Abstract:
A method of preparing a fiber including electro-spinning onto a substrate polymer solutions from a plurality of jets to form a network of filaments, wherein at least one jet sprays onto the substrate a first chemical mixture including a carbon fiber precursor compound, and at least one other jet sprays onto the substrate a second chemical mixture comprising a sacrificial polymer and a precursor compound of a functional material; and processing the filaments on the substrate, thereby forming an arrangement of carbon fibers having the functional material deposited thereon.

Description:
BACKGROUND 
     Carbon materials such as graphite, glassy carbon, carbon black, graphene, and carbon nanotubes (CNT) have been widely used as electrodes of electrochemical devices due to their conductivity, abundance and electrochemical stability. While carbon itself can be used for electrode applications, it can be hybridized with functional materials such as catalysts (e.g., metals, semiconductors, etc.) to improve performance. Typically the hybridization of functional materials with carbon has been done using common material deposition methods such as, for example, vacuum deposition, solution deposition, spraying, electroplating, and high energy irradiation. 
     SUMMARY 
     Drawbacks of common material deposition methods described above include, for example, the need for a high vacuum environment, multiple process steps, and the large amounts of energy to run the material deposition process. Sometimes the material deposition process employed limits the candidate materials for hybridization. 
     In general, the present disclosure describes a simple multiple jet electrospinning method of fabricating nanostructured carbon materials having a surface decorated and/or at least partially coated with a functional material. 
     In one embodiment, the invention is directed to a method of preparing a fiber, including electro-spinning onto a substrate polymer solutions from a plurality of jets to form a network of filaments, wherein at least one jet sprays onto the substrate a first chemical mixture including a carbon fiber precursor compound, and at least one other jet sprays onto the substrate a second chemical mixture including a sacrificial polymer and a precursor compound of a functional material. The filaments are processed on the substrate, thereby forming an arrangement of carbon fibers having the functional material deposited thereon. 
     In another embodiment, the present disclosure is directed to a method including: 
     (a) spraying from at least one first orifice a first solution onto a surface of a substrate, wherein the first solution includes a carbon fiber precursor compound; 
     (b) spraying from at least one second orifice a second solution onto a surface of the substrate, wherein the second solution includes a sacrificial polymer and a precursor compound of a functional material; 
     (c) forming a first filament from the first jet, and forming a second filament from the second jet; 
     (d) rotating the substrate and collecting the first filaments and the second filaments to form a mat including a network of the first and the second filaments; 
     (e) heating the mat to a first temperature above a glass transition temperature of the sacrificial polymer; and 
     (f) heating the mat to a second temperature above the first temperature and sufficient to:
         (i) form the functional material from the precursor compound of the functional material, and   (ii) carbonize the carbon fiber precursor compound to form an arrangement of carbon fibers; wherein the functional materials contact a surface of the carbon fibers.       

     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of an embodiment of a multiple jet electrospinning apparatus that can be used to make carbon hybrid materials according to this disclosure. 
         FIG. 2A  is a scanning electron microscope (SEM) image of a carbon web structure formed by single jet electrospinning of polyacrylonitrile (PAN). 
         FIG. 2B  is a SEM image of a carbon web structure formed using a two jet electrospinning process with PAN and polyethylene oxide (PEO) as described in Example 1. 
         FIG. 3A  is a low magnification SEM image of a web of surface-decorated carbon fibers prepared by two jet electrospinning of PAN and PEO+manganese acetoactonate (MnAc), and subsequently heat treated for carbonization as described in Example 1. 
         FIG. 3B  is a high magnification SEM image of the carbon fibers of  FIG. 3A , which shows nanoparticles of manganese oxide formed on the surface of the carbon fibers. 
         FIG. 4  is a cross-sectional transmission electron microscope (TEM) image of the carbon nanofibers of  FIG. 3A , which shows surface decoration of manganese oxide on the carbon fiber surface. 
         FIG. 5  is energy dispersive X-ray (EDX) data of the carbon fibers of  FIG. 3A . 
         FIG. 6  is a SEM image of carbon fiber soaked into a metal oxide precursor solution and thermally treated. 
         FIG. 7  is a SEM image of carbon fibers that have been surface decorated with graphite particles as described in Example 2. 
         FIG. 8  is a schematic, exploded view of a SWAGELOK type lithium-oxygen battery utilized in Example 3. 
