4-fluid bipolar plate for fuel cell

Abstract

To provide a bipolar plate that provides improved delivery of humidified reactants and better removal of produced water.SOLUTION: A bipolar plate 100 for a fuel cell includes a non-porous subplate 102 and a porous subplate 104. The non-porous subplate 102 includes a water management surface 122, a reactant surface on the opposite side, and an internal coolant passage between the two surfaces. The porous subplate 104 includes a reactant surface and a water management surface on the opposite side. The reactant surface includes a first reactant flow field. The water management surface is fluidically connected to the water management surface of the non-porous subplate.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims priority and the benefit of U.S. Patent Application No. 17 / 344,377, titled "FOUR - FLUID BIPOLAR PLATE FOR FUEL CELL", filed on June 10, 2021, and the entire disclosure of which is incorporated herein by reference in its entirety.

Background Art

[0002] The present disclosure generally relates to fuel cell bipolar plates, and more specifically to bipolar plate structures that provide improved delivery of humidified reactants and better removal of generated water.

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

[0004] A single fuel cell includes a membrane electrode assembly (MEA) that includes a membrane electrolyte sandwiched between a pair of electrodes (anode and cathode), and a conductive plate that defines a reactant gas flow field adjacent to each electrode on the side opposite the membrane electrolyte. A typical flow field plate conducts reactant gases through the gas diffusion layer and the microporous layer to their respective electrodes. In some designs, the flow field plate can also carry water by-products out of the cell.

[0005] To increase the electrical output of an electrochemical conversion assembly or a fuel cell, multiple fuel cells are generally arranged and connected in series in a stack. In this arrangement, two adjacent cell units can share a common bipolar plate, which functions as the anode and cathode of two adjacent cell units connected in series. Such a bipolar plate is generally referred to as a "bipolar plate".

Summary of the Invention

Means for Solving the Problems

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

[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, the bipolar plate for a fuel cell includes a non-porous sub-plate having a water management surface and a reactant surface. The reactant surface includes a first reactant flow field. The bipolar plate further includes a porous sub-plate having a reactant surface and an opposite water management surface. The reactant surface includes a second reactant flow field. The water management surface of the porous sub-plate is fluidly connected to the water management surface of the non-porous sub-plate.

[0009] The features described herein can be better understood with reference to the drawings described below. The drawings are not necessarily to scale and emphasis has been placed on illustrating the principles of the present invention. In the drawings, like numbers are used to indicate like parts throughout the various figures.

Brief Description of the Drawings

[0010]

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[0011] FIG. 1 shows a typical polymer electrolyte membrane (PEM) fuel cell 10, which generally includes an anode 12 and a cathode 14 separated by an ionomer membrane 16. An anode catalyst layer 18 a and a cathode catalyst layer 18 c are formed on respective faces of the planar membrane and convert hydrogen and oxygen reaction gases into electricity and water. This assembly is typically referred to as a membrane electrode assembly (MEA) 20. The catalyst layers 18 a 18 c may be the same for the anode 12 and the cathode 14, but are usually different. For example, the anode catalyst layer 18a may have the function of splitting hydrogen atoms into hydrogen ions and electrons, while the cathode catalyst layer 18 c may have the function of reacting oxygen gas with electrons to form water.

[0012] The reactants (i.e., hydrogen and air) are directed to the MEA 20 by a flow field plate 22 that typically contains reactant flow channels (shown in dashed lines). The flow field plate 22 is shown as a bipolar plate and contains reactant flow channels for both fuel and oxidant. The reactants are transported from the channels to a gas diffusion layer (GDL) 24 adjacent to the flow field plate 22. a ,twenty four c and then through the GDL and each catalyst layer 18 a , 18 c Microporous layer (MPL) 26 located between a , 26 c The GDL may have several functions, such as diffusing the reactant gas stream to the catalyst layer, transporting the liquid and vapor water by-product from the catalyst layer to the cathode gas channel (which is carried by the gas stream), collecting the electric current generated from the electrochemical reaction, and imparting mechanical strength to support and protect the catalyst-coated membrane. The GDL is typically a highly porous (e.g., 60%-90%) carbon fiber nonwoven paper or woven carbon fiber fabric, approximately 0.25-0.35 mm thick, with pore sizes on the order of hundreds of microns, and may be treated with a variety of proprietary substances to improve performance. The MPL functions to minimize the contact resistance between the GDL and the catalyst layer, helping to improve water transport. The MPL typically consists of a thin layer of carbon powder and PTFE particles coated on the GDL, with pore sizes on the order of one micron. Some fuel cells are assembled to produce a membrane electrode assembly (MEA), microporous layer (MPL), and gas diffusion layer (GDL) into a unitary assembly, known as a unitized electrode assembly (UEA).

[0013] Figure 2 shows a general fuel cell power plant 30 that uses the stack of the fuel cell 10 as described in Figure 1. Fuel such as hydrogen (H 2 ) is supplied to the fuel inlet 32, flows through the anode flow field plate, and is distributed to the anode catalyst layer. Unconsumed fuel flows out from the fuel outlet 34, returns to the fuel inlet 32 through a recycle pump (not shown), and may be purged to the ambient environment periodically. An oxidant such as air is supplied to the air inlet 36 by a blower (not shown), flows through the cathode flow field plate, and is distributed to the cathode catalyst layer. Excess process air humidified by water by-products flows out from the air outlet 38 and may pass through a radiator and / or a condenser (not shown) before being discharged to the environment.

