Hydrogen bond network connectivity in the electric double layer dominates the kinetic pH effect in hydrogen electrocatalysis on Pt

Abstract

The origin of the large kinetic pH effect in hydrogen electrocatalysis, that is, the approximately two orders of magnitude decrease in reaction kinetics when moving from acid to alkaline, remains far from having a consensus. Here we show that it is the significantly different connectivity of hydrogen-bond networks in electric double layers that causes the large kinetic pH effect. This result has been obtained by meticulously comparing the electric double layers of acid and alkaline interfaces from ab initio molecular dynamics simulations, and the computed vibrational density of states of water molecules in the interfaces simulated with ab initio molecular dynamics, with the results of in situ surface-enhanced infrared absorption spectroscopy. Using a Pt–Ru alloy as a model catalyst, we further reveal an unanticipated role of OH adsorption in improving the kinetics of alkaline hydrogen electrocatalysis, namely, by increasing the connectivity of hydrogen-bond networks in electric double layers rather than by merely affecting the energetics of surface reaction steps. These findings highlight the key roles of electric double layer structures in electrocatalysis. The hydrogen evolution and oxidation reactions on Pt electrocatalysts exhibit much more favourable kinetics in acidic than in alkaline electrolytes. Now, by combining theoretical simulations and spectroscopic measurements, it is demonstrated that the different connectivity of hydrogen-bond networks in the electric double layer is responsible for such an effect.