Pseudo-capacitor device for aqueous electrolytes

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 A2[B2-xPbx]O7-y, where A=Pb, Bi, and B=Ru, Ir, and O<x</=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 DEG -80 DEG 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.