GB2467148A - Redox Fuel Cells - Google Patents

Abstract

A redox fuel cell comprising an anode and a cathode separated by an ion selective polymer electrolyte membrane; means for supplying a fuel to the anode region of the cell; means for supplying an oxidant to the cathode region of the cell; means for providing an electrical circuit between the anode and the cathode; a catholyte solution comprising a modified ferrocene species comprising at least one electron withdrawing substituent on at least one cyclopentadienyl ring, the substituent being separated from the ring by a spacer group, the modified ferrocene species being at least partially reduced at the cathode in operation of the cell, and at least partially re-generated by reaction with the oxidant after such reduction at the cathode.

Description

FUEL CELLS

The present invention relates to fuel cells, in particular to indirect or redox fuel cells which have applications as power sources for: portable products such as portable electronics products; for transport vehicles such as automobiles, both main and auxiliary; auxiliary power for caravans and other recreational vehicles, boats etc; stationary uses such as uninterruptible power for hospitals computers etc and combined heat and power for homes and businesses.

The invention also relates to certain catholyte solutions for use in such fuel cells.

Fuel cells have been known for portable applications such as automotive and portable electronics technology for very many years, although it is only in recent years that fuel cells have become of serious practical consideration. In its simplest form, a fuel cell is an electrochemical energy conversion device that converts fuel and oxidant into reaction product(s), producing electricity and heat in the process. In one example of such a cell, hydrogen is used as fuel, and air or oxygen as oxidant and the product of the reaction is water.

The gases are fed respectively into catalysing, diffusion-type electrodes separated by a solid or liquid electrolyte which carries electrically charged particles between the two electrodes. in an indirect or redox fuel cell, the oxidant (and/or fuel in some cases) is not reacted directly at the electrode but instead reacts with the reduced form (oxidized form for fuel) of a redox couple to oxidise it, and this oxidised species is fed to the cathode (anode for fuel).

There are several types of fuel cell characterised by their different electrolytes.

The liquid electrolyte alkali electrolyte fuel cells have inherent disadvantages in that the electrolyte dissolves CO2 and needs to be replaced periodically.

Polymer electrolyte or PEM-type cells with proton-conducting solid cell membranes are acidic and avoid this problem. However, it has proved difficult in practice to attain power outputs from such systems approaching the theoretical maximum level, due to the relatively poor electrocatalysis of the oxygen reduction reaction.

In addition expensive noble metal electrocatalysts are often used, It would be preferable to use a less costly inert electrode, such as one formed of or coated with carbon, nickel or titanium. However, prior art cells in which inert electrodes have been utilised have produced unsatisfactory power output.

Is Many current fuel cell technologies employ cathodes where oxygen gas is flowed directly to the electrode where it then reacts with a catalyst to produce water, In many cases the catalyst employed is platinum, a precious metal.

Not only does this increase the costs of the overall fuel cell, but the inefficiency of the reaction leads to a loss in available power.

US-A-3152013 discloses a gaseous fuel cell comprising a cation-selective permeable membrane, a gas permeable catalytic electrode and a second electrode, with the membrane being positioned between the electrodes and in electrical contact only with the gas permeable electrode. The electrodes are formed of platinum, iridium or other noble metal electrocatalysts. An aqueous catholyte is provided in contact with the second electrode and the membrane, the catholyte including an oxidant couple therein. Means are provided for supplying a fuel gas to the permeable electrode, and for supplying a gaseous oxidant to the catholyte for oxidising reduced oxidant material. The preferred catholyte and redox couple is HBr/KBr/Br2. Nitrogen oxide is disclosed as a preferred catalyst for oxygen reduction, but with the consequence that pure oxygen was required as oxidant, the use of air as oxidant requiring the venting of noxious nitrogen oxide species.