[0014] The power plant 30 may further include a coolant loop 40 for removing heat from the fuel cell. In many automotive applications, the coolant is a mixture of water and ethylene glycol to prevent freezing of the coolant in cold regions. The pump 42 supplies the coolant to the coolant inlet 44, where it is then guided through a cooling plate etc. (not shown but typically placed between the fuel cells 10) and distributed on the surface of the plate. The fuel cell 10 transfers sensible heat to the circulating coolant, so the coolant gets warmer but no phase change occurs. When leaving the stack at the coolant outlet 46, the coolant passes through the heat exchange device 48, where the sensible heat is removed before being circulated back to the inlet 44. In one example, the heat exchange device 48 is a radiator. A flow control valve or an orifice 50 may be used to regulate the flow of the coolant.

[0015] As shown in FIG. 1, the reactant flow field plate 20 is a bipolar plate. Many bipolar plate designs use solid materials, and in a very small number of designs, porous materials are used on both the anode side and the cathode side. Each design has its own advantages and disadvantages. As the name indicates, solid bipolar plates are excellent at separating reactant gases because they do not allow hydrogen fuel to permeate. Further, solid bipolar plates in a stack are relatively easy to seal because of their impermeability. Thus, the power plant stack can be pressurized, which improves cell performance and reduces cell degradation. Another advantage of solid bipolar plates is that due to their impermeable nature, coolant types such as water / ethylene glycol mixtures (WEG) can be used within the stack. This type of coolant is very beneficial for batteries operating in low-temperature environments such as automotive applications. However, since WEG contaminates the membrane electrode assembly, care must be taken to isolate WEG from the MEA.

[0016] Solid bipolar plates can be manufactured from metals such as stainless steel and titanium. Metal plates can potentially be mass-produced inexpensively because the flow field shape can be formed by conventional mass production methods such as stamping. Solid bipolar plates may also be manufactured from non-porous carbon or from polymer (composite) materials. Solid carbon or composite plates can be mass-produced, for example, by molding, and can generally maintain tighter dimensional tolerances than metal forming plates. However, solid carbon or composite plates are more costly to manufacture than metal plates.

[0017] Solid bipolar plates are useful and can be advantageous in certain applications, but they have drawbacks. One drawback of metal plates is that they are prone to corrosion because air and water are present at very high electrochemical potentials. The corrosion layer is non-conductive, and as the plate corrodes, the performance of the fuel cell degrades. Coatings have been developed and applied to the plates to reduce corrosion, but this technology also has operational limitations.

[0018] In particular, the automotive industry may aim to have a fuel cell operating life of 5,000 hours. Some coatings on metal plates are said to have achieved this goal. However, in the large vehicle industry, an operating life of 30,000 hours may be required. Current automotive coatings and structures are not close to their limits. Therefore, in the large vehicle industry, it is necessary to develop fuel cells with much longer operating limits, perhaps up to 30,000 hours.

[0019] Another drawback of solid plates is that they do not have inherent water management capabilities. In the operation of a PEM fuel cell, it is important to maintain an appropriate water balance between the rate at which water (including water generated from proton drag through the PEM electrolyte) is produced at the cathode electrode and the rate at which water is removed from the cathode or supplied to the anode electrode. In the case of a PEM fuel cell, if insufficient water is returned to the anode electrode, the adjacent parts of the PEM electrolyte dry out, thereby reducing the migration rate of hydrogen ions passing through the PEM, causing crossover of the returned fluid, and resulting in local overheating. Similarly, if insufficient water is removed from the cathode, the cathode electrode floods, substantially restricting the supply of oxidant to the cathode and potentially reducing the current. Furthermore, if too much water is removed from the cathode, the PEM dries out, limiting the ability of hydrogen ions to pass through the PEM and potentially reducing battery performance. Usually, for solid plates, external water management means such as an external humidifier are required to prevent drying out and cracking of the MEA.

[0020] The porous bipolar plate, also called the water transport plate, is a porous separator plate used on both the cathode side and the anode side of the electrodes of a fuel cell. The porous bipolar plate precisely controls the pore size, creating a bubble barrier that enables the movement of liquid water through the pores into the liquid water cavities during the operation of the fuel cell while preventing the movement of the reaction gas. Liquid movement enables membrane hydration and the removal of the water generated on the cathode side resulting from the electrochemical reaction within the fuel cell. By preventing the movement of the reaction gas, it prevents the fuel and oxidant gases from leaking into the liquid water cavities.

[0021] The porous plate sucks up excess water in the flow field channels and moves it to areas where water is being lost by evaporation, thereby maintaining the hydration of the membrane electrode assembly and providing an excellent water balance. The porous bipolar plate is exposed to the water flow field to maintain the desired operation of the fuel cell. In local regions of the cell where the reaction gas flows from a low-temperature region to a high-temperature region, water evaporates from the porous plate and the gas flow becomes saturated with water vapor; in regions where the reaction gas flows from a high-temperature region to a low-temperature region, the porous plate can suck up the water generated by the electrochemical reaction and the liquid water condensed from the cooling gas flow. As a result, one of the advantages of a fuel cell system equipped with a porous bipolar plate is that they have very high durability. Another advantage is that a system equipped with a porous bipolar plate does not require the use of an external humidifier, thus reducing weight and complexity.

[0022] Typically, a pump-driven circulating water loop can be used to provide the driving force not only for the cooling function of the cell but also to move water through the pores of the water transport plate to remove the generated water.

[0023] The porous bipolar plate has advantages but also has disadvantages. For example, it is difficult to manufacture plates with specific pore sizes, so the cost can be high when mass-producing. Another disadvantage is that the porous plate is difficult to seal, which can lead to reliability problems in the pressurization system. Another major disadvantage is that a fuel cell using a porous bipolar plate cannot use an antifreeze-type coolant such as WEG in the water cooling loop in order to avoid the coolant being absorbed into the pores and contaminating the MEA.

