Ab initio study of interfacial correlations in polymer electrolyte membranes for fuel cells at low hydration

Polymer electrolyte membranes (PEMs) are the critical components of polymer electrolyte fuel cells (PEFCs). Proper operation of current PEMs hinges on sufficient amounts of water as the medium for proton conduction. Membrane dehydration, thus, causes failure of the fuel cell. For the design of advanced PEMs it is of foremost interest, whether high-proton mobility could be attained at low hydration and elevated temperature (>100 °C). Under such conditions structural correlations and interfacial proton transport at acid-functionalized hydrated polymer aggregates are vital for membrane operation. We consider a minimally hydrated, densely packed array of proton-binding surface groups as a model of microscopic interfacial elements in PEMs. Terminating carbon atoms of these surface groups are fixed at the positions of a regular hexagonal array. We explore the role of density, chemical architecture, and conformational flexibility of surface groups on interfacial correlations and acid dissociation. The transition from highly ordered to clustered conformations occurs at the same critical density of surface groups for all systems. For longer polymeric sidechains, the formation energy at the most stable conformation decreases slightly, while the range of 2D correlations extends to markedly reduced densities of surface groups.