WO1993011574A1

WO1993011574A1 - Fuel cell

Info

Publication number: WO1993011574A1
Application number: PCT/GB1992/002172
Authority: WO
Inventors: Brian Charles Hilton Steele
Original assignee: Imperial College Of Science, Technology & Medicine
Priority date: 1991-11-25
Filing date: 1992-11-25
Publication date: 1993-06-10
Also published as: GB9125012D0; AU2951192A

Abstract

An electrolyte structure for a fuel cell particularly a ''solid oxide'' fuel cell. A structural support is formed from an impermeable mixed conductor such as lanthanum cobalt manganese oxide in which conduction occurs by self-diffusion rather than by molecular diffusion through pores, which forms one electrode of the cell and carries the solid oxide electrolyte as a coating of a material such as Zr0.9Y0.1O1.95. This enables the electrolyte layer to be made very thin, as is required for relatively low temperature operation whilst providing good structural integrity. Multi-cell structures can be formed as ''stacks'' or planar arrays.

Description

"Fuel Cell"

This invention relates to fuel cells, and particularly although not exclusively to fuel cells of the so-called "solid oxide" type. Fuel cells of this type are commonly constructed in stacks as a series of layers, as illustrated in Figure 1, which shows a cell comprising an end plate 2 which forms one terminal of the stack, beneath which are positioned an anode 4, an electrolyte matrix 6, a cathode 8, and a "bipolar separator plate" 10, which separates the top cell from a further cell assembly beneath it. It will be appreciated that a series of such cells will be provided in the stack, depending on the required output voltage. In the type of construction shown, which is a "cross-flow" type, the separator plate has a first series of parallel channels 12 running in one direction on its upper surface, which carry the oxidant (usually air) for the upper cell, and a second series of parallel channels 14 on its lower surface at right angles to the first, carrying fuel which, in the case of the SOFC, is usually a hydrocarbon fuel, for the lower cell. The end plate 2 also carries corresponding channels 14 on its underside only through which fuel passes across the top of the upper cell. In action, electricity is produced at the "interfacial three-phase boundary" where the electrode, the electrolyte and one of the reacting phases are in intimate contact. In the case of the solid oxide fuel cell illustrated, the electrolyte is a ceramic solid oxide with a typical composition of ^Zro.9^Y0.1°1.95_' ^{τhe an}i°ⁿ vacancies created by the introduction of aliovalent Y³⁺ anions on the Zr⁴⁺ site produce an oxygen ion conductor which has a conductivity comparable to liquid electrolytes in the temperature range 900 to 1000°C. Because the electrolyte is solid the electrolyte management problems associated with other fuel cell types are eliminated. Moreover, the high temperature operation allows insitu reforming of hydrocarbon fuels and so electricity/fuel conversion efficiencies should approach 60%.

A number of successful cells have also been constructed with a tubular configuration, basically comprising a similar series of layers arranged concentrically, an internal air flow, and an external fuel flow. However there are difficulties involved in scaling these up to large sizes, and there are also complications involved in the fabrication of the assembly with its various layers. In practice, it is much simpler to make an assembly of planar layers as illustrated in Figure 1, partly because of the simple configuration of the individual elements, and partly because of the ease with which relatively compact large stacks can be designed.

Another type of recently proposed electrochemical cell construction (with different types of electrodes) is that shown in Figures 1(b) and 1(c). As can be seen from the perspective view (lb) , this is essentially a planar construction in which individual cells are formed side-by- side rather than in a stacked arrangement, which simplifies the constructional problems in some respects, and also enables more efficient use to be made of the fuel which passes successively over a series of cells. As indicated in the cross section of Figure 1(c), this involves the cells being arranged in a "side-by-side" relationship with the individual electrodes overlapping the interstices. Such an arrangement may also be adapted for use with solid oxide fuel cells, as will be explained in more detail below.

The level of performance and ease of construction of a solid oxide fuel cell do, of course, depend very much on the properties of the materials used, and it is, in fact, rather difficult to find materials for both the electrodes and the electrolyte which have the required characteristics at the necessary operating temperatures. For example, the current zirconia solid oxide electrolyte (Zr_0>gY_0#10_1>95) requires operating temperatures in excess of 900°C to achieve conductivities comparable to liquid electrolytes for electrodes of 200-300 microns thick, i.e. with adequate structural properties. It would be most desirable to reduce the operating temperature, so as to be able to use stainless steel (for example) in the construction of the cell.

