Flow field plate for electrochemical fuel cell

Abstract

A flow field plate comprises a first flow field; an opposing second flow field; and at least one flow channel formed in the first flow field, the at least one flow channel comprising: a first side and an opposing second side separated by an open-faced top and a bottom; and a first side channel formed in a portion of the open-faced top and in a portion of the first side along a continuous length of the at least one flow channel, the first side channel comprising a first side wall and a first bottom wall; wherein the first side wall of the first side channel and the first bottom wall of the first side channel form an obtuse angle in cross-section; and a depth of the bottom of the at least one flow channel is greater than a depth of the bottom wall of the first side channel.

Claims

1. A flow field plate for an electrochemical fuel cell comprising: a first flow field surface; an opposing second flow field surface; and at least one flow channel formed in the first flow field surface, the at least one flow channel comprising: a first side and an opposing second side separated by an open-faced top and a bottom surface having a depth (D.sub.f) relative thereto; and a first side channel formed in a portion of the open-faced top and in a portion of the first side along a continuous length of the at least one flow channel, the first side channel comprising a first side wall and a first bottom wall having a depth (D.sub.s) relative to an open-faced top of the first side channel such that the first side wall is adjacent to and in contact with the first flow field surface; wherein the first side wall of the first side channel and the first bottom wall of the first side channel form an obtuse angle in cross-section relative to the at least one flow channel; the depth of the bottom surface of the at least one flow channel (D.sub.f) is greater than the depth of the first bottom wall of the first side channel (D.sub.s).

2. The flow field plate of claim 1, wherein a ratio of a cross-sectional width of the open-faced top of the at least one flow channel to a cross-sectional width of the open-faced top of the first side channel is at least 5:1.

3. The flow field plate of claim 1, wherein a ratio of a cross-sectional width of the open-faced top of the at least one flow channel to a cross-sectional width of the open-faced top of the first side channel is at least 10:1.

4. The flow field plate of claim 1, wherein a ratio of the depth of the bottom of the at least one flow channel (D.sub.f) to the depth of the first bottom wall of the first side channel (D.sub.s) is less than 2:1.

5. The flow field plate of claim 1, wherein a cross-sectional width of the at least one flow channel decreases in a direction away from the open-faced top and towards the bottom surface.

6. The flow field plate of claim 1, wherein a cross-sectional area of the at least one flow channel is greater than a cross-sectional area of the first side channel.

7. The flow field plate of claim 6, wherein a cross-sectional area of the at least one flow channel is at least 3 times of a cross-sectional area of the first side channel.

8. The flow field plate of claim 6, wherein a cross-sectional area of the at least one flow channel is at least 5 times of a cross-sectional area of the first side channel.

9. The flow field plate of claim 1, wherein the opposing second flow field is a coolant flow field.

10. The flow field plate of claim 1, wherein the first flow field is a cathode flow field and the opposing second flow field is an anode flow field.

11. The flow field plate of claim 1, further comprising a graphitic, carbonaceous or metallic material, or combinations thereof.

12. The flow field plate of claim 1, wherein the first flow field surface comprises a hydrophilic material on a surface thereof.

13. The flow field plate of claim 1, further comprising: a second side channel formed in a portion of the open-faced top and in a portion of the opposing second side along the continuous length of the at least one flow channel, the second side channel comprising a second side wall and a second bottom wall having a depth (D′.sub.s) relative to an open-faced top of the second side channel; wherein the second side wall of the second side channel and the second bottom wall of the second side channel form an obtuse angle in cross-section relative to the at least one flow channel; and the depth of the bottom surface of the at least one flow channel (D.sub.f) is greater than the depth of the second bottom wall of the second side channel (D′.sub.s).

