Effect of pretreatment of electrocatalyst for direct methanol fuel cell applications S....

Effect of pretreatment of electrocatalyst for

direct methanol fuel cell applications

S. Chandravathanam

(CY02D015)

Ph. D. Viva voce

2

1. Introduction

2. Experimental methods

3. Effect of carboxylic acid functionalization of CDX975 carbon black support

of Pt/CDX975 catalyst for methanol electro-oxidation studies

4. Effect of sulfonic acid functionalization of CDX975 carbon black support of

Pt/CDX975 catalyst for methanol electro-oxidation studies

Part A: Grafting of ethylsulfonic acid using 2-aminoethanesulfonic acid

Part B: Grafting of sulfonic acid group using ammonium sulphate

Part C: Grafting of methylsulfonic acid group using sodium sulphite

and formaldehyde

5. Effect of aminopyridine functionalization of CDX975 carbon black support of

Pt/CDX975 catalyst for methanol electro-oxidation studies

6. Methanol oxidation studies of Pt/CDX975 catalyst prepared using citrate as

the capping agent

7. Summary and conclusions

Contents

3

Schematic of energy conversion in Fuel cells and Internal Combustion Engines (ICE)

What are Fuel Cells ?

Chemical energy of fuels Electrical Energy

Thermal Energy Mechanical Energy

Fuel Cell

ICE-1

ICE-2

ICE-3

Schematic of Direct Methanol Fuel Cell

External circuit Load

H+

Platinised carbon electrodes

Polymer electrolyte membrane(PEM)

e- e-

AnodeCH3OH + H2O

CO2 + 6H+ + 6e-

- ve

cathode3/2O2 + 6H+ + 6e-

H2O

+ ve

5

Relationship between current densities for hydrogen evolution and M – H adsorption enthalpies

Why Pt ?Why Pt ?

• Good resistance to corrosion

• High exchange current density

M – H adsorption enthalpies, KJ /mol

Exc

han

ge c

urr

ent

for

H2 e

volu

tion

log

I (A

cm

2 )

PtPd

Rh

Ta

W

FeNi

Au

Al

TlIn

> go. > 0

H - 0

> go. > 0

H - 1

6

Catalyst Particle size

Supported 9.4 Å unsupported 21 Å

Supported catalyst: 20 wt % PtRu (50:50)/C ( 0.46 mg/cm2 )

Un-Supported catalyst: 20 wt % PtRu (50:50) ( 2 mg/ cm2)

Temperature C

Mass Specific Activity (mA/ mg)

Supported unsupported

0.30 V 0.35 V 0.30 V 0.35V

50 4.6 9.2 3.5 8.7

60 9.2 21.0 6.2 14.5

70 28.3 68.4 10.0 27.0

80 55.3 121.0 19.5 44.3

90 106.6 217.1 35.5 -

Three fold increase in the specific activities on supporting the catalysts

Liu.L. et al., Electrochim. Acta, 43 (24) (1998) 3657-3663

Effect of the catalyst support on the

performance of the catalyst

7

Support materials for fuel cell catalysts

Carbon black

Carbon nanotubes

Transition metal oxides

Organic polymers

Mesoporous carbon

8

Chemical properties

- Good corrosion resistance

Electrical Properties

- Good Conductivity

Mechanical Properties

- Dimensional & Mechanical stability

- Light weight & adequate strength

Why carbon is used as an electrocatalyst support ?Why carbon is used as an electrocatalyst support ?

9Schematic of the surface groups on the carbon black support

10

Chapter-3. Effect of carboxylic acid functionalization of CDX975

carbon black support of Pt/CDX975 catalyst for

methanol electro-oxidation studies

11

0.5 mg CDX975

i) 20 or ii) 40 ml of 1N or conc. HNO3 is added

Stirred at 70-80 °C for i) 60 or ii) 95 min.

