FUEL CELL STACK WATER MANAGEMENT

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of US Provisional Patent Application filed on August 7, 2006 and bearing serial number 60/835,906.

TECHNICAL FIELD

The present invention relates to the field of fuel cells, and more particularly to the design of a membrane electrode assembly (MEA) to optimize water management inside the fuel cell stack.

BACKGROUND OF THE INVENTION

In the fuel cell industry, operating a fuel cell stack with a low relative humidity (RH) (≤50%) of either or both fuel and oxidant reactants is highly desired for automotive, stationary, back up power, and portable applications. However, short stack lifetime is a major challenge for low RH operation due to the membrane's accelerated failure since membranes are not fully saturated during fuel cell operation .

To maximize membrane water saturation, flow-field designs with different configurations may be implemented, by which the water transfer between the higher RH side and the lower RH side can be enhanced. In many cases, however, the membrane is still in sub- saturation due to the water transfer limitations between the anode side and the cathode side .

Therefore, there is a need to provide other designs for MEAs that will enhance water transfer between anode and cathode in a fuel cell stack.

SUMMARY OF THE INVENTION

An MEA design is described to enhance the water transfers across the MEA and thus improve both membrane water saturation and water content uniformity over the MEA area 5 during low RH fuel cell operation. The fuel cell stack performance, reliability and lifetime may be improved.

In accordance with a first broad aspect of the present invention, there is provided a method for facilitating water transfer in a membrane electrode assembly for a fuel 0 cell stack, the method comprising: supplying at least one of an anode and a cathode with low relative humidity reactants,- and providing a membrane having a non-uniform capability of water transfer, the non-uniform capability being an intrinsic characteristic of the membrane.

5 In accordance with a second broad aspect of the present invention, there is provided a membrane electrode assembly comprising: an anode electrode,- a cathode electrode; and a membrane between the anode and the cathode electrode, the membrane having a non-uniform capability of water transfer, 0 the non-uniform capability being an intrinsic characteristic of the membrane.

When referring to the anode side, it should be understood that was is meant is the combination of anode flow fields, and/or anode gas diffusion layer (GDL) and anode electrode. 5 What is meant by the cathode side is the combination of cathode flow fields, and/or cathode GDL and cathode electrode .

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention 0 will become apparent from the following detailed

description, taken in combination with the appended drawings, in which:

Fig. 1 shows the prior art membrane electrode assembly having a cathode side, an anode side, and a uniform 5 membrane ;

Fig. 2a shows an embodiment of the present invention having a non-uniform membrane that gets thinner from anode inlet to anode outlet;

Fig. 2b shows an embodiment of the present invention having 0 a membrane that has an ion exchange capability getting lower from cathode inlet to cathode outlet;

Fig. 3 shows an embodiment of the present invention having a non-uniform membrane that gets thinner from cathode inlet to cathode outlet;

5 Fig. 4 shows an embodiment of the present invention having a non-uniform membrane having alternating thin and thick zones,- and

Figs. 5a-5d show membranes having different gradually varying thickness.

0 It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

Figure 1 illustrates the prior art, where the membrane of an MEA is not equally humidified because of the constant 5 width of the membrane. The unit cell 10 comprises an anode side 12 (anode electrode + anode gas diffusion layer + anode flow field) , a cathode side 14 (cathode electrode + cathode gas diffusion layer + cathode flow field) and a

proton exchange membrane 16 which is of uniform thickness throughout. In the case of counter- flow designs for flow field, the anode reactant enters the anode side 12 following the direction of arrow 18 and exits the anode 5 side 12 following arrow 20 while the cathode reactant enters and exits the cathode side 14 following arrows 24 and 22, respectively. In the case of counter-flow designs, the water contents are higher at the anode 32 and cathode 26 outlet portions and they are lower at the anode 28 and 0 cathode 34 inlet portions. The excess water is transferred from the anode outlet to the cathode inlet following arrow 36 and from the cathode outlet to the anode inlet following arrow 30.

