Intrinsic Lability of NiMoO₄ to Excel the Oxygen Evolution Reaction

Nickel-based bimetallic oxides such as NiMoO₄ and NiWO₄, when deposited on the electrode substrate, show remarkable activity toward the electrocatalytic oxygen evolution reaction (OER). The stability of such nanostructures is nevertheless speculative, and catalytically active species have been less explored. Herein, NiMoO₄ nanorods and NiWO₄ nanoparticles are prepared via a solvothermal route and deposited on nickel foam (NF) (NiMoO₄/NF and NiWO₄/NF). After ensuring the chemical and structural integrity of the catalysts on electrodes, an OER study has been performed in the alkaline medium. After a few cyclic voltammetry (CV) cycles within the potential window of 1.0-1.9 V (vs reversible hydrogen electrode (RHE)), ex situ Raman analysis of the electrodes infers the formation of NiO(OH)_ED (ED: electrochemically derived) from NiMoO₄ precatalyst, while NiWO₄ remains stable. A controlled study, stirring of NiMoO₄/NF in 1 M KOH without applied potential, confirms that NiMoO₄ hydrolyzes to the isolable NiO, which under a potential bias converts into NiO(OH)_ED. Perhaps the more ionic character of the Ni-O-Mo bond in the NiMoO₄ compared to the Ni-O-W bond in NiWO₄ causes the transformation of NiMoO₄ into NiO(OH)_ED. A comparison of the OER performance of electrochemically derived NiO(OH)_ED, NiWO₄, ex-situ-prepared Ni(OH)₂, and NiO(OH) confirmed that in-situ-prepared NiO(OH)_ED remained superior with a substantial potential of 238 (±6) mV at 20 mA cm^-2. The notable electrochemical performance of NiO(OH)_ED can be attributed to its low Tafel slope value (26 mV dec^-1), high double-layer capacitance (C_dl, 1.21 mF cm^-2), and a low charge-transfer resistance (R_ct, 1.76 Ω). The NiO(OH)_ED/NF can further be fabricated as a durable OER anode to deliver a high current density of 25-100 mA cm^-2. Post-characterization of the anode proves the structural integrity of NiO(OH)_ED even after 12 h of chronoamperometry at 1.595 V (vs reversible hydrogen electrode (RHE)). The NiO(OH)_ED/NF can be a compatible anode to construct an overall water splitting (OWS) electrolyzer that can operate at a cell potential of 1.64 V to reach a current density of 10 mA cm^-2. Similar to that on NF, NiMoO₄ deposited on iron foam (IF) and carbon cloth (CC) also electrochemically converts into NiO(OH) to perform a similar OER activity. This work understandably demonstrates monoclinic NiMoO₄ to be an inherently unstable electro(pre)catalyst, and its structural evolution to polycrystalline NiO(OH)_ED succeeding the NiO phase is intrinsic to its superior activity.