CA1319171C - Dry fuel cell stack assembly - Google Patents

Abstract

14 54,197 ABSTRACT OF THE DISCLOSURE
A fuel cell is comprised of first and second opposed plates. A pair of opposed electrodes are positioned between the first and second plates. A matrix is positioned between the electrodes. The matrix includes a layer of paper having sufficient porosity so as to enable the fuel cell to be assembled before the electrolyte is added to the matrix. Passageways are provided for supplying electrolyte to the matrix.

Description

13t9171 1 5~,197 DRY FUEL CELL STACK ASSEMBLY

BACXGRO~ND OF THE INVENTION
Field of the Invention:
The present invention relates qenerally to a stack of fuel cells which are capable of converting the latent chemical energy of a fuel into electricity and, more particularly, to a type of fuel cell which can be assembled dry.
Description of_the Prior Art:
Fuel cells used to convert the latent chemical energy of a fuel directly into electricity are well-known in the art. For example, see U.S. Patent No. 4,463,068.
Such cells may be based on a variety of electrochemical reactions. One well-known reaction is based on using hydrogen as a fuel which reacts with oxygen to generate electricity.

2 54,197 One common form for constructing a hydrogen-oxygen cell is a laminated structure wherein an anode electrode and a cathode electrode are spaced apart by a porous layer of material which holds an electrolyte such as concentrated phosphoric acid. The hydrogen is guided by passageways behind the active region of the anode and the oxygen is guided by passageways behind the active region of the cathode. Both the anode and the cathode have a catalyst, such as platinum, deposited thereon.
At the anode, the hydrogen gas dissociates into hydrogen ions plus electrons in the presence of the catalyst. The hydrogen ions migrate through the electrolyte to the cathode in a process constituting ionic current transport while the electrons travel through an external circuit to the cathode. At the cathode, the hydrogen ions, electrons, and molecules of oxygen combine to produce water.
It is known that during operation the electrolyte contained within the matrix may expand or contract due to changes in operating conditions. It is also anticipated that over time, a certain amount of the electrolyte will be expended thereby reducing the volume of electrolyte in the matrix. Problems associated with changes in the volume of the electrolyte as well as problems associated with the consumption of the electrolyte have been addressed in the past. For example r in the aforementioned U.~. Patent NoO 4,463,068 an electrolyte distribution and supply system is disclosed.
A set of electrolyte containers are joined to respective fuel cells. The electrolyte is separately stored so as to provide for electrical isolation between the electrolyte of the individual cells of a stack. Individual storage compartments are coupled by tubes containing wicking fibers with the ends of the respective tubes terminating

3 54~197 on the means for drawing electrolyte in each of the respective fuel cells. Each tube is heat shrunk to tiqhtly bind the fibers therein. In this manner, electrolyte can be fed to the matrix in order to compensate for shrinkaqe or consumption, but it cannot accept acid if an operational upset causes the acid volume to swell.
~ nother e~mple o:E an electrolyte supply system i8 disclosed in Canadian Patent l~pplic&tion Serial No. 553,834, Filed December 8, 1987, entit;Led ~Improved I~ternal Electrolyte Supply System for Reliable Transport Throuqhout Fuel Cell Stack" which is assigned to the same assiqnee as the present invention. (W.E. 52,955) Although several embodiments of an electrolyte supply system are disclosed in the aforementioned application, all of the embodiments provide for the internal feedinq of the electrolyte to the fuel cells in the stack, multiple accessinq of the electrolyte grooves in each cell alonq the stack height, and limiting-~e developed head pressure~
Althouqh the prior art has addressed the problem of supplyinq electrolyte to the matrix to compensate ror electrolyte shrinkage and consumption, none of the pxior art systems is capable of enabling the fuel cell stack to be assembled dry. There are several advantages to be gained by assembling the fuel cell stack dry. First, where an acid electrolyte is used, during assembly the concentration and volume of acid may change due to humidity in the air. It is, therefore, currently necessary to assemble fuel cell stacks in an environment wherein the humidity is precisely controlled. Such a requirement complicates assembly and adds to the cost of the fuel cell stack. Additionally, if fuel cell stacks could be assembled dry, the assembly time could be .. . . .

4 54,197 substantially reduced and, perhaps, less skilled workers used for the assembly. Finally, where a fuel cell stack has been assembled dry, storage for an indefinite period of time presents no problems. Currently, with fuel cell stacks being assembled wet, if the fuel cell stack is to be stored for any length of time, environmental controls must be imposed to ensure that the acid concentration and volume are not upset and that carbon corrosion is minimized. Such complicated storage problems add unnecessary expense to the fuel cell stack and detract from the marketing of such stacks.

