Abstract:
A fuel cell system is specially adapted to provide enhanced air purification for stationary and mobile applications. An air purification subsystem may be installed along a cathode flow path to enhance air purification by utilization of fuel cell operating conditions. Synergistic automotive, residential, commercial and agricultural applications are thus provided. Air purification subsystems may include, for example, a multi-purpose platinum-based catalyst configuration adapted to convert carbon monoxide into carbon dioxide and ozone into diatomic oxygen.

Description:
BACKGROUND 
     This invention relates to integrated power generation and air purification and conditioning systems for stationaryand mobile applications. Specifically, an air purification subsystem may be installed in a fuel cell system across a cathode gas diffusion layer or along a cathode flow path to enhance air purification by utilization of fuel cell operating conditions. 
     A fuel cell is a device which converts chemical energy of a fuel into electrical energy, typically by oxidizing the fuel. In general a fuel cell includes an anode and a cathode separated by an electrolyte. When fuel is supplied to the anode and oxidant is supplied to the cathode, the cell electrochemically generates a useable electric current which is passed through an external load. The fuel typically supplied is hydrogen and the oxidant typically supplied is oxygen. In such cells, oxygen and hydrogen are combined to form water and to release electrons. The chemical reaction for a fuel cell using hydrogen as the fuel and oxygen as the oxidant is shown in equation (1). 
     
       
         H 2 +½O 2 →H 2 O  (1) 
       
     
     This process occurs through two half-reactions which occur at the electrodes: Anode Reaction 
     
       
         H 2 →2H + +2e −   (2) 
       
     
     Cathode Reaction 
     
       
         ½O 2 +2H + +2e − →H 2 O  (3) 
       
     
     In the anode half-reaction, hydrogen is consumed at the fuel cell anode releasing protons and electrons as shown in equation (2). The protons are injected into the fuel cell electrolyte and migrate to the cathode. The electrons travel from the fuel cell anode to cathode through an external electrical load. In the cathode half-reaction, oxygen, electrons from the load, and protons from the electrolyte combine to form water as shown in equation (3). 
     The directional flow of protons, such as from anode to cathode, serves as a basis for labeling an “anode” side and a “cathode” side of the fuel cell. 
     Fuel cells are classified into several types according to the electrolyte used to accommodate ion transfer during operation. Examples of electrolytes include aqueous potassium hydroxide, concentrated phosphoric acid, fused alkali carbonate, stabilized zirconium oxide, and solid polymers, e.g., a solid polymer ion exchange membrane. 
     An example of a solid polymer ion exchange membrane is a Proton Exchange Membrane (hereinafter “PEM”) which is used in fuel cells to convert the chemical energy of hydrogen and oxygen directly into electrical energy. A PEM is a solid polymer electrolyte which when used in a PEM-type fuel cell permits the passage of protons (i.e.,H +  ions) from the anode side of a fuel cell to the cathode side of the fuel cell while preventing passage of reactant fluids such as hydrogen and oxygen gases. 
     A PEM-type cell includes an electrode assembly disposed between an anode fluid flow plate and a cathode fluid flow plate. An electrode assembly usually includes five components: two gas diffusion layers; two catalysts; and an electrolyte. The electrolyte is located in the middle of the five-component electrode assembly. On one side of the electrolyte (the anode side) a gas diffusion layer (the anode gas diffusion layer) is disposed adjacent the anode layer, and a catalyst (the anode catalyst) is disposed between the anode gas diffusion layer and the electrolyte. On the other side of the electrolyte (the cathode side), a gas diffusion layer (the cathode gas diffusion layer) is disposed adjacent the cathode layer, and a catalyst (the cathode catalyst) is disposed between the cathode gas diffusion layer and the electrolyte. 
     Several PEM-type fuel cells may be arranged as a multi-cell assembly or “stack.” In a multi-cell stack, multiple single PEM-type cells are connected together in series. The number and arrangement of single cells within a multi-cell assembly are adjusted to increase the overall power output of the fuel cell. Typically, the cells are connected in series with one side of a fluid flow plate acting as the anode for one cell and the other side of the fluid flow plate acting as the cathode for an adjacent cell. 
