Treated Carbon Nanofibers for Storing Energy in Aqueous KOH

NASA Tech Briefs, Nov 2004

Treatment can increase specific capacitance by as much as 400 percent.

A surface treatment has been found to enhance the performances of carbon nanofibers as electrode materials for electrochemical capacitors in which aqueous solutions of potassium hydroxide are used as the electrolytes. In the treatment, sulfonic acid groups are attached to edge plane sites on carbon atoms.

The treatment is applicable to a variety of carbon nanofibers, including fibrils and both single- and multiple-wall nanotubes. The reason for choosing nanofibers over powders and other forms of carbon is that nanofibers offer greater power features.

In previous research, it was found that the surface treatment of carbon nanofibers increased energy-storage densities in the presence of acid electrolytes. Now, it has been found that the same treatment increases energy-storage densities of carbon nanofibers in the presence of alkaline electrolytes when the carbon is paired with a NiOOH electrode. This beneficial effect varies depending on the variety of carbon substrate to which it is applied.

It has been conjectured that the sulfonic acid groups, which exist in a deprotonated state in aqueous KOH solutions, undergo reversible electrochemical reactions that are responsible for the observed increases in energy-storage capacities. The increases can be considerable: For example, in one case, nanofibers exhibited a specific capacitance of 34 Farads per gram before treatment and 172 Farads per gram (an increase of about 400 percent) after treatment.

The most promising application of this development appears to lie in hybrid capacitors, which are devices designed primarily for storing energy. These devices are designed to be capable of (1) discharge at rates greater than those of batteries and (2) storing energy at densities approaching those of batteries. A hybrid capacitor includes one electrode like that of a battery and one electrode like that of an electrochemical capacitor. For example, a hybrid capacitor could contain a potassium hydroxide solution as the electrolyte, a carbon capacitor electrode, and a nickel hydroxide battery electrode. By making the capacitor electrode of treated carbon nanofibers instead of another carbon material, one could obtain greater energy-storage capacity.

This work was done by David W. Firsich of Inorganic Specialists, Inc. for Johnson Space Center.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Inorganic Specialists, Inc.

720 Mound Ave.

Miamisburg, OH 45343-0181

Refer to MSC-23321, volume and number of this NASA Tech Briefs issue, and the page number.

Lyndon B. Johnson Space Center, Houston, Texas