Electrolytes for hybrid carbon–MnO2 electrochemical capacitors

Different aqueous-based electrolytes have been tested in order to improve the electrochemical performance of hybrid (asymmetric) carbon/MnO2 electrochemical capacitor (EC). Chloride and bromide aqueous solutions lead to the formation of Cl2 and Br2 respectively upon oxidation of the corresponding salt, thus limiting the useful electrochemical window of the MnO2 electrode and producing gas evolution (in the case of chloride salts) detrimental to the cycling ability of an hybrid device. For sulfate and nitrate salts, MnO2 electrode exhibits a 20% increase in capacitance when lithium is used as the cation compared to sodium or potassium salts, probably due to partial lithium intercalation in the tunnels of α-MnO2 structure. The higher ionic conductivity and solubility of LiNO3 has led to the investigation of this electrolyte in carbon/MnO2 supercapacitor compared to standard hybrid cell using K2SO4. A lower resistance increase was evidenced when the temperature was decreased down to −10 °C. Long term cycling ability of carbon/MnO2 supercapacitor was also evidenced with 5 M LiNO3 electrolyte.