Graphene-based electrode for a supercapacitor

[0001] The invention described herein was made in the performance of work under a NASA contract and by an employee of the United States Government and is subject to the provisions of Public Law 96-517 (35 U.S.C. §202) and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore. In accordance with 35 U.S.C. §202, the contractor elected not to retain title.

[0002] This invention relates to nanofabrication of an electrode suitable for use in a supercapacitor.

[0003] A supercapacitor requires use of a substance that has a relatively high power density, which can be achieved with some materials, and simultaneously a relatively high energy density. Achievement of both of these conditions with a single material has not been possible in the prior art.

[0004] The invention meets these needs by providing a procedure for nanofabrication of an electrode mechanism, for use in a supercapacitor (SC) that has relatively high power density (100-200 KW/Kgm or higher) and simultaneously has relatively energy density (20 KW-hr/KGM or higher). The procedure includes the steps of: (1) providing a porous, electrically conductive substrate that has first and second, spaced apart surfaces; (2) depositing one or more selected metal oxides, including at least one of MnO₂ and Co₃O₄, on the first substrate surface; and (3) depositing an assembly of chemically reduced graphene oxide (rGO) on the second substrate surface, to thereby provide an electrical double layer associated with the substrate.

[0008] FIG. 1 is a flow chart of a procedure for fabricating an electrode mechanism according to an embodiment of the invention. In step 11, a porous, electrically conductive substrate is provided that has spaced apart, first and second, surfaces. The substrate can be a metal, such as stainless steel or thin metal foil, a doped semiconductor, such as Si doped at 10¹⁹ cm⁻³ or higher, carbon paper, such as Toray paper or Bucky paper, an electrically conducting polymer, or a cellulose substance (optionally including photocopy paper) impregnated with a combination of carbon nanotube (CNT) ink and sodium dodecylbenzesesulfonate, the carbon paper having a thickness in a range 500-2000 nm, or another conductive substance.

[0009] In step 12, one or more selected metal oxides, including at least one of MnO₂ and Co₃O₄, is deposited on the first substrate surface. The metal oxide has an associated specific capacity of around 300 Farads/gm.

[0010] In a third step 13, an assembly of flakes or chips of reduced graphene oxide (rGO) is deposited on the second substrate surface, with individual flake thicknesses in a range 1-15 μm, depending upon the electrolyte. Deposition of the rGO can, for example, be achieved by deposit of graphene oxide (GO) on the substrate second surface, followed by electrophoretic deposition (EPD), such as cathodic deposition using an electrolyte such as a room temperature ionic liquid (RTIL).

[0011] An RTIL is a room temperature, liquid, solvent-free electrolyte. Examples of RTILs are 1-butyl-1-methylpyrrolldinium bis(trifluoromethylsulfonyl)imide, Methyltrioctylammonium bis(trifluoromethylsulfonyl)imide, ammonium ions, and methyl imidazole.

[0012] The reducing agent may be hydrazine, sodium borohydride, or a similar substance. The rGO helps provide an electrical double layer associated with the substrate.

[0013] FIG. 2 illustrates a three-layer, substantially planar embodiment 20t of the invention provided by the procedure discussed in FIG. 1, comprising a metallic oxide layer 21, a substrate 22 and an rGO layer 23, with associated electrical terminals 24 and 25. The electrolyte may be an RTIL or a combination of RTIL and a solid polymer (SP) that manifests at least some piezoelectric behavior.

[0014] This fabrication procedure also provides a three-layer (more generally, a multi-layer) cylindrical embodiment 30 in FIG. 3 for storage or transfer of an electrical charge with relatively high power density and energy density. The energy density and power density can be increased substantially by rolling this three-layer system into a 3N-layercylindrical system 30, as indicated in an end view in FIG. 3, with N>>1. In FIG. 3, the geometric structure is illustrated with N approximately equal to 2. The first layer 31 contains metallic oxides, the second layer 32 is a thicker substrate layer, and the third layer 33 includes rGO and a selected electrolyte. The substrate may have a thickness as small as 1-5 μm and as large as 150 μm depending upon the substrate material and the maximum permissible curvature of the geometric structure illustrated in FIG. 3. For a given substrate material, the smaller the substrate thickness, the greater the geometric structure that can be supported without substrate cracking or crazing. The layers illustrated in FIG. 3 are preferably rolled into a structure resembling a cylinder.

[0015] In an embodiment, illustrated in FIG. 2 or FIG. 3, the (solid or gel-like) electrolyte comprises a combination of an RTIL and a solid polymer SP, such as polyvinylidene fluoride or difluoride, preferably β-phase.

[0016] Ideally, the system should operate at temperatures in a range −40° C.<T<100° C., should operate at voltages in a range −3 to +7 Volts, and should rely upon non-toxic materials for fabrication. The system should be mechanically flexible and have a relatively small form factor.

[0017] It will be appreciated to those skilled in the art that the preceding examples and embodiment are exemplary and not limiting to the scope of the present invention. It is intended that all permutations, enhancements, equivalents, combinations, and improvements thereto that are apparent to those skilled in the art upon a reading of the specification and a study of the drawings are included within the true spirit and scope of the present invention.