxidation on catalytic layers containing Nafion

treated Pt40%/C and cesium salts of Keggin-type heteropolyacids, particularly12-

phosphotungstic acid (Cs2.5H0.5PW12O40), 12-phosphomolybdic acid

131

(Cs2.5H0.5PMo12O40), 12-silicotungstic acid (Cs2.5H1.5SiW12O40) and 12-silicomolybdic

acid (Cs2.5H1.5SiMo12O40), respectively, as a matrix and commercial unmodified

electrocatalyst (Pt40%/Vulcan XC-72 carbon) as a reference. Presence of

Cs2.5H0.5PMo12O40 matrix in the system results in some increase of ethanol

electrocatalytic currents (compare curves for unmodified and modified systems in Fig.

73).

0.4 0.6 0.8 1.0

0

2

4

6

8

10

12

j / m

A c

m-2

E / V vs. RHE

Fig.73. Staircase voltammetric current densities for the ethanol (0.5 mol dm-3) oxidation

recorded every 25 mV (between 0.25 and 1.07 V) following application of 50-s potential steps at

the Nafion-containing layer of (--) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H0.5PW12O40, (--) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PMo12O40, (--)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiW12O40, (-♦-) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H1.5SiMo12O40 and (--) unmodified Pt40%/Vulcan XC-72 carbon.

Electrolyte: argon saturated 0.5 mol dm-3 H2SO4..Temperature, 240C.

It is important to note that electrocatalytic activities (shown as the current densities at

corresponding peaks) for all modified electrode materials are higher than that of Cs2.5-

HPAs - free system. To express more clearly, the mass activity (MA, mA mg-1), defined

by peak current density per unit of catalyst loading, was used to evaluate the

electrocatalytic activity of the system for ethanol electrooxidation. The MA was

calculated according to the following equation[213]:

MA = jp/md x 103 (89)

132

where jp (mA cm-2) is the peak current density and the md (µg cm-2) is the loading mass

of Pt. The obtained values are presented in Table 16.

Catalysts j p /

mA cm-2

LPt /

mg cm-2

MA /

mA mg-1

Pt40%/C

Pt40%/C-Cs2.5H0.5PW12O40

Pt40%/C-Cs2.5H0.5PMo12O40

Pt40%/C-Cs2.5H1.5SiW12O40

Pt40%/C-Cs2.5H1.5SiMo12O40

8.770

10.13

12.02

10.73

9.570

100

100

100

100

100

87.70

101.3

120.2

107.3

95.70

Table 16. The mass activity of the investigated catalysts (24 0C).

The highest MA value was obtained for catalytic layer modified by

Cs2.5H0.5PMo12O40 zeolite type matrix (Table 16). The mass activity value for this

system is almost 40% higher in comparison to platinum electrode (Pt40%/C) while for

the other investigated electrode materials the MA value was from 9 to 22 % higher than

for unmodified system. This can be explained by the more exposed nanoparticles of Pt

in the zeolite matrix (Cs2.5-HPAs), which result in increasing specific surface and also

effect in higher electrocatalytic activity of Pt/C-zeolite (Cs2.5-HPAs) catalyst. Moreover,

zeolite material with special porous structure provides relatively high permeability and

good micromedia for ethanol oxidation.[7] We should to point out that for Pt/C - Cs2.5-

HPAs catalyst, Pt act as the main catalyst for catalyzing the dehydrogenation of ethanol

during the oxidation reaction and the oxygen containing species can be provided by the

framework oxygen sites or surface hydroxyls of the zeolite particles. What’s more these

oxygen-containing species strongly react with CO-like intermediate species on Pt

surface to release the active sites for further ethanol oxidation.[7]

9.3 Chronoamperometry (CA) measurements

Chronoamperometric experiments were carried out to observe the stability and

possible poisoning of the catalysts under short-time continuous operation. Fig. 74 shows

the current-time curves recorded for the several Pt containing electrode systems in

argon saturated 0.5 mol dm-3 C2H5OH + 0.5 mol dm-3 H2SO4 solutions at a fixed

potential of 0.47 V versus RHE. The initial current density decay can be addressed to a

133

great extent to the increasing surface coverage with partially oxidized intermediates.[214]

Meanwhile, that initial decay was much less pronounced in the case of ethanol

oxidation for Pt40%/C-Cs2.5PW12 composite suggesting less poisoning of the electrode

surface than for the other electrode materials.

0 200 400 600 800 1000

0.2

0.3

0.4

0.5

0.6

E / V vs. RHE

j / m

A c

m-2

Fig. 74. Chronoamperometric curves recorded for the ethanol oxidation at the Nafion-

containing layer of (—) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PW12O40, (—)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PMo12O40, (—) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H1.5SiW12O40, (—) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H1.5SiMo12O40 and (—) unmodified Pt40%/Vulcan XC-72 carbon upon application of (A)

0.47 V. Electrolyte: argon saturated 0.5 mol dm-3 H2SO4.. Temperature, 240C.

Most of the current densities present a quasi-stationary behavior after 400 seconds of

polarization. This slow decay of the current densities is due to the poisoning of the

material during the advance process.[215] However, it is important to note that the current

density for the layer containing Cs2.5PMo12 zeolite matrix after initial decay start to

increases with time what suggest that the presence of PMo12 into the system help to

cleaning Pt surface from CO-like intermediate species.

134

2 4 6 8 100,22

0,23

0,24

0,25

0,26

0,27

0,28

0,29

0,30

Pt40%

/C-C

s 2.5H 1.

5SiM

o 12O 40

Pt40%

/C-C

s 2.5H 1.

5SiW 12

O 40

Pt40%

/C-C

s 2.5H 0.

5PM

o 12O 40

Pt40%

/C-C

s 2.5H 0.

5PW 12

O 40

j / m

A c

m-2

Pt40%

/C

Fig. 75. Current densities of ethanol electrooxidation as a function of electrode composition.

