WO2006072782A2

WO2006072782A2 - Tetracyanoquinodimethane composite material and use in electrodes

Info

Publication number: WO2006072782A2
Application number: PCT/GB2006/000017
Authority: WO
Inventors: Danny O'hare
Original assignee: Imperial Innovations Ltd
Priority date: 2005-01-07
Filing date: 2006-01-04
Publication date: 2006-07-13
Also published as: WO2006072782A3; GB0500289D0

Abstract

A composite material comprising a charge transfer complex of a tetracyanoquinodimethane (TCNQ) molecule of general formula (I), a binder substance and an isoalloxazine molecule is provided, in which the TCNQ molecule of general formula (I) is represented by Formula (I); in which R is represented by H, (C1-C6) alkyl, halide, cyanide, nitro, nitroso, amino, sulfonyl, thiol, sulfonate, silanyl, sulfate or (C1-C6) alkyl substituted with one or more of these functional groups or substituted by one or more electron donor compounds including: tetrathiafulvalene (TTF) in which the sulfur atom is optionally replaced by O, Se or Te, N-methyl-phenazinium, hexamethylene-tetratellurofulvalene, tetramethylammonium, methyltriphenyl-phosphonium, methoxytrifluoro- phosphonium, methoxytrifluoro-methylphenylacetic acid, hexamethylene- tetrathiafulvalene, ethylenedithio-tetrathiafulvalene (BEDTTF), BTP. Such materials may be used in the composition of electrodes.

Description

TETRACYANOOUINODIMETHANE COMPOSITE MATERIAL AND USE IN

ELECTRODES

The present invention relates to electrodes which have application in fuel cells using organic substrate as fuel.

Carbon pastes are suitable electrode material for enzyme-based biosensors and have been used since the 1970s (Kissinger et al Brain Research, 55, 209 (1973)). Unique to carbon pastes is the method of enzyme immobilisation where the enzyme mediators and enzyme stabilisers can be mixed into the carbon composite. However, the oil in the paste is not suitable for in vivo use (Ormonde & O'Neill J. Electroanal Chem., 261, 463 (1989)). Replacing the oil with solid or high viscosity insulator makes robust and reusable sensors with good biocompatibility and the same advantages as bulk-modified carbon paste sensors.

Biofuel-powered fuel cells using glucose as a fuel, amongst others, have been described in WO 02/47806, US 4,294,891 and US 5,660,940. However, such devices suffer from problems of suitability for in vivo use and technical issues resulting from current density or stability, and a requirement for a significant "over-voltage" to be present in order for the oxidation reaction to proceed. Such devices are of intricate, frequently layer-by-layer manufacture and expensive to produce on a large scale.

The present invention provides a means to overcome such problems as follows. A means has been found to increase the current density and stability in a fuel-cell powered by the oxidation of glucose. The invention also provides for modified composite electrodes in which the requirement for an "over-voltage" has been lowered, thus avoiding losses in the fuel cell and avoiding interference from other electrochemical reactions proceeding in an in vivo situation.

According to a first aspect of the present invention, there is provided a composite material comprising a charge transfer complex of a tetracyanoquinodimethane (TCNQ) molecule of general formula (I), a binder substance and an isoalloxazine molecule, in which the TCNQ molecule is the electron acceptor of general formula (I) is represented by:

in which R is represented by H, saturated or unsaturated (C₁-C₆) alkyl, halide, cyanide, nitro, nitroso, amino, sulfonyl, thiol, sulfonate, silanyl, sulfate or (C₁-C₆) alkyl substituted with one or more of these functional groups or substituted by one or more electron donor compounds including but not restricted to: tetrathiafulvalene (TTF), N-methyl-phenazinium, hexamethylene-tetratellurofulvalene, tetramethylammonium, methyltriphenyl-phosphonium, methoxytrifluoro- phosphonium, methoxytrifluoro-methylphenylacetic acid, hexamethylene- tetrathiafulvalene, ethylenedithio-tetrathiafulvalene (BEDTTF), BTP.

