OH$^-$-Enhanced Alkaline Hydrogen Evolution Reaction at the Au(111) Electrode

Abstract: The hydrogen evolution reaction (HER) in alkaline media suffers from sluggish kinetics but the origin of the pH-dependent activity remains debated. This study investigates the role of hydroxide ions (OH) in enhancing the alkaline HER at Au(111) by systematically varying the pH and the NaOH concentration both with and without fixing the total Na concentration.Contrary to conventional cation-centric models of alkaline HER, we demonstrate a notable anion effect by showing that the HER activity increases monotonically with pH and OH concentration, even at extremely high NaOH concentrations(up to 9 M).Tafel slopes decrease from 181 mV/dec at pH=10 to 124 mV/dec at pH=13 and to 111 mV/dec for the case with 9 M NaOH, indicating accelerated kinetics.Infrared spectroscopy reveals that interfacial OH strengthens the hydrogen-bond network, which is expected to lower the activation energy for the Volmer step,the rate-determining step of HER on Au(111). Hence,OH enhances alkaline HER kinetics by strengthening the hydrogen bond network and its connectivity at the electrochemical interface;this allows us to propose a unified mechanism for the electrolyte effects on alkaline HER where structure-making ions (Li,K,and OH) improve the reaction kinetics by optimizing the interfacial hydrogen bond network.