Patent Application
20110286490
The invention relates to a method for producing activated carbon material, suitable for use as an electrode in electrochemical cells and supercapacitors, or as an adsorbent of for example volatile organic compounds (VOCs).
Carbon materials such as carbon fibre materials may be activated to create pores in and thus increase the surface area of the material, making the materials useful for various applications including as electrodes in cells and supercapacitors and as adsorbents, by “physical activation” with steam or carbon dioxide at temperatures around 1000K or by “chemical activation” by for example aqueous alkali solutions.
In the arc-discharge method for producing carbon nanotubes such as described in our international patent application WO 03/082733, current flows between carbon electrodes creating an arc between them. Evaporation of the carbon electrodes forms a vapour-cluster-nanoparticle suspension which condenses as nanoscale carbon fibrils or nanotubes.
In broad terms the invention in one aspect comprises a method for producing an activated material, including moving a carbon-containing substrate within a reaction chamber either through an electric arc in a gap between two electrodes or past an electrode so that an electric arc exists between the electrode and the substrate at a temperature and time effective to activate the carbon-containing substrate substantially without causing nanotubes to form on the substrate.
In broad terms in another aspect the invention comprises a method for producing an activated carbon material, including moving a carbon-containing substrate within a reaction chamber either through an electric arc in a gap between two electrodes or adjacent an electrode so that an electric arc exists between the electrode and the substrate to heat the substrate to a substrate surface temperature effective to activate the carbon-containing substrate and above about 3750K.
In broad terms the invention in a further aspect comprises a method for producing an activated carbon material, including moving a carbon-containing substrate within a reaction chamber either through an electric arc in a gap between two electrodes or past an electrode so that an electric arc exists between the electrode and the substrate to activate the substrate, the arc having a sufficient voltage and/or current ripple to activate the substrate substantially without causing nanotubes to form on the substrate.
In broad terms in another aspect the invention comprises a method for producing an activated carbon material within a reaction chamber, including causing relative movement between a carbon-containing substrate and an electric arc in a gap between two electrodes or past an electrode so that an electric arc exists between the electrode and the substrate to activate the substrate at a speed such that the substrate has a residence time in the arc of less than three seconds and/or at a speed of more than 3 mm per second.
By “activation” is meant the creation of pores typically of nanoscale and typically up to 50 nm in diameter, and typically also coarser corridor pores up to 100 nm in diameter in the material, or on the surface of the material, by the arc treatment, and by vaporising or removing in the arc some matter of the carbon substrate and preferably non-graphitic carbon or a sufficient part or a major part of the non-graphitic carbon of the substrate. The interior pores can be termed “internal activation” to distinguish from the surface generated by exterior nanostructures which may be deposited by the arc (e.g. nanotubes).
Either an arc may be formed between two electrodes and the substrate moved through the arc or alternatively the arc may exist between one electrode and the substrate, which is most conveniently earthed. Another electrode may be used to initiate the arc, and may then be withdrawn leaving an arc between one electrode and the earthed substrate.
Typically one or both electrodes will be carbon electrodes such as graphite electrodes, but it may be possible that the electrodes or electrode are formed of a non-carbon material (of sufficient refractory nature that it does not generate impurities at the reactor temperatures) and that only the substrate itself is carbon.
The substrate may be moved at a substantially steady speed through the arc or in steps.
The substrate may be composed of carbon fibres and may comprise a tape or belt woven from carbon fibres or a paper of carbon fibres for example. Preferred substrate materials include carbon fabric derived from rayon, polyacrylonitrile, phenol resin, and pitch materials.
Preferably the substrate is moved at a speed such that the substrate has a residence time in the arc of less than three seconds. Preferably the substrate is moved at a speed of greater than 3 mm per second.
Preferably the method includes flushing an inert gas through the reaction chamber, or an otherwise inert gas which contains a low amount of oxygen sufficient to react with other species such as carbon species without destructively oxidising the substrate on cool down. Most preferably a flow of gas is directed to cool one or both of the electrodes and/or the substrate, and particularly to cool the substrate after it has passed through the arc. Alternative to the gas containing a low concentration of oxygen, the substrate after exiting the reactor chamber may be moved through an oxygen-containing gas in a separate lower temperature heating stage e.g. a resistive heating stage, to separately provide a further micropore activation. The arc activation gives larger pores (without the <2 nm pores) than does activation with an oxygen-containing gas, and for many uses the arc activation is optimal, but a further activation to provide 2 nm pores can be desirable.
