Patent Application
20080031802
The terms “nanotube”, “nanofiber” and “fibril” are used interchangeably to refer to single walled or multiwalled carbon nanotubes. Each refers to an elongated structure preferably having a cross section (e.g., angular fibers having edges) or a diameter (e.g., rounded) less than 1 micron (for multiwalled nanotubes) or less than 5 nm (for single walled nanotubes). The term “nanotube” also includes “buckytubes”, and fishbone fibrils.
“Multiwalled nanotubes” as used herein refers to carbon nanotubes which are substantially cylindrical, graphitic nanotubes of substantially constant diameter and comprise cylindrical graphitic sheets or layers whose c-axes are substantially perpendicular to the cylindrical axis, such as those described, e.g., in U.S. Pat. No. 5,171,560 to Tennent, et al.
“Single walled nanotubes” as used herein refers to carbon nanotubes which are substantially cylindrical, graphitic nanotubes of substantially constant diameter and comprise a single cylindrical graphitic sheet or layer whose c-axis is substantially perpendicular to their cylindrical axis, such as those described, e.g., in U.S. Pat. No. 6,221,330 to Moy, et al.
The term “functional group” refers to groups of atoms that give the compound or substance to which they are linked characteristic chemical and physical properties.
A “functionalized” surface refers to a carbon surface on which chemical groups are adsorbed or chemically attached.
“Graphenic” carbon is a form of carbon whose carbon atoms are each linked to three other carbon atoms in an essentially planar layer forming hexagonal fused rings. The layers are platelets only a few rings in diameter or they may be ribbons, many rings long but only a few rings wide.
“Graphitic” carbon consists of graphenic layers which are essentially parallel to one another and no more than 3.6 angstroms apart.
The term “aggregate” refers to a dense, microscopic particulate structure comprising entangled carbon nanotubes.
The term “micropore” refers to a pore which has a diameter of less than 2 nanometers.
The term “mesopore” refers to pores having a cross section greater than 2 nanometers and less than 50 nanometers.
The term “surface area” refers to the total surface area of a substance measurable by the BET technique.
The term “accessible surface area” refers to that surface area not attributed to micropores (i.e., pores having diameters or cross-sections less than 2 nm).
The term “isotropic” means that all measurements of a physical property within a plane or volume of the structure, independent of the direction of the measurement, are of a constant value. It is understood that measurements of such non-solid compositions must be taken on a representative sample of the structure so that the average value of the void spaces is taken into account.
The term “untreated” when used in comparison to “ozone treated” carbon nanotubes, aggregates or any other carbon nanotube structures mean that that the carbon nanotubes, aggregates, or structures have not been specifically treated with ozone. It does not preclude carbon nanotubes, aggregates, or structures which have been subjected to other non-ozone treatments before the treatment with ozone.
In the present invention, the surface properties and characteristics of carbon nanotubes are modified by subjecting them to treatments with ozone in accordance with the preferred embodiment. One desirable result of treating the fibrils in accordance with the preferred embodiment is that the surface of the fibrils become functionalized with oxygen containing moieties.
Additionally, the resulting ozone treated nanotubes can be easily dispersed in both organic and inorganic solvents, and especially in water. The ozone treated nanotubes can be placed in matrices of other materials, such as plastics, or made into structures useful in catalysis, chromatography, filtration systems, electrodes, capacitors and the like. Furthermore, the ozone treated nanotubes can also be used to form other useful carbon nanotube structures as discussed in the subsequent sections.
In the preferred embodiment, the carbon nanotubes are substantially cylindrical, graphitic carbon fibrils of substantially constant diameter and are substantially free of pyrolytically deposited carbon. The nanotubes include those having a length to diameter ratio of greater than 5 with the projection of the graphite layers on the nanotubes extending for a distance of at least two nanotube diameters.
The single walled and multiwalled carbon nanotubes useful for the methods of the present invention have been more specifically described under the previous heading “Carbon Nanotubes.” In a preferred embodiment, the multiwalled nanotubes are prepared according to U.S. Pat. No. 5,171,560 to Tennent, et al. or U.S. Pat. No. 6,696,387 to Moy, et. al, both of which are incorporated herein by reference. The multiwalled carbon nanotubes preferably have diameters no greater than one micron, more preferably no greater than 0.2 micron. Even more preferred are multiwalled carbon nanotubes having diameters between 2 and 100 nanometers, inclusive; most preferably between 3.5 and 75 nanometers.
Alternatively, preferred single walled carbon nanotubes are prepared as disclosed in U.S. Pat. No. 6,211,330 to Moy, et. al, incorporated herein by reference. The single walled carbon nanotubes preferably have diameters no greater than 5 nanometers, more preferably between 0.6 and 5 nanometers.
Ozone used to treat the carbon nanotubes may be delivered in the form of gaseous ozone, liquid ozone, or ozone dissolved in aqueous solvent such as water. Ozone containing gas may be optionally diluted with gases such as oxygen, air, nitrogen, noble gases and mixtures thereof. Any conventionally or commercially available ozone generator may be used to produce the ozone or ozone containing gas. The ozone generator may be fed with gases such as air or pure oxygen in order to generate the ozone or ozone containing gas. The use of air as the feed gas will often yield an ozone containing gas which also contains oxides of nitrogen that may react with a medium such as water to create nitric acid. This extra oxides of nitrogen or nitric acid byproduct in the ozone containing gas may affect the desired functionalization of the carbon nanotube. On the other hand, the use of pure oxygen as the feed gas will yield a purer ozone containing gas, without any or with much smaller amounts of extra by products such as oxides of nitrogen which may affect the desired functionalization of the carbon nanotubes.
In one embodiment, ozone may be introduced to the carbon nanotubes at a rate of 250 mg/hr. Preferably, ozone is introduced to the carbon nanotubes at a rate of 200 to 300 mg/hr. One skilled in the art will understand that the actual flow rate of the ozone containing gas will depend on the amount of ozone in the gas and the quantity of carbon nanotubes to treat.
The ozone may be introduced to the carbon nanotubes using any known conventional reactors, processes or methods, including through vertical reactors such as through the use of a vertical tube reactor, packed bed, fluidized bed, etc. Non-vertical or horizontal reactors may also be utilized. For example, ozone may be fed through a sparger into a tumbling reactor or rotating drum reactor wherein the vessel housing the carbon nanotubes is rotated to more evenly distribute the carbon nanotube's exposure to the ozone and result in more even functionalization of the carbon nanotubes.
