The present invention relates to fuel cells, in particular to indirect or redox fuel cells which have applications in stationary, back-up and combined heat and power (chp) contexts, as well as in fuel cells for the automotive industry and in microfuel cells for electronic and portable electronic devices.

Fuel cells have been known for portable applications such as automotive and portable electronics technology for 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.

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. 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 C0 ₂ 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. A further disadvantage of fuel cells in which liquid electrolytes are employed is the difficulty of maximising the flow of the electrolyte through those cells while also ensuring a high degree of chemical interaction between the electrodes and the electrolyte. Maximising flow rate is especially challenging for fuel cells in which liquid electrolyte is passed through porous electrodes, rather than being flowed adjacent to electrodes.

Additionally, when attempts are made to scale-up the energy output of liquid electrolyte fuel cells, a loss in fuel cell performance is generally observed. It is believed that this is partly because when the size of the fuel cell is increased, the total flow of electrolyte through the cell must be proportionally increased to maintain electrolyte stoichiometry. However, when the fuel cell and volume is scaled up in this way, a drop in flow pressure is observed, resulting in sub- optimal fuel cell performance. The need to balance these conflicting requirements gives rise to inefficiencies in cell performance, particularly in automotive applications and in combined heat and power.

It is an object of the present invention to overcome or ameliorate one or more of the aforesaid disadvantages. It is a further object of the present invention to provide an improved redox fuel cell structure providing improved energy output in larger scale fuel cells.

According to a first aspect of the present invention, there is provided a redox fuel cell comprising:

an anode and a cathode assembly, the anode and the cathode assembly separated by an ion selective polymer electrolyte membrane;

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 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 closed at one end, and wherein the at least one cathode region is provided along substantially the entirety of the walls defining the flow channels.

5

Where the "cathode" or "cathodes" of the fuel cell of the present invention are mentioned below, reference is made to the one or more cathode regions.

A preferred form of arrangement in the present invention is one in which at least 10 one flow channel wall is provided by the porous cathode region, any remaining walls being provided by a non-porous material. For example, the porous cathode region may overlie a non-porous substrate (a bi-polar plate for example) provided with flow channels, the porous cathode region thereby forming the upper wall of the flow channels. In this embodiment the porous cathode is provided along i s substantially the entirety of the walls defining the flow channels by overlying substantially the whole of the flow channel substrate, or at least that part of the substrate in which flow channels are provided.

By "catholyte inlet channel" is meant a channel which carries catholyte into the 20 cathode assembly and not merely directs catholyte into the assembly. Thus, an inlet port provided on the outside of a cathode assembly chamber wall, directing catholyte into the assembly, would not itself be considered to be a catholyte inlet channel. However, a channel fed by such a port, even if not aligned with the inlet port, which carries catholyte into the assembly, would be considered as a catholyte inlet channel.

In operation of the cell, the catholyte is provided flowing in fluid communication with the cathode through the cathode assembly of the cell. The redox mediator couple is 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. In a preferred embodiment of the present invention, the cell will be arranged such that the catholyte is distributed through the at least one porous cathode region, resulting in the redox mediator couple being at least partially reduced. The flow channels are closed at one end, so as to force the catholyte flowing into the flow channel through the path of least resistance, i.e. through the at least one porous cathode region; thus reducing the flow length in the porous region.

The use of one or more flow channeis which are non-aligned with the catholyte inlet channel effectively creates multiple short flow paths for the catholyte through the cathode, which typically forms the porous cathode region and typically overlies the flow channels, forming the top wall thereof. A loss of velocity of the catholyte is observed as the catholyte passes through the non-linear flow path. Advantageously, this loss of catholyte velocity reduces the drop in fluid pressure observed in prior art fuel cells when catholyte is passed through cathode assemblies. By minimising the pressure drop, the interaction between the catholyte and the at least one cathode region is maximised, increasing energy output. Further, as the loss of flow velocity is offset by the reduction in flow distance through the (generally overlying and parallel) porous regions, a high overall flow rate of catholyte through the cell can be maintained, meaning that a given current can be generated using a lower percentage of catholyte than in conventional cells.