It is believed that the arc discharge takes place by an electron and ion flow between both electrodes and/or between one electrode and the substrate. Free electrons and ions are accelerated by the voltage difference between the electrodes. The electrons collide with gas atoms, leading to excitation of the atoms and causing emission of radiation. Atoms and molecules are ionised via collisions involving the electrons. Mainly N+, N2+, Cn+ and Cn− ions occur in the arc when the discharge is performed in nitrogen. The collisions raise the arc temperature. Non-graphitic carbon of the substrate vaporizes leaving graphitic carbon so that nanoscale pores are formed in the substrate (by the loss of (mostly) non-graphitic carbon), activating the substrate.
It has been found that activated carbon material produced by the method of the invention may have high quickly accessible adsorbency in the gas phase, to for example VOCs, and high surface capacitance, useful for porous electrodes in cells. Also abnormally large pores of size distribution in the range 2 to 10 nm and larger may be developed. This hierarchy of pores allows reactants and ions to diffuse easily and quickly to the pore surface even in the centre of fibres of the material from the outer surface of the carbon fibres (e.g. over a distance of around 5 micron).
The activated carbon material produced by the method of the invention may have increased electrical conductivity relative to the material before activation, and after activation may be a good conductor (similar to polycrystalline graphite).
In broad terms in a further aspect the invention comprises a supercapacitor or battery or fuel cell comprising one or more high surface area electrodes comprising an activated carbon material produced by the method(s) defined above and described herein.
By “supercapacitor” is meant a capacitive energy storage device housing capacitance of at least 1 Farad.
Temperature values given in this specification refer to blackbody temperature values measured by observing the arc-facing surface of the substrate with an optical pyrometer.
The term “comprising” as used in this specification means “consisting at least in part of”. When interpreting each statement in this specification that includes the term “comprising”, features other than that or those prefaced by the term may also be present. Related terms such as “comprise” and “comprises” are to be interpreted in the same manner.
The invention is further described with reference to the accompanying figures by way of example wherein:
In
Carbon-substrate 8 passes between electrodes 2 and 3 and through the arc during operation of the reactor, as shown. This is shown in more detail in
During operation the interior of the reactor is preferably at or slightly above atmospheric pressure, and the gas flow exiting the reactor through slits 12 and 13 is extracted via a fume hood or similar. An inert gas such as nitrogen, argon or helium for example is flushed through the reaction chamber at a rate between 3-10 L/min, and it is preferred this is done by introducing a controlled gas flow inside the reaction chamber 1 through one of the openings 11 at the base of the reactor. Additionally or alternatively a gas flow may also be directed through the tungsten tube 7 via a porous carbon anode 3 to flush away carbon vapour and/or cool the substrate during arc treatment.
The cooling through porous carbon 3 assists in avoiding burn-through of the substrate and removal of excessive carbon vapour during arc discharge, whereas the operation of the other inlet 11 serves to control oxidation.
The anode as well as the spool which drives the tape are preferably earthed. Any take up mechanism for collecting the substrate after it has passed through the reactor chamber is also preferably earthed, as is also the reactor shell.
Referring to
The substrate may be of any desired type but it is believed that best results may be achieved with a substrate composed of carbon fibres such as a tape or belt woven from carbon fibres or a paper of carbon fibres for example. Preferably the substrate and the electrodes have a high carbon purity since any impurities will vaporise or partially vaporise at the temperatures within the reactor. In particular it is desirable to avoid too high hydrocarbon impurities which can disrupt the fibres on their rapid heating. Typically the electrodes and substrate should have a carbon purity of at least 95% and preferably in excess of 99%.
In some embodiments the electrode spacing i.e. the inter-electrode gap, is less than 5 mm or above 8 mm; and may be in the range 2 to 5 or the range 8 to 12 mm.
The current density should be sufficiently low to substantially avoid structural damage to the substrate (i.e. damage which would significantly affect a graphitic part of and thus the structural integrity of the substrate) but sufficient to achieve a current density at the contact point of the arc on the substrate (and the arc tends to spread at the contact point on the substrate) which is sufficient to vaporise a major fraction of non-graphitic carbon (but no more than a minor fraction graphitic carbon) and activate the substrate. In some embodiments the arc current is below or above 16 Amps, more preferably is in the range from above 16 Amps to about 20 Amps or from about 10 Amps to below 16 Amps. In some embodiments the current density is above 1 Amps/mm2 for example. It is an advantage of the method of the invention that the arc tends to spread over the substrate, which is advantageous for activating as broad an area of the substrate as possible in a non-destructive manner. A transition to a highly destructive arc mode occurs above a certain current, which may be 16 A or 20 A, depending on the substrate.
It is preferred that gas flushed through the reactor chamber contains sufficient oxygen to react with other carbon species present without oxidising the carbon fibres destructively on cool down. Oxygen concentrations of about 800 and 6000 ppm have been found effective.
The method may be carried out in the presence of an introduced catalyst. Suitable catalysts may be metal catalysts such as Ni—Co, Co—Y, Ni—Y catalysts or alternatively lower cost metal catalysts such as Fe or B catalysts for example.
