Theoretical Elucidation of Water-Mediated Multiple Proton Transport in the Sulfonates

Abstract

Proton transport, a fundamental process in electrochemical energy conversion and biological signaling, faces significant limitations due to its strong humidity dependence. This sensitivity is demonstrated in state-of-the-art Nafion membranes (Nafion 117, 80°C), where proton conductivity decreases 100-fold from 0.1 S/cm (100% RH) to 0.001 S/cm (20% RH), highlighting the crucial role of water networks. While this correlation is established, the molecular mechanisms of water-facilitated proton transport remain unclear. In this work, through systematic investigation of proton-conducting materials, we identify sulfonate/water coordination as a key structural feature governing proton transfer. Our results show that the water-mediated Triple Proton Synergetic Hopping (TPSH) facilitates a reduction in the activation energy, from 0.339 eV to 0.269 eV. This reduction stems from water molecules weakening the proton donor bond energy through charge redistribution. Vibrational circular dichroism spectroscopy further reveals enhanced proton-detection sensitivity in sulfonate groups and water molecules compared to benzene rings, attributable to their intrinsic chirality. This provides a new approach for the scientific detection of proton conduction in the future. This study offers new mechanistic insights at the molecular level of cluster models for water-enhanced proton conduction in sulfonate complexes.