WO2006061639A2

WO2006061639A2 - A fuel cell

Info

Publication number: WO2006061639A2
Application number: PCT/GB2005/004744
Authority: WO
Inventors: John Thomas Sirr Irvine; John Lewis Bradley; John Barry Lakeman
Original assignee: The Secretary Of State For Defence
Priority date: 2004-12-08
Filing date: 2005-12-08
Publication date: 2006-06-15
Also published as: WO2006061639A3; GB0426879D0

Abstract

An electricity producing fuel cell comprises a cathode (22), a solid electrolyte (14) and a composite anode (13) which anode comprises, at an operating temperature of the cell, a dispersion of a solid fuel material in a liquid electrolyte.

Description

A FUEL CELL

The present invention is generally concerned with a fuel cell comprising a composite anode of a solid fuel material dispersed in a liquid electrolyte. The invention is particularly, although not exclusively, directed to fuel cells in which the composite anode comprises carbon as the solid fuel material.

As used herein, the term "fuel cell" also refers to semi-batteries and semi-fuel wherein the fuel is stored.

Carbon fuel cells are of interest as portable or compact power sources in that they offer the potential for high energy storage and power efficiencies when compared to alternative fuel cells, for example, those based on hydrogen, methanol and methane fuel. Although carbon fuel cells comprising a solid carbon anode and molten carbonate or hydroxide electrolytes are well known, the power available from such cells has to date been limited by inherent low current density and problems with corrosion of the cathode or electrolyte poisoning. Recent improvements have sought to address these inefficiencies.

For example, US patent application 2002/0106549 (Cooper et al) discloses a corrosion resistant carbon fuel cell based on molten carbonate electrolyte. The cell uses a highly reactive and expensive form of carbon (pyrolysed carbon) in order to obtain improved current density and power output (-100 mWcm^"2). In the carbon fuel cell described by US patent 6 200 697 (Pesavento), acidic oxide is added to a molten hydroxide electrolyte and a humidified air feed is added to the cathode in order to avoid carbonate poisoning of the cell.

US patent application 2002/0015877 (Tao) discloses a carbon fuel cell in which a solid state cathode reduces oxygen to oxygen anions (O^2") and passes them to a solid state or a melt electrolyte in contact with a carbon-containing anode. In one arrangement, the carbon anode is particulate in form and the electrolyte is a solid metal oxide. In another arrangement, the electrolyte comprises molten carbonate, which contacts a solid carbon anode or is admitted to a housing containing particulate carbon. The current density of each arrangement is, however, low (power output ~ 1 mWcm^"2) on account of solid-solid boundary surfaces or sluggish reactivity of carbon with carbonate ion.

Notwithstanding these improvements, there remains a desire for a low cost and low weight carbon fuel cell capable of providing high power outputs.

International patent applications WO 01/80335 (Celltech) and WO 03/044887 (Celltech) disclose electrochemical devices capable of switching between a battery mode and a fuel cell mode. These devices utilise a liquid metal anode and a metal- metal oxide anode cycle in which a reductant fuel, such as carbon, regenerates the anode by reaction with accumulated metal oxide. The present invention, however, generally aims to impart a liquid character to an anode which is solid at the operating temperature of a fuel cell whereby to maximise contacting solid-solid surfaces between anode and electrolyte.

The present invention, therefore, provides an electricity producing fuel cell comprising a cathode, a solid electrolyte and a composite anode, which anode comprises, at an operating temperature of the cell, a dispersion of a solid fuel material in a liquid electrolyte.

References herein to "liquid" refer to the property of flow or a tendency to flow in response to an applied force, which flow is within a time scale perceptible to the human eye.

It will be understood that the cathode and/or solid electrolyte is capable of generating and/or passing a species oxidising the solid fuel material of the composite anode. The fuel cell may, for example, comprise means providing for or supplying a precursor for the species to the cathode.

