Abstract:
An method of producing an electronic device, including identifying a graphene sheet, functionalizing the graphene sheet to yield a functionalized sheet, attaching respective vanadium oxide molecules to respective functional groups to define an impregnated graphene sheet, removing organic solvents from the impregnated graphene sheet to define a composite sheet, and positioning the composite sheet onto a metallic substrate to yield a capacitor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of, and claims priority to, co-pending U.S. patent application Ser. No. 13/664,847, filed on Oct. 31, 2012. 
     
    
     TECHNICAL FIELD 
       [0002]    The novel technology relates generally to the field of electronic materials, and, more particularly, to a high capacitance material including alternating vanadium oxide monolayers or multilayers, each supported by a single graphene sheet substrate. 
       BACKGROUND 
       [0003]    Supercapacitors are useful for many applications because of their high power density, long cycle life and the potential applications on both military and commercial devices. For example, supercapacitors are important to the designs of portable laser systems and electric vehicles. Two mechanisms are associated with energy storage in a supercapacitor, namely electrical double layer charge storage and pseudo-capacitance charge storage. The capacitance of the former comes from the charge accumulation at the electrode/electrolyte interface, and therefore highly depends on the pore structure of the electrode, including such parameters as pore size and accessible surface area to the electrolyte molecules. The latter capacitance mechanism arises from to the fast reversible faradic transitions (electrosorption or surface redox reactions) of the electro-active species of the electrode, including surface functional groups, transition metal oxides and conducting polymers and this type of supercapacitor is also called electrochemical supercapacitors. The pseudo-capacitance from reversible faradic reactions of an electro-active material offers a higher power storage capacity than the electrical double layer capacitance mechanism. 
         [0004]    Transition metal oxides have typically been considered to have a great potential to increase the capacitance in the electrochemical supercapacitors. Amorphous hydrated RuO 2  has attracted particular interest as a supercapacitor electrode material with a capacitance over 700 F/g having been achieved, significantly higher than that has been observed with an electrical double layer capacitor. Unfortunately, hydrated RuO 2  is too rare and expensive to be commercially viable as a supercapacitor material. Supercapicitors utilizing nano-crystalline vanadium nitride materials have exhibited capacitance of 1340 F/g at a 2 mV/s scan rate, which is far more than that of the hydrated RuO 2  based supercapacitors. Such a high capacitance is believed to be caused by a series of reversible redox reactions on few atomic layers of vanadium oxide on the surface of the underlying nitride nanocrystals, which exhibit a metallic electronic conductivity (σ bulk =1.67×10 6 Ω −1  m −1 ). 
         [0005]    Thus, there remains a need to supercapacitor material having even higher capacitance and using more readily available materials. The present invention addresses this need. 
       SUMMARY 
       [0006]    The present novel technology relates to energy storage devices supporting vanadium oxide dielectric layers on graphene substrates. 
         [0007]    One object of the present novel technology is to provide an improved capacitor device. Related objects and advantages of the present novel technology will be apparent from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  graphically illustrates a graphene/vanadium oxide composite dielectric material according to a first embodiment of the present novel technology, having vanadium oxide molecular monolayers connected to both sides of a graphene sheet. 
           [0009]      FIG. 2  graphically illustrates the process of attaching vanadium oxide layers to functionalized graphene according to a second embodiment of the present novel technology. 
           [0010]      FIG. 3  is a photomicrograph of graphene as synthesized through thermal expansion according to the embodiment of  FIG. 2 . 
           [0011]      FIG. 4  schematically illustrates the functionalization of a carbon atom according to the embodiment of  FIG. 2 . 
           [0012]      FIG. 5  chemically illustrates the process of  FIG. 1 . 
           [0013]      FIG. 6  graphically illustrates a capacitor according to a third embodiment of the present novel technology. 
           [0014]      FIG. 7  is a partial perspective view of a plurality of composite sheets on a metallic substrate defining a thin layer. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    For the purposes of promoting an understanding of the principles of the novel technology and presenting its currently understood best mode of operation, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, with such alterations and further modifications in the illustrated device and such further applications of the principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates. 
         [0016]    As illustrated in  FIGS. 1-6 , the present novel technology relates to capacitors, specifically capacitor devices  10  with nano-structured vanadium oxide molecules present as thin, ultrathin, or mono-layers  15  and supported on electrically conductive, typically carbonaceous, support structures  20 . The carbonaceous support structure is typically one or more graphene sheets, although other morphologies of carbon, such as diamond, may be used. Such capacitors  10  may approach an extremely high theoretical capacitance of 4577 F/g and exhibit high electric conductivity and a low time constant. In contrast, the current state-of-art capacity of RuO 2  is only 700 F/g. The instant capacitors  10  represent a significant increase in supercapacitor energy storage for high power density applications, such as laser systems and electric vehicle (EV)/hybrid electric vehicle (HEV) systems. 
