Abstract:
A fuel cell system including a humidification system is described. The humidification system employs a recycling system that recycles relatively humid gas exhausted from a multistage fuel cell stack, either on the anode and/or cathode side, and sends this relatively humid gas back to be combined with relatively dry supply gas, such as but not limited to hydrogen and/or air. The humidified supply gas mixture is then reintroduced into the first stage of the multistage fuel cell stack. A recirculation device, such as but not limited to a pump and/or an ejector, can be used to aid in moving the humid exhaust gas back through a recycle gas line to be combined with the supply gas.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to fuel cell systems, and more particularly to new and improved gas flow recycle systems for use in fuel cell systems. 
     BACKGROUND OF THE INVENTION 
     Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In PEM-type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements, sometimes referred to as the gas diffusion media components, that: (1) serve as current collectors for the anode and cathode; (2) contain appropriate openings therein for distributing the fuel cell&#39;s gaseous reactants over the surfaces of the respective anode and cathode catalysts; (3) remove product water vapor or liquid water from electrode to flow field channels; (4) are thermally conductive for heat rejection; and (5) have mechanical strength. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (e.g., a stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the MEA described earlier, and each such MEA provides its increment of voltage. 
     In PEM fuel cells, hydrogen (H 2 ) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O 2 ), or air (a mixture of O 2  and N 2 ). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation. 
     Examples of technology related to PEM and other related types of fuel cell systems can be found with reference to commonly-assigned U.S. Pat. No. 3,985,578 to Witherspoon et al.; U.S. Pat. No. 5,272,017 to Swathirajan et al.; U.S. Pat. No. 5,624,769 to Li et al.; U.S. Pat. No. 5,776,624 to Neutzler; U.S. Pat. No. 6,103,409 to DiPierno Bosco et al.; U.S. Pat. No. 6,277,513 to Swathirajan et al.; U.S. Pat. No. 6,350,539 to Woods, III et al.; U.S. Pat. No. 6,372,376 to Fronk et al.; U.S. Pat. No. 6,376,111 to Mathias et al.; U.S. Pat. No. 6,521,381 to Vyas et al.; U.S. Pat. No. 6,524,736 to Sompalli et al.; U.S. Pat. No. 6,528,191 to Senner; U.S. Pat. No. 6,566,004 to Fly et al.; U.S. Pat. No. 6,630,260 to Forte et al.; U.S. Pat. No. 6,663,994 to Fly et al.; U.S. Pat. No. 6,740,433 to Senner; U.S. Pat. No. 6,777,120 to Nelson et al.; U.S. Pat. No. 6,793,544 to Brady et al.; U.S. Pat. No. 6,794,068 to Rapaport et al.; U.S. Pat. No. 6,811,918 to Blunk et al.; U.S. Pat. No. 6,824,909 to Mathias et al.; U.S. Patent Application Publication Nos. 2004/0229087 to Senner et al.; 2005/0026012 to O&#39;Hara; 2005/0026018 to O&#39;Hara et al.; and 2005/0026523 to O&#39;Hara et al., the entire specifications of all of which are expressly incorporated herein by reference. 
     The membranes of PEM fuel cells should be kept in humid conditions in order to achieve high performance and durability. Therefore, if operated at elevated temperatures, fuel cell systems usually require a humidification device for the feed gases, air and/or hydrogen. It has been shown that the fuel gas, which is fed to the anode of the fuel cell stack, requires humidification in order to prevent the fuel cell stack from drying at the fuel inlet. Along the internal channels of the fuel cell stack, there is an increase in water content that causes a humidity gradient in the electrolyte membrane, and inhomogeneous power distribution. The inhomogeneous power distribution might lead to hot spots in some areas, and to excessive water accumulation in other areas, which again has a negative affect on performance and durability. Furthermore, humidification devices have several disadvantages, especially for automotive applications of the fuel cell stack, as they are heavy, expensive, and sometimes, due to the water they contain, subject to freezing at low ambient temperatures. 