If the temperature is to be reduced, however, it is necessary to reduce the thickness of the electrolyte layer, in order to achieve the same conductivity, and unfortunately the structural strength of the zirconia solid oxide ceramic electrolyte is insufficient to enable this to be done in anything but the very smallest constructions. An alternative approach is to use a different material as a structural support and a thin layer (e.g. 20-25 microns) of zirconia as the ionic conductor. However, it is necessary for the structural part to allow the passage of gas, and previous proposals have relied on a porous structure which is unfortunately, inherently weak.

Accordingly, the present invention provides an electrolyte structure for a fuel cell, comprising a structural support material carrying a thin layer of ceramic electrolyte as an ionic conductor, in which the structural material is impermeable and in which conduction occurs by self diffusion rather than molecular diffusion through pores.

Preferably the support material is arranged to form the cathode or anode of the fuel cell, thus avoiding the necessity for separate support means for each of the electrolyte and corresponding electrode structures.

This construction is particularly suitable for solid oxide fuel cells but the same principle of a strong impermeable electrode supported thin film electrolyte structures can be extended to different systems, for example, composite Bi₂0₃ - Tb₂0₃ mixed cathode with Ce_Q 9^G^o.l°1.95 thick film electrolytes can be used at 400°- 500°C in methanol fuel cells in transport applications for heavy lorries, buses, fleet vehicles, etc.

The properties of a number of suitable materials are discussed in reference (1) . In particular the oxygen transport properties of various fluorite and fluorite related systems, and perovskite and perovskite related systems, are described in that reference. Examples of some of the possible structures are shown in Figure 5 and 6 of the accompanying drawings.

Preferably, in the case of a solid oxide fuel cell, the support material is one having a lattice structure that allows oxygen ions to diffuse through, such as lanthanum cobalt manganese oxide which has good gas diffusion properties at a temperature of 800°C.

Figure 2 shows some examples of the conductivities of materials of this kind, at 800°C. Other suitable materials include solid solutions of Uo₂-Y₂0₃ (U₀.₇Y₀.₃O₂-x) or Ce0₂-Tb₂0₃ (Ce_0#7 b_0>3O₂-__χ) and composite materials which comprise ionic/mixed conductors e.g. (Ce0₂-Gd₂0₃) + ZnO(Fe).

In addition impermeable mixed conductivity cathode materials such as -L _Q>6Se_0#4Co_0>gFe_{0 2}0₃__χ can be incorporated into the heat exchange stage of a fuel cell to separate oxygen from air. Pure oxygen is then fed to the fuel cell and the excess oxygen can be burnt with unused fuel to provide thermal energy for heat exchanger, steam turbines, process heat etc. The advantage of combusting excess fuel with pure oxygen is that the pollutant N0_χ will not be produced.

The invention also extends to a solid oxide fuel cell construction comprising: a housing; a metal bipolar separator plate: an open structured resilient support means supported on said bipolar plate; a porous anode plate supported on said resilient support means; a compound cathode/electrolyte structure comprising a cathode of a mixed conductor material allowing both ionic and electronic conduction and a coating of ceramic solid oxide forming the electrolyte, on the side of the cathode facing the anode; a further separator plate above the cathode; means for introducing air on the cathode side, and means for introducing fuel on the anode side, whereby oxygen in the air passes through the cathode material and is ionically conducted through the electrolyte to react with the fuel at the anode. Alternatively the construction in respect of the anode and cathode may be reversed, i.e. with a separate porous cathode plate, and an impervious compound anode/electrolyte structure.

The invention also extends to a planar multi-cell solid oxide fuel cell comprising a rigid frame structure formed with a plurality of side-by-side apertures each of which is adapted to receive a cell assembly; each assembly comprising a compound first electrode/electrolyte plate structure, in which the first electrode is of a mixed conductor material and the electrolyte is a coating of ceramic solid oxide, and a second electrode plate facing the electrolyte, the frame also providing means for insulating the edges of adjacent cells from one another, means for electrically connecting the anode of one cell to the cathode of the next, and means for passing air over the cathode side of the frame and fuel over the anode side so as to pass over each cell in turn.

Some embodiments of the invention will now be described by way of example with reference to Figures 3 to9 of the accompanying drawings, in which

Figure 3 is a vertical cross-section of a first type of fuel cell in accordance with the invention, Figure 4 is a cross-section on line A-A of Figure

3.

