The history of electrochemical energy storage devices, especially capacitors and batteries, has involved attempts to reduce the size, including both weight and volume, and to increase the electrical energy storage capacity while at the same time increasing the voltage required for dielectric breakdown. Recent advances in battery design have included improvements in life, efficiency and energy density by making improved lead-acid, nickel-cadmium, nickel-zinc and various primary cells. However, although many of the devices embracing the recent technological advances have filled a need, there continues to be a requirement for efficient high power density devices which withstand the rigors of continuous use and virtually unlimited cycling in electrical circuits.
The occurrence, under certain conditions, of large electrochemical capacitance, including pseudocapacitance, is well established. Recent technological advances in capacitors have included aluminum electrolytic capacitors, tantalum capacitors, ceramic capacitors and supercapacitors.
The supercapacitor is an electrochemical cell or combination of cells consisting of two electrodes, an electrolyte and a container. The electrodes are composed of one or more oxides of ruthenium, tantalum, rhodium, iridium, cobalt, nickel, molybdenum, tungsten or vanadium deposited on a metal foil. The electrolyte may be acidic, basic or neutral, such as sulfuric acid, potassium hydroxide or sodium sulfate. The supercapacitor is made by laminating electrodes onto a separator. Supercapacitors typically employ stacks of laminated electrodes consisting of a separator between the electrodes. Ion permeable membranes have been used as separators, the particular configuration depending upon the application of the battery. Current-collector grids or meshes are also employed in the electrode assembly, if desired.
A prior art electrode as taught by Craig in Canadian Patent 1,196,683 is made by dipping a sheet of a conductive metal such as titanium into a solution of the metal oxide in order to deposit the metal oxide onto the surface of the metal sheet. The coated metal sheet is then dried, and the dipping and drying process is repeated to build another thin oxide layer. This process is continued until the oxide layer is of a sufficient thickness to function as an electrode. Fabricating a supercapacitor electrode by depositing oxide layers onto a metal substrate as described in the prior art is costly and very lengthy, requiring repeated dipping of the electrode in order to build up an coating of sufficient thickness.
Clearly, the present method of forming an active electrode for supercapacitors is slow and laborious, requiring much time, and is not a method that can be relied upon to achieve high quality, due to the need for repeated dipping of the electrode. A need exists for an improved electrode composition that is easier and faster to fabricate.