Four-fluid bipolar plate for fuel cell

Abstract

A four-fluid bipolar plate for a fuel cell includes an oxidant flow field, a fuel reactant flow field, a dedicated coolant passage, and a water management flow field. The bipolar plate includes at least one porous layer. The first side of the porous layer is fluidly connected to the water management flow field via a plurality of pores that act as a bubble barrier. An opposing second side of the porous layer includes a fuel reactant flow field or an oxidant flow field. In one example, a dedicated coolant passage is located inside the bipolar plate and may be configured to flow antifreeze-type coolant.

Description

This application is a Divisional of PCT Patent Application entitled “FOUR-FLUID BIPOLAR PLATE FOR FUEL CELL,” filed June 4, 2022, by Applicant NIBUS POWER SYSTEMS LLC, having Application Number 202280037078.3 (International Application Number PCT / US2022 / 032283). Cross Reference to Related Applications

[0001] This application claims priority to and the benefit of U.S. Patent Application Serial No. 17 / 344,377, filed June 10, 2021, entitled “FOUR-FLUID BIPOLAR PLATE FOR FUEL CELL,” which is incorporated by reference herein in its entirety. TECHNICAL FIELD

[0002] The present disclosure relates generally to fuel cell bipolar plates, and more particularly to a bipolar plate structure that provides improved delivery of humidified reactants and better removal of product water. BACKGROUND

[0003] In a proton exchange membrane (PEM) fuel cell, hydrogen fuel is supplied to the negative electrode (anode), where it is catalytically decomposed into protons and electrons according to the oxidation reaction H2→ 2H + + 2e - The protons (H + ) pass through a membrane electrolyte to the positive electrode (cathode), while the electrons (e - ) are conducted through an external path, creating an electric current through an external load between the anode and the cathode. At the cathode, the protons and electrons recombine in the presence of oxygen to form water according to the reduction reaction: O2+ 4e - + 4H + → 2H2O. The byproducts of the PEM fuel cell reaction are water and heat; the heat requires the fuel cell to be cooled to maintain an acceptable internal temperature.

[0004] A single fuel cell includes a membrane electrode assembly (MEA) that includes a membrane electrolyte interposed between a pair of electrodes (anode and cathode), and a conductive plate adjacent to each electrode opposite the membrane electrolyte that defines a flow field for the reaction gas. A typical flow field plate directs the reaction gas through a gas diffusion layer and a microporous layer to their respective electrodes. In some designs, the flow field plate can also transport byproduct water away from the cell.

[0005] A plurality of fuel cells are typically arranged in series and connected in a stack to increase the electrical output of the electrochemical conversion assembly or fuel cell. In such an arrangement, two adjacent cell units can share a common bipolar plate that acts as both an anode and a cathode for two adjacent cell units connected in series therewith. Such a bipolar plate is commonly referred to as a "bipolar plate." SUMMARY

[0006] In one embodiment, a bipolar plate for a fuel cell includes a non-porous sub-plate including at least one water management side and an internal coolant passage. The bipolar plate further includes a porous sub-plate including a reactant side and an opposing water management side. The reactant side of the porous sub-plate includes a first reactant flow field and the water management side is fluidly connected to the water management side of the non-porous sub-plate.

[0007] In another embodiment, a bipolar plate for a fuel cell includes an oxidant flow field, a fuel reactant flow field, a dedicated coolant passage, and a water management flow field.

[0008] In yet another embodiment, a bipolar plate for a fuel cell includes a non-porous sub-plate including a water management side and a reactant side. The reactant side includes a first reactant flow field. The bipolar plate further includes a porous sub-plate including a reactant side and an opposing water management side. The reactant side includes a second reactant flow field. The water management side of the porous sub-plate is fluidly connected to the water management side of the non-porous sub-plate. BRIEF DESCRIPTION OF DRAWINGS

[0009] The features described herein can be better understood with reference to the drawings described below. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the drawings, like reference numerals are used to refer to like parts throughout the various views.

[0010] Figure 1 depicts a schematic cross-sectional exploded view of a typical fuel cell;

[0011] Figure 2 depicts a schematic cross-sectional view of a typical fuel cell power plant;

[0012] Figure 3 depicts an exploded perspective view of the anode side of a bipolar plate according to one embodiment of the invention;

[0013] Figure 4 depicts Figure 3 an exploded perspective view of the cathode side of the bipolar plate shown;

[0014] Figure 5 depicts Figure 3 another exploded view of the bipolar plate shown;

[0015] Figure 6depicted Figure 4 another exploded view of the bipolar plate shown;

[0016] Figure 7 depicted Figure 3 perspective cross-sectional view of the cathode side of the bipolar plate shown;

[0017] Figure 8 depicted Figure 7 enlarged cross-sectional view of the bipolar plate shown;

[0018] Figure 9 depicted Figure 3 another perspective cross-sectional view of the cathode side of the bipolar plate shown;

[0019] Figure 10 depicted Figure 9 enlarged cross-sectional view of the bipolar plate shown;

[0020] Figure 11 depicted

[0021] Figure 12 depicted

[0022] Figure 13 depicted

[0023] Figure 14 depicted

[0024] Figure 15 depicted

[0025] Figure 16 depicted

[0026] Figure 17 depicted

[0027] Figure 18 depicted

[0028] Figure 19 depicted DETAILED DESCRIPTION

[0029] Figure 1 A typical polymer electrolyte membrane (PEM) fuel cell 10 is shown, which generally includes a negative electrode (anode) 12 and a positive electrode (cathode) 14 separated by an ionomer membrane 16. Anode catalyst layer 18 a and cathode catalyst layer 18 c are formed on respective sides of the generally planar membrane to convert hydrogen and oxygen reaction gases into electricity and water. This assembly is typically referred to as a membrane electrode assembly (MEA) 20. Catalyst layers 18 a , 18 c may be the same for anode 12 and cathode 14, but typically they are different. For example, anode catalyst layer 18 a may be used to split hydrogen atoms into hydrogen ions and electrons, while cathode catalyst layer 18 c may be used to react oxygen and electrons to form water.

