Patent Publication Number: US-2010119911-A1

Title: Liquid electrolyte fuel cell having high permeability wicking to return condensed electrolyte

Description:
TECHNICAL FIELD 
     Liquid electrolyte fuel cells include fine pore, high permeability wicking between each electrode substrate and the separator plate on the anode side and/or the cathode side to enhance transfer of condensed electrolyte from a condensation zone back through the remainder of the cell on the respective side. 
     BACKGROUND ART 
     There are two approaches to providing acid to a phosphoric acid fuel cell to replenish acid loss with time, due to evaporation into the reactant streams. There are known acid addition approaches where acid in a liquid or vapor form is continuously or periodically added to the cells. These approaches are complicated and expensive. The more preferred approach is a passive approach where sufficient acid to satisfy the life requirements of the cell is incorporated into porous components in the cell during the initial assembly of the cell. 
     Conventional phosphoric acid fuel cell power plants typically comprise stacks  7  of fuel cells  8 , as shown in  FIG. 1 , the temperature of the fuel cells being controlled by a coolant that passes through cooler plates  9  interposed between groups of between five and ten fuel cells. Referring to  FIG. 2 , each fuel cell  8  comprises an acid retaining matrix  11  having anode catalyst  12  on one side and cathode catalyst  13  on the other side. The catalysts are respectively supported by a porous anode substrate  16  and a porous cathode substrate  17 . Porous anode substrate  16  and porous cathode substrate  17  are hydrophilic as is known in the art. The fuel cells (except at the ends of the stack or adjacent to cooler plates) share non-porous, hydrophobic separator plate assemblies  19  which include fuel channels  20  adjacent the anode substrate  16  and air (or other oxidant) channels  21  adjacent the cathode substrate  17 . The reactant gases in the channels  20 ,  21  diffuse through respective substrates  16 ,  17 ; hence the reference to gas diffusion layers (GDLs). Adjacent a cooler plate  9 , the fuel flow channels  20  may be formed in a fuel flow field plate  23  which does not have air flow channels therein; similarly for the cathode side. 
     The terms “non-porous” and “hydrophobic”, as used herein with respect to the separator plates  19 , mean that the separator plates  19  are sufficiently non-porous and hydrophobic so that substantially no liquid electrolyte penetrates the separator plates. 
     As shown in  FIG. 2 , the traditional phosphoric acid fuel cell has a substrate  16  adjacent the anode catalyst  12  which is of substantially the same thickness as the substrate  17  adjacent the cathode catalyst  13 . However, the anode substrate may be thicker than the cathode substrate, as disclosed by Breault in PCT/US06/42495, filed Oct. 27, 2006. 
     In normal operation of a liquid electrolyte fuel cell stack, electrolyte is evaporated into both of the reactant gas streams, as the reactant flows from the inlet to the exit. In order to retain acid for an extended life of the fuel cell power plant, the condensation of vaporized liquid electrolyte is accomplished near the exit of the reactant gas, so as to recover substantially all of the electrolyte. 
     In U.S. Pat. No. 4,345,008, retention of the liquid electrolyte is improved significantly by provision of a condensation zone to recover electrolyte vapor that has evaporated into one or both of the reactant gas flows. 
     Referring to  FIG. 3 , an exemplary fuel cell power plant  6  has a stack  7  which includes fuel cells  8 , each having a condensation zone  27 . In  FIG. 3 , the dotted line demarcates the extent of the catalyst  12 ,  13  and the dash lines demarcate the three groups of fuel flow channels through which the fuel flows in succession. Therein, the matrix  11  extends throughout the overall planform  28 , but the catalysts  12 ,  13  extend over only a portion of the overall planform forming an active area  29 , leaving an inactive area in the remainder of the overall planform, which constitutes an acid condensation zone  27 . 
     Alternatively, the anode catalyst may extend over the whole planform while the cathode catalyst  13  extends over only a portion of the planform, as disclosed by Breault et al in WO2006071209A1. 