Minimising the pressure drop of catholyte flowing through a cathode assembly is especially advantageous in assemblies adapted to direct the flow of catholyte through porous cathode regions (i.e. porous cathodes). If there is any pressure drop in the flow of catholyte, the flow resistance of the catholyte through the porous cathode region will be increased. This will have the effect of reducing the overall flow rate of catholyte through the cathode assembly. As the viability of fuel cells relies on a high flow rate of catholyte therethrough, the rate of catholyte flow through porous cathodes, and thus the pressure of that flow, is critical.

Linear flow channels can be effectively employed in the cathode assemblies employed in the fuel cells of the present invention. However, in alternative embodiments, the flow channels are non-linear and include one or more corners and/or angles. Similarly, although the aims of the present invention are met by an arrangement in which some, but not all of the flow channels, are non-aligned with the catholyte inlet channel, performance will be improved in most embodiments when the highest possible proportion of flow channels are non-aligned with the catholyte inlet channel.

The cathode in the redox fuel cell of the invention may comprise as cathodic material carbon, gold, platinum, nickel, metal oxide species. However, it is preferable that expensive cathodic materials are avoided, and therefore preferred cathodic materials include carbon, nickel and metal oxide. Especially preferred cathodic materials are porous, for example reticulated vitreous carbon, carbon fibre based electrodes such as carbon felt, woven carbon fabric or 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 assembly is designed to create maximum flow of catholyte solution to the porous 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 or by other well known techniques such as sputter coating or other vacuum deposition methods on the surface of the electrode.

In one embodiment, the one or more flow channels extend from the catholyte inlet channel. It will be appreciated that one or more of the non-aligned flow channels will extend from the catholyte inlet channel at an angle. It is this angle which delineates the catholyte inlet channel and the flow channel/s. In preferred embodiments, the one or more flow channels extend from the catholyte inlet channel at an angle of 135 or less, 120° or less, or most preferably 90° or less. For the avoidance of doubt, the angle of projection of the one or more flow channels is measured from the longitudinal axis of the catholyte inlet tube immediately prior (i.e. upstream) of the flow channel. in an alternative arrangement, the catholyte inlet channel may terminate in a catholyte deposit zone, rather than having flow channels extending therefrom. In such an arrangement, the one or more flow channels may extend from the catholyte deposit zone. It is preferred that the flow channels be generally parallel. In especially preferred embodiments of the present invention, a plurality of parallel flow channels extend perpendicularly from the catholyte inlet channel. For the avoidance of any doubt, a flow channel is not considered to be aligned with the catholyte inlet channel merely if it is parallel to that channel.

The cathode assembly employed in the fuel cell of the present invention preferably comprises a catholyte collection zone. The catholyte collection zone is at least partially defined by the outer walls of the one or more flow channels. The walls defining the catholyte collection zone may comprise cathode region/s. The catholyte will gather in the catholyte collection zone after it has exited the one or more flow channels. The catholyte collection zone may comprise collection channels which are preferably formed by the outer walls of the flow channels. In a preferred embodiment, collection channels will be provided between a plurality of parallel flow channels, to provide an interdigitated structure. Cathode regions may be provided in the walls defining the collection channels.

The cathode assembly is preferably housed in a chamber. The chamber walls may partly define one or more of the catholyte inlet channel, the one or more flow channels, the one or more collection channels and the catholyte collection zone. The cathode assembly is preferably provided with a catholyte outlet channel. In arrangements in which a catholyte collection zone is present, the catholyte outlet channel is preferably in fluid communication with that collection zone. In arrangements in which no catholyte collection zone is present, the catholyte outlet channel is preferably in fluid communication with the at least one flow channels.

In a preferred embodiment of the present invention, following reduction of the redox couple within the cathode assembly, that couple is re-oxidised in a regeneration zone. The regeneration zone is preferably separate from the membrane assembly. Thus, the fuel cell of the invention preferably comprises means for supplying at least partially reduced redox mediator couple from the cathode assembly of the cell to the regeneration zone.

