NiO/Co₃O₄ composite: A noble-metal-free bifunctional catalyst for oxygen evolution reaction and oxygen reduction reaction applications

Oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are critical for the efficient functioning of fuel cells, electrolyzers, metal–air batteries, etc. It is well known that Pt-based catalysts are excellent for ORR but are poor for OER; similarly, Ru,Ir-based materials are excellent for OER but poor for ORR. However, these materials (Pt, Ir, Ru) are expensive and less abundant. Therefore, there is a need to develop cheaper and sustainable alternatives that are good at both ORR and OER (bifunctional) so that they can be used in regenerative fuel cells, which can operate as a fuel cell and water electrolyzer. Herein, we report a simple synthesis method for the formation of a NiO/Co₃O₄ composite, as confirmed by a multitude of characterization techniques, namely X-ray diffraction, Raman scattering, X-ray photoelectron spectroscopy (XPS), etc. The noble-metal-free composite, i.e., NiO/Co₃O₄ composite, was tested as a bifunctional catalyst for OER and ORR. The composite exhibited excellent performance in ORR and OER with very low catalyst loading. An onset potential of 0.73 V vs. RHE at 0.1 mA/cm² for ORR and an overpotential of 0.48 V vs. RHE at 10 mA/cm² for OER were obtained for the composite. The redox couples Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ and oxygen defects in the composite are attributed to the enhanced OER and ORR performances. These results are quite encouraging given that the synthesis of the composite is simple, and the precursors used for the same are environmentally benign.