Multifunctional high entropy metal/metal oxide hybrid nanostructures-embedded carbon nanofiber composites for electrochemical energy storage

Carbon nanofibers (CNF) containing high-entropy metal/metal oxide hybrid (HEMMOs) nanostructures are fabricated by electrospinning to improve the overall electrochemical performance. The HEMMO nanostructures have an unique configuration that represents the formation of a single phase metallic alloy nanoparticles consisting of the Fe, Co, Ni, and Cu elements on the surface of manganese oxide-CNF composites structure. The porosity and crystallinity of HEMMOs in the HEMMO/CNF composites were carefully controlled by varying the concentration of metal acetic acid precursors and the oxidation stabilization time, and their electrochemical performances were compared. The optimized HEMMO/CNF hybrid materials deliver a specific capacitance of 215 Fg^-1 at 1 mAcm⁻² with 75 % specific capacitance retention in a symmetric two-electrode cell using a 6 M KOH electrolyte. It also shows a maximum energy density of 26.0 Whkg⁻¹ at power densities of 400 Wkg^-1 along with excellent cycling stability of 90 % retention at 1 mAcm⁻² after 10,000 cycles. An asymmetric device exhibits a high energy density of 41 Whkg⁻¹ at a power density of 400 Wkg^-1, and a stable energy density of 23 Whkg⁻¹ even at a high power density of 10,000 Wkg^-1 when the operating voltage was increased to 1.4 V. The porous structure of HEMMO/CNF containing crystalline high entropy metal nanoparticles significantly enhances the stored charge capacity due to the synergistic effects of more redox-active sites and unique 3D diffusion channels into the electrolyte.