PNNL

Abstract

We describe molecular orbital theory and ab initio molecular dynamics studies of acid/base equilibria of clusters AH:(H2O)n ? A-:H+(H2O)n in low hydration regime (n=1-4), where AH is a model of perfluorinated sulfonic acids, RSO3H, encountered in polymeric electrolyte membranes of fuel cells. Free energy calculations on the neutral and ion pair structures for n=3 indicate that the two configurations are close in energy and are accessible in the fluctuation dynamics of proton transport. For n=1,2 the only relevant configuration is the neutral form. This was verified through ab initio metadynamics simulations. These findings suggest that bases are directly involved in the proton transport at low hydration levels. In addition, the gas phase proton affinity of the model sulfonic acid RSO3H was found to be comparable to the proton affinity of water. Thus, protonated acids can also play a role in proton transport under low hydration conditions and under high concentration of protons. This work was supported by the Division of Chemical Science, Office of Basic Energy Sciences, US Department of Energy (DOE under Contract DE-AC05-76RL)1830. Computations were performed on computers of the Molecular Interactions and Transformations (MI&T) group and MSCF facility of EMSL, sponsored by US DOE and OBER located at PNNL. This work was benefited from resource of the National Energy Research Scientific Computing Centre, supported by the Office of Science of the US DOE, under Contract No. DE-AC03-76SF00098.