Potential-Dependent Competitive Adsorption of Electrolytes Kinetically Inhibits High-Current Hydrogen Oxidation in Non-Aqueous Media

Abstract

Performing electrochemical transformations in non-aqueous media can unlock new reactivities and selectivities unattainable in aqueous media. For instance, non-aqueous media (e.g. acetonitrile (MeCN), dimethylformamide (DMF), dimethylsulfoxide (DMSO)) allow the dissolution of organic compounds and enable water-incompatible organic electrosynthesis reactions. On the other hand, the lithium-mediated nitrogen reduction reaction (NRR) carried out electrochemically in tetrahydrofuran (THF) is one of the most promising routes towards electrifying the Haber-Bosch reaction and thus eliminating the need for hundreds of atmospheres of pressure and high temperatures. However, many cathodic non-aqueous electrochemical reactions either do not operate with well-defined anodic reactions (e.g. unproductive solvent and/or electrolyte oxidation) or are often coupled with the sacrificial oxidation of a dissolving metal electrode (e.g. magnesium, zinc). Hence, the development of a non-sacrificial anodic reaction is essential for engineering practical, sustainable non-aqueous electrochemical transformations. In this study, we investigate the hydrogen oxidation reaction (HOR) in a variety of non-aqueous solvents (MeCN, DMF, DMSO, THF) and buffers (AcO-/AcOH, Cl-/HCl, etc.) whose base serves as the proton acceptor for HOR. The reason why HOR can be sustainable is because it produces protons, which can be paired with a number of proton-demanding electro-reductions such as NRR, carbon dioxide reduction (CDR) to form oxalic acid or carboxylic acids, and more. Hydrogen is also stoichiometrically cheaper and less greenhouse-emitting than sacrificial metals, making HOR both cheaper and greener. First, we demonstrate a fabricated gas diffusion electrode (GDE) architecture that supports high H2 transport-limited current densities in non-aqueous solvents at ambient pressure. We find that the maximum HOR current density obtained is not H2 transport-limited but instead depends on the kinetics influenced by the pKa and steric profile of the buffer electrolytes present. By also varying the buffer electrolyte concentration and H2 partial pressure and conducting voltametric studies in the absence of H2, we demonstrate that the HOR current density reaches a maximum due to a competitive adsorption mechanism of the buffer electrolytes, limiting the surface concentration of adsorbed hydrogen atoms from H2 dissociative adsorption and thus inhibiting HOR. Based on these findings, we outline strategies for mitigating HOR inhibition via this competitive adsorption mechanism to unlock higher HOR current densities.