Washed and dried at air oven at 90 °C for 15-20 h

HNO3 Treatment of CDX975

12

S.no. Sample pH

1 Untreated CDX 6.1

2 1N HNO3 treated CDX-60 min 5.0

3 conc. HNO3 treated CDX - 60 min 4.5

4 1N HNO3 treated CDX - 95 min 4.1

5 conc. HNO3 treated CDX – 95 min 3.7

pH of the unmodified and nitric acid modified carbon blacks

30 mg of the catalyst in 7 ml of dist. H2O

Ultrasonication for 5 mins.

Measurement of pH

13

FT- IR of the unmodified and the nitric acid modified CDX975

14

a) Thermogravimetric analysis in N2 at 10°C/min and b) NH3-TPD of

the unmodified and the nitric acid modified CDX975

a b

15

N2 Adsorption/ Desorption of the unmodified

and the nitric acid modified CDX975

S.no. Sample BET surface area (m2/g)

Pore volume-BJH desorptive(cm3/g)

1 Untreated CDX 211 0.54

2 1N HNO3 treated CDX-60 min 194 0.44

3 conc. HNO3 treated CDX-60 min 136 0.42

4 1N HNO3 treated CDX-95 min - -

5 conc. HNO3 treated CDX-95 min 139 0.40

Textural properties of the unmodified and the nitric acid modified CDX975

C(002)

XRD patterns of the unmodified and

nitric acid modified CDX975

16

Loading of 10% Pt on nitric acid treated CDX by H2 reduction method

120 mg of CDX

0.69 ml of 5 wt % H2PtCl6 solution made into 2.4 ml is added

Stirred at 60 - 65 C

Evaporated to dryness

Cooled and kept for H2 redn. at 350 C for 4 h

17

Sample Crystallite size (nm)

Untreated 13.5

1N HNO3-60 min

21.2

1N HNO3-95 min

12.9

conc. HNO3-60 min

6.6

conc. HNO3-95 min

13.4

a) XRD patterns of the 10% Pt supported on unmodified and nitric acid

modified CDX975 b) Crystallite size determined from XRD by Scherrer

equation

a bC(002)

Pt(111)

Pt(200) Pt(220) Pt(311) Pt(222)

18

Sample EAS (m2/g)

10% Pt/untreated CDX 18.1

10% Pt/1N HNO3 treated CDX-60 min

24.8

10% Pt/1N HNO3 treated CDX-95 min

10.2

10% Pt/conc. HNO3 treated CDX-60 min

9.0

10% Pt/conc. HNO3 treated CDX-95 min

5.2

a) Cyclic voltammogram in 1M H2SO4 and b) EAS of unmodified and nitric acid modified CDX975

a b

19

a) Methanol oxidation activity in 1M H2SO4 and 1M methanol and b) Chronoamperometry at 0.6 V

a b

Sample EAS (m2/g) Mass specific methanol oxidation activity (A/g)

10% Pt/untreated CDX 18.1 214

10% Pt/1N HNO3 treated CDX-60 min 24.8 311

10% Pt/conc. HNO3 treated CDX-60 min 9.0 406

10% Pt/1N HNO3 treated CDX-95 min 10.2 269

10% Pt/conc. HNO3 treated CDX-95 min 5.2 265

20

Summary of Chapter-3

The increase of oxidative treatment conditions with nitric acid of the carbon black support show an increase in the concentration of carboxylic acid functional groups on the support.

XRD studies show smaller particle size of Pt for the catalyst treated with conc. HNO3

for 60 minutes optimum concentration of the carboxyl functional groups on the surface of the support.

Further increase of oxidative condition leads to the aggregation of the Pt nanoparticles the lesser diffusion of the Pt nanoparticles to the inner pores of the support.

Methanol oxidation studies follow the XRD crystallite size values rather than Electrochemical Active surface area (EAS). This shows that the increase in activity is due to the enhanced dispersion of the Pt nanoparticles as a result of the incorporation of optimum concentration of carbonyl functional groups. The deviation of EAS values can be attributed to the disturbance from the pseudocapacitance value, which results from the incorporation of charged carboxylic functional groups.

