Cathode Catalyst Layer Degradation in Proton Exchange Membrane Fuel Cells under Idle and Load Cycling Conditions

Proton exchange membrane fuel cells (PEMFCs) are viewed as an important technology for medium- and heavy-duty vehicles. However, the oxygen reduction reaction (ORR) at the cathode catalyst layer (CCL) is kinetically limited and occurs in a highly oxidizing and corrosive environment. Therefore, to achieve the necessary power density, catalysts are required that consist of expensive Pt or bimetallic Pt (Pt-M) nanoparticles (NPs) supported on a carbon support. Consequently, the lifetime of the fuel cell is intimately tied to the durability of the CCL and this, along with the associated costs of the catalyst, is a paramount concern. The scope of the review is as follows. First, state-of-the-art CCL materials and highlights of the recent development of ORR Pt-based catalysts are covered. This is followed by a description of metrics that are used to compare the performance of these catalysts. Detailed discussion of degradation mechanisms is then provided to grant an overview of current knowledge along with specific examples of studies related to the degradation of both pure and bimetallic Pt catalysts. It is shown that degradation in general is heterogeneous with significant differences in the degradation spatially across the CCL in both in-plane and through-plane directions. Electron microscopy (EM)-based techniques have been imperative to spatially deconvolute different degradation mechanisms, while synchrotron X-ray techniques have also provided important information about heterogeneous changes across the CCL. While many issues related to degradation are revealed throughout literature, the fundamental understanding of degradation mechanisms and their interplay and identification of the current gaps are imperative for development of improved materials and devices and transition of the technology to large-scale adoption.