WO2011073621A1

WO2011073621A1 - Fuel cell system

Info

Publication number: WO2011073621A1
Application number: PCT/GB2010/002277
Authority: WO
Inventors: Michael Alexander Priestnall
Original assignee: Michael Alexander Priestnall
Priority date: 2009-12-15
Filing date: 2010-12-15
Publication date: 2011-06-23
Also published as: GB201010672D0; GB0921881D0

Abstract

An integrated fuel cell and carbon capture system, operating on carbon- containing reactants including a liquid, wherein CO₂ in the fuel cell reacts or dissolves in the liquid; and a means for sequestering the reacted or dissolved CO₂ in the liquid by forming a stable carbonate compound and regenerating the liquid. The means for sequestering the reacted or dissolved CO₂ may be a source of group II metal ions, commonly found as finely divided waste mineral materials.

Description

Fuel Cell System

Field of the Invention

This invention relates to a fuel cell in which Carbon Dioxide (C0₂) is captured and sequestered as a solid mineral during fuel cell operation.

Background to the Invention

Stored chemical energy in carbon-based fuels can be converted to electrical energy using conventional thermo-mechanical power conversion technologies or, less commonly, electrochemically in a fuel cell. Both methods involve oxidation and the consequent formation of product C0₂. The latter approach has the general advantage of higher electrical conversion efficiency, so producing less C0₂ per unit of electrical energy, but generally has an economic disadvantage associated with higher system capital costs. These higher capital costs are not currently sufficiently offset by lower use of fuel or the value ascribed to reduced C0₂ emissions.

Carbon capture and storage (CCS) encompasses a range of approaches by which the C0₂ that would otherwise be emitted during oxidation of hydrocarbon fuels is prevented from release to the atmosphere. Near-term adoption of such technologies is generally considered to be essential if fossil fuels are to continue to be widely used while at the same time global emissions of C0₂ are to be reduced. Economic considerations dictate that only those effective CCS processes that add the least cost to total cost of electrical energy generated are likely to be adopted. At present one of the lowest cost processes involves capturing pure C0₂ from combustion exhaust gases; liquefaction of the C0₂; transport and transfer of the liquid C0₂ to a suitable high-pressure, high- integrity underground or undersea repository; and long-term geological storage of the liquid C0₂. Unfortunately such processes present uncertainties as to reliability of long-term storage and risks of gradual or sudden release of C0₂ to the atmosphere which, if releases were to happen, could (a) accelerate global climate change, (b) endanger flora, fauna and human populations local to the C0₂ release site, and (c) result in financial liabilities to organisations in the CCS supply chain. Such CCS processes are also generally only considered economically and technically appropriate to large centralised sources of C0₂, such as power stations; no CCS solutions have been proposed with relevance to distributed, portable or small-scale hydrocarbon power conversion systems.

An alternative approach to CCS mimics natural geological weathering and sedimentary processes and involves chemical reaction of captured C0₂ with metal ions (e.g. magnesium and calcium) to form stable solid carbonate minerals which can be readily stored long-term without risk of C0₂ release and may even have value as industrial materials. CCS processes using alkaline hydroxide solutions to convert carbon dioxide gas into solid carbonates and bicarbonates have been described in e.g. US20060185985 and

WO2008018928 and IEA Greenhouse Gas R&D Programme Report Number 2005/11 "Carbon dioxide storage by mineral carbonation". The background, state-of-art and technical issues relating to the use of mineralisation and aqueous phase mineralisation as it is applied to carbon capture and storage processes is described further in WO2009139813, WO2008142017,

WO2008101293, WO2007106883, WO2006108532, US20081 12868, US2009010827, US2009214408, US2005180910, US2007261947,

US20050002847, US2004219090, and US20040213705.

Unfortunately, the costs and energy requirement of supplying chemicals in an appropriate form for reaction with C0₂ (e.g. pre-processing of magnesium and calcium silicate minerals through heat or grinding), outweighs any value from the product carbonate minerals and currently makes the overall process of CCS via carbonate mineralisation less economically attractive than CCS via C0₂ liquefaction and storage. Such C0₂-mineralisation processes are also generally considered as only being appropriate to large scale, point sources of C0₂ where, typically >0.1 million-tonnes C0₂ per year is produced, these sources generally being considered to account for ~60% of global C0₂ emissions.

Alkaline fuel cells (AFCs) have been established since 1910 (Teitelbaum) as being an efficient means of converting the chemical energy in hydrocarbon fuels, even gasoline or coal, directly to electrical energy. The ability to operate alkaline fuel cells on non-precious metal electrocatalysts also offers a cost advantage over other low-temperature fuel cell types, such as proton-exchange membrane (PEM) fuel cells, that require platinum-based anode and cathode electrocatalysts. Unfortunately, AFCs, which currently operate using liquid aqueous hydroxide electrolytes, such as potassium and sodium hydroxides, are sensitive to carbonation by both C0₂ in the ambient air used as oxidant at the cathode of the AFC and any C0₂ generated at the anode of the AFC by electrochemical oxidation of carbon-containing fuels. Partial dissolution of C0₂ in the hydroxide electrolyte and reaction with it forms carbonates which, at sufficient concentration, can precipitate out and form unwanted and potentially damaging deposits. The reduction in hydroxide concentration can reduce electrolyte conductivity and cell efficiency. For these reasons, AFCs are generally operated on high-purity hydrogen and often incorporate some C0₂ scrubbing system to remove C0₂ from inlet air fed to the cathode. This situation is summarised by Prof. Elton Cairns in Ch.17 vol.1 pp.301 of The Handbook of Fuel Cells (ISBN 0471499269): "Sodium or potassium electrolytes cannot be used directly with ambient air or with organic fuels, since they react with C0₂ to yield carbonate, eventually converting the hydroxide electrolyte to a carbonate electrolyte. At the concentrations normally used, this results in the precipitation of sodium or potassium carbonate and/or bicarbonate, damaging the electrodes and rendering the cell useless.".

