WO2011006216A1

WO2011006216A1 - Electrode for electrochemical cells

Info

Publication number: WO2011006216A1
Application number: PCT/AU2010/001184
Authority: WO
Inventors: Bjom Winther-Jensen; Maria Forsyth; Douglas Robert Macfarlane
Original assignee: Monash University
Priority date: 2009-07-14
Filing date: 2010-07-14
Publication date: 2011-01-20

Abstract

The invention relates to an electrode for- oxygen reduction comprising a porous non-organic material and at least one electro-catalytic inherently conducting polymer such as a charge transfer complex or a conductive polymer, optionally combined with a non-conducting polymer. The electrode is suitable for use with an ion-conducting membrane and fuel to form a fuel-cell. The electrode is also suitable for use with an anode, such as a reactive metal and an electrolyte to form a battery.

Description

ELECTRODE FOR ELECTROCHEMICAL CELLS

Field of the Invention

This invention relates to electrochemical cells such as batteries and fuel cells. Even more particularly the present invention relates to electrochemical cells having an electrode comprising a porous non-organic material and an inherently conducting polymer electro-catalyst.

Background of the Invention

In this specification where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned.

While the present invention will be principally described with reference to use of a porous non-organic anode formed of carbon nanotubules it will be readily appreciated that the present invention is not so limited, but can be extended to electrochemical cells having a wide range of porous non-organic anodes such as other carbon nanostructures, foamed metals or metal coatings on foamed or fibre supports.

In their broadest sense, electrochemical cells store or convert chemical energy and make it available in an electrical form. Electrochemical cells include energy sources such as batteries and fuel cells.

Batteries

The term 'battery' typically refers to two, or more electrochemical cells connected in series, however the term is also used to refer to a single cell.

Batteries typically comprise an anode, a cathode and electrolyte in a sealed container and are directly delivering electrical current,.

There are myriad batteries commercially available. One of the best known types of commercial battery is the zinc-carbon battery which is packaged in a zinc container that also serves as an anode. Typically the cathode is a mixture of manganese dioxide and carbon powder. The electrolyte is a paste of zinc chloride and ammonium chloride dissolved in water. One of the many patents directed to batteries includes US 5,718,986, which teaches the use of batteries having a magnesium or aluminium anode, an inert cathode and a chlorite or hypochlorite based electrolyte for large scale applications such as providing power to cars.

A novel type of 'paper' battery has recently been developed at Rensellaer

Polytechnic Institute (Flexible Energy Storage Devices Based on Nanocomposite Paper, 13 August 2007, Proc, Nat. Acad. Sci). The nanoengineered battery is lightweight, ultra thin and completely flexible and comprised of paper infused with aligned carbon nanotubes. An ionic ^'liquid is used as the electrolyte. The nanotubes act as electrodes and allow the device to conduct electricity - functioning as both a lithium-ion battery and a supercapacitor. . The paper batteries can be stacked to boost the total power output. Paper is extremely biocompatible and these new batteries are potentially useful as power supplies for devices implanted in the body. The paper batteries have also been shown to work without added electrolyte - the naturally occurring electrolytes in human sweat, blood and urine being suitable to activate the paper battery.

There is growing interest in compact, light weight, thin film (less than 1 mm thick) batteries for biomedical and bionic applications. Biocompatible batteries are in demand to power out a range of biological devices including devices for controlling release of hormones, providing electrical stimulation of cell-growth, operating artificial retinas, or releasing electrical stimulus through a heart pacemaker.

Many of these applications do not require high discharge currents but rather flexibility in shape and size. The strategy in developing such a device involves selecting materials which themselves, and any reaction products, are biocompatible. The accepted definition of biocompatibility is 'the ability of a material to perform with an appropriate host response in a specific application' (Williams D. F., ed, Definitions in Biomaterials. Progress in Biomedical Engineering, 4, Amsterdam, Elsevier Publishers 1987). Biocompatibility is a convolution of certain characteristics of materials. For example the material must exhibit characteristics such as low toxicity, and the physical and mechanical design must be suitable for the specific application and have long life, preferably matching the lifespan of the recipient, so they do not need to be surgically replaced.

Fuel cells

A fuel cell, like a battery, converts chemical energy to electrical energy. However a battery typically holds a limited fuel supply in a sealed container whereas a fuel cell uses an ongoing supply of fuel to create a continuous flow of electricity. The external supply of fuel (the anode) and oxidant (the cathode) react in the presence of the electrolyte. Typically, reactants flow in and react to form reaction products which then flow out of the cell. The electrolyte remains in the cell.

