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

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.