An acknowledged problem concerning electrochemical fuel cells is that the theoretical potential of a given electrode reaction under defined conditions can be calculated but never completely attained. Imperfections in the system inevitably result in a loss of potential to some level below the theoretical potential attainable from any given reaction. Previous attempts to reduce such imperfections include the selection of mediators which undergo oxidation-reduction reactions in the catholvte solution. For example, US-A- 3294588 discloses the use of quinones and dyes in this capacity. However, despite the electrodes being coated with platinum, relatively low output was obtained during running of the cell. Another redox couple which has been tried is the vanadate/vanadyl couple, as disclosed in US-A-3279949. In this case, the slow rate of reduction and oxidation of the vanadium couple reduces its performance. This problem is exacerbated by the insolubility of the vanadium couple. The same vanadium couple was used in US4396687, According to U&A-3540933, certain advantages could be realised in electrochemical fuel cells by using the same electrolyte solution for both catholyte and anolyte. This document discloses the use of a liquid electrolyte containing more than two redox couples therein, with equilibrium potentials not more than 0.8V apart from any other redox couple in the electrolyte.

The matching of the redox potentials of different redox couples in the electrolyte solution is also considered in US-A-3360401, which concerns the use of an intermediate electron transfer species to increase the rate of flow of io electrical energy from a fuel cell. The use of platinum coated electrodes is also disclosed.

US 3607420 discloses an electrolyte in which the only soluble redox species present is the catalyst species. The electrolyte comprises a CuWICu(hI) catalyst.

WO-A-2006/057387 discloses a bio fuel cell making use of a material which participates in the donation and receiving of electrons, the cell being said to exhibit an enhanced output power density. The material comprises an electron conductor of a specified external surface area, a redox polymer and a bio catalyst.

USA-2003i0152823 discloses a fuel cell having an anode and a cathode with an anode enzyme disposed on the anode and a cathode enzyme disposed on the cathode.

US-A-200110028977 discloses a method for preparing a high energy density electrolyte solution for use In ore-vanadium redox cells.

s Prior art fuel cells all suffer from one or more of the following disadvantages: They are inefficient they are expensive and/or expensive to assemble; they use expensive and/or environmentally unfriendly materials; they yield inadequate and/or insufficiently maintainable current densities and/or cell potentials; they are too large in their construction; they operate at too high a temperature; they produce unwanted by-products and/or pollutants and/or noxious materials; they have not found practical, commercial utility in portable applications such as automotive and portable electronics.

ft is an object of the present invention to overcome or ameliorate one or more of the aforesaid disadvantages. ft is a further object of the present invention to provide an improved catholyte solution for use in redox fuel cells.

PCT/GB2007/050420 describes the use of modified ferrocene species as effective mediators in PEM fuel cells. Ferrocene itseff is not ideally suited to this purpose as It is uncharged, insufficiently soluble and the Fe(lll) form has a positive charge which makes ft unsuitable for use in PEM fuel cells employing a cation exchange membrane. PCT/GB2007/050420 describes the chemical modification of ferrocene to improve its solubility and allow the charge of the speciestobemanipulated. Iftheferrocenespeciesistobeusedina catholyte in a PEM fuel cell comprising a cation exchange membrane, it will preferably be non-ionic in its oxidised form, or more preferably anionic.

PCT/GB2007/050420 discloses that anionic charge can be introduced by modifying ferrocene with anionic charge inducing groups such as carboxylate, phosphate or phosphonate groups. Stronger acid groups such as sulphonate and sulphate can also be introduced.

It would however be advantageous to further increase the redox potential of the ferrocene based mediator. Additionally, it would also be advantageous to further increase the solubility of ferrocene based mediators whilst also increasing the anionic charge present for use in a PEM fuel cell with a cationic exchange membrane.

Accordingly, the present invention provides a redox fuel cell comprising an anode and a cathode separated by a cation selective polymer electrolyte membrane; means for supplying a fuel to the anode region of the cell; means for supplying an oxidant to the cathode region of the cell; means for providing an electrical circuit between the anode and the cathode; a catholyte solution comprising at least one non-volatile catholyte component flowing in fluid communication with the cathode, the catholyte solution comprising a modified ferrocene species comprising at least one electron withdrawing substituent on at least one cyclopentadienyl ring, the substituent being separated from the ring by a spacer group, the modified ferrocene species being at least partially reduced at the cathode in operation of the cell, and at least partially regenerated by reaction with the oxidant after such reduction at the cathode; by direct reaction with the oxidant, or by indirect reaction therewith using a redox catalyst catalysing the regeneration of the ferrocene mediator.