Efforts have been made to address the problem of carbonation of electrolyte in alkaline fuel cells, one example of which is described in GB1213777. The system described in GB1213777 forces potassium carbonate out of solution in potassium hydroxide by adding additional hydroxide to the solution in a separate chamber outside the fuel cell. The system exploits the reduced solubility of the carbonate in highly concentrated hydroxide so that solid carbonate is separated out. A problem with this approach is that it consumes the hydroxide electrolyte. Fresh hydroxide must be continually added for the system to keep operating. This is economically unattractive. The system described in GB1213777 also fails to sequester the carbon dioxide into a geologically stable carbonate, as potassium and sodium carbonates will readily dissolve in rainwater. The invention aims to address problems associated with the costs, scale and risks of carbon capture and storage (CCS) technologies. The invention also aims to address the costs and electrolyte-carbonation of fuel cells, and the efficiency of conversion of hydrocarbon fuels to electrical energy. Summary of the Invention

The present invention is defined in the appended independent claims, to which reference should be made. Preferred features are set out in the dependent claims. The invention may thus relate to a fuel cell in which C0₂ is produced as a reaction product and/or in which C0₂ enters the cell, for example mixed with air being used as an oxidant or contained in flue gases from a fossil fuel power station or other reactor. The C0₂ dissolves in or reacts with a liquid in the fuel cell, which may be a carrier liquid for a fuel supplied to the cell, a carrier liquid for an oxidant supplied to the cell, and/or the electrolyte of the cell. Depending on the design of the cell, the liquid may serve one, two or all of these functions, as described below. The C0₂ dissolved in or reacted with the liquid in the fuel cell may then be sequestered or converted into an insoluble carbonate, by reaction with group II metal ions or another suitable reagent such as Fe.

In one aspect, the invention comprises a fuel cell, consisting of an anode, a cathode and an electrolyte, operating on fuel and oxidant reactants, either or both of which contain carbon. The same or different liquids may be used for electrolyte and for dispersion/dissolution of oxidant and fuel. Alternatively, the electrolyte may be a solid. C0₂ (which may be contained in a fluid, such as ambient air, entering the fuel cell or may be generated within the fuel cell as a result of electrochemical oxidation of fuel) is at least partially prevented from release to the atmosphere from the exhaust of the fuel cell by dissolution in or reaction with one or more of the liquid or liquids. The C0₂ captured in this way is concurrently or subsequently converted into a solid carbonate or bicarbonate that is geologically stable. Group II metal ions, typically calcium ions or magnesium ions or both calcium and magnesium ions, can be used to react with the dissolved or reacted C0₂ to form these stable carbonates. It is also or alternatively possible to use other means to react with the dissolved or reacted C0₂ to form the stable carbonates, such as iron. The fuel cell system is able to capture and sequester C0₂ in the air or other fluids fed to the fuel cell, or C0₂ or other carbon-containing oxidation products that are produced by electrochemical oxidation of a carbon-containing fuel, or any combination of these. Three basic types of implementation of the invention are envisaged: a fuel cell in which the carbon-containing reactant is dispersed in the liquid electrolyte and the C0₂ is initially captured by reaction with or dissolution in the liquid electrolyte; a solid-membrane-type fuel cell in which the reactant is supplied to the fuel cell dispersed in a carrier liquid which reacts with or dissolves the C0₂; a fuel cell in which C0₂ supplied to or generated at one electrode is transported through the electrolyte as a carbonate or bicarbonate ion to react with or dissolve in a carrier liquid at the other electrode.

Preferably, the invention uses a liquid electrolyte (such as the hydroxide electrolyte of an alkaline fuel cell, or the carbonate electrolyte of an aqueous or molten carbonate fuel cell) to capture and convert into carbonate the majority of the carbon dioxide or other carbon-containing oxidation products that are generated from electrochemical oxidation of a carbon-containing fuel and C0₂ that is present with the oxygen in the air that is fed to the fuel cell to oxidise the fuel. Partially or fully consumed electrolyte is replenished by means of a secondary or simultaneous reaction that generates a solid carbonate or bicarbonate precipitate. Exemplar reactions include:

C0₂[g] + 20H [aq] = C0₃ ^2"[aq] + H₂0

then

2C0₃ ^2"[aq] + Mg₂Si0₄[s] + 2H₂0 = 2MgC0₃[s] + Si0₂[s] + 40H [aq]

Sequestration of product C0₂ as a solid may be used to increase the Gibbs free energy of the fuel cell reaction and thereby increase the electrical conversion efficiency of the fuel cell. Electrochemical reaction of C0₂ within the fuel cell to form carbonates may be used to extract useful electrical energy from the mineralisation process, for example by supplying C0₂ to one electrode and metal ions or mineral species to the other electrode. Continuous removal of the reaction product C0₂ (e.g. by reaction with a metallic species to form a solid carbonate precipitate) in a fuel cell shifts the equilibrium of the fuel oxidation half-cell reaction further to the right, in accordance with Le Chatelier's principle. This increases the free energy of the reaction and the overall cell potential. This results in an increase in electrical conversion efficiency, a reduction in the proportion of C0₂ generated per unit of electrical energy and an improvement in the cost effectiveness of a fuel cell system. Exemplar reactions include:

