Microporous Layer 3D Porosity Distribution in Fuel Cell ...

WIR SCHAFFEN WISSEN – HEUTE FÜ R MORGEN

Microporous Layer 3D Porosity Distribution in Fuel Cell Gas Diffusion Layers

Yen-Chun Chen, Chrysoula Karageorgiou, Anne Berger, Salvatore De Angelis, Tobias Schuler, Thomas J. Schmidt, and Felix N. Büchi

October 2021 / ECS 240th Conference

Water management is one of the key challenges of PEFCs

• Water balance plays a crucial role for PEFC to be operated at optimal conditions. • One way to improve water management is through gas diffusion layer (GDL) structure engineering. • Proper structural design of GDL as well as its microporous layer (MPL) coating improves PEFC performance and

reduces mass transport losses 1,2,3.

Microporous layer (0.02 – 0.5 μm pore size) Gas diffusion layer substrate (1 – 40 μm pore size)

1 Energy Conversion and Management 243 (2021) 1143192 International Journal of Heat and Mass Transfer 175 (2021) 121340 3 Journal of The Electrochemical Society, 166 (13) F1022-F1035 (2019)

Effective DiffusivityLimiting current densityReactant Gas Flux

GDL: 150-300 μm

Microporous layer (20 – 50 μm)

Functions of the gas diffusion layer (GDL)

• The gas diffusion layer (GDL) provides PEFCs with Electricity conduction Gas transport Water drainage Heat conduction Mechanical support

• Microporous layer functions Mechanical protection of the catalyst coated membrane Reduce saturation in the GDL

• More debated Prevent catalyst layer flooding Thermal conduction/insulation Structure-dependent functions (cracks, porosity, pore size, surface wettability) If water transport via MPLs in liquid state

Tomogram (GDL with MPL) modified from: Q. Meyer et al. Electrochimica Acta Volume 211, 1 September 2016, 478-487

Microporous Layer (20-50 μm)

Catalyst coated membrane (CCM)

800 μm

H2 H2

O2O2

Anode

CathodeWater out Reactants in

Gas Diffusion Layer (150 – 250 μm)

Water Management

J. Power Sources 353 (2017) 221-229J. Electrochem. Soc., 157 10 (2010) B1456-B1464 Mater. Today, 9 (2018) 187-197 Electrochem. Commun. 41 (2014) 72–75J. Power Sources. 353 (2017) 221-229J. Electrochem. Soc., 165 (6) (2018) F3271-F3280Electrochem. Commun. 11 (2009) 576–579

https://www.sciencedirect.com/science/journal/00134686

https://www.sciencedirect.com/science/journal/00134686/211/supp/C

A common microporous layer • consists of carbon black + fluoropolymer binder• is hydrophobic (net) • has peak pore size between 20 – 2000 nm• has porosity between 40 – 85%

• Due to the difficulty of determination, MPL micro-porosity is often assumed to be homogeneous and represented by a constant value.

Microporous layer composition and characteristics

Electrochimica Acta 268 (2018) 366-382

Carbon black (e.g. acetylene black, Vulcan)

J. Power Sources 319 (2016) 82-89Photo credits: https://en.wikipedia.org/wiki/Carbon_black

Fluoropolymer (e.g. PTFE or FEP)

or

M. Andisheh-Tadbir et al. / Journal of Power Sources 310 (2016) 61-69

1.5 μm μm

https://en.wikipedia.org/wiki/Carbon_black

X-ray tomographic microscopy

Radiography = 2D Tomography = 3D image in greylevel

• Non-destructive: operando water distribution can be investigated• High image quality and μm-scale resolution• Compatible with liquid water (vacuum condition is not required)

Advantages of tomography

10-310-610-9

X-ray micro-tomography

Neutron radiography

Cryo-SEM

X-ray nano-tomography (Unit: m)

MPL pore size

Extending the use of polychromatic X-ray source for quantitative greyscale analyses

• Tomography greylevel value is proportional to the linear attenuation coefficient at that voxel. • Access to monoenergetic X-ray tomography is limited. • Use of laboratory computed tomography (CT) scanners for quantitative greylevel analyses extends the

application of XTM for quantitative greylevel analyses.

