Patent Application
20090098432
There exists a need, in many technological applications, to effectively transfer and distribute mixtures of liquids and gases within chemical and engineering devices. In general, the performance of heat exchangers and chemical reactors critically depends on the distribution of the fluids flowing therethrough.
An electrochemical reactor such as a fuel cell is a specific example where the distribution of the reactants therein and the removal of products therefrom require special attention. In its most basic configuration, a fuel cell comprises a pair of electrodes supported on the two opposing faces of a thin proton exchange membrane, wherein the resulting membrane electrode assembly is interposed between a pair of current collector plates. The fuel, which is generally pure hydrogen or diluted alcohol, and the oxidant, which may be either oxygen or air, are continuously supplied to the cell from outside, and are allowed to react at the anode and the cathode, respectively. The membrane electrode assembly generally comprises suitable catalytic surfaces, to accelerate the reduction and oxidation reactions. Electrons released on the anode become a power source while traveling via an external current conductor under the redox voltage of the electrodes towards the cathode, in order to react with excess protons and the oxidant to form water. The respective chemical reactions for direct methanol fuel cell are the following:
CH3OH+H2O→6H++CO2+6e− (anode reaction)
3/2O2+6H++6e−+3H2O (cathode reaction)
For many practical utilities it is necessary to stack together a plurality of individual cells in series. In the resulting arrangement, known in the art as a “fuel cell stack”, oppositely charged electrodes of each pair of adjacent cells are separated by one or more electronically conductive plates. Throughout this application, the terms “bipolar plate”, “bipolar unit” and “bipolar assembly” are interchangeably used to designate the aforementioned one or more electronically conductive plates disposed between adjacent cells in a fuel cell stack.
A bi-polar plate used in a fuel cell stack is ideally intended to serve several functions: 1) collecting and conducting the electrical current; 2) providing an effective flow field for the reactants and the products, namely, directing the reactants, which are delivered to the bi-polar plate from external sources through suitable passages provided within the fuel cell stack, and bringing the same into contact with the adjacent electrodes, while allowing an effective removal the reaction products therefrom at a preferred flow regime with minimal pressure drops; 3) mechanically supporting the entire fuel cell stack arrangement; 4) allowing efficient heat transfer; 5) contributing to the resiliency of the stack, mainly during an operation involving temperature changes; and 6) providing a low volume, light weight structure.
The art proposed various suitable designs for a bi-polar plate assembly, in an attempt to meet the aforementioned requirements. The following discussion relates to various combinations comprising a plurality of structurally modified metal sheets, which are placed between adjacent cells in a fuel cell stack.
U.S. Pat. No. 4,855,193 discloses a fuel cell, wherein an electrically conductive screen is placed between a separator sheet and a wet-proofed carbon sheet contacting the electrode surface.
U.S. Pat. No. 6,007,933 describes the use of a perforated metal sheet in combination with a separator plate having serpentine flow channels defined thereon.
WO 2003/0203272 describes a system for uniformly distributing gaseous fuel over the anode surface of a fuel cell, according to which a first plate, which is perforated with holes, is disposed between the anode and a second plate provided with bumps thereon, wherein said second plate faces the cathode and is in electrical contact therewith due to said bumps. The plenum defined between said first and second plates is used to receive the fuel, which reaches the anode surface through the aforementioned holes.
U.S. Pat. No. 6,872,482 describes a fuel cell stack which comprises a leaf spring in the form of a metal sheet, capable of undergoing elastic deformation when placed under a load and upon removal of said load. The leaf spring is interposed between a pair of metal plates defining the separator according to the cell disclosed in said publication.
It is an object of the present invention to provide an improved flow distributor, which may be suitably used, inter alia, as a component of a mono and/or bi-polar unit in an electrochemical cell or fuel cell stack or in heat exchangers, particularly in a direct methanol fuel cell stack and heat exchangers which require improved and controlled flow fields, as well as improved resiliency and electrical conductance.
It is another object of the present invention to provide a flow distributor, and a bi-polar unit based thereon, which are easy and inexpensive to produce and effectively fulfill the combination of functional and structural requirements mentioned above.
It is yet another object of the present invention to provide a fuel cell, or a fuel cell stack, comprising the novel flow distributor and a bi-polar unit based thereon, respectively.
In a first aspect, the present invention provides a flow distributor plate, which is particularly suitable for use in electrochemical cells and heat exchangers, wherein said plate comprises an electronically conductive region perforated by a plurality of apertures, wherein one face of said perforated region is provided with electronically conductive elastically displaceable baffles distributed thereon and extending therefrom.
