Electrodeposited ZnCo sulfide composite for balance-performance electrochromic supercapacitors with enhanced charge storage and kinetics

Electrochromic supercapacitors (ECSCs) offer a promising route toward integrated energy storage and optical modulation. However, simultaneously achieving high charge storage capacity and rapid charge-transfer kinetics remain challenging. Rather than prioritizing energy density alone, this study emphasizes the regulation of charge storage kinetics through compositional engineering to achieve stable multifunctional performance. In this work, composition-tunable zinc cobalt sulfide (ZnCoS) electrodes are synthesized via a direct electrodeposition approach, enabling controlled growth of nanostructured films on conductive substrates. By systematically varying the Zn:Co:S ratio, three compositions (ZnCoSa, ZnCoSb, and ZnCoSc) are developed to establish the relationship between composition, structure, and electrochemical behavior. Structural and morphological analyzes reveal the formation of mixed-phase ZnCoS heterostructures with hierarchical porous architectures and uniform elemental distribution, which facilitate electrolyte accessibility and charge transport. Among the samples, ZnCoSc exhibits a well-developed interconnected hierarchical nanostructure and the lowest charge-transfer resistance (1.11 Ω), promoting efficient ion diffusion and electron transport. Kinetic analysis indicates that charge storage is predominantly diffusion-controlled (78%), with an increased capacitive contribution in ZnCoSc, suggesting enhanced redox accessibility. ZnCoSc delivers a high coloration efficiency of 402.14 cm² C⁻¹, an optical contrast of 78.88%, and rapid coloration/bleaching response times of 13.6/15.7 s, demonstrating excellent optical modulation and reversibility. The assembled ZnCoSc//AC asymmetric device demonstrates stable electrochemical performance, including high capacitance retention (93.1% after 10,000 cycles) and good rate capability. The improved performance is associated with optimized compositional balance and microstructure, which collectively enhance electron transport and ion diffusion pathways. These findings highlight the role of compositional engineering in tuning charge storage kinetics and provide mechanistic insight into the role of compositional engineering in regulating electrochemical-electrochromic coupling.