Flow field plate for improved coolant flow

Abstract

A bipolar plate arrangement comprising an internal coolant flow field for a fuel cell, wherein the bipolar plate arrangement comprises: an anode plate comprising inlet-side and outlet-side openings for respective fluids in the form of a fuel, an oxidizer, and a coolant; a fuel flow field comprising a transition region and an active region on the anode side of the anode plate, wherein the transition region comprises a plurality of transition channels for the fuel, wherein the active region comprises a plurality of active fuel channels, and wherein the transition channels for the fuel fluidically connect the active fuel channels to a fuel opening; and a coolant flow field on the internal coolant side of the anode plate;a cathode plate comprising: inlet-side and outlet-side openings for respective fluids in the form of a fuel, an oxidizer, and a coolant; an oxidizer flow field comprising a transition region and an active region on the cathode side of the cathode plate, wherein the transition region comprises transition channels for the oxidizer, wherein the active region comprises active oxidizer channels, and wherein the transition channels for the oxidizer fluidically couple the active oxidizer channels to an oxidizer opening; and a coolant flow field on the inner coolant side of the cathode plate, wherein the inner coolant side of the anode plate is connected to the inner coolant side of the cathode plate;and characterized in that the active fuel channels are aligned with the active oxidant channels and the transition channels for the fuel are arranged offset from the transition channels for the oxidant, such that the offset creates larger, continuous spaces between the transition channels for the fuel and the transition channels for the oxidant for the transverse coolant flow.

Description

background Field of invention The invention relates to designs of flow field plates for improving coolant flow by reducing the pressure drop in the coolant transition areas between the openings and the flow fields in bipolar plate arrangements for solid polymer electrolyte fuel cell stacks. Description of the related prior art Fuel cells, such as solid-polymer electrolyte (SPI) or proton exchange membrane (PEM) fuel cells, electrochemically convert reactants—a fuel (such as hydrogen) and an oxidant (such as oxygen or air)—to generate electrical power. SPI fuel cells generally use a proton-conducting solid-polymer membrane electrolyte between cathodic and anodic electrodes. A structure comprising a solid-polymer membrane electrolyte positioned between these two electrodes is called a membrane electrode assembly (MEA).In a typical fuel cell, flow field plates are provided on each side of the MEA (Mechanical Electrode Assembly). These plates incorporate numerous fluid distribution channels for the reactants, distributing the fuel and oxidant to the respective electrodes and removing byproducts of the electrochemical reactions occurring within the fuel cell. Water is the primary byproduct in a fuel cell operating with hydrogen and air as reactants. Because the output voltage of a single cell is on the order of 1V, multiple cells are typically stacked in series for commercial applications to provide a higher output voltage. For use in automotive applications and similar scenarios, these fuel cell stacks can be further interconnected in series and / or parallel groups. Along with water, heat is a significant byproduct of the electrochemical reactions that take place within the fuel cell. Therefore, cooling means are generally required for a fuel cell stack. Stacks designed to achieve high power density (for example, automotive stacks) typically circulate a liquid coolant through the stack to dissipate heat quickly and efficiently. To achieve this, coolant flow fields, comprising numerous coolant channels, are typically integrated into the flow field plates of the cells within the stacks. The coolant flow fields can be formed on the electrochemically inactive surfaces of the flow field plates, thus distributing the coolant evenly within the cells while reliably keeping the coolant separated from the reactants. Bipolar plate arrangements comprising an anodic flow field plate and a cathodic flow field plate, connected and suitably sealed to form a sealed coolant flow field between the plates, are therefore commonly used in the prior art. The plates forming the arrangement can optionally be metallic and have suitable corrosion-resistant coatings, and are typically manufactured by stamping the desired features into sheets of suitable metallic materials (for example, certain stainless steels). Alternatively, the plates can be carbonaceous and are typically manufactured by forming features into plates made of suitable, malleable, carbonaceous materials (for example, polymer-impregnated expanded graphite). To supply and discharge both reactants and coolant to and from the individual cells in the stack, a series of openings are generally provided at opposite ends of each cell, forming manifolds for these fluids when the cells are stacked. Further design features required include passages to distribute the majority of the fluids to and from the various channels in the flow field channels for the reactants and coolant within the plates. The areas associated with the coolant are referred to as the coolant transition zones. These coolant transition zones can, in turn, comprise numerous fluid distribution channels, such as transition channels for the oxidant and / or the fuel. For ease of manufacture and other reasons, a common stack design uses a stack of generally rectangular, planar fuel cells whose flow plates include numerous straight flow channels for the reactants and the coolant, running from one end of the plates to the other. Furthermore, it can be