Comparing the differential capacitance of two ionic liquid electrolytes: Effects of specific adsorption

Abstract

The differential capacitance/potential curves of two ionic liquid (IL) electrolytes, 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIM+/PF6−) and N-butyl-N-methyl-pyrrolidinium hexafluorophosphate (Pyr14+/PF6−) on a glassy carbon (GC) electrode were measured experimentally. The differential capacitance of BMIM+/PF6−/GC is higher in the negative polarization, while the differential capacitance of Pyr14+/PF6−/GC is higher in the positive polarization, although both ILs are composed of common anions, with cations of similar ionic structures and diameters. Such an opposite trend may be understood in terms of the specific adsorption between BMIM+ and the GC electrode, caused by the π-stacking interaction between the aromatic imidazolium ring and the sp2 graphite surface. The specific adsorption effectively shortens the electric double layer (EDL) thickness on the negatively charged electrode but elongates the EDL thickness on the positively charged electrode. Such an effect is manifested in the differential capacitance, with a higher value on the negative polarization branch than on the positive polarization branch. The impact of the specific adsorption is also seen from the positive shift of the potential of zero charge of BMIM+/PF6−/GC in comparison with that of Pyr14+/PF6−/GC.