DE102011108435A1 - Carbon-carbon composite, useful e.g. as electrode, catalyst support and adsorber, comprises large- and open porous support of carbon or organic precursor coated with nanoporous carbon material of high specific surface - Google Patents

Abstract

Carbon-carbon composite comprises a large- and open porous support made of carbon or an organic precursor, preferably charcoal, porous graphite plates or flakes, carbon- or graphite fiber felts, carbon foams, or their organic precursors, coated with a nanoporous carbon material of high specific surface and whose specific Brunauer-Emmett-Teller (BET) surface area is greater than 50 m 2>/g. An INDPENDENT CLAIM is also included for producing the carbon-carbon composite, comprising: (a) soaking substrate made of carbon or organic precursor, preferably polyacrylonitrile or pitch-based fibers and fabrics with a liquid comprising hydroxybenzene-aldehyde, furfuryl alcohol, polyacrylonitrile or melamine-aldehyde; (b) producing by a sol-gel method; and (c) drying and pyrolyzing at 500[deg] C.

Description

The invention relates to a carbon-carbon (C / C) composite, characterized in that a carbon carrier material is coated with a nanoporous carbon material of high specific surface, in which case pores smaller than 1 μm are to be understood as meaning nanopores.

For the process, a carbon support material (eg charcoal, graphite, carbon and graphite felts, carbon foams or the organic precursors of these types of materials) is chosen. The coating of the carrier material takes place via a liquid precursor in a sol-gel process, the coating initially being present as organic precursor (eg porous duromer). Since the precursor of the C / C composite still contains organic components, the C / C composite is produced by carbonation in a pyrolysis step.

[State of the art]

Carbon-carbon (C / C) composite materials consisting of a carbon support material and a pyrocarbon coating are known from various sources and state of the art. The pyrocarbon layer is characterized by a high structural order of the carbon atoms (graphitic) and an increased resistance to oxidation. The specific surface area of the coating is well below 10 m ² / g and the pyrocarbon coating has no significant nanostructuring and no nanoporosity (structures <1 μm). In addition, the coating with pyrocarbon leads via chemical vapor deposition (CVD) to a C / C composite of very low specific surface area. If the carbon support itself has a high specific surface area and relevant microporosity (pores <2 nm), the coating with pyrolytic carbon leads to a significant reduction in the specific surface area, in particular no micropores (definition of micropores in: Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA, Rouquerol J, et al. Reporting physisorption data for gas / solid systems with special reference to the determination of surface area and porosity. Pure and Applied Chemistry 1985; 57: 603 ) are detected more.

Out JP6143469AA Carbon and graphite fiber forms are known which are coated with dense graphite via a CVD process and whose layer thickness is 10 μm-1000 μm.

In JP10045473AA and JP10045474AA For example, a carbon fiber-reinforced carbon or graphite-based molded article is subjected to CVD treatment to form a pyrolytic carbon coating having a thickness of 10 μm-150 μm. The dense pyrocarbon layer has low impurities and is more resistant to oxidation.

In JP61219708A the production of a graphite-based material is described, which is coated with pyrolytic carbon via a CVD process. The coating is deposited with hydrocarbons or halogenated hydrocarbons as precursors at temperatures of 600 ° C to 2400 ° C on the base material.

In EP1961701A1 Graphite particles of size 5 μm-50 μm are provided with a carbon layer via a thermal CVD process. The composite has a specific surface area of less than 5 m ² / g.

In CN101209837A For example, a macromolecular polymer is dissolved in a corresponding organic solvent whose boiling point is below the softening point of the macromolecular polymer. The solution is mixed with graphite particles and the graphite coated by distillation and curing of the macromolecular polymer. Subsequently, a carbonization is carried out. The coating is smooth and the coated graphite particles do not agglomerate.

Out JP60025163A An electrode for redox flow batteries is known, which consists of conductive carbon fabric with homogeneously distributed activated carbon and is introduced into the cavity of a graphite plate.

In US005626977A thin composite layers are shown, wherein a carbon keratin based on resorcin-formaldehyde is used as binder material for a granulated second carbon (carbon foam, fibers, airgel).

There are some combinations of different carbons, but no large- and open-pored carbon carrier material is known in which the inner surface of the carrier material is coated with a nanoporous carbon or carbon-containing component having a high specific surface area.

