In search of an appropriate ionic liquid as electrolyte for macroporous manganese oxide film electrochemistry

Macroporous manganese oxide films were produced via template-assisted electrodeposition methodology employing polystyrene spheres with different diameters as template, thereby producing films with different pore sizes. The obtained films were true casts of the template(s) employed, as evidenced by Field Emission Scanning Electron Microscopy. The influence of pore size on capacitive behavior was assessed by cyclic voltammetry experiments in conventional propylene carbonate/LiClO4. Besides the influence of pore size, four different ionic liquids (ILs) were studied as electrolytes, showing that the physico-chemical nature of IL strongly affects the capacitive behavior of macroporous manganese oxide films, particularly in terms of stability throughout successive charge/discharge cycles. Optimal performance was obtained using 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([BMMI][Tf2N]), in which a constant increase in capacitance was observed for the first 700 charge/discharge cycles. This capacitance remained constant, at least until the 1,000 cycle, and was higher than the capacitance achieved using conventional organic solvents. Moreover, the electrochemical window obtained using [BMMI][Tf2N] was widened (from 0.9 V to 1.5 V), corresponding to a significant gain (up to 75%) in terms of energy density. Thus, the development of electrochemical capacitors may significantly benefit from macroporous manganese oxide film electrochemistry employing ionic liquids as electrolytes.