Reversible insertion of a trivalent cation onto MnO₂ leading to enhanced capacitance

Supercapacitor properties of MnO₂ are studied generally in aqueous alkali metal salt solutions, often in a Na salt solution. During electrochemical discharge-charge processes, Na⁺ ions from the electrolyte get reversibly inserted/deinserted on the surface of MnO₂ particles, which leads to redox reaction between MnOONa and MnO₂. In the present study, it has been shown that MnO₂ exhibits enhanced capacitance behaviour in a rare earth metal salt solution, namely, La(NO ₃)₃ solution in comparison with NaNO₃ and Mg(NO₃)₂ aqueous solutions. The specific capacitance increases with an increase in charge on the solution cation (Na⁺, Mg²⁺ and La³⁺). It is proposed that the number of surface sites for adsorption of cations remains unaltered in all solutions. The surface insertion of cation facilitates the reduction of Mn⁴⁺ in MnO ₂ to Mn³⁺ equivalent to the charge present on the cation. As the specific capacitance is related to the quantity of charge that is exchanged between the solid MnO₂ and the aqueous solution, the trivalent cation (La³⁺) provides greater specific capacitance than in Mg(NO₃)₂ and NaNO₃ electrolytes. Accordingly, the number of Mn(IV)/Mn(III) redox pairs involved in the neighbourhood of the adsorption site is one, two and three when Na⁺, Mg²⁺ and La³⁺ ions, respectively, are adsorbed.