Sieving effects in electrical double-layer capacitors based on neat [Al(hfip)(4)](-) and [NTf2](-) ionic liquids

We present an electrochemical investigation into ionic liquids (ILs) based on [Al(hfip)4](-) and [NTf2](-) anions as electrolytes in supercapacitors. The synthesis of [Al(hfip)4](-) ILs was improved to minimize the water content for electrochemical measurements. Several electrodes with different pore sizes from micropores to macropores were prepared. Impedance and cyclic voltammetry studies of eight ILs were conducted with the electrodes and revealed that, despite their close-to-ideal transport properties, [Al(hfip)(4)](-) ILs suffer from a sieving effect in the case of micropores, which leads to a sharp drop in capacitance compared with [NTf2](-) ILs. When the influence of the size is abrogated at meso- or macroporous electrodes, the aluminate salts become competitive in terms of capacitance and show that, without a sieving effect, their basic properties in principal make them suitable electrolytes as well.