For all the TTF compounds, the sulfur may be replaced by any chalcogen. Chalcogens are the name for the periodic table group 6 (old-style: VIB or VIA) elements in the periodic table. The chalcogen group consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), the radioactive polonium (Po).

A suitable charge transfer complex of a tetracyanoquinodimethane (TCNQ) molecule of general formula (I) is TTF-TCNQ.

The term (C₁-C₆) alkyl refers to a straight, branched or cyclic hydrocarbon chain having from one to six carbon atoms which may be methyl, ethyl, isopropyl, n-propyl, butyl, t-butyl, pentyl, or n-hexyl. The group may be saturated or unsaturated at one or more locations in the carbon chain, for example characterised by the groups allyl, vinyl and/or other mono-or multiply unsaturated hydrocarbon chains. The term halide refers to an element of the halogen group of the periodic table, which includes fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

Optionally, the substituent group R may be independently substituted at more than one location on the ring in the TCNQ molecule of general formula (I) at positions 2, 3, 5 and/or 6

The TCNQ molecule of general formula (I) may be present in the composite material in a concentration of from 10% to 95%, suitably 15% to 80%, preferably of from 30% to 75%, and optimally at around 55% to 65%, or 60%.

The binder substance may be an inert insulating binder, an electrochemically active binder or a binder with a specific affinity. Inert insulating binders may be particularly suitable since such materials permit exploitation of microelectrode array-like behaviour of dilute dispersions near the insulator conductor transition. Generally, such binders will be able to resist the ingress of water, seal well to the conducting particles, have high breakdown voltages, and be chemically stable in water and common solvents. Suitable binders include, but are not limited to, epoxy resin, polytetrafluoroethylene (PTFE), polyvinylchloride (PVC), polymethylmethacrylate, polyester, polyimide, polyamide, silicone elastomers, KeI F, polyethylene, polypropylene and/or block or graft co-polymers of the foregoing. Epoxy resin may be a suitable binder material in some instances.

The isoalloxazine molecule may be isoalloxazine, or a derivative thereof, including but not limited to riboflavin, flavin mononucleotide (FMN) and/or flavin adenine dinucleotide (FAD).

The isoalloxazine molecule may be present in the material in an amount of from 0.1% w/w to 10% w/w, suitably, from 1% to 5%. Suitably the concentration of the isoalloxazine molecule in the material can be in the range of from ImM to 10OmM. According to a preferred embodiment of this aspect of the invention there is provided a composite material in accordance with the first aspect, which additionally comprises an enzyme which catalyses the oxidation of an oxidisable substance, for example a carbohydrate molecule.

The carbohydrate molecule may be a hexose or pentose sugar molecule, such as glucose or fructose, or a compound comprising such sugar molecules, such as sucrose. Other potential oxidisable substances suitable fur use as biofuel molecules include, but are not limited to, lactate or ethanol.

Where the carbohydrate molecule is glucose, the enzyme may one or more of glucose oxidase (E.C. 1.1.3.4), glucose dehydrogenase (PQQ dependent) (E.C. 1.1.99.7), quinoprotein glucose dehydrogenase (E.C. 1.1.5.2), PQQ-dependent glucose dehydrogenase from Erwinia sp. (E.C.1.1.1.1.47). Suitably, the enzyme may be glucose oxidase and the carbohydrate molecule oxidised may be glucose.

Alternatively, where fructose is the biofuel to be oxidised, the enzyme may be galactose oxidase. For ethanol, the enzyme may be alcohol oxidase or ethanol dehydrogenase, and for lactate the enzyme may be lactate dehydrogenase. Sucrose may be hydrolysed by invertase, followed by oxidation of the component sugar residues by glucose oxidase and/or galactose oxidase.

All of the enzymes referred to above may be wild-type or modified by means of techniques of recombinant molecular biology for enhanced stability and/or activity.