[0030] Reactants (i.e., hydrogen and air) are directed to MEA 20 by flow field plates 22, which typically include reactant flow channels (indicated by dashed lines). Flow field plates 22 are shown as bipolar plates, which include reactant flow channels for both fuel and oxidant. The reactants pass from these channels through gas diffusion layers (GDLs) 24 a , 24 c , which abut flow field plates 22, then through microporous layers (MPLs) 26 a , 26 c , which are located between the GDLs and respective catalyst layers 18 a , 18 c . The GDLs can have several functions, including diffusing the reactant gas flow to the catalyst layers, transporting liquid and vapor water byproducts from the catalyst layers to the cathode gas channels (where they are carried away by the gas flow), collecting the electrical current produced by the electrochemical reactions, and providing mechanical strength to support and protect the catalyst-coated membrane. The GDLs are typically highly porous (e.g., 60-90%) non-woven carbon fiber paper or woven carbon fiber cloth, about 0.25-0.35 mm thick, with pore sizes on the order of hundreds of microns, and can be treated with various proprietary substances to improve performance. The role of the MPL is to minimize the contact resistance between the GDL and the catalyst layer, and to help improve water transport. The MPL is typically composed of a thin layer of carbon powder and PTFE particles coated onto the GDL, with pore sizes on the order of one micron. Some fuel cells are manufactured to produce the membrane electrode assembly (MEA), microporous layer (MPL), and gas diffusion layer (GDL) as a one-piece assembly, referred to as a unitized electrode assembly (UEA) 28.

[0031] Figure 2 A fuel cell stack 30 is shown using a unitized electrode assembly (UEA) 28 as Figure 1The fuel cell 10 described herein is a stack of general-purpose fuel cell power generation devices 30. Fuel (e.g., hydrogen (H2)) is supplied to fuel inlet 32 ​​and flows through an anode flow field plate to be distributed to the anode catalyst layer. Unconsumed fuel flows out of fuel outlet 34 and may flow back to fuel inlet 32 ​​via a recirculation pump (not shown), and may be periodically purged into the environment. Oxidant (e.g., air) is supplied to air inlet 36 by a blower (not shown) and flows through a cathode flow field plate to be distributed to the cathode catalyst layer. Excess process air, which has been humidified by byproduct water, flows out from air outlet 38 and may pass through a radiator and / or condenser (not shown) before being discharged into the environment.

[0032] The power generation unit 30 may further include a coolant circuit 40 for removing heat from the fuel cell. In many automotive applications, the coolant is a mixture of water and ethylene glycol to prevent it from freezing in cold climates. A pump 42 supplies coolant to a coolant inlet 44, where it is then directed through radiator plates (not shown, but typically located between the fuel cells 10) and distributed across the surface of the plates. The fuel cell 10 transfers sensible heat to the circulating coolant, thus making it hotter without undergoing a phase change. As the coolant exits the stack at a coolant outlet 46, it passes through a heat exchange device 48, whereby the sensible heat is expelled before circulating back to the inlet 44. In one example, the heat exchange device 48 is a radiator. The coolant flow can be regulated using a flow control valve or orifice 50.

[0033] like Figure 1 As shown, the reactant flow field plate 20 is a bipolar plate. Most bipolar plate designs use solid materials, with only a small percentage using porous materials on both the anode and cathode sides. Each design has its own advantages and disadvantages. As the name suggests, solid bipolar plates are impermeable to hydrogen fuel and therefore perform excellently in maintaining the separation of reactant gases. Furthermore, due to the impermeability of solid bipolar plates, sealing them in a stack is relatively simple. Therefore, the power generation stack can be pressurized, which improves battery performance and reduces battery degradation. Another advantage of solid bipolar plates is that their impermeability allows the use of antifreeze coolants, such as water / ethylene glycol mixtures (WEG), in the stack. This antifreeze coolant is highly beneficial for fuel cells operating in cold environments (such as automotive applications). However, WEG can poison the membrane electrode assembly (MEA), so care must be taken to isolate the WEG from the MEA.

[0034] Solid bipolar plates can be made of metal, such as stainless steel or titanium. Since the flow field geometry can be formed by conventional, high volume manufacturing methods, such as stamping, etc., metal plates can be produced in bulk at low cost. Solid bipolar plates can also be made of non-porous carbon or polymeric (composite) materials. Solid carbon or composite plates can be produced in bulk by molding, etc., and dimensional tolerances can typically be held tighter than with metal formed plates. However, solid carbon or composite plates are more expensive to produce than metal plates.

[0035] While solid bipolar plates can be useful and can be advantageous for certain applications, they also have disadvantages. One disadvantage of metal plates is that they are susceptible to corrosion from the presence of air and water, which have very high electrochemical potentials. The corrosion layer is not electrically conductive, and as the plate continues to corrode, the fuel cell loses performance. Coatings have been developed and applied to the plates to mitigate corrosion, but even this technology has operational limits.

[0036] In particular, the automotive industry can target a useful life of a fuel cell of 5,000 hours. Some coatings on metal plates have reportedly achieved this target. However, the heavy duty vehicle industry can require a useful life of 30,000 hours. Current automotive coatings or constructions are not close to this limit. Thus, the heavy duty vehicle industry requires the development of fuel cells with longer operational limits, possibly up to 30,000 hours.

