Nickel oxide/nickel nanohybrids for oxygen and hydrogen evolution in alkaline media

Ni-based materials are cost-efficient electrocatalysts for the oxygen and hydrogen evolution reactions (OER and HER). Specifically, high-valence nickel oxides have been recently identified as highly active for both reactions; however, the origin of their activity during operation, particularly towards the HER, is still undefined. Herein, electrodeposition was used to produce Ni-based electrocatalysts supported on carbon fiber paper, followed by UV/O3 treatment to oxidize and modify their surface chemistry. The resulting electrodes were composed of nanoclusters formed by a metallic nickel core and ultrathin sheets of a high-valence nickel oxide whose crystalline structure was similar to NiO2, with Ni2+/Ni3+ oxidation states. Upon investigating the effect of the electrochemical conditioning of these high-valence nickel oxide/nickel electrodes, confirming the formation of surface β-Ni(OH)2. This surface layer improved the performance of the electrode by providing active sites for H2O adsorption and dissociation, as indicated by detailed density functional theory (DFT) calculations. The origin of the higher HER activity of β-Ni(OH)2 (001) surface compared to NiO2 (2D), and Ni (111) surfaces is attributed to its unique electronic structure. The high valence nickel oxide/nickel electrodes possessed robust long-term OER and HER stability over 24h. Finally, the potential for modifying the structural composition of these electrodes and their use as bifunctional electrocatalysts for the water-splitting reaction was demonstrated by using the resulting electrodes in an electrolyzer coupled with/without a photovoltaic cell.