Further, the fuel cell of the invention preferably comprises means for supplying at least partially regenerated redox mediator couple from the regeneration zone to the cathode assembly. The catholyte solution comprising the redox mediator couple therefore circulates in operation of the cell from the regeneration zone in at least partially regenerated (oxidised) form to the cathode assembly where it is at least partially reduced and thereafter returns to the regeneration zone where it reacts (directly or indirectly, when a redox catalyst is present) with the oxidant before returning to the cycle.

At any convenient location in the cycle one or more pumps may be provided to drive circulation of the catholyte solution. Preferably, at least one pump is situated between the downstream end of the regeneration zone and the upstream end of the cathode assembly.

The regeneration zone preferably comprises one or more of: a chamber in which the regeneration reaction takes place; a first inlet port for receiving into the chamber reduced redox mediator couple from the cathode assembly of the cell; a first outlet port for supplying oxidised redox mediator couple to the cathode assembly of the cell; a second inlet port for receiving a supply of oxidant; and a second outlet port for venting gas, water vapour and/or heat from the chamber.

To reduce, and possibly eliminate, any loss in catholyte solution, one or more demisters may be provided in or upstream of the second outlet port.

Additionally, to prevent excessive evaporation of water from the catholyte, condensers may be provided in or upstream of the second outlet port. If a condenser is employed in the fuel cell of the present invention, it is preferably arranged such that a predetermined amount of condensate will be returned to the system. Prior to being passed back into the cathoiyte, the condensate is preferably passed through the demister/s.

The fuel cell of the invention, when used in a chp application, may be provided 5 with heat transfer means associated with the regeneration zone for transferring heat generated in the regeneration zone to an external target such as a domestic or commercial boiler for example. Heat transfer means may work under standard principles of heat exchange, e.g. with close-contacting pipework, fins and vanes for increasing surface area contact between a cold pipe and a warm pipe, for 10 example.

While reference is made above to the "anode" and "cathode assembly" (in the singular) it will be appreciated that the fuel cell of the invention will typically comprise more than one membrane electrode assembly and thus more than one i s anode and cathode assembly. Each membrane electrode assembly is preferably separated by bipolar separation plates, in what is commonly known in the art as a fuel cell stack.

The redox mediator couple and/or the redox catalyst when present may comprise 20 a polyoxometallate compound, as described in our co-pending PCT/GB2007/050151 The redox mediator couple and/or the redox catalyst when present may comprise a polyoxometallate compound with a divalent counterion, as described in our copending PCT/GB2008/050857 The redox mediator couple and/or the redox catalyst when present may comprise an N-donor compound, as described in our co-pending PCT/GB2007/050421 .

The redox mediator couple and/or the redox catalyst when present may comprise a multi-dentate N-donor ligand comprising at least one heterocyclic substituent selected from pyrrole, imidazole, 1 ,2,3-triazole, 1 ,2,4-triazole, pyrazole, pyridazine, pyrimidine, pyrazine, indole, tetrazole, quinoline, isoquinoline and from alkyl, alkenyl, aryl, cycloalkyl, alkaryl, alkenaryl, aralkyl, aralkenyl groups substituted with one or more of the aforesaid heterocyclic groups, as described in our co-pending PCT/GB2009/050065.

The redox mediator couple and/or the redox catalyst when present may comprise a multidentate macrocyclic N-donor ligand, as described in our co-pending PCT/GB2009/050067. The redox mediator couple and/or redox catalyst when present may comprise a modified ferrocene species as described in our co-pending PCT/GB2007/050420. The redox mediator couple and/or redox catalyst when present may comprise a modified ferrocene species comprising a bridging unit between the cyclopentadienyl rings as described in our co-pending PCT/GB2009/050066. Generally, the redox mediator couple will comprise ligated transition metal complexes. Specific examples of suitable transition metals ions which can form such complexes include manganese in oxidation states II - V, Iron l-IV, copper I- III, cobalt l-lll, nickel l-l l l , chromium (ll-VII), titanium ll-IV, tungsten IV-VI, vanadium II - V and molybdenum ll-VI. Ligands can contain carbon, hydrogen, oxygen, nitrogen, sulphur, halides, and phosphorus. Ligands may be chelating complexes include Fe/EDTA and Mn/EDTA, NTA, 2- hydroxyethylenediaminetriacetic acid, or non-chelating such as cyanide.