         FIG. 9  is a discharge-charge plot of the lithium-oxygen battery shown in  FIG. 8  and used in Example 3. 
     
    
    
     Like symbols in the figures indicate like elements. 
     DETAILED DESCRIPTION 
     This disclosure is directed to a method for making nanostructured carbon hybrid materials by multiple-jet electrospinning. In this method at least a first jet in a multiple-jet array sprays onto a surface of a disk-like substrate a first solution including a carbon fiber precursor compound. At least a second jet in the multiple-jet array sprays onto the surface of the substrate a second solution including a functional material precursor compound and a sacrificial polymer. The jets and/or the substrate are rotated with respect to one another, and outputs of the first and the second jets form on the substrate a web-like composite mat-like network of filaments. 
     This filamentous mat is then thermally treated at a temperature sufficient to transform the carbon fiber precursor compound into carbon fibers. The thermal treatment also converts the functional material precursor compound into a functional material. The sacrificial polymer flows at a temperature above its glass transition temperature (T g ) to deliver the functional material on the surface of the carbon fibers, and then the sacrificial polymer subsequently substantially decomposes. A web of carbon fibers is formed having a surface at least partially coated with the functional material. 
       FIG. 1  illustrates an embodiment of a multiple jet electrospinning apparatus  100  that can be used to make nanostructured carbon hybrid materials according to the present disclosure. The apparatus  100  includes an array  102  of jets each configured with a suitably-sized orifice to emit a fine stream of a liquid when a suitable voltage is applied. In the embodiment shown in  FIG. 1 , the array  102  includes at least two jets  104  and  106 , but it will be understood that the array  102  could include any suitable number of jets necessary for a particular application. In the embodiment of  FIG. 1 , the first jet  104  is configured to spray a fine stream of a first liquid including a carbon fiber precursor compound  110 . The second jet  106  is configured to spray a fine stream  120  of a second liquid including a sacrificial polymer compound and at least one functional material precursor compound. 
     After emerging from the jets  104  and  106 , the streams  110  and  120  form thin, elongate filaments  122 , which are collected on a surface  130  of a spinning, disk-like collector  132  that rotates about a shaft  134 . The collected filaments  122  form a mat-like composite network of filaments  140 . 
     In some embodiments, the first liquid  110  sprayed from the first jet  104  is a first chemical mixture, preferably a solution, including a carbon fiber precursor compound. As used herein carbon fiber precursor compound refers to a polymeric material that may be thermally treated and carbonized to form a carbon fiber. Suitable examples of carbon fiber precursor compounds include, but are not limited to, polymeric materials such as polyacrylonitrile (PAN), cellulosic precursors, pitch precursors, non-heterocyclic aromatic polymer precursors (such as phenolic polymers, phenol formamide resincs, polyacenaphtahlene, polyacrylether, certain polyamides, and polyphenylene), aromatic heterocyclic polymer precursors (such as polyimides, polybenzimidazole, polybenzimidazonium salt, polytriadizoles), polyvinyl chloride, polymethyl vinyl ketone, polyvinyl alcohol and poly vinyl acetate. 
     The carbon fiber precursor compound may optionally be mixed with or dissolved in a suitable solvent such as, for example, N-N′-dimethylformamide (DMF), dimethylacetate (DMAc), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), or trifluoro acetic acid. 
     The amount of the carbon fiber precursor compound in the first solution may vary widely, but is typically about 1 wt % to about 20 wt %, or about 5 wt % to about 10 wt %, or about 8 wt %. 
     The second chemical mixture  120 , preferably a solution, sprayed from the second jet  106  includes a functional material precursor compound and a sacrificial polymer compound. The term sacrificial polymer as used herein refers to polymer that at least partially decomposes when the mat-like network  140  is heated, and preferably substantially decomposes if the network  140  is heated to a sufficiently high temperature. The decomposing sacrificial polymer compound initially flows at temperatures above its glass transition temperature (Tg) to distribute the dispersed functional material precursor compound or the functional material itself onto the surface of the filaments. As the filaments are further heated, the carbon fiber precursor compound carbonizes to form an arrangement of carbon fibers. In some embodiments the functional material remains randomly distributed on the surface of the carbon fibers after the mat-like-network of filaments  140  is carbonized, while in other embodiments the functional material partially or completely coats the carbon fibers. 