14. An electrochemical fuel cell, comprising: a membrane electrode assembly comprising an anode, a cathode, and a proton exchange membrane interposed therebetween; and a flow field plate comprising: a first flow field surface; an opposing second flow field surface; and a flow field adjacent the anode or the cathode and at least one flow channel formed in the flow field adjacent the anode or the cathode, the at least one flow channel comprising: a first side and an opposing second side separated by an open-faced top and a bottom surface having a depth (D.sub.f) relative thereto; and a side channel formed in a portion of the open-faced top and in a portion of the first side along a continuous length of the at least one flow channel, the side channel comprising a side wall and a bottom wall having a depth (D.sub.s) relative to the open-faced top of the side channel such that the side wall is adjacent to and in contact with the first flow field surface; wherein the side wall of the side channel and the bottom wall of the side channel form an obtuse angle in cross-section relative to the at least one flow channel; the depth of the bottom surface of the at least one flow channel (D.sub.f) is greater than the depth of the bottom wall of the side channel (D.sub.s).

15. The electrochemical fuel cell of claim 14, further comprising: a second flow field plate comprising a flow field adjacent the other of the anode or the cathode.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1 to 4 show a fuel cell stack configuration according to the prior art.

(2) FIG. 5 shows a cross-sectional view of a flow channel configuration according to the prior art.

(3) FIG. 6 shows a cross-sectional view of another flow channel configuration according to the prior art.

(4) FIG. 7 shows a cross-sectional view of a flow field plate according to one embodiment.

(5) FIG. 8 shows an exploded cross-sectional view of a flow channel configuration according to one embodiment.

(6) FIG. 9 shows an exploded cross-sectional view of a flow channel configuration according to one embodiment.

(7) FIG. 10 shows the difference in pressure drop variation at different coolant outlet temperatures for five fuel cells operating at the same conditions.

(8) FIG. 11 shows the difference in pressure drop stability at different coolant outlet temperatures for five fuel cells operating at the same conditions.

(9) FIG. 12 shows the pressure drop stability over time at different coolant outlet temperatures for two fuel cells operating at the same conditions.

DETAILED DESCRIPTION

(10) In the following description, certain specific details are set forth in order to provide a thorough understanding of the various embodiments. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures associated with fuel cells, fuel cell stacks, and fuel cell systems have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments.

(11) Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is, as “including, but not limited to”. Also, reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

(12) In the present context, “continuous flow channel” means that the flow channel spans from the fuel cell inlet to the fuel cell outlet without any discontinuities, thereby allowing fluids to flow therein from the fuel cell inlet to the fuel cell outlet. Similarly, “continuous side channel” means that the side channel spans from the fuel cell inlet to the fuel cell outlet without any discontinuities, thereby allowing fluids to flow therein from the fuel cell inlet to the fuel cell outlet.

(13) As mentioned in the foregoing, the presence of liquid water in the flow channels typically creates random and varying flow resistance in fuel cells because liquid water blocks or clogs the gas flow passage. To address this problem, the flow field plates are designed with continuous small side channels formed in an upper top portion of the flow channel adjacent the gas diffusion layer (GDL) to attract liquid water away from the surface of the GDL from under the landings of the flow field plate and/or from the main flow channel and to an area that does not significantly hinder gas flow in the main flow channel.

(14) It has been discovered that fuel cells with such continuous side channels improve flow resistance stability in comparison to fuel cells with conventional flow field channel designs. This is demonstrated by improved pressure drop stability measured by the standard deviation of the change in pressure drop over time. Without being bound by theory, a higher stability in pressure drop means that there is less water in the flow field channel that, if present, would result in poor flow characteristics (i.e., more pressure drop “noise”), as well as voltage variability from cell to cell due to reactant starvation. For example, it is estimated that with a standard deviation of 3.5%, the minimum required oxidant stoichiometry to sustain a given load can vary by more than 12%. Thus, the minimum oxidant stoichiometry that is supplied to the fuel cell must also be increased by at least 12% to account for pressure drop instability. Furthermore, it was observed that the pressure drop variation at different temperatures (and, thus, water content in the fuel cell) was also low for fuel cells utilizing such side channels. Typically, a low variance in pressure drop is desired so that the oxidant blower or compressor will experience more constant conditions and, thus, the oxidant blower or compressor can be more accurately sized. For example, if the oxidant blower or compressor is too big, parasitic losses would be unnecessarily high. On the other hand, if the oxidant blower or compressor is too small, reactant starvation would occur in the cells with higher flow resistance.