     In the example of  FIGS. 1-3 , the fuel cell power plant includes a source of fuel  30  applied through a fuel inlet manifold  31  to fuel flow fields ( 20 ,  FIG. 2 ), the fuel flowing through a portion of each fuel cell to the right as shown in  FIG. 3  to a turn manifold  32  and then flowing to the left as shown in  FIG. 3 . Then the fuel flows through a second turn manifold  32  and to the right through the remaining portion of each of the fuel cells to a fuel exit manifold  33 , where the fuel flows out, to either a fuel recycle arrangement, fuel processing, or ambient. 
     The fuel cell power plant  25  also includes a pump  37  for causing an oxidant-containing gas such as air to flow from an air inlet manifold  38  through all of the fuel cells to an air exit manifold  39 . The air may then be provided to further processing, such as an enthalpy exchange device, fuel processing apparatus, or ambient. The condensation zone  27  coincides with the last pass of fuel through the cells, and is at the exit end of the air flow channels  21  ( FIG. 2 ). Typically, cooling may be concentrated near the condensation zone so as to provide a low enough temperature for adequate condensation to recover substantially all of the electrolyte, as is known in the art. 
     Phosphoric acid fuel cell stacks have a significant temperature distribution along the air flow path. This results in phosphoric acid evaporating into the gas streams towards the inlet of the cell and condensing out of the gas streams towards the cell exit. Acid is continuously wicked, by the influence of capillary flow, through the porous cell components, from the cooler condenser zone back to the hotter evaporator zone. This internal reflux must be sustained to prevent dryout of the matrix and seals, which would lead to cell failure. 
     There are competing requirements for the electrode support substrates in liquid electrolyte fuel cells. Generally speaking, large pores and high porosity are desired to maximize the amount of electrolyte that can be stored therein. Large pores and high porosity are also favorable to the diffusion of reactant gases from the reactant flow channels to the catalysts. The pore size, porosity and design fill level are chosen to provide maximal electrolyte storage with more than adequate diffusion of reactant gases. 
     There is a concern that the rate of backflow of acid will be inadequate at low electrolyte fill levels representative of cells that are 5 to 10 years old. Advanced designs, where both the anode and cathode porous electrolyte reservoir plates are replaced by dense graphite-Teflon® flow fields, will tax in-plane acid transfer even further. 
     Proton-conducting liquid electrolytes which may be used as alternatives to phosphoric acid are known. U.S. Pat. No. 5,344,722 discloses an electrolyte which is a mixture of phosphoric acid and a fluorinated compound or a mixture of phosphoric acid and siloxanes. U.S. Publication No. 2006/0027789 discloses a proton-conducting liquid electrolyte where the anion is a fluoroborate or fluoroheteroborate 
     SUMMARY  
     The subject improvement takes into account the fact that while large pores reduce resistance to flow of liquid, small pores increase the capillarity, and therefore the capillary pressure that can move the liquid through the pores. 
     While the use of wicks to transport fluids, in fuel cells that contain solid flow field plates and wetproofed substrates (gas diffusion layers), from a liquid condensing zone to a liquid evaporation zone is known, the use of wicks in cells with hydrophilic (wettable) substrates is not known and has unique requirements. 
     In cells with wettable substrates, there are several parallel paths that can wick acid from an acid condensation zone to an acid evaporation zone. Those paths are the anode substrate, the cathode substrate, and the electrolyte retaining matrix. The amount of acid that is wicked through a particular path is dependent on its cross-sectional area and permeability. The characteristics of any additional wick which is incorporated into the cell must be established relative to the characteristics of the existing materials for it to be effective. 
       Porous Media: Fluid Transport and Pore Structure,  Second Edition, Dullien, Academic Press, San Diego, 1992 shows that the permeability is a complicated function of pore size, porosity and the degree of saturation of the porous media with liquid. The equation presented by Dullien for permeability is: 
     
       
         
           
             k 
             = 
             
               
                 