CH₃OH(l) + 60H (aq) = C0₂(g,aq) + 5H₂0 + 6e

removal of C0₂:

C0₂(g,aq) + 20H^"(aq) = C0₃ ^2"(aq) + H₂0

precipitation of solid carbonate and regeneration of hydroxide:

Ca(OH)₂ + C0₃ ^2"(aq) + 2H₂0 = CaC0₃(s) + 20H (aq)

CaSi0₃(s) + C0₃ ² (aq) + H₂0 = CaC0₃(s) + Si0₂(s) + 20H^"(aq)

In the case of a carbonate fuel cell, where carbonate or bicarbonate (herein referred to as carbonate) species are formed electrochemically, it is possible to extract additional electrical energy from the reactions of C0₂ with the electrolyte, or with additives to the electrolyte, or with reactants supplied to the electrodes. In principle, this enables C0₂ to be used as a reactant for the fuel cell in which the Gibbs free energy of reaction to form carbonates is partially converted to electrical energy. Suitable reactants for such a fuel cell could include the exhaust gases of fossil-fuelled power stations and the C0₂ that forms part of ambient air. In the latter case, a fuel cell with at least one selective electrocatalyst can be fed with ambient air where the C0₂ component of the air is one reactant and oxygen or mineral species supplied to the fuel cell is another reactant. Such a fuel cell could be used to sequester carbon dioxide directly from the atmosphere while producing useful electricity. Continual removal of product C0₂ and continual regeneration of hydroxide electrolyte enables an alkaline fuel cell to operate continuously and efficiently with carbon-containing fuels and ambient air, reducing fuel cell operating costs, reducing risks of damage by electrolyte carbonation and increasing operating time between servicing. Further, it enables C0₂ to be captured and

sequestered as a stable solid, and the risks associated with CCS via C0₂ liquefaction and underground or undersea storage to be avoided.

The use of a fuel cell as a means to capture C0₂ offers an advantage in the inherent separation of product C0₂ from the air used in the electrochemical oxidation. This avoids energy being expended in separating or concentrating the C0₂ as is required in conventional CCS processes. Inherent dissolution of C0₂ in the electrolyte of an AFC offers an advantage in that additional processes, materials or energy are not required to separate or dissolve the C0₂ from exhaust gases. This advantage is enhanced if the carbon-containing fuel is dissolved or dispersed in the electrolyte and the C0₂ generated is released directly into the electrolyte. The process is particularly suited to a mixed- reactant or single-chamber type fuel cell in which fuel and oxidant (and, optionally, electrolyte) are combined together in one mixture. Additionally, the temperature, pH and composition of the electrolyte of the fuel cell can be selected to optimise the dissolution and/or reaction of C0₂ and the precipitation of mineral carbonate.

As fuel cells are typically modular devices - where costs and efficiencies are largely independent of scale - integration of CCS via C0₂ mineralisation within an AFC enables, for the first time, CCS to be applied to small-scale and to portable electrical power generation systems, not just large centralised facilities.

Continuous or periodic regeneration of the electrolyte enables hydroxide concentration to be maintained and fuel cell efficiency and power output to be maintained, without being compromised by carbonation.

(Bi)carbonate precipitates in the electrolyte can also be separated and reacted with suitable minerals to regenerate hydroxide and to form stable carbonates through reactions such as:

NaOH(aq) + C0₂(g, aq) = NaHC0₃(s)

NaHC0₃(s) + Ca(OH)₂(s) = CaC0₃(s) + NaOH(aq) + H₂0(l) Addition of mineral species, such as magnesium silicate and/or calcium silicate, which are suitable for reaction with dissolved carbonate and bicarbonate species, to the electrolyte enables regeneration of the hydroxide through reactions such as:

C0₂(g, aq) + OH (aq) = HC0₃ ^"(aq)

Mg₂Si0₄(s) + 2HC0₃ ^"(aq) = 2MgC0₃(s) + Si0₂(s) + 20H^"(aq)

Finely-divided waste mineral materials are available from a variety of industrial processes such as spoil materials from metals mining, ashes from combustion processes, slags from metal refining and cement and concrete wastes. Many of these materials are available at low or negative cost and with suitably small particle sizes that will enable them to react rapidly with C0₂ to form stable carbonates. Use of such waste feedstock materials in conjunction with a fuel cell system may improve the overall economic case for CCS. Likewise the chemistry of the CCS mineralisation process may be selected to produce mineral carbonate species that have industrial utility and value.

In another aspect, the invention provides a fuel cell system comprising:

a fuel cell comprising an anode, a cathode, fuel, oxidant reactant, at least one of the fuel and oxidant reactant containing carbon, and a liquid, wherein C0₂or a carbon-containing oxidation product in the fuel cell reacts or dissolves in the liquid; and a means for regenerating the liquid by converting the reacted or dissolved C0₂ or carbon-containing oxidation product to a geologically stable carbonate.

In yet another aspect, the invention provides a fuel cell system comprising: a fuel cell comprising an anode, a cathode, fuel, oxidant reactant, at least one of the fuel and oxidant reactant containing carbon, and a liquid, wherein C0₂ or a carbon-containing oxidation product in the fuel cell reacts or dissolves in the liquid; and a means for sequestering the reacted or dissolved C0₂ or carbon- containing oxidation product and regenerating the liquid.

Brief Description of the Drawings

Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a fuel cell system in accordance with a first embodiment of the invention;

Figure 2 is a schematic diagram illustrating the industrial application of a fuel cell as shown in Figure 1 , for producing power;

Figure 3 is a schematic diagram of a fuel cell system in accordance with a second embodiment of the invention;

Figure 4 is a schematic diagram of a fuel cell system in accordance with a third embodiment of the invention; and

Figure 5 is a schematic diagram of a fuel cell system in accordance with a fourth embodiment of the invention.

