WO2007103422A1

WO2007103422A1 - Mesoporous carbon fiber with a hollow interior or a convoluted surface

Info

Publication number: WO2007103422A1
Application number: PCT/US2007/005809
Authority: WO
Inventors: Rudyard Lyle Istvan; John M. Kennedy
Original assignee: Clemson University
Priority date: 2006-03-07
Filing date: 2007-03-06
Publication date: 2007-09-13

Abstract

Convoluted activated carbon fibers and methods for their preparation are described. The activated carbon materials are engineered to increase the proportion of activated porous material proximate to external surfaces and decrease the proportion of activated porous material internal and distance from these surfaces. The convoluted activated carbon fibers may be used in all manner of devices that contain high surface area carbon materials, including but not limited to various electrochemical devices (e.g., capacitors, batteries, fuel cells, and the like), hydrogen storage devices, filtration devices, catalytic substrates, and the like.

Description

MESOPOROUS CARBON FIBER WITH A HOLLOW INTERIOR OR A

CONVOLUTED SURFACE

BACKGROUND OF THE INVENTION

The present invention relates to fibrous activated carbons and to the shapes in which they are made. The fibrous activated carbons are engineered to maximize effective surface proximate to exterior electrolytes, other liquids, or gasses and may be used in all manner of devices that contain activated carbon materials, including but not limited to various electrochemical devices (e.g., capacitors, batteries, fuel cells, and the like), hydrogen storage devices, filtration devices, catalytic substrates, and the like.

In many emerging technologies, electric vehicles and hybrids thereof, there exists a pressing need for capacitors with both high energy and high power densities. Much research has been devoted to this area, but for many practical applications such as hybrid electric vehicles and fuel cell powered vehicles current technology is marginal or unacceptable in performance and too high in cost. This remains an area of very active research, such as that sponsored by the Department of Energy in their FreedomCar initiative. (See DOE Progress Report for Energy Storage Research and Development, funding year 2004 (Jan. 2005)).

Electric double layer capacitors (EDLCs or ultracapacitors) are one type of capacitor technology that has been studied for such applications. The primary remaining challenges according to DOE include improving the energy density, and lowering cost.

Electric double layer capacitor designs rely on very large electrode surface areas, which are usually made from "nanoscale rough" metal oxides or activated carbons coated on a current collector made of a good conductor such as aluminum or copper foil, to store charge by the physical separation of ions from a conducting electrolyte into a region known as the Helmholtz layer. This Helmholtz layer, which forms for a few Angstroms beyond the solid material surface, typically corresponds to the first two (unsolvated, partly solvated, and solvated) ions from the surface, and consists of the inner (ion with polarity opposite that of the surface) and outer (solvated counter-ion) Stern layers together comprising the Helmholtz double layer. There is no distinct physical dielectric in an EDLC. Nonetheless, capacitance is still based on a physical charge separation across an electric field. Because the electrodes on each side of the cell and separated by a porous membrane store identical but opposite ionic charges at their surfaces while the electrolyte solution in effect becomes the opposite plate of a conventional capacitor, this technology is called electric double layer capacitance (See U.S. 3,288,641). However, large commercial EDLCs (sometimes called ultracapacitors) are presently too expensive and insufficiently energy dense for many applications such as hybrid vehicles and are used instead in small sizes primarily in consumer electronics for fail-soft memory backup.

It is generally accepted that EDLC pore size should be at least about 1-2 nm for an aqueous electrolyte or about 2-3 nm for an organic electrolyte to accommodate the solvation spheres of the respective electrolyte ions in order for the pores to contribute their surface for Helmholtz layer capacitance (J. Electrochem. Soc. 148(8):A910-A914 (2001); Electrochem & Solid State Letters 8 (7):A357-A360 (2005)). Pores also should be open to the external surface for electrolyte exposure and wetting, rather than closed and internal. At the same time, the more total open pores there are just above this threshold size the better, as this maximally increases total surface area. Substantially larger pores are undesirable because they comparatively decrease total available surface. It has been shown that pores much above 13nm, although contributing capacitance, reduce surface (See Carbon 39:937-950 (2001); Eurocarbon Abstracts 841-842 (1998)). Conventional activated carbons used in such ELDC devices have many electrochemical Iy useless micropores (i.e., below 2 nm according to the IUPAC definition). The pore size must be approximately the sphere of solvation of electrolyte ions, or larger, for the Helmholtz layer to form. (See U.S. 6,491,789). For organic electrolytes, these pores should ideally be larger than 3 -4 nm. In the best highly activated electrochemical carbons reported in the literature, actual measured EDLC is less than 20% of theoretical due to suboptimal pore size distributions, with a large fraction (typically more than a third to half) being micropores that cannot contribute capacitance and a growing fraction of macropores above 50 nm (depending on degree of activation) that reduce overall surface area. See U.S. 6,737,445. A separate problem with highly activated carbons in electrochemical devices is their increased brittleness; they tend to form small irregular particles that contribute to higher electrode ESR due to the many poorly contacting grain boundaries, with experimentally determined conductivity as low as 7 S/cm.

Several alternative approaches to producing a carbon material suitable for operation with organic electrolytes at their desired higher operating voltages have been undertaken. These include physical activation using carbon dioxide or steam, chemical activation, carbon aerogels, various templating techniques, and applications of carbon nanotubes.

