Abstract:
We disclose a system that includes a fuel cell which during operation exhausts a fluid composition that includes an acid or a derivative of the acid, and an acid trap arranged to receive the fluid composition and configured to reduce a concentration of the acid or the derivative of the acid in the fluid composition.

Description:
STATEMENT AS TO FEDERALLY SPONSORED RESEARCH  
       [0001]     This invention was made with Government support under NIST Cooperative Agreement Number 70NANB1H3065. The Government has certain rights in this invention. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates to fuel cells and fuel cell systems that include acid traps.  
       BACKGROUND  
       [0003]     A fuel cell can convert chemical energy to electrical energy by promoting electrochemical reactions between two reactants.  
         [0004]     One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.  
         [0005]     Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.  
         [0006]     The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.  
         [0007]     During operation of the fuel cell, one of the gases (the anode gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas (the cathode gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.  
         [0008]     As the anode gas flows through the channels of the anode flow field plate, the anode gas diffuses through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode gas flows through the channels of the cathode flow field plate, the cathode gas diffuses through the cathode gas diffusion layer and interacts with the cathode catalyst.  
         [0009]     The anode catalyst interacts with the anode gas to catalyze the conversion of the anode gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode gas and the anode reaction intermediates to catalyze the conversion of the cathode gas to the chemical product of the fuel cell reaction.  
         [0010]     The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.  
         [0011]     The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.  
         [0012]     Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load, to the cathode flow field plate, and to the cathode side of the membrane electrode assembly.  
         [0013]     Electrons are formed at the anode side of the membrane electrode assembly, indicating that the anode gas undergoes oxidation during the fuel cell reaction. Electrons are consumed at the cathode side of the membrane electrode assembly, indicating that the cathode gas undergoes reduction during the fuel cell reaction.  
         [0014]     For example, when hydrogen and oxygen are the gases used in a fuel cell, hydrogen flows through the anode flow field plate and undergoes oxidation. Oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3. 
 
H 2 →2H + +2e −   (1) 
 
½O 2 +2H + +2e − →H 2 O  (2) 
 
H 2 +½O 2 →H 2 O  (3) 
 
         [0015]     As shown in Equation 1, hydrogen forms protons (H + ) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in Equation 2, the electrons and protons react with oxygen to form water. Equation 3 shows the overall fuel cell reaction.  
         [0016]     In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.  
         [0017]     Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.  
         [0018]     To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.  
       SUMMARY  
       [0019]     In one aspect, the invention features a system that includes a fuel cell which during operation exhausts a fluid composition that includes an acid or a derivative of the acid, and an acid trap arranged to receive the fluid composition and configured to reduce a concentration of the acid or the derivative of the acid in the fluid composition.  
         [0020]     Embodiments of the system can include any of the following features.  
         [0021]     The acid can be phosphoric acid.  
         [0022]     The acid trap can include a first region of a first material, and a second region of a second material different from the first material. The first material can include channels that have a mean diameter d 1  and that extend through a length of the first material, and the channels can form an array extending in a direction of flow of the first fluid composition along the length of the first material. The second material can include channels that have a mean diameter d 2  and that extend through a length of the second material. Mean diameter d 1  can be larger than mean diameter d 2 .  
         [0023]     The first material can be a ceramic material. The ceramic material can be coated with at least one of an activated carbon material and a silica material.  
         [0024]     The first material can be a zeolite material.  
         [0025]     The second material can be a ceramic material. The ceramic material can be coated with at least one of an activated carbon material and a silica material.  
         [0026]     The second material can be a zeolite material.  
         [0027]     The acid trap can be arranged so that the first fluid composition flows through the first region and then through the second region. The first material can be configured to adsorb the acid or derivative of the acid from the first fluid composition.  
         [0028]     The fuel cell can be a component of a fuel cell stack. For example, the acid trap can form a portion of a gas diffusion layer in the fuel cell stack. Alternatively, or in addition, the acid trap can form a portion of a flow field plate in the fuel cell stack.  
         [0029]     In another aspect, the invention features a method that includes directing a fluid composition exhausted from a fuel cell to flow through an acid trap, where the fluid composition includes an acid or a derivative of the acid, and the acid trap reduces a concentration of the acid or the derivative of the acid in the fluid composition.  
         [0030]     Embodiments of the method can include any of the following features.  