In some instances, it may be convenient to include a stabiliser molecule in the composite material in addition to an enzyme as described above. Such stabilisers include, but are not limited to, polyelectrolyte stabilisers such as polyethyleneimine (PEI), or dithiothreitol, or other molecules reactive with cysteine moieties.

According to a second aspect of the invention, there is provided an electrode which comprises a composite material of the first aspect of the invention. According to a third aspect of the invention, there is provided a fuel cell comprising an electrode of the second aspect of the invention as an anode, and a cathode electrode which comprises a metallised carbon composite material comprising carbon, a transition metal, platinum group metal or a rare earth metal, and a binder material.

The carbon may be present in the isoform of graphite or diamond and may be present as carbon particles (e.g. microparticles or nanoparticles), carbon fibres, carbon nanotubes, or boron-doped diamond. The proportion of carbon present in the cathode material may be up to 40%, up to 50%, up to 60%, up to 70% or up to 80%, or higher.

The metal may be Rhodium (Rh), Platinum (Pt), Palladium (Pa), Ruthenium (Ru), Iridium (Ir) or Nickel (Ni).

The cathode material may further comprise other modifier molecules, such as an isoalloxazine molecule as defined above, porphyrin, phthalocyanine Schiff bases such as salen or salphen compounds or a transition metal salt of such compounds.

A fuel cell may be composed of appropriate materials for its working environment. For non-medical or non-veterinary uses, the fuel cell may be composed any generally suitable material. For uses requiring implantation into the body of a live subject, the choice of materials may rely more on biocompatible materials, non-allergenic materials etc. Alternatively, the fuel cell may be suitable for operation on the skin of a human or animal subject.

In a medical use, the fuel cell may be used to power a device such as a pacemaker, a drug delivery device, a valve, a shunt, a pump or other device that requires power to operate. Such devices may be implanted into the body of a patient, or may be external to the body when in use.

The fuel cell may be particularly suitable for internal use in a biological environment where the oxygen is dissolved in the liquid environment of the fuel cell. In such embodiments, a permeable membrane may be used to achieve a physical separation of the electrodes from the biological environment in which the fuel cell is situated. In other embodiments, the electrodes may be covered by a gel or other substance which allows oxygen from the atmosphere to permeate through to the electrodes.

In a preferred embodiment of the invention, there is provided a fuel cell comprising an anode and a cathode, in which the anode electrode comprises a composite material comprising a charge transfer complex of a tetracyanoquinodimethane (TCNQ) molecule of general formula (I) as defined above, a binder substance, an isoalloxazine molecule, an enzyme which catalyses the oxidation of an oxidisable substance, and a stabiliser substance, and in which the cathode electrode comprises a metallised carbon composite material comprising carbon, a transition metal or a rare earth metal, and a binder material.

Suitably, the anode may be composed of TTF-TCNQ, epoxy resin, FAD, glucose oxidase and PEL The cathode may be composed of a composite material of rhodium and carbon with epoxy resin, in which carbon is present at 60% w/w.

The fuel cell may be constructed from any suitable material as described above with respect to the end use of the fuel cell.

Preferred features for the second and subsequent aspects of the invention are as for the first aspect mutatis mutandis.

The present invention will now be further described by way of illustration with reference to the accompanying Examples and Drawings which are not to be construed as being limiting on the invention. In the drawings, reference is made to a number of Figures in which,

FIGURE 1 shows detailed view (right) and schematic representation (left) of a total epoxy encapsulation electrode (made with protocol 1). FIGURE 2 shows detailed view (right) and schematic representation (left) of a plastic tubing encapsulated electrode (made with protocol X).

FIGURE 3 shows hollow Teflon™ electrode filled with the composite enzyme paste

FIGURE 4 shows amperometric curves of the 12 additions of 1 M glucose solution for a 70 % TTF-TCNQ, 10 % glucose oxidase + PEI, 1 % FAD hollow Teflon™ electrode.