Kyotani, Carbon 38: 269-286 (2000) summarized available methods for obtaining mesoporous carbon. Lee et al., Chem. Commun. 2177-2178 (1999), described a mesoporous carbon film for use with electrochemical double-layer capacitors. Most commercial electrocarbons from suppliers such as Kuraray in Japan (BP20), Kansai Coke in Korea (MSP20), or MeadWestvaco in the United States, use conventional physical activation for cost reasons. One example of chemical activation is potassium hydroxide. (See Carbon 40(14):2616-2626 (2002) for KOH activation of a commercial mesopitch; J. Electrochem. Soc. 151(6):E199-E2105 (2004) for KOH activation of PVDC). However, these KOH activated carbons produce specific capacitances ranging from 30-35 F/g (two electrode cell basis) or 120-140 F/g (three electrode reference system basis). That is not appreciably different that conventional physically activated carbons that may have capacitance of 100 to 140 F/g (3 electrode reference basis) with BET surface areas ranging from about 1500 to 2000 square meters per gram. {Reports of Res. Lab. Asahi Glass Co LTD 54: 35 (2004) reporting on their experimental ultracapacitor development for Honda Motors). Honda itself in conjunction with Kuraray has announced commercial introduction of a KOH activated mesopitch with activation based on U.S. 5,877,935 using a precursor mesopitch based on -A-

U.S. 6,660,583. This material is reported to have 40 F/g in two electrode cells, equivalent to 160 F/g in a reference system. It is, however, more expensive than simple physical activation. It has a pore distribution with about one fourth of pores below 1.3 nm (See Proceedings of the 15th International Seminar on Double Layer Capacitors, Dec 5-7, 2005, p. 79).

A second approach has been various forms of carbon aerogel. However, the supercritical drying step-whether by carbon dioxide, isopropyl alcohol, or cryogenic extraction (freeze drying)- makes these carbons relatively expensive but with at best only modest performance improvements. (See J. Appl. Polym. Sci. 91 :3060-3067 (2004). They are also usually limited in surface area to between about 400 and 700 meters, although more of this surface is accessible to electrolyte. Depending on pore distribution, they can be subject to local depletion under charge caused by aperture restriction by the Helmholtz layer. Therefore, their commercial performance is not substantially different that conventional physically activated carbons (consider for example, MarkeTech International, Inc.'s. Carbon Nanofoam Electrode Grade 2 with 600 m²/g and 28-30 F/g in two electrode cells). Cooper has introduced an ultracapacitor based on aerogels formulated according to U.S. 5,626,977.

A third approach is to use some sort of a template or structure to form pores of suitable dimension and connection geometry. One method uses aluminosilicate nanoparticles of various types, for example as described in U.S. application 2004/0091415. These are presently even more expensive than aerogels because of the need to prepare the template and then at the end to remove it, usually by dissolving in hydrofluoric acid. Many of these carbons have demonstrated disappointing capacitance even in aqueous sulfuric acid, let alone in organic electrolytes with larger solvated ions. See Hyeon's summary overview of Korean experimental work in J. Mater. Chem. 14:476-486 (2004). One of the best experimental carbons according to this method used aluminosilicate nanospheres averaging 8nm as the template; the carbon achieved only 90 F/g in a three electrode reference system despite a 1510m² BET surface due to aperture restriction and local depletion (See Electrochimica Acta 50(14):2799-2805 (2005)). Another approach uses carbide particles (for example, Electrochem. and Solid State Letters 8(7):A357-A360 (2005)). Carbons made by one version of this carbide approach (described in PCT/EE2005/000007) only ranged from 115 to 122 F/g in a three electrode reference system (See Proceedings of the 15th International Seminar on Double Layer Capacitors Dec 5-7, 2005, pp. 249-260). Yet another approach uses surfactant nanomicelles. TDA carbons made according to U.S. 6,737,445 were reported at the 2002 National Science Foundation Proceedings with 140-170 F/g, but have proved difficult to scale to commercial quantities despite substantial federal funding support. Yet another approach uses liquid crystal materials in a carbon electrodeposition according to U.S. 6,503,382. These carbons, however, have the disadvantages of being thin films with rather large pores, so only limited surface areas for electrochemical purposes in capacitors and batteries.

Yet another alternative approach is to use some form of carbon nanotube (also known as fibril), either single wall or multiwall, either grown separately and applied as an entangled fibrous material or grown in situ in a vertically aligned fashion. Examples of electrodes made from separate fibrils include U.S. 6,491 ,789 and U.S. 6,934,144. Vertically aligned carbon nanotube ultracapacitors are being investigated among others by MIT with sponsorship from Ford Motor Company. Entangled CNT have two serious drawbacks. First, the material is very expensive, tens of dollars a gram (rather than kilogram). Second, the material has a Young's modulus of elasticity nearly equivalent to that of diamond at around 2000 (extremely stiff), and is therefore extremely difficult to compact to take full advantage of the surface presented by the very fine fibers. Not surprisingly, Frackowiak et al. reported that ELDC devices made using mesopores from multi- walled carbon nanotube "entanglement" had capacitance ranging widely from 4 to 135 F/g, highly dependent on multi-walled carbon nanotube density and post processing (densification) {Applied Physics Letters 77(15):2421-2423 (2000)). The best reported capacitances vary from about 120 F/g to about 187 F/g (See J. Mater. Chem. 15(5):548-550 (2005)). Vertically aligned CNT grown in situ using CVD in a vacuum overcome the density problem, but would be extremely expensive with present semiconductor like manufacturing technology, when the industry needs thousands of kilograms of material. Others have explored using carbonized electrospun fibers as carbon nanotubes equivalents in order to reduce cost, for example U.S. patent publication 2005/0025974 Al.

