Khantesh Agrawala, Saroj Chaudharyb, Damaraju Parvatalub, and Venugopal Santhanama

1

Khantesh Agrawal

Email: agrawal@iisc.ac.in

(Session - I01D-1164: Polymer Electrolyte Fuel Cells & Electrolyzers 21 (PEFC&E 21) Catalyst Activity/Durability for Hydrogen(-Reformate) Acidic Fuel Cells)

240th ECS Meeting (October 10-14, 2021)

a Department of Chemical Engineering, Indian Institute of Science, Bangalore, Karnataka 560012, India b ONGC Energy Centre, Phase-II IEOT Complex, ONGC Panvel, Maharashtra 410221, India

Durability of Platinum Overlayers

formed by

Self-Terminating Electrodeposition

A representative FCV

(Toyota Mirai)

https://simanaitissays.com/2015/06/27/toyota-mirai-tidbits/ (Accessed on 4th Sept, 2021)

https://www.statista.com/statistics/200002/international-car-sales-since-1990/ (Accessed on 3rd Oct, 2021)

o FCV cost is ~x2.5 than GVs

Required platinum loading to match the ultimate DOE target of

$30/kW and comparable with GVs = 50 µg cm-2

o FCVs use x6 platinum (30g) than GVs

Platinum amount comparisonCost comparison of Sedan

(Toyota Mirai vs Toyota corolla)

Road to mass-adaptation of Fuel Cell Vehicles

(FCVs)

https://www.numbeo.com/cost-of-living/country_price_rankings?itemId=206 (Accessed on 3rd Oct, 2021)

A representative GV

(Toyota Corolla)

Thompson, Simon T., et al. (2018) Journal of Power Sources 399: 304-313Toyota Mirai Sales Figures | GCBC (goodcarbadcar.net) (Accessed on 3rd Oct, 2021)

2

A representative FCV

(Toyota Mirai)

https://simanaitissays.com/2015/06/27/toyota-mirai-tidbits/ (Accessed on 4th Sept, 2021)

https://www.statista.com/statistics/200002/international-car-sales-since-1990/ (Accessed on 3rd Oct, 2021)

o FCV cost is ~x2.5 than GVs

Required platinum loading to match the ultimate DOE target of

$30/kW and comparable with GVs = 50 µg cm-2

o FCVs use x6 platinum (30g) than GVs

Platinum amount comparisonCost comparison of Sedan

(Toyota Mirai vs Toyota corolla)

Road to mass-adaptation of Fuel Cell Vehicles

(FCVs)

https://www.numbeo.com/cost-of-living/country_price_rankings?itemId=206 (Accessed on 3rd Oct, 2021)

A representative GV

(Toyota Corolla)

Thompson, Simon T., et al. (2018) Journal of Power Sources 399: 304-313

5.08

o Annual vehicles sold every year ~ 70 million

o Total FCVs sold in 7 years ~ 20,000

Y. Wang et al., Materials Today, (2019)

Fuel cell stack cost break-up (at 1% of total vehicle share)

o Economies of scale makes platinum cost prominent

o Catalyst cost reduction is the need of the hour

Toyota Mirai Sales Figures | GCBC (goodcarbadcar.net) (Accessed on 3rd Oct, 2021)3

2020s

Pt/C: 0.3 mgPt/cm2

2010s

Corrosion of carbon base

Sinha et al. Journal of The Electrochemical Society, 158 (7), (2011) 4https://www.hydrogen.energy.gov/pdfs/review17/fc144_kongkanand_2017_o.pdf (Accessed on 2nd Oct, 2021)

1970s

Pt black: 10 gPt/cm2

Patel A et al. Hydrometallurgy 157 (2015)

1 µm

Evolution of platinum electrocatalyst for PEMFC

Degradation of MEA (from Toyota Mirai)

PtCo/C: 0.075 mgPt/cm2

ANL/ESD-18/12, Argonne National Lab.(ANL), Argonne, IL, United States (2018).