In some embodiments the arc has a sufficient voltage and/or current ripple to activate the substrate substantially without causing nanotubes to form on the substrate. Preferably the power supply should have a peak to peak ripple of more than one volt and/or more than 0.5 Amps. It has been found that nanotubes may form with lower levels of ripple. In these embodiments where the power supply has sufficient ripple to activate the substrate without causing nanotubes to form on the substrate, the arc may be operated to generate any substrate surface temperature effective to activate the carbon-containing substrate, and typically at any temperature above about 3600K. The temperature range also is constricted by achievement of stable arc operation.
In some embodiments moving the substrate (or the arc) at a speed of more than 3 mm per second and/or such that the substrate has a residence time in the arc of less than three seconds has been found to activate the substrate without causing nanotubes to form on the substrate at any substrate surface temperature, and typically at any temperature above about 3600K.
As stated it has been found that activated carbon material produced by the method of the invention has high rapid absorbency to for example VOCs. Also pores of size distribution in the range 2 to 10 nm or larger may be developed. This allows reactants and ions to diffuse easily and quickly to the pore surface even in the centre of fibres of the material from the outer surface of the carbon fibres (e.g. over around 5 micron). Optionally the arc-activated material may subsequently be given a short CO2 or H2O activation to etch further short pores for example of less than 2 nm off the corridor pores produced by the arc activation.
In an alternative the single electrodes 2 and 3 may each be replaced by a number of adjacent anodes and cathodes to generate multiple arcs adjacent to each other for processing a wider substrate.
The invention is further illustrated by the following description of experimental work which is given by way of example and without intending to be limiting.
A Rayon-based woven carbon fibre tape UVIS TR-3/2-22 manufactured by Carbonics GmbH, Germany was used as a substrate for Run 1. The tape was a cross weave knitted fabric, the specific weight of the tape was 470 g/m2, its thickness was 1 mm with an average filament diameter of 8-10 μm, and it had a carbon content of 99.9%. The tape was cut into strips of width 25 mm.
A PAN based woven carbon fibre tape CW1001 manufactured by TaiCarbon, Taiwan sold under the brand name KoTHmex was used as a substrate for run 2. The tape was a woven fabric, the specific weight of the tape was 300 g/m2, its thickness was 0.7 mm with an average filament diameter of 6-7 μm, and it had a carbon content of 99.98%. The tape was cut into strips of width 25 mm.
The tape strips were fed into a reactor similar to that described with reference to
During operation the reactor was flushed with nitrogen or a nitrogen-air mixture at a rate set to 10 L/min, and cooling water was circulated through cooling coils around the electrode supports. To strike the arc, the cathode was moved forward until the discharge took place, then the cathode was withdrawn slightly to establish the arc. The current was set to approximately 20 A for run 1 and 16 A for run 2. The tape was fed through in one run at a speed of 3 mm/second for run 1 and 4 mm/second for run 2.
An additional cooling gas was introduced through a porous carbon anode 3 to cool the tape close to the arc attachment zone (as shown in
The tape samples were examined with a LEO Leica scanning electron microscope. The substrate was activated with the creation of nanoscale pores (by the loss of mostly non-graphitic carbon), on the tape for both runs.
The arc treated tapes were also found to have increased electrical conductivity significantly (despite losing some carbon). For example, an 0.5 mm thick PAN-derived carbon tape also processed as described above was found to have increased conductivity by about 30 times, with the sheet resistance dropping from around 8.5 Ohm per square as supplied to 0.3 Ohm per square.
The arc activated PAN derived substrate of run 2 was also found to have increased capacitance by around 15 times compared to the activated rayon tape of run 1. Electrochemical experiments indicated a specific capacitance of 165 F/g or 2.5 F/cm2. These experiments were performed in an aqueous electrolyte (5 M KOH) and with untreated carbon fibre as the counter electrode and a pseudo Ag/AgCl reference electrode.
A series of runs were conducted passing a Rayon-based carbon fibre tape as described in example 1 through the arc reactor similar to-that described with reference to
For each combination of current and cathode diameter a 2 metre length of rayon tape was loaded onto the feed spool of the reactor, tensioned into place with contact to the anode support, and touched with the cathode to begin the arc. The cathode was withdrawn until the image projected onto an external surface showed the desired gap, and the tape fed through at a rate of 3 mm/s by setting the control on the power supply to the motor driving spool 9 in
The foregoing describes the invention including preferred forms thereof and alterations and modifications as will be obvious to one skilled in the art are intended to be incorporated in the scope thereof as defined in the accompanying claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 573247 | Nov 2008 | NZ | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NZ2009/000271 | 11/30/2009 | WO | 00 | 8/10/2011 |
