Comparative study of heterogeneous and homogeneous catalyst-layer models of PEM fuel cells

Heterogeneous and homogeneous catalyst-layer models are used for microscopic and macroscopic simulations of proton exchange membrane fuel cells, respectively. However, their differences remain unclear. In this study, a comprehensive comparison of both models under various catalyst-layer structures and operating conditions is conducted to clarify the scenarios that yield a significant difference. A heterogeneous model is created by constructing a finely meshed 3D structure and upscaling it while extracting the local volume fraction and tortuosity. Subsequently, a homogeneous model is created by incorporating the average values of the transport properties and component amounts of the heterogeneous model. Numerical simulations are conducted using both models to analyze the contribution of each process of reaction and mass transport to the cathode overvoltage comparatively. Although the cathode overvoltages of both models are qualitatively similar, the overvoltage difference depends on the catalyst-layer structures and operating conditions. For the same Pt content and current density, the activation contribution to the overvoltage difference is significantly smaller than the mass-transport contribution. Oxygen-transport and proton-transport losses are greater in unfavorable catalyst-layer structures and operating conditions for each mass transport, with more pronounced effects in the heterogeneous model. Although the normalized overvoltage difference, interpreted as the error ratio, is typically below 0.10, it reaches a maximum of 0.16 under mass-transport-unfavorable catalyst-layer structure and operating conditions. The overvoltage difference is analyzed via linear regression using two dimensionless moduli. The findings enable precise and efficient analysis of physicochemical phenomena within the catalyst layer by appropriately selecting between the two models.