Electrochemical impedance characterisation of a nanostructured (mesoporous) platinum film

Nanostructured platinum films with an hexagonal array of uniform, regularly-sized pores were grown on platinum substrates by electrochemical plating from a liquid crystalline electrolyte. The films were characterised as models for nanoporous electrodes, first by cyclic voltammetry in dilute sulfuric acid, showing enhanced currents due to the internal area of the pore structure. Complex impedance spectroscopy was then used to determine the access time of the capacitance and pseudocapacitance in the double layer and hydrogen adsorption regions, respectively. The results were analysed in terms of a transmission line model, showing capacitance at low frequencies and diffusive behaviour above 1 kHz. Capacitance values were as expected from the internal surface area and in agreement with the cyclic voltammetry results. The pore resistance was higher than predicted from a straight pore model, and showed apparent tortuosity factors of about 10. Nevertheless, the nanostructured material gave a very impressive performance as a supercapacitor, with a volumetric capacitance of 110 F cm(-3) and an effective diffusion coefficient of 5 x 10(-5) cm(2) s(-1) in the double layer region.