In the examples given, in which a C / C composite was produced by means of a coating, the carbon coating is exclusively smooth, highly graphitic carbon layers of low specific surface area without nanoporosity, usually referred to as pyrocarbon. These pyrocarbon coatings serve as protection against thermal and oxidative influences and are unsuitable for use, for example, as an electrode material due to their low specific surface area.

OBJECT OF THE INVENTION

The object of the invention is, in a large and porous material made of carbon and its organic precursor while maintaining the large pores, the specific surface of the total material significantly, d. H. by at least an order of magnitude.

For this purpose, initially an open-pore carrier material made of carbon or of an organic precursor, in particular charcoal, porous graphite plates or flakes, carbon or graphite fiber felts, carbon foams or their organic precursors, via the infiltration of a liquid phase in a sol-gel process with a carbon Precursor coated. The coating can be carried out, for example, by infiltrating the support material with a solution comprising polymer constituents, the polymer constituents depositing on the support material in the wet-chemical reaction or during the subsequent removal of the solvent by drying and forming a polymeric precursor of the carbon coating. The decisive factor here is the formation of a polymeric, nanostructured (eg nanoporous) layer via the sol-gel process on the inner surface of the support material. The resulting coated material remains open-pored.

Whereas in the pyrocarbon deposition by thermal decomposition of a carbonaceous gas on the surface of the support material, a nonporous carbon layer is directly produced, in the present invention, by contrast, the support material is infiltrated or soaked with a liquid. The fluid contains molecular carbonaceous constituents which, by polymerization at the surface or in the pores of the carrier material, result in a coating thereof. The polymer coating already has substantial structural properties which are characteristic of the C / C composite according to the invention, on, for. For example, see spherical components of the coating (right) and , as well as structural units in the range <1 μm (eg pores).

Furfuryl alcohol, polyacrylonitrile (PAN) and combinations of hydroxybenzenes (eg phenol, resorcinol) with aldehydes (eg formaldehyde, furfural) or melamine-aldehydes which are dissolved in a solvent (eg Water, alcohols, ketones) are dissolved and, to accelerate the polymerization, mixed with a catalyst (base or acid) or a hardener (eg hexamethylenetetramine HMTA). The support material is then infiltrated with the diluted polymer solution; the polymerization can be thermally or chemically controlled until it is complete.

After completion of the polymerization, the remaining solvent is removed by drying from the coated composite.

When using hydroxybenzenes and aldehydes, the coating takes place via the attachment of the polymer building blocks to the inner surface of the support material. The decisive factor here is that in the composite actually comes exclusively to a coating of the carrier material, and the usual sol-gel process, which would lead to a filling of the interstices of the carrier material is prevented by a suitable choice of the process parameters.

Since the composite still contains organic constituents (polymers of the coating and, for example, polyacrylonitrile (PAN) or pitch-based fiber felts), carbon conversion takes place by thermal decomposition of the organic constituents at temperatures between 500 ° C. and 1100 ° C. under an oxygen-free atmosphere ( Pyrolysis) to produce the C / C composite.

To further increase the specific surface area of the C / C composite, an activation step may occur at 500 ° C-1500 ° C (physically, eg with CO ₂ or H ₂ O or chemically eg with alkali hydroxides / carbonates ) connect. To increase the electrical conductivity of the C / C composite, in particular the coating, a high-temperature treatment at temperatures above 1100 ° C offers. For a combination of high specific surface area and good electrical conductivity, depending on the application, a combination of activation and high-temperature treatment may be required. Here, for example, the specific surface of the C / C composite would first be enlarged by activation and then the electrical conductivity increased by a high-temperature treatment of the C / C composite. This is also possible in reverse order.

To improve wettability and reactivity of the electrode, the carbon surface may be provided with functional groups (eg, -C-OH, -C = O, -C-OOH or surface groups containing nitrogen, or other polar surface groups). This can be done by gas phase reactions at temperatures of 100-1000 ° C. or of 100-700 ° C. in air or under gas flow with a defined composition (eg ammonia, oxygen, CO ₂ , H ₂ O with or without chemically inert carrier gas (eg N ₂ , Ar)) or by chemical treatment after immersion in appropriate solution containing one or more reactive components (eg KOR, NaOH, K ₂ CO ₃ , Na ₂ CO ₃ , ZnCl 2, or H ₂ SO ₄ , HNO ₃ ) at room temperature or at temperatures up to 200 ° C. Furthermore, one is electrochemical oxidation of the carbon electrode in an electrochemical cell and a suitable electrolyte (e.g., H ₂ SO ₄ or aforementioned chemicals) to modify the surface of the carbon component. In addition, a direct treatment of the surface by oxygen plasma or ozone is possible to achieve the above-mentioned effect. The treatment times range in all the aforementioned methods from one minute to several hours or days, in particular from 1 minute to 40 hours. The consequence of a corresponding treatment is the formation of polar surface groups, which on the one hand improve the wettability / hydrophilicity of the surface of the electrode and increase the contact area between an electrolyte or another liquid and the electrode surface. On the other hand, the reaction kinetics, ie the exchange current density of the corresponding reaction at the electrode (eg for V ^{+ 3 / + 2} , V ^{+ 5 / + 4} , Fe ^{+ 3 / + 2} ) can be supported by supporting the reaction process Increase redox reaction by the functional surface groups.

