Patent Application
20070003814
The present invention generally relates to fuel cells, and more particularly to a polymer electrolyte membrane fuel cell stack having improved fuel concentration across an anode, and efficient cooling across the cathode.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. A typical fuel cell comprises a fuel electrode (anode) and an oxidant electrode (cathode) separated by an ion-conducting electrolyte. The electrodes are connected electrically to a load (such as an electronic circuit) by an external circuit conductor. In the circuit conductor, electric current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H+) in acid electrolytes, or the hydroxyl ion (OH−) in alkaline electrolytes. In theory, any substance capable of chemical oxidation that can be supplied continuously (as a gas or fluid) can be oxidized galvanically as the fuel at the anode of a fuel cell. Similarly, the oxidant can be any material that can be reduced at a sufficient rate. Gaseous hydrogen has become the fuel of choice for most applications, because of its high reactivity in the presence of suitable catalysts and because of its high power density. Similarly, at the fuel cell cathodes the most common oxidant is gaseous oxygen, which is readily and economically available from air for fuel cells used in terrestrial applications. When gaseous hydrogen and oxygen are used as fuel and oxidant, the electrodes are porous to permit the gas-electrolyte junction area to be as great as possible. The electrodes must be electronic conductors, and possess the appropriate reactivity to give significant reaction rates. At the anode, incoming hydrogen gas is oxidized to produce hydrogen ions (protons) and electrons. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via an external electrical circuit. At the cathode, oxygen gas is reduced and reacts with the hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The byproduct water is typically expelled as vapor at elevated temperatures. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction released directly as electrical energy. The difference between this available free energy and the heat of reaction is produced as heat at the temperature of the fuel cell. It can be seen that as long as hydrogen and oxygen are supplied to the fuel cell, the flow of electric current will be sustained by electronic flow in the external circuit and ionic flow in the electrolyte.
In practice, a number of these unit fuel cells are normally stacked or ‘ganged’ together to form a fuel cell assembly. A number of individual cells are electrically connected in series by abutting the anode current collector of one cell with the cathode current collector of its nearest neighbor in the stack. Fuel and oxidant are introduced through manifolds into respective cells. The fuel and oxidant flow across the anode and cathode, respectively. One known fuel cell disclosed in U.S. Patent Publication 2004/0038112 A1 shows the fuel and oxidant flowing in serpentine channels across the anode and cathode. However, fuel is consumed as it progresses along the anode, creating an uneven fuel concentration and distribution of power across the anode. Furthermore, the oxidant tends to cool the cells it first contacts much more that the remainder of cells in the stack, causing uneven cooling of the fuel cell assembly resulting in uneven power distribution across the stack. Ideally, the temperature of the cells at the ends of the stack are the same as the cells in the center of the stack
Accordingly, it is desirable to provide a polymer electrolyte membrane fuel cell stack having evenly distributed fuel concentration from fuel flowing across an anode and improved heat distribution within the fuel cell stack. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
A polymer electrolyte membrane fuel cell stack comprises a plurality of membrane electrode assemblies having evenly distributed fuel concentration across an anode, and efficient cooling from the oxidant that flows across a cathode. A first plate has a fuel side with a plurality of serpentine channels formed therein for distributing fuel across the anode, and a second plate has an oxidant side with oxidant channels formed therein for distributing an oxidant across the cathode. The membrane electrode assembly has an even fuel concentration thereacross and the oxidant is routed through the cell for absorbing heat prior to being distributed across the cathode.
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.
A polymer electrolyte membrane fuel cell is disclosed that operates at high temperature using membranes that don't require humidification to work. More specifically, the preferred embodiment of the present invention comprises a stack for operating elevated temperature (120-250° C.) polymer electrolyte membrane fuel cells that is compact and lightweight. Low and medium temperature polymer electrolyte membrane stacks (<120° C.) are complicated by water management and cooling issues. Water management is not an issue in polymer electrolyte membrane fuel cells operating at temperatures above 120° C., as long as the membrane is capable of proton conduction independent of humidity. Cooling issues are significantly different when using elevated temperature membrane electrode assemblies compared to “standard” Nafion-type membrane electrode assemblies. The problems that need to be resolved in an elevated temperature polymer electrolyte membrane fuel cell stack include: ensuring even fuel and thermal distribution across the stack, and sufficient thermal management such that operation at current densities <600 mA/cm2 requires no additional stack-related “balance of plant” (other components required to cool or maintain the temperature in a fuel cell stack, e.g., fans, pumps, cooling channels, coolant fluid/gas, and control circuitry). Also, the stack should have very low pressure drop so that demand on system balance of plant is minimized, and also compact and lightweight to meet certain power density specs. In the stack disclosed herein, stack temperature is controlled by the rate, or stoichiometry, at which the cathode oxidant is supplied. During startup, or when the stack is operated at low load, cathode oxidant is supplied at rates corresponding to a fuel stoichiometry in the range of 1-3. During operation at higher temperatures/load, the rate of the oxidant flow is increased, which withdraws heat more quickly from the stack and reduces temperature. Typically, cathode oxidant flow rates corresponding to reactant stoichiometry 3-10 are sufficient to control the stack temperature when operating at the parameters anticipated for a portable power system. It is important for the temperature of the cells to be even across the length of the stack in order to ensure even power distribution, particularly when operating the stack using reformate fuel containing carbon monoxide impuritites. In practice, it is preferred that the temperature gradient along the length of the stack be less than 30° C. In the stack disclosed herein, the oxidant is passed through the edges of the stack assembly prior to passage across the cathode side of the fuel cell. This method has the advantage of preventing the ends of the stack from becoming excessively cooled during operation, and thus maintain an even stack temperature.