[0037] Another disadvantage of solid plates is that they have no inherent water management capability. In the operation of a PEM fuel cell, it is critical that an appropriate water balance be maintained between the rate at which water is produced at the cathode electrode (including water produced by the dragging of protons through the PEM electrolyte) and the rate at which water is removed from the cathode electrode or supplied to the anode electrode. For a PEM fuel cell, if not enough water is returned to the anode electrode, adjacent portions of the PEM electrolyte can dry out, reducing the rate at which hydrogen ions can be transferred through the PEM, and also causing crossover of the reducing fluid, resulting in localized overheating. Similarly, if not enough water is removed from the cathode, the cathode electrode can become flooded, effectively limiting the oxidant supply to the cathode, and thus reducing the current. Furthermore, if too much water is removed from the cathode, the PEM can dry out, limiting the ability of hydrogen ions to cross the PEM, thus reducing cell performance. Solid plates typically require external water management means, such as external humidifiers, to prevent the MEA from drying out and cracking.

[0038] Porous bipolar plates (sometimes referred to as water transport plates) are porous separators used on the cathode side and anode side of electrodes in fuel cells. The porous bipolar plates strictly control pore size to form a bubble barrier that allows liquid transport through the pores into a liquid water cavity but prevents reaction gas transport during fuel cell operation. Liquid transport allows membrane hydration and enables removal of product water generated on the cathode side by electrochemical reactions within the fuel cell. Preventing reaction gas transport stops fuel gas and oxidant gas from escaping into the liquid water cavity.

[0039] The porous plate provides excellent water balance to wick away excess water in the flow field channels and migrate to areas that have lost water due to evaporation, while maintaining membrane electrode assembly hydration. The porous bipolar plate is exposed to the water flow field to maintain the desired operation of the fuel cell. In local areas of the cell where reaction gas flows from lower temperature areas to higher temperature areas, water evaporates from the porous plate, saturating the gas stream with water vapor; in areas where reaction gas moves from higher to lower temperature, product water formed in the electrochemical reaction and liquid water condensed from the cooling gas stream can be wicked away by the porous plate. As a result, one advantage of fuel cell systems having porous bipolar plates is that they exhibit very high durability. Another advantage is that systems having porous bipolar plates do not require the use of an external humidifier, which can reduce weight and complexity.

[0040] Typically, a pump driven recirculation water loop can be utilized to provide both the cell cooling function and the driving force to move water through the pores of the water transport plate to remove product water.

[0041] Despite the advantages of porous bipolar plates, they also have disadvantages. For example, porous bipolar plates can be costly to mass produce due to the difficulty of manufacturing plates with specific pore sizes. Another disadvantage is that porous plates are difficult to seal, which can cause reliability issues in pressurized systems. Another significant disadvantage is that fuel cell systems utilizing porous bipolar plates cannot use an anti-freeze type coolant (such as WEG) in the water cooling loop because the coolant would be drawn into the pores of the plate and poison the MEA.

[0042] Embodiments of the disclosed invention address many of the foregoing problems of bipolar plates by utilizing a four-fluid plate construction that provides a fuel reactant flow field, an oxidant flow field, a water management flow field, and a dedicated coolant passageway for an anti-freeze type coolant. Embodiments include a non-porous plate portion and a porous plate portion, which are intelligently selected to capture the best aspects of both designs while reducing or eliminating the associated disadvantages. The four-fluid bipolar plate can be easily manufactured to reduce costs.

[0043] Reference is made to Figure 3 and Figure 4A bipolar plate 100 for a fuel cell includes a non-porous sub-plate 102 and a porous sub-plate 104. In one embodiment of the invention, the non-porous sub-plate 102 includes a reactant side 106 (as shown in Figure 3 ) and an opposing water management side 108 (as shown in Figure 4 ). As shown, the reactant side 106 supplies hydrogen gas to the anode side of the MEA via a fuel flow field. Non-limiting examples of flow fields include cavities, porous substrates, or fuel flow field channels 110 as shown in the illustrated embodiment. The non-porous sub-plate 102 further includes internal coolant passages 112 Figure 8 and Figure 11 that isolate the freeze-resistant coolant (such as WEG) from other components in the fuel cell. Other general features of the non-porous sub-plate 102 can include internal manifolds 114 for fuel supply 114a and return 114b, oxidant supply 114c and return 114d, water management supply 114e and return 114f, and WEG coolant supply 114g and return 114h. Sealing means 116 allow multiple fuel cells to be sealed and operated under pressure.

[0044] Figure 4 The opposing side of the bipolar plate 100 is shown. The water management side 108 of the non-porous sub-plate 102 includes a water flow field. Non-limiting examples of flow fields include cavities, porous substrates, or water channels 118 as shown in the illustrated embodiment that form part of an external circulating water management loop 150 Figure 13 , which allows for proper water management of the cathode flow field, as discussed in detail below. Water enters the plate channels through the water management supply manifold 114e and exits through the water management return manifold 114f.

[0045] The porous sub-plate 104 includes a reactant side 120 and an opposing water management side 122. The reactant side 120 supplies oxidant (e.g., air) to the cathode side of the MEA via an oxidant flow field. Non-limiting examples of flow fields include cavities, porous substrates, or oxidant flow field channels 124 as shown in the illustrated embodiment. The water management side 122 Figure 3 ) is featureless in this embodiment (e.g., flat), but plays a critical role in maintaining optimal cell performance and durability.

[0046] The porous sub-plate 104 can be made of graphite or other carbon-based materials, and can also be made of metals such as titanium or stainless steel. Features such as channels can be formed by hydro-forming, casting, thermo-forming, 3D printing / additive manufacturing, or milling / machining.