The fuel cell of the invention may operate straightforwardly with a redox couple catalysing in operation of the fuel cell the reduction of oxidant in the cathode assembly. However, in some cases, and with some redox couples, it may be necessary and/or desirable to incorporate a catalytic mediator in the catholyte solution. In one preferred embodiment of the invention, the ion selective PEM is a cation selective membrane which is selective in favour of protons versus other cations. 51014

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, perfluorosulfonic 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 α,β,β triflurostyrene 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 polyimides; styrene/ethylene-butadiene/styrene triblock copolymers; partially sulphonated polyarylene ether sulphone; partially sulphonated 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 as described in our copending PCT/EP2006/060640. According to a further aspect of the present invention, there is provided a process for operating a redox fuel cell comprising:

providing an anode and a 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 non- aligned with the catholyte inlet channel, wherein one or more of the flow channels are closed at one end, and wherein the at least one cathode region is provided along substantially the entirety of the walls defining the flow channels;

providing a catholyte solution comprising at least one catholyte component, the catholyte solution comprising a redox mediator couple; and

supplying a flow of catholyte solution to the cathode assembly via the catholyte inlet channel, the catholyte solution flowing into the one or more flow channels and contacting the one or more cathode regions;

supplying an oxidant to the cathode assembly;

supplying a fuel to the anode assembly;

providing an electrical circuit between respective anodes and cathode assembly of the cell. It will be appreciated that the process of the invention may be used to operate the fuel cell of the invention in all its various embodiments, preferences and alternatives, and that when this specification describes any feature of such a fuel cell it also specifically envisages that that feature may also be a preference or alternative process feature in the process 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 (e.g. 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 region.

It may be desirable in certain applications of the cell to provide a fuel humidifier configured to humidify the fuel, e.g. hydrogen. The cell may then comprise a fuel supply device configured to supply the humidified fuel to the anode region.

An electricity loading device configured to load an electric power may also be provided in association with the fuel cell of the invention. Preferred fuels include hydrogen; metal hydrides (for example borohydride which may act as a fuel itself or as a provider of hydrogen), ammonia, low molecular weight alcohols, aldehydes and carboxylic acids, sugars and biofuels as well as LPGLNG or gasoline. Preferred oxidants include air, oxygen and peroxides

The anode in the redox fuel cell of the invention may for example be a 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 bio-fuel cell 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.

According to a further aspect of the present invention, there is provided a cathode assembly comprising:

a catholyte inlet channel;

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 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 closed at one end, and wherein the at least one cathode region is provided along substantially the entirety of the walls defining the flow channels;

a catholyte collection zone defined, at least partly, by the outer walls of the flow channels; and a catholyte outlet channel in fluid communication with the cathoiyte collection zone.

For the avoidance of any doubt, any of the features of the cathode assembly of 5 the fuel cell of the present invention which are provided above may be employed in the cathode assembly of this aspect of the present invention.

Also provided in accordance with the invention is the use of a fuel cell as described herein to provide motive power to a vehicle.

10

Also provided in accordance with the invention is the use of a fuel cell as described herein to generate power in an electronic component.

The invention also provides a combined heat and power system comprising at i s least one fuel cell as described herein.

The invention also provides a vehicle comprising at least one fuel cell as described herein.

20 The invention also provides an electronic component comprising at least one fuel cell as described herein. Various aspects of the present invention will now be more particularly described with reference to the following figures which illustrate embodiments of the present invention: Figure 1 a illustrates a comparative example, being a schematic view of a cathode assembly including an open-ended serpentine arrangement of flow channels.

Figure 1 b illustrates a comparative example, being a schematic view of a cathode assembly including an open-ended parallel arrangement of flow channels.

Figure 1c illustrates a comparative example, being a schematic view of a cathode assembly including an open-ended discontinuous arrangement of flow channels.

Figure 2 illustrates a schematic view of a cathode assembly according to the invention including an interdigitated arrangement of flow channels.