The data were obtained from chronoamperometric curves after 1000 seconds of electrodes

polarization at 0.47 V in argon saturated 0.5 mol dm-3 C2H5OH + 0.5 mol dm-3 H2SO4.

Temperature, 24 0C.

Values of the current density for ethanol oxidation measured after 1000 second at 0.47

V are plotted against the electrode composition in Fig. 75. It can be observed that

electrodes with Cs2.5H1.5SiW12O40 and Cs2.5H1.5SiMo12O40 matrix exhibit lower activity

than unmodified system. Layer containing cesium salt of 12-phosphotungstic acid after

1000 seconds of oxidation of ethanol at 0.47 V shows the same current densities like

unmodified Pt40%/C electrode. The highest current density for EOR was found for

Pt40%/Vulcan XC-72 carbon containing Cs2.5PMo12 salt as a matrix, which is in good

agreement with CV and SV results.

9.4. Electrochemical impedance spectroscopy

for ethanol electrooxidation

In order to understand the characteristics of the electrocatalysis reaction on

modified electrodes, additional analysis of the ac impedance behaviour was carried out.

The electrochemical impedance spectra of Pt modified electrodes recorded in argon

saturated 0.5 mol dm-3 H2SO4 + 0.5 mol dm-3 C2H5OH at 0.75 V are shown in Fig. 76.

135

0 500 1000 1500 2000-300

0

300

600

900

-Z

imag

/ oh

m

Zreal

/ ohm

Fig. 76. Nyquist plots recorded in Ar saturated 0.5 mol dm-3 C2H5OH + 0.5 mol dm-3 H2SO4

aqueous solution for Nafion-containing (--) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H0.5PW12O40,(--) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H0.5PMo12O40, (--)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiW12O40, (-♦-) Pt40%/Vulcan XC-72

carbon modified with Cs2.5H1.5SiMo12O40 and (--) unmodified Pt40%/Vulcan XC-72 carbon

catalysts at 0.75 V. The frequency range is from 0.05Hz to 100kHz. Each electrode contained

100 µg cm-2 of the platinum.

The diameters of the semicircles (Fig. 76.) decreased in order Pt40%/C > Pt40%/C +

Cs2.5SiMo12 > Pt40%/C + Cs2.5SiMo12 > Pt40%/C + Cs2.5SiMo12 > Pt40%/C +

Cs2.5PMo12 > Pt40%/C + Cs2.5PW12. This behavior can be related to the charge transfer

resistances, which decrease with increasing effective surface area for the charge transfer

reaction. A significantly different x-axis intercept were observed for the Pt40%/C and

Pt40%/C-Cs2.5SiMo12. This behavior is due to the higher film resistivity for these

samples in comparison to the others investigated materials. The impedance data from

Fig. 76 suggest inductive behavior of the spectrum, which has been well studied by

several researchers.[208,216] They showed that an inductive loop appeared if the reactions

step of an intermediate species is the rate determining. It is considered that CO forms on

the electrocatalyst as a strongly adsorbed intermediate and that the electro-oxidation of

COads to CO2 is a rate determining step.

136

9.5 Electrochemical stability of investigated materials

containing Cs2.5-HPAs

Electrochemical stability of the catalytic layers containing Cs2.5-HPAs zeolite

matrix during long-term ethanol oxidation was shown in Fig. 77.

0 200 400 600 8000

5

10

15

20

25

30

j p / m

A c

m-2

t / min

Fig. 77. Long-term stability test of the Nafion-containing (--) Pt40%/Vulcan XC-72 carbon

modified with Cs2.5H0.5PW12O40, (--) Pt40%/Vulcan XC-72 carbon modified with

Cs2.5H0.5PMo12O40, (--) Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiW12O40, (-♦-)

Pt40%/Vulcan XC-72 carbon modified with Cs2.5H1.5SiMo12O40 and (--) unmodified

Pt40%/Vulcan XC-72 carbon electrodes in argon saturated 0.5 mol dm-3 H2SO4 containing 0.5

mol dm-3 C2H5OH solution at 50 mV s-1. Temperature, 240C. Same Pt loadings mounted in all

cases (LPt = 100 µg cm-2).

As a reference stability test for unmodified Nafion treated Pt40%/Vulcan XC-72 carbon

electrode is also presented. It is evident that the worst performance shows system

containing Cs2.5SiMo12 as a matrix. The final current density for this electrode material

at the end of the test was 32% lower than the onset potential.

The drop of the current densities during long-term stability test for ethanol

electro-oxidation on the investigated layers is presented in Table 17.

137

Catalysts Current

density drop

Pt40%/C

Pt40%/C-Cs2.5H0.5PW12O40

Pt40%/C-Cs2.5H0.5PMo12O40

Pt40%/C-Cs2.5H1.5SiW12O40

Pt40%/C-Cs2.5H1.5SiMo12O40

22.3 %

13.5 %

22.8 %

29.7 %

32.4 %

Table. 17. Current density drops during long-term stability test.

The data obtained from the stability test indicate that the best performance

towards ethanol oxidation in the long term shows catalytic layer modified by the

Cs2.5PW12. However, the highest current densities values during all experiment were

obtained for the Nafion-containing Pt40%/Vulcan XC-72 carbon modified with

Cs2.5PMo12 matrix.

138

Conclusions

This thesis is focused on the development of stable, proton highly-conductive,

meso-microporous matrices for Pt that can be used to increase the CO tolerance of

PEMFC, DMFC, and DEFC anodes.