advantageous to use a stack design in which certain openings are located on the side of the plates and thus not in line with the flow channels. However, such a design necessitates routing the associated fluid transversely to the flow channels in order to fluidically connect the opening to the flow channels in the coolant transition zones. This can be achieved by forming passages in these coolant transition zones that are transverse to all the reactant transition channels.The presence of such passages can impede the flow of other fluids in the coolant transition area, as will become clearer when the figures below are discussed. Therefore, it may be necessary to balance the flow through such passages with the flow through other transition channels. This can be particularly important in designs of high-power-density stacks that include coolant openings located on the sides of the plates and coolant passages in the coolant transition zone that run transversely to the flow fields. To achieve the highest power densities, fluid channels are often designed at the limits of reliable manufacturability and tolerances. Because it is a liquid, the coolant flow is subject to greater pressure drops than a gaseous reactant when flowing through passages or channels of a given size. As a result, the coolant pressure drop, especially in the coolant transition zones, can be significant in such high-power-density stacks, and particularly in wider cells where longer transverse coolant passages are required.This can lead to an uneven distribution of the coolant on, and consequently, within the coolant flow channels in the active areas of the fuel cell. This, in turn, can increase the risk of overheating (steady-state temperature spikes) and accelerated drying in the cells on hot days. Furthermore, it can lead to the formation of wet spots in the cells, making it difficult to prepare the stack for shutdown when conditions are below freezing, and it can also make it difficult to recover from sub-zero conditions during startup. Additionally, a significant coolant pressure drop necessitates the use of a larger, more powerful coolant pump. The pressure drop can be reduced to some extent by sacrificing space allocated for reactant flow in the coolant transition zone. However, depending on the design of the opening and transition, this can result in an unacceptable blockage of the flow of one or both reactants. Alternatively, the thickness of the individual fuel cells can be increased, thereby increasing the height of the coolant passages in the coolant transition zones. However, this undesirably reduces the power density of the stack, and this is accompanied by a potential, undesirable increase in the stack's mass. US Patent 2012 / 0295178A1 discloses a flow field plate design for improved coolant flow and reduced pressure drop associated with coolant flow in the coolant transition zones of fuel cell stacks. The pressure drop is reduced by increasing the height of the coolant passages in the transition zone of the flow field plate, extending them beyond the plane of the plate. Reducing the pressure drop results in improved coolant flow distribution. This height adjustment can be achieved by offsetting the passages in adjacent cells within the stack. However, this design utilizes non-planar MEAs (Mechanical Energy Applications), which are not commonly used in this area. US Patent 2011 / 0 165 493 A1 discloses a design of a flow field plate with fuel channels, each comprising a first upper flow section, a subsequent second upper flow section, and a subsequent lower flow section. The first upper flow sections of the fuel channels are arranged adjacent to the transition channels of the oxidation channels, the second upper flow sections of the fuel channels intersect the transition channels of the oxidation channels, and the lower flow sections overlap the transition channels of the oxidation channels. Despite the progress achieved so far, there remains a need for ever higher power densities in fuel cell stacks and more efficient designs of flow field plates. The present invention describes one option to meet these needs and provides further related advantages. Summary The present invention provides improved coolant flow and reduced pressure drop in the transition regions of a bipolar plate array without adversely affecting the reactant flow in these regions, while maintaining the overall stack volume. This is achieved by offsetting the fuel transition channels from the oxidant transition channels in the transition regions. Consequently, this can be accomplished using conventional planar MEAs. Specifically, a bipolar plate arrangement according to the invention comprises an anode plate, a cathode plate, and an internal coolant flow field for a fuel cell. The anode plate includes inlet-side and outlet-side openings for respective fluids in the form of fuel, oxidizer, and coolant; a fuel flow field comprising a transition region and an active region on the anode side of the anode plate, wherein the transition region comprises a plurality of transition channels for the fuel, the active region comprises a plurality of active fuel channels, and the transition channels for the fuel fluidically connect the active fuel channels to a fuel opening; and a coolant flow field on the internal coolant side of the anode plate.Similarly, the cathode plate comprises inlet-side and outlet-side openings for respective fluids in the form of fuel, oxidizer, and coolant; an oxidizer flow field comprising a transition region and an active region on the cathode side of the cathode plate, wherein the transition region comprises transition channels for the oxidizer, wherein the active region comprises active oxidizer channels, and wherein the transition channels for the oxidizer fluidically connect the active oxidizer channels to an