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 (Li₂SO₄, Na₂SO₄, Rb₂SO₄, and Cs₂SO₄) is reported. The EC with a less conductive 1 mol L^-1 Li₂SO₄ solution demonstrates the best long-term performance (214 h floating test) compared to the EC with a highly conductive 1 mol L^-1 Cs₂SO₄ 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, Li₂SO₄ 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 Li₂SO₄ and Na₂SO₄ 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 Li₂SO₄). Therefore, two successful pathways for improving sulfate-based ECs are demonstrated.