[0024] Embodiments of the invention of the present disclosure solve many of the aforementioned problems with bipolar plates by using a four-fluid plate structure that provides a fuel reactant flow field, an oxidant flow field, a water management flow field, and a dedicated coolant passage for an antifreeze-type coolant. The embodiments include both a non-porous plate portion and a porous plate portion, and are thoughtfully selected to obtain the best aspects of both designs while reducing or eliminating the associated disadvantages. The four-fluid bipolar plate can be easily manufactured and the cost can be reduced.

[0025] Referring to FIGS. 3 and 4, the bipolar plate 100 for a fuel cell includes a non-porous subplate 102 and a porous subplate 104. In one embodiment of the present invention, the non-porous subplate 102 includes a reactant surface 106 (shown in FIG. 3) and a water management surface 108 (shown in FIG. 4) on the opposite side. As shown, the reactant surface 106 supplies hydrogen to the anode side of the MEA through a fuel flow field. Non-limiting examples of the flow field include a cavity, a porous substrate, or, as shown in the illustrated embodiment, fuel flow field channels 110. The non-porous subplate 102 further includes internal coolant passages 112 (FIGS. 8 and 11) that separate a coolant of the antifreeze type such as WEG from other components in the fuel cell. Other common features of the non-porous subplate 102 may include internal manifolds 114 for the fuel supply section 114a and the fuel return section 114b, for the oxidant supply section 114c and the oxidant return section 114d, for the water management supply section 114e and the water management return section 114f, and for the WEG coolant supply section 114g and the WEG coolant return section 114h. The sealing means 116 enables sealing of a plurality of fuel cells and operation under pressure.

[0026] FIG. 4 shows the opposite side of the bipolar plate 100. The water management surface 108 of the non-porous subplate 102 includes a water flow field. Non-limiting examples of the flow field include a cavity, a porous substrate, or, as shown in the illustrated embodiment, water channels 118 that form a part of an external circulating water management loop 150 (FIG. 13) and enable proper water management of the cathode flow field, as will be described in detail below. Water enters the plate channel through the water management supply manifold 114e and exits through the water management return manifold 114f.

[0027] The porous subplate 104 includes a reactant surface 120 and an opposite water management surface 122. The reactant surface 120 supplies an oxidant (e.g., air) to the cathode side of the MEA through an oxidant flow field. Non-limiting examples of the flow field include a cavity, a porous substrate, or, as shown in the illustrated embodiment, in this embodiment, an oxidant flow field channel 124. In this embodiment, the water management surface 122 (FIG. 3) is featureless (e.g., flat), but plays an important role in maintaining optimal cell performance and durability.

[0028] The porous subplate 104 may be manufactured from graphite or other carbon-based materials, or may be manufactured from a metal such as titanium or stainless steel. Features such as channels may be formed by hydroforming, casting, thermoforming, 3D printing / additive manufacturing, or milling / machining.

[0029] As described above, the pores of the porous subplate 104 are sized to form a bubble barrier during the operation of the fuel cell. The pore size is determined by specific fuel cell operating conditions and pressures. For graphite or other carbon-based materials, pores may be formed in the plate by known methods. For example, U.S. Patent No. 6,197,442 details a manufacturing process in which graphite powder, reinforcing fibers, cellulose-based fibers, and a thermosetting resin are mixed with a liquid to form a slurry, showered onto a screen to form a planar sheet, dried to form paper, cut to the desired size, and laid up. The layup is laminated with pressure and heat, carbonized, and graphitized to form a water transport plate for later machining as needed. The completed porous plate exhibits excellent physical properties with respect to bubble pressure, water permeability, median pore diameter, porosity, in-plane resistivity, and compressive yield strength. In the case of a metal porous plate, the pores can be formed, for example, by a punch press or laser drilling.

[0030] Figures 5 and 6 show further exploded views of the non-porous subplate 102 according to the first embodiment of the present invention. The non-porous subplate 102 can be formed from two half-plates 102A and 102B that are easily manufactured and joined together. For example, the half-plates can be manufactured from metals such as stainless steel or titanium, the flow channels and other features can be formed by metal stamping, etc., and these two half-plates can be joined together by welding.

[0031] Other non-limiting examples of joining methods include, for example, laser welding, brazing, thermoplastic adhesion, or adhesives. The half-plate 102A in the illustrated embodiment includes a fuel flow field channel 110 on the side facing the reactants (Figure 5) and a WEG coolant half-channel 126A on the opposite side (Figure 6). The half-plate 102B includes a water channel 118 on the water management surface 108 (Figure 6) and a WEG coolant half-channel 126B on the opposite side (Figure 5).

[0032] Further details can be found by referring to Figures 7 and 8, where Figure 7 shows a cross-sectional view of the cathode side of the bipolar plate 100 along approximately the position shown in Figure 4, and Figure 8 shows an enlarged view of the plate shown in Figure 7. Referring to Figure 8, the non-porous subplate 102 and the porous subplate 104 are shown in more detail. The half-plates 102A, 102B are shown separately (e.g., before joining) for clarity. Each half-plate may include a row of raised surfaces 128 and valleys 130, 132 therebetween, and can define fluid flow channels on the outer surface of the non-porous plate. The raised surface 128 on one side of the plate defines a recess 134 on the opposite side of the same plate. The recess can define an internal cavity 136 when the two half-plates 102A, 102B are joined together. In one example, the valley 130 on the half-plate 102B defines the water management channel 118, the valley 132 on the half-plate 102A defines the fuel flow field channel 110, and the internal cavity 136 defines an internal antifreeze coolant passage.

