Abstract:
An example method of controlling fluid distribution within a fuel cell includes adjusting a flow of a reactant moving within a fuel cell to increase water within a portion of the fuel cell. Another example method of controlling fluid distribution within a fuel cell includes adjusting a flow of fuel entering a fuel cell, a velocity of air entering the fuel cell, or both, so that a first amount of water exiting the fuel cell in a fuel stream is about the same as a second amount of water exiting the fuel cell in an airstream.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national phase of PCT/US2010/047202, filed Aug. 31, 2010. 
     TECHNICAL FIELD 
     This disclosure relates generally to fuel cells. In particular, this disclosure relates to hydrating a fuel cell. 
     DESCRIPTION OF RELATED ART 
     Fuel cell assemblies and power plants are well known. One example fuel cell assembly includes multiple individual fuel cells arranged in a stack. Each individual fuel cell has an anode and a cathode positioned on either side of a polymer electrolyte membrane. A fuel, such as hydrogen, is supplied to the anode side of the polymer electrolyte membrane. An oxidant, such as air, is supplied to the cathode side of the polymer electrolyte membrane. The fuel and the oxidant initially move through channels established within plates. Gas diffusion layers distribute some of the hydrogen and the air from the channels to other portions of the fuel cell. Molecules of the hydrogen are oxidized in the catalyst layer located on the anode side of the polymer electrolyte membrane. The hydrogen protons resulting from the oxidizing move through the polymer electrolyte membrane to the cathode side. The electrons resulting from the oxidizing travel to the cathode side through an external circuit, which provides electrical current. 
     Increasing the humidity of the gases entering the fuel cell desirably increases the conductivity of the polymer electrolyte membrane. Humidifiers are often used to humidify the air entering the fuel cell. In some working environments, such as in automotive applications, it is desirable to eliminate the external humidifiers to reduce complexity and weight. As can be appreciated, eliminating the humidifiers results in less humid air entering the fuel cell. The conductivity of polymer electrolytes decreases as the humidity of air entering the fuel cell decreases, which can affect the performance and durability of the fuel cell. 
     SUMMARY 
     An example method of controlling fluid distribution within a fuel cell includes adjusting a volumetric flow of a reactant moving within a fuel cell to increase the amount of water within a portion of the fuel cell. 
     Another example method of controlling fluid distribution within a fuel cell includes adjusting a volumetric flow of fuel entering a fuel cell, volumetric flow of air entering the fuel cell, or both, so that a first amount of water exiting the fuel cell in a fuel stream is about the same as a second amount of water exiting the fuel cell in an air stream. 
     An example fuel cell device includes an electrode assembly and a first plate adjacent the electrode assembly that is configured to communicate a fuel from a fuel inlet to a fuel outlet. A first amount of water and at least some of the fuel are exhausted from the fuel cell at the fuel outlet. A second plate is adjacent the electrode assembly on an opposite side of the electrode assembly from the first plate. The second plate is configured to carry an oxidant from an oxidant inlet to an oxidant outlet. A second amount of water and at least some of the oxidant is exhausted from the fuel cell at the oxidant outlet. The first amount of water is about the same as the second amount of water in one example. 
     Another example fuel cell device includes an electrode assembly of a fuel cell and the first plate adjacent the electrode assembly. The first plate has at least one first channel configured to communicate a fuel from a fuel inlet to a fuel outlet. A volumetric flow of fuel communicating through the first channel is increased to redistribute water within the fuel cell. A second plate is adjacent the electrode assembly on an opposite side of the electrode assembly from the first plate. The second plate has at least one second channel configured to carry an oxidant in from an oxidant inlet to an oxidant outlet. A volumetric flow of oxidant communicating through the at least one second channel is adjustable to redistribute water within the fuel cell. 
     These and other features of the disclosed examples can be best understood from the following specification and drawings. The following is a brief description of the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a perspective, schematic view of an example fuel cell assembly. 
