1. Field of the Invention
This invention relates to fuel cells and more particularly to a stack of fuel cells.
2. Description of the Prior Art
A basic fuel cell comprises an anode electrode spaced apart from a cathode electrode with an electrolyte disposed therebetween in a compartment formed between the two electrodes. Typically each electrode comprises a thin catalyst layer adjacent to the electrolyte and disposed upon a layer of support material usually called the electrode substrate. Behind the substrate is a reactant gas compartment. The substrate is gas porous perpendicular to its thickness so that reactant gas which is fed into the compartment behind the electrode substrate diffuses therethrough to the catalyst layer. An electrochemical reaction occurs at the gas/electrolyte/catalyst interface whereby ions travel from one electrode to the other through the electrolyte.
Commercially useful amounts of electric power require stacking a plurality of cells and connecting them electrically in series. Electrically conductive gas impermeable plates separate the anode of one cell from the cathode of the next adjacent cell. These separator plates include the ribs (or other protrusions) on each side thereof which contact the electrode substrates. The ribs provide paths for the current to flow from one cell to the next while defining reactant gas compartments (such as channels) behind each substrate. In this manner gas is distributed over the back surface of each electrode. The ribs or protrusions also provide structural rigidity to the stack of cells and support to the electrodes which are usually made as thin as possible. A fuel cell stack constructed in accordance with the foregoing description is shown in commonly owned U.S. Pat. No. 3,994,748 to H. R. Kunz and C. A. Reiser.
Ribbed gas separator plates are expensive to make; and the ribs (or any other type of protrusions) create other problems, such as maldistribution of the reactant gas to the catalyst layer. For example, direct perpendicular passage (through plane) of the reactant gas to the catalyst layer through the areas of contact between the separator ribs and electrode substrate is blocked. Reactant gas must diffuse in plane through the substrate under the ribs to reach catalyst disposed on the substrate directly beneath the ribs. This diffusion is made more difficult because the substrate layer is somewhat compressed directly under the ribs and may be only several mils thick prior to compression.
The voltage across a stack of fuel cells is the sum of the voltage gains across the individual cells, which is a function of the current produced by each cell. The current passes perpendiclar to the plane of the electrodes from one end of the stack to the other. The current density through a stack of cells is equal to the current divided by the cross-sectional area through which the current passes at any particular cross-sectional plane. It is a constant at any one particular plane for any one particular power setting. If the cross-sectional area through a plane is reduced and total electric power generated is held constant, the current density must increase in that plane. Voltage losses are directly proportional to the current density; thus, at constant power, voltage is lost whenever the cross-sectional area through which the current passes is reduced. Such an area reduction occurs at the interface between the electrodes and the ribs or other protrusions of the separator plates since the contact area between the plates and the electrodes may only be on the order of 50% of the electrode cross-sectional area. Because perfect contact even between flat mating surfaces is impossible to achieve, there are also contact losses at every interface between adjacent components, particularly if they are not bonded together.
Satisfactory solutions for eliminating the above-discussed problems are continually being sought, but until the present invention have not been found.