Patent Application
20090162733
The present invention relates generally to a flow field separator including gas distribution features that reduce blockage of gases in the flow field plate.
A typical fuel cell system includes a power section in which one or more fuel cells generate electrical power. A fuel cell is an energy conversion device that converts hydrogen and oxygen into water, producing electricity and heat in the process. Each fuel cell unit may include a proton exchange member (PEM) with gas diffusion layers on either side of the proton exchange member. Anode and cathode catalyst layers are respectively positioned between the gas diffusion layers and the PEM. This unit is referred to as a membrane electrode assembly (MEA). Separator plates (also referred to herein and flow field plates or bipolar plates) are respectively positioned on the outside of the gas diffusion layers of the membrane electrode assembly. This type of fuel cell is often referred to as a PEM fuel cell.
The reaction in a single MEA typically produces less than one volt. Therefore, to obtain operating voltages useful in most applications, a plurality of the MEAs may be stacked and electrically connected in series to achieve a desired voltage. Electrical current is collected from the fuel cell stack and used to drive a load. Fuel cells may be used to supply power for a variety of applications, ranging from automobiles to laptop computers.
The efficiency of the fuel cell power system depends on the flow of reactant gases across the surfaces of the MEA as well as the integrity of the various contacting and sealing interfaces within individual fuel cells of the fuel cell stack. Such contacting and sealing interfaces include those associated with the transport of fuels, coolants, and effluents within and between fuel cells of the stack. Proper positional alignment of fuel cell components and assemblies within a fuel cell stack is critical to ensure efficient operation of the fuel cell system.
Embodiments of the invention involve a flow field plate for a fuel cell having features that enhance gas distribution in the flow field channels. The flow field plate includes an input area providing fluid communication between an input manifold and the flow field channels of the flow field plate. The flow field channels are disposed on at least a first surface of the flow field plate and are configured to distribute the reactant gas substantially evenly over a gas diffusion layer. The input area includes one or more input channels which are defined by input channel walls. The input channels direct the reactant gas from the input manifold to one or more of the flow field channels. One or more features of the input area enhance distribution of the reactant gas to the flow field channels. The gas distribution enhancement features may provide support for a sealing element to reduce blockage of the channels and/or may provide an alternate a path for fluid communication between adjacent input channels in the event a blockage occurs.
The gas distribution enhancement features may be located in a seal region of the input area. In some embodiments, the gas distribution enhancement features comprise seal support features. For example, the seal support features may be positioned within one or more of the input channels and/or aligned relative to gaps in the input channel walls and/or may be positioned within the gaps in the input channel walls. According to one aspect, a first group of the seal support features differs from a second group of seal support features. For example, one group of the seal support features may differ in one of both of cross sectional area and shape from a second group of the seal support features.
In some embodiments, the gas distribution enhancement features include gaps in one or more input channel walls. The gaps provide a path for gas flow between adjacent input channels. In one implementation, at least one channel wall of each input channel includes multiple discontinuities. In one implementation, at least two channel walls include the gaps and the gaps are staggered.
Another embodiment of the invention is directed to a fuel cell assembly. The fuel cell assembly includes a fuel cell membrane electrode assembly (MEA), comprising first and second gas diffusion layers (GDLs), and a membrane provided between anode and cathode catalytic layers. A sealing system is arranged relative to a periphery of the MEA. The fuel cell assembly includes first and second flow field plates arranged relative to the MEA and the sealing system.
Gas flow to the flow field of each flow field plate is facilitated by a manifold. A pattern of flow field channels is disposed on at least one surface of the flow field plates, the flow field channels are arranged in a pattern to distribute a reactant gas substantially evenly over an adjacent GDL. The flow field plates include an input area between the manifold and the flow field channels. The input area comprises one or more input channels each defined by input channel walls. The input channels are configured to direct the reactant gas from the input manifold to one or more flow field channels. The input area also includes one or more features that enhance distribution of the reactant gas to the flow field channels. The gas distribution enhancement features provide one or both of support for a sealing element to reduce blockage of the input channels and a path for fluid communication between adjacent input channels.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.
While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It is to be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.
In the following description of the illustrated embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, various embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made to the illustrated embodiments without departing from the scope of the present invention.
Embodiments of the invention are directed to flow field plates that incorporate features providing enhanced gas distribution in the reactant gas input area of the flow field plate. In some embodiments, the features include input channel wall discontinuities resulting in gaps that allow flow of reactant gases or other fluids between adjacent input channels. In some embodiments, the features include support features within the channel and/or within the channel wall gaps that provide support for a sealing element, such as a gasket or o-ring. In embodiments that include discontinuous channel walls, the arrangement of the features may provide for both supporting the sealing element while also allowing gas flow between adjacent channels. It will be appreciated that many configurations of features for enhancing gas distribution are possible. Accordingly, the specific illustrative embodiments described below are for purposes of explanation, and not of limitation.
A flow field plate of the present invention may be incorporated in fuel cell assemblies and stacks of varying types, configurations, and technologies. For example, a flow field separator including features for enhancing gas distribution can be employed in proton exchange membrane (PEM) fuel cell assemblies. PEM fuel cells operate at relatively low temperatures, have high power density, can vary their output quickly to meet shifts in power demand, and are well suited for applications where quick startup is required, such as in automobiles for example.
Although generally illustrated herein in conjunction with PEM fuel cells, flow field separators in accordance with embodiments of the invention may also be employed in other types of fuel cells, including direct methanol fuel cells (DMFC). Direct methanol fuel cells are similar to PEM cells in that they both use a polymer membrane as the electrolyte. In a DMFC, however, the anode catalyst itself draws the hydrogen from liquid methanol fuel, eliminating the need for a fuel reformer.