                   CD 
                   p 
                   2 
                 
                  
                 
                   E 
                   3 
                 
                  
                 
                   S 
                   3.3 
                 
               
               
                 
                   ( 
                   
                     1 
                     - 
                     E 
                   
                   ) 
                 
                 2 
               
             
           
         
       
     
     where k=permeability, D p =pore size, E=porosity, C=constant and S=% saturation with liquid. 
     It has now been found that an effective wick must have a high degree of liquid saturation relative to the electrode substrate, and further, that this dictates that the mean pore size of the wick should be less than about 50% of the mean pore size of the substrate, and preferably less than about 25% of the mean pore size of the substrate. 
     Substrates used in typical fuel cells have mean pore size on the order of 20 to 50 microns with approximately 30 microns being preferred. To improve backflow of liquid electrolyte in a fuel cell, in addition to the wicking provided by the substrates, wicking is accomplished by means of additional porous hydrophilic material having mean pore size less than about one-half the mean pore size of the substrates, disposed between each separator plate and one or both of the substrates. 
     In one form, the additional wicking material is disposed in grooves which are interspersed with every third or fourth (or other number) of reactant gas grooves in the separator plate. In another form, the additional wicking material is disposed in zones extending from a surface of a separator plate into a substrate; the zones may preferably extend only part way through each substrate plate, but may extend completely through the substrate plate; the zones may preferentially be formed so as to match face-to-face with the ribs (between grooves) in the adjacent reactant gas flow field of the separator plate. In another form, the additional wicking material may be disposed on a base surface of reactant gas grooves, leaving adequate cross-sectional area for sufficient reactant gas flow. In another form, the additional wicking material is disposed between the surface of at least one of the substrates and the facing surface of the ribs between reactant gas flow field channels in the separator plates. In still another form, wicking material is disposed on dense, planar, hydrophobic separator plates to form ribs, the spaces between the formed wicking material ribs comprising the reactant gas flow field channels for either or both of the anode and the cathode reactant gases. 
     The wicking material can be disposed by well-known processes, such as screen printing. The wick material must be wettable and chemically compatible with the fuel cell electrolyte and operating conditions, and may consist of well-known materials such as silicon carbide or carbon or graphite in various forms such as particulates, flakes and fibers. The pore size, particle size, porosity and percent coverage in the various forms should be established so that the wick is nearly saturated when the electrolyte reservoirs (the substrates) are nearly empty, thereby to ensure good in-plane transfer. While the electrolyte transfer path starts in the condensation zone, the specific end point will be determined by the particular stack design and its associated evaporation zone. 
     Other improvements, features and advantages will become more apparent in the light of the following detailed description of exemplary embodiments, as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified, schematic, side elevation view of a phosphoric acid fuel cell stack known to the prior art. 
         FIG. 2  is a fragmentary, simplified, sectioned side elevation view of a pair of fuel cells and a cooler plate in a phosphoric acid fuel cell stack known to the prior art, not to scale, with sectioning lines omitted for clarity. 
         FIG. 3  is a simplified, stylized top plan view of a fuel cell power plant known to the prior art. 
         FIGS. 4-8  are fragmentary, simplified, sectioned side elevation views of a pair of phosphoric acid fuel cells having various forms of the improvement herein, not to scale, with sectioning lines omitted for clarity, and with stippling to emphasize the position of wicking material of the present improvement. 
     
