Patent Publication Number: US-2005130017-A1

Title: Fuel cell plate

Description:
TECHNICAL FIELD  
      The invention relates to fuel cell plates, and related systems and methods.  
     BACKGROUND  
      A fuel cell can convert chemical energy to electrical energy by promoting electrochemical reactions of two reactants.  
      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 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.  
      Each reactant 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 reactants to the membrane electrode assembly.  
      The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane) between a first catalyst and a second catalyst. One diffusion layer is between the first catalyst and the anode flow field plate, and another diffusion layer is between the second catalyst and the cathode flow field plate.  
      During operation of the fuel cell, one of the reactants (the anode reactant) 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 reactant (the cathode reactant) 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.  
      As the anode reactant flows through the channels of the anode flow field plate, some of the anode reactant passes through the anode diffusion layer and interacts with the anode catalyst. Similarly, as the cathode reactant flows through the channels of the cathode flow field plate, some of the cathode reactant passes through the cathode diffusion layer and interacts with the cathode catalyst.  
      The anode catalyst interacts with the anode reactant to catalyze the conversion of the anode reactant to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode reactant and the anode reaction intermediates to catalyze the conversion of the cathode reactant to the chemical product of the fuel cell reaction.  
      The chemical product of the fuel cell reaction flows through a 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.  
      The electrolyte provides a barrier to the flow of the electrons and reactants 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.  
      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 and to the cathode flow field plate, and the cathode side of the membrane electrode assembly.  
      Because electrons are formed at the anode side of the membrane electrode assembly, the anode reactant undergoes oxidation during the fuel cell reaction. Because electrons are consumed at the cathode side of the membrane electrode assembly, the cathode reactant undergoes reduction during the fuel cell reaction.  
      For example, when molecular hydrogen and molecular oxygen are the reactants used in a fuel cell, the molecular hydrogen flows through the anode flow field plate and undergoes oxidation. The molecular 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) 
 
1/2O 2 +2H + +2e − →H 2 O   (2) 
 
H 2 +1/2O 2 →H 2 O   (3) 
 