[0047] As previously mentioned, the size of the pores in the porous sub-plate 104 are designed to create a bubble barrier during fuel cell operation. The pore size is determined by the specific fuel cell operating conditions and pressures. For graphite or other carbon-based materials, the pores can be formed in the plate by known processes. For example, U.S. Patent No. 6,197,442 details a manufacturing process in which graphite powder, reinforcing fibers, cellulose fibers, and a thermoset resin are mixed with a liquid to form a slurry and the slurry is sprayed onto a web to form a planar sheet, the sheet is dried to form a paper. The paper is cut to the desired size and stacked. The stack is laminated, carbonized, and graphitized by pressure and heat to form a water transport plate, which is subsequently machined as needed. The finished porous plate exhibits superior physical properties in terms of bubble pressure, water permeability, median pore size, porosity, through-plane resistivity, and compressive yield strength. For metal porous plates, the pores can be formed by, for example, a punch press or laser drilling.

[0048] Figure 5 and Figure 6 depicts another exploded view of the non-porous sub-plate 102 according to the first embodiment of the present application. The non-porous sub-plate 102 can be formed from two half-plates 102A and 102B that are easily manufactured and then joined together. For example, the half-plates can be made from a metal such as stainless steel or titanium, the flow channels and other features can be formed by metal stamping or the like, and the two half-plates are joined together by welding. Other non-limiting examples of joining methods include, for example, laser welding, brazing, thermoplastic bonding, or adhesives. The half-plate 102A in the illustrated embodiment includes the fuel flow field channels 110 Figure 5 ) on the reactant-facing side and the WEG coolant half-channels 126A Figure 6 ) on the opposite side. The half-plate 102B includes the water channels 118 Figure 6 ) on the water management side 108 and the WEG coolant half-channels 126B Figure 5 ) on the opposite side.

[0049] Reference is made to Figure 7 and Figure 8 for further details, wherein, Figure 7 depicts a cathode-side cross-sectional view of the bipolar plate 100 taken approximately at Figure 4 depicts an enlarged view of the plate shown at Figure 8 depicts a cathode-side cross-sectional view of the bipolar plate 100 taken approximately at Figure 7 depicts an enlarged view of the plate shown at Figure 8The non-porous sub-plate 102 and the porous sub-plate 104 are shown in more detail. For clarity, the half-plates 102A, 102B are shown separated (e.g., prior to joining). Each half-plate can include a row of raised surfaces 128, and the valleys 130, 132 between these raised surfaces can define fluid flow channels on the outer surface of the non-porous plate. The raised surfaces 128 on one side of the plate define recesses 134 on the opposite side of the same plate. When the two half-plates 102A, 102B are joined together, the recesses can define an internal cavity 136. In one example, the valleys 130 on the half-plate 102B define the water management channels 118, the valleys 132 on the half-plate 102A define the fuel flow field channels 110, and the internal cavity 136 defines the internal freeze protection coolant passage 112.

[0050] The reactant side 120 of the porous sub-plate 104 includes oxidant flow field channels 124 to supply air to the MEA. In one example, the channels 124 are transverse to the fuel flow field channels 110. The water management side 122 of the porous sub-plate 104 is positioned against the flat raised surface 128 of the half-plate 102B. In this way, when deionized (DI) water is circulated through the water channels 118, the porosity within the porous sub-plate 104 is in fluid communication with the deionized water, allowing the sub-plate 104 to become and remain fully saturated with liquid.

[0051] The desired porosity in the porous sub-plate 104 can be achieved by any suitable method known in the fuel cell art. For example, the porous sub-plate 104 can be constructed as a water transport plate (WTP), a net shape molded from a slurry with appropriate particle size, or laser-drilled to achieve the desired pore size.

[0052] Figure 9 An alternative cross-sectional view of the bipolar plate 100 is depicted; Figure 10 A portion of this view is enlarged to illustrate one possible construction. Turning to Figure 10 , the cross-sectional view includes the half-plate 102A, the half-plate 102B, and the porous sub-plate 104. Similar to Figure 8 , the half-plates 102A and 102B are shown slightly separated for clarity. Also shown are the recesses 134 in the half-plate 102A, which form the WEG coolant half-channels 126A.

[0053] The porous sub-plate 104 can be sealed to the non-porous sub-plate 102 by conventional means to prevent gas leakage or water leakage. For example, the sealing means 116 can include adhesive, nesting, interference fit, or a groove for receiving a molded compression seal, gasket, or O-ring. In one example, the porous sub-plate 104 can nest into a recess 138 formed in the water management side 108 of the non-porous sub-plate 102. The recess 138 spans the entire planform of the porous sub-plate 104 to effectively capture the plate and ensure proper alignment during assembly. In some examples, the recess 138 can reduce the overall thickness of the bipolar plate 100, as the porous sub-plate 104 is essentially recessed into the thickness of the other plate and only minimally increases the overall thickness dimension.

[0054] Figure 11 A cross-sectional view of a proton exchange membrane (PEM) fuel cell 140 having a bipolar plate 100 according to a first embodiment of the present invention is depicted, Figure 12 A stack of such fuel cells is depicted, and Figure 13 A cross-section of a fuel cell power plant 144 having the disclosed bipolar plate 100 is depicted. In the example shown, the oxidant flow field channels 124 are shown as parallel to the fuel flow field channels 110, but this is for illustrative purposes and the convention will be followed for other embodiments. The fuel cell 140 includes the bipolar plate 100 between an upper combined electrode assembly 28 (UEA) and a lower combined electrode assembly (UEA). The bipolar plate 100 abuts each UEA 28.

