Possible improvements in making carbon electrodes for organic supercapacitors

Properties of an Electrical Double Layer Capacitor depend both on the technique used to prepare the electrode and on the current collector structure. Capacitors can be built in a similar way to prismatic cells, with several electrodes connected in parallel for each polarity. Our Electrical Double Layer Capacitors included several carbon/carbon electrodes and all the components (electrodes and separators) were wetted with an organic liquid solvent containing a quaternary ammonium salt as electrolyte. In the present work, electrodes were prepared by two different ways: the first one consisted in spraying a liquid suspension of the electrode materials on a nickel foil, and the second one which consisted in filtering and pressing the electrode materials on to nickel collectors. The first technique allowed us to build seven capacitors of 600 F-2.5 V, with time constants of 12 s. Two banks with series connected supercapacitor cells, one as a 12 V-100 F bank and the other one as a 15 V-85 F were tested on cycling. In our experiments to test these banks of supercapacitors, we also coupled the 100 F-12 V bank to a 12 V-7 Ah secondary lead-acid battery in order to demonstrate the contribution of the supercapacitors during power peaks. Comparing the two techniques used to make the electrodes in terms of performances obtained on the supercapacitors prepared, we obtained the best results by using the second method of electrode preparation. Moreover, two kinds of nickel collectors were studied: expanded nickel grids-and various grades of nickel foams, nickel foams giving the best results. The Equivalent Series Resistance of the electrodes prepared with nickel foams depends on nickel foam grade; it is observed to be 1.75 Ohm for a capacitance of 1.37 F in our experimental set up, leading to a time constant of 2.4 s. The mechanical properties of the electrodes were improved as well. (C) 1999 Elsevier Science S.A. All rights reserved.