Optimal surface/diffusion-controlled kinetics of bimetallic selenide nanotubes for hybrid supercapacitors

Nickel chalcogenides, as electrode materials, often have low rate capability and poor cycling stability because of their sluggish diffusion-controlled electrochemical kinetics in aqueous alkaline electrolytes. In this study, Co ions serving as both morphological and electronic modulators were introduced to synthesize Ni_xCo_1-xSe₂ nanotubes. The chemical composition of these nanotubes can be designed purposefully. Moreover, their electrochemical kinetics and pseudocapacitive behavior can be controlled using a typical two-step hydrothermal strategy by tuning the feed amount of Co²⁺. The optimized Ni_0.67Co_0.33Se₂ electrode provided a distinguished specific capacity (1157F g^-1 at 1 A g^-1), cycling stability (91% after 2000 cycles), and excellent rate capability (600F g^-1 at 20 A g^-1). Through calculations, we explain that the obtained Ni_0.67Co_0.33Se₂ electrode has both battery and capacitance behavior and a synergistic effect that combines the complementary advantages of metal Ni and Co ions, thus addressing a massive boost in the specific capacity and cycle performance. Density functional theory (DFT) was used to verify the benefit of substituting Co ions for partial Ni ions in nickel selenides in enhancing charge transfer and accelerating OH^- adsorption and deprotonation/protonation reactions.