      As shown in equation 1, the molecular 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 the molecular oxygen to form water. Equation 3 shows the overall fuel cell reaction.  
      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.  
      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.  
      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  
      In one aspect, the invention features an article that includes a fuel cell flow plate having a sulfonic acid moiety covalently bonded thereto. The sulfonic acid moiety has the formula RSO 3 H, where R is /, an alkyl moiety, an alkenyl moiety, an alkynyl moiety, an aryl moiety, or a heteroaryl moiety.  
      In another aspect, the invention features a fuel cell that includes two fuel cell flow plates and an electrolyte between the fuel cell flow plates. A sulfonic acid moiety is covalently bonded to at least one of the fuel cell flow plates. The sulfonic acid moiety has the formula RSO 3 H, where R is /, an alkyl moiety, an alkenyl moiety, an alkynyl moiety, aryl moiety, or a heteroaryl moiety.  
      In a further aspect, the invention features an article that includes a fuel cell flow plate and an acidic moiety covalently bonded to the fuel cell flow plate. The acidic moiety has the formula R—X. R is /, an alkyl moiety, an alkenyl moiety, an alkynyl moiety, an aryl moiety, or a heteroaryl moiety. X is selected from SO 3 H, PO 3 H 2 , AsO 3 H 2  and COOH.  
      As used herein, the symbol “/” refers to a direct bond between the sulfur atom in the sulfonic acid moiety and the fuel cell flow plate.  
      As used herein, the term “alkyl moiety” refers to an unsaturated hydrocarbon chain that may be a straight chain or branched chain. In general, the number of carbon atoms in an alkyl moiety can be varied as desired (e.g., one to ten carbon atoms, one to six carbon atoms, one to three carbon atoms). An alkyl moiety can be substituted (e.g., substituted with one or more halogens) or unsubstituted.  
      As used herein, the term “halogen” refers to any radical of fluorine, chlorine, bromine or iodine.  
      As used herein, the term “alkenyl moiety” refers to a hydrocarbon chain having at least one carbon-carbon double bond. In general, the number of carbon atoms in an alkenyl moiety can be varied as desired (e.g., two to ten carbon atoms, two to six carbon atoms, two to three carbon atoms). An alkenyl moiety can be substituted (e.g., substituted with one or more halogens, substituted with one or more alkyl) or unsubstituted.  
      As used herein, the term “alkynyl moiety” refers to a hydrocarbon chain having at least one carbon-carbon triple bond. In general, the number of carbon atoms in an alkynyl moiety can be varied as desired (e.g., two to ten carbon atoms, two to six carbon atoms, two to three carbon atoms). An alkynyl moiety can be substituted (e.g., substituted with one or more halogens, substituted with one or more alkyl) or unsubstituted.  
      As used herein, the term “aryl” refers to a carbon-containing moiety having at least one aromatic ring. For example, an aryl moiety can contain at least one 6-carbon monocyclic aromatic ring and/or at least one 10-carbon bicyclic aromatic ring system. The atoms in the ring of an aryl moiety can be substituted (e.g., substituted with one or more halogens, substituted with one or more alkyl) or unsubstituted.  
      As used herein, the term “heteroaryl” refers to a carbon-containing moiety that has at least one aromatic ring with at least one non-carbon atom (e.g., O, S, N) in the ring. Examples of heteroaryls include: aromatic 5-8 membered monocyclic rings with at least one O, S and/or N in the ring; 8-12 membered bicyclic rings with at least one O, S and/or N in the ring; and 11-14 membered tricyclic rings with at least one O, S and/or N in the ring. Generally, for monocyclic ring systems, the number of non-carbon atoms in the ring is one, two or three; for bicyclic rings, the number of non-carbon atoms in the ring is one, two, three, four, five or six; for tricyclic rings, the number of non-carbon atoms in the ring is one, two, three, four, five, six, seven, eight or nine.  
      Embodiments can have one or more of the following features.  
      In some embodiments, R is / (i.e., the sulfonic acid moiety is directly bonded to the fuel cell flow plate).  
      In certain embodiments, R is an alkyl moiety that is substituted with one or more halogens.  
      In some embodiments, R is an aryl moiety that is substituted with one or more halogens and/or one or more alkyl moieties.  
      In certain embodiments, the fuel cell flow plate is formed of carbon (e.g., graphite).  
      In some embodiments, the hydrophilicity of the article is greater than the hydrophilicity of the fuel cell flow plate.  
      In certain embodiments, the fuel cell flow plate is porous. In such embodiments, the sulfonic acid moiety can be present in pores of the fuel cell flow plate.  
      In some embodiments, the fuel cell flow plate is non-porous.  
      In certain embodiments, the fuel cell flow plate is a bipolar flow plate, a monopolar plate or a coolant plate.  
      In some embodiments, at least about 90% of the surface area of the fuel cell flow plate has the sulfonic acid moiety covalently bonded thereto.  
      In certain embodiments, the surface of the flow-field channel portion of the fuel cell flow plate is covalently bonded to the sulfonic acid moiety.  
      In some embodiments, the surface of the land portion of the fuel cell flow field plate is substantially devoid of the sulfonic acid moiety.  
      In certain embodiments, at least a portion of the surface of the article has an initial contact angle with water that is less than about 75°.  
      In some embodiments, at least a portion of the surface of the article has an initial contact angle with diiodomethane that is less than about 40°.  
      In certain embodiments, at least a portion of a surface of the article has an initial contact angle with water that is at least about 15° less than an initial contact angle with water of the fuel cell flow plate.  
      In some embodiments, at least a portion of a surface of the article has an initial contact angle with water that is at least about 20% less than an initial contact angle with water of the fuel cell flow plate.  
      In some embodiments, the fuel cell is a proton-exchange-membrane fuel cell or a direct-feed liquid fuel cell (e.g., a direct alcohol fuel cell, such as a direct methanol fuel cell).  
      Embodiments can provide one or more of the following advantages.  
      In some embodiments, the fuel cells exhibit reduced flooding during use. Without wishing to be bound by theory, it is believed that the flooding may be reduced because the sulfonic acid moiety can increase the hydrophilicity of portions of a fuel cell flow plate that are in contact with water. It is believed that the increased hydrophilicity can cause water to spread more easily on the portions of the fuel cell flow plate in contact with the water, thereby increasing the surface area of the water and allowing for quicker evaporation of the water.  
      In certain embodiments, the sulfonic acid moiety is short enough so that the sulfonic acid moiety does not substantially impede the-flow of electrons within the fuel cell.  
      In some embodiments, the presence of the sulfonic acid moiety can enhance the durability of the fuel cell flow plate, thereby increasing the useful life of the fuel cell flow plate and a fuel cell having the fuel cell flow plate incorporated therein.  
      Other features and advantages will be apparent from the description, drawings and from the claims. 
    