     The anode and cathode fluid flow plates are typically made of an electrically conductive material, typically metal or compressed carbon, in various sizes and shapes. Fluid flow plates may act as current collectors, provide electrode support, provide paths for access of the fuels and oxidants to the electrolyte, and provide a path for removal of waste products formed during operation of the cell. 
     The cell also includes a catalyst, such as platinum on each side of the electrolyte for promoting the chemical reaction(s) that take place in the electrolyte in the fuel cells. The fluid flow plates typically include a fluid flow field of open-faced channels for distributing fluids over the surface of the electrolyte within the cell. 
     Fluid flow plates may be manufactured using any one of a variety of different processes. For example, one technique for plate construction, referred to as “monolithic” style, includes compressing carbon powder into a coherent mass which is subjected to high temperature processes to bind the carbon particles together, and to convert a portion of the mass into graphite for improved electrical conductivity. The mass is then cut into slices, which are formed into the fluid flow plates. Typically, each fluid flow plate is subjected to a sealing process (e.g., resin impregnation) in order to decrease gas permeation therethrough and reduce the risk of uncontrolled reactions. 
     Fluid flow plates may also have holes therethrough which when aligned in a stack form fluid manifolds through which fluids are supplied to and evacuated from the stack. Some of the fluid manifolds distribute fuel (such as hydrogen) and oxidant (such as air or oxygen) to, and remove unused fuel and oxidant as well as product water from, the fluid flow fields of the fluid flow plates. Additionally, other fluid manifolds circulate coolant to control the temperature of the stack. For example, a PEM fuel cell stack may be maintained in a temperature range of from 60° C. to 200° C. The temperature of the anode and cathode exhaust streams may also be within this range as they leave the fuel cell. Cooling mechanisms such as cooling plates are commonly installed within the stack between adjacent single cells to remove heat generated during fuel cell operation. 
     PEM fuel cell systems using hydrogen as a fuel may include a fuel processing system such as a reformer to produce hydrogen by reacting a hydrocarbon such as natural gas or methanol. Many such fuel processing systems are well known in the art. Where a reformer is used, the reformed fuel gas is referred to as reformate, and may typically contain predominantly hydrogen, carbon dioxide and water. In some cases, reformate may also a relatively small amount of carbon monoxide. Since carbon monoxide, even in trace amounts, acts as a poison to most fuel cell catalysts, for example platinum-based catalysts, methods have been developed to minimize or eliminate carbon monoxide in reformate streams. Such methods include, for example, using a preferential oxidizer system to convert carbon monoxide into non-poisoning carbon dioxide, or optimizing fuel processor operating conditions such as temperature and air flow to minimize the production of carbon monoxide in the reformer. 
     Typically only a portion of the reactants (e.g., reformate containing hydrogen on the anode side, and air containing oxygen on the cathode side) flowing through a fuel cell will react. For example, the amount of reactants in the anode and cathode streams that are reacted may depend on factors including temperature, pressure, residence time, and catalyst surface area. For this reason, it may be desirable to feed excess reactants to a fuel cell in order to increase the reaction level to a point corresponding to a desired power output of the fuel cell. For example, it may be that 100 standard liters per minute (slm) of hydrogen must be reacted in a fuel cell to achieve a desired power output, but it is determined that 140 slm of hydrogen must be fed to the fuel cell to achieve this reaction of 100 slm of hydrogen. This system may be said to be running at 40% excess hydrogen at the anode inlet. In other terminology, this system may also be characterized as running at a stoichiometry of 1.4. For similar reasons, it may be desirable to supply the cathode side of the fuel cell with an excess of oxidant. The stoichiometry of the anode and cathode flows may be selected independently. 
     It will thus be appreciated that by reacting hydrogen from the anode stream, the fuel cell provides an anode exhaust stream that is concentrated in its non-hydrogen components. Likewise, by reacting oxygen from the cathode stream, the fuel cell provides a cathode exhaust stream that is concentrated in its non-oxygen components. 