SUMMARY OF THE INVENTION
The present invention is directed to a fuel cell capable of being assembled dry. The fuel cell is comprised of first and second opposed plates. A pair of opposed electrodes is positioned between the first and second plates. A matrix is positioned between the pair of electrodes. The matrix includes a layer of silicon carbide and a layer of paper having sufficient porosity so as to enable the fuel cell to be assembled before the electrolyte is added to the matrix. The fuel cell also includes a mechanism for supplying the electrolyte to the matrix.
It has been found that the matrices of most prior art fuel cells have a low porosity such that there is low lateral electrolyte mobility~ That low lateral mobility prevents electrolyte from being introduced into a dry matrix from an edge thereof. The present invention solves that problem by providing a matrix with sufficient porosity such that good lateral transport of electrolyte into a dry matrix from an edge thereof is achieved. This has been achieved while maintaining the other character-54,197istics of the matrix, i.e. ability to withstand pressure differentials between process losses, low internal resistance, etc~ within acceptable limits.
The fuel cell of the present invention lends itself to a method of assembling a stack of dry fuel cells. That method is comprised of the steps of positioning a first electrode on top of a plate. A highly porous matrix is positioned on top of the first electrode.
A second electrode is positioned on top of the matrix and a plate is positioned on top of the second electrode.
These steps are repeated until a stack of the desired number of cells is assembled.
The apparatus and method of the present invention represent a substantial advance over the art.
Because the fuel cell stack can be assembled dry, the stack need not be assembled in an environmentally controlled area. Assembly of the stack dry requires less time and the possibility exists for using less skilled workers. After the stack has been assembled, it may be stored under normal storage conditions. Such advantages lower -the cost of the fuel cell stack as well as making the technology more marketabIe. These and other advantages and benefits of the present invention will be apparent from a description of a preferred embodiment hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS
In order that the present invention may be clearly understood and readily practiced, a preferred embodiment will now be described, by way of example only, with reference to the accompanying figures wherein:
Figure 1 is an exploded view of a fuel cell; and Figure 2 illustrates the various passages within the fuel cell of Figure 1;

6 _ 54,197 Figure 3 illustrates a fuel cell stack of a preferred embodiment of the present invention; and Figure 4 is a plan view of the top anode side of one bipolar plate of the stack of Figure 3.

DESCRIPTION OF THE PREFERRED E:MBODIMENT
A single fuel cell 10 constructed according to the teachings of the present invention is illustrated in Figure 1. Fuel cell 10 generally includes a top bipolar plate 12 and a bottom bipolar plate 14, between which are sandwiched an anode electrode 16, a matrix 18, and a cathode electrode 20. Plates 12 and 14 may be comprised of a material such as compression molded graphite-resin composite. The electrodes 16 and 20 may be made of a porous graphite material provided with porous fiber backing (not shown) for added structural integrity. The matrix 18 is comprised of a highly porous layer of paper 22. The layer of paper 22 must have sufficient porosity such that an electrolyte, for ëxample phosphoric acid, can be transported laterally into a completely dry matrix 18 from one end thereof. It has been discovered that a commercially available paper about four mils thick (.1 mm) known as TORA~ TGP 30 can be used for the layer 22, The layer of porous paper 22 must be kept very thin in order to prevent cell voltage loss due to internal ionic resistance. The matrix 18 is also comprised of a porous layer of a silicon carbide 24 on the order of five mils (.127 mm).
Various passages within the fuel cell 10 are illustrated in Figure 2. An oxidant such as air, or other oxygen containing material, flows through oxidant paths such as channels 26. A fuel which is hydrogen rich flows through fuel channels 28. When such fuel cells are stacked one atop another, the bipolar plate 12 between the * Denotes Trade-mark i 7 54,197 cells provides oxidant gas to one cell from one of its.
sides and fuel gas to the adjacent cell from its other side. Manifolds (not shown) are typically utilized to provide oxidant to oxidant inlets 30 of the oxidant channels 26 and to receive oxidant from oxidant outlets 34 of oxidant channels 26. Similar manifolds (not shown) are used to supply fuel to fuel inlets 38 of fuel channels 28 and to receive fuel from fuel outlets 42 of fuel channels 28.
Referring now tci Fig. 3, there is shown a fuel cell stack 58 of the improved internal electrolyte supply system of the present invention. The stack 58 is composed of a multiplicity of repeating fuel cells. The fuel cell includes top and bottom bipolar plates 12 and 14. Although not shown in Figure 3, between the plates 12, 14, there are sandwiched the lower anode electrode 16, an electrolyte-containing porous matrix and an upper electrolyte-containing porous matrix (both of which have been shown as matrix 18 in Fig. 1) and the upper cathode electrode 20.
Gaskets are ordinarily provided for sealing the peripheries of the electrodes.
The improved internal electrolyte supply system in the stack 58 shown in Fig~ 3, interconnects the pairs of fill holes 72 in opposite end portions of the top compression plate 74 and the pairs of drain holes 76 in the opposite ~nd portions of the bottom compression plate 74 of the stack. Ths supply system routes electrolyte through the fuel cell stack 58 along a series of first paths each extending horizontally and directly through one of the fuel cells between the bipolar plates 12, 14 thereof so as to expose electrolyte to the matrix 18 and along a series of second paths extending vertically through the stack 58 adjacent to opposite ends of the first horizontal paths.
The second paths are in ~ 1319171 8 54,197 communicative flow relation to the first paths and adapted to supply electrolyte directly to the respective first paths.
More particularly, the bipolar plates 12 and 14 of the stack 58 of Fig. 3 include two pairs of dual electrolyte flow grooves, primary grooves 78A and auxiliary grooves 78B, as shown in Fig. 4. Auxiliary grooves 78B extend generally parallel to and are interconnected with the primary grooves 78A by a series of spaced apart cross flow channels 82 defined in the plates 12 and 14 by intermittent walls 92 so as to provide the communicative flow relation therebetween. Also, primary grooves 78A are covered by a gasket. Further, an electrolyte transport wick 94 is disposed in the auxiliary grooves 78B of each cell (although only one wick is depicted in Fig. 4) and is engaged with and supports the matrix 18 thereof for facilitating transfer of electrolyte to the matrix 18.
The bipolar plates 12, 14 of the stack 58 include means defined therein which supply electrolyte downwardly through the stack in a by-pass fashion and to the primary grooves 78A so as to produce a cascading electrolyte flow. Such means include electrolyte flow passages 80 longitudinally aligned and spaced outwardly from the opposite ends of the auxiliary grooves 78B and extending through the plates 12 and 14 of the cells in spaced communicative flow relation with the opposite ends of the primary grooves 78A, and a dam or step 88 defined in the plates between the respective electrolyte flow passage 80 and primary groove 78A. The steps establish the communicative flow relation and produces the cascading electrolyte flow between the passages and primary grooves 78A.

