Nickel foam supported Cr-doped NiCo₂O₄/FeOOH nanoneedle arrays as a high-performance bifunctional electrocatalyst for overall water splitting

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.