Crystalline and amorphous cobalt-based metal oxides for water oxidation reaction – structure, composition, morphology and electrochemical

Cu, Ni, and Fe-substituted cobalt-based amorphous materials (CoOx, CuCo2Ox, Ni0.5Cu0.5Co2Ox, Fe0.1Cu0.9Co2Ox, and Fe0.1Ni0.9Co2Ox) were prepared by thermal decomposition method and employed as electrocatalysts for oxygen evolution reaction (OER). These materials were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). CuxCo3-xO4 (0 ≤ x ≤ 1), NixCu1-xCo2O4 (0 ≤ x ≤ 0.75) and FeyCux-yCoxO4 (x=1, 1.5 and y = 0.1, 0.15) were also by thermal decomposition method. Surface area measurements were performed using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The catalytic activity of spinel oxides and amorphous metal oxides for OER was analyzed by CV, while kinetic studies were carried out using a rotating disc electrode (RDE).

TEM and HR-TEM analysis were performed on polarized and nonpolarized CuCo2O4. The study clearly indicates that the as-prepared CuCo2O4 possesses a higher degree of crystallization compared to the polarized sample. The crystallite size of the samples measured by TEM and XRD analysis is very similar. Surface area measurements indicate that the incorporation of Cu and Ni increased the surface area of amorphous and crystalline samples. The incorporation of Fe increased the surface area of the amorphous samples, while an opposite trend was observed in the case of spinel oxides. XPS analysis indicates that the surface of both amorphous and crystalline samples contains different species and the metal ions exist in different oxidation states. Catalytic activity was measured as a function of geometric and real surface area. Both amorphous and spinel oxides were found to be active for OER. However, when corrected for real surface area, spinel oxides provided much higher current density compared to amorphous samples. Our study indicates that spinel oxides outperform amorphous samples for OER.