     In most environments, fuel cell cathode air streams will not contain significant carbon monoxide levels because carbon monoxide is not generally present in fresh atmospheric air. However, in polluted environments, such as might be seen by fuel cells, for example, in or near automotive and commercial environments, the ambient air fed to the fuel cell may contain carbon monoxide as well as other air contaminants such as ozone. Carbon monoxide is a well known poison to humans and animals. Ozone is known to cause a variety of health problems including lung damage. Tropospheric ozone is also known to cause substantial damage to agricultural crops. Various other organic and non-organic pollutants may also be present in such environments, such as those emitted from automotive exhaust and the evaporation of organic solvents. 
     SUMMARY 
     In general, in one aspect, the invention provides a fuel cell system with an air purification subsystem located along the cathode flow path, wherein the air purification subsystem utilizes heat from the fuel cell to react air pollutants. In this context, the cathode flow path refers to the oxidant flow through the fuel cell system, starting with the oxidant inlet (e.g., an air blower or compressor), and ending with the cathode exhaust as it leaves the fuel cell system. In one possible embodiment, the air purification subsystem may include a multi-purpose catalys bed suitable for converting carbon monoxide into carbon dioxide, and converting ozone into diatomic oxygen. The air purification subsystem may also be effective to oxidize organic pollutants. In other embodiments, the catalyst bed may be selected to abate a specific air pollutant. As examples, suitable catalysts may include precious metals and alloys thereof, being supported by conventional means known in the art such as alumina and zeolite monoliths, and carbon black catalyst support systems. In one possible embodiment, the electrodes of the fuel cell may include a multi-purpose platinum-based catalyst structure and composition that is optimized to provide enhanced air purification in addition to the hydrogen oxidation required to power the fuel cell. In general, the air purification subsystem may be located up or downstream of the fuel cell, or along the cathode flow plates of the fuel cell, or on the fuel cell cathode or cathode side gas diffusion layer. 
     In another aspect, the invention contemplates systems for providing purified air to an interior air space of a building or a vehicle, as examples, by treating the cathode air flow of a fuel cell system with the catalytic air purification subsystem, and then supplying the interior air space with the cathode exhaust from the fuel cell. The decrease in oxygen in the cathode stream may be referred to as the cathode oxygen depletion. Additional embodiments may include a means for selecting the cathode flow rate of the fuel cell system to control the cathode oxygen depletion of the cathode exhaust. For example, the stoichiometry of the cathode flow may be set to provide a cathode oxygen depletion of less than 10% or less than 5%, as examples. A by-pass may also be provided to control the flow of the cathode exhaust into the interior air space. A heat exchanger may also be provided to control the temperature of the cathode exhaust. A dehumidifier may also be provided to control the humidity of the cathode exhaust. 
     In another aspect, the air purification subsystem can include a catalyst bed adapted to react carbon monoxide at an operating temperature of the fuel cell system. For example, carbon monoxide may be reacted at a temperature of less than 200° C. for certain fuel cell systems, or less than 100° C. or less than 80° C. for other fuel cell systems. In another aspect, the air purification subsystem can include a catalyst bed adapted to react ozone at such operating temperatures. As an example, for a system adapted to focus on carbon monoxide removal, it may be desirable for the purification subsystem to be located in the cathode flow path at a point upstream from the fuel cell. As another example, for a system adapted to focus on ozone removal, it may be desirable for the purification subsystem to be located in the cathode flow path at a point downstream from the fuel cell. 
    
    
     Other advantages and features of the invention will be apparent from the description, drawings and claims. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of an air circulation system according to an embodiment of the invention. 
     FIG. 2 is a building with a fuel cell system and air purification subsystem according to an embodiment of the invention. 
     FIG. 3 is a vehicle with a fuel cell system and air purification subsystem according to an embodiment of the invention. 
     FIG. 4 is a schematic diagram of a fuel cell system that may be used in accordance with the present invention. 
