The Emerging Heteroepitaxial NiSe₂/Nb₂C: A Two‐In‐One Bi‐Functional Material Architecture for Electrocatalytic and Supercapattery Application

Abstract The rational design of electrode materials is pivotal for optimizing the performance of energy storage and conversion systems such as supercapattery and electrocatalysis. In this study, a one‐step supercritical fluid synthesis is presented to craft a unique heteroepitaxial composite, incorporating nickel di‐selenide nanoparticles on niobium carbide MXene nanosheets in various ratios. The integration of 2D nanosheets enhances the kinetics of the electrode reaction process, while NiSe₂ facilitates efficient charge transport in MXene. The composite demonstrated superior performance with the 1:2 ratio of NiSe₂/Nb₂CTx demonstrates an impressive specific capacitance of 1309 C g⁻¹ at 2 A g⁻¹ current density in a three‐electrode system. Furthermore, the asymmetric supercapattery, featuring NiSe₂/Nb₂CTx 1:2 (positive electrode) and bio‐derived activated carbon (AC) (negative electrode) achieves a gravimetric capacitance of 205 C/g at 0.5 A g⁻¹ current density, a power density of 10,800 W kg⁻¹ at the energy density of 64 Wh kg⁻¹, with 90% retention after 12 000 cycles. As a catalyst for water oxidation, the NiSe₂/Nb₂CTx 1:2 exhibits the lowest over potential of 359 mV at the current density 10 mA cm⁻² and Tafel slope of 105 mV dec⁻¹. This investigation introduces an innovative approach for developing high‐performance supercapattery electrode materials and electrocatalysts using MXene.