The large-pore C / C composite according to the invention has a nanoporous carbon coating on the entire inner surface of the original carrier material. If the pure carrier material itself has only a small specific surface area (eg charcoal, pressed expanded graphite, graphite fiber felt or carbon foam), then the C / C composite has a significantly increased specific surface area, although the difference can amount to several orders of magnitude. In addition to the increase in the specific surface, which is mainly due to the microporosity (pores <2 nm) of the carbon coating, the inner surface of the C / C composite also increases by the spherical components of the carbon layer, as can be seen from SEM images (please refer right and ).

The large-pored C / C composite is characterized z. B. by low density, an open-pore superstructure with pores greater than 1 micrometer, a low flow resistance, a carrier material made of carbon with high electrical conductivity (> 0.1 S / cm, in particular> 1.0 S / cm), and a nanoporous carbon coating with high specific surface area.

mit V dem pro Zeiteinheit durch das Komposit transportierte Flüssigkeitsvolumen bei einer Querschnittsfläche A senkrecht zum Volumenstrom und einem Druckabfall Δp über der Dicke l des Komposits in Richtung des Stromes. n ist die Viskosität der Flüssigkeit.Therefore, the C / C composite according to the invention is suitable for various applications, preferably as electrode material in electrochemical stores, in particular in redox flow batteries (eg in the systems vanadium / vanadium, vanadium / bromine, bromine / polysulfide and cerium / zinc, zinc / Bromine, etc.). Here is a C / C composite of electrically highly conductive carbon high, readily accessible porosity, with simultaneously large pores in the range of 1 micron-5 mm (eg graphite, pressed expanded graphite, carbon or graphite fiber felt, carbon foam) As a carrier material important to allow high flow rates of the electrolyte through the electrode even at low pressure differences. The permeability of the composite for liquids can be quantified by the permeability coefficient k:

with V the volume of liquid transported through the composite per unit time at a cross-sectional area A perpendicular to the volume flow and a pressure drop Δp across the thickness l of the composite in the direction of the flow. n is the viscosity of the liquid.

The materials used hitherto have a low mass and volume-specific surface and thus also a low active surface area relative to the electrolyte (eg vanadium ions), which limits the performance of the cell per volume.

The nanoporous carbon coating, which has a high surface area, significantly increases the electrode surface area active to the electrolyte without significantly affecting the flow rate of the electrolyte through the electrode. Overall, this results in an increased power density per volume compared to an uncoated carbon electrode.

The C / C composite also lends itself to use as electrode material in other electrochemical applications and other battery forms, for which a coarse and open porous electrode material with high conductivity and high surface area is advantageous, e.g. B. in metal-air batteries (Zn-air, Li-air).

In addition, the use of the C / C composite as a catalyst carrier offers.

The C / C composite with its high specific surface area and large pores, which enable a good flow rate, is also ideal as an adsorber for filtration.

[Examples]

Embodiment 1

Pressed expanded graphite with a density of 0.03 g / cm ³ and one from nitrogen sorption are used as carbon carrier material DIN ISO 9277: 2003-05 specific surface area S _BET of 45 m ² / g.

For the coating resorcinol, aqueous 37% formaldehyde solution (stabilized with about 10% methanol), deionized water and 0.1 N Na ₂ CO ₃ solution mixed together in a beaker on a magnetic stirrer. The molar ratio of formaldehyde to resorcinol is F / R = 2, the molar ratio of resorcinol to the catalyst (Na ₂ CO ₃ ) is R / C = 1000 and the mass of resorcinol and formaldehyde in the total mass of the solution is 20%.