         [0031]     The acid can be phosphoric acid.  
         [0032]     The fluid composition can be directed to the fuel cell after the fluid composition has flowed through the acid trap.  
         [0033]     Embodiments may include one or more of the following advantages. For example, a filter (e.g., an acid trap) can reduce an amount of an undesirable compound (e.g., an acid) present in exhaust gases from a fuel cell system, thereby reducing undesirable emissions from a fuel cell system into the environment.  
         [0034]     Embodiments can also feature fuel cell systems with improved durability relative to comparable fuel cell systems that do not include a filter. Filters can reduce the amount of undesirable compounds/contaminants that are present within the fuel cell system, thereby reducing the amount of damage to the fuel cell system due to the compound. For example, corrosive impurities, such as acids, can degrade metal conduits used to transport gases, and can also degrade other metal and non-metal components of fuel cell systems such as flow channels, valves, and housings. In addition, some impurities can deposit on process catalysts used to enable various chemical reactions in fuel cell systems. For example, impurity materials can deposit on catalysts present in a reformer and used to convert fuel gas to reformate. The deposition of impurities on reformer catalyst surfaces can lead to accelerated deactivation or “poisoning” of the catalysts and less efficient operation of the fuel cell system. Using a filter, such as an acid filter, to reduce the amount of impurities in a fuel cell system can reduce these adverse affects, thereby prolonging the operational lifetime of the system or of components of the system.  
         [0035]     Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.  
         [0036]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will be apparent from the description, drawings, and claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0037]      FIG. 1  is a schematic diagram of an embodiment of a fuel cell system.  
         [0038]      FIG. 2  is a schematic diagram of an embodiment of an acid trap.  
         [0039]      FIG. 3  is a cross-sectional view of an embodiment of a fuel cell.  
         [0040]      FIG. 4  is a schematic diagram of another embodiment of a fuel cell system. 
     
    
       [0041]     Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0042]     Referring to  FIG. 1 , a fuel cell system  200  includes a fuel cell stack  202  (including one or more fuel cells), a reformer  204 , and a burner  236 . Fuel cell system  200  is configured so that fuel cell stack  202  applies a voltage across an external load  226 .  
         [0043]     Fluid flow between various components of fuel cell system  200  may be controlled using one or more regulators (not shown in  FIG. 1 ).  
         [0044]     During operation, a fuel (e.g., methane or methanol) enters fuel cell system  200  through a fuel inlet  208 . Inlet  208  directs the fuel to reformer  204 , which produces a reformate (e.g., a H 2 -rich reformate) from the fuel and directs the reformate to fuel cell stack  202  via a conduit  220 .  
         [0045]     A cathode gas (e.g., air) enters fuel cell stack  202  through an inlet line  224 . Inside the fuel cell stack  202 , the anode and cathode gases react, producing electrical power that flows through external load  226 . Fuel cell stack  202  also produces one or more chemical byproducts (e.g., water). The exhaust gas from the anode in fuel cell stack  202  exits fuel cell stack  202  through a conduit  228 , which directs the gas to a first acid trap  206 . The exhaust gas from the cathode in fuel cell stack  202  exits via a conduit  244 , which directs the gas to a second acid trap  207 .  
         [0046]     First and second acid traps,  206  and  207  reduce the concentration of acid (and/or a derivative of an acid) in the gases exhausted from fuel cell stack  202 . These components are discussed in more detail below. Gas exiting acid trap  206  is conveyed via a conduit  252  to a burner  236  which oxidizes the gas exhausted from the fuel cell stack anode before exhausting the gas via an exhaust conduit  258  into the environment. During operation, burner  236  draws air from through an inlet  238 . Gas exiting acid trap  207  is convey via a conduit  218  back to reformer  204 , where it is added to the reformate produced by reformer  204 .  
         [0047]     In general, the acid or acid derivative may come from a variety of sources in fuel cell system  200 . For example, in some embodiments, phosphoric acid and/or its chemical derivatives may leech out from ion exchange membranes in one or more of the fuel cells in fuel cell stack  202 . Sources of phosphoric acid and its derivatives are discussed below. These compounds can poison catalysts used in reformer  204  and can corrode conduits, fixtures, and other elements of system  200 . Further, these compounds can be vented to the environment surrounding fuel cell system  200  through vent  258 , with adverse health consequences for humans and other living entities.  
