Abstract:
A method for transporting a gas to an electrode in a fuel cell is provided, whereby the gas is dissolved in an emulsion comprising a fluorinated hydrocarbon, a surfactant and an aqueous electrolyte with a pH of at most 4 or at least 9, and the emulsion is contacted with the electrode.

Description:
BACKGROUND 
     In typical proton exchange membrane (PEM) fuel cells, a proton conducting membrane is sandwiched between the anode and cathode and is often called a membrane electrode assembly (MEA). The membrane serves multiple purposes. It acts as an insulator for electron conduction, while conducting positive and negative charges. It also provides a solid support for the catalytic layers and separates the fuel from the oxidant feed, so that mixing or crossover does not take place. 
     The fuel may be delivered to the electrode in the gaseous form, for example molecular hydrogen, or liquid form, for instance methanol or formic acid dissolved in water. Oxygen, however, typically enters the cell in gaseous form, as a component of air or as pure oxygen. Due to the chemical nature of the membrane, this gives rise to logistical problems that lower cell performance. When both the fuel and the oxidant feeds are gaseous, the gasses need to be humidified so as not to dry out the PEM. If the PEM dries out, the cell performance drops considerably. In addition, the PEM needs to be kept at low temperatures, whereas the catalyst at the cathode and anode perform best at high temperatures. A cooling apparatus for the membrane is thus often needed. 
     Problems arise with liquid fuels as well. For instance, when methanol is introduced as an aqueous based fuel, the membrane is slightly permeable to it and crossover of the fuel to the cathode takes place. The crossover causes consumption of fuel at the cathode without production of electricity, and results in a mixed potential at the cathode, causing a considerable drop in potential. 
     Laminar flow fuel cells avoid the need for a PEM. In this type of cell, parallel laminar flow between two streams of liquid creates an interface between the streams, which replaces the PEM or salt bridge of conventional devices. When the first stream, containing an oxidizer, comes into contact with the first electrode, and the second stream, containing the fuel, comes into contact with the second electrode, a current is produced, while charge migration from the anode to the cathode occurs through the interface. This cell design minimizes crossover by maximizing consumption of the fuel before it diffuses into the oxidant stream. 
     However, in laminar flow fuel cells both fuel and oxidant are delivered in liquid form, and both the fuel and oxidant fluids must be proton conductive. This limits the applicability of oxygen as the oxidant, because this gas is characterized by a low solubility in water and aqueous solutions of electrolytes. 
     SUMMARY 
     In a first aspect, the invention provides a method for transporting gas, whereby the gas is dissolved in an emulsion comprising a fluorinated hydrocarbon, a surfactant and an aqueous electrolyte with a pH of at most 4 or at least 9. The emulsion then comes into contact with an electrode. 
     In a second aspect, the invention provides emulsions for delivering gas and ions to an electrode, comprising a fluorinated hydrocarbon, a surfactant, and an aqueous electrolyte with a pH of at most 4 or at least 9. 
     In a third aspect, the invention provides a fuel cell for the generation of electricity, comprising an anode and a cathode wherein at least one of the anode and the cathode is in contact with an emulsion comprising a fluorinated solvent, a surfactant and an aqueous electrolyte with a pH of at most 4 or at least 9. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  depicts the cyclic voltamogram of Example 1. 
         FIG. 2  depicts the current voltage plot of Example 1. 
         FIG. 3  depicts the current voltage plot of Example 3. 
         FIG. 4  depicts the current voltage plot of Example 4. 
         FIG. 5  depicts the current voltage plot of Example 5. 
         FIG. 6  shows a cross-sectional schematic illustration of a PEM fuel cell. 
         FIG. 7  shows a cross-sectional schematic illustration of a laminar flow fuel cell. 
     
