BACKGROUND

Technical Field

The present disclosure relates to electrochemical fuel cells and, particular, to flow field plates for air-cooled fuel cells.

Description of the Related Art

Fuel cells convert reactants, namely, fuel and oxidant, to generate electric power and reaction products. Polymer electrolyte membrane fuel cells ("PEM fuel cell") employ a membrane electrode assembly ("MEA"), which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes.

Separator plates, or flow field plates for directing the reactants across one surface of each electrode substrate, are disposed on each side of the MEA.

In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, multiple cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. End plate assemblies are placed at each end of the stack to hold the stack together and to compress the stack components together. Compressive force effects sealing and provides adequate electrical contact between various stack components. Fuel cell stacks can then be further connected in series and/or parallel combinations to form larger arrays for delivering higher voltages and/or currents. Figures 1 -4 (prior art) collectively illustrate a typical design of a conventional MEA 5, with electrodes 1 ,3 sandwiching a proton exchange membrane 2 therebetween (Figure 1 ); an electrochemical cell 10 comprising an MEA 5 between flow field plates 1 1 , 12 (Figure 2); a stack 50 of electrochemical cells 10 (Figure 3); and stack 50 compressed between endplates 17, 18 (Figure 4). Figures 1 -4 each also illustrate manifolds 30 for delivering and removing reactants and products to and from the fuel cells during operation.

For fuel cell stacks designed for higher current density operation, such as automotive applications, a separate cooling stream is desirable because such fuel cell stacks tend to operate at higher temperatures and produce more liquid water. A separate coolant stream, therefore, provides better control of the conditions of the fuel cell stack during operation but requires additional balance of plant components, such as a coolant reservoir and pumps, as well as additional maintenance. For liquid-cooled fuel cell stacks, a bipolar flow field plate is formed by joining two flow field plates together, namely, an anode flow field plate and a cathode flow field plate, so that an anode flow field is formed on one surface of the anode flow field plate while a cathode flow field is formed on one surface of the cathode flow field plate. A coolant flow field is formed between opposing surfaces of the anode flow field plate and the cathode flow field plate when the anode and cathode plates are bonded, glued or welded together to form a bipolar flow field plate. The coolant, which may be water, glycol, or a mixture thereof, is circulated as a separate stream in the coolant flow field.

For fuel cell stacks designed for lower current density operation, such as for back-up power applications, an air-cooled fuel cell stack can be used because control of the oxidant stoichiometry and temperature of the fuel cell stack is less stringent. Air-cooled stacks are desirable as they typically operate with reactants at near ambient oxidant temperatures and pressures, and do not require a separate cooling system because air is used as the oxidant and the coolant, thereby simplifying the system design. Air-cooled stacks also often operate with a dead-ended anode, that is, the anode is dead- ended or closed for at least a portion of the time during operating and periodically purged to remove primarily impurities and liquid water, such as the method of operating a fuel cell as described in U.S. Patent No. 7, 153,598. The cathode flow field channels are typically very large compared to the anode flow field channels to allow for a very high air stoichiometry, which also allows for sufficient cooling of the fuel cell during operation. The cathode flow field channels tend to be very short, for example, running along the width of the flow field plate (i.e. , running perpendicular to the anode flow field channels) and open to air on both ends, to reduce pressure drop, which reduces the need for more powerful air blowers. For air-cooled fuel cell stacks, the bipolar flow field plate is typically a single plate that has an anode flow field on one surface and a cathode flow field on the opposing surface.

Flow field plates need to be electrically and thermally conductive and, therefore, typically comprise graphitic and/or metallic materials, such as graphite and stainless steels that are coated with an electrically conductive material. Metal plates are desirable as they are usually made thinner than graphite plates, which is advantageous for automotive applications which require high power density, and can be easily formed by stamping such that the negative features of the anode flow field plate can be used as the positive features of the coolant flow field channels. For metal plates for air-cooled fuel cell stacks, however, the negative features of the anode flow field plate need to be suitable as the positive features of the cathode flow field channels. This may not be possible as the anode and cathode flow field channels may be running in different directions, for example, if the anode flow field channels are serpentine flow channels and the cathode flow field channels are straight flow channels, such as that shown in U.S. SIR registration number H002241 .

Additionally, different fuel and air stoichiometric requirements may make it difficult to find a compromise in the anode and cathode flow field channel dimensions.

Thus, it is difficult to create a flow field design in which the negative features of the anode flow field plate are suitable to use as the positive features of the cathode flow field plate for air-cooled fuel cell stacks.

Accordingly, there remains a need for improved flow field plate design for bipolar flow field plates for air-cooled fuel cell stacks. The present disclosure addresses this need and provides further related advantages. BRIEF SUMMARY

Briefly summarized, a flow field plate for an air-cooled fuel cell comprises: an anode flow field comprising a serpentine anode flow channel with a plurality of passes, wherein each pass is separated by an anode landing and each pass is fluidly connected to an adjacent pass by a recess formed in the anode landing; and an opposing cathode flow field comprising a plurality of cathode flow channels, wherein each cathode flow channel is separated by a cathode landing; wherein the anode landing of the anode flow field forms the cathode flow field channels of the cathode flow field; and a depth of the recess in the anode landing is less than a depth of the cathode flow channels.

In some embodiments, the depth of the recess in the anode landing is less than half the depth of the cathode flow channel. In further embodiments, the depth of the recess in the anode landing is less than a third of the depth of the cathode flow channel. In yet further embodiments, the depth of the recess in the anode landing is less than a quarter of the depth of the cathode flow channel.

These and other aspects will be evident upon reference to the following detailed description and attached drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Figures 1 to 4 show a fuel cell stack configuration according to the prior art.

