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• A facile dual-sites method is developed to modify MoO 2 nanospheres. • The optimal Ni and Fe co-doped MoO 2 shows low overpotential and good stability. • The synergistic catalytic mechanism for OER is illustrated. Oxygen evolution reaction (OER) is of critical importance in the area of electrocatalysis as it is frequently involved in a diversity of electrochemical devices. Molybdenum-based materials, thanks to their metallic character induced high conductivity, have been widely studied as catalysts for a variety of reactions but few reports for oxygen evolution reaction (OER). Herein, we report a dual-sites modifying strategy to modulate electrocatalytic properties of metallic MoO 2 nanospheres toward OER, which is implemented by a one-pot synthesis process to prepare nickel and iron co-doped molybdenum oxide (Mo 0.9 Ni 0.05 Fe 0.05 O 2). The introduction of dual sites (i.e. , Ni and Fe) on MoO 2 nanospheres surface can tune the surface electronic structure and local-chemical environment with generation of high-valence Mo and hydroxyl-rich surface, synergistically contributing to the improved electrocatalytic performance in terms of stability and activity. Density functional theory (DFT) calculation indicates Fe-doping site favors for enhancing the turnover efficiency (TOF) while the Ni-doping site facilitates the first proton coupled electron transfer (PCET) upon electrocatalysis of OER. The Mo 0.9 Ni 0.05 Fe 0.05 O 2 requires only 249 mV of overpotential to reach a current density of 10 mA cm−2 with excellent stability for 80 h. This study expands the research on molybdenum-based OER catalysts and gains insight into the origin of both the activity and stability of Ni/Fe-doping reactive sites. [ABSTRACT FROM AUTHOR]

A bifunctional electrocatalyst has been developed through integrating nickel–iron (oxy)hydroxide nanoparticles (NiFeO x (OH) y) and MoS 2 nanosheets on reduced graphene oxide. It has been deduced that MoS 2 is oxidized during oxygen evolution reaction, and the oxidized Mo species modulate the electronic structure of NiFeO x (OH) y , while NiFeO x (OH) y serves as water dissociation promoters to facilitate hydrogen evolution reaction, resulting in high electrochemical activity and durability for overall water splitting in alkaline electrolyte. • The composites consisting of NiFeO x (OH) y and vertically-oriented MoS 2 nanosheets on reduced graphene oxide has been prepared. • The promoting roles of MoS 2 nanosheets on oxygen evolution reaction have been proposed. • The integrated electrocatalyst shows high activity and superior durability for water splitting. Developing cost-effective methodologies for fabricating efficient bifunctional electrocatalysts is pivotal to boost electrochemical water splitting. Herein, we report an integrated composite, which is composed of NiFeO x (OH) y nanoparticles and vertically-oriented MoS 2 nanosheets on reduced graphene oxide (rGO) matrix (NiFeO x (OH) y @MoS 2 /rGO), for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). It has been deduced that MoS 2 is oxidized during OER, and the oxidized Mo species modulate the electronic structure of NiFeO x (OH) y to facilitate chemisorption of the oxygen species in OER. Meanwhile, NiFeO x (OH) y speeds up sluggish water dissociation in HER, and thus significantly improving HER activity of MoS 2 in alkaline electrolyte. The integrated composite shows high electrocatalytic activity and superior durability for HER and OER with low overpotentials of 170 mV at 10 mA cm−2 and 250 mV at 20 mA cm−2, respectively. This work provides new approaches to develop electrocatalysts for overall water splitting and other applications. [ABSTRACT FROM AUTHOR]