Measurements of the double layer capacitance for electrodes in supercritical CO2/acetonitrile electrolytes

•Impedance measurements were made in supercritical CO2 with 11 wt% acetonitrile.•Measurements were made at a range of T, p, potential and concentration.•[NBu4n][BF4], [NBu4n][B{3,5-C6H3(CF3)2}4] and [NMe4][BF4] were studied.•The impedance data fit a simple RC equivalent circuit.•The double layer capacitance can be described by a simple Helmholtz layer model.

Electrochemical impedance measurements were made in the single supercritical phase of CO2 with 11 wt% acetonitrile as the co-solvent at temperatures and pressures between 306 and 316 K and 15.5 and 20.2 MPa over the potential range from −0.45 to +0.75 V vs. Pt. Gold, platinum and glassy carbon electrodes were studied together with the effects of electrolyte concentration for [NBu4n][BF4]. Two other electrolytes, [NBu4n][B{3,5-C6H3(CF3)2}4] and [NMe4][BF4], were also studied. We find that the impedance data can be described by a simple RC equivalent circuit where the uncompensated solution resistance is independent of the electrode potential and is consistent with earlier measurements for the electrolyte conductivity. The results for the double layer capacitance can be described by a simple Helmholtz layer model and are very similar to those found for similar electrolytes in non-aqueous solvents such as propylene carbonate. For glassy carbon the double layer capacitances show a parabolic like potential dependence which we attribute to the lower density of states near the Fermi level.