    
    
     DETAILED DESCRIPTION 
     The performance of both PEM and laminar flow cells would benefit from transporting oxygen to the cathode with a fluid having a very low affinity for the fuel and a high affinity for oxygen and carbon dioxide. In a PEM cell, crossover could be minimized, while the fluid would remove heat from the cathode in a more efficient fashion than a gaseous stream. In a laminar flow cell, oxygen dissolved in the liquid could be used as the oxidant at high concentrations, while fuel crossover could be further reduced. 
     Fluorinated solvents, such as fluorinated hydrocarbons (viz., perfluorinated fluids such as perfluorodecalin available from F2 Chemicals Ltd., Preston, UK) have a very high affinity for oxygen and carbon dioxide and have been successfully used in respiration-type fluids for medicinal applications, such as artificial blood (Clark et al., Journal of Fluorine Chemistry, Vol. 9, pp. 137–146, 1977). However, fluorinated solvents by themselves would not be good choices as they are very strong insulators with a low dielectric constant and not good at charge separation or ionic conduction. 
     The present invention provides for liquid-phase delivery and removal of a gas to an electrode, and in particular of oxygen to the cathode of a fuel cell, and CO 2  from its anode, by making use of fluorinated solvent emulsions. These fluids are also proton conductive and maintain the flow of positive charges to the cathode, or OH −  conductive and maintain the flow of negative charges to the anode. Furthermore, they are usually inert to electrocatalysts, and have a high capacity for carbon dioxide. Fuel crossover is also minimized, and the cooling effect exerted by these emulsion reduces the need for active cooling. 
     The invention includes emulsions of fluorinated solvents in aqueous electrolytes, the compositions including a fluorinated solvent, an aqueous electrolyte and a surfactant. These emulsions combine the gas transporting capabilities of fluorinated hydrocarbons with the charge conductivity of aqueous electrolytes. Alternatively, the aqueous phase may be emulsified in the fluorinated solvents, yielding a reverse phase micellar structure, where the fluorinated solvent is the continuous phase. 
     Fluorinated solvents, including hydrochlorofluorocarbons, perfluorocarbons, hydrofluorocarbons, hydrofluoroethers, hydrofluoroesters, and fluorosylanes, are characterized by a low chemical reactivity and a high affinity for oxygen. 
     Preferred fluorinated solvents include: (C n F 2n+1 )Si(OCH 3 ) 3 ; (C n F 2n+1 ) 2 Si(OCH 3 ) 2 ; (C n F 2n+1 )CH 2 OC(O)CH 3 ; CF 3 [OCF 2 CF 2 ] n OCF 3 ; CF 3 [OCF 2 CF 2 ] n OCF 2 Cl; CF 3 [OCF 2 CF 2 ] n OCF 2 Br; CF 3 [OCF 2 CF 2 ] n CF 2 H; CF 3 [OCF 2 CF 2 ] n F; CF 3 [OCF 2 CF 2 ] n Cl; CF 3 [OCF 2 CF 2 ] n Br; CF 3 [OCF 2 CF 2 ] n H; CF 3 CF 2 [OCF 2 CF 2 ] n F;CF 3 CF 2 [OCF 2 ] n Cl; CF 3 CF 2 [OCF 2 CF 2 ] n Br; CF 3 CF 2 [OCF 2 CF 2 ] n H; CF 3 CHF[OCF 2 CF 2 ] n F; CF 3 CHF[OCH 2 CF 2 ] n Cl; CF 3 CHF[OCF 2 CF 2 ] n Br; CF 3 CHF[OCF 2 CF 2 ] n H; CF 3 CHF[OCF 2 CF( CF 3 )] n F; (CF 3 ) 2 CF(CF 2 ) n F; (CF 3 ) 2 CF(CF 2 ) n Cl; (CF 3 ) 2 CFO(CF 2 ) n Br; (CF 3 ) 2 CFO(CF 2 ) n H; (CF 3 ) 2 CFO(CF 2 ) n F; (CF 3 ) 2 CFO(CF 2 ) n Cl; (CF 3 ) 2 CFO(CF 2 ) n Br; (CF 3 ) 2 CFO(CF 2)   n H; C n F 2n+2 ; CF 3 (CF 2 ) n Cl; CF 3 (CF 2 ) n HCF 3 (CF 2 ) n Br; N(C n F 2n+1 ) 3  wherein n is 1 to 20; C 6 F m H 6−m ,C 6 F m Cl 6−m , C 6 F m Br 6−m , C 6 F m (CF 3 ) 6−m , wherein m is 1 to 6; and mixtures thereof. 
     Particularly preferred fluorinated solvents include: CF 3 (CF 2 ) 7 Br; (CF 3 ) 2 CF(CF 2 ) 4 Cl; (CF 3 ) 2 CFO(CF 2 ) 6 F; perfluorobutyltetrahydrofuran; perfluoropropyltetrahydropyran; C 8 F 18 ; CF 3 CFBrCF 2 Br; (CF 3 ) 2 CF(CF 2 ) 4 Br; [(CF 3 ) 2 CFOCF 2 CF 2 ] 2 ; C 9 F 20 ; C 6 F 6 ; CF 3 CHF[OCF 2 CF(CF 3 )] 3 F; (CF 3 ) 2 CF(CF 2 ) 6 Cl; C 10 F 16 ; CF 3 CHF[OCF 2 CF(CF 3)]   4 F; perfluorotetrahydrodicyclopentadiene; [(CF 3 ) 2 CFO(CF 2 ) 4 ] 2 ; perfluorodecalin; CF 3 CHF[OCF 2 CF(CF 3 )] 5 F; perfluorodimethyladamantane; N(C 4 F 9 ) 3 ; perfluoromethyldecalin; C 6 H 4 (CF 3 ) 2 ; and CF 3 CHF[OCF 2 CF(CF 3 )] 9 F. 
     Preferably, the fluorinated solvent is inert to the materials in the fuel cell. For example, when the cathode is platinum, as is the case in most fuel cells, and methanol or formic acid is the fuel, the use of perfluorodecaline (PFD) is particularly indicated. This fluorinated solvent is inert to the catalyst and has minimal affinity for either fuel. 
     The surfactant emulsifies the fluorinated solvent in the water phase, and is preferably inert to the electrode in order not to cause poisoning. Thus, compatibility between surfactant and electrode should be determined. A simple test for this determination is as follows: a first cyclic voltamogram is run on the electrode in a test solution, followed by a second voltamogram in the same solution with a quantity of the surfactant. The occurrence of discrepancies between the former and the latter will reveal electrode poisoning. 