The face of the flow distributor plate that is provided with the elastically displaceable baffles thereon is designated “the rough face”, whereas the opposing face thereof is referred to as “the non-rough face”. It should be noted that the baffles may be either randomly or orderly distributed on the rough face of the flow distributor plate, according to a pre-determined pattern.
Most preferably, the aforementioned elastically displaceable baffles are in the form of metallic tabs, wherein each of said metallic tabs is associated with one of the apertures in the perforated region of the flow distributor plate, said tab and said aperture associated therewith having a common boundary. This preferred embodiment of the flow distributor plate of the present invention is conveniently formed by bending out of the plane of a metal plate a plurality of individual sectors, to obtain a plurality of apertures, wherein said sectors are caused to extend from one face of said plate to provide a plurality of baffles in the form of metallic tabs distributed on said face.
The preferred shape, structure and purpose of the aforementioned apertures and baffles, which constitute an important structural feature according to the present invention, are described in more detail hereinbelow.
It should be noted that when placed within a fuel cell, the flow distributor plate of the invention fulfills additional functions, namely, the collection and the conductance of the electrical current. Accordingly, the term “current collector flow distributor plate” is hereinafter used to indicate the specific embodiment of the invention intended for use in electrochemical cells. When intended for use in electrochemical cells and especially, in a fuel cell stack, the perforated region of the current collector flow distributor plate is advantageously surrounded by a peripheral region defined by a sealable surface having openings incorporated therein. Briefly, the current collector flow distributor plates of the present invention are positioned within a fuel cell, such that the non-rough face of each of said plates is parallel to, and in intimate contact with, a gas diffusion layer provided on the membrane electrode assembly of said cell, whereas the opposing, rough face of said current collector flow distributor plate is affixed to, or has an intimate contact with, a metallic separator sheet to form a space therebetween. In operation, the externally supplied reactants fed into the fuel cell are delivered to the spaces defined between the metallic separator sheet and the rough faces of the current collector flow distributor plates affixed thereto. The flow of the fluids (namely, the fuel or the oxidant, and/or the products, e.g., water or carbon dioxide, as well as non-reacted materials such as nitrogen and water, either in a gaseous or a liquid phase) within said spaces is directed by the baffles provided therein, which distribute said flow such that the fuel and the oxidant are effectively brought into contact with the adjacent anode and cathode, respectively, either through a direct contact or, most preferably, through a diffusion layer interposed therebetween, and products are removed therefrom. It may be appreciated that the baffles extended from the surface of the current collector flow distributor plate are at least partially pressed against the metallic separator sheet affixed thereto, such that, in view of their electron conductivity and resiliency properties, the baffles contribute to the current collection, fluid flow management and the compressibility of the fuel cell stack.
The current collector flow distributor plate 1 is provided in the form of a square or rectangular metal sheet, having a thickness in the range between 0.05 and 5 mm. The central region 2 of the rectangular metal sheet is perforated by a plurality of apertures 3, and is concentrically positioned within said sheet. Typically, the area of the central region 2 constitutes about 50 to 90 percent, and more preferably about 75 to 85 percent, of the total area of the current collector flow distributor plate 1.
The current collector flow distributor plate 1 provided by the present invention may be suitably used in electrochemical cells and more preferably in a fuel cell, and the central, perforated region 2 of said current collector flow distributor plate 1 is accordingly designed such that it corresponds in geometrical form and size to the active area of the membrane electrode assembly placed within said fuel cell. Typically, the area of the central, perforated region 2 of the current collector flow distributor plate 1 may range from several squared centimeters to about few thousands of squared centimeters, depending on the current that is to be generated by the cell. In practice, the central region of particularly large current collector flow distributor plate may be sub-divided, to form a plurality of separated zones each of which being perforated by apertures and having baffles distributed on one of its faces, said separated zones preferably having dimensions in the range of 5×5 and 50×50 cm.
According to the embodiment shown in
The central region of the current collector flow distributor plate according to the present invention is perforated by a plurality of apertures, such that the combined area of the apertures constitutes about 15% to 70%, and more preferably about 25% to 50%, of the area of said central region. It should be understood that the term “aperture”, as used herein, encompasses a slot or a hole defined by any closed, arbitrary curve or polygon. The geometrical form of the apertures is preferably selected from the group consisting of circles, triangles, squares, rectangles, parallelograms, trapezoids or other n-polygons, wherein n is an integer between 5 to 12, ellipses and stars, sections of the aforementioned shapes and combinations thereof.
