Long-term electrochemical behaviors of manganese oxide aqueous electrochemical capacitor under reducing potentials

Long-term electrochemical behaviors of hydrated MnO2 electrochemical capacitor (EC) electrode in alkali chloride (KCl(aq)) electrolyte have been studied by using potential cycling for thousands of cycles within different potential windows spanning from 0.8 V (versus Ag/AgCl(aq)) to varied lower-end potentials below the open-circuit potential. Three potential ranges resulting in different cycling behaviors of the oxide EC have been identified. Range I: cycling above 0.2 V results in no change in either microstructure or surface chemistry of the oxide electrode, and no capacitance reduction has been observed. Range II: cycling down to 0.0 V leads to extensive morphological reconstruction and limited reduction of surface Mn ions, while the electrode capacitance remains stable. Range III: cycling with lower-potential end below 0.0 V results in obvious capacitance reduction, along with different morphological reconstruction and Mn reduction from those in Range II. For each selected lower-end potential in Range III, the capacitance descends to a plateau within first thousand cycles, and the extent of the capacitance reduction increases as the lower-end potential decreases.