     Fluorinated surfactants, alone or in combination with non-fluorinated surfactants, are preferred for the emulsification of fluorinated solvents. Preferred fluorinated surfactants include: F(CF 2 CF 2 ) y (CH 2 CH 2 O) x H, wherein y is 1 to 10, and x is 0 to 25; ((F(CF 2 CF 2 ) z CH 2 CH 2 ) x P(O)(ONH 4 ) y , wherein x is 1 or 2, y is 1 or 2, x+y is 3, and z is 1 to 8; F(CF 2 CF 2 ) x CH 2 CH 2 SCH 2 CH 2 CO 2 Li, wherein x is 1 to 10; F(CF 2 CF 2 ) x CH 2 CH 2 SO 3 Y, wherein x is 1 to 10, and Y is H +  or NH 4   + . Other surfactants may be found in Fluorinated Surfactants, Synthesis, Properties, Applications (Eric Kissa; Marcel Dekker Publisher, 1993). For example, when the fluorinated solvent is PFD, CF 3 (CF 2 ) 5 CH 2 CH 2 SO 3 X (X═H or NH 4 ), commercially available as ZONYL® FS-62 (DuPont, Wilmington, Del.) is a particularly preferred surfactant. 
     The electrolyte enhances charge conductivity of the emulsion, but should not poison the electrode. Also, a high concentration of charges improves the current from the fuel cell. For fuel cells operating in acidic conditions, acidic electrolytes such as solutions of H 2 SO 4 , HNO 3 , HClO 4 , H 3 PO 3 , H 3 PO 4 , HCl, HBr, HI, CH 3 CO 2 H, CCl 3 CO 2 H, CF 3 CO 2 H, and mixtures thereof, are preferred. For fuel cells operating in alkaline conditions, alkaline electrolytes such as solutions of LiOH, NaOH, KOH, Rb(OH), CsOH, Mg(OH) 2 , Ca(OH) 2 , Sr(OH) 2 , and Ba(OH) 2  are mixtures thereof are preferred. 
     Additional electrolytes, for example inorganic salts, may be added. When PFD is the fluorinated solvent, and CF 3 (CF 2 ) 5 CH 2 CH 2 SO 3 X (X═H or NH 4 ) the surfactant, H 2 SO 4  is a particularly preferred electrolyte. 
     Fluorinated solvent: aqueous electrolyte volume-to-volume ratios can vary greatly, since direct and reverse micellar structures can both act as charge conductive oxygen carriers. Preferred ratios range from 1:24 to 24:1, more preferably 3:24 to 12:24, yet more preferably 1:6 to 5:7, and most preferably from 2:9 to 4:9. The preferred amount of surfactant may vary from 0.07% to 3% by weight of the total weight of the emulsion, more preferably 0.125% to 2%, and most preferably 0.5% to 1%. 
       FIG. 6  shows a cross-sectional schematic illustration of a (PEM) fuel cell  2 . Fuel cell  2  includes a high surface area anode  4  that acts as a conductor, an anode catalyst  6  (typically platinum alloy), a high surface area cathode  8  that acts as a conductor, a cathode catalyst  10  (typically platinum), and a PEM  12  that serves as a solid electrolyte for the cell. The PEM  12  physically separates anode  4  and cathode  8 . Fuel in the gas and/or liquid phase is brought over the anode catalyst  6  where it is oxidized to produce protons and electrons in the case of hydrogen fuel, and protons, electrons, and carbon dioxide in the case of an alcohol fuel. The electrons flow through an external circuit  16  to the cathode  8  where air, oxygen, or another is being constantly fed. Protons produced at the anode  4  selectively diffuse through PEM  12  to cathode  8 , where oxygen is reduced in the presence of protons and electrons at cathode catalyst  10  to produce water. 
     In the laminar flow cell  20 , as seen in  FIG. 7 , both the fuel input  22  (for example an aqueous solution containing MeOH) and the oxidant input  24 , a solution containing oxygen dissolved in one of the emulsions of the invention and a proton source, are in liquid form. By pumping the two solutions into the channel  26 , parallel laminar flow induces the interface  28  that is maintained during fluid flow. Rapid proton diffusion  29  completes the circuit of the cell as protons are produced at the anode  30  and consumed at the cathode  32 . In this case, the interface  28  prevents the two solutions from mixing and allows rapid proton conduction to complete the circuit. In addition, methanol crossover is minimized by the very low affinity of the emulsion of the invention for methanol. If fuel crossover into the emulsion occurs, the emulsion is recovered, the fuel is separated and the solution may be re-circulated in the cell. Furthermore, since both liquids are excellent heat exchangers, an external cooling system is not required. 
     The fluids may also be used to deliver oxidant gases other than oxygen, for example N 2 O and O 3 . Likewise, the fluid compositions of the invention may also be used to transport fuel gases, for example H 2 , to the anode. 
     EXAMPLES 
     1) Emulsions 
     PFD was emulsified using a mixture of CF 3 (CF 2 ) 5 CH 2 CH 2 SO 3 H and CF 3 (CF 2 ) 5 CH 2 CH 2 SO 3 NH 4  (ZONYL® FS-62, DuPont) in aqueous 0.5 M H 2 SO 4  via a ultrasonic homogenizer. The emulsion was prepared at the concentrations of PFD reported in Table 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Volume 0.5 M 
                 Volume of 
                 Amount of 
                   