In the first experimental part of this thesis (Chapter 6), the IR spectra clearly

show that the primary Keggin structure remains unaltered even when the protons form

parental heteropolyacid are substituted by the cesium or rubidium cations. This is

important from the point of view of the systems CO tolerance that shall be attributed to

the presents of tungstate and molybdenum units in the structure. The SEM results

presented here clearly suggest that morphology of heteropolyacids changes dramatically

when the protons in heteropolyacid are substituted by cesium or rubidium cations. This

substitution supports the existence of small spherical particles. The cyclic

voltammograms recorded for investigated materials deposited on the glassy carbon

electrodes show that some of the heteropolyacid salts exhibit behavior similar to that

characteristic of their analogues in solution. This phenomenon can be explained in terms

of the partial solubility in solution, particularly ammonium salts. On the basis of the

results obtained in Chapter 6, the salts of Keggin-type heteropolyacids containing 2.5

moles of cesium and rubidium cations in 1 mole of the heteropolyacid salt (such as

Cs2.5H0.5PW12O40, Rb2.5H0.5PW12O40, Cs2.5H0.5PMo12O40, Rb2.5H0.5PMo12O40,

Cs2.5H1.5SiW12O40, Rb2.5H1.5SiW12O40, Cs2.5H1.5SiMo12O40, Rb2.5H1.5SiMo12O40) can be

applied as matrices for Pt catalysts.

The next part of the thesis (Chapter 7) is devoted to the hydrogen oxidation

reaction on the Pt/Vulcan catalysts modified with Cs2.5H0.5PW12O40 prepared by two

methods (mixing and electrochemical deposition).

We demonstrate that incorporation and activation of catalytic Pt centers in these

conductive high-surface-area zeolite-type robust matrices is feasible. Immobilization of

platinum nanoparticles within the micro-mesoporous hybrid material was achieved

through electrochemical deposition of platinum by corrosion of Pt counter electrode.

HRTEM investigation shown that the particles have spherical sizes and their diameters

range 1 – 2 nm and 3 - 4 nm for Cs2.5PW12 modified and free system, respectively. The

difference in particle size may be due to fact that Pt deposition, on the Cs2.5H0.5PW12O40

containing composite, occurs inside the nanochannels of the porous tertiary structure of

139

the salts that posses characteristic diameters, while in the case of pure Vulcan the

particles grow on the surface where are more prone to grow. The electrochemically

active area obtained from CO stripping voltammetry combined with the results from

HRTEM allows estimating the loading of Pt, ca. 2.6 µg cm-1.

Comparison was made to the electrocatalytic system produced by simple mixing

of Vulcan XC-72 supported platinum (10 wt %) nanoparticles with the cesium

heteropolytungstate salt. In both cases, regardless the method of preparation and the

nature of immobilization of Pt sites, clear enhancement of the platinum activity were

observed during hydrogen oxidation.

The next chapter of the thesis (Chapter 8) deals with the applications of the

cesium and rubidium salts of the Keggin-type heteropolyacids as matrices for Pt/Vulcan

during methanol oxidation reaction. The results obtained with cyclic voltammetry and

staircase voltammetry methods clearly show that the modification of commercial Pt/C

with cesium and rubidium salts of HPAs significantly increases the electrocatalytic

currents (compare to unmodified Pt/C), and shifts the onset potential of the methanol

oxidation reaction to the less positive values. This suggests that the presence of Cs2.5-

HPAs or Rb2.5-HPAs matrix into the catalytic layer may facilitate the electrooxidation

of intermediate species such as the CO that is adsorbed on the Pt catalyst surfaces

leading to suppression of the poisoning effect on Pt catalysts by CO or CO-like

intermediates.

The results are also consistent with the data obtained from CO stripping

voltammetry, where the modification of Pt catalyst by the Cs2.5-HPAs or Rb2.5-HPAs

results in shifting onset potential of CO oxidation to the less positive values. An

important issue is that, among all examined catalysts, the Pt/C-Cs2.5SiMo12 and Pt/C-

Rb2.5SiMo12 shows the lowest onset and peak potentials for CO oxidation, while the

Pt/C exhibits the highest parameters, respectively. Long-term stability test confirm

earlier obtained results that modification of commercial Pt/C with zeolite matrix

increase catalytic activity of the system towards methanol electro-oxidation. The highest

value of the jp (peak current densities of methanol electrooxidation) was obtained for the

Rb2.5SiMo12 containing catalytic layer. However, the most stable during long-term

cyclic voltammetry stability test is the system modified with Rb2.5PW12 and Rb2.5PMo12

matrices.

The results presented in chapter 9 clearly show that the Pt/C modified with Cs2.5-

HPAs catalysts studied display electrocatalytic activity with respect to ethanol oxidation

140

as evidence by the voltammetric and chronoamperometric measurements. The data

obtained from the stability test indicate that the best performance towards ethanol

oxidation (in the long term experiment) is exhibited by the catalytic layer modified by

the Cs2.5PW12. However, the highest current densities values during all experiment were

obtained for the Pt40%/Vulcan XC-72 carbon modified by Cs2.5PMo12 system.

The results presented in the thesis suggest that cesium and rubidium

heteropolyacids salts should be considered as good matrices for anodes in the low

temperature fuel cells (PEMFC, DMFC, DEFC). It is reasonable to expect that the

amounts of precious Pt (or Ru) can be somewhat diminished during practical

applications.

We should to admit that the all experiments presented in the thesis were reproducible

within 5-7% in different measurements.

The results presented in Chapter 7 of the thesis are the part of the Italian and

international patent.[203,204]

141

Bibliography

[1] C. Arbizzani, S. Beninati, E. Manferrari, F. Soavi, M. Mastragostino, J. Power Sources 161 (2006)

826. [2] S.S. Gupta, J. Datta, J.Electroanal. Chem. 594 (2006) 65. [3] M.S. Volkmar, I. Remo, P. Elena, G. Sergio, J. Pchys. Chem. 100 (1996) 17901. [4] S.Q. Song, W.J. Zhou, Z.H. Zhou, L.H. Jiang, G.Q. Sun, Q. Xin, V. Leontidis, S. Kontou, P.

Tsiakaras, Int. J. Hydrogen Energy 30 (2005) 995. [5] W.J. Zhou, B. Zhou, W.Z. Li, Z.H. Zhou, S.Q. Song, G.Q. Sun, Q. Xin, S. Douvartzides,M. Goula, P.