Detailed Description

Figure 1 shows a generic schematic of a first aspect of the invention. Figure 1 shows a fuel cell operated with carbon-containing reactants and fabricated according to the current art, which consists of an electrically-conductive anode electrode 1 and an electrically-conductive cathode electrode 2, separated by, and in contact with an electrolyte 3. The electrolyte may be a solid membrane but in this example is a liquid, which may be constrained within a solid matrix or may be free-flowing and circulated through the fuel cell via an entry port 11 and an exit port 14 (e.g. for cooling purposes or to prevent crossover of fuel from anode to cathode). A carbon-containing liquid or gaseous fuel 9 is supplied to the anode electrode 1 , via an anode chamber 4, and an oxidant 10, typically air containing ~0.04% C0₂, is supplied to the cathode 2, via a cathode chamber 5. The anode electrode 1 includes an electrically-conductive substrate 8, typically a porous mesh, cloth or paper of carbon or metal, to which is bonded an electrocatalyst 7, active towards the electro-oxidation of fuel, and in contact with both the fuel 9 and electrolyte 3. Similarly, the cathode contains an

electrocatalyst active towards the electro-reduction of oxidant. When an electrical load 6 is applied across the anode and cathode, electricity is generated as a result of the spontaneous electro-oxidation of the fuel to form cations and electrons and the electro-reduction of the oxidant by electrons to form anions and the transfer of anions through the electrolyte. Spent oxidant 12 is removed from the fuel cell to atmosphere via a cathode exhaust port 15 and depleted fuel 13 may exit via an anode exhaust port 16. Depending upon the nature of the electrolyte and the design of the fuel cell, the reaction products, typically H₂0 and gaseous C0₂, will exhaust the fuel cell from ports 15 or 16. Those skilled in the art of fuel cells will appreciate that many design variants and options on materials, component design, electrolyte types, operating temperatures and reactants are encompassed within this description.

Additional to the above components and shown in dotted lines, the fuel cell system of Figure 1 incorporates a liquid 17, supplied with any combination of the fuel 9 or the oxidant 10 or the electrolyte 11 , or supplied as the electrolyte, that reacts with C0₂ within the fuel cell and exits the fuel cell, in substantially carbonated form 18, via ports 12 and/or 13 and/or 14; and, secondly, a reaction chamber 19 in which the carbonated liquid 18 is reacted with a group II metal ion 20 to form a solid group II metal carbonate precipitate 21 and to regenerate liquid 17.

The cathode 2 typically contains an active oxygen-reduction catalyst such as silver, platinum, nickel, manganese dioxide or transition metal carbide (and combinations thereof) and the anode 1 typically contains a fuel oxidation electrocatalyst, such as a transition metal oxide decorated with precious metal particles, nickel, palladium or platinum alloy (or combinations thereof). The electrolyte 3 is preferably an alkaline liquid aqueous hydroxide, such as sodium or potassium hydroxide, or other ionically-conducting fluid which will dissolve or react with C0₂. Alternative electrolytes are possible such as aqueous or molten salts including carbonates. The fuel is preferably carbon-containing and a liquid, such as methanol. In an alternative embodiment, the fuel may be dissolved or dispersed in the electrolyte. The fuel or electrolyte-fuel mixture is supplied to the anode 1 of the fuel cell and the electrolyte 3 or fuel-electrolyte mixture is circulated between and past the anode 1 and cathode 2, so that the anode and cathode have a continuous ionic connection between them. Air or other appropriate oxidant 10 is supplied to the cathode. The anode and cathode of the cell are connected electrically to an external circuit and electricity is generated when fuel is electrochemicaiiy oxidised at the anode and oxidant is electrochemicaiiy reduced at the cathode.

C0₂ generated by fuel oxidation at the anode, and also any C0₂ entrained in the oxidant supplied to the cathode, reacts with or dissolves in the flowing electrolyte, which comprises liquid 17 in this embodiment. For example, the liquid 17 may be sodium hydroxide and the C0₂ reacts with flowing sodium hydroxide to generate dissolved or dispersed sodium carbonate and/or sodium bicarbonate.

In a further example, in the case of an aqueous or molten carbonate electrolyte, fuel such as methanol can react electrochemicaiiy with carbonate or

bicarbonate ion at the anode to generate product carbon dioxide which can dissolve in the electrolyte to form (bi)carbonate ions and/or further react with dissolved carbonate to form bicarbonate ions. Hydroxide ions formed at the cathode of the fuel cell from electrochemical reduction of oxygen can react with bicarbonate to form carbonate ions or oxygen may react directly with C0₂ at the cathode to form carbonate ions. Reactions occurring at electrodes and within the electrolyte may include:

CH₃OH + 3C0₃ ²- = 4C0₂ + 2H₂0 + 6e

3C0₂ + 3H₂0 + 3C0₃ ^2" = 6HC0₃- 30₂ + 6H₂0 + 12e = 120H^"

60H- + 6HC0₃ ^" = 6CO₃ ^2" + 6H₂0

0₂ + 2C0₂ +4e = 2C0₃ ^2"

In one embodiment, particles of magnesium or calcium silicates dispersed in the electrolyte react with the dissolved or dispersed C0₂ or (bi)carbonate to form a solid precipitate of magnesium or calcium carbonates and silica and to regenerate hydroxide ions. These precipitates are continuously or periodically settled and separated from the electrolyte.

