WO2009149848A1

WO2009149848A1 - Leak proof fuel cell stack

Info

Publication number: WO2009149848A1
Application number: PCT/EP2009/003947
Authority: WO
Inventors: Niels Erikstrup
Original assignee: Haldor Topsøe A/S
Priority date: 2008-06-13
Filing date: 2009-06-03
Publication date: 2009-12-17

Abstract

Interconnect for solid oxide fuel cells comprising an oxidant side and opposite the oxidant side a fuel side, in which the oxidant side contains a plurality of oxidant gas channels and the fuel side contains a plurality of fuel gas channels. The fuel side is further provided with at least one purge channel within which oxidant flows and into which leaking fuel gas is mixed.

Description

Title: Leak Proof Fuel Cell Stack

This invention relates to a novel interconnect in which un- desired leak of fuel to the surroundings is prevented by means of channels provided therein. In particular, the invention relates to Solid Oxide Fuel Cell stacks (SOFC) comprising such interconnects in which fuel leaking from the fuel side of the interconnect is trapped in channels ar- ranged in said fuel side, where it is mixed, combusted and removed with oxidant gas.

A Solid Oxide Fuel Cell (SOFC) comprises a solid electrolyte that enables the conduction of oxygen ions, a cathode where oxygen is reduced to oxygen ions and an anode where hydrogen is oxidised. The overall reaction in a SOFC is that hydrogen and oxygen electrochemically react to produce electricity, heat and water. In order to produce the required hydrogen, the anode normally possesses catalytic ac- tivity for the steam reforming of hydrocarbons, particularly natural gas, whereby hydrogen, carbon dioxide and carbon monoxide are generated. Steam reforming of methane, the main component of natural gas, can be described by the following equations:

CH₄ + H₂O -► CO + 3H₂ CH₄ + CO₂ -► 2CO + 2H₂ CO + H₂O →- CO₂ + H₂

During operation an oxidant such as air is supplied to the solid oxide fuel cell in the cathode region. Fuel such as hydrogen is supplied in the anode region of the fuel cell. Alternatively, a hydrocarbon fuel such as methane is supplied in the anode region, where it is converted to hydrogen and carbon oxides by the above reactions. Hydrogen passes through the porous anode and reacts at the anode/- electrolyte interface with oxygen ions generated on the cathode side that have diffused through the electrolyte. Oxygen ions are created in the cathode side as a result of the acceptance of electrons from the external electrical circuit of the cell.

To increase voltage, several cell units are assembled to form a stack and are linked together by interconnects. Interconnects serve to separate the anode and fuel sides of adjacent cell units and at the same time enable current conduction between the adjacent cells. Interconnects are normally provided with a plurality of channels for the passage of fuel gas on one side of the interconnect and oxidant gas on the opposite side. As fuel gas passes through the channels on one side of the interconnect it may, due to diffusion and pressure gradients, tend to escape to the surroundings of the cell stack. This is a circumstance that conveys a significant safety hazard as fuel gas is put in contact with surrounding air in the immediate vicinity of the stack. This problem may be alleviated by providing gas- tight seals along the edges of the fuel side of the interconnect, but the incorporation of such seals is not trivial and the seals may show defects during operation which make them incapable of fully restricting fuel leakage.

Methods for eliminating risks associated with fuel leakage from fuel cells such as fire or explosion rely on the simple principle of dilution. A fuel cell stack and accompany- ing manifolds for fuel and oxidant inlets and outlets are provided within a common case through which dilution air is passed. Any fuel gas leaking from the stack is thereby immediately diluted by the larger flow of dilution air pass- ing the stack surfaces. Such systems are, however, bulky and thereby inexpedient.

US 7,285,351 describes an apparatus for dilution of discharged fuel from a fuel cell stack, whereby purged hydro- gen gas is subjected to a dilution process before it is discharged into the open air. The dilution is conducted in an apparatus in the form of a box-like container that encompasses a reservoir for storage of the incoming hydrogen gas from the anode of the cell. The hydrogen is diluted with cathode exhaust gas by means of a cathode pipe line penetrating the reservoir.

Our EP-A-I, 447, 869 describes an interconnect provided with a channel system including so-called collection channels for the distribution of fuel gas over the entire surface of the cell and thereby creating a system of many small electrochemical cells on one fuel cell. In this manner thermal gradients are reduced. The problems associated with fuel leakage from the prescribed path in the fuel side is solved by firmly sealing along the edges of the interconnect.

It is an object of the invention to provide a fuel cell stack with no emission of unburnt fuel through the outer surfaces of the stack.