From the different /conditions/degree of oxidative treatment, conc. HNO3-60 min can be stated as the optimum condition, as it leads to smaller particle size with the maximum activity, which as stated earlier is due to the incorporation of optimum concentration of carboxyl functional groups.

21

Chapter-4 Effect of sulfonic acid functionalization of CDX975 carbon

black support of Pt/CDX975 catalyst for methanol electro-oxidation

studies

Part A: Grafting of ethylsulfonic acid using 2-aminoethanesulfonic acid

22

Schematic illustration of the internal structure of the electrocatalyst

Schematic illustration of the three-phase boundary structure

23

polymer composite (polypyrrole/polystyrenesulfonic acid and

poly-3,4-ethylenedioxythiophene/polystyrenesulfonic acid)

Fast loosing of electronic conductivity

Oxidizing the carbon using nitric acid Increased wettability and

proton conductivity of the surface of carbon particles

carboxylic acid is a weak acid and a poor proton conductor

Approach to make the support to conduct both electrons and protons

Short chain sulfonic or phosphonic acid groups

chemically linked onto carbon-supported catalysts

24

Anticipated structure

Schematic illustration of the internal structure of

the a) Existing and b) Anticipated electrocatalyst

25

Sulfonating Agents

acrylamide tertiary butyl sulfonic acid (ATBS) with HCHO

and ceric ammonium nitrate

p-amino phenyl sulfonic acid and sodium nitrite

2-aminoethanesulfonic acid with isobutyl nitrite

(NH4)2SO4

4-styrene sulfonate

HCHO and Sodium sulfite

26

Sulfonating Agents

acrylamide tertiary butyl sulfonic acid (ATBS) with HCHO

and ceric ammonium nitrate

p-amino phenyl sulfonic acid and sodium nitrite

2-aminoethanesulfonic acid with sodium nitrite

(NH4)2SO4

4-styrene sulfonate

HCHO and Sodium sulfite

27

Preparation of 10% Pt/CDX

2.25 g of CDX

13 ml of 5% H2PtCl6 in 25 ml of ethylene glycol

Stirring for 30 mins

Addition of 125 ml of ethylene glycol

Heating to 120 ° C

Heating at 120 °C 4 h

Filtration, washing with 150 ml of acetone and dist. H2O

Drying in air oven for 30 min followed by drying in vacuum oven for 36 h at 70 °C

Allowed to stand for 2 h

28

Sulfonation of 10% Pt/CDX catalyst with 2-aminoethanesulfonic acid

Filtered and washed with warm water

180 or 160 mg of 10% Pt/CDX catalyst in the 5 ml aqueous solution of 10 or 20 % (20 or 40 mg) of

2-aminoethanesulfonic acid

Heated to 80 °C

ice cooled (10 °C) aqueous solution of NaNO2 is added

Stirred for 30 min at 20 °C

Cooled to 20 °C (10 °C)

Allowed to stand for 45 min

Dried in vacuum oven at 70 °C for 20 hrs

Conc. HNO3 (2 ml) is added in drops and heated at 80 °C for 10 min.

Zhiqiang Xu, Zhigang Qi and Arthur Kaufman, Electrochemical and Solid-State Letters, 6(9) A171-A173 (2003)G. Selvarani, A.K. Sahu, N.A. Choudhury, P. Sridhar, S. Pitchumani, A.K. Shukla, Electrochimica Acta 52 4871–4877 (2007)

29

pH of 10% Pt/CDX catalyst with and without sulfonation with 2-aminoethanesulfonic acid

S.No Catalyst pH

1 0% 2-aesa/10% Pt/CDX 5.9

2 10% 2-aesa/ 10% Pt/CDX 4.4

3 20% 2-aesa/ 10% Pt/CDX 4.2

FT-IR of the unmodified and the 2-aminoethanesulfonic acid modified 10% Pt/CDX catalysts