In the preferred embodiment, carbon nanotubes are treated with ozone at temperatures ranging from 0° C. to 100° C. Preferably, the nanotubes are contacted with ozone at or about room temperature. More preferably, the nanotubes are contacted with ozone at a temperature range between 0° C. and 60° C. and most preferably between 20° C. and 50° C.
In the exemplary embodiment, this ozonolysis process at the preferred temperatures may be carried on for over 24 hours. Preferably, the reaction is permitted to continue between 3 to 8 hours, or 10 to 45 hours, and more preferably between 15 to 25 hours. One skilled in the art will understand that the run time of this process depend on factors such as the size and temperature of the reactor, the amount of carbon fibrils needed for treatment, the rate of ozone being introduced to the fibrils, the manner by which ozone is introduced to the fibrils (i.e, in a gaseous or liquid medium), the desired acidity, etc.
Carbon nanotubes treated in accordance with this preferred embodiment result in a number of unexpected benefits.
It has been unexpectedly found that the ozone treated carbon nanotubes of the preferred embodiment experience a weight gain instead of a weight loss which is what would have been expected when treating carbon nanotubes with oxidizing agents. The increase in weight may be greater than 1%, and preferably greater than 5%. In one embodiment, the ozone treated carbon nanotubes of the preferred embodiment exhibit a 5 to 20% weight gain, more preferably a 10 to 15% weight gain.
Without being bound to any theory, it is believed this overall weight gain can be attributed to the ozone at the recited temperature ranges preferentially engaging in surface treatment of the carbon nanotube surface (thereby resulting in the attachment, formation, substitution or deposition of functional groups onto the surface of the carbon nanotubes and aggregates) as opposed to engaging in uncapping, stripping or shortening reactions (thereby resulting in either minimal or no measurable or visual loss of carbon mass) that are more typical with other oxidizing agents at higher temperatures (for example nitric acid—degrades aggregates to form weathered rope structures). On a more atomic level, ozone treatment at moderate temperatures in accordance with the preferred embodiment is believed to proceed via an addition reaction by which ozone forms a live membered “ozonide” ring with the double bonded surface carbon atoms which then decomposes into COOH and C═O. This result is surprising since ozone is a strong oxidant at higher temperatures, and thus would have been expected to aggressively uncap, strip or shorten the carbon nanotubes as with other strong oxidants such as nitric acid. The mechanisms of ozonolysis is described in Murray, Robert W., “The Mechanism of Ozonolysis,” Accounts of Chemical Research, Volume 1, pp. 313-320 (October, 1968), herein incorporated by reference.
Consistent with the weight gain, it has also been unexpectedly found that treatment of carbon nanotubes with ozone in accordance with the preferred embodiment yields carbon nanotubes with higher amounts of functional groups (especially acidic groups) on the nanotube surface than treatments using other oxidizing agents. (As a result, in the specification and claims, the term “functionalized” nanotubes may be used interchangably with “ozone treated carbon nanotubes” where the nanotubes were treated with ozone). Specifically, carbon nanotubes treated with ozone can reach a higher acid titer (i.e., have more acidic groups) than those treated with other oxidizing agents such as nitric acid and hydrogen peroxide. Acid titer can be measured using the methods described in the application, such as the method described under the heading entitled “Method of Measuring Titer” in the Examples section.
Ozone treated carbon nanotubes of the preferred embodiment can reach an acid titer greater than 2 meq/g, preferably in the range of 1.6 to 2.2 meq/g. Alternatively, ozone treated carbon nanotubes can reach an acid titer in the range of 0.0040-0.0080 meq/m2. As acid titer measurements are an indication of the quantity of acid reacting oxygen containing moieties such as COOH, phenolic OH, lactone, etc. which have been deposited onto the carbon nanotubes, the exceedingly high acid titer reached confirms that the ozone in the method of the preferred embodiment is reacting with the surface side walls of the carbon nanotubes and not just the end caps.
It has also been found that ozone can be used in accordance with the preferred embodiment to further treat carbon nanotubes which have already been oxidized or functionalized by other oxidizing agents. For example, a single or multi walled carbon nanotube mixture which contain amorphous carbon can first be oxidized with nitric acid using conventional methods to, inter alia, purify the mixture (i.e., to remove the amorphous carbon via oxidation). These oxidized carbon nanotubes will be consequently functionalized with some oxygen containing moieties, and thus will yield a low acid titer upon titration. When further treating these oxidized carbon nanotubes with ozone in accordance with the preferred embodiment, the ozone treated carbon nanotubes was discovered to exhibit upon titration an increase in acid titer by at least 1.5 meq/g and preferably greater than 2 meq/g. For example, the increase in acid titer may be between 1.5 meq/g to 3 meq/g, or between 2 meq/g to 2.5 meq/g. (These acid titer increases may also apply to ozone treated carbon nanotubes when comparing to as made carbon nanotubes which have not previously been treated with oxidizing agents). The final ozone treated carbon nanotubes may themselves exhibit upon a titration an acid titer greater than 2.5 meq/g or between 2.5 meq/g to 3.5 meq/g.
Additionally, the use of ozone to treat carbon nanotubes at the recited temperature range also results in a more efficient generation (higher percentage) of surface acidic groups compared to carbon nanotubes which have been oxidized with nitric acid or hydrogen peroxide. Carbon nanotubes treated with ozone at the recited temperature range have higher oxygen content than those oxidized with nitric acid or hydrogen peroxide. The surface of the ozone treated carbon nanotubes of the preferred embodiment can have an oxygen content greater than 4 percent (e.g., 4-10%), preferably greater than 6 percent (e.g., 6-10%). Preferred functional groups resulting from the ozone treatment include carboxyl, anhydride and ketone.
It has been further discovered that ozone treatment of carbon nanotubes in accordance with the preferred embodiment also results in less destruction to the carbon nanotube itself as compared to other oxidizing agents. Experimental data and scanning electronic micrographs such as those shown in
In addition to the surface and structural benefits on the carbon nanotube itself (i.e., more functional groups, less carbon destruction), a further benefit in using the preferred ozone treatment is the energy and cost savings imparted as a result of being able to carry out the ozone oxidation treatment at a lower temperature range such as at room temperature. Thus, compared other oxidation processes, additional heating equipment is not necessarily required.