oxidizer opening; and a coolant flow field on the inner coolant side of the cathode plate.In the bipolar plate arrangement, the inner coolant side of the anode plate is connected to the inner coolant side of the cathode plate in such a way that the active fuel channels are aligned with the active oxidant channels and the transition channels for the fuel to the transition channels for the oxidant are offset. In a simple embodiment, the fuel and oxidant transition channels in the transition region(s) are essentially straight and parallel, and the active fuel and oxidant channels are essentially straight and parallel in the active regions. However, the fuel and oxidant channels necessarily exhibit a bend at the interfaces between the transition channels and the active channels and are therefore not straight in these short interface regions between the transition regions and the active region in this embodiment. In certain embodiments, the fuel flow field may include transition areas at both ends of the active area on the anode side of the anode plate, and the oxidizer flow field may include transition areas at both ends of the active area on the cathode side of the cathode plate. Thin bipolar plate assemblies with anodic and cathodic plates made of metal can benefit from the invention through improved coolant flow distribution in the transition zones. Bipolar plate assemblies with anodic and cathodic plates made of carbon can also benefit from the invention, as it allows the use of thinner plates. The bipolar plate assemblies are particularly suitable for use in a solid-polymer electrolyte fuel cell stack, in which rows of membrane electrode assemblies are separated from one another by a succession of bipolar plate assemblies. In such a stack, the bipolar plate assemblies include anodic shoulders between the fuel transition channels and cathodic shoulders between the oxidant transition channels, and the membrane electrode assemblies in the row stack can be compressed between the anodic and cathodic shoulders of the bipolar plate assemblies on each side of the membrane electrode assemblies. For this purpose, the anodic shoulders of each bipolar plate assembly can be desirablely aligned with the cathodic shoulders of the adjacent bipolar plate assembly in the row stack.This can be achieved by using a set of bipolar plate assemblies, wherein the set comprises assemblies of a first design and assemblies of a second design, the bipolar plate assemblies in the set being arranged such that the assemblies of the first design alternate with the assemblies of the second design in the stack. In an alternative embodiment, however, the assemblies of the first design can have the same shape as the assemblies of the second design, but the assemblies of the second design are rotated 180° relative to the stack axis with respect to the assemblies of the first design when installed in the stack. The bipolar plate assemblies can be easily manufactured using standard techniques known to those skilled in the art. After a suitable design for the assemblies has been determined, the anodic and cathodic plates are first manufactured, and then the inner coolant side of the anode plate is connected to the inner coolant side of the cathode plate. These and other aspects of the invention are evident with reference to the attached figures and the following detailed description. Brief description of the drawings Figures 1a, b, c, and d show schematic top views of the coolant side of an anodic flow field plate, the coolant side of a cathodic flow field plate, and the fuel side of the anodic flow field plate or the oxidizer side of the cathodic flow field plate of a prior art solid polymer electrolyte fuel cell stack, which includes lateral coolant inlet openings and cross-cut coolant inlet passages in the coolant transition areas of the flow field plates. (These figures are reproductions of the figures from US 2012 / 0295178A1.)Figure 2 shows a schematic, longitudinal sectional view of the coolant transition zone and coolant passages of a prior art solid polymer electrolyte fuel cell stack, which includes lateral coolant inlet openings and a plurality of transverse coolant passages in the coolant transition zone of the flow field plates. Figures 3a, 3b, and 3c show several schematic, width-oriented sectional views of some cells in different fuel cell stacks. Figure 3a shows the view in the active region of the fuel cells in a prior art fuel cell stack. Figure 3b shows the view in the transition zone of fuel cells in a prior art fuel cell stack. Figure 3c compares the view in the transition zone of fuel cells in a fuel cell stack according to the invention. Figures 4a and 3c show the view in the transition zone of fuel cells of a fuel cell stack according to the invention.Figure 4b shows schematic top views of the cathode sides of a bipolar plate arrangement of a first design and of a bipolar plate arrangement of a second design. Together, these bipolar plate arrangements can be used to create a fuel cell stack in which the MEAs are compressed between the anodic and cathodic steps in the transition regions. Detailed description In this description, words such as "a" and "encompasses" should be understood in an open sense, and should be understood to mean at least one but not limited to just one. In the present context, the expression “approximately” should be understood as lying in the range of up to plus 10% and up to minus 10%. Throughout this description, "active regions" refers to the areas within the fuel cell where electrochemical reactions take place. "Transition regions" refers to the areas within the fuel cell adjacent to the active regions, through which fluids (a reactant and / or a coolant) flow, but where electrochemical reactions do not occur. "Active reactant channels" therefore refers to the sections of