Pristine Bi \(_2\) Se \(_3\) is a promising pseudocapacitive electrode material owing to its layered topological insulator structure and narrow band gap, but its moderate bulk conductivity ( \(10^{2}\) – \(10^{3}\) S m \(^{-1}\) ) and limited redox-active site density restrict its practical energy storage performance. While composite and multi-metal strategies have improved Bi \(_2\) Se \(_3\) -based electrodes, the effect of systematic, lattice-level Co \(^{2+}\) substitution on the structural and electrochemical properties of Bi \(_2\) Se \(_3\) is underexplored. Here, we synthesize a series of Co-doped Bi \(_2\) Se \(_3\) nanostructures (0–20% Co) by hydrothermal method and establish a direct correlation between Co \(^{2+}\) substitution at Bi \(^{3+}\) sites, Co–Se covalent bond formation confirmed by Fourier electron density mapping and XPS peak shifts, and enhanced pseudocapacitive performance. At the optimal 10% Co loading identified by the concurrent maximum in Fourier electron density (33.9 e \(^{-}\) /Å \(^{3}\) ), minimum charge-transfer resistance, and peak specific capacitance across the doping series, the specific capacitance reaches 856 F g \(^{-1}\) (a 2.1-fold increase over pristine Bi \(_2\) Se \(_3\) ), with an energy density of 24.07 Wh kg \(^{-1}\) , coulombic efficiency of 98.7%, and 90.0% capacitance retention over 5000 cycles. Rietveld refinement confirms lattice contraction, while EIS reveals a minimised charge-transfer resistance of 2.21 \(\Omega\) . These results establish controlled Co doping as an effective and scalable strategy for engineering high-performance Bi \(_2\) Se \(_3\) -based pseudocapacitive electrodes.
