WO2017129994A1

WO2017129994A1 - Fuel cell, electrolyser or battery

Info

Publication number: WO2017129994A1
Application number: PCT/GB2017/050217
Authority: WO
Inventors: Shanwen Tao; Rong LAN
Original assignee: University Of Warwick
Priority date: 2016-01-28
Filing date: 2017-01-27
Publication date: 2017-08-03
Also published as: GB201601580D0; GB2546770A

Abstract

A fuel cell, electrolyseror battery comprises a liquid electrolyte and a cathode. The cathode comprises a plurality of pipes arranged, in use, to be at least partially within the electrolyte, each pipe being arranged to allow oxygen to flow therethrough and having a permeable section, the permeable section being arranged, in use, to allow the oxygen to come into contact with the liquid electrolyte. The apparatus also includes an anode, wherein the anode comprises a plurality of elongate projections arranged, in use, to be at least partially within the electrolyte.

Description

FUEL CELL, ELECTROLYSER OR BATTERY

The invention relates to a fuel cell, electrolyser or battery. In particular, but not exclusively, the fuel cell, electrolyser or battery may have a liquid electrolyte. Specifically, and again not exclusively, the fuel cell, electrolyser or battery may have a molten carbonate or other molten salt electrolyte, an ionic liquid electrolyte and/or a salt solution electrolyte.

As would be understood by the skilled person, in reversible battery and/or fuel cell systems, which electrode is the anode and which is the cathode reverses when the battery or fuel cell is operated in reverse - i.e. when the battery is being recharged or the fuel cell being operated in electrolysis mode . The terms "air electrode" and "counter electrode" may be used more generally for reversible systems, to avoid having to change terminology when the direction of operation of the system is reversed.

The term "cathode" is used herein to denote the air electrode. In battery /fuel cell mode (electricity generation), the air electrode is the cathode . In recharging/ electrolysis mode (using electricity), the air electrode becomes the anode .

The term "anode" is used herein to denote the counter electrode . In battery/fuel cell mode, the counter electrode is the anode. In recharging/electrolysis mode, the counter electrode becomes the cathode . The term "fuel cell" is used herein to describe cells which are or may be reversible between a fuel cell mode and an electrolysis mode or which are operable in fuel cell mode only. The skilled person will appreciate that all of the structures, systems and embodiments described in terms of a fuel cell can equally be applied to electrolysers, which may or may not be reversible . The features discussed with respect to fuel cells should therefore be construed as also being applicable to electrolysers.

Whilst the field of molten electrolyte fuel cells and batteries has been developed for over half a century, the limited power densities and stringent sealing requirements, in particular with respect to circulating hot, corrosive carbon dioxide (C0₂), have led to low market penetration. Further, most such fuel cells and batteries have been designed for stationary applications only, due to these limitations.

When molten carbonate is used as an electrolyte in a direct carbon fuel cell or battery, for example, the avoidance of combustion and use of carbon as a fuel lead to low or zero emissions of particulates and nitrogen oxides (NO_x) . In addition, the efficiencies of molten carbonate fuel cells are higher than those of conventional combustion power plants. The technology is therefore beneficial in combatting smog, as well as providing more electricity per unit of fuel, even if the same fuel is used as in conventional power stations.

In the paper of Stuart Licht, et al , "Molten air-a new, highest energy class of rechargeable batteries. " Energy & Environmental Science 6. 12 (2013): 3646-3657, metal, carbon and VB₂ molten air batteries are presented, in which each electrode is formed as an approximately planar disc by coiling a wire .

The use of planar electrodes - often an anode plate and a cathode plate - is commonplace in the prior art. Figure 1 shows a standard direct carbon fuel cell structure 100, comprising a molten electrolyte 102 sandwiched between two electrodes 104, 106. The anode 104 is the fuel electrode and the cathode 106 is the air electrode. The electrolyte 102 is often supported by a ceramic matrix (such as LiA10₂). The ceramic matrix serves to contain the liquid electrolyte 102 between the electrodes 104, 106. At the fuel electrode 104, fuel (not shown) is supplied to an anode chamber 1 10 adjacent to the anode 104 through an entry point 1 14, as marked by arrow A. This could be coal, charcoal, oil or natural gas, to name but a few examples. At the anode 104, the fuel is oxidised, converting the carbon to carbon dioxide, which leaves the anode chamber 1 10 via a gas conduit 1 12c, as marked by arrow A' .

At the air electrode 106, air (not shown) is supplied to a cathode chamber 108 adjacent to the cathode 106 through a gas conduit 1 12a, as marked by arrow C. Atmospheric air, oxygen, a mixture of carbon dioxide and air/oxygen, or another oxidant could be used. At the cathode 106, oxygen is reduced to O^2" anions (or other oxygen anions, such as 0₂ ^~ or 0₂ ^2~) which diffuse into the electrolyte 102. The remaining air leaves the cathode chamber 108 via a gas conduit 1 12b, as marked by arrow C .

The oxygen anions diffuse through the electrolyte to the anode 104, where they oxidise the carbon in the fuel. Further, carbon dioxide (C0₂) may be provided to the cathode along with the air or oxygen. Carbonate ions, C0₃ ^2~, may then be formed at the cathode from the carbon dioxide supplied thereto and take part in the reactions. The skilled person would appreciate that the ionic conductivity of C0₃ ^2~ ions within a molten carbonate electrolyte 102 is high, so supplying C0₂ directly to the cathode can generate carbonate ions to be used as the charge carriers, reducing resistance within the electrolyte 102, so increasing power output and/or allowing a larger electrode separation to be used for the same power output.

Additionally or alternatively, carbon dioxide generated at the anode may dissolve into the electrolyte and travel to the cathode to react with the oxygen there so as to form carbonate ions as charge carriers for the fuel cell, electrolyser or battery.

Charge carrying species can therefore include O^2", 0₂ ^2~, 0₂ ^~, C0₃ ^2~, HC0₃ ^~ and OH^" among others.

In a battery, the structure 100 is very similar - the anode 104 and anode chamber 1 10 become a single component which is gradually exhausted as no new fuel is provided. Similarly, the cathode 106 and cathode chamber 108 may become a single component which is gradually exhausted as no new oxygen is provided. However, a cathode chamber 108 of sorts may still be used in a metal-air (or metal - 0₂) battery for which atmospheric air or 0₂ is used, for example.

According to a first aspect of the invention, there is provided a fuel cell, electrolyser or battery comprising:

a liquid electrolyte;

a cathode, wherein the cathode comprises a plurality of elongate projections arranged, in use, to be at least partially within the electrolyte

an anode, wherein the anode comprises a plurality of elongate projections arranged, in use, to be at least partially within the electrolyte. The elongate projections forming the cathode may be hollow. More specifically, the cathode may comprise a plurality of pipes, each pipe being arranged, in use, to allow oxygen to flow therethrough. Each pipe may have a permeable section, the permeable section being arranged, in use, to allow the oxygen to come into contact with the liquid electrolyte.

Optionally, the permeable section is a porous section. The porous section may allow the passage of gaseous oxygen, and/or of the liquid electrolyte, therethrough.

Additionally or alternatively, the permeable section may comprise an ionic conducting material which allows the passage of oxygen anions therethrough. The ionic conducting material may be a mixed ionic-electronic conducting material.

The cathode pipes may be completely non-porous, and be made from a mixed ionic- electronic conducting material.

The permeable section may be arranged, in use, to be completely covered by the electrolyte.

Each pipe may have at least one non-permeable section. At least one of the one or more non-permeable sections of each pipe may be located in an end region of the pipe .

In some cases, each end of at least some of the pipes is open. Each pipe with two open ends may comprise two non-permeable sections, one at each end of the pipe. The permeable section may be between the two non-permeable sections.

In some cases, at least some of the pipes may have one open end and one closed end. Each pipe with one open end may comprise one non-permeable section at the open end of the pipe.

For any pipes with one or more non-permeable section, the one or more non- permeable sections of each pipe may be arranged to allow sealing and/or manifolding of the pipe. For any pipes with one or more non-permeable section, the at least one non-permeable section may be arranged to be partially within/covered by the electrolyte, and to extend out of the electrolyte. The electrolyte may be contained within a chamber. Each pipe may extend through at least a portion of the chamber.

At least one end of each pipe may protrude out of the chamber. The one or more sections of each pipe which protrude through a face of the chamber and out of the chamber may be non-permeable.

The pipes and the elongate projections may extend substantially across the chamber.

