CURRENT DISTRIBUTORS OF SINTERED METALS AND FUEL CELLS USING THEM

Abstract: Current distributors comprising sintered metals and fuel cells employing such current distributors (14, 18, 24).

Description: CURRENT DISTRIBUTORS OF SINTERED METALS AND FUEL CELLS USING THEM This invention relates to fuel cells, in particular to current distributors comprising sintered metals, and to fuel cells employing such current distribution. Fuel cells produce electrical energy by the reaction of fuel and oxidant. In recent years, solid polymer fuel cells (SPFCs) have become a commercially viable proposition due to a number of fundamental scientific advances in the field. SPFCs usually employ hydrogen as a fuel gas and oxygen as the oxidant. An SPFC typically comprises a solid polymer electrolyte or ion exchange membrane having platinum catalyst located thereon. The polymer electrolyte or ion exchange membrane is interposed between two sheet electrodes, typically of a porous carbon composition. The catalyst may be located on these electrodes. The combination of polymer electrolyte or ion membrane and electrodes is generally referred to as the membrane electrode assembly (MEA). The MEA is conventionally sandwiched between a pair of current distributors. The current distributors perform a number of roles, acting as current collectors, structural support, and providing passages for conveying fuel and oxidant to the MEA and removing water formed during the oxidation process. Commercial fuel cells to date have been of the prismatic design, wherein multiple fuel cells are stacked, usually in series. In this arrangement, the current distributors act as "bipolar plates" : a single current distributor is in contact with two MEAS, one face contacting an anode side of a first MEA, the other face contacting a cathode side of a second MEA. However, prismatic bipolar stacking arrangements have a number of significant limitations : a) the bipolar plate must, for obvious reasons, be impermeable to hydrogen, thereby restricting the choice of materials and the manufacturing route. b) as stacking density increases, integral coding requirements increase. The result is additional weight and peripheral complexity. c) operation on air requires high volume flow rates which, with thin plates and tortuous flow paths, leads to high pressure drops and significant parasitic losses. Second generation SPFCs must address these issues and provide products which are reliable, and exhibit competitive performance and lifetime at an appropriate cost. In particular, it is desirable that emphasis is placed upon : i) low component cost; ii) high volume manufacturing techniques; iii) operation on ambient pressure air; iv) high systems voltages; and v) low system weight. The present invention provides materials and fuel cell configurations which address the abovementioned considerations. According to the first aspect of the invention there is provided a current distributor comprising a sintered metal for use in a fuel cell. The current distributor may be sufficiently porous that i) oxidant and/or fuel gases may permeate from a first face of the distributor to a second face thereof; and ii) water generated in the fuel cell can permeate from the second face of the distributor to the first face thereof. The topography of the current distributor may be defined during the sintering process. The topography of the distributor may consist of holes, hills, ridges, or combinations thereof. The current distributor may comprise sintered stainless steel powder or mesh, which may comprise sintered 316 stainless steel powder or mesh. According to a second aspect of the invention there is provided a fuel cell arrangement comprising at least one current distributor according to the first aspect of the invention. The arrangement may be a back to back arrangement comprising : a fuel gas manifold having a fuel gas inlet and outlet, and a fuel gas distribution assembly connecting said inlet to said outlet; a first sintered metal anode current distributor cooperating with a first face of the manifold; a first MEA cooperating with the first sintered metal anode current distributor; a first sintered metal cathode current distributor cooperating with the first MEA; a second sintered metal anode current distributor cooperating with a second face of the manifold; a second MEA cooperating with the second sintered metal anode current distributor; and a second sintered metal cathode current distributor cooperating with the second MEA. Alternatively, a fuel cell may comprise: an anode assembly; a membrane electrode assembly cooperating with said anode assembly; and a sintered metal cathode current distributor cooperating with said membrane electrode assembly; the anode assembly having a fuel gas inlet and outlet, a fuel gas distribution assembly connecting said inlet to said outlet, and a recess, defined by one or more side walls, in which the membrane electrode assembly and cathode current distributor are disposed, the edge portions of the side wall or walls being inwardly folded to retain said membrane electrode assembly and cathode current distributor in the recess. Current distributors and fuel cells in accordance with the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is an exploded view of a back-to-back fuel cell arrangement; Figure 