One of the best known fuel cells is the polymer electrolyte membrane fuel cell (PEMFC) which comprises a proton-conducting polymer membrane (the electrolyte) which has an anode side and cathode side. At the anode side, hydrogen diffuses to an anode catalyst which causes the hydrogen to dissociate into protons and electrons. The protons flow through the proton-conducting polymer membrane to the cathode. Meanwhile, because the membrane is electrically insulating, the electrons travel in another circuit, thus supplying power. On the cathode catalyst, oxygen molecules react with the electrons (that have passed through the circuit) and protons, to form water. The water then flows out of the cell.

The working of fuel cells is principally based on catalysis, separating the electrons and protons of the reactant fuel, and forcing the electrons to travel through a circuit, thus creating electrical power. The catalyst is typically comprised of particulate platinum group metal or alloy. One of the problems associated with fuel cells is that platinum is expensive and the construction of fuel cells is typically complex. Furthermore, these cells suffer from problems including drifting of the particles of platinum catalyst leading to significant, rapid diminution of the catalytic effect.(Yu et a\, J. Power Sources 172 (2007) 145-154; Shao ef a/, J.Power Sources 171 (2007) 558-566)

Accordingly, there is an ongoing need for electrochemical cells that have optimised power output. There is also an ongoing need for electrochemical cells that meet the design and energy requirements of tomorrow's devices, implantable medical equipment and transportation vehicles. Summary of the.lnvention

The present invention provides an air-eleςtrode comprising a porous nonorganic material and at least one inherently conducting polymer (ICP) electro- catalyst.

. In a first embodiment, the present invention provides an electrochemical cell comprising an encapsulating means that encloses:

(a) an electrode comprising a porous non-organic material and at least one inherently conducting polymer electro-catalyst,

(b) an anode, and

(c) an electrolyte intermediate the electrodes.

The surprising and novel aspect of the present invention is the use of a porous non-organic material such as metal or carbon nano-structures.

In a second embodiment, the present invention provides an electrochemical cell comprising an encapsulating means that encloses:

(a) an electrode comprising a porous carbon nano-structure material and at least one inherently conducting polymer electro-catalyst,

(b) an anode, and

(c) an electrolyte Intermediate the electrodes.

A further surprising and novel aspect of the present invention is the use of the aforesaid electrode in combination with an anode in an electrochemical cell, such as a metal-air battery, or a fuel-cell.

The electrolyte intermediate the electrodes may be in any state. Typically, when the electrochemical cell is a battery, the electrolyte is a solid, liquid, gel or solution.

Accordingly, in a third embodiment of the present invention there is provided an electrochemical cell in the form of a fuel cell, the fuel cell comprising an encapsulating means that encloses:

(b) an anode, and

(c) an electrolyte comprising a gas, intermediate the electrodes. In a fourth embodiment of the present invention there is provided an electrochemical cell in the form of a fuel cell, the fuel cell comprising an encapsulating means that encloses:

(a) an electrode comprising a porous carbon nano-structure material and at least one inherently conducting polymer electro-catalyst;

(b) an anode, and

(c) an electrolyte comprising a gas, intermediate the electrodes.

The present invention further provides an electrode comprising a porous non-organic material and an inherently conducting polymer electro-catalyst which can succesfully be used in metal/air batteries and in fuel-cells or, for example, a hydrogen fuel-cell or a direct methanol fuel-cell.

Electrolyte

The electrolyte may be any known ion-conducting material in any suitable state - solid, liquid or gas or combinations thereof. This includes conventional aqueous or solvent based systems, ionic liquids (including protic ionic liquids), ionic plastic crystals, solid state electrolytes (eg Li-polyethylene oxide based systems) and proton (and hydroxyl ion) conducting membranes such as Nafioη®.

Typically, when the electrochemical cell is a battery, the electrolyte is a liquid, gel or solution. Alternatively, when the electrochemical cell is a fuel cell, the electrolyte comprises a gas or vapour or is in the form of an ion-conducting membrane such as Nation®.

For example, preferably the electrochemical cell is a battery having an electrolyte comprising one or more metal salts, including alkali metal or alkaline earth metal salts, such as halides or nitrates. The electrolyte is typically aqueous, and/or may comprise a gel. The gel could be formed, for example, from polyethylene oxide. The electrolyte may alternatively be non-aqueous such as, for example, an ionic liquid or ionic liquid gel.