    
     DESCRIPTION OF DRAWINGS  
       FIG. 1  is a cross-sectional view of an embodiment of a fuel cell.  
       FIG. 2  is an elevational view of an embodiment of an anode flow field plate.  
       FIG. 3  is an elevational view of an embodiment of a cathode flow field plate  
       FIG. 4  is an elevational view of an embodiment of a coolant flow field plate  
       FIG. 5   a  is a diagram of the location on a fuel cell flow plate where contact angle measurements were obtained.  
       FIG. 5   b  is a pictorial showing the contact angle θ of a water drop on a surface of a fuel cell flow plate.  
       FIG. 6   a  is a picture of an unsulfonated composite graphite plate taken using scanning electron microscopy.  
       FIG. 6   b  is a picture of a sulfonated composite graphite plate taken using scanning electron microscopy.  
       FIG. 7   a  is a plot of an X-ray emission analysis of an unsulfonated composite graphite plate.  
       FIG. 7   b  is a plot of an X-ray emission analysis of an sulfonated composite graphite plate. 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  shows a fuel cell  100  that includes a cathode flow field plate  110 , an anode flow field plate  120 , an electrolyte  130 , catalysts  140  and  150 , and diffusion layers  160  and  170 .  
      Cathode flow field plate  110  is made from a carbon containing material (e.g., graphite, such as porous graphite or non-porous graphite), and surfaces  111  of open-face channels  116  of cathode flow field plate  110  have covalently bonded thereto sulfonic acid moieties (RSO 3 H), where R is /, an alkyl moiety, an alkenyl moiety, an alkynl moiety, aryl moiety, or a heteroaryl moiety.  
      In some embodiments, a sulfonic acid moiety can be directly covalently bonded to surface  111  (R is /).  
      In certain embodiments, a sulfonic acid moiety can be indirectly covalently bonded to surface  111 . In such embodiments R represents a linker moiety that is present between surface  111  and the sulfur atom in the sulfonic acid moiety. In general, a linker moiety can be any moiety capable of covalently bonding —SO 3 H to surface  111 . In embodiments in which plate  110  is formed of carbon, the structure can be represented by C-linker-SO 3 H. Examples of linker moieties include alkyl moieties (e.g., C 1 -C 10  alkyl, C 1 -C 6  alkyl, C 1 -C 3  alkyl), alkenyl moieties (e.g., C 2 -C 10  alkenyl, C 2 -C 6  alkenyl, C 2 -C 3  alkenyl), alkynyl moieties (e.g., C 2 -C 10  alkynyl, C 2 -C 6  alkynyl, C 2 -C 3  alkynyl), aryl moieties, and heteroaryl moieties. In some instances, multiple linker moieties can be used with one or more heteroatoms, such as, for example, O, N, S, P, or halogen, between linker moieties (e.g., alkyl-O-alkyl, alkyl-S-alkyl, etc.).  
      A sulfonic acid group can be covalently bonded to surface  111  in a variety of ways.  
      In some embodiments, a sulfonic acid group can be directly covalently bonded to surface  111  by reacting one or more surface carbons of surface  111  with chlorosulfonic acid as depicted in the reaction scheme below: 
 
C—H+ClSO 3 H→C—SO 3 H+HCl. 
 
