Urea Oxidation Electrocatalysis on Nickel Hydroxide: The Role of Disorder

The urea oxidation reaction (UOR) is an important challenge in electrocatalysis, in the context of electrolyzers, fuel cells, medical sensing, and bio-waste treatment. The leading electrocatalysts for the UOR are nickel hydroxide surfaces, operating by the Botte mechanism: NiII(OH)2 is oxidized to NiIIIOOH, and the latter drives the UOR. However, these materials have several polymorphs, with α- and β-Ni(OH)2, and γ- and β-NiOOH, as described by the Bode diagram. We now report a systematic study of a series of nickel hydroxides, varying gradually from α-rich to β-rich materials. The crystallinity, order, hydration, intercalation, and morphology of these materials have been characterized by XRD, Raman, ATR-FTIR, XPS, and HRSEM and linked to their electrochemical activity. The disordered α phase was found to be superior to the β phase as a UOR electrocatalyst, with an oxidation onset potential of 0.31 V vs. SCE, better reversibility, and higher anodic current densities. Moreover, the perfect match in onset potentials between UOR electrocatalysis and the Ni2+/Ni3+ oxidation was observed for both phases, suggesting that the Botte mechanism is true for both the α-Ni(OH)2/γ-NiOOH couple and the β-Ni(OH)2/β-NiOOH couple. We further characterize the dynamic behavior of these materials (activation and aging), allowing us to propose a unified Botte-Bode diagram of UOR on Ni(OH)2/NiOOH.