All of these benefits were also discovered to apply when treating any carbon nanotube structure (e.g., fibril aggregates, mats, assemblages, three dimensional networks, rigid porous structures, etc.) with ozone. The ozone treated structures remained intact despite high degree of surface oxidation and all of the above recited weight gain, oxygen content and acid titer characteristics apply as well.
Furthermore, as explained in the subsequent sections, similar benefits are also conferred upon carbon nanotube structures which are made from these ozone oxidized carbon nanotubes.
Carbon nanotube structures include, but are not limited to the following groups: aggregates, assemblages, networks and rigid porous structures.
a. Aggregates are dense microscope particulate structures of entangled carbon nanotubes and may resemble the morphology of birds nest, cotton candy, combed yard or open net.
b. Assemblages are carbon nanotube structures which have relatively uniform properties in along one, preferably two and most desirably three dimensional axis of the three dimensional assemblage. (E.g., U.S. Pat. No. 5,691,054 hereby incorporated by reference). Generally, assemblages are formed by de-aggregating the carbon nanotube aggregate structure, and then reassembling them to form assemblages.
C. Networks are formed by linking individual functionalized carbon nanotubes together by using a linking molecule between the functionalized groups located on the surface of the carbon nanotubes. (E.g., PCT/US97/03553 or WO 97/32571, hereby incorporated by reference).
d. Rigid porous structures are formed by either linking the individual functionalized carbon nanotubes together without the use of a linking molecule, or by gluing carbon nanotube aggregate structures together with a gluing agent. (E.g., U.S. Pat. No. 6,099,965, hereby incorporated by reference).
In the preferred embodiment, ozone treated carbon nanotube structures may be made by forming the respective structure first and then subjecting that structure to ozone treatment as described above in “Methods Of Treating Carbon Nanotubes With Ozone.” Alternatively, ozone treated carbon nanotube structures may be formed from the ozone treated carbon nanotubes themselves.
It has also been discovered that improved aggregates of carbon nanotubes can be formed by treatment with ozone in accordance with the preferred embodiment. As defined previously, aggregates are dense microscopic particulate structures of entangled carbon nanotubes and can be made using any of the procedures previously described or incorporated by reference in the section entitled “Aggregates of Carbon Nanotubes and Assemblages.” Preferred aggregates have diameters less than 50 microns.
Consistent with the non-destructive treatment of carbon nanotubes discussed above, it has been further discovered that improved aggregates of carbon nanotubes may be formed from the ozone treated carbon nanotubes described. Alternatively, improved aggregates of carbon nanotubes may formed by subjecting the untreated aggregates to ozone treatment in accordance with the preferred embodiment.
When forming aggregates of carbon nanotubes from ozone treated carbon nanotubes, the individual carbon nanotubes are first subjected to ozone treatment as disclosed in the previous section “Method Of Treating Carbon Nanotubes With Ozone” to form ozone treated carbon nanotubes. The ozone treated carbon nanotubes are then formed into ozone treated aggregates of carbon nanotubes using any method disclosed in the references incorporated by reference in the previous section entitled “Aggregates Of Carbon Nanotubes And Assemblages.”
Alternatively, if aggregates of untreated carbon nanotubes have already been formed, then those untreated aggregates may be subject to ozone treatment in the same manner and conditions described in the previous section “Method Of Treating Carbon Nanotubes With Ozone.”
In either embodiment, ozone treated aggregates of carbon nanotubes were discovered to have a number of similar benefits as ozone treated carbon nanotubes. For example, the ozone treated aggregates exhibit an unexpected weight gain compared to untreated aggregates of carbon nanotubes. The increase in weight of the ozone treated aggregates of carbon nanotubes may be greater than 1%, and preferably greater than 5%, by comparison to the untreated aggregates of carbon nanotubes. In one embodiment, the ozone treated carbon nanotube aggregates of the preferred embodiment exhibit a 5 to 20% weight gain, more preferably a 10 to 15% weight gain.
Moreover, it has also been unexpectedly found that the ozone treatment in accordance with the preferred embodiment yields aggregates of carbon nanotubes with higher amounts of functional groups (especially acidic groups) on the nanotube surface than treatments using other oxidizing agents. Specifically, ozone treated aggregates of carbon nanotubes can reach a higher acid titer (i.e., have more acidic groups) than those treated with other oxidizing agents such as nitric acid and hydrogen peroxide. Ozone treated aggregates of carbon nanotubes can reach an acid titer between 1 meq/g to 2 meq/g.
Even more surprising is the discovery that the ozone treated aggregates of carbon nanotubes substantially retain the structure of the original untreated aggregates while reaching acid titers between 1 meq/g to 2 meq/g. This substantial retention of structure again confirms the non-destructive effect of ozone when used in accordance with the preferred embodiment. Aggregates of carbon nanotubes which have been subjected to treatment with other oxidizing agents such as nitric acid are unable to reach such a high acid titer since those oxidizing agents will engage in uncapping, stripping and shortening reactions on the carbon nanotubes and thereby cause the aggregate structure to unravel and come apart.
Experiments further confirm that the use of ozone to treat aggregates of carbon nanotubes at the recited temperature range results in a more efficient generation (higher percentage) of surface acidic groups compared to carbon nanotubes which have been oxidized with nitric acid or hydrogen peroxide. Ozone treated aggregates of carbon nanotubes have higher oxygen content than those oxidized with nitric acid or hydrogen peroxide. The surface of the ozone treated aggregates of carbon nanotubes can have a oxygen content greater than 4 percent (e.g., 4-10%), preferably greater than 6 percent (e.g., 6-10%). Experiments also showed that preferred functional groups resulting from the ozone treatment were carboxyl, anhydride and ketone.
Consistent with the previous section entitled “Methods Of Treating Aggregates Of Carbon Nanotubes With Ozone,” other improved carbon nanotube structures may be formed from the ozone treated carbon nanotubes. Alternatively, the untreated carbon nanotube structure may be subjected to ozone treatment in accordance with the preferred embodiment. Ozone treatment may be performed in gaseous or liquid phase.
Ozone treated carbon nanotubes can also be used to functionalize high quality extrudates which can be formed by using a small amount of water soluble binder. In the preparation of extrudates, the functionalized surface of the nanotubes allows for improved binder dispersion during the mixing stage and minimizes the segregation of binder in the subsequent heating step.
