The present invention relates to an apparatus and method for generating foam. In particular, but not exclusively, the apparatus may be used for generating foam in a fuel cell system.

Published international patent applications WO2010/128333, WO2011/107794 and WO20 1/107795 disclose redox fuel cells in which a regeneration zone is provided for the oxidative regeneration of a redox active species which has been reduced at the cathode in operation of the cell. These fuel cells may use hydrogen as a fuel with air or oxygen as oxidant. The operation of indirect redox fuel cells of these types is such that the oxidant is not supplied directly to the electrode but instead reacts with the reduced form of a redox couple in the catholyte to oxidise the redox couple, and this oxidised species is fed to the cathode of the fuel cell. In the regenerator, the catholyte comes into contact with an oxidant such as air or oxygen and is regeneratively oxidised before flowing back to the fuel cell. The oxidant may be a mixture of gases including oxygen, such as air, and the term "air" will be used when describing embodiments of the invention for convenience and brevity. However, a person of ordinary skill in the art will recognise that embodiments are not limited to the use of air as an oxidant.

FIG. 1 is a simple flow diagram illustrating the redox reactions occurring in these types of fuel cell. WO2011/107794 is to some extent concerned with the oxidative regeneration of the redox active species in a regeneration zone and describes the basic principles of liquid flow past a porous element through which gas is injected into the liquid thus creating high interfacial area between the gas and liquid phases by generating many small bubbles, each having a gas-iiquid interfacial membrane.

There are a number of constraints on this step of oxidising the redox couple. Oxidation of the redox couple should occur as rapidly as possible as a reduction in flow rate of the catholyte through the cathode will reduce the rate of energy production. The rate of energy production will also be reduced if oxidation of the redox couple is not as complete as possible, i.e. if a significant proportion of the redox couple remains unoxidised. The provision of apparatus which rapidly and completely oxidises redox couples present in catholyte solutions is made challenging by the need to ensure that the energy consumed when the oxidation step is taken is relatively low, otherwise the overall power generation performance of the fuel cell will be reduced. Additionally, the apparatus used to oxidise the redox couple should be as compact as possible, especially when the fuel cell is intended for use in portable or automotive applications. A very large interfacial surface area between the liquid catholyte and the oxidative gas promotes a higher oxidation rate.

Aspects and embodiments in accordance with the present invention were devised with the foregoing in mind.

Viewed from a first aspect there is provided a foam generator comprising, in a foam generation region, a porous member including first and second surfaces, and arranged to supply a gas to a first surface of the porous member and a liquid to the second surface of the porous member; wherein the porous member is configured to communicate gas incident at the first surface through the porous member to the second surface of the porous member; the foam generator further comprising a gas supply modulator arranged to modulate supply of the gas to the first surface and wherein the foam generator is arranged to receive the gas from a gas supply common to at least one other foam generator.

Viewed from a second aspect there is provided a method for generating foam, comprising:

providing plural ones of a foam generator of which a foam generator includes in a foam generation region a porous member comprising first and second surfaces; modulating a supply of gas from a gas supply common to at least one other of the plural ones of a foam generator to a first surface of the porous member;

supplying a liquid to the second surface of the porous member; and

configuring the porous member to communicate gas incident at the first surface through the porous member to the second surface of the porous member.

One or more embodiments in accordance with the first and second aspects may provide restriction and/or control of the size of bubbles in the foam, and/or the possibility of producing a greater volume of foam using foam generators of practical size, and/or good coupling of modulator power to foam across plural foam generators while avoiding standing waves in the gas flow and/or in the foam generators. The need to adjust coupling dimensions of a gas supply and the dimensions of individual foam generators to eliminate or at least reduce standing waves is avoided.

One or more embodiments in accordance with the first and second aspects may provide for integration of one or more individual modulators or oscillators within the foam generator, e.g. at least one modulator or oscillator for each foam generator. The inventors have found that doing this may provide for optimised coupling of the gas in each element to the gas input of the foam generator. In particular, this may provide for optimised coupling of the porous members to the air injection point (input) of the foam generator. This may be achieved by using plural, preferably small-scale, modulators/oscillators, one associated with each foam generator. The modulator/oscillator of each foam generator may be integrated with the foam generator, within the foam generator, or coupled to the foam generator. Typically, the modulator/oscillator of each foam generator is at the gas input end of the foam generator.

The porous member is configured to introduce the gas into the liquid to generate bubbles and the pores of the porous member may be configured at the surface adjacent the foam generation region to promote bubbles of a desired size.