    
    
     MODE(S) OF IMPLEMENTATION 
     A first form of the invention shown in  FIG. 4  provides wicking material  49  in every fourth fuel channel  20  or air channel  21 . Although illustrated in  FIG. 4  as if the wicking material  49  were simply inserted in the air or fuel channels, channels of different configuration than the air and fuel channels may be utilized in any given implementation of the improvement. Furthermore, the periodicity may be other than 1 in 4, such as one channel having wicking material for every N reactant gas channels, where N is a positive integer greater than one. 
     In a typical phosphoric acid fuel cell stack, the mean pore size of the substrates  16 ,  17  may be on the order of between 20 and 50 microns; wicking material utilized for the present improvement has, in contrast, mean pore size less than about one-half of the mean pore size of the substrates and preferably about 25% of the mean pore size of the substrate. 
     A second form of the invention illustrated in  FIG. 5  includes zones  53  formed in the substrates  16 ,  17 , with wicking material  54  disposed therein. The zones  53  may be formed by screen printing an ink containing a silicon carbide or carbon particle into the electrode substrate by known techniques. In the example of  FIG. 5 , the zones  53  do not extend completely through the substrates  16 ,  17 , but zones with wicking which do extend fully through the substrates  16  and  17  may be utilized in any implementation of the improvement herein. The zones  53  are shown in the example of  FIG. 5  as being disposed in a face-to-face relationship with the ribs  50  of the separator plates  19 ,  23 ; this provides a minimal interference with the flow of reactant gas from the reactant gas flow channels  20 ,  21  to the electrodes  12 ,  13 . However, the improvement herein may be implemented with the zones  53  disposed in a random fashion, or any other fashion with respect to the ribs  50 . 
     In the improvement illustrated in  FIG. 6 , the wicking material  58  is disposed at the base surface of the reactant gas channels  20 ,  21 . Since the material provides no structural or electrical function, the material may be deposited utilizing known screen printing techniques. In this form, the reduced cross section of reactant gas flow channels will typically result in a higher pressure drop across the reactant flow fields. This in turn will result in a slightly higher parasitic load on the fuel cell power plant to provide the additional pressure in the oxidant channels, but is typically easily provided in the fuel cell channels by a simple adjustment of a fuel pressure control valve. Alternatively, the channels may be made deeper or wider to maintain adequate flow cross section. 
     In the forms of the improvements of  FIGS. 4-6 , the wicking material porosity may be in excess of 50% or 60% and the pore size and porosity may be selected simply for the desired acid flow characteristics, since the wicking performs no structural or electrical function whatsoever. 
     In the form illustrated in  FIG. 7 , the wicking material  62  is disposed between the substrates  16 ,  17  and the facing surface of the corresponding ribs  50  of the separator plates  19 ,  23 . In this instance, the wicking material  62  must provide high electrical conductivity and have sufficient strength so as to withstand compression in the stack between the end plates. Therefore, the porosity may have to be below 50% and the thickness limited to on the order of 125 microns (0.005 inches). Since adding material to the rib faces of the separator plates  19 ,  23  will result in larger air channels, the width of the air channels may be reduced so that the ribs are wider, thereby making it easier to facilitate the design of wick material  62  that will withstand the mechanical stresses, while preserving sufficient porosity to provide the improved electrolyte backflow described herein. 
     An extension of the form described with respect to  FIG. 7  is illustrated in  FIG. 8 . Therein, the wicking material  65  is formed on dense, hydrophobic separator plates  19   a,    23   a  which are planar, that is, both surfaces being flat. In this case, the wicking material  65  must supply the electrical and mechanical requirements of ribs and may comprise carbon or graphite particles screen printed onto the planar, dense, hydrophobic separator plates  19   a,    23   a.  The planar separator plates may be electrically conductive carbon-plastic composite, or otherwise. 
     Throughout the foregoing disclosure, the provision of wicking material is disclosed on both the anode side and the cathode side of the fuel cells. Even though the larger quantity of electrolyte evaporation occurs within the air flow, and therefore the greatest proportion of condensed electrolyte appears in the air flow channels within the condensation zone, nonetheless there is significant evaporation and condensation of electrolyte within the fuel flow channels. However, in some instances, it may be possible to utilize a lesser amount of wicking material (e.g.  62 ,  65 ) with respect to the fuel flow channels  20  than is required with respect to the air flow channels  21  or vice versa. This may assist in limiting the bulk size of the fuel cell stack and improve the electrical and mechanical properties thereof.