Electrolyte Engineering in Redox-Enhanced Electrochemical Capacitors with Zn Anodes: The Role of Colorimetric Indicators

Abstract The increasing demand for high-performance energy devices has prompted the exploration of advanced electrolytic solutions for aqueous energy storage. Redox-enhanced Zn-ion capacitors (RZICs) overcome the limitations of conventional electrochemical capacitors by integrating redox-active molecules into the electrolyte, which enables higher energy density and expanded voltage windows. In this study, we developed organic dye-based colorimetric indicators for the fabrication of functional electrolytes in RZICs. The structural responsiveness of these dyes, driven by proton–electron transfer through electrochromic dynamics, allows real-time monitoring and optimization of the RZICs. The acid–base equilibrium of colorimetric indicators supports pH buffering, resulting in an extended lifespan of Zn||Zn cells up to 4,000 h. The conjugated aromatic structure of the indicators enhances their adsorption onto activated carbon, thereby minimizing the self-discharge in RZICs. Additionally, the phenol–quinone transformation increases the capacity of RZICs to 152.4 mAh g −1 within an optimized voltage window of 0.2–1.6 V, while promoting electrochemical kinetics and suppressing anode degradation. The results advance the design and customization of redox electrolytes with colorimetric properties for supercapacitive energy storage.