Subsequently, the graphite compact is infiltrated in a desiccator with the resorcinol-formaldehyde solution by means of vacuum pressure infiltration. The sample is heated to 90 ° C for 24 hours to achieve coating of the carbon support material with resorcin-formaldehyde polymers. Subsequently, the liquid is removed by convective drying. To convert the polymer coating to a carbon coating, the sample is pyrolyzed under nonoxidizing atmosphere (argon) for one hour at 800 ° C. shows for comparison a SEM image of the pressed expanded graphite (left) with the coated sample, the C / C composite (right). The deposited nanoporous carbon material on the graphite surface can be clearly seen.

The C / C composite has a density of 0.08 g / cm ³ . The by nitrogen sorption after DIN ISO 9277: 2003-05 certain specific surface has increased by a factor of about 10 to S _BET = 458 m ² / g. The micropore volume according to the t-plot method ( DIN 66135-2 ) is 0.16 cm ³ / g, the external surface 39 m ² / g. The significantly increased specific surface area of the carbon-coated C / C composite is also evident in the adsorbed gas quantities of the nitrogen sorption isotherms at 77 K of pure carbon support material (pressed expanded graphite) and the C / C composite in ,

Embodiment 2:

A graphite hard felt of density 0.1 g / cm ³ and one from nitrogen sorption after DIN ISO 9277: 2003-05 determined specific surface S _BET of 0.4 m ² / g with the dimensions 10 cm × 10 cm × 1.5 cm is soaked with a dilute polymer solution at room temperature and atmospheric pressure. This solution consists of 27.2 g of resorcinol, 39.5 g of an aqueous 37% formaldehyde solution (stabilized with about 10% methanol), 82.7 g of deionized water and 0.6 g of a 0.1 N Na ₂ CO ₃ solution. The sample is heated at 85 ° C for 24 hours to provide a coating of the carbon support material with resorcin-formaldehyde polymers. Subsequently, the gel is dried convectively at 40 ° C in air at atmospheric pressure for 36 hours. The organic precursor of the C / C composite thus obtained has a density of 0.355 g / cm ³ . The organic precursor of the C / C composite is then pyrolyzed for 3 hours at 800 ° C under a non-oxidizing atmosphere (argon). The C / C composite thus obtained has a density of 0.226 g / cm ³ and a BET surface area DIN ISO 9277: 2003-05 of 389 m ² / g. The micropore volume according to the t-plot method ( DIN 66135-2 ) is 0.15 cm ³ / g, the external surface 2 m ² / g. shows an SEM image of the coated C / C composite. The spherical structures of the nanoporous carbon coating with a high specific surface can be clearly seen. The permeability coefficient k is 7 × 10 ^-11 m ² .

Embodiment 3

Pressed expanded graphite with a density of 0.03 g / cm ³ and one from nitrogen sorption are used as carbon carrier material DIN ISO 9277: 2003-05 specific surface area S _BET of 45 m ² / g.

For the coating, phenol, aqueous 37% formaldehyde solution (stabilized with about 10% methanol), n-propanol and 37% HCl are mixed together in a beaker on a magnetic stirrer. The molar ratio of formaldehyde to phenol is F / P = 2, the molar ratio of phenol to the catalyst (HCl) is P / C = 3 and the mass of phenol and formaldehyde in the total mass of the solution is 15%.

With the help of vacuum pressure infiltration, the graphite compact is infiltrated in a desiccator with the phenol-formaldehyde solution. The sample is heated for 24 hours at 90 ° C to achieve a coating of the carbon support material with phenol-formaldehyde polymers. Subsequently, the liquid is removed by convective drying. To convert the polymer coating to a carbon coating, the sample was pyrolyzed under nonoxidizing atmosphere (argon) for one hour at 800 ° C.

The C / C composite has a density of 0.09 g / cm ³ . The by nitrogen sorption after DIN ISO 9277: 2003-05 certain specific surface area is S _BET = 267 m ² / g.

Embodiment 4

Graphite felt consisting of predominantly amorphous carbon fibers (viscose-based) and partly graphitic carbon fibers (PAN-based) serves as carbon carrier material. The felts are activated in hot concentrated sulfuric acid over 30 minutes. Both by the activation and beyond by the execution as C / C Composite increases the power density measured in a vanadium redox-battery test cell. This treatment leads to an improvement in the wettability (drip entest / wetting angle). The power density of the felts improved by the said treatment by 38% and 19% compared to the unactivated felt at a tension efficiency of 90%.

Compared to the best activated felt, the corresponding C / C composite achieves an increase in power density of 25% from 0.086 W / cm ² to 0.0108 W / cm ² with a 90% voltage efficiency.