The chamber may have a first face, and a second face. The elongate projections and the cathode pipes may all extend through the chamber and protrude from the chamber through the first face and through the second face .

Each pipe may have at least one non-permeable section adjacent to at least one of the first face and the second face. The or each non-permeable section of each pipe may extend through the face of the chamber to which it is adjacent.

The chamber may contain a gap not filled by the electrolyte. The permeable portion of each pipe may be arranged, in use, to remain covered by the electrolyte during normal movement of the chamber.

The chamber may have at least one conductive face. The or each conductive face may be arranged, in use, to form an electrical interconnection. Each end of each pipe which protrudes out of the conductive face of the chamber may be in electrical connection with that face .

In embodiments with or without a chamber, the plurality of pipes of the cathode may be arranged in rows interleaved with rows of the elongate projections of the anode .

The plurality of pipes of the cathode may be arranged in grids interleaved with grids of the elongate projections of the anode . The anode elongate projections may be made of metal, carbon, silicon or a conductive ceramic material. The anode elongate projections may be made of one or more of the following:

(i) nickel;

(ii) iron;

(iii) tin;

(iv) vanadium;

(v) zinc; and

(vi) carbon.

In the case of fuel cells or electrolysers, the anode elongate projections may comprise a plurality of anode pipes. Each anode pipe may have a porous section and may be arranged, in use, to allow flow of a solid, liquid or gaseous fuel therethrough.

The plurality of anode pipes may have any of the features of the cathode pipes described above.

The fuel cell, electrolyser or battery may further comprise one or more barriers arranged, in use, to separate the cathode pipes from the anode elongate projections. Regions of the barriers arranged to always be within the electrolyte may be porous, and regions of the barriers arranged to extend beyond the electrolyte may be non- porous. A safety margin may be provided such that a barrier region near the electrolyte surface is non-porous.

When barriers are used in conjunction with an electrolyte chamber, the barriers may be arranged to segment the chamber into a plurality of anode and cathode portions.

According to a second aspect of the invention, there is provided a fuel cell power system comprising:

a fuel cell having any, some or all of the features described above;

a heater arranged, in use, to raise the fuel cell to its operating temperature; a cathode manifold arranged, in use, to supply air or oxygen to each cathode pipe;

an anode manifold arranged, in use, to supply fuel to each anode pipe; and two or more electrical interconnections arranged, in use, to connect the cathode pipes and the anode pipes to an external circuit.

The gas supplied to the cathode may also be a mixture of C0₂ and air/0₂.

The fuel cell power system may further comprise one or more sensors. The sensors may be arranged, in use, to monitor fuel cell system performance.

The one or more sensors may comprise at least one of:

(i) a temperature sensor;

(ii) a pressure sensor;

(iii) a chemical sensor; and/or

(vi) a current or voltage sensor. The fuel cell power system may further comprise a control system. The control system may be arranged, in use, to effect a change in fuel cell system operation in response to a sensor reading.

The skilled person would understand that features described with respect to one aspect of the invention may be applied, mutatis mutandis, to the other aspect of the invention.

There now follows by way of example only a detailed description of embodiments of the present invention with reference to the accompanying drawings in which:

Figure 1 (prior art) shows a traditional direct carbon fuel cell structure;

Figure 2a and 2b show electrode pipes according to two embodiments; Figure 3 shows gas flow along the electrode pipe of Figure 2a and/or 2b;

Figure 4 shows a fuel cell, electrolyser or battery structure according to an embodiment, comprising a plurality of electrode pipes as shown in Figures 2a and 3 ; Figure 5 shows an alternative fuel cell, electrolyser or battery electrode arrangement according to an alternative embodiment, comprising a plurality of electrode pipes as shown in Figures 2a and 3 ;

Figure 6 shows an alternative fuel cell, electrolyser or battery electrode arrangement according to an alternative embodiment, comprising a plurality of electrode pipes as shown in Figures 2a and 3 ;

Figure 7 shows a further alternative fuel cell, electrolyser or battery electrode arrangement according to an alternative embodiment, comprising a plurality of cathode pipes as shown in Figures 2a and 3 ;

Figures 8a and 8b show a fuel cell, electrolyser or battery structure according to an embodiment, comprising a plurality of electrode pipes as shown in Figures 2b and 3 ;

Figures 9a, 9b, 9c, 9d and 9e show a fuel cell, electrolyser or battery structure according to various embodiments further comprising barriers between anode and cathode portions; and

Figures 10a and 10b show two alternative battery structures.

In the embodiment being described with respect to Figures 2 to 5, the cell is a fuel cell 200 and the liquid electrolyte 202 is a molten carbonate . The fuel cell 200 can therefore be termed a molten carbonate fuel cell (MCFC). The electrolyte 202 is solid at room temperature, but liquefies on heating to its operating temperature .

The skilled person would understand that the structural features described with respect to the Figures can equally be applied to other kinds of fuel cell or electrolyser 200 with liquid electrolytes 202, and/or to batteries 300 with liquid electrolytes 202.

In the embodiments of the invention described herein, each electrode (anode and cathode) comprises a plurality of elongate projections 204, 206a, 304. The elongate projections 204, 206a, 304 are at least partially within/covered by the liquid electrolyte 202. In the embodiment being described, the elongate projections 204, 206a, 206b, 304 are made of metal. The skilled person would understand that alternative materials, such as ceramics, may also be used, as is discussed in more detail below, without changing other features of the embodiment.

The elongate projections 206a, 206b (denoted 206 herein where either or both of 206a and 206b is intended) forming the cathode are hollow so as to allow the passage of air or oxygen therethrough. The hollow elongate projections 206 may therefore be described as pipes. The flow of air or oxygen through the pipes is as marked by arrow C in Figure 3. As mentioned above, C0₂ may also be provided along with the air or oxygen - mentions of air or oxygen to be supplied to the cathode herein are therefore also intended to cover mixtures of C0₂ with air or oxygen. In some embodiments, as shown in Figure 2a, the pipes 206a are each open at both ends. In such embodiments, the oxygen or air may flow into one end of each pipe 206a and out of the other end. In other embodiments, as shown in Figure 2b, the pipes 206b are each open at one end and closed at the other end. The closed end may be curved, as shown in Figure 2b, or flat, as shown in Figures 8a and 8b and Figure 9d. In such embodiments, oxygen or air (in some cases mixed with carbon dioxide) is supplied to the open end of each pipe 206b. Any unused oxygen, or other unused or waste product gases present, may leave the pipe 206b via the same open end. Oxygen which is used passes into the electrolyte 202. In alternative embodiments, the pipes 206 may branch such that each pipe 206 has more than two ends. The skilled person would understand that the same principles could be applied in such cases.

In the embodiment being described, the pipes 206 are substantially cylindrical (i.e. have a substantially circular cross-section). In alternative or additional embodiments, the pipes may be substantially elliptical, triangular, square, rectangular or hexagonal in cross-section, or may take any other suitable shape as would be understood by one skilled in the art. The range of suitable shapes may depend upon the choice of material and mechanical strength required, amongst other considerations. The skilled person would understand that pipe shape may vary between pipes 206 within the same embodiment. Further, in some embodiments, pipes 206a which are open at both ends may be used alongside pipes 206b which have a closed end. Additionally or alternatively, the shape of the cross-section of a pipe 206 may vary along its length. In some embodiments, the diameter of each pipe 206 (or width in cases of non-circular pipes) may vary along its length.

In the embodiment being described, each pipe 206 is a single component. In alternative embodiments, each pipe 206 may be composed of two or more separate pipe sections which are joined together to form a continuous hollow structure. In such embodiments, the separate pipe sections may or may not have the same diameter, and may or may not be made of the same material or materials. Each pipe 206 has a permeable section 220. The permeable section 220 is arranged, in use, to allow oxygen flowing within the pipe 206 to come into contact with the electrolyte 202 surrounding the pipe 206.

In various embodiments disclosed herein, the permeable section 220 is at least one of the following:

(i) porous, such that the gaseous oxygen flowing within the pipe 206 can travel through the wall of the pipe 206 via channels or pores 250 through the pipe walls;

(ii) porous, such that the liquid electrolyte around the pipe 206 can enter into the channels or pores 250; and

(iii) a mixed ionic-electronic conductor (MIEC), such that the gaseous oxygen within the pipe 206 can pick up electrons at the internal pipe surface and form oxygen anions which diffuse through the MIEC material to the other side of the pipe wall, so coming into contact with the electrolyte 202 surrounding the pipe 206.