(15) FIG. 7 illustrates a flow field plate in cross-section according to one embodiment (side channel features not shown for ease of understanding). Flow field plate 100 includes a first flow field surface 102 and an opposing second flow field surface 104. (In a fuel cell configuration, a GDL is placed adjacent first flow field surface 102.) First flow field surface 102 includes continuous flow channels 106 defined by a first side 108 and an opposing second side 110 that are separated by open-faced top 112 and bottom surface 114. Each of flow channels 106 are separated by landings 116. For better manufacturability, the cross-sectional width of the flow channel should generally decrease in a direction away from open-face top 112 and towards bottom surface 114 (i.e., the cross-sectional width of open-faced top 112 is greater than the cross-sectional width of bottom surface 114). Similarly, second flow field surface 104 may also include flow channels (not shown). Although not shown in FIG. 7, a typical flow field plate will have manifolds for supplying and removing reactants, such as manifolds 30 shown in FIGS. 1 to 4. The manifolds may be internal manifolds (formed in the plate) or external manifolds (attached to the plate). One skilled in the art will appreciate that flow field channels typically extend continuously from an inlet manifold to an outlet manifold of the flow field plate.

(16) With reference to FIG. 8, which is an exploded view of flow channel 106 in cross-section, flow channel 106 includes a first side channel 118 formed in a portion of open-faced top 112, which is typically adjacent and fluidly connected to a GDL (not shown), and in a portion of first side 108 along the continuous length of flow channel 106 as depicted in FIG. 7. First side channel 118 is fluidly connected to the main flow channel section of flow channel 106 and is defined by a first side wall 120 and a first bottom wall 122. Similarly, second side channel 124 is formed in a portion of open-faced top 112, which is also typically adjacent and fluidly connected to the GDL (not shown), and in a portion of second side 110 (as shown in FIG. 7) along the continuous length of flow channel 106. Second side channel 124 is also fluidly connected to the main flow channel section of flow channel 106 and is defined by a second side wall 126 and a second bottom wall 128. Preferably, first side wall 120 and first bottom wall 122 form an obtuse angle δ in cross-section to help facilitate manufacturing processes, such as embossing, machining, and molding. Similarly, second side wall 126 and second bottom wall 128 preferably form an obtuse angle δ in cross-section.

(17) Without being bound by theory, capillary-driven flow can occur in a corner provided that the Concus-Finn condition is met, where the Concus-Finn condition is defined by formula (I):
θ<π/2−α  (I)
where θ is the static contact angle of the surface and α is the half-angle of the corner. Therefore, the first and second side channels are designed to be a particular size, shape and location to wick water away from the surface of the GDL and the main channel by creating capillary-driven corner flow. Since water is wicked away into an area outside of the main flow channel area, flow resistance variability is decreased and pressure drop stability is improved.

(18) For example, the cross-sectional depth of first and second side channels 118, 124 (D.sub.s) should be deep enough so that liquid water is drawn slightly away from the gas diffusion layer and to side channels 118, 124 (i.e., outside of the main flow channel area of flow channel 106), thereby removing liquid water droplets on the surface of the GDL that may contribute to flow resistance variation and/or gas diffusion blockage. In addition, the cross-sectional depth of first and second side channels 118, 124 (D.sub.s) should be less than the cross-sectional depth of flow channel 106 (D.sub.f) so that water can be drawn more easily to the side channel from the surface of the GDL than into the main flow channel area. Without being bound by theory, as mentioned in the foregoing and as shown in FC-5 in the Examples section, it is believed that water droplets typically do not wick into a corner formed by the flow field plate and the gas diffusion layer because the gas diffusion layer is generally hydrophobic, which makes it a poor corner to contain water (i.e., violates the Concus-Finn condition). However, by providing a corner that is located away from the surface of the gas diffusion layer as well as the bulk of the flow channel and is appropriately sized and shaped, liquid water can wick away from the surface of the GDL by capillary-driven corner flow into the corner without having water accumulate in the bulk of the flow channel. As a result, the depth of flow channel 106 is not limited because it only functions to provide reactant flow. Furthermore, first and second side channels 118, 124 are preferably continuous and fluidly connected to the outlet or outlet manifold of the fuel cell so that excess liquid water can be collected everywhere in first and second side channels 118, 124 along the continuous length of flow channel 106, and excess water can flow and be removed from first and second side channels 118, 124 at the outlet or outlet manifold of flow channel 106.

