Pseudocapacitive behavior of Mn oxide in aprotic 1-ethyl-3-methylimidazolium-dicyanamide ionic liquid

The electrochemical behavior of anodically deposited Mn oxide was studied in three ionic liquids (ILs): 2-methylpyridine-trifluoroacetic acid (P-TFA), 1-ethyl-3-methylimidazolium-dicyanamide (EMI-DCA), and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4). In the aprotic and low-viscosity EMI-DCA IL, ideal pseudocapacitive behavior of the oxide electrode was observed; the specific capacitance, measured using cyclic voltammetry at a sweep rate of 5 mV/s, was 72 F/g. The operation potential window was as wide as 2 V, which is double that found in traditional aqueous electrolytes. Moreover, electrochemical stability of the Mn oxide electrode in EMI-DCA IL was excellent; after 600 redox cycles, the capacitance barely decayed. The charge storage mechanism of Mn oxide in the IL was examined using X-ray photoelectron spectroscopic (XPS) analyses. The results reveal that DCA(-), instead of EMI+, is the primary working ion that penetrates into the oxide and compensates the Mn valent state variation. This is the first study that provides a detailed explanation of the pseudocapacitive properties of Mn oxide in IL.