FIGURE 5 shows calibration curve of a 70 % TTF-TCNQ, 10 % glucose oxidase + PEI, 1 % FAD hollow Teflon™ electrode.

FIGURE 6 shows parallel amperometric recording of the enzyme electrode (red curve) and of the rhodium-carbon electrode (blue curve) initially in 20 ml of O₂-saturated PBS. 380 μl of 1 M glucose solution were added at 100 seconds and 1950 μl at 400 seconds. The final concentration of the solution is 38 mM of glucose.

FIGURE 7 shows polarization curve (left) and power curve(right) for a biofuel cell composed of: a 2 mm thick 60 % rhodium-carbon cathode with a polymerized FAD film and a 70 % TTF-TCNQ anode containing 10 % glucose oxidase + PEI mixture and 1 % of FAD. Both electrodes were in a 0.1 M air-saturated glucose solution.

FIGURE 8 shows simple design of a complete biofuel cell. Both discs are wired with insulated silver wire and stuck together with epoxy.

Materials and Methods For the conductive part of electrodes, different materials were tested: micrometric graphite, Ultra F' purity graphite and glassy carbon spherical powder were bought from ALFA, rhodium (5 % wt) on activated carbon and platinum (5 % wt) on carbon were purchased from ALDRICH. The other conductor used in composite electrodes was TTF-TCNQ salt: tetrathiafulvalene (TTF) was purchased from ALDRICH and tetracyanoquinodimethane (TCNQ) from FLUKA. These crystals were synthesised following Bartlett's protocol [17].

The non-conductive part was low viscosity epoxy resin CIBA-GEIGY Araldite CY 1300 GB and Hardener HY 1301 GB from (Robnor, Swindon).

Enzyme glucose oxidase (β-D-Glucose:oxygen 1-oxidoreductase, EC 1.1.3.4) type II- s from Aspergillus niger and flavin adenine dinucleotide (FAD) were purchased from SIGMA and poly(ethylenimine) (PEI) was bought from ALDRICH.

Preparation of TTF-TCNO crystals

1 g of TTF powder was weighed and dissolved in 50 ml of acetonitrile. 1 g of TCNQ powder was weighed and put it in 50 ml of acetonitrile. The two solutions were gently heated and stirred on a heating plate under the hood until the powders were completely dissolved. The two solutions were mixed (without spilling the solutions on the hot plate) with continuous stirring; black crystals appeared instantaneously in a dark green solution. The solution was left overnight at 4 °C and then the crystals were filtered under vacuum and washed two- or three-times with acetonitrile. The washed crystals were spread in a Petri dish and allowed to dry overnight.

Preparation of FAD solution for electropolvmerisation

To make 10 ml of a 2.5 mM solution of FAD in HCl, 20.5 mg of FAD crystals were weighed and dissolved in a vial with a 0.1 M HCl solution. The resulting solution had a bright yellow colour and was kept at 4 ⁰C.

Preparation of benzoquinone in KCl solution from non-crystalline benzoquinone Pure benzoquinone crystals were obtained by sublimation from raw benzoquinone (black powder). Some benzoquinone was put in a Petri dish and spread all over the surface of the dish, then covered it with a bigger Petri dish. The whole thing was gently heated with a heating plate and the reaction carefully controlled because of the risk of explosion. While heating, yellow crystals (straw shaped) were observed to start growing at the lower surface of the second Petri dish. The reaction was kept going until the whole surface was covered with crystals.

Example 1 Construction of Composite electrodes

Two different protocols were devised. The first one was based on total epoxy encapsulation which allows having robust, well insulated, easy-made, reliable and good-looking electrodes. For this method, the total curing time was at least 4 days. The second protocol was a plastic tubing encapsulation.

