Electrode engineering strategies to advance polymer electrolyte fuel cells — recent progress and opportunities

Porous electrodes — typically referred to as catalyst layers — are critical components in polymer electrolyte fuel cells and several electrochemical technologies, where they determine the performance, durability, and cost of the system. The electrodes are responsible for sustaining electrochemical reactions, delivering reactants and removing products, and providing electronic and ionic transport. Simultaneously providing these functions necessitates sophisticated control over material properties across multiple length scales, making electrode design an important field of research. Here, we review recent trends in electrode engineering with a focus on optimizing complex mass transport phenomena to advance polymer electrolyte fuel cells. We first describe approaches to produce hierarchically organized electrode structures. Then, we discuss methods to control the support morphology, followed by strategies to functionalize the support chemical composition. We then highlight emerging trends in ionomer engineering and conclude with recommendations for standardized testing and the need to assess the end-of-life performance of novel electrode structures.