30

a) TGA profiles in N2 atmosphere and b) XRD patterns of the unmodified and the 2-aminoethanesulfonic acid modified 10% Pt/CDX catalysts

S. No. Sample Crystallite size (nm)

Wt% of Pt from ICP

1 0 % 2aesa / 10% Pt / CDX 8.6 6.1

2 10 % 2aesa / 10% Pt / CDX 8.0 7.0

3 20 % 2aesa / 10% Pt / CDX 7.9 -

Average crystallite size and wt.% of Pt for 10% Pt/CDX catalyst with and without 2-aminoethanesulfonic acid modification

C(002)

Pt(111)

Pt(200)

Pt(220) Pt(311)

Pt(222)

ba

31

Cyclic Voltammetry in A) 1M H2SO4 and B) Methanol oxidation activity in 1M H2SO4 and 1M Methanol of unmodified and 2-aminoethanesulfonic acid modified 10% Pt/CDX catalysts

A B

S. No.

Sample Peak Current Density (mA/ cm2)

Mass specific methanol oxidation activity (A/g)

1 0 % 2aesa / 10% Pt / CDX 35.7 125

2 10 % 2aesa / 10% Pt / CDX 78.6 275

3 20 % 2aesa / 10% Pt / CDX 82.9 290

Comparison of methanol oxidation activity for 10% Pt/CDX catalyst with and without 2-aminoethanesulfonic acid modification

32

Chapter-4 Part B: Grafting of sulfonic acid group using ammonium sulphate

33

Zhiqiang Xu, Zhigang Qi and Arthur Kaufman, Electrochemical and Solid-State Letters, 8 A313-A315 (2005)

(NH4)2SO4 2NH3 + H2O + SO3

Carbon-H + SO3 Carbon-SO3H

235°C

34

Sulfonation of 10% Pt/CDX catalyst with ammonium sulphate

160 or 180 mg of 10% Pt/CDX

i) 10 and ii) 20 % of (NH4)2SO4 in 10 ml of distilled water

Stirred in room temp. for 2 min.

filtered and washed with distilled water

Dried in vacuum oven at 70 °C for 20 h

Stirred at preheated stirrer for 30 min.

Cooled for 45 min.

35FT-IR of the unmodified and the ammonium sulfate modified 10% Pt/CDX catalysts

1100 cm-1 SO2 stretching

S.No Catalyst pH

1 0% am sul/10% Pt/CDX 6.0

2 10% am sul/ 10% Pt/CDX 5.1

3 20% am sul/ 10% Pt/CDX 4.9

pH of 10% Pt/CDX catalyst with and without sulfonation with ammonium sulfate

36

S.No.

Catalyst Crystallite size (nm)

Wt% of Pt from ICP

1 0% am sul/10% Pt/CDX 9.9 -

2 10% am sul/ 10% Pt/CDX 9.9 -

3 20% am sul/ 10% Pt/CDX 10.2 6.9

a) TGA profiles in N2 atmosphere and b) XRD patterns of the unmodified and the ammonium sulfate modified 10% Pt/CDX catalysts

Average crystallite size and wt.% of Pt for 10% Pt/CDX catalyst with and without ammonium sulfate modification

C(002)

Pt(111)

Pt(200)

Pt(220) Pt(311)

Pt(222)

ba

37

Cyclic Voltammetry in A) 1M H2SO4 and B) Methanol oxidation activity in 1M H2SO4 and 1M Methanol of unmodified and ammonium sulfate modified 10% Pt/CDX catalysts

A B

38

S. No

Sample Peak Current Density (A/cm2)


1 0 % am. sul. / 10% Pt / CDX 35.7 125

2 10 % am. sul. / 10% Pt / CDX 64.3 225

3 20 % am. sul. / 10% Pt / CDX 48.6 170

Chronoamperometry at 0.6 V of

10% Pt/CDX catalyst with and

without 2-aminoethanesulfonic

acid modification

Comparison of methanol oxidation activity for 10% Pt/CDX catalyst with and without 2-aminoethanesulfonic acid modification