In particular, the present invention may provide a fuel cell comprising a cathode capable of generating oxygen anion from an oxygen-containing gas, means providing for or supplying an oxygen-containing gas to the cathode, a solid electrolyte capable of passing oxygen anion and a composite anode which comprises, at an operating temperature of the cell, a dispersion of a solid fuel material in a liquid electrolyte. The fuel cell may use air as a source of the oxygen-containing gas. The fuel cell may, in particular, comprise an oxygen gas permeable cathode, which is simply exposed to air.

The solid fuel material may comprise any suitable anode material capable of undergoing oxidation at the operating temperature of the cell. Suitable solid fuel materials include, in particular, magnesium, aluminium, carbon, silicon, zinc, iron, sulphur or materials containing these elements.

Preferably, the solid fuel material comprises carbon or a carbon-containing material.

The solid fuel material is preferably comprised in a form, such as powder, particulate or shavings, which presents a high surface area. For example, a suitable carbon comprises one or more of graphite, quasi-graphite, coal, coke, fullerenes, carbon black, activated carbon, decolourised carbon and pyrolysed carbon. Commercially available mixtures known as Super-S or Super-P carbon are particularly suitable.

Preferably, the liquid electrolyte of the composite anode comprises a melt electrolyte. Particularly suitable liquid electrolytes include highly conducting, molten metal salts, in particular, molten metal oxides, metal hydroxides, metal carbonates or mixtures thereof. It will be understood that the composite anode generally comprises, at the operating temperature of the cell, a slurry of the solid fuel anode material in the liquid electrolyte.

As used herein the term "operating temperature" refers to a temperature at which the cell produces electricity. It will be appreciated, however, that the cell will produce electricity over a wide temperature range and that there is no requirement to maintain any particular temperature within that range.

In certain embodiments the minimum operating temperature of the cell is that necessary to achieve, at least in part, a molten electrolyte. In any case, however, it is preferred that the operating temperature or temperature range is such that there is minimum resistance in the fuel cell.

In preferred embodiments of the present invention, the operating temperature lies between about 300⁰C and about 1000⁰C. In particularly preferred embodiments, the operating temperature lies between about 575⁰C and about 75O⁰C.

The melt or solid electrolyte may suitably include any of the following metal oxides: lead (II) oxide, bismuth (III) oxide, bismuth (V) oxide, molybdenum oxide, caesium (I) oxide, caesium (III) oxide, antimony (III) oxide, antimony (IV) oxide, antimony (V) oxide, copper (II) oxide, copper (I) oxide, germanium (II) oxide, germanium (IV) oxide, lithium oxide, palladium oxide, potassium (I) oxide, sodium (I) oxide, sodium peroxide, rubidium (II) oxide, rubidium (III) oxide, tin (II) oxide, tin (FV) oxide, tellurium (II) oxide, tellurium (I) oxide, tellurium (III) oxide, vanadium pentoxide, arsenic (III) oxide, arsenic (V) oxide, indium (I) oxide, indium (III) oxide, boron oxide and mixtures thereof.

The melt electrolyte may suitably include any of the following metal hydroxides: Group IA and Group IIA metal hydroxides, such as potassium hydroxide, sodium hydroxide, lithium hydroxide, strontium hydroxide, calcium hydroxide, magnesium hydroxide and mixtures thereof.

Preferably, however, the melt electrolyte includes a metal carbonate such as lithium carbonate, sodium carbonate, potassium carbonate, strontium carbonate, barium carbonate, magnesium carbonate, calcium carbonate, beryllium carbonate, caesium carbonate, rubidium carbonate or mixtures thereof.

In a particularly preferred embodiment of the present invention, the composite anode comprises a melt electrolyte of about 62% lithium carbonate and about 38% potassium carbonate.

