Evidence for enhanced capacitance and restricted motion of an ionic liquid confined in 2 nm diameter Pt mesopores

Abstract

The organised nanostructure of mesoporous platinum deposited from the H_I phase of a lyotropic liquid crystal template contains a regular, hexagonal array of uniform nanometre diameter cylindrical pores. This structure is ideally suited to the investigation of the interfacial capacitance and properties of ionic liquids confined within small pores of the type found in the high surface area electrodes favoured for supercapacitors and batteries. Cyclic voltammetry experiments for BMIM-PF₆ show a large capacitance for the mesoporous Pt electrode, confirming that the ionic liquid fills the 2 nm pores. The value of the specific capacitance, normalised to the total surface area, for the ionic liquid within the pores is approximately twice as large as the corresponding value at a flat Pt surface. Impedance measurements, using a small amplitude perturbation, give a value for the capacitance about one order of magnitude less than that from cyclic voltammetry where the amplitude of the perturbation is much larger. The impedance measurements show that the conductivity of the ionic liquid within the pores is at least three orders of magnitude lower than that in the bulk indicating highly restricted mobility for the ions in these narrow pores. The implications of these results for applications in supercapacitors and batteries are discussed.