Influence of pH and Proton Donor/Acceptor Identity on Electrocatalysis in Aqueous Media

Abstract

Understanding the effects of solution pH on the rates and mechanisms of multiproton/electron transfer reactions at aqueous electrolyte/electrode interfaces has been an active area of research for many decades. Recent interest in this topic has been driven by observations that the reaction selectivity and rates of electrocatalytic processes for energy storage and renewable fuel synthesis can profoundly change with electrolyte pH. Further, a subset of these reactions, such as CO and CO2 reduction, are often carried out under near-neutral bulk pH conditions. Such conditions, in combination with insufficient mass transport and limited pH buffer capacity, can lead to substantial deviations of the near-electrode pH from the bulk pH of the electrolyte. Such pH gradients, together with electrodes whose surface chemistry is highly dependent on pH, can give rise to complex feedback between the reactions that are catalyzed on the surface and the local pH conditions and surface speciation. In this Perspective, we discuss representative studies that characterize and quantify the effects of (local) pH on electrocatalysis with innovative experimental and theoretical methods. We further highlight possible future directions of investigation.

Proton-donating and chemistry-dependent buffering capability of amino acids for the hydrogen evolution reaction

“…5,6 As a result, the kinetics and selectivity of such reactions show a dependence on acid/base equilibria, where the acid/base properties of the electrolyte and/or the surface play a key role in the efficiency of many reduction reactions. 6,7 Water electrolysis (2H 2 O -H 2 + O 2 ) demonstrates a strong sensitivity to acid/base chemistry where, on the cathodic side, the hydrogen evolution reaction (HER: 2H + + 2e À -H 2 ) occurs. Platinum (Pt) remains the state-of-the-art catalyst for HER, despite advancements in developing novel catalysts for HER, such as, transition metal carbides and transition-metal chalcogenides.…”

Impact of Reversible Proton Insertion on the Electrochemistry of Electrode Materials Operating in Mild Aqueous Electrolytes: A Case Study with TiO2

“…These different aspects were well-identified in energy-conversion systems involving irreversible PCETs, especially in electrochemical cells used for water electrolysis or photoelectrochemical water splitting. [9][10][11][12][13][14][15][16][17] However, it has never been clearly established in PICET-based energy storage systems. Indeed, the question of the source of protons in batteries operating in mild unbuffered aqueous electrolytes has been largely overlooked, often with the assumption that water plays the role of an inexhaustible source of protons, which has never been unambiguously demonstrated and also is not necessarily true.…”

Effect of Adsorbed Sulfate on the Product Selectivity of Ethanol Oxidation on Pt Nanoparticles in Acidic Solution

“…43 pH also impacts the interaction of surface adsorbates by influencing the configuration and stability of surface phases 19,41 and influences the identity and concentration of proton donors and acceptors via acid−base equilibrium. 44,45 The effect of adsorbed sulfate on product selectivity has been scantly reported until now. Herein, we used pH modulation, cyclic voltammetry (CV), and chronoamperometry with online high-performance liquid chromatography (CA-HPLC) to systematically investigate the sulfate-induced change in product selectivity by comparing with nonspecifically adsorbed perchlorate electrolyte.…”