In embodiments wherein the pipe 206 can conduct oxygen anions, the pipe 206 may be fully dense (i.e. not porous). Advantageously, the lack of pores 250 in these pipes 206 reduces the risk of the liquid electrolyte 202 getting inside the pipes 206, which could lead to leakage in some embodiments. When the pipe 206 is fully dense, the pipe walls are not permeable to molecular oxygen, but only to oxygen anions. Further, in embodiments wherein the permeable section 220 is an oxygen anion conductor, it may also be porous. In embodiments wherein the pipe 206 can conduct oxygen anions and is also porous, the pipe walls are permeable to both molecular oxygen and oxygen anions.

In embodiments with closed pipes 206b, the closed end may itself be permeable (e.g porous, as shown in Figure 2b and Figure 9d) or may be non-permeable (i.e. fully dense and not an oxygen anion conductor) .

In the embodiments being described with respect to the Figures, the permeable section 220 is porous. In the embodiments being described, the permeable section 220 is not an oxygen ion conductor. The skilled person would understand that any of the points discussed below with respect to the porous section 220 may also be applied to porous permeable sections 220 of MIEC materials, and also to non-porous permeable sections 220 of MIEC materials (with the exception of discussions of pore size and porosity).

The porous section 220 is arranged, in use, to allow oxygen flowing within the pipe 206 to come into contact with the electrolyte 202 surrounding the pipe 206, as marked by the arrows D in Figure 3, via the pores 250. The arrows D show the oxygen passing through the pores 250. However, in additional or alternative embodiments, the electrolyte 202 may enter the pores 250, such that the contact between oxygen and pores occurs at the internal entrance to each pore 250, within the pipe walls, and/or within the pores 250 themselves. In the embodiments currently being described, the electrolyte 202 does not enter into the body of the pipe 206a, 206b. An anti-wetting coating, as described below, may be used to prevent electrolyte 202 ingress beyond the pores 250 and into the pipe 206, and/or to prevent electrolyte 202 ingress into the pores 250 at all. In the embodiments being described, the porosity (volume percentage of void in the total volume) of the porous section 220 is typically in the range of 5 - 90%, and is preferably between 20% and 70%. The pore size of the pores in the porous section 220 is typically between 0. 1 μιη and 5 mm, and preferably between 1 μιη and 1 mm. The skilled person would understand that, for sufficiently small pore sizes (as could be calculated based on the choice of electrolyte 202), the liquid electrolyte 202 would be held inside the pores 250 by the capillary effect, and would be unable to enter the body of the pipe 206. Under these circumstances, an anti-wetting coating may not be required to prevent the electrolyte 202 from entering into the body of the pipe 206.

In the embodiment being described, the porous section 220 constitutes 90% of the length of the cathode pipe 206 which is in contact with the electrolyte 202. In alternative embodiments, the porous section 220 may constitute at least 95 %, 80%, 70% or 60% of the length of the cathode pipe 206 which is in contact with the electrolyte 202. The skilled person would understand that the porous section 220 is advantageously contained within the depth of the electrolyte 202 so as to prevent escape of the air/oxygen flowing through the pipes 206a around the electrolyte 202 through the pores 250 of the porous section 220.

The size of the pores 250 in the porous section 220 may be controlled to reduce, minimise or prevent ingress of the liquid electrolyte 202 into the pipes 206.

Additionally or alternatively, an anti-wetting coating may be used in the pores 250 to further reduce the chance of electrolyte 202 ingress . Suitable anti-wetting materials include metal and ceramic materials; the skilled person would understand that the choice of coating may depend on the properties of the electrolyte 202.

In embodiments wherein a molten carbonate electrolyte 202 is used, potential materials for the anti-wetting coating include Bi₂0₃ or doped Bi₂0₃ such as

Bi .₅Yo.₃Sm₀.₂0₃-5 (BYS) . Properties of the electrode 206 onto which the anti-wetting material is coated should also be considered; for example, a mis-match in thermal expansion coefficient between the electrode 206 and the coating could lead to delamination of the coating . In at least one embodiment, Bi₂0₃ or doped Bi₂0₃ may be mixed with metal and/or oxide materials to form a composite . The composite may then be coated/deposited on the outer and/or inner surface of the pipes 206. The skilled person would understand that coating the outer surface of the pipes 206 may prevent the electrolyte 202 entering the pores 250 at all, whereas coating only the inner surface may allow the electrolyte 202 to enter the pores 250, but not to pass through the pores 250 and into the main body of the pipe 206. In embodiments wherein an ionic liquid or salt solution is used as the electrolyte 202, operating temperatures may be lower than for molten carbonate electrolytes 202 (perhaps below 300^°C). A wider range of materials, such as plastics materials, may then be used as the anti-wetting coating. For example, aqueous potassium hydroxide (KOH), quaternary ammonium hydroxide solution or another aqueous salt solution may be used. A hydrophobic polymer such as PTFE may be used as the anti-wetting coating in such embodiments. In particular, the porous section 220 arranged to be within the liquid electrolyte 202, may be coated in such embodiments, whether or not the remainder of the pipe 206 is coated.

In some embodiments, the porous section 220 may extend along substantially the whole of the length of the pipe 206. In the embodiment being described, each open end of each pipe 206 has a non-porous section 230a, 230b. The pipe 206b with only one open end therefore has a single non-porous section 230a, whereas the pipe 206a with two open ends has two non-porous sections 230a, 230b. Advantageously, the non-porous sections 230a, 230b may provide one or more of the following features:

• increased ease of sealing;

• reduced risk of gas escaping;

• increased ease of manifolding;

· increased ease of forming electrical interconnections; and

• protection from formation of air bubbles and/or gas cross-over in the event of a change of volume of the electrolyte 202.

In the embodiment being described, the electrolyte 202 is contained within an electrolyte chamber 240. The pipes 206a with two open ends pass through the chamber 240, with each end of each pipe protruding from a face 242, 244, 246 of the chamber 240. In embodiments where some or all of the pipes 206b have a closed end, the open end of each pipe 206b protrudes from a face 242, 244, 246 of the chamber 240 and the closed end may be within the electrolyte 202. In the embodiment being described with respect to Figure 2a and Figures 4 to 7, the chamber 240 has three faces and each pipe 206a passes through two opposing faces 242, 244 of the chamber 240, with one end of each pipe 206a exiting the chamber 240 on the opposite side of the chamber 240 from the other. In alternative embodiments, the chamber 240 may have more or fewer sides 242, 244, 256, and may for example be cubic, cuboid, cylindrical or spherical in shape .

In additional or alternative embodiments, the pipes 206a may be shaped such that both ends of each pipe 206a leave the chamber 240 through the same face 242, 244 of the chamber 240 (for example, U-shaped pipes), or through a different face 246 of the chamber 240 (for example arc-shaped, or L-shaped pipes).

In additional or alternative embodiments, the pipes 206 may have multiple bends and/or be spiral-shaped. Advantageously, this may increase the available surface area for reactions within the electrolyte 202.

The faces 242, 244 of the chamber 240 through which the cathode pipes 206 and/or anode projections 204 pass may be made of a ceramic material. In such embodiments, a non-conductive ceramic such as yttria-stabilised zirconia (YSZ), cerium-gadolinium oxide (CGO), alumina, any porcelain or Macor® may be used. If the operating temperature is not too high, plastic materials may be used, for example PTFE

(polytetrafluoroethylene), PE (polyethylene) or the likes. Advantageously, the non- conductivity of these materials means that the risk of short-circuiting between the electrodes 204, 206 is reduced. In such embodiments, an anode current collector (not shown) may be connected to the plurality of anode projections 204, and a cathode current collector (not shown) may be connected to the plurality of cathode pipes 206, where the pipes/projections 204, 206 extend beyond the chamber 240.

In alternative embodiments, the faces 242, 244 of the chamber 240 through which the cathode pipes 206 and/or anode projections 204 pass may be made of metal, such as stainless steel, nickel or nickel alloy, silver or silver alloy, aluminium, or of a conductive ceramic such as lanthanum strontium manganite (LSM) or lanthanum strontium cobalt ferrite (LSCF). In embodiments wherein the faces 242, 244 are conductive, and wherein both anode projections 204 and cathode pipes 206 pass through the same face 242, 244, a sealing material between at least either the anode projections 204 or the cathode pipes 206 is an insulating material to avoid a short circuit. In these embodiments, the conductive face 242, 244 can be used as a current collector for whichever electrode 204, 206 is not insulated from it. For example, in an embodiment wherein the cathode pipes 206 passing through face 242 are insulated from face 242 by the insulating sealing material, and the anode projections 204 passing through face 242 are not insulated from it, face 242 acts as the anode current collector. In embodiments wherein the face 242, 244 of the chamber and the anode projections 204 or cathode pipes 206 are both made of metal, the anode projections 204 or cathode pipes 206 may be welded to the face 242, 244 so as to provide both sealing and an electrically conductive connection.