It will also be understood that in the preferred embodiments, the composite anode will at some point contain carbonate anion. However, unlike the prior art, the maintenance of carbonate, oxide or hydroxide anion levels is not a requirement for functioning of the fuel cell. The fuel cell may be provided with a current collector comprising one or more of a wire, mesh, filings or powder. Preferably, the current collector is resistant to corrosion by the action of carbonate. The current collector material may also be chosen to catalyse the anode reaction.

Alternatively, or additionally, the composite anode may also comprise a catalyst which catalyses the anode reaction and/or improves conductivity of the electrolyte. Preferred collector materials and catalysts include nickel or precious metals such as platinum or palladium, metal alloys such as FeCrAlY or certain metal oxides and others known to those skilled in the art of molten carbonate fuel cells.

The solid electrolyte may comprise a solid state metal oxide. It may, in particular, comprise a solid state electrolyte, such as those based on doped ceramic materials of general formula (ZrO₂)(HfO₂)a(TiO₂)_b(Al₂O₃)_c(Y₂O₃)_d(M_xO_y)e where a is about 0 to 0.2, b is about 0 to 0.5, c is about 0 to 0.5, d is about 0 to 0.5, e is about 0 to 0.15 and M is a divalent or trivalent metal such as manganese, iron, cobalt, nickel, copper or zinc.

In some preferred embodiments, the solid state electrolyte comprises an yttrium stabilised zirconia (YSZ). However, zirconias doped with lanthanides such as gadolinium, ytterbium, praeseodymium and cerium ion are also suitable. A lanthanide-doped ceria, for example, GdCeo.sO_t.g may be particularly suitable in that it is very stable to lithium carbonate. Other suitable solid electrolytes may be based on lanthanum gallate, lanthanum aluminate, ABO₃ perovskites, calcium aluminate (C₁₂Al₁₄O₃₃) and La_9J3SiOO₂₆ type apatites.

The resistivity of the solid electrolyte, which should have substantially zero continuous porosity, is largely a function of its thickness (the distance between a surface contacting the cathode and a surface contacting the anode). The solid electrolyte may, in particular, have thickness from about 10 μm to about 4 mm. In one embodiment of the present invention, the solid electrolyte is tubular and has thickness of about 3 mm.

A preferred cathode arrangement comprises a porous platinum paste sintered to the inner surface of a tubular solid electrolyte, which is open, at one end, to the atmosphere. Alternatively, the cathode material may comprise a complex ceramic oxide, such as those known to the art under the formula (La_1-xSr_x)_1-yBO₃._δ, where B is manganese, iron, cobalt or a combination thereof.

The composite anode may be contained within a compartment of the cell such that it is out of contact with the cathode surfaces of the cell. In one embodiment, the compartment is defined by the inner surfaces of a tubular nickel body and the outer surface of the tubular solid electrolyte referred to above.

The present invention also provides for a method of producing electricity comprising i) supplying an oxygen-containing gas to a cathode of a fuel cell comprising a solid electrolyte capable of passing oxygen anion and a composite anode, which anode comprises, at an operating temperature of the cell, a dispersion of a solid fuel material in a liquid electrolyte and ii) heating the fuel cell to the operating temperature.

Embodiments of the method of the present invention will be apparent from the foregoing description of apparatus.

hi particular, the solid fuel material may comprise carbon or a carbon-containing material. The solid fuel material may, in particular, comprise carbon in one or more of the forms referred to above.

As mentioned above, in certain embodiments, the heating of the fuel cell should achieve a molten electrolyte. Preferably, the fuel cell is heated to the minimum operating temperature necessary to ensure a molten electrolyte. In any case, the fuel cell is preferably heated to an operating temperature at which the resistance of the solid electrolyte is a minimum.

In the preferred embodiment, the fuel cell is heated to an operating temperature lying between about 300⁰C and about 1000⁰C. In a particularly preferred embodiment, in which the solid electrolyte comprises YSZ, the fuel cell is heated to an operating temperature lying between about 575⁰C and about 700⁰C.