Both methods have the same first step: making the composite paste by mixing in the desired ratio epoxy resin (proportion used was 2.6:1 araldite:hardener by volume), conductive material (graphite or TTF-TCNQ crystals), enzyme and optional additives (FAD, PEI, catalysts, etc). Depending on the ratio, this manual mixing may be really hard (especially with 60 % of conductive material). This step can be improved by gently heating (60 ⁰C) the araldite (to decrease its viscosity) before adding the conductive material and finally the hardener.

Then, the paste is packed into plastic tubing of 4 mm internal diameter and cured at ambient temperature for 24 hours followed by 2 hours at 60 ⁰C (for enzyme electrodes curing is at least 72 hours at 2 ⁰C).

Once cured, the composite rods were unpacked and depending on the protocol, they were repacked in epoxy (protocol 1) or directly sliced (protocol 2) with a rotating diamond saw (from BUEHLER).

The influence of thickness of electrodes was compared by cutting the rods in slices of 2 mm, 1 mm and 0.5 mm; the precision of the slice was 0.01 mm. Then the composite disks were wired to a 0.25 mm diameter silver wire (from JMC) using conductive silver loaded epoxy resin (from CIRCUIT WORKS). Depending on the protocol, the wired discs were repacked in epoxy (protocol 1) or insulated with heat-shrink sleeving and tightly fitted in 4 mm plastic tubing (protocol 2). The electrodes shown in Figure 1 and Figure 2 were obtained.

A range of TTF-TCNQ/epoxy electrodes, with concentration of conducting salt varying from 10 % to 80 %, were built in order to assess the conductivity of the organic conducting salt composite. The effect of thickness and polishing were also assessed.

Enzyme electrodes were cast using protocol 1 but the initial paste was cured for 4 days at 2 °C. The composition used was 60 % of TTF-TCNQ conducting salt, 1 % of Flavin Adenine Dinucleotide (FAD) and 1.5 % of enzyme-stabilizer mixture (Glucose Oxidase from SIGMA with 2 % poly(ethylenimine) (PEI) as mentioned by Khurana et al. (in Electroanalysis, 12, 1023-1030 (2003)) all mixed in epoxy.

To avoid the cutting and cleaning procedure, a silicone mould was made where 8 composite discs (of the same size as the previous ones) could directly be cast (4 x 2 mm thick, 2 x 1 mm thick and 2 x 0.5 mm thick). They were at least made of 70 % of TTF-TCNQ and different concentrations of glucose oxidase and FAD were tested: from 1 % to 10% of glucose oxidase (always with PEI). TTF-TCNQ-Enzyme discs were then wired with conductive epoxy to a silver wire and sealed in plastic tubing with quick curing epoxy glue.

AU the graphite-based electrodes were first roughly polished by hand using 600 and 2500 grit emery paper with water and fine polishing was achieved using alumina powder with grain size of 1 μm, 0.3 μm and finally 0.05 μm mixed with water on polishing paper until reaching mirror-like surface. However, the polishing was quickly assessed with magnifying glasses under light by looking at the epoxy surface around the electrode.

Polished electrodes were then surface-activated in two steps (only graphite electrodes were activated, not the TTF-TCNQ ones): 1. Cyclic voltammetry in 0.1 M H₂SO₄ solution between -1.5 V and 1.5 V at 1 V s^"1 for 15 minutes.

2. Applying fixed potential of -1.5 V in the same H₂SO₄ solution for 15 minutes.

Example 2: Studies on electrochemistry of electrodes in use

All the measurements were achieved with a three electrode set up (an Ag | AgCl home-made reference electrode, a platinum wire counter-electrode and composite electrodes) in a 30 ml electrochemical cell using a CH Instruments 1030 Multi- Potentiostat and CHI1030 Software.

• 0.1 M PBS pH 7.4 solution, to perform oxygen reduction.

• 2.5 niM FAD in 0.1 M HCl solution was used to perform electrochemical polymerisation of FAD on composite electrodes using the protocol described by Chi and Dong, J. Electroanalytical Chemistry, 369, 169-174 (1994) • Glucose (D-(+)-glucose purchased from SIGMA) stock solutions of 0.1 M and 1 M were made in 0.1 M PBS buffer and let overnight at 4 °C. They were then used for enzyme electrodes calibration.