It is apparent from the forgoing discussion as well as from the many ongoing research efforts that high capacitance, low cost carbon materials are a large unmet need.

Physical and chemical activation of carbons are generally thought to be the lowest cost methods for creating high surface area carbons due to relative simplicity and least number of process steps. Such carbons are used in large quantities for water filtration, gas separation, food and drug purification, lithium ion batteries, and the like. Both physical and chemical carbon activation have been shown to create two kinds of surfaces. Traditionally, it has been thought that most surface enhancement comes from enlarging preexisting micropores caused by the disordered graphite crystallite graphene sheets (or equivalent) microstructure on subnanometer scales in pyrolyzed turbostratic non-graphitizing carbons (See U.S. 5,877,935). The actual microstructures of many carbons contain surprisingly little graphene and a lot of curved subunits because of the presence of SP3 bonds and pentagonal and heptagonal carbon rings as well as the conventional hexagonal units (for a current overview, see Harris, Critical Reviews in Solid State and Mat. ScL 30:235-253 (2005)). Therefore, they contain little in the way of microslit pores even when the precursor carbon is a highly ordered polymer such as a phenolic novoloid like Kynol (See Proceedings of the 8th polymers for Advanced Technology International Symposium in Budapest Sept. 11-14, 2005). This highly tortuous pore structure is widened by activation, and beyond some size will allow solvated ions to enter and use at least a portion of the surface for double layer capacitance (See J. Phys. Chem B 105(29):6880-6887 (2001)). For most filtration carbons, the small size of the molecules being filtered means the pore size distribution is not a significant issue, and what is therefore sought is maximum surface. There are however, larger molecules, frequently biological ones, where mesoporous rather than microporous surface is desired. Such filtration carbons have issues similar to those of electrocarbons, even though the solvated electrolyte ions of ultracapacitor and lithium ion batteries may be toward the large pore extreme of most filtration requirements.

The second kind of surface is additional exterior surface as nanoparticles of carbon are weathered away by convergence of micropores during physical activation. These nanoparticles tend to be around 100 nm or less in diameter, and tend to form aggregates that "decorate" the exterior surface of the larger particles (typically a few microns in diameter)(See DOE Project DE-FG-26_ 03NT4J796, June 2005). Similar nanoparticles have been observed with chemical activation (See J. Electrochem. Soc. 151(6):E199-E205 (2004)). The result is a substantial amount of exterior surface simply caused by roughness and nanoscale particles. This roughness can be quite substantial and may account for a few hundred square meters of surface; some of these carbon surfaces have been micrographed using STEM and may represent 30 to 50 fold increases over the unactivated carbon precursor (See Proceedings of the 8th polymers for Advanced Technology International Symposium in Budapest Sept. 11-14, 2005). It has been known for some years that "chemically roughened" gold and platinum electrodes with no interior pores can increase surface from by 30 (gold) to 1000 (platinized platinum) fold (See J. Electroanal. Chem. 367:59-70 (1994)).

It has also been shown that in at least some carbons, the exterior surface can contribute several times (four fold measured) the capacitance per square meter of surface of the interior pores (See Electrochimica Acta 41 (10): 1633- 1630 (1996)). This is so for two fundamental reasons, although neither was explicitly discussed in the paper that reported the finding. First is the probability of access to internal pores. Activated carbon pores always exist in some size distribution, i although the peak of the distribution may shift to larger pores and the shape may change with activation conditions (See, for example, Electrochimica Acta 41(10):1633-1630 (1996); J. Electrochem. Soc. 149(1 l):A1473-1480 (2002); J. Electrochem. Soc. 151(6):E199-E205 (2004)). Normally, a substantial proportion of the distribution remains micropores under 2 nm. Since the size of solvated electrolyte ions range from about 5.8 to 10.0 to 11.9 to 16.3 to about 19.6 angstroms in diameter depending on salt and solvent (see J. Electrochem. Soc. 148(8):A910-914 (2001); Carbon 40:2623-2626 (2002)), these ions may be blocked or sieved out (molecular sieving) by small pores and thereby prevented from using interior pore surface for capacitance. Ionic sieving where the smaller of the two solvated electrolyte ions is not sieved and the larger is has been well demonstrated both in aqueous (SeeJ. Phys. Chem. B 105(29):6880-6887 (2001) and in organic electrolytes (See Carbon 43:1303-1310 (2005)). Any pore below the critical size will prevent access to all pore surface interior to that point accessible through that point; therefore the probability of access declines with depth in a way stochastically dependent on the pore size distribution. For example, in a random distribution of pores half of which are to small to permit access, the probability of access to a second interior level of randomly distributed but connected pores is 1/4, and in general declines as (1/2) ^N with N levels of sequential pore to the interior, as a simple consequence of independent multiplicative probabilities.

It is known that the larger of the two ions in the electrolyte acts as a kinetic control on the system, as the net charges on the opposite sides of a device are roughly equal (See J. Electrochem. Soc. 148(8):A910-914 (2001)). Therefore, the probabilistic limitation on access to interior pores is a significant constraint in organic electrolytes with large solvated ions of disparate sizes, since the larger of the two determines aggregate pore accessibility and therefore the electrochemical performance of the device. Organic electrolytes as desirably used in lithium ion batteries and high energy density ultracapacitors have larger solvated ion sizes than aqueous electrolytes, and so are more constrained by this phenomenon. Identical activated carbons therefore exhibit 2 to 3 times more capacitance in aqueous electrolytes than in organics.