Tsiakaras, J. Power Sources 126 (2004) 16. [6] E. Passalacqua, F. Lufrano, G. Squadrito, A. Patti, L. Giorgi, J. New. Mat. Electrochem. Syst. 3

(2000) 131. [7] P. V. Samant, J., B. Fernandes, J. Power Sources 125 (2004) 172 [8] H. Pang, J. Chen, L. Yang, B. Liu, X. Zhong, X. Wei, J. Solid State Elechtrochem. 12 (2008) 237. [9] M. T. Pope, A. Muller, Angew. Chem. Int. Ed. Engl. 30 (1991) 34. [10] I. V. Kozhevnikov, Catal. Rev. – Sci. Eng. 37 (1995) 311. [11] C. L. Hill, C. M. Prosse-McCartha, Coord. Chem. Rev. 143 (1995) 407. [12] A. Corma, Chem. Rev. 95 (1995) 559. [13] T. Okuhara, N. Mizuno, M. Misono, M. Adv. Catal. 41 (1996) 113. [14] N. Mizuno, M. Misono, M. Chem. Rev. 98 (1998) 199. [15] M. Sadakane, E. Steckham, Chem. Rev. 98 (1998) 219. [16] C.L. Hill, Chem. Rev. 98 (1998) 1. [17] D.E. Katsoulis, Chem. Rev. 98 (1998) 359. [18] I.V. Kozhevnikov, Chem. Rev. 98 (1998) 171. [19] R. Wlodarczyk, M. Chojak, K. Miecznikowski, A. Kolary, P. J. Kulesza, R. Marassi, J. Power

Sources 159 (2006) 802. [20] R. Wlodarczyk, A. Kolary-Zurowska, R. Marassi, M. Chojak, P. J. Kulesza, Electrochim. Acta 52

(2007) 3958. [21] M. Chojak, A. Kolary-Zurowska, R. Wlodarczyk, K. Miecznikowski, K. Karnicka, B. Palys, R.

Marassi, P.J. Kulesza, Electrochim. Acta 52 (2007) 5574. [22] K. Karnicka, M. Chojak, K. Miecznikowski, M. Skunik, B. Baranowska, A. Kolary, A. Piranska, B.

Palys, L. Adamczyk, P.J. Kulesza, Bioelectrochemistry 66 (2005) 79. [23] N. Giordano, P. Staiti, S. Hocevar, A.S. Arico, Electrochim. Acta 41 (1996) 397. [24] Y.S. Kim, F. Wang, M. Hickner, T.A. Zawodzinski, J.E. McGrath, J. Membr. Science 212 (2003)

263. [25] V. Ramani, H.R. Kunz, J.M. Fenton, J. Membr. Science 232 (2004) 31. [26] T. Moiyalagan, B. Viswanathan, J. Power Sources 175 (2008) 789. [27] R.J. Stanis, M.-C. Kuo, J.A. Turner, A.M. Herring, J. Electrochem. Soc. 155 (2008) B155. [28] J.R. Ferrell, M.-C. Kuo, J.A. Turner, A.M. Herring, Electrochim. Acta 53 (2008) 4927. [29] R.J. Stanis, M.-C. Kuo, J.A. Turner, A.M. Herring, Electrochim. Acta 53 (2008) 8277. [30] J. Shim. C.-R. Lee, H.-K. Lee, J.-S. Lee, E.J. Cairns, J. Power Sources, 102 (2001) 172. [31] S. Soled, S. Miseo, G. Mc Vicker, W.E. Gates, A. Gutierrez, J. Paes, Catal. Today 36 (1997) 441. [32] S. Soled, S. Miseo, G. McVicker, W.E. Gates, A. Gutierrez, J. Paes, Chem. Eng. J. 64 (1996) 247. [33] M. Misono, Mater. Chem. Phys. 17 (1987) 103. [34] G. Hoogers, Fuel cell technology handbook, 2003 [35] hydrogencommerc.com [36] K.R. Cooper, V. Ramani, J.M. Fenton, H.R. Kunz, Experimental Methods and data analyses for

polymer electrolyte fuel cell, 2005. [37] www.fuelcell.no [38] J. Larminie, A. Dicks, Fuel Cell System Explained, 2003. [39] www.fuelcells.org [40] S. Shuldiner, J. Electrochem. Soc. 107 (1960) 452. [41] V. S. Bagotzky, N. V. Osetrova, Electroanal. Chem. 43 (1973) 233. [42] W. Vogel, J. Lundquist, P. Ross, P. Stonehart, Electrochim. Acta 20 (1975) 79. [43] F.T.A. Vork, L.J.J. Janssen, E. Barendecht, Electrochim. Acta 31 (1986) 1569. [44] R.M.Q. Mello, E.A. Ticianelli, Electrochim. Acta 42 (1997) 1031. [45] N.M. Marković, B.N. Grugur, P.N. Ross, J. Phys. Chem. B 101 (1997) 5405. [46] J. Maruyama, M. Inaba, K. Katakura, Z. Ogumi, Z. Takehara, J. Electroanal. Chem. 447 (1998) 201.

142

[47] M.J. Croissant, T. Napporn, J.-M. Léger, C. Lamy, Electrochem. Acta 43 (1998) 2447. [48] T.J. Schmidt, H.A. Gasteiger, G.D. Stäb, P.M. Urban, D.M. Kolb, R.J. Behm, J. Electrochem. Soc.