Those skilled in the art will realise that the present invention provides a composite anode which imparts a liquid character to a solid fuel anode material so minimising solid-solid boundaries between the anode material and a solid electrolyte and improving current densities.

The carbon fuel cell of the present invention offers other significant advantages over, in particular, molten carbonate based carbon fuel cells. The avoidance of a requirement for circulation of carbon dioxide means that the oxygen-containing gas flow remains undiluted by carbon dioxide thereby increasing the available cell potential. In addition, a lighter and more compact design is available. Further, a suitable cathode material is not limited by a necessity for corrosion resistance since it is never in contact with molten carbonate. The solid electrolyte avoids the need for regulation of a carbonate electrolyte level and allows a less complicated cathode reaction in which oxygen is directly reduced to oxygen anion. In other words, there is no requirement for the delivery of carbon dioxide before conversion of carbon can commence.

The fuel cell of the present invention may be provided with an interconnect allowing it to be used in a series-parallel stack with other fuel cells according to the present invention. The stack may, in particular, be used to recharge a battery such as a lithium ion rechargeable battery.

Accordingly, the present invention also provides for a hybrid battery device, comprising one or more fuel cells according to the present invention and one or more lithium ion rechargeable batteries. In preferred embodiments the device comprises one or more carbon fuel cells according to the present invention.

The present invention will now be described by reference to the following drawings and Example in which

Figures 1 (a) and (b) are section perspective views of a fuel cell according to one embodiment of the present invention;

Figure 2 is a plan view of the embodiment of Figure 1 adapted for inclusion in a stack;

Figure 3 is a graph showing the open circuit voltage of the embodiment of Figure 1 at two operating temperatures;

Figures 4 (a) and (b) are graphs showing the impedance characteristics of the embodiment of Figure 1; and

Figures 5 (a) and (b) are graphs showing the current developed by the embodiment of Figure 1 at two operating temperatures.

Referring now to Figures 1 (a) and 2, a fuel cell 11 according to one embodiment of the present invention comprises a half-closed cylinder container 12. The cylinder 12, which comprises a corrosion resistant nickel body, contains a 8:1 (w/w) solid mixture of carbonate salt (62% lithium carbonate 38 % potassium carbonate) and Super-S carbon powder 13. A tube, comprising YSZ ceramic 14, is maintained immersed in the solid mixture 13. The ceramic 14 may include an interconnecting portion 15 which projects beyond an aperture in provided in the cylinder 12.

The tube is open to the atmosphere and its wall (3 mm thickness) carries a sintered coating of a porous platinum paste 22 across the whole of its inner surface 16 (35 cm²). A connecting platinum contact wire 17, provided with an alumina protective covering 18, of diameter substantially less than the tube, contacts the paste.

Referring now to Figure 1 (b) a nickel mesh 19 (1 cm²) is arranged in the solid mixture so that it contacts the outer surface 20 of the ceramic tube. A gold, current collecting, wire 21 is connected to the mesh.

At the chosen operating temperature of the fuel cell between 575⁰C to 700⁰C, at which the solid mixture becomes a slurry of carbon in carbonate melt, oxygen is

reduced at the platinum cathode according to the reaction O₂ + 4e -> 2O²\ The

oxygen anions are transported across the ceramic wall. The nickel mesh allows free circulation of the carbon containing carbonate melt and a series of reactions occur which directly convert carbon to carbon dioxide.

The reactions at the anode include inter alia C + 2O^2" -» CO₂ + 4e and C + 2CO₃ ^2' ->

3CO₂ + 4e. The reaction 2CO₃ ^2" -> O₂ + 2CO₂ + 4e may be suppressed at least whilst

levels of carbon are high. Example

The cell is operated at a potential of 500 mV and ran over successive periods of 24 h at temperatures between 600⁰C and 725⁰C. The cell develops an electrical power output of about lOmWcm^"2. The specific resistance (resistance per cm² of anode) is

about 17 Ωcm^'2 of which 4 Ωcm^"2 is attributed to the electrolyte and the remainder to

the anode.

