Environmentally friendly recycling of spent Ni–MH battery anodes and electrochemical characterization of nickel and rare earth oxides obtained by sol–gel synthesis

In this study, Ni and rare earths (REs) were recycled from anodes of spent Ni–MH batteries by means of a sol–gel process in a leachate solution of anode material and citric acid. The metal–organic precursor material was heat treated at 1123 (HT1), 1373 (HT2), and 1473 K (HT3), resulting in mixed Ni and RE oxides, such as LnNiO3 and LnO. Phase proportions varied according to heat treatment temperature. Scanning electron microscopy of Ni and RE oxides revealed porous particles containing pores and macropores. Electrochemical properties were assessed using cyclic voltammetry, galvanostatic cycling, and impedance spectroscopy. The cyclic voltammograms of HT1, HT2, and HT3 oxides exhibited double-layer capacitive behavior. Galvanostatic charge/discharge cycling at 0.5 A g−1 showed that oxides obtained by HT1, HT2, and HT3 had specific capacitances of 8.48, 4.23, and 1.28 F g−1, respectively.