Tunable construction of Fe_xCo_3-xSe₄ nanostructures as advanced electrode for boosting capacity and energy density

Hierarchical nanoarchitectures with large void space, unique porous networks, and numerous active sites for advanced supercapacitor (SC) electrodes has attracted great attention in modern electronics. Herein, a novel strategy is established to rational design of hierarchical iron cobalt selenide (Fe_xCo_3-xSe₄) with tunable nanostructure and morphology by adjusting the stoichiometric ration of Fe: Co in order to enhance the energy density of SCs. The optimal FeCo₂Se₄ nanosheet arrays (NAs) enhances the electrical conductivity, electroactive sites, and intrinsic reactivity, achieving an ultra-high specific capacity of ~398.5 mA h g⁻¹ at a current density of 1 mA cm⁻² with an exceptional rate capability (~304.2 mA h g⁻¹ at a current density of 50 mA cm⁻²), and ultra-long cycle life (~98.2% retention after 10,000 cycles). Taking advantage of FeCo₂Se₄ NAs positive electrode, we have successfully assembled solid-state asymmetric SC (ASC) with Fe₂O₃@NG hydrogel as the negative electrode. Impressively, the solid-state ASC achieves high operating voltage window upto 1.6 V, and thus delivers ultrahigh energy density of ~84.1 Wh kg⁻¹ at a power density of 0.69 kW kg⁻¹, and exceptional cycling stability (5.5% of capacity decay after 10,000 cycles), which outperform the recently reported metal selenide-based ASCs. These consequences clearly designate FeCo₂Se₄ NAs as an advanced electrode for next-generation energy storage technologies.