GB2515725A - Fuel cells - Google Patents

Abstract

The present invention concerns a redox fuel cell with a porous non-conductive separator. In one particular aspect, the fuel cell comprises an anode and a cathode separated by the porous non-conductive separator, 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, and a non-volatile catholyte solution in fluid communication with the cathode, the catholyte solution comprising a redox couple 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 reaction at the cathode. In another aspect, the cathode assembly comprises a catholyte inlet channel and flow channels with at least one porous region. In a still further aspect the fuel cell includes a regeneration zone separate from the electrode assemblies, with oxidant being supplied to the regeneration zone.

Description

tM:;: INTELLECTUAL

PROPERTY OFFICE

Application No. 0B1307880.3 RTM Date:31 October 2013 The following terms are registered trade marks and should be read as such wherever they occur in this document: Nafion.

Intellectual Properly Office is an operaling name of Ihe Patent Office www.ipo.gov.uk

FUEL CELLS

The present invenUon relates to fu& cells, in particular to indftect or redox fuel cells, including those which have applicafions as micro fu& ceHs, in electronic and portable electronic environments, and also as larger fuel cells for the automotive industry and for stationary appllcations.

A fuel cell is an electrochemical energy conversion device which converts fuel and oxidants into reaction products, producing electricity in the process.

iO Fuel cells have certain simfiarities with batteries, but are significanUy different in several ways, for example in b&ng able to functhn, and provide electrical energy, on a continuous basis and theoretically for as long as fuel and oxidant are supplied. In contrast, batteries have a finite amount of stored energy, and consequently different design principles apply and several is components are different, There are three fundamental parts to a fuel cell: the anode, the cathode, and the electrolyte. A fuel cell allows a chemical reaction to be spllt into two half reactions, one taking place at the anode, and the other taking place at the cathode, such that cations or anions are carried through the electrolyte between the two electrodes, whereas electrons are unable to flow through the fuel cell but instead flow through an external circuit, thereby providing an electric current.

Historically different types of fuel ceO have been categorised by their different electrolytes.

Perhaps the most common types of fuel cells to date have been solid oxide fuel cells (SOFCs), molten carbonate fuel cells (MCFCs) and PEM (polymer electrolyte membrane or proton exchange mechanism) fuel cells.

SOECs and MSFCs have applications in some fields; they both however suffer from the disadvantage that they need to be operated at high temperatures.

PEM fuel cells or PEM-type fuel cells are currently of considerable interest for both portable (including transport) applications and stationary applications.

They have a relatively low operating temperature range.

The overall effect of one of the basic designs of PEM fuel cell is to indirectly combine hydrogen with oxygen to produce water thereby providing an electrical current. In one type of PEM fuel cell, the hydrogen fuel and oxidant (oxygen or air) are fed respectively to catalysing, diffusion-type electrodes separated by a membrane electrolyte which carries ions between the two electrodes. At the anode, hydrogen gas Is catalytically converted to protons and electrons. The protons pass through the polymer electrolyte membrane to reach the cathode side. The electrons also reach the cathode, but need to do so via an external circuit (thereby generating electricity) because the fuel cefl is electricafly insulating. At the cathode, oxygen gas reacts with the protons and electrons to produce water.

Other types of PEM fuel cefl are known, including those which use, methanol and other alcohols such as ethanol and ethylene glycoL formic acid, borohydride. or other fuels.

An indirect or redox fuel cell has a redox cycle between the oxidant and the cathode. Thus, for example, the electrons at the cathode do not react with oxygen dfrectly, but instead reduce a redox-active component, and the resultant reduced form of the redox-active component then reacts with oxygen and is thereby regenerated.

From a first aspect the present invention provides a redox fuel cefl IS comprising an anode and an cathode separated by a porous non-conductive separator; means for supplying a fuel to the anode region of the cell; means for supplying an oxidant to the cathode region of the ceH; means for providing an electrical circuit between the anode and the cathode; and a non-volatile catholyte solution in fluid communication with the cathode. the catholyte solution comprising a redox couple 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 reaction at the cathode.

The present invention stems from a surprising discovery during experiments carried out by the applicant in the development of PEM-type fuel cells.

The present applicant has carried out considerable research and development in the area of PEM-type redox fuel cells, and is developing several commercial products. Some of the applicant's work In this area is S disclosed in the following international patent publications: WO 20061097438, WO 2007/110663, WO 2007/122431, WO 2008/009993, WO 2008/009992, WO 2009/037513, WO 2009/040577, WO 2009/060419, WO 2009/093081, WO 2009/093082, WO 2009/093080, WO 2010/128333, WO 2011/01 5875.

WO 2011/148198, WO 2011/107795, WO 2012/085542, WO 2012/175950 and WO 201 2/175997.

The membranes used previously in PEM-type fuel cells have typically been ionomer membranes, for example per-fluorinated lonomer membranes made from Nation. They conduct cations and allow protons to cross the membrane, e.g. by moving between different acId sttes. At the same time they do not conduct anions or electrons and they are impermeable to gases (the most common gases in these type of fuel cells being hydrogen gas and oxygen gas or air), and therefore they keep the reactants separate.

Separation of oxygen and hydrogen Is extremely desirable for safety reasons: these gases can react to form water explosively under fuel cell reaction conditions. Similarly, other fuels and other oxidants can react together in an unsafe manner.

Historically, for reasons given above, It has always been assumed that the conductive membrane within these types of fuel cells is an essential component, During operation of one of the applicant's prototypes, it became apparent that cross-over of some material through email holes in the membrane (which were unintentionally caused during the stack build) surprisingly did not affect the operational performance of the system. The basIs of the present invention Is therefore that porous separators can be used in place of conductive membranes, in indirect PEM-type fuel cells.

The conductive membranes commonly used in PEM-type fuel cells are quite expensive and relatively complex to make. In contrast the porous separators of the present invention can be made from commodity plastics more simply and at much lower cost. The present Invention allows a fuel cell to function without needing a polymer electrolyte membrane, and therefore said polymer electrolyte membrane may be absent from the fuel cell. Nevertheless, in some embodiments polymer electrolyte membranes or conductive membranes may still be present.

A conductive membrane has an ionic resistance which causes a significant IR drop even if the membrane is thin, and therefore contributes to parasitic losses in the system. The present invention uses a non-conductive porous separator and the ionic conducting path is through the catholyte, which typically has a higher proton conductivity and lower ionic resistance than a membrane. The present invention enables lower cost fuel cells to be built because the it enables cell performance to be maintained or even surpassed at the kind of levels associated with the use of specialised and generally

S

expensive materials (such as Naflon, etc,) with alternative materials which are more cost-effective.

There are further advantages In avoiding the use of conductive membranes.

$ To work effectively some conductive membranes require a significant uptake of water which limits the operational temperatures to below 100 (* PEM materials appear to suffer from interaction with some catholyte or anolyte materials, which could be attributed to an increase in resistance due to Donnan exclusion. Certain species can also penetrate into, and interfere with the performance of, membranes (M. Vljayakumar at aL, Journal of Membrane Science 2011, 366, 325-334).

The non-conductive separator used In the fuel cell of the invention Is porous, optionally microporous or nanoporous. Cross-over of the catholyte will not cause major problems because due to the indirect or redox nature of the cell the fuel does not come into contact directly with the oxidant. The risk of explosive reaction between fuel and oxidant is removed because oxidant is not supplied directly to the cathode but rather reacts indirectly via the redox couple(s).

Preferably, oxidation of the catholyte after reduction at the cathode takes place in a regeneration zone separated from the cathode chamber of the fuel cell so that the oxidant supplied to the cathode region in this connection does not come Into contact with the porous separator.

Throughout this specification by the qualifying phrase "non-conductive" we mean that the material to which the qualifying phrase is applied is non-conductive to ions, particularly cations, particularly protons. Also or alternatively we mean preferably that the material to which the qualifying phrase is applied has a conductivity (e.g. to ions, particularly cations, particularly protons) of I S/rn or lower, or 0.1 S/rn or lower, or 0.01 S/m or lower, under the operating conditions of the cell, for example at 30°C.

Generally, the porous separator of the present invention is not an ion-conductor. It is not made from conventional PEM materials which comprise cation-conductIng (usually proton-conducting) materials or materials having cation-exchange capability. Nor is it made from conventional anion-conducting or anion-exchange materials. Whilst the porous separator is preferably non-conducting to cations and anions (there being no need to incorporate expensive conducting materials), nevertheless It may have a low conductivity with respect to cations or anions to the extent that this does not adversely affect the operation of the cell.

Previously it had been thought that one of the reasons for needing a membrane was to avoid shorting between the electrodes. In the present invention, the porous separator provides an inert physical baffler between the anode and cathode, and the catholyte. Therefore the present invention avoids the problem of short-circuiting without needing a membrane.