(19) In some embodiments, flow channel 106 is much larger than first and second side channels 118, 124. With reference to FIG. 9, the ratio of cross-sectional depth of the flow channel, D.sub.f (i.e., distance from the open-faced top of the flow channel to the bottom of the flow channel), to the cross-sectional depth of the of the first side channel, D.sub.s (i.e., distance from the open-faced top of the side channel to the bottom of the side channel), is greater than about 1:1 but less than about 2:1. Further, the cross-sectional width of the open-faced top of flow channel 106, W.sub.f, should be greater than that of the open-faced top of first and second side channels 118, 124, W.sub.s. In some embodiments, the ratio of the cross-sectional width of the open-faced top of the flow channel, W.sub.f, to the cross-sectional width of the open-faced top of first side channel, W.sub.s, is at least about 5:1, and in further embodiments, at least about 10:1. In other embodiments, the cross-sectional area of the main flow channel, A.sub.f, is at least 3 times the total cross-sectional area of each of the side flow channels, A.sub.s, and in further embodiments, at least 5 times the cross-sectional area of each of the side flow channels. In some embodiments, obtuse angle δ of first and second side channels 118, 124 may be the same or may be smaller than obtuse angle γ of flow channel 106. Without being bound by theory, by employing an obtuse angle δ in first and second side channels 118, 124 that is smaller than obtuse angle γ of flow channel 106, water is more likely to be drawn into side channels 118, 124.

(20) In one embodiment, first major flow field surface 102 may be an anode or cathode flow field and second major flow field surface 104 may be a coolant flow field. In another embodiment, first major flow field may be an anode or cathode flow field, and second major surface may be the other of the cathode or anode flow field. Flow field plate 100 may be any suitable material, such as, but not limited to, graphitic, carbonaceous, or metallic, and combinations thereof. The first major flow field may comprise a coating on its surface, such as a hydrophilic coating. Without being bound by theory, such a coating will lower the surface tension (i.e., decrease contact angle) of any water droplets that form in the flow channel so that water can be wicked more easily into the side channels.

(21) As one of ordinary skill in the art will appreciate, the flow field plate may be incorporated into an electrochemical fuel cell by placing a membrane electrode assembly between two flow field plates such that the gas diffusion layers of the membrane electrode assembly are adjacent to the flow field plate. Further incorporation of the electrochemical fuel cell into a fuel cell stack and system will be easily determined by one of ordinary skill in the art.

(22) Although the corners of the side channels are shown to be sharp corners, rounded corners may be used without affecting the function of the side channels, so long as the side channels comprise a side wall and a bottom wall that form an obtuse angle and the depth of the bottom wall is less than the depth of the flow channel. Furthermore, first side channel and second side channel may have the same or different dimensions. For example, width of second side channel, W.sub.s, and depth of second side channel, D′.sub.s, may be different from the width of first side channel, W.sub.s, and the depth of first side channel, D.sub.s. In addition, while two side channels are shown in FIGS. 8 and 9, it is contemplated that water management may still be improved even if the flow field channel incorporates only one of such side channels.

(23) The following examples are provided for the purpose of illustration, not limitation.