It is another object of the invention to provide a fuel cell stack which is simple and compact whilst at the same time being able to reduce the risk of fire or explosion due to uncontrolled fuel emission from the stack.

It is further an object of the invention to provide a fuel cell comprising interconnects with reduced dependency on the quality of fuel gas seals.

These and other objects are achieved by the invention as described below.

Accordingly, we provide an interconnect for solid oxide fuel cells comprising an oxidant side and opposite the oxidant side a fuel side, in which the oxidant side contains a plurality of oxidant gas channels and the fuel side con- tains a plurality of fuel gas channels, the oxidant and fuel gas channels are open at both ends and have an inlet side for the passage of gas from an inlet manifold at one open end and an outlet side for the passage of gas to an outlet manifold at the other open end, wherein the fuel side is further provided with at least one purge channel within which oxidant flows and into which leaking fuel gas is mixed, said at least one purge channel is in the form of an elongated groove open at both ends that at least extends along one perimeter edge of the fuel side of the intercon- nect, and in which said at least one purge channel has an inlet side for the passage of oxidant gas from oxidant gas inlet manifold at one open end and an outlet side for the passage of gas to outlet oxidant gas manifold at the other open end.

It would be understood that the fuel side and oxidant side of the interconnect correspond, respectively, to the cell anode and cathode side. Thus, one face of the interconnect defines the oxidant side and the opposite face defines the fuel side.

According to the invention only oxidant is provided on the oxidant side of the interconnect, while oxidant and fuel are provided on the fuel side of the interconnect.

Any fuel leaking from the fuel side of the interconnect is trapped in the purge channel and thereby entrained in the oxidant stream flowing through this channel rather than being emitted to the surroundings of the stack. At the temperatures prevailing in the stack, in which oxidant gas such as air normally enters the stack at temperatures above 600⁰C and leaves at temperatures of 800⁰C or higher, the fuel combusts spontaneously in the purge channel and the resulting gas is carried away through this channel towards the outlet oxidant gas manifold.

Seen in cross section the purge and oxidant gas channels may have any shape such as trapezoid, semicircular or rectangular shape. The depth and width of the elongated groove serving as purge channel is preferably in the range 0.1 to 10 mm.

The interconnect has preferably a rectangular or square planar geometry. Such a geometry defines four perimeter edges in the interconnect. On the surface of these perimeter edges a fuel gas seal is conventionally provided. It would be understood that although the geometry is said to be rectangular or square, such geometries involve instances in which the corners of the interconnect are rounded or bend at other angles than 90°, for instance at 45°, thus forcing the interconnect to slightly deviate from a strict rectangular or square planar geometry.

In a particular embodiment the interconnect has rectangular or square planar geometry and at least one purge channel extends along all perimeter edges of the fuel side of the interconnect. This embodiment may correspond to an interconnect with internal fuel and oxidant manifold design in which oxidant gas is passed through the purge channel arranged along the four perimeter edges of a square or rectangular interconnect. Any leaking fuel leaving the fuel gas channels and travelling towards any perimeter edge of the interconnect enters the purge channel and is mixed with the oxidant gas running therein.

Alternatively, instead of a single purge channel in the form of one continuous elongated groove around the periphery, the interconnect may be provided with discrete or separate purge channels around the periphery which are in fluid communication with each other.

In another embodiment of the invention, the interconnect has rectangular or square planar geometry and said at least one purge channel extends from one perimeter edge of the fuel side of the interconnect to the opposite perimeter edge of the fuel side of the interconnect, and optionally it runs in the same direction as the oxidant flowing through the oxidant gas channels on the oxidant side of the interconnect. This embodiment corresponds to an interconnect with an external air manifold design. Thus, the at least one purge channel runs along the whole width or length of one or two perimeter edges of the fuel side of the interconnect.

In the external manifold design, the extension direction of the two perimeter edges of the fuel side, which also corresponds to two perimeter edges of the interconnect, is at right angle with respect to the perimeter edges along which oxidant flows in and out of the stack. Any fuel that leaks towards the perimeter edges along which oxidant flows in and out is burnt by the oxidant gas, such as air, in the inlet or outlet oxidant manifold. A perimeter edge (side length) of the interconnect having a rectangular planar geometry is preferably in the range 60-500 mm. A square or rectangular planar geometry with two purge channels running each at opposite perimeter edge of the fuel side is preferred. This enables cheap and fast interconnect construction since the required channel pattern is simple and straightforward to fabricate. Thus, the fuel cell stack becomes safer and much simpler than prior art fuel cell sys- terns. Outer (external) containers for external dilution of emitted fuel become completely unnecessary.