Methods for producing a network of carbon nanotubes comprising treating carbon nanotubes with ozone for a period of time sufficient to functionalize the surface of the carbon nanotubes, contacting the ozone treated carbon nanotubes with a reactant suitable for adding a secondary functional group to the surface of the carbon nanotube, and further contacting the secondarily treated nanotubes with a cross-linking agent effective for producing a network of carbon nanotubes. A preferred cross-linking agent is a polyol, polyamine or polycarboxylic acid. A useful polyol is a diol and a useful polyamine is a diamine.
In one aspect of the invention a network of carbon nanotubes is obtained by first oxidizing the as-produced carbon nanotubes with ozone (alternatively, liquid phase ozone may be used), followed by subjecting the ozone treated nanotubes to conditions which foster crosslinking. For example, heating the ozone treated nanotubes in a temperature range from 180° C. to 650° C. results in crosslinking the ozone treated nanotubes together with elimination of the oxygen containing moieties of the ozone treated nanotubes.
The invention also includes three-dimensional networks formed by linking the ozone treated nanotubes of the preferred embodiment. These complexes include at least two surface-modified nanotubes linked by one or more linkers comprising a direct bond or chemical moiety. These networks comprise porous media of remarkably uniform equivalent pore size. They are useful as adsorbents, catalyst supports and separation media.
Stable, porous 3-dimensional networks or structures with meso- and macropores (pores>2 nm) are very useful as catalysts or chromatography supports. Since nanotubes can be dispersed on an individualized basis, a well-dispersed sample which is stabilized by cross-links allows one to construct such a support. Ozone treated nanotubes are ideal for this application since they are easily dispersed in aqueous or polar media and the oxygen-containing moieties present on the oxidized nanotubes provide cross-link points. Additionally, the oxygen containing moieties also provide points to support the catalytic or chromatographic sites. The end result is a rigid, 3-dimensional structure with its total surface area accessible with secondary group sites on which to support the active agent.
Although the interstices between these nanotubes are irregular in both size and shape, they can be thought of as pores and characterized by the methods used to characterize porous media. The size of the interstices in such networks can be controlled by the concentration and level of dispersion of nanotubes, and the concentration and chain lengths of the cross-linking agents. Such materials can act as structured catalyst supports and may be tailored to exclude or include molecules of a certain size. Aside from conventional industrial catalysis, they have special applications as large pore supports for biocatalysts.
Typical applications for these supports in catalysis include their use as a highly porous support for metal catalysts laid down by impregnation, e.g., precious metal hydrogenation catalysts. Moreover, the ability to anchor molecular catalysts by tether to the support via the secondary groups combined with the very high porosity of the structure allows one to carry out homogeneous reactions in a heterogeneous manner. The tethered molecular catalyst is essentially dangling in a continuous liquid phase, similar to a homogeneous reactor, in which it can make use of the advantages in selectivities and rates that go along with homogeneous reactions. However, being tethered to the solid support allows easy separation and recovery of the active, and in many cases, very expensive catalyst.
These stable, rigid structures also permits carrying out heretofore very difficult reactions, such as asymmetric syntheses or affinity chromatography by attaching a suitable enantiomeric catalyst or selective substrate to the support. The rigid networks can also serve as the backbone in biomimetic systems for molecular recognition. Such systems have been described in U.S. Pat. No. 5,110,833 and International Patent Publication No. WO93/19844. The appropriate choices for cross-linkers and complexing agents allow for stabilization of specific molecular frameworks.
Rigid porous structures are prepared by first preparing ozone treated nanotubes as described above, dispersing them in a medium to form a suspension, separating the medium from the suspension to form a porous structure, wherein the ozone treated nanotubes are further interconnected to form a rigid porous structure, all in accordance with methods more particularly described in U.S. Pat. No. 6,099,965 entitled “Rigid Porous Carbon Structures, Methods of Making, Methods of Using and Products Containing Same” filed on May 15, 1997, hereby incorporated by reference.
The hard, high porosity structures can be formed from carbon nanotubes or nanotube aggregates. In order to increase the stability of the nanotube structures, it is also possible to deposit polymer at the intersections of the structure. This may be achieved by infiltrating the assemblage with a dilute solution of low molecular weight polymer cement (i.e., less than about 1,000 MW) and allowing the solvent to evaporate. Capillary forces will concentrate the polymer at nanotube intersections. It is understood that in order to substantially improve the stiffness and integrity of the structure, only a small fraction of the nanotube intersections need be cemented.
The nanotubes may be uniformly and evenly distributed throughout the structure or in the form of aggregate particles interconnected to form the structure. When the former is desired, the nanotubes are dispersed thoroughly in the medium to form a dispersion of individual nanotubes. When the latter is desired, nanotube aggregates are dispersed in the medium to form a slurry and said aggregate particles are connected together with a gluing agent to form said structure.
The medium used may be selected from the group consisting of water and organic solvents. Preferably, the medium comprises a dispersant selected from the group consisting of alcohols, glycerin, surfactants, polyethylene glycol, polyethylene imines and polypropylene glycol.
The medium should be selected which: (1) allows for fine dispersion of the gluing agent in the aggregates; and (2) also acts as a templating agent to keep the internal structure of the aggregates from collapsing as the mix dries down.
One preferred embodiment employs a combination of polyethylene glycol (PEG) and glycerol dissolved in water or alcohol as the dispersing medium, and a carbonizable material such as low MW phenol-formaldehyde resins or other carbonizable polymers or carbohydrates (starch or sugar). Once the rigid porous structure has been prepared, it can then be treated with ozone in accordance with the preferred embodiment in preparation for use in electrochemical capacitors, for example.
In other words, a preferred embodiment includes a method for forming a catalyst support comprising the steps of: forming a rigid porous structure comprising carbon nanotubes, and contacting said rigid porous structure with ozone at a temperature range between 0° C. to 100° C. under conditions sufficient to form a functionalized rigid porous structure which is greater in weight than said rigid porous structure.
In another embodiment, if ozone treated carbon nanotubes or aggregates are employed to form the rigid porous structure, the nanotubes are treated with ozone in accordance with the preferred embodiment prior to dispersing in the medium and are self-adhering forming the rigid structure by binding at the nanotube intersections. The structure may be subsequently pyrolized to remove oxygen. A useful pyrolysis temperature range is from about 200° C. to about 2000° C. and preferably from about 200° C. to about 900° C.