[0033] The reactant surface 120 of the porous subplate 104 includes an oxidant flow field channel 124 for supplying air to the MEA. In one example, the channel 124 is transverse to the fuel flow field channel 110. The water management surface 122 of the porous subplate 104 is disposed against the flat raised surface 128 of the half plate 102B. In this way, when deionized (DI) water is circulated through the water channel 118, the pores in the porous subplate 104 are in fluid communication with the DI water, and the subplate 104 is completely saturated with the liquid and that state is maintained.

[0034] The desired porosity in the porous subplate 104 may be achieved by any suitable method known in the fuel cell art. For example, the porous subplate 104 may be assembled as a water transport plate (WTP), net shape molded from a slurry having an appropriate particle size, or laser drilled to achieve the desired pore size.

[0035] FIG. 9 shows another cross-sectional view of the bipolar plate 100, a portion of which is enlarged in FIG. 10 to illustrate one possible structure. Looking at FIG. 10, the cross-sectional view includes the half plate 102A, the half plate 102B, and the porous subplate 104. Similar to FIG. 8, the half plates 102A and 102B are shown somewhat separated for clarity. Also shown is the recess 134 of the half plate 102A that forms the WEG coolant half channel 126A.

[0036] The porous subplate 104 may be sealed to the non-porous subplate 102 by conventional means to prevent gas or water leakage. For example, the sealing means 116 may include an adhesive, nesting, interference fit, or a groove for receiving a molded compression seal, gasket, or O-ring. In one example, the porous subplate 104 may be nested in a recess 138 formed in the water management surface 108 of the non-porous subplate 102. The recess 138 extends across the entire plane of the porous subplate 104, effectively capturing the plate and ensuring proper alignment during assembly. In some examples, the porous subplate 104 is substantially embedded within the thickness of the other plate and the overall thickness dimension increase is minimal, so the recess 138 can reduce the overall thickness of the bipolar plate 100.

[0037] FIG. 11 shows a cross-sectional view of a proton exchange membrane (PEM) fuel cell 140 with the bipolar plate 100 of the first embodiment of the present invention, FIG. 12 shows a stack of such fuel cells, and FIG. 13 shows a cross-sectional view of a fuel cell power plant 144 with the disclosed bipolar plate 100. In the illustrated example, the oxidant flow field channel 124 is shown parallel to the fuel flow field channel 110, but this is for illustrative purposes and is the same for other embodiments. The fuel cell 140 includes a bipolar plate 100 between upper and lower unitized electrode assemblies 28 (UEAs). The bipolar plate 100 is in contact with each UEA 28.

[0038] During operation, hydrogen is introduced into inlet 114a and flows through fuel flow field channels 110 in non-porous subplate 102 to reach the anode side of UEA 28. Air is introduced into inlet 114c and flows through oxidant flow field channels 124 in porous subplate 104 to reach the cathode side of UEA 28. Water pump 146 circulates water through desalination device 148 in water management loop 150. Desalinated water, or deionized (DI) water, passes through water management supply 114e and flows into stack 144 through channel 118 formed by non-porous subplate 102 and porous subplate 104. The pores of porous subplate 104 are filled with water, and the subplate functions as a sponge to hold water and maintain the water content of UEA 28. Porous subplate 104 can directly transport the liquid to UEA 28, or it can evaporate the water and move the water vapor to the UEA through the air flow. Porous subplate 104 can also remove the water produced by the reaction at the cathode from UEA 28. The water produced in liquid form can be directly fed into the pores of porous subplate 104 by maintaining the pressure in water management loop 150 lower than the pressure of the reactants. If the water produced is in the form of vapor, it can be condensed on the porous subplate, where it is absorbed and returned to the circulating water loop.

[0039] Thermal management is mainly controlled by a dedicated, isolated coolant loop 152. A coolant pump 154 flows coolant through coolant supply 114g to stack 144 and out of stack 144 through coolant return 114h. During this process, in some configurations, the coolant is distributed across the entire surface of cells 140. In the illustrated embodiment, the coolant flows through internal passageways 112 formed by joining half plates 102A and 102B (FIG. 10). When leaving the stack at coolant return 114h, the coolant passes through heat exchanger 156 where sensible heat is dissipated and then is circulated back to supply 114g. In one example, heat exchanger 156 is a radiator. A flow control valve or orifice 158 can be used to regulate the flow of coolant.

[0040] The impermeability of non-porous sub-plates 102 eliminates the need for separate coolant tubes and allows coolant passages to be located inside sub-plates 102, saving space compared to some designs that add separate cooling plates. As described above, this design allows the use of coolant types such as antifreeze liquids like water / ethylene glycol mixtures (WEG), which is beneficial for fuel cells operating in low-temperature environments.

[0041] In the illustrated embodiment, the coolant flows through internal passageways formed by joining half plates 102A and 102B. However, other means of distributing the coolant are also envisioned within the scope of the present invention. For example, the internal coolant passageways can be defined by cavities containing a porous substrate for distributing the coolant.

[0042] In most situations, an external humidifier is not required in the disclosed embodiments, although there are scenarios where adding an external humidifier can be beneficial to the system. For example, if the bipolar plate 100 uses only a passive water management function and is operated in a particularly hot and dry environment, water may evaporate from the porous subplate faster than the fuel cell can produce generated water. In such an environment, it may be advantageous to add an external humidifier 159 (FIG. 13) to the system rather than incorporating an active cooling function as detailed in other embodiments herein.