Figure 5 shows the relationship between various types of fluorite and fluorite related oxygen ion conductors;

Figure 6 shows the relationship between various types of perovskite and perovskite related oxygen ion conductors;

Figure 7 shows a cross-section through a planar multi-cell structure; and

Figure 8 shows a separator/support plate for the structure of Figure 7.

Referring firstly to Figure 3, the base of the cell illustrated is formed by a metal bipolar plate 20 having a chamber 22 to which the fuel, typically a hydrocarbon gas, is introduced from inlet 40 (Figure 4) . The upper surface 24 of plate 20 is perforated to allow the gas to enter the anode side 26 of the cell, and a resilient support member such as a felt or spring 28 rests on the perforated surface to support an anode plate 30. A cathode/electrolyte structure 32, 34 is in turn supported on the anode, the cathode member 32 comprising a mixed conductor such as lanthanum cobalt manganese oxide or lanthanum strontium cobalt ferric oxide (La_{0 6}Sr_0φ4Co_0>8Fe_0>20₃__χ) which have good gas diffusion properties at 800°C (see Figure 2) .

The electrolyte layer 34 comprises a zirconia solid oxide such as •Z-r_0#9Y_0#ιO_1<g5 coated onto the cathode (e.g. by sputtering) or chemical vapour deposition and which can thus be made very thin e.g. 20-25 microns.

Thick electrolyte films (l-50μm) can also be fabricated by a variety of techniques including pulsed laser depositions MOCVD, CVD, calendering tape-casting, electrophoresis etc. For some processing routes it will be necessary for dense electrode substances to be co-fired with the solid electrolyte which requires high temperature chemical compatibility.

A chamber 36 above the cathode carries air which is introduced at an inlet 38, positioned transversely relative to the fuel inlet 40, Figure 4 , which supplies the chamber 22 as mentioned above. The oxygen in the air thus enters the cathode and oxygen ions are transported through it by self-diffusion to the electrolyte, where they react with the hydrocarbon fuel in a known fashion.

Good electrical contact must be maintained between the individual fuel cell components. This can be ensured in part by the force exerted by the resilient felt or spring 28. However it may also be necessary to ensure that the impermeable cathode member 32 is also bonded to the second metal bi-polar plate 41 (Fig.3). This would also provide improved structural strength of the fuel cell assembly.

Whilst the impermeable cathode member 32 provides oxygen ion transport to the solid cathode/electrolyte interface it also has to act as an electro-catalyst, for example, for the Faradaic reduction of oxygen,

1/2 0₂ + 2e~ -> 0²~ For high current density operation it may be necessary to provide an additional thin porous coating of an appropriate electro-catalyst (e.g. noble metal alloy) at the air/cathode interface to enhance the reactivity of this interface.

As mentioned above, some other types of oxygen ion conductors are illustrated in Figures 5 and 6 which illustrate possible fluorite and perovskite derived structures.

The embodiment of the invention described above refers to an impervious cathode structure. It will be appreciated that a similar structure could be arranged for an impervious anode structure. Moreover for certain designs it may be appropriate to have both impermeable cathode and anode structures.

As it is now possible to fabricate solid electrolyte films of only 2—3/zm thickness the same principles of construction can be extended to fuel cells operating around 400°C using methanol fuel. These direct methanol fuel cells will have major applications in transport sector for heavy lorries, buses, and fleet vehicles etc.

An alternative type of planar multi-cell structure as illustrated in Figure 7, in which the cells are arranged side-by-side which helps to simplify the arrangements for supplying fuel to one side (the top as illustrated) , and air to the other side of the cells (underneath as illustrated) . In this construction a metal interconnecting frame 50, shown separately in Figure 8, comprises a separator plate 52 which is arranged to separate adjacent cell locations, and a pair of perforated support plates 54, 56 which extend at right angles from its opposite sides. One support plate 54 is arranged near the lower end of the separator plate 52 so as to extend underneath the cell which is to the right of the separator plate, whilst the other support plate 56 is arranged near the upper end of the separator plate so as to extend above the left-hand cell. Flanges 58 also extend outwardly from the top and bottom edges of plate 52 so as to locate the adjacent edges of the cells and co-operate with the adjacent frame 50, and thus it will be appreciated that a multi-cell structure of a large number of cells can be built up in this way. The electrical components of a cell of this kind will typically comprise: (60) Mixed conducting cathode as a dense impermeable structural support component (« 200μm thick)