the fluid channels for the reactant in the flow field plates located within the active regions and opposite the active electrode surfaces in the fuel cell. "Transition channels for a reactant" refers to the sections of the fluid channels for a reactant in the flow field plates that are adjacent to and fluidically connected to the active reactant channels. However, "transition channels" are not opposite the active electrode surfaces. The term "offset" is used here to describe the alignment of different sets of channels. A given channel is considered to be offset from other channels if it is not directly aligned with any of the other channels. Although it may be desirable for some reasons, an offset channel need not be centered, for example, at the midpoint between two other channels. Furthermore, an offset channel may overlap with another channel to some extent (for example, if it is wider than the available space between the other channels or if it is not centered between the other channels). For the purposes of this discussion, a given channel is considered to be offset from another channel if the amount of overlap of the given channel is 50% or less than half the width of the other channel. In the description, the expression "essentially straight and parallel" was used to describe the channel geometry in certain embodiments in which the channels run straight and parallel over most, but not all, of the area in question. Certain channels must necessarily exhibit a bend over short intervals in every embodiment of the invention. A PEMFC stack design suitable for automotive applications typically comprises a row stack of generally rectangular, planar PEM fuel cells. The fuel used is usually pure hydrogen, although other fuels may be considered. Air is typically supplied as the oxidizer. The individual PEM fuel cells comprise a membrane electrode assembly (MEA) of a polymer membrane electrolyte and two catalyst layers, usually based on noble metals, on each side of the membrane electrolyte, serving as the anode and cathode, respectively.Gas diffusion layers are typically placed adjacent to the catalyst layers in the MEA for various purposes, such as distributing the reaction gases uniformly across the electrodes, removing byproduct fluids, providing an electrical connection to the electrodes, and offering mechanical support. These gas diffusion layers are engineered, porous, and electrically conductive structures and typically comprise carbon fibers, binders, and materials that control the wetting properties of the layers.Flow field plates are then provided adjacent to the anodic and cathodic gas diffusion layers to distribute the majority of the fluids to and from the gas diffusion layers, to provide mechanical support, to provide a manifold structure for the fluids supplied to and discharged from the cell, and also to provide a structure that allows the circulation of a liquid coolant to the fuel cells. Other special layers or intermediate layers may also be provided in the structure for various purposes (for example, between the electrode and the gas diffusion layer or between the gas diffusion layer and the flow field plate). Figures 1a, b, c, and d show schematic top views of flow field plates according to the prior art, suitable for a high-power-density, automotive-scale PEMFC stack. There are two types of plates involved: one for the anode side of the cell and one for the cathode side. Figures 1a and 1c show views of the opposing main surfaces of an anodic flow field plate 100, and Figures 1b and 1d show views of the opposing main surfaces of a cathodic flow field plate 101. (Note: Figures 1a, b, c, and d were reproduced from US 2012 / 0295178 A1.) Both flow field plates have openings located at opposite ends, serving as inlet and outlet openings for the various fluids supplied to and discharged from the cells. Seals are used around these openings so that multiple openings align and tightly connect when the majority of these cells are stacked in series, forming manifolds for the various fluids within the stack. Figures 1a, b, c, and d show the various openings: a fuel inlet 102, a fuel outlet 103, an oxidizer inlet 104, an oxidizer outlet 105, two coolant inlets 106, and two coolant outlets 107. (For simplicity and clarity, the sealing structures involved have been omitted from these figures.) Figures 1c and 1d show the main surfaces of the anodic flow field plate 100 and the cathodic flow field plate 101, which face the electrochemically active anode and cathode, respectively, in the MEA. Flow fields for reactants are formed into each of these plates, comprising a plurality of essentially straight flow field channels extending from one end of the plate to the other. Figure 1c shows numerous flow field channels 108 for the fuel, which are opposite the active region of an anode when installed in a fuel cell. Figure 1d shows numerous flow field channels 109 for the oxidant, which are opposite the active region of a cathode when installed in a fuel cell. Transition zones for the coolant are provided between the flow field channels for the reactants in these plates and the various openings in the plates.In these coolant transition zones, various structures are provided to fluidically connect the different openings with their corresponding flow field channels. In Fig. 1c, the coolant transition zones are designated 110. As shown, linear flow field channels 108 extend from each end of the flow field plate 100 into the coolant transition zones 110. These sections of the channels are designated as fuel transition channels 111a, 111b. (In actual cells, other distribution structures are typically formed in the coolant transition zones 110 between the fuel transition channels 111a, 111b and between the fuel inlets and outlets 102, 103. However, these are not shown in Fig. 1c for