A typical proton exchange member fuel cell is depicted in
In operation, hydrogen fuel is introduced into the anode portion of the fuel cell 110, passing over the first flow field separator 112 and through the GDL 114. At the interface of the GDL 114 and the CCM 120, on the surface of the catalyst layer 115, the hydrogen fuel is separated into hydrogen ions (H+) and electrons (e−).
The electrolyte membrane 116 of the CCM 120 permits only the hydrogen ions or protons and water to pass through the electrolyte membrane 116 to the cathode catalyst layer 113 of the fuel cell 110. The electrons cannot pass through the electrolyte membrane 116 and, instead, flow through an external electrical circuit in the form of electric current. This current can power an electric load 117, such as an electric motor, or be directed to an energy storage device, such as a rechargeable battery.
Oxygen flows through the second GDL 118 at the cathode side of the fuel cell 110 via the second flow field separator 119. On the surface of the cathode catalyst layer 113, oxygen, protons, and electrons combine to produce water and heat.
Individual fuel cells, such as the fuel cell shown in
Sealing the fuel, oxidant, coolants, and other fluids within each fuel cell in a stack is critical to the efficient operation of the fuel cell stack. Gaskets are typically deployed around the perimeter of the active area of the electrolyte membrane. Gaskets may be placed on one or both surfaces of the electrolyte membrane and/or on one or both catalyst layers and/or on one or both surfaces of the gas GDLs. The gaskets are important to seal against leaks in the peripheral areas and/or edges of the electrolyte membrane, GDLs and the flow field plates that face the GDLs. In some configurations, a sealing system may include both gaskets and o-rings. The sealing system can bridge the gas input areas of the flow field plates as described in more detail below.
In one configuration, a membrane layer 222 is fabricated to include an anode catalyst coating 230 on one surface and a cathode catalyst coating 232 on the other surface. This structure is often referred to as a catalyst-coated membrane or CCM. The GDLs 224, 226 can be fabricated to include or exclude a catalyst coating. In one configuration, an anode catalyst coating 230 can be disposed partially on the first GDL 224 and partially on one surface of the membrane 222, and/or a cathode catalyst coating 232 can be disposed partially on the second GDL 226 and partially on the other surface of the membrane 222.
In the particular embodiment shown in
In the configuration depicted in
The edge seal systems 234, 236 provide the necessary sealing within the fuel cell to isolate the various fluid (gas/liquid) transport and reaction regions from contaminating one another and from inappropriately exiting the fuel cell 220, and may further provide for electrical isolation and/or hard stop compression control between the flow field plates 240, 242. The term “hard stop” generally refers to a nearly or substantially incompressible material that does not significantly change in thickness under operating pressures and temperatures. More particularly, the term “hard stop” refers to a substantially incompressible member or layer in a membrane electrode assembly (MEA) which halts compression of the MEA at a fixed thickness or strain.
The sealing systems 234, 236, may employ one or more gaskets, sub-gaskets and/or O-rings to effect sealing of the edges of the MEA 225 and sealing between and around the MEA 225 and the flow field separators 240, 242. In one configuration, the sealing systems 234, 236 include a gasket system formed from one, two or more layers of various selected materials employed to provide the requisite sealing within the fuel cell 220. Such materials include, for example, TEFLON, fiberglass impregnated with TEFLON, an elastomeric material, UV curable polymeric material, surface texture material, multi-layered composite material, sealants, and silicon material. Other configurations employ an in-situ formed seal system.
In certain embodiments, a fuel cell stack may use bipolar flow field plates, as illustrated in
Similarly, MEA 325b includes a cathode 362b/membrane 361b/anode 360b layered structure sandwiched between GDLs 366b and 364b. GDL 364b is situated adjacent a flow field end plate 354, which is configured as a unipolar flow field plate. GDL 366b is situated adjacent a second flow field surface 356b of bipolar flow field plate 356. Sealing systems 371b and 372b are deployed to provide sealing for MEA 325b and flow field end plate 354 and MEA 325 and bipolar flow field plate 356, respectively.
It will be appreciated that in other configurations, N number of MEAs 325 and N−1 bipolar flow field plates 356 can be incorporated into an N-cell fuel cell stack.
The fuel cell and/or stack configurations shown in
Embodiments of the invention are directed to features provided in the input area 408 of the flow field plate 400 that enhance distribution of gases that flow between the openings 406 the flow field 418. The input area is shown in enlarged detail in
Features incorporated into the input area 408 in accordance with various embodiments involve gaps 401 in the input channel walls 411 that provide a path for gas to flow between adjacent input channels. In the event that one input channel becomes blocked, an adjacent, unblocked input channel in the input area 408 can provide reactant gas to the blocked channel.
Embodiments of the invention illustrate various configurations of features arranged in the input area of a flow field plate that provide enhanced gas distribution to the flow field. In some embodiments, as illustrated in the plan view of
In some embodiments, the features which enhance gas distribution comprise discontinuities in the input channel walls.
In some embodiments, as illustrated in
The support features may take on any shape that provides support for the seal and allow low resistance gas flow. In some configurations, a first group of support features may differ from another group of support features in shape and/or cross sectional area. One or both of the support feature groups may be aligned relative to gaps in the input channel walls.
The use of support features and/or discontinuous channel walls within the input area of a flow field plate allows more even distribution of reactant gases to the flow field. If an input channel is blocked or restricted, then regions of the fuel cell are starved of reactants and will start to drop in voltage or become unstable. In a blockage situation, the voltage of the blocked cell can drop below that of the neighboring cells causing premature failure of the entire fuel cell stack. The use of gas distribution enhancement features, including seal support features and/or discontinuous input channel walls, advantageously reduces the occurrence of blockages and/or provides alternate paths for supplying gas to the flow field in the event an input channel becomes blocked.
The foregoing description of the various embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
