Hydration-Dependent Local Proton Transfer in Confined Ionomer Clusters: Connectivity and Dynamics of the Hydrogen Bond Network.

Local proton transfer within the confined ionomers of catalyst layers is critical for proton accessibility in polymer electrolyte membrane electrochemical devices, yet its mechanism remains poorly understood, particularly under varying hydration conditions. Here we employ ab initio molecular dynamics simulations to investigate hydration-dependent local proton transfer within ionomer clusters, with emphasis on proton-state evolution, elementary hopping events, and hydrogen bond network (HBN) dynamics. Our simulations reveal that the local proton transfer ability exhibits a nonmonotonic dependence on hydration within ionomer clusters: the Grotthuss-type proton hopping frequency first increases and then decreases with increasing hydration. Under low hydration, poor HBN connectivity confines protons within isolated water molecules around the sulfonate group and suppresses hopping. At intermediate hydration, a connected and dynamically reorganized HBN facilitates frequent proton hopping, resulting in the fastest proton transfer. At high hydration, although locally connected water channels emerge, excessive water-induced stronger HBN fluctuations increase the energy barrier for Grotthuss-type proton hopping, impeding efficient proton transfer. These results clarify the local proton transfer mechanism in confined ionomers and establish a structure-property relationship linking hydration level, HBN behavior, and proton-hopping performance.