[0043] In the illustrated embodiment, there is no porous medium in the anode channel 110. Under some operating conditions, such as the presence of locally cold regions, moisture can condense within the anode channel and water can accumulate. To prevent degradation of the anode electrode performance, it is necessary to periodically remove the water. Prior art solutions to this problem include attempts to blow the water away, which require additional operating procedures and consume parasitic power. In one embodiment, as shown in FIGS. 11 and 12, one or more small drain holes 142 can be drilled from the bottom of the hydrogen channel to communicate with the DI water cavity 118. The drain holes 142 can be sized as a bubble barrier for transporting excess water from the fuel channel 110 to the water channel 118 without allowing the reaction gas to escape. The pressure of the DI water loop can be maintained lower than the pressures of the anode and cathode. In this way, the pressure difference causes the accumulated water to be fed into the cavity 118 through the drain holes 142, where the water is returned to the DI water loop.

[0044] As described above, under typical operating conditions, the thermal management of a fuel cell power plant is mainly controlled by the coolant loop 152, and the sensible heat is transferred to the circulating coolant passing through the coolant flow field. To a lesser extent, part of the cell cooling can also be provided by evaporative cooling when the generated water in the pores evaporates, but the evaporative cooling effect is usually not considered as a control parameter in the sensible heat coolant flow system.

[0045] Evaporative cooling utilizes the latent heat of vaporization to improve the cooling effect per volume of water by up to 100 to 1 compared to the sensible heat coolant flow method. The inventors of the present disclosure have confirmed that under certain conditions, enhanced cooling can be achieved by evaporation. Thus, in one aspect of the present invention, independent operation of the water management loop and the coolant loop can be utilized to operate in a thermal boost mode or a water recovery / accumulation mode.

[0046] In the thermal boost mode, additional cooling is required for a finite duration, such as when the stack is demanding a large amount of power. In fuel cell vehicles (especially trucks), the thermal boost mode can be useful when climbing steep hills, long grades, operating at high power on hot days, or in any other scenario where the radiator is not large enough to adequately handle the cooling requirements. In the thermal boost mode, the heat management method transitions from sensible heat cooling to evaporative cooling, providing a greater cooling capacity. Evaporative cooling can account for the majority of the total cooling function in the thermal boost mode and may account for over 90% in some design scenarios.

[0047] During operation, when additional cooling is required or calculated to be required, in the first step, the coolant flow rate (i.e., WEG) is decreased, thereby reducing the sensible heat cooling capacity. As a result, the temperature of the stack begins to rise, the evaporation rate of water from the pores increases, and significant evaporative cooling is achieved. Next, in the second step, the temperature of the fuel cell is increased or maintained to enhance the degree of evaporative cooling. In the third step, to compensate for the increased evaporation of water and prevent the pores from drying out and losing the bubble barrier, the flow rate of water through the water management flow field can be increased. In one example, the increase in the flow rate of water is achieved by providing a pump-driven circulating water management loop in fluid communication with the water management flow field and increasing the flow rate of water by the pump.

[0048] The disclosed evaporative cooling method provides a better short-term thermal management control strategy because it has a greater ability to handle large short-term heat demands. The coolant flow rate can be adjusted to a low value to achieve an appropriate level of evaporative cooling and the desired stack temperature.

[0049] The disclosed thermal boost mode depletes the amount of water in the water management loop beyond what can be simultaneously replenished by the formation of generated water. Thus, the thermal boost mode is intended for a relatively short period. However, in another aspect of the invention, the independent operation of the water management loop and the coolant loop may be utilized to operate a water recovery / accumulation mode. In the water recovery / accumulation mode, the coolant flow rate (i.e., WEG) is increased above the normal rate, evaporative cooling is reduced, and excess water is generated within the cell by condensation. The excess generated water can be collected and retained for use in future thermal boost modes.

[0050] In one embodiment, the water recovery / accumulation mode can also be operated during a portion of the cycle where the stack is not required and the airflow through the radiator provides sufficient cooling, such as when the vehicle is traveling on flat ground. In a first step, when additional generated water is needed or calculated to be needed, the coolant flow rate (i.e., WEG) in the coolant loop is increased to increase sensible heat cooling. As a result, the stack temperature decreases, the amount of generated water evaporating from the pores decreases, and instead, condensed water is formed. In a second step, the temperature of the fuel cell is lowered to condense the excess generated water, but the temperature is maintained. To compensate for the decrease in water evaporation and prevent self-flooding, in a third step, the flow rate of water through the water management flow field may be decreased. In one example, the decrease in the water flow rate may be achieved by providing a pump-driven circulating water management loop in fluid communication with the water management flow field and using the pump to decrease the water flow rate.

[0051] In another embodiment, the fuel cell controller may receive sensor inputs or environmental inputs to determine whether the thermal boost mode, or the water recovery / accumulation mode, is guaranteed and, if so, to what extent. Non-limiting examples of sensor inputs can include air flow rate, cathode exhaust temperature, cathode exhaust pressure, total water storage capacity, water inventory, water temperature, ambient temperature, coolant return temperature, and water loop outlet pressure. The controller can command the flow rate settings of the coolant pump and / or the water pump according to the sensor input values.

[0052] The fuel cell controller may also receive inputs from external environmental factors. Non-limiting examples include payload timing, vehicle route, GPS coordinates, road grade, weather forecast, time, and driver behavior. In one example, the controller may receive GPS route data indicating that a steep or extended road grade is approaching. The controller can command the stack to operate in the water recovery / accumulation mode well in advance, collect the generated water, and hold it in the water storage tank. Then, when the vehicle encounters the grade, the controller can command the stack to operate in the thermal boost mode.

