Patent Application
20050221157
The present invention relates to a fuel cell layer made of one or more unit fuel cells, wherein at least one unit fuel cell has at least one reactor frame. Each reactor frame has a process layer that facilitates a transport process between the reactant plenums in the reactor.
Fuel cells are comprised of chemical reactors. The size of the chemical reactors put constraints on the ability to reduce the size of a fuel cell to micro-dimensions.
Existing fuel cells generally are a stacked assembly of individual fuel cells, with each stack producing high current at low voltage. The typical reactor construction involves reactant distribution and current collection devices brought into contact with a layered electrochemical assembly consisting of a gas diffusion layer, and a first catalyst layer. With the exception of high temperature fuel cells, such as molten carbonate cells, most proton exchange membrane, direct methanol, solid oxide or alkaline fuel cells have a layered planar structure where the layers are first formed as distinct components and then assembled into a functional fuel cell stack by placing the layers in contact with each other.
One major problem with the layered planar structure fuel cell has been that the layers must be held in intimate electrical contact with each other, which if intimate contact does not occur the internal resistance of the stack increases, which decreases the overall efficiency of the fuel cell.
A second problem with the layered planar structured fuel cell has been that with larger surface areas, problems occur to maintain consistent contact with both cooling and water removal in the inner recesses of the layered planar structured fuel cell. Also, if the overall area of the cell becomes too large then there are difficulties creating the contacting forces needed to maintain the correct fluid flow distribution of reactant gases over the electrolyte surface.
Existing devices also have the feature that with the layered planar structure fuel cell since both fuel and oxidant are required to flow within the plane of the layered planar structured fuel cell, at least 4 and up to 6 distinct layers have been required to form a workable cell, typically with a first flowfield, a first gas diffusion layer, a first catalyst layer, a first electrolyte layer, a second catalyst layer, a second gas diffusion layer, a second flowfield layer and a separator. These layers are usually manufactured into two separate fuel cell components and then a fuel cell stack is formed by bringing layers into contact with each other. When contacting the layers, care must be taken to allow gas diffusion within the layers while preventing gas leaking from the assembled fuel cell stack. Furthermore, all electrical current produced by the fuel cell in the stack must pass through each layer in the stack, relying on the simple contacting of distinct layers to provide an electrically conductive path. As a result, both sealing and conductivity require the assembled stack to be clamped together with significant force in order to activate perimeter seals and reduce internal contact resistance.
Electrical energy created in the fuel cell has to travel between layers of material compressed together before it can be used. These layers include membrane electrode assemblies, gas diffusion layers, and separator plates. The resistance to the transfer of electrical energy through each layer and between layers also affects the performance of the fuel cell. The contact pressure and contact area that can be achieved between the layers of the fuel cell stack are directly proportional to the conductivity of these components and hence the performance of the fuel cell stacks.
Laying out layers of material and compressing them together using the brute force approach of traditional fuel cell stacks is inefficient and expensive. In addition, such designs suffer from long term performance degradation because of thermal and mechanical cycles that occur during the operation of the fuel cells. A need has existed for less expensive and more efficient fuel cell layers.
In manufacturing fuel cell stack assemblies using this typical layering approach of all the components, it is difficult to accurately align the layers. Inaccurate alignment has a detrimental effect on the performance and durability of the fuel cell stacks.
A need has existed for a micro, or small fuel cells having high volumetric power density. A need has existed for micro fuel cells capable of low cost manufacturing because of having fewer parts than the layered planar structure fuel cell. A need has existed for a micro fuel cell having the ability to utilize a wide variety of electrolytes. A need has existed for a micro fuel cell, which has substantially reduced contact resistance within the fuel cell. A need has existed for a micro fuel cell, which has the ability to scale to high power has long been desired. A need has existed for micro fuel cells having larger reactant surface areas. A need has existed for fuel cells capable of being scaled to micro-dimensions. A need has existed for fuel cells capable of being connected together without the need for external components for connecting the fuel cells together.
A need has existed for a compact fuel cell with high aspect ratio cavities. The aspect ratio of the fuel cell is defined as the ratio of the fuel cell cavity height to the width. Increasing this aspect ratio is beneficial for increasing the efficiency of the fuel cell.