     FIG. 5 is an exploded view of an air purification subsystem according to an embodiment of the invention. 
     FIG. 6 is an air purification subsystem according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, an embodiment of an air circulation system  10  includes a fuel cell system  14  that supplies electrical power (via electrical lines  16 ) to a building having an air space  12  and purifies the air in the air space  12 . The air space may be the interior air space of a building, such as a home or office building, or as another example, the interior air space of a vehicle. The air space may also have a vent  6 , for example to vent to the atmosphere. It will also be appreciated that where the fuel cell system  14  is vented to the atmosphere, for example, through line  4 , the present invention may also be used to purify the ambient air around the fuel cell system  14 , as fed to the fuel cell for example through line  2 . 
     The fuel cell system  14  may include a fuel cell stack  50  that consumes both oxygen and hydrogen to produce the electrical power. The oxygen may be taken, for example from the air around the fuel cell system  14 , or from the interior air space  12 . The hydrogen may be provided, for example, from a natural gas reformer (not shown) in the fuel cell system  14 . As other examples, the hydrogen may be provided from hydrogen tanks or other hydrogen storage systems such as hydrogen storage alloys. 
     In the embodiment shown in FIG. 1, air is drawn into the fuel cell stack  50  through cathode inlet conduit  18 . Within the stack  50 , the air passes over the fuel cell cathode (not shown), and passes out of the stack  50  as cathode exhaust through cathode outlet conduit  20 . In this particular embodiment, the air purification subsystem consists of the modifications made to the cathode to optimize its air purification capabilities over what would normally be required just for fuel cell operation (see discussion below). Inside the fuel stack  50 , the stream of air passes across the modified cathode of each fuel cell of the stack  50 , and as a result, the air flowing through the fuel cells is purified, for example as pollutants are oxidized. Therefore, the air that returns to the air space  12  from the fuel cell system  14  may be cleaner than the air that exits the air space  12 . Thus, due to this arrangement, the fuel cell system  14  may provide dual functions: a power function in which the fuel cell system  14  supplies electrical power and an air purification function in which the fuel cell system  14  purifies the air in the air space  12 . 
     Referring to FIG. 2, as a more specific example, the fuel cell system  14  may reside outside of the air space  12  and may be connected into an air circulation path with an interior region of the air space  12  via an cathode inlet conduit  18  and a cathode outlet conduit  20 . In this manner, the cathode inlet conduit  18  may receive air from the interior region of the air space  12  and direct a stream of air into the fuel cell system  14 . The fuel cell system  14 , in turn, uses oxygen from the stream of air to promote the cathodic reactions of its fuel cells. At the same time, the catalysts of the fuel cells produce a purified stream of air that is routed through the cathode outlet conduit  20  to a region inside the air space  12 . For example, in some embodiments, the cathode outlet conduit  20  may be connected to an air intake return duct  24  for a climate control unit  22  of a climate control system of the air space  12 . 
     The climate control system may maintain the interior of the air space  12  at a specified climate, such as a specified temperature and/or humidity. As examples, the climate control unit  22  may be an interior unit of an air conditioning system, a heating system, a humidifying system or a dehumidifying system, as just a few examples. Thus, in some embodiments, the purified air may be injected into the return duct  24  to be heated, cooled, humidified or dehumidified by the climate control unit  22  before exiting an output duct  26  of the climate control unit  22  to propagate throughout the air space  12  through additional ducts (not shown). The climate control unit  22  may be located within air space  12 , or may also be located within fuel cell system  14 . In this context, the terms ducts and conduits are used interchangeably to refer to the way the cathode flow path of the fuel cell system  14  is connected, for example to air space  12 . The invention is not limited according to the means by which the cathode flow path is connected. 
