Efficient and robust noble-metal-free bifunctional electrocatalysts for overall water splitting (OWS) is of great importance to realize the large-scale hydrogen production. Herein, we report the growth of undoped and Cr-doped NiCo₂O₄ (Cr-NiCo₂O₄) nanoneedles (NNs) on nickel foam (NF) as bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). We demonstrate that Cr-doping significantly improves activity for HER and OER by increasing the conductivity of NNs and allowing more active sites on NNs electrochemically accessible. When amorphous FeOOH is electrodeposited on the surface of Cr-NiCo₂O₄ NNs, the resulting FeOOH/Cr-NiCo₂O₄/NF exhibits itself as an excellent bifunctional catalyst for OWS. In the two-electrode cell where FeOOH/Cr-NiCo₂O₄/NF is used both as cathode and anode for OWS, a cell voltage of only 1.65 V is required to achieve an electrolysis current density of 100 mA·cm^-2. In addition, the catalyst shows a very high stability for OWS, the two-electrode cell can operate at a consist current density of 20 mA·cm^-2 for 10 h OWS with the cell voltage being stable at ca. 1.60 V. These results demonstrate that FeOOH/Cr-NiCo₂O₄/NF possesses an OWS performance superior to most of transition-metal based bifunctional electrocatalysts working in alkaline medium. The excellent bifunctional activity and stability of FeOOH/Cr-NiCo₂O₄/NF are attributed to the following reasons: (i) The NN structure provides a large specific surface area; (ii) the high conductivity of Cr-NiCo₂O₄ enables more active centers on the far-end part of NNs to be electrochemically reached; (iii) the deposition of FeOOH supplies additional active sites for OWS.

A facile electron-charging and reducing method was developed to prepare Au/WO₃ nanocomposites for plasmonic solar water splitting. The preparation method involved a charging step in which electrons were charged into WO₃ under negative bias, and a subsequent reducing step in which the stored electrons were used to reductively deposit Au on the surface of WO₃. The electron-charged WO₃ (c-WO₃) exhibited tunable reducibility that could be easily controlled by varying the charging parameters, and this property makes this method a universal strategy to prepare metal/WO₃ composites. The obtained Au/WO₃ nanocomposite showed greatly improved photoactivity toward the oxygen evolution reaction (OER) when compared with WO₃. After Au decoration, the OER photocurrent was improved by a percentage of over 80% at low potentials (< 0.6 V vs. SCE), and by a percentage of over 30% at high potentials (> 1.0 V vs. SCE). Oxygen evolution measurements were performed to quantitatively determine the Faraday efficiency for OER, which reflected the amount of photocurrent consumed by water splitting. The Faraday efficiency for OER was improved from 74% at the WO₃ photoanode to 94% at the Au-8/WO₃ composite photoanode, and this is the first direct evidence that the Au decoration significantly restrained the anodic side reactions and enhanced the photoelectrochemical (PEC) OER efficiency. The high photoactivity of the composite photoanode toward OER was ascribed to the plasmon resonance energy transfer (PRET) enhancement and the catalytic enhancement of Au nanoparticles (NPs).