Nickel-based bimetallic oxides such as NiMoO 4 and NiWO 4 , 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, NiMoO 4 nanorods and NiWO 4 nanoparticles are prepared via a solvothermal route and deposited on nickel foam (NF) (NiMoO 4 / NF and NiWO 4 /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 NiMoO 4 precatalyst, while NiWO 4 remains stable. A controlled study, stirring of NiMoO 4 /NF in 1 M KOH without applied potential, confirms that NiMoO 4 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 NiMoO 4 compared to the Ni−O−W bond in NiWO 4 causes the transformation of NiMoO 4 into NiO(OH) ED .A comparison of the OER performance of electrochemically derived NiO(OH) ED , NiWO 4 , 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, NiMoO 4 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 NiMoO 4 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.

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In the present era, electrochemical water splitting has been showcased as a reliable solution for alternative and sustainable energy development. The development of a cheap, albeit active, catalyst to split water at a substantial overpotential with long durability is a perdurable challenge. Moreover, understanding the nature of surface-active species under electrochemical conditions remains fundamentally important. A facile hydrothermal approach is herein adapted to prepare covellite (hexagonal) phase CuS nanoplates. In the covellite CuS lattice, copper is present in a mixed-valent state, supported by two different binding energy values (932.10 eV for CuI and 933.65 eV for CuII) found in X-ray photoelectron spectroscopy analysis, and adopted two different geometries, that is, trigonal planar preferably for CuI and tetrahedral preferably for CuII. The as-synthesized covellite CuS behaves as an efficient electro­(pre)­catalyst for alkaline water oxidation while deposited on a glassy carbon and nickel foam (NF) electrodes. Under cyclic voltammetry cycles, covellite CuS electrochemically and irreversibly oxidized to CuO, indicated by a redox feature at 1.2 V (vs the reversible hydrogen electrode) and an ex situ Raman study. Electrochemically activated covellite CuS to the CuO phase (termed as CuSEA) behaves as a pure copper-based catalyst showing an overpotential (η) of only 349 (±5) mV at a current density of 20 mA cm–2, and the TOF value obtained at η349 (at 349 mV) is 1.1 × 10–3 s–1. A low R ct of 5.90 Ω and a moderate Tafel slope of 82 mV dec–1 confirm the fair activity of the CuSEA catalyst compared to the CuS precatalyst, reference CuO, and other reported copper catalysts. Notably, the CuSEA/NF anode can deliver a constant current of ca. 15 mA cm–2 over a period of 10 h and even a high current density of 100 mA cm–2 for 1 h. Post-oxygen evolution reaction (OER)-chronoamperometric characterization of the anode via several spectroscopic and microscopic tools firmly establishes the formation of crystalline CuO as the active material along with some amorphous Cu­(OH)2 via bulk reconstruction of the covellite CuS under electrochemical conditions. Given the promising OER activity, the CuSEA/NF anode can be fabricated as a water electrolyzer, Pt(−)//(+)­CuSEA/NF, that delivers a j of 10 mA cm–2 at a cell potential of 1.58 V. The same electrolyzer can further be used for electrochemical transformation of organic feedstocks like ethanol, furfural, and 5-hydroxymethylfurfural to their respective acids. The present study showcases that a highly active CuO/Cu­(OH)2 heterostructure can be constructed in situ on NF from the covellite CuS nanoplate, which is not only a superior pure copper-based electrocatalyst active for OER and overall water splitting but also for the electro-oxidation of industrial feedstocks.

Due to the inferior conductivity and lability to dissolution during electrocatalysis, iron catalysts lack superior electrochemical performance. However, recent studies on transition-metal oxyhydroxides depict that iron is the active site for water oxidation. Herein, a heterobimetallic ferberite iron-tungstate nanostructure has been employed as an efficient anode material not only for alkaline oxygen evolution reaction (OER) involving water and ethanol oxidation but also as a non-noble metal-based anode for overall water splitting (OWS). The presence of tungstate in the nanostructure improves the efficiency of OER, as reflected in the overpotential value of 282 (±3) mV at 10 mA cm–2 and the Tafel slope of 54 mV dec–1, which is far better compared to that of pure iron-oxyhydroxides as well as some noble metal-based catalysts. A fair activity of the FeWO4 anode further helped to construct a water electrolyzer coupled with a commercial Pt cathode, giving a cell potential of only 1.66 V to reach 10 mA cm–2 current density. The strong binding of [FeO6] with the corner- and edge-shared [WO6] presumably provides facile electron conduction as well as robustness in the structure, which results in long durability during OER and OWS. This study showcases a facile approach to design a stable anode relying on earth-abundant metal precursors, which has remained a perdurable challenge so far.

Due to poor conductivity, electrocatalytic performance of independently prepared iron oxy-hydroxide (FeO(OH)) is inferior whereas in-situ derived FeO(OH) from the iron based electro(pre)catalyst shows superior oxygen evolution reaction (OER). Use...

While bulk or surface modification of transition-metal-based pre-catalysts is the most obvious reason of their superior activity during alkaline oxygen evolution reaction (OER), identification of electroactive species and accurately establishing the in situ evolution pathway of such species remain challenging, albeit fundamentally important to correlate the electronic structure with their inherent activity. Given that, a detailed electrochemical OER study with two bimetallic Fe II M VI O 4 (M = Mo and W) type electrocatalysts has been performed, and the influence of M in the electronic structure at the molecular level to control the energetics of the electroactive species formation has been studied. FeMoO 4 turns out to be better compared to its isotypic FeWO 4 which is due to facile electrokinetics to evolve the electroactive species. Post-chronoamperometric characterization and time-dependent quasi in situ Raman analyses reveal a potentialdriven hydrolytic dissolution of [MoO 4 ] 2from FeMoO 4 to form α-FeO(OH) as the electroactive species. Poor lattice stability (formation enthalpy; ΔH f ), weak Fe−O−Mo bonding, and low decomposition enthalpy (ΔH D ) of the FeMoO 4 lattice favor a facile electrocatalytic decomposition to evolve the highly reactive FeO(OH) surface in which the peroxo (O−O) bond formation is rate limiting with a minimum potential barrier of 38 kJ mol −1 obtained from the variable temperature OER study. Under a very similar electrochemical condition, FeWO 4 is relatively robust, with negligible [WO 4 ] 2− leaching due to a high ΔH D value and less ionic character of the Fe−O−W bonds. This combined experimental and theoretical study establishes how the material's electronic structure, that is, the bonding between the anionic counterpart and the active metal, can play an intriguing role to influence the OER activity which is so far relatively less well-explored.

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