[0053] The operation of the thermal boost mode and the water recovery / accumulation mode is not limited to the disclosed hybrid bipolar plate. The inventors assume that the disclosed operating method may be possible and beneficial in any 4-fluid fuel cell power plant where an antifreeze-type coolant loop operates independently of the water management loop, as disclosed in U.S. Patent No. 7,135,247. The '247 patent discloses separate individual cooling plates arranged among all other fuel cells.

[0054] The disclosed thermal boost mode and water recovery / accumulation mode provide several advantages and benefits over prior art three-fluid stacks. One advantage is that the thermal boost mode reduces parasitic power by actually tuning down, rather than increasing, the radiator and fan during high power output. In prior art stacks, running the radiator and fan aggressively decreases efficiency. Conversely, tuning down the radiator increases efficiency.

[0055] Another advantage of the disclosed thermal boost mode is that the size of the radiator can potentially be reduced because there are alternative cooling means achievable within the fuel cell. In prior art three-fluid designs, much larger radiators are used, which are more expensive and increase the weight of the vehicle, thereby degrading performance. This is particularly true for fuel cell trucks.

[0056] FIG. 14 shows 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. The half plate 102A may have the same structure as that shown in FIG. 11, but the half plate 102B is replaced by a simple flat plate 202B. The flat plate can be formed from the same material as the half plate 102A. The porous subplate 204 in this embodiment includes an oxidant flow field channel 224 on a first surface of the plate and a DI water channel 218 on an opposite second surface. In this structure, the non-porous subplate 102 does not have water channels. One advantage of this embodiment is that it is thin, reducing the height and weight of the stack. The size of the WEG coolant passage 212 is also reduced by half, but this can be compensated for by increasing the coolant flow rate.

[0057] FIG. 15 shows a cross-sectional view of a fuel cell 340 having a four-fluid bipolar plate 300 according to a third embodiment of the present invention. In this embodiment, DI water does not circulate throughout the stack, but only within the cell 340. The half plate 102A may have the same structure as that shown in FIG. 11, but the half plate 102B is replaced by a simple flat plate 302B. The flat plate can be formed from the same material as the half plate 102A. The subplate 304, which can be constructed as a water transport plate, functions as a porous substrate for DI water and is effectively a DI water "sponge": it collects the generated water and humidified water from the air, circulates it, and returns it to the inlet of the cell reactant channel 324 to moisten the UEA 28. The in-cell circulation occurs by capillary wicking such that when the water in the pores evaporates at the inlet of the reactant channel, new water is drawn up from further downstream in the channel 324 where the pores are still saturated. This cycle continues passively, with evaporation occurring at the inlet of the channel and condensation occurring at the outlet of the channel. This embodiment provides the advantage of passive water management, which is not as complex, saves the cost of external pumps and piping, and does not consume parasitic power.

[0058] FIG. 16 shows a cross-sectional view of a fuel cell 440 having a four-fluid bipolar plate 400 according to a fourth embodiment of the present invention. In this embodiment, the configuration is essentially the same as that shown in FIG. 11, except that an additional separator plate 460 divides the internal WEG coolant passage into two separate channels (shown as WEG1 and WEG2). The individual channels can be used to equalize the thermal distribution throughout the cell, i.e., additional cooling capacity can be added where needed. In one example, the two separate channels can carry coolants of different compositions or completely different fluids.

[0059] FIG. 17 shows a cross-sectional view of a fuel cell 540 having a four-fluid bipolar plate 500 according to a fifth embodiment of the present invention. In this embodiment, the cathode-side structure and the WEG internal coolant passage are essentially the same as those shown in FIG. 9, but on the anode side, a porous subplate 562 is used to supply hydrogen to the UEA 28. The non-porous subplate 102 is unchanged, but instead of the trough 132 of the subplate 102A that defines the fuel reactant channel (FIG. 8), in this embodiment, these define water channels 518 to maintain the hydration of the porous anode subplate 562. Similar to the cathode side, the porous anode subplate 562 includes fuel flow field channels 510 adjacent to the UEA 28.

[0060] FIG. 18 shows a cross-sectional view of a fuel cell 640 having a bipolar plate 600 according to a sixth embodiment of the present invention. This embodiment is a three-fluid system because it does not include an internal coolant passage for the WEG coolant. The bipolar plate includes a non-porous subplate 602 and a porous subplate 104. The porous subplate is essentially the same as that shown in FIG. 11. The non-porous subplate 602 includes a single plate and is different from the previous embodiment in that there is no flat plate welded or joined to it in another way. Thus, the subplate 602 includes a water management surface that defines water channels 618 and a reactant surface on the opposite side that defines fuel flow field channels 610.

[0061] FIG. 19 shows 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. In this embodiment, the bipolar plate 700 includes a porous subplate 704 on the cathode side and a hybrid subplate 766 on the anode side. The subplate 704 is essentially the same as the subplate 204 (FIG. 14), having an oxidant flow field 724 on one side and a water flow field 718 on the opposite side. The hybrid subplate 766 includes a porous portion and a non-porous portion. The non-porous portion defines an internal coolant passage 712 that isolates the coolant from being exposed to other cell components. The coolant may be an antifreeze-type coolant such as WEG. The porous portion defines a plurality of pores 768 that fluidly connect the fuel reactant flow field 710 to the water flow field 718. The pores 768 are sized as a bubble barrier for transporting excess water from the fuel flow field 710 to the water flow field 718 without allowing hydrogen gas to escape into the water voids.