         [0048]     Either or both of the anode exhaust gas and the cathode exhaust gas can include concentrations of phosphoric acid and/or its derivatives that are higher than a determined concentration limit for the safe and reliable operation of system  200  (e.g., that reduce the operational lifetime of fuel cell system  200  due to corrosion, and/or exceed emissions standards). Acid traps  206  and  207  can be used to reduce a concentration of phosphoric acid and/or its derivatives in a gas stream to a levels that fall under these concentration limits.  
         [0049]     In general, the structure of acid traps  206  and  207  may vary as desired. Referring to  FIG. 2 , in some embodiments, acid trap  206  includes a container  302  having an influent conduit  304  and an effluent conduit  306 . A first flow portion  308  of filter  206  includes a first filter material  310 . A second flow portion  312  of filter  206  includes a second filter material  314 . Influent gas  316  is directed to flow into influent conduit  304 , through first filter material  310 , through second filter material  314 , and subsequently out of effluent conduit  306  as filtered gas  318 .  
         [0050]     Each of first filter material  310  and second filter material  314  include flow channels or pores. The channels or pores extend through the length of the material and are substantially oriented in a direction parallel to the flow of influent gas  316 . First filter material  310  includes channels having a mean cross-sectional diameter d 1  that is larger than a mean cross-sectional diameter d 2  of the channels in second filter material  314 . In certain embodiments, d 1 &gt;d 2 . For example, in some embodiments, d 1  can be about 1.5×d 2  or more (e.g., about 2×d 2  or more, about 3×d 2  or more, about 4×d 2  or more, about 5×d 2  or more, about 10×d 2  or more).  
         [0051]     Selected components in an influent gas stream generally adsorb onto the walls of the channels in the first and second filter materials. Once a monolayer of adsorbed component material covers the channel walls, further component material is adsorbed atop the already-deposited component material. The diameter of channel openings decreases as the build-up of component material on the walls of the channels increases, thereby reducing the flow capacity of the channels.  
         [0052]     Due to their larger mean channel diameter, the channels in first filter material  310  can adsorb relatively large quantities of one or more impurity components before gas flow through the channels is unduly restricted. In contrast, due to their smaller mean channel diameter, the channels in second filter material  314  can adsorb relatively small quantities of one or more impurity components before gas flow through the channels is unduly restricted. However, due to their smaller mean channel diameter, the channels in second filter material  314  collectively provide a larger surface area for adsorption of impurity components, and therefore more efficiently reduce a concentration of impurity components in an influent gas. Thus, the first material acts as a coarse filter, which reduces the contaminant concentration to the second filter material. The second filter material then acts as a fine-polish while remaining unclogged longer due to the lower feed contaminant concentration. Overall, the combination of the two materials provides longer life and better filtering characteristics than a single material can provide.  
         [0053]     Embodiments of acid traps generally use two or more filter materials to cooperatively reduce a concentration of one or more particular components in influent gas  316 . For example, first filter material  310 , due to its large adsorption capacity, functions as a “coarse” filter in order to adsorb a relatively large amount of one or more impurity components present in a relatively high concentration in influent gas  316  flowing through the channels of first filter material  310 . Passage through first filter material  310  generates an intermediate gas from influent gas  316 , where the intermediate gas has a concentration of one or more impurity components that is reduced by a relatively large amount compared with influent gas  316 . Second filter material  314 , due to its relatively large channel surface area and relatively small adsorption capacity, functions as a “fine” filter in order to adsorb a relatively small amount of one or more impurity components which are present in a relatively low concentration in the intermediate gas flowing through the channels of second filter material  314 . Passage through second filter material  314  generates filtered gas  318  from the intermediate gas, where filtered gas  318  has a concentration of one or more impurity components that is reduced by a relatively small amount compared with the intermediate gas. By using first filter material  310  to adsorb a relatively large amount of one or more impurity components, a concentration of these impurity components in filtered gas  318  can be reduced without severely impeding the flow of influent gas  316  through acid trap  206 . By using second filter material  314 , a concentration of the impurity components in filtered gas  318  can be reduced even further without clogging or obstructing the channels of second filter material  314  too severely. First filter material  310  and second filter material  314  can therefore be used cooperatively to provide the dual advantages of significantly reducing a concentration of one or more impurity components in influent gas  316 , and maintaining a flow rate of influent gas  316  that is sufficiently high so that operation of the fuel cell system is not impaired.  