In some embodiments, such as those shown in Figures 9c and 9d, the cathode pipes 206 and anode pipes 205 may pass through different faces 242, 244 of the chamber 240. In such embodiments, all pipes/projections 204, 304, 206 may be electrically connected to the face(s) through which they pass, so forming a current collector. The faces 242, 244, 246 of the chamber 240 may be electrically isolated from each other so as to prevent short circuits. In the embodiments being described, wherein the pipes 204, 206 are made of metal, the pipes may all be welded to the faces 242, 244, 246 of the chamber 240 through which they pass, when the chamber 240 is made of metal. In selecting materials for the faces 242, 244, 246 of the container 240, ease of sealing between the faces 242, 244, 246 may be considered. In particular, in at least some embodiments, at least one face 242, 244, 246 may be arranged to be removable from the container 240, so acting as a lid 810, as is shown in Figures 8a and 8b. Advantageously, use of a lid 810 may facilitate periodic purification or replacement of the electrolyte 202. The skilled person would understand that periodic purification or replacement of the electrolyte 202 may be of use when impurities (for example, from the fuel) build up within the electrolyte 202. In the embodiment being described, the non-porous section 230a, 230b at each end of each pipe 206a is arranged to pass through one face 242, 244 of the chamber 240. Advantageously, the lack of porosity in these sections 230a, 230b improves sealing around the face 242, 244 of the chamber, so reducing the risk of gas escaping.

Further, the portion of the non-porous section 230a, 230b outside of the chamber 240 can be used for manifolding without loss of gas. The skilled person would appreciate that the same points apply to the one open end of pipes 206b with one open end and one closed end. In the embodiment being described, there is a portion of the non-porous section 230a, 230b at one or each end of each pipe 206a inside the chamber 240. Advantageously, this may reduce the risk of air bubbles forming if the electrolyte 202 does not completely fill the chamber 240, as the porous section 220 is arranged not to come into contact with any part of the chamber 240 not filled with the electrolyte 202.

The skilled person would understand that, when a molten salt electrolyte 202 is used, for example, there is a volume change as the electrolyte 202 is heated. The length of the non-porous section 230a, 230b within the chamber 240 may therefore be set to allow for a certain change in volume, for example 5%, 10%, 15% or 20%, during heating and cooling. The skilled person would understand that the % change in volume will vary dependent on temperature range and electrolyte 202 composition. Advantageously, the porous section 220 is arranged to be always covered by the electrolyte 202, despite the expansion and contraction of the electrolyte 220. The length of the porous section 220 is selected such that the porous section 220 remains fully covered by the electrolyte 202 even when the electrolyte is at its smallest volume. The flow of oxygen through the pores 250 of the porous section 220 is therefore limited by its ability to diffuse into the electrolyte 202, and the gas cannot leak directly from the pipe 206a into a space 610 above or below the electrolyte 202 from the porous section 220.

In at least some embodiments wherein a gap 610 not filled with the electrolyte 202 is present within the chamber 240, and/or wherein the volume of the electrolyte 202 is expected to change, a pressure management mechanism (not shown) is incorporated into the chamber 240. The skilled person would appreciate that, if the pressure within the chamber 240 increased significantly, electrolyte 202 might be forced through the pores 250, and/or the pipes 206 may be damaged. The pressure management mechanism may comprise a pressure release valve to allow any build-up of gas to escape. Additionally or alternatively, the pressure management mechanism may comprise a bellows-type mechanism, such that a volume of gas can leave the chamber 240 whilst keeping the system sealed. Any gas leaving the chamber 240 and entering the bellows-type mechanism may then be returned to the chamber 240 when the pressure starts to decrease, so maintaining a substantially constant pressure .

In the embodiments shown in Figures 4 and 5, face 242 of the chamber 240 is designed to be the uppermost face of the chamber 240 and the pipes/projections 204, 206a are substantially vertical. In such embodiments, the non-porous section 230a, 230b arranged to be near the uppermost face 242 of the chamber 240 may be longer than that for the other non-porous section 230b, 230a. The skilled person would understand that the action of gravity on the electrolyte 202 may keep the lower part of the pipe 206a covered with the electrolyte 202 throughout expansion and contraction, so reducing the risk of leaks/air bubble formation.

Advantageously, the entirety of the porous section 220 is in contact with/exposed to the electrolyte 202. The non-porous section 230a or 230b may be substantially not exposed to the electrolyte 202 (for example, above the electrolyte 202 in the gap 610, and/or extending through and beyond the faces 242, 244, 246 of the chamber 240) . An overlap of the non-porous section 230a or 230b with the electrolyte 202 may provide a safety margin as described above. In alternative embodiments, the pipes 206 may be arranged substantially horizontally instead of vertically, or at any angle between. Further, in some embodiments, pipe orientation may vary between pipes 206 within the same cell 200.

Air, oxygen or the like may be supplied from either end of the pipes 206a.

In the embodiment being described, a metal or conductive oxide coating is used on the cathode pipes 206. In additional or alternative embodiments, different or additional catalytic coatings may be used to increase reaction rates. In the embodiment being described, the diameter of each pipe 206 is in the range of 2- 3mm. In alternative or additional embodiments, the internal pipe diameter may be between 0.5 and 20 mm, and preferably between 2 and 10 mm. The wall thickness of each pipe 206 is substantially 1 mm in the embodiment being described. In other embodiments, the wall thickness of the pipes 206a may be between 0.5 and 3 mm.

In large-scale examples such as power stations, pipe 206 diameters and wall thicknesses may both be larger - with diameters of up to 200 mm and wall thicknesses of up to 10 mm being envisaged.

The skilled person would understand that any shape or size of anode elongate projections 204 can be used, provided that these do not contact the cathode pipes 206, which would create a short circuit. In the embodiments shown in Figures 4 and 5, the elongate projections 204 forming the anode are also pipes 204 like the cathode pipes 206 described above. Instead of air passing through the pipes, the anode pipes 204 carry fuel. The fuel may be solid (e.g. powdered or pulverised biomass, coal, charcoal, graphite, paper waste or food waste), liquid (e.g. (bio)oil, petrol, diesel, (bio)methanol or (bio)ethanol) or gas (e .g. natural gas, biogas, methane, hydrogen, ammonia, propane, butane, LPG, carbon monoxide or coal gas).

In embodiments wherein a carbon-containing fuel is used, the carbon dioxide generated may be collected and stored. Additionally or alternatively, some or all of the carbon dioxide produced may be mixed with the gas provided to the cathode 206 and so re-used within the system.

The diameter of the anode pipes 204 may be selected depending upon the type of fuel used. In the embodiment being described, a gaseous fuel is used and an internal diameter of 2-3 mm is chosen. The skilled person would understand that for gaseous fuels or liquid fuels (which are likely to become gaseous at fuel cell operating temperatures), similar size requirements would apply to the anode pipes 204 as to the cathode pipes 206. In embodiments wherein solid fuels are used, or wherein solids such as carbon are formed at the anode due to pyrolysis of carbon-containing liquid or gaseous fuels, larger diameters may be desired. For example, anode pipes 204 may have internal diameters in the range of 1 mm to 200 mm, and preferable in the range of 3 mm to 50 mm. The skilled person would understand that diameters towards the upper end of the range may be useful in larger-scale operations, such as stationary power plants.

In the embodiments being described, the anode pipes 204 and the cathode pipes 206 have the same size and shape . In fuel cells 200 of other embodiments, the anode pipes 204 may have a different size and/or shape from the cathode pipes 206, and sizes and shapes may also vary between anode pipes 204. For example, the cathode pipes 206a may each have two open ends and the anode pipes 204b may each have one open end and one closed end.

The nature of the fuel used can vary between embodiments, so anode pipe 204 shape may be varied accordingly. The anode pipes 204 may have any, some or all of the features described herein with respect to the cathode pipes 206.

