FeNi (oxy)hydroxides embedded with high-valence Mo atoms: A efficient and robust water oxidation electrocatalyst.

The incorporation of high-valence transition metal atoms into FeNi (oxy)hydroxides may be a promising strategy to regulate the intrinsic electronic states, thereby reducing the thermodynamic barrier and accelerating oxygen evolution reaction (OER). Here, a high-valence Mo atoms doping route is proposed by an efficient self-reconstruction strategy to prepare MoFeNi (oxy)hydroxides for efficient alkaline OER. By using borides (MoNiB) as sacrificial template and Mo source, FeNi (oxy)hydroxides nanoflakes embedded with high-valence Mo atoms (MoFeNi) is successfully synthesized, which can modulate the electron coordination to improve the intrinsic catalytic activity. Remarkably, the obtained MoFeNi exhibits extremely low overpotential (η100 = 252 mV and η500 = 288 mV) and small Tafel slope (18.35 mV dec-1). The robust catalyst can run stably for hours at 500 mA cm-2. Characterization results and theoretical calculations confirmed that the addition of high-valence Mo effectively modulated the intrinsic electronic structure of metal sites and optimized the adsorption/desorption energy of the intermediates, accelerating OER reactions kinetics. By coupling MoFeNi anode with Pt/C cathode, anion exchange membrane (AEM) electrolyser can operate stably at 500 mA cm-2 with about less than 2.2 V. This research introduces a novel approach to develop ideal electrocatalysts through the incorporation of high-valence molybdenum species.

High-Valence Mo Doping and Oxygen Vacancy Engineering to Promote Morphological Evolution and Oxygen Evolution Reaction Activity.

The rational design of high-efficiency, low-cost electrocatalysts for electrochemical water oxidation in alkaline media remains a huge challenge. Herein, combined strategies of metal doping and vacancy engineering are employed to develop unique Mo-doped cobalt oxide nanosheet arrays. The Mo dopants exist in the form of high-valence Mo6+, and the doping amount has a significant effect on the structure morphology, which transforms from 1D nanowires/nanobelts to 2D nanosheets and finally 3D nanoflowers. In addition, the introduction of vast oxygen vacancies helps to modulate the electronic states and increase the electronic conductivity. The optimal catalyst MoCoO-3 exhibits greatly increased active sites and enhanced reaction kinetics. It gives a dramatically lower overpotential at 50 mA cm-2 (288 mV), much smaller than that of the undoped counterpart (418 mV) and comparable to those of the recently reported electrocatalysts. Density functional theory results further verify that the increased electronic conductivity and optimized adsorption energy toward oxygen evolution reaction intermediates are mainly responsible for the enhanced catalytic activity. Moreover, the assembled two-electrode electrolyzer (MoCoO-3||Pt/C) exhibits superior performance with the cell potential decreased by 233 mV to reach a current density of 50 mA cm-2 with respect to the benchmark counterpart catalysts (RuO2||Pt/C). This work might contribute to the rational design of effective, low-cost electrocatalyst materials by combining multiple strategies.

Boosting oxygen evolution by nickel nitrate hydroxide with abundant grain boundaries via segregated high-valence molybdenum.

High-valence metal doping and abundant grain boundaries (GBs) have been proved to be effective strategies to promote the oxygen evolution reaction (OER). However, the reasonable design of the two to facilitate OER collaboratively is challenging. Herein, a convenient and novel one-step molten salt decomposition strategy is proposed to fabricate segregated-Mo doped nickle nitrate hydroxide with substantial GBs on MoNi foam (Mo-NNOH@MNF). When processed in molten salt, the Mo species on the conductive substrate migrates unevenly to the surface of Mo-NNOH@MNF, which not only induces the formation of abundant GBs to modulate electronic structure, but also improves the intrinsic activity as high-valence dopants, synergistically elevating OER activity. As verification, the optimized Mo-NNOH@MNF-10h exhibits low overpotential of 150 mV at 10 mA cm-2, which can be attributed to the reduced valence charge transition energy of Ni by high-valence Mo dopant, coupled with the fine-tuning of d-band center bond and corresponding local electron density by induced GBs and Mo doping, as DFT calculations revealed. Moreover, the intrinsic robustness and strong adhesion ensure the long-term stability of 6 h at 500 mA cm-2. This work provides a promising molten salt decomposition approach to synthesize advanced materials with unique structures.