Figure 5 shows an isometric view of an anode flow field side of a bipolar flow field plate according to one embodiment.

Figure 6 shows an isometric view of a cathode flow field side of a bipolar flow field plate according to one embodiment.

Figure 7 shows a cross-sectional view of two bipolar flow field plates according to one embodiment. DETAILED DESCRIPTION

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.

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.

In one embodiment, a bipolar flow field plate for an air-cooled fuel cell comprises an anode flow field on one surface having a serpentine anode flow channel and a cathode flow field on the opposing surface having a plurality of cathode flow channels. Figure 5 shows the anode flow field side of an exemplary bipolar flow field plate. Bipolar flow field plate 100 comprises an anode flow field having serpentine flow channel 104 which is fluidly connected to fuel inlet manifold 106 via fuel feed channel 108. Serpentine flow channel 104 includes a number of passes 1 10a, 1 10b, 1 10c that traverse across the width of bipolar plate 100 towards to the oxidant outlet manifold (not shown). One skilled in the art will appreciate that anode landings 1 12a, 1 12b, 1 12c are formed between passes 1 10a, 1 10b, 1 10c to separate each of the passes while also forming the cathode flow channels on the opposing side of bipolar flow field plate 100.

In order to fluidly connect passes 1 10a, 1 10b, 1 10c, to their respective adjacent passes, recesses 1 14a, 1 14b, 1 14c are formed in the end portions of anode landings 1 10a, 1 10b, 1 10c, thereby forming a single serpentine flow channel 104. The depth of recesses 1 14a, 1 14b, 1 14c are less than the depth or height of anode landings 1 12a, 1 12b, 1 12c, so as not to significantly block air flow through the cathode flow channels on the opposing side of bipolar flow field plate 100. One skilled in the art will appreciate that the depth of the recesses or "vias" 1 14a, 1 14b, 1 14c should be such that the pressure drop of the cathode flow channels is not significantly increased.

In one embodiment, the depths of recesses 1 14a, 1 14b, 1 14c are less than one half of the depth or height of anode landings 1 12a, 1 12b, 1 12c. In specific embodiments, the depths of recesses 1 14a, 1 14b, 1 14c are less than one third of the depth or height of anode landings 1 12a, 1 12b, 1 12c. In yet further embodiments, the depths of recesses 1 14a, 1 14b, 1 14c are less than one quarter of the depth or height of anode landings 1 12a, 1 12b, 1 12c. In further embodiments, the depths of recesses 1 14a, 1 14b, 1 14c are less than one eighth of the depth or height of anode landings 1 12a, 1 12b, 1 12c.

As mentioned, anode landings 1 12a, 1 12b, 1 12c formed on the anode flow field side of the bipolar plate form cathode flow channels 1 18a, 1 18b, 1 18c on the cathode flow field side of bipolar flow field plate 100, as shown in Figure 6. In some embodiments, cathode flow channels 1 18a, 1 18b, 1 18c may be oversized to further reduce the pressure drop therein.

During operation of an air-cooled fuel cell stack, the air stoichiometry of the oxidant flowing through the cathode flow channels may range from about 10 to about 200 during fuel cell operation. While the air stoichiometry can be designed to be very high, the fuel stoichiometry is still desirably low for fuel efficiency reasons. For example, the fuel stoichiometry may range from 1 .0 to about 2.0, typically about 1 .1 to about 1 .5. A serpentine anode flow channel that traverses the anode flow field is desirable to maximize the length of the anode flow channel in order to create enough pressure drop within the channel to remove impurities and liquid water, particularly when operating with a dead-ended anode. In some embodiments, a single

serpentine flow channel is used on the anode flow field.

Recesses 1 14a, 1 14b, 1 14c should be sized such that they allow for liquid water removal. In addition, recesses 1 14a, 1 14b, 1 14c are not necessarily the same size. For example, the recesses near the outlet could be slightly bigger in cross-sectional area than the recesses at the inlet to help with liquid water removal at the outlet. Again, one skilled in the art will be able to determine the size of the cathode flow channels to allow for different recess sizes while reducing pressure drop in the cathode flow channels.

As discussed, to form a fuel cell stack, a number of fuel cells are stacked together such that the MEA is interposed between two bipolar plates. However, for air-cooled fuel cells, one skilled in the art will appreciate that when two identical flow field plates are stacked on top of each other and separated by an MEA between the two flow field plates, the landings and channels will overlap one another and create an "egg-box" effect, which will damage the MEA due to flexure.

As shown in Figure 7 (wherein the MEA is removed between the plates), the flow field channels and landings of one flow field plate are offset with the flow field landings and channels of the adjacent flow field plate so that the MEA can be compressed without any flexural movement. In this manner, the anode landing aligns and overlaps with the cathode landing of the opposite plate. Furthermore, the flow field plate is asymmetrical such that the flow field features of the flow field plate are biased towards one manifold of the plate, so that when the flow field plate is rotated 180 degrees in the x-y plane, the features of the adjacent plate will overlap one another such that the anode landing overlaps with the cathode landing of the opposite plate. This allows only one flow field plate that needs to be formed, thereby reducing production costs. Flow field plate 100 may be any suitable material, such as, but not limited to, graphitic, carbonaceous, or metallic, and combinations thereof. For example, flow field plate 100 may be a stainless steel metal coated with an electrically conductive coating. The advantage of using a metallic material is that the plate may be formed simply, for example but not limited to, by stamping or hydroforming. However, flow field plate 100 may also be, for example, a graphite plate or an expanded graphite plate with molded or machined features.

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, and U.S. Provisional Patent Application No.

62/478,440, filed March 29, 2017, are incorporated herein by reference, in their entirety. The various embodiments described above can be combined to provide further embodiments.

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.