         FIG. 2  shows another example fuel cell assembly having a fuel recycle loop. 
         FIG. 3  shows a highly schematic view of water distribution within a fuel cell of the  FIG. 1  assembly. 
         FIG. 4  shows a graph of fuel cell performance verses. fuel utilization 
         FIG. 5  shows a section view of an example fuel cell. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an example proton exchange membrane fuel cell  10  includes multiple individual fuel cells  14  arranged in a stack. Each fuel cell  14  includes an anode plate  18  and a cathode plate  22  on opposing sides of a membrane electrode assembly  26 . The membrane electrode assembly  26  includes a proton exchange membrane  30  positioned between catalyst layers  32  and  34 . The typical cell package, as shown, has a solid separator plate  20  between the anode plate  18  and cathode plate  22 . The solid plate  20  may be a separate element or it may be an integral part of either  18  or  22 . While not shown for clarity reason each cell or number of cells may include a cooler plate. 
     A cathode side gas diffusion layer  38  is arranged between the cathode plate  22  and the membrane electrode assembly  26 . The membrane electrode assembly  26  includes the anode catalyst layer  34 , the proton exchange membrane  30 , and the cathode catalyst layer  32 . An anode side gas diffusion layer  42  is arranged between the anode plate  18  and the membrane electrode assembly  26 . In this example, a unitized electrode assembly  58  of the fuel cell  14  includes the cathode side gas diffusion layer  38 , the anode side gas diffusion layer  42 , and the membrane electrode assembly  26 . 
     A fuel source  46  supplies a fuel, such as hydrogen, to the fuel cell assembly  14 . A controller  64  controls a valve  62  to control delivery of fuel. In the  FIG. 2  example, fresh fuel from source  46  is mixed with fuel recycled from the exit of the cell assembly  14  at a mix point  48 , and the resulting mixture is fed to the to the fuel channels  50  (or fuel flow fields) within the anode plate  18 . The valve  62  controls fuel flow from the source  46  to the mix point  48 . A purge valve  63  controls fuel flow from the fuel cell assembly  14 . The example of  FIG. 1  does not utilize recycled fuel. 
     In one example, the fuel  46  is dry (does not contain water), while the recycled fuel may contain a fraction of the water generated in the fuel cell  14 . Ejectors (not shown) or a recycle blower  49 , which may be variable speed, may be used to return the recycled fuel to the mix point  48 . An oxidant source  53  supplies an oxidant, such as air, to oxidant channels  54  (or oxidant flow fields) within the cathode plate  22 . Air is supplied with a variable speed blower or a compressor  66 . If oxygen is the oxidant source, then the flow is typically controlled by a valve (not shown). 
     In this example, the fuel moves within the fuel cell  14  through the fuel channels  50 , which extend from a plurality of fuel inlets  51  to a plurality of fuel outlets  52 . The gas diffusion layer  42  distributes some of the fuel from the fuel channels  50  to the catalyst layer  42 . The oxidant moves within the fuel cell  14  through the oxidant channels  54 , which extend from a plurality of oxidant inlets  55  to a plurality of oxidant outlets  56 . The gas diffusion layer  38  distributes some of the oxidant from the oxidant channels  54  to the catalyst layer  34 . 
     The example fuel cell  14  of  FIG. 1  is a counter-flow fuel cell. That is, the fuel moves through the fuel channels  50  in a direction X that is generally opposite to a direction Y of oxidant movement through the oxidant channels  54 . In this example, the direction X is directly opposite to the direction Y. The hydrogen and the air are the reactants in the example fuel cell  14 . As the reactants move through the fuel cell  14 , the catalyst layer  32  separates the hydrogen to provide protons, and the catalyst layer  34  separates the oxygen molecules in the air to provide reactive oxygen intermediates that reside on the surface of the cathode catalyst. The electrons from the separated hydrogen ions are used to power a load  60 . The protons pass across the proton exchange membrane  30  and react with the oxygen intermediates at the cathode, which forms water and produces thermal energy. 