clarity.)) The associated fuel cell is supplied with fuel at the fuel inlet 102, and this is fed through inlet-side return openings 130a for the fuel to the adjacent transition channels 111a for the fuel, then guided through the fuel flow field channels 108 and thus directed to the channels 108 which are adjacent to the electrochemically active anode, and any remaining fuel and by-products of the reaction are fed from the transition channels 111b for the fuel through an outlet-side return opening 130b for the fuel to the adjacent outlet-side opening 103 for the fuel. In the same way as in Fig. 1c, the transition zones for the coolant in the cathode plate 110 are designated 112. Linear flow field channels 109 extend into the transition zones 112 for the coolant on each side of the flow field plate 110. These sections of the channels are designated as transition channels 113a, 113b for the oxidizing agent.The oxidizing agent is accordingly supplied to the associated fuel cell at the inlet-side opening 104 for the oxidizing agent, is supplied through an inlet-side return-flow opening 140a for the oxidizing agent to the adjacent transition channels 113a for the oxidizing agent, is guided through the flow field channels 109 for the oxidizing agent and thus supplied to the electrochemically active cathode, which is adjacent to the channels 109, and any remaining oxidizing agent and byproducts of the reaction are supplied from the transition channels 113b for the oxidizing agent through an outlet-side return-flow opening 140b for the oxidizing agent to the adjacent outlet-side opening 105 for the oxidizing agent. As is typically done in the prior art, the other sides of the flow field plates 100 and 101 are used to interact to create coolant flow fields for the cells in the stack. Specifically, for a given cell in the stack, the main surface on the coolant side of its anodic flow field plate 100 (shown in Fig. 1a) interacts with the main surface on the coolant side of the cathodic flow field plate 101 (shown in Fig. 1b) of an adjacent cell in the stack to create a coolant flow field. Typically, pairs of anodic flow field plates 100 and cathodic flow field plates 101 are connected to form flow field plate assemblies before the rest of the fuel cell stack is assembled. The view in Fig. 1a of the anodic flow field plate 100 corresponds to that of Fig. 1c, except that it is rotated 180° about a longitudinal axis of the plate. Linear flow field channels 114 for the coolant are visible here, and these extend into the transition regions 110 for the coolant at each end of the flow field plate 100. These sections of the channels are designated as transition channels 115a, 115b for the coolant. Similarly, the view of the cathodic flow field plate 101 in Fig. 1b corresponds to that of Fig. 1d, except that it is rotated 180° about the longitudinal axis of the plate. Linear flow field channels 118 for the coolant are visible here, and these extend into the transition regions 112 for the coolant at each end of the flow field plate 110. These regions of the channels are designated as transition channels 117a, 117b for the coolant. In adjacent cells of the stack, the two surfaces shown in Fig. 1a and Fig. 1b fit together and interact to form a coolant flow field, which is suitably sealed from the rest of the cell and from the surrounding environment. The coolant is supplied at the interface of these adjacent cells through coolant inlet openings 106 and must be fed to the adjacent coolant transition channels 115a, 117a. It is then guided through the coolant flow field channels 114, 118 and subsequently from the coolant transition channels 115b, 117b at the other end of the cells to the adjacent coolant outlet openings 107. As can be seen from Figs. 1a, b, c and d, the flow field plates 100, 101 have a design with lateral inlets for all fluids in the form of fuel, oxidizer and coolant. This means that the openings 102, 103 for the fuel, the openings 104, 105 for the oxidizer and the openings 104, 105 for the coolant are arranged on the sides of opposite ends of the plates 100, 101 and therefore do not align with the linear flow fields 108, 109, 114, 118. As shown, a wide path is available in the transition areas 110 for the coolant for the fuel, which is directed from the inlet-side return opening 130a for the fuel to the transition channels 111a for the fuel at the inlet-side end, and which is directed from the transition channels 111b for the fuel to the outlet-side return opening 130b for the fuel at the outlet-side end of the plate 110.However, no such comparably wide paths are available to fluidically connect the coolant openings 106, 107 with the coolant transition channels 115a, 115b at each end of the plate 100. Instead, coolant passages 119a, 119b (in the anodic flow field plate 100) and 120a, 120b (in the cathodic flow field plate 101) are provided to establish a fluidic connection between the coolant openings 106, 107 and the coolant transition channels 115a, 115b, 117a, 117b. The coolant passages 119a, 119b, 120a, and 120b are necessary to provide a reasonably sized path for the coolant fluid flowing transversely to the flow field channels 114 and 118, ensuring that the coolant is distributed reasonably well to and from all channels. However, the presence of the coolant passages obstructs the transition channels 111 and 113 for the fuel and oxidizer, and vice versa, as is better illustrated with reference to Fig. 2. Fig. 2 shows a schematic, longitudinal sectional view of the coolant transition areas 110, 112 near one end of some cells in the fuel cell stack. However, unlike Fig. 1, this prior art embodiment uses a plurality of transverse coolant passages. In Fig. 2, the vertical line represents the stack direction and the horizontal line the direction of the linear flow field channels 108, 109, 