               
               
                   
                 H 2 SO 4  solution 
                 PFD 
                 ZONYL ® 
                 Volume of PFD 
               
               
                 Emulsion 
                 (mL) 
                 (mL) 
                 FS-62 
                 emulsified (mL) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1A 
                 30 
                 0 
                 0 
                 0 
               
               
                 2A 
                 20 
                 10 
                 5 drops 
                 3 
               
               
                 3A 
                 20 
                 10 
                 5 drops 
                 10 
               
               
                   
               
             
          
         
       
     
     The resulting emulsions were exposed to a Pt catalyst and were found to be inert, as seen in the cyclic voltamogram of  FIG. 1 . The emulsions were then saturated with oxygen for ½ hour and tested in a laminar flow fuel cell with formic acid as the reductant. As seen in the current voltage plot of  FIG. 2 , the emulsion with the largest PFD content showed the largest current when compared to aqueous streams that were saturated with oxygen in the same way and introduced in the cell at the same flow rate (0.3 mL/min). 
     2) Concentration Effects of ZONYL® FS-62 on Emulsions 
     Emulsion experiments were performed with different concentrations of ZONYL® FS-62 to determine the ability to emulsify and the stability of the emulsion. As can be seen from the results of Table 2, a minimum of 0.25 wt % ZONYL® FS-62 in 20 mL 0.5 M sulfuric acid was needed to emulsify 10 mL (1:3) of PFD under ultrasonication conditions. Higher ZONYL® FS-62 concentrations led to better, and more stable emulsions. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Volume 0.5 
                   
                   
                 Volume of 
                   
               
               