145 (1998) 2354. [49] J. Jiang, A. Kucernak, J. Electroanal. Chem. 567 (2004) 123. [50] P.M. Quaino, M.R.G. de Chialvo, A.C. Chialvo, Phys. Chem. Chem. Phys. 6 (2004) 4450. [51] A.F. Innocente, A.C.D. Ângelo, J. Power Sources 162 (2006) 151. [52] B.M. Babić, L.M. Vračar, V. Radmilović, N.V. Krstajić, Electrochim. Acta 51 (2006) 3820. [53] A. śurowski, M. Łukaszewski, A. Czerwiński, Electrochem. Acta 51 (2006) 3112. [54] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Applied Catalysis B: Environmental 56

(2005) 9. [55] M. Łukaszewski, A. śurowski, M. Grdeń, A. Czerwiński, Electrochem. Commun. 9 (2007) 671. [56] M. Łukaszewski, A. śurowski, A. Czerwiński, J. Power Sources 185 (2008) 1598. [57] A. śurowski, M. Łukaszewski, A. Czerwiński, Electrochimica Acta 53 (2008) 7812 [58] V. A. Paganin, E. A. Ticianelli, E.R. Gonzales, J. Applied Electrochem. 26 (1996) 297. [59] T. Frelink, W. Visscher, J.A.R. van Veen, J. Electroanal. Chem. 382 (1995) 65. [60] L.W. Niedrach, I.B. Weinstock, Electrochem. Technol. 3 (1965) 270. [61] J.H. Choi, K.-W. Park, B.-K. Kwon, Y.-E. Sung, J. Electrochem. Soc. 150

(2003) A973. [62] A.L. Ocampo, M. Miranda-Hernández, J. Morgado, J. A. Montoya, P.J. Sebastian, J. Power Sources

160 (2006) 915. [63] A. Hamnett, Catal. Today 38 (1997) 445. [64] S. Wasmus, A. Kuver, J. Electroanal. Chem. 461 (1999) 14. [65] D. Lee, S. Hwang, I. Lee, J. Power Sources 145 (2005) 147. [66] M. Mastragostino, A. Missiroli, F. Soavi, J. Electrochem. Soc. 151 (2004)

A1919. [67] C. He, H.R. Kunz, J.M. Fenton, J. Electrochem. Soc. 144 (1997) 970. [68] A.S. Aricó, P. Cretí, P.L. Antonucci, J. Cho, H. Kim, V. Antonucci, Electrochim. Acta 43 (1998)

3719. [69] D. Chu, R. Jiang, Solid State Ionics 148 (2002) 591. [70] P.J. Kulesza, B. Grzybowska, M.A. Malik, M. Chojak, K. Miecznikowski, J. Electroanal. Chem. 512

(2001) 110. [71] C. Arbizzani, M. Biso, E. Manferrari, M. Mastragostino, J. Power Sources 178 (2008) 584. [72] C. Arbizzani, S. Beninati, F. Soavi, A. Varzi, M. Mastragostino, J. Power Sources 185 (2008) 615. [73] J.M. Macak, P.J. Barczuk, H. Tsuchiya, M.Z. Nowakowska, A. Ghicov, M. Chojak, S. Bauer, S.

Virtanen, P.J. Kulesza, P. Schmuki, Electrochem. Communications 7 (2005) 1417. [74] M. Chojak, M. Mascetti, R. Wlodarczyk, R. Marassi, K. Karnicka, K. Miecznilowski, P.J. Kulesza,

J. Solid State Electrochem. 8 (2004) 854. [75] F. Sen, G. Gökağaç, J. Phys. Chem. C 111 (2007) 5715. [76] L.C. Ordóñez, P. Roquero, P.J. Sebastian, J. Ramírez, Catalysis Today 107-108 (2005) 46. [77] A.S. Aricò, S. Srinivasan, V. Antonucci, Fuel Cells 1 (2001) 133. [78] J. Prabhuram, R. Manoharan, J. Power Sources 74 (1998) 54. [79] F. Viger, C. Coutanceau, F. Hahn, E.M. Belgsir, C. Lamy, J. Electroanal. Chem. 563 (2004) 81. [80] D. Yuan, S. Tan,Y. Liu, J. Zeng, F. Hu, X. Wang, P. Sen, Carbon 46 (2008) 531. [81] T. Schultz, S. Zhou, K. Sundmacher, Chem. Eng. Technol. 24 (2001) 12. [82] M. Umeda, H. Sugii, I. Uchida, J. Power Sources 179 (2008) 489. [83] M. Umeda, M. Kokubo, M. Mohamedi, I. Uchida, Electrochim. Acta 48 (2003)1367. [84] T. Iwastia, Electrochim. Acta 47 (2002) 3663. [85] R. Inada, K. Shimazu, H. Kita, J. Electroanal. Chem. 277 (1990) 315. [86] M. Shen, S. Roy, K. Scott, J. App. Electrocham. 35 (2005) 1103. [87] G. Girishkumar, T.D. Hall, K. Vinodgopal, P.V. Kamat, J. Phys. Chem. B 110 (2006) 107. [88] Y.T. Kim,T. Mitani, J. Catal. 238 (2006) 394. [89] M.H. Seo, S.M. Choi, H.J. Kim, J.H. Kim, B.K. Cho, W.B. Kim, J. Power Sources 179 (2008) 81. [90] Z. He, J. Chen, D. Liu, H. Tong, W. Deng, Y. Kuang, Materials Chem. Phys. 85 (2004) 396. [91] Z. Jusys, T.J. Schmidt, L. Dubau, K. Lasch, L. Jörissen, J. Garche, R.J. Behm, J. Power Sources 105

(2002) 297. [92] J.M. Léger, S. Rousseau, C. Coutanceau, F. Hahn, C. Lamy,Electrochim. Acta 50 (2005) 5118. [93] W.S. Li, L.P. Tian, Q.M. Huang, H. Li, H.Y. Chen, X.P. Lian, J. Power Sources 104 (2002) 281. [94] P. Waszczuk, J. Solla-Gullón, H.S. Kim, Y.Y. Tong, V. Montiel, A. Aldaz, A. Wieckowski, J. Catal.

203 (2001) 1. [95] Y. Morimoto, E.B. Yeager, J. Electroanal. Chem. 444 (1998) 95.

