Abstract:
A purging system for removing oxygen from a fuel cell system during a shutdown period for the fuel cell system. The purging system includes a separator having an inlet and an outlet; a first exhaust line for communicating a first exhaust gas stream from an outlet of the fuel cell system to the separator inlet during the shutdown period of the fuel cell system; and a second exhaust line for communicating a second exhaust gas stream to an inlet of the fuel cell system for delivering the second exhaust gas stream to the fuel cell system during the shutdown period. The separator removes oxygen from the first exhaust gas stream such that the first stream nitrogen molar volume is lower than the second steam nitrogen molar volume and the first stream oxygen molar volume is higher than the second stream oxygen molar volume.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/780,101 filed Jul. 19, 2007, the disclosure of which is incorporated in its entirety by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    One aspect of the present invention relates to oxygen removal systems for fuel cells during shutdown. Another aspect of the present invention relates to oxygen introduction prevention systems for fuel cells during shutdown. 
         [0004]    2. Background Art 
         [0005]    Fuel cells are electrochemical devices that convert the chemical energy of a fuel into electricity and heat without fuel combustion. In the one type of fuel cell hydrogen gas and oxygen gas are electrochemically combined to produce electricity. The hydrocarbon used in this process may be obtained from natural gas or methanol while air provides the oxygen source. The only by products of this process are water vapor and heat. Accordingly, fuel cell-powered electric vehicles reduce emissions and the demand for conventional fossil fuels by eliminating the internal combustion engine (e.g., in completely electric vehicles) or operating the engine at only its most efficient/preferred operating points (e.g., in hybrid electric vehicles). However, while fuel cell-powered vehicles have reduced harmful vehicular emissions, they present other drawbacks. 
         [0006]    PEM fuel cells comprise an anode and a cathode which are separated by a polymeric electrolyte or proton exchange membrane (“PEM”). Each of the two electrodes may be coated with a thin layer of platinum. At the anode, the hydrogen is catalytically broken down into electron and hydrogen ions. The electrons provide the electricity as the hydrogen ions move through the polymeric membrane toward the cathode. At the cathode, the hydrogen ions combine with oxygen from the air and electrons to form water. 
       SUMMARY 
       [0007]    According to one aspect of the present invention, during shutdown of a PEM fuel cell, purging oxygen from the fuel cell can minimize hydrogen and oxygen mixing during startup of the fuel cell, which can increase the lifetime of the fuel cell. According to another aspect of the present invention, the lifetime of a PEM fuel cell can be increased by preventing the introduction of oxygen into the fuel cell during shutdown so that hydrogen and oxygen mixing during startup is minimized. 
         [0008]    According to one embodiment of the present invention, a purging system for removing oxygen from a fuel cell system during a shutdown period for the fuel cell system is disclosed. The purging system includes a separator having an inlet and an outlet; a first exhaust line for communicating a first exhaust gas stream from an outlet of the fuel cell system to the separator inlet during the shutdown period of the fuel cell system; and a second exhaust line for communicating a second exhaust gas stream to an inlet of the fuel cell system for delivering the second exhaust gas stream to the fuel cell system during the shutdown period. The first exhaust gas stream includes oxygen at a first oxygen molar volume and nitrogen a first nitrogen molar volume. The second exhaust gas stream includes oxygen at a second oxygen molar volume and nitrogen at a second nitrogen molar volume. The separator removes oxygen from the first exhaust gas stream such that the first nitrogen molar volume is lower than the second nitrogen molar volume and the first oxygen molar volume is higher than the second oxygen molar volume. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]      FIG. 1  depicts a system including a fuel cell and a separator according to one embodiment of the present invention; 
           [0010]      FIG. 2  depicts a system including a fuel cell, a separator, and an oxygen depleted air storage device according to another embodiment of the present invention; 
           [0011]      FIG. 3  depicts a system for preventing the introduction of oxygen into the fuel cell during shutdown according to an embodiment of the present invention; and 
           [0012]      FIGS. 4A ,  4 B,  4 C and  4 D depict polarization curves for quantifying fuel cell degradation under specified experimental conditions. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Except where expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the present invention. Practice within the numerical limits stated is generally preferred. 
         [0014]    The description of a single material, compound or constituent or a group or class of materials, compounds or constituents as suitable for a given purpose in connection with the present invention implies that mixtures of any two or more single materials, compounds or constituents and/or groups or classes of materials, compounds or constituents are also suitable. Also, unless expressly stated to the contrary, percent, “parts of,” and ratio values are by volume. Description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among constituents of the mixture once mixed. The first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property. 