Example 3: Electropolymerzation of FAD film on working electrodes As described by Chi and Dong (cited above) a 2.5 mM solution of flavin adenine dinucleotide (FAD)was prepared in 0.1 M HCl solution. Then, an initiation process was started by a cyclic scanning (3 cycles) of the working electrode between -1.2 V and 1.6 V at 50 mV s^"1 in the previous solution. Film growth was finally achieved by scanning continuously between -0.8 V and 0.5 V in the same solution at the same scan rate. The thickness of the FAD layer is controlled by the number of cycle and it was decided to do 25 cycles and 50 cycles in order to have a FAD film effective enough to promote oxygen reduction, which seems to have been successful. Modified working electrodes were then rinsed with distilled water.

Example 4: Enzyme electrode

Enzyme composite electrodes were first built following protocol. The initial rods (made of at least 60 % TTF-TCNQ, glucose oxidase, PEI and FAD) were cured at 4 ⁰C. The first set of electrodes was made using the same enzyme mixture as described by Khurana et at. (in Electroanalysis, 12, 1023-1030 (2003)) (1 % FAD and 1.5 % of the glucose oxidase-PEI mixture).

A further batch of electrodes was prepared based on a 60 % TTF-TCNQ composite paste with a proportion of 5 % glucose oxidase (and PEI) but with no FAD. This paste was used to cast 3 needle electrodes (made by casting the paste in a small piece of tubing of 0.5 mm in diameter, directly in contact with a silver wire encased in the same piece of tubing) and one 'plastic' electrode (see Figure 3) (paste was directly cast in the hole of a hollow Teflon™ electrode).

To assess the behaviour of the enzyme electrodes, a calibration of these electrodes in a PBS solution was performed. Aliquots of a 1 M solution of glucose (prepared the day before the experiment and let at 4 ⁰C overnight (Khurana et al Electroanalysis, 12, 1023-1030 (2003)) were added so as to cover the concentration range from 0 to 40 mM in glucose. Between each addition, the solution was stirred for 2 minutes; then the current at electrode was recorded for 1 minute (a potential of 0 V was applied). The amperometric curve (Figure 4) and the calibration curve below (Figure 5) were obtained.

The calibration curve has a classic shape for an enzyme electrode: a hyperbolic increase of the current at the beginning leading to a saturation (starting at the concentration of 10 mM) corresponding to the actual saturation of the enzyme. From the linear part of the curve (between 0 and 2 mM), the limit of detection was calculated (three times the standard deviation of the error in the slope carried out in the linear region of the graph) of the glucose electrode of the invention with the method described by Miller and Miller (in Statistics and chemometrics for analytical chemistry fourth edition, Pearson, Prentice Hall, Chapter 7, 128-213 (200)) . Z value of 0.187 mM of glucose was found. Example 5: Biofuel cell

Following preparation of a working anode and a working cathode, a whole cell was attempted. The first attempt was to record simultaneously the amperometric activity of a hollow Teflon™ electrode containing 80 % TTF-TCNQ with 10 % glucose oxidase + PEI on one channel, and the amperometric activity of a 60 % rhodium- carbon electrode with polymerized FAD film on a second channel. The curves shown in (Figure 6) were obtained.

It can be clearly seen that there was an increase of the current at the cathode while the oxygen in the solution is being reduced. At the anode, the first addition of the glucose causes an increase of the current until the saturation of the enzyme, and then the current decreases as the glucose is consumed. For the second addition, the current at the anode starts increasing again until reaching a constant value.

