Electrochemical capacitors are a class of high-rate energy storage/discharge devices which use electrolytes and electrodes of various kinds in a system similar to that of conventional batteries. Electrochemical capacitors, like batteries, are essentially energy storage devices. However, unlike batteries, they rely on charge accumulation at the electrode/electrolyte interface to store energy. Charge storage in electrochemical capacitors therefore is a surface phenomenon. Conversely, charge storage in batteries is a bulk phenomenon occurring within the bulk of the electrode material.
Electrochemical capacitors can generally be divided into two subcategories: Double layer capacitors in which the interfacial capacitance at the electrode/electrolyte interface can be modeled as two parallel sheets of charge; and pseudocapacitor devices in which charge transfer between the electrolyte and the electrode occurs over a wide potential range. These charge transfers are the result of primary, secondary, and tertiary oxidation/reduction reactions between the electrode and the electrolyte. These types of electrochemical capacitors are being developed for high-pulse power applications.
Most of the known pseudocapacitor active materials are based on noble metal elements such as ruthenium and iridium. These materials are generally quite expensive. Material expense thus poses a significant hurdle to the wide-spread commercialization of this technology. Other less expensive materials have been tried, but have been less than successful. For example, workers in the field have attempted to fabricated devices using pressed powder cobalt and cobalt oxide electrodes. However, these types of electrodes have failed for numerous reasons including, for example, poor life cycle performance, and inability to achieve desired electrochemical performance characteristics.
Accordingly, there exists a need for pseudocapacitive electrode materials which deliver good performance in terms of energy storage, power density, and cycle life. Moreover, such materials should be abundant in nature, inexpensive in cost, readily processable into devices, and relatively benign environmentally.