         [0015]    The lifetime of a proton exchange membrane (“PEM”) fuel cell can be shortened by exposure to air during the transition of the PEM fuel cell from off to a startup. As applied to automotive vehicles, this transition is commonly referred as vehicle startup. During vehicle startup, if hydrogen fuel is introduced into the anode that is exposed to air, the simultaneous mixing of hydrogen and oxygen within the anode layer may cause local potential gradients within the same catalyst layer, which may degrade the catalyst, the catalyst support, and/or the membrane. In certain applications, degradation is a predominant problem in the cathode catalyst layer. 
         [0016]    Each fuel cell cycle, i.e. a cycle including the transition between idle and start up, can present an opportunity in which the simultaneous mixing of hydrogen and oxygen may occur, thereby contributing to the degradation of the fuel cell. According to conventional systems, the problem of simultaneous mixing of any oxygen with hydrogen was identified as substantially degrading the fuel cell over time. 
         [0017]    Embodiments of the present invention include the discovery of the problem of having the simultaneous mixing of greater than 5% oxygen in air and balance hydrogen, as supported by the experimental data presented here. Unexpectedly, insignificant degradation occurs when cycling includes the simultaneous mixing of 0-5% oxygen in air and hydrogen. Therefore, embodiments of the present invention provide a new solution to a new problem. 
         [0018]    In light of the foregoing, an oxygen removal system is needed for minimizing the scenario of hydrogen and air mixing in the same catalyst compartment. What is also needed is a method for preventing catalyst exposure to oxygen in the fuel cell during shutdown. 
         [0019]      FIG. 1  depicts a system  10  including a fuel cell  12  and a separator  14 , according to one embodiment of the present invention. The fuel cell  12  includes an anode compartment  16 , an electrolyte compartment  18 , and a cathode compartment  20 . The electrolyte compartment  18  is disposed between the anode compartment  16  and the cathode compartment  20 . In at least one embodiment, the fuel cell  12  is a proton exchange membrane (PEM) fuel cell. As known in the art, the PEM fuel cell transforms chemical energy liberated during the reaction of hydrogen (H 2 ) and oxygen (O 2 ) to electrical energy. 
         [0020]    In one embodiment of the present invention, the fuel cell  12  performs this transformation during an operational mode, i.e., an “on” mode. During the operational mode, a hydrogen stream is fed into the anode compartment  16  through an anode inlet  22 . A hydrogen fuel source  24  for the hydrogen stream can be produced by a hydrogen generator. The system  10  includes an anode inlet line  28  situated between the hydrogen fuel source outlet  26  and the anode inlet  22 . The hydrogen fuel source travels through the inlet line  28  as the hydrogen stream, which is fed into the anode compartment  16  through the anode inlet  22 . 
         [0021]    A hydrogen stream valve  30  is positioned on inlet line  28  to control the supply of the hydrogen stream to the anode compartment  16 . During the operational mode, the hydrogen valve  30  is at least partially open to allow the hydrogen stream to flow into the anode compartment  16 . During a shutoff mode, i.e., the time period in which the fuel cell  12  is not operating, and otherwise referred to as the “off” mode, the hydrogen stream valve  30  is closed to prevent the flow of hydrogen into the anode compartment  16 . 
         [0022]    During the operation mode, the hydrogen introduced into the anode compartment  16  is catalytically split into protons and electrons. Excess hydrogen fuel exits the anode compartment  16  through an anode outlet  32  to the anode exhaust line  34 . The newly formed protons permeate the electrolyte compartment  18  to the cathode compartment  20 . The electrons travel along an external load circuit (not shown) to the cathode compartment  20 , thereby creating a current output of the fuel cell  12 . 
         [0023]    During the operational mode, an air stream including oxygen and nitrogen is fed into the cathode compartment  20  through a cathode inlet  36 . An air source  38  can be an atmospheric source in which air is supplied through a cathode inlet line  40  to the cathode inlet  36  for use within the cathode compartment  20 . An air stream valve  39  is positioned on inlet line  40  to control the supply of air to the cathode compartment  20 . 
         [0024]    The oxygen in the air stream reacts with the protons permeating through the electrolyte compartment  18  and the electrons arriving through the external circuit (not shown) to form water molecules. The water molecules and heat exit the cathode compartment  20  through a cathode outlet  42  to the cathode exhaust line  44 . 
         [0025]    In one embodiment of the present invention, the separator  14  is positioned at the end of the cathode exhaust line  44 . In at least one embodiment, the separator  14  includes a cathode exhaust input  45  for receiving the cathode exhaust in the cathode exhaust line  44 . In another embodiment, the separator  14  includes an atmospheric air inlet  47  for receiving air from the atmosphere. In yet another embodiment, the separator  14  includes both inputs  45  and  47 . 