2 is a graph of potential vs current for six back-to-back fuel cell arrangements connected in series; Figure 3 shows power output as a function of time for a single back- to-back fuel cell arrangement; Figure 4 shows a) components of a circular fuel call; and b) a cross sectional view of the circular fuel cell; and Figure 5 shows a) a first embodiment, and b) a second embodiment of a square fuel cell. The invention comprises a current distributor comprising a sintered metal for use in a fuel cell. Sintered metals are continuous metal structures produced from powdered metals or other forms of metals by appropriate treatment at elevated temperatures and pressures. Sintered metal current distributors exhibit a number of advantageous properties, such as high mechanical strength, ductility, good heat and electrical conductivity, corrosion resistance at low cost. Of particular relevance to the present invention is ability to produce a material of defined porosity and controllable permeability. More specifically, sintered metal current distributors have been produced which are sufficiently porous that : i) oxidant and/or fuel gases can permeate from a first face of the distributor to a second face thereof, and ii) water generated in the fuel cell can permeate from the second face of the distributor to the first face thereof. In this way, an efficiently self-regulating fuel cell, operable, if desired, on ambient air may be produced. Another important aspect of the present invention is that the topography of the sintered metal current distributor can be defined during the sintering process. In other words, topographical features such as holes, hills, ridges or combinations thereof may be produced during the sintering process. Such features may be incorporated, for example, to provide gas inlets and outlets, and gas feed manifolds. In this way, costly and time consuming machining of the current distributor is either reduced or completely eliminated. There are numerous commercially available sintered metals, and the invention is not limited in this regard. Sintered metals fabricated from stainless steel powder or meshes are preferred; an example which has proven extremely useful is sintered metal fabricated from water quenched AISI 316 stainless steel powder (GKN Ltd, Lichfield, UK). Sintered meshes are also available from this manufacturer. However, the optimal combination of metal and sintering conditions is dependent upon factors such as the precise operational characteristics of the fuel cell in question and the nature of the MEA employed. Sintered metal mesh may be used in the manufacture of current distributors, and it is an advantage of such an approach that the pore size may be precisely controlled. Furthermore, adjacent bonding of layers of differing mesh sizes can produce abrupt or gradual changes in pore size and structure. Figure 1 shows a first embodiment of a fuel cell arrangement 10 comprising at least one current distributor of the present invention. The arrangement 10 is a back to back one comprising : a fuel gas manifold 12 having a fuel gas inlet 12a and outlet 12b, and a fuel gas distribution assembly 12c connecting said inlet 12a to said outlet 12b; a first sintered metal anode current distributor 14 cooperating with a first face of the manifold 12; a first MEA 16 cooperating with the first sintered metal anode current distributor 14; a first sintered metal cathode current distributor 18 cooperating with the first MEA 16; a second sintered metal anode current distributor 20 cooperating with a second face of the manifold 12; a second MEA 22 cooperating with the second sintered metal anode current distributor 20; and a second sintered metal cathode current distributor 24 cooperating with the second MEA 22. The first and second MEAs 16, 22 comprise carbon cloth electrodes having platinum catalyst, thermally bonded to a Nafion ® 112 membrane. The sintered metal current distributors 14, 18,20, 24 comprise sintered 316 stainless steel powder. The cell arrangement operates on ambient air, which is introduced to the MEAs 16, 22 principally through a plurality of relatively large pores/holes 18a, 24a located in the first and second sintered metal cathode current distributors 18, 24. These relatively large pores/holes 18a, 24a may be formed by drilling or, preferably, in the initial manufacturing step as described earlier. Pore size can range between 10 μm and 1 mm or greater. The use of a mesh is also feasible. The hydrogen gas enters the fuel cell arrangement 10 via fuel gas inlet 12a and exits via fuel gas outlet 12b. The sintered metal current distributors 14, 18, 20, 24 also possess a multitude of micropores (produced by the sintering process). The microporous nature of these sintered components leads to a capillary attraction into the bulk, which enables efficient removal of moisture, primarily on the cathode side of the MEAs 16, 22, to be achieved. In this regard, it is advantageous that the face of the cathode current distributors which moisture is transported towards is open to atmosphere, since this results in a continuous transpiratory pull. Thus water may be continuously extracted at a rate which is regulated by both thermal and convective influences on evaporation. It will be apparent from the foregoing discussion that the term "porous" can