[0055] In operation, hydrogen is introduced at the inlet 114a and passes through the fuel flow field channels 110 in the non-porous sub-plate 102 to the anode side of the UEA 28. Air is introduced at the inlet 114c and passes through the oxidant flow field channels 124 in the porous sub-plate 104 to the cathode side of the UEA 28. A water pump 146 circulates water through a water management circuit 150 including a water softener 148. The softened water, or deionized (DI) water, passes through the water management supply 114e and through the channels 118 formed by the non-porous sub-plate 102 and the porous sub-plate 104 into the stack 144. The porosity in the porous sub-plate 104 is filled with water, and the sub-plate acts as a sponge to hold water and maintain hydration of the UEA 28. The porous sub-plate 104 can transport the liquid directly to the UEA 28, or the porous sub-plate can evaporate the water and the water vapor can migrate to the UEA through the air stream. The porous sub-plate 104 can also remove product water formed by the reaction at the cathode from the UEA 28. By maintaining the pressure in the water management circuit 150 below the pressure of the reactants, the product water in liquid form can be driven directly into the porosity of the porous sub-plate 104. If the product water is in vapor form, it condenses on the porous sub-plate and is absorbed back into the circulating water circuit.

[0056] Thermal management is controlled primarily by a dedicated and isolated coolant loop 152. A coolant pump 154 flows coolant through a coolant supply 114g into the stack 144 and out of the stack 144 through a coolant return 114h. Between the coolant supply and the coolant return, in some configurations, the coolant is distributed across the surface of the cells 140. In the illustrated embodiment, the coolant flows through internal passages 112 formed by the joining of half-plates 102A and 102B. Figure 10 ). When the coolant exits the stack at the coolant return 114h, the coolant passes through a heat exchange device 156, whereby sensible heat is rejected before being circulated back to the supply 114g. In one example, the heat exchange device 156 is a radiator. The coolant flow can be regulated using a flow control valve or orifice 158.

[0057] The impermeable nature of the non-porous sub-plate 102 eliminates the need for separate coolant plumbing and allows the coolant passages to be located inside the sub-plate 102, which saves space compared to some designs that add a separate cooler plate. As previously mentioned, this design allows the use of a non- freezing type coolant, such as a water / glycol mixture (WEG), which is beneficial for fuel cells operating in cold environments.

[0058] In the illustrated embodiment, the coolant flows through internal passages formed by the joining of half-plates 102A and 102B. However, other means of distributing the coolant are within the scope of the present invention. For example, the internal coolant passages can be defined by a cavity containing a porous substrate that distributes the coolant.

[0059] In most cases, an external humidifier is not required in the disclosed embodiments, but there are situations where adding an external humidifier can be beneficial to the system. For example, if the bipolar plate 100 is using only passive water management features and is operating in a particularly hot and dry environment, water can evaporate from the porous sub-plate faster than product water is being generated by the fuel cell. In such an environment, it can be advantageous to add an external humidifier 159 Figure 13 ) to the system, rather than incorporating active cooling features as detailed in other embodiments herein.

[0060] In the illustrated embodiment, there is no porous media present in the anode channel 110. Under certain operating conditions, such as when there are localized cooling regions, moisture can condense in the anode channel, causing water to accumulate. The water must be periodically removed to prevent performance degradation at the anode electrode. Prior art solutions to this problem include attempts to blow the water out, which involves additional operational steps and consumes parasitic power. In one embodiment, as Figure 11 and Figure 12As shown, one or more small weep holes 142 can be drilled from the bottom of the hydrogen gas channel to communicate with the DI water cavity 118. The weep holes 142 can be sized as bubble barriers to transport excess water from the fuel channel 110 to the water channel 118 without allowing the reaction gases to escape. The DI water loop pressure can be maintained below the pressure of the anode and cathode. In this way, the pressure differential will drive accumulated water through the weep holes 142 into the cavity 118, where the water returns to the DI water loop.

[0061] As noted above, under typical operating conditions, fuel cell power plant thermal management is dominated by the sensible heat cooling loop 152, in which sensible heat is transferred to the circulating coolant passing through the coolant flow field. To a lesser extent, some cell cooling can be provided by evaporative cooling as product water in the pores evaporates, but the evaporative cooling function is typically not considered a control parameter in the sensible heat coolant flow system.

[0062] In contrast to the sensible heat coolant flow approach, evaporative cooling utilizes the heat of vaporization to increase the cooling efficiency of each volume of water by up to one hundred-to-one. The inventors of the present disclosure have determined that enhanced cooling can be achieved via evaporation in certain situations. Thus, in one aspect of the invention, independent operation of the water management loop and the coolant loop can be used to operate a thermal boost mode or a water recovery / accumulation mode.

[0063] In the thermal boost mode, additional cooling is required for a limited duration, such as when the stack is required to deliver a large amount of power. In fuel cell vehicles, especially trucks, the thermal boost mode can be beneficial when climbing steep or long road grades, or operating at high power in hot weather, or any other situation in which the radiator is not large enough to handle the cooling demand. In the thermal boost mode, the thermal management strategy shifts from sensible heat cooling to evaporative cooling to provide greater cooling capacity. In the thermal boost mode, evaporative cooling can account for a large fraction of the total cooling function, and in certain design scenarios can account for 90% or more.

[0064] In operation, when additional cooling is required or calculated to be required, in a first step the coolant flow rate is reduced (i.e., the WEG is decreased), which reduces the sensible heat cooling capacity. As a result, the stack temperature begins to rise, the rate of water evaporation from the pores increases, and significant evaporative cooling is achieved. Then, in a second step, the fuel cell is allowed to warm up or maintain temperature to increase the degree of evaporative cooling. To compensate for the increased water evaporation and prevent the pores from drying out and losing their bubble barrier, in a third step the water flow rate through the water management flow field can be increased. In one example, the increase in water flow rate can be achieved by providing a pump-driven circulating water management loop in fluid communication with the water management flow field and utilizing the pump to increase the water flow rate.

