Impact of carbon pores size on ionic liquid based-supercapacitor performance

A comprehensive comparison of symmetrical supercapacitors assembling carbon electrodes with exclusively microporous, mesoporous or combined micro-mesoporous networks provides a critical outlook on the influence of pores size on the performance with ionic liquid-based electrolyte 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI) dissolved in acetonitrile. Contrary to widespread claims, the results for an electrodes set involving carbons of different origin indicate that the presence of large pores does not ensure a better supercapacitor performance. At low current density, the capacitance is basically determined by the surface in pores above 0.8 nm, regardless of the pore size distribution. In addition, the beneficial effect of large pores on the response rate of the supercapacitor cannot be concluded in a straightforward manner. On the contrary, wide porosity in electrodes has detrimental effects that should not be underestimated as far as the competitiveness of the final device is concerned. The greater amount of electrolyte required by larger pores will increase both the weight and the cost of the cell. More importantly, the widening of carbon pores (even in the range of micropores) notably reduces the density of the corresponding electrodes and, consequently, the supercapacitor performance in volumetric terms may not be suitable for practical applications.