Unraveling The Hidden Conditions in NiOOH for Electrocatalytic Oxidation of Methanol to Formaldehyde with Unity Faraday Efficiency and Selectivity

Abstract

Nickel oxyhydroxide (NiOOH) has been widely applied in electrocatalytic water splitting. Limited by the sluggish kinetics of the oxygen evolution reaction, the electrocatalytic oxidation of methanol has emerged as an alternative reaction. Methanol oxidation to higher value-added formaldehyde is of great significance in organic synthesis. However, the selectivity of formaldehyde remains ambiguous, whereas formic acid as the further oxidised product has been frequently reported, raising the question of whether NiOOH enables selective formaldehyde formation. This study investigates the reaction pathway of formaldehyde formation from methanol oxidation using NiOOH electrocatalysts. The ~ 100% selectivity and Faraday efficiency of formaldehyde formation requires low applied potential and high concentration of methanol. Operando spectroelectrochemical techniques are employed to reveal the mechanism of formaldehyde formation. Low potential facilitates the NiOOH(3+) as the oxidation species and 90% methanol prohibits the activation of second C-H bond to formic acid as overoxidation. H/D isotope exchange measurements indicate that methanol oxidation is sensitive to the C-H bond activation which is the primary reason of unreported formaldehyde formation and overoxidation to formic acid in literature. Therefore, this study reveals the hidden condition for NiOOH oxidizing methanol to higher-value formaldehyde, thus enabling further application in formaldehyde production on a practical scale.