encompass relatively large pores which might be introduced after the initial sintering process has been completed. These relatively large pores have been found to enhance the transport of air to the MEA. However, it should be noted that the capillary action which results in water removal from the MEA is due to the presence of the micropores, which are formed in the sintering process. In fact, the back-to-back fuel cell arrangement will operate if the relatively large pores are not present, i.e. if air is transported to the MEA solely through the micropores. However, this arrangement is not an optimal one since slow flooding of the sintered metal matrix results in eventual oxygen starvation. The optimum range of pores sizes and pore distribution is likely to be device and application specific. For example, the MEA employed is likely to influence the choice of sintered metal employed, since different membranes exhibit different water capacities, affinities and electro osmotic drag characteristics, and different gas diffusion electrodes have different water rejection capabilities. Further factors include the power output of the cell and the air flow into the cell. At present, a mean current of 2A at 1.4V per back-to-back fuel cell arrangement has been achieved, figures which are limited by thermal rather than activity considerations. Figures 2 and 3 show the results of tests on fuel cell arrangements of the above description. Figure 2 shows output potential vs current for six back-to-back fuel cell arrangements connected in series. Hydrogen at 1 bar pressure is used in conjunction with naturally convected air. A platinum dispersion of 2mg cm '2 is employed. Figure 3 shows the results of an (ongoing) long term test of the power output of a single ambient air "breathing" back-to-back fuel cell arrangement. Hydrogen at 1 bar pressure is used, and the cell temperature is maintained at 35°C. Excellent reproducibility is exhibited over a period in excess of 80 days continuous operation. The flat, back-to-back arrangement of Figure 1 maximises the available volume for cathode air passage. Furthermore, by providing a low current and high voltage, inter-cell electrical conductors of small cross-section can be used. It should be noted that gas permeable sintered metal current distributors are not suitable, for obvious reasons, to act as traditional bipolar plates. However, it is possible to produce higher density sintered metal current distributors, which are not permeable to oxidant and fuel gases, and therefore are suitable for use as bipolar plates. Such bipolar plates do not possess the ability to remove moisture. However, the considerable advantage of being able to define the size and topography of the current distributor during the forming process (the sintering) is retained. Figures 4 and 5 show further embodiments of fuel cells incorporating sintered metal current distributors. Figure 4b shows a fuel cell comprising: an anode assembly 40; a membrane electrode assembly 42 cooperating with said anode assembly 40; and a sintered metal cathode current distributor 44 cooperating with said membrane electrode assembly 42; the anode assembly 40 having a fuel gas inlet 46 and outlet 48, a fuel gas distribution assembly 50 connecting said inlet 46 to said outlet 48, and a recess 52, defined by one or more side walls 54, in which the membrane electrode assembly 42 and cathode current distributor 44 are disposed, the edge portions 54a of the side wall or walls 54 being inwardly folded to retain said membrane electrode assembly 42 and cathode current distributor 44 in the recess 52. Insulating means, such as an insulating gasket 56, is disposed between the inwardly folded edge portions 54a and the cathode current distributor 44. This fuel cell is inexpensive and convenient to manufacture. The anode assembly 40 is conveniently manufactured in stainless steel, although other conductive materials would suggest themselves to the skilled person. The fuel gas distribution assembly 50, as shown in Figure 4, comprises a serpentine fuel distribution channel conveniently formed in the stainless steel back face of the anode assembly. Other types of fuel gas distribution assemblies are within the scope of the invention. For example, a channel might allow a "zig zag" path rather than a serpentine path. Furthermore, the anode assembly 40 need not necessarily comprise a single, integral unit : rather, the fuel distribution channel might be formed in a separate anode assembly plate. It is possible to employ conductive carbon electrode layers between the membrane electrode assembly and either or both of the anode assembly and the cathode current distributor. Such electrode layers are sometimes referred to as macrodiffusion layers, and might comprise woven carbon cloth or carbon paper. The cathode current distributor 44 is formed from a sintered metal which, as described previously, may comprise sintered powder or mesh. A further advantage of the fuel cell arrangements of the type shown in Figure 4 is that they are conveniently stacked together. By providing a stacking arrangement, a plurality of standard fuel cells can be used to provide a desired voltage output. A suitable stack holder can be provided to retain the fuel cells. The fuel cells in the stacking arrangement may be