Determining the Viability of Hydroxide-Mediated Bifunctional HER/HOR Mechanisms through Single-Crystal Voltammetry and Microkinetic Modeling

Abstract

The slow kinetics of the hydrogen oxidation and hydrogen evolution reactions (HER and HOR) in alkaline compared to acidic media remain a fundamental conundrum in modern electrocatalysis. Recent efforts have proposed that OH, as well as H, must bind optimally for improved kinetics, but the exact role of adsorbed OH is not yet known. In this work, we combine steady-state single-crystal voltammetry and microkinetic modeling to determine the roles of adsorbed hydroxide and the so-called bifunctional mechanism in alkaline HER and HOR kinetics. We consider both a direct Volmer mechanism, in which H and OH compete for sites on Pt (110), and an OH-mediated mechanism, in which Pt ( 111) adsorbs H while transition metal clusters adsorb OH. Our experimental and computational results show that on a thermodynamic coverage basis, increasing OH adsorption strength cannot promote faster HER/HOR kinetics. Only changes to the kinetic rate constants can explain experimental observations. We speculate that adequate electrocatalyst design in alkaline media additionally requires manipulation of interfacial water structure to lower energetic barriers for HER and HOR.

The role of adsorbed hydroxide in hydrogen evolution reaction kinetics on modified platinum

“…This selective promotion of the HER and not the HOR (as well as the promotion of both HER and HOR via a stronger OH* binding (catalyst (a) in Supplementary Fig. 14)) has been observed experimentally 3,18,22,36 , and the reaction model we present here in Supplementary Fig. 14 may explain this phenomenon.…”

“…There has since been substantial debate 12,13 on the role of the electric field 14 , PZC/PME 15 and near-surface water 16 , with studies that either support [17][18][19][20] or refute 5,21,22 a bifunctional mechanism that involves adsorbed hydroxide, in the HER/HOR. However, the majority of this work was performed using complex polycrystalline or metal (alloy) nanoparticles, which made it difficult to connect changes in activity to an atomic-scale picture of the reaction mechanism, as the surface structure and composition, especially in the electrochemical environment, are unknown.…”

pH Effects on Hydrogen Evolution and Oxidation over Pt(111): Insights from First-Principles

“…9,10,11 2. The proton donor is pH dependent 12,13,14,15 . This means that the proton donor can change from hydronium ions (H3O + ) in acidic conditions to water (H2O) in alkaline conditions, and buffer molecules may act either as proton donors or alleviate transport limitations of H3O + to the interface.…”

Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum

“…[21,22] Thes hift in the (apparent) hydrogen binding energy has long been recognized as ak ey descriptor in predicting the performance of HOR/HER catalysts. [1-3, 10, 14, 18, 19] H upd peaks have also been proposed to be the result of the exchange of Hand OH on the surface, [4,5,17,[23][24][25][26] with the peak shifts with the pH of the electrolyte attributed to the change in the PtÀOH bond strength due to the specific adsorption of cations. [4,5,17] Both hypotheses recognize that the diverging trends of the electrode potential on the SHE and the RHE scales with changing pH are key to understanding the impact of the electrolyte pH on the H upd peak and HOR/HER activities,but the proposed molecular level pathways are quite different.…”