Brazed bipolar plates for PEM fuel cells

#This invention relates to PEM fuel cell stacks, and more particularly to bipolar plates for separating one cell from the next in such stacks.

#Fuel cells have been proposed as a power source for electric vehicles, and other devices. One such fuel cell is the PEM (i.e., proton exchange membrane) fuel cell as it has potentially high energy, as well as low weight and volume. PEM fuel cells are well known in the art and include in each cell thereof a so-called "membrane-electrode-assembly" comprising a thin, proton-conductive, polymer, membrane-electrolyte having an anode electrode film formed on one face thereof and a cathode electrode film formed on the opposite face thereof. Such membrane-electrolytes are well known in the art and are described in such U.S. Pat. No. as 5,272,017 and 3,134,697, as well as in the Journal of Power Sources, Volume 29 (1990) pages 367-387, inter alia. In general, such membrane-electrolytes are made from ion exchange resins, and typically comprise a perflourinated sulfonic acid polymer such as NAFION.TM. available from the E.I. DuPont de Nemours & Co. The anode and cathode films, on the other hand, typically comprise (1) finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton-conductive material (e.g., NAFION.TM.) intermingled with the catalytic and carbon particles, or (2) catalytic particles dispersed throughout a polytetrafluroethylene (PTFE) binder. One such membrane-electrode-assembly and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 and assigned to the assignee of the present invention.

#The membrane-electrode-assembly (hereafter MEA) for each cell is sandwiched between a pair of electrically conductive elements which serve as current collectors for the anode/cathode and contain an array of grooves in the faces thereof for distributing the fuel cell's gaseous reactants (e.g., H.sub.2 and O.sub.2 /air) over the surfaces of the respective anode and cathode. In a fuel cell stack, a plurality of the cells are stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive, bipolar plate referred to as a bipolar plate. The bipolar plate serves as an electrically conductive separator element between two adjacent cells, and (1) has reactant gas distributing grooves on both external faces thereof, (2) conducts electrical current between the anode of one cell and the cathode of the next adjacent cell in the stack, and (3) in most cases, has internal passages therein which are defined by internal heat exchange faces and through which coolant flows to remove heat from the stack.

#The invention will better be understood when considered in the light of the following detailed description of certain specific embodiments thereof which is given hereafter in conjunction with the several figures in which:

#FIG. 1 is a schematic, isometric, exploded illustration of a liquid-cooled PEM fuel cell stack;

#FIG. 2 is an isometric, exploded view of a bipolar plate useful with PEM fuel cell stacks like that illustrated in FIG. 1;

#FIG. 1 depicts a two cell, bipolar PEM fuel cell stack having a pair of membrane-electrode-assemblies (MEAs) 4 and 6 separated from each other by an electrically conductive, liquid-cooled, bipolar plate 8. The MEAs 4 and 6, and bipolar plate 8, are stacked together between stainless steel clamping plates 10 and 12, and end contact elements 14 and 16. The end contact elements 14 and 16, as well as the bipolar plate 8, contain a plurality of grooves and openings 18, 20, 22, and 24 for distributing fuel and oxidant gases (i.e., H.sub.2 & O.sub.2) to the MEAs 4 and 6. Nonconductive gaskets 26, 28, 30, and 32 provide seals and electrical insulation between the several components of the fuel cell stack. Gas permeable carbon/graphite diffusion papers 34, 36, 38 and 40 press up against the electrode faces of the MEAs 4 and 6. The end contact elements 14 and 16 press up against the carbon/graphite papers 34 and 40 respectively, while the bipolar plate 8 presses up against the carbon/graphite paper 36 on the anode face of MEA 4, and against carbon/graphite paper 38 on the cathode face of MEA 6. Oxygen is supplied to the cathode side of the fuel cell stack from storage tank 46 via appropriate supply plumbing 42, while hydrogen is supplied to the anode side of the fuel cell from storage tank 48, via appropriate supply plumbing 44. Alternatively, air may be supplied to the cathode side from the ambient and hydrogen to the anode from a methanol reformer or the like. Exhaust plumbing (not shown) for both the H.sub.2 and O.sub.2 /air sides of the MEAs will also be provided. Additional plumbing 50, 52 and 54 is provided for supplying liquid coolant to the bipolar plate 8 and end plates 14 and 16. Appropriate plumbing for exhausting coolant from the plate 8 and end plates 14 and 16 is also provided, but not shown.

#FIG. 2 is an isometric, exploded view of a bipolar plate 56 comprising a first exterior metal sheet 58, a second exterior metal sheet 60, and an interior spacer metal sheet 62 interjacent the first metal sheet 58 and the second metal sheet 60. The exterior metal sheets 58 and 60 are made as thin as possible (e.g., about 0.002-0.02 inches thick), may be formed by stamping, by photo etching (i.e., through a photolithographic mask) or any other conventional process for shaping sheet metal. The external sheet 58 is formed so as to provide a plurality of lands 64 which define therebetween a plurality of grooves 66 through which the fuel cell's reactant gases (i.e., H.sub.2 or O.sub.2) flow in a tortuous path from one side 68 of the bipolar plate to the other side 70 thereof. When the fuel cell is fully assembled, the lands 64 press against the carbon/graphite papers 36 or 38 (see FIG. 1) which, in turn, press against the MEAs 4 and 6 respectively. For drafting simplicity, FIG. 2 depicts only two arrays of lands and grooves. In reality, the lands and grooves will cover the entire external faces of the metal sheets 58 and 60 that engage the carbon/graphite papers 36 and 38. The reactant gas is supplied to grooves 66 from a header or manifold groove 72 that lies along one side 68 of the fuel cell, and exits the grooves 66 via another header/manifold groove 74 that lies adjacent the opposite side 70 of the fuel cell. As best shown in FIG. 3, the underside of the sheet 58 includes a plurality of ridges 76 which define therebetween a plurality of channels 78 through which coolant passes during the operation of the fuel cell. As shown in FIG. 3, a coolant channel 78 underlies each land 64 while a reactant gas groove 66 underlies each ridge 76.

#Metal sheet 60 is similar to sheet 58. The internal (i.e., coolant side) of sheet 60 is shown in FIG. 2. In this regard, there is depicted a plurality of ridges 80 defining therebetween a plurality of channels 82 through which coolant flows from one side 69 of the bipolar plate to the other 71. Like sheet 58 and as best shown in FIG. 3, the external side of the sheet 60 will have a plurality of lands 84 thereon defining a plurality of grooves 86 through which the reactant gases pass. An interior metal spacer sheet 62 is positioned interjacent the exterior sheets 58 and 60 and includes a plurality of apertures 88 therein to permit coolant to flow between the channels 82 in sheet 60 and the channels 78 in the sheet 58 thereby breaking laminar boundary layers and affording turbulence, and hence more effective heat exchange with the inside faces 90 and 92 of the exterior sheets 58 and 60 respectively.