In another embodiment, the electrolyte containing dissolved/dispersed C0₂ or (bi)carbonates and/or depleted in hydroxide ions is replaced or topped-up with a fresh source of hydroxide or other appropriate ions. The electrolyte is then reacted with an appropriate source of metal ions to form a stable carbonate precipitate.

In a still further embodiment, the electrolyte containing dissolved/dispersed CO2 or carbonates is circulated through a bed of metal silicates or other appropriate oxide, hydroxide, silicate or other species which are capable of reacting with the dissolved carbonates to form solid carbonates and to regenerate hydroxide ions.

A direct-oxidation fuel cell operating on a carbon-based fuel generates carbon dioxide as a reaction product. According to an embodiment of the present invention, the fuel, preferably a (first) liquid, is dissolved or dispersed in a (second) liquid, preferably an alkaline electrolyte, which is supplied to the anode of the fuel cell. The (second or carrier) liquid dissolves and/or reacts with the product C0₂. Product C0₂ is captured as a solid mineral precipitate through reaction with a metal ionic species, preferably a magnesium, and/or calcium ionic species, which is preferably in solution and preferably in a solution formed of the (second) liquid. The solid carbonate precipitate is removed and the (second) liquid, to which additional fuel is added, is recirculated to the anode of the fuel cell. The purpose of the process and system is to prevent gaseous C0₂ from being released to the atmosphere during the electrochemical oxidation of a carbon-containing fuel and the simultaneous sequestration of the CO2 in a mineral precipitate.

In another embodiment, a carbon containing fuel, such as methanol or particles of coal, is mixed with a carrier liquid(s), such as sodium hydroxide or an amine, and supplied to the anode of a polymeric- or ceramic-membrane-electrolyte fuel cell. The membrane electrolyte is ionically conducting to hydroxide ions and/or to protons and/or to carbonate ions and/or to oxide ions. C0₂ generated by the electrochemical oxidation of the fuel at the anode dissolves in the carrier liquid(s) or reacts with carrier liquid(s) to form carbonate species. Mineral solids (such as metal carbonates, oxides, hydroxides and/or silicates) dispersed in the carrier liquid(s), or confined within a reactor through which carrier liquid(s) is circulated, react with C0₂ and/or carbonate to form a solid stable carbonate or bicarbonate. In yet another embodiment, C0₂ reacting at one electrode is transported through the electrolyte in the form of carbonate or bicarbonate ions to the other electrode, where it reacts again to form a solid carbonate precipitate. Electrical energy is generated as a consequence of the transport of (bi)carbonate ions from one electrode to the other.

Specific examples of the electrochemistry used in a fuel cell system in accordance with the invention will now be described.

Example 1

A first example is a direct-methanol alkaline fuel cell of the type shown in Figure 1 in which aqueous KOH is used both as the liquid to react with the product C0₂ in the fuel cell and also as the fuel cell electrolyte. CaO is used as the source of group II metal ions to precipitate calcium carbonate from the carbonated electrolyte (K₂C0₃ and KHC0₃) and to regenerate KOH.

Anode and cathode electrodes 1 ,2 are formed from a conductive porous carbon cloth laminated to a PTFE-bonded mixture of carbon black and nanoparticulate platinum (such as "BiPlex PlaXC" anode material available from Gaskatel GmbH).

The fuel 9 is an aqueous methanol solution (CH₃OH), typically industrially synthesised from coal or natural gas.

The oxidant 10 is simply air and in particular oxygen found in air.

The electrolyte 3 is 3 molar potassium hydroxide (KOH) in water.

The method of operation is as follows. Electrolyte and fuel are each supplied to the fuel cell at a rate of at least 1 mole (56g) of KOH per mole (32g) of CH₃OH that is oxidised. Any methanol that is supplied to, but not consumed (oxidised) in, the fuel cell is recycled for subsequent supply to the fuel cell. Oxygen (in air) is supplied to the fuel cell at a rate at least three moles (96g) of 0₂ per two moles (64g) of CH₃OH that is oxidised. Excess air (or air partially depleted of oxygen and C0₂) is exhausted to atmosphere. Products of the fuel cell reactions are exhausted from the fuel cell in solution with the electrolyte.

At the anode, methanol is oxidised to form water (H₂0) and carbonate ions (C0₃ ² ) and/or bicarbonate ions (HC0₃ ^~) in the electrolyte according to the following reactions:

CH₃OH + 80H^'(aq) = C0₃ ² (aq) + 6H₂0 + 6e

CH₃OH + 70H^"(aq) = HC0₃ ^"(aq) + 5H₂0 + 6e^"

Bicarbonate ion formation is favoured at low rates of electrolyte supply (low ratio of KOH: CH₃OH), while carbonate ion formation is favoured at high rates of KOH supply.

At the cathode, oxygen is reduced to form hydroxide (OH ) ions in the electrolyte; and trace C0₂ (~400ppm) in the supplied air reacts with hydroxide to form carbonate or bicarbonate ions in the electrolyte:

1.50₂(g) + 3H₂0 + 6e = 60H (aq)

C0₂ + 20H (aq) = C0₃ ² (aq) + H₂0

C0₂ + OH^'(aq) = HC0₃ ^"(aq)

The net reaction within the fuel cell for carbonate formation is: CH₃OH + 1.50₂(g) + 20H (aq) = 0O₃ ² (aq) + 3H₂0 For bicarbonate formation, the net fuel cell reaction is: CH₃OH + 1.50₂(g) + OH (aq) = HC0₃ (aq) + 2H₂0