                   
                 M H 2 SO 4   
                 Volume 
                 Amount of 
                 PFD 
               
               
                   
                 solution 
                 of PFD 
                 ZONYL ® 
                 emulsified 
               
               
                 Emulsion 
                 (mL) 
                 (mL) 
                 FS-62 
                 (mL) 
                 Stability 
               
               
                   
               
             
             
               
                 1B 
                 20 
                 10 
                    1 wt % 
                 10 
                 Weeks 
               
               
                 2B 
                 20 
                 10 
                  0.5 wt % 
                 10 
                 Days 
               
               
                 3B 
                 20 
                 10 
                  0.25 wt % 
                 10 
                 2–3 days 
               
               
                 4B 
                 20 
                 10 
                 0.125 wt % 
                 3–4 
                 0 
               
               
                   
               
             
          
         
       
     
     3) PFD Concentration Effects on Fuel Cell 
     To test the concentration effects of the PFD in the laminar flow fuel cell, the emulsions listed in Table 3 were prepared, saturated with oxygen for ½ hour before being inserted into a laminar flow cell at a flow rate of 0.3 mL/min. As seen in the current voltage plot of  FIG. 3 , emulsion 3C shows the highest current profile which correlates with it having the highest oxygen content. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Volume of 0.25 wt % 
                   
                   
               
               
                   
                 ZONYL ® in 0.5 M 
                 Volume of  
               
               
                 Emulsion 
                 H 2 SO 4  solution (mL) 
                 PFD (mL) 
                 Stability 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1C 
                 27.5 
                 2.5 
                 Week 
               
               
                 2C 
                 25 
                 5 
                 Days 
               
               
                 3C 
                 20 
                 10 
                 2–3 days 
               
               
                   
               
             
          
         
       
     
     4) Oxygen Concentration Effects on Fuel Cell. 
     Emulsion 1B was exposed to oxygen in air and run through the laminar flow fuel cell. The same experiment was repeated, this time exposing the emulsion to pure oxygen. As seen in  FIG. 4 , the increase in oxygen concentration resulted in an increased current output, especially at lower voltages. 
     5) Limit of Currents Obtainable with the Emulsions. 
     To find the limit of the current that could be reached with the laminar flow fuel cell, another series of emulsions were made with varying amounts of PFD, as can be seen in Table 4. All of these emulsions were stable and are assumed to have a reversed micellar structure at PFD concentrations greater than 50%. All of the emulsions were saturated with pure oxygen for ½ hour and run through a laminar flow fuel cell, as previously done. As seen in  FIG. 5 , the current for each emulsion followed nearly the same trend, although the oxygen concentration was undoubtedly increasing through the series. One possible explanation may be that some other factor, for example high resistance of the cell, was limiting the current that the cell could produce and the cell performance was no longer related to oxygen concentration. Nevertheless, these experiments proved that reverse micellar structures can also act as both oxygen carriers and proton conductors. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                 Volume of 2 wt % 
                   
                   
               
               
                   
                 ZONYL in 0.5 M 
                 Volume of 
               
               
                 Emulsion 
                 H 2 SO 4  solution (mL) 
                 PFD (mL) 
                 Stability 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1D 
                 20 
                 10 
                 Week 
               
               
                 2D 
                 17.5 
                 12.5 
                 Week 
               
               
                 3D 
                 15 
                 15 
                 Week 
               
               
                 4D 
                 12.5 
                 17.5 
                 Week 
               
               
                   
               
             
          
         
       
     
     Other experiments have been performed with the PFC emulsions in conventional fuel cells, and the emulsions have been found to function in much the same way as in the laminar flow fuel cell. They showed no deleterious effect on this type of device, proving that the fluid may be used in conventional PEM based fuel cells as well. 
     6) Use of PFD-Based Emulsion in a PEM Fuel Cell (Prophetic Example). 
     In a PEM fuel cell, the anode compartment is filled with a gas transporting emulsion, and H 2  is bubbled in the emulsion. Both gas transporting emulsions are obtained by emulsifying 10 ml of PFD in 20 ml of 0.5 M sulfuric acid, with an amount of ZONYL® FS-62 equivalent to 1% of the total weight of the emulsion. The cathode compartment is also filled with a gas transporting emulsion, and O 2  is bubbled in the emulsion. The apparatus may also run on a continuous stream of oxidant and fuel solutions, for example in a laminar flow fuel cell.