     It will be appreciated that the cathode exhaust of the fuel cell system  14  will be at about the same temperature as the fuel cell stack  50 , for example, an operating temperature in the range of 60° C. to 200° C. Thus, where it is desired to heat the air space  12 , as might be the case with a building or vehicle of such a system in a cold climate, the heat from the fuel cell stack  50  may be used to efficiently provide air that is not only purified, but also pre-heated. It will also be appreciated that where it is desired to cool the air space  12 , as might be the case with a building or vehicle of such a system in a hot climate, it may be undesirable to introduce the warm cathode exhaust into the air space  12 . For example, in such a case the cathode inlet might instead be taken from the outside air around the fuel cell system  14  to preserve the cool air in the air space  12 , and similarly the cathode exhaust might be vented to the atmosphere. In such a configuration, the purification fumction of the fuel cell system  14  would be directed to the atmosphere around the fuel cell system  14 . 
     Referring to FIG. 3, an embodiment of an air circulation system  100  includes a fuel cell system  114  that supplies electrical power to a vehicle  130 , for example to propel the vehicle  130 , and also provides purified air to the air space  112 A of the vehicle  130 . The fuel cell system  114  may draw oxygen, for example from the air space within the vehicle  112 A through cathode inlet conduit  118 A, or from the air space outside the vehicle  112 B through cathode inlet conduit  118 B. As previously discussed with respect to FIGS. 1 and 2, the air passing through the fuel cell system  114  is purified, and may be flowed into the vehicle air space  112 A through cathode outlet conduit  120 A, or may be vented to the atmosphere around the vehicle  112 B through cathode outlet conduit  120 B. For example, the fuel cell system  114  of vehicle  130  may be used to clean pollutants from the air space outside the vehicle  130  as the vehicle is driven and air from air space  112 B circulates through the fuel cell system  114 . Such a vehicle  130  could provide the advantage of removing more pollutants from the air  112 B than produced by the vehicle  130 , and thus could be referred to as a sub-zero emission vehicle. 
     FIG. 4 depicts one of many possible embodiments of the fuel cell system  14 . As shown in FIG. 4, the fuel cell system  14  may include a particulate filter  74  that is connected to the cathode inlet conduit  18  to filter particles from the incoming air before the air reaches an air blower  72 . The air blower  72 , in turn, may be controlled by an electrical control unit  68  (via one or more electrical control lines  69 , for example) to regulate a rate at which the air flows from the blower  72  through an air conduit  52  into control valves  56 . As an example, the rate of air flow out of the air blower  72  may be dependent on the output power (as indicated by an output current, for example) that is currently being provided by the fuel cell system  14 . Through a fuel input conduit  54 , the control valves  56  also receive hydrogen (the fuel). The control valves  56  maintain the appropriate flow rates of the hydrogen and air into respective conduits  58  and  60  that direct the hydrogen and air into the fuel cell stack  50 . 
     In some embodiments, the fuel cell system  14  may include an air control valve  78  that is coupled between the cathode outlet conduit  20  and a conduit  62  that is connected to an air exhaust port of the fuel cell stack  50 . In this manner, the electrical control unit  68  may regulate the valve  78  to control when the purified air exits the fuel cell system  14  and enters the air space  12 . For example, the climate control unit  22  (see FIG. 2) during its normal course of operation may turn on and off (turn its air blower on and off, for example) as needed to regulate the climate of the air space  12 . Thus, it may be desirable for the purified air to enter the air intake duct  24  when the climate control unit  22  is turned on (otherwise referred to as the climate control unit  22  being in an “on state”), and it may be desirable to prevent the purified air from entering the return duct  24  when the climate control unit  22  is off (otherwise referred to as the climate control unit  22  being in an “off state”). To accomplish this, in some embodiments, the control unit  68  may be electrically coupled to the climate control unit  22  to receive a signal from the climate control unit  22  to indicate whether the blower of the unit  22  is turned on (i.e., the climate control unit  22  is turned on) or off (i.e., the climate control unit is turned off. 