114, 118. The right side of Fig. 2 is located near one end of the stack (the actual edge of the stack is not shown in Fig. 2). Each cell comprises a MEA 1, a fuel transition channel 111, and an oxidizer transition channel 113.Anodic flow field plates 100 are connected to cathodic flow field plates 101 to form flow field plate assemblies, and a plurality of closed coolant passages 119 / 120 are formed within these flow field plate assemblies. (Note: The coolant passage 119 of the anodic flow field plate 100 aligns with the coolant passage of the cathodic flow field plate 101 to create the closed passages shown in Fig. 1. The internal height of the created closed passage is thus determined by the sum of the depths of the passage formed in plate 100 plus the depth of the passage formed in plate 101.) As can be seen in Fig. 2, the entire vertical height dedicated to the size of the closed coolant passages 119 / 120 serves to restrict the flow of reactants in the transition channels 111 for the fuel and the transition channels 113 for the oxidizer, and vice versa. Therefore, when attempting to create a fuel cell stack with the highest power density (and consequently the smallest size), a balance must be struck between the flow capacity of the reactants and the flow capacity of the coolant in the coolant transition areas. In practice, a plurality of coolant passages 119 / 120 can be used to obtain a satisfactory flow of the coolant transversely to the various linear flow fields without unacceptably impeding the flow of reactants in the transition channels 111, 113. In the flow field plates according to the prior art, which are shown in Fig. 1 and Fig. 2, the surfaces of the plates are generally flat throughout, and as partially shown in Fig. 2, the MEAs 1 in the individual cells are also generally flat over the entire cells. Figures 3a and 3b show representative, schematic, width-oriented sectional views of a prior art fuel cell stack in the active region and the transition region, respectively (two sets of cell components in the stack are shown). Using the same reference numerals from Figures 1a to d to identify components, the view in Figure 3a shows MEAs 1, anodic flow field plates 100, and cathodic flow field plates 101. Active flow field channels 108 for the fuel are located opposite the anode side of the MEAs 1, and active flow field channels 109 for the oxidizer are located opposite the cathode side of the MEAs 1. Flow field channels 114 and 118 for the coolant are aligned to form coolant flow fields between pairs of plates 100 and 101. As can be seen from Fig. 3a, there is no possibility for a transverse flow of the coolant in this active area (i.e. a cross-flow between adjacent coolant channels), since the bottoms 108a of the active flow field channels for the fuel are in contact with the bottoms 109a of the active flow field channels for the oxidizer. To nevertheless ensure at least a moderate transverse flow of the coolant in the transition region, either one or both of the transition channels for the fuel or the oxidizer are designed to be shallower, as shown in the view of Fig. 3b. In Fig. 3b, both the flow field transition channels 111 for the fuel and the flow field transition channels 113 for the oxidizer are shallower than the channels shown in Fig. 3a. Therefore, gaps 120 exist between the bottoms 111c of the flow field transition channels for the fuel and the bottoms 113c of the flow field transition channels for the oxidizer. These gaps 120 thus allow a modest, restricted transverse flow of the coolant, which is generally indicated by the arrow.Preferably, however, a desired transverse flow of the coolant is obtained without restricting the flow in the channels for the fuel and the oxidizer in this area. Fig. 3c shows a schematic sectional view of the transition region of a fuel cell stack according to the invention. As before, MEAs 1 appear in Fig. 3c. However, two differently designed bipolar plate arrangements are used. In the first design, an anodic flow field plate 200 is combined with a cathodic flow field plate 201. In the second design, an anodic flow field plate 300 is combined with a cathodic flow field plate 301. Both the first and second designs contain flow field transition channels 211 and 311, respectively, for the fuel and flow field transition channels 213 and 313, respectively, for the oxidant. However, in these designs, the channels in each flow field plate are offset from the channels in its partner plate in the bipolar plate arrangements. For example, the flow field transition channels 211 for the fuel are offset from the flow field transition channels 213 for the oxidant and are located between them. As described in Fig. 3c, the flow field transition channels for the fuel in each set of bipolar plate arrangements are offset such that they are centrally located between the flow field transition channels for the oxidant of their partner cathode plates. As also shown, there is nevertheless some overlap between these channels in the regions around the channel walls.For example, channel walls 211c of the flow field transition channels 211 for the fuel overlap with channel walls 213c of the flow field transition channels 213 for the oxidizer. In other embodiments, however, the channels may not be arranged centrally offset, and there may be a greater or lesser overlap of the channels. Essentially, the approach according to the invention provides greater possibilities for achieving a transverse coolant flow with less required restriction in the channels for the reactants. As shown in Fig. 3c, larger, continuous "intervals" 220, 320 are now formed between the plates of the first and second designs, respectively. Furthermore, the use of suitable first and second designs of the bipolar plate arrangements ensures a configuration in which the MEAs can