Highly active and stable bi-functional NiCoO2 catalyst for oxygen reduction and oxygen evolution reactions in alkaline medium

Single step solution combustion technique was used to synthesize NiO, Co3O4 and NiCoO2 mixed metal oxide with good crystallinity and uniform properties. XRD spectrum indicates the existence of cubic NiCoO2 phase without any impurities. SEM results indicate the presence of porous structures in all the three cases, a typical characteristic of combustion synthesized samples, which is due to the evolution of gases during the synthesis process. TEM along with the phase mapping shows the presence of well dispersed elements Ni, Co and O throughout the sample. All the three catalysts were evaluated for their bifunctionality towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline medium. NiCoO2 shows the highest number of electron transfer in the overall reaction mechanism with the maximum kinetic current density of 12.3 mA/cm2. The kinetics of NiCoO2 towards ORR and OER was analyzed using Tafel plot and compared with the mono-metal oxides. The catalytic stability was evaluated for 24 h using continuous chronoamperometric (CA) runs, where NiCoO2 shows exceptionally stable performance without any significant decay in current. The highest activity of NiCoO2 could be due to the presence of higher oxidation states of Ni and Co and because of the existence of the oxygen defects acting as active sites for the oxygen adsorption/desorption during the electrocatalytic reactions. Based on the activity and stability trends, NiCoO2 is found to be a promising bifunctional oxygen electrocatalyst for long-term applications.