     Suitable sacrificial polymers include, but are not limited to, poly(ethylene oxide) (PEO), poly(methyl methacrylate) (PMMA), polystyrene (PS), polypropylene oxide (PPO), polyacrylates, polyvinylfluoride, poly(butyl methacrylate), polycarprolactam, polylactides, polyacrylic acid, polyvinyl pyridine, polyvinyl benzyl alcohol, polyesters, polyamides, and polycarbonates. These polymeric materials can be homopolymers or copolymers such as random copolymers, block copolymers or graft copolymers. 
     The sacrificial polymer may optionally be combined with a suitable solvent as necessary for a particular application, such as, for example, DMF, DMSO, DMAc, toluene, anisole, chloroform, cyclohexane, THF, diethylamine, diethyl ether, ethyl acetate, formamide, isopropyl alcohol, tirfluoro acetic acid, and/or pyridine. 
     The functional material precursor compound may be selected from any material that can be thermally decomposed at or below the carbonizing temperature to form a functional material. Examples of functional materials include, but are not limited to, metal oxides, semiconductors, metals, carbons and the like. For example, functional material precursor compounds that can be used to deposit a metal (such as Pt, Pd, Ag, Au, Ru, Ni, Co, Mn, Cr, Sn, W, Ta, Ti, Mo, Rh, Re, Ir, Hf, Zr, Fe and combinations thereof), or a metal oxide (for example, MnOx, MoOx, TiOx, PbOx, WOx, RuOx, ReOx, NiOx, FeOx, TcOx, RhOx, IrOx, CrOx, CeOx, ZrOx, SnOx and combinations thereof) on the surface of a carbon fiber include, but are not limited to, metal acetates, metal hydroxide, metal acetylacetonate, metal nitrates, metal sulfates, metal carbonates, metal chloride and the like. 
     After the mat-like construction  140  of filaments  122  is fully formed, the construction  140  is optionally removed from the surface  130  of the disk-like collector  132  and thermally treated. The thermal treatment initially heats the filaments  122  to a temperature sufficient to cause the sacrificial polymer to flow and randomly distribute and/or at least partially coat the functional material precursor compound onto the surface of the filamentous strands of the carbon fiber precursor compound. With further heating to a temperature sufficient to carbonize the filaments of the carbon fiber precursor compound and transform the functional material precursor compounds into functional materials, the sacrificial polymer substantially fully decomposes, leaving a web-like mat of carbon fibers having their surfaces randomly decorated and/or coated with the functional materials. 
     The thermal treatment protocol will of course vary depending on the materials selected for use in the process, but typically includes a first heating step in which the mat-like construction of filaments  140  is heated from room temperature to a temperature above the glass transition temperature (Tg) of the sacrificial polymer compound, typically to about 200° C. to about 500° C., and in some embodiments to about 250° C. The filaments  122  in the mat-like construction  140  are then heated in a second heating step to a temperature sufficient to cause: (1) the functional material precursor compounds to form functional materials, and (2) the carbon fiber precursor compounds to form carbon fibers. This second heating step, which is typically conducted at about 500° C. to about 1000° C., in some embodiments at about 850° C., also decomposes the sacrificial polymer and randomly deposits and/or coats the functional materials on the surface of the carbon fibers as said fibers are formed. 
     The hybrid carbon fibers with functional materials randomly distributed on their surfaces may then be cooled to room temperature and further processed for a particular end use application. The amount and morphology of the functional materials may be controlled by controlling the mixing composition of the functional material precursor and the sacrificial polymer. The functional material decorating the surface of the carbon fiber is in intimate contact with the surface of the carbon fiber. For example, the interface between the functional material and the carbon fiber may in some embodiments be sufficiently good to transport carriers such as electrons, which can make the decorated carbon fibers useful as catalysts. In some embodiments the functional materials may fully or partially coat the carbon fibers. 
     The first and the second heating steps may be conducted in air or in an inert atmosphere as necessary. Optionally, after the first heating step and/or the second heating step, the mat-like filament construction  140  may be maintained at a selected temperature for an extended time. In some embodiments, suitable extended heating times of about 0.5 hours to 2 hours may be used between each of the first and the second heating steps, preferably about 1 hour. 
