Non-woven fibrous materials and electrodes therefrom.

(54) Title: FIBROUS MATERIALS NON-WOVEN AND ELECTRODES FROM THESE. (54) Tltle: NON-WOVEN FIBROUS MATERIALS AND ELECTRODES THEREFROM.

(57) Summary

Fibrous materials composed of activated carbon fibers and methods for their preparation are described; Electrodes comprising the fibrous materials are also described.

(57) Abstract

Fibrous materials composed of activated carbon fibers and methods for their preparation are described. Electrodes comprising the fibrous materials are also disclosed.

FIBROUS NON-WOVEN MATERIALS AND ELECTRODES FROM

THESE

TECHNICAL FIELD

The present invention relates to fibrous materials composed of activated carbon fibers and to methods for their preparation. Activated carbon fibers can be used in all manner of devices containing active carbon materials, including without restriction various electrochemical devices (eg capacitors, batteries, fuel cells and the like), hydrogen storage devices, filtration devices, substrates catalytic and the like.

Introduction

Electric double layer capacitor designs are based on very large electrode surface areas, which are usually made of nanoscale roughness metal oxides or activated carbons coated in a current collector made of a good conductor such as aluminum foil or copper, to store charge by physically separating ions from a conductive electrolyte salt in a region known as the Helmholtz layer. This Helmhotz layer, which is formed with a few

Angstroms beyond the electrode surface, typically correspond to the first two or three surface molecules. There is no distinct physical dielectric in an EDLC, which is rather provided by the electromagnetically determined Helmholtz layer. However, capacitance is still based on a separation of physical charge through an electric field. Since the electrodes on each side of the cell store identical but opposite Ionic charges on their surfaces as the electrolyte between them (but beyond the Helmholtz layer) is depleted and, in effect, becomes the opposite plate of a conventional capacitor This technology is known as electrical double layer capacitance. The electrodes are physically separated by a porous thin film separator similar to electrolytic capacitors or lithium ion batteries. The present EDLCs have frequency response constants (response curve or RC) on the scale of milliseconds to seconds. However, commercial EDLCs (sometimes called ultracapacitors) today are very expensive and insufficiently energy-dense for applications such as hybrid vehicles, and are used primarily in consumer electronics for partial functionality memory backup.

It is generally accepted that the carbon surface pore size of EDLC should be at least 1-2nm for an aqueous electrolyte or about 2-3nm for an organic electrolyte to house the soldering spheres of the corresponding electrolyte Ions for that the pores contribute to the surface available for the Helmholtz layer capacitance. Pores should also be open to the 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 the total surface area. Substantially larger pores are undesirable because they comparatively decrease the total available area. Research by others has shown that capacitance improves as the average pore size increases from 4 to about 20nm.

Conventional activated carbons and used in such ELDC devices have many electrochemically useless micropores (i.e. below 2nm according to the IUPAC definition). The pore size must be approximately the electrolyte ion soldering sphere or larger for the Helmholtz layer to form. For organic electrolytes, these pores should ideally be greater than 3 to 4 nm. In the best highly activated electrochemical carbons reported in the literature, the actual measured EDLC is less than 20% of theory due to sub-optimal pore size distributions, with a large fraction (typically more than one-third to one-half) being Micropores that cannot contribute capacitance and an increasing factor of macropores (depending on the degree of activation) that reduce the overall surface area. In contrast, certain tempered carbons with optimal pore sizes and shapes governed by the precursor template material have shown theoretical values ​​approaching capacitance except for losses introduced by the formation of electrode materials made from the carbon particles.

Performance can be optimized by increasing the usable carbon footprint enough that both more capacitance and less cost (from less material required) are achieved. In the main, there are two ways that the capacitance can be increased. The primary form is an improved effective carbon surface. The IUPAC definition of nanotechnology is potentially useful for the conventional wisdom of organic electrolytes about characteristic dimensions: micropores are <2nm, mesopores are> 2 and <50nm, and macropores are> 50nm. The conventional goal is therefore generally to maximize the mesoporous surface.

The secondary form is the formation of electrode materials that take full advantage of the existing carbon surface. For reasons of cost and density, activated carbon powder is commonly compacted onto the current collector sheet at some depth, typically 50 to 150 microns. To maximize the surface, the dust particles need to be compacted randomly as compact as possible. Conventionally, this is done with irregular shapes ground into a scale particle size distribution of various increments in diameter (5 to 20 microns native to Kuraray PB20, or 3 to 30 microns with d₅₀8 microns according to Maxwell patent 6643119) so that smaller particles fill gaps between larger particles.