A need has existed to develop fuel cells topologies or fuel cell architectures that allow increased active areas to be included in the same volume, i.e. higher density of active areas. This will allow fuel cells to be optimized in a manner different than being pursued by most fuel cell developers today.
The present invention meets these needs.
The present invention relates to a fuel cell layer with a central axis that includes one or more unit fuel cells, a front fuel plenum, and a back oxidant plenum. The first and second unit fuel cells are disposed adjacent to each other to form a front side and a back side of the fuel cell layer. The front fuel plenum communicates with the front side, and the back oxidant plenum communicates with the back side.
Each unit fuel cell is made of a front and back process layer, a front and back cavity, and a front and back perimeter barrier. The front cavity is formed between the front and back process layers and a back cavity formed between the back process layers.
A front perimeter barrier is disposed on the back process layer substantially surrounding the back cavity and a back perimeter barrier disposed on the front process layer substantially surrounding the front cavity. The front cavity is in communication with the front side and the back cavity is in communication with the back side.
At least one of the process layers facilitates a transport process between the fuel and oxidant plenums and wherein at least one of the unit fuel cells comprises at least one frame formed from one of the process layers; at least one of the perimeter barriers, at least one of the cavities. Each cavity is in communication with one side of the fuel cell layer and the process layer facilitates a transport process between the plenums.
Each unit fuel cell is made of one or more reactor frames. The reactor frames include one or more of the process layers, one or more of the perimeter barriers disposed on the process layer, and one or more cavities are formed in each reactor frame. The resulting assemblage is configured so the cavities are in communication with one side of the fuel cell layer.
The fuel cell layer can be attached to an electrical appliance. The fuel cell layer is a source of power for running the electrical appliances.
In the detailed description of the preferred embodiments presented below, reference is made to the accompanying drawings.
The present embodiments are detailed below with reference to the listed Figures.
Before explaining the present embodiments in detail, it is to be understood that the embodiments are not limited to the particular descriptions and that it can be practiced or carried out in various ways.
The use of frames in the fuel cell layer simplifies the overall design, reduces the number of components needed in construction, and simplifies the construction steps needed to make the device. The frame construction also increases the precision of alignment between process layers and perimeter barriers used in the unit fuel cells.
When composite frames are used, the frame based design simplifies the task of sealing layers by only having to bond similar materials to each other.
With reference to the Figures,
The fuel cell layer (10) can have a variable thickness; variable thickness can mean that one individual unit fuel cell can be thicker than an adjacent unit fuel cell.
The fuel cell layer (10) is constructed of two or more unit fuel cells. The design of the fuel cell layer (10) can range from 2 unit fuel cells to 50,000 or more unit fuel cells to be adjoined to create the fuel cell layer. Preferably, between 2 unit fuel cells and 300 unit fuel cells are used in the fuel cell layer with a preferred embodiment of between 2 unit fuel cells and 100 unit fuel cells.
The first and second unit fuel cells are disposed adjacent each other and form a front side (32) and a back side (34) for the fuel cell layer. The front side (32) communicates with the oxidant plenum (36) and the back side (34) communicates with the fuel plenum (38). In another version of the invention, the unit fuel cells can be oriented to form the fuel plenum (38).
The fuel cell layer (10) includes an oxidant plenum (36) and a fuel plenum (38). The oxidant plenum (36) is enclosed by a structure (40). The structure can either be a closed container or open to ambient atmosphere.
The fuel plenum (38) is enclosed by a device (42) which is similar to structure (40). The device (42) can be a closed container or open to ambient atmosphere. When the device (42) is open to the atmosphere, it adds structural support to the fuel plenum.
In the embodiment depicted in
The process layer material could be an electrolyte, an ion exchange membrane, an electrical conductor, and combinations of these. For example, a workable ion exchange membrane would be Nafion™ available from E.I. DuPont DeNamours of Wilmington, Del.