At least one embodiment of the present invention 5 homogenizes the humidification of a membrane and anode/cathode reactants. At least one embodiment of the invention facilitates water transfer in at least a portion of a membrane. This is achieved by providing a membrane having a non-uniform capability of water transfer, the 0 capability being an intrinsic characteristic of the membrane. This should be understood to mean that the nonuniform capability of water transfer can be the result of the special shape of the membrane and/or the physical properties of the materials of which the membrane is made. 5 It is understood that the membrane can comprise an assembly of membranes having different shapes and/or an assembly of different materials.

The amount of water transferred is determined by the membrane's thickness and other physical parameters, as 0 described by the following equation:

where Q is water transfer rate, D is water diffusion coefficient, S is water solubility in the membrane, L is membrane thickness, p _x and p ₂ are H ₂ O partial pressure from both sides of the membrane, respectively. Hence, if at a

5 point of the membrane the thickness of the membrane is too large or if the difference of the two partial pressures

(P ₁ -P ₂ ) is too small, the water transfer rate is not enough to properly humidify the membrane and the reactants as well. Several embodiments are possible to remedy the 0 problem. A first one consists in providing a membrane that has an enhanced water diffusion coefficient D and/or an enhanced water solubility S. Another one is to decrease the thickness L of the membrane in a specific and non-uniform manner .

5 Figure 2a illustrates one possible embodiment of the present invention to improve the water management in the fuel cell stack. In the present example, one wants to increase the humidification degree of the membrane and reactant at the cathode inlet side of a counter- flow 0 configuration for flow field design. The embodiment illustrated in figure 2a facilitates water transfer between the anode outlet and the cathode inlet. Figure 2a is a cross-sectional side view of a unit cell 50 comprising an anode side 52, a cathode side 54 and a membrane 56 having a 5 non-uniform thickness. In this embodiment, the anode reactant enters the anode side 52 following the direction of arrow 62 and exits the anode side 52 following arrow 64 while the cathode reactant enters and exits the cathode side 54 following arrows 68 and 66, respectively. In the 0 present example, the membrane thickness 60 at the cathode outlet is larger than the membrane thickness 58 at the cathode inlet. As a result, the transfer of water from the water-rich anode region 76 to the water-lean cathode region

78, illustrated by arrow 80, is larger than the water transfer from the water-rich cathode region 72 to the water-lean anode region 70, illustrated by arrow 74. Using a membrane such as membrane 56 enables an enhanced water 5 transfer from the anode outlet side to the cathode inlet side so that the humidification of the membrane as well as the reactants are more homogenous along its length, thereby increasing membrane conductibility and lifetime.

Another embodiment to improve water transfer between anode 0 outlet and cathode inlet consists in using higher ion exchange capacity (IEC) materials at the anode outlet side, as illustrated in figure 2b. In figure 2b, unit cell 90 comprises an anode side 91, a cathode side 92 and a membrane 93 of uniform thickness therebetween. The membrane 5 93 has a higher ion exchange capability at the cathode inlet 94 than that at the cathode outlet 95. This will increase the water transfer from the anode outlet 96 to the cathode inlet 94 with respect to the water transfer between the cathode outlet 95 and the anode inlet 97. Yet another 0 embodiment used to achieve the same objective is to use higher water diffusion and/or water solubility materials at the anode outlet side of the membrane.

It should be understood that the non-uniform variation of either the ion exchange capacity, the water diffusion, or 5 the water solubility can be gradual or discrete along the membrane. In one embodiment, a plurality of membranes having different characteristics (i.e. different ion exchange capacity and/or water diffusion and/or water solubility) can be assembled together to obtain a membrane 0 having a substantially gradual variation of its capability of water transfer. It should be understood that other alternative membranes having a non-uniform capability of

water transfer are possible while still falling within the scope of the present invention.

