Pseudocapacitance of MnO2 originates from reversible insertion/desertion of thiocyanate anions studied using in situ X-ray absorption spectroscopy in ionic liquid electrolyte

Charge storage mechanisms of electrodeposited MnO2 in various aprotic ionic liquids (ILs) are studied using in situ X-ray absorption spectroscopy (XAS). The analytical results show that a unique thiocyanate (SCN−) anion can reversibly insert/desert into/from the tunnels between the [MnO6] octahedral subunits depending on the applied potential. This process charge compensates the Mn3+/Mn4+ redox transition upon charging–discharging and thus contributes to an ideal pseudocapacitive behavior of the MnO2 electrode. The present work would open up a route for developing a novel oxide-based supercapacitor, with high cell-voltage, high thermal stability, and high safety, incorporating IL electrolytes.