Anodic activation-mediated formation of a crystalline–amorphous heterojunction of CoNi(O)OH for improved urea oxidation activity

Abstract

The urea oxidation reaction (UOR) has been proposed as an alternative to anodic oxygen evolution for an improved cell voltage and cost-effective H₂ production by water electrolysis. However, the progress in this field is limited by the lack of efficient UOR electrocatalysts. Herein, we report two different active catalysts, [CoNi(O)OHs], formed by the in situ anodic activation of the cyanide-bridged polymer [CoNi(O)OH-1] and CoNi-layered double hydroxide [CoNi(O)OH-2] for the electrochemical UOR. In CoNi(O)OH-1, the Co-leaching during the anodic activation produced a Ni-rich CoNi(O)OH phase, forming a crystalline–amorphous heterojunction with a highly disordered structure. The defect-rich structure of CoNi(O)OH-1 facilitates the oxidation of Ni²⁺ to Ni³⁺, stabilizes the Ni³⁺ valence state, and thereby provides dynamically accessible active sites for the UOR. As a result, CoNi(O)OH-1 achieved a current density of 400 mA cm⁻² at a potential of 1.34 V vs. RHE, far better than that of CoNi(O)OH-2 (1.45 V vs. RHE) and Ni(O)OH (1.57 V vs. RHE). Furthermore, operando Raman spectroscopy reveals the adsorption of urea molecules on the catalyst surface, followed by dehydrogenation, and C–N bond cleavage during the UOR.