         [0054]     As an example, first filter material  310  can include an extruded ceramic monolith material (available, for example, from Corning Inc., Corning, N.Y.) having about 150 cells per square inch (CPSI) or less (e.g., about 100 CPSI or less, about 75 CPSI or less, about 50 CPSI or less, about 25 CPSI or less). Second filter material  314  can include an extruded ceramic monolith material (also available from Corning) having about 250 CPSI or more (e.g., about 300 CPSI or more, about 350 CPSI or more, about 400 CPSI or more, about 500 CPSI or more, about 600 CPSI or more). The material can also be metallic monolith, which operates to absorb contaminants since contaminants would react with metals and thus begin the adsorption cake formation. Each of first filter material  310  and second filter material  314  can be provided in the form of a solid brick, e.g., a rectangular brick having dimensions 6″×6″ on an end face (oriented substantially perpendicular to a direction of flow of influent gas  316 ) and 12″ long (in a direction substantially parallel to a direction of influent gas flow). Other shape bricks, such as round or oval bricks, can also be used. Generally, the dimensions of the brick cross-sections can be free variables for design capacity. The brick length is typically a function of the filtering level desired. The two materials can be encased in a container  302  such as a steel canister, and positioned therein such that the channels in each of first filter material  310  and second filter material  314  are oriented substantially in a direction of flow of influent gas  316 . Further, first filter material  310  is positioned within container  302  in first flow portion  308  such that it is adjacent to influent conduit  304 , and second filter material  314  is positioned in second flow portion  312  adjacent to effluent conduit  306 . The two filter materials provide for sequential filtering of influent gas  316 . The flow portions are generally designed so that the flow velocity through the materials are relatively even throughout the cross-sectional area. Sufficient flow transition space can also be provided prior to the gas exiting into subsequent piping Typically, each material reduces the gas concentration of contaminants by an approximately fixed percentage per unit length. This percentage is approximately inversely proportional to flow rate, and approximately proportional to the surface area of the monolith. So, filtration at half flows or half-power of the fuel cell, would provide approximately twice the filtration level—so that the initial concentration may be reduced to 0.5% of the original concentration. At full flows (full power of the fuel cell), the concentration would then be reduced to about 1% of the original concentration. In some embodiments, first filter material  310  can reduce a concentration of one or more contaminants (e.g., acid or acid derivative) to about 20% or less (e.g., about 10% or less, about 5% or less, about 2% or less) of its initial concentration at full flow. In certain embodiments, second filter material  314  can reduce a concentration of one or more contaminants (e.g., acid or acid derivative) to about 2% or less (e.g., about 1% or less, about 0.5% or less, about 0.1% or less) of its initial concentration at full flow. In certain embodiments, second filter material  314 . In combination, both filter material  310  and filter material  314  can provide a contamination reduction to about 0.5% or less (e.g., about 0.2% or less, about 0.1% or less, about 0.05% or less, about 0.02% or less) of the initial contaminant concentration in influent gas  316  at full flow.  
         [0055]     Monolithic filter materials, such as the ceramic monoliths discussed above, used in combination can provide a number of advantages with respect to more conventional pelletized adsorbents or single monolithic adsorbent materials. First, monolithic materials generally do not impede the flow of influent gas as strongly as pelletized materials, due to the presence of channels in the structure of monolithic materials. As a result, the pressure drop introduced by an acid trap based on a combination of monolithic materials is generally less than the pressure drop introduced by a filter having a similar adsorptive capacity and based on a pelletized adsorbent such as alumina pellets or extrudates. For example, the difference between the pressure of influent gas  316  and filtered gas  318  introduced by acid trap  206  can be about 5 mbar or less (e.g., about 3 mbar or less, about 1 mbar or less). As the assembly becomes saturated, the pressure drop may increase.  
         [0056]     Second, monolithic materials generally provide a larger available surface area for adsorption of components in an influent gas than is provided by a bed of pelletized absorbent. For example, an acid trap  206  constructed as described above, including coarse and fine monolithic ceramic adsorbents, may provide a large adsorbent surface area, so that about 50% or more (e.g., about 60% or more, about 70% or more, about 80% or more) of the volume of the adsorbent is available for adsorbing one or more components from the influent gas. By contrast, using a similar volume of a pelletized alumina adsorbent provided in an adsorbing bed, only about 35% of the volume of the adsorbent material may be available for adsorbing components from the influent gas.  