[0062] In one example, the subplate 766 may include a half plate 766A (similar to 102A in FIG. 8) joined to a half plate 766B (similar to 202B in FIG. 14) to form the internal coolant passage 712. The flat plate 766B may be formed from the same material as the half plate 766A. The half plates 766A, 766B may be joined by any of the aforementioned techniques such as welding, laser welding, brazing, thermoplastic adhesion, or an adhesive. After joining, the pores 768 may be formed by any suitable technique such as laser drilling.

[0063] A further embodiment may be realized by exchanging the fuel and oxidant reactants. For example, in the previous embodiment, it was described that air flows through the channels of the porous subplate 104 and hydrogen flows through the channels of the non-porous subplate 102. It is also considered within the scope of the present invention to exchange the positions, that is, for hydrogen to flow through the channels of the porous subplate 104 and air to flow through the channels of the non-porous subplate 102.

[0064] One of the improvements of the disclosed fuel cell system is the prevention of electrolytic corrosion on the non-porous metal subplate. Electrolytic corrosion may occur at the interface 164 (Figs. 11 and 13) between the porous carbon subplate and the metal subplate due to their potential difference. When the metal begins to oxidize, the oxide layer becomes non-conductive, and the cell begins to degrade in performance. As a prior art solution to this problem (when the system contains non-porous carbon), applying a coating to the metal plate to prevent corrosion can be mentioned. The disclosed fuel cell system can still benefit from the coating, but the system may not need to utilize the coating because the desalination / deionized water loop sweeps the interface 164 between the metal and carbon and removes any corrosion products that may normally accumulate and render the interface non-conductive. In fact, the water circulating through the interface prevents the accumulation of oxides.

[0065] Samples of the methods described herein are as follows:

[0066] (1) A method for preventing corrosion of a carbon / metal interface in a fuel cell, the method comprising the following steps:

[0067] Providing a bipolar plate comprising a metal subplate and a porous subplate, the metal subplate having at least one water management surface, the porous subplate having a water management surface opposite the reactant surface, and the water management surface of the porous subplate being adjacent to the water management surface of the metal subplate so as to form an interface;

[0068] Providing a unitized electrode assembly adjacent to the bipolar plate;

[0069] Flowing fuel and oxidant reactants from a reactant flow field on the bipolar plate to the unitized electrode assembly to initiate an electrochemical reaction;

[0070] To sweep away corrosion products formed at the interface, the step of flowing water through a water management loop to the water management surfaces of the metal subplate and the porous subplate; and,

[0071] The step of deionizing and desalinating the water flowing in the water management loop.

[0072] (2) A method for preventing corrosion at the carbon / metal interface in the fuel cell according to (1) above, further comprising the step of forming an internal coolant passage in the bipolar plate and flowing an antifreeze-type coolant through the internal coolant passage.

[0073] (10) A method for operating a four-fluid fuel cell in thermal boost mode, comprising the following steps:

[0074] Providing a four-fluid fuel cell including an oxidant flow field, a fuel reactant flow field, a water management flow field, and an independent circulation coolant loop operable to remove sensible heat, the coolant loop being in fluid communication with a coolant flow field;

[0075] Reducing the flow rate of the coolant in the coolant loop to reduce the sensible heat cooling capacity; and

[0076] Maintaining or increasing the temperature of the fuel cell to increase evaporative cooling.

[0077] (11) A method for operating the four-fluid fuel cell according to (10) above, wherein the coolant is an antifreeze-type coolant.

[0078] (12) A method for operating the four-fluid fuel cell according to (10) above, wherein at least one of the oxidant flow field and the fuel reactant flow field includes a plurality of pores fluidly connected to the water management flow field, the pores being configured as a bubble barrier.

[0079] (13) A method of operating a four-fluid fuel cell according to (10) above, wherein the step of providing the 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.

[0080] (14) A method of operating a four-fluid fuel cell according to (10) above, further including the step of increasing the flow rate of water through the water management flow field to compensate for increased evaporation.

[0081] (15) A method of operating a four-fluid fuel cell according to (14) above, wherein the step of providing the four-fluid fuel cell further includes providing a circulating water management loop in fluid communication with the water management flow field.

[0082] (20) A method of accumulating and holding generated water in a four-fluid fuel cell, including the following steps:

[0083] Providing a four-fluid fuel cell including an oxidant flow field, a fuel reactant flow field, a water management flow field, and an independent circulating coolant loop operable to remove sensible heat, the coolant loop being in fluid communication with a coolant flow field;

[0084] Increasing the flow rate of the coolant in the coolant loop to increase the sensible heat cooling capacity; and

[0085] Maintaining or reducing the temperature to condense excess generated water.

[0086] (21) The method of accumulating and holding generated water in a four-fluid fuel cell according to (20) above, further including the step of providing a water storage tank for storing excess generated water, the water storage tank being in fluid communication with the water management loop.

[0087] (22) The method for accumulating and retaining generated water in a four-fluid fuel cell according to (20) above, further comprising the step of reducing the flow rate of water passing through the water management flow field, accumulating excess generated water, and compensating for the reduced evaporation.

[0088] (23) The method for accumulating and retaining generated water in a four-fluid fuel cell according to (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.

[0089] (24) The method according to any one of (10) or (20) above, wherein the controller commands the flow rate settings of the coolant pump and the water pump according to the sensor data, and the sensor data includes at least one of air flow rate, cathode exhaust temperature, cathode exhaust pressure, total water storage capacity, water inventory, water temperature, ambient temperature, coolant return temperature, and water loop outlet pressure.