The spacer group is preferably conjugated.

The electron withdrawing substituent is preferably also an anionic substituent.

Thus, here we describe that further to our previously reported chemical modifications of ferrocenes for use as mediators in PEM fuel cells systems (described in PCT/GB20071050420), the use of conjugated linkages between the ferrocene and electron withdrawing substituent can create an increase in the operating redox potential of the modified ferrocene mediator used.

Furthermore, these electron withdrawing groups may also be negatively charged, thus increasing the overall negative charge of the mediator, as well as increasing the aqueous solubility.

In one preferred form of fuel cell in accordance with the invention, the modified ferrocene species may be represented by the formula: -R-4X) /RfY) wherein: X and Y are, independently, any electron withdrawing substituent; R and R' are, independently, any spacer group; and n and m can independently be any number from 1-10.

X and Y may be the same or different, and are preferably independently selected from nitro, halide, nitrile, carbonyl, ketone, aldehyde, ester, acyl chloride, protonated amine, quaternary amine, hydroxy, acyloxy, sulphate, carboxylic acid, carboxylate, sulphonate, suiphonic acid, phosphonic acid, phosphonate, phosphate, thiol, alkoxycarbonyl, alkoxy, aryloxy, and from alkyl, alkenyl, aryl, cycloalkyl, alkaryl, alkenaryl, aralkyl, aralkenyl groups substituted with one ore more of the aforesaid functional groups.

X and Y are more preferably both electron withdrawing and capable of providing negative charge. For example, hydroxy, acyloxy, su!phate, carboxylic acid, carboxylate, suiphonate, suiphonic acid, phosphonic acid, phosphonate, phosphate, thiol, alkoxycarbonyl, alkoxy, aryloxy, and from is alkyl, alkenyl, aryl, cycloalkyl, alkaryl, alkenaryl, aralkyl, aralkenyl groups substituted with one ore more of the aforesaid functional groups.

R and R' may be the same or different and either one or both are preferably conjugated. Preferably the modified ferrocene species contains a conjugated spacer group selected from alkenyl, cycloalkenyl, alkynyl, aryl, phenyl, heteroaryl, carbonyl, a,J3-unsaturated carbonyl, amide, imine, azo, sulfoxide.

The or each spacer group may be further spaced from the ferrocene ring by any suitable number of spacer elements, for example, alkyl, alkenyl, aryl, cycloalkyl, alkaryl, alkenaryl, acetylene. etc. The spacer group may also be straight or branched and may comprise a combination of the aforesaid units.

There may be any number from 1-5 R-X substituents, in which case each R and X substituent may be the same or different. There may be any number from 1-5 R'-Y substituents, in which case each R' and Y substituent may be the same or different.

io Any ferrocene sites without R-X or R'-Y substituents may be independently substituted with hydrogen or with functional groups comprising halogen, hydroxy, amino, imino, nitro, cyano, acyl, acyloxy, sulphate, sulphonyl, sulphinyl, alkylamino, carboxy, carboxylic acid, ester, ether, amido, sulphonate, sulphonic acid, sulphonamide, phosphonic acid, phosphonate, phosphonic acid, phosphate, alkylsuiphonyl, arylsulphonyl, alkoxycarbonyl, alkylsulphinyl, arylsuiphinyl, alkylthio, arylthio, alkyl, alkoxy, oxyester, oxyamido, aryl, arylamino, aryloxy, heterocycloalkyl, heteroaryl, (C2-C5)alkenyl, (C2-C5)alkynyl, azido phenylsulphonyloxy or amino acid conjugates having the formula -CO-W-OH, where W is an amino acid, and from alkyl, alkenyl, aryl, cycloalkyl, alkary alkenaryl, aralkyl, aralkeny groups substituted with one or more of the aforesaid functiona' groups.