         [0026]    During the shutoff period, the separator  14  can be utilized to remove oxygen from the cathode exhaust line  44  (and/or atmospheric air fed from the atmospheric air inlet), which exits the separator  14  through oxygen outlet line  48 , and to produce a nitrogen-rich gas stream (otherwise referred to as an oxygen depleted gas stream), which exits the separator  14  through outline line  48 . The nitrogen-rich gas stream can be fed to a junction  50 , which allows the nitrogen-rich gas to flow into the anode return line  52  and cathode return line  54 . The anode return line  52  is connected to valve  30  and the cathode return line is connected to valve  39 , according to at least one embodiment. Non-limiting examples of junctions include orifices and/or pipes. In another embodiment, the system  10  does not include a junction  50 , and all of the nitrogen-rich gas travels through anode return line  52 . This embodiment is signified by dotted line  54 . 
         [0027]    During the shutoff period, valves  30  an/or  39  are in a position to allow the flow of nitrogen-rich gas into the anode inlet line  28  and the cathode inlet  40 . The nitrogen-rich gas then flows into the anode and cathode compartments  16  and  20  from the anode inlet line  28  and/or the cathode inlet  40 , thereby removing air from the fuel cell  12  through anode and cathode exhaust lines  34  and  42 . This purging of nitrogen-rich gas into the fuel cell  12  during shutdown ameliorates the mixing of oxygen and hydrogen at startup. The nitrogen purge minimizes simultaneous mixtures of hydrogen and oxygen within the same catalyst layer of the fuel cell  12 . 
         [0028]    In at least one embodiment, the nitrogen molar volume % in the nitrogen-rich gas stream is 80 to 100%. In another embodiment, the nitrogen molar volume % in the nitrogen-rich gas stream is 91 to 100%. In yet another embodiment, the nitrogen molar volume % in the nitrogen-rich gas stream is 95% to 100%. In one embodiment, the nitrogen molar volume % in the nitrogen-rich gas stream is 99% to 100%. 
         [0029]    In at least one embodiment, the oxygen molar volume % in the nitrogen-rich gas stream is 0 to 20%. In another embodiment, the oxygen molar volume % in the nitrogen-rich gas stream is 0 to 9%. In yet another embodiment, the oxygen molar volume % in the nitrogen-rich gas stream is 0% to 5%. In one embodiment, the oxygen molar volume % in the nitrogen-rich gas stream is 0% to 1%. 
         [0030]    In at least one embodiment, the ratio of nitrogen molar volume in the cathode exhaust line  44  to the oxygen depleted air line  48  is in the range of 80:81 to 80:100 and the ratio of oxygen molar volume in the cathode exhaust line  44  to the oxygen depleted air line  48  is in the range of 20:19 to 0. 
         [0031]    In at least one embodiment, the separator  14  is an oxygen separating chamber, and in other embodiments, the separator  14  is a nitrogen generator. In certain embodiments, the present invention utilizes the nitrogen generator uniquely to automotive fuel cell applications with any adaptations necessary. 
         [0032]    In at least one embodiment, the separator  14  is a molecular sieve. In one embodiment, a molecular sieve available from Universal Industrial Gases, Inc., Easton, Pa. is utilized. Examples of adsorbent materials that can be utilized in the molecular sieve, include, but are not limited to, aluminosilicate minerals, clays, porous glasses, microporous charcoals, zeolites, active carbons, and/or synthetic compounds that have open structures through which small molecules, such as nitrogen can diffuse. In at least one embodiment, the molecular sieve can be regenerated during the operational period via a chamber regenerator  58 . Non-limiting examples of chamber regenerators  58  include exhaust heat from the fuel cell  12 , a nitrogen carrier gas, or dilute hydrogen bleed or vacuum. 
         [0033]    In other embodiments, oxygen removal can be achieved by pressure swing adsorption. In one embodiment, a unit for pressure swing adsorption is the Parker-Balston High Flow Nitrogen Generator Model AGS200. U.S. Pat. Nos. 4,440,548 and 4,439,213 disclose examples of pressure swing adsorption, and are incorporated herein in their entirety. In yet other embodiments, a membrane separation device can be utilized for oxygen removal from the anode exhaust line  34  during the shutoff mode. In one embodiment, nitrogen membrane systems available from Universal Industrial Gases, Inc. are utilized. In other embodiments, the Parker-Balston Model N2-80 can be utilized. U.S. Pat. Nos. 5,439,507 and 5,302,189 disclose examples of membrane separation devices, and are incorporated herein in their entirety. 
         [0034]    In at least one embodiment, the gas content within the fuel cell  12  can be circulated through the separator  14  during the shutoff mode. The circulation can be provided through the cathode exhaust line  44  and the anode return line  52  and/or the cathode return line  54 . Increasing the circulation time can increase the removal of oxygen from the fuel cell  12  during the shutoff mode, thereby minimizing the amount of oxygen present in the fuel cell  12  during the startup mode. In at least one embodiment, the circulation time can encompass the entire shutdown period. 
