Anderson-Newns Hamiltonian molecular dynamics approach to understanding electrochemical double layer effects in hydrogen electrocatalysis

Abstract

Today, green hydrogen is at the cusp of widespread adoption. However, to meet critical CO2 emission reduction targets sooner, faster scale up of green hydrogen infrastructure is needed. To that end, progress in device design is needed to reduce the material costs in fuel cell and electrolyzer production. ☐ The development of hydroxide exchange membrane fuel cells (HEMFCs) and electrolyzers (HEMELs) is a promising route towards lower device costs. However, design challenges remain in the way of commercialization. The hydrogen oxidation (HOR) and evolution reaction (HER) kinetics are much slower at high pH. Several theories have been proposed to explain pH-dependent kinetics, however, debate is ongoing. Changes in interfacial electrostatic potential and the presence of supporting electrolyte ions greatly affect HOR and HER kinetics but are not well understood. ☐ To clarify how interfacial effects are related to kinetic trends in HOR and HER, we developed a composite model to simulate the rate limiting Volmer reaction step. We embed an Anderson-Newns Hamiltonian electronic model for the redox event into classical molecular dynamics simulation of the Pt (111) – aqueous electrified interface. We show how solvent reorganization energy is not sensitive to interfacial electric fields as was previously thought. Instead, changes in the interfacial electrostatic potential affect the redox activation barrier by changing the local potential of interfacial H+. Second, we show that slower kinetics in Cs+-containing electrolyte versus Na+-containing electrolyte are governed by increased solvent reorganization energy in the Cs+ case as well as increased entropy loss as H+ approaches the surface.