Charge storage mechanism of manganese dioxide for capacitor application: Effect of the mild electrolytes containing alkaline and alkaline-earth metal cations

Effect of the electrolyte particularities on manganese dioxide in the neutral electrolytes containing alkaline (Li+, Na+, or K+) and alkaline-earth (Mg2+, Ca2+, or Ba2+) metal cations for electrochemical capacitor is investigated by cyclic voltammetry and electrochemical impedance spectroscopy. A very high capacitance of 357.9 F g−1 is measured when Ca2+ ion is used as electrolyte cation. The results show that cations of the electrolyte instead of proton or anions are responsible for the pseudo-capacitive behavior of manganese dioxide. The capacitance of manganese dioxide is found to depend strongly on the electrolyte particularities, for example, pH value, cation and anion species, and salt concentrations. A logarithmic dependence of capacitance of MnO2 on cation activity is obtained for all alkaline and alkaline-earth metal cations. The capacitance of MnO2 can be doubled by replacing univalent cation with bivalent cation at a close cation activity.

► The effect of the electrolyte on MnO2 has been investigated systematically by using alkaline (Li+, Na+, or K+) and alkaline-earth (Mg2+, Ca2+, or Ba2+) metal cations as an ion probe.
► A very high capacitance of 357.9 F g−1 has been obtained when Ca2+ ion is used as electrolyte cation.
► The reversible insertion–extraction of cation instead of proton or anions is responsible for the pseudo-capacitive behavior of manganese dioxide.
► The capacitance of MnO2 has been doubled by replacing univalent cation with bivalent cation at a close cation activity.