Adsorbed Hydroxide Does Not Participate in the Volmer Step of Alkaline Hydrogen Electrocatalysis

Abstract

The sluggish kinetics of the alkaline hydrogen electrode have been attributed to the need to adsorb both H and OH optimally. In this work, single-crystal voltammetry and microkinetic modeling show that an OH-mediated mechanism is not viable on Pt(110). Only a direct Volmer step can explain observed kinetic trends with OH adsorption strength in KOH and LiOH electrolytes. Instead, OH behaves as a rapidly equilibrated spectator species that decreases available surface sites and slows hydrogen kinetics. These results identify kinetic barriers from interfacial water structure, not adsorption energies, as key to explaining changes in hydrogen kinetics with pH.

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.…”

“…We have considered the following possible elementary steps in our micro-kinetic model, including both electrochemical and chemical steps, as shown The net rate, ri, for each elementary reaction described by Equations (1)-(7) is written below in Equations (8)-(14), where θi represents the surface coverage of adsorbate i, ki+/irepresent the rate constants of the forward and backward reactions, respectively, Pj represents the pressure of a gas phase species j, and Ck represents the concentration of solution phase species k42 . Rate constants are given by k = Q×e -Ga/(kbT) , where Q represents the reaction pre-factor 42 of 2 /ℏ (~ 1×10 13 s -1 ), Ga represents the activation barrier, kb represents the Boltzmann constant and T represents the reaction temperature.…”

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.…”

Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum

“…The shift in the (apparent) hydrogen binding energy has long been recognized as a key descriptor in predicting the performance of HOR/HER catalysts . H upd peaks have also been proposed to be the result of the exchange of H and OH on the surface, with the peak shifts with the pH of the electrolyte attributed to the change in the Pt−OH bond strength due to the specific adsorption of cations . 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.…”