Abstract: The present invention aims at improving the catalytic activity of the metallic platinum deposited on the substrate of the counterelectrode (i.e., the cathode), so as to obtain a high value of the catalytic activity on the reduction reaction of triiodide to iodide essentially independent of the nature of the solvent used in the electrolyte. To that effect, the manufacturing process according to the invention is characterized in that said metallic platinum is deposited in the form of a plurality of spheroidal microcrystallite clusters each having a size of less than about 100 nanometers, dispersed over the surface of said substrate, and in that said deposited metallic platinum is submitted to a thermal treatment, carried out at a temperature in the range of about 375.degree. to 400 C., so as to enhance the catalytic properties of the platinum on the oxido-reduction of iodine and triiodide 3I.sub.2 +2e.fwdarw.2I.sub.3 in the redox system iodine/iodide.