In situ Mn K-edge X-ray absorption spectroscopy studies of electrodeposited manganese oxide films for electrochemical capacitors

In situ Mn K-edge fluorescence X-ray absorption spectroscopy (XAS) was used to analyze the manganese oxides electrodeposited on a porous carbon paper substrate for use in electrochemical capacitors in order to determine the local and electronic structural changes in the material as a function of the applied potential in a neutral electrolyte. Within the potential range from +0.1 to +0.8 V us SCE (reversible region), the cyclic voltammogram (CV) showed ideal capacitive characteristics. On the other hand, large current tails were observed at near both ends of the potential window in the CV when the upper and lower potential limits were expanded to +1.0 and -0.3 V vs SCE (irreversible region), which is indicative of an irreversible reaction. According to the in situ X-ray absorption near-edge structure (XANES) results, the capacitive currents of the manganese oxides in 2 M KCl in the reversible region originated from the Faradaic pseudocapacitance. The average oxidation state and local structure of the manganese oxide changed reversibly during charging/ discharging within the reversible region. On the other hand, the local and electronic structure of manganese oxide changed in an irreversible manner in the irreversible region, particularly during the redox reaction within the potential range between +0.1 to -0.3 V vs SCE. This irreversible feature of the local and electronic structure changes was attributed to the formation of the electrochemically irreversible low valence manganese oxides such as Mn₂O₃ and Mn₃O₄, and the dissolution of Mn species from the electrode.