However, importantly, the catholyte is an ionic conductor, thus allowing hydrogen ions to pass from the anode to the cathode side of the cell.

Without wishing to be bound by theory, the cathoyte soution may traverse or occupy the pore spaces in the porous separator, and the separator may be adapted for examp'e through the dimensionaL geometrical, structural and/or chemical characteristics of its pores to permit the cathoyte solution to enter the matrix of the separator (permitting &ectrolyhc contact with the anode side of the separator in the arrangement of the ecU). OptionaUy the characteristics may be such as to not permit substantial through-flow of the catholyte between the cathode &de and the anode side thereof, or between the anode side and the cathode side thereof.

The present invention is particulary appUcable to proton exchange mechanism fuel ceUs. in other words fuel cefls where direct or indirect reaction with a fuel (e.g. hydrogen but optionafly other fu&s) at the anode results in protons which travel through proton-conducting material (in this case the catholyte) to the cathode, where there is direct or indirect reaction with oxidant (e.g. oxygen or air).

Therefore, from a further aspect the present invention provides an indirect proton exchange mechanism fuel ceU comprising a porous non-conductive separator.

In the present invention indirect reaction occurs at the cathode, in other words a redox couple cycle is used between the cathode and the oxidant; optionafly one or more mediator cycle may also be used, and/or there may be more than one redox couple cycle.

Keeping the external oxidant separate from the fuel cell enhances safety and convenience, and means that reactive oxidant gases do not pass through the porous separator.

The porous separator may be made from polymer(s), plastic(s) or any other suitable material, for example cellulolosic material, which has the physical o and chemical properties suitable for and stable under fuel cell conditions.

Preferably the separator is made from or comprises polyolefinic material, for example polyethylene or polypropylene, e.g. HDPE.

Is Optionally the porous separator may be adapted to permit the ingress of catholyte solution into the porous separator but to prevent or hinder substantial through-flow of the solution across the porous separator.

Optionally the pore size is adapted to be suited to and compatible with the catholyte used. Optionally, suitable average pore sizes include for example those withIn any of the ranges 0.1 -2D00 nm, 0.2-1000 nm, Ito 500 nm, or I to 100 nm. Preferably the separator Is a microporous separator.

Due to the use of a porous separator rather than a membrane higher temperatures can be used and therefore the fuel cell has an enhanced operating range. Furthermore enhanced proton conductivty can occur at higher temperatures.

The separator a more treatab'e wfth a variety of materials than a typica membrane whch a less inert and more susceptible to inacflvation.

edoxcoue The redox coupe may comprise any suitable redox-active material.

I C

The redoxacUve material may comprise one or more transition metal.

The redox couple may comprise one or more polyoxometalate (POM), These are suited to the fuel ce conditions and oxidant reactMty so that they are effective redoxactive materials, aflowing indirect reaction of the oxidant.

OptionaHy mediator cycles and/or other redoxacflve mate1als and/or other components may also be used.

The catholyte solution may also comprise at least one anciflary redox species. The anclUary redox species may be selected from ligated transitbn metai complexes, polyoxometalate species, other species, and combinations thereof.

The polyoxometalate may optionally be represented by the formula: X44McOdJ wherein: S X is selected from hydrogen, alkali metals, alkaline earth metals, ammonium or alkyl ammonium and combinations of two or more thereof; Z is selected from B, P, 5, As, Si, Ge, Ni, Rh, Sn, Al Cu, I, Br, F, Fe, Go, Cr, Zn, H2, Te, Mn and So and combinations of two or more thereof; M is a metal selected from Mo, W, V, Nb, Ta, Mn, Fe, Go, Cr, Ni, Zn Rh, Ru, TI, Al, Ga, In and other metals selected from the 1M,2S and 3 transition metal series and the lanthanide series, and combinations of two or more thereof a is a number of X necessary to charge balance the (4MCOd] anion; bisfromoto2O,; cisfromlto4O;and disfromltol8O.

The polyoxometalate may be In the Keggin form, though other types of polyoxometalate can also be used.

In one embodiment the polyoxometalate may have the formula X4Z1M12O40I.

U

Optionauy the catholyte solution comphses at least about 0075M polyoxometalate.

Optionafly the polyoxometalate structure may be as disclosed in our earUer patent pubflcaflons WO 20071110663, WO 2009/040577 or WO 2012/085542.

Opfionafly the catholyte comprises a polyoxometalate s&ected from one of the foHowing groups. *I0

ygxometaIateroup The polyoxornetaate may be represented by the formula: XZbMcOd] wherein: X is selected from hydrogen, alkaU metals. aIkane earth metals, ammonium and combinations of two or more thereof; Z is s&ected from B, P, S, As, Si, Ge, Ni, Rh, Sn, Al, Cu. 1, Br, F: Fe, Co, Cr! Zn, H2, Te, Mn and Se and combinations of two or more thereof; M is a metal selected from Mo, W, V. Nb, Ta, Mn, Fe; Cc, Cr, Ni, Zn Rh, Ru, TI, Al, Ga, In and other metals selected from the 12 and 3 transition metal series and th.e anthanide series, and combinations of two or more thereof; a Is a number of X necessaiy to charge balance the [MoOd] anion; bisfromoto20,; cis from Ito 40; and d is from Ito 180. $

Preferred ranges for b are from 0 to 15, more preferably 0 to 10, still more preferably 0 to 5, even more preferably 0 to 3, and most preferably 0 to 2.

Preferred ranges for c are from 5 to 20, more preferably from 10 to 18, most preferably 12.

Preferred ranges for d are from 30 to 70, more preferably 34 to 62, most preferably 34 to 40.

Vanadium and molybdenum, and combinations thereof, are particularly preferred for M. Phosphorus is particularly preferred for Z. ii A combination of hydrogen and an alkali metal and/or alkaline earth metal is particularly preferred for X. One such preferred combination is hydrogen and sodium.

Specific examples of polyoxometalates include molybdophosphoric acid, H3PMo12 O4 and molybdovanadophosphosphoric acid, H5PMo10V2O.,0.

In a preferred embodiment of the present invention, the polyoxometalate comprises vanadium, more preferably vanadium and molybdenum.

Preferably the polyoxometalate comprises from 2 to 4 vanadium centres.

Thus, particularly preferred polyoxometalates include H3Na2PMo10V2O40, H3Na3PMo9V3O40, or H3Na4PMogVO40, and compounds of intermediate composition.. In addition, a mixture of these or other polyoxometalate catalysts is also envisaged. For this embodiment, preferably, at least one X is hydrogen. However, it is also preferred that not all X be hydrogen. More preferably, at least two of X are not hydrogen. X comprising at least one hydrogen and at least one other material selected from alkali metals, alkaline earth metals, ammonium and combinations of two or more thereof is preferred.

The concentration of the polyoxometalate in solution is preferably at least about O.08M, more preferably at least about 0.1 M, still more preferably at least about O.125M and most preferably at least about 0.15M.

Poyoxornetaate group B The countehons of the poyoxornetaate may comprise at east uric thvaent S ion.

The or each dvalent on s preferaby s&ected from Ca Mg, Mn, Fe, Cu, NL Cu, Zn, Sr Ba, Be, Cr, Cd, Hg. Sn and other sthtabe ons from the 2nd and 3rd tran&tion series or from the anthandes, or from combinations of two or more thereof; more preferaby from Ca Mg, Mn, Fe, Co, Ni, Cu, Zn, or from combinaflons of two or more thereof.

The poyoxometaate and assoSted counterion may be represented by the formula: X[ZbMOd] wherein: X is seected from hydrogen, akaU meta's, alkane earth metas, ammoniurn, transition meta' ions arid combinations of two or more thereof, but wherein at east one X is a divaent ion; Z is s&ected from B, P. 5, As, SL Ge, Ni, Rh: Sn, A, CU: , Br, F, Fe, Co, Cr, Zn, H2. Te. Mn and Sc and combinaUons of two or more thereof; M is a metal selected from Mo, W, V, Nb, Ta. Mn, Fe, Go, Cr, Ni, Zn Rh, Ru, TI, Al, Ga, In and other metals selected from the 1st, 2nd and 3rd transition metal series and the lanthanide series, and combinations of two or more thereof; s a is a number of X necessary to charge balance the [MOOd] anion; b Is from 0 to 20; cis from ito 40; and d is from ito 180.

At least one X is preferably selected from Ca Mg, Mn, Fe, Co, Ni, Cu, Zn, Sr Ba, Be, Cr, Cd, Hg, Sn and other suitable ions from the 2nd and 3rd transition series or from the lanthanides, or from combinations of two or more thereof; more preferably from Ca Mg, Mn, Fe, Go, Ni, Cu, Zn, or from combinations of two or more thereof.

