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Manganese dioxide exhibits the ideal capacitive behavior in aqueous electrolytes containing alkaline-earth cations (Mg[sup2+], Ca[sup2+] or Ba[sup2+]). A specific capacitance value as high as 325 F g[sup-1] was obtained in 0.1 mol L[sup-1] Mg(NO[sub3])[sub2] electrolyte at 2 mV s[sup-1]. The ideal capacitive behavior, high specific capacitance, good coulombic efficiency, and rate ability of manganese dioxides in aqueous electrolytes containing bivalent cations (Mg[sup2+], Ca[sup2+], or Ba[sup2+]) indicate that these alkaline-earth cations might be good alternatives for the state-of-the-art univalent alkaline cations. Moreover, activated carbon/MnO[sub2] asymmetric capacitors with 2 V operating voltage were built up based on the aqueous electrolytes containing these alkaline-earth cations. The energy density of the AC/MnO[sub2] asymmetric capacitor with Ca[sup2+] cation at a current density of 0.3 A g[sup-1] was found to be 21 Wh Kg[sup-1]. [ABSTRACT FROM AUTHOR]