[0065] Because the disclosed evaporative cooling scheme has greater capacity to handle large, short duration heat demand, it provides a better short term thermal management control strategy. The coolant flow rate can be adjusted to a reduced value to achieve an appropriate level of evaporative cooling and desired stack temperature.

[0066] The disclosed thermal boost mode consumes more water in the water management circuit than can be simultaneously replenished by product water formation. Thus, the thermal boost mode is intended to last for a relatively short duration. However, in another aspect of the invention, independent operation of the water management circuit and the coolant circuit can be used to operate a water recovery / accumulation mode. In the water recovery / accumulation mode, the coolant flow (i.e., WEG) is increased above its normal rate to reduce evaporative cooling and produce excess water through condensation within the cell. The excess product water can be collected and retained for future use in the thermal boost mode.

[0067] In one implementation, the water recovery / accumulation mode can be operated during a portion of the cycle when the stack is not demanded (such as when the vehicle is driving on level ground) and air flow through the radiator provides sufficient cooling. In a first step, when additional product water is needed or calculated to be needed, the coolant flow rate in the coolant circuit (i.e., WEG) is increased to increase sensible cooling. As a result, the stack temperature drops, less product water is evaporated via the pores, and instead condensation water formation. In a second step, the fuel cell is allowed to cool down or maintain temperature to condense the excess product water. To compensate for the reduced water evaporation and prevent flooding of the cell, in a third step, the water flow rate through the water management flow field can be reduced. In one example, the reduction in water flow rate can be achieved by providing a pump driven circulation water management circuit in fluid communication with the water management flow field and utilizing the pump to reduce the water flow rate.

[0068] In another implementation, the fuel cell controller can receive sensor inputs or environmental inputs to determine whether the thermal boost mode or the water recovery / accumulation mode is warranted and, if so, to what extent. Non-limiting examples of sensor inputs can include air flow, cathode exhaust temperature, cathode exhaust pressure, water reservoir total capacity, water inventory, water temperature, ambient temperature, coolant return temperature, and water circuit outlet pressure. The controller can command coolant pump and / or water pump flow settings in response to the sensor input values.

[0069] The fuel cell controller can also receive inputs from external environmental factors. Non-limiting examples include payload timing, vehicle route, GPS coordinates, road grade, weather forecast, time of day, and driver behavior. In one example, the controller can receive GPS route data indicating that a steep or long road grade is being approached. The controller can command the stack to operate the water recovery / accumulation mode far enough in advance to collect product water and retain it in the reservoir. Then, when the vehicle encounters the grade, the controller can command the stack to operate the thermal boost mode.

[0070] The operation of the thermal boost mode and the water recovery / accumulation mode are not limited to the disclosed hybrid bipolar plate. The inventors envision that the disclosed methods of operation are possible and beneficial in any four-fluid fuel cell power plant in which an antifreeze-type coolant circuit operates independently of a water management circuit, such as disclosed in U.S. Patent No. 7,135,247. The '247 patent discloses separate individual cooler plates disposed between every other fuel cell.

[0071] The disclosed thermal boost mode and water recovery / accumulation mode provide several benefits and advantages over prior art 3-fluid stacks. One benefit of the thermal boost mode is that parasitic power is reduced because the radiator and fan are actually dialed down instead of up during high power spikes. In prior art stacks, running the radiator and fan severely compromises efficiency. In contrast, dialing down the radiator temperature increases efficiency.

[0072] Another advantage of the disclosed thermal boost mode is that the size of the radiator can be reduced because there is an alternative cooling means available that can be implemented within the fuel cell. Prior art 3-fluid designs utilize larger radiators that are more costly and add weight to the vehicle, resulting in performance losses. This is especially true for fuel cell trucks.

[0073] Figure 14 A cross-sectional view of a fuel cell 240 having a four-fluid bipolar plate 200 according to a second embodiment of the present invention is depicted. The half-plate 102A can have the same construction as the depicted construction, but the half-plate 102B is replaced by a simple flat plate 202B. The flat plate can be formed of the same material as the half-plate 102A. The porous sub-plate 204 in this embodiment includes oxidant flow field channels 224 on a first side of the plate and DI water channels 218 on an opposite second side. In this construction, the non-porous sub-plate 102 does not have water channels. One advantage of this embodiment is that it is lower profile, which reduces stack height and weight. The size of the WEG coolant passageway 212 is also reduced by half, but this can be compensated for by increasing coolant flow. Figure 11

[0074] Figure 15 ​A cross-sectional view of a fuel cell 340 with a four-fluid bipolar plate 300 according to a third embodiment of the application is depicted. In this embodiment, DI water is not circulated throughout the stack, water is only circulated in the cell 340. The half-plate 102A can have the same configuration as Figure 11 the configuration depicted in FIG. 3, but the half-plate 102B is replaced with a simple flat plate 302B. The flat plate can be formed of the same material as the half-plate 102A. The sub-plate 304 can be configured as a water delivery plate and function as a porous substrate for DI water; in effect a DI water “sponge”: the sub-plate collects product water and humidification water from the air, circulating them back to the inlet of the cell reactant channels 324 and hydrating the UEA 28. The intra-cell circulation occurs by wicking through the porosity, as water in the porosity evaporates at the reactant channel inlet, fresh water is wicked from further down the channel 324 where the porosity is still saturated. This circulation continues passively, with evaporation occurring at the channel inlet and condensation occurring at the channel outlet. This embodiment provides the advantages of passive water management, which is less complex, saves the expense of external pumps and plumbing, and does not consume parasitic power.