As for the cathode pipes 206, the anode pipes 204 have at least one permeable section 220. The permeable section 220 of the anode pipes 204 may be porous so as to allow the fuel to come into contact with the electrolyte 202. Alternatively, the permeable section 220 of the anode pipes 204 may be an oxygen anion conductor, which allows oxygen anions in the electrolyte 202 to pass through the pipe walls so as to come into contact with the fuel. Any or all of the features of the cathode pipes 206 discussed above may therefore be applied to the anode pipes 204. When solid fuel is used, arranging the anode pipes 204 vertically within the chamber 240, as for the embodiment shown in Figure 5, allows increased ease of fuelling as the fuel can be fed from the top of the chamber 240 and be moved through the anode pipes 204 by gravity. In alternative or additional embodiments, the anode pipes 204 are positioned at an angle to vertical. The skilled person would understand that pipe angle can be adjusted as appropriate to fit system requirements.

Use of solid fuel within the embodiment shown in Figure 5 may also be advantageous for safety. The solid fuel cannot cross the holes in the porous section 220 of the anode pipes 204, so cannot directly react with air/oxygen at the cathode 206. Any leaking air or oxygen at the cathode 206 will diffuse/bubble to the top of the chamber 240 (above the liquid electrolyte 202), and therefore cannot come into contact with and react with the solid fuel, which is inside the anode pipe 204.

When liquid or gaseous fuels are used, pressure (or another indicative variable, such as oxygen content in a gap 610 within the electrolyte chamber 240) should be carefully monitored and controlled. Leaking of both the fuel from the anode 204 and the air/oxygen from the cathode 206 could lead to a mixture of fuel and air forming at the top of the chamber 240 at high temperature, which could be explosive.

In the embodiments being described, an equal number of anode projections 204 and of cathode pipes 206 is provided. In alternative embodiments, the numbers may not be equal; i.e. the ratio of anode projections 204 to cathode pipes 206 may not be 1 : 1. The skilled person would understand that the ratio can be adjusted according to which electrode 's reaction rate is limiting the overall reaction rate . For example, if the reaction rate is limited by available cathode surface area, advantageously more cathode pipes 206 may be provided than anode projections 204 so as to increase the available cathode surface area.

Similarly, in alternative or additional embodiments, diameter of the anode projections 204 and of cathode pipes 206 may be varied, for example to increase the surface area of whichever electrode is limiting reaction rates. In the embodiment shown in Figure 4, the anode projections 204 and cathode pipes

206 are arranged in a grid, with alternating rows of anode projections 204 and cathode pipes 206.

In the embodiment shown in Figure 5, the anode projections 204 and cathode pipes 206 are arranged in a grid, alternating anode-cathode-anode-cathode along each row.

The skilled person would understand that many different configurations may be used without departing from the scope of the invention. For example, the pipes/projections 204, 206 may be arranged in interleaved rows, interleaved grids, in a hexagonal pattern, in radial lines or in concentric circles. In some embodiments, the electrode pipes/projections 204, 206 may be arranged such that the two, three or four nearest neighbours of each anode projection 204 are cathode pipes 206, and vice versa.

In some embodiments, the plurality of pipes of the cathode 206 and the plurality of elongate projections of the anode 204, 304 are arranged such that the distance between each pipe and its nearest elongate projection is substantially equal to the diameter of the pipe or to the width of the elongate projection.

In still further embodiments, the electrode pipes/projections 204, 206a may be arranged such that the anode projections 204 are at an angle to the cathode pipes 206 (see Figures 9c and 9d). For example, the anode projections 204 and cathode pipes 206 may be perpendicular, and/or may pass through different faces 242, 244, 246 of the chamber 240. Advantageously, having the cathode pipes 206a passing through different faces of the chamber 240 from the anode projections 204 may facilitate manifolding and/or interconnections: manifolds and interconnections (not shown) for the cathode pipes 206 and for the anode projections 204 on different faces 242, 244, 246 of the chamber 240 may be easier to manufacture and maintain in some embodiments.

In the embodiment being described, the spacing between adjacent anode projections 204 and cathode pipes 206 is substantially equal to the diameter of each pipe 206 (in this case, 2-3 mm). In alternative embodiments, the spacing may be substantially 0.5, 1 , 2, 3, 4, or 5 times the pipe 206 diameter, or not directly related to the pipe diameter. Examples of possible spacing width ranges include 0.5 mm to 20 mm, and preferably 1 mm to 10 mm.

The skilled person would understand that, in some embodiments, deposits may accumulate on one or both electrodes. For example, in a metal-air battery, metal is often deposited on the cathode 206 during charging. The spacing between anode projections 204 and cathode pipes 206 should therefore be selected to avoid contact between cathode 206 and anode 204, 304, which would cause short circuits, resulting from this size change. This may be a particular problem for zinc-air batteries, in which the zinc has a tendency to form dendritic protrusions, such that even a relatively small volume of deposited material may lead to a short circuit.

Figures 9a to 9e illustrate the use of barriers 900a, 900b, 900c (900) between the cathode pipes 206 and anode pipes/projections 204, 304. The barriers 900 may also be referred to as separators.

In the embodiments being described, the barriers 900 are in the form of sheets or plates inserted between each row of anode projections/pipes 204, 304 and the adjacent row of cathode pipes 206. In alternative embodiments wherein the arrangement of pipes/projections 206, 204, 304 differs, the skilled person would understand that barriers 900 of a different shape may be used accordingly.

The barriers 900 are solid, so forming physical obstacles to reduce the likelihood of, for example, dendritic growth from the anode 204, 304 reaching the cathode 206.

The barriers 900 are porous to allow movement of the electrolyte 202 through the barriers 900. Porosities chosen for the barriers 900 may be the same as for the porous section 220 of the pipes 206, as discussed above.

In the embodiment being described, the material chosen for the barriers 900 is not electrically conductive, or at least has a low electrical conductivity. Advantageously, this reduces the chance of the barrier 900 itself forming part of a short circuit. Advantageously, in the embodiment being described, the barriers 900 are made of a ceramic material.

The barriers 900 may be made of LiA10₂, Lanthanum strontium gallate magnesite (LSGM), doped Ce0₂ or YSZ, for example. Advantageously, when the barriers 900 are made of a material which conducts oxide ions (e .g. YSZ or Ce0₂), the ions carried by the electrolyte may also be transmitted through the barrier itself.

In alternative or additional embodiments, electrically conductive materials may be used for the barriers 900. For example, porous metals can be used for the barrier 900a; suitable metals include nickel foam sheet or the like . The skilled person would appreciate that the barrier 900 should be arranged not to come into contact with more than one electrode.

In embodiments with lower operating temperatures, polymeric materials may be used for the barriers 900.

In embodiments wherein the pipes/projections 206, 204, 304 are arranged vertically (e.g. Figures 8a and 8b, and Figure 9e), and/or arranged in vertical rows (e.g. Figure 9a, 9d) such that one cathode pipe 206 lies directly above another cathode pipe 206, the barriers 900a, 900c are vertical. The top section 910 of the barriers 900a, 900c may not be covered by the electrolyte 202 in such embodiments. In the embodiment being described with respect to Figure 9a, the top section 910 of the barriers 900a is non-porous so as to prevent gas flow through the top section 910. Advantageously, this may reduce the risk of any escaped fuel gathering in the gap 610 mixing with any escaped air or oxygen gathering in the gap 610, so reducing the risk of fire or explosion. In this way, the barriers 900a may segment the gap 610 into multiple isolated gaps, each of the isolated gaps corresponding wither to an anode portion or to a cathode portion of the chamber 240. As would be understood by the skilled person, any escaped oxygen or air should only gather in the gaps corresponding to cathode portions and any escaped fuel should only gather in the gaps corresponding to anode portions. In embodiments wherein the barriers 900a may segment the gap 610 into multiple isolated gaps, the one or more porous sections 220 of the pipes 206, 204 may extend into the gap 610. The skilled person would understand that, in embodiments wherein the fuel and air/oxygen would still be isolated from each other, leaks from the pipes 204, 206 within the chamber 240 are less of a security concern. Further, as the electrolyte 202 expands, the gas may be driven back into the corresponding pipes 204, 206 through the pores 250 therein.

In alternative embodiments wherein the pipes/projections 206, 204, 304 are arranged vertically, and/or arranged in vertical rows, such as that shown in Figure 9e, the top section 910 of each barrier 900e is also porous. Gases from the anode and cathode may therefore mix within the gap 610 if there is a leak. In such embodiments, the porous portions 220 of each pipe 204, 206 are advantageously fully covered by the electrolyte 202 to reduce the chance of leakage . In embodiments such as that shown in Figure 6, Figure 9b and Figure 9c, the anode and cathode pipes 204, 206 are arranged horizontally within the chamber 240. In Figure 6, a gap 610 within the chamber 240 can be seen above the electrolyte 202. In the embodiment shown in Figure 6, the porous section 220 of each pipe 204, 206a extends across substantially all of the length of the pipes 204, 206a contained within the chamber 240. In alternative or additional embodiments with some or all of the pipes 204, 206a arranged substantially horizontally, the upper surface of the uppermost pipes 206 may be non-porous, and the porous section 220 may extend substantially the full length of the underside of the uppermost pipes 206. The skilled person would understand that the portions of pipe 204, 206 selected to be non-porous may be varied depending upon configuration of the pipes.

