Microstructural Analysis and Transport Resistances of Low-Platinum-Loaded PEFC Electrodes

Abstract

In this study, we present microstructural characterization for polymer electrolyte fuel cell (PEFC) cathodes with low platinum loadings (low-PGM). The characterization results are used to quantify the contribution of mass transport resistances to cell voltage losses observed in polarization curve data. Three-dimensional pore morphology and ionomer distribution are resolved using nano-scale X-ray computed tomography (nano-CT). Electrode structural properties are reported along with analysis of the impact of microstructure on the effective charge and reactant transport properties. These characterizations are incorporated with a two-dimensional multi-physics model that accounts for energy, charge, and mass transport along with the effect of liquid water flooding. Defining a total mass transport resistance for the whole polarization curve, contributions of transport mechanisms are identified. Analysis of the experimental polarization curves at different operating pressures and temperatures indicates that the mass transport resistance in the cathode is dominated by the transport processes in the electrode. It is shown that flooding in the electrode is a major contributor to transport losses especially at elevated operating pressures while the pressure-independent resistance at the catalyst surface due to transport through the ionomer film plays a significant role, especially at low temperatures and low catalyst loading. In addition, by performing a parametric study for varying catalyst loadings, the importance of electrode roughness (i.e, electrochemically-active surface area/geometric electrode area) in determining the mass transport losses is highlighted.