Effects of Porous Carbon Morphology, Agglomerate Structure and Relative Humidity on Local Oxygen Transport Resistance

The morphology and ionomer distribution in the polymer electrolyte membrane fuel cell (PEMFC) cathode electrode (used in Toyota Mirai) are quantified with nano-scale resolution X-ray computed tomography (nano-CT). Using the nano-CT data, different shapes, sizes and compositions of agglomerates are extracted. Statistical information from multiple techniques, including transmission electron microscopy (TEM), ultra-small angle X-ray scattering (USAXS), and Brunauer-Emmett-Teller (BET) gas adsorption porosimetry are combined to reconstruct the high surface-area porous carbon (HSC) support, exterior (on the surface of carbon) and interior (inside carbon pores) catalysts, ionomer, and primary pores in the extracted agglomerates. Application of capillary condensation theory to the reconstructed agglomerate structure is shown to accurately represent the experimentally-observed relative humidity (RH) dependence of the electrochemically-active surface area (ECA). Direct numerical simulations (DNS) show that the high local O-2 transport resistance (R-O2) under dry conditions is mainly associated with the reduced ECA. We demonstrate that the agglomerate shape and size affect R-O2 only if the primary pores are poorly accessible (i.e., capillary-condensed water filled primary pores result in lower R-O2 for lower aspect ratio agglomerates). We also estimate R-O2 associated with the location of catalyst (whether inside or on the surface of HSC). (C) The Author(s) 2019. Published by ECS.