In one preferred embodiment, one or more apertures have a geometrical form which is centrically symmetric. According to a particularly preferred embodiment, the geometrical shape of one or more apertures is defined by a segment, hereinafter sometimes referred to as the base, the end points of which are connected by a polygonal line or a curved line, e.g. an arc. Preferably, the aforementioned shape is the figure obtained by bisecting a centrically symmetric figure, such as a circle, an ellipse and a polygon. The advantages associated with this particular type of apertures will become apparent as the description proceeds. Briefly, individual sectors within the central region of the current collector flow distributor plate that are bound by a segment, the end points of which are connected by a polygonal line or a curved line, may be bent along said segment out of the plane of said plate, resulting in the formation of apertures, while concurrently providing elastically displaceable baffles in the form of metallic tabs on one surface of the current collector flow distributor plate. For example, the aforementioned apertures may be in the form of isosceles trapezoids, positioned within the central region of the current collector flow distributor plate in pairs, such that the bases of the trapezoids belonging to the same pair are parallel to, and spaced apart from, each other.
The dimension of the aperture is preferably within the range of 0.2 mm to 12 mm average diameter while slots-like apertures may have a length longer than 12 mm. The size of the apertures is most preferably optimized in order to improve the electronic conductivity in plane and through the plane as well as fluid transfer between the central region of the current collector flow distributor metal plate and the membrane electrode assembly, which is intended to be in contact therewith in the fuel cell, without reducing the mechanical strength of the plate, as will be explained in more detail below.
The apertures may be randomly positioned within the central region of the current collector flow distributor plate, or may be orderly arranged therein in a predetermined form, such as an array defined by rows of said apertures. It is preferred, however, that the apertures are distributed in a well-considered layout within the central region of the current collector flow distributor plate, in order to control the distribution of the flow of the fluids and the collection of the current, as will be discussed in more detail below.
The peripheral region 4 of the rectangular plate, which surrounds the perforated, central region, is provided by a surface 5 that is used for sealing and supporting the current collector flow distributor plate within the fuel cell stack, as will be discussed in more detail hereinbelow. In the preferred embodiment shown in the figure, the peripheral region 4 comprises two pairs of openings, designated (6a, 7a) and (6b, 7b), respectively, wherein openings of the same pair are located in opposing sides of said peripheral region, such that they are separated by the central region 2. According to the embodiment shown in the figure, all the four openings disposed within the peripheral region of the current collector flow distributor plate have rectangular shape, with openings belonging to the same pair having the same dimensions. However, these structural characteristic are not mandatory, and the openings (6a, 7a) and (6b, 7b) may have different geometrical shapes and sizes. As will become apparent as the description proceeds, passages longitudinally extending within the fuel cell stack for delivering the externally supplied reactants onto the electrode active area, and for removing the products therefrom, are formed upon affixing together suitable members of said fuel cell stack, which members comprise in their margins corresponding openings, by the apposition of such openings. Preferably, the combined area of the openings (6a, 7a) and (6b, 7b) constitutes about 2% to 20% of the total area of the current collector flow distributor plate 1. However, it should be noted that according to an alternative embodiment of the invention, the peripheral region 4 may comprise only one pair of openings, such that the resulting pair of longitudinal passageways will be used for feeding and removing the anodic materials. In such a case, the other externally supplied reactants (the air) may be directly delivered into each of the plurality of cathodic active areas from the atmosphere.
As set forth above, a particularly preferred current collector flow distributor of the present invention is formed by bending out of the plane of the central region thereof a plurality of individual sectors, to obtain a plurality of apertures corresponding in geometrical form and size to said sectors, which sectors are caused to extend from one face of said plate to provide a plurality of baffles in the form of metallic tabs distributed on said face. One possible preparation procedure will now be illustrated in respect to
a and 2b show two preferred patterns that may be suitably used in the preparation of the perforated central region of the current collector flow distributor plate according to the present invention. The preparation involves the processing of the central region of a metal plate, for example, a thin stainless steel sheet (SS302-FH) having a thickness of about 0.05 to 0.50 mm, by techniques well known in the art, including, for example, laser cutting or wet-etching, to produce full-depth incisions in the form of a curved line 3a or a polygonal line 3b, having a width in the range of 0.05 and 2 mm, such that said curved or polygonal line define, together with the imaginary segment connecting the end points of said line, a sector that may be easily bent out of the plane of the metal plate along said segment. The sector may have any desired geometrical shape, which depends, of course, on the particular curved or polygonal lines 3a and 3b. According to one embodiment, the sector has a geometrical shape possessing an axial symmetry, obtained by bisecting a centrically symmetric figure. Thus, as shown in
Having produced the desired pattern of incisions on the central region of the metal plate, the plate is placed on a suitable working surface, shaped to incorporate the bent baffles in such a way that the bending lays on a solid material line and the plurality of sectors 9 are all caused to bent out of the plane of the metal plate in one direction, that is, either upward or downward with respect to said plane, following which the central region of said metal plate becomes perforated with a plurality of apertures, while a plurality of elastically displaceable metallic leaves, or tabs, each of which corresponding in geometrical shape and size to said sector 9, extend from one face of said metal plate. Thus, according to a particularly preferred embodiment of the invention, the baffles distributed on one face of the current collector flow distributor metallic plate and extending therefrom are provided in the form of elastically displaceable metallic tabs, which have been bent out of said plate.