[0075] Figure 16 A cross-sectional view of a fuel cell 440 with a four-fluid bipolar plate 400 according to a fourth embodiment of the application is depicted. In this embodiment, the configuration is substantially the same as Figure 11 depicted in FIG. 3, with the exception that an additional barrier 460 separates the internal WEG coolant passageway into two separate channels (shown as WEG1, WEG2). The separate channels can be used to distribute heat evenly across the cell, i.e., to add more cooling capacity where needed. In one example, the two separate channels can carry different compositions of coolant or completely different fluids.

[0076] Figure 17 A cross-sectional view of a fuel cell 540 with a four-fluid bipolar plate 500 according to a fifth embodiment of the application is depicted. In this embodiment, the cathode side configuration and WEG internal coolant passageway are substantially the same as Figure 9 depicted in FIG. 3, but the anode side utilizes a porous sub-plate 562 to supply hydrogen gas to the UEA 28. The non-porous sub-plate 102 is unchanged, but the valleys 132 in the sub-plate 102A define fuel reactant channels Figure 8 ), in this embodiment, these valleys define water channels 518 to keep the porous anode sub-plate 562 hydrated. Similar to the cathode side, the porous anode sub-plate 562 includes fuel flow field channels 510 abutting the UEA 28.

[0077] Figure 18A cross-sectional view of a fuel cell 640 having a bipolar plate 600 according to a sixth embodiment of the invention is depicted. This embodiment is a 3-fluid system because it does not include an internal coolant passage for WEG coolant. The bipolar plate includes a non-porous sub-plate 602 and a porous sub-plate 104. The porous sub-plate... Figure 11 The description is substantially the same as that in the previous embodiment. The difference between the non-perforated subplate 602 and the previous embodiment is that it comprises a single plate and has no flat plates welded to or otherwise joined thereto. Accordingly, the subplate 602 includes a water management side defining a water channel 618 and a corresponding reactant side defining a fuel flow field channel 610.

[0078] Figure 19 A cross-sectional view of a fuel cell 740 having a four-fluid bipolar plate 700 according to a seventh embodiment of the present invention is depicted. In this embodiment, the bipolar plate 700 includes a porous sub-plate 704 located on the cathode side and a hybrid sub-plate 766 located on the anode side. The sub-plate 704 can be coupled with sub-plate 204 ( Figure 14 The mixing subplate 766 is essentially the same, having an oxidant flow field 724 on one side and a water flow field 718 on the opposite side. The mixing subplate 766 includes porous and non-porous portions. The non-porous portion defines internal coolant passages 712 that isolate the coolant from exposure to other battery components. The coolant can be an antifreeze coolant, such as WEG. The porous portion defines multiple pores 768 that fluidly connect the fuel reactant flow field 710 to the water flow field 718. The pores 768 are sized to act as bubble barriers to transport excess water from the fuel flow field 710 to the water flow field 718 without allowing hydrogen to escape into the water chamber.

[0079] In one example, the daughterboard 766 may include a halfboard 766A (similar to...) Figure 8 102A in the middle), the half plate is joined to half plate 766B (similar to ... Figure 14 766B (as in 202B) forms an internal coolant passage 712. The plate 766B can be formed from the same material as the half-plate 766A. The half-plates 766A and 766B can be joined using any of the aforementioned techniques, such as welding, laser welding, brazing, thermoplastic bonding, or adhesives. After joining, the aperture 768 can be formed using any suitable technique, such as laser drilling.

[0080] Further embodiments can be achieved by exchanging the fuel reactants and oxidant reactants. For example, previous embodiments described air flowing through channels in the porous plate 104 and hydrogen flowing through channels in the non-porous plate 102. Within the scope of the invention, the exchange of positions is contemplated, meaning that hydrogen flows through the channels in the porous plate 104, while air flows through the channels in the non-porous plate 102.

[0081] One of the improvements of the disclosed fuel cell system is to prevent galvanic corrosion on the non-porous metal sub-plate. Galvanic corrosion can occur at the interface 164 between the porous carbon sub-plate and the metal sub-plate due to the potential difference between the porous carbon sub-plate and the metal sub-plate Figure 11 and Figure 13 ). When the metal begins to oxidize, the cell begins to lose performance because the oxide layer is not conductive. The prior art solution to this problem, where the system includes non-porous carbon, includes applying a coating to the metal plate to prevent corrosion. Although the disclosed fuel cell system can still benefit from coatings, the system can not need to utilize them because the brine / deionized water circuit sweeps across the interface 164 between the metal and the carbon and carries away any corrosion products that would normally accumulate and make the interface non-conductive. In fact, the water circulating at this interface prevents the oxide from accumulating.

[0082] An example of the method described herein is as follows:

[0083] (1) A method of preventing corrosion at a carbon / metal interface in a fuel cell, the method comprising the steps of:

[0084] providing a bipolar plate comprising a metal sub-plate having at least one water management side and a porous sub-plate having a reactant side and an opposing water management side, the water management side of the porous sub-plate abutting the water management side of the metal sub-plate to form an interface;

[0085] providing a combined electrode assembly abutting the bipolar plate;

[0086] causing a fuel reactant and an oxidant reactant to flow from a reactant flow field on the bipolar plate to the combined electrode assembly to initiate an electrochemical reaction;

[0087] causing water to flow through a water management circuit to the water management sides of the metal sub-plate and the porous sub-plate to sweep away corrosion products formed at the interface; and

[0088] deionizing and desalting the water flowing in the water management circuit.