The electrons released at the anode 1 by the electro-oxidation of the methanol are transferred through an external electrical circuit, doing useful work as electricity, before returning to the cathode 2 where they take part in the electro- reduction of the oxygen. The exhaust carbonated electrolyte from the fuel cell containing dissolved product carbonate and/or bicarbonate ions is reacted with lime (calcium oxide, CaO) in at least the proportion of 1 mole (56g) of CaO per mole (60g) of dissolved (bi)carbonate ion. This may be done in a continuous mode by circulating the electrolyte through a packed bed of solid lime granules in the reaction chamber 19, before returning the electrolyte to the fuel cell; or by dosing batches of the carbonated electrolyte with an appropriate quantity of lime. The KOH electrolyte is regenerated and solid calcium carbonate is precipitated, according to the following reactions:

CaO(s) + C0₃ ^2"(aq) + H₂0 = CaC0₃(s) + 20H (aq)

CaO(s) + HC0₃ ^"(aq) = CaC0₃(s) + OH (aq)

The solids are separated from the solution (e.g. by filtration or settling), excess product water from the fuel cell reaction is removed as hydration water in the solid carbonate (or by evaporation) and the regenerated electrolyte is recycled for use again as input to the fuel cell.

The net reaction of the combined fuel cell and carbonate precipitation processes is:

CH₃OH + 1.50₂(g) + CaO(s) = CaC0₃(s) + 3H₂0

Where the methanol fuel has been originally synthesised from coal, the thermodynamically favourable oxidation of carbon to aqueous carbonate and regeneration of the alkaline capture liquid (carrier liquid) by precipitation of solid carbonates can be represented as:

C + 0₂ + 20H- = C0₃ ^2" + H₂0 = (+ CaO) = CaC0₃ + 20H^"

Little or no C0₂ is released to the atmosphere by the oxidation of the fuel to solid carbonate and, furthermore, C0₂ (g) that enters the fuel cell as a trace constituent (~0.04%) of air is also sequestered as a solid carbonate.

Figure 2 illustrates how the fuel cell chemistry described in Example 1 can be utilised for a 500MW power station. The group II metal ions can be provided from a number of economically viable sources, such as olivines, serpentines, industrial waste materials, mine and quarry fines, oil drilling cuttings or combustion ash. As shown in Figure 2, a proportion, typically between 5 and 30% of energy output from the power station, may be needed to process the source of metal ions. Figure 2 shows the approximate quantities of raw material required and generated by the power station. Figure 2 also illustrates that the system provided savings in treating waste material.

Example 2

In a minor variant of Example 1 , the methanol fuel 9 is supplied to the fuel cell dissolved in the aqueous KOH electrolyte 3, rather than as a separate supply to the anode, in a similar mode to that described in US5004424. To avoid direct chemical oxidation of methanol and oxygen at the cathode, the platinum is replaced with an electrocatalyst such as silver or manganese oxide that is inactive toward methanol oxidation. A suitable cathode is a conductive porous nickel mesh laminated to a PTFE-bonded mixture of carbon black and nanoparticulate silver (such as "BiPlex Oxag" cathode material available from Gaskatel GmbH).

Reaction sequences are the same as in Example 1. Any excess, unreacted methanol fuel that exits the fuel cell in the electrolyte exhaust stream may be directly recycled or recovered by distillation when the carbonated electrolyte is regenerated and excess product water removed.

Example 3

In another variant of Example 1 , an impermeable anion-conducting solid polymer electrolyte membrane (PEM) 30, is used instead of KOH as electrolyte. This is illustrated in Figure 3, which is otherwise of the same layout as the system of Figure 1. Ethanol, rather than methanol, is used as fuel 9 and the ethanol is dissolved in a carrier fluid 17 consisting of an aqueous solution of ammonia/ammonium hydroxide (NH₄OH) where the molar ratio of ammonium hydroxide to ethanol is at least 2:1. An anode electrocatalyst 7, suitable for the alkaline electro-oxidation of ethanol, such as Hypermec series 3020 non- platinum catalyst from Acta S.p.A., is used in place of platinum. Anode and cathode electrodes 1 ,2 are bonded directly to either side of the alkaline PEM 30.

Electro-oxidation of ethanol at the anode 1 by hydroxide ions conducted through the PEM generates carbon dioxide which reacts with the NH₄OH in the carrier liquid to form ammonium bicarbonate, thereby avoiding C0₂ release to the atmosphere:

C₂H₅OH(aq) + 120H (aq) = 2C0₂(aq) + 9H₂

C0₂(aq) + NH₄OH(aq) = NH₄HC0₃(aq)

Subsequent reaction of the ammonium bicarbonate in the carrier liquid 18 with a high pH ammoniacal solution of magnesium salts, such as magnesium sulphate, causes magnesium carbonate to precipitate:

NH₄HC0₃(aq) + MgS04(aq) + NH₄OH(aq) = MgC0₃(s) + (NH₄)₂S0₄(aq) + H₂0

After removal of the solid carbonate precipitate, evaporation of the solution enables unreacted ethanol fuel to be recovered for return to the fuel cell and solid product ammonium sulphate. The latter is thermally decomposed to regenerate ammonia for the aqueous ethanol carrier fluid and ammonium hydrogen sulphate which can be used, as per the method described in

US3338667, to extract magnesium as magnesium sulphate from a suitable mineral feedstock, such as serpentine rock:

(NH₄)₂S0₄(s) = NH₄HS0₄(s) + NH₃(g)

NH₃(g) + H₂0 = NH₄OH(aq)

6NH₄HS0₄(aq) + Mg₃Si₂0₅(OH)₄(s) = 3MgS0₄(aq) + 2Si0₂(s) + 5H₂0 + 3(NH₄)₂S0₄(aq)

Example 4

A further example of the invention uses a mixed-reactant, single-feed fuel cell with a porous flow-through polymer membrane electrolyte, as shown in Figure 4 (which is of the type described in US2003165727). A low-grade oxygen- contaminated reformate gas is used as fuel, exhaust gases from an air-rich combustion process are used as oxidant, aqueous ammonia are used as the liquid to be carbonated and a suspension of magnesium hydroxide particles is used as an integral source of group II ions.