         [0057]     Third, the use of two monolithic materials can increase the usable lifetime of the filter, relative to the usable lifetime of a filter having a single monolithic filter material. For example, in fuel cell systems, a cathode exhaust gas leaving a fuel cell stack may include concentrations of a component such as phosphoric acid (and/or one or more of its chemical derivatives) of parts per million (ppm). In order to ensure safe and reliable operation of the fuel cell system, the concentration of phosphoric acid may need to be reduced to less than 30 ppb before the cathode exhaust gas is combined with reformer oxidant gas and directed into a fuel reformer. In order to reduce the concentration of phosphoric acid to less than 30 ppb, a relatively fine monolithic material may be required (e.g., having about 300 CPSI or more). However, given the relatively high initial concentration of phosphoric acid in the cathode exhaust gas, the pores in a fine monolithic material may become rapidly obstructed with adsorbed phosphoric acid, impeding the flow of influent gas through the filter material, and necessitating replacement of the monolithic material with fresh filter material having unobstructed channels.  
         [0058]     A filter material that includes both coarse and fine monolithic adsorbents can have a significantly longer lifetime. The coarse adsorbent material can be used to adsorb a relatively large amount, such as about 80% or more (e.g., about 90% or more, about 95% or more, about 97% or more) of the phosphoric acid present in an influent gas. The fine adsorbent material can then be used in sequential fashion to adsorb about 90% or more (e.g., about 95% or more) of the remaining phosphoric acid in the influent gas. Due to the action of the coarse adsorbent material, the amount of phosphoric acid adsorbed by the fine adsorbent material for a given flow rate of influent gas is much less than the amount of phosphoric acid adsorbed by a fine adsorbent material acting alone, and therefore a filter material that includes both coarse and fine adsorbents can have a significantly longer lifetime.  
         [0059]     Embodiments of acid trap  206  can also include other monolithic adsorbent materials. For example, in addition or as alternatives to the ceramic materials discussed above, filter  206  can include corrugated metal monolithic adsorbent materials (available, for example, from Johnson Matthey PLC, London, UK). Metal monolithic materials can include flow channels or passages for gas flow that provide for even less obstruction than the channels in ceramic monoliths, and may therefore contribute an even smaller pressure drop when a filter  206  that includes these materials is incorporated into a fuel cell system. Both coarse and fine metal monoliths can be used in combination in acid trap  206  to provide the same advantages as those associated with ceramic monoliths. In other embodiments, for example, acid trap  206  can include materials such as zeolites, activated carbon, silica, and other porous materials. Filter  206  can further include one or more adsorbent materials coated on channel walls in porous materials such as ceramic monoliths or foams. For example, filter  206  can include a ceramic monolith having adsorptive surfaces coated with particles of activated carbon or silica in order to further enhance the adsorptive capability of the filter and/or in order to preferentially adsorb a particular component in an influent gas.  
         [0060]     As shown in  FIG. 2 , acid trap  206  includes a first filter material  310  and a second filter material  314 . In general, however, embodiments of filter  206  can include more than two filter materials. For example, filter  206  can include three or more filter materials (e.g., four or more filter materials, five or more filter materials, ten or more filter materials) in order to reduce a concentration of one or more components in an influent gas to a desired concentration in effluent conduit  306  of the filter, and in order to extend the useful operating lifetime of the filter in a fuel cell system. In some embodiments, a filter includes three filter portions each formed from a different material. The third portion includes channels with a mean cross-sectional diameter d 3 . In some cases, d 3 &lt;d 2 . Alternatively, d 3 =d 2 . The third filter portion can include a material that is coated for very fine filtration of trace contaminants.  
         [0061]     In general, acid trap  206  is positioned to receive a gas such as cathode exhaust gas and to reduce a concentration of one or more components in the gas. In some embodiments, such as the embodiment of  FIG. 2  for example, filter  206  can be positioned to receive a mixture of reformer oxidant gas and cathode exhaust gas, and may filter this mixture of gases to remove one or more components such as phosphoric acid and/or one or more of its chemical derivatives. Due to the poisoning effects of phosphoric acid with respect to reformer catalysts, acid trap  206  is generally positioned upstream from reformer  204  along a gas flow path in a fuel cell system, in order to reduce a concentration of phosphoric acid that is introduced into the reformer.  