[0089] (2) The method of preventing corrosion at a carbon / metal interface in a fuel cell as described in (1) above, the method further comprising the step of forming an internal coolant passage within the bipolar plate and flowing an antifreeze-type coolant through the internal coolant passage.

[0090] (10) A method of operating a four-fluid fuel cell in a thermal boost mode, the method comprising the steps of:

[0091] providing a four-fluid fuel cell comprising an oxidant flow field, a fuel reactant flow field, a water management flow field, and a separate circulating coolant circuit operable to remove sensible heat, the coolant circuit being in fluid communication with the coolant flow field;

[0092] reducing the flow rate of the coolant in the coolant circuit to reduce the sensible heat cooling capacity; and

[0093] allowing the fuel cell to maintain or increase temperature to increase evaporative cooling.

[0094] (11) The method of operating a four-fluid fuel cell of (10) above, wherein the coolant is an antifreeze type coolant.

[0095] (12) The method of operating a four-fluid fuel cell of (10) above, wherein at least one of the oxidant flow field and the fuel reactant flow field includes a plurality of apertures fluidly connected to the water management flow field, the apertures configured as a bubble barrier.

[0096] (13) The method of operating a four-fluid fuel cell of (10) above, wherein the step of providing a four-fluid fuel cell includes providing a hybrid bipolar plate including an oxidant flow field, a fuel reactant flow field, an internal coolant passage, and a water management flow field.

[0097] (14) The method of operating a four-fluid fuel cell of (10) above, further comprising the step of increasing the flow of water through the water management flow field to compensate for increased evaporation.

[0098] (15) The method of operating a four-fluid fuel cell of (14) above, wherein the step of providing a four-fluid fuel cell further includes providing a circulating water management circuit in fluid communication with the water management flow field.

[0099] (20) A method of accumulating and retaining product water in a four-fluid fuel cell, the method comprising the steps of:

[0100] providing a four-fluid fuel cell including an oxidant flow field, a fuel reactant flow field, a water management flow field, and a separate circulating coolant circuit operable to remove sensible heat, the coolant circuit in fluid communication with the coolant flow field;

[0101] increasing the flow of coolant in the coolant circuit to increase the sensible heat cooling; and

[0102] allowing the fuel cell to maintain or decrease temperature to condense excess product water.

[0103] (21) The method of accumulating and retaining product water in a four-fluid fuel cell of (20) above, further comprising the step of providing a water reservoir to store excess product water, the water reservoir in fluid communication with the water management circuit.

[0104] (22) The method of accumulating and retaining product water in a four-fluid fuel cell as described in (20) above, further comprising the step of reducing the flow of water through the water management flow field to accumulate excess product water and compensate for reduced evaporation.

[0105] (23) The method of accumulating and retaining product water in a four-fluid fuel cell as described in (22) above, wherein the step of providing a four-fluid fuel cell further comprises providing a circulating water management loop in fluid communication with the water management flow field.

[0106] (24) The method as described in (10) or (20) above, wherein the controller commands coolant pump and water pump flow settings in response to sensor data including at least one of air flow, cathode exhaust temperature, cathode exhaust pressure, water reservoir total capacity, water inventory, water temperature, ambient temperature, coolant return temperature, and water loop outlet pressure.

[0107] (25) The method as described in (10) or (20) above, wherein the controller commands coolant pump and water pump flow settings in response to environmental factors including at least one of duty cycle, vehicle route, GPS coordinates, road grade, weather forecast, time of day, and driver behavior.

Claims

1. A bipolar plate for a fuel cell, comprising: a non-porous sub-plate, the non-porous sub-plate comprising a water management side, an opposite reactant side, and an internal coolant passage therebetween; and a porous sub-plate, the porous sub-plate comprising a reactant side and an opposite water management side, the reactant side comprising a first reactant flow field, and the water management side fluidly connected to the water management side of the non-porous sub-plate.

2. The bipolar plate of claim 1, wherein the reactant side of the non-porous sub-plate comprises a second reactant flow field.

3. The bipolar plate of claim 2, wherein the water management side of the non-porous sub-plate comprises a water flow field.

4. The bipolar plate of claim 3, wherein the water flow field comprises channels.

5. The bipolar plate of claim 1, wherein the internal coolant passage of the non-porous sub-plate is subdivided into a primary path and a secondary path.

6. The bipolar plate of claim 5, wherein the non-porous sub-plate further comprises a separator to separate the primary path from the secondary path.

7. The bipolar plate of claim 2, wherein the first reactant flow field in the porous sub-plate comprises an oxidant channel, and the second reactant flow field in the non-porous sub-plate comprises a fuel channel.

8. The bipolar plate of claim 1, wherein the water management side of the porous sub-plate comprises a water flow field.

9. The bipolar plate of claim 8, wherein the water flow field comprises channels.

10. The bipolar plate of claim 8, wherein the water flow field comprises a pore structure configured as a water reservoir to facilitate passive water migration across the fuel cell.

11. The bipolar plate of claim 8, wherein the water flow field comprises a bubble barrier pore structure adapted to allow transport of liquid through the pore structure, and prevent transport of reactant gas through the pore structure.

12. The bipolar plate of claim 1, wherein the water management side of the non-porous sub-plate comprises a recessed perimeter adapted to provide a nested seal with the porous sub-plate.

13. The bipolar plate of claim 1, wherein the non-porous sub-plate comprises a first half-plate joined with a second half-plate.

14. The bipolar plate of claim 13, wherein the internal coolant passage is defined by the joined first and second half-plates.

15. The bipolar plate of claim 1, wherein the non-porous sub-plate further comprises at least one weep hole fluidly connecting the reactant side to the water management side, the at least one weep hole configured as a bubble barrier to transport excess water from the reactant side to the water management side while inhibiting transport of reactant gas.

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