The porous member may be configured to communicate the gas between the first and second surface at a rate of up to 100 litres per minute, in particular up to 50 litres per minute or more particularly between 1 and 20 litres per minute. Suitably, the gas supply modulator is configured to modulate the supply of gas at a frequency to generate bubbles having a diameter less than about 100 microns, and more particularly configured to modulate the supply of gas at a frequency to generate bubbles having a diameter of about 50 microns. This may be achievable by configuring the gas supply modulator modulate the supply of gas at a frequency between about 1 kHz and 5 kHz. Additionally, the modulation frequency may be chosen to avoid the formation of standing waves downstream of the oscillator, in particular between the oscillator and the porous member. A suitable gas supply modulator is an oscillator such as a fluidic oscillator.

Arranging the foam generator such that the liquid travels in a direction transverse to gas flow at the second surface assists in removal of bubbles from the second surface of the porous member.

In one or more embodiments, the foam generator may be coupled to a reaction volume for reacting gas in bubbles of foam generated in the foam generation region with the liquid. Typically, the reaction volume comprises a fluid conduit in fluid communication with the foam generation region and extending therefrom. The modulation of the gas supply may be chosen to avoid the formation of standing waves in the connection between the oscillator and the porous member or to promote the formation of standing waves such that an antinode of a standing wave is incident at the porous member. In an embodiment in which the formation of standing waves is promoted, the modulation of the gas supply may be chosen such that an area around the antinode of a pressure wave is incident at the porous member. That is to say, the antinode of the standing wave need not be precisely incident at the porous member.

In one or more embodiments, the fluid conduit is coupled to a foam destruction region at an end distal from the foam generation region. Destruction of the foam assists in transfer of the liquid from the foam generator and reaction volume.

Suitably, the foam destruction region comprises a foam destruction material such as a low surface energy material. In one or more embodiments, the plural foam generators may be configured as an assembly. In such an assembly, respective foam generators are configured to operate substantially in parallel with each other and optionally or additionally configured to be substantially similar to each other. Typically, such an assembly may further comprise an output manifold including plural inlet ports, respective inlet ports coupled to respective reaction volumes of the plural foam generators; one or more of the plural inlet ports at least partially comprising the foam destruction region. Typically, the one or more plural inlet ports at least partially comprise foam destruction material, such as a low surface energy material.

In an assembly, typically plural ones of the foam generator are placed adjacent each other; suitably the plural ones of the foam generator are assembled to form a stack. An assembly may comprise a gas inlet manifold arranged to distribute gas to each of the plural foam generators for supply to the first surface of respective porous members. Suitably, the assembly is arranged to allow a total air flow through the assembly in excess of 1 ,000 litres per minute.

Respective gas supply modulators of the assembly may be configured to modulate the supply of gas at a frequency to generate a total number of bubbles in the assembly of the order of 2 E11 (2 x 10 ¹¹ ) bubbles per second.

One or more embodiments may comprise a fuel cell comprising a foam generator as set out above. One or more embodiments may comprise a fuel cell comprising an assembly as set out above.

A fuel cell such as disclosed in the foregoing may be used in electronic or automotive equipment, or for generating combined heat and power.

One or more embodiments may comprise an electronic, automotive, or combined heat and power equipment comprising a fuel cell as disclosed in the foregoing. Locating a modulator/oscillator towards the input end of each foam generator, or at least in a similar or the same location within each respective foam generator, may provide good coupling of the modulator or oscillator to the foam within each foam generator and similar, or well-matched, coupling across plural foam generators may be achieved.

If the modulator/oscillator is coupled to the foam generator (rather than being integral with the foam generator), then it may be closely coupled, in close proximity to or adjacent to the gas input of the foam generator and closely coupled, in close proximity to or adjacent to the porous member. This may enhance matching of the modulator/oscillator to the gas inputs of the foam generator and therefore enhance coupling of modulator's/oscillator's power to each foam generator and to thus to the foam in generated by each foam generator. If the modulator/oscillator is coupled, but not closely coupled, i.e. it is not adjacent to or close to the porous member, then the modulator and porous member may be spaced apart, along the flow direction of the gas, by substantially the same distance for each gas-liquid contacting element. This may also result in enhanced matching of the modulator/oscillator to the gas inputs of the foam generators, good coupling of the oscillator output to the foam-generating region of each foam generator, and therefore good coupling of the modulator/oscillator power to the foam in each foam generator. In both of the above cases when periodic oscillation is used, the phase difference of oscillation between the oscillator output of an element and the gas input of the element may be substantially the same for each foam generator. In operation of a foam generator as gas is forced across the porous member by the pressure difference across the porous member, bubbles form in the liquid on the lower-pressure (liquid-containing) side of the porous member. For example, if the member is a holed (apertured) plate, bubbles will form at the holes of the plate. The bubbles will grow in volume as gas continues to pass from the porous member into the liquid.

