WO2004008556A2

WO2004008556A2 - Planar seal-less fuel cell stack

Info

Publication number: WO2004008556A2
Application number: PCT/GB2003/002908
Authority: WO
Inventors: Thomas Gordon Lindsay
Original assignee: Thomas Gordon Lindsay
Priority date: 2002-07-10
Filing date: 2003-07-07
Publication date: 2004-01-22
Also published as: WO2004008556A3; GB2390739A; AU2003244838A8; GB2390739B; AU2003244838A1; GB0216063D0

Abstract

Construction of an SOFC stack by connecting individual solid oxide fuel cells using a novel metallic interconnect plate, typically alloy UNS 544400, into which alloy tubes are brazed. As these tubes extend from the centre of the interconnect and pass through the hot zone walls, their ends are therefore at ambient temperatures allowing polymer tubing connections to reactant supplies. The alloy tubes also function to remove power from the stack, and, uniquely, allow faulty cells to be ‘shorted out' avoiding stack failure. The alloy tube to each anode can contain a porous bed of reforming agent to convert fuel gases to hydrogen and carbon monoxide for direct use by the cell. A novel fugitive pore former, combined with electrically conducting cement, joins the cells to the metallic interconnects, and functions to distribute the feed gases uniformly over the active cell surfaces.

Description

A Novel Planar Seal-less Fuel Cell Stack

Technical Field

1 This invention relates to a stacked arrangement of planar interconnects and their use in a solid oxide fuel cell stack for the production of combined heat and power from readily available liquid or gaseous fuels and air.

Background

2 A fuel cell comprises an electrolyte, with a porous cathode material on one of its sides, and a porous anode on the other. Feeding a fuel gas to the anode, and oxygen (air) to the cathode, results in a voltage being developed across the cell. Many cells are required to make a useful power source (i.e. a stack), and this is accomplished by joining one cell to the next one. This cannot be done directly, as, for a series connection, the anode of one cell would require to be in contact with the cathode of the next. As one surface requires oxygen, and the other a fuel gas, an interconnect has therefore to be placed between to keep the gas streams apart. The difficulty then is to introduce these gas streams between the respective electrode (anode/cathode) surface, and the interconnect surface, and this is a continuing industrial problem. This has been overcome in this design through the use of a metallic interconnect, with tubes introduced into it, which feed gases directly to the centre (or any other desired part) of the interconnect. This also removes another problem associated with many stack designs - that of sealing the stack. In the present arrangement, this becomes unnecessary.

Solid oxide fuel cells are brittle, and hence liable to fracture. This can lead to an open circuit condition that can render the stack inoperable. In this design, these faulty cells can be shorted out electrically to give continued stack operation. The gas flows to these faulty cells can also be individually terminated. Traditional difficulties of connecting individual cells to the interconnect plate, i.e. that of poor mechanical support, poor gas distribution, and resistive electrical connections, have also been innovatively overcome.

The design is sufficiently flexible to allow planar cells from various manufacturers to be fitted. In general, most competing designs are unable to do this

The stack is unusually compact, and the interconnect (and stack) can be easily mass-produced with simple equipment. This results in an inexpensive design.

Careful choice of material for the interconnect that must work at temperatures up to 850 degrees Celsius, solves industrial problems associated with high levels of heat.

Essential Technical Features and Examples

3 Four drawings are used to illustrate the present approach:

Figure 1 shows the metallic interconnect tubes in place in the metallic interconnect plate.

Figure 2 is a cross-section through the centre of the drawing of Figure 1 , and shows the exit holes for the gases.

Figure 3 shows one interconnect arrangement used to form a stack

Figure 4 shows another interconnect arrangement to make the stack.

4 The interconnect plate, Fig. 1, (1) is fed with gas by means of a pair of alloy tubes brazed into the plate, Fig. 1, (2) & (3), one tube being opened upwards (to feed fuel to cell 1 anode), Fig 2, (4), the other opened downwards (to feed air to cell 2 cathode), Fig 2, (5). The other ends of the tubes run through the wall of the furnace, Fig 3, (6) to the 'outside', and remain at ambient temperatures for making the gas connections. At this position, also, tabs brazed onto the tubes, Fig 3, (7) allow faulty cells to be bypassed using a shorting link across the faulty cell. They also allow the final power to be drawn from the stack. Where it is necessary to reform the fuel gas (for example, methane), this can be accomplished by embedding a suitable catalyst in the interconnect tube leading to the anode of the cell, thus avoiding present problems with carbon deposition in the anode which result from internal reforming reactions.