Activated carbon pore distributions are randomly distributed based on experimental evidence. Therefore computing sequential probability of access according to the standard laws of probability is mathematically and physically correct. One method of demonstrating pore randomness uses filtered TEM images of the turbostratic carbons in which the pores exist as defects (See Critical Reviews in Solid State and Mat. ScU 30:235-253 (2005)). Another uses fast Fourier transforms of TEM pixels of a carbon cross section (See J. Electrochem. Soc. 148(8):A910-A914 (2001)). Another method uses STM (See Proceedings of 8th Polymers for Advanced Technology International Symposium, Budapest, Sept 2005; Carbon 33(3):344-5 (1995)). Experimental results have been correlated with recent simulation models based on first principals of turbostratic carbon formation (See Langmuir 16(13):5761-5773 (2000)); Critical Reviews in Solid State and Mat. Sci. 30:235-253 (2005).

The pore size distributions of activated carbons (with either physical or chemical activation) ordinarily contain a substantial proportion (up to half or more) of pores smaller than a size capable of accepting solvated ions, particularly with organic electrolyte systems. For typical pore distributions in physically activated carbons see Electrochimica Acta 41(10):1633-1630 (1996); in carbonized PVDC see MoI. Cryst Liq Cryst. 3S6:61-72 (2002); in carbonized PVDC copolymers see J. Electrochem. Soc. 149(11 ): Al 473- 1480 (2002); in KOH activated PVDC see J. Electrochem Soc. 151(6):E199-E205 (2004); for KOH activated mesopitch see Carbon 40(14):2613-2626 (2002), in double activated carbon fiber see Korean J. Chem. Eng. 17(2):237-240 (2000), for comparisons of commercial electrocarbons such as Kuraray BP20 (see also U.S. 6,642,119) and Kansai Coke MSP20 see Korean Electrochem. Soc. 4(3): 113 (2001).

The result of any such pore size distribution is that a substantial proportion of the interior is not accessible, since the probability of a continuous suitably large pathway declines as a direct mathematically computable function of the pore size distribution. The function is approximately equal to the probability of a pore being too small (that fraction of the area under the total pore size distribution raised to the Nth power, where N is the average pore distance toward the interior of the carbon particle from an external surface). The exterior carbon surfaces are therefore disproportionately important, as are those interior regions proximate to the exterior and therefore with a higher probability of access.

Surprisingly, no purposeful consideration of certain material geometries mitigating this inevitable stochastic effect has been heretofore made in commercial or experimental electrocarbons of conventional (micron scale) particle dimensions. Carbon nanotubes have a central hollow that may or may not be large enough to accept solvated ions; the principal scientific motivation for nanotubes is simply more exterior surface from finer fibers since even multi-wall nanotubes by definition don't have a pore structure internal to the carbon itself. Finely electrospun fibers on the order of tens or hundreds of nanometers have the same scientific motivation, increased total exterior surface as a pure function of fineness. It has been surprisingly been discovered that a simple alternative approach, spinning convoluted cross sectional shapes in conventional diameter fibrous material, can provide a several fold increase in activated surface proximate to the exterior of a material and therefore having high probability of access. Optionally, such precursors to such activated carbon fibers may also be spun hollow, further increasing effective exterior surface since the foregone central interior volume of a solid fiber is least likely to have probability of access and is largely unusable un the applications herein envisioned. Its interior pore surface loss therefore does not matter, while the hollow center offers more "exterior" for the remaining carbon pores to have increased probability of access since N is smaller.

Such convoluted and/or hollow activated carbon fibers are contrary to the principal purpose for which carbon fibers are presently manufactured, which is as a structural material valued for its tensile strength. Convoluted and/or hollow carbon fibers would have diminished carbon volume and tensile strength per unit length. They would also require more resin in the resulting composites, and possibly would introduce voids. These factors are undesirable in structural composites. The principal uses of convoluted carbon fibers are therefore as herein envisioned, activated as electrocarbons, specialized filtration carbons, and the like.

BRIEF SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a carbon fiber having a hollow interior and/or a convoluted surface and having mesopores that range in size from about 2 nm to about 20 nm, preferably 2 nm to 10 nm. A second aspect of the present invention is a material comprising a carbon fiber having a hollow interior and/or a convoluted surface and having mesopores that range in size from about 2nm to about 20 nm. The material can be woven or non-woven.

A third aspect of the present invention is a capacitor comprising at least some of the inventive carbon material.

A fourth aspect of the present invention is an electrode comprising a current collector; and the inventive carbon material in electrical contact with the current collector.

Another aspect of the present invention is a method of preparing a carbon fiber having a hollow interior and/or convoluted exterior and having mesopores that range in size from about 2 nm to about 20 nm.

DETAILED DESCRIPTION OF THE INVENTION

Activated carbon fibers maximizing external surface and probability of proximate pore access have been discovered and are described herein. The materials have very high external surface area proportions especially well- suited for use in ultracapacitors, and may be prepared by methods including solvent spinning, melt spinning, or electrospinning. The preparation methods described herein provide control over the relative proportion of proximate external surface compared to the interior carbon of such fibers. Activated carbon fibers according to this invention have characteristics tailor-made for specific applications including, but not limited to, electric double layer capacitors and lithium ion batteries.