When the modulator inhibits or prevents the passage of gas from the porous member into the liquid, by reducing or eliminating the pressure difference across the porous member from the gas input to the liquid, the gas bubbles stop increasing in volume, and at least most of the bubbles are detached from the porous member and are swept, by the flowing liquid, along the major direction of flow of liquid relative to the porous member. Typically such flow of liquid is, close to the porous member, mainly tangential to the outer surface of the porous member. The intermittent limiting of the gas flow and the movement of liquid past the porous member surface together act to limit the average volume of bubbles generated.

From the above and from the more detailed description of embodiments provided below, it will be evident to the person of ordinary skill that one or more embodiments of the invention may provide for one or more of the following features: a. The provision of plural individual fluid oscillators, an oscillator in each one of a foam generator of an assembly for a liquid catholyte fuel cell;

b. Provision of multiple oscillators within one foam generator to improve coupling of oscillations to supplied gas over a large input area of the gas liquid contacting element;

c. A stack of foam generators and reaction volumes with common air and liquid manifolds forming an efficient mass transfer device. This may provide for more efficient oxidation of catholyte in a redox fuel cell;

d. The use of the above-described foam generator for efficient gas liquid mass transfer in other industrial applications such as a bioreactor.

The above and further aspects and advantages of embodiments in accordance with the invention will become clearer from the following detailed description, given by way of example only and with reference to the appended drawings. It should be understood that the scope of protection sought by the applicant is defined by the appended claims and should not be considered as limited to any one of the described embodiments. In the drawings:

FIG. 1 is a flow diagram illustrating the redox reactions occurring in one type of redox fuel cell;

FIG. 2 is a schematic diagram of a redox fuel cell system;

FIG. 3 is a side sectional view of bubble-generating apparatus according to an embodiment;

FIG. 4(a) is an illustration of an example of a fluidic oscillator showing the main components; FIG. 4(b) is a schematic illustration illustrating the function of a fluidic oscillator;

and

FIG. 5 shows both plan and side, transparent sectional views of a bubble-generating apparatus according to an embodiment of the invention.

FIG. 2 is a schematic diagram of a liquid electrolyte-based fuel cell system 101. The system 101 comprises two major components: fuel cell stack 201 and regenerator section 301. Fuel stack 201 as illustrated comprises four half-membrane electrode assemblies 401. Each membrane electrode assembly and cathode 401 is separated from its neighbouring membrane electrode assembly and cathode by a bipolar plate which will comprise flow channels for allowing hydrogen fuel (in the case of the anode side) to diffuse across the electrode surface in operation of the cell and a well to site the cathode electrode and catholyte (in the case of the cathode side), in a manner which is well known in the art. At each end of the fuel cell stack unipolar separating plates are provided (meaning that diffusion channels are provided on only one side thereof for the anode; the side facing the electrode, and a cathode well for the cathode). Figure 2 does not attempt to show these plates, since their configuration, assembly and function are well known in the art.

Catholyte channels 601 are also schematically shown in Figure 2 and the arrows indicate the direction of fuel flow around the cell.

Catholyte is supplied to the fuel stack in line 501 through recycle pump 701 and is recovered, the redox mediator couple component of the catholyte having been at least partially reduced at the cathode in operation of the cell. The catholyte containing at least partially reduced redox mediator couple is recovered and supplied to regeneration chamber 801 through first inlet port 901. Regeneration chamber 801 is further supplied in second inlet port 1001 with a flow of oxidant; in this case air. The oxidant passes through a porous member 1201 into the interior of an adjacent channel (not shown) and contacts the at least partially reduced catholyte passing therethrough. The redox couple flowing in solution in the regeneration chamber in operation of the cell is used as a catalyst for the reduction of oxygen.

Catholyte solution containing regenerated oxidised redox couple is recovered from regeneration chamber 801 through first outlet port 1 101 and may be supplied directly into line 501 through recycle pump 701. Some or all of the water vapour may be condensed in a condenser and returned to the catholyte solution, via a demister (not shown) in order to assist in maintaining the humidity balance in the cell.