According to another embodiment, the nanotubes are dispersed in said suspension with gluing agents and the gluing agents bond said nanotubes to form said rigid structure. Preferably, the gluing agent comprises carbon, even more preferably the gluing agent is selected from a material that, when pyrolized, leaves only carbon. Accordingly, the structure formed with such a gluing may be subsequently pyrolized to convert the gluing agent to carbon. Additional methods and details for forming rigid porous structures are found in U.S. Pat. No. 6,099,965, hereby incorporated by reference.
Preferably, the gluing agents are selected from the group consisting of acrylic polymers, carboxylic polymers, cellulose, carbohydrates, polyethylene, polystyrene, nylon, polyurethane, polyester, polyamides, polyvinyl acetate/alcohol emulsions or resins, amino resins, epoxy resins and phenolic resins.
According to further embodiments of the invention, the step of separating comprises filtering the suspension or evaporating the medium from said suspension.
According to yet another embodiment, the suspension is a gel or paste comprising the nanotubes in a fluid and the separating comprises the steps of:
Isotropic slurry dispersions of nanotube aggregates in solvent/dispersant mixtures containing gluing agent can be accomplished using a Waring blender or a kneader without disrupting the aggregates. The nanotube aggregates trap the resin particles and keep them distributed.
These mixtures can be used as is, or can be filtered to remove sufficient solvent to obtain cakes with high nanotube contents (5-20% dry weight basis). The cake can be molded, extruded or pelletized. The molded shapes are sufficiently stable so that further drying occurs without collapse of the form. On removing solvent, disperant molecules, along with particles of gluing agent are concentrated and will collect at nanotube crossing points both within the nanotube aggregates, and at the outer edges of the aggregates. As the mixture is further dried down and eventually carbonized, nanotube strands within the aggregates and the aggregates themselves are glued together at contact points. Since the aggregate structures do not collapse, a relatively hard, very porous, low density particle is formed. The resulting rigid porous structure can then be subjected to ozone treatment under the same manner and conditions described in the “Method Of Treating Carbon Nanotubes With Ozone.”
As set forth above, the rigid, porous structures may also be formed using ozone treated nanotubes with or without a gluing agent. Carbon nanotubes become self-adhering after being oxidized by ozone. Very hard, dense mats are foamed by highly dispersing the oxidized nanotubes (as individualized strands), filtering and drying. The dried mats have densities between 1-1.2 g/cc, depending on oxygen content, and are hard enough to be ground and sized by sieving. Measured surface areas are about 275 m2/g.
The ozone treated nanotubes may also be used in conjunction with a gluing agent. Ozone treated nanotubes are good starting materials since they have attachment points to stick both gluing agents and templating agents. The latter serve to retain the internal structure of the particles or mats as they dry, thus preserving the high porosity and low density of the original nanotube aggregates. Good dispersions are obtained by slurrying ozone treated nanotubes with materials such as polyethyleneimine cellulose (PEI Cell), where the basic imine functions form strong electrostatic interactions with carboxylic acid functionalized fibrils. The mix is filtered to form mats. Pyrolizing the mats at temperatures greater than 650° C. in an inert atmosphere converts the PEI Cell to carbon which acts to fuse the nanotube aggregates together into hard structures. The result is a rigid, substantially pure carbon structure, which can then be further treated again with ozone if desired.
Solid ingredients can also be incorporated within the structure by mixing the additives with the nanotube dispersion prior to formation of the structure. The content of other solids in the dry structure may be made as high as fifty parts solids per part of nanotubes.
According to one preferred embodiment, nanotubes are dispersed at high shear in a highshear mixer, e.g., a Waring Blender. The dispersion may contain broadly from 0.01 to 10% nanotubes in water, ethanol, mineral spirits, etc. This procedure adequately opens nanotube bundles, i.e. tightly wound bundles of nanotubes, and disperses the nanotubes to form self-supporting mats after filtration and drying. The application of high shear mixing may take up to several hours. Mats prepared by this method, however, are not free of aggregates.
If the high shear procedure is followed by ultrasonication, dispersion is improved. Dilution to 0.1% or less aids ultrasonication. Thus, 200 cc of 0.1% fibrils, for example, may be sonified by a Bronson Sonifier Probe (450 watt power supply) for 5 minutes or more to further improve the dispersion.
To achieve the highest degrees of dispersion, i.e., a dispersion which is free or virtually free of nanotube aggregates, sonication must take place either at very low concentration in a compatible liquid, e.g., at 0.001% to 0.01% concentration in ethanol or at higher concentration e.g., 0.1% in water to which a surfactant, e.g., Triton X-100, has been added in a concentration of about 0.5%. The mat which is subsequently formed may be rinsed free or substantially free of surfactant by sequential additions of water followed by vacuum filtration. The mat thus formed can then be treated with ozone in accordance with the preferred embodiment.
Particulate solids such as MnO2 (for batteries) and Al2O3 (for high temperature gaskets) may be added to the ozone treated nanotube dispersion prior to mat formation at up to 50 parts added solids per part of nanotubes.
Reinforcing webs and scrims may be incorporated on or in the mats during formation. Examples are polypropylene mesh and expanded nickel screen.
Carbon nanotube structures such as carbon nanotube aggregates, three dimensional networks or rigid porous structures can be used as catalyst supports, including but not limited to catalyst supports for catalysts which catalyze the formation of carbon nanotubes. Of these choices, rigid porous structures offer the preferred combination of size, strength and surface area as catalyst supports.
Thus, the preferred embodiments include forming catalyst supports by functionalizing carbon nanotube structures such as aggregates, rigid porous structures or three dimensional networks via treatment with ozone. Aggregates may be prepared by any methods discussed previously, including those disclosed in U.S. Pat. No. 5,165,909 to Tennent; U.S. Pat. No. 5,456,897 to Moy et al.; Snyder et al., U.S. Pat. No. 5,707,916, filed May 1, 1991, and PCT Application No. US89/00322, filed Jan. 28, 1989 (“Carbon Fibrils”) WO 89/07163, and Moy et al., U.S. Pat. No. 5,456,897 filed Aug. 2, 1994 and PCT Application No. US90/05498, filed Sep. 27, 1990 (“Battery”) WO 91/05089, and U.S. Pat. No. 5,500,200 to Mandeville et al., filed Jun. 7, 1995 and U.S. Pat. No. 5,456,897 filed Aug. 2, 1994 and U.S. Pat. No. 5,569,635 filed Oct. 11, 1994 by Moy et al, all of which are hereby incorporated by reference.