Most activated carbon pore size distributions contain a proportion of pores sufficiently small to block or sieve out at least some solvated ions. Therefore, the probability of accessing pores large enough to accept such solvated ions declines as a stochastic function of the shape of the pore size distribution as a pore exists further away from the exterior of the material and further into the interior. Increasing the exterior surface therefore increases the proportion of the total pores with better probability of access. There is simply more outside and less inside. Surprisingly, carbon fibers, more particularly activated carbon fibers such as those described by Carbon 33(8):1085-1090 (1995), and especially electrocarbon fibers such as those described by Electrochimica Acta 41 (10):1633-1630 (1996), Korean J. Chem. Eng. 17(2):237-240 (2000), or by Carbon 43 1303-1310 (2005), or as offered commercially by firms such as American Kynol are conventional round or elliptical (roughly circular cross section) solid fibers.

It is well known in the art that polymeric fibers can be spun with a convoluted external shape that increases the extraluminal surface of the fiber. See for example U.S. publication T105,401. It is also well known that fibers can be spun to be hollow, typically through a c shaped spinneret such that the c cross section "heals" into a tubular form before the fiber finishes forming or by using water or some other immiscible liquid to form the hollow core during the spinning process. (See for example U.S. 6,991 ,727). It is also well known that convoluted hollow fibers can be spun (See, for example, U.S. 6,805,730). Such polymeric fibers are conventionally used in sophisticated biomedical filtration equipment such as for blood separation and kidney dialysis and are spun in comparatively large cross sections, for example to accommodate cells. However, such fibers are ordinarily not spun in fine diameters or made of materials suitable for carbonizing and activating to create high exterior surface mesoporous carbons. Nor are convoluted carbonized and activated fibers used in electrochemical applications such as ultracapacitors and lithium ion batteries.

A circular cross section is the worst possible geometric shape mathematically for creating maximum external fiber surface, or maximum interior material proximate to the external surface. In three dimensions, spheres maximize volume and minimize surface; that is why bubbles, droplets, and planets are approximately spheres. The circle in two dimensions is equivalent to the sphere in three. Therefore the swept volume of a circular cylinder (a fiber of some length) minimizes surface and maximizes volume for the geometry. Any other shape of equivalent cross sectional area will have more exterior circumference and therefore more proximate exterior material volume. This is easy to visualize and trivial to prove. For example, a circle with radius 1 has an area of π and a circumference of 2π (pi times a diameter of 2 radii of 1 ). Any triangle has an area of (1/2) base times height. An equilateral triangle (all three sides of equal length, equal internal comer angles of 60 degrees) has an area of (1/4)(^/3) a² where a is the length of any side. For an area equivalent to π, a therefore equals V((4π)/V3) or 1.128. The exterior "circumference" of the triangle is 3a or about 3.385, 7.7% greater than that of the equivalent area circle. A more convoluted shape than a triangle, such as an asterisk, will have substantially more external circumference. Depending on the number, depth, and shape of the convolutions, an exterior "circumference" several times that of an equivalent area circle may be obtained. Moreover higher external circumference can be obtained with convoluted shapes inscribed into the diameter of a circle of equivalent longest cross section. This is readily demonstrated by construction. Upon activation, material proximate to these surfaces has the same probability of access of before, but there is proportionately more of it relative to the total. Therefore, proportionately more of the total pore surface of the material is probabilistically accessible given any pore size distribution. Yet the external dimensions of the material given by cross section will not be different, and therefore other material considerations such as the aspect ratio of the fiber for optimally dense packing of electrode material remain substantially unaffected.

Surprisingly, eliminating the central region volume and the associated pore surface of a fiber will further increase the total accessible and therefore effective pore surface for most activated carbon pore size distributions for the normal size range of solvated ions. For example, it is possible to spin a 7 micron diameter fiber with a 3 micron hollow core. The wall thickness of the hollow fiber is therefore two microns, equivalent to the smallest particle diameters in conventional electrocarbon powder polydispersions. The outside circumference of such a round fiber is 7π. The interior circumference is 3π, the total is therefore 10π or 43% more. That will directly increases the total effective surface and the accessible pore fraction by more than 43%, since the probability of access to interior material from the outside or from the hollow inside does not change with a random activated pore size distribution. With pores on the order of nanometers and material thicknesses on the order of microns, or three orders of magnitude larger, the probability of unrestricted access to deep interior pores is still small. The actual effective surface will increase by more than this amount through the erosion and spalling of nanoparticles from the interior hollow surface during activation, increasing its roughness. Such roughness only exists when the nanoparticles have been cleaved off an exterior surface by consolidation of enlarging micropores. It can easily increase such a surface by more than 30 times as described above for metals with no internal pore structure. The interior hollow's 3π "exterior" is capable of this surface enhancing roughness, further increasing probability of access to internal pores proximate to the increased roughened surface.

Further supporting the utility of the extra surface caused by hollowness, such fibers are optimally processed for electrode materials into relatively short aspect ratios, for example from 2 to 5, compared to the shortest commercially available aspect ratios at 15 or higher (150 microns is the typical shortest milled fiber length commercially available, since the material is used as a strengthening reinforcement in plastics and shorter lengths defeat the purpose). With such small aspect ratios, wetting of the full internal hollow's surface is not a problem, nor is ion transport to the end of the fiber (at most 1/2 the length of the fiber). The interior surface made accessible in this fashion will not substantially affect the RC or other characteristics of the device, while increasing its effective surface.