39

Chapter-4 Part C: Grafting of methylsulfonic acid group using sodium sulphite and formaldehyde

40

Reaction scheme for grafting of methylsulfonic acid groups onto catalyst-supported carbons

41

Flowchart of Grafting of methylsulfonic acid on 10% Pt/CDX

Protonated with 1N HCl

0.261 mg 10% Pt/CDX in dist. H2O

Addition of HCHO and Na2SO3

Refluxing at 383 K for 12 h

Washed with dist.H2O

Washed with dist.H2O

Extracted with rotaevaporator

Dried at 343 K for 24 h in a vacuum oven

42

FT-IR spectra of unmodified and methylsulfonic acid modified 10% Pt/CDX catalyst

1100 cm-1 - SO2 stretching

43

a) TGA profiles and b) XRD patterns of 10% Pt/CDX catalyst with and without methylsulfonic acid modification in N2 at 10 C/min

S.No Catalyst pH Crystallite size XRD (nm)

1 Unmodified 10% Pt/CDX 3.3 13.5

2 Methylsulfonic acid modified 10% Pt/CDX

3.3 4.4

Comparison of pH and crystallite size for unmodified and methylsulfonic acid modified catalysts

a b

44

Cyclic Voltammetry in a) 1M H2SO4 and b) 1M Methanol in 1M H2SO4 at 25 mV/s with and without sulfomethyl modification

a b

S. No.

Catalyst Wt% of Pt from ICP

EAS (m2/g)

Peak Current Density (mA/ cm2)


1 Unmodified 10% Pt/CDX 9.1 17.9 61.1 213

2 Methylsulfonic acid modified 10% Pt/CDX

10.0 41.5 138.4 484

Comparison of EAS and methanol oxidation activity for unmodified and methylsulfonic acid modified catalysts

45

Chronoamperometry at 0.6 V of 10% Pt/CDX catalyst with and without sulfomethyl modification

46

Studies by FT-IR and pH measurements supports the successful grafting

of the sulfonic acid groups on the carbon black support with three

different sulfonating agents .

XRD studies show the redistribution of the Pt nanoparticles upon sulfonic

acid grafting.

TGA profiles show enhanced thermal stability of the sulfonated catalysts.

The methanol oxidation activity studies show enhanced activities for all

the three cases of sulfonating agents. The increase in activity can be

attributed to two factors:

(i) Pt nanoparticle redistribution to smaller particles, thereby

increasing the electrochemical active surface area

of Pt and

(ii) increased protonic conductivity inside the catalyst support.


47

Chapter-5 Effect of aminopyridine functionalization of CDX975

carbon black support of Pt/CDX975 catalyst for methanol

electro-oxidation studies

48

Modification of the Carbon Support

Carbon sample Nitrogen content wt

%

Sulfur content

wt%

Mean particle

size from TEM (nm)

Methanol oxidation potential

at +50 mA/cm2 (V)

Untreated (U) 0 0.3 2.5 604

Nitrogen functionalized (N) 0.7 0.1 1.5 554

Sulfur functionalized (S) 0 5.6 1.0 633

Current – potential curve for Sulfur functionalized (S), Nitrogen functionalized (N) and un-functionalized (U) carbon supports

Roy.S.C. et al., J. Electrochem. Soc., 144 (1997) 2323 – 2328

49

Aminopyridine modification of CDX975 with 2-amino-5-chloropyridine (acp)

500 mg CDX975

i) 1%

ii) 5% of acp in THF is added

Refluxed for 3 h at 353 K

Extracted the unreacted with THF in rotavapour

Washed with methanol and dist. H2O

Dried at 353 K in air oven for 15 h

50

1482 cm-1

- shows the characteristic stretching vibrations of C=C and C=N of pyridine at 1482 cm-1 to 1648 cm-1 and - N-H stretching of primary amine at 1619 cm-1, indicative of the incorporation of the aminopyridine group on the CDX975 carbon black support.

a) FT- IR spectra and b) Elemental analysis results of unmodified, 1% and 5% of 2-amino-5-chloro-pyridine (acp) modified CDX975 carbon black support

1648 cm-1

1619 cm-1

1482 cm-1

S. No.

Sample C % H % N %

1 0% acp/ cdx 95.33 0.6 0.15

2 1% acp/ cdx 96.02 0.96 0.23

3 5% acp/ cdx 94.96 0.45 0.84

a

(b)

51

S.No.