     Previous solutions of the humidification problem involved membrane humidifiers and water injection methods, as well as humid gas recirculation, including those described in U.S. Pat. No. 5,478,662 to Strasser and U.S. Pat. No. 5,935,726 to Chow et al., the entire specifications of which are expressly incorporated herein by reference. 
     Recirculation methods take advantage of the fact that gases at the fuel cell outlets are humidified with the water produced in the fuel cell, and can be fed back at the fuel cell inlet in order to bring the humidity there without having liquid water involved. A disadvantage is the need for a recirculation pump, the power consumption of the pump and the humidity gradient in a stack along the channel. 
     Also, the switching of oxidizing feed gas between cathode gas inlets and outlets of the fuel cell has been proposed. The advantage provided by that system is the better homogeneity of humidity in the fuel cell as the dry feed gas is alternating in one and the other direction in the channel. The feed gas, however, is suggested to be the oxidant, and is dry, which might lead to performance degradation at both gas inlets. 
     Accordingly, there exists a need for new and improved gas flow recycle systems that provide improved humidification distribution within the fuel cell. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a fuel cell system is provided, comprising: (1) a first fuel cell stack having a fuel or oxidant gas inlet operable to receive an amount of fuel or oxidant gas; (2) a second fuel cell stack having an exhaust gas outlet operable to exhaust an amount of exhaust gas, wherein the first and second fuel cell stacks are in fluid communication; and (3) a gas recirculation system in fluid communication with the first and second fuel cell stacks, wherein the gas recirculation system is operable to receive the exhaust gas from the exhaust gas outlet and reintroduce the exhaust gas into the first fuel cell stack, wherein the exhaust gas has a higher moisture level than the fuel or oxidant gas. 
     In accordance with a first alternative embodiment of the present invention, a fuel cell system is provided, comprising: (1) a first fuel cell stack having a fuel or oxidant gas inlet operable to receive an amount of fuel or oxidant gas and an exhaust gas outlet operable to exhaust an amount of exhaust gas; and (2) a gas recirculation system in fluid communication with the first fuel cell stack, wherein the gas recirculation system is operable to receive the exhaust gas from the exhaust gas outlet and reintroduce the exhaust gas into the first fuel cell stack, wherein the exhaust gas has a higher moisture level than the fuel or oxidant gas. 
     In accordance with a second alternative embodiment of the present invention, a method of operating a fuel cell system is provided, comprising: (1) providing a first fuel cell stack having a fuel or oxidant gas inlet operable to receive an amount of fuel or oxidant gas; (2) providing a second fuel cell stack having an exhaust gas outlet operable to exhaust an amount of exhaust gas, wherein the first and second fuel cell stacks are in fluid communication; and (3) providing a gas recirculation system in fluid communication with the first and second fuel cell stacks, wherein the gas recirculation system is operable to receive the exhaust gas from the exhaust gas outlet and reintroduce the exhaust gas into the first fuel cell stack, wherein the exhaust gas has a higher moisture level than the fuel or oxidant gas. 
     In accordance with a third alternative embodiment of the present invention, a method of operating a fuel cell system is provided, comprising: (1) providing a first fuel cell stack having a fuel or oxidant gas inlet operable to receive an amount of fuel or oxidant gas and an exhaust gas outlet operable to exhaust an amount of exhaust gas; and (2) providing a gas recirculation system in fluid communication with the first fuel cell stack, wherein the gas recirculation system is operable to receive the exhaust gas from the exhaust gas outlet and reintroduce the exhaust gas into the first fuel cell stack, wherein the exhaust gas has a higher moisture level than the fuel or oxidant gas. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a schematic illustration of a three stack fuel cell system having a humidification/gas recirculation system operably associated with the anode side, in accordance with the general teachings of the present invention; 
         FIG. 2  is a schematic illustration of a two stack fuel cell system having a humidification/gas recirculation system operably associated with the anode side, in accordance with the general teachings of the present invention; 
         FIG. 3  is a schematic illustration of a three stack fuel cell system having a humidification/gas recirculation system operably associated with the cathode side, in accordance with the general teachings of the present invention; and 
         FIG. 4  is a schematic illustration of a two stack fuel cell system having a humidification/gas recirculation system operably associated with the cathode side, in accordance with the general teachings of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     By “fluid,” as that term is used herein, it is meant any gaseous and/or liquid material, such as but not limited to liquid water, water vapor, and combinations thereof. 