      The degree of sulfonation can generally be controlled by modifying appropriate reaction conditions, such as, for example, the length of reaction time, reaction temperature, concentration of reagents, and solvent. As an example, if a high degree of sulfonation is desired, surface  111  can be treated with neat or concentrated chlorosulfonic acid. As another example, if a lower degree of sulfonation is desired, surface  111  can be treated with dilute chlorosulfonic acid (e.g., chlorosulfonic acid diluted in an organic solvent such as dichloromethane or acetic acid).  
      Other reagents can also be used to covalently bond a sulfonic acid moiety to a carbon containing cathode flow field plate  110 . For example, cathode flow field plate  110  can be treated with sulfuric acid (e.g., fuming sulfuric acid), chloromethylsulfonic acid, or other chemical species that can react with surface  111  to provide a sulfonic acid group covalently bonded to surface  111 .  
      In general, the presence of the sulfonic acid groups on the surface of plate  110  increases the hydrophilicity of plate  110 . As an example, in some embodiments, the initial contact angle of a cathode flow field plate with water after covalently bonding the sulfonic acid moiety is at least about 20% less (e.g., at least about 25% less, at least about 30% less) than the initial contact angle of the cathode flow field plate with water before covalently bonding the sulfonic acid moiety. As another example, in certain embodiments, the initial contact angle of a cathode flow field plate  110  with water after covalently bonding the sulfonic acid moiety is at least about 15° less (e.g., at least about 20° less, at least about 25° less) than the initial contact angle of the cathode flow field plate with water before covalently bonding the sulfonic acid moiety.  
      In some embodiments, the portion of plate  110  having the sulfonic acid covalently bonded thereto has an initial contact angle with water that is less than about 75° (e.g., less than about 70°, less than about 65°, less than about 62°). The contact angle of the surface of an article with water is measured as described below.  
      In certain embodiments, the portion of plate  110  having the sulfonic acid covalently bonded thereto has an initial contact angle with diiodomethane that is less than about 40° (e.g., less than about 35°, less than about 30°, or less than about 28°). The contact angle of the surface of an article with diiodomethane is measured as described below with respect to measuring the contact angle of an article with water, except that diiodomethane is substituted for the water in the measurement.  
      Typically, anode flow field plate  120  is formed of a carbon material (e.g., graphite, such as porous graphite or nonporous graphite).  
      Electrolyte  130  should be capable of allowing ions to flow therethrough while providing a substantial resistance to the flow of electrons. In some embodiments, electrolyte  130  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, electrolyte  130  can also be prepared from the commercial product GORE-SELECT, available from W.L. Gore &amp; Associates (Elkton, Md.).  
      Catalyst  140  can be formed of a material capable of interacting with hydrogen to form protons and electrons. Examples of such materials include, for example, platinum, platinum alloys, such as platinum-ruthenium, and platinum dispersed on carbon black. Catalyst  140  can further include an electrolyte, such as an ionomeric material, e.g., NAFION, that allows the anode to conduct protons. Alternatively, a suspension is applied to the surfaces of diffusion layers (described below) that face electrolyte  130 , and the suspension is then dried. In some embodiments, a catalyst material (e.g., platinum) can be applied to electrolyte  130  using standard techniques. The method of preparing catalyst  140  may further include the use of pressure and temperature to achieve bonding.  
      Catalyst  150  can be formed of a material capable of interacting with oxygen, electrons and protons to form water. Examples of such materials include, for example, platinum, platinum alloys, and noble metals dispersed on carbon black. Catalyst  150  can further include an electrolyte, such as an ionomeric material, e.g., NAFION, that allows the cathode to conduct protons. Catalyst  150  can be prepared as described above with respect to catalyst  140 .  
      In general, diffusion layers  160  and  170  are electrically conductive so that electrons can flow from catalyst  140  to flow field plate  120  and from flow field plate  110  to catalyst  150 . GDLs can be formed of a material that is both gas and liquid permeable. It may also be desirable to provide the GDLs with a planarizing layer, for example, by infusing a porous carbon cloth or paper with a slurry of carbon black followed by sintering with a polytetrafluoroethylene material. Suitable GDLs are available from various companies such as Etek in Somerset, N.J., SGL in Valencia, Calif., and Zoltek in St. Louis, Mo.  
       FIG. 2  shows an embodiment of cathode flow field plate  110 , which is used to provide a flow path that allows the molecular oxygen to interact with catalyst  150  during use of fuel cell  100 . Cathode flow field plate  110  has an inlet  112 , an outlet  114  and open-faced channels  116  that define a flow path for an oxidant from inlet  112  to outlet  114 . As the oxidant flows along channels  116 , the molecular oxygen contained in the oxidant permeates diffusion layer  170  to interact with catalyst  150 , electrons and protons to form water. The water can pass back through diffusion layer  170 , enter the oxidant stream in channels  116 , and exit fuel cell  100  via outlet  114 .  
       FIG. 3  shows an embodiment of anode flow field plate  120 , which is designed to provide a flow path for a fuel that allows the molecular hydrogen to interact with catalyst  140  during use of fuel cell  100 . Anode flow field plate  120  has an inlet  222 , outlet  224  and open-faced channels  226  that define a flow path for a fuel from inlet  222  to outlet  224 . The protons pass through solid electrolyte  130 , and the electrons are conducted through diffusion layer  160  to anode flow field plate  120 , ultimately flowing through an external load to cathode flow field plate  110 . The unreacted fuel exits fuel cell  100  via outlet.  
      The heat produced during the fuel cell reaction can be removed by flowing a coolant through the fuel cell via a coolant flow field plate.  FIG. 4  shows an embodiment of a coolant flow field plate  300  having an inlet  310 , an outlet  320  and open-faced channels  330  that define a flow path for coolant from inlet  310  to outlet  320 . The coolant enters fuel cell  100  via inlet  310 , flows along channels  330  and absorbs heat, and exits fuel cell  100  via outlet  320 .  
      The following examples are illustrative and not intended to be limiting.  
     EXAMPLES  
      A piece of graphite composite plate (2.0 cm×11.0 cm) was sulfonated under the following conditions: 5 times dilution of chlorosulfonic acid by dichloromethane, 5-minute contact time between this diluted mixture and the plate at room temperature.  
      The contact angle measurement was performed using a Rame-Hart Goniometer Model # 100-00 by following ASTM D724-99: standard test method for surface wettability of paper. The plate was measured using three water drops purposely applied onto the top right corner, middle center, and bottom left corner (See  FIG. 5   a ). Each angle was an average of the left contact (θ 1 ) and right contact (θ 2 ) of the water drop with the plate (See  FIG. 5   b ).  
      Table 1 shows results averaged from the three different locations for the graphite composite plate before and after sulfonation at different times. Initial refers to a measurement. that was taken less than one minute from the moment the water drop was laid onto the plate. Two subsequent measurements were taken at 5 and 15 minutes, respectively.  
               TABLE 1                          Measurements of Contact Angle                         Contact Angle (°)                                     Plate       5 minutes   15 minutes           Conditions   Initial   later   later                                                 Unsulfonated   85   77   57           Sulfonated   60   48   6                      
 