Electrolyte additives

Various additives may be added to optimise the electrolyte characteristics. For example, additives may be chosen from the group comprising solvents that act as 'swelling agents', non-solvents, ionic-liquids and phosphates. The role of these additives is to enhance the interaction between the electrolyte and the conducting polymer, that is, to help optimise the three phase interface. For example, the additives may improve the structure of the conducting polymer by causing it to swell.

Anode

Typically, the anode will contain metal, but the person skilled in the art will appreciate that other types of anodes can also be used in the electrochemical cell of the present invention. For example, the electrochemical cell may have a catalytically active anode or a non-metal anode.

When the electrochemical cell of the present invention is intended for in vivo use, typically the anode comprises a bio-compatible metal or metal alloy, such as magnesium or magnesium alloy. If the electrochemical cell is not intended for use in vivo the metal alloy can be chosen from any metal that has a suitable electrochemical potential when compared to the ICP chosen for the cathode. The anode could include, for example, magnesium, aluminium, zinc, iron or lithium metals or their alloys.

In another embodiment of the present Invention, the anode may comprise a metal catalyst in combination with other materials, or alternatively, the anode may not have any metal content whatsoever. For example, if the electrochemical cell is a common fuel cell, the anode could be mainly carbon with a platinum catalyst. Furthermore, if the electrochemical cell is a 'blo-battery^* (which is actually a bio-fuel cell) the anode could be an enzyme providing the catalytic

'function¹. Ultimately, organic catalysts could be used as anodes in fuel-cells of this type.

Normally reduced conjugated polymers such as polyterthiophen or poly-3- methyl-thiophene may also be suitable for use as an anode in the electrochemical cell of the present invention.

The combination of an anode and an electrode comprising an ICP may provide a higher electromotive output than the use of an ICP for both the anode and cathode. Without wishing to be bound by theory, It is likely that certain metals such as magnesium, when used as an anode may cause the ICP to remain in a partly oxidised state, thus maintaining sufficient conductivity to work as cathode.

Cathode

(i) Porous non -organic material Preferably the porous non-organic material comprises carbon or one or more metals. It is important that the porous material permits diffusion of air/oxygen through to the ICP electro-catalyst and provides a three-phase interface over a large surface area. The person skilled In the art will appreciate that this may be achieved by careful control of the pore size and/or hydrophobicity of the material.

The porous non-organic material can be made of any suitable conducting material having sufficiently low electrical resistance and high surface area. In a preferred embodiment the non-organic material is based on a metal foam, or carbon structure. Suitable carbon structures may include, for example, fullerenes, endohedral fullerenes, graphenes, carboranes, nanosheets, nanotubes, nanoribbons, single walled nanotubes, nanowhiskers, nanonails, other carbon nanostructures, nanostructure encapsulating fullerenes (e.g. 'peapods') or metalofullerenes and nanostructures on graphenes.

Optimally, the structure, particularly the pore size of the porous nonorganic material can be readily controlled. For example, single-wall carbon nanotubules (which are quite straight) can be combined with multi-wall carbon nanotubules (which can include many bends) to control structure, including pore size.

The porous non-organic material may be included on a support such as a fabric, polymer or ceramic support. The support is inert, that is, it does not contribute to the functioning of the cathode. In a particularly preferred embodiment the porous non-organic material is formed by metal coated on a fabric, polymer or ceramic. In another preferred embodiment the porous non- organic material is formed by depositing carbon structures on a fabric, cellulosic, polymeric or ceramic support.

Suitable polymeric supports may, for example, be based on polypropylene, polyvinylidene fluoride (PVDF) or polyethylene polymers although in some applications cellulosic polymers, such as paper, may be suitable. In a particularly preferred embodiment the porous material is chosen from Goretex®, CelGard® K880, Nafion® or a PVDF membrane such as those marketed by Millipore. Goretex® is a material comprising a microstructure of node and fibrils of polytetrafluoroethylene and described in US-3,953,566. Goretex® has 1.4 billion pores per cm². CelGard® K880 is a polyethylene membrane having similar pore size and structure to Goretex. The Millipore PVDF membrane has significantly smaller pores. Nation® is a proton conducting membrane composed of sulfonated tetrafluoroethylene based fluoropolymer-copolymer.

Coating or deposition of the porous non-organic material onto the support may be achieved by any means known in the art.