An electrical conductor which is contemplated for use in the invention would be a thin film of metal, such as copper, stainless steel, aluminum or tin, or a silver filled epoxy such as model number TF12202 from Tech Film of Peabody, Mass.
Alternatively, each process layer can be made of a filled metal composite, a filled micro-structure of polymer, filled epoxy composite, graphite composite, or combinations of these materials. Filled metal composites would be a stainless steel filled with carbon, such as those available from Angstrom Power Inc. of Vancouver, Canada. Filled micro-structures of polymers include Primea™ membrane available from Gore Industries of Elktown, Md. Filled epoxy composites include those available from Tech Film of Peabody, Mass. Graphite composites include Grafoil™ available from Graftek of Wilmington, Del.
It is also contemplated that the fuel cell layer can have a first process layer that performs a different process from the second process layer, for example, the first process layer can be an electrolyte and the second process layer can be an electrical conductor.
At least one process layer must be ionically conductive in order to facilitate the transport of ions. Optionally, at least one process layer may be made electronically conductive to transport electrons between fuel cells. The ion transporting process layer can be made from a proton exchange membrane, an electrolyte filled microporous structure, a liquid electrolyte trapped in a mesh, and combinations of these. The electron transporting process layer can be made from an electrical conductor, a filled metal composite, a filled micro-structure of a polymer, a filled epoxy composite, a graphite composite, or combinations thereof. In both cases, the process layer should be for a bas barrier to prevent the mixing of fuel and oxidant uncontrollably.
Returning to
The perimeter barriers ensure that reactant from one reactant plenum which connects to one of the cavities, does not migrate into another reactant plenum which connects to the other cavity. More specifically, when the fuel cell layer functions with fuel cells as the unit fuel cells, with oxidant in one of the reactant plenums and fuel in the other reactant plenum, the perimeter barriers prevent the uncontrolled mixing of fuel and oxidant.
The perimeter barriers keep the reactant from migrating by the material and/or the form of the perimeter barriers. Usable materials for the perimeter barriers include metal, such as stainless steel; silicone such as RTV™ those available from Dow Corning of Midland, Mich.; a rubber in the form of seals such as those available from the Apple Rubber Company of Lancaster, N.Y.; a polyamide, such as nylon, such as a nylon 6 or a nylon 6,6 available from DuPont; synthetic rubber such as BUNA available from Edegem, Belgium; epoxy, such as those available from EPO Tech of Billerica, Mass.; polytetrafluoroethylene, also available as Teflon™ from various sources; polyvinyldiflouride, known as Kynar™, available from Atofina Chemicals of Philadelphia, Pa.; composites thereof, laminates thereof, alloys thereof, and blends thereof. Usable forms for the perimeter barriers include micro-structures or three-dimensional structures that create a tortuous path for the reactant. In some cases, the perimeter barriers can employ both the use of materials and form to prevent the migration of the reactant to another reactant plenum.
One or more of the cavities can be filled partially or completely with a material to aid in the transport of reactant, by- product of the reaction caused by the reactants, or transport of attributes of reactant. A porous media, such as those available from Angstrom Power Inc. of Vancouver, Canada can be used to partially or completely fill the cavities. In a preferred embodiment, the cavity is filled 100% with the porous media although the cavity can be filled as little as 5% with the porous media.
Continuing with
Examples of fuels usable in this invention include hydrogen, liquid phase hydrocarbons, gas phase hydrocarbons, by-products of the reaction and combinations of these. Hydrogen is a typical fuel reactant when coupled with oxygen as the oxidant. Liquid phase hydrocarbons which can serve as reactants include methanol, ethanol, butanol, and formic acid. Gas phase hydrocarbons include propane, butane, methane, and combinations of these.
A typical fuel cell layer has an overall length between 1 millimeter and 100 centimeters; an overall width of the fuel cell layer is between 1 millimeter and 50 centimeters; and an overall thickness between the front face and the back face of the fuel cell layer is between 100 nanometers and 5 centimeters.
The fuel cell layer assembled according to the present invention provides high surface area process layers which are in communication with the reactant plenums through the front or back cavities. The front or back cavities have high aspect ratio's, wherein the distance from the front or back face to the opposite side of the cavity is much larger than the height of the front or back perimeter barrier.
