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

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.