Preferred ranges for b are from 0 to 15, more preferably 0 to 10, still more preferably 0 to 5, even more preferably 0 to 3, and most preferably 0 to 2.

Preferred ranges for care from 5 to 20, more preferably from 10 to 18, most preferably 12.

Preferred ranges for d are from 30 to 70, more preferably 34 to 62, most preferably 34 to 40.

Vanadium and molybdenum, and combinations thereof, are particularly S preferred for M. Phosphorus is particularly preferred for Z. A combination of hydrogen and an alkali metal and/or alkaline earth metal is particularly preferred for X, provided that at least one X is one or more divalent ions. One such preferred combination is hydrogen and sodium with one or more divalent ions. In each case the or each divalent ion is preferably selected from Ca Mg, Mn, Fe, Co, Ni, Cu, Zn, Sr Ba, Be, Cr, Cd, Hg, Sn and other suitable ions from the 2nd and 3rd transition series or from the is lanthanides, or from combinations of two or more thereof; more preferably from Ca Mg, Mn, Fe) Co, Ni, Cu, Zn, or from combinations of two or more thereof.

Specific examples of polyoxometalates include molybdophosphoric acid, H3PMo12O40 and molybdovanadophosphosphoric acid, H5PMo10V2O10, wherein the protons are at least partially replaced by one or more divalent ions, preferably selected from Ca Mg. Mn, Fe, Co. Ni, Cu, Zn, Sr Ba, Be, Cr, Cd, Hg, Sn and other suitable ions from the 2nd and 3rd transition series or from the anthanides, or from combinations of two or more thereof: more preferably from Ca Mg, Mn, Fe, Ca, Ni, Cu, Zn, or from combinations of two or more thereof.

In a preferred embodiment of the present invenflon, the polyoxometalate comprises vanadium, more preferably vanadium and molybdenum.

Preferably the polyoxometalate comprises from 2 to 4 vanadium centres.

Thus, parUculary preferred polyoxometalates include H3Na2PMo13V2O40, H3Na3FMo9V3O43, or H3Na4FMo5V4O43, wheren sodium ions are at east partiaHy replaced by one or more divalent ions, and compounds of intermediate composition. In addition, a mixture of these or other polyoxometalate catalysts is also envisaged. For this embodiment, preferably, at east one X is hydrogen. However, it is also preferred that not is all X be hydrogen. More preferabiy, at least two of X are not hydrogen. X comprising at least one hydrogen and at least one other material s&ected from alkak metals, alkaline earth metals. ammonium and combinations of two or more thereof is preferred, provided that at least one X is one or more divalent ions, preferably selected from Ca Mg, Mn, Fe. Ca: Ni, Cu: Zn, Sr Ba.

Be, Cr, Cd, Hg, Sn and other suftabe ions from the 2nd and 3rd transition series or from the ianthanides, or from combinations of two or more thereof: more preferably from Ca Mg, Mn, Fe, Co. Ni, Cu, Zn. or from combinations of two or more thereof.

The concentration of the polyoxometalate n solution is preferably at least about O08M, more preferably at least about 0.IM, stifl more preferably at east about 0.125M arid most preferably at least about 0.15M.

Pooxonuetalat.grp.yp.Q The polyoxometalate may represented by the formula: X3[ZbMcOd] wh crc X is selected from hydrogen, alkaU metals, alkaUne earth metals, ammonium or alkyl ammoniuni and combinations of two or more thereof; Z is selected from B. P, S, As, Si, Ge, NL Rh, Sn, Al, Cu, I, Br, F, Fe, Co, Cr.

Zn, H2. Te, Mn and Se and combinations of two or more thereof; M comprises at least one V atom, and M is a metal selected from Mo, W, V, Nb: Ta, Mn, Fe. Co, Cr, Ni. Zn Rh, Ru, TI, Al, Ga, In and other metals seected from the 1st2 and 3 transition metal series and the lanthanide series and combinations of two or more thereof; a is a number of X necessary to charge balance the [4MOd] anion; b is from 0 to 20; c is from I to 40; and d is from ito 180.

The catholyte may optionally comprise not only a polyoxometalate of this group (Group C) but also a vanadium(IV) compound.

Preferably. the composition is an aqueous based solution.

Preferred polyoxometalate compounds have a Keggin structure with general formula x4z1M1204 Preferred metals for M are molybdenum, tungsten and vanadium and combinations of two or more of these, provided that the polyoxometalate must have at least one of M being vanadium. Preferably 2 -5 of M are vanadium, more preferably 3 or 4, and most preferably 4.

The remaining M is preferably either molybdenum or tungsten or a combination of both.

Phosphorous is particularly preferred for Z. X is preferably s&ected from hydrogen, aikaU metS, alkafine earth metals, ammonium or alkyl ammoniuni and combinations of two or more thereof.

Parculariy preferred examples nckjde hydrogen, sodium, lithium and combinations thereof Specific nonHmlling examples of catholytes of the present invention comprise H3+ePMO12VeO4 and H3+ePW12eVeO4a, Where C = 2-5; HfX9PMO12.

eVeO4o, HrXPWi2eVeO4c where e = 2-5, f+g = 3+e and X = Na. Li or combinations thereof.

The concentration of the polyoxometalate in the composftion is preferably at least about O,1M, more preferably at east about 015M and most preferably at least about 0.2DM.

Also included as part of the composition may be a vanadiumQV) compound.

Any vanadium(IV) containing compound can be used hut specific examples include V02, V204, VOSO4, VO(acac)2, VO(Ci04)2, VO(BF4)2, and hydrated versions of these materials. Particularly preferred examples are V02, V204 and VOSO4.xH2O.

Preferably the concentration of the vanadium (IV) compound is at least about 0.05M or at least about 0.IM or at least about O.15M or at least about 0.2M or at least about 0.25M or at least about 0.3M.

Preferably the molar ratio beten the polyoxometalate and the vanadium(IV) compound is at least about 1:10 or at least about 1.5:10 or at least about 2:10 or at least about 2.5:10 or at least about 3:10.

Particularly preferred combinations Include H340PMOI24VeO4O or H3÷PW,2.

O eVeO4o where e = 2-5 with added V204, and l-lrXPMoi2.eVeOcor HfXgPWI2..

where e = 2-5, f+g = 3+e and X = Na, Li or combinations thereof with added V0804.

Preferred ranges for b are from 0 to 15, more preferably 0 to 10, still more preferably 0 to 5, even more preferably 0 to 3, and most preferably 0 to 2.

Preferred ranges for c are from 5 to 20, more preferably from 10 to 18, most preferably 12.

Preferred ranges for d are from 30 to 70, more preferably 34 to 62, most preferably 34 to 40.

A combination of hydrogen and an alkali metal and/or alkaline earth metal is particularly preferred for X. One such preferred combination is hydrogen and sodium.

In a preferred embodiment of the present invention, the polyoxometalate comprises vanadium, more preferably vanadium and molybdenum.

Preferably the polyoxometalate comprises from 2 to 4 vanadium centres, Thus, particularly preferred polyoxometalates include H3Na2PMo10V2O40, H3Na3PMo9V3O40, or H3Na4PMo8V4Oc, and compounds of intermediate composition.. In addition, a mixture of these or other polyoxometalate catalysts is also envisaged. Preferably at least one X Is hydrogen, and in some embodiments all X will be hydrogen. However, in some cases it may be preferred that not all X be hydrogen, for example in that case that at least is two of X are not hydrogen. For example in that case, X may comprise at least one hydrogen and at least one other material selected from alkali metals, alkaline earth metals, ammonium and combinations of two or more thereof.

Polvoxometalate group D Optionally the polyoxometalate may be represented by the formula: X44MCOdJ wherein: X is selected from hydrogen, alkali metals, alkaline earth metals, ammonium and combinations of two or more thereof Z is selected from B, P, S. As, Si, Ge, Ni, Rh, Sn, Al, Cu, I, Br, F, Fe, Go, Cr, Zn, H2, Te, Mn and Se and combinations of two or more thereof: M comprises W and optionally one or more of Mo, V, Nb, Ta, Mn, Fe, Go, Cr, NI, Zn Rh, Ru, TI, Al, Ga, in and other metals selected from the 2 and 3td transition metal series and the lanthanide series; to a is a number of X necessary to charge balance the [4McOd]° anion; b is from 0 to 5; c is from S to 30; and d is from Ito 180.

is It is to be understood that such formulae used herein are generic formulae and that a distribution of related species may exist in solution.

Preferred ranges for b are from 0 to 5, more preferably 0 to 2.