a and 3b provide top view of the rough face of the current collector flow distributor plate obtained following the procedure described hereinabove in relation to
a and 4b provide a perspective view and a side section view, respectively, showing the elastically displaceable metallic tabs 12 extending from the plane of the metal plate 1. In the specific embodiment shown in these figures, the metallic tabs 12 are formed by bending out of the surface of the metal plate 1 individual sectors thereof having a geometrical shape of isosceles trapezoid, in accordance with the procedure illustrated in
The altitude (a), the long base (b) and the short base (c) of the trapezoid are each preferably within the range of 0.2 to 12 mm. The distance (d) between the bases of the trapezoidal tabs belonging to the same pair is typically about 1 to 25 mm. The dihedral angle θ defined between the plane of the metal plate 1 and the tab 12 is typically in the range between 5 to 175 degrees, and more preferably in the range 20 to 60 or 120 to 170 degrees, as mentioned hereinabove. Preferably, the height (h) of the tab 12, which is defined by the distance between the uppermost point of the free end of said tab 12 (the end which is not connected to the metal plate 1) and the surface of the metal plate 1 is in the range of 0.15 to 5 mm.
The plurality of elastically displaceable metallic tabs 12 that extend from one face of the current collector flow distributor plate 1, which metallic tabs have been bent out of said plate, as described herein above in relation to
The elastically displaceable tabs 12 serve to compensate for any manufacturing tolerance, as well as assembling and operating conditions causing changes in the original dimensions of the fuel cell in which the plate is positioned. Furthermore, the tabs 12 are back-holding the required pressure imposed by the current collector flow distributor plate onto the gas diffusion layer and through that onto the catalyzed active area and the membrane in the case of a chemical reactor or a fuel cell, and between other plates or fluid conductors in the case of heat exchangers or coalescers. The spring constant [k] of the elastically displaceable tab 12, defined as the deformation imposed by a specific applied force and expressed by [k]=deflection [microns]/force [kg] is affected by the material of which the tab is made and its optional post treatment (i.e. hardening), as well as the shape and dimensions of the tab, and the means by which it was formed. The elastically displaceable tab 12 is designed to allow a required deflection under a specified force. Thus, the elastically displaceable tab 12 has a free end which is not connected to the current collector flow distributor plate, for allowing the desired deflection. The deflection of the tab 12 is measured by the reduction of the height (h) of the tab, where higher flexibility is associated with larger deflection under the application of the same force. Preferably, in the case of presently used fuel cells, a pressure in the range of 2 to 50 kg/cm2 and more specifically 5-25 kg/cm2 will result in a deflection in the range of 0.005 to 1 mm and more specifically 0.02 to 0.2 mm; in the case of heat exchangers and coalescers the deflection will be closer to the low limits of the aforementioned ranges.
The distribution of the metallic tabs 12 within the central region of the flow distributor current collector plate, and their inclination relative to said plate, depend, inter alia, on the relative position of the tabs in respect to the expected flow patterns of the reactants and products. Thus, for examples, the density of tabs in the vicinity of the anode inlet, where a single phase flow exists, may be greater than the density of the tabs in the vicinity of the anode outlet, where a bi-phase fluid flow exists and the gas bubbles require a less dense flow plenum. Thus, according to one embodiment, the distribution, and optionally also the size and shape of the baffles on the rough face of the current collector flow distributor are position dependent.
If desired, some holes may be applied onto the metallic tab, in order to affect the tabs strength and their resiliency, while also contributing another degree of freedom in the design and control of the fluid flow towards the electrodes.