In the system shown in Figure 4, the fuel 9, oxidant 10, carrier liquid 17 and group II ion source 20 are introduced into the fuel cell in a single feed, and spent oxidant and depleted fuel exits together with carrier liquid and a carbonate precipitate 21. The carbonate precipitate is separated from the carrier liquid in a chamber 19 and the carrier liquid is returned to the fuel cell together with fresh fuel and oxidant.

An anion-conducting polymer electrolyte membrane 3 is used (such as available commercially from Solvay, Fumatec, Dupont or Tokayama), which is multiply perforated to allow a fluid with entrained particles to pass through it with minimal pressure drop. A flow-through fuel cell is constructed using membrane electrode assemblies formed by bonding porous anode and cathode electrodes 1 ,2 to each side of the perforated membrane. The anode contains an electrocatalyst 7 such as tungsten oxide that is selective toward oxidation of fuel while the cathode contains an electrocatalyst such as silver that is selective toward reduction of oxygen. The fuel cell contains a single chamber with the membrane electrode assembly or assemblies arranged across it.

The single-feed for the fuel cell is prepared consisting of a three-phase mixture of bubbles of reformate gases (containing hydrogen, carbon monoxide, oxygen, nitrogen and carbon dioxide), air and exhaust combustion gases in a liquid aqueous ammonia solution with suspended solid particles of magnesium hydroxide. The mixture is pumped through the single chamber of the fuel cell where the fuel components of the mixed feed are electro-oxidised at the anode and the oxygen is electro-reduced at the cathode. Electricity is generated by these electrochemical reactions and used in an external circuit. The

perforations and porosity of the membrane electrode assemblies are large enough, and the flow-rates high enough, to avoid clogging of the electrode and electrolyte pores by the suspended particles in the feed mixture.

C0₂ entering the fuel cell as a component of the mixed feed, and also C0₂ generated by the oxidation reactions in the fuel cell, reacts chemically with the aqueous ammonia to form ammonium carbonate. The ammonium carbonate reacts with the suspended particles of magnesium hydroxide and with any magnesium ions in solution to form suspended particles of solid magnesium carbonate and to regenerate ammonium hydroxide solution. C0₂ may also react directly with the suspended magnesium hydroxide particles to form magnesium carbonate and hydroxide ions. Little or no gaseous CO₂ exits the fuel cell.

After exiting the fuel cell chamber, the solid, gaseous and liquid phases of the mixture are separated and the ammonium solution returned for re-use in the fuel cell. Any unreacted fuel may be recycled to the fuel cell or combusted. The solid carbonate product phases can be filtered and consolidated and used as construction materials.

Example 5

Another variant of example 1 is a fuel cell in which C0₂ is supplied to one electrode and is transported through the electrolyte as a carbonate or bicarbonate ion to react with a liquid at the other electrode.

In this case, as illustrated in Figure 5, the cathode 2 of the fuel cell is supplied with a gas mixture 50 containing C0₂ and 0₂, such as air or a flue gas. The anode 1 is supplied with limewater, a solution containing calcium hydroxide, which, in this example, is liquid 17 and fuel 51 and source of group II ion 20, i.e. all one and the same, so that the anode chamber inside the fuel cell is also the reaction chamber where regeneration of the liquid 17 and precipitation of solid calcium carbonate 21 takes place. (An alternative version of this example could instead, at the anode, use a hydroxide solution, such as NaOH or NH₄OH, as fuel 51 and liquid 17; and regenerate the liquid 17 with a group II ion source 20 in a reaction chamber 19 external to the fuel cell). A separate electrolyte 3 such as aqueous caesium carbonate is provided to separate the cathode and anode compartments and to conduct carbonate ions between them. Anodes, cathodes and their respective electrocatalysts, such as carbon-supported platinum dispersed on carbon cloth, are chosen as suitable for the alkaline environment and to accelerate the reactions at each electrode. Oxygen is electrochemically reduced at the cathode electrode to form carbonate ion in combination with the C02:

2C0₂ + 0₂ + 4e = 2C0₃ ^2"

At the anode, oxygen is generated when hydroxide ion is oxidised to water and carbonate ion reacts with the liquid. E.g. for lime water:

2C0₃ ^2" + 2Ca(OH)₂ = 2CaC0₃ + 0₂ + 2H₂0 +4e

The fuel cell of this example is able to extract electrical energy directly from the exothermic oxidation of carbon dioxide to carbonate. The direct formation of a solid carbonate also has advantages of driving the electrochemical reaction and increasing electrical conversion efficiency according to le Chatelier's principle, i.e. through removal of a reaction product.

Chemistries for C0₂ sequestration other than those described as examples herein are possible such as those described in WO2009139813,

WO2008142017, WO2008101293, WO2007106883, WO2006108532, US2008112868, US2009010827, US2009214408, US2005180910,

US2007261947, US20050002847, US2004219090, US20040213705, all of which are hereby included in full as relevant sequestration approaches and chemistries in support of the present invention.

