A unified catalyst layer design classification criterion on proton exchange membrane fuel cell performance based on a modified agglomerate model

Abstract

The main goal of catalyst layer (CL) optimization is decreasing the platinum in proton exchange membrane fuel cells (PEMFCs) without a deterioration in performance; this, however, is hindered by a lack of precise knowledge regarding the cell performance at various CL design criterions. In this study, a modified CL agglomerate model is developed to consider the coupled local oxygen and ionic transport and subsequently incorporated into a multi-phase, non-isothermal PEMFC model. A general classification criterion for CL design is proposed, which is classified into four CL modes: dynamic volume fraction mode, constant ionomer volume fraction mode, constant carbon volume fraction mode, and constant pore volume fraction mode. The respective contribution of local oxygen and ionic transport under four CL design modes are elucidated. It is found that the oxygen concentration at Pt surface is determined by oxygen transport resistance and local current density, and their impacts are synergetic. For fixed platinum/carbon and ionomer/carbon ratios, an increase in Pt loading results in a decrease in the ohmic loss by improving the ionic transport capacity. On the condition of fixing the volume fraction of each CL composition, the ionic conduction essentially remains unchanged as the Pt loading, whereas the oxygen transfer is enhanced. There exists a threshold value, beyond which the dominant loss shifts from concentration loss to activation loss with an increase in Pt loading. An increase in the ionomer and carbon volume fractions help improve the local membrane hydration, whereas a high porosity facilitates the oxygen concentration at Pt surface. This study is expected to provide deeper insights into CL design and performance optimization for PEMFCs.