Facile synthesis of novel NiCo2O4-Mn1-xCrxCo2O4 (x = 0.2, 0.6, 0.8) composites for oxygen evolution and methanol oxidation in alkaline medium

Abstract

The electrochemical properties of spinel oxide-spinel oxide composites [NiCo₂O₄-Mn_1-xCr_xCo₂O₄ (x = 0.2, 0.6, 0.8)] with regard to oxygen evolution reaction (OER) and methanol oxidation reaction (MOR) have been investigated. Co-precipitation and sol–gel techniques were utilized to synthesize the materials. Fourier-transform infrared (FTIR) spectroscopy, X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were employed to analyse the physicochemical properties of the synthesized materials. Electrochemical examinations such as cyclic voltammetry (CV) and Tafel experiment were performed in 1 M KOH with and without methanol at 25 °C. The electrocatalytic performance of the materials was assessed via anodic polarization curves and NiCo₂O₄-Mn_0.2Cr_0.8Co₂O₄ was found to be most active towards the OER (j = 33.5 mAcm⁻²) and NiCo₂O₄-Mn_0.8Cr_0.2Co₂O₄ was found to be most active towards the MOR (j = 74.3 mAcm⁻²) at 750 mV. The electrodes’ durability was investigated using chronoamperometric studies and electrochemically active surface area (ECSA) evaluations using Electrochemical Impedance Spectroscopy (EIS) offered further understanding into the enhancement in electrocatalytic activity. From the EIS experiment, the roughness factor for NiCo₂O₄-Mn_0.8Cr_0.2Co₂O₄ and NiCo₂O₄-Mn_0.2Cr_0.8Co₂O₄ was calculated and found to be 6.48 & 22.48, respectively in 1 M KOH. Using chronoamperometry experiment, the retained current (%) for NiCo₂O₄-Mn_0.8Cr_0.2Co₂O₄ and NiCo₂O₄-Mn_0.2Cr_0.8Co₂O₄ was found to be 27% and 41%, respectively in 1 M KOH. The interplay in the thermodynamic properties of the electrocatalyst has been also investigated for OER and MOR. It was found that the nanocomposite NiCo₂O₄-Mn_0.8Cr_0.2Co₂O₄ is well optimized to demand only 35 kJ mol⁻¹ of \(\Delta H_{el}^{\circ\#}\) at 650 mV for OER and the nanocomposite NiCo₂O₄-Mn_0.6Cr_0.4Co₂O₄ demands only 45 kJ mol⁻¹ of \(\Delta H_{el}^{\circ\#}\) at 550 mV for MOR.