Advanced α-phase transition metal hydroxide nanostructures and their composites for energy storage electrode materials

Abstract

Transition metal hydroxides (TMH) have become more popular than transition metal oxides (TMO) as electrode materials for energy storage because these materials have higher specific capacitance. Among TMH materials, Fe-Co hydroxides have garnered significant attention due to their exceptional charge storage capability, abundance, and non-toxic nature. In this study, the co-precipitation method has been used to synthesize micro-flower-like bimetallic alpha-FeCo₂(OH)₆ (α-FCH) and its nanocomposite with reduced graphene oxides (rGO + α-FCH). Synchrotron X-ray diffraction (SXRD) analysis has confirmed the presence of the alpha phase in the synthesized FCH nanostructure. The cyclic voltammetry plot confirmed the pseudocapacitive behavior of these electrodes. Specifically, high specific capacities of α-FCH and the nanocomposite rGO + α-FCH were observed to be 608 mAh/g and 1335 mAh/g, respectively, at a current density of 0.1 A/g. After the next higher current density, an initial dip in the electrode performance was observed for both the samples, but the performance was recovered (self-healing) after further charging. Degradation of performance and self-healing were attributed to the transition of the electrode material from α-FCH to β-FCH phase on the first few charging-discharging cycles, and back to α-FCH (self-healing). To assess the stability of the α-FCH electrodes, a cyclic stability test was performed for 500 cycles at a current density of 2.2 A/g. The results indicated that the α-FCH electrode retained 65% of its initial performance, enhancing its durability and potential for practical energy storage applications.