     In this example, the flow of fuel moving through the fuel channels  50  and the flow of air moving through the oxidant channels  54  are adjusted relative to each other to increase the amount of water within the fuel cell  14 . The amount of water within the fuel cell  14  peaks when the flow of fuel and the flow of air entering the fuel cell  14  are approximately equal. In other words, the amount of water in the fuel cell  14  peaks when the ratio of the entering fuel flow divided by entering air flow is approximately one. The amount of water in the cell decreases, which causes the performance to decrease, when this ratio is significantly less than one or significantly greater than one. If the example were cross-flow the results would be less effective and very ineffective for or co-flow configuration. 
     In some examples, a fuel cell operates on hydrogen and air, and the hydrogen flow entering the fuel cell is 1.2 times the stoichiometric amount, and the oxygen flow is 2 times the stoichiometric amount. For pure hydrogen and air fed at these stoichiometric ratios, the ratio of entering fuel flow to entering air flow is approximately 0.25. This is considerably lower than the desired ratio of 1. 
     One example technique for achieving the desired ratio of flows includes increasing the flow of fuel while holding constant the flow of air. The example adjustment causes water near the area of the cell air outlets  55  to move through the polymer electrolyte membrane  30  toward the fuel inlets  51 . 
     Since air contains only 20% oxygen, decreasing the air flow while holding the fuel flow constant to yield a flow ratio of one is not possible in this case since it causes significant performance loss from a lack of oxygen. Thus, increasing the fuel flow will generally be the preferred approach. In the case where the fuel flow is a dilute hydrogen mix, air adjustments could play a more significant role. 
     In this example, the variable speed recycle blower  49  is linked to a controller  65 . The variable speed recycle blower  49  is used to control the flow of recycled fuel to the mix point  48  and subsequently the fuel inlets  51 . The speed of the recycle blower  49  increases to allow more fuel to move to the fuel inlets  51 , for example, which increases the flow of fuel moving through the fuel channels  50 . More of the water tends to move from the unitized electrode assembly  58  into the fuel channels  50  when the flow of the fuel moving through the fuel channels  50  is increased. The water is then more easily convected into other areas of the fuel cell  14 . 
     More water typically exits the fuel cell  14  at the oxidant outlets  56  because water is produced on the cathode side of the membrane electrode assembly  26 , and also because of electroosmotic drag of water. As known, the oxidant channels  54  facilitate the movement of water to the oxidant outlets  56 . 
     In this example, a variable speed air compressor  66  linked to a controller  68  is used to control the flow of oxidant from the oxidant source  53  to the oxidant inlets  55 . The air compressor  66  adjusts to hold the oxidant flow to the fuel cell  14 , at a constant utilization rate through the oxidant channels  54 . When the flow of the oxidant moving through the oxidant channels  54  is about equal to the fuel flow, more of the water tends to distribute into areas of the fuel cell  14  rather than continue to move through the oxidant channels  54 . 
     Generally, increasing the flow of fuel moving through the fuel channels  50  to achieve a nearly equal fuel to air flow ratio and maintaining the flow of air, to hold a constant utilization, moving through the oxidant channels  54  circulates water throughout the fuel cell  14 . An example of the path of water circulation through the fuel cell  14  is shown in  FIG. 3 . Such circulation is especially beneficial when the oxidant source  53  is not humidified before entering the fuel cell  14 , such as when the fuel cell assembly  10  lacks a connection to an external humidifier and the fuel cell contains at least one solid separator plate. 
     Referring to  FIG. 4  with continuing reference to  FIGS. 1 and 2 , in a specific example, the fuel cell  14  is a solid plate fuel cell operating with a coolant exit temperature of 80° C. with hydrogen fuel and oxidant air exiting the fuel cell  14  at 40 kPag. The dew point at the fuel inlets  51  is 53° C. The cathode humidifier temperature is room temperature (about 25° C.) and the relative humidity at the oxidant inlets  55  is about 7%. 