Surprisingly, utilizing convoluted and/or hollow fiber shapes increasing the effective available surface of activated carbon fibers for enhanced mesoporous access are not heretofore known.

Throughout this description and in the appended claims, the following definitions are to be understood:

The term "activated" as used in reference to a carbon describes either physical activation by means such as carbon dioxide or steam at elevated temperatures, or chemical activation by means such as potassium hydroxide at elevated temperatures, or by other or by combined means done to enlarge and enhance preexisting microporosity in the carbon material

The phrase "pore size distribution" as used in reference to a carbon refers to a distribution measured by number of pores, pore volume, or pore surface by size of pore varying by width, diameter, radius, or other cross section.

The phrase "fiber" used in reference to polymers and carbons refers to filamentous material of fine diameter, such as diameters less than about 20 microns, and preferably less than about 10 microns, such as the type that may be obtained using conventional polymeric spinning processes or unconventional spinning processes such as electrospinning to produce carbonizable fiber.

In presently preferred embodiments, the precursor polymeric material to be spun, carbonized, and activated may come from any source of sufficient purity and precursor chemistry to be used as an activatible electrocarbon, including various isotropic and anisotropic pitches, specialized pitch precursors such as described by U. S.6, 660, 583, or polymeric materials such as polyacrylonitrile (PAN), phenolic resins, novoloid phenolics, polyvinylidene chloride (PVDC), Kevlar, or cellulose acetate. Although a specialized carbon precursor material is conventionally desirable, the present invention is not limited thereto but comprises any precursor capable of being spun, carbonized, and activated.

Although catalytic mesoparticle activation is preferable because of the mesopore geometries that are created, the present invention is not limited thereto but comprises activation by any means.

In some preferred embodiments, the activated fibers prepared according to the present invention are further processed by milling or by equivalent means to aspect ratios of less than 15, where by aspect ratio is meant the ratio of the length of a fiber fragment to its average cross sectional diameter. In some embodiments the aspect ratio is less than 5, in other embodiments the aspect ratio is less than 3, and in other embodiments the aspect ratio averages 2. In some embodiments, the fibrous materials made from such fiber fragments contain a plurality of substantially similar aspect ratios. In some embodiments, a bimodal distribution comprising a plurality of a first, substantially similar aspect ratio is combined with a minority of a second, higher aspect ratio. In some embodiments, a plurality of a first aspect ratio in accordance with this invention is combined with a minority of a second longer aspect ratio of fiber fragments derived by any means.

This process can provide a material according to the present invention compatible with conventional particulate carbon electrode coating processes as described in U.S. 6,627,252 and 6,631 ,074, the entire contents of both of which are incorporated herein by reference, except that in the event of any inconsistent disclosure or definition from the present application, the disclosure or definition herein shall be deemed to prevail. Optionally the activated material may be further milled or otherwise processed to a fiber fragment size distribution best suited to the needs of a particular electrode manufacturing process and device requirement.

An electrode embodying features of the present invention, suitable for use in a capacitor or other electrochemical devices, includes a current collector foil, covered with a substantially activated high surface fibrous carbon material. EDLC electrodes are typically made of activated carbon - bonded directly or indirectly onto a metal foil current collector, although metal oxides can be used or admixed. In accordance with the present invention, activated carbon fiber materials prepared by the methods described herein may be applied to current collectors together with additional metal oxides or the like for enhanced hybrid characteristics including enhanced pseudocapacitance.

A capacitor embodying features of the present invention includes at least one electrode of a type described herein. In some embodiments, the capacitor further comprises an electrolyte, which in some embodiments is aqueous, in other embodiments is organic. In some embodiments, the . capacitor exhibits electric double layer capacitance. In some embodiments, particularly when residual catalytic metal oxide is present on or in connection with the surface of the activated carbon fibrous material, the capacitor further exhibits pseudocapacitance.

Conventional carbon EDLCs with organic electrolytes use either propylene carbonate or acetonitrile organic solvents and a standard fluoroborate salt. Some carbon and most commercial metal oxide EDLCs use aqueous electrolytes based on sulfuric acid (H₂SO₄) or potassium hydroxide (KOH). Any of these electrolytes or the like may be used in accordance with the present invention.

Activated high external surface carbon fiber materials, or their respective fragments, embodying features of the present invention may be incorporated into all manner of devices that incorporate conventional activated carbon materials or that could advantageously be modified to incorporate activated mesoporous carbon materials. Representative devices include but are not limited to all manner of electrochemical devices (e.g., capacitors; batteries, including but not limited to one side of a nickel hydride battery cell and/or both sides of a lithium ion battery cells; fuel cells, and the like). Such devices may be used without restriction in all manner of applications, including but not limited to those that potentially could benefit from high energy and high power density capacitors or the like. By way of illustration, devices containing activated carbons embodying features of the present invention may be included in all manner of vehicles (e.g., as elements in capacitors and/or batteries, or electrical combinations thereof, which may optionally be coupled to one or more additional components including but not limited to capacitors, batteries, fuel cells or the like); electronic devices (e.g., computers, mobile phones, personal digital assistants, electronic games, and the like); any device for which a combination of battery and capacitor features is desirable ( combining the energy density of batteries with the power densities of capacitors) including an uninterrupted power supply (UPS) in order to accommodate power surges and power failure ride-throughs, cordless drills, and the like; any device that may advantageously contain a conventional batcap (i.e., a system of devices that provide a capacitor for handling power density and a battery for providing energy density, wired in parallel); and the like. In some embodiments, a device embodying features of the present invention comprises a capacitor used in a vehicle, including but not limited to an electric vehicle and hybrids thereof. Representative vehicles for use in accordance with the present invention include but are not limited to automobiles, motorcycles, scooters, boats, airplanes, helicopters, blimps, space shuttles, human transporters such as that sold under the tradename SEGWAY by Segway LLC (Manchester, NH), and the like.