Durability is key

Space application

BOL EOL

NSTF Pt: 0.05 mg/cm2

Complete coverage of platinum by

thermal evaporation on the organic

whiskers (developed by 3M)

Mark K Debe Journal of The Electrochemical Society, 160 (6), (2013)5

Noncorrodible catalyst

synthesis using scalable

approach is desirable

Strategies to avoid carbon corrosion

Platinum thin film on Carbonless

support (developed by Toyota R&D)

Self-Terminating Electrodeposition (STED)

Pt < 0.05 mg/cm2

Electrocatalysis (2020) 11:14–24

STED protocol for platinum atomic layer deposition (Adapted from Liu et al. 2012)*

* Science, 338(6112), 1327-1330..

o Scalable electrochemical synthesis at room temperature and ambient pressure

o Excellent control over platinum film thickness

6

Self-terminating Pt electrodeposition (STED):

A scalable approach for Pt ML deposition

International Journal of Hydrogen Energy 45.16 (2020): 9603-961

Science advances 1.8 (2015): e1400268.ACS Catal. 2015, 5, 2124−2136

❖ Potentiostatic experiments (500 s duration) confirm the thin film

deposition regime

LSV and potentiostatic deposition

for platinum thin film synthesis

❖ Linear Sweep Voltammetry (from 0.95 V)

0.87 V

0.07 V

-0.33 V

7

Platinum deposition profile

o Onset of platinum reduction at 0.45 V

o Remarkable quenching at ~ -0.2 V

o Bulk deposition ~ 0.1 V

Experimental conditions: 3 mM K2PtCl4 +0.5 M KCl (pH=4) at 2

mV/s scanrate

8

How many platinum cycles are required for a durable

electrocatalyst for FCV?

Chronoamperometry

Current spikes corresponding to platinum

electrodeposition and self-termination

Pt deposition

Hupd oxidation

Quenching of the deposition within a second

n = 1,4,8,12,20

Gold thin film formed

by e-bam evaporation

on silicon substrate

Potential cycles for platinum

electrodeposition

9

Systematic study with the number of Pt electrodeposition cyclesEffect of number of platinum cycles on durability

o Increasing current spikes magnitude with platinum cycles

o More than 4 cycles give complete coverage of platinum

overlayers on evaporated gold (roughness ~ 3 nm)

AuO

reduction

Surface coverage of platinum overlayers

o Comparison• Liu et al. (2012) reported 85% coverage on flame

annealed gold (typical roughness <1 nm)

*Science 338.6112 (2012): 1327-1330.

Surface coverage of platinum overlayers

Surface coverage of platinum

monolayer on flame annealed gold

(Adapted from Liu et al. 2012)*

10

Possible growth mechanism of platinum overlayers

on evaporated gold

Evaporated Au

1st cycle

2nd cycle

8th cycle

11

Chronoamperometry

Possible growth mechanism for platinum overlayers on evaporated gold

Surface coverage of platinum overlayers

AuO

reduction

4th cycle

Evaporated Au

12

Platinum loading calculation

Number of platinum cycles Platinum loading (µg/cm2)

1 0.41 ± 0.04

4 3.63 ± 0.24

8 7.34 ± 0.82

12 18.84 ± 0.78

20 39.99 ± 0.48

o Increased platinum loading per cycle with increase in number of deposition cycles

o Measured platinum loading using ICP OES for 20 cycles (41.94 ± 8.18 µg/cm2) corroborates with

chronoamperometry

Calculated platinum loading from chronoamperometry

XPS signals confirms the successive build-up of platinum overlayers

Pt 4f7/2

Pt 4f5/2

13

XPS characterization of successive platinum

overlayers buildup

Pt

Au~10

nm

x1

0-X

14

Estimation of Pt film thickness from XPS

• Key assumptions:

o Uniform overlayer

o Single scattering

o Straight-line paths from creation to emission

𝐼𝐴 = 𝐼𝐴∞{1 − 𝑒𝑥𝑝[−

𝑑

λ𝑖𝑚𝑓𝑝𝐴 𝐸𝐴 𝑐𝑜𝑠𝜃

]}

𝐼𝐵 = 𝐼𝐵∞{1 − 𝑒𝑥𝑝[−

𝑑

λ𝑖𝑚𝑓𝑝𝐵 𝐸𝐵 𝑐𝑜𝑠𝜃

]}

• 𝐼𝐴∞ and 𝐼B

∞are proportional to the sensitivity factors 𝑠𝐴 and 𝑠𝐵

• For electrons with similar K.E (valid for Pt4f and Au4f electrons), λ𝑖𝑚𝑓𝑝𝐴 𝐸𝐴 = λ𝑖𝑚𝑓𝑝