nevertheless be compressed between the anode and cathode shoulders in the transition regions (as exemplified, for example, by MEA 1, which is compressed between shoulder 200d of the anode and shoulder 301d of the cathode in Fig. 3c). The offset or positioning between the channels, the depth of the flow field transition channels for the fuel and oxidizer, and other channel dimensions can be adjusted to improve certain properties at the expense of others, as will be apparent to a person skilled in the art. The appropriate adjustment will depend on individual circumstances and can be expected by the average person skilled in the art. The active area of ​​a fuel cell stack according to the invention can be manufactured essentially identically to that of a stack according to the prior art (for example, having the same cross-section, which is shown in Fig. 3a). Figs. 4a and 4b show schematic top views of the entire cathode sides of the bipolar plate arrangements, which appear in Fig. 3c. Facing the reader are the cathodic flow field plates 201 and 301 in Figs. 4a and 4b, respectively. The flow field transition channels 213 for the oxidant and the active flow field channels 109 for the oxidant are visible on the cathodic flow field plate 201. The flow field transition channels 313 for the oxidant and the active flow field channels 109 for the oxidant are visible on the cathodic flow field plate 301.As shown, these channels are essentially straight and parallel throughout, except where they necessarily exhibit a bend at the interfaces 3 between the transition areas and the active areas on the plates. To illustrate the relative alignment of the oxidant channels with respect to the fuel channels, the contours of the fuel channels located on opposite sides of the bipolar plate assemblies are shown in dashed lines in Figures 4a and 4b. Specifically, the fuel flow field transition channels 211 and the active fuel flow field channels 108 on the anodic flow field plate are shown in dashed lines. Similarly, the fuel flow field transition channels 311 and the active fuel flow field channels 108 on the anodic flow field plate are shown in dashed lines. Together, these bipolar plate assemblies can be used to create a fuel cell stack with improved flow distribution in the transition regions.This can be achieved while maintaining an adequate flow of reactants and while still compressing the MEAs between the anodic and cathodic steps in the transition regions. Although two different designs of bipolar plate arrangements are required to produce such a stack, it is evident from Figures 4a and 4b that the arrangements can otherwise have the same design. The arrangements in these two figures have the same design, but the arrangement in Figure 4b is rotated 180° perpendicular to the side (that is, around the axis of the fuel cell stack) relative to the arrangement in Figure 4a. The following example illustrates the invention, but should not be interpreted as limiting in any way. Example Analyses were performed to compare the expected coolant pressure drop in a conventional fuel cell with the expected pressure drop in an exemplary fuel cell according to the invention. The conventional fuel cell was assumed to have a design similar to that shown in Figures 2, 3a, and 3b, and to be intended for use in a high-power-density automotive application. The fuel cell according to the invention was assumed to be similar to the conventional fuel cell, except that the fuel transfer channels are offset from the oxidant transfer channels, as shown in Figures 3c and 4. During operation, reactant and coolant flows were assumed to be typical for such high-power-density automotive applications. Computational fluid dynamics methods were then used to determine the expected coolant pressure drops in the transition and active regions of the two fuel cell designs. In both fuel cells, a coolant pressure drop of approximately 44 mB was determined in the active region. In the conventional fuel cell, the pressure drop in each transition region at each end of the cell was approximately 93 mB (implying that the total pressure drop in the conventional fuel cell was approximately 230 mB). In the fuel cell according to the invention, the pressure drop in each transition region at each end of the cell was approximately 51 mB (implying that the total pressure drop in the fuel cell according to the invention was approximately 145 mB). The coolant pressure drop in the transition regions of the fuel cell according to the invention was therefore approximately 45% lower than in the transition regions of the conventional fuel cell, resulting in a significantly improved coolant distribution to the channels in the active region. Furthermore, the total coolant pressure loss in the active region of the fuel cell according to the invention was approximately 37% lower than that of the coolant in the conventional cell, thus reducing the required pressure to be supplied by the coolant pump in the associated automotive fuel cell system. This, in turn, allows the use of a smaller, less expensive coolant pump that requires less power. While certain elements, embodiments, and applications of the present invention have been shown and described, it is naturally understood that the invention is not limited thereto, since modifications can be made by a person skilled in the art without departing from the essence and scope of the present disclosure, particularly in light of the foregoing teachings. For example, although the foregoing description has been mainly directed to embodiments comprising carbon-containing flow field inserts for an oxidizing agent, it may be desirable for other reasons to consider embodiments comprising carbon-containing flow field inserts for a fuel. Such modifications are to be considered within the scope and extent of the following claims.

