Modeling Nanoscale Ohmics in Carbon Supports of Fuel Cell Cathodes

Reducing platinum (Pt) loading in polymer electrolyte fuel cells (PEFCs) while meeting performace requirements is critical to their widespread deployment. However, significant polarization losses manifest at higher current densities in cathodes with lower Pt content. The morphology of the carbon supports in PEFC cathodes affects the location of Pt deposition into the micropits or onto the surface of the carbon support, translating into different kinetic and transport resistances. In this work, we derive an agglomerate scale model that differentiates the sink terms for Pt on the surface and in the pits of carbon supports. We develop an approach to assess the impact of nanoscale ohmic resistance to Pt in the micropits arising from weakly ionic solution in the carbon support on PEFC performance. Effectiveness factors relating the actual reaction rate to the maximum reaction rate (had all the Pt been exposed) are derived and embedded into a one-dimensional catalyst layer model. Parameters in the catalyst layer model are tuned based on experimental local oxygen transport resistances. Subsequently, we estimate bounds for the micropore resistances based on geometric and physical arguments. Lastly, polarization curves are simulated to assess the effect of the micropore resistance in fully-humidified and oxygen-rich environments.

Automated summary

Reducing the platinum (Pt) loading in polymer electrolyte fuel cells (PEFCs) is important for their widespread use, but it leads to polarization losses at higher current densities. The morphology of carbon supports in PEFC cathodes affects the deposition of Pt, resulting in different kinetic and transport resistances. In this study, we develop a model to differentiate the sink terms for Pt on the surface and in the pits of carbon supports, and assess the impact of nanoscale ohmic resistance on PEFC performance. We estimate the micropore resistances based on experimental local oxygen transport resistances and simulate polarization curves to evaluate the effect of micropore resistance.