The individual processing acts used in the methods embodying features of the present invention — spinning a high external surface convoluted and/or hollow fiber, carbonization, activation, and milling — are well understood in the art and have been thoroughly described in the references cited herein. Each of the patents, patent publications, and non-patent literature references cited is incorporated herein by reference in its entirety, except that in the event of any inconsistent disclosure or definition from the present application, the disclosure or definition herein shall be deemed to prevail.

For example, U.S. 6,746,230 describes a method for forming hollow fibers with varying widths. U.S. 6,805,730 describes methods for forming porous hollow fibers with at least one convoluted surface.

The techniques of carbonization and activation described above may be implemented using any of the well-known techniques described in the literature. By way of example, various processes that may be used in accordance with the present invention include but are not limited to those described in U.S. 6,737,445 (Bell et a/.); 5,990,041 (Chung et a/.); 6,024,899 (Peng et a/.); 6,248,691 (Gadkaree et a/.); 6,228,803 (Gadkaree et a/.); 6,205,016 (Niu); 6,491,789 (Niu); 5,488,023 (Gadkaree et a/.); U.S. Publication Nos. 2004/0047798 A1 (Oh et a/.); 2004/0091415 A1 (Yu era/.); 2004/0024074 A1 (Tennison et al.). Additional description is provided in Chemical Communications 2177-2178 (1999); and Journal of Power Sources 134(2): 324-330 (2004).

By way of illustration of the utility of the invention described herein, it is known that the total capacitance of an ELDC is a direct linear function of suitable available surface area, defined as the total area of surface features greater than at least one, and for full coverage twice the sphere of solvation, or approximately 2-3 nm for organic electrolytes. The governing equation is: C/A = e/(4^*π^*d) (eq 1) where C is capacitance, A is usable surface area, e is the relative dielectric constant of the electrolyte, and d is the distance from the surface to the center of the ion (Helmholtz) layer in the electrolyte. For any given electrolyte solvent and salt, e and d are fixed, so the right side of the equation is some constant k. Substituting and rearranging, C = kA (eq 2)

Thus, doubling usable surface area effectively doubles capacitance.

Korean experimenters achieved the equivalent of 632 F/g (three electrode reference) with Espun PAN fibers averaging 200-400nm diameter. They achieved a BET surface of only 830 square meters but with 62% mesopores (and with high probability of access given the comparatively small fiber diameter and limited material interior, the relatively greater fiber exterior proportion, and smaller ion sizes of the KOH aqueous electrolyte used){Applied Physics Letters 83(6):1216-1218 (2003)). The 76μF/cm² that was measured is about the theoretical maximum possible with two spheres of solvation in their potassium hydroxide electrolyte. Given the plane packing limit of circles or spheres for the outer Helmholtz layer equal to (1/6)τW3 or 0.968996821... and the solvated potassium ion dimension (with two spheres of solvation) of about 10 angstroms, the alternative international definition of the coulomb as 6.24125...E+18 elementary charges computes a capacitance (ignoring any contribution of the exponential decline in the diffuse region of the Debye distance beyond the outer Stern or Helmholtz plane) of >74μF/cm². Therefore approaching the theoretical maximum is experimentally proven possible with a surface that is mostly external, and with pores with higher probability of access to external electrolyte that will not cause ionic sieving or local depletion under charge.

A first example of the novelty of the present invention is a recent paper on chemically activated (by potassium hydroxide) carbon microbeads specifically for improved ultracapacitors. This 2004 paper by Shen et a of the Institute for Carbon Fibers at Beijing University appears in English on the Chinese government website www.portalenergy.com. The researchers prepared carbon microbeads of similar diameter by three different methods. One method they labeled onion because of the concentric shells of carbon visualized in the published SEM micrographs of fractured microbead cross sections. One they labeled graphene because of the conventional activated carbon appearance. One they labeled as wrinkled/corrugated because of the exterior surface appearance, which obviously had the highest external surface. The convoluted surface of these wrinkled beads was an incidental artifice of the emulsion method used to synthesize them. All three activated carbons had substantially similar BET surface of about 2500 square meters. All three had substantially the same proportion of micropores below 2nm, ranging from a low of 62.4% to 64.1%. All three pore distributions were bimodal, with peaks at about O.δnm and 2.0 to 2.5nm, and with a pore size range from 0.6nm to about 6nm. Surprisingly, the higher external surface wrinkled microbeads of method three had 387 F/g in KOH aqueous electrolyte compared to 156 F/g for method 1 and 124 F/g for method two. In Carbon 43:1303-1310 (2005), the authors analyzed the surprisingly near constant capacitance of three differently activated Kynol carbon fiber cloths ranging from a nominal 1500 square meters per gram to 2600 square meters per gram. The paper corrected for any error from BET surface estimation methodology by using DFT theory to calculate surface, and computed the pore size distributions that were all below 2nm. The probability of access to the interior for organic electrolyte ions larger than 1.5 nm is relatively small in these distributions, less than 1/4 even for pores immediately adjacent to the exterior surface, but that was not considered at all. The paper rejected the idea posited by Shi (Electrochimica Acta 41(10):1633-1630 (1996)) that external surface may be disproportionately important. Since the fibers were identical except for degree of activation and therefore degree of internal porosity, the researchers concluded that the constant capacitance despite varying total surface had to be a function of carbon space charge limitations (the density of available Fermi states for charge transport) in the carbon's interior as the carbon between internal pores became thinner with higher activation. This is appears wrong, since other carbons corrected for BET error by using DFT have not shown such limitations, including those just introduced by Honda with activation based on U.S. 5,877,935 using a precursor mesopitch based on U.S. 6,660,583.