Claims

A bipolar plate arrangement comprising an internal coolant flow field for a fuel cell, wherein the bipolar plate arrangement comprises: an anode plate comprising inlet-side and outlet-side openings for respective fluids in the form of a fuel, an oxidizer, and a coolant; a fuel flow field comprising a transition region and an active region on the anode side of the anode plate, wherein the transition region comprises a plurality of transition channels for the fuel, wherein the active region comprises a plurality of active fuel channels, and wherein the transition channels for the fuel fluidically connect the active fuel channels to a fuel opening; and a coolant flow field on the internal coolant side of the anode plate;a cathode plate comprising: inlet-side and outlet-side openings for respective fluids in the form of a fuel, an oxidizer, and a coolant; an oxidizer flow field comprising a transition region and an active region on the cathode side of the cathode plate, wherein the transition region comprises transition channels for the oxidizer, wherein the active region comprises active oxidizer channels, and wherein the transition channels for the oxidizer fluidically couple the active oxidizer channels to an oxidizer opening; and a coolant flow field on the inner coolant side of the cathode plate, wherein the inner coolant side of the anode plate is connected to the inner coolant side of the cathode plate;and characterized in that the active fuel channels are aligned with the active oxidant channels and the transition channels for the fuel are arranged offset from the transition channels for the oxidant, such that the offset creates larger, continuous spaces between the transition channels for the fuel and the transition channels for the oxidant for the transverse coolant flow. Bipolar plate arrangement according to claim 1, wherein the transition channels for the fuel and the transition channels for the oxidant are essentially straight and parallel in the transition regions, and wherein the active fuel channels and the active oxidant channels are essentially straight and parallel in the active regions. Bipolar plate arrangement according to claim 1, wherein the fuel flow field comprises transition regions at both ends of the active region on the anode side of the anode plate, and wherein the oxidizing agent flow field comprises transition regions at both ends of the active region on the cathode side of the cathode plate. Bipolar plate arrangement according to claim 1, wherein the anode plate and the cathode plate are made of metal. Solid polymer electrolyte fuel cell stack comprising a series stack of membrane electrode assemblies separated from each other by a sequence of the bipolar plate assemblies according to claim 1. Solid polymer electrolyte fuel cell stack according to claim 5, wherein the bipolar plate arrangements comprise anodic steps between the transition channels for the fuel and cathodic steps between the transition channels for the oxidant, and wherein the membrane electrode arrangements in the row stack are compressed between the anodic steps and the cathodic steps of the bipolar plate arrangements on each side of the membrane electrode arrangements in the transition regions. Solid polymer electrolyte fuel cell stack according to claim 6, wherein the anodic shoulders of each bipolar plate arrangement are aligned with the cathodic shoulders of the adjacent bipolar plate arrangement in the row stack. Solid polymer electrolyte fuel cell stack according to claim 7, wherein the bipolar plate arrangements comprise arrangements having a first design and arrangements having a second design, and wherein the bipolar plate arrangements are arranged such that the arrangements of the first design alternate with the arrangements of the second design in the row stack. Solid polymer electrolyte fuel cell stack according to claim 8, wherein the arrangements of the first type differ in their design from the arrangements of the second type. Solid polymer electrolyte fuel cell stack according to claim 8, wherein the arrangements of the first type have the same design as the arrangements of the second type, and wherein the arrangements of the second type are rotated 180 degrees around the stack axis relative to the arrangements of the first type. A method for improving coolant flow in a bipolar plate arrangement having an internal coolant flow field for a fuel cell, wherein the bipolar plate arrangement comprises: an anode plate comprising inlet-side and outlet-side openings for respective fluids in the form of a fuel, an oxidizer, and a coolant; a fuel flow field comprising a transition region and an active region on the anode side of the anode plate, wherein the transition region comprises a plurality of transition channels for the fuel, wherein the active region comprises a plurality of active fuel channels, and wherein the transition channels for the fuel fluidically connect the active fuel channels to a fuel opening; and a coolant flow field on the internal coolant side of the anode plate.a cathode plate comprising: inlet-side and outlet-side openings for respective fluids in the form of a fuel, an oxidizer, and a coolant; an oxidizer flow field comprising a transition region and an active region on the cathode side of the cathode plate, wherein the transition region comprises transition channels for the oxidizer, wherein the active region comprises active oxidizer channels, and wherein the transition channels for the oxidizer fluidically couple the active oxidizer channels to an oxidizer opening; and a coolant flow field on the inner coolant side of the cathode plate;wherein the method comprises the following steps: manufacturing the anode plate and the cathode plate such that the active fuel channels are aligned with the active oxidant channels and that the fuel transition channels are offset from the oxidant transition channels, such that the offset creates larger, continuous spaces between the fuel transition channels and the oxidant transition channels for the transverse coolant flow; and connecting the inner coolant side of the anode plate to the inner coolant side of the cathode plate.

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