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Electrochemical CharacterizationDaniele Procaccio

Nernst Equations

Redox potentials (E0) are defined at standard

conditions (ex: 1M concentrations, 1 bar, 1 atm)

Nernst equation relates equilibrium potentials

at standard conditions to real system conditions

Takes into account changes in activites,

concentrations, partial pressures, etc..

Ex: OER depends on pOH, partial pressure of

O2, and water activity (a0)𝐸𝑎𝑛 = 𝐸0 −

𝑅𝑇

4𝐹l𝑛

𝑎𝐻𝑂−4

𝑝𝑂2

𝐸𝑐𝑎𝑡 = 𝐸0 +𝑅𝑇

4𝐹l𝑛 𝑎𝐻𝑂−

4 𝑝 𝐻22

Nernst Equations

Redox potentials (E0) are defined at standard

conditions (ex: 1M concentrations, 1 bar, 1 atm)

Nernst equation relates equilibrium potentials

at standard conditions to real system conditions

Takes into account changes in activites,

concentrations, partial pressures, etc..

Ex: OER depends on pOH, partial pressure of

O2, and water activity (a0)𝐸𝑎𝑛 = 𝐸0 −

𝑅𝑇

4𝐹l𝑛

𝑎𝐻𝑂−4

𝑝𝑂2

𝐸𝑐𝑎𝑡 = 𝐸0 +𝑅𝑇

4𝐹l𝑛 𝑎𝐻𝑂−

4 𝑝 𝐻22

Anode 4 𝐻𝑂− → 2 𝐻2𝑂 + 𝑂2(𝑔) + 4 𝑒−

Cathode 4 𝐻2𝑂 + 4 𝑒− → 4 𝐻𝑂− + 2 𝐻2(𝑔)

Three main contributions

Ohmic Drop

Mainly due to to the resistance of ion conduction and

electron transfer through the MEA structure. It increase

linearly with current following Ohm’s law.

η𝛀 = 𝐢 𝐑𝛀

Overpotentials

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 0.2 0.4 0.6 0.8 1

Ove

rpo

ten

tia

ls( η

) /

V

Current density ( j ) / A cm-2

Ohmic

𝑈𝑐 = 𝐸 − 𝑏 ∙ log𝑗

𝑗0− 𝑖𝑅 + 𝑎 ∙ log 1 −

𝑗

𝑗𝑙𝑖𝑚


Mass transport

Caused by the transport of the reactant species from

the bulk solution toward the electrodes.

η 𝑴𝑻 = 𝒂 𝐥𝐨𝐠 𝟏 −𝒋

𝒋𝒍𝒊𝒎

Overpotentials

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1

Ove

rpo

ten

tia

ls( η

) /

V


Transport


𝑗0− 𝑖𝑅 + 𝑎 ∙ log 1 −

𝑗

𝑗𝑙𝑖𝑚


Kinetic or Activation

Generally due to the electron-transfer from the

electrode to the reactant species.

Overpotentials

0

0.05

0.1

0.15

0.2

0.25

0 0.2 0.4 0.6 0.8 1

Ove

rpo

ten

tia

ls( η

) /

V


Kinetic


𝑗0− 𝑖𝑅 + 𝑎 ∙ log 1 −

𝑗

𝑗𝑙𝑖𝑚


𝑗0− 𝑖𝑅 + 𝑎 ∙ log 1 −

𝑗

𝑗𝑙𝑖𝑚

1.50

1.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

1.95

0 200 400 600 800 1000

Ove

rpo

ten

tia

ls( η

) /

V


Ohmic drop

Three main contributions shown in a polarization curve

ET MT

MTOhmic dropET

Butler-Volmer

j = 𝑗0 𝑒α𝑛𝐹𝑅𝑇 η − 𝑒 1−α

𝑛𝐹𝑅𝑇 η

The model is valid for a one step process, with a single electron transfer

O + 𝒆− ⇄ 𝑹

Low overpotentials, so mass transfer phenomena can be

neglected

The electrode material itself is inert and it is not undergoing

any chemical reactions (Outer sphere mechanism)

j = 𝑗0 𝑒α𝑛𝐹𝑅𝑇

η − 𝑒 1−α𝑛𝐹𝑅𝑇

η

j : current density [mA cm-2]