     Referring to  FIG. 1 , there is shown a schematic illustration of a three stack fuel cell system  10  having a humidification/gas recirculation system  12  operably associated with the stack anode flow  14  side, in accordance with one embodiment of the present invention. 
     The fuel cell system  10  includes three fuel cell stacks,  16 ,  18 ,  20 , respectively, that are in fluid communication with one another, or at least with an adjacent fuel cell stack. The fuel cell stacks  16 ,  18 ,  20 , respectively, are comprised of a number of individual fuel cell elements  22 . The number of fuel cell elements  22  can be varied among the fuel cell stacks  16 ,  18 ,  20 , respectively. Conduits  16   a  and  18   a  can be provided to provide fluid communication between fuel cell stack  16 /fuel cell stack  18  and fuel cell stack  18 /fuel cell stack  20 , respectively. 
     Fuel cell stack  16  is provided with a fuel gas inlet  24  (e.g., an stack anode inlet) operable to receive an amount of fuel gas, and fuel cell stack  20  is provided with an exhaust outlet  26  (e.g., a stack anode outlet) operable to exhaust an amount of exhaust gas. Fuel gas inlet  24  is in fluid communication with a flow control valve  28  which is in fluid communication with the humidification/gas recirculation system  12 . The humidification/gas recirculation system  12  can include a humidification/gas recirculation device  30 . The humidification/gas recirculation device  30  can include a pump, ejector, injector (e.g., a Venturi injector) and combinations thereof. 
     The humidification/gas recirculation device  30  is in fluid communication with a fuel gas storage/supply source  32 , e.g., via conduit  32   a . By way of a non-limiting example, the fuel gas can be comprised of hydrogen. Exhaust outlet  26  is in fluid communication with a pressure control valve  34 , e.g., via conduit  34   a . The humidification/gas recirculation system  12 , and more specifically the humidification/gas recirculation device  30 , can be in fluid communication with either the exhaust outlet  26  and/or conduit  34   a , e.g., via conduit  30   a.    
     Because the exhaust gas exiting the fuel cell system  10  is typically higher in moisture than the fuel gas being introduced into the fuel cell system  10 , the present invention employs the humidification/gas recirculation system  12 , and more specifically the humidification/gas recirculation device  30 , to provide a method for reintroducing the relatively moist and/or humid exhaust gas (e.g., from conduit  30   a ) back into the first fuel stack  16  after it has exiting the third fuel cell stack  20 . However, it should be appreciated that the exhaust gas can be re-circulated after it has exited either of the first and/or second fuel cell stacks  16 ,  18 , respectively. 
     Due to the fact that the exhaust gas leaves the first fuel stack  16  (and subsequent fuel cell stacks) at a lower pressure than the incoming fuel gas, the humidification/gas recirculation system  12 , and more specifically the humidification/gas recirculation device  30 , employs a device to mix both of the gas streams (i.e., the exhaust gas and the fuel gas). As previously described, this device can be a pump, ejector, injector, or a combination thereof, that are operable to increase the pressure of the re-circulated exhaust gas to the required value. In this manner, the relatively dry incoming fuel gas can be mixed with at least a portion of the relatively moist and/or humid exhaust gas of either the first, second and/or third fuel cell stacks  16 , 18 ,  20 , respectively. Once mixed, the gas mixture (i.e., the relatively moist and/or humid exhaust gas and the relatively dry fuel gas) can then be introduced back into the fuel cell system  10 , and more specifically, the first fuel cell stack  16 . 