In operation of the cell, electrons generated at the anode by the oxidation of fuel gas flow in an electrical circuit (not shown) and are returned to the cathode, in a manner well known. Using a conventional porous 'sparge' with a 2-micron pore size, such as supplied by Mott Corporation, the smallest size of bubble that can be created is around 150- microns or micrometres (μητι).

Investigations have been carried out into the use of engineered surfaces with a controlled array of small holes typically produced by laser drilling a thin metal foil. European Patent Application Publication EP2542332 discloses the use of such engineered surfaces for fine bubble generation, and in particular the use of tapered holes to restrict the naturally rapid rate of growth of small bubbles. An alternative approach towards restricting the size of bubbles is the use of oscillatory air flow to limit bubble growth. This has been described in "On the Design and Simulation of an Airlift Loop Bioreactor with Microbubble Generation by Fluidic Oscillation", by W Zimmerman, B Hewakandamby, V Tesar, H Bandulasena and O Omotowa (Zimmerman et al). The approach involves the use of an oscillating air flow comprising high pressure pulses. The volume of air in each high pressure pulse restricts the size of bubble formed.

The applicant has experimented with applying fluidic oscillation to the production of fine bubbles, specifically in a catholyte regeneration system of a fuel cell. Experiments have concentrated on air pulses produced at relatively low frequencies in a porous plate for generation of the bubbles. The porous plate has of the order of 1 ,000 holes and typically less than 1 ,000 holes.

The inventors have found that frequency is the main discriminator for bubble size. At higher frequencies needed for the higher gas flow rates there are shorter wavelengths so coupling and avoidance of standing waves becomes an issue

The inventors have found that it is desirable to generate a total air flow in excess of 1 ,000 litres per minute (16.66 litres per second). In order to achieve high enough rates of mass transfer it is desirable to create bubbles having diameters of between around 50 and 100 microns. It has been found to be desirable to generate bubbles having sizes of around 50 microns. This translates to generating of the order of 2 E 11 (2 x 10 ¹¹ ) bubbles per second. In order to limit the size of bubbles, the inventors have found that it is desirable to oscillate, or pulse, the air at a frequency or rate of 2 kHz or more, and further desirable to oscillate the air at a rate in excess of 3 kHz.

To achieve the required very small bubbles, high gas flow velocities in each gas- liquid contacting element are used, typically 1 to 20 litres per minute, that is around 17 to 330 cm ³ per second.

When implementing a fluid oscillator in a fuel cell, the inventors have found that it is beneficial to divide the oscillatory air flow between many individual gas-liquid contacting elements. If only a single element were used, it would be exceedingly large. Therefore, it is beneficial to restrict the size of such a gas-liquid contacting element by arranging the gas-liquid contacting element as a planar element and stacking multiple such planar elements in parallel. However, it has been found that doing this causes standing waves in the air flow. This becomes a particular problem when oscillation frequencies are increased into the kilohertz range suitable to generate the high air flow rates (and therefore high air flow velocities), and the very small bubbles, to achieve efficient mass transfer as explained above.

High frequencies of between 1 to 5 kHz, suitable to achieve the very small bubbles at such high gas flow velocities, result in standing waves whose dimensions are of the order of a few centimetres (cm). Due to these standing waves, matching of oscillating air flow to multiple gas-liquid contacting elements is complex and uses significant physical space. For example, at a frequency of 3 kHz, standing waves in any connecting fluid channel or pipework will have a physical length of around 5 cm. In this example, a length of fluid channel or piping may therefore be increased by up to 5cm so as to avoid or remove such a standing wave.

Alternatively, the frequency and/or the distance between the oscillator output and the porous member may be configured, in particular optimised, to generate a standing wave between the oscillator output and the porous member so that an antinode of the standing wave is incident on the porous member, i.e. where the foam is generated. This is to promote the maximum air pressure modulation to be incident on the porous member thereby to enhance, in particular optimise, the generation of bubbles using a standing wave.

In a practical implementation, as suggested above, it will be beneficial to have multiple gas-liquid contacting elements or modules, typically stacked in parallel and operating in parallel, each having a porous region and a chamber where mass transfer is allowed to complete via gas-liquid contacting.

This arrangement of multiple (e.g. stacked) modules may present problems in a physical implementation in which large numbers of porous plates must be provided with oscillating air. This is because the length of each pipe connecting the oscillator to each of the individual elements (operating in parallel) should be selected, adjusted or tuned to take account of standing waves, resulting in a very complex design. For example, as many as 100 elements may be stacked in a complete system. In addition, such a design will only be effective at a single frequency. Thus, the inventors have devised apparatus and method for generating a foam comprising very small bubbles at very high gas flow rates. One or more embodiments aim to achieve this while avoiding generation of standing waves within the foam.

