Synthesis and evaluation of electrochemical and photocatalytic properties of rare Earth, Ni and Co mixed oxides recycled from spent Ni–MH battery anodes

This study presents a new environmentally favorable hydrometallurgical approach to the recycling of nickel–metal hydride (Ni–MH) battery anodes into new materials based on mixed oxides of Ni, Co, Mn, and lanthanides, which were applied as photocatalysts and electrodes in alkaline media. The starting material was characterized by X-ray diffraction (XRD) and inductively coupled plasma optical emission spectroscopy (ICP-OES). Tartaric acid was used as leaching agent for anode metals, and the resulting tartrate salts were converted to oxides via thermal treatment. Characterization of the synthesized materials was performed by XRD, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Cyclic voltammetry and electrochemical impedance spectroscopy were used to analyze the electrochemical properties of oxides, which were found to behave as electric double-layer capacitors. The recycled materials were used as catalysts in the decolorization of methylene blue in the presence of H₂O₂ under UV radiation. A decolorization efficiency greater than 90% was achieved in a shorter time than that taken for the non-catalyzed reaction to achieve 80% color removal (maximum decolorization). The results showed that methylene blue photocatalysis and photolysis differ in optimal pH conditions and that the catalyzed reaction follows pseudo-first-order kinetics.