     Referring to  FIG. 2 , there is shown a schematic illustration of a two stack fuel cell system  110  having a humidification/gas recirculation system  112  operably associated with the stack anode flow  114  side, in accordance with a first alternative embodiment of the present invention. 
     The fuel cell system  110  includes two fuel cell stacks,  116 ,  118 , respectively, that are in fluid communication with one another. The fuel cell stacks  116 ,  118 , respectively, are comprised of a number of individual fuel cell elements  122 . The number of fuel cell elements  122  can be varied among the fuel cell stacks  116 ,  118 , respectively. Conduit  116   a  can be provided to provide fluid communication between fuel cell stack  116  and fuel cell stack  118 . 
     Fuel cell stack  116  is provided with a fuel gas inlet  124  (e.g., an stack anode inlet) operable to receive an amount of fuel gas, and fuel cell stack  118  is provided with an exhaust outlet  126  (e.g., a stack anode outlet) operable to exhaust an amount of exhaust gas. Fuel gas inlet  124  is in fluid communication with a flow control valve  128  which is in fluid communication with the humidification/gas recirculation system  112 . The humidification/gas recirculation system  112  can include a humidification/gas recirculation device  130 . The humidification/gas recirculation device  130  can include a pump, ejector, injector (e.g., a Venturi injector) and combinations thereof. 
     The humidification/gas recirculation device  130  is in fluid communication with a fuel gas storage/supply source  132 , e.g., via conduit  132   a . By way of a non-limiting example, the fuel gas can be comprised of hydrogen. Exhaust outlet  126  is in fluid communication with a pressure control valve  134 , e.g., via conduit  134   a . The humidification/gas recirculation system  112 , and more specifically the humidification/gas recirculation device  130 , can be in fluid communication with either the exhaust outlet  126  and/or conduit  134   a , e.g., via conduit  130   a.    
     As with the previous embodiment, because the exhaust gas exiting the fuel cell system  110  is typically higher in moisture than the fuel gas being introduced into the fuel cell system  110 , the present invention employs the humidification/gas recirculation system  112 , and more specifically the humidification/gas recirculation device  130 , to provide a method for reintroducing the relatively moist and/or humid exhaust gas back into the first fuel stack  116  (e.g., from conduit  130   a ) after it has exiting the second fuel cell stack  118 . However, it should be appreciated that the exhaust gas can be re-circulated after it has exited the first fuel cell stack  116 . 
     Due to the fact that the exhaust gas leaves the first fuel stack  116  (as well as the second fuel cell stack  118 ) at a lower pressure than the incoming fuel gas, the humidification/gas recirculation system  112 , and more specifically the humidification/gas recirculation device  130 , employs a device to mix both of the gas streams (i.e., the exhaust gas and the fuel gas). As previously described, this device can be a pump, ejector, injector, or a combination thereof, that are operable to increase the pressure of the re-circulated exhaust gas to the required value. In this manner, the relatively dry incoming fuel gas can be mixed with at least a portion of the relatively moist and/or humid exhaust gas of either the first and/or second fuel cell stacks  116 ,  118 , respectively. Once mixed, the gas mixture (i.e., the relatively moist and/or humid exhaust gas and the relatively dry fuel gas) can then be introduced back into the fuel cell system  110 , and more specifically, the first fuel cell stack  116 . 
     Referring to  FIG. 3 , there is shown a schematic illustration of a three stack fuel cell system  210  having a humidification system  212  operably associated with the stack cathode flow  214  side, in accordance with a second alternative embodiment of the present invention. 
     The fuel cell system  210  includes three fuel cell stacks,  216 ,  218 ,  220 , respectively, that are in fluid communication with one another, or at least with an adjacent fuel cell stack. The fuel cell stacks  216 ,  218 ,  220 , respectively, are comprised of a number of individual fuel cell elements  222 . The number of fuel cell elements  222  can be varied among the fuel cell stacks  216 ,  218 ,  220 , respectively. Conduits  216   a  and  218   a  can be provided to provide fluid communication between fuel cell stack  216 /fuel cell stack  218  and fuel cell stack  218 /fuel cell stack  220 , respectively. 
