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

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 (UeNiCo) 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., eta(50) = 518 mV for U-NiO and eta(50) = 622 mV for RuO2). (C) 2019 Elsevier Ltd. All rights reserved.