New directions in perfluoroalkyl sulfonic acid-based proton-exchange membranes

Perfluoroalkyl sulfonic acid (PFSA) polymer membranes have enabled a variety of electrochemical energy storage and con- version devices such as fuel cells and flow batteries and material conversion devices such as electrolyzers to produce hydrogen and chlorine. For decades, a small group of simple copolymers were relied on in these applications. The practical limits for conductivity through lower equivalent weight are being approached for these polymers. Recently, the community has recognized the need for new PFSA ionomers with properties tailored to the specific needs of each application. An emerging area of importance is the manipulation of the diffusion properties of these polymers. For example, fuel cell membranes need to maintain good barrier properties as thicknesses are reduced, while ionomers with increased oxygen diffusion rates are sought for fuel cell electrodes. In recent years, the class of PFSA ionomers has expanded to include new side chain structures, new comonomers, and even hydrocarbon hybrid structures.