Rigid porous structures may be made using any methods discussed previously, including those disclosed in U.S. Pat. No. 6.432,866 to Tennent et al., hereby incorporated by reference. In summary, rigid porous structures such as those described above or in U.S. Pat. No. 6,432,866, are typically prepared by oxidizing the nanotubes and then heating them to cause cross-linking between the nanotubes or are prepared by mixing nanotubes with a gluing agent and heating to carbonize the gluing agent. Three dimensional networks may be made using any methods disclosed in U.S. Pat. No. 5,968,650 to Tennent et al., hereby incorporated by reference.
One important aspect of a catalyst support is that the support must be able to hold onto the catalyst during the course of the catalytic reaction, whether via chemical bonding, adhesion, or any other forces which permit the catalyst to remain on the support itself. To promote stable and sufficient bonding between the catalyst and the support, it is preferred that the support itself contain a number of functional groups on its surface which the catalyst will bind to or react with so as to establish the desired bonding between the catalyst and the support.
Another important aspect of a catalyst support is that the support be able to maintain its structure during the course of a reaction instead of deteriorating or breaking apart.
In the preferred embodiment, catalyst supports are created by treating or contacting the carbon nanotube structure such as carbon nanotube aggregates, three dimensional networks or rigid porous structures with ozone at a temperature range between 0° C. to 100° C., preferably between 0° C. to 60° C., most preferably between 20° C. to 50° C. or at about room temperature. Ozone is especially preferred since it has been discovered to add functional groups onto the carbon nanotube structures without weakening or destroying the crosslinking, glue or other forces or bonds which hold those structures together (i.e., no observable change in structure).
Other strong oxiding agents such as nitric acid can also be used to add functional groups onto the surface of these structures, however, it has been discovered that nitric acid is more likely to hydrolyze crosslinked bonds or dissolve the glue bonds or diminish other bonds so as to either weaken the integrity of the original support structure. Conversely, weak oxidizing agents such as hydrogen peroxide, while the) may not weaken the integrity of the support structure, do not result in enough functional groups being added to the support surface (i.e., low acid titers) to be of commercially practical use.
Thus, to obtain catalyst support structures with sufficient structural integrity and sufficient functional groups, it is preferred to prepare catalyst support structures by contacting the carbon nanotube structures such as carbon nanotube aggregates, three dimensional networks or rigid porous structures with ozone as the preferred oxidizing agent, since that will result in catalyst supports with higher concentration functional groups (i.e., higher acid titers, and thus better retention of the catalyst) and stronger structural integrity as compared to treating those structures with other oxidizing agents.
Preferred catalyst supports comprise ozone treated carbon nanotube structures which exhibit upon titration an acid titer grater than 1 meq/g such as from 1 to 2 meq/g. These ozone treated carbon nanotube structures further exhibit no observable change in structure in comparison to the original untreated carbon nanotube structures. The ozone treated carbon nanotube structure can also exhibit a weight gain of greater than 1%, preferably 5 to 20% weight gain, more preferably a 10 to 15% weight gain. Other acid titer, oxygen contents, and weight gain characteristics as described earlier apply to the functionalized structures used as catalyst supports as well.
Carbon nanotubes are electrically conductive. Electrodes and their use in electrochemical capacitors comprising carbon nanotubes and/or functionalized carbon nanotubes have been described in U.S. Pat. No. 6,031.711 entitled “Graphitic Nanofibers in Electrochemical Capacitors,” filed on May 15, 1997 and is incorporated herein by reference.
Further details about electrochemical capacitors based on catalytically grown carbon nanotubes are disclosed in Chumming Niu et al., “High Power Electrochemical Capacitors based on Carbon Nanotube Electrodes,” in Applied Physics Letters 70(11), pp. 1480-1482, Mar. 17, 1997 incorporated herein by reference.
The quality of sheet electrode depends on the microstructure of the electrode, the density of the electrode, the functionality of the electrode surface and mechanical integrity of the electrode structure.
The microstructures of the electrode, namely, pore size and size distribution determines the ionic resistance of electrolyte in the electrode. The surface area residing in micropores (pore diameter<2 nm) is considered inaccessible for the formation of a double layer (2). On the other hand, distributed pore sizes, multiple-pore geometries (dead end pores, slit pores, cylindrical pores, etc.) and surface properties usually give rise to a distributed time constant. The energy stored in an electrode with a distributed time constant can be accessed only with different rates. The rapid discharge needed for pulsed power is not feasible with such an electrode.
The density of the electrode determines its volumetric capacitance. An electrode with density less than 0.4 g/cc is not practical for real devices. Simply, the low-density electrode will take up too much electrolyte, which will decrease both volumetric and gravimetric capacitance of the device.
The surface of the carbon nanotubes is related to the wetting properties of electrodes towards electrolytes. The surface of as-produced, catalytically grown carbon nanotubes is hydrophobic. The hydrophobic surface properties of the as produced carbon nanotubes can be changed to hydrophilic by treatment of the as produced carbon nanotubes or aggregates of carbon nanotubes with ozone in accordance with the preferred embodiment. Furthermore, the capacitance can be increased by further attaching redox groups on the surface of the carbon nanotubes.
Finally, the structural integrity of the electrodes is critical to reproducibility and long term stability of the device. Mechanical strength of electrodes incorporating carbon nanotubes is determined by the degree of entanglement of the carbon nanotube and bonding between carbon nanotubes in the electrode. A high degree of entanglement and carbon nanotube bonding can also improve the conductivity, which is critical to the power performance of an electrode. The specific capacitance (D.C. capacitance) of the electrodes made from gas-phase treated fibrils was about 40 F/g.
One aspect of the present invention relates to preparing electrodes and electrochemical capacitors from ozone treated carbon nanotubes. Broadly, as prepared carbon nanotubes have been treated with ozone in accordance with the preferred embodiment to provide surface oxidized, single walled or multiwalled carbon nanotubes which can be used to prepare the electrodes of the invention.
