Hollow Prussian blue analogue–derived (CoFe)Se2 composites: Structural designing for preparing flexible, high-performance solid-state supercapacitors

Selenides show higher corrosion and oxidation resistance than conventional materials. Moreover, bimetallic selenide materials typically show higher electrical conductivity than monometallic materials, which accelerates their electron transfer kinetics during charge–discharge cycling. The combination of two different metals in bimetallic selenides improves the energy-storage capacity and provides a stable electrochemical environment resulting from their unique electronic structure and interaction with the electrolyte. Among various bimetallic selenides, cobalt–iron selenides are especially noted for their excellent electrocatalytic properties. In this study, we synthesised co-doped hollow Prussian blue (PB) analogue nanocubes with large sizes, high specific surface areas and many active sites. When employed as the positive electrode in a flexible solid-state supercapacitor, bimetallic (CoFe)Se ₂ nanocomposites obtained at 500°C in nitrogen showed high supercapacitor performance, with a capacitance of 1710 F g ⁻¹ at 1 A g ⁻¹; extended cycle life (maintaining 98.5 % of their initial capacitance after 3000 cycles at 5 A g ⁻¹) and near 100 % Coulombic efficiency. Further, a flexible asymmetric supercapacitor ((CoFe)Se ₂//activated carbon (AC)) was developed using AC and (CoFe)Se ₂ as the negative and positive electrodes, respectively. (CoFe)Se ₂//AC achieved a Coulombic efficiency of 90 % and outstanding cycling stability, maintaining 93.5 % of its original capacity after 10,000 cycles at 5 A g ⁻¹. The specific capacitance and energy density were 40 F g ⁻¹ at 1 A g ⁻¹ and 750 W kg ⁻¹ (10.41 W kg ⁻¹), respectively. This research paves the way for rational and environmentally friendly synthesis of bimetallic selenides with distinct sizes and topologies. By varying the metal dopant, our method can potentially produce bimetallic selenides for diverse applications.