It should be understood that the preparation method described hereinabove, involving the formation of a plurality of full-depth incisions within the central region of the metal plate and the subsequent bending of the plurality of sectors, each of which being defined by an incision and the segment connecting the end points of said incision, is provided for the purpose of illustration only. The perforation of the central region of the metal plate 1, for providing the plurality of metallic tabs 12 on one face of said metal plate 1 may be effectively accomplished using one or more of the following metal processing techniques well known to those skilled in the art: cutting, drilling, punch-cutting, punching, etching, laser cutting, forming, roll forming, embossing, shaping, magnetic shaping, rubber body shaping, fluid pressure shaping, embedding, die pressing and forging.
Additional various pattern examples that may be suitably used for perforating the central region of the current collector flow distributor plate according to the present invention are given for the purpose of illustration in
As explained hereinabove, the current collector flow distributor plate of the present invention may be suitably positioned within a fuel cell, such that the non-rough face of said plate is parallel to, and is in intimate contact with, a gas diffusion layer provided on the membrane electrode assembly of said cell, whereas the opposing, rough surface of said current collector flow distributor plate faces an electronically conductive separator sheet to form a space therebetween, wherein said space contains the baffles deposited on said rough surface of said current collector, which space is intended for receiving the externally supplied reactants and distributing the same, by means of said baffles, to allow an effective contact with the surface area of the adjacent electrode. As will become apparent as the description proceeds, in order to increase the distance between the current collector flow distributor plate and the metallic separator sheet affixed thereto, suitable spacer elements are disposed therebetween, to provide an effective flow space for the reactants. The spacers may be provided in the form of separate, suitably designed sheets, or, alternatively, may form integral part of either the metallic separator or the current collector flow distributor plate. It may be also appreciated that when assembled to form a bi-polar unit, the flow distributor current collector plates and the separator interposed therebetween are suitably arranged to allow the introduction of the fuel and the oxidant to the anodic and cathodic spaces, respectively, and the removal of reaction products therefrom, through the openings that are peripherally incorporated in said plates.
Thus, according to one preferred embodiment, the present invention provides a bi-polar assembly which comprises a first current collector flow distributor plate and a second current collector flow distributor plate, wherein each of said plates has an electronically conductive region perforated by a plurality of apertures, wherein one face of said region is provided with elastically displaceable, electronically conductive baffles distributed thereon and extending therefrom, and wherein said perforated region is surrounded by a peripheral region defined by a sealable surface having at least one pair of openings incorporated therein, wherein openings of the same pair are located in opposing sides of said peripheral region, such that they are separated by said perforated region, wherein said plates are placed in parallel to and spaced apart from each other, with their faces having the baffles distributed thereon facing one another; and
An electronically conductive separator interposed between said pair of current collector flow distributor plates, the geometric form and size of said separator being identical to the form and size of said current collector flow distributor plates, said separator having in its margins at least one pair of openings which are substantially aligned with the openings located in the peripheral region of said current collector flow distributor plates, with respect to position, geometric form and size;
wherein said current collector flow distributor plates and the separator interposed therebetween are being affixed together and are preferably circumferentially sealed to define two internal separated spaces, wherein the first space is bound by the rough face of the first current collector flow distributor plate and the separator, and the second space is bound by the rough face of said second current collector flow distributor plate and the separator, and wherein the openings located in the peripheral regions of said pair of current collector flow distributor plates and the openings in the margins of said separator are contiguously arranged to form at least one pair of continuous passageways that extend perpendicularly throughout said bi-polar assembly, such that passageways of the same pair are located in opposing sides of said bi-polar assembly, wherein each pair of perpendicularly extending passageways is capable of being in fluid communication with either said first space or with said second space.
The following figures illustrate various modes of assembling together the current collector flow distributor plates, the separator sheet and various spacer elements. It should be noted that the two current collector flow distributor plates may not be necessarily identical, and they may differ from one another by, for example, the size, geometrical shape and distribution of the baffles distributed thereon.
In the embodiment shown in
In addition to the pair of current collector flow distributor metallic plates 1A, 1B and the separator sheet 21, the bi-polar plate assembly further comprises a pair of metallic spacers 24A and 24B, each of which having a geometric form and size that are essentially identical to the form and size of the current collector flow distributor metallic plates 1A, 1B and the separator 21. Thus, according to the embodiment illustrated in
Each of the spacers 24A, 24B is in the form of a planar frame 25A, 25B, such that continuous open areas 26A, 26B are bound by said frames, respectively.
In the spacer 24A, the open area 26A is identical, in geometrical form, position and size, to the area obtained by combining together the central region 2 of the current collector flow distributor metal plate 1A, the first pair of openings 6a, 7a located on the peripheral region of said metal plate, and the sections separating said central region and said openings. The frame 25A of the spacer 24A is perforated by a pair of holes 27b, 28b which coincide with the second pair of openings 6b, 7b located in the peripheral region of said first metallic plate 1A, with respect to position, geometric form and size.