Sample XRD crystallite

size [TEM] (nm)

1 10% Pt/ 0% acp/ cdx 18.5 [7.6]

2 10% Pt/ 1% acp/ cdx 13.4

3 10% Pt/ 5% acp/ cdx 9.4

Average Pt crystallite size of 10% Pt catalyst loaded on unmodified and aminopyridine modified CDX975 carbon black support

a) TGA in N2 atmospere and b) XRD patterns of 10% Pt loaded on (a) unmodified (b) 1% and (c) 5% 2-amino-5-chloropyridine modified CDX975 carbon black support

ba

ab

TEM of a) unmodified (7.6 nm) b) 5% aminopyridine modified (3.4 nm) 10% Pt/CDX975 catalysts

52

Cyclic voltammograms in (A) 1M H2SO4 and (B) in 1M methanol and 1M H2SO4 of 10% Pt loaded on (a) unmodified, (b) 1% and (c) 5% of aminopyridine modified CDX975 carbon black catalysts at the scan rate of 25 mV/s

Sample EAS (m2/g)

Peak methanol oxidation activity

(mA/ cm2)

Mass specific methanol oxidation

activity (A/g)

10% Pt/ 0% acp/ cdx 19.3 44.3 155

10% Pt/ 1% acp/ cdx 38.7 77.1 270

10% Pt/ 5% acp/ cdx 60.0 118.6 415

Electrochemical active surface area and methanol oxidation activity of 10% Pt loaded on unmodified and aminopyridine modified CDX975 catalysts

53

The aminopyridine modification of the carbon black support has been succesfully

carried out as supported by Elemental analysis and FT-IR studies.

XRD and TEM results show a decrease in particle size of Pt nanoparticles as a

result of better dispersion on the aminopyridine modified CDX975.

The electrochemical studies in sulphuric acid show an enhancement in

electrochemical active surface area upon aminopyridine modification of the carbon

black support, also confirms the better dispersion of the Pt nanoparticles on the

aminopyridine modified support.

Methanol oxidation studies has shown enhanced activity for the catalyst prepared

with aminopyridine modified carbon black support compared to the unmodified one,

which can be attributed to the smaller particle size of Pt nanoparticles as a result of

enhanced dispersion on the modified carbon black support.


54

Chapter-5 Methanol oxidation studies of Pt/CDX975

catalyst prepared using citrate as the capping

agent

55

Nanoparticles are thermodynamically unstable and their apparent stability comes from an acquired kinetic hindrance to agglomeration.

To produce desired sizes of Pt nanoparticles with uniform dispersion on the carbon support, some kind of stabilizing agents, such as

- Surfactants (SB12)

- Polymers (poly(N-vinyl-2-pyrrolidone) (PVP))

- Citric Acid/ Acetic Acid

usually employed during the preparative process to prevent particles in

close proximity from being coalesced .

56

A schematic illustration for

(a) an electrostatically stabilized metal (M) particle (i.e., one stabilized by the adsorption of ions and the resultant electrical double layer

(b) a sterically stabilized metal particle (i.e., one stabilized by the adsorption of polymer chains)

Saim Ozkar and Richard G. Finke J. AM. CHEM. SOC. 124(20) 2002, 5796-5810

Blocking of the active sites by polymer molecules

Removal of polymer by thermal methods

Agglomeration of noble metal particles

Limitations of steric stabilization

57

0.225 g of CDX

i) 0ii) 2.4iii) 6iv) 12v) 24 mole ratio of citrate/Pt in dist. H2O

+

Ethylene glycol

Stirred at 393-403 K for 80 min.