[0090] (25) The method according to any one of (10) or (20) above, wherein the controller commands the flow rate settings of the coolant pump and the water pump according to environmental factors, and the environmental factors include at least one of payload timing, vehicle route, GPS coordinates, road gradient, weather forecast, time, and driver behavior. 〔1〕The following: A non-porous subplate, the non-porous subplate including a first water management surface and a second water management surface opposite to the first water management surface, the following: A fuel supply internal manifold through-passage and a fuel return internal manifold through-passage; An oxidant supply internal manifold through-passage and an oxidant return internal manifold through-passage; A water management supply internal manifold through-passage and a water management return internal manifold through-passage; A coolant supply internal manifold through-passage and a coolant return internal manifold through-passage; and An internal coolant passage that is in fluid communication with the coolant supply internal manifold through-passage at one end and in fluid communication with the coolant return internal manifold through-passage at the other end, the internal coolant passage extending across a region between the fuel supply internal manifold through-passage and the fuel return internal manifold through-passage and between the oxidant supply internal manifold through-passage and the oxidant return internal manifold through-passage A non-porous subplate that defines; A first porous subplate including a reactant surface and a water management surface opposite thereto, the reactant surface including a first reactant flow field that is in fluid communication with one of the fuel supply internal manifold through-passage and the oxidant supply internal manifold through-passage, and the water management surface being in fluid communication with the first water management surface of the non-porous subplate; and A second porous subplate including a reactant surface and a water management surface opposite thereto, the reactant surface including a second reactant flow field that is in fluid communication with the other of the fuel supply internal manifold through-passage and the oxidant supply internal manifold through-passage, and the water management surface being in fluid communication with the second water management surface of the non-porous subplate A bipolar plate for a fuel cell including. 〔2〕The bipolar plate according to 〔1〕, wherein at least one of the first porous subplate and the second porous subplate forms a nested seal within a recessed perimeter of the non-porous subplate. 〔3〕The at least one surface of the non-porous subplate defines a water management flow field, the bipolar plate according to 〔1〕. 〔4〕The water management flow field includes water flow field channels, the bipolar plate according to 〔3〕. 〔5〕The non-porous subplate includes a first half plate joined to a second half plate, the bipolar plate according to 〔1〕. 〔6〕The internal coolant passage is defined by the joined first half plate and second half plate, the bipolar plate according to 〔5〕. 〔7〕The internal coolant passage is compatible with an antifreeze type coolant, the bipolar plate according to 〔1〕. 〔8〕At least one of the first porous subplate and the second porous subplate has a bubble barrier pore structure that enables transfer of liquid through the pore structure and is suitable for preventing transfer of reaction gas through the pore structure, the bipolar plate according to 〔1〕 including a bubble barrier pore structure. 〔9〕Both the first porous subplate and the second porous subplate have a bubble barrier pore structure that enables transfer of liquid through the pore structure and prevents transfer of reaction gas through the pore structure, the bipolar plate according to 〔8〕 including a bubble barrier pore structure. 〔10〕A non-porous subplate for a fuel cell bipolar plate assembly, including a first water management surface and a second water management surface on the opposite side of the first water management surface, the non-porous subplate has the following: A fuel supply internal manifold through-passage and a fuel return internal manifold through-passage; An oxidant supply internal manifold through-passage and an oxidant return internal manifold through-passage; A water management supply internal manifold through-passage and a water management return internal manifold through-passage; A coolant supply internal manifold through-passage and a coolant return internal manifold through-passage; and An internal coolant passage that is in fluid communication with the coolant supply internal manifold through-passage at one end and in fluid communication with the coolant return internal manifold through-passage at the other end, the internal coolant passage extending across a region between the fuel supply internal manifold through-passage and the fuel return internal manifold through-passage and between the oxidant supply internal manifold through-passage and the oxidant return internal manifold through-passage defining a non-porous subplate. 〔11〕The non-porous subplate according to 〔10〕, wherein the first water management surface includes a first recessed peripheral portion suitable for receiving the first porous subplate. 〔12〕The non-porous subplate according to 〔11〕, wherein the first recessed peripheral portion is further suitable for providing a nested seal with the first porous subplate. 〔13〕The non-porous subplate according to 〔11〕, wherein the second water management surface includes a second recessed peripheral portion suitable for receiving the second porous subplate. 〔14〕The non-porous subplate according to 〔13〕, wherein the second recessed peripheral portion is further suitable for providing a nested seal with the second porous subplate. 〔15〕The non-porous subplate according to 〔10〕, further including a first half-plate joined to the second half-plate, thereby defining an internal coolant passage.

Claims

1. a non-porous subplate including a water management side and an opposing reactant side with internal coolant passages therebetween; and a porous subplate including a reactant surface and an opposing water management surface, the reactant surface including a first reactant flow field, the water management surface being in fluid communication with the water management surface of the non-porous subplate; 13. A bipolar plate for a fuel cell comprising:

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

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

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

5. The bipolar plate of claim 1 , wherein the interior coolant passages of the non-porous subplate are subdivided into first and second passages.

6. The bipolar plate of claim 5 , wherein the non-porous subplate further comprises a divider for separating the first passage from the second passage.

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

8. The bipolar plate of claim 1 , wherein the water management surface of the porous subplate comprises a water flow field.

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

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 movement across the fuel cell.

11. 11. The bipolar plate of claim 10, wherein the water flow field includes a bubble barrier pore structure adapted to allow transport of liquid through the pore structure and prevent transport of reactant gases through the pore structure.

12. 10. The bipolar plate of claim 1, wherein the water management surface of the non-porous subplate includes a recessed perimeter adapted to provide a nested seal with a porous subplate.

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

14. The bipolar plate of claim 13 , wherein the interior coolant passages are defined by the first and second half-plates joined together.

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

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