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–1Li2SO4solution demonstrates the best long-term performance (214
h floating test) compared to the EC with a highly conductive 1 mol
L–1Cs2SO4solution (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 theSBETfade. Interestingly,
carbonate formation is observed as a minor cause of aging. Two strategies
for optimizing sulfate-based ECs are proposed. In the first approach,
Li2SO4solutions 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 Li2SO4and Na2SO4at an equal concentration. This concept allows
the operational time to be significantly prolonged, up to 648 h (+200%
compared to 1 mol L–1Li2SO4). Therefore, two successful pathways for improving sulfate-based
ECs are demonstrated.