NiO–NiMoO4 Nanocomposites on Multi-Walled Carbon Nanotubes as Efficient Bifunctional Electrocatalysts for Total Water Splitting

Multicomponent synergistic regulation and defect design have been effective strategies to enhance the electrocatalytic activity of transition-metal oxides. In this work, NiO and NiMoO4 nanocomposites on multi-walled carbon nanotubes (termed NiO-NiMoO4/mCNTs) are synthesized through a two-step method. Physical characterizations show NiO-NiMoO4/mCNTs have a well-defined NiO-NiMoO4 structure, large specific surface area, and abundant oxygen vacancies. For oxygen evolution reaction (OER), NiO-NiMoO4/mCNTs deliver lower overpotential (277 mV) than NiO/mCNTs, NiO, and commercial RuO2 nanocrystals at 10 mA cm-2. For hydrogen evolution reaction (HER), NiO-NiMoO4/mCNTs still show the best HER activity, manifested by the smallest onset potential and the lowest Tafel. Density functional theory calculations show that the adsorption energies of hydrogen- and oxygen-containing intermediates on the NiO-NiMoO4/mCNTs surface have changed, which can lower the energy barriers required for HER and OER. The excellent electrocatalytic activity of bifunctional NiO-NiMoO4/mCNTs for OER and HER can be attributed to the synergy effect between NiO, NiMoO4, and mCNTs. A symmetrical two-electrode water electrolyzer with NiO-NiMoO4/mCNTs as both the cathode and anode are constructed, which can reach a current density of 10 mA cm-2 and only requires 1.57 V.