         [0062]     Acid trap  207  may be the same or different than acid trap  206 .  
         [0063]     Referring to  FIG. 3 , an embodiment of a fuel cell  100  includes a cathode flow field plate  102 , an anode flow field plate  104 , a membrane electrode assembly  106  having an ion exchange membrane  108 , cathode catalyst layer  110 , and anode catalyst layer  112 . Gas diffusion layers  114  and  116  separate membrane electrode assembly  106  from flow field plates  102  and  104 . During operation of the fuel cell, anode gas is directed to flow through channels  120  in anode flow field plate  104  and cathode gas is directed to flow through channels  118  in cathode flow field plate  102 . The anode gas passes through anode gas diffusion layer  116  and interacts with anode catalyst layer  112 . The anode catalyst catalyzes the conversion of anode gas to reaction intermediates. For example, an anode gas including hydrogen gas can be converted to protons and electrons. The cathode gas passes through cathode gas diffusion layer  114  and interacts with cathode catalyst layer  110 . The cathode catalyst catalyzes the conversion of cathode gas to a chemical product of the fuel cell reaction. For example, for an anode gas including hydrogen and a cathode gas including oxygen, the chemical product of the fuel cell reaction can be water.  
         [0064]     Ion exchange membrane  108  provides a barrier to the flow of electrons and gases from one side of membrane electrode assembly  106  to the other. However, membrane  108  allows ionic reaction intermediates, such as protons, to flow from the anode side of the membrane electrode assembly to the cathode side. In order to provide this selectivity, ion exchange membrane can include significant quantities of phosphoric acid and its chemical derivatives (e.g., dihydrogen phosphate anions, H 2 PO 4   − , monohydrogen phosphate anions, H 2 PO 4   2− , phosphate anions, PO 4   − , and the like). The relatively large numbers of negative charges in these substances assist the flow of positively charged ions, such as protons, from the anode side of membrane electrode assembly  106  to the cathode side through ion exchange membrane  108 . In contrast, the negative charges assist in preventing the flow of negatively charged species such as electrons and neutral species such as anode and cathode gases through membrane  108 .  
         [0065]     During operation, phosphoric acid and its chemical derivatives may leech out from ion exchange membrane  108  and be combined with gases flowing out of fuel cell  100  from either or both of the anode and cathode sides.  
         [0066]     While an embodiment of a fuel cell system is described above, in general, other configurations are also possible. For example, in some embodiments, acid trap  207  can be used to filter fuel, in addition to filtering exhaust gas from fuel cell stack  202 . For example,  FIG. 4  is a schematic diagram of an embodiment of a fuel cell system  400  that is similar to fuel cell system  200 . In this embodiment fuel is passed through acid trap  207  and combined with exhaust gas from the cathode of fuel cell stack  202 . The filtered fuel mixture is directed to reformer  204  via conduit  410 .  
         [0067]     In some embodiments, the fuel gas is at a lower temperature than the exhaust gas returning from fuel cell stack  202 , and the temperature difference is used to promote precipitation of one or more components such as phosphoric acid in the gas mixture onto the walls of the channels in acid trap  207 . By regulating the temperature of the fuel gas, it is possible in some embodiments to adjust the precipitation rate of one or more components of the gas mixture.  
         [0068]     In the embodiments shown, the acid traps  206  and  207  are shown being distinct components of the fuel cell systems. In general, however, the acid trap can also be physically combined with other components of a fuel cell system. For example, an acid trap can be incorporated as a portion of either or both of anode gas diffusion layer  116  and cathode gas diffusion layer  114 . Either or both of these gas diffusion layers can have an outer portion comprising coarse and fine filter materials, with channels therein aligned substantially in a direction of gas flow through the diffusion layers. In other embodiments, for example, the acid trap materials can be combined with channels in either or both of the anode and cathode flow field plates in order to reduce concentrations of components such as phosphoric acid and/or its chemical derivatives that leech out from the proton exchange membrane of a fuel cell.  
         [0069]     A number of embodiments have been described. Other embodiments are within the scope of the following claims.