In the embodiment shown in Figure 6, each pipe 204, 206a has a non-porous section 630a, 630b at each end. Approximately one third of each non-porous section 630a, 630b is located within the chamber 240, and the remainder extends through a face of the chamber 240. The non-porous section 630a, 630b may facilitate sealing and/or manifolding.

In alternative embodiments, the pipes 204, 206 may be entirely porous and additional cuffs or coatings (not shown) may be used to block selected pores 250, for example at each end of each pipe 204, 206a. For example, in the case of porous metal pipes, 204, 206, metal cuffs could be welded to the inside or outside of sections of the pipe. The combination of cuffs with a fully-porous pipe may be considered equivalent to the pipe 204, 206a with porous sections 220 described above.

In embodiments with cathode pipes 206 made of a material which conducts oxygen anions, either sections of the pipe 206 are made of a different material which does not conduct oxygen anions, or a cuff, coating or other material covering is used to block oxygen transport through certain regions, so as to provide the one or more non- permeable sections 230.

In further alternative embodiments, such as those shown in Figure 7, Figure 10a and Figure 10b, the elongate projections 304 forming the anode are not hollow, so not described as pipes. The elongate projections 304 are rods in the embodiments shown, although the shape may differ in other embodiments. The skilled person would understand that embodiments wherein the anode projections 304 are not pipes may be more suited to batteries 300 than to fuel cells 200, although solid anode elongate projections 304 which are replenished over time (for example by moving a bar of solid fuel further into/through the chamber 240) could be used for a fuel cell 200.

In some embodiments, carbon-containing fuels may be compressed to form some or all of the elongate projections 304. The compressed carbon elongate projections 304 (e .g. rods) may be put directly into the liquid electrolyte 202. The carbon rod 304 itself may be sufficiently conductive that it can be used as the anode current collector.

Additionally or alternatively, a conductive strip, wire or the likes may be provided along the length of the elongate projection 304 to increase conductivity.

After the carbon rod 304 is consumed, it can be replaced by another rod 304.

Additionally or alternatively, the carbon rod 304 may be moved further into the electrolyte 202 as it gets consumed. The carbon rod 304 may be mechanically fixed on a holder, which may be conductive (e .g. made of metal) and which may also serve as a current collector. A carbon-air fuel cell according to such embodiments could be used for coal-fired power stations, although mobile applications such as vehicle power are also considered. The replacement of the carbon rod 304 may be thought of as mechanically charging a battery, although such embodiments can also be thought of as a kind of fuel cell (in particular, embodiments wherein the rod 304 is gradually moved into the electrolyte 202).

In some embodiments, these elongate anode projections 304 are at least partially porous to maximise the available surface area for reactions. For example, a metal sponge or foam 1000 may be used for at least a portion of the elongate projection 304. The sponge or foam may comprise nickel, stainless steel, or any other suitable metal or alloy as would be understood by one skilled in the art. Additionally or

alternatively, ceramic materials may be used in some embodiments. The elongate anode projections 304 may have a non-porous portion at one or each end to facilitate sealing and/or manifolding.

In alternative embodiments, the anode projections 304 are not porous. In

embodiments wherein the anode projections 304 are not porous, a porous coating 1000 may be provided on at least a portion of the anode projection 304. The elongate anode projections 304 in the embodiment being described comprise a solid metal rod 304 with a surface coating 1000 of porous metal. The surface coating of porous metal does not cover the full length of the anode projections 304, but rather a portion thereof within the chamber 240.

In the embodiment shown in Figure 7, the battery 300 is a metal-air battery. Suitable metals for the elongate anode projections 304 of metal-air batteries include nickel, iron, tin, cobalt, vanadium (or vanadium boride), antimony, tungsten molybdenum, niobium, titanium, aluminium, copper, zinc, chromium, magnesium and lead, amongst others. In alternative embodiments, the same principles may be applied to carbon-air or silicon-air batteries.

The cathode pipes 206 may supply atmospheric air, or air or oxygen from another source . The air or oxygen may be mixed with carbon dioxide .

In the embodiment being described, the elongate anode projections 304 (rods) have substantially the same diameter as the cathode pipes 206; around 2-3 mm. In alternative embodiments, the diameters of the anode projections 304 and the cathode pipes 206 may differ. Anode projection 304 diameters may be between 1mm and 200 mm, and preferably between 1 mm and 10 mm. The skilled person would understand that the diameter selected may depend on the material from which the anode projections 304 are made, so as to maintain a desired mechanical strength. A catalyst coating on the anode may again be used to increase reaction rate . Catalyst coatings may advantageously by porous to increase the surface area available for reactions. Examples of suitable catalyst materials for the cathode and/or anode include a porous nickel coating 1000, a porous coating of a metal such as silver or a silver alloy, a mixed electronic and ionic conductor (MIEC) such as LSCF, or a mixture of YSZ with a compatible electronically conducting material such as CGO.

The use of MIEC materials for the cathode catalyst facilitates the transport of both electrons and O^2" ions and can effectively increase the available surface area for reactions. The different chamber 240 sizes and shapes, and the different configurations and optional properties of anode pipes 204 and cathode pipes 206 discussed with respect to, and shown in, Figures 4, 5, 8 and 9 can equally be applied to the anode elongate projections 304 and cathode pipes 206a of the embodiments shown in Figure 7, Figure 10a and Figure 10b.

In embodiments wherein the anode pipes 204 are closed pipes 204b, as for the examples shown in Figures 8a, 8b and 9d, the anode pipes 204b may comprise a mesh and/or have pores 250 large enough that the electrolyte 202 can enter into the anode pipes 204. In the embodiments shown, the closed anode pipes 204b are arranged vertically such that gravity prevents the electrolyte 202 from escaping from the cell 800 via the pipes 204b. In embodiments wherein the anode pipes 205b are not vertical, and/or wherein gravity is insufficient, other sealing means may be required. Advantageously, in embodiments wherein a solid fuel is used, allowing the electrolyte 202 to enter into the anode pipes 204b increases the contact surface area for reactions. The skilled person would understand that many solid fuels (e .g. coal) are less dense than many liquid electrolytes 202, and would therefore float on top of the liquid electrolyte 202 within the pipes 204b in the absence of any means for pushing the fuel downwards. A screw, pressure plate, or the likes may be used to force the fuel down the pipe 204b and into the electrolyte 202.

In at least some embodiments with closed anode pipes 204b, the carbon dioxide produced will exit the pipe 204b through the open end by which fuel enters. One or more small pipes (not shown) may be provided within the anode pipe 206b to facilitate the escape of carbon dioxide.

In embodiments with closed anode pipes 204b in which non-pure fuels are used, soot or other non-used or non-usable material may accumulate within the closed pipe 204b. Periodic emptying out of the closed pipe may therefore be required.

Fuel cells 200 as described above can be used in fuel cell power systems.

The fuel cell power system comprises a heater which is used to raise the fuel cell 200 to its operating temperature. In the embodiment being described, the heater is an electrical heating element. In alternative embodiments, fuel may be burned to raise the temperature, or a different form of heater may be provided.

In the embodiment being described, insulation is provided around the electrolyte chamber 240. Advantageously, the insulation facilitates maintenance of the operating temperature once it is reached.

The fuel cell power system comprises a cathode manifold arranged to supply air or oxygen or a mixture of C0₂ and air/0₂ to each cathode pipe 206. The cathode manifold may comprise a branching pipe, one branch of which connects to each cathode pipe 206, an injection block which receives the open end of each of the cathode pipes 206, or the likes. An outlet cathode manifold may also be provided in embodiments with cathode pipes 206a with two open ends, through which unused air or oxygen, and any reaction products present in the cathode can leave the cathode pipes 206a.

