Electrocatalytic evaluation of Co3O4 and NiCo2O4 rosettes-like hierarchical spinel as bifunctional materials for oxygen evolution (OER) and reduction (ORR) reactions in alkaline media

In this work, we have successfully synthetized Co3O4 spheres and NiCo2O4 rosettes by a simple chemical bath. The Ni incorporation to Co to form the NiCo2O4 spinel decreased the crystallite size from 32.4 (Co3O4) to 15.3 nm (for NiCo2O4). The NiCo2O4 spinel displayed a rosette-like hierarchical shape, the Co3O4 presented a semispherical shape. Additionally, NiCo2O4 material presented 1.7-fold higher BET surface area than Co3O4. These physicochemical changes had a direct impact on the electrocatalytic activity for ORR/OER in alkaline media. The NiCo2O4 displayed an onset potential of 0.91 V vs. RHE during the ORR, which is 100 mV more positive to that obtained by the Co3O4 (0.81 V), and 110 mV lower to that displayed by Pt/C as reference material (1.02 V). Additionally, the NiCo2O4 material showed superior activity for OER in contrast to Co3O4 and Pt/C displaying a potential difference of 155 mV@10 mA cm(-2). Electrochemical impedance spectroscopy (EIS) revealed that the improved activity of NiCo2O4 was not only related to surface area differences, but also to the improvement of the electrical properties because NiCo2O4 displayed a lower charge transfer resistance than the Co3O4 material (785.1 vs. 921.5 Omega cm(2)).