Carbon Nanotube Foams and Nanoscale Manganese Oxide Make Improved Supercapacitor Electrodes Say Navy Scientists

The United States of America Navy (Washington, DC) garnered U.S. Patent 7,724,500 for a composite made of nanoscale manganese oxide (MnO2) on ultraporous carbon nanoarchitecture which can be used to make improved supercapacitor electrodes.

Inventors Jeffrey W. Long, Anne E. Fischer and Debra R. Rolison developed a composite made of a porous carbon structure with a surface and pores and a coating of MnO2 on the carbon surface, in which the coating does not completely fill or obstruct a majority of the pores. The coating is formed by self-limiting electroless deposition.

Electrochemical capacitors (also denoted as supercapacitors or ultracapacitors) are a class of energy-storage materials that offer significant promise in bridging the performance gap between the high energy density of batteries and the high power density derived from dielectric capacitors. Energy storage in an electrochemical capacitor is accomplished by two principal mechanisms: double-layer capacitance and pseudocapacitance. 

Nanostructured MnO2-carbon nanoarchitecture hybrids can be designed as electrode structures for high-energy-density electrochemical capacitors that retain high power density. Homogeneous, ultrathin coatings of nanoscale MnO2 can be incorporated within porous, high-surface-area carbon substrates (such as carbon nanofoams) via electroless deposition from aqueous permanganate under controlled pH conditions.

The resulting hybrid structures exhibit enhanced gravimetric, volumetric, and area-normalized capacitance when electrochemically cycled in aqueous electrolytes. This design can be extended to other mesoporous and macroporous carbon forms possessing a continuous pore network.

The performance limitations of MnO2 for electrochemical capacitors can be addressed with a hybrid electrode design, by incorporating discrete nanoscale coatings or deposits of MnO2 onto porous, high-surface-area carbon structures (see FIG. 1).

FIG. 1 shows a schematic of a hybrid electrode structure comprising a highly porous carbon nanostructure coated with nanoscopic MnO2 deposits. 

  In such a configuration, long-range electronic conduction is facilitated through the carbon backbone and solid-state transport distances for ions through the MnO2 phase can be minimized by maintaining a nanoscopic carbon .parallel.MnO2.parallel. electrolyte interface throughout the macroscopic porous electrode. Such a design can be realized using various types of porous carbon substrates including but not limited to aerogels/nanofoams, templated mesoporous carbon, and nanotube/nanofiber assemblies. 

FIG. 2 shows a schematic of electrodeposition on an ultraporous electrode structure in (i) a poorly controlled manner in which the pores are ultimately blocked by the growing film and (ii) a controlled, self-limiting deposition. 

The synthesis and electrochemical characterization of MnO2-carbon composites has been reported and primarily focused on incorporating nanoscale MnO2 deposits onto carbon nanotubes using a variety of approaches including simple physical mixing of the components, chemical deposition using such precursors as KMnO4, and electrochemical deposition.

 In these cases, the incorporation of MnO2 improves the capacitance of the electrode structures that contain the MnO2-modified carbon nanotubes; however, the overall specific capacitance for the composite structures is typically limited to <200 F g^-1, even for electrodes with high weight loadings of MnO2.   

FIG. 3 shows scanning electron micrographs of (a and b) 4-h acid-deposited MnO2-carbon, (c and d) 4-h neutral-deposited MnO2-carbon, and (e and f) bare carbon nanofoam.