The fuel cell power system comprises an anode manifold arranged to supply fuel to each anode pipe 204. The anode manifold may comprise a branching pipe, one branch of which connects to each anode pipe 204, an injection block which receives the end of each of the anode pipes 204, or the likes. An outlet anode manifold may also be provided in embodiments with anode pipes 204 with two open ends, through which unused fuel and carbon dioxide (or other waste products) can leave the anode pipes 204. An electrical interconnection is also provided for each electrode . As described above, one or more faces 242, 244, 246 of the chamber 240 may be used as part of the interconnector. The interconnector allows the passage of current between the electrodes and an external circuit. In the embodiment being described, sensors are provided to monitor fuel cell performance. The sensors may be used to detect short circuits, any temperature fluctuations and/or leaks. Pressure sensors or chemical sensors may be used for leak detection. Chemical sensors may be selected to detect oxygen, any component of the fuel supplied to the anode, and/or any component of the air or other oxygen- containing mixture supplied to the cathode . A control system programmed to recognise one or more triggers receives data from the sensors. If a trigger condition is identified in the sensor data, the control system can raise an alert (e.g. sounding an alarm, illuminating a light or causing a message to be sent to a registered user) and/or effect a change in system operation (e.g. shutting off the heater, increasing the power supplied to the heater, shutting off the air/oxygen supply and/or the fuel supply, and/or opening a release valve).

For example, in the embodiment being described, the fuel is methane and a methane sensor is located in the gap 610 within the electrolyte chamber 240. If methane is detected in the gap 610, the fuel cell 200 is shut down. Changes in pressure within the gap 610 and/or within the pipes 204, 206a could also be used as a leak warning trigger. For example, pressure sensors could be present in each pipe 204, 206. A sudden pressure drop in a pipe 204, 206 may indicate leakage, and the fuel and air supplies to the fuel cell 200, or to the pipe or pipes 204, 206 in question, may be shut off in response.

In the embodiment being described, the heater is controlled in response to readings from a temperature sensor. If the electrolyte temperature drops below a predetermined value (for example, its ideal operating temperature), the heater is activated.

The skilled person would understand that, in at least some embodiments, the fuel cell power system can be run in reverse as an electrolytic cell or electrolyser. Cells 200 as described above may therefore be electrolytic cells as well as fuel cells.

Electrolytic cells 200 as described above can be used in electrolyser systems, wherein power is supplied to the electrolyser system to cause a chemical compound to break down. The fuel cell reactions occur in reverse . For example, instead of converting a carbon-containing fuel and oxygen into carbon dioxide and obtaining energy in the process, carbon dioxide may be converted into carbon (or carbon monoxide, CO) and oxygen, using input electrical energy. In such embodiments, the electrolyser system may be used as a carbon capture system. When a molten carbonate, or a molten salt containing carbonate, such as Li₂C0₃, is used within an electrolyser, carbon is deposited at the counter electrode and Li₂C0₃ is partially converted into Li₂0. On exposure to air, Li₂0 in the electrolyte reacts with C0₂ in air to regenerate Li₂C0₃. To continuously operate the electrolysis and C0₂ capture process, C0₂ can be captured from the air to be converted into carbon with the input of electricity.

Examples of Suitable Electrode and Electrolyte Materials

The skilled person will understand that the examples of suitable materials provided herein are not intended to be limiting, and are provided for illustrative purposes.

In the embodiments described above, the cathode pipes 206 and, where applicable, anode pipes 204 comprise metal pipes. The metal pipes 206, 204 may be made of Ni, Ni alloy, Ag, Ag alloy, Fe, iron alloy, e.g. stainless steels such as Kanthal (which is an FeCrAl alloy), and/or other alloys known to the skilled person as typically being used as metal interconnectors (current collectors) for solid oxide fuel cells. For the cathode pipes 206, a layer of metal or oxide material may be coated onto the surface of the metal pipe. The metal or oxide material is catalytically active, facilitating the oxygen reduction reaction at the cathode.

Examples of suitable metals include silver or silver alloy. For oxide coatings, two groups of oxide materials may be used:

• Group A: typical conductive oxide materials as known in the art of solid oxide fuel cells having perovskite or perovskite-related structures (such as K₂NiF₄ structure), or a spinel or pyrochlore structure. One specific example of a suitable oxide in this group is Sm_0.5Sr₀.5Feo.8Cuo_.20₃-5;

· Group B : typical conductive oxide materials as known in the art of molten carbonate fuel cells, with the general formula of A_xM0₂ where x <1.2; A = Li, Na, K or mixed Li/Na/K etc., M = Ti, V, Cr, Fe, Co, Ni, Mn, Zn etc., or a mixture of those elements. If an ionic liquid, molten hydroxide, molten halide or hydroxide solution is used as the electrolyte 202, a simple oxide such as manganese oxide or cobalt oxide can be coated on cathode 206 to be used as the cathode catalyst. In alternative embodiments, the cathode pipe 206 is made of a typical oxide cathode material for solid oxide fuel cells and molten carbonate fuel cells (Groups A and B above), such as Smo_.sSro_.sFeo_.sCuo^Os-e or Li_xNi0₂ as described above. MIEC materials may be used. Oxide materials such as manganese or cobalt oxides can also be used for the cathode . These may be provided as a coating on the cathode pipes 206, if they are not used to make the pipes 206 themselves. Reactivity of the cathode material with the electrolyte 202 should be considered in selecting a suitable cathode material.

For anode pipes 204 or projections 304 in metal - air battery embodiments, metals or metal alloys can be used directly as the anode active surface . No coating is therefore necessary, although coatings (sometimes of the same material) can be used, for example to increase surface area. Boron-air, carbon-air and silicon-air batteries are further examples of anode materials. For metal air batteries, typical metals for the anode are: Li, Na, K, Mg, Ca, Sr, Al, Ti, V, Cr, Mn, Fe, Co, Ni. Cu, Zn, Y, Zr, Nb, Mo, W, Sn, Sb.

Preferred elements to use for battery anodes are: Fe, Zn, V, Ti, Sn, Mn, Ni. In fuel cell embodiments 200, metals or metal alloys are also suitable anode materials. For example, Ni is known to be a suitable anode reaction catalyst for solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs). Therefore, nickel or a nickel alloy can be used as the anode . In order to increase the surface area available for reactions, a thin layer of porous Ni or Ni alloy can be coated onto the outer and/or inner surface of the anode pipe 204, or onto the surface of the anode projection 304, as applicable.

In embodiments wherein an anti-wetting coating is used, nickel can also form a part of a composite non-wettable barrier coating, together with doped Bi₂0₃ as discussed above. In alternative embodiments, conductive oxides as described above in relation to the cathode pipes 206a can be used to form part or all of the anode projections 204, 304 In the embodiments described above, three main groups of materials can be used for the electrolyte 202:

Group A: Molten Carbonates

Molten carbonates may be used, in particular molten carbonates comprising at least one of Li₂C0₃, Na₂C0₃, K₂C0₃, Cs₂C0₃, and Rb₂C0₃.

In addition, one or more additives may be provided as part of the electrolyte 202. Examples of suitable additives include oxides (typically La₂0₃, Li₂0, Na₂0, K₂0), halides (LiCl, NaCl, KC1, CsCl, MgCl₂, CaCl₂ and the likes, and the bromide, iodide or fluoride, equivalents thereof), hydroxides (such as LiOH, NaOH, KOH, or CsOH) or sulphate, nitrates - if the melting point of the molten salts is below the

decomposing temperature of the added nitrates - such as LiN0₃, NaN0₃, KN0₃, CsN0₃ etc., or sulphates - such as Li₂S0₄, Na₂S0₄, K₂S0₄, Cs₂S0₄ and the likes.

The skilled person would appreciate that the added oxide or hydroxide may change the composition, and therefore the melting point, of the molten carbonate if they are converted into the corresponding carbonates (for example due to the presence of C0₂ within the chamber 240).

Group B: Molten Hydroxides

Molten hydroxides may be used, in particular molten carbonates comprising LiOH, NaOH, KOH, RbOH, CsOH etc.

In addition, nitrate, sulphate, halide and/or carbonate additives may be used.

In some embodiments, solutions of these hydroxides are used as the liquid electrolyte 202, in the place of molten hydroxides. Aqueous solutions may be used, but the skilled person would understand that other solvents may also be used. Ionic liquids may also be used, as may mixtures of ionic liquids and salt solutions.

The melting points of hydroxides are generally lower than those of the carbonates, making the operating temperature of the fuel cell or battery lower. However, this type of electrolyte 202 is sensitive to the presence of carbon, so C0₂ should be removed if air is being used at the cathode 206a and carbon-containing fuels should be avoided to avoid poisoning of the anode . The skilled person would understand that a KOH or Ca(OH)₂ aqueous solution or other material can be used to remove C0₂ from air, and that fuels such as hydrogen or ammonia could be used.

