Insights into the Pseudocapacitive Mechanism of Selenium-Doped Nickel Cobaltite for Supercapacitors with Improved Electrochemical Performance

Doping engineering has been widely regarded as an efficient way to optimize the electrochemical characteristics of transition metal oxides. In this study, we have adopted a facile hydrothermal method to prepare a multifunctional material, which is uniformly composed of Se-doped NiCo₂O₄ and significant amounts of SeO₂ nanophase (referred to as NCO/Se) as a promising electrode material. The influence of selenium doping is systematically studied based on both experimental and theoretical approaches. As a result, the optimized selenium-doped NCO/Se exhibits significantly enhanced electrochemical performance (a specific capacity of 333.9 mA h g^-1 at 1 A g^-1) in comparison to the pristine NiCo₂O₄ (a specific capacity of 188.1 mA h g^-1 at 1 A g^-1). Density functional theory calculations further validate that Se doping effectively narrows the band gap, elevates the charge carrier concentration, and optimizes the adsorption energy of OH^- ions, which consequently enhances electrical conductivity, facilitates ion diffusion, and accelerates the electrochemical kinetics. Additionally, the Se-doped crumpled sea-urchin-like morphology contributes to increased specific surface area, offers abundant ion transport pathways, and significantly improves the availability of active sites. Furthermore, a high-performance NCO/Se//activated carbon supercapacitor device was constructed, achieving an outstanding energy density of 59.8 W h kg^-1 at a power density of 800 W kg^-1. After 10,000 charge-discharge cycles, the device preserves 84.8% of its original capacitance, indicating outstanding cycling stability. This study offers valuable theoretical and experimental insights into the development of advanced atom-doped transition metal oxides for electrochemical energy storage applications.