Examples of such modified ferrocene species for use in the fuel ceD of the invention include: Fe-SO3H /so3H H03S HO3S\_ SO3H H O3S SO3H HO3S HO3S7 HO3S Thus, it can be seen that the charge of the modified ferrocene species of the present invention can be easily modified.

The incorporation of electron withdrawing and/or negatively charged substituents attached to the ferrocene via a conjugated spacer group allows the properties of the ferrocene to be tailored to the particular conditions of the cell with which it is to be used. For example, it can be tailored to the potential of the catholyte catalyst, the pH of the catholyte, and the charge of the exchangeable ions in the membrane.

Also provided in accordance with the invention is a catholyte solution for use in such a red ox fuel cell, The concentration of the modified ferrocene species in the catholyte solution is preferably at least about O.000IM, more preferably at least about O.005M and most preferably at least about O.OO1M.

In a fuel cell containing a cation selective membrane which is selective in favour of protons versus other cations, the pH of the catholyte is preferably below 7, more preferably below 4, even more preferably below 2 and most preferably below 1.

The cation selective polymer electrolyte membrane may be formed from any suitable material, but preferably comprises a polymeric substrate having cation exchange capability. Suitable examples include fluororesin-type ion exchange resins and non-fluororesin-type ion exchange resins. Fluororesin-type ion exchange resins include perfluorocarboxylic acid resins, Is perfluorosulphonic acid resins, and the like. Perfluorocarboxylic acid resins are preferred, for example "Nafion" (Du Pont Inc.), "Flemion" (Asahi Gas Ltd),"Aciplex" (Asahi Kasei Inc), and the like. Non-fluororesin-type ion exchange resins include polyvinyl alcohols, polyalkylene oxides, styrene-divinylbenzene ion exchange resins, and the like, and metal salts thereof.

Preferred non-fluororesin-type ion exchange resins include polyalkylene oxide-alkali metal salt complexes. These are obtainable by polymerizing an ethylene oxide oligomer in the presence of lithium chlorate or another alkali metal salt, for example. Other examples include phenolsulphonic acid, polystyrene sulphonic, polytriflurostyrene sulphonic, sulphonated trifluorostyrene, sulphonated copolymers based on afttriflurostyrene

H

monomer, radiation-grafted membranes. Non-fluorinated membranes include sulphonated poly(phenylquinoxalines), poly (2,6 diphenyl-4-phenylene oxide), poly(arylether sulphone), poly(2,6-diphenylenol); acid-doped polybenzimidazole, sulphonated polyim ides; styrene/ethylene-butadiene/styrene triblock copolymers; partially sulphonated polyarylene ether suiphone; partially suiphonated polyether ether ketone (PEEK); and polybenzyl suphonic acid siloxane (PBSS).

In some cases it may be desirable for the ion selective polymer electrolyte membrane to comprise a bi-membrane. The bimembrane if present will generally comprise a first cation selective membrane and a second anion selective membrane. In this case the bimembrane may comprise an adjacent pairing of oppositely charge selective membranes. For example the bi-membrane may comprise at least two discreet membranes which may be is placed side-by-side with an optional gap therebetween. Preferably the size of the gap, if any, is kept to a minimum in the redox cell of the invention. The use of a bi-membrane may be used in the redox fuel cell of the invention to maximise the potential of the cell, by maintaining the potential due to a pH drop between the anode and catholyte solution. Without being limited by theory, in order for this potential to be maintained in the membrane system, at some point in the system, protons must be the dominant charge transfer vehicle. A single cation-selective membrane may not achieve this to the same extent due to the free movement of other cations from the catholyte solution in the membrane.

In this case the cation selective membrane may be positioned on the cathode side of the bimembrane and the anion selective membrane may be positioned on the anode side of the bimembrane. In this case, the cation selective membrane is adapted to allow protons to pass through the membrane from the anode side to the cathode side thereof in operation of the cell. The anion selective membrane is adapted substantially to prevent cationic materials other than protons from passing therethrough from the cathode side to the anode side thereof. In this case protons may pass from anode to cathode.

io A representative example of a useful bipolar membrane, the arrangement used with the anionic-selective membrane on the anode side is that sold under the trademark Neosepta(R) BP-1, available from Tokuyama Corporation.

