A class of developmental asymmetric electrochemical capacitors is based on (1) nonpolarizing positive electrode made of nanotructured γ-MnO2, (2) aqueous electrolyte , and (3) polarizing negative electrodes made of activated carbon. Like other state-of-the-art electrochemical capacitors, these can store several hundred times as much energy per unit weight or volume as can traditional electrolytic capacitors. In comparison with other, symmetric state-of-the-art electrochemical capacitors, these asymmetric electrochemical capacitors are expected to be capable of higher energy densities. In addition, by virtue of the high surface areas of the nanostructured γ-MnO2 electrodes, these capacitors are expected to be capable of high power densities. These capacitors could be particularly useful as load-leveling devices in conjunction with batteries, or as replacements for batteries, in portable electronic devices that rely on short-term intermittent or pulsed high power.

A large Voltage Window is expected to arise through polarization at the negative electrode as a capacitor of the present type is charged. The potential window is limited by the onset of the evolution of hydrogen and oxygen at the electrodes. The potential of the metal oxide electrode remains essentially unchanged during both charge and discharge.

The energy densities of asymmetric electrochemical capacitors can be made to exceed those of symmetric electro- chemical capacitors for several reasons:

  • The two electrodes of an electrochemical capacitor, having capacitances C1 and C2, are electrically connected in series. One of the electrodes is made of a material with a specific charge capacity and thus a capacitance greater than that of the other electrode. The capacitance of the series combination [given by CT = C1 C2/(C1+C2)] is almost equal to the smallest of C1 or C2. In contrast, the capacitance of a symmetric electrochemical capacitor is only about half that of either electrode.
  • Because the specific charge capacity of one electrode material greatly exceeds that of the other electrode material, the mass and volume of one electrode can be made smaller than that of the other electrode. The decrease in volume translates to an increase in the energy density of the overall capacitor.
  • Unlike asymmetric electrochemical capacitor, an asymmetric electrochemical capacitor with an aqueous electrolyte can reliably operate at voltages above 1.22 V without evolution of gas, whereas commercially available symmetric aqueous electrochemical capacitors are limited to about 0.9 V. Inasmuch as energy density is propotional to voltage squared, the energy density of an asymmetric capacitor can be correspondingly higher.

In an asymmetric electrochemical capacitor of the present type, the positive electrode is expected to act as pseudocapacitor: it is expected to store charge through the faradaic reaction

γ-MnO2 +H20 +e- ↔ α-MnOOH + OH-

with a reversible potential of l.23 V versus a standard hydrogen electrode. The negative electrode is expected to store charge at the electrode/electrolyte interface (double-layer capacitance).

To make it possible to obtain high energy density, the positive electrode must exhibit a low degree of polarization; that is, the potential of the positive electrode must change little from its reversible potential during the passage of current. In addition, nonfaradaic processes must be minimized and faradaic procesess must occur at high reaction rate across the surface of the positive electrode. On the other hand, the negative electrode should be highly polarizable and, under ideal condition, should develop a large window of potential change during charge and discharge. The figure qualitatively depicts the expected changes in potential at each electrode during charge and discharge.

This work was done by David E. Reisner and T. Danny Xiao of us Nanocorp, Inc.; John R Miller of]ME, Inc.; and Stephen M. Lipka of Florida Atlantic University for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasat.ech.com under theElectronic Components and Systems category.

Inquiries concerning rights for the commercial use o f this invention should be addressed to

NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland, Ohio 44135.

Refer to LEW-16930.

Electronics Tech Briefs Magazine

This article first appeared in the August, 2000 issue of Electronics Tech Briefs Magazine.

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