Synergistic analysis of oxygen transport resistance in polymer electrolyte membrane fuel cells

Abstract

The oxygen transport resistance of polymer electrolyte membrane fuel cells operated under various conditions (e.g., temperature and relative humidity) was separated into molecular diffusion, Knudsen diffusion, and ionomer film (IF) resistances using the catalyst agglomerate model, dissection of oxygen transport resistance, and distribution of relaxation time analysis. Simultaneously, an analysis of resistance, including charge transfer, proton transfer, and high-frequency resistances, was performed. The Knudsen diffusion resistance of the catalyst layer was calculated by assessing the effects of relative humidity on porosity and pore size. Oxygen transport resistance was analyzed to establish a correlation between temperature, relative humidity, and IF resistance. Water negligibly impacted performance at low oxygen levels at all examined current densities. The fractional contributions of molecular diffusion, Knudsen diffusion, and IF resistances obtained using oxygen transport analysis could be effectively applied to mass transport resistance in the distribution of relaxation time analysis. The IF resistance in the catalyst layer was up to eight times higher than the Knudsen diffusion resistance and 150 times higher than the proton transfer resistance across all current densities, thus most strongly contributing to the catalyst layer resistance. In the gas diffusion layer, the molecular diffusion resistance was up to four times higher than the Knudsen diffusion resistance. Thus, we examined the relationship between the mass transport resistances of individual elements and IF behavior under different operating conditions, revealing that the design of the IF in the catalyst should be considered alongside the relationship between the gas diffusion layer and membrane for optimal performance.