Electrochemical Capacitors

Abstract

Electrochemical capacitors using porous silicon nanostructures were prepared and studied. Capacitors are favored over batteries for certain applications because they can provide higher power, albeit at lower energy densities, and do not degrade significantly over many thousands of charging cycles. These properties are a direct result of the fact that capacitors do not rely on chemical reactions to store energy. In this work, porous silicon nanostructures that have exceedingly high surface-to-volume ratios with channel sizes 20 nm to 100 nm are combined with an ionic electrolyte to create an electrochemical capacitor. The devices are fabricated using silicon process methods with the potential to be incorporated into integrated circuits.Initial measurements from the prepared porous silicon capacitors reveal that high capacitances can be achieved, contrary to previously reported early studies [1, 2]. Unpassivated silicon structures, however are not stable over a long period of time due to oxidation/reduction reactions with the electrolyte, resulting in rapidly decreasing capacitance over repeated charge-discharge cycles. Our work has focused on optimizing the porous silicon nanostructure (i.e. surface area, pore morphology, etc.) and passivating the surfaces. As part of this, the pore sizes and size distributions were optimized and modeled to make the total surface area high to obtain large capacitance.[1] Rowlands, S. E., Latham, R. J. & Schlindwein, W. S. Supercapacitor devices using porous silicon electrodes. Ionics 5, 144–149 (1999).[2] S. Desplobain, G. Gautier, J. Semai, L. Ventura and M. Roy, "Investigations on porous silicon as electrode material in electrochemical capacitors", Phys. Stat. Sol. (c), 4, 2180, 2007.