Technically these are polydisperse random compactions. The compaction density can be adjusted in some way by the shape of the size distribution. Fine particles reduce vacuum / volume porosity. Such material gaps are commonly thought of as three orders of magnitude greater than the macropores of the IUPAC definition, although technically within it. To eliminate terminological confusion in this discussion, void scale material gaps are called material pores (material porosity) to distinguish them from nanoscale surface pores on or within carbon particles.

It is useful in this context to introduce a concept, intrinsic capacitance, and the term, loss of compaction. Intrinsic capacitance is the ideal capacitance of the total effective carbon surface when in full double layer. The closest measurement to this idea is "carbon Helmholtz capacity" or C_H in pF / cm². Kinoshita compiled many CH measurements reported in Carbon Electrochemical and Physical Properties (1988); More recent documents place the scale on organic electrolytes between 3 and 20. Conventional wisdom is that the orientation of edges exposed to graphite crystallites plays a role in explaining these differences. Most (if not all) of these C measurements_H they are based on three reference capacitance electrode measurements and a BET surface estimate and therefore include both loss of compaction and any surface measurement errors introduced by the BET methodology. Surprisingly, since many activated carbon surfaces consist of micro pores that cannot contribute to substantial organic electrolyte capacitance, some of the observed experimental variations in different carbons have to come from the material porosity of the electro formed material.

Compaction loss is the difference (in F / g, F / cc or percentage) between the intrinsic capacitance of a carbon and the traditional specific capacitance of an otherwise formed electrode used as an industry parameter. Industry experts estimate compaction loss on a scale from a low of around 30% to over 80%. The actual figure will also vary with the thickness of the electrode for any given material.

Compaction losses originate from at least five separate phenomena. First, random compacting of different sized particles results in highly variable material voids. Such gaps are at best long and tortuous and at worst completely cut out of the electrolyte by random constraints (unmoistened surface). Any restrictions less than about 6nm - easily arising from the conjunction of irregularly formed interlocking particles ranging from a few tenths of a nanometer to a few microns in diameter - can show results of full compaction and are therefore blocked by soldered electrolyte ions. adjacent once a load is placed on the device. An additional mass transport or diffusion of electrolyte is then not possible. Recent research has surprisingly shown that a substantial proportion of a typical activated carbon is actually agglomerations of finer particles produced by activation. A study shows that they average less than 1400 nm, but to group on (due to Van der Waal forces) or to decorate ”larger particles the size of microns. The result is that such a restricted region is locally depleted of ions, since there is no possibility of further mass transport to the region. The surface of the region is subutllized. Second, compacting smaller carbon particles in material voids for more surface area results in the electrolyte moving from within the material to beyond it, increasing ionic conductivity and mass transport requirements from beyond the electrode surface. , for example from the separator region. In the worst case this limits the effective capacitance. In the best case it increases RC and undesirably slows down the frequency response of the device. Third, smaller particles increase the number of grain boundaries through which electricity must flow at the electrode. This undesirably reduces the conductivity of the electrode, undesirably increases its ESR, and therefore increases its RC. Fourth, to overcome the conductivity problem introduced by many small particles, it is common to add a proportion of conductive carbon particles that do not contribute to effective surface. Fifth, to bind said polydispersity of fine Irregular particles together, it is common to add a proportion of a binder such as PFTE that does not contribute to the effective surface. Experimental electrodes reported in the scientific literature can have as much as 10% each of conductive and binder carbon, meaning that only 80% of the elector mass is capable of contributing to the effective capacitance surface.

It is desirable to maximize the performance of EDLC.

BRIEF DESCRIPTION OF THE INVENTION

The present inventor has found that by forming a fibrous material from activated carbon fiber fragments of substantially similar diameter and alpha spectrum ratio that the performance of EDLC can be Increased.

In another aspect, the present inventor has found that EDLC performance can be Increased using a fibrous material formed from a mixture of (a) 50 to 95 +% of a first population of activated carbon fiber fragments and (b) a second population of carbon fiber fragments with diameters substantially similar to or equal to the first population and of greater length than the first population.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph illustrating final volume fractions Φ for amorphous compacts as a function of aspect ratio a. The solid line is a theoretical fit of the random contact equation Φα = 5.1. The insert shows a magnified view of the same graph in a low aspect ratio. Graph reproduced from Physical Review E 67 051301, 051301-5 (2003).