Figure 3 illustrates an embodiment used to facilitate water transfer between cathode outlet and anode inlet. The 5 membrane shown in figure 2a has been reversed such that the thin side is now at the cathode outlet side, while the thicker side is at the cathode inlet side. Figure 3 presents a unit cell 100 comprising an anode side 102, a cathode side 104 and a membrane 106 of varying thickness 0 therebetween. In this embodiment, the anode reactant enters the anode side 102 following the direction of arrow 108 and exits the anode side 102 following arrow 110 while the cathode reactant enters and exits the cathode side 104 following arrows 114 and 112, respectively. In the present 5 example, the membrane thickness 107 at the cathode inlet is larger than the membrane thickness 105 at the cathode outlet. As a result, the transfer of water from the water- rich cathode region 118 to the water- lean anode region 116, illustrated by arrow 120, is larger than the water transfer 0 from the water-rich anode region 122 to the water-lean cathode region 124, illustrated by arrow 126. Alternatively to the non-uniform membrane, or in combination with it, higher IEC materials may be used at the cathode outlet side. Also alternatively or in combination with both 5 previous techniques, higher water diffusion and/or solubility materials may be used at the cathode outlet side of the membrane .

Figure 4 is a design which facilitates water transfer between anode/cathode outlets and inlets, respectively. 0 Figure 4 illustrates a unit cell 150 comprising an anode side 152, a cathode side 154 and a non-uniform membrane 156 therebetween. The thickness of the membrane 156 varies to

improve the water transfer. The thickness 158 of the membrane at the cathode outlet and the thickness 162 of the membrane 156 at the cathode inlet is inferior to the thickness 160 at substantially the middle of the membrane 5 156. As a result of this design, the transfer of water from the water-rich cathode region 172 to the water-lean anode region 170, illustrated by arrow 174, and the transfer of water from the water-rich anode region 176 to the water- lean cathode region 178, illustrated by arrow 180 is 0 enhanced compared with the water transfer occurring at substantially the middle of the membrane 156. Also used, either individually or in combination with the non-uniform membrane, is higher IEC materials at both the anode and cathode outlet sides. In addition, higher water diffusion 5 and/or solubility materials can be used at both anode and cathode outlet sides of the membrane.

The non-uniform membrane may be provided in different forms/shapes. For the non-uniform membrane varying in thickness, the variation may be very abrupt as illustrated 0 by membranes 56, 106, and 156, or gradual as illustrated by the different membranes of figure 5. Figure 5a illustrates a membrane of which the thickness gradually decreases from top and bottom to the middle so that the transfer of water is increased in the middle of the membrane. Figures 5b and 5 5c present membranes of which the thickness gradually increases from top and bottom to the middle so that the transfer of water is decreased in the middle of the membrane. Figures 5d illustrates a membrane of which the thickness gradually and linearly decreases from top to 0 bottom so that the water transfer is enhanced at the bottom of the membrane. In order to achieve such thickness -varying membranes, one can assemble together several membranes of different thickness or machine a single membrane to give it

the desired shape. It should be understood that other alternative shapes are also possible while still falling within the scope of the present invention.

With respect to the materials used for the varying ion

5 exchange capacity throughout the membrane, an example of suitable material is proton exchange materials, including organic materials (such as Nafion™ membrane), inorganic materials (such as ceramic materials) or their combinations

(Nafion + in-organics) . This type of membrane can also be 0 made by combining two different membranes into a single membrane, or by a fabrication process that will allow the variance in ion exchange capacity to be gradual throughout the membrane. Other materials can be used and are readily known by a person skilled in the art.

5 As for the membrane that has varying water diffusion and/or solubility, the same principle can be applied. It should be understood that a combination of any of the above-described embodiments is possible.

There may be other cases in which one of the reactants, 0 either fuel or oxidant, is supplied in low RH. In these cases a variety of MEA designs having thinner and thicker portions, larger or lower IEC portions, and/or having portions of higher and lower water diffusion or solubility material, as illustrated in figures 2 to 4, may be 5 implemented in conjunction with different flow field configurations including co-flow, cross-flow, and counter- flow, to achieve optimized water management in the fuel cell stack.

Using the present design, the membrane and the reactants 0 are saturated via enhanced water transfer between the water-rich side and the water-lean side across the MEA. The

membrane conductivity and MEA performance are increased. The membrane/MEA lifetime is also improved.

The embodiments of the invention described above are intended to be exemplary only. The scope of the invention 5 is therefore intended to be limited solely by the scope of the appended claims.