Mechanisms of proton transfer in Nafion®: elementary reactions at the sulfonic acid groups

Abstract

Proton transfer reactions at the sulfonic acid groups in Nafion® were theoretically studied, using complexes formed from triflic acid (CF₃SO₃H), H₃O⁺ and H₂O, as model systems. The investigations began with searching for potential precursors and transition states at low hydration levels, using the test-particle model (T-model), density functional theory (DFT) and ab initio calculations. They were employed as starting configurations in Born–Oppenheimer molecular dynamics (BOMD) simulations at 298 K, from which elementary reactions were analyzed and categorized. For the H₃O⁺–H₂O complexes, BOMD simulations suggested that a quasi-dynamic equilibrium could be established between the Eigen and Zundel complexes, and that was considered to be one of the most important elementary reactions in the proton transfer process. The average lifetime of H₃O⁺ obtained from BOMD simulations is close to the lowest limit, estimated from low-frequency vibrational spectroscopy. It was demonstrated that proton transfer reactions at –SO₃H are not concerted, due to the thermal energy fluctuation and the existence of various quasi-dynamic equilibria, and –SO₃H could directly and indirectly mediate proton transfer reactions through the formation of proton defects, as well as the –SO₃⁻ and –SO₃H₂⁺ transition states.