According to another aspect of the present invention, there is provided a method of operating a cation exchange membrane fuel cell comprising the steps of: a) forming H ions at an anode situated adjacent to a cation exchange membrane; b) supplying the catholyte of the invention with its modified ferrocene species in an oxidised state to a cathode situated oppositely adjacent to the cation exchange membrane; and c) allowing the modified ferrocene species to become reduced upon contact with the cathode concomitantly with H ions passing through the membrane to balance charge.

In another embodiment, the catholyte is supplied from a catholyte reservoir.

The method of the above aspect may additionally comprise the step of: d) passing the catholyte from the cathode to a reoxidation zone wherein the modified ferrocene species is reoxidised by the catalyst reacting with the oxidant.

In another embodiment, the method of the above aspect comprises the step of: e) passing the catholyte from the reoxidation zone to the catholyte reservoir.

In this embodiment, the cell is cyclic and the modified ferrocene species in the cathode can be repeatedly oxidised and reduced without having to be replaced.

An electricity loading device configured to load an electric power may also be provided in association with the fuel cell of the invention.

The fuel cell of the invention may comprise a reformer configured to convert available fuel precursor such as LPG, LNG, gasoline or low molecular weight alcohols into a fuel gas (eg hydrogen) through a steam reforming reaction.

The cell may then comprise a fuel gas supply device configured to supply the reformed fuel gas to the anode chamber.

-14 -Preferred fuels include hydrogen; metal hydrides, for example borohydride which may act as a fuel itself or as a provider of hydrogen, low molecular weight alcohols, aldehydes and carboxylic acids, sugars and biofuels as well as LPG, LNG or gasoline.

Preferred oxidants include air, oxygen and peroxides The anode in the redox fuel cell of the invention may for example be a io hydrogen gas anode or a direct methanol anode; other low molecular weight alcohols such as ethanol, propanol, dipropylene glycol; ethylene glycol; also aldehydes formed from these and acid species such as formic acid, ethanoic acid etc. In addition the anode may be formed from a blo-fuel cell type system where a bacterial species consumes a fuel and either produces a mediator is which is oxidized at the electrode, or the bacteria themselves are adsorbed at the electrode and directly donate electrons to the anode.

The cathode in the redox fuel cell of the invention may comprise as cathodic material carbon, gold, platinum, nickel, metal oxide species, However, as a result of the advantageous catholyte of the present invention, the use of such cathodes is not necessary to achieve satisfactory power output. Thus, the preferred cathodic materials include carbon, nickel, titanium and other metals inert in the specific catholyte and metal oxide or sulphide. One preferable material for the cathodes is reticulated vitreous carbon or carbon fibre based electrodes such as carbon felt. Another is nickel foam or mesh, or titanium foam or mesh. The cathodic material may be constructed from a fine dispersion of particulate cathodic material, the particulate dispersion being held together by a suitable adhesive, or by a proton conducting polymeric material. The cathode is designed to create maximum flow of catholyte solution to the cathode surface. Thus it may consist of shaped flow regulators or a three dimensional electrode; the liquid flow may be managed in a flow-by arrangement where there is a liquid channel adjacent to the electrode, or in the case of the three dimensional electrode, where the liquid is forced to flow through the electrode. It is intended that the surface of the electrode is also io the electrocatalyst, but it may be beneficial to adhere the electrocatalyst in the form of deposited particles on the surface of the electrode.

The modified ferrocene species flowing in solution in the cathode chamber in operation of the cell is used in the invention as a mediator which acts as an is electron sink for electrons formed during the fuel cell reaction. Following this reduction of the mediator, it is reoxidised by the catalyst reacting with the oxidant.

The modified ferrocene species, and any catalyst redox couple, utilised in the fuel cell of the invention should be non-volatile, and be preferably soluble in aqueous solvent. Preferred catalyst couple species should react with the oxidant at a rate effective to generate a useful current in the electrical circuit of the fuel cell, and react with the oxidant such that water is the ultimate end product of the reaction.

