Primary hydration and proton transfer of electrolyte acids: An ab initio study

Abstract

Acids dissolved in solution or tethered to a polymer backbone play an important role in the performance of the electrolytes utilized in energy storage and conversion technologies. The relative acidity and acid-base interactions are influential in the design of advanced electrolyte materials such as acid-base complexes and electrospun ionomers that involving two or more types of acids. Here we present a first principles based study of the primary hydration and proton transfer of ten acid molecules commonly employed in electrolytes. Clusters consisting of a single molecule with up to five water molecules were considered and the effect of a dielectric continuum solvation model was explored. For CF3SO3H, FSO3H, CH3SO3H, H2SO4, HNO3, HF, H3PO2, H3PO3, H3PO4, and CF3SO2NHSO2CF3 acids a minimum number of 3, 3, 3, 4, 5, >5, 5, >5, >5, and 3 water molecules were required, respectively, to effect proton transfer in the gas phase. It was determined that stronger acids or clusters with a continuum solvation model require fewer number of water molecules to first witness proton dissociation. The hydronium ion further separated from the conjugate anion as the number of water molecules was increased. Potential energy surface scans indicated that the proton transfer energetics are controlled by the hydration structure, acid strength, and the continuum solvation model. The calculated energy barrier to transfer a proton from HF to a water molecule was determined 27kcal/mol, more than twice the barrier for all the other acids (10–16kcal/mol). This fundamental study may serve the purpose of designing or screening novel electrolytes featuring one or more of these acids.