Ambiguous Role of Cations in the Long-Term Performance of Electrochemical Capacitors with Aqueous Electrolytes

[Image: see text] A comprehensive comparison of electrochemical capacitors (ECs) with various aqueous alkali metal sulfate solutions (Li(2)SO(4), Na(2)SO(4), Rb(2)SO(4), and Cs(2)SO(4)) is reported. The EC with a less conductive 1 mol L(–1) Li(2)SO(4) solution demonstrates the best long-term performance (214 h floating test) compared to the EC with a highly conductive 1 mol L(–1) Cs(2)SO(4) solution (200 h). Both the positive and negative EC electrodes are affected by extensive oxidation and hydrogen electrosorption, respectively, during the aging process, as proven by the S(BET) fade. Interestingly, carbonate formation is observed as a minor cause of aging. Two strategies for optimizing sulfate-based ECs are proposed. In the first approach, Li(2)SO(4) solutions with the pH adjusted to 3, 7, and 11 are investigated. The sulfate solution alkalization inhibits subsequent redox reactions, and as a result, EC performance is successfully enhanced. The second approach exploits so-called bication electrolytic solutions based on a mixture of Li(2)SO(4) and Na(2)SO(4) at an equal concentration. This concept allows the operational time to be significantly prolonged, up to 648 h (+200% compared to 1 mol L(–1) Li(2)SO(4)). Therefore, two successful pathways for improving sulfate-based ECs are demonstrated.