     The fuel utilization within the fuel cell  14  was then decreased while the air utilization was fixed at 67% to cause an increase in the flow of fuel moving through the fuel channels  50  while maintaining the flow of air moving through the oxidant channels  54 . 
     As the fuel utilization decreases from 90% to 30% the cell performance increases as predicted until the utilization reaches the predicted maximum performance, which is at about 30% utilization in this example. This reflects an improved water distribution with the cell package including the membrane. Below this 30% H 2  utilization, the performance again decreases due to increasing water imbalance. 
       FIG. 5  shows a schematic cross-sectional view of an example fuel cell  114  having a plurality of fuel channels  150  and a plurality of oxidant channels  154 . The fuel channels  150  are configured to have a shorter path length for transport of water from the UEA than the air channels. This configuration favors transportation of water to the fuel stream over transport of water to the air stream. This reduction in transport resistance helps move water to the fuel stream and can be combined with the optimum flow strategy. 
     As can be appreciated from  FIG. 5 , the channel depth of the oxidant channels  154  is three times that of the depth of the fuel channels  150 . Specifically, in this example, the depth of the oxidant channels  154  is 0.75 mm and the depth of the fuel channels  150  is 0.25 mm. The width of the oxidant channels  154  is about the same as the width of the fuel channels  150 . The fuel channels  150  thus have a smaller cross-section than the oxidant channels  154 . Accordingly, an equal flow of gas would move through the fuel channels  150  faster than the oxidant channels  154 . This higher fuel flow velocity and lower channel depth enhances water movement through the membrane and into the relatively dry fuel gas at the fuel inlet. In this example, the ratio of oxidant channel depths to fuel channel depth is best at about 3:1. Other examples include other ratios. 
     In another example, the width of the oxidant channels  154 , the fuel channels  150 , or both are adjusted to change the transport resistance for water moving from the UEA to the fuel or air channels. 
     In yet another example, the width and the depth of the of the oxidant channels  154 , the fuel channels  150 , or both are adjusted to change the relative transport lengths for water of reactants moving through the fuel cell  114 . 
     Referring again to the example of  FIG. 2 , the speed of the recycle blower  49  is dependent on the flow of fuel and the flow of air. In this example, the speed of the recycle blower  49  is adjusted so that the flow of fuel matches the flow of air. The method may turn the recycle blower  49  on when the fuel cell  14  is operated in a condition that would cause the membrane to dry. Such a condition is identified by calculations or by measuring resistance of the membrane, for example. 
     The method may also measure water content to determine when to operate the recycle blower  49 . For example, if the amount of water carried by the oxidant exiting the fuel cell  14  is about the same as the amount of water carried by the fuel exiting the fuel cell, no adjustments are made. If the amount of water carried by the oxidant is not about the same as the amount of water carried by the fuel, the method adjusts the flow of fuel moving through the fuel cell  14 . The flow of fuel moving through the fuel channels  50  is adjusted so that the amount of water carried by the air exiting the fuel cell  14  at the oxidant outlets  56  approximately matches the amount of water carried by the fuel exiting at the fuel outlets. 
     In another example, the flow of fuel moving through the fuel channels  50  is adjusted so that the amount of water carried by the fuel exiting the fuel cell  14  matches the amount of water carried by the oxidant exiting the fuel cell  14 . Adjusting oxidant flow is not utilized in some examples as the adjustments can cause performance to decrease and cannot be decreased enough to compensate typical fuel flows. 
     Features of this invention include operating a fuel cell stack assembly with drier reactants in a relatively uncomplicated manner. Another feature of this invention is operating a fuel cell stack assembly without requiring an external humidifier. 
     Thus, we find that increasing the flow of relatively dry fuel to the fuel cell results in improved hydration of the membrane, which is counter to expectation and would not occur in a fuel cell with co-flow fuel and air. 
     Although a preferred embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.