In another aspect of the invention, the ozone treated nanotubes can be further treated with a reactant suitable to react with moieties present on the oxidized nanotubes to form nanotubes having secondary groups on its surface which are also useful in preparing the electrodes of the present invention.
Electrodes are assembled by simple filtration of slurries of the ozone treated nanotubes. Thickness is controlled by the quantity of material used and the geometry, assuming the density has been anticipated based on experience. It may be necessary to adjust thickness to get selfsupporting felts.
The electrodes are advantageously characterized by cyclic voltammetry, conductivity and DC capacitance measurement.
The following examples serve to provide further appreciation of the invention but are not meant in any way to restrict the effective scope of the invention.
As an initial matter, measuring an acid titer can be effected in a number of ways. In one embodiment, 0.10 g of fibrils are transferred to a Waring Laboratory Blender containing 350-400 CC of D.I. water. The fibrils are blended at slow speed for 10-15 minutes until all fibrils in the water phase appeared to be homogenously black. 10 CC of standard 0.10 N sodium hydroxide solution is added to the blender containing fine fibrils slurry and again blended for 4-5 minutes. The slurry is then transferred to a beaker containing a stirring rod and a pH electrode. A solution of hydrochloric acid of known strength is then added gradually to neutralize the fibrils slurry at pH 7.0. The volume of hydrochloric acid used to neutralize fibrils slurry is noted and used in calculation of meq/g of fibrils. The results of titer measurement may be expressed in either milliequivalence per nanotube weight (i.e., meq/g) or milliequivalence per nanotube surface area (i.e., meq/m2).
Ozone was generated via an air purifier made by Del Industry, San Luis Obispo, Calif., which can generate ozone at a rate of 250 mg/hr. A mixture of ozone and air (0.29% ozone) at a flow rate of 1200 mL/min was then passed though a 1-inch (OD) reactor tube packed with dry as-made fibrils. The weight of fibrils before and after ozone treatment were recorded. The reaction was allowed to proceed for a period of 3 to 45 hours at room temperature.
In a separate experiment, 20 grams of as-made fibrils were placed in a flask containing 500 mL 30% or 60% nitric acid. The reaction flask was then heated to reflux temperature of 95-120° C. for 4-6 hours. After the reaction was stopped, the fibrils were cooled to room temperature, filtered, and washed with water until neutral. In another separate experiment, 20 grams of as-made fibrils were placed in a flask containing 376.2 grams of 30% H2O2 in molar ratio of 1:2. The temperature was set to be in the vicinity of 35° C., but it rose to reflux temperature quickly. A water bath was applied to maintain the reaction temperature at 30° C. After 2-hour reaction, the slurry was filtered and washed until all the residual H2O2 was removed.
The results are reported below:
Experimental data confirmed that oxidation of carbon nanotubes using ozone at room temperature as compared to other oxidizing agents such as nitric acid or hydrogen peroxide resulted in a surprising difference in weight of the final product. Namely, carbon nanotubes subjected to ozone treatment at room temperature posted a significant weight gain instead of weight loss as compared to carbon nanotubes which have been subjected to other oxidation treatment. This is surprising given that ozone is a strong oxidant.
With respect to using nitric acid as the oxidizing agent, the nitric acid strength and reaction condition affected the degree of weight loss The weight loss is attributed to the oxidation of carbon to form CO or CO2, which evolute from the reactor system.
3 grams of dry fibrils was placed in a vertical reactor and ozone-containing air was passed through them at room temperature. The reactor was shut off periodically at every hour and the total weight of fibrils plus the reactor tube were measured on an electric balance. The weight gain of fibrils against reaction time was then obtained after tarring off the reactor weight.
The results of this measurement are displayed in FIG. 7, which shows that the sample's weight increased over the course of reaction and leveled off after approximately 15 hours of reaction lime.
Various oxidized fibrils prepared according to the methods in Example 1 such as Samples 1-3 and 5-7 were measured to determine their relative amount of acidic groups through titration. 0.25 gram of each sample was placed into a flask containing 300 mL D.I. water and the slurry was titrated with 0.1N NaOH. The consumption of NaOH was translated into the quantity of total surface acidic groups as meq/g.
As shown in Table 3, more acidic groups can be deposited onto the surface of carbon nanotubes when oxidized with ozone at room temperature than with other oxidizing agents such as nitric acid or hydrogen peroxide. In other words, treatment of carbon nanotubes with ozone at room temperature will yield nanotubes with a higher titer (i.e., an indicator of acidic groups) than nanotubes resulting from treatment with other oxidizing agents.
Additionally, even though carbon nanotubes treated with nitric acid was able to reach a titer within the range of the lowest titer yielded by ozone treatment, such nitric acid treated nanotubes may be unsuitable for certain applications due to the significant weight loss that accompany such titer range.
Table 3 further confirms that ozone treatment at room temperature yields carbon nanotubes which are significantly more acidic than those by weak acids such as hydrogen peroxide or diluted nitric acid.
To compare the degree of damage done to the carbon nanotube surface by the oxidation treatments, the surface area of the treated fibrils was measured using an Autosorb-1 instrument. For the room temperature ozone oxidized fibrils, the average BET surface area at −196° C. were all in the range of 240-250 m2/g, which is very close to as-made untreated fibrils. On the other hand, the average BET surface area (measured at 196° C.) of oxidized fibrils treated with nitric acid increased from 250 m2/g to 341 m2/g.
Such increase in surface area is believed to be caused by the stripping of layers or uncapping of nanotubes. Therefore, minimal or no measurable damage was generated on the surface of carbon nanotubes during ozone treatment at room temperature.
XPS (X-ray Photoelectron Spectroscopy) was applied to characterize the fibril surface of fibril aggregates treated using nitric acid, hydrogen peroxide, gaseous ozone, (all as described earlier) and liquid ozone (as described later) as compared to plain untreated fibril aggregates.
Three sets of characteristics were measured: the atomic concentration of surface oxygen; carbon functionality and oxygen functionality.
As expected, plain fibril had the least amount of oxygen on the surface, which is generally created upon exposure to air after production. Table 4A further confirms that oxidation yields more oxygenated groups onto the fibril surface as the oxygen content was increased for all fibrils treated with oxidizing agents. Table 4A and 4B both confirm that the highest oxygen content was exhibited in gaseous ozone treated carbon nanotubes at room temperature.