(H) Inherently Conducting Polymer electro-catalyst

^' ICPs can be divided into two general classes namely (1 ) charge transfer complexes and (2) conductive polymers including polyacetylenes, polypyrroles, polythiophenes, polyanilines, polyfluorenes, poly(3-hexylthiophene), polynaphthalenes, poly(3,4-ethylenedioxythiophene), poly(p-phenylene sulphide), poly(para-phenylenevinylenes) and their derivatives. In a particularly preferred embodiment the conductive polymer cathode is chosen from class (2). A cathode comprising a polymer in the oxidised state (polaron conductivity) is preferable; a polymer in the reduced state (exhibiting semi-conductor behaviour) typically has higher resistance which may overly limit the performance of the battery of the present invention.

It will be readily apparent to the person skilled in the art that the choice of ICP will depend on the nature of the cell. For example, polypyrroles and polythiophenes have a tendency to degrade in some environments. The suitability of an ICP for a particular application can typically be gauged by using cyclic voltammetry. For example, cyclic, voltammetry indicates that polyacetylenes tend to have an optimal range of capacitance for reducing O₂ at a required voltage.

In a particularly preferred embodiment, the ICP is a polypyrrole or poly(3,4- ethylenedioxythiophene) (PEDOT), The electro-catalytic mechanism of operation of PEDOT is only understood on a very basic level but its redox capability is believed to play a key role. This suggests a- totally different mechanism of catalysis as compared with metals - without the well-known drawbacks associated with metals such as poisoning by CO. PEDOT is particularly preferred due to its long-term stability as an electro-catalyst, which is a characteristic not seen in other conducting polymers. Furthermore, PEDOT exhibits conversion currents and over-potential that are comparable to Pt, which is surprising for an organic material.

Synthesis of ICP's including PEDOT by base inhibited oxidative polymerisation of thlophenes and anilines using Fe(III) salts has been previously described in WO 2005/103109. Other preferred embodiments of the ICP include derivatives of PEDOT, Changes in the basic PEDOT structure by relatively simple substitutions may change the ICP properties. For example a substituted

PEDOT (Formula I) and ProDOT (Formula II) may be suitable for use with the electrode of the present invention.

Formula I

Formula Il

Substituent 'A' of Formula I can include a wide range of moieties, but preferably the substituent increases the hydrophilicity of the molecule and any polymer formed there from without compromising the conjugation of the polymer. For example, 'A' may constitute an alkane chain linking PEDOT and OH (PEDOT- (CH_∑J_n-OH, where n is typically between 0 and about 12. Alternatively, 'A' may for example comprise COH, or a moiety comprising a glycol oligomer such as (PEDOT-(C)_p-(O(CH₂)_m)n-X, where typically m and p can be 1 to 4, n can be 0 to about 12 and X can be OH, OCH₃, OOH, COOH, COONa, SO₃Na.

In yet another alternative, the PEDOT may be substituted in both position 3 and 4 of the di-oxy ring, (e.g. HO-(CH₂)m-PEDOT-(CH₂)n-OH, where n and m may be the same or different.

The two substituents ¹X' of Formula Il may be the same or different and can include a wide range of moieties. Preferably, each substituent X, is independently chosen from the group comprising halides, H, alkanes, aromatics, ethers, aldehydes, carboxylic acids. In a particularly preferred embodiment each substituent X is independently chosen from the group comprising halides, H₁ CH₃, C₆H₅ and OCH₃. When choosing an optimal ICP for use in an electrode, consideration must be given to the nature of the ICP. For example it is anticipated that in accordance with their order of decreasing hydrophilicity, the PEDOT-COH, PEDOT and ProDOT ICPs will perform in the order PEDOT-COH > PEDOT > ProDOT irrespective of the fact that PEDOT-COH has a conductivity that is only about 1% of the conductivity of PEDOT. Furthermore, it must be kept in mind that PEDOT- COH is unstable in alkaline solutions, thus limiting its range of application to electrochemical cells having neutral and acidic electrolytes.

The electrode of the present invention may contain one or more ICPs. For example, the electrode may include two or more, ICPs in a physical mixture or a layered structure or an interpenetrating network. Furthermore, the ICP for use in the present invention may comprise one or more ICPs in combination with one or more non-conducting polymers; The combination of non-conducting polymer and ICP may provide characteristics that are preferable to the characteristics of the ICP alone. Thus mixing or blending the two may be carried out to 'tune^* the electro-catalytic layer by adjusting its characteristics such as the hydrophobicity, hydrophilicity, phase interface, current density, rheological characteristics or diffusivity (of O₂, OH^' or the like). For example, a non-conducting polymer based on polyethylene glycol (PEG) may be added to provide improved hydrophilicity, phase interface, current density or rheological characteristics compared with the pure ICP. Typically this would be achieved by blending non-conducting polymer into the ICP as the latter undergoes polymerisation.