Liquid volume derived from XTMLiquid volume

White, R.T. et al., Sci Rep 9, 1843 (2019). Calibration curve forgreyscale value and linear attenuation of materials to be used in themodel calculation. ACS Appl. Mater. Interfaces 2021, 13, 14, 16227–16237

Poro

sity

of M

PL

Error: ±0.15

In-house made MPL “Li100”1

Compare the XTM image results with MIP and FIB-SEM

1 Journal of The Electrochemical Society, 164 (14) F1697-F1711 (2017)ACS Appl. Mater. Interfaces 2021, 13, 14, 16227–16237

Mercury intrusionporosimetry1


Li100 totalporosity

80% 79%

Commercial MPL Freudenberg H23C6

Error: ±0.23

Poro

sity

of

MPL

Focused ion beam-scanning electron microscopy


H23C6 (commercial) total porosity

57% 62%

FIB-SEM: Michele Bozzetti, EPFL

Voxel edge length: 3.6 μm

Three-dimensional porosity image of SGL 36BB MPL

Segmented SGL 36BB MPL through-plane porosity

SGL 36BB GDL through-plane view

SGL 36BB GDL in-plane view (MPL side)

SGL 36BB in-plane MPL porosity

Slicing

MPL micro-porosity

MPL cracks

76%

XTM derived porosity of MPL

MPL porosity (including cracks)

MPL micro-porosity (excluding cracks)

SGL 36BB 77% 74%

70%


Three-dimensional porosity image of Freudenberg H23C4 MPL

H23C4 GDL in-plane view (MPL side)

H23C4 in-plane MPL porosity

1000 μm

Slicing


MPL porosity (includingcracks)


Freudenberg H23C4

60% ==60%

Segmented Freudenberg H23C4 MPL through-plane porosity

Freudenberg H23C4 GDL through-plane view

MPL intrusion65% 59%

Average thickness: 46 μm


Three-dimensional porosity image of MPL from FuelCellStore

The MPL coating on a Toray 060 carbon paper from FuelCellStore

The MPL coating’s porosity

1000 μm

Slicing


MPL porosity (including cracks)


MPL coating from FuelCellStore

72% 66%

Segmented MPL coating through plane view porosity

MPL coating from FuelCellStore on Toray 060

MPL micro-porosity

MPL cracks

65% 65% 65%64%


• XTM is a high resolution (μm scale) tool with mm scale field of view that provides 3D porosity distribution of the MPL coatings from commercial gas diffusion layers (pore size independent).

• The major error source is the X-ray quantum mottle. This leads to a probabilistic interpretation of the porosity values. The accuracy is high for large area-averaged porosity. For smaller areas, discernability criteria depend on the spatial domain size of interest and image quality. (Y.-C. Chen et al., ACS Appl. Mater. Interfaces 2021, 13, 14, 16227–16237).

• With the most recent scans, there is a 95% confidence level to judge that two arbitrary regions of 1296 μm2 (=36 μm × 36 μm) area in the MPL have different porosity, if the image shows >6% porosity differences between the two regions.

Conclusion

• The demonstrated method was recently successfully applied to operando systems to quantify water distribution in the MPL.

• Owing to sufficient field of view, the XTM method is ready to be applied to characterize MPL porosity change due to flow field compression.

• A manuscript will be submitted shortly with porosity characterization of 16 commercial gas diffusion layers.

Outlook

Wir schaffen Wissen – heute für morgen

My thanks go to

• Dr. Jens Eller

• Michele Bozzetti

• Hong Xu

• Christoph Csoklich

• Adrian Mularczyk

• Cordelia Gloor

Speaker contact information

[email protected]

OVGA.005 PSI

CH-5232 Villigen, Switzerland

mailto:[email protected]

mailto:[email protected]

Microporous Layer 3D Porosity Distribution in Fuel Cell ...

Documents

Transcript of Microporous Layer 3D Porosity Distribution in Fuel Cell ...