The theoretical maximum capacitance computation equivalent to that of paragraph 51 for the most common electrolyte salt in acetonitrile solvent (as used by these investigators) is >25μF/cm². Only about 375 square meters of fully accessible surface is required to produce this capacitance in the Kynol activated carbon fibers. Prof. Economy (an inventor of Kynol) has recently shown that activated Kynols have a very rough exterior extending into the material for a depth of about 10 nanometers (See Proceedings of the tf^h Polymers for Advanced Technology International Symposium in Budapest 11- 14 Sept. 2005). A typical unactivated external surface of a few square meters per gram can be multiplied more than 30 fold if it just reaches the surface roughness of chemically roughened gold. That by itself produces over a hundred square meters of surface. Moreover, all the pores proximate to this enhanced exterior surface have an increased probability of access, even though the same is not true for pores further to the interior. Therefore, the most likely explanation for the similar capacitance of the Kynols in this new study is that they all had similar roughened exteriors and pores proximate to the exterior that produced substantially all of the observed capacitance. The greatly differing internal pore surface did not change the measured capacitance because almost none of it was accessible in any of the samples. Although these researchers may not have had access to the external surface STM images of activated Kynols, this very recent work shows the power of conventional wisdom that most capacitance must arise from the high interior surface area of internal pores. Conventional wisdom fails to consider probability of access, and therefore does not consider how maximizing external surface morphology will increase that probability. The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.

Claims

1. A carbon fiber having a hollow interior and/or a convoluted surface and having mesopores that range in size from about 2 nm to about 20 nm.

2. The carbon fiber of claim 1 , wherein the majority of mesopores range in size from about 2 to about 10 nm.

3. The carbon fiber of claim 1 , wherein the maximum cross section of the fiber is about 10 microns or less.

4. The carbon fiber of any of the preceding claims, wherein the fiber has a hollow interior that comprises at least about 10 percent of the area of the fiber.

5. The carbon fiber of any of the preceding claims, wherein the carbon fiber has a porosity volume comprised of greater than about 35% mesopores.

6. The carbon fiber of claim 5, wherein the carbon fiber has a porosity volume comprised of greater than about 50% mesopores.

7. The carbon fiber of any of the preceding claims, which has both a hollow interior and at least one convoluted surface.

8. A material comprising a carbon fiber having a hollow interior and/or a convoluted surface and having mesopores that range in size from about 2nm to about 20 nm.

9. The material of claim 8, wherein the carbon fiber has both a hollow interior and at least one convoluted surface.

10. The material of claim 8 or 9, which is a non-woven material comprising randomly packed fragments of the carbon fiber.

11. The material of claim 8, 9 or 10, further comprising a binder.

12. The material of claim 8, which is woven.

13. A capacitor comprising at least some. of the carbon material of any one of claims 8-12, or fragments thereof.

14. An electrode comprising:

a current collector; and ^'

the material of any one of claims 8-12 in electrical contact with the current collector.

15. A method of preparing a carbon fiber having a hollow interior and/or convoluted exterior and having mesopores that range in size from about 2 nm to about 20 ran, comprising the steps of:

providing at least one fiber which is either carbon or a carbon precursor and which has a hollow interior and/or convoluted surface;

^■ coating the fiber with organometallic nanoparticles;

if the fiber is a carbon precursor, then carbonizing the fiber;

catalytically activating the carbon fiber to form a mesoporous carbon fiber with mesopores that range in size from about 2nm to about 20 nm.

16. The method of claim 15, wherein the organometallic nanoparticles are suspended in solvent, and wherein the method further comprises the step of evaporating the solvent prior to the catalytically activating step.

17. The method of claim 15, further comprising milling the mesoporous carbon fibers.

18. The method of claim 15 or 17, further comprising the step of forming a layer by depositing a slurry or solution of a plurality of mesoporous carbon fiber fragments and a binder on a surface and evaporating the solvent.

19. The method of claim 17, further comprising the step of compacting the layer.

20. A method of preparing a mesoporous carbon fiber comprising the steps of:

providing at least one fiber which is either carbon or a carbon precursor and which has a hollow interior and/or a convoluted surface;

coating the fiber with precursor to a organometallic nanoparticle; and

if the fiber is a carbon precursor, then carbonizing the fiber to form a carbon fiber coated with organometallic nanoparticles and then catalytically activating the carbon fiber to form mesopores that range in size from about 2nm to about 20 nm; or

if the fiber is a carbon fiber, then activating the fiber to concomitantly form an organometallic nanoparticle which in turn forms a mesoporous carbon fiber with mesopores that range in size from about 2 nm to about 20 nm.