Proton exchange membranes with functionalized sulfonimide and phosphonic acid groups for next-generation fuel cells operating at 120 °C

Proton exchange membrane fuel cells (PEMFCs) face a significant challenge when operating in high-temperature and low-humidity environments, the hydrocarbon-based polymers that are sulfonated or phosphonated can’t meet the demand for achieving high performance. Modifying perfluorosulfonyl fluoride (PFSF) through meticulous design of side-chain chemical structures and introducing functionalized sulfonimide and phosphonic acid groups. Membranes derived from a perfluorocarbon backbone and polyacid side chain structures exhibit distinct microphase separation. The sulfonimide group exhibits hyper-acidic properties, whereas the phosphonic acid group functions as proton acceptor and donor. These two groups work synergistically to establish a distinct proton transport channel. The robust adsorption between acidic functional groups and water molecules enhances the diffusion of water molecules, resulting in an increase in proton conductivity. Moreover, Density Functional Theory (DFT) calculations reveal that the phosphonic acid group of the side chain has difficulty forming anhydrides. The prepared perfluorosulfonimide-phosphonic acid (PFSNPA) proton exchange membrane demonstrates an impressive conductivity of 263 mS·cm−1 at 120 °C. In H2/Air fuel cell tests, PFSNPA achieves an outstanding output power of 1076.1 mW·cm−2 at 120 °C and 40 % RH. This study introduces a new approach for designing and fabricating proton exchange membrane materials for high-temperature fuel cells.