143

[96] C. Lamy, J.-M. Léger, S. Srinivasan, Direct methanol fuel cells-from a 20th century electrochemists dream to a 21st century emerging tehnology, in: J.O.M. Bockris, B.E. Conway (Eds.), Modern Aspects of Electrochemistry, Vol. 34, Plenum Press, New York, 2000 (Chapter 3), pp.53-117.

[97] C. Lamy, A. Lima, V. LeRhun, F. Delime, C. Coutanceau, J.-M. Léger, J. Power Sources 105 (2002) 283.

[98] V.S. Silva, J. Schirmer, R. Reissner, B. Ruffmann, H. Silva, A. Mendes, L. M. Madeira, S. P. Nunes, J. Power Sources 140 (2005) 41.

[99] J.O`M. Bockris, S. Srinivasan, Fuel Cells: Their Electrochemistry, McGraw – Hill Cook Company, New York, 1969.

[100] E. Herro, W. Chrzanowski, A.Wieckowski, J. Phys. Chem. 99 (1995) 10423. [101] E. Antolini, J.R.C. Salgado, E.R. Gonzalez, Appl. Catal. B 63 (2006) 137. [102] K. Sundmacher, T. Schultz, S. Zhou, K. Schott, M. Ginkel, E.D.Gilles, Chem. Eng. Sci. 56 (2001)

333. [103] L.D. Burke, J.K. Casey, Electrochim. Acta 37 (1992) 1817. [104] C. Lamy, E.M. Belgsir, J.-M. Léger, J. Appl. Electrochem. 31 (2001) 799 [105]. R. Parsons, T. Vandernoot, J. Electroanal. Chem. 257 (1988) 9. [106] K.L. Ley, R.Liu,C. Pu, Q.Fan, N. Leyarovska, C. Segre, E.S. Smotkin, J. Electrochem. Soc. 144

(1997) 1543. [107] H.A. Gasteiger, N. Markovic, P.N. Ross, E.J. Cairnes, J. Phys. Chem. 98 (1994) 617. [108] B.Gurau, R. Viswanathan, R.Liu, T.J. Lafrenz, K.L. Ley, E.S.Smotkin, E. Reddington, A. Sapienza,

B.C. Chan, T.E. Mallouk, S. Sarangapani, J. Phys. Chem. B 102 (1998) 9997. [109] M. Watanabe, S. Motoo, J. Electroanal. Chem. 60 (1975) 267. [110] K.A. Friedrich, K.P. Geyzers, U. Linke, U. Stimming, J. Stumper, J. Electroanal. Chem. 402 (1996)

123. [111] Y. Tong, H.S. Kim, P.K. Babu, P. Waszczuk, A. Wieckowski, E. Oldfield, J. Am. Chem. Soc. 124

(2002) 468. [112] T. Freelnik, W. Visscher, J.A.R. van Veen, Langmuir 12 (1996) 3702. [113] J. Mc Breen and S. Mukerjee, J. Electrochem. Soc. 142 (1995) 3399. [114] A.S. Aricò, V. Antonucci, N. Giordano, A.K. Shukla, Barman, D.D. Sarma, J. Power Sources 50

(1994) 295. [115] N. Markovic, A. Widelov, P.N. Ross, O.R. Monteiro, I.G. Brown, Catal. Lett. 43 (1997) 161. [116] A.S. Arico, P. Cretí, P.L. Antonucci, V. Antonucci, Electrochem. Solid State Lett. 1 (1998) 66. [117] F. Vigier, S. Rousseau, C. Coutanceau, J.-M. Leger, C. Lamy, Topics in Catalysis 40 (2006) 111. [118] W.J. Zhou, S.Q. Song, W.Z. Li, Z.H. Zhou, G.Q. Sun, Q.Xin, S. Douvartzides, P. Tsiakaras, J.

Power Sources 140 (2005) 50. [119] S. Song, P. Tsiakaras, App. Catal. B: Environmental 63 (2006) 187. [120] T. Iwanista, B. Rasch, E. Cattaneo, W. Vielstich, Electrochim. Acta 34 (1989) 317. [121] V.M. Schmidt, R. Ianniello, E. Pastor, S. Gonzalez, J. Pchys. Chem. 100 (1996) 17901. [122] M.H. Shao, R.R. Adzic, Electrochim. Acta 50 (2005) 2415. [123] J.P.I. De Souza, S.L. Queiroz, K. Bergmaski, E.R. Gonzalez, F.C. Nart, J. Phys. Chem. B 106

(2002) 9825. [124] A.O. Neto, M.J. Giz, J. Perez, E.A. Ticianelli, E.R. Gonzalez, J Electrochem. Soc. 149 (2002)

A272. [125] A.C.C. Tseung, K.Y. Chen, Catal. Today 38 (1997) 439. [126] P.K. Shen, K.Y. Chen, A.C.C. Tseung, J. Electrochem. Soc. 142 (1995) L85. [127] K.Y. Chen, P.K. Shen, A.C.C. Tseung, J. Electrochem. Soc. 142(1995) L185. [128] K.Y. Chen, Z. Sun, A.C.C. Tseung, Electrochem. Solid-State Lett. 3 (2000) 10. [129] C. Roth, M. Goetz, H. Fuess, J. Appl. Electrochem. 31 (2001) 793. [130] W. Xu, C. Liu, W. Xing, T. Lu, Electrochem. Commun. 9 (2007) 180. [131] C. Lamy, S. Rousseau, E.M. Belgsir, C. Coutanceau, J.M. Léger, Electrochim. Acta 49 (2004)

3901. [132] A.O. Neto, E.G. Franco, E. Arico, M. Linardi, E.R. Gonzalez, J. European Ceramic Society 23

(2003) 2987. [133] X. H. Xia, H.-D. Liess, T. Iwastia, J. Electroanal. Chem. 437 (1997) 233. [134] R. Alcala, J.W. Shabaker, G. W. Huber, M. A. Sanchez-Castillo, J. A. Dumestic, J. Phys. Chem. B

109 (2005) 2075. [135] E. Antolini, J. Power Sources 170 (2007) 1. [136] L. Colmenares, H. Wang, Z. Jusys, L. Jiang, S. Yan, G.Q. Sun, R. J. Behm, Electrochim. Acta 52

(2006) 221 [137] Z. Liu, L. Hong, J. App. Electrochem. 37 (2007) 505.