Turning to FIG. 3, an apparatus for generating foam is shown, in which air is fed through a common inlet manifold 210 of a stack 200 of plural gas-liquid contacting elements 220, 230, 240, the manifold 210 distributing the air to each of the plural elements in the stack (one such element 230 being shown completely and two elements 220, 240 being shown partly in FIG. 2). The plural elements together form a multi element gas-liquid contacting device.

Each element has a gas supply input, or gas feed 250, and a liquid supply input 255, each shown to the left of the figure and feeding into a foam generator 261. Gas passes via the gas feed 250 through a porous member 260 into a foam generation region 262 and then onwards in the form of a foam into a liquid-containing region 270 (which can be termed a reaction volume) of the element. Liquid is supplied into the foam generation region 262 and onto the liquid-containing region 270 from a liquid inlet manifold 505. The gas at the gas output region of the porous element (upper side of the porous element, as shown) forms bubbles in the liquid proximal to the surface of the porous member 260 in foam generation region 262. The bubbles begin to grow in volume due to the continued injection of gas through the porous element 260. The liquid is injected through the liquid inlet manifold 505 at a rate sufficient to cause the injected liquid to flow rapidly onto, across and/or along the (upper) surface of the porous member 260 in the foam generation region 262. This flow of liquid acts to separate, or dislodge, some of the recently formed bubbles from the surface. The bubbles collect to form foam and the foam passes (from left to right in FIG. 3) into the liquid-containing region 270. The liquid-containing region 270 can also be termed a reaction chamber 270 since, in this region, gas-liquid contacting occurs and oxygen from the gas reacts with the reduced catholyte to oxidise it. To the right of FIG. 3, the foam passes into a mesh 280 comprising low-surface energy material. The mesh 280 acts to break or burst the bubbles of the foam, resulting in separation of the gas and liquid phases of the foam. The broken-down, or destroyed, foam passes into an outlet manifold 290. The use of the low-surface energy material assists in breaking or bursting the bubbles. An example of such an arrangement is disclosed in the Applicant's co-pending international patent application number PCT/GB2013/050174 incorporated herein by reference.

The supply of gas via the gas supply input 250 of each element is modulated by means of a gas supply modulator (not shown in FIG. 3), which is located either proximal to, or within the gas supply input 250. The modulator is arranged to urge input gas towards the porous element with a varying pressure, typically cyclically varying. It should be understood that the variation may have a constant or non- constant period. When the pressure differential, i.e. the difference between the pressure of the gas at the gas input side of the porous member 260 and the pressure of the liquid on the other side of the porous member 260, is above a threshold value, gas passes into the liquid and causes a bubble to form or grow. When the pressure differential is below the threshold value, bubbles at the porous member 260 substantially do not form or grow. The period during which the pressure differential is below the threshold is known as the "dwell time". During the dwell time bubbles are substantially static and if not already separated from the porous member are removed (swept away) from the porous member by liquid flowing past the porous member. Bubbles formed this way are likely to be very small since their growth has been inhibited by the reduction of the gas pressure under the influence of the modulator.

In this case, because each element has its own modulator or oscillator associated with it (either proximal to the element or within the element) the matching of the oscillation of gas to the porous element may be achieved for each individual element 220, 230 and 240. Each element may extend in transverse directions to provide a large reaction volume 270 which would likely result in a large area of porous material.

Fluidic oscillators of the type described by Zimmerman et al have a substantially planar configuration and may be particularly suitable as modulators for one or more embodiments in accordance with the present invention in which relatively low profile foam generation apparatus is utilised. Such fluidic oscillators are not limited to use with low profile foam generation apparatus. An example of such a fluidic oscillator is illustrated in FIG. 4.

FIG. 4(a) illustrates a model of a so-called fluidic jet-deflection amplifier, details of which are disclosed in Tesa ^' r, V., Hung, C.-H. and Zimmerman, W.B., 2006, No moving part hybrid synthetic jet mixer Sensors and Actuators A, 125(2): 159-169, which forms the basic component of a fluidic oscillator. A steady supply of air is input to a supply terminal 402 of the fluidic amplifier 400. The supply of air to one of the two output terminals 408 and 410 is under control of the control signals (air flow) sent to control terminals 404 and 406. The control signals applied to control terminals 404 and 406 deflects the jet of air issuing from the main nozzle into one or other of output terminals 408 and 410.

