Reciclagem verde de níquel e terras raras de ânodos de baterias exauridas de Ni-MH e suas aplicações como pseudocapacitores e fotocatalisadores

Abstract

In this work, a hydrometallurgical and environmentally friendly process was used to recycle Ni-MH battery anodes with the objective of synthesizing new materials based on mixed oxides of nickel, cobalt, manganese and lanthanides, which applications were studied as electrochemical capacitors and photocatalysts. The predominant metallic alloy in the anode was identified through XRD. Tartaric acid and acetic acid were used as leaching agents for metals in the anode. Metal concentrations in leach solutions were determined by ICP-OES. Metal tartrate and acetate salts (precursor materials) were characterized by FTIR, TG, and DTG. These analyzes showed the complexation of leaching agents with metals, and that their thermal decomposition occurs at up to 500 ºC. The calcination of the precursor materials at different temperatures originated the oxides T500, T700, T900, A500, A700 and A900. The oxides were characterized using XRD, FTIR, SEM and EDS techniques. The techniques of cyclic voltammetry, galvanostatic chronopotentiometry and electrochemical impedance spectroscopy were used to study the electrochemical properties in basic medium of the produced materials, and it was shown that they behave like electric double layer capacitors and have specific capacitance values of up to 28 F∙g-1 , which decreases as the synthesis temperature increases. The electrical and diffusional resistances of the oxides increased according to the calcination temperature, in agreement with the increase in particle size shown by XRD. The synthesized oxides were used as catalysts in the decolorization process of methylene blue solutions in the presence of H2O2 under ultraviolet radiation. Photocatalysis tests showed that it is possible to decolorize more than 90% of methylene blue in up to 60 min, reaching 96% in 150 min, against a maximum of 80% in 150 min in the noncatalyzed reaction. Photocatalysis was favored by a more acidic condition, while photolysis was favored by a more alkaline condition. It was proposed that the catalyzed reaction with the studied materials follows a pseudo-first order kinetics.