Combination of porous non-organic material and ICP

The porous non-organic material and ICP of the present invention may be combined in any convenient manner. For example, the ICP can be coated onto the porous non-organic material by any appropriate method. The ICP may be applied in the form of a solid, liquid or geL Preferred application methods include in-situ chemical or electro-chemical polymerisation of the ICP, coating from a solution or suspension of the ICP, photo-polymerisation, vapour phase polymerisation, plasma-polymerisation (eg low power AC plasma-polymerisation) or any other means known. Good control over the thickness of the applied layers is necessary in order to avoid the ICP blocking the pores in the porous material. The assembled 3-dimensional porous non-organic structure can be coated. Alternatively, or in addition, constituent parts (eg fibres or particles) of the structure can be coated and then assembled (eg by weaving, pressing, filtering, sintering or self-assembly) into the 3-dimensional porous structure. The large surface area provided by the 3-dimensional porous structure means that it is possible to achieve current densities (measured in current per geometric area) and low ohmic losses required for applications such as fuel cells in cars and high power zinc-air batteries for cochlear implants.

By virtue of the pores in the non-organic material, the ICP, electrolyte and air are in a close three-phase contact on the μm scale. Control of the interface can be obtained by several different means including (i) controlling the hydrophilicity of the ICP (eg by blending non-conducting polymers into the ICP), (ii) controlling the nature of the ICP, and/or (ιii) applying an additional coating, for example, to obtain a gradient of hydrophilicity through the coating.

Encapsulating means

The encapsulating means may be constructed of any convenient material. When the electrochemical cell is a battery, the principal purpose of the encapsulating means is to contain the electrolyte intermediate the metal anode and conducting polymer cathode. When the battery is intended for In vivo use it also forms a barrier between the components of the battery and living tissue so at least the outermost part of the encapsulating means is preferably constructed of bio-compatible material. In a preferred embodiment the entire electrochemical cell is made of bio-compatible, bio-degradable material and the encapsulating means is the first component of the battery to degrade. When the electrochemical cell is a fuel cell, the encapsulating material will be adapted to allow inflow of reactant and outflow of reaction products.

The electrical contact to the porous non-organic structure can be established by any convenient means such as wires, via a conducting frame or any other means suitable for maintaining the functionality of the cathode.

Other features

The electrochemical cell according to the present invention may comprise a battery that provides a direct current (DC). However by combining two batteries appropriately wired, and switching between the two, it is possible to provide an alternating current (AC). This would be particularly advantageous for many in- vivo applications where the use of DC causes damaged caused by electrophoresis.

In another preferred embodiment the electrochemical cell of the present invention can be switched on and off using a magnetic switch. This would be particularly advantageous for in vivo applications so that the electrochemical cell can be activated and turned off by a magnetic switch located outside the body. Examples

The present invention will be further illustrated with reference to the following non-limiting examples and drawings in which;

• Figure 1 is a Scanning electron micrograph of carbon nanotubule paper (CNT-paper) coated w^'rth PEDOT according to Example 1 (magnification 1000Ox, 15kV with no additional conducting coating),

• Figure 2 is a schematic drawing of an electrochemical cell including the electrode of Figure 1 ,

• Figure 3 is a plot of I (mA/cm²) against Ewe (V vs SCE) illustrating oxygen (from air) conversion current as a function of potential for the electrodes of:

Comparative Example 1(a) ((i) - PEDOT on AU/Goretex O₂ reduction, 1 M H₂SO₄),

- Comparative Example 1(b) ((H) - Pt on Au/Goretex O₂ reduction, 1 M

H₂SO₄), and

Example 1 ((Ui) - PEDOT on CNT-paper, 1 M H₂SO₄), and

• Figure 4 is a plot of I (mA/crn²) against Ewe (V vs SCE) illustrating steady state measurements of oxygen reduction in alkaline solutions for the electrodes of:

Comparative Example 1(a) ((i) - Pt o.n AU/Goretex, 1M KON), Comparative Example 1(b) ((ii) - Pt on Goretex/Au, 1 M KOH), and Example 1 ((Hi) - PEDOT on CNT₁ 4x spin-coated, 0.5M NaOH)., Example 1

Electrode comprising carbon nanotubes

CNT-paper was made by filtering a suspension of multi-walled carbon nanotubules in toluene through a PVDF membrane (0.1 μm pores). The carbon nanotubules formed a "mat" or "paper" on the membrane, and were lifted off as a freestanding 3-dimensional material The thickness of the carbon nanotubule- paper was 0.1 mm and the resistance 4 ohm/square.