EXAMPLES

(24) Six cathode flow field plates were machined with the flow field channel dimensions specified in Table 1. The five oxidant flow field plates FC-1 to FC-5 had side channels on both sides of the flow channel while the Baseline did not contain any side channels. In addition, the side channels of FC-5 do not include a side wall and a bottom wall in its side channels, but instead has a sloped wall (see FIG. 6). The flow field plates were assembled into fuel cells with the same anode flow field plate and MEA for evaluating their operational characteristics.

(25) TABLE-US-00001 TABLE 1 Flow field channel geometries Name Baseline FC-1 FC-2 FC-3 FC-4 FC-5 Number of Channels 28 11 18 17 28 28 Landing Width, W.sub.l (mm) 1.03 1.39 1.00 1.39 0.50 0.65 Channel Width, W.sub.f (mm) 0.64 2.86 1.53 1.29 1.07 1.02 Channel Depth, D.sub.f (mm) 0.77 0.78 0.68 0.78 0.78 1.00 Side channel width, W.sub.s N/A 0.21 0.26 0.24 0.24 0.25 (mm) Side channel depth, D.sub.s N/A 0.35 0.45 0.45 0.45 0.25 (mm)

(26) The fuel cells of Table 1 were operated in a vertical orientation (reactant inlet at the top, reactant outlet at the bottom) at 160 Amps with different coolant temperature differentials to evaluate the oxidant differential pressure exhibited by the fuel cell during operation. The operating conditions are set out in Table 2.

(27) TABLE-US-00002 TABLE 2 Operating conditions Temperature 65° C. coolant inlet; 70° C., 75° C., or 78° C. coolant outlet Inlet Dew Point 67° C. (fuel and oxidant) Fuel 75% hydrogen/balance nitrogen Oxidant air (21% oxygen/79% nitrogen) Reactant inlet pressure 50-200 mbarg (fuel and oxidant) Reactant stoichiometry 1.3 for fuel, 1.8 for oxidant

(28) FIG. 10 shows the difference in pressure drop at different coolant temperature differentials to observe the pressure drop variation at different operating conditions. (A larger coolant temperature differential indicates higher coolant outlet temperatures and, thus, typically less water accumulation in the fuel cell.) FC-1 and FC-2 showed much lower pressure drop variation than the Baseline, FC-3, and FC-4. Without being bound by theory, it is believed that the side channels of FC-3 and FC-4 were too similar to that of the main flow channel dimensions and, thus, did not show a marked improvement in pressure drop variation. For example, the cross-sectional area of the main channel of FC-3 was less than 2.5 times the total cross-sectional area of the side channels. Similarly, the cross-sectional area of the main channel of FC-4 was less than 2 times the total cross-sectional area of the side channels. Conversely, the cross-sectional area of the main channel of FC-1 was almost 10 times the total cross-sectional area of the side channels, and cross-sectional area of the main channel of FC-2 was more than 3 times the total cross-sectional area of the side channels. In addition, the cross-sectional width of the main channel of FC-3 and FC-4 were less than 2 times the total cross-sectional area of their respective side channels.

(29) However, despite this observation, both FC-3 and FC-4 still showed similar pressure drop stability as FC-1 and FC-2, as measured by the standard deviation of the pressure drop (see FIG. 11). Therefore, although FC-3 and FC-4 showed some variation in pressure drop at different coolant temperature differentials, FC-3 and FC-4 still exhibited improved pressure drop stability compared to Baseline.

(30) A more detailed look at the results of the Baseline fuel cell and FC-2 is shown in FIG. 12. It is evident that the oxidant differential pressure stability for both fuel cells increased as the coolant temperature differential decreased, which is expected because there is typically less liquid water at higher operating temperatures (i.e., larger coolant temperature differential). However, FC-2 exhibited greater stability in oxidant differential pressure, as measured by the standard deviation of the pressure drop, as well as lower variability in the oxidant differential pressure at all coolant temperature differentials in comparison to the Baseline.

(31) All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety.

(32) From the foregoing, it will be appreciated that, although specific embodiments have been described herein for the purpose of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not limited except by the appended claims.