Electrochemical capacitors with KCl electrolyte

Potassium manganese dioxide KxMnO2+delta.n H2O and amorphous MnO2 in a mild 2 M KCl aqueous electrolyte prove to be excellent electrodes for faradaic electrochemical capacitors. The KxMnO2+delta.n H2O materials were prepared by direct thermal decomposition of KMnO4 and contained a large amorphous/crystalline ratio. A sample decomposed at 550 degrees C gave a specific cyclic capacitance between -0.2 and +1.0 V/SCE of 240 F.g(-1), which corresponds to nearly one-third of the Mn(IV) ions participating in the faradaic reaction. Excellent cyclability at 12 mA.cm(-2) was found for 100 cycles. On short-circuit, K0,31MnO2,12.0,63 H2O in 2 M KCl and pH 10.6 aqueous solution gave an initial current density of 0.58 A.cm(-2) and a total released charge of 4.6 C.cm(-2) compared with 0.32 A.cm(-2) and 11.1 C.cm(-2) for RuOOH.n H2O in 5.3 M H2SO4. Similar results obtained with amorphous MnO2 demonstrate that alkali ions can be used as the working ion in a faradaic supercapacitor, which frees the search for new materials from the constraint of working in a strong-acid aqueous medium. (C) 1999 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.