𝐵 𝐸𝐵

𝑑 = λ𝑖𝑚𝑓𝑝 𝑐𝑜𝑠𝜃{1 +𝐼𝐴/𝑠𝐴

𝐼𝐵/𝑠𝐵]}

Surface And Interface Analysis, Vol. 25, 430-436 (1997)

o Estimated thickness for 20 cycles of platinum overlayers by XPS: 5.08 ± 0.25 nm

Estimated thickness from XPS

Platinum film thickness estimation from XPS

15

Pt film thickness corroboration

16

o Nominal thickness of Au ~150 nm

o Estimated thickness of 20 cycles of platinum overlayers by EDS = 4.86 ± 0.78 nm

Estimated thickness for 20 cycles of platinum overlayers from EDS

XPS EDS d-spacing

20 cycles of Pt thin film 5.08 ± 0.25 nm 4.86 ± 0.78 nm 4.6 nm

Estimated Pt film thickness using different approaches

ECSA measurement at intermittent stages of the durability analysis gives

the degradation profile

ECSA =QH

WPt∗0.21

1.00 V

0.65 V 0.65 V0.65 V

1.00 V

Durability cycle profile

Max load

No load

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Ex-situ Accelerated Durability test: setup and protocol

3-electrode setup Figure of merit

18

Ex-situ durability analysis with number of Pt cycles

o More than 4 cycles of platinum are needed for meeting

DOE target for durability (< 40% ECSA loss)

o < 10 µg cm-2Pt and < 5 nm thickness

Number of

platinum cycles

Hupd Charge

(µC/cm2)

BOL ECSA

(m2/gPt)

% ECSA retention

after 30k cycles

20 395.3 ± 23.9 4.7 ± 0.3 62.9 ± 4.9

12 252.1 ± 41.3 6.5 ± 1.5 68.2 ± 2.1

8 148.7 ± 15.6 9.9 ± 1.0 68.1 ± 4.4

4 111.0 ± 14.8 14.7 ± 1.2 49.2 ± 19.9

1 37.4 ± 6.2 44.9 ± 11.4 5.6 ± 2.1

Ex-situ durability for 8 cycles of platinum

Phys. Chem. Chem. Phys., 2014, 16, 5301--5311

ECSA and durability trend

o Further optimization to achieve complete coverage with even lesser loadings on rough substrates

AuO

reduction

Successive build-up of

platinum film

Coverage of platinum

overalyers

Conclusions

Pt 4f7/2

Pt 4f5/2

19

Proof of concept demonstration of durable platinum overlayers

formed using Self-Terminating Electrodeposition

o Durable platinum overlayers with < 10 µg cm-2 and < 5 nm thickness for FCVs

Number of

platinum cycles

BOL

ECSA

% ECSA

retention

20 4.7 ± 0.3 62.9 ± 4.9

12 6.5 ± 1.5 68.2 ± 2.1

8 9.9 ± 1.0 68.1 ± 4.4

4 14.7 ± 1.2 49.2 ± 19.9

1 44.9 ± 11.4 5.6 ± 2.1

ECSA and durability trend

Acknowledgements

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EMR/2016/005343ONGC/MCE/VS/001

PM/MHRD-19-15970

Khantesh Agrawala, Saroj Chaudharyb, Damaraju Parvatalub, and Venugopal Santhanama

21

(Session - I01D-1164)

240th ECS Meeting (October 10-14, 2021)

a Department of Chemical Engineering, Indian Institute of Science, Bangalore, Karnataka 560012, India b ONGC Energy Centre, Phase-II IEOT Complex, ONGC Panvel, Maharashtra 410221, India

Durability of Platinum Overlayers

formed by

Self-Terminating Electrodeposition

Khantesh Agrawal

Email: agrawal@iisc.ac.in