(62) Solid ceramic electrolyte e.g. ^Zr ₍y₎ ° t2-y) ' ^e(^G()⁰2-x'

(«5-20μm thick)

(64) Porous anode structure e.g. Zr0₂-Ni (« 20μm thick) In order to provide the necessary electrical isolation between adjacent cells, a large insulator 66 (for example of MgO or Mg A1₂0₄) is positioned on the left hand side of each separator plate 52 so as to insulate it from the whole body of the next cell to the left, and a small insulator 68 of the same material is positioned at the top right hand side of the plate, so that the adjacent anode 64 to the right is also insulated from the plate. It will thus be appreciated that the frame structure 50 which is preferably of a high-Cr alloy, then acts to connect all the cells in series.

Reference

(1) Material Science & Engineering B13 (1992) 79-87 (Paper presented during Symposium A2: Solid State Ionics (Chairman: M. Balkanski, T. Takahashi, H.L. Tuller) at the International Conference on Advanced Materials, Strasbourg, France, May 27-31, 1991.

Claims

1. An electrolyte structure for a fuel cell, comprising a structural support material carrying a thin layer of ceramic electrolyte as an ionic conductor, in which the structural material is impermeable and in which conduction occurs by self diffusion rather than molecular diffusion through pores.

2. An electrolyte structure according to claim 1 in which the said support material is adapted to form the cathode or anode of the fuel cell.

3. An electrolyte structure according to claim 1 or claim 2 in which the said support material has a lattice structure which allows oxygen ions to diffuse through it.

4. An electrolyte structure according to claim 2 or claim 3 in which the support material comprises a solid solution of U0₂-Y₂0₃ (U_0<7Y_{Q 3}0₂~ ) or

Ce0₂-Tb₂0₃(Ce_0>7Tb_0>30₂__χ)

5. An electrolyte structure according to claim 2 or claim 3 in which the support material comprises a composite ionic/mixed conductor.

6. An electrolyte structure according to claim 5 in which the support material comprises (Ce0₂-Gd₂0₃) + ZnO(Fe) .

7. A solid oxide fuel cell having an electrolyte structure according to any preceding claim.

8. A solid oxide fuel cell comprising a housing; a metal bipolar separator plate: an open structured resilient support means supported on said bipolar plate; a porous anode plate supported on said resilient support means; a compound cathode/electrolyte structure comprising a cathode of a mixed conductor material allowing both ionic and electronic conduction and a coating of ceramic solid oxide forming the electrolyte, on the side of the cathode facing the anode; a further separator plate above the cathode; means for introducing air on the cathode side, and means for introducing fuel on the anode side, whereby oxygen in the air passes through the cathode material and is ionically conducted through the electrolyte to react with the fuel at the anode.

9. A solid oxide fuel cell comprising a housing; a metal bipolar separator plate: an open structured resilient support: means supported on said bipolar plate; a porous cathode plate supported on said resilient support means; a compound anode/electrolyte structure comprising an anodeof a mixed conductor material allowing both ionic and electronic conduction and a coating of ceramic solid oxide forming the electrolyte, on the side of the anode facing the cathode; a further separator plate above the anode; means for introducing air on the cathode side, and means for introducing fuel on the anode side, whereby oxygen in the air passes through the cathode material and is ionically conducted through the electrolyte to react with the fuel at the anode.

10. A multi-cell structure comprising a stack of cells in accordance with claim 8 or claim 9.

11. An electrolyte structure according to claim 1 suitable for use in a methanol fuel cell and comprising a support structure of Bi₂0₃ - b₂0₃ forming an electrode and carrying a film of ^Ceo.g^Gdo.1°1 ₉5 comprising the electrolyte.

12. A planar multi-cell solid oxide fuel cell comprising a rigid frame structure formed with a plurality of side-by-side apertures each of which is adapted to receive a cell assembly; each assembly comprising a compound first electrode/electrolyte plate structure, in which the first electrode is of a mixed conductor material and the electrolyte is a coating of ceramic solid oxide, and a second electrode plate facing the electrolyte, the frame also providing means for insulating the edges of adjacent cells from one another, means for electrically connecting the anode of one cell to the cathode of the next, and means for passing air over the cathode side of the frame and fuel over the anode side so as to pass over each cell in turn.