DETAILED DESCRIPTION OF THE INVENTION

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

The term "aspect ratio" as used in reference to a carbon or fiber fever refers to the length of the fiber divided by the diameter of a fiber.

The term "compaction loss" as used to refer to electrodes refers to the difference (in F / g, F / cc, or percentage) between the intrinsic capacitance of a total effective carbon surface and the traditional measured specific capacitance.

The term "intrinsic capacitance" refers to the ideal capacitance of the total effective carbon surface when it is in full double layer.

The term "mesoporous" as used in reference to a carbon fiber or fiber describes a surface characteristic pore size distribution where at least about 20% of the total pore volume is from about 2 to about 50 nm in size. .

The phrase "catalytically activated" as used in reference to a carbon or fiber fever refers to its pore-containing surface where the pores have been introduced by catalytically controlled activation (eg, etching). In some embodiments, metal oxide particles of a chosen average size serve as suitable catalysts and at least a portion of the metal oxides 5 remain in or on the fibers after the activation process.

The term fiber used with reference to polymers and carbon refers to fine diameter filamentary material, such as diameters less than about 20 microns, and preferably less than about 10 microns, as the type that can be obtained using conventional spinning processes.

The term "nanofiber" using in reference to polymers and carbons refers to a filamentary material of very fine diameter less than a miera and preferably at nanoscale (100 nanometers or less in diameter) as the type that can be obtained using an electrospinning process.

Activated carbon fibers

The carbon fibers embodying features of the present invention can be prepared by any known process. In general carbon fibers are prepared by polymerizing a monomer to form a polymer fiber and carbonizing at least a portion of the polymer fiber to provide a carbon fiber.

The carbon fibers can be activated using any known methods. For example, Kyotani, Carbón, 2000, 38: 269286, have summarized available methods for obtaining mesoporous carbon fibers. Hong et al., Korean J. chem. Eng., 2000, 17 (2), 237-240, described a second activation of carbon fibers previously activated by catalytic gasification. Preferred methods of preparing carbon fibers with controlled mesoporosity are described in US Patent Application Serial No. 11/211, 894, filed August 15, 2005; The full contents of the application are incorporated herein by reference. Ideally, one should control the activation of the carbon fiber to ensure the formation of mesopores, as described in the US application series no. 11 / 211,894. However, activated carbon fibers formed from other preparation methods can also be used in this invention.

In some embodiments, the activated carbon fibers of the present invention comprise diameters of about 10 microns or less, in other embodiments of about 5 microns or less, in another embodiment of about 1 micron or less, in other embodiments of about 500 nm or less, in other modes around 100 nm or less. The preferable diameter depends on the process used to create a fibrous material.

The activated carbon fibers of the present invention have pores (ie they are not smooth surfaces). The pore size is introduced into the fiber surfaces and into the fibers during activation depending on the process and a preferred embodiment is the catalytic activity of a nanoparticle metal oxide catalyst, its quantity, and / or the size of its nanoparticles. as well as the activation conditions. In general it is desirable to select pore sizes large enough to accommodate the particular electrolyte used at an optimal but substantially larger surface compaction to prevent unnecessary reductions in total fiber surface area.

The average pore size is typically on the scale of about 1nm to about 20nm. Ideally, the average pore size is around 3nm to 15nm, preferably 6Ί0nm.

Homogeneous carbon fiber fragments

The present invention is based on the embodiment that a reasonably homogeneous population of rod-like fragments of carbon fibers can be used to maximize both the surface area and the porosity of a fibrous material formed therefrom. A first surprising aspect of the invention is that both mathematical models and experimental evidence show that low a-fibrous materials (short rods, cylinders, or fibers) can operate randomly as densely as spheres. The theoretical three-dimensional random compaction limit for spheres is 0.64, known as the Bernal limit. Empirically, the Bernal limit is measured at around 0.63 due to non-homogeneous experimental materials. Surprisingly, cylinders with an aspect ratio α of 2 have a compaction density φ of around 0.62.

Conveniently for certain materials such as electrodes, around the same random compacting contacts (5.4 ± 0.2, empirically verified in many experiments) as spheres of equivalent volume (and less than for irregular shapes) but they also have more than double the surface for a material of equal diameter, so that proportionally smaller points of contact by means and potential surface occlusion by unit surface.

