Quantum chemical analysis of the deprotonation of sulfonic acid in a hydrocarbon membrane model at low hydration levels

Abstract

We conducted a quantum chemical analysis of the deprotonation reaction of the sulfonic group (SO3H) in a model of a hydrocarbon (HC) membrane, which has been proposed as a new proton conductor for polymer electrolyte membranes (PEMs) of polymer electrolyte fuel cells (PEFCs). By comparison with perfluorosulfonic acid (PFSA) species, activation energies are higher at all hydration levels. When deprotonation occurs in the PFSA at hydration level three, the activation energy in the model HC is still higher than the thermal energy of the PEFC operation temperature and therefore it is difficult or impossible to overcome it. Moreover, at hydration level three, the deprotonated state is not stable, in contrast to PFSA, and deprotonation of SO3H and protonation of the sulfonate (SO3−) should occur with the same probability. Because the activation energy is high and the deprotonation state is unstable, it is difficult to deprotonate the SO3H of the model HC to SO3− at hydration level three. Moreover, a bond-order analysis shows that SO3− is more strongly connected to H3O+ than it is for PFSA. These appear to be the main causes of the remarkably reduced proton conductivity in HC membranes at low hydration levels.