Preferred ranges for c are from S to 30, preferably from 10 to 18 and most preferabIy 12.

Preferred ranges for d are from 1 to ISa, preferably from 30 to 70, more s preferaby 34 to 62 and most preferably 34 to 40.

The polyoxometalate preferably contains from I to 6 vanadium centres.

Example formue therefore include XaZiWi2.VxOio] where x = 1 toG. In one embodiment of the present invenUon, the polyoxometalate has the formula X[Z1WV3O4c]. in another embodiment, the polyoxometalate has the formula XZiW1iViO4a].

B, P, 8, As, Si, Ge. Al, Co, Mn or So are particuiarly preferred for Z, with F, S. Si, AI or Co being most preferred. The successfu use of such a range of atoms would not he possible with a polyoxometalate that contains, for example, molybdenum, as outlined in the prior art. In particular, the use of siUcon and aluminium in combination with tungsten in the polyoxometalates of the present invention has surprisingly been shown to significantly improve the performance of the fuel cells. For example, tungsten polyoxometalates with aluminium or shicon demonstrate more reversible electrochemical properties at a higher potential compared to the polyoxornetalates commonly

found in the prior art

M preferably consists of I to 3 different &ements. In one embodiment, M is a combination of tungsten, vanadium and/or molybdenum The polyoxometalate may be absent of molybdenum, and further may be absent of any metals other than tungsten or vanadium. The polyoxornetalate may atternativeiy consist of tungsten. M preferably includes more than two, more than four or more than six tungsten atoms.

Hydrogen, or a combination of hydrogen and an alkali metal and/or akaline earth metai are particularly preferred examples for X. X preferably comprises a hydrogen ion or a combination of a hydrogen ion and an alkah metal ion, and more preferably comphses one or more of H4. Nat K4 or Lit Preferred combinations include hydrogen, hydrogen with sodium and hydrogen with potassium is In a preferred embodiment, the polyoxometalate may be H6[AIW11V1O40] Afternatively the polyoxometalate may be X7fSIW9V5O40] where, as an example, X can give rise to the gener& formula K2H5(SiW9V3O40J Further, a mixture of these or other polyoxometalate catalysts is also envisaged.

o The concentration of the polyoxometalate in solution may opuonally be between 0.01 M and O.6M. If the polyoxometalate of the present invention is the major conslituent of the solution, a concentration range of OAM -0.6M is preferred, whilst O.15M 0,4M is most preferred.

A te maUve or add Wonm2&@twe species or mediators The catholyte may comprise a mediator. The mediator may act as a redox-active component or catalyst or redox couple and/or may be ancifiary to a redox couple.

n another embodiment of the present invention, the fuel ceU may comprise a catholyte solution comprising a polyoxometalate redox couple being at least to partially reduced at the cathode in operation of the ceH, and at east partiaUy re-generated by reaction with the oxidant in a regeneration zone after such reduction at the cathode, the catholyte solution further comprising one or more vanadium species that result from the speciation of the polyoxometalate at an &evated temperature and/or pressure. The &evated temperature is preferably above 80°C and/or below the boffing point for the cathoMe solution. The elevated pressure is preferably above ambient pressure.

Cathoiyte&.gpmrisinLm N-donarjgjjds n one possible system according to the invention, the catholyte solution comprises a complexed multidentate N-donor igand.

The complexed rnutidentate Ndonor igand may be a compiexed multidentate macrocyclic Ndonor igand.

Optionally the multidentate N-donor ligand comprises at least one heterocyclic substituent selected from pyrrole, imidazole, I,2,3-triazole, 1,2,4-triazole, pyrazole, pyridazine, pyrimidine, pyrazine, indole, tetrazole, s quinoline, Isoquinoilne and from alkyl, alkenyl, awl, cycloalkyl, alkaryl, aikenaryl, aralkyl, aralkenyl groups substituted with one or more of the aforesaid heterocyclic groups.

Therefore, from a lurther aspect, the present invention provides a redox fuel cell comprising an anode and a cathode separated by a porous separator; 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 is communication with the cathode, the catholyte solution comprising a redox mediator which is at least partially reduced at the cathode in operation of the cell, and at least partially regenerated by, optionally indirect, reaction with the oxidant after such reduction at the cathode, the catholyte solution comprising a complexed multidentate N-donor ligand as said redox mediator and/or as a redox catalyst catalysing the regeneration of the said mediator.

The N-donor Ilgand and other catholyte components may for example be as disclosed in WO 2008/009993, WO 2009/093082 or WO 2009/093080.

Catholytes comprising ferrocene soecies Other possible types of redox couple in the cathdyte include ferrocene species, for example modified ferrocene species.

Therefore, in accordance with a further embodiment of the present invenUon a catholyte solution comprises a modified ferrocene species being at east parfiaHy reduced at the cathode in operation of the celL and at east partiay regenerated by reaction with the oxidant after such reduction at the cathode.

The ferrocene species may react with oxidant directly or indirectly (via a further redoxactive component).

One suitable class of modified ferrocene species is represented by the form ul a: wherein: X and Y are independenfly selected from hydrogen and from functional groups comprising halogen, hydroxy, amino, protonated amino, imino, nitro.

cyano, acyl, acyloxy, sulphate, suiphonyl, sulphinyl, alkylamino, protonated alkylamino, quaternary alkylammonium carboxy, carboxylic acid, ester, ether, amido, suiphonate, suiphonic acid, sulphonamide, phosphonic acid, phosphonate, phosphonic acid, phosphate, alkylsulphonyl, arylsulphonyL alkoxycarbonyl, alkyisuiphinyL arylsulphinyl, alkyfthio, arylthio, alkyl, alkoxy, oxyester, oxyamido, aryl, arylamino, aryloxy, heterocycloalkyl, heteroary!, (C2C5)alkenyL (C2-C5)alkynyl, azido phenylsulphonyloxy or amino acid coniugates having the formula -CO-W-OH, where W is an amino acid, and s from alkyl, alkenyl, aryl, cycloalkyL alkaryl alkenaryl, aralkyL araikenyl groups substituted with one or more of the aforesaid functional groups.

The catholyte solution may comprise a modified ferrocene species comprising at east one bridging unit between the cyciopentadienyl rings, the modified ferrocene species being at east partiaUy reduced at the cathode in operation of the c&l, and at east partiaHy regenerated by reaction with the oxidant after such reducUon at the cathode by direct reaction with the oxidant or by indirect reaction therewith u&ng a redox cataiyst catalysing the regeneration of the ferrocene mediator.

One suitable dass of bridged modified ferrocene species is represented by the formula: Fe çB)n wherein: A-B)nC together comprise a divalent heteroannular bridging group; n is from Ito 6; and each B (B6) may be the same or different.

The catholyte sokjtion may additionay or afternatively comprise a modified ferrocene species compri&ng at east one &ectron withdrawing subsfituent on at east one cydopentadienyl ring, the subsfltuent b&ng separated from the ring by a spacer group, the modified ferrocene species being at least partiafly reduced at the cathode in operation of the ceU, and at east partiaHy regenerated by reacflon with the oxidant after such reduction at the cathode by direct reaction with the oxidant, or by indirect reaction therewfth u&ng a redox catalyst catalysing the regeneration of the ferrocere mediator.

A further suitable class of modified ferrocene species is represented by the formula: RfX) wherein: X and V are, independently, any electron withdrawing substituent; R and ft are, independently, any spacer group; and n and rn are, independently, any number from 1 10, Other examples of suitable ferrocene species are disdosed in WO 2008/009992 and WO 2009/093081.

