Bi₂O₃ thin films prepared by sol-gel processing are attractive for electrochromic energy-storage electrodes because the same reversible charge-insertion reaction can generate visible color change and capacitive response. In this work, Bi₂O₃ films with thicknesses of approximately 70, 100, 130, and 160 nm were deposited on fluorine-doped tin oxide glass by repeated spin coating of a citrate-assisted Bi precursor sol, followed by annealing in air at 400 ℃. The thickness series was used to clarify how active material loading and transport accessibility jointly control the light-yellow-to-black electrochromic response, in which the films switch reversibly between a light-yellow transparent bleached state and a black colored state. X-ray diffraction confirmed crystalline Bi₂O₃, while scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy indicated thickness-dependent morphology, Bi-O bonding, and surface oxygen-containing species. The X-ray photoelectron spectroscopy binding energies are reported with the uncertainty associated with adventitious-carbon charge referencing, and the elemental compositions obtained from these measurements are treated as semi-quantitative rather than definitive stoichiometric values. Among the investigated films, the 130 nm sample gave the strongest redox response, the largest optical modulation at 633 nm (63.5%), bleaching/coloring times of 39.8/40.4 s, a coloration efficiency of 9.3 cm² C⁻¹, and the highest areal capacitance (20.3 mF cm⁻² at 0.1 mA cm⁻²), corresponding to an areal energy density of 0.0176 mWh cm⁻² and an areal power density of 0.125 mW cm⁻². It also showed the most favorable areal-capacitance trend and transmittance changes synchronized with galvanostatic charge/discharge. The results suggest that, among the four thicknesses investigated, the 130 nm film provides the best balance among film continuity, surface accessibility, and ion/electron transport. The optical modulation decreased from 63.537% to 32.59% after 100 switching cycles, corresponding to a retention of 51.29%, so long-term durability is not claimed and remains to be established.