The highly prioritized objective of research on polymer electrolyte fuel cells is to make highly performing and stable catalyst layers with drastically reduced platinum loading. Achieving this objective demands an understanding of the impact of composition and porous structure of the electrode layers on the water balance in the cell. Experimental studies ^1-3 have shown a marked increase in the resistance to oxygen diffusion when the Pt content of the cathode catalyst layer was lowered ⁴. We present a water balance model to explain these trends. Figure 1 illustrates modeling domain and processes considered. The set of 1D continuity and flux equations is formulated and solved for water distribution and fluxes in catalyst layers, diffusion media and flow fields. Model solutions reveal the impact of structure, composition, and operating conditions on the water transport phenomena and performance. Reducing the Pt loading induces a drastic shift of the balance between rates of water production and vaporization. This leads to a build-up of the liquid water pressure in catalyst layer and gas diffusion layer on the cathode side that raises the liquid water accumulation and thereby drastically diminishes the effective oxygen diffusivity. Our model unravels the fine details of this interplay and it helps identify strategies for minimizing the Pt loading without running into this water trap

References:

[1] A. Kongkanand et al., ACS Catal., 6, 1578-1583, (2016)

[2] J. Owejan et al. J. Electrochem. Soc. 160, F824-F833, (2013)

[3] M. Wilson, J. Electrochem. Soc., 139. L28-L39, (1992)

[4] T. Muzaffar et al., “Advanced Energy Materials”, in Review