A pseudo-capacitor having a high energy storage capacity develops a double layer capacitance as well as a Faradaic or battery-like redox reaction, also referred to as pseudo-capacitance. The Faradaic reaction gives rise to a capacitance much greater than that of the typical ruthenate oxide ultracapacitor which develops only charge separation-based double layer capacitance. The capacitor employs a lead and/or bismuth/ruthenate and/or iridium system having the formula A.sub.2 [B.sub.2-x Pb.sub.x ]O.sub.7-y, where A=Pb, Bi, and B=Ru, Ir, and O<x.ltoreq.1 and O<y<0.5 and limits the amount of ruthenate and/or iridium in the electrodes while increasing the energy storage capacity. The ruthenate can be synthesized at low temperatures (40.degree.-80.degree. C.) to form a compound with a high surface area and high electronic conductivity which, in combination with the increased pseudo-capacitance, affords high energy/power density in the pseudo-capacitor. The amount of expensive ruthenate and iridium can be substantially reduced in the pseudo-capacitor by increasing the lead content while improving energy storage capacity.
CONTRACTUAL ORIGIN OF THE INVENTION
The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy and The University of Chicago representing Argonne National Laboratory.