Allowed to stand for 1 h 15 min

Filtered and washed with acetone and dist. H2O

Dried in vacuum oven for 20 h at 343 K

Preparation of 10% Pt/CDX975 catalyst with different mole ratio of citrate/Pt

0.225 g of CDX

i) 0ii) 6iii) 12iv) 18v) 24 mole ratio of citrate/Pt in dist. H2O

+

Ethylene glycol

Stirred at 393-403 K for 80 min.

Allowed to stand for 1 h 15 min

Filtered and washed with acetone and dist. H2O

Dried in vacuum oven for 20 h at 343 K

Preparation of 20% Pt/CDX975 catalyst with different mole ratio of citrate/Pt

58

Pt(111)

Pt(200)

Pt(220)Pt(311)

Pt(111)

Pt(200)

Pt(220)Pt(311)

XRD patterns of 10 and 20% Pt/CDX975 catalysts with citrate and without citrate

TEM images of 10% Pt/CDX975 catalyst a) with out citrate (7.5 nm) and b) with citrate (2.7 nm)

TEM images of 20% Pt/CDX975 catalyst a) without citrate (5.6 nm) and b) with citrate (3.2 nm)

a b a b

59

C(002)Pt(111)

Pt(200)

Pt(220)Pt(311)

C(002)

Pt(111)

Pt(200)

Pt(220)Pt(311)

Mole ratio of citrate/Pt

Crystallite size (nm) [TEM]

0 4.8 [5.6]

6.0 3.2 [3.7]

12.0 3.0 [3.2]

18.0 2.8 [3.6]

24.0 2.3


Crystallite size XRD [TEM] (nm)

0 9.1 [7.5]

2.4 4.1 [3.0]

6.0 3.7

12 3.3 [2.7]

24 3.1

5 nm

5 nm

citrate/Pt = 2.4

citrate/Pt = 12

Crystallite size of 10% Pt/CDX catalysts with varying mole ratios of citrate/Pt

Crystallite size of 20% Pt/CDX catalysts with varying mole ratios of citrate/Pt

5 nm

5 nm

citrate/Pt = 6

citrate/Pt = 18

60


% Pt(0) % Pt(II)

0 73.3 26.7

2.4 83.7 16.3

6.0 79.3 20.8

12.0 74.4 24.5

24.0 73.3 26.7


% Pt(0) % Pt(II)

0 - -

6 74.3 25.8

12 - -

18 67.4 32.7

24 75.0 25.0

Oxidation State of Pt by XPS for 10 % Pt/CDX975 catalyst with varying mole ratios of citrate/Pt

Oxidation State of Pt by XPS for 20 % Pt/CDX975 catalyst with varying mole ratios of citrate/Pt

X-ray photoelectron spectra for Pt 4f of A) 10% and B) 20% Pt/CDX975 prepared with the citrate/Pt mole ratio of 12.0 and 18.0

A B

61

Cyclic Voltammograms of 10 % Pt/CDX975 catalyst for varying mole ratios of citrate/Pt at 25 mV/s in A) 1M H2SO4 and B) 1M 1M H2SO4 and 1M Methanol

A B

S.No. Mole ratio of citrate/Pt

EAS m2/g

Methanol Oxidation Activity (mA/cm2)

1 2.4 7.25 94

2 6.0 12.41 96

3 12.0 17.68 202

4 24.0 11.83 148Chronoamperometry of 10% Pt/CDX975 a) with b) without citrate and c) 10% Pt/Vulcan E-TEK catalyst in 1M H2SO4 and 1M Methanol at 1.0 V

62

S.No. Mole ratio of citrate/Pt

EAS m2/g Methanol Oxidation Activity (mA/cm2)

