Low Nickel-doped copper as the electrocatalyst for oxidation of formaldehyde and evolution of hydrogen

As copper electrode can catalyze formaldehyde (HCHO) oxidation reaction (FOR) and oxygen reduction reaction in 0.1 M KOH, these redox systems can be utilized to assemble a HCHO fuel cell with an ideal rest potential of 0.8 V. As formaldehyde, a liquid at room temperature, can release hydrogen upon oxidation at a Cu electrode, this system can serve as a H₂ portable source. This study employs home – made single crystal electrodes of Cu(111) and Cu_100-xNi_x (111) (x = 1.5 and 4.3) to highlight the surprising boosting effect of the Ni dopant (∼1.5% molar ratio) in a Cu electrode toward FOR in 0.1 M KOH. These ordered electrodes are made by an annealing and quenching method performed in the ambient air (without the involvement of vacuum). Their FOR activities in KOH are scrutinized by voltammetry and chronoamperometry, from which the Cu_98·5Ni_1.5 (111) electrode is found to be 12 folds more active than the Cu(111) electrode. An increase of Ni content from 1.5 to 4.3% in the Cu crystal does not substantiate the FOR activity. The rate-determining steps of FOR at these electrodes are probed in media of KOH and KOD, revealing a 50% lower FOR peak current at the Cu_98·5Ni_1.5 (111), but not the Cu(111), electrode in 0.1 M KOD + 0.1 M HCHO. The effect of Ni dopant on Cu(111) is considered theoretically by using density functional theory (DFT), showing that the adsorption of H₂COOH⁻, the main species of alkaline HCHO, is increased by the Ni dopant in the Cu matrix. A tiny amount of Ni significantly modifies the electronic structure of the Cu substrate.