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 a further oxidised product, has been frequently reported, raising the question of whether NiOOH enables selective formaldehyde formation. This study investigates the reaction pathway for formaldehyde formation during methanol oxidation in the presence of NiOOH electrocatalysts. The ∼100% selectivity and Faraday efficiency for formaldehyde formation require a low applied potential and high concentration of methanol. Operando spectroelectrochemical techniques are employed to reveal the mechanism of formaldehyde formation. Low potential facilitates the formation of NiOOH(3+) as the oxidation species; additionally, 90% methanol prohibits the activation of the second C–H bond, avoiding overoxidation to formic acid. H/D isotope exchange measurements indicate that methanol oxidation is sensitive to the C–H bond activation, which is the primary reason for unreported formaldehyde formation and overoxidation to formic acid in the literature. Therefore, this study reveals the previously overlook conditions for NiOOH to oxidise methanol to higher-value formaldehyde, thus enabling further application in formaldehyde production on a practical scale.