Referring now to Figure 3, the open circuit potential of the fuel cell at 625⁰C is about 700 to 800 mV and increases with run number. At an operating temperature above 665⁰C (up to 725⁰C), the potential is constant at 850 mV with run number.

Figures 4 (a) and (b) show the impedance characteristics of the fuel cell with run number at a number of temperatures. As may be seen the impedance of the cell is initially diffusion controlled. However, the change in line shape in Figure 4 (b) at 665⁰C of the second run suggests that deposited nickel on the solid electrolyte inhibits the diffusion of oxygen anions to the mesh. After the second run, the build up of nickel becomes sufficient for efficient current collection and the resistance of the cell has dropped to its original value.

Referring now to Figure 5 (a) the current collected from the cell at 665⁰C during the third run 22 mA shows a gradual decay over time. The current can be restored to its steady state by slight agitation of the cell suggesting that it is due to the viscosity of the carbon containing carbonate and trapped carbon dioxide bubbles. Figure 5 (b) shows that the effect is not seen when the operating temperature is 700⁰C.

Claims

1. An electricity producing fuel cell comprising a cathode, a solid electrolyte and a composite anode which anode comprises, at an operating temperature of the cell, a dispersion of a solid fuel material in a liquid electrolyte.

2. A fuel cell according to Claim 1 comprising means providing for or supplying an oxygen-containing gas to the cathode and in which the cathode is capable of generating oxygen anion from an oxygen-containing gas and the solid electrolyte is capable of passing oxygen anion to the composite anode.

3. A fuel cell according to Claim 1 or Claim 2, in which the solid fuel material comprises carbon or a carbon-containing material.

4. A fuel cell according to Claim 3, in which the solid fuel material comprises one or more of graphite, quasi-graphite, coal, coke, fullerenes, carbon black, activated carbon, decolourised carbon and pyrolised carbon.

5. A fuel cell according to any of Claims 1 to 4, in which the liquid electrolyte comprises a melt electrolyte.

6. A fuel cell according to Claim 5, in which the melt electrolyte comprises a molten metal salt.

7. A fuel cell according to Claim 6, in which the salt comprises a metal oxide, metal hydroxide, metal carbonate or mixtures thereof.

8. A fuel cell according to any preceding Claim, further comprising a current collector which catalyses the anode reaction.

9. A fuel cell according to any preceding Claim, in which the composite anode contains a catalyst which catalyses the anode reaction.

10. A fuel cell according to any preceding Claim, in which the solid electrolyte comprises a solid state metal oxide.

11. A fuel cell according to any preceding Claim, in which the thickness of the solid electrolyte ranges from 10 μm to 4 mm.

12. A fuel cell according to Claim 11, in which the thickness of the solid electrolyte is 3 mm.

13. A fuel cell according to any preceding Claim, in which the power output is about lO mWcm^'2.

14. A fuel cell according to any preceding Claim, having an inherent resistance of

about 17 Ωcm^"2.

15. A fuel cell according to any preceding Claim, in which the cathode comprises a porous platinum paste or a complex ceramic oxide of formula (La_1-xSr_x)_1-yBO_3-δ, where B is manganese, iron, cobalt or a combination thereof.

16. A method for generating electricity comprising i) supplying an oxygen- containing gas to the cathode of a fuel cell according to any preceding Claim and ii) heating the fuel cell to its operating temperature.

17. A method according to Claim 16, comprising heating the fuel cell to a temperature lying between about 300⁰C and about 1000⁰C.

18. A method according to Claim 17, comprising heating the fuel cell to a temperature ranging lying between about 575⁰C and about 700⁰C.

19. A battery device comprising a fuel cell according to any of Claims 1 to 15, and a lithium ion rechargeable battery.

20. A fuel cell substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

21. A method substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.