     In other embodiments, as another example, the control unit  68  may be coupled to a sensor  21  that is positioned inside the cathode outlet conduit  20 . Due to this arrangement, the control unit  68  may use the sensor  21  to determine when the climate control unit  22  has its blower turned on. For example, the sensor  21  may be an acoustic sensor that the control unit  68  uses to recognize an acoustic signature (that propagates through the duct work) to indicate that the blower of the climate control unit  22  has been turned on. Alternatively, the sensor  21  may be, for example, a pressure sensor to detect a slight vacuum that indicates the climate control unit&#39;s blower has been turned on. Other arrangements are possible. 
     Among the other features of the fuel cell system  14 , a power conditioning circuit  64  may receive a DC voltage from the fuel cell stack  50  and furnish to the electrical lines  16  one or more AC voltages. The fuel cell system  14  may also include a water humidification system  66  to vaporize deionized water and circulate the vapor through the fuel cell stack  50  to humidify the air and hydrogen. In this manner, the humidified air and hydrogen keep membranes of the fuel cell stack  50  from drying out. A reformer  70  may receive propane or natural gas (as examples) and convert the gas into the hydrogen fuel that is consumed by the fuel cell stack  50 . In some embodiments, when the air control valve  78  blocks communication between the conduits  62  and  20 , the valve  78  establishes communication between the conduit  62  and a conduit  76  that is connected to the reformer  70 . In this manner, when the purified air is not being furnished to the air space  12 , the purified air may be used to, as an example, oxidize carbon monoxide (CO) that is a byproduct of the reactions that occur in the reformer  70 . 
     It will be appreciated that as the cathode air stream flows through the stack  50 , due to the reaction of oxygen from the cathode stream to form product water, the cathode exhaust may be significantly concentrated in carbon dioxide and saturated with humidity. Such an exhaust stream may be used as generally shown in FIGS. 1 and 2 to provide an ideal air feed system for a greenhouse or other agricultural applications, since the utilization of both moisture and carbon dioxide is generally essential to plant life. For example, cathode air might be taken from outside a greenhouse (to provide normal amounts of oxygen to the fuel cell), and then the humid cathode exhaust could be supplied to a greenhouse. The carbon dioxide rich anode exhaust could also be supplied to the greenhouse. 
     Where a fuel cell system such as shown in FIGS. 1 and 2 is used to supply purified air to an internal air space  12 , it will be appreciated that the inherent cathode oxygen depletion of such a system may serve to undesirably reduce the amount of oxygen in the air space  12 . For example, in the Earth&#39;s atmosphere, while the relative concentration of oxygen in the atmosphere does not significantly change with altitude, at higher altitudes there is less pressure and therefore less oxygen available for breathing. Atmospheric pressure at sea level is about 14.7 psi, while the pressure at 18,000 feet is about 7.3 psi. Thus, for example, at 10,000 feet, an average person&#39;s blood might contain only 90% of its normal oxygen level. The reduced availability of oxygen at high altitudes may be taken as a rough analogy to the reduced availability of oxygen in an oxygen deplete fuel cell cathode exhaust stream. 
     To minimize the degree of oxygen depletion in a cathode exhaust stream to acceptable levels, for example 2% to 10%, it may therefore be desirable to operate the cathode stream at a higher flow rate with respect to the stoichiometric requirements of the fuel cell system  14 . For example, in a system such as shown in FIGS. 1 and 2, the cathode outlet conduit  18  might be equipped with an oxygen sensor to control the cathode flow rate at a level corresponding to an acceptable level of oxygen depletion (less than 5% or 2% as examples). It will be appreciated that a higher cathode flow rate may tend to lower the operating temperature of the fuel cell stack  50 . Thus, it is contemplated that a cooling subsystem (not shown) of the fuel cell system  14  may be responsive to the above cathode flow modulations to control or optimize the operating temperature of the stack  50 . 
