Proton transfer reactions and dynamics of sulfonic acidgroup in Nafion®

Abstract

Proton transfer reactions and dynamics of the hydrophilic group (–SO₃H) in Nafion® were studied at low hydration levels using the complexes formed from CF₃SO₃H, H₃O⁺ and nH₂O, 1 ≤ n ≤ 3, as model systems. The equilibrium structures obtained from DFT calculations suggested at least two structural diffusion pathways at the –SO₃H group namely, the “pass-through” and “pass-by” mechanisms. The former involves the protonation and deprotonation at the –SO₃H group, whereas the latter the proton transfer in the adjacent Zundel complex. Analyses of the asymmetric O–H stretching frequencies (ν^OH) of the hydrogen bond (H-bond) protons showed the threshold frequencies (ν^OH*) of proton transfer in the range of 1700 to 2200 cm⁻¹. Born–Oppenheimer Molecular Dynamics (BOMD) simulations at 350 K anticipated slightly lower threshold frequencies (), with two characteristic asymmetric O–H stretching frequencies being the spectral signatures of proton transfer in the H-bond complexes. The lower frequency (ν^OH,MD_A) is associated with the oscillatory shuttling motion and the higher frequency (ν^OH,MD_B) the structural diffusion motion. Comparison of the present results with BOMD simulations on protonated water clusters indicated that the –SO₃H group facilitates proton transfer by reducing the vibrational energy for the interconversion between the two dynamic states (Δν^OH,MD_BA), resulting in a higher population of the H-bonds with the structural diffusion motion. One could therefore conclude that the –SO₃H groups in Nafion® act as active binding sites which provide appropriate structural, energetic and dynamic conditions for effective structural diffusion processes in a proton exchange membrane fuel cell (PEMFC). The present results suggested for the first time a possibility to discuss the tendency of proton transfer in H-bond using Δν^OH,MD_BA and provided theoretical bases and guidelines for the investigations of proton transfer reactions in theory and experiment.