Understanding the effect of porosity and pore size distribution on low loading catalyst layers

Stochastic reconstructions, generated using an overlapping sphere algorithm with different particle sizes, were used to understand the role of the catalyst layer (CL) pore size distribution and porosity on the gas transport, local saturation and electrochemical performance of a low loading cathode. Statistical functions were used to characterize the morphology of the CLs and numerical simulations were performed to study the effective transport properties and electrochemical performance under dry and wet conditions. Results show that an increase in pore size increases the dry effective diffusivity but lowers the partially-saturated diffusivity at a given capillary pressure due to higher local saturation in the CL. Under dry conditions, porosity and particle size had negligible effect on the electrochemical performance of low loading CLs despite substantial changes in the ionomer distribution. Electrochemical simulation results at different liquid pressures show that CLs with moderate porosity and small particle size would maximize performance at a given capillary pressure due to lower liquid water accumulation, higher evaporation driven water transport and lower probability of water breakthrough to the diffusion media.