5 To obtain maximum power transfer, it is essential that good electrical contact is made between electrode (anode or cathode) and interconnect plate, and in the present invention, this is accomplished using an intermediate layer that also has a controlled pore structure to give uniform and controlled gas distribution. This property results from the use of a combustible mesh embedded in a preferably non-drying paste, which is applied to each side of the cell, before the interconnect plates are placed in position. When heated to working temperature (~850C) under light load, the mesh burns out leaving a network of connected pores to aid gas distribution, and the paste hardens to become electrically conducting, and act as a mechanical support for the cell itself. This innovative approach also ensures a minimum of cell breakage.

6 The above paste for the anode side of the cell will preferably be based on nickel oxide incorporating a suitable ceramic filler and non-drying organic vehicle, while the cathode paste will preferably be based on a lanthanum manganite or variant and a similar filler and vehicle. (A variation of this invention is to use an air drying paste, assemble the stack, and oven dry, prior to placing the stack in its furnace.).

7 To form a stack, the repeating arrangement of interconnect, cell, interconnect is replicated as necessary. This is shown in outline detail in Fig 3.

8. Each successive interconnect is placed at 180 degrees, such that accidental tube-to-tube contact is avoided, and room is provided for electrical and gas connections. The electrical tabs, Fig 3, (7) are also staggered for similar reasons.

9. The metal tubes are fed with the appropriate gases from central manifolds, Fig. 3, (8) to which they are connected with polymer tubing, Fig. 3, (9) which passes through a variable tube constrictor, like an electrical 'chocolate block' connector - essentially a metal tube, with a screw passing through its wall, and bearing on the gas tubing - where the gas flows to each cell can be adjusted for optimum electrical performance, or closed off completely in the case of a faulty cell. The stack electrical output is available from the tabs, Fig. 3, (7) attached to the tubes running from the outermost interconnect plates.

10 A much simpler arrangement of cells connected in parallel is an alternative, with the major penalty being a low voltage, high current output, and higher electrical losses. In this arrangement only a single tube, Fig. 4, (3) is needed per interconnect plate, and this can be joined (by brazing) directly into an alloy manifold gas feed tube, Fig. 4, (8) which also carries the electrical output. To allow control of gas flows, a similar approach to that described for the series arrangement could be adopted, and all the tabs on one side electrically connected to give one pole of the 'battery', and all the tabs on the other side to give the other pole.

Claims

1. A stacked arrangement of metallic interconnects, each of which comprises a flat plate with integral metallic tubes welded or brazed into it, which is used to feed gases (reactant and oxidant) from outwith the stack hot zone, directly into the interconnect, and thence via a hole, or holes, in the tubes, preferably located near the centre of the interconnect area adjacent to the appropriate active surfaces of the cells which lie in contact with the interconnect.

2. A stacked arrangement of metallic interconnects as claimed in claim 1 , where the tubes extend out from each interconnect through the hot zone thermal insulation surrounding the stack, enabling the ends of the tubes to remain near ambient temperature to allow connection of the reactant (fuel and oxidant) supplies using polymer (or ceramic or metal) tubing and where the stack has cells connected in series, this connection must be electrically non-conducting ie polymer or ceramic but where the stack has cells arranged to work in parallel, this connection can be electrically conducting.

3. A stacked arrangement of metallic interconnects as claimed in claim 2, where the reactant supply streams to failed cells may be selectively terminated outwith the hot zone, by constriction of the polymer (or metallic) tubing linking the manifold reactant supply lines to the interconnect tubes.

4. A stacked arrangement of metallic interconnects as claimed in claim 2 where the reactant supply streams may be adjusted to optimise the electrical characteristics of each individual cell comprising the stack, by controlled constriction of the polymer tubing.

5. A stacked arrangement of metallic interconnects as claimed in 1, from which the electrical power generated by the cells in contact with each of the interconnects, can be removed from the hot zone through its thermal insulation, to ambient temperatures, along the said tubes, with final electrical connections being made to the cool end of these tubes.

6. A stacked arrangement of metallic interconnects as claimed in claim 5, each of whose tubes (at ambient temperature) may be electrically joined with the tubes (at ambient temperature) of another (generally adjacent) interconnect in the stack, for the purpose of 'shorting out' a faulty cell(s) contained between them, to allow continued operation of the stack.

7. A stack of cells, using the interconnects as claimed in claim 1 , where uniform reactant distribution across the faces of the cells is achieved by incorporating a fugitive mesh in the electrically conductive compliant 'cements' used to join cell to interconnect.

8. Interconnects as claimed in claim 1, whose (sheet) material of construction is preferably UNS S44400, to withstand stack operating conditions to 850 degrees Celsius.

9. Interconnects as claimed in claim 1, in which a porous bed of reforming agent may be incorporated in the interconnect tubes carrying the fuel gases.