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 FTO glass by repeated spin coating of a citrate-assisted Bi precursor sol, followed by annealing in air at 400°C. The thickness series was used to clarify how active material loading and transport accessibility jointly control the white-to-black-brown electrochromic response. XRD confirmed crystalline Bi₂O₃, while SEM/EDS and XPS indicated thickness-dependent morphology, Bi-O bonding, and surface oxygen-containing species. 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, and a coloration efficiency of 9.3 cm²·C⁻¹. It also showed the most favorable areal-capacitance trend and transmittance changes synchronized with galvanostatic charge/discharge. Figure 1 summarizes the key electrochromic and charge-storage behavior of the optimized film. The results suggest that the optimized thickness balances film continuity, surface accessibility, and ion/electron transport in sol-gel-derived Bi₂O₃ electrodes.
 

Sol-gel; Thin film; Electrochromism; Energy storage; Surface chemistry