-16 -The fuel cell of the invention requires the presence of at least about 0,0001M of a modified ferrocene species in the catholyte solution. However, catalyst redox couples should be included in the catholyte solution in addition to the modified ferrocene species. There are many suitable examples of such catalyst redox couples, including ligated transition metal complexes and polyoxometallate species. Specific examples of polyoxometal late catalyst species which are useful in the fuel cell of the present invention are disclosed in the co-pending UK patent application, GB 0605878.8. Specific examples of suitable transition metals ions which can form such complexes include io manganese (II -V), iron (I -IV), copper (I -Ill), cobalt (I -Ill), nickel (I -Ill), chromium (II -VII), titanium (II -IV), tungsten (IV -VI), vanadium (II -V) and molybdenum (II -VI). Ligands for ligated transition metal complexes can contain carbon, hydrogen, oxygen, nitrogen, sulphur, halides and/or phosphorus. Ligands may be chelating including EDTA, for example bound to iron or manganese metal centres, NTA, 2-hydroxyethylenediamifletriaCetic acid, or non-chelating such as cyanide.

Alternative catalysts which may be useful in the present invention are complexes of multidentate N-donor ligands. Such Iigands are described in PCT/GB2007/050421 and in our co-pending applications GB (P509433GB and P509434GB) filed 23 January 2008 and may be coordinated with any suitable metal or metals, for example transition metals. Examples of such N-donor ligands can be selected from N4Py and derivatives thereof, pydien or derivatives thereof, and trilen and tpen and derivatives thereof. Iron complexes of these example N-donors are found to be effective catalysts for the oxidation of redox mediators in fuel cell systems.

Various aspects of the present invention will now be more particularly described with reference to the following figure which illustrate embodiments of the present invention: Figure 1 illustrates a schematic view of the cathode compartment of a fuel cell in accordance with the present invention.

Referring to Fig. 1 there is shown the cathode side of fuel cell 1 in accordance with the invention comprising a polymer electrolyte membrane 2 separating an anode (not shown) from cathode 3. Cathode 3 comprises in this diagram reticulated carbon and is therefore porous. Polymer electrolyte membrane 2 comprises cation selective Nafion 112 membrane through which protons generated by the (optionally catalytic) oxidation of fuel gas (in this case hydrogen) in the anode chamber pass in operation of the cell. Electrons 11 generated at the anode by the oxidation of fuel gas flow in an electrical circuit (not shown) and are returned to cathode 3. Fuel gas (in this case hydrogen) is supplied to the fuel gas passage of the anode chamber (not shown), while the oxidant (in this case air) is supplied to oxidant inlet 4 of cathode gas reaction chamber 5. Cathode gas reaction chamber 5 (the catalyst reoxidation zone) s provided with exhaust 6, through which the by-products of the fuel cell reaction (e.g. water and heat) can be discharged.

A catholyte solution comprising a catalyst and the oxidised form of the modified ferrocene species is supplied in operation of the cell from catholyte reservoir 7 into the cathode inlet channel 8. The catholyte passes into reticulated carbon cathode 3, which is situated adjacent membrane 2. As the catholyte passes through cathode 3, the modified ferrocene species and catalyst are reduced and are then returned to cathode gas reaction chamber 5 via cathode outlet channel 9.

Due to the advantageous composition of the catholyte of the present to invention, reoxidation of the modified ferrocene species and the catalyst occurs very rapidly, which allows the fuel cell to produce a higher sustainable

current than with cathotytes of the prior art.