conveniently separated by gaskets, electrical connection between adjacent cells being made via simple electrodes which are conveniently retained between the folded edge portions of the anode assembly side wall and the cathode current distributor. Figures 5a and 5b illustrate another embodiment of a fuel cell suitable for use in a stacking arrangement. This embodiment may be employed, for example, when the fuel cell is of square or rectangular cross-section (when the use of separating gaskets is perhaps less convenient). In Figure 5a, two fuel cell arrangements 60, 68 are shown, each arrangement 60, 68 having an anode current distributor 62, 70 a membrane electrode assembly 64, 72, and a sintered metal cathode current distributor 66, 74. The sintered metal cathode current distributors 66, 74 are of the type described previously which allows oxidant gases and generated water to permeate through its structure. The anode current distributors 62, 70 are formed from a conductive material which is not permeable in this way, such as stainless steel or a high density sintered metal. Furthermore, the anode current distributor 62, 70 have a series of crenellations formed on one face thereof. For convenience, the fuel gas distribution system is not shown in Figure 5. However, it is understood that many such systems, which might involve the use of serpentive or zig-zag feed channels formed on the surface of the anode current distributors 62, 70, might be employed. The use of conductive carbon electrode layers, as previously described, positioned between the membrane electrodes assemblies and the current collectors, is also within the scope of the invention. The fuel cells 60, 68 shown in Figure 5a may be conveniently stacked together, the crennelations of the anode current distributor 70 of one fuel cell 68 abutting the cathode current distributor 66 of the adjacent fuel cell 60 to form channels along which generated water may be removed. It should be noted that the crennelations need not be of rectangular cross section : other cross sectional configurations, such as semicircular or 'V shaped, may be employed. Figure 5b shows an alternative embodiment of a fuel cell 76 suitable for stacking. In identical fashion to Figure 5a, the fuel cell 76 comprises an impermeable anode current distributor 78, a membrane electrode assembly 80 and a sintered metal cathode current distributor 82. In this embodiment, the cathode current distributor 82 comprises a series of crenellations, which, when fuel cells are stacked, abut the anode current distributors of adjacent fuel cells, thereby providing water removal channels.

Claims: CLAIMS 1. A current distributor comprising a sintered metal for use in a fuel cell. 2. A current distributor according to claim 1 sufficiently porous that i) oxidant and/or fuel gases may permeate from a first side of the distributor to a second face thereof; and ii) water generated in the fuel cell can permeate from the second face of the distributor to the first face thereof. 3. A current distributor according to claim 1 or claim 2 in which the topography of the current distributor is defined during the sintering process. 4. A current distributor according to claim 3 in which the topography consists of holes, hills, ridges or combinations thereof. 5. A current distributor according to any previous claims comprising sintered stainless steel powder or mesh. 6. A current distributor according to claim 5 comprising sintered 316 stainless steel powder or mesh 7. A fuel cell arrangement comprising at least one current distributor according to any of claims 1 to 6. 8. A back-to-back fuel cell arrangement according to claim 7 comprising: a fuel gas manifold having a fuel gas inlet and outlet, and a fuel gas distribution assembly connecting said inlet to said outlet; a first sintered metal anode current distributor cooperating with a first face of the manifold; a first membrane electrode assembly cooperating with the first sintered metal anode current distributor; a first sintered metal cathode current distributor cooperating with the first membrane electrode assembly; a second sintered metal anode current distributor cooperating with a second face of the manifold; a second membrane electrode assembly cooperating with the second sintered metal anode current distributor; and a second sintered metal cathode current distributor cooperating with the second membrane electrode assembly. 9. A fuel cell according to claim 7 comprising : an anode assembly; a membrane electrode assembly cooperating with said anode assembly; and a sintered metal cathode current distributor cooperating with said membrane electrode assembly; the anode assembly having a fuel gas inlet and outlet, a fuel gas distribution assembly connecting said inlet to said outlet, and a recess, defined by one or more side walls, in which the membrane electrode assembly and cathode current distributor are disposed, the edge portions of the side wall or walls being inwardly folded to retain said membrane electrode assembly and cathode current distributor in the recess.

Inventors: MITCHELL PHILIP JOHN, MITCHELL, PHILIP, JOHN
Applicants: MITCHELL PHILIP JOHN, UNIV LOUGHBOROUGH, LOUGHBOROUGH UNIVERSITY INNOVATIONS LIMITED, MITCHELL, PHILIP, JOHN
Application year: 1998
Main classification: H01M8
IPC codes: H01M4, H01M8