     The process of this disclosure will now be more fully illustrated by the following non-limiting examples. 
     EXAMPLES 
     Example 1 
     PAN, PEO and manganese acetylacetonate (MnAc) were purchased from Sigma-Aldrich Corp., St. Louis, Mo., and used as received. The carbon fiber precursor PAN was dissolved in N-N′-dimethylformide (DMF) to make an 8 wt % solution. The sacrificial polymer PEO and the functional material precursor MnAc were dissolved in DMF to make a mixture. These polymer solutions (i.e., PAN and PEO+MnAc) were each loaded into 12 ml disposable syringes. A blunt tip needle was used as an orifice. High voltage (e.g., 15 kV) was applied to the needles to form polymer jets. 
     Electrospun filaments were collected on a rotating disc type collector that was electrically grounded. After electro-spinning, the composite membrane (or web-like mat) on the collector was dried under ambient conditions. The web-like mat was carbonized using a cylinder type furnace in a controlled environment. 
     The thermal profile used for carbonization was: RT to 250° C. under air, iso at 250° C. for 1 hr under air, heating to 850° C. under nitrogen, iso at 850° C. for 1 hr under nitrogen, and then cooling down to RT. 
       FIG. 2A  shows carbon fibers prepared from a pure PAN solution, and  FIG. 2B  shows carbon fibers produced from a two jet process (PAN and PEO each having a dedicated jet).  FIG. 2B  shows that the PEO jet affects the connectivity of carbon fibers after carbonization. While not wishing to be bound by any theory, this is likely due to the flow of PEO during thermal treatment, which moves the PAN fibers and binds them together. 
       FIGS. 3A and 3B  are SEM micrographs of carbon nanofibers prepared by the multi-jet process of Example 1 (PAN and PEO+MnAc, each having a dedicated jet). The sample was heat-treated under the conditions mentioned above. It is clear that the surface of the carbon fibers contains numerous nanoparticles which were delivered from the PEO+MnAc mixture during the thermal treatment. 
       FIG. 4  shows a cross-sectional TEM micrograph of the carbon fiber hybrid. It clearly shows the manganese oxide nanoparticles are located selectively at the surface of the carbon fibers, which indicates that the PEO fibers deliver the metal oxide precursor on the surface of PAN and eventually onto the carbon fibers. 
       FIG. 5  shows EDX data of the carbon hybrid of  FIG. 3A . These data show a strong peak of Mn at 5.9 keV, which corresponds to 5 at % of Mn in the sample. 
     As a comparison, a carbon fiber web was soaked with metal oxide solution and subsequently thermally treated. In this case, as shown in  FIG. 6 , the metal oxide precursors are aggregated macroscopically and form large domains between the fibers. 
     Example 2 
     Samples were prepared using the same method described in Example 1. Instead of using manganese acetylacetonate as a functional material precursor, graphite nanoflakes (from Asbury Carbon, Asbury, N.J.) were dispersed into a PEO solution in DMF. The sample was thermally treated to convert the carbon fiber precursor compound PAN into carbon and decompose the sacrificial polymer PEO.  FIG. 7  is a SEM micrograph of the carbon fiber hybrid, which clearly shows the surface of carbon fibers are decorated with graphite nanoflakes, resulting in a very bumpy surface morphology on the surface of the carbon fiber. 
     Example 3 
     This example describes using a nanostructured carbon hybrid as an electrode of a lithium-oxygen battery, which is a high energy density metal-oxygen battery. A SWAGELOK type cell structure as shown in a schematic exploded view in  FIG. 8  was used to prepare a battery cell. 
     As shown in  FIG. 8 , the battery cell  200  included a tube  202  through which oxygen gas flows into a carbon cathode  204 , a separator and electrolyte  206 , a Li anode  208 , a stainless steel separator disk  210 , a union assembly  212 , a compression spring  214  and a retaining nut and rod  216 . 
     The battery was discharged to 2.0V with a current density of 200 μA/cm 2  and recharged to 4.7V with the same current density. 1 M lithium bis(trifluoromethane sulfonyl imide) (LiTFSI) in dimethoxy ethane (DME) was used as an electrolyte. 
       FIG. 9  shows a potential-capacity plot of the lithium-air battery during a discharge and charge process. 
     Various embodiments of the invention have been described. These and other embodiments are within the scope of the following claims.