         [0035]    In yet another embodiment of the present invention, as depicted in  FIG. 2 , a system  100  including a storage device  56 , e.g. a tank with a check valve, can be utilized. The storage device  56  can store nitrogen-rich air during shutdown, which can be purged into the fuel cell  12  through the anode inlet line  28  and/or cathode inlet line  40 . In at least one embodiment, the purging occurs just prior to startup of the fuel cell  12 . 
         [0036]    In another embodiment, during operation of the fuel cell  12 , the separator  14  can produce an oxygen depleted air stream from atmospheric air being fed through the atmospheric air inlet  47 . The oxygen depleted air stream can be stored in the storage device  56  for use in the purging operation during shutoff. 
         [0037]    In at least one embodiment, the nitrogen molar volume % of the stored oxygen depleted air is 80 to 100%. In another embodiment, the nitrogen molar volume % of the stored oxygen depleted air is 91 to 100%. In yet another embodiment, the nitrogen molar volume % of the stored oxygen depleted air is 95% to 100%. In one embodiment, the nitrogen molar volume % of the stored oxygen depleted air is 99% to 100%. 
         [0038]    In at least one embodiment, the oxygen molar volume % of the stored oxygen depleted air is 0 to 20%. In another embodiment, the oxygen molar volume % of the stored oxygen depleted air is 0 to 9%. In yet another embodiment, the oxygen molar volume % of the stored oxygen depleted air is 0% to 5%. In one embodiment, the oxygen molar volume % of the stored oxygen depleted air is 0% to 1%. 
         [0039]      FIG. 3  depicts a system  150  according to the present invention in which air introduction into the fuel cell  12  is prevented during shutdown via a device  152 , for example, a mechanical device (e.g., check valve) or chemical means (e.g., oxygen scavenger). 
         [0040]    During the transition between the operational mode and the shutoff mode, otherwise referred to as the shutdown mode, oxygen may leak back into the fuel cell  154  and the anode and cathode compartments, through outlet line  156  (collectively referring to the anode and/or cathode outlet lines). Furthermore, the oxygen can become trapped in the anode exhaust line and/or cathode exhaust line. This trapped oxygen may leak into the anode and/or cathode compartments during the shutoff period. During the transition between the shutoff mode and the operational mode, the oxygen that leaked into one or both of the compartments during shutoff may simultaneously mix with the hydrogen entering through the anode inlet  158 . This mixing of hydrogen and oxygen within the anode compartment  16  may lead to local potential gradients within the same electrode, which may attack the catalyst and catalyst support within the anode compartment  16  and/or cathode compartment  18 , as well as the electrolyte compartment  18 . The device  152  prevents or at least minimizes the reintroduction of oxygen into the fuel  154  during shutdown. 
       Experimental Data 
       [0041]    A test stand was constructed for cycling various gases through a fuel cell for different time periods. Air, air/nitrogen mixtures, hydrogen, and nitrogen enter the system through pneumatically controlled valves that can be programmed to open and close to control the type of gas flowing to the fuel cell. Three gas mixtures were selected for this experiment (i) 21% oxygen (100% air), 5% oxygen/95% nitrogen, and 10% oxygen/90% nitrogen. All gas pressures were 10 psig and flowed at a rate of 800 sccm (“Standard Cubic Centimeters per Minute”). All gas cycles consisted of a 60 second “on” and a 60 second “off” time. Following each gas cycle, polarization curves were completed to quantify MEA (membrane electrode assembly) degradation. 
         [0042]      FIGS. 4A ,  4 B,  4 C and  4 D illuminate the degradation effects of cycling the anode gas.  FIG. 4A  indicates minimal cell performance degradation.  FIG. 4B  is a typical representation of cell degradation observed when cycling the anode between air and hydrogen. The loss in cell performance is significant after 20 cycles of hydrogen to air. Cycling the anode between a gas mixture of 5% oxygen in air and hydrogen ( FIG. 4C ) demonstrates an insignificant loss in performance similar to  FIG. 4A .  FIG. 4D  demonstrated that cycling between 10% oxygen in air and hydrogen degrades the cell performance in a similar fashion as when cycling between air and hydrogen. The polarization curve data indicates that using an oxygen depleted gas to purge the anode during shutdown can be as beneficial as using a 100% nitrogen gas as the purge gas. 
         [0043]    As required, detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely exemplary of an invention that may be embodied in various and alternative forms. While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. 
         [0044]    In accordance with the provisions of the patent statute, the principle and mode of operation of this invention have been explained and illustrated in its various embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.