144

[138] R. Alcalá, M. Mavrikakis, J.A. Dumestic, J. Catal. 218 (2003) 178. [139] V. Rao, C. Cremers, U. Stimming, L. Cao, S. Sun, S.Yan, G. Sun, Q. Xin, J. Electroanal. Soc. 154

(2007) B1138. [140] S. Liu, D.G. Kurth, B. Bredenkötter, D. Volkmer, J. Am. Chem. Soc. 124 (2002) 12279. [141] S. Liu, D. Volkmer, D.G. Kurth, Journal of Cluster Science 14 (2003) 405. [142] J.T. Rhule, C.L. Hill, D.A. Judd, Chem. Rev. 98 (1998) 327. [143] E. Papaconstantinou, Chem. Rev. 18 (1989) 1. [144] M.S. Kaba, I.K. Song, D.C. Duncan, C.L. Hill, M.A. Barteau, Inorg. Chem. 37 (1998) 398. [145] D.R. Park, H. Kim, J.C. Jung, S.H. Lee, I.K. Song, Catayst Communicaton 9 (2008) 293. [146] A.J. Bridgeman, Chemical Physics 287 (2003) 55. [147] L. C. W. Baker, D. C. Glick, Chem. Rev. 98 (1998) 3. [148] I. Kozhevnikov, “Catalysts for fine chemical synthesis. Catalyst by polyoxometallates” 2000. [149] A. Yamada, J.B. Goodenough, J. Electrochem. Soc. 145 (1998) 737. [150] M.J. Janik, R.J. Davis, M. Neurock, J. Phys. Chem. B 108 (2004) 12292. [151] I.K. Song, M.A. Barteau, J. Mol. Catal. A: Chemical 182 (2002) 185. [152] M.T. Pope, Prog. Inorg. Chem. 39 (1991) 181. [153] S. Liu, D.G. Kurth, H. Möhwald, D. Volkmer, Adv. Mater. 14 (2002) 225. [154] I. Moriguchi, J.H. Fendler, Chem. Mater. 10 (1998) 2205. [155] M. Kozik, C.F. Hammer, L.C. W. Baker, J. Am. Chem. Soc. 108 (1986) 2748. [156] M. Kozik, N. Casan-Pastor, C. F. Hammer, L. C. W. Baker, J. Am. Chem. Soc. 110 (1988) 7697. [157] W.A. England, M.G. Cross, A. Hamnett, P.J. Wiseman, J.B. Goodenough, Solid State Ionics, 1

(1980) 231. [158] B.B. Bardin, S.V. Bordawekar, M. Neurock, R.J. Davis, J. Phys. Chem. B 102 (1998) 10817. [159] C. Rong, F.C. Anson, Anal. Chem. 66 (1994) 3124. [160] T. Okuhara, H. Watanabe, T. Nishimura, K. Inumaru, M. Misono, Chem. Mater. 12 (2000) 2230. [161] W.B. Kim, T. Voilt, G.J. Rodriguez-Rivera, J.A. Dumesic, Science 305 (2004) 1280. [162] T. Okuhara, Applied Catalysis A: General 256 (2003) 213. [163] G. Koyano, K. Ueno, M. Misono, Applied Catalysis A: General 181 (1999) 267. [164] T. Okuhara, T. Nakato, Catal. Surv. from Japan 2 (1998) 31. [165] Y. Kamiya, T. Okuhara, M. Misono, A. Miyaji, K. Tsuji, T. Nakajo, Catal. Surv. Asia 12 (2008) 101. [166] T. Okuhara, T. Nishimura, M. Misono, in: Proceedings of the 11th International Congress on

Catalysis, Stud. Surf. Sci. Catal., 101 (1996) 559. [167] M. Yoshimune, Y. Yoshinaga, T. Okuhara, Micropor. Mesopor. Mater. 51 (2002) 165. [168] J. Wang, Analytical Electrochemistry, second edition, 2001. [169] Z. Galus, Fundamentals of electrochemical analysis, second edition, 1994. [170] H.H. Willard, L.L. Merritt, J.A. Dean, F.A. Settle, Instrumental methods of analysis, 7th edition,

1988. [171] R. Varma, J.R. Selman, Techniques for characterization of electrodes and electrochemical process,

1991. [172] Z. Stojek, Electroanalytical Methods. Guide to experiments and Applications, F. Scholz-Editor,

Springer, Heidelberg 2002, p.102. [173] E. Barsoukov, J.R. Macdonald, Impedance spectroscopy theory, experimental, and Applications,

second edition, 2005. [174] R. Marassi, F. Nobili, chapter in Encyclopaedia of electrochemical power sources, 2008. [175] Y. Ding, Fundamental Theory of Transmission Electronic Microscopy.

www.nanoscience.gatech.edu [176] D. Kealey, P.J. Haines, Analytical Chemistry, 2002. [177] N. Essayem, Y.Y. Tong, H. Jobic, J.C. Vedrine, Applied Catal. A. General., 194-195 (2000) 109. [178] N. Essayem, A. Holmqvist, P.Y. Gayraud, J.C. Vedrine, Y. Ben Taarit, J. Catal., 197 (2001) 273. [179] N. Essayem, Y. Ben Taarit, P.Y. Gayraud, G. Sapaly, C. Naccache, J. Catal., 204 (2001) 157. [180] N. Essayem, G. Coudurier, J.C. Vedrine, D. Habeamacher, J. Catal., 183 (1999) 199. [181] C. Rocchiccioli-Deltcheff, R. Thouvenot, R. Franck, Spectrochim. Acta A 32 (1976) 587. [182] Q.J. Shan, J. Gong, L.Y. Qu, J. Coord. Chem. 55 (2002) 319. [183] L.R. Pizzio, M.N. Blanco, App. Catal. A General 255 (2003) 265. [184] N.K. Mizuno, K.Y. Yoneda, M. Misono, J. Catal. 83 (1983) 384 [185] M. Fourier, C. Rocchiccioli-Deltchef, L.P. Kazansky, Chem. Phys. Lett. 223 (1994) 297. [186] M. Lira-Cantú, P. Gómez-Romero, Chem. Mater. 10 (1998) 698.

