WIR SCHAFFEN WISSEN – HEUTE FÜR MORGEN

Capillary Pressure Evolution in Operating Polymer Electrolyte Fuel Cells

Adrian Mularczyka, Qingyang Linb, Daniel Niblettc, Jens Elleraa Paul Scherrer Institut, b Imperial College London, UK & Zhejiang University, CNc University of Manchester, UK

240th ECS Meeting, October 10 – 14, 2021

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Working principle of Polymer Electrolyte Fuel Cells

H2O

500 µm 20µm200 µm

GDLMEM CLCLGDL Flow fieldFlow field

H2 2 H+ + 2 e- 0.5 O2 + 2 H+ + 2 e- H2O

Liquid water on the micro-scale influencesoverall PEFC performance and efficiency

Flückiger et al., 2008

Gas Diffusion Layer (GDL)ε0 = 0.8

Dp = 20 μm ε0 = 0.4

Dp = 60 nm

Catalyst Layer (CL)

Metal FF (automotive)

Toyota.com

Graphitic FF (stationary)

Swisshydrogen.ch

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(adapted from www.ak-tomographie.de)

X-ray Tomographic Microscopy Principle

Take up to 1501 projections

Rotate sampleover 180º

Greek: ‚tomos‘ <> slice

Reconstruct 3D material

distribution via 2D slices

(using eg. filtered backprojection)

X-ray

visible light

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• Determine realistic water distribution

• Influence of current density

• Feed gas humdification• Channel-Rib variations• Transient phenomena

Classification of experimental setups

Dry GDL Liquid injection Evaporation Running Fuel Cell

P H2

O2

Ex situ Operando

• Identify limiting process

• GDL thickness• Gas type &

velocity• Size of injection

area

• Determine pore (de-)wetting

• Capillarypressure vssaturation(WRC)

• 20 – 80 oC

• Determine pore structure

• GDL type• PTFE content• Compression

P

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I II

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Water

Water injection cell

Injection rate: 350 nL/min Injection diameter: 500 µm

Temperature: 25°CGas: Nitrogen (RH 0%)Flow rate: 6 m/s

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Water injection – Droplet release cycle stages

Mularczyk et al 2020 J. Electrochem. Soc. 167 084506

M. Andersson, A. Mularczyk et al., J. Power Sources, 404, 159, 2018

XTM VoF Simulation

D. Niblett, A. Mularczyk et al., J. Power Sources, 471 (2020) 228427,

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Droplet release cycle influences GDL saturation

Before droplet formation

After droplet formation

Fiber

• Water retracts in GDL after droplet detachment• GDL saturation changes by just 2 %

Mularczyk et al 2020 J. Electrochem. Soc. 167 084506--- Do not redistribute ---

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Curvature analysis to determine capillary pressure

Mularczyk et al 2020 J. Electrochem. Soc. 167 084506--- Do not redistribute ---

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Operando cluster formation

Mode Current jump (~2 A/cm²)

Scan procedure 0.25 s every 1 s

Scans 180

Active area: 200 µm * 250 µm

Material TGP-H 060 (10wt% PTFE)

Temperature: 25°CGas: Oxygen (RH 100%)Flow rate: 6 m/s

--- Do not redistribute ---Mularczyk et al 2021 ACS Appl. Mater. Interfaces 2021, 13, 29, 34003–34011, DOI: 10.1021/acsami.1c04560

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Water cluster – droplet interactions

• Droplet feed is unstable• Curvature based capillary pressure correlates with pore scale events

--- Do not redistribute ---Mularczyk et al 2021 ACS Appl. Mater. Interfaces 2021, 13, 29, 34003–34011, DOI: 10.1021/acsami.1c04560

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Thoughts on high capillary pressure values

• Volume of fluids simulation confirms high capillary pressure level• Pc of individual clusters may not follow pc-saturation curves determined by ex-

situ experiments on large representative GDL areasXTM VoF Simulation

--- Do not redistribute ---Mularczyk et al 2021 ACS Appl. Mater. Interfaces 2021, 13, 29, 34003–34011

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Conclusion

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• Curvature analysis provides insight into capillarypressure of running polymer electrolyte fuel cells

• Individual clusters grow at high pc than what typicalpc-sat curves tell us

• How common are such percolation path instabilities?

I01A-1026Insights into the Stability and Formation of Water Droplets Using Operando X-Ray Tomographic MicroscopyT. Dörenkamp, M. Sabharwal, J. Eller (PSI)

Acknowledgement

Thank You for Your Attention!

• Electrochemistry Lab F. Büchi, C. Chen, C. Csoklich, C. Gloor, T. Gloor,S. de Angelis, J. Halter, A. Lamibrac, M. Sabharwal,A. Schuller, T. Schuler, M. Striednig, T. Rosén,J. Roth, A. Vasile, H. Xu

• ENE A. Hamburger, U. Ludgate, T. Schmidt

• TOMCAT Beamline M. Bührer, F. Marone, C. Schlepütz, M. Stampanoni

• Funding Swiss National Science Foundation

Paul Scherrer Institute (PSI) - Switzerland

Thank You for Your Attention!