In the second spacer 24B, the open area 26B is identical, in geometrical form, position and size, to the area obtained by combining together the central region 2 of the current collector flow distributor metal plate 1B, the second pair of openings 6b, 7b located on the peripheral region of said metal plate, and the sections disposed between the central region and said openings. The frame 25B of the spacer 24B is perforated by a pair of holes 27a, 28a which coincide with the first pair of openings 6a, 7a located in the peripheral region of said first metallic plate 1B, with respect to position, geometric form and size.
b shows how the aforementioned metal plates and sheets 1A, 24A, 21, 24B, 1B are assembled together to afford a bi-polar unit. The current collector flow distributor metal plate 1A, the spacer 24A, the separator 21, the spacer 24B and the current collector flow distributor metal plate 1B are successively arranged in parallel, such that said members, all having rectangular shape of the same size, are caused to overlap each other, with the surfaces of the current collector flow distributor metal plates 1A,1B having the baffles distributed thereon facing said spacers 24A, 24B, respectively, wherein the separator 21 is interposed between said spacers. As a result, two internal separated spaces are formed within the bi-polar unit, wherein the first space is bound by the rough face of the first current collector flow distributor metal plate 1A and the separator 21, and the second space is bound by the rough face of the second current collector flow distributor metal plate 1B and said separator 21. It may be appreciated that two pairs of passages perpendicularly extending within the bi-polar unit are also formed upon affixing together the aforementioned members in the manner described hereinabove, by the apposition of the openings (6a, 6b, 7a, 7b) located in the peripheral regions of said metallic plates 1A, 1B, the openings 22a, 22b, 23a, 23b located in the margins of said separator 21 and the holes 27a, 27b and 28a, 28b located in the spacers 24A and 24B. Passages of the same pair are located in opposing sides of the bi-polar assembly, wherein one pair of passages is connected to the first space and the other pair of passages is connected to the second space.
The bi-polar assembly is circumferentially sealed where needed to prevent fluid leakage during the operation of the fuel cell stack. Various sealing techniques may be used, including, for example, fit-pressing, folding and overlapping, riveting, welding (Electrode, TIG/MIG, laser, friction, compression, vacuum, magnetic, etc.), soldering, brazing, fusion, use of flat or shaped cross-section gaskets (i.e.: o-rings), adhesives, glues, dry or non-dry sealants, whether pre-made or applied in-situ or ex-situ. The sealing procedure is aimed to prevent internal and external leakage, while preferably improving the electrical contact between the bipolar sides (anode and cathode current collectors). For example, in the embodiment shown in
It may be appreciated that the production of the bi-polar unit illustrated in
The rectangular metal sheet 41 is further processed in the boundary lines 47 separating between the aforementioned sections, for example, by forming columns of small holes in said boundary lines, such that said sections may be easily folded along said lines, to form the bi-polar plate unit described in
a provides a top view of a metallic separator comprising the spacer elements as an integral part thereof. The separator 71 is in the form of a rectangular metal plate having a thickness in the range of 0.2 to 5.0 mm. The peripheral range of the plate is perforated with two pairs of openings (72a, 73a and 72b, 73b) such that openings belonging to the same pair are positioned on opposing sides of the plate. The plate is provided, on each of its two opposing faces, with recessed central regions 74a and 74b, respectively, surrounded by a region 75 of said plate which is elevated in respect to said recessed regions, wherein the first recessed central region 74a, defined on the first face of the separator plate, is surrounded by an elevated region 75a and is continuous with the first pair of openings (72a, 73a), while being separated from the second pair of openings (72b, 73b) by means of a portion of said elevated region 75a, whereas the second recessed central region 74b, defined on the second face of the separator plate (and therefore not shown in
As set forth above, in operation, the separator 71 will be interposed between two current collector flow distributor plates according to the invention, such that the peripherally-placed openings will serve for delivering the externally supplied reactants into the spaces confined between said separator and the current collectors flow distributor plates attached thereto, and removing the products therefrom. Accordingly, in a particularly preferred embodiment of the invention shown in
As explained hereinabove, the aforementioned structural modifications introduced into the recessed region of the separator plate (namely, the narrow dikes and the raised bosses or dimples) are intended improve the flowability of the reactants within the spaces defined between the separator and the two current collector flow distributors plates affixed to the opposing faces thereof and, in addition, to function as local supportive and/or conductive members, for example, for supporting a gasket sheet placed in the opposing side of the electrochemical cell.