      The initial contact angles were 85° and 60° for the original and sulfonated plates, respectively, demonstrating that covalently bonding the sulfonic acid moiety increased the hydrophilicity of the surface. While the contact angles decreased with time for both sulfonated and unsulfonated plates, the decrease was greater for the sulfonated plate.  
      Scanning Electron Microscopy (SEM) was performed using a JEOL 6400 SEM with an IXRF 500 EDS Digital Processor to examine if the surface morphology of the plate was changed after sulfonation. Referring to  FIGS. 6   a  and  6   b,  the sulfonated plate appeared rougher than the unsulfonated counterpart. Without wishing to be bound by theory, it is believed that a possible explanation for the change in surface morphology is that some the resin at the surface of the plate was modified or destroyed by chlorosulfonic acid.  
      The linkage of —SOH 3  onto the plate was confirmed by X-ray emission analysis. A much larger S peak (about 10 times larger) was observed for the sulfonated plate than for the unsulfonated counterpart, as shown by the spectra in  FIGS. 7   a  and  7   b  and the data in Tables 2 and 3 below.  
               TABLE 2                          Unsulfonated Composite Graphite Plate                                                 Intensity                   Element   Line   (counts/s)   Atomic %                                                     C   Ka   691.77   98.529               F   Ka   11.99   1.376           S   Ka   10.09   0.094                       100.000   Total                      
 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                   
               
               
                 Sulfonated Composite Graphite Plate 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Intensity 
                   
                   
               
               
                   
                 Element 
                 Line 
                 (counts/s) 
                 Atomic % 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 C 
                 Ka 
                 608.11 
                 98.878 
                   
               
               
                   
                 F 
                 Ka 
                 2.73 
                 0.270 
               
               
                   
                 S 
                 Ka 
                 107.50 
                 0.852 
               
               
                   
                   
                   
                   
                 100.000 
                 Total 
               
               
                   
                   
               
            
           
         
       
     
      While certain embodiments have been described, other embodiments are possible.  
      As an example, while covalently bonding a sulfonic acid moiety has been described with respect to the surfaces of the channel portions of a cathode flow field plate, other portions of a cathode flow field plate can also have a sulfonic acid moiety covalently bonded thereto. In certain embodiments (e.g., when the flow field plate is formed of a porous material, such as porous graphite), a sulfonic acid moiety may be covalently bonded within pores in the plate. In some embodiments, surfaces  117  of the lands of the cathode flow field plate can have a sulfonic acid moiety covalently bonded thereto. It is to be noted that, in such embodiments, the sulfonic acid moiety does not substantially impede the flow of electrons within the fuel cell. In some embodiments, at least about 90% (e.g., at least about 95%, at least about 97%) of the surface area of a cathode flow field plate has sulfonic acid moiety covalently bonded thereto.  
      As another example, while covalently bonding a sulfonic acid moiety has been described with respect to cathode flow field plates, a sulfonic acid moiety can also be covalently bonded to an anode flow field plate and/or a coolant plate.  
      As a further example, while flow field plates have been described as being formed of carbon materials, other materials can also be used.  
      As an additional example, while proton exchange fuel cells have been described, other fuel cells can also be used. Examples of fuel cells include solid oxide fuel cells and direct-feed liquid fuel cells. Examples of direct-feed liquid fuel cells include direct alcohol fuel cells, such as direct methanol fuel cells, direct ethanol fuel cells and direct isopropanol fuel cells.  
      In some embodiments, the acidic moiety has the formula R—X, where R is as described above, and X is an acidic moiety. Examples of such acidic moieties include SO 3 H, PO 3 H 2 , AsO 3 H 2  and COOH.  
      The fuel cells can be used in a variety of applications, including, for example, in automobiles or stationary systems (e.g., systems designed to power a home).  
      Other embodiments are in the claims.