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.