Boosted urea electrooxidation activity by dynamic steady blending CoOOH–Ni(OH)2 nanoclusters for H2 production in a pH-asymmetric electrolyzer

Abstract

Electrochemical urea oxidation reaction (UOR) is a promising alternative to the oxygen evolution reaction for reducing the overall potential of the hydrogen evolution reaction during water electrolysis. The theoretical potential for the UOR is only 0.37 V versus reversible hydrogen electrode (RHE). However, the kinetics of the six-electron transfer process involved in the UOR are inherently sluggish, resulting in high overpotential during the reaction. This study designed an active catalyst with a lower kinetic barrier in the UOR by fabricating blending CoOOH–Ni(OH)₂ nanoclusters through the structural transformation of amorphous Co–Ni hydroxide films. This structural transformation was investigated using high-angle annular dark-field scanning transmission electron microscopy, corresponding energy-dispersive X-ray spectroscopy, and in situ X-ray absorption spectra. The blending CoOOH–Ni(OH)₂ nanoclusters exhibited superior electrocatalytic activity in the UOR in an alkaline environment, achieving a low onset potential of 1.24 V (vs. RHE) in 1 M KOH with 0.5 M urea. We employed the CoOOH–Ni(OH)₂ nanoclusters as anodic electrocatalysts in a two-cell electrolyzer for asymmetric electrocatalysis. Hydrogen could be produced at a remarkable current density of 10 mA cm⁻² at a low applied potential of only 0.45 V. Density functional theory calculations revealed that blending CoOOH–Ni(OH)₂ nanoclusters with more oxygen vacancies exhibited a lower Gibbs free energy for the intermediate reaction pathway of NCONH₂ → NCONH, compared with the fine structure of CoNiO_x (x = 2–3). This study lays down a novel pathway for developing new blending electrocatalysts to be used in electrochemical reactions.