Percolation threshold and interface optimization in a HUP/C doublelayer supercapacitor

Abstract

Energy consumption of electronic devices has decreased gradually with progress in integrated circuits; thus, the durable voltage required for a capacitor has become lower and use of the electric double layer developed at a highly polarizable blocking electrode/electrolyte interface is possible. The demand for miniaturized high energy density and low leakage current capacitors as, for example, a stand-by power source for RAM devices or for actuators offers new opportunities for these devices. The first attempts to achieve such a device were made in the seventies [1], but electrical performances were poor and potential applications were restricted. More recently, a double layer capacitor using a liquid electrolyte (H2SO4) and a carbon electrode was developed and commercialized by NEC [2]. Use of a solid electrolyte opens new possibilities: (i) manufacture of microdevices using a collective process, (ii) a better long-time stability and, owing to the use of pure ionic conductor as separator, a high charge capability, (iii) the slow rate of electrochemical reactions in the solid state prevents degradation due to transient overvoltages. All these potential advantages require a high interface area between the electrolyte and the electrode. The double layer capacity of a typical electrode/electrolyte interface falls into the 10 to 40/~F cm 2 range per effective interface area, indeed. Use of a liquid electrolyte facilitates: contacts with area of carbon. On the other hand, a double percolation (between the solid electrolyte itself and the carbon particulates itself) it necessary to combine in the all solid system a high interface area and a (relatively) low serial resistance [3]. Here we present results concerning the optimization of percolation conditions for two kinds of black activated carbon in the case of an all solid HUP (H3OUO2PO 4. 3H20)/C double layer capacitor. H 3 O U O z P O 4 ° 3H20 is among the best known-fast proton solid conductors [4] and is used in microionic devices such as electrochromic displays [5], sensors [6] or batteries [7]. The HUP structure consists in a quasitwo-dimensional network of water molecules and H3 O+ ions, alternately with layers of uranyl and phosphate ions. The high protonic conductivity occurs via the formation of a quasi-liquid state of H30 + and H20 species between the ( U O z P O 4 ) n slabs [81. Fig. 1 shows a schematic representation of the *To whom all correspondence should be addressed.