Ionomeric binders in polymer electrolyte fuel cells: roles, challenges, and advances

Polymer electrolyte fuel cells (PEFCs) represent a sustainable technology for converting chemical energy into electrical energy, playing a pivotal role in achieving carbon neutrality. At the core of PEFCs lie membrane electrode assemblies (MEAs), wherein ionomers perform dual functions as both ion exchange membranes (IEMs) and catalyst layer (CL) binders. This review focuses on ionomers used as CL binders, an underexplored yet critical component that significantly influences PEFC performance and durability. We systematically examine the roles of ionomers in enabling multiphase mass transport within CLs, including oxygen transport, water management, and ion conduction, alongside design principles for optimizing their molecular structures. Additionally, we investigate the interactions between ionomers and catalysts, and their implications for PEFC performance and longevity. The review also addresses the long-term stability of PEFCs, analyzing mechanisms of ionomer chemical degradation and physical aging, as well as potential mitigation strategies. By offering comprehensive analyses of ionomer functionalities, structure-property relationships, and their impact on PEFC performance, this review aims to inform future research and development efforts toward more efficient and durable ionomers. These insights are also applicable to other emerging MEA-based electrochemical devices, such as water and carbon dioxide electrolyzers.