Intrinsic Lability of NiMoO4 to Excel the Oxygen Evolution Reaction.

Abstract

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 (Cdl, 1.21 mF cm-2), and a low charge-transfer resistance (Rct, 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.