Separator plate 71 may be conveniently prepared by processing a metal sheet having a thickness in the range indicated above using etching techniques, and specifically, photo etching procedures, in order to remove the unwanted portions therefrom according to a predetermined design complying with the structural requirement described hereinabove, as illustrated in more detail in the examples below.
a, 9b and 9c further illustrate several preferred modes of using the current collector flow distributor plate discussed hereinabove to form a bi-polar plate. The alternative embodiments shown in
In accordance with one embodiment of the present invention, a metallic separator sheet 21 is interposed between a first current collector flow distributor plate 1A and a second current collector flow distributor plate 1B, each of said first and second plates having an electronically conductive central region perforated by a plurality of apertures, the geometric form and size of said central region being essentially identical to the form and size of said gas diffusion layer contacting the same. As set forth above, one face of each of said first and second current collector flow distributor plates is provided, on its central region, with electronically conductive elastically displaceable baffles distributed thereon and extending therefrom, wherein said faces of said first and second current collector flow distributor plates having the baffles thereon are affixed to the two opposing surfaces of said metallic separator sheet 21 to form a first space bound between said first current collector flow distributor plate 1A and said separator, and a second space bound between said second current collector flow distributor plate 1B and said separator, said first and second spaces being connected to passages provided within said fuel cell stack for delivering the fuel and the oxidant therein, respectively. In
According to one embodiment of the invention, a fine wire mesh is placed between the current collector flow distributor plate 1A (and/or 1B), and the adjacent gas diffusion layer. To this end, a stainless steel wire mesh having a wire diameter of 0.009 inch and 18 meshes per linear inch and an open area of about 70% is particularly useful. It should be noted that the open area of the aforementioned wire mesh is sufficiently large, and the diameter of the wire is sufficiently small, such that said mesh may be substantially embedded in the carbon sheet contacting the same, thus improving the mechanical strength and the electrical conductivity of the gas diffusion layer.
In the specific design shown in
When used in heat exchangers or in coalescers, the flow distributor plate provided by the present invention improves the fluid turbulence without increasing the pressure-drop associated with a high speed flow, which high speed flow may be otherwise required in order to achieve the desired flow pattern in the heat exchanger and/or coalescer. In addition, the resilient flow distributor plate of the present invention applies a desired, controllable force upon adjacent plates, thus contributing to the mechanical properties of the heat exchanger and/or coalescer in which it is placed.
In the Drawings:
a and 2b illustrate possible patterns for use in the design and preparation of the perforated region of the current collector flow distributor plate of the present invention.
a and 3b provide a side-elevated view and a top view, respectively, of two different embodiments of the rough face of a current collector flow distributor plate of the present invention.
a and 4b provide a perspective view and a side section view, respectively, of an individual baffle extending from the current collector flow distributor plate according to the invention.
a to 5c show possible patterns for the perforated region of the current collector flow distributor plate of the present invention.
a to 6c illustrate the construction of a preferred five layered bi-polar plate according to the invention.
a, 7b and 7c provide a top view and a sectional view, respectively, of a separator sheet suitable for use in combination with the current collector flow distributor plates of the present invention.
a and 8b illustrate an exploded view and a sectional view, respectively, of a three layered bipolar plate according to one preferred embodiment of the invention.
a, 9b and 9c provide a top view and a sectional view, respectively, of a three layer bi-polar assembly according to one preferred embodiment of the invention.
Two stainless steel sheets (SS302-FH, commercially available) having thickness of 0.15 mm were wet etched to form two pairs of peripheral rectangular inlet and outlet flow openings, as shown in
A stainless steel sheet (SS316) having a thickness of 0.15 mm was used. Peripheral fluid inlet and outlet openings, were cut by laser based on a design as presented in
A metal of thickness of 0.50 mm (SS 316) was cut to shape as per the design in
To prevent leaks, gasket frames were introduced between each metal sheet contact. The gasket frames are made of Mylar® and have a thickness of 0.05 mm. The frames are cut by means of a specially designed punch-cutter, which is laid on top of the raw material atop of plastic sheet in a press, allowed to be pressed between two bed-plates of the press machine, until cut to the pre-designed shape. The general dimensions of the gasket sheets and the sizes of the fluid openings correspond to those described hereinabove in respect to the spacer sheets.