1 6 10.57 177

2 12 9.39 176

3 18 13.13 227

4 24 7.08 150

Cyclic Voltammograms of 20 % Pt/CDX975 catalyst for varying mole ratios of citrate/Pt at 25 mV/s in A) 1M H2SO4 and B) 1M 1M H2SO4 and 1M Methanol

Chronoamperometry of 20% Pt/CDX975 catalysts a) with and b) without citrate in 1M H2SO4 and 1M Methanol at 1.0 V

63


% Pt(0) % Pt(II)Crystallite size (nm)

Particle size TEM

(nm)

EAS m2/g

Methanol Oxidation

peak current density

(mA/cm2)

0 - - 4.8 5.6 - 66.4

6 74.3 25.8 3.2 3.7 10.57 177

12 - - 3.0 3.2 9.39 176

18 67.4 32.7 2.8 3.6 13.13 227

24 75.0 25.0 2.3 - 7.08 150

Comparison of oxidation State of Pt by XPS, Pt particle size and methanol oxidation activity for 20 % Pt/CDX975 catalyst with varying mole ratios of citrate/Pt


% Pt(0) % Pt(II) Pt Crystallite size XRD (nm)

Particle size TEM (nm)

EAS m2/g Peak Methanol Oxidation current density (mA/cm2)

0 73.3 26.7 9.1 7.5 - 148

2.4 83.7 16.3 4.1 3.0 7.3 94

6.0 79.3 20.8 3.7 - 12.4 96

12.0 74.4 24.5 3.3 2.7 17.7 202

24.0 73.3 26.7 3.1 - 11.8 148

Comparison of oxidation State of Pt by XPS, Pt particle size and methanol oxidation activity for 10 % Pt/CDX975 catalyst with varying mole ratios of citrate/Pt

64

XRD and TEM results show the decrease of Pt particle size with the addition of

citrate as the stabilizing agent.

Calculation of Electrochemical Active Surface Area (EAS) evidences the better

dispersion of the Pt nanoparticles on the carbon black support in presence of

citrate.

The existence of Pt in the Pt(II) state through XPS results also supports the

existence of the Pt nanoparticles in the presence of the electrostatic stabilizing

agent.

Both the 10% and 20% Pt/CDX975 catalysts show enhanced activity for methanol

oxidation for the catalyst having higher percentage of Pt in Pt(II) state. Therefore

this study shows the experimental evidence for the participation/ promoting effect

of Pt in higher oxidation state for methanol oxidation as suggested by the earlier

reports.


65

Conclusions

The carboxylic acid functionalization of the carbon black support upto

an optimum concentration, results in better dispersion of the Pt

nanoparticles. However the thermal stability of the catalyst is reduced.

The introduction of sulfonic acid groups on the carbon black support

seems to be experimentally simple. It enhances the dispersion of the Pt

nanoparticles as evidenced from smaller Pt particles. It also increases the

oxidation activity for methanol, which may partly be attributed to the

increase of protonic conductivity inside the pores of the carbon black

support. The thermal stability is improved as observed from TGA profiles.

Aminopyridine functionalization proves to be effective in the dispersion

of Pt nanoparticles on the carbon black support. This has resulted in the

enhancement of oxidation activity for methanol.

The utilization of citrate as the capping agent proves to be effective in

stabilizing the Pt nanoparticles supported on carbon black. Experimental

evidence has been provided for the participation of Pt in higher oxidation

state for the electro-oxidation of methanol.

66

Grateful thanks are due to

• Prof. B. Viswanathan (Research Guide)

• Prof. T.K. Varadarajan (Research Co-Guide)

• The Heads of Department of Chemistry and Deans

• The Doctoral committee members and faculty of the Department of

Chemistry

• The authorities for providing the various facilities

• The supporting staff, fellow research scholars and friends

• CSIR and Columbian Chemicals Company, USA.

AcknowledgementsAcknowledgements

67

Thank YouThank You

Effect of pretreatment of electrocatalyst for direct methanol fuel cell applications S....

Documents

Transcript of Effect of pretreatment of electrocatalyst for direct methanol fuel cell applications S....