Claims

1. A fuel cell system comprising: a fuel cell comprising an anode, a cathode, the fuel cell operating with a fuel and an oxidant reactant, at least one of the fuel and the oxidant reactant containing carbon, and a liquid, wherein C0₂ or a carbon-containing oxidation product in the fuel cell reacts or dissolves in the liquid; and a source of group II metal ions for converting the reacted or dissolved C0₂ or carbon-containing oxidation product into an insoluble carbonate to regenerate the liquid.

2. A fuel cell system according to claim 1 , wherein the liquid acts as an electrolyte in the fuel cell.

3. A fuel cell system according to claim 1 , further comprising an electrolyte.

4. A fuel cell system according to claim 3, wherein the electrolyte is a solid membrane.

5. A fuel cell system according to any preceding claim, wherein the liquid is an aqueous hydroxide.

6. A fuel cell system according to any of claims 1 to 4, wherein the liquid is a molten salt.

7. A fuel cell system according to any of claims 1 to 4, wherein the liquid is an amine.

8. A fuel cell system according to any of claims 1 to 4, wherein the liquid is an ionic liquid.

9. A fuel cell system according to any preceding claim, wherein the liquid is a carbonate.

10. A fuel cell system according to any preceding claim, wherein the liquid contains ammonia or an ammonium salt.

1 1. A fuel cell system according to any preceding claim, wherein the fuel cell is a mixed-reactant or single-chamber fuel cell, in which fuel and oxidant are in direct contact in the fuel cell.

12. A fuel cell system according to any preceding claim, wherein the carbon-containing fuel and/or the oxidant reactant is dissolved or dispersed in the liquid.

13. A fuel cell system according to any preceding claim, wherein the source of group II metal ions is provided in a reaction chamber external to the fuel cell.

14. A fuel cell system according to claim 13, wherein the reaction chamber is a bed, column or other suitable containment device containing mineral solids capable of reacting with dissolved CO2 or carbonate or bicarbonate species to form a solid stable carbonate precipitate, through which the liquid is circulated.

15. A fuel cell system according to any of claims 1 to 12, wherein the source of group II metal ions is provided within the fuel cell.

16. A fuel cell system according to claim 15, wherein the group II metal ions are obtained from a mineral source in a chemical extraction process integrated with the fuel cell.

17. A fuel cell system according to any preceding claim, wherein the group II metal ions are calcium or magnesium ions.

18. A fuel cell system according to any one of claims 1 to 12, wherein the source of group II metal ions is a dispersion of solid mineral particles within the liquid capable of reacting with dissolved C0₂ or carbonate ions to form a solid stable carbonate precipitate.

19. A fuel cell system according to any preceding claim, wherein the fuel is mixed with or dispersed in the electrolyte.

20. A fuel cell system according to any preceding claim, wherein the fuel is an alcohol.

21. A fuel cell system according to any preceding claim, wherein the fuel is coal.

22. A fuel cell system according to any preceding claim, wherein at least one of the fuel and oxidant reactant incorporates the exhaust gases of a combustion process.

23. A fuel cell system according to any preceding claim, wherein the carbon-containing reactant is atmospheric air.

24. A method of operating a fuel cell, the fuel cell comprising an anode, a cathode, fuel, and oxidant reactant, at least one of the fuel and the oxidant reactant containing carbon, comprising the steps of: introducing a liquid into the fuel cell, C0₂ or a carbon-containing oxidation product in the fuel cell reacting with or dissolving in the liquid; and converting the reacted or dissolved CO₂ or carbon-containing oxidation product into an insoluble carbonate by reaction with group II metal ions.

25. A method according to claim 24, wherein the group II metal ions are calcium or magnesium ions.

26. A method according to any of claims 24 or 25, wherein the step of converting is carried out in a reaction chamber separate from the fuel cell.

27. A method according to claim 26, wherein the step of converting comprises passing the liquid through a bed, column or other suitable containment device containing mineral solids capable of reacting with dissolved C0₂ or carbonate species to form a solid, stable carbonate precipitate.

28. A method according to claim 26 or 27, wherein the liquid is circulated through the fuel cell and the reaction chamber.

29. A method according to claim 24 or 25, wherein the group II metal ions are contained within the liquid in the fuel cell.

30. A method according to claim 28, wherein the group II metal ions are obtained from a mineral source in a chemical extraction process integrated with the fuel cell.

31. A method according to claim 28, wherein the group II metal ions are obtained from a dispersion of solid mineral particles within the liquid.

32. A method according to any of claims 24 to 31 , wherein the liquid acts as an electrolyte in the fuel cell.

33. A method according to any of claims 24 to 32, wherein the fuel and/or the oxidant reactant is dissolved or dispersed in the liquid.

34. A method according to any of claims 24 to 33, wherein the fuel cell is a mixed-reactant or single-chamber fuel cell, in which the fuel and the oxidant are in direct contact in the fuel cell.

35. A method for capturing carbon using a fuel cell, the fuel cell comprising an anode, a cathode, fuel, and oxidant reactant, at least one of the fuel and oxidant reactant containing carbon, comprising the steps of: introducing a liquid into the fuel cell, C0₂ or a carbon-containing oxidation product in the fuel cell reacting with or dissolving in the liquid; and sequestering the reacted or dissolved C0₂ or carbon-containing oxidation product by reaction with group II metal ions to form a stable solid carbonate.

36. A method according to claim 35, wherein the oxidant reactant is air containing CO2.

37. A sequestration apparatus for use with a fuel cell, comprising:

a reaction vessel having an input configured for connection to the outlet of the fuel cell to receive reagents from the fuel cell, the reaction vessel containing a source of group II metal ions, and an output configured to supply liquid back to the fuel cell.