LIST OF REFERENCE NUMBERS

1

Carrier material made of carbon

2

nanoporous carbon

3

big, open pores

k

permeability coefficient

V

per unit of time through the composite transported liquid volume

A

Cross-sectional area perpendicular to the volume flow

Ap

pressure drop

l

Thickness of the composite in the direction of the liquid flow

η

Viscosity of the liquid.

S _BET

specific BET surface area

p / p ₀

relative pressure

STP

"Standard temperature and pressure"

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• JP 6143469 [0004]
• JP 10045473 [0005]
• JP 10045474 [0005]
• JP 61219708 A [0006]
• EP 1961701 A1 [0007]
• CN 101209837 A [0008]
• JP 60025163 A [0009]
• US 005626977A [0010]

Cited non-patent literature

• Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA, Rouquerol J, et al. Reporting physisorption data for gas / solid systems with special reference to the determination of surface area and porosity. Pure and Applied Chemistry 1985; 57: 603 [0003]
• DIN ISO 9277: 2003-05 [0030]
• DIN ISO 9277: 2003-05 [0033]
• DIN 66135-2 [0033]
• DIN ISO 9277: 2003-05 [0034]
• DIN ISO 9277: 2003-05 [0034]
• DIN 66135-2 [0034]
• DIN ISO 9277: 2003-05 [0035]
• DIN ISO 9277: 2003-05 [0038]

Claims (16)

Carbon-carbon (C / C) composite, characterized in that a large and open-pored carrier material of carbon or an organic precursor, in particular charcoal, porous graphite plates or flakes, carbon or graphite fiber felts, carbon foams or their organic precursors, with a coated nanoporous carbon material of high specific surface area and whose BET specific surface area is greater than 50 m ² / g. C / C composite according to claim 1, characterized in that it has a porosity> 50%. C / C composite according to one of claims 1 or 2, characterized in that it has large, open pores in the range 1 micron-5 mm with a proportion of the total porosity of over 70%. C / C composite according to one of claims 1, 2 or 3, characterized in that it has an electrical conductivity> 0.1 S / cm, in particular> 1.0 S / cm. C / C composite according to one of claims 1 to 4, characterized in that the uncoated carrier material has a low specific BET surface area less than 50 m ² / g. C / C composite according to one of claims 1 to 4, characterized in that it has a permeability coefficient greater than 10 ^-12 m ² . C / C composite according to one of claims 1 to 4, characterized in that the reaction rate of the desired battery reaction / redox reaction is increased by modification of the surface of the C / C composite by gas phase or liquid phase reactions and / or electrochemically assisted reactions. C / C composite according to one of claims 1 to 4, characterized in that the wettability, hydrophilicity and / or polarity and / or catalytic activity of the electrode surface is increased by modification of the surface of the C / C composite by gas phase or liquid phase reactions and / or electrochemically assisted reactions. A method for producing a composite according to any one of claims 1 to 6 according to the invention characterized in that i. a carrier material of carbon or an organic precursor, in particular charcoal, porous graphite plates or flakes, carbon or graphite fiber felts, carbon foams or their organic precursors, in particular polyacrylonitrile (PAN) or pitch-based fibers and fabrics, ii. is impregnated with a liquid which comprises hydroxybenzene-aldehyde, furfuryl alcohol, polyacrylonitrile (PAN) or melamine-aldehyde, iii. produced via a sol-gel process, iv. dried and pyrolyzed at temperatures above 500 ° C. A method according to claim 9, characterized in that the drying is carried out sub-critical, in particular at temperatures between 0 ° C and 120 ° C in air at atmospheric pressure (about 1013 mbar). Method according to one of claims 9 to 10, characterized in that the C / C composite is activated physically, in particular with an oxygen-containing gas, or chemically, in particular with an alkali metal hydroxide or alkali carbonate. Method according to one of claims 9 to 11, characterized in that the C / C composite is subjected to a high-temperature treatment at temperatures above 1100 ° C. Method according to one of claims 9 to 11, characterized in that the surface of the C / C composite is modified by gas phase or liquid phase reactions and / or electrochemically assisted reactions. Use of a C / C composite according to one of the above claims as an electrode, preferably in metal-air batteries or in redox flow batteries, in particular in vanadium redox flow batteries. Use of a C / C composite according to the invention according to one of the above claims as a catalyst support. Use of a C / C composite according to one of the above claims as adsorber, in particular for filtration.

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