F : Faraday constant = 96485.3 [C mole—1]

j0 : exchange current density [mA cm-2]

T : working temperature [K]

n : number of electrons exchanged [mole- mol-1]

𝜂 : overvoltage [V]

𝛼 : transfer coefficient

R : Universal gas constant = 8.314 [J-1 K-1 mol-1]

Butler-Volmer equation

Bard, Allen; Faulkner, Larry (2000). Electrochemical Methods: Fundamentals and Applications. J. Wiley and Sons.

Tafel approximation

Tafel slope change in according to the reaction

mechanism

At large value of overpotentials (generally >120 mV)

only one part of the current dominate, so the other

one can be neglected

η = ±𝑏 log10𝑗

𝑗0

Catalyst characterization

Rotating Disk Electrode (RDE)

• The rotation let to control the thickness of the limit diffusive layer toward the electrode surface, minimizing the Mass Transport contribution. Controlled Hydrodinamic conditions.

• The rotation acts as a pump, bringing the reactant species from the bulk solution towardthe electrode surface.

• Laminar flow over the eletrode surface


Hydrogen Evolution Reaction (HER)

INK preparation:

HER Catalyst was suspended in H2O/EtOH and 1:100 anionic-exchange ionomer related to catalyst mass

Low loadings ( 5-15 µg cm-2 ) were used with a GC TIP for RDE

Half cell: Pt counter electrode, Hg/HgO REF and H2 saturated solution 0.2 M KOH

Bio-logic VMP3 multichannel potentiostat with EIS

iR correction by 90% of Re(Z) at 100 kHz.

Techniques:

1) Cyclic Voltametry (CV) Record at different sweeprates in order to confirm reversibility of HER/HOR and calculate ElectroChemical Surface Area (ECSA) from hydride adsorption/desorption peaks.

• Reversible hydride adsorption/desorption peaks;

• Constant specific ECSA of ca. 10 m2/g Pt;

• Lower ECSA at higher catalyst loadings

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-0.77 -0.67 -0.57 -0.47 -0.37

j / m

A c

m-2

E vs Hg/HgO / V

Benchmark Pt/C

* Area normalized for the GC electrode

Techniques:

1) Cyclic Voltametry (CV) Record at different sweeprates in order to confirm reversibility of HER/HOR and calculate ElectroChemical Surface Area (ECSA) from hydride adsorption/desorption peaks.

2) Staircase Voltametry (SV)RDE experiments at 400-2500 RPM, 180s per point on the following ranges:

a) 0.025÷0.025V vs RHE for Butler-Volmer;b) 0.5V for Koutecký–Levich;c) -0.05÷ -0.12V vs RHE for Tafelapproximation.

Benchmark Pt/C

y = -0.1234x - 0.0711R² = 0.998

-0.12

-0.11

-0.1

-0.09

-0.08

-0.07

-0.06

-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30

E R

HE

/ V

log j / mA cm-2

Main Issues:

Bubble formation toward the electrode that can lead to partial coverage of the active area

Anionic-exchange ionomer (instead of Nafion), in order to have a double-layer similar to the workingcell

Ionomer to catalyst ratio was selected as best trade-off between stability and ionomersegregation, leading to lower ECSA;

Low loadings of the catalyst layer led to reproducible ECSA and rotation-speed constantdata after mass-transport correction.

Bubble formation on GC with catalyst loading > 40 µg cm-2

Several studies of HER on Pt in acid using RDE report Tafel slopes around 30 mV/dec and exchange current densities, which are almost two orders of magnitude lower than the results obtained both from other techniques that removes the mass transport contributions. (i.e H2 pump or UME).

Mass transport overvoltages usually not corrected for HER in alkaline due to the ‘sluggish’ kineticscompared to the acidic media;

Two approaches have been compared:

1) Tafel approximation at high overvoltages;

2) Koutecký-Levich correction for HOR:

𝒋−𝟏 = 𝒋𝒌−𝟏 + 𝑩𝝎−𝟎.𝟓

*Levich, V. G. (1962). Physical Chemical Hydrodynamics. N.J.:Prentice-Hall.