A regular number of contacts on longer conductive elements with reduced total grain limit numbers through the material to the collector sheet improves electrical conduction and reduces ESR. Long narrow vacuum channels in cylinder compacts have electrolyte diffusion and ionic conductivity advantages, similar to carbon fiber cloth, but without the same material density limitations and at a lower cost since tissue passage is avoided. Ordinary carbon papers or felts are comprised of a high polydisperse aspect ratio distribution of most of the longer fibers which cannot achieve the same random compacted density and total surface area.

Ordinarily, the expense of manufacturing fibers is rationalized by using their length (for example, for tensile strength or conductive continuity). Surprisingly, the present invention proposes to take advantage only of cylindrical geometry in short lengths. Since these random compacting properties are scale invariant, they can be predictably extended to a second generation of finer fibrous materials.

During activation, the carbon fibers can fragment. For the present invention, the fibers are further fragmented so that the average fiber length is relatively homogeneous. The fibers can be fragmented using any means known as chemical or mechanical shredding, and sieved by means such as advanced air classifiers into particle distributions without excessive polydispersion, for example an aspect ratio distribution from 1 to 5 but concentrated within 2 a 3. In contrast, a typically commercial dispersion of activated carbon particles is from 3 to 30 microns with an average of 8 microns; it is highly polydisperse. The many smaller particles are intended to fit in the gaps among the few larger ones to maximize the total surface area but giving rise to loss of compaction.

The carbon fibers embodying features of the present invention can be fragmented into shorter fragments (eg after carbonization and during or after activation) and then applied to a substrate (eg by suspension) to form a similar layer to non-woven paper. A particle-like short fiber fragment powder can be made from the longest volume material by grinding, grinding, chopping, abrasion, chemical grinding, etc., with a fragment length distribution designed for subsequent coating on a substrate ( for example an electrode surface).

In general, the population of fragments for maximum random compaction has an average length of 1 to 5 times the diameter; that is, an aspect of 1 to 5. Aspect ratios less than 1 constitute fine particles that can “clog” pores of material; older aspect ratios are not packed as densely. An aspect ratio can be selected for a specific device feature; for example, for dust density more porosity of materials is desirable to allow the transport of electrolyte mass (larger ratio), while for energy density more surface area from a denser compaction could be desirable (lower ratio). In practice, the milling and sieving processes result in a particle distribution with some dispersion around the engineering design goal.

In general, as the fiber diameter decreases, increasing its total surface area, it becomes more difficult but less important to achieve single-digit aspect ratios. A somewhat lower compaction density is offset by a larger individual fiber surface. To maintain conductivity along the fiber axis and not to introduce many grain limits, a practical minimum average length is contemplated. In some modalities such as with a fiber diameter of 7 microns this length can be 15 microns in an aspect ratio of about 2. In some modalities with fibers with a diameter of 5 microns it can be a length of 10 microns also in aspect ratio 2. For electrospun nanofibers below one micron in diameter, a preferred length may remain a few microns for conductivity, resulting in aspect ratios that increase as the fiber diameter decreases. In general, however, aspect ratios for the average material should remain below 20 to achieve a reasonable material density greater than about 50%. Engineering disadvantages are illustrated in Figure 1 which shows theoretical and experimental results for monodisperse packaging (taken from Physical Review

E27 051301 (2003).

The aspect ratio distribution of the resulting fibrous powder will result in a material of predictable average density and porosity in accordance with these principles of random compaction.

Heterogeneous mixture of carbon fiber fragments.

It is also within the scope of the present invention to have a multimodal sample of activated carbon fiber fragments. The first population would comprise fragments with reasonably homogeneous lengths and diameters. Other populations of activated fiber fragments would contain substantially the same diameter as the same population, but would have longer lengths and higher aspect ratios.

Since density and total area do not critically depend on some long fragments (at substantially higher aspect ratios) on a mixture of relatively homogeneous smaller fragments, it is possible to have a second population with fiber fragments of greater lengths than the first population without substantially affect the density or surface. Technically this is a bimodal or multimodal polydispersion. Moderate proportions of longer fibers can be mixed, each averaging 5.4 contacts for every few length diameters. This would have major advantages for conductivity or material and ESR by providing semi-continuous conductance pathways and further reducing grain boundary interfaces.

In one embodiment, the heterogeneous mixture contains from about 50 to 95% of a first population of substantially homogeneous (not highly polydisperse) fragments and balance fragments of diameter substantially similar to the first population, but with greater lengths.

In one embodiment the length of the fibers in the second population is greater than about twice the length of the first population, in another embodiment the fiber is five times as long. In another embodiment, the longest fibers are 50, 100, 150 or 200 microns on average in length regardless of the first population, said lengths corresponding to the desired average thickness of the electrode material.