Table 4B further displays the breakdown of those oxygenated contents on the fibril surfaces and confirms that different oxidizing agents yields different quantities of different functional groups. For example, carbon nanotubes treated with H2O2 treatment created more —OH groups while nitric acid generated more —COOH as a percentage increase of the total functional groups generated. The fibrils treated with gaseous ozone at room temperature yielded the greatest —COOH increase.
Further analysis on the oxygen content so as to determine more information on the surface functional groups of the fibrils are shown in Table 4C and FIG. 8. FIG. 8 showed the oxygen spectra at the 1 s region. The raw data are represented by the plotted dots while the fitted data and deconvoluted signals are represented by the solid lines. The peaks at 531 eV, 533 eV and 535 eV are assigned to the C—O, C═O and H2O configuration. The data in FIG. 8 was summed up in Table 4C below:
As shown in Table 4C, the oxygen groups of plain fibril generally are in a single bond “O—” conformation. Fibrils subjected to H2O2 oxidization yielded more oxygen groups in the form of —OH and C═O. However, since the single bond —O confirmation still dominated, this may explain the lower titer results of nanotubes treated with hydrogen peroxide.
On the other hand, nitric acid or ozone oxidation yielded carbon nanotubes in which the surface oxygen group was switched from being predominately single bond —O conformation to predominantly double bond ═O conformation. Furthermore, fibrils subjected to gas phase ozone treatment resulted in less moisture content as well.
Ozone treated fibrils at room temperature were examined using SEM. Analysis of the fibril aggregate structure under the scanning electron microscope as shown in FIG. 9 confirmed that the fibril aggregates structure remained intact after the ozone treatment at room temperature.
Differences between untreated fibril aggregates, fibril aggregates oxidized with nitric acid and fibril aggregates oxidized with ozone were demonstrated by the following test. Three sets of fibril aggregates were prepared in the same manner. The first fibril aggregates were left untreated after preparation. The second fibril aggregates were subsequently treated with nitric acid at room temperature. The third fibril aggregates were subsequently treated with ozone at room temperature. The three sets of fibril aggregates were dispersed in water and the following differences were observed:
Thus, fibril aggregates treated with ozone have different characteristics from untreated fibril aggregates (i.e., hydrophobic and non wetting v. hydrophillic and wetting) as well as from fibril aggregates treated with strong oxidants such as nitric acid (i.e., retains structure v. breaks apart).
The following experiments were performed using the procedure described in Example 1, except a heating device such as heating tape was applied to the 1-inch reactor tube. Ozone treatment of dry fibril aggreagates was carried out at room and slightly elevated temperatures. The following data was obtained.
These experiment confirm that conditions such as temperature and reaction time can affect the resulting ozone treatment on the carbon nanotubes. Reactions carried out at temperatures between 0° C. to 100° C. result in the efficient generation of abundant surface functional groups without the resultant destruction of fibril aggregate mass. Experiments carried out at 100° C. or greater for extended periods of time can lead to more substantial reactions between ozone with carbon to form CO and CO2 resulting in loss of fibril aggregate mass.
1 gram of nitric acid treated fibrils exhibiting a titer of 0.93 meq/g was subject to ozone treatment at room temperature for a period of five hours. The treatment followed the same procedure as described in Example 1. The resulting materials exhibited a weight gain of 10.6% and its titer was improved to 1.84 meq/g. This titer level is unexpected and surprising since it could not be achieved with the nitric acid oxidation without substantially damaging and disassembling the carbon nanotube structures.
An impure mixture of single wall carbon nanotubes and amorphous carbon was subjected to selective oxidation treatment with nitric acid to reduce soot and amorphous carbon. The resulting oxidized mixture as analyzed by electron micrograph was approximately 20-50% impure and exhibited upon titration an acid tier of 0.9 meq/g. Using the procedure described in Example 1, the oxidized mixture was subjected to ozone treatment at room temperature for approximately 5 hours. The ozone treated mixture exhibited upon titration an acid titer of 2.86 meq/g.
The ozone treated fibrils were further blended with water to form a slurry with solid content of 15˜25% and then subject to extrusion through a ⅛ inch die. Cylindrical extrudates of oxidized fibrils were obtained and dried at 180° C. for 12 hours. The extrudates appeared to be very strong similar to those generated with nitric acid oxidized fibrils. It is believed that the abundant surface functional groups may cross-link to self-assemble a rigid porous structure. As-made fibrils cannot form this type of structures without proper binding materials.
20 grams of as-made CC nanotubes were placed in a Waring blender. 60 grams gluing agent containing 10% polymer, e.g. polyacrylic acid (MW=15000), in aqueous solution, was added into the carbon nanotubes and the mixture was blended till uniform. The resulting material was then push through a ⅛″ tie via a Brabender extruder followed by drying and calcination at temperatures between 300 and 600° C. in argon. The resulted extrudates were then treated with ozone for 20 hours using the procedure described in Example 1. After ozone treatment, the extrudates exhibited an acid titer of 0.79˜0.93.
10 grams of as-made fibrils were placed in cyclohexane in a reaction flask. Ozone containing gas was then passed through the slurry with vigorous agitation. The reaction was allowed to proceed for 8-10 hours under ambient conditions. After reaction, fibrils was filtered, washed and dried. Suitable liquid media could, but not limited to be, water, saturated alcohol, saturated hydrocarbon or cyclic hydrocarbons, etc. The reaction can also be carried out at elevated temperatures and pressures up to supercritical conditions.
1/16 inch carbon nanotube extrudates were placed into rotating drum reactors. Ozone containing gas generated from the feed of pure oxygen gas into an ozone generator was introduced via a sparger into the rotating drum reactors at rates of 1 L/min and 2.5 L/min. Two samples were titrated from each reactor at selected times to compare the functionalization uniformity. The following results were obtained:
The terms and expressions which have been employed are used as terms of description and not of limitations, and there is no intention in the use of such terms or expressions of excluding any equivalents of the features shown and described as portions thereof, it being recognized that various modifications are possible within the scope of the invention.
This application claims priority to and the benefit of U.S. Provisional Application Ser. No. 60/720,806, filed Sep. 26, 2005, and of U.S. Provisional Application Ser. No. 60/621,132, filed Oct. 22, 2004, all of which are hereby incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60720806 | Sep 2005 | US | |
| 60621132 | Oct 2004 | US |