Q?in9bescompnsna other aromatic compounds A further type of redox catayst and/or mediator which may be used in the cathoyte soution of the fu& oeM of the present invention is of the foMowing formula: R? R2 N wher&n: X is selected from hydrogen and from functional groups comprising halogen, hydroxyl, amino, protonated amino, imino, nitro, cyano, acyL acyloxy, sulphate, suifonyl, sulfinyL alkyamino, protonated akylamino, quaternary alkylammonium, carboxy, carboxyMc acid, ester, ether, amido, suffonate, sulfonic acid suphonarnide, phosphonic acid, phosphonate, phosphate, alkylsulfonyL arylsuFfonyl, alkoxycarbonyL kylsulfiny, arylsulflnyl, alkylthio, aryithio, alkyl, alkoxy, oxyester, oxyamido, aryL fusetharyl, arylarnino, aryloxy, heterocycloalkyl, heteroaryl, fuse&heteroaryi, (C2.C5)aikenyE, (02- 0C5)akynyl, azido, phenylsulfonyloxy, amino acid or a combination thereof; are independently selected from hydrogen, halogen, hydroxyL amino, protonated amino, irnino, nitro, cyano, acyl, acyloxy, sulphate, sulfonyl, suifinyl, alkyarnino, protonated alkylamino, quaternary alkyiarnmonium, carboxy, carhoxylic acid, ester, ether, amido, sulfonate, sulfonic acid, sulphonamide, phosphonic acid, phosphonate, phosphate, alkylsuifonyL arysuIfony, akoxycarbonyi, akysulfinyL arysuftinyL akyfthio, aryfthio, aUcyL akoxy, oxyester, oxyamido, aryL fused-aryL aryamino, arybxy, heterocydoalkyL heteroaryL fused-heteroary, (C2C5)akeny, (C2- 0C5)allcynyl, azido, phenysuftonyloxy, amino add or a combination thereof: wherein R1 and X and/or R5 and X may together form an optionaUy wherehi R1 and R2 and/or R2 and R7 and/or R3 and R4 and/or R4 and R8 and/or R and R7 and/or R7 and R and/or R and R5 may together form an optionaHy subsUMed ring structure: wherein (L) indicates the optional presence of a ftking bond or group between the Iwo neighbouring aromatic rings of the structure, and when present may form an optionaHy substftuted ring structure with one or both of R4 and R8; and wherein at beast one substituent group of the structure is a charge-Preferabty, in the compound of the above formula, X is represented by: (So) R9 wherein R93 are independently selected from hydrogen and from functional groups comprising halogen, hydroxy, amino, protonated amino, imino, nitro, cyano. acyl, acyloxy, sulphate. suiphonyl, su]phinyL akylamino, protonated alkylamino, quatemary alkylammonium, carboxy, carboxylic acid, ester, ether, amido, suiphonate, suiphonic acid, suiphonamide, phosphonic acid, phosphonate, phosphonic acid, phosphate, alkylsuiphonyl, arylsuiphonyl, alkoxycarbonyl, alkylsuiphinyl, arylsulphinyl, alkylthio, aryithio, alkyl, alkoxy, oxyester, oxyamido, aryl, arylamino, aryioxy, heterocycloalkyl. heteroaryl, (C2-C5)alkenyl, (C2-C5)alkynyi, azido phenylsuiphonyloxy or amino acid conjugates having the formula -CO-W-OH, where W is an amino acid, and from aikyl, alkenyl, aryl, cycloalkyl, alkaryi alkenaryl, aralkyl, aralkenyl groups substituted with one or more of the aforesaid functional groups; wherein R9 together with R1 and/or R'3 together with R5 may form a linking group, preferably selected from 0, N, S, imino, suifonyl, sulfinyl, alkylamino, protonated alkylamino, quatemary alkylammonium, carbonyl, ester, ether, amido, suiphonamide, phosphonate, phosphate, aikysulfonyl, aikenylsulfonyl arylsulfonyl, alkylsulflnyi, alkenylsulfinyl, arysulfinyi, alkylthio, alkenylthio, is arylthio, oxyester, oxyamido, aryl, cycloalkyl, heteroaryl, (CCalkyi, (Cr C5)aikenyl, (CC5)alkynyl, or amino acid; and wherein (Sp) indicates the optional presence of a spacer group.

Optionally the compound may be selected from the following: i/ \ / I 1' / ) /1 -1--c /7 -/ 1 77 1 / 1' / rm,7Th 7 / // 1' H 1 i / // -,-I it 3) \ I K: /4 / / // /7 / \\ 7 1 \ \1 / r / / // / / // \ I / / /7 -/ 7/ -c\ /7 c 7/ / c // ; / \_-.// -c / *__?__ -c -/ -/ c-. \t/ c N --_ 7 / N -.

/ -c / / / --ct-I.-_c / /

I

7 -1* U 4' / " >-_ /; \ / --.

7/-/ --k / 7 -t / -:-== 7/ 1' / / I / / C H / -c 7 7,11 1' 4 c\ /1 7 \ ,4 t-1' ,7-' fi I 7 &-/ / cc I // / / / \ /7 / /7. /7 -\ /1 7/ N / :-----"=7 -7 /1 // -cc-/ / cc C, cc \/ cc --// /7 N \ / cc" C) Further examples of suitable redox-active components which may be used In the catholyte of the fuel cell of the present invention are those described in ther ootional features of the fuel cell In a further embodiment, the redox fuel cell of the present invention may comprise a cathode assembly comprising a catholyte inlet channel and one or more flow channels in fluid communication with the catholyte inlet channel, to the flow channels being defined by flow channel walls comprising at least one porous cathode region, at least one of the flow channels being non-aligned with the catholyte inlet channel, wherein one or more of the flow channels are dosed at one end, and wherein the at least one cathode region is provided along substantially the entirety of the walls defining the flow channels. This is brings catholyte flow enhancements. Optionally the cathode assembly may be configured as specified In WO 2011/148198.

As discussed above, the fuel may be hydrogen or other materials, such as alcohols e.g. methanol. Optionally, the anode region of the cell is supplied with an alcoholic fuel, and the redox potential of the redox couple is from 0.OIV to 0.6V different from the potential of oxygen and the redox couple and/or the concentration of the redox couple in the catholyte solution is selected so that the current density generated by the cell in operation Is substantially unaffected by the oxidation of alcoholic fuel due to crossover of the alcoholic fuel from the anode region of the cell to the cathode region of the cell across the porous separator.

In the redox fuel cell of the present invention, wherein there Is indirect oxidant reaction at the cathode via a catholyte, the cathode region may additionally be supplied in use thereof with a liquid low molecular weight fuel (for example an alcohol, e.g. methanol), wherein at least some of the liquid low molecular weight fuel in use crosses the porous separator to supply the anode region with liquid low molecular weight fuel. The anode need not have a separate fuel supply and can therefore be solely fuelled by fuel crossing the separator.

The catholyte which is reduced by reaction at the anode is regenerated by oxidation, for example in a specific regeneration zone. Thus the fuel cell may have a regeneration zone separate from the electrode assembly. Optionally the volume of catholyte solution in the regeneration zone is from about 25% to about 90% of the total combined volume of catholyte solution in the regeneration zone and the cathode chambers. There may be cooling means upstream or downstream of the cathode chambers for cooling the catholyte solution. Optionally the regeneration zone Is maintained in operation of the cell at an operating temperature at least 2°C higher than the cathode chambers of the cell andlor at a higher operating temperature than that of cooling means.

The redox fuel cell of the present invention may also comprise a regeneration zone comprising a catholyte channel and a porous member having an active surface, the catholyte channel being arranged to direct a flow of catholyte solution adjacent to or towards the active surface, the means for supplying an oxidant to the cell being adapted to supply the oxidant to the porous member.

Preferably, the average pore size in the active surface of the porous member is 20 to 50 microns and/or the active surface is formed of or coated with a hydrophilic material. The flow rate of the catholyte through the regeneration zone is also optionally at least 0.2 mIs.

The redox fuel cell may comprise a plural stack of electrode assemblies.

Therefore, from a further aspect the present Invention provides a redox fuel cell comprising: a plural stack of electrode assemblies, each electrode assembly IS comprising an anode and a cathode separated by a non-conductive porous separator; an anode chamber adjacent the anode of each electrode assembly; a cathode chamber adjacent the cathode of each electrode assembly; means for supplying a fuel to the anode chambers of the cell; means for supplying an oxidant to the cell; means for providing an electrical circuit between respective anodes and cathodes of the cell; a catholyte solution comprising at least one non-volatile catholyte component, the catholyte solution comprising a redox mediator couple; and a regeneration zone separate from the membrane electrode assemblies, the means for supplying an oxidant to the cell being adapted to supply the oxidant to the regeneration zone.

S In a related aspect the present invention provides a redox fuel cell comprising: a plural stack of electrode assemblies, each electrode assembly comprising an anode and a cathode separated by a non-conductive porous separator, and each electrode assembly comprising an anode chamber adjacent the anode of that assembly and a cathode chamber adjacent the cathode of that assembly; means for supplying a fuel to the anode chambers of the cell; means for supplying an oxidant to the cell; means for providing an electrical circuit between respective anodes and cathodes of the cell; a catholyte solution comprising at least one non-volatile catholyte component, the catholyte solution comprising a redox mediator couple; optionally, means upstream andlor downstream of the cathode chambers for cooling the catholyte solution; and a regeneration zone separate from the electrode assemblies, the means for supplying an oxidant to the cell being adapted to supply the oxidant to the regeneration zone, the regeneration zone being maintained in operation of the cell at an operating temperature at least 2C higher than the cathode chambers of the cell and/or at a higher operating temperature than 2$ that of the cooling means.

The component features of the multiple stack fuel cells, in particular the nature of the redox-active materials, may optionally be the same as In the individual fuel cells as described above.