The membrane electrode assembly 12 comprises an ion exchange membrane 22 of a solid polymer electrolyte interposed between an anode 24, and a cathode 26 on a side opposed to the anode 24. The ion exchange membrane 22 is any ionically conductive and not electron conductive material capable of operating up to 250° C. independent of external humidification, for example, Celtec® membrane electrode assembles from PEMEAS, Inc., preferably between 2 and 8 mils in thickness. The gasket 18 forms an opening 23 wherein the anode 24 is positioned against a side 28 of the end plate 14 and the gasket 20 forms an opening 25 wherein the cathode 26 is positioned against a side 30 of the end plate 16. The gasket preferably would have a thickness for which a sufficient compression and sealing is achieved.
Backing plate 13 is positioned adjacent to the end plate 14 and backing plate 15 is positioned adjacent the end plate 16 on opposed ends of the cell (or a stack of cells as described hereinafter). The backing plates 13, 15 have internal passages for the flow of fuel and an oxidant, respectively, as described hereinafter with reference to
The backing plates 13, 15, end plates 14, 16, gaskets 18, 20, and ion exchange membrane 12 are held together, for example, by tightening bolts (not shown) inserted through holes 54 formed in opposed corners and secured by nuts (not shown).
Referring to
Referring to
The ideal performance of a fuel cell is defined by its Nernst potential, E, or the ideal cell voltage. The overall reactions for a hydrogen fuel cell is as follows:
Anode: H2→2H++2e−
Cathode: ½O2+2H++2e−→H2O
Overall Cell Reaction: H2+½O2→H2O
Nernst Equation: E=E°+(RT/2F) In [PH2/PH2O]+(RT/2F) In [P62]
At the anode, the reaction releases hydrogen ions (protons) and electrons whose transport is crucial to energy production. The protons build up on the anode creating a positive potential which promotes their transfer through the electrolyte (membrane) either by remaining connected through an attraction to a water or phosphoric acid molecule which travels through the electrolyte, or by transferring between water or phosphoric acid molecules. The oxygen side of the water molecule contains a slight negative charge which attracts the protons and may become attached to it, but the attraction is weak so any forces made are easily broken. The actual method of transfer varies depending on the type of electrolyte, but is based on the thickness of the membrane, the amount of water or phosphoric acid in the membrane, and the number of protons transported. Thus, the anode contains a net positive charge while the cathode, towards which the ions drift, contains a negative potential.
The acid functional groups in the electrolyte serve to provide structure for the electrolyte as well as a barrier to electrons. It is conducive for electrons to flow through materials whose electrons are held loosely (conductive materials) because of the process of electron transport. Thus, electrons move from the reactions sites on the anode through the gas diffusion section of the electrode, through the anode current collector, through a load to do work, across the cathode current collector, through the gas diffusion section of the electrode on the cathode and then to the catalyzed reaction sites on the cathode. The electrons do not move through the electrolyte because the acid chains hold their electrons tightly and thus constitute an electric insulator. Other criteria for selecting an electrolyte are its structural stability, low resistance to ionic movement and low porosity.
As the fuel is distributed and consumed through the fuel flow channels 40 (
Referring to
A fuel, such as hydrogen, is fed into the fuel inlet passage 44 at the backing plate 13, traversing each of the cells 10 in the stack 52 through the fuel inlet passage 44 in the z direction. At end plate 14 and each bipolar plate 53, some of the fuel is diverted through fuel flow channels 40 to the fuel outlet passage 46 and out of the fuel cell stack 52 at the backing plate 15.
Referring to
The distribution of air through the air inlet passage 48 in the z direction and then back through the air passage 60 in the −z direction absorbs heat from the cells 10 of the stack 52. This both cools the stack 52 and preheats the air so the cells first exposed to the air are not cooled significantly more than the rest of the cells. By using the air to cool the stack 52 as well as for cathode oxidant, an additional cooling plate previously used in known fuel cells is avoided.
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.