Fibrous materials

The fibers of the present invention can be further processed to provide a material according to the present invention compatible with conventional particulate carbon coating processes as described in US Patent No. 6,627,252 and 6,631,074, the contents of both of which are incorporated herein by reference, except that in the case of any inconsistent description or definition of this application, the description or definition herein will be deemed to prevail.

The density of the resulting “paper-like fibrous material, as coated on a current collector sheet, is a designed property of the length of the fiber fragments compared to their diameter (their aspect ratio, polydispersed distribution of lengths compared with the average diameter, and optionally post deposition densification (for example by pressure). If the length approaches the diameter, then the fragments will be more like conventional particles and compact more densely with less porosity in the resulting material. If the length is much greater than the diameter, then the aspect ratio will be high and the compaction less dense (that is, a ratio of more porous vacuum to volume in material). The aspect ratio by length to diameter can be adjusted and / or mixtures of different ratios can be used to provide any desired material porosity (void / volume ratio) within the limits of random compaction principles. In some embodiments, at least about 50% of the total number of carbon fiber fragments has a scale length of from about 5 to about 30 microns equivalent to some particulate materials of activated carbon. In other modes, at least about 50% of the total number of fragments have aspect ratios less than 30. In other modalities, average aspect ratios are less than 20. In other embodiments, aspect ratios are less than 10. In other embodiments, where the fiber fragment diameters at or below 100 nm more closely resemble carbon tubes, at least about 50% of the total number of fragments Carbon fiber are less than one length in length with aspect ratios less than 20.

In some embodiments, the density of the fibrous material can be increased additionally (for example by simple pressure lamination to the desired thickness or the like). In some modalities, the density is increased prior to charring and / or activation, and in other modalities, the density is increased after charring and / or activation. In some embodiments, the thickness of a dense fibrous material is less than or equal to about 200 microns, in other modalities less than or equal to about 150 microns, and in other modalities, less than or equal to about 100 microns.

Capacitors

EDLC electrodes are typically made of activated carbon attached directly or indirectly to a sheet metal current collector, although metal oxides can be used. In accordance with the present invention, activated carbon materials prepared by the methods described herein can be applied to current collectors along with additional metal oxides or the like for hybrid characteristics including improved pseudocapacitation.

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

Conventional carbon EDLCs with organic electrolytes use either propylene carbonate or acetonitrile organic solvents and a standard fluoroborate salt. Some carbons and most of the

Commercial metal oxide EDLCs use aqueous electrolytes based on sulfuric acid (H2SO4) or potassium hydroxide (KOH). Any of these electrolytes or the like can be used in accordance with the present invention.

Since organic electrolytes have lower conductivity than aqueous electrolytes, they have slower RC characteristics and higher ESR contributions, and they reach mass transport pore constraints at substantially larger geometries since they are much larger solvate ions. However, since they have fragmentation voltages above 3V compared to 1V with aqueous electrolytes, organics produce higher total energy density since total energy is a function of voltage squared. Carbon pores and organic optimized materials would optionally work for aqueous electrolytes as well, since the aqueous solder spheres are smaller. This would allow, for example, ultracapacitor devices to be adjusted to RC requirements and respectively carbon fabrication by changing the compacted electrode density by aspect ratio, and by changing electrolyte. Hybrid devices would naturally have a broader scale of characteristics than total RC characteristics since they combine EDLC with PC capacitive phenomena. The practical range for use in hybrid electric vehicles is less than about one second to about 15 seconds, and for distributed power less than about 0.01 seconds to about 1 second.

Activated mesoporous carbon fibers or fibers, or their corresponding fragments, embodying features of the present invention can be incorporated in all manner of devices incorporating conventional activated carbon materials or which could conveniently be modified to incorporate fibrous carbon materials of designed material geometry , surface, porosity and productivity. Representative devices include but are not restricted to all manner of electrochemical devices (eg capacitors;

batteries, including without restriction one side of a nickel hydride battery cell and / or both sides of a lithium ion battery cell; fuel cells, and the like). Such devices can be used without restriction in all manner of applications, including without restriction those that could potentially benefit from high energy and high energy density capacitors or the like.

The foregoing detailed description has been provided by way of illustration and illustration and is not intended to limit the scope of the appended claims. Many variations on the presently preferred embodiments illustrated herein will be apparent to one skilled in the art, and will remain within the scope of the appended claims and their equivalents.