9 _ 54,197 Finally, upper cooling plates 84 have the passages 80 in their left end portions and the groove 78A, passages 80 and steps 88 in their right end portions, whereas lower cooling plates 85 have only passayes 80 in their left and right end portions.
The offset or displacemen~ of the passages 80 from primary grooves 78A via the steps 88 provides a by-pass type arrangement and causes small pools of electrolyte to form in the grOGves 78A and 78B at every bipolar plate 12, 14 which overflow to the next lower plate. In this arrangement, no head pressure exists in the flow of electrolyte.
The exact mechanism by which electrolyte is provided to an edge of the matrix 18 is not considered to be an important feature of the present invention so long as some mechanism is provided for insuring a sufficient quantity of electrolyte can be easily supplied to the edge of the matrix 18. The configuration of electrolyte passages disclosed ~n the afor~mentioned Canad~an Patent Applicatlon No. 553,834, Piled ~ecember 8, 1987 and entitled ~Improved Internal Electr~lyte Supply System for Reliable Transport Throughout Fuel Cell Stack'.

The present inventionl by providing a matrix having sufficient porosity such that a fuel cell stack can be assembled before the electrolyte is added, provides a significant advantage over the prior art. Dry assembly results in labor savings. Additionally, no dry room requirements are needed for acid concentration control during assembly or subsequent acid addition.
In the prior art, if too much acid is needed prior to compression of the stack, or distribution of the acid is poor, then the hydrostatic pressure caused by compression of the stack can exceed the wet proofability -~ 1319171 10 54,197 (1 psi or less) of the cell due to very poor lateral transport and the matrix's inability to allow the excess acid to easily squeeze out the edge of the cell into the electrolyte grooves. Such problems are eliminated in the present invention by the porous nature of the matrix 18.
A similar problem exists due to acid volume expansion caused by operational upsets. Because of the very porous nature of the matrix 18 of the present invention, it is possible to eliminate the build up of cell transverse hydrostatic pressures caused by such operational upsets because the acid will expand laterally with ease into the electrolyte grooves.
Finally, construction of a dry stack enables the stack to be stored for any length of time using only normal storage procedures. No strict environmental control is necessary as with prior art stacks. These advantages and benefits result in lower manufacturing and storage costs as well as improving the marketability of such technology.
One feature of the matrix 18 which may be sacrificed by providing a highly porous matrix, is the matrix's ability to withstand a high pressure differential (bubble pressure). Based on system design, a five psi pressure differential is required. However, the silicon carbide layer 24 can withstand approximately an eight psi pressure differential when filled with acid. Therefore, the advantages of the present invention can be obtained without jeopardizing other desirable characteristics of the matrix 18.
While the present invention has been described in connection with an exemplary embodiment thereof, it will be understood that many modifications and variations 11 54,197 will be readily apparent to those of ordinary skill in the art. This disclosure and the following claims are intended to cover all such modifications and variations.

Claims (4)

1. A fuel cell, comprising:
first and second opposed pressure plates;
a pair of opposed electrodes positioned between said first and second plates, at least one opposed plate having a groove;
matrix means positioned between said pair of electrodes, said matrix means including paper means having sufficient porosity so as to enable the fuel cell to be assembled before the electrolyte is added to said matrix means; and means in said groove and engaged with said matrix means for supporting said matrix means and for supplying said electrolyte to said matrix means.

2. The fuel cell of claim 1 wherein said paper means includes a sheet of TORAY TGP 30 paper.

3. The fuel cell of claim 1 wherein said matrix means additionally comprises a layer of silicon carbide.

4. The fuel cell of claim 1 wherein said means for supplying said electrolyte supplies said electrolyte at the edge of said matrix means.