As with the internal manifold design, it is also possible to provide several separate purge channels in fluid commu- nication with each other along the one or two perimeter edges of the interconnect, i.e. along the whole width or length of the two perimeter edges.

In a preferred embodiment of the invention, a fuel gas seal is provided along the perimeter edges of the fuel side of the interconnect. This combination of fuel gas seal and purge channel is used to further increase safety. Hence, independent of the geometry of the interconnect, if a fuel seal is provided and there still are defects in the seal at the edges of the fuel side, the escaping fuel is not emitted to the surroundings, but instead flows into the purge channel being burnt and flushed with oxidant gas.

The interconnect of the invention may then be connected to the anode side of a cell on its fuel side and to the cathode side of a cell on its oxidant side, and be assembled into a solid oxide fuel cell stack. Accordingly, the invention encompasses solid oxide fuel cells as set forth in claim 5. The invention encompasses also a solid oxide fuel cell stack comprising at least two of such solid oxide fuel cells as recited in claim 6.

It would be appreciated that the interconnect of the present invention is for use in solid oxide fuel cells. In this type of cells the problems associated with fuel leakage due to poor fuel sealing are normally encountered and the operating temperatures of the fuel cell stack are 600⁰C or higher. The leaking fuel is normally hydrogen, but also methane or carbon monoxide may be present on the anode side and can potentially leak.

The main advantages of the interconnects prepared according to the invention may be summarised as follows:

- Increased safety. Risk of leakage of unburnt fuel and explosion is eliminated.

- Expensive and bulky air diluting container outside the fuel cell stack is no longer needed.

- Less dependency on expensive gas-tight fuel seals along the edges of the interconnect. - Simple construction. Easy to fabricate. Reduced costs.

- No need for a separate fan or blower for purge air. The purge air is provided by the process air blower adapted to the air manifold.

- The purge air is supplied at the very surface of the stack, whereby the amount of purge air is smaller than when purging a bulky container.

The invention is further illustrated by the accompanying drawings .

Fig. Ia shows a perspective view of the interconnect with an external manifold design and seen from above the fuel side of the interconnect.

Fig. Ib shows details of the purge channel of Fig. Ia.

Fig. 2 shows a plan view of the oxidant side of the inter- connect of Fig. 1.

Fig. 3a shows a perspective view of the interconnect with an internal manifold design and seen from above the fuel side of the interconnect.

Fig. 3b shows details of the purge channel of Fig. 3a.

Fig. 4 shows a plan view of the oxidant side of the interconnect of Fig. 3. Fig. Ia shows interconnect 100 in the form of a rectangular plate defining four interconnect perimeter edges. The interconnect has an oxidant side 102 on one face of the interconnect and fuel side 103 on the opposite face. Oxidant gas as depicted by arrows 104 such as air from inlet oxidant manifold (not shown) is introduced to oxidant gas channels 105 which are open at both ends and with inlet sides arranged along one perimeter edge 106 of the interconnect 100. The oxidant gas leaves as gas flow 107 at the outlet sides of the same channels 105 at the opposite edge 108 of the interconnect. Fuel enters through aperture 109 near one perimeter edge 108 and is directed to fuel gas channels 110 arranged on the fuel side 103. The fuel gas channels 110 provide for an overall co-current or counter- current flow with respect to the oxidant flow underneath. The fuel flows on the fuel side towards a second aperture 111 located at the opposite end near perimeter edge 106 and in fluid communication with fuel gas outlet manifold (not shown) . On the fuel side 103 of the interconnect purge channels 112 are arranged along the remaining perimeter edges 113, 114. These perimeter edges extend at a substantially right angle with respect to the perimeter edges 106, 108 along which oxidant gas channels 105 are arranged. The purge channels 112 are in the form of elongated grooves and have an inlet side for the passage of oxidant gas from oxidant gas inlet manifold at one open end 115 and outlet side for the passage of gas to outlet oxidant gas manifold at the other open end 116. The purge channels 112 run along the whole length of the two opposite perimeter edges 113, 114 of the fuel side and in the same direction as that of the oxidant gas flow 104 passing through oxidant channels 105. Fig. Ib shows an expanded view of purge channel 112 being supplied with oxidant gas 104 at open end 115. Along the perimeter edge 113 any fuel gas 117 that escapes from the flow path dictated by fuel gas channels 110 is burnt by the oxidant flow 104 running through purge channel 112 instead of leaking to the surroundings.