The CNT-paper was coated with PEDOT using vapour phase polymerisation. The Fe(III)PTS (PTS = para-toulene-sulfonate) oxidant in n- butanol solution (40% from Bayer AG + 0.8% pyridine) was soaked into the CNT- paper for 1 minute. The CNT-paper was taken out of the solution and the excess of solution was thereafter allowed to drain out of the CNT-paper. After drying in the oven at 7O⁰C for five minutes the CNT-paper/Fe(lll)PTS was exposed to PEDOT vapour for 1 hour at 70⁰C, washed twice in ethanol and dried over night at room temperature.

The PEDOT coated CNT-paper was laminated with a Goretex membrane using a conventional office laminator. (As an alternative, a PVDF membrane could be used to obtain the same result.) Before laminating an area was cut out of the laminate to allow access of electrolyte and air to the PEDOT/carbon nanotubule and Goretex side respectively.

Figure 1 shows the micrograph of the CNT-paper coated with PEDOT. The porous structure of the CNT-paper is clearly seen indicating a very thin PEDOT coating over the entire CNT area.

Comparative Example 1 (a)

A comparative electrode that was constructed by coating PEDOT onto one side of a sheet of Goretex® (commercially available from Gore Inc.). This process provides a plasma-polymerised poly-acid layer on one side of the Goretex that provides good bonding to the PTFE and ensures that the oxidant (Fe(III)PTS) stays on that side, with the PEDOT only polymerised on one side during vapour phase polymerisation (VPP).

Specifically, an acid monomer such as maleic anhydride is plasma polymerised on one side of the Goretex using a low power AC plasma discharge operating in a plasma chamber. The plasma parameters were tuned to ensure good binding between the plasma-polymer and the Goretex substrate. For electrodes including a metallic current conductor, the plasma chamber may be further equipped with a magnetron working as a sputter unit allowing the plasma polymerised material and sputtered layer to be applied in the same chamber. An oxidant for the polymerisation, iron(lll) para-toluenesulfonate (Fe(III)PTS) obtained from H. C. Starck in a 40% solution in butanol, was then applied to the polyacid. Vapour phase polymerisation of the conducting polymer was then carried out. Once the polymerisation was complete, the Fe(II) and excess of anion was washed out with ethanol. The layer of PEDOT was typically about 400 nm thick (equivalent to about 0.05 mg/cm²) but for other embodiments the optimal thickness will change with pore size, shape and other characteristics of the porous material. When a current conductor was included in the electrode the layer thickness was optimised to give the desired surface resistance on the PTFE membrane. For example, the optimal surface resistance was between 12 and 15 Ohm/sq,

The electrode also includes a thin layer (approx. 20 nm) of gold between the ICP and Goretex, the gold acting as a conductor.

Comparative Example 1(b)

A PEDOT-Au/Pt-Goretex electrode was created by sputtering a 45 nm Pt layer onto the Au layer. The thickness of the Pt was measured on a glass slide exposed to same Pt sputter procedure. Although the thicknesses are different for the R (45 nm) and PEDOT (400 nm) layers the differences in their densities (21 .1 g/cm³ for Pt and approx 1.2 g/cm³ for PEDOT) means that the mass loading of active material is actually lower in the PEDOT case by a factor of about 2.

The CNT-paper electrode of Example 1 was then- included in an electrochemical cell of the type depicted at Figure 2 and used in a series of experiments to characterise the present invention and compare its performance with other electrode constructions described in Comparative Examples 1(a) and 1(b).

In Figure 2 the reference electrode (10), platinum counter (11 ), gold connector (12) and electrode (13) of Example 1 (or alternatively Comparative Example 1(a) or 1(b)) can be clearly seen. A 1 x 1 cm² window in the laminate allows access for air from the bare side of the porous material and for electrolyte from the ICP coated side when mounted on the test cell. Phosphate buffer electrolytes were used to maintain pH values.

