Fabrication and characterisation of a mixed oxide-covered mesh electrode composed of NiCo2O4 and its capability of generating hydroxyl radicals during the oxygen evolution reaction in electrolyte-free water

Abstract

A mixed oxide-covered mesh electrode composed of NiCo2O4 (MOME-NiCo2O4) was prepared on a stainless-steel substrate using thermal decomposition (slow-cooling rate method). Surface, bulk and electrochemical properties of MOME were studied using different techniques, namely scanning electron microscopy (SEM), X-ray diffraction (XRD), cyclic voltammetry (CV) with determination of the electrochemical porosity (ϕ) and morphology factor (φ) parameters, quasi-stationary polarisation curves (PC) and electrochemical impedance spectroscopy (EIS). SEM images revealed a good coverage of the metallic wires by a compact oxide layer (absence of cracks). XRD analysis confirmed the formation of the spinel NiCo2O4 with the presence of NiO. The ‘in situ’ surface parameters denoted as ϕ and φ exhibited values of 0.39 and 0.33, respectively, revealing that the electrochemically active surface area is mainly confined to the ‘outer/external’ surface regions of the oxide layer. The PC was characterised by two Tafel slopes distributed in the low (b 1 = 46 mV dec−1) and high (b 2 = 59 mV dec−1) overpotential domains. The corresponding apparent exchange current densities were j 0(1) = (3.43 ± 0.11) × 10−6 A cm−2 and j 0(2) = (6.70 ± 0.08) × 10−6 A cm−2, respectively. The EIS study accomplished in the low-overpotential domain revealed a Tafel slope (b 1) of 51 mV dec−1. According to the spin-trapping reaction using N,N-dimethyl-p-nitrosoaniline (RNO), the MOME-NiCo2O4 electrode exhibited good performance for the generation of weakly adsorbed hydroxyl radicals (HO•) during the OER in electrolyte-free water.