One way of enhancing regenerabifity of the catholyte in the presence of oxidant (for example air or oxygen) is to atomize it, or convert it to spray or droplets, thereby increasing the surface area for reaction. This method is applicable to a range of catholytes including those which contain catalysts, redox-active components, redox couples, mediators or redox mediator couples. For example catholytes comprising polyoxometalates can be regenerated in this way.

Therefore, one method of regenerating a catholyte solution in a redox fuel cell may comprise the steps of: -providing a redox fuel cell comprising a catholyte solution; -providing droplet formation means for catholyte solution atomisation; -atomising, by way of the droplet formation means or other means, the catholyte solution, thereby generating a mist of fine droplets; -feeding the mist of fine droplets into an oxidant stream; -regulating the oxidant stream flow so that time of flight of the droplets is sufficient to accomplish required mass transfer; a reaching sufficient time of flight; and -providing separation means for separating the mist of fine droplets from the oxidant stream.

Therefore the redox fuel cell of the present invention may comprise a regeneration zone which enhances the reactivity of the reduced catholyte with oxidant.

in one embodiment the regeneration zone comprises: droplet formation means for catholyte solution atomisation operable to generate a mist of fine droplets; means for supplying an oxidant stream to the cathode region of the cell; means for feeding the mist of fine droplets into the oxidant stream; means for regulating the oxidant stream flow so that time of flight of the droplets is sufficient to accomplish required mass transfer and separation means for separating the mist of fine droplets from the oxidant stream.

Optionally there may be multiple dropiet formation means for catholyte solution atomisation. Suitable dropiet formation means include spray nozzles, spinning disks, acoustically driven oscillators, and microfluidic driven oscillators.

Suitable separation means include cyclone separators, for example spiral cyclone separators.

Other features of the regeneration zone and the regeneration method may be as disclosed In WO 201 2/1 75950.

In general the cathode may be any cathode, preferably one suitable for use In an indirect proton exchange mechanism fuel cell. in one embodiment, the cathode of the fuel cell comprises a cathodic material and a proton-conducting polymeric material.

In one embodiment the fuel cell cathode electrode may comprise a porous skeletal medium. In one embodiment the fuel cell cathode electrode comprises a porous skeletal medium, the surface of which medium is modified or otherwise arranged or constructed to induce enhanced activated behaviour, wherein the enhanced activated behaviour is induced by means of increasing the surface area for a given volume of the electrode and/or by increasing the number and/or availability of reactive sites on the electrode.

This may be done using surface modification methods such as carbon is coating, chemical vapour deposition or infiltration, liquid chemical modification, gas-phase chemical modification, plasma etching or heat treatment. Additionally or alternatively, the electrode may be compressed.

Further possible types of porous skeletal medium cathode electrodes are as described in WO 201 Vi 75997.

In another embodiment, the fuel cell of the present invention may comprise a liquid electrolyte regenerator including a separator, the separator comprising a helical channel in the form of a fluid channel formed on a helix and arranged to conduct a gas-liquid mixture and separate a liquid from the gas-liquid mixture.

In a further embodiment, the fuel cell of the present invention may comprise: -a catholyte solution comprising a redox catalyst and/or mediator couple; -means for contacting the redox catalyst and/or mediator couple with the oxidant to generate a foam comprising oxidised catalyst and/or mediator couple; and -means for supplying the foam to a separator for separating the gas and liquid phases of a foam, the separator comprising a first side and a second side and having through-flow means provided therein for permitting a foam or a foam phase to pass from the first side to the second side, the separator further comprising at least one foam contacting surface having a low surface energy, and means for recovering at least one separated foam phase from the foam.

The redox fuel cell of the present invention may further comprise a device for generating fine bubbles, comprising a substrate having holes therethrough, each hole comprising a gas inlet and a gas outlet, wherein the width of the gas outlet is greater than the width of the gas inlet, wherein the average gas inlet width ranges from about 2 to 10 microns and/or wherein the average gas outlet width ranges from about 5 to 100 microns and/or wherein the inlet diameter is 1/10th to 1/5th of the outlet diameter. Preferably, the hole tapers regularly or irregularly from the gas inlet to the gas outlet.

According to another aspect of the present invention, there is provided a method of operating a proton exchange mechanism fu& oeM comprising the steps of: a) forming H ions at an anode situated adjacent to a porous separator; s b) supplying the catholyte of the invention wfth its redox couple in an oxidised state to a cathode situated opposit&y adjacent to the porous separator: and c) aHowing the catalyst to become reduced upon contact with the cathode concomitanfly with H ions passing through the porous separator to balance charge.

In a preferred embodiment, the catholyte is suppMed from a catholyte reservoir.

is The method of the above aspect may additionafly comprise the step of: d) passing the catholyte from the cathode to a reoxidation zone wherein the catalyst is reoxidised.

in an especiafly preferred embodiment, the method of the above aspect comprises the step of: e) passing the catholyte from the reoxidation zone to the catholyte reservoir.

In a further embodiment, the oeM is cyclic and the catalyst in the cathode can 2S be repeatedly oxidised and reduced without having to be replaced.

The fu& ceH of the invenUon may comprise a reformer configured to convert avaable fuel precursors such as LPG, LNG, gasone or ow molecular weight alcohols into a fuel gas (e.g. hydrogen) through a steam reforming reaction. The cefl may then comprise a fuel gas supply device configured to supply the reformed fuel gas to the anode chamber.

It may be desirable in certain applications of the ecU to provide a fu& humidifier configured to humidify the fuel, e.g. hydrogen. The cefl may then o comprise a fuel supply device configured to supply the humidified fuel to the anode chamber.

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

Preferred fuels include hydrogen: ow molecur weight alcohols, dehydes and carboxyfic acids, sugars and biofu&s as weH as LPG, LNG or gasoline.

Preferred oxidants include air, oxygen and peroxides.

The anode in the redox fuel ceV of the invention may for example react with hydrogen gas or methanol: other low molecular weight alcohols such as ethanol or propanol; dipropylene glycol; ethylene glycol; aldehydes formed from these; and acid species such as formic acid, ethanoic acid etc. In addition, the anode may be formed from a bio4uel ecU type system where a bacterial species consumes a fuel and either produces a mediator 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 a cathodic material such as carbon, gold, platinum, nickel or metal oxide species.

However, it is preferable that expensive cathodic materials are avoided and therefore preferred cathodic materials include carbon, nickel and metal oxide.

One preferable material for the cathodes is reticulated vitreous carbon or carbon fibre based electrodes such as carbon felt. Another is nickel foam.

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 is 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 the electrocatalyst, but it may be beneficial to adhere the electrocatalyst in the form of deposited particles on the surface of the electrode.

The redox couple flowing in solution in the cathode chamber in operation of the cell is used in the invention as a catalyst for the reduction of oxygen in the cathode chamber, ri accordance with the foflowing (wherein Sp is the redox coupe species): 02 + + 4Ht -2H20 + 4Spç< The poyoxometSte redox couple, and any other anciuary redox coup'e, utHised in the fue ceH of the nvenflon should be non-volatile and is preferably soluble in aqueous solvent. Freferred redox couples should react with the oxidant at a rate effective to generate a useful current in the electrical circuit ci of the fuel ceu, and react with the oxidant such that water is the ultimate end product of the reaction.

The fuel ecU of the invenUon may operate straighiforwardly with a redox couple catalysing in operation of the fuel cefi the reduction of oxidant in the cathode chamber. However, in some cases, and with some redox couples, it may be necessary and/or desirable to incorporate a catalytic mediator in the cathode chamber.

Also provided is the use of a fuel ecU as described herein to produce electricity.

The contents of WO 2006/097433, WO 2007/110663, WO 2007/122431. WO 20081009993, WO 2008/009992, WO 2009/037513, WO 2009/040577, WO 2009/060419, WO 2009/093081, WO 2009/093082, WO 2009/093080, WO 2010/128333, WO 2011/015875, NO 2011/148198, WC) 2011/107795, WO 20121085542, WO 2012/175950 and WO 2012/175997 are applicable, mutatis mutandis, to the present invention. For example, the catholyte, redox-active component, redox couple, ancillary redox couple, cathode, regenerator and other components may be as described In these documents.

Various aspects of the present invention will now be described in further non-limiting detail with reference to the following Figure 1 which shows a polarisatlon curve for a fuel cell using a Nation membrane, and using a microporous separator in accordance with the present invention. to

Examøle In Figure 1 the use of porous separator gives results comparable with those when a Nation membrane is used.

