Phase-Dependent Reactivity of Nickel Molybdates for Electrocatalytic Urea Oxidation

Identifying the influence of the phase of a catalyst on its reactivity is crucial for guiding the rational design of highly active electrocatalysts. Herein, we unveil the phase-dependent reactivity of nickel molybdates (NiMoO4) for the electrocatalytic urea oxidation reaction (UOR). Various NiMoO4 phases, namely, α-NiMoO4, β-NiMoO4, and the hydrate NiMoO4·xH2O, were synthesized, and their structural characteristics and electrochemical properties were related to their electrocatalytic performance for the UOR. The NiMoO4 phase was found to determine its reactivity, and phase-dependent UOR activities were observed. In particular, β-NiMoO4 exhibited a higher activity and faster kinetics than NiMoO4·xH2O and α-NiMoO4, which was attributed to the large electrochemical surface area, low Tafel slope, and small charge-transfer resistance of β-NiMoO4. Moreover, hydrogen generation via β-NiMoO4-catalyzed urea electrolysis achieved a much lower cell voltage (1.498 V to reach 10 mA cm-2) than that required for water electrolysis (1.633 V to reach 10 mA cm-2). This work provides insights into design strategies for high-activity electrocatalysts for energy-efficient hydrogen production.