Abstract
There are two parts in this research. In the first part, partially substituted MxNi1-xCo2O4 (M = Mn, Fe, Cu, and Zn, x=0.1) spinel oxides, labeled as MNCO-01, FNCO-01, CNCO-01, and ZNCO-01, were synthesized via a facilely hydrothermal method. The effects of the metal partially substitution in the Ni site of the spinel oxide on the morphology and electrocatalytic properties of the materials toward the ORR and OER were investigated and compared by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), inductively coupled plasma-mass spectrometer (ICP-MS), X-ray photoelectron spectroscopy (XPS), surface area analyzer and electrochemical techniques including cyclic voltammetry (CV), linear sweep voltammetry (LSV), tafel plot, and galvanodynamic charge-discharging. As a result, the iron-substituted nickel-cobalt based oxides (FNCO-01) displayed a significant electrocatalytic activity towards both the ORR and OER in comparison with the Mn-substituted, Cu-substituted, and Zn-substituted nickel-cobalt based oxides. In the second part, the spinel-type ternary transition metal oxides of nickel, cobalt, and iron with the composition FexNi1-xCo2O4 (0 ≤ x ≤ 1) were prepared and tested as promising electrocatalysts for the ORR and OER in alkaline solution. The structural, morphological and electrocatalytic performances confirmed that substitution of Ni by Fe increases the electrocatalytic activity of the resulting material significantly. The highest activity was achieved for FexNi1-xCo2O4 with 0.1 ≤ x ≤ 0.3. The practicality of the catalyst were corroborated by testing in a realistic rechargeable zinc-air battery utilizing atmospheric air in ambient conditions, where FexNi1-xCo2O4 (x=0.3) demonstrated superior charge and discharge voltages and long-term cycle stability with virtually no battery voltage fading. The excellent electrochemical results presented in this study highlight the FexNi1-xCo2O4 as highly efficient and commercially viable bifunctional catalyst for rechargeable metal-air battery application.