For measuring the resistance of the ICP during operation (to calculate the electronic conductivity) a special laminated layout was used. Here a 0.5 x 1 cm² window was used, the porous material membrane was cut to a 0.6 x 2 cm² and two gold connectors were connected to each end of the membrane. The resistance was measured between these gold connectors. The gold connectors were not in contact with the electrolyte during the measurement.

For the electrochemical testing a multi-channel potentiostat (VMP2 from

Princeton Applied Research) was used to apply potential and measure the resulting conversion current, Steady-state measurements of the conversion current were obtained after one hour at the given potential. A saturated calomel reference electrode was used to control potentials; the internal structure of the electrode presents an unknown internal resistance (and hence a potential shift) in these measurements. Potentials have therefore been used for comparison purposes only.

Example 2

The laminate electrodes of Example 1(CNT), Comparative Example 1(a) (PEDOT-Au/Goretex) and Comparative Example 1 (bX Pt-Au/Goretex) were each mounted on a container with electrolyte and connected as working electrode (Pt counter electrode and SCE reference electrode). Figure 3 shows steady-state measurements of the conversion current from the oxygen reduction at various potentials in 1 M H₂SO₄. This pH is selected for simulating PEMFC conditions with a proton conducting membrane. Steady state was obtained after 30min at each potential step. From Figure 3 it is clearly seen that the CNT-paper based electrode performs significantly better than the Goretex based PEDOT or Pt. However, based on "real" surface area the increase should have been even more significant.

Example 3

Example 1 was repeated using a thinner layer of CNT coated on the Goretex membrane and it was not lifted off, but allowed to remain on the membrane. The resulting layer had a resistance of 18 ohm/square indicating a thickness in the 20 μm range. The coating with PEDOT, laminating and testing was done in similar way as described in Examples 1 and 2.

At -30OmV vs. SCE an oxygen conversion current of 3mA/cm² was obtained. This lower value is reflecting the thinner CNT-paper as well as the higher resistance of the CNT-paper. Example 4

Example 1 was repeated with the CNT-paper being coated with PEDOT by adding 4 layers of PEDOT onto the CNT-paper using spin-coating of the oxidant followed by vapour-phase polymerisation of PEDOT. This procedure ensured that the thickness of the PEDOT coated onto the individual CNTs decreased through the thickness of the CNT-paper. The coated CNT-paper was laminated with a gold wire and used without further treatment as air-electrode under alkaline conditions.

Figure 4 shows steady-state measurements of the conversion current from the oxygen reduction at various potentials in alkaline electrolytes typical for metal- air battery conditions. Steady state was obtained after 30 min at each potential step.

Figure 4 also shows the conversion currents for the CNT-paper electrode compared with the currents for the electrodes of Comparative Example 1(a) (PEDOT on Goretex/Au) and Comparative Example 1(b) (Pt on Goretex/Au).

The very significant increase in conversion current shown in Figure 4 is due to the increase in active catalytic area provided by the CNT-PEDOT combination. It should be mentioned that currents in the range of 10-30 mA/cm² are comparable with the currents generated by prior art zinc-air batteries.

Example 5 '

Electrode comprising a mixture of carbon nanotubes

CNT-paper was made by filtering a suspension of single-walled and multi- walled carbon nanotubes (ratio 1 :3) in water with TritonX surfactant through a PTFE membrane (5 μm pores). The carbon nanotubes formed a paper on the membrane which was not lifted off, but allowed to remain on the membrane. The CNT-paper was washed twice with ethanol to remove surfactant. The thickness of the CNT-paper was 0.02 mm and the resistance 20 ohm/square.

The CNT-paper was coated with PEDOT using vapour phase polymerisation. The Fe(III)PTS oxidant in n-butanol solution (10% in 1-butanol from Bayer AG + 0.2% pyridine) was spin-coated onto the CNT-paper at 1500 rpm followed by vapour phase polymerisation (VPP) of PEDOT at 70⁰C. After 30min of polymerisation the spin-coating/VPP procedure was repeated twice before the CNT-paper/PEDOT was washed in ethanol to remove Fe(II) and excess of PTS and dried in air.

When tested as air-electrode (see Example 3) in 1 M NaOH solution at - 40OmV vs. SCE an oxygen conversion current of 30mA/cm² was obtained.