145

[187] M. Skunik, B. Baranowska, D. Fattakhova, K. Miecznikowski, M. Chojak, A. Kuhn, P.J. Kulesza, J. Solid State Electrochem. 10 (2006) 168.

[188] A. Bielanski, J. Datka, B. Gil, A. Malecka-Lubanska, A. Micek-Ilnicka, Cat. Lett. 57 (1999) 61. [189] G.M. Brown, M.R. Noe-Spirlet, W.R. Busing, H.A. Levy, Acta. Cryst. B 33 (1977) 1038. [190] A. Bielanski, A. Lubanska, J. Pozniczek, A. Micek-Ilnicka, App. Catal. A: General 256 (2003) 153. [191] C. Danumah, S. Vaudreuil, L. Bonneviot, M. Bousmina, S. Giasson, S. Kaliaguine, Microporous Mesoporous Mater. 44-45 (2001) 241. [192] M.I. Ahmad, S.M.J. Zaidi, S.U. Rahman, S. Ahmed, Microporous Mesoporous Mater 91 (2006) 296. [193] D. Ingersoll, P.J. Kulesza, L.R. Faulkner, J. Electrochem. Soc. 141 (1994) 140. [194] A. Lewera, M. Chojak, K. Miecznkiowski, P.J. Kulesza, Electroanalysis 17 (2005) 1471. [195] W.L. Latimer, The Oxidation States of the Elements and Their Potentials in Aqueous Solutions, 2nd Edition, Prentice-Hall, INC. Englewood Cliffs, N. J, 1952 (Chapter12). [196] H.A. Gasteiger, N.M. Marković, P.N. Ross Jr., J. Phys. Chem. 99 (1995) 8290. [197] A.J. Bard, L. Faulkner, Electrochemical Metchods, Fundamentals and Applications, John Wiley &

Sons INC., New York, 2001 (Chapter 3, chapter 9). [198] Z.D. Wei, S.H. Chan, L.L. Li, H.F. Cai, Z.T. Xia, C.X. Sun, Electrochim. Acta 50 (2005) 2279. [199] A. Pozio, M. De Francesco, A. Cemmi, F. Cardellini, L. Giorgi, J. Power Sources 105 (2002) 13. [200] M. Ciureanu, H. Wang, J. Electrochem. Soc. 146 (1999) 4031. [201] M.J. Weaver, S.C. Chang, L.W.H. Leung, X. Jiang, M. Rubel, M. Szklarczyk, D. Zurawski, A.

Wieckowski, J. Electroanal. Chem. 327 (1992) 247. [202] F. Gloaguen, J.M. Léger, C. Lamy, J. Appl. Electrochem. 27 (1997) 1052. [203] B. Scrosati, R. Marassi, A. Kolary, A. Zurowski, P.J. Kulesza, S. Dsoke “Catalizzatori a basso contenuto di Platino per celle a combustibile” RM 2007A000228, April 2007. [204] B. Scrosati, R. Marassi, A. Kolary, A. Zurowski, P.J. Kulesza, S. Dsoke, “ Catalysts having low Pt content for fuel cells”, PCT/IB2008/051462, April 2008. [205] V.E. Kazarinov, G.Y. Tysyachnava, V.N. Andreev, J. Electroanal. Chem. 65 (1975) 391. [206] G.R. Mundy, R.J. Potter, P.A. Christensen, A. Hammett, J. Electroanal. Chem. 27 (1990) 257. [207] Y. Zhu, H. Uchida, T. Yajima, M. Watanabe, Langmuir 17 (2001) 146. [208] R.E. Melnick, G.T.R. Palmore, J. Phys. Chem. B 105 (2001) 1012. [209] G. Wu, L. Li, B.Q. Xu, Electrochim. Acta 50 (2004) 1. [210] W. Sugimoto,K.Aoyama, T. Kawaguchi, Y. Murakami, Y. Takasu, J. Electroanal. Chem. 576

(2005) 215. [211] P.K. Shen, A.C.C. Tseung, J. Electrochem. Soc. 141 (1994) 3082. [212] M. Watanabe, S. Motoo, J. Electroanal. Chem. 60 (1975) 275. [213] H. Tang, J.H. Chen, L.H. Nie, D.Y. Liu, W. Deng, Y.F. Kuang, S.Z. Yao, J. Colloid Interface Sci.

269 (2004) 26 [214] T. Iwasita, E. Pastor, Electrochim. Acta 39 (1994) 531. [215] H. Hoster, T. Iwasita, H. Baumgartner, W. Vielstich, J. Electrochem. Soc. 148 (2001) A496. [216] J. Otomo, X. Li, T. Kobayashi, C. Wen, H. Nagamoto, H. Takahashi, J. Electroanal. Chem. 573

(2004) 99. [217] J.J. Wang, G.P. Yin, J. Zhang, Z.B. Wang, Y.Z. Gao, Electrochim. Acta 52 (2007) 7042. [218] D.A. Harrington, B.E. Conway, Electrochim. Acta 32 (1987) 1703. [219] C.N. Cao, Electrochim. Acta 35 (1990) 831. [220] R.E. Melnick, G.T.R. Palmore, J. Phys. Chem. B 105 (2001) 9449. [221] V.S. Bagotzky, Y.B. Vassilyew, Electrochim. Acta 12 (1967) 1323. [222] Z. Hou, B. Yi, H. Yu, Z. Lin, H. Zang, J. Power Sources 123 (2003) 116.