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

-0.03 -0.02 -0.01 0.00 0.01 0.02 0.03

Log

j /

mA

E vs RHE / V

Uncorrected

Irreversiblecorrected

Comparable results exchange current densities Tafel and Koutecký Levich correction;

Both approaches led to values closer to the H2 pump experiments* due to the sluggish kinetic of Pt in alkaline media, compared to acidic media.

Analysis has been repeated for the HER using Tafel approximation at different temperature deriving the Ea from an Arrhenius Plot: 43.77 kJ mol-1

This data will be used for estimate the HER overvoltage at operating temperature and catalystloading in the single cell.

*J. Electrochem. Soc. 2015 volume 162, issue 14, F1470-F1481

y = -4.7899x + 12.728R² = 0.9225

-5.00

-4.50

-4.00

-3.50

-3.00

-2.50

3.2 3.25 3.3 3.35 3.4 3.45 3.5 3.55 3.6

ln (

ma

ss a

ctiv

ity)

/ A

mg

-1ca

t

1000 / T

α Tafel slope / mv dec-1 j0 / mA cm-2

Tafel Approximation - 130 3.34

Irreversible correction HOR 0.56 131 3.20


Oxygen Evolution Reaction (OER)

Ink preparation:

Ink prepared using Enapter OER catalyst suspended in EtOH/H2O and 1:500 anionic-exchange ionomer to catalyst ratio;

Low loadings (5-40 µg/cm2) were deposited on a GC RDE tip;

Half cell: O2 saturated 0.2M KOH solution at room temperature and 35°C, using graphite rod and Hg/HgO ascounter and reference electrodes. Bio-logic VMP3 multichannel potentiostat with EIS;

iR correction by 90% of Re(Z) at 100 kHz.

Techniques:

1) ChronoAmperometry (CA) - Activation at 0.7 V vs Hg/HgO until stable current (30’-1h)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1 2 3 4 5 6 7 8 9 10 11 12 13

j / m

A c

m-2

Time / min

Techniques:


2) Staircase Voltammetry (SV)

RDE experiments at 1600 RPM, 180s per point between1.33÷1.42 V vs RHE for Tafel approximation.

y = 0.0409x + 1.5086R² = 0.9983

1.37

1.38

1.39

1.40

1.41

1.42

1.43

-3.0 -2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 -2.2 -2.1

E vs

RH

E /

V

Log|j| / mA cm-2

Techniques:


2) Staircase Voltammetry (SV)

RDE experiments at 1600 RPM, 180s per point between1.33÷1.42 V vs RHE for Tafel approximation.

y = 0.0409x + 1.5086R² = 0.9983

1.37

1.38

1.39

1.40

1.41

1.42

1.43

-3.0 -2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 -2.2 -2.1

E vs

RH

E /

V

Log|j| / mA cm-2

Main Issues:

Similar to the HER (low loading, anionic-exchange ionomer, optimization Ionomer/catalyst ratio);

Catalyst ‘activation’ at 0.7 V vs Hg/HgO required to obtain stable data.

* No variation changing catalyst loading or rotation speed

At potential higher than 1.39V, Tafel slope for OER of ca. 41 mV/dec*;

Exchange current density (j0) proportional to catalyst loading;

y = -17.757x + 47.407R² = 0.979

-18

-16

-14

-12

-10

-8

3.20 3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60 3.65

ln (

ma

ss a

ctiv

ity)

/ A

mg

-1ca

t

1000 / T

Different test performed using catalyst loading 5-40 µg cm-2

and different temperature between 5-40 °C

Mass activity @ 25°C 7.071 E-05 A mg-1

J0 @25 °C 4.124 E-04 mA cm-2

Activation Energy 147.64 KJ mol-1

This data will be used for estimate the OER overvoltageat operating temperature and catalyst loading in the single cell.

Thank you for your kind attention!

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