Influence of imidazolium-based ionic liquid electrolytes on the performance of nano-structured MnO₂ hollow spheres as electrochemical supercapacitor

Use of room temperature ionic liquids (ILs) as electrolytes with a wider potential window offers scope for developing high energy density supercapacitors for efficient energy storage. In this work, a comparative study on the performance of nanostructured MnO₂ hollow spheres as electrochemical supercapacitor was made by fabricating activated carbon (AC)//MnO₂ asymmetric supercapacitor cells using imidazolium-based ILs as electrolytes. Mesoporous MnO₂ hollow spheres were synthesized through a simple low temperature (80 °C) solvothermal method by reduction of KMnO₄ under acidic condition in presence of Cu₃(1,3,5-benzenetricarboxylate)₂ metal organic framework (MOF) in the precursor solution, which acts as the source of Cu²⁺ playing a crucial role for the formation of the hollow spheres. Four different ILs were investigated from combinations of two different cations [1-ethyl-3-methylimidazolium (EMI⁺) and 1-butyl-3-methylimidazolium (BMI⁺)] and four different anions [hexaflurophosphate (PF₆^-), tetrafluoroborate (BF₄^-), trifluromethanesulfonate (Otf^-) and bis(trifluromethylsulfonyl)imide (TFSI^-)]. Influences of the physico-chemical properties such as ionic size, nucleophilicity, viscosity of the IL on the electrochemical properties are discussed. A high energy density of 163 W h kg^-1, which is comparable to the energy density of a lithium-ion battery, could be achieved with EMIMBF₄ as electrolyte. The present findings would help in further research for developing IL-based supercapacitors.