The reactor preferably is made wherein the aspect ratio of at least one cavity is >1 cm/cm, more preferably is between 1 cm/cm and 100 cm/cm and most preferably is between 2.5 cm/cm and 15 cm/cm.
The selection of the aspect ratios of the cavities must be carefully chosen to accommodate the properties of the porous media which has been utilized in the cavities. For example, the transport of fuel and oxidant from the plenums to the gas diffusion electrode formed in the cavities is primarily by diffusion the aspect ratio must be maintained so that the concentration of reactants is sufficiently large to sustain the reactions throughout the electrode. At least one low aspect ratio cavity can be at least partially filled with a catalyst to promote the function of the fuel cell.
In operation reactants move from the reactant plenums into the front or back cavities of the unit fuel cells to come into contact with the process layers. In a preferred embodiment the reactants move in and out of the cavities through diffusion only.
In an alternative embodiment the reactant transport into and out of the cavities is aided by forced convection or by the forced flow through a micro-structure embedded within at least on of the front or back cavities.
Each unit fuel cell can be made of one or more frames (62 and 64). The frames can be made of one material, so that the frames can function as both a perimeter barrier and as a process layer.
The frames which are made from one material can be made by stamping, embossing, ablating, machining, molding, casting, water jetting, or otherwise gouging, or chemically etching a substrate. Typical substrates can be stainless steel, Nafion™ a composite, a metal filled composite, electrolyte filled composites, or combinations of these.
The frames can selectively be porous and used within the scope of this invention. Preferably, the frames are the same dimension as the components of the unit fuel cells which they replace. Two types of frames can be used on the fuel cell, an electronically conducting frame and an ionically conducting frame. The electronically conducting frame is made from an electronically conducting material or alternatively, is made conductive by filling a porous region with a nonporous conductive material. The ionically conductive frame is made from an ionic conductor, such as Nafion™ from DuPont and if Nafion™ is used, then the perimeter barrier if also formed from Nafion™. Alternatively, the frame material can be made from electrically insulating material such as polyethylene with a porous region that has been filled with Nafion™ to render the region ionically conductive. The frames can be made of identical materials or the frame can each have a different material.
The frames are typically one piece structures to advantageously reduce the number of parts. The one piece construction also makes it simpler to align a fuel cell to form the fuel cell layer making the process for making fuel cell layers cheaper and quicker than those currently available. By using a one piece construction of frames, there is no need for the extra step of bonding dissimilar materials together such as bonding a perimeter barrier material to a process layer material. Thus a fuel cell layer using frames will have better integrity and fewer maintenance issues than multipart constructions.
Tonically conductive process layer (48) and conductive process layer (156) are used in the unit fuel cells.
Undulating in the context of this application refers to non-planar process layers, such as layers which are sinusoidal in shape, or arcs, or irregular in some other manner. It is contemplated that some of the process layers can be undulating while remaining process layers can be planar and still form a usable fuel cell layer.
In a bipolar configuration, as shown in
The catalyst layer of either the bipolar or edge collected configuration can be composed of a noble metal catalyst, a transition metal catalyst, alloys thereof and combinations thereof. The catalyst layer can be a carbon supported catalyst or a thin film catalyst formed by spraying, sputtering, electroplating, printing, pulsed laser deposition, or combinations thereof. Alternatively, the catalyst layer can be cracked.
In this embodiment, the unit fuel cells each comprise one or more process layers of electrolyte (73). One or more of the cavities include a first catalyst (74) forming at least one anode (80). One or more other cavities include a second catalyst (78) forming at least one cathode (76). The anode (80) and the cathode (76) are disposed on either side of the electrolyte (73). The frame (62) serves as a separator between unit fuel cells as well as forming the two perimeter barriers (56 and 58).
One of the reactant plenums (38) contains an oxidant (82), such as oxygen, and the other reactant plenums (36) contain a fuel (84), such as hydrogen. The anode (80) and the cathode (76) connect to the electrical appliance and provide power.
The embodiments have been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the embodiments, especially to those skilled in the art.
