Hierarchical nanowire and nanoplate-assembled NiCo2O4–NiO biphasic microspheres as effective electrocatalysts for oxygen evolution reaction

Abstract In this study, NiCo2O4–NiO nanomaterials with two different morphologies were synthesized using urea and cetyltrimethylammonium chloride (CTAC) as structure directing agents. A combination of analytical techniques including XRD, XPS, SEM-EDX, TEM, and BET were utilized for the characterization of the materials. Microscopy images show that urea-stabilized NiCo2O4–NiO (U–NiCo) has urchin-like morphology assembled by nanowires, and CTAC stabilization (C–NiCo) led to the formation of microspheres with nanoplate assembly. In addition, both wires and plates are formed by the assembly of individual NiCo2O4–NiO nanoparticles. Then, glassy carbon electrodes were modified with these hierarchical nanomaterials, and their electrochemical behavior in oxygen evolution reaction was investigated in 0.1 M KOH solution. Although both of the materials had similar onset potentials (1.56 V vs RHE for U–NiCo and 1.57 V vs RHE for C–NiCo), U–NiCo could drive 10 mA cm−2 of current density at a lower overpotential value (387 mV) than C–NiCo (430 mV). Both of the materials demonstrated impressive kinetic performance with low Tafel slopes (49 mV dec−1 for U–NiCo and 44 mV dec−1 for C–NiCo) and good stability during long-term constant potential electrolysis. In addition, NiCo2O4–NiO nanomaterials outperformed RuO2 at high catalytic current densities (i.e., η50 = 518 mV for U–NiO and η50 = 622 mV for RuO2).