Estudos físico-químicos e desenvolvimento de baterias de inserção de prótons a partir de hexacianoferratos

Abstract

This work involved the synthesis and physicochemical characterization, including electrochemical studies, of hexacyanoferrates (HCFs), CuHCF, VOHCF, ZnHCF, CoHCF, NiHCF and MnHCF, for application in energy storage devices by means of proton insertion reactions in aqueous media. The results showed that both the synthesis carried out by coprecipitation at a temperature of 25-30°C and that carried out in an autoclave at 120 and 150°C obtained particles of very similar morphology and size. The specific discharge capacities at 0.5 C were 80, 65 and 50 mAh g-1 and at 120 C 40, 4 and 15 mAh.g-1, respectively for CuHCF, CuHCF.120 and CuHCF.150. The VOHCF, VOHCF.120 and VOHCF.150 compounds showed values of 120, 76 and 82 mAh g-1 at 0.5 C and 40, 29 and 19 mAh g-1 at 120 C, respectively. Furthermore, ex-situ XRD studies showed that after 500 cycles at a rate of 20 C, the CuHCF kept its crystal structure fairly well preserved, in the cubic phase. However, after the first ionic insertion into the VOHCF (oxidized state), a new phase of greater crystallinity appears, never reported in the literature. After 500 cycles (oxidized state), the material has degraded considerably; while in its reduced state it displays a mixture of phases, with the initial cubic phase being fairly well preserved. The galvanostatic intermittent titration (GITT) studies carried out on CuHCF and VOHCF, with temperature variations and using ordinary and deuterated sulphuric acid electrolytes, showed that the reactions are dependent on the interaction of these ions with the water molecules inside the structure and suggest that the predominant mechanism for proton transport is that of Grotthuss. The CoHCF, ZnHCF and MnHCF compounds did not perform well. NiHCF showed good electrochemical performance at high rates, with a specific capacity at 0.5 C of 47 mAh g-1 and at 100 C of 24 mAh g-1. These results confirmed that this class of compounds, especially CuHCF, VOHCF and NiHCF, could be promising for the development of rechargeable proton insertion batteries. These devices can be cheap, safe, with good energy density and power density.