     Fuel cell stack  216  is provided with an oxidant gas inlet  224  (e.g., an stack cathode inlet) operable to receive an amount of oxidant gas, and fuel cell stack  220  is provided with an exhaust outlet  226  (e.g., a stack cathode outlet) operable to exhaust an amount of exhaust gas. Oxidant gas inlet  224  is in fluid communication with at least one compressor  228 . Compressor  228  can also be operably associated and/or in fluid communication with another compressor  230 . Compressors  228 ,  230 , respectively, can be powered by motor  232 . 
     Compressors  228 ,  230 , respectively, can be in fluid communication with a flow meter  234 , via conduit  234   a , which is in fluid communication with an oxidant gas storage/supply source  236 , e.g., via conduit  236   a . By way of a non-limiting example, the oxidant can be comprised of air, e.g., ambient air. 
     In this embodiment, the compressors  228 ,  230 , respectively, and any associated components as previously described, comprise the humidification/gas recirculation system  212 . However, instead of employing pumps, ejectors and/or injectors, the humidification/gas recirculation system  212  employs the compressors  228 ,  230 , respectively to accomplish the recirculation function. 
     Compressors  228 ,  230 , respectively, are in fluid communication, either directly or indirectly, with any of the fuel cell stacks,  216 ,  218 ,  220 , respectively, e.g. through conduits  216   b ,  218   b ,  220   b , respectively. A recycle flow control valve  240  can be disposed in conduit  216   c  and flow control valves  242 ,  244 , respectively, can be disposed in conduits  218   b ,  220   b , respectively. In this manner direct inputs  246 ,  248 , respectively, can be provided for fuel cell stacks  218 ,  220 , respectively, from humidification/gas recirculation system  212 , and more specifically compressors  228 ,  230 , respectively. Additionally, a pressure control valve  250  can be provided that is in fluid communication with exhaust outlet  226 , e.g., via conduit  250   a.    
     Because the exhaust gas exiting the fuel cell system  210  is typically higher in moisture than the oxidant gas being introduced into the fuel cell system  210 , the present invention employs the humidification/gas recirculation system  212 , and more specifically the compressors,  228 ,  230 , respectively, to provide a method for reintroducing the relatively moist and/or humid exhaust gas (e.g., from conduit  216   c ) back into the first fuel stack  216  after it has exiting the first fuel cell stack  216 . However, it should be appreciated that the exhaust gas can be re-circulated after it has exited either of the second and/or third fuel cell stacks  218 ,  220 , respectively. 
     Due to the fact that the exhaust gas leaves the first fuel stack  216  (and subsequent fuel cell stacks) at a lower pressure than the incoming oxidant gas, humidification/gas recirculation system  212 , and more specifically the compressors,  228 ,  230 , respectively, are employed to mix both of the gas streams (i.e., the exhaust gas and the oxidant gas). The compressors,  228 ,  230 , respectively, are operable to increase the pressure of the re-circulated exhaust gas to the required value. In this manner, the relatively dry incoming oxidant gas can be mixed with at least a portion of the relatively moist and/or humid exhaust gas of either the first, second and/or third fuel cell stacks  216 ,  218 ,  220 , respectively. Once mixed, the gas mixture (i.e., the relatively moist and/or humid exhaust gas and the relatively dry oxidant gas) can then be introduced back into the fuel cell system  210 , and more specifically, either of the first, second and/or third fuel cell stacks  216 ,  218 ,  220 , respectively. 
     Referring to  FIG. 4 , there is shown a schematic illustration of a two stack fuel cell system  310  having a humidification system  312  operably associated with the stack cathode flow  314  side, in accordance with a third alternative embodiment of the present invention. 
     The fuel cell system  310  includes two fuel cell stacks,  316 ,  318 , respectively, that are in fluid communication with one another. The fuel cell stacks  