Ultracapacitors:

Ultracapacitors are prime candidates for use as the load-leveling power source in electric and hybrid vehicles, premium power systems and battery-powered electronics because they can be charged and discharged far faster than batteries, and can be cycled many thousands of times without degradation. Ultracapacitors store energy in a polarized liquid layer only a few angstroms thick at the interface between an ionically conducting electrolyte solution and an electronically conducting electrode. The figure at right illustrates double layer capacitance in a porous carbon-based ultracapacitor. The separation of charge in the ionic species at the interface (called a double layer) produces a standing electric field. The larger the electrode surface area the more charge can be stored.

Electrodes for double-layer capacitors are generally made from porous carbons because of their low cost and high surface area (which results in high energy storage densities). Since the capacitance of the material increases linearly with the specific surface area, a carbon material with a capacitance of 20 mF/cm² and a surface area of 1000 m²/g should have a capacitance of 200 F/g if all of the surface were electrochemically accessible. However, since high surface area porous carbons made from woods and coal typically have a high fraction of micropores which are so small that they cannot be easily reached by the liquid, only a fraction of the surface of the carbon is effectively utilized. Generally, the measured capacitance values with standard activated carbons are only about 20% of what is theoretically possible.

As a result, there has been considerable work to make better performing carbons from synthetic precursors. In fact, some carbons with excellent performance have been made, but they use expensive precursors (carbon cloths), expensive processing steps (supercritical extraction) or both. This leaves ultracapacitor manufacturers with a choice between expensive, high performance carbons or inexpensive, low performance activated carbons; this dilemma has stifled the growth of the ultracapacitor industry.

To address this problem we have developed carbons that performs as well as or better than the best carbons available and are far less expensive because they are made from very low cost precursors using conventional processing methods. As shown in the table below [carbon electrode performance data in organic electrolyte (1 M NEt⁴BF4 in 1:1 DME/PC)], these carbons show very good gravimetric and volumetric capacitance in organic electrolytes.

This work has been supported by the National Science Foundation.

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