Group C: Molten Halides

Molten halides, particularly chlorides, may be used, in particular molten chlorides comprising at least two of LiCl, NaCl, KC1, CsCl, RbCl, MgCl₂, SrCl₂, CaCl₂, BaCl₂, FeCl₂, FeCl₃, CoCl₂, MnCl₂, NiCl₂, CuCl₂, ZnCl₂ and the likes.

Bromide, iodide and/or fluoride salts of the same metals may also be used, alone or in combination with each other and/or with the listed chlorides Where molten halides are used, nitrates, carbonates and/or sulphates may be added to increase the oxygen anion conductivity of the electrolyte .

Group C electrolytes 202 generally have lower melting points than molten carbonates, so allowing lower operating temperatures. In addition, Group C electrolytes are generally chemically compatible with C0₂ and carbon-containing fuels, as for molten carbonates.

For batteries, oxides such as magnesium oxide, aluminium oxide, calcium oxide, boron oxide, silicon oxide, titanium oxide, vanadium oxide, chromium oxide, manganese oxide, iron oxide, cobalt oxide, copper oxide, zinc oxide, niobium oxide, molybdenum oxide, tungsten oxide, tin oxide and/or antimony oxide may also be added to the liquid electrolyte, which may typically be a molten carbonate or molten halide (chloride, fluoride, bromide iodide or their mixture) as described above. Additionally or alternatively, some hydroxide salts, such as LiOH, NaOH, KOH, RbOH, CsOH, Mg(OH)₂, Ca(OH), Sr(OH)₂, Ba(OH)₂ may be added to the molten salts. The added oxides and hydroxides may react in the molten electrolyte 202 forming new oxides which can be dissolved in the liquid electrolyte 202 as the source for each metal for the metal-air battery.

Some complex oxides of the form A_xB_yO_z, with A = Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba or the likes, and B = B, Al, Si, Sn, Sb, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ce or the likes can be added and dissolved to the molten salts to provide a source of metal for the metal-air battery.

The skilled person would appreciate that the material(s) chosen for the chamber 240 should be chemically compatible with the electrolyte 202, and able to withstand the cell' s operating temperature .

One example of suitable materials is ceramics. Alumina is one example, and the use of pure alumina is not required; porcelains can also be used. Advantageously, these materials are chemically compatible with the corrosive molten salts described above.

As a second example, metals can be used. Examples of suitable metals include stainless steel, nickel or a nickel alloy may be used. Advantageously, metals can resist thermal shock if the temperature of the battery/fuel cell increases or drops suddenly. However, the chemical compatibility is generally not as good as for a ceramic container, such as alumina.

As a third example, plastics may be used. Examples of suitable plastics include polytetrafluoroethene (PTFE), Low-density polyethylene (LDPE) or high-density polyethylene (HDPE) may be used. Plastics materials are generally suitable only if the operating temperature of the electrolyte 202, which is often at or near its melting point, is not very high (for example, below 300°C).

Claims

1. A fuel cell, electrolyser or battery comprising:

a liquid electrolyte;

a cathode, wherein the cathode comprises a plurality of pipes arranged, in use, to be at least partially within the electrolyte, each pipe being arranged to allow oxygen to flow therethrough and having a permeable section, the permeable section being arranged, in use, to allow the oxygen to come into contact with the liquid electrolyte; and

an anode, wherein the anode comprises a plurality of elongate projections arranged, in use, to be at least partially within the electrolyte.

2. The fuel cell, electrolyser or battery of claim 1 wherein the permeable section is a porous section.

3. The fuel cell, electrolyser or battery of claim 1 or claim 2 wherein the permeable section comprises an ionic conducting material which allows the passage of oxygen anions therethrough.

4. The fuel cell, electrolyser or battery of any preceding claim, wherein each pipe has at least one non-permeable section.

5. The fuel cell, electrolyser or battery of claim 4 wherein at least one of the one or more non-permeable sections of each pipe is located in an end region of the pipe .

6. The fuel cell, electrolyser or battery of claim 4 or claim 5 wherein each end of each pipe is open, and wherein each pipe comprises two non-permeable sections, one at each end of the pipe, and wherein the permeable section is between the two non- permeable sections.

7. The fuel cell, electrolyser or battery of claim 4 or claim 5 wherein each pipe has one open end and one closed end, and comprises one non-permeable section at the open end of the pipe.

8. The fuel cell, electrolyser or battery of any of claims 3 to 7 wherein the one or more non-permeable sections of each pipe are arranged to allow sealing and/or manifolding of the pipe.

9. The fuel cell, electrolyser or battery of any of claims 3 to 8 wherein the at least one non-permeable section is arranged to be partially within the electrolyte, and to extend out of the electrolyte.

10. The fuel cell, electrolyser or battery of any preceding claim wherein the permeable section is arranged, in use, to be completely within the electrolyte.

1 1. The fuel cell, electrolyser or battery of any preceding claim wherein the electrolyte is contained within a chamber, and wherein each pipe extends through at least a portion of the chamber, and wherein at least one end of each pipe protrudes out of the chamber.

12. The fuel cell, electrolyser or battery of claim 1 1 , wherein the one or more sections of each pipe which protrude through a face of the chamber and out of the chamber are non-permeable.

13. The fuel cell, electrolyser or battery of claim 1 1 or claim 12, wherein the pipes and the elongate projections extend substantially across the chamber.

14. The fuel cell, electrolyser or battery of any of claims 1 1 to 13 wherein the chamber has a first face, and a second face, and wherein the elongate projections and the cathode pipes all extend through the chamber and protrude from the chamber through the first face and through the second face .

15. The fuel cell, electrolyser or battery of claim 14 wherein each pipe has at least one non-permeable section adjacent to at least one of the first face and the second face, and wherein the or each non-permeable section of each pipe extends through the face to which it is adjacent.

16. The fuel cell, electrolyser or battery of any of claims 1 1 to 15, wherein the chamber contains a gap not filled by the electrolyte, and wherein further the permeable portion of each pipe is arranged, in use, to remain within the electrolyte during normal movement of the chamber.

17. The fuel cell, electrolyser or battery of any of claims 1 1 to 16, wherein the chamber has at least one conductive face arranged, in use, to be an electrical interconnection, and wherein each end of each pipe which protrudes out of the conductive face of the chamber is in electrical connection with that face.

18. The fuel cell, electrolyser or battery of any preceding claim wherein the plurality of pipes of the cathode are arranged in rows interleaved with rows of the elongate projections of the anode.

19. The fuel cell, electrolyser or battery of any preceding claim wherein the plurality of pipes of the cathode are arranged in grids interleaved with grids of the elongate projections of the anode.

20. The fuel cell, electrolyser or battery of any preceding claim wherein the cathode pipes are non-porous and comprise an ionic or mixed ionic-electronic conducting material.

21. A fuel cell according to any preceding claim, wherein the elongate projections comprise a plurality of anode pipes, each anode pipe having a porous section and being arranged, in use, to allow flow of a solid, liquid or gaseous fuel therethrough.

22. The fuel cell of claim 21 wherein the plurality of anode pipes have any of the features of the cathode pipes described in claims 2 to 20.

23. The fuel cell, electrolyser or battery of any preceding claim, wherein the anode elongate projections are made of one or more of the following:

(i) nickel;

(ii) iron;

(iii) tin;

(iv) vanadium;

(v) zinc; and

(vi) carbon.

24. The fuel cell, electrolyser or battery of any preceding claim, further comprising one or more barriers arranged, in use, to separate the cathode pipes from the anode elongate projections, wherein regions of the barriers arranged to always be within the electrolyte are porous, and regions of the barriers arranged to extend beyond the electrolyte are non-porous.

25. A fuel cell power system comprising:

a fuel cell according to any of claims 1 to 24;

26. The fuel cell power system of claim 25, further comprising one or more sensors arranged, in use, to monitor fuel cell system performance.

27. The fuel cell power system of claim 26, wherein the one or more sensors comprise at least one of:

(i) a temperature sensor;

(ii) a pressure sensor;

(iii) a chemical sensor; and/or

(vi) a current or voltage sensor.

28. The fuel cell power system of claim 26 or claim 27 further comprising a control system, wherein the control system is arranged, in use, to effect a change in fuel cell system operation in response to a sensor reading.

29. The fuel cell power system of any of claims 25 to 28, wherein the cathode manifold is arranged to supply carbon dioxide to each cathode pipe, in addition to the air or oxygen.