Claims (14)

1. CLAIMSA redox fuel cell comprising: an anode and a cathode separated by an ion selective polymer electrolyte membrane; means for supplying a fuel to the anode region of the cell; means for supplying an oxidant to the cathode region of the cell; means for providing an electrical circuit between the anode and the cathode; a catholyte solution comprising at least one non-volatile catholyte component flowing in fluid communication with the cathode and comprising a modified ferrocene species comprising at least one electron withdrawing substituent on at least one cyclopentadienyl ring, the substituent being separated from the ring by a spacer group, the modified ferrocene species being at least partially reduced at the cathode in operation of the cell, and at least partially regenerated by reaction with the oxidant after such reduction at the cathode by direct reaction with the oxidant, or by indirect reaction therewith using a redox catalyst catalysing the regeneration of the ferrocene mediator.
2. 2. A redox fuel cell according to claim 1 wherein the spacer group is conjugated.
3. 3. A redox fuel cell according to claim 1 or claim 2 wherein the electron withdrawing substituent is also anionic.
4. 4. A redox fuel ce!l according to any one of claims 1 to 3 wherein the modified ferrocene species is represented by the formula: wherein: X and Y are, independently, any electron withdrawing substituent; R and R' are, independently, any spacer group; and n and mare, independently, any number from 1-10.
5. 5. A redox fuel cell according to claim 4 wherein X and Y are the same or different, and are independently selected from nitro, halide, nitrile, carbonyl, ketone, aldehyde, ester, acyl chloride, protonated amine, quaternary amine, hydroxy, acyloxy, sulphate, carboxylic acid, carboxylate, sulphonate, sulphonic acid, phosphonic acid, phosphonate, phosphate, thiol, alkoxycarbonyl, alkoxy, aryloxy, and from alkyl, alkenyl, aryl, cycloalkyl, alkaryl, alkenaryl, aralkyl, aralkenyl groups substituted with one ore more of the aforesaid functional groups.
6. 6. A redox fuel cell according to claim 4 or claim 5 wherein X and Y are both electron withdrawing and capable of providing negative charge. 2I
7. 7. A redox fuel cell according to claim 6 wherein X and Y are independently selected from hydroxy, acyloxy, sulphate, carboxylic acid, carboxylate, sulphonate, sulphonic acid, phosphonic acid, phosphonate, phosphate, thiol, alkoxycarbonyl, alkoxy, aryloxy, and from alkyl, alkenyl, aryl, cycloalkyl, alkaryl, alkenaryl, aralkyl, aralkenyl groups substituted with one ore more of the aforesaid functional groups.
8. 8. A redox fuel cell according to any one of claims 4 to 7 wherein R and R' are the same or different and either one or both are conjugated.
9. 9. A redox fuel cell according to claim 8 wherein R and R' are independently selected from alkenyl, cycloalkenyl, alkynyl, aryl, phenyl, heteroaryl, carbonyl, a3-unsaturated carbonyl, amide, imine, azo, sulfoxide.
10. 10. A redox fuel cell according to any one of claims 4 to 9 wherein the or each spacer group is further spaced from the ferrocene ring by any suitable number of spacer elements.
11. 11, A redox fuel cell according to claim 10 wherein each spacer element is independently selected from alkyl, alkenyl, ary, cycloalkyl, alkaryl, alkenaryl, and acetylene. 22 -
12. 12. A redox fuel cell according to any one of claims 4 toll wherein one ferrocene ring is substituted by from I to 5 R-X(n) substituents, and wherein each R, X and n may be the same or different when more than one R-X(n) substituent is present
13. 13. A redox fuel cell accordkig to any one of claims 4 to 12 wherein one ferrocene ring is substituted by from I to 5 R'-Y(m) substituents, andwhereineach R',Vand mmaybethesameordifferentwhen more than one R'-Y(m) substituent is present.