Figure 3 illustrates a schematic view of a comparative cathode assembly which is included for exemplary purposes only.

Figure 4 illustrates a schematic view of a cathode assembly including interdigitated arrangement of flow channels in accordance with the invention. Figure 5 is a line graph illustrating the pressure drop of catholyte flowed through the cathode assemblies illustrated in Figures 3 and 4.

Figure 6 shows a perspective view of the assembly similar to the one schematically illustrated in Figure 4.

In the arrangements illustrated in Figures 1 a to 1 c, which are comparative examples, the flow channels are open ended at both ends. This type of arrangement is preferred where the cathode regions are non-porous. The catholyte inlet channel 12 brings catholyte into the assembly 10. The catholyte is then flowed through the flow channels 14 until the catholyte reaches the catholyte collection zone 18. The catholyte then exits the assembly via a catholyte outlet port (not shown). The catholyte inlet channel 12, flow channels 14 and catholyte collection zone 18 are all partly defined by the assembly chamber wall 16 and partly defined by flow channel walls 20. Cathode regions (not shown) are provided on the flow channel walls. In certain embodiments, the entirety of the flow channel wall may constitute a cathode region.

In figure 2, an alternative assembly type in accordance with the invention is shown. In the illustrated assembly 10, the catholyte inlet channel 12 has four flow channels 14 extending perpendicularly therefrom. The ends 22 of those flow channels 14 are closed. Parts (not shown) of the flow chamber walls 20 are formed of porous cathode material. By providing closed ends 22 of the flow chambers, a build up of pressure is caused in the flow channels 14, resulting in the catholyte being forced through the porous cathode regions into the catholyte collection zone 18. In the illustrated arrangement, the catholyte collection zone 18 is provided with collection channels 24 which extend between the flow channels 14 to form an interdigitated arrangement. In most embodiments the porous cathode region will overlie the depicted flow channels, hence forming the top wall of each flow channel.

The performance of a fuel cell including a cathode assembly comprising an interdigitated arrangement (illustrated in Figure 4) was tested against the performance of a similar cell including a comparative cathode assembly which is illustrated in Figure 3. The arrangement shown in Figure 3 was an early attempt to provide an improved cathode assembly. That arrangement possessed acceptable strength and was compatible with conventional anode assemblies. The illustrated cathode assembly 100 includes manifolds 102, 104 which retain porous cathodes 106. The catholyte in fig 3 flows as a single layer as indicated by the arrows. In the case of figure 4 the catholyte flows through the channels indicated by the arrows and crosses between the flow channels through a single sheet of porous electrode which lies above the interdigitated non porous plate. Catholyte is flowed through an inlet port in the first manifold 102 and passes through the porous cathodes 106, before exiting through an outlet port in the second manifold 104. The performance of these two assemblies was tested by flowing cathoiyte through them at comparable flow rates. The velocity of the catholyte through the catholyte collection channels shown in Figure 4 was 1/4 (four being the number of catholyte collection channels) and the velocity of the catholyte through the flow channels was 1/5 (five being the number of flow channels). The flow length through the assemblies shown in Figures 3 and 4 is approximately equal. The overall flow rate of catholyte through the two cells was comparable as was the catholyte stoichiometry. A lower pressure drop in catholyte flow through the assembly shown in Figure 4 was observed than when the assembly shown in Figure 3 was operated, and this can be clearly seen from Figure 5. From that figure, it can be seen that the pressure drop observed in catholyte flow through the assembly illustrated in Figure 3 is at least a third greater than that observed for the assembly illustrated in Figure 4, regardless of flow rate. Figure 6 shows in perspective view an interdigitated design according to the invention, and very similar to the arrangement schematically depicted in Figure 4. The lower non-porous plate 1 contains an inlet manifold 3 and interdigitated channels 4 at 90 degrees to the inlet manifold 3. Catholyte flows from the interdigitated channels 4 through the porous cathode electrode 2 into the outlet interdigitated channels 6 and then to the outlet manifold 5. The interdigitated channels 4 and 6 and the inlet and outlet manifolds 3 and 5 are formed on three sides by the lower plate 1 and on the upper surface by the porous electrode 2.