All layers from above were assembled layer by layer to create a carefully aligned bi-polar plate according to the following order:
Each of the two faces of a 1.5 mm thick 316SS plate (see Example 1(b) for general characteristics thereof) was wet etched to form a pool-like recessed central region and two pairs of peripheral fluid inlet and outlet openings. On each face, “mini-dikes” in the boundary region between one pair of openings and the central region, as shown in
b) Preparation of the current collector flow distributor sheet was done following the same procedure as had been described hereinabove in part (a) of Example 1, while the gaskets were made as per the description in part (d) of same example.
All layers of Example 2 were assembled layer by layer to create one bi-polar plate, according to the following order:
a) bi-Polar Plate Assembly
A set of three metal layers as described in Example 2 hereinabove, consisting of a pool like separator and two current collectors flow distributor sheets, were welded together in order to create a bipolar plate. Welding eliminates any need for gaskets and/or sealing, and further reduces internal electrical resistance within the cell-plate.
Welding is carried out in the midst of those places where the gasket were intended to be placed. The welding procedure was carried out by a laser YAG machine, after adjusting for sufficient penetration to assure a good seal while not cutting through the plates. Weld was carried out through the current collector flow distributor while this was pressed to place between two plates.
The assembly order of the bi-polar plate was:
A three metal layered bipolar plate, wherein the two current collectors flow distributor plates and the separator are all made of a single metal sheet, is described hereinbelow. Hence, the current collector flow distributor plates and the separator are made in this case of the same stainless steel type, and will have identical thickness.
The non-smooth faces of the current collector flow distributor, that is, the faces having the baffles thereon, were structured on opposing sides, to enable the right folding (see
The assembly order of the bi-polar plate was:
The dimensions and other structural details are similar to those described hereinabove in example 1.
Three layered MEA (which is also known as Catalyst Coated Membrane or ‘CCM’, commercially available from DuPont™ (material code D-GABAA based on Nafion® 117)) was cut to the desired dimensions and configuration by means of a punch-cutter-tool.
Gas diffusion layers made of carbon paper (type 31bc and 31da, 90×63 mm by SGL, for anode and cathode sides respectively) were also cut according to the desired shape and size by punch-cutter tool.
A five layered MEA is then prepared by heat pressing two pieces of the precut gas diffusion media (GDM) precut pieces aligned to fit in midst of two seal-frames (described as ‘e’ herein above) one set on each side of the precut three layered MEA (140° C., under 800 psi for 5 minutes) between two beds of a press machine.
Pre cutting of the three layers MEA, the gas diffusion media and the frames was carried out by cutting those with a punch, made to drawing, where the outer design meets with the plate design, and the catalyzed area is centered to overlap the flow-distributor. For the membrane the cutting tool is applied to allow the membrane to extend from the active area, forming together with the frame-seals a membrane-barrier between the anode and cathode sides of each cell in the assembly. In some cases the GDM was allowed to extend from the active area through the connector towards the external fluid flow feed perforation (headers), in order to assist the support of the stacked parts, and hold it in shape with the connector held open to the required fluid while being kept sealed toward the other fluid.
Multiple cells stack was assembled using the above MEAs, and the different bi-polar plates from the Examples above. An indexing tool was made in order to facilitate the operation, through which the different parts are being stacked one on top of the other in the following order: a first end-plate, (and a gasket provided thereon), MEA, alternately arranging bi-polar plates and an identical number of MEA's, another gasket and finally, a second end-plate. The entire stack is now compressed by tightening stud bolts (4 polyethylene sleeve-coated steel, M6, isolated from the end plate assemblies by means of plastic sleeves and contact isolation discs), until a defined spacing is achieved (i.e. 0.450-0.750 mm per MEA, depending on GDM and gasket frame thickness).
The fuel cell stack was operated as follows. An aqueous methanol solution (1M) at 80° C. is circulated through the stack anodes using an adjustable flow peristaltic pump, and ambient air was streamed from the top side into the stack via the end plate assemblies, which directs the air into the cathodes. The fluid flow is set to follow the operating current and calculated for about 2-8 times stoichiometric flow on both the anode and cathode sides, about 4-12 cc/min. methanol solution and 0.5-1.5 lpm per cell when operated at 14 Amperes. The stack was maintained at 80° C., using temperature controlled heat pads at both end plate assemblies, in order to compensate for heat losses to the atmosphere. As a variable load, heavy duty variable resistors as well as an electronic load were used, and the current and voltage, as well as other operation conditions were recorded and controlled through a computerized control board.
| Number | Date | Country | Kind |
|---|---|---|---|
| 173539 | Feb 2006 | IL | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IL2007/000144 | 2/4/2007 | WO | 00 | 12/4/2008 |