14. 14. A fuel cell according to any one of claims 4 to 13 wherein any site on one ferrocene ring not occupied by a R-X(n) substituent is independently hydrogen or is substituted with one or more functional groups selected from halogen, hydroxy, amino, lmino, nitro, cyano, acyl, acyloxy, sulphate, suiphonyl, sulphinyl, alkylamino, carboxy, carboxylic acid, ester, ether, amido, sulphonate, sulphonic acid, sulphonamide, phosphonic acid, phosphonate, phosphonic acid, phosphate, alkylsulphonyl, arylsulphonyl, alkoxycarbonyl, alkyisuiphinyl, aryisulphinyl, alkylthio, arylthio, alkyl, alkoxy, oxyester, oxyamido, aryl, arylamino, aryloxy, heterocycloalkyl, heteroaryl, (C2-C5)alkenyl, (C2-C5)alkynyl, azido phenyisulphonyloxy or amino acid conjugates having the formula -COW-OH, where w is an amino acid, and from alkyl, alkenyl, aryl, cycloalkyl, alkaryl alkenaryl, aralkyl, aralkenyl groups substituted with one or more of the aforesaid functional groups. -23 -15. A fuel cell according to any one of claims 4 to 14 wherein any site on one ferrocene ring not occupied by a R'-Y(m) substituent is independently hydrogen or is substituted with one or more functional groups selected from halogen, hydroxy, amino, imino, nitro, cyano, acyl, acyloxy, sulphate, sulphonyl, suiphinyl, alkylamino, carboxy, carboxylic acid, ester, ether, amido, suiphonate, suiphonic acid, sulphonamide, phosphonic acid, phosphonate, phosphonic acid, phosphate, alkylsulphonyl, arylsulphonyl, alkoxycarbonyl, alkylsulphinyl, arylsulphinyl, alkylthio, arylthio, alkyl, alkoxy, oxyester, oxyamido, aryl, arylamino, aryloxy, heterocycloa!ky!, heteroaryl, (C2-C5)a Ikenyl, (C2-C5)a lkynyl, azido phenylsulphonyloxy or amino acid conjugates having the formula -CO-W-OH, where W is an amino acid, and from alkyl, alkenyl, aryl, cycloalkyl, alkaryl alkenaryl, aralkyl, aralkenyl groups substituted with one or more of the aforesaid functional groups.16. A fuel cell according to any one of claims 1 to 15 wherein the modified ferrocene species is selected from: SO3H Fe SOH Fe be HO3S HO3S\_ SO3H T2SO3H -k J 1/ flU33 _-\ j SO3H HO3S HO3S 17. A redox fuel cell according to any one of claims 1 to 16 wherein the ion selective polymer electrode membrane is cation and/or proton selective.18. A redox fuel cell according to any one of claims 1 to 17 wherein the catholyte is acidic.19. A redox fuel cell according to claim 17 or claim 18 wherein the modified ferrocene species is nonionic or anionic in its oxidised form.20. A redox fuel cell according to claim 19 wherein the modified ferrocene species is anionic in its oxidised form.21. A redox fuel cell according to any one of claims I to 20 wherein the ion selective polymer electrode membrane is a bi-membrane with the cation exchange membrane on the cathode side.22. A redox fuel cell according to any one of claims 1 to 21 wherein the modified ferrocene species is present in the catholyte at a concentration of at least 0000IM.-25 - 23. A redox fuel cell according to any one of claims 1 to 22 wherein the modified ferrocene species is present in the catholyte at a concentration of at least O.005M.24. A redox fuel cell according to any one of claims 1 to 23 wherein the modified ferrocene species is present in the catholyte at a concentration of at least 0.001 M. 25. A redox fuel cell according to any one of claims 1 to 24 wherein the catholyte additionally comprises a catalyst redox species.26. A redox fuel cell according to claim 25 wherein the catalyst redox species is selected from ligated transition metal complexes, polyoxometallate species, and combinations thereof.27. A redox fuel cell according to claim 26 wherein the transition metal(s) in the transition metal complexes are selected from manganese (II -V), iron (I -IV), copper (I -Ill), cobalt (I -Ill), nickel (I -Ill), chromium (II -VII), titanium (II -IV), tungsten (IV -VI), vanadium (II -V) and molybdenum (II VI).28. A redox fuel cell according to claim 25 wherein the catalyst redox species comprises a multidentate Ndonor ligand. 26 -29. A redox fuel cell according to claim 28 wherein the N-donor ligand comprises one or more pyridine substituents.30. A redox fuel cell according to claim 29 wherein the catalyst redox species is an iron complex of N4Py, pydien, trilen or derivatives thereof.31. A catholyte solution for use in a redox fuel cell according to any one of Claims 1 to 30.-27 -