The next experiment was to directly put the two previous electrodes in a beaker containing 20 ml of a PBS solution constantly stirred and to follow the evolution of the potential between them with a voltmeter. The initial voltage was below the millivolt (out of the range of the voltmeter) and when 20 ml of a 0.1 M solution of glucose (concentration sufficient to reach the saturation of the enzyme) was added, the voltage started to increase and reached 250 mV in about 5 minutes. Pure oxygen was then bubbled in the solution and the voltage increased faster. 24 hours later, the voltage was about 320 mV and stopped increasing and it remained above 300 mV for more than five days (no more glucose added, continuously stirred and bubbled with oxygen).

Depending on the cathode used, different maximum voltages were obtained: the voltage observed using a 60 % rhodium-carbon electrode (of any thickness) with a polymerized FAD film was higher than the voltage of any 50 % rhodium-carbon electrode. It was also observed that there were higher voltage for cathodes that did not contain FAD directly in the composite. To see the influence of FAD in the enzyme electrode, other composite discs were cast containing 70 % of TTF-TCNQ, 10 % of the mixture glucose oxidase with PEI, and 1 % FAD. For the same experiment as presented above, a voltage of 430 mV was reached for one of the electrode made with those discs. For this cell, a polarisation curve was recorded (Figure 7). The maximum output power is 0.21 μW (between 1.8 and 2 μA, i.e. at around 100 mV) for an effective electrode surface of 25 mm².

Claims

1. A composite material comprising a charge transfer complex of a tetracyanoquinodimethane (TCNQ) molecule of general formula (I), a binder substance and an isoalloxazine molecule, in which the TCNQ molecule of general formula (I) is represented by:

in which R is represented by H, (C₁-C₆) alkyl, halide, cyanide, nitro, nitroso, amino, sulfonyl, thiol, sulfonate, silanyl, sulfate or (C₁-C₆) alkyl substituted with one or more of these functional groups or substituted by one or more electron donor compounds including: tetrathiafulvalene (TTF) in which the sulfur atom is optionally replaced by

O, Se or Te, N-methyl-phenazinium, hexamethylene-tetratellurofulvalene, tetramethylammonium, methyltriphenyl-phosphonium, methoxytrifluoro- phosphonium, methoxytrifluoro-methylphenylacetic acid, hexamethylene- tetrathiafulvalene, ethylenedithio-tetrathiafulvalene (BEDTTF), BTP.

2. A composite material as claimed in claim 1, in which the charge transfer complex of tetracyanoquinodimethane (TCNQ) molecule of general formula (I) is TTF-TCNQ.

3. A composite material as claimed in claim 1 or claim 2, in which the binder substance is an inert insulating binder, an electrochemically active binder or a binder with a specific affinity.

4. A composite material as claimed in claim 3, in which the inert insulating binder is epoxy resin, polytetrafluoroethylene (PTFE), polyvinylchloride (PVC), polymethylmethacrylate, polyester, polyimide, polyamide, silicone elastomers, KeI F, polyethylene, polypropylene and/or block or graft co-polymers thereof.

5. A composite material as claimed in any preceding claim in which the isoalloxazine molecule is isoalloxazine, or a derivative thereof,

6. A composite material as claimed in claim 5, in which the derivative of isoalloxazine is riboflavin, flavin mononucleotide (FMN) and/or flavin adenine dinucleotide (FAD).

7. A composite material as claimed in any preceding claim which additionally comprises an enzyme which catalyses the oxidation of an oxidisable substance.

8. An electrode which comprises a composite material as claimed in any one of claims 1 to 7.

9. A fuel cell comprising an electrode as claimed in claim 8 as an anode.

10. A fuel cell comprising an electrode as claimed in claim 8 as an anode and a cathode electrode which comprises a metallised carbon composite material comprising carbon, a transition metal, platinum group metal or a rare earth metal, and a binder material.

11. A fuel cell as claimed in claim 9, in which the cathode material further comprises a modifier molecule, such as an isoalloxazine, phthalocyanine (or its derivatives), porphyrin (or its derivatives) or transition metal complexes of one or more of these compounds.