         [0017]    The thin layer or, typically, monolayer  15  of vanadium oxide molecules  17  supported on a graphene substrate  20  defines a V 2 O 5 /graphene composite  25 . The structure of the composite  25  allows respective vanadium oxide (V 2 O 5 ) molecules to avail themselves to electrolytes with high surface area accessibility for ions in the electrolytes, which in turn allows each V 2 O 5  molecule to participate in the redox reaction and facilitates the fast mass transport of ions. The high capacitance of the composite material  25  appears to arise from the 3-electron redox reactions of vanadium oxide (V 2 O 5 ) (V 5+ →V 4+ +1e − ; V 4+ →V 3+ +1e − ; and V 3+ →V 2+ +1e − ). The V 2 O 5  molecules in the monolayer  15  may directly electrically communicate with the carbon atoms in the graphene layer  20 . Consequently, the electron transfers in the V 2 O 5 /graphene composite  25  primarily involve the direct transfer of electrons from the carbon atoms to the V 2 O 5  molecules. Alternately, carbon spacers (such as amorphous carbon, diamond, graphite, partially graphitized carbon, and combinations thereof) or the like may be positioned between the graphene substrate layers  20  and/or the substrate layers  20  and the vanadium oxide layer  15 . The slow electron transfer between V 2 O 5  molecules (which causes the extremely low electronic conductivity, 8.7×10 −7  S cm −1 , and, consequently, limits the application of vanadium oxide in supercapacitors requiring low time constant) is thus minimized or eliminated. Accordingly, the electronic conductivity of V 2 O 5 /graphene composite  25  is greatly increased, resulting in a greatly reduced the time constant. In addition, the positioning of the V 2 O 5  monolayer  15  on graphene  20  provides a very high mass ratio of active material to supporting material, typically at least 3:1, and more typically 3.83:1 (V 2 O 5 :graphene=3.83:1), which is typically about fifteen times that of vanadium oxide/vanadium nitrides composites (V 2 O 5 /VN) (V 2 O 5 :VN=0.251). Vanadium oxide benefits from an electrically conducting support due to its low electronic conductivity, and the single carbon layer of graphene  20  is ideal, providing carbon support with minimized space constraints. 
         [0018]    The nano-structured vanadium oxide monolayer  15  is formed and supported on graphene  20 , and a thin film electrode  30  is typically fabricated thereupon to allow each V 2 O 5 /graphene composite sheet  25  to enjoy good electric communication or conduction. The synthesis of nano-structured vanadium oxide monolayer  15  supported on graphene  20  is typically achieved through the functionalization  40  of the graphene sheet  20  and the subsequent removal of benzene rings or the like from the functionalized graphene  20 , following the attachment of vanadium ions/vanadium oxide monolayer  15  on the graphene substrate  20 . 
         [0019]    Graphene, a single-atom-thick sheet of hexagonally arrayed sp2-bonded carbon atoms, is a two-dimensional macromolecule exhibiting extremely high surface area (2600 m 2 /g). The in-plane electronic conductivity (10 9 Ω −1  m −1 ) of graphene is much higher than that of the vanadium nitride. Single sheet graphene  20  is a very good candidate for support of the vanadium oxide monolayer  15 , as it has both good in-plane electrical conductivity as well as physical strength, as the in-plane carbon-carbon bonds are stronger than those in diamond. Graphene sheets  20  may be synthesized, such as by the thermal expansion method or the like, and hydroxyl groups (—OH)  43  may be chemically attached to the surface of graphene  20  through the diazonium reaction  45 . The attachment  49  of a vanadium oxide layer  15  onto the functionalized graphene  47  is typically carried out by a hydrothermal technique, such as has been used to produce vanadium oxide monolayer on alumina, silica, magnesia, and titania supports. Vanadium ions may be attached to the functionalized graphene-OH  47  by impregnation of the same with vanadyl triisobutoxide, and then typically purified such as by vacuum distillation (typically b.p. 414-415 K at 1.07 kPa). The use of an isobutyl alcohol derivative of vanadium offers the advantage of a monomeric nature, as compared to the methoxide. Alternately, the vanadium oxide layer  15  may be deposited  49  by other convenient means, such as atomic layer deposition or the like. The functionalized graphene  47  is then typically impregnated with a solution of vanadyl triisobutoxide in anhydrous nhexane. After a predetermined period of time (typically about 24 hours) the solution is removed and the mixture is washed, typically several times, with solvent. The impregnated graphenes  50  are subsequently calcined  55  for a predetermined period of time (typically several hours, more typically about three hours) at elevated temperatures (typically, about 300° C.) in a stream of dry air to form the vanadium oxide monolayer  15  on graphene  20 . In this calcination step  55 , organic solvents  60  such as benzene and the like are removed and the vanadium oxide monolayer  15  is directly formed on the graphene substrate surface  20 . The composite sheets  25  may then be then positioned  65  onto a (typically metallic) substrate  80 . The reaction scheme is shown in  FIGS. 2, 4 and 5 . 
         [0020]    To make the high performance capacitor  10  characterized by extremely high capacitance, each vanadium oxide monolayer  15 /graphene sheet  20  in the electrode layer  30  typically participates in the charging/discharging process. This participation arises because the electronic conduction between each vanadium oxide monolayer  15 /graphene sheet  20  is maintained. Such conduction may benefit from the provision of an appropriately conductive electrode layer  30  structure. The structure of the desired electrode layer  30  typically has the graphene edges of vanadium oxide monolayer/graphene sheet composite  25  physically in contact with each other, or contacting through conductive metal substrates. For example, the synthesized vanadium oxide monolayer/graphene composites  25  may be dispersed in organic solvents along with a binder to form a uniform dispersion. This dispersion may then be coated  65  onto a nickel substrate  80  to form a thin layer  70  of composite sheets  25 , each sheet  25  in metallic contact with another sheet  25  and/or the nickel substrate  80 . 
         [0021]    While the novel technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the novel technology are desired to be protected.