Experimental details for Figure 1: IV polarization curve for a fuel cell with a 25cm2 active area using Dupont Nation NR212 PEM (supphed by Ion Power mc) or a HDPE microporous separator (supplied by Entek Teklon Gold LP) showing comparable performance when using catholyte -03M Na3H4PMo$V4040].H20. The cathode reaction occurred on SGL GFD25.2 graphite felt with a catholyte flow of 15OmVmin The anode was 0.4mg/cm2 Pt on SGL GDL34BC. The catholyte temperature was 80°C, the cell temperature was set at 80°C, and the H2 pressure at the anode was -Ibar g.

Claims (13)

1. CLAIMS1. A redox fuel cell compnsng an anode and an cathode separated by a porous nonconducfive separator; means for supplying a fu& 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; and a nonvolatile catholyte solution in fluid commumcaton with the cathode, the catholyte solution comprising a redox couple being at east oartially reduced at the cathode in operation of the cell, and at least partially regenerated by reaction with the oxidant after such reaction at the cathode.
2. 2. A redox fuel cell as dairned in claim 1 further comprising a regeneration zone which is separate from the cathode so that in use oxidant does not come into contact with the cathode but instead regenerates the redox couple in the regeneration zone.
3. 3, A redox fuel cell as claimed in claim I or claim 2 in which the separator is microporous.
4. 4, A redox fuel cell as claimed in any preceding claim which is a proton exchange mechanism fuel cell.
5. 5, A redox fuel cefl as claimed in any preceding cairn wherein the porous separator is polyolefinic.
6. 6. A redox fuel ecU as claimed in any preceding claim wherein the catholyte comprises one or more tran&tion metaL
7. 7. A redox fuel ecU as claimed in any preceding daim wherein the redox couple comprises a polyoxometalate
8. 8. A redox fuel cell as claimed in claim 9 wherein the polyoxometalate is a Keggin4ype polyoxometalate.
9. 9. A redox fuel ccii as claimed in any preceding claim wherein the catholyte solution comprises at least about O.075M polyoxometifiate.
10. 10. A redox fuel cell as claimed in any preceding claim wherein the catholyte comprises a polyoxometalate represented by the formula: XJZbMCOd] wherein: X is selected from hydrogen; alkaN metals, alkaline earth metals, ammonium or alkyl ammonium and combinations of two or more thereof; Z is selected from B, P, 8, As, Si, Ge, Ni, Rh, Sn, Al, Cu, I, Br, F, Fe, Cc, Cr, Zn, H2, Te, Mn and Se and combinations of two or more thereof; M is a metal selected from Mo, W, V. Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn Rh, Ru, TI, Al, Ga, In and other metals selected from the l,2' and 3 transition metal series and the lanthanide series, and combinations of two or more thereof; a is a number of X necessary to charge balance the (4McOd] anion; b is from 0 to 20,; c is from Ito 40; and d is from Ito 180.
11. ii. A redox fuel cell as claimed in any preceding claim wherein the catholyte comprises a polyoxometalate represented by the formula: X44MCOdJ wherein: X is selected from hydrogen, alkali metals, alkaline earth metals, ammonium and combinations of two or more thereof Z is selected from B, P, S. As, SI. Ge, Ni, Rh, Sn, Al, Cu, I, Br, F, Fe, Co, Cr, Zn, H2, Te, Mn and Se and combinations of two or more thereot M is a metal selected from Mo, W, V. Nb, Ta, Mn, Fe, Go, Cr, Ni, Zn Rh, Ru, TI, Al, Ga: In and other metals selected from the I',2°' and 3td transition metal series and the lanthanide series and combinations of two or more thereof; a Is a number of X necessary to charge balance the anion; bisfrom0to2O; o is from I to 40; and disfroml to 180.
12. 12. A redox fuel cell as claimed in any of claims 7 to 11, wherein the counterion for the redox couple comprises one or more divalent ion.
13. 13. A redox fuel ceH as claimed in claim 12 wherein the or each divalent ion is selected from Ca Mg, Mn, Fe, Go, Ni, Cu, Zn, Sr Ba, Be, Cr, Cd, Hg, Sn and other suitable ions from the 2nd and 3rd transition series or from the lanthanides, or from combinations of two or more thereat 14, A redox fuel cell as claimed in any preceding claim wherein the catholyte comprises a polyoxometalate represented by the formula: X$4McOd] wherein: X is selected from hydrogen, alkafi metals, alkaline earth metals, ammonium or aikyl ammonium and combinations of two or more thereof; Z is selected from B, P, S, As, Si, Ge, Ni, Rh, Sn, At, Cu, I, Br, F, Fe, Co, Cr, Zn, 112, Ta, Mn and Se and combinations of two or more thereof; M comprises at least one V atom, and M is a metal selected from Mo, W, V, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn Rh, Ru, TI, Al, Ga, In and other metals selected from the 1st,2M and 3 transition metal series and the Ianthanide series and combinations of two or more thereof; a is a number of X necessary to charge balance the (4McOdl anion; bisfromoto2O; cisfromlto4O;and d is from Ito 180.15. A redox fuel cell as claimed in claim 14 wherein the cathoiyte further comprises a vanadiumQV) compound.16. A redox fuel cell as claimed in any preceding claim wherein the catholyte comprises a polyoxometalate represented by the formula: X4ZbMCOd] wherein: X is selected from hydrogen, alkali metals, alkaline earth metals, ammonium and combinations of two or more thereof; Z is selected from B, P. 5, As, Si, Ge, Ni, Rh, Sn, Al, Cu, I, Br, F, Fe, Co, Cr, Zn, H2, Te, Mn and Se and combinations of two or more thereof; M comprises W and optionally one or more of Mo, V. Nb, Ta, Mn, Fe, Co, Cr, NI, Zn Rh, Ru, TI, Al, Ga, In and other metals selected from the 1, 2d and 3 transition metal series and the lanthanide series; a Is a number of X necessary to charge balance the (4MOt anion; bisfromOto5; cisfrom5to3o;and d is from Ito 180.17. A redox fuel cell as claimed in any preceding claim wherein the catholyte comprises a mediator.18. A redox fuel cell as claimed in any preceding claim wherein the catholyte comprises a complexed multidentate N-donor ligand.is. A redox fuel ceO comprising an anode and a cathode separated by a porous separator 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 redox mediator which is at least partially reduced at the cathode In operation of the cell, and at least partially regenerated by, optionaily indirect, reaction with the oxidant after such reduction at the cathode, the catholyte solution comprising a complexed multidentate N-donor ligand as said redox mediator and/or as a redox catalyst catalysing the regeneration of the said mediator.20. A redox fu& ceU as daimed in daim 18 or daim 19 wherein the complexed muftdentate N-donor igand is a complexed muitidentate macrocycUc N-donor flgand.21. A redox tue cefl as claimed in claim 18 or daim 19 wherein the muffidentate N-donor Ugand comprises at east one heterocychc substituent seiected from pyrrole, imidazoe, 1,2,3-triazole, 1,2,4-triazole, pyrazoe, pyridazine, pyrimidine, pyrazine. indole, tetrazole, quinoUne, isoquinoUne and from ky keny, ary!. cydoaiky, akaryL akenaryb. araky, arakenyb groups substituted with one or more of the aforesaid heterocycc groups.22. A redox fu& ce as claimed in any preceding claim wherein the redox couple compdses a ferrocene species.23. A redox fuel c& as claimed in any preceding daim wherein the cathoyte comprises a ferrocene species of formula: cx Ie wherein: X and V are independently selected from hydrogen and from functional groups comprising haogen. hydroxy, amino, protonated amino, imino, nitro, cyano, acyL acyloxy, sulphate, sulphonyL sulphinyL aikylamino, protonated aikylamino, quaternary akylammonium, carboxy, carboxyc acid, ester, ether, amido, sulphonate, sulphonic acid, sulphonamide, phosphoric acid, phosphonate, phosphonic acid, phosphate, alkylsulphonyl, arylsulphonyi, akoxycarbonyl, alkylsuiphinyL arylsulphinyL alkyfthio, arylthio, alkyL alkoxy, oxyester, oxyamido, aryl, arylarnino, aryloxy, heterocycbalkyL heteroaryL (CC5)alkenyl, (CC5)alkynyl, azido phenylsulphonyloXY or amino acid conjugates having the formula -COWOH, where W is an amino acid, and from alkyl, alkenyh aryl. cycloakyl, alkaryl alkenaryl, aralkyl, aralkenyl groups substituted with one or more of the aforesaid functional groups.24. A redox fuel cefl as claimed in any preceding claim wherein the catholyte comprises a ferrocene species having at least one bridging unit between the cyclopentadienyl rings.25. A redox fuS cell as claimed in any preceding daim wherein the catholyte comprises a ferrocene species represented by the formula: o Fe B) wher&n: A(B)nC together comprise a divaent heteroannuar bñdgng group; n s from 1 to 6; and each B (B1) are the same or different.26. A redox fu& ceh as dairned n any precedThg daim wher&n the cathdyte comphses a ferrocene species represented by the formua: wherein: X and Y are, independenUy, any