Exploiting Interfacial Cl^–/Cl⁰ Redox for a 1.8-V Voltage Plateau Aqueous Electrochemical Capacitor

The ideal way to boost the energy storage of an aqueous-electrolyte electrochemical capacitor (EC) is to increase its capacity at high voltage. However, oxygen evolution of aqueous electrolytes and poor reversibility exclude high-potential redox reactions, resulting in an unsatisfying plateau voltage and a limited energy density. To address this issue, this study arms aqueous ECs with regulated anionic hydration structure and intensified interfacial affinity for realizing the high-potential Cl–/Cl0 redox reaction, which has rarely been utilized before. Experiments and simulations show that the hetero-ion invaded solvation structure can put chlorine redox potential below oxygen evolution potential, and negatively charged functional groups on the wall of two-dimensional nanochannels increase the energy barrier of the Heyrovsky step and decrease the tendency for Cl2 desorption. Furthermore, we demonstrate the advantage of chlorine redox in the bi-redox activity of a chloride ion-containing electrolyte, methyl viologen dichloride, which delivers a sustained discharging voltage plateau of 1.8 V and a high energy density.