The arrangement illustrated in FIG. 4(a) is known as an amplifier because powerful output flow through output terminals 408 and 410 is controlled by much weaker control flows input into control terminals 404 and 406.

Operation of a fluidic oscillator is illustrated with reference to FIG. 4(b) in which a fluidic amplifier 400 is schematically illustrated having a feedback loop 412 coupled between control terminals 404 and 406. Air supplied at terminal 402 attaches to a wall of either one of the output channels leading to respective output terminals 408 and 410 due to the so-called Coanda effect. The deflection of air into one or other of the output channels causes a pressure gradient across the airflow. For the situation illustrated in FIG. 4(b), a decrease in pressure at control port 404 is caused which then draws air through the feedback loop 412 from the opposite control terminal 406 where pressure is higher. Once airflow in the feedback loop 212 has gained momentum the control flow in control terminal 404 switches the main airflow from the channel leading to terminal 408 and diverts it to the channel leading to terminal 410. Due to the device being substantially symmetrical, following a delay for the feedback airflow to gain sufficient momentum in an opposite direction, the main airflow is diverted back to the channel leading to terminal 408 thus leading to an oscillation of airflow between respective output terminals 408 and 410.

The length of feedback loop 412 will determine the periodicity of the oscillation. Additionally, although the schematic illustration of FIG. 4(b) shows a feedback loop extending a considerable distance proud of the substantially planar configuration of the fluidic amplifier 400, the feedback loop may be made with a lower profile by appropriately configuring the feedback loop conduit. Typically, the feedback loop length would be equivalent to about or equal to half the wavelength at the frequency of oscillation.

In order to gain effective coupling of the air from the oscillator to a large area of porous material, two or more individual oscillators may be incorporated in each element 220, 230, and 240 of the stack 200.

FIG. 5 shows such an arrangement or apparatus in which plural oscillators 310 are incorporated in an element 300. The element 300 illustrated in FIG. 5 has five modulators or oscillators 310 within it, distributed along a width of the element and each fed by a port of gas inlet manifold 320.

As should be apparent from the above, and from FIG. 5, the alternating air/gas outputs or supplies are fed to individual isolated areas of the porous member 260, preventing internal cancellation due to standing wave(s). Optionally, respective oscillators or groups of oscillators may be modulated at different rates.

The apparatus shown in FIG. 5 (upper portion of the figure) has plural oscillators, as suggested above, integrated into a single planar (low profile) gas-liquid contactor element 300. The height of the element (direction out of the page in the upper portion of FIG. 5 and upward in the lower portion of FIG. 5) is small relative to the width of the element (up-down in the upper portion of FIG. 5, into/out of the page in the lower portion of FIG. 5). Thus, each such element can form one element of a multi-element stack.

As used herein any reference to "one embodiment" or "an embodiment" means that a particular element, feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of the phrase "in one embodiment" or the phrase "in an embodiment" in various places in the specification are not necessarily all referring to the same embodiment.

As used herein, the terms "comprises," "comprising," "includes," "including," "has," "having" or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, "or" refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

In addition, use of the "a" or "an" are employed to describe elements and components of the invention. This is done merely for convenience and to give a general sense of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention. For example, although embodiments in accordance with the invention has been described with reference to a liquid catholyte, the principles disclosed herein may be applicable to liquid electrolytes in general to facilitate reaction with other components and particularly where a large mass transfer is desirable. Furthermore, embodiments in accordance with the present invention are not limited to fuel cell technologies or the particular fuel cell configurations described herein.

Although embodiments have been described utilising air, another gas mixture comprising oxygen or oxygen alone may be suitable. For other applications, other gas mixtures comprising different gases may be utilised. Although a fluidic oscillator of the jet-diverter type has been described, other fluidic oscillators may be utilised in one or more embodiments in accordance with the present invention. Furthermore, other mechanisms for modulating airflow may be utilised such as a swiftly switched valve or intermittent pump arrangement. However, such devices which can operate at the required frequencies are likely to require external drive force and hence connections and additional energy requirements which mitigates against their use in a fuel cell. The scope of the present disclosure includes any novel feature or combination of features disclosed therein either explicitly or implicitly or any generalisation thereof irrespective of whether or not it relates to the claimed invention or mitigates against any or all of the problems addressed by the present invention. The applicant hereby gives notice that new claims may be formulated to such features during prosecution of this application or of any such further application derived therefrom. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in specific combinations enumerated in the claims.