Example 6

Electrode comprising metal coated fabric

Cu/Ni coated fabric (Laird Technologies no. 3055-213) was used as a model materia! for manufacture of high surface area metal electrodes. One side of the fabric was coated with a hydrophobic, polyethylene-like coating using plasma-polymerisation of dodecene at 10Pa and 1 W/l in a 50Hz AC plasma chamber (see also US6628084 and US2004050493 for further details) for 3 min. The uncoated side of the fabric was then coated with a PEDOT/polyethyleneglycol (PEG) blend according to the following procedure: 6.5ml of lron(lll)PTS (40% in butanol solution with 0.8% pyridine) was mixed with 1 ml of PEG (20% in water (MW 20000)) at room temperature. The mixture was applied to the metal-coated fabric by spin-coating at 1500rpm followed by vapour phase polymerisation (VPP) of PEDOT. Three layers of PEDOT were applied by

VPP in the same way as described in Example 5, followed by an ethanol wash.

When tested as air-electrode (using the, testing procedure disclosed in Example 3) in 1 M NaOH solution at -40OmV vs. SCE (with the poly-dodecene coated side away from the electrolyte) an oxygen conversion current of 8 mA/cm² was obtained. This result reflects the fact that a large interface area was obtained by the procedure, albeit lower than in the case of carbon nano tubes. When compared to similar electrodes prepared with PEDOT (not PEDOT/PEG), the conversion current using the PEDOT/PEG blend was twice as high. This is attributed to the higher diffusion rate of O₂ and/or OH^' when PEG is present in the catalytic layer.

The word 'comprising' and forms of the word 'comprising' as used in this description does not limit the invention claimed to exclude any variants or additions. Modifications and improvements to the invention will be readily apparent to those skilled in the art. Such modifications and improvements are intended to be within the scope of this invention.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1 A cell comprising an encapsulating means that encloses:

(b) an anode, and

(c) an electrolyte intermediate the electrodes.

2. A cell comprising an encapsulating means that encloses:

(a) an electrode comprising a porous carbon nano-structure material and at least one inherently conducting polymer electro-catalysl,

(b) an anode, and

(c) an electrolyte intermediate the electrodes,

3. A cell according to claim 1 or claim 2 chosen from the group comprising electrochemical cells and fuel cells.

4. A cell according to claim 1 or claim 2 wherein the porous non-organic material is chosen from the group comprising metal or carbon.

5. A cell according to claim 1 or claim 2 wherein the inherently conducting polymer electro-catalyst additionally includes a non-conducting polymer.

6. A cell according to claim 4 or claim 5 wherein the porous non-organic material is included on a support.

7. A cell according to claim 6 wherein the support is a polymeric support chosen from the group comprising polypropylene, polyvinylidene fluoride, polyethylene polymers, cellulosic polymers or combinations thereof.

8. An electrode suitable for use in the cell of claim 1 or claim 2 wherein the electrode comprises inherently an conducting polymer electro-catalyst, a porous non-organic material and a support.

9. An electrode according to claim 8 wherein the Inherently conducting polymer electro-catalyst is chosen from the group comprising polyacetylenes, polypyrroles, polythiophenes, polyanilines, polyfluorenes, poly(3-hexylthiophene), polynaphthalenes, poly(3,4-ethylenedioxythiophene), poly(p-phenylene sulphide), poly(para-phenylenevinylenes) and their derivatives and is coated on a laminate comprising a support and a porous non-organic material chosen from carbon structures and metal.

10. An electrode according to claim 8 wherein the porous non-organic material is a carbon structure chosen from the group comprising fullerenes, endohedral fullerenes, graphenes, carboranes, nanosheets, nanotubes, nanoribbons, single walled nanotubes, nanowhiskers, nanonails, nanostructures encapsulating fullerenes, metalofullerenes, nanostructures on graphenes and combinations thereof.

11. An electrode according to claim 8 wherein the porous non-organic material comprises carbon nanotubule-paper, the carbon nanotubules being chosen from single-wall carbon nanotubules, multi-wall carbon nanotubules and combinations thereof.

12. An electrode according to claim 8 wherein the porous non-organic material is a metal coated on fabric support.

13. A method of forming the electrode of claim 8 which includes a step comprising one or more manufacturing technique chosen from the group comprising coating, vapour depositing or laminating any one or more of the Inherently conducting polymer electro-catalyst, porous non-organic material or the support.

14. A cell according to claim 1 or claim 2 and substantially as herein described with reference to the examples.

15. An electrode according to claim 8 and substantially as herein described with reference to the examples.