The depth of the purge channel 112 is such that sufficient oxidant gas 104 is able to flow through the oxidant gas channels 105 and burnout any leaking fuel 117 escaping from the fuel channels 110. On perimeter edge surfaces 113, 114 of the fuel side a conventional fuel gas seal is provided.

The oxidant side 102 of the interconnect in this external manifold design is shown in Fig. 2, where oxidant gas flow 104 is introduced to the interconnect via oxidant channels 105. Oxidant gas leaves as flow 107 at the opposite end, while fuel gas is introduced through aperture 109 and with- drawn through aperture 111. The purge channels are provided on the fuel side 103 of the interconnect device and are therefore not seen in this plan view. A co or counter-flow configuration can be made by switching either the fuel or the oxidant inlets and outlets.

Fig. 3a shows interconnect 301 in the form of a rectangular plate defining four interconnect perimeter edges. The interconnect has an oxidant side 302 on one face of the interconnect and fuel side 303 on the opposite face. Oxidant gas as depicted by arrows 304 such as air is introduced at perimeter edge 307 through aperture 305 to purge channel 306. The purge channel 306 is in the form of an elongated groove arranged along the perimeter edges 307, 308, 309 and 310 of the interconnect 301. The purge channel 306 is shown here as a single channel forming 90° bends at the four corners of the interconnect. As described before there are also instances in which the corners of the interconnect may be rounded or bend at other angles than 90°.

The oxidant gas running through the purge channels leaves at the opposite perimeter edge 309 through aperture 311. Fuel enters through apertures 312 at one perimeter edge 309 of the interconnect and is directed to fuel gas channels 313 arranged on the fuel side 303. The fuel gas channels 313 provide for an overall co-current or counter-current flow with respect to the oxidant flow on the opposite face of the interconnect (Fig. 4) . The fuel flows towards second apertures 314 located at the opposite end along perimeter edge 307 and in fluid communication with fuel gas outlet manifold (not shown) .

Fig. 3b shows an expanded view of purge channel 306. Along the perimeter edge 310 any fuel gas 315 that escapes from the flow path dictated by fuel gas channels 313 is burnt by the oxidant flow 304 passing through purge channel 306 instead of leaking to the surroundings.

The oxidant side 302 of the interconnect in this internal manifold design is shown in Fig. 4, where oxidant gas flow 304 is introduced to the interconnect at apertures 316 and thereby passed through oxidant channels 318 arranged in the interconnect. Oxidant gas enters at apertures 316 adapted to inlet oxidant manifold as well as through aperture 311 adapted to the open end of purge channel 306. The oxidant gas leaves at the opposite end at apertures 317 adapted to outlet oxidant manifold as well as via oxidant aperture 305. Fuel gas is for a co-flow configuration introduced through apertures 312 and withdrawn through apertures 314. The purge channels 306 are provided on the fuel side 303 of the interconnect and are therefore not seen in this plan view.

Claims

1. Interconnect for solid oxide fuel cells comprising an oxidant side and opposite the oxidant side a fuel side, in which the oxidant side contains a plurality of oxidant gas channels and the fuel side contains a plurality of fuel gas channels, the oxidant and fuel gas channels are open at both ends and have an inlet side for the passage of gas from an inlet manifold at one open end and an outlet side for the passage of gas to an outlet manifold at the other open end, wherein the fuel side is further provided with at least one purge channel within which oxidant flows and into which leaking fuel gas is mixed, said at least one purge channel is in the form of an elongated groove open at both ends that at least extends along one perimeter edge of the fuel side of the interconnect, and in which said at least one purge channel has an inlet side for the passage of oxidant gas from oxidant gas inlet manifold at one open end and an outlet side for the passage of gas to outlet oxidant gas manifold at the other open end.

2. Interconnect according to claim 1, wherein the interconnect has rectangular or square planar geometry and the at least one purge channel extends along all perimeter edges of the fuel side of the interconnect.

3. Interconnect according to claim 1, wherein the interconnect has rectangular or square planar geometry and the at least one purge channel extends from one perimeter edge of the fuel side of the interconnect to the opposite perimeter edge of the fuel side of the interconnect.

4. Interconnect according to any of claims 1 to 3, wherein a fuel gas seal is provided along the perimeter edges of the fuel side.

5. Solid oxide fuel cell stack comprising an interconnect according to any of claims 1 to 4.

6. Solid oxide fuel cell stack comprising at least two solid oxide fuel cells according to claim 5.