     Referring to FIG. 5, an air purification subsystem  200  is shown that could be used in an embodiment of the invention. Housing  210  has an inlet  220 , an outlet  230 , and a central portion  240  housing a catalytic monolith  250 . The subsystem  200  could be located, for example, in the cathode flow path of a fuel cell system (not shown) immediately downstream from a fuel cell stack, so that inlet  220  receives cathode exhaust from the fuel cell stack. The exhaust is then passed through monolith  250 , which due to heating from the cathode exhaust passing through it (as an example), is at a temperature corresponding to the operation temperature of the fuel cell stack (e.g., about 80-200° C., depending on the particular system). Monolith  250  can be a ceramic or zeolite substrate  260  coated with a catalyst  270  suitable for oxidizing carbon monoxide at the temperature of the fuel cell stack. In this respect, the housing  210  and its contents may be generally referred to as a catalyst bed, though the term catalyst bed may also refer to other configuration, such as non-monolith arrangements (e.g., catalyst coated ceramic spheres, etc.). The catalyst  270  can be, for example, a platinum based material such as the Premair® catalyst available from Engelhard Corporation. As another example, the catalyst  270  can be pure platinum, or platinum alloys asknown in the art for oxidizing carbon monoxide. Such materials may also be selected specifically to optimize the reaction of ozone from the cathode-stream. 
     As another example, the catalyst  270  can comprise a cement carrier including activated carbon and an alkali, the cement being impregnated with palladium or a palladium alloy. Such a catalyst arrangement is taught by Maki, et al., U.S. Pat. No. 4,212,854, which is hereby incorporated by reference. Another possible catalyst arrangement suitable for temperatures above about 80° C., is a hopcalite type catalyst consisting of manganese dioxide mixed with the oxide of a metal such as copper, iron, cobalt or silver. Another possible catalyst arrangement is a tin (IV) oxide support with a thin precious metal layer as taught by Wright, et al., U.S. Pat. No. 4,524,051, which is hereby incorporated by reference. Another possible catalyst arrangement is an A type zeolite impregnated with the cathode stream. For example, a ceramic honeycomb monolith containing MnO 2 , NiO, CuO, or Ag 2 O may be used as noted in Yoshimoto, et al., U.S. Pat. Nos. 5,212,140 and 5,221,649, which are hereby incorporated by reference. Discussion of the utilization of various ozone catalyst systems, both new and well known arrangements is provided in Galligan, et al., U.S. Pat. No. 5,620,672, Campbell, et al., U.S. Pat. No. 5,888,924, and Sassa, et al., U.S. Pat. No. 5,891,402, which are also hereby incorporated by reference. 
     Referring to FIG. 6, another air purification subsystem  300  is shown that could be used in an embodiment of the invention. As previously discussed, PEM fuel cells typically utilize a five layer configuration consisting of a polymeric membrane  310  having a catalyst layer  320  on either side, the catalyst layer being further enclosed by gas diffusion layers  330  on either side. Membrane  310  is a solid polymer (e.g., a solid polymer ion exchange membrane), such as a solid polymer proton exchange membrane (e.g., a solid polymer containing sulfonic acid groups). Such membranes are commercially available from E.I. DuPont de Nemours Company (Wilmington, Del.) under the trademark NAFION. Alternatively, membrane  310  can also be prepared from the commercial product GORE-SELECT, available from W.L. Gore &amp; Associates (Elkton, Md.). As another example, the membrane may be made from a polybenimidazole material, such as taught by Onorato, et al., U.S. Pat. No. 5,945,233, which is hereby incorporated by reference. Catalyst layers  320  can include, as examples, platinum, platinum alloys, platinum dispersed on carbon black, and other materials known in the art. Also as known in the art, gas diffusion layers  330  can be formed of porous conductive materials such as carbon paper or carbon cloth. 
     The cathode side  340  of the membrane electrode assembly  350  shown in FIG. 6 has an additional air purification catalyst layer  360 . Air purification catalyst layer  360  may consist of the catalyst materials described with respect to FIG.  5 . While in the embodiment shown in FIG. 6, the catalyst layer  360  is shown as an external layer to gas diffusion layer  330 , it will be appreciated that catalyst layer  360  may also be applied as an additional layer to fuel cell catalyst layer  320 . Alternatively, layer  360  may be combined into layer  320  to achieve multiple functions from the same catalyst. 
     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.