eectron withdrawing substituent; R and R are. independenfly, any spacer group; and n and mare, independenfly, any number from i 10.27 A redox fu& ce as daimed in any preceding daim wherein the cathdyte comprises a compound of the tbrmua:Jwherein: X is s&ected from hydrogen and from functiona' groups comprising habgen, hvdroxyL amino, protonated amino, imino, nitro, cyano, acyL acyloxy, sulphate. suftonyL sulfinyl, alkyamino, protonated aIkyamino, quaternarg alkylammonium, carboxy, carboxyHo acid, ester, ether, amido, sulfonate, suifonic acid, suiphonamide, phosphonic acid, phosphonate.phosphate, aikylsuftonyL arysufonyl, alkoxycarbonyL alkylsultinyl, arysuftinyl, akylthio, arythio, alkyl, alkoxy, oxyester, oxyamido, ayl, fused aryl, arylamino, aryloxy, heterocycloalkyl, heteroaryl, fusedheteroaryL (Cr C5)alkenyl, (C2-OCL5)akynyL azido, phenylsuftonyloxy, amino acid or a combination thereof; are independently s&ected from hydrogen, halogen, hydroxyL amino, protonated amino, imino, nitro, cyano, acyl, acyloxy, suphate, sulfonyL sulfinyl. aficyamino, protonated aUcylamino, quaternary alkylammonium, carhoxy, carboxyflc acid, ester, ether, amido, suftonate, sulfonic acid, suiphonamide, phosphonic acid, phosphonate, phosphate, alkylsuftonyL asulfonyl, alkoxycarbonyl, aikylsuffinyl, arylsulfinyl, alkyfthio, aryithio, alkyl, &koxy, oxyester, oxyamido, aryL fusedaryL arylamino, aryloxy, heterocycoalkyl, heteroaryl, fuse&heteroaryL (C2-C)alkenyi, (Cr OC5)kynyl, azido, phenylsuffonyloxy, amino acid or a combination thereof; wherein R1 and X and/or R5 and X may together form an optionafly wherein R1 and R2 and/or R2 and R3 and/or R3 and R4 and/or R4 and R6 and/or RB and R7 and/or R' and R6 and/or Rh and R5 may together form an optionaHy substituted ring structure; wherein (L) indicates the optional presence of a linking bond or group between the two neighbouring aromatic rings of the structure, and when present may form an optionally substituted ring structure with one or both of R4 and R'; and wherein at least one substituent group of the structure Is a charge- 28. A redox fuel cell as claimed in claIm 27 wherein X is represented by: (Sp) R1°Rwherein R13 are independently selected from hydrogen and from functional groups comprising halogen, hydroxy, amino, protonated amino, imino, nitro, cyano, acyl, acyloxy, sulphate; suiphonyl, suiphinyl, alkylamino, protonated alkylamino, quatematy alkylammonium, carboxy, carboxylic acid, ester, ether, amklo, sulphonate, sulphonic acid, sulphonamide, phosphonic acid, phosphonate, phosphonic acid, phosphate, alkylsulphonyl, arylsulphonyl, alkoxycarbonyl, aikylsulphinyl, 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, alkaryl aflcenaryl, aralkyl. aralkenyl groups substituted with one or more of the aforesaid functional groups; wherein R9 together with R' and/or R13 together with R5 may form a linking group, preferably selected from 0, N, 8, imino, sulfonyl, sulfinyl, alkylamino, protonated alkylamino, quatemary alkylammonium, carbonyl, ester, ether, amido, sulphonamide, phosphonate, phosphate, alkysulfonyl.alkenylsulfonyl, arylsulfonyl, alkylsulfinyl, alkenylsulfinyl, arysulfinyl, alkylthio, alkenylthio, arytthio, oxyester, oxyamido, aryl, cycloalkyl, heteroaryl, (CCalkyl, (C2-C5)alkenyl, (CC5)alkynyl, or amino acid; and wherein (Sp) indicates the optional presence of a spacer group.29. A redox fuel cell comprising: an anode and a cathode assembly, the anode and the cathode assembly separated by a non-conductive porous separator; means for supplying a fuel to the anode assembly; means for supplying an oxidant to the cathode assembly; means for providing an electrical circuit between respective anodes and cathodes of the cell and; a catholyte solution comprising at least one catholyte component, the catholyte solution comprising a redox mediator couple; the cathode assembly comprising a catholyte inlet channel and one or more flow channels in fluid communication with the catholyte inlet channel, the flow channels being defined by flow channel walls comprising at least one porous cathode regions, at least one of the flow channels being nonahgned with the catholyte inlet channel, wherein one or more of the flow chann&s are closed at one end, and wherein the at least one cathode region is provided along substantiay the enfirety of the was defining the flow chann&s.30. A redox fuel ce as claimed in any preceding claim, the anode region of the ceu being supphed with an alcoholic fu&, the redox potential of the redox couple being from 0.OIV to O.6V different from the potential of oxygen arid the redox couple and/or the concentration of the redox couple in the catholyte sohition being selected so that the current density generated by the ce in operation is substantiaUy unaffected by the oxidation of alcoholic fuel due to crossover of the alcoholic fuel from the anode region of the ceH to the cathode region of the cell across the porous separator.31.A redox fuel cell as claimed in any preceding daim, wherein the cathode region is supplied in use thereoF wfth a liquid low molecular weight fueL wherein at least some of the liquid low rnoiecuar weight fuel in use crosses the porous separator to supply the anode region with liquid ow molecular weight fuel.32. A redox fuel ecU comprising: a plural stack of electrode assemblies, each electrode assembly comprising an anode and a cathode separated by a porous non-conductive separator: an anode chamber adjacent the anode of each electrode assembly; a cathode chamber adjacent the cathode of each electrode assembly; means br supplying a fuel to the anode chambers of the cell; means for supplying an oxidant to the cell; means for providing an electrical circuit between respective anodes and cathodes of the cell; a catholyte solution comprising at least one non-volatile catholyte component, the catholyte solution comprising a redox mediator couple; and a regeneration zone separate from the electrode assemblies, the means for supplying an oxidant to the cell being adapted to supply the oxidant to the regeneration zone.33. A redox fuel call as claimed in claim 32 wherein in use the volume of catholyte solution in the regeneration zone is from about 25% to about 90% of the total combined volume of catholyte solution in the regeneration zone and the cathode chambers.34. A redox fuel cell comprising: a plural stack of electrode assemblies, each electrode assembly comprising an anode and a cathode separated by a porous non-conductive separator, and each electrode assembly comprising an anode chamber adjacent the anode of that assembly and a cathode chamber adjacent the cathode of that assembly; means for supplying a fuel to the anode chambers of the cell; means for supplying an oxIdant to the cell; means for providing an electrical circuit between respective anodes and cathodes of the cell; a catholyte solution comprising at least one non-volatile catholyte component, the catholyte solution comprising a redox mediator couple; optionally, means upstream and/or downstream of the cathode chambers for cooling the catholyte solution; and a regeneration zone separate from the electrode assemblies, the means for supplying an oxidant to the cell being adapted to supply the oxidant to the regeneration zone, the regeneration zone being maintained in operation of the cell at an operating temperature at least 2°C higher than the cathode chambers of the cell and/or at a higher operating temperature than that of the cooling means.35. A redox fuel cell as claimed In any preceding claim comprising a regeneration zone with droplet formation means or atomisatlon means for the catholyte.36. A redox fuel cell as claimed in any preceding claim comprising a regeneration zone with means to separate catholyte after regeneration.37. A redox fu& ecU as claimed in any precedng caim wherein the cathode electrode comprises a porous skeleta medium.38. A redox fuel ecU as claimed in any preceding cairri further comprising means for contacting the redox cata'yst and/or mediator couple with the oxidant to generate a foam comprising oxidised catalyst and/or mediator couple.39, A redox protonexchangernechaniSrfl fuei ecU comprising a porous non conductive separator.40. The use of a redox fuel ecU as daimed in any preceding claim to produce &ectricity.41. A method of operating a protonexchangen1eChaniSrn fuel ecU comprising the steps of: a) forming H" ions at an anode situated adjacent to a porous non conductive separator; b) supplying a catholyte as defined in any preceding claim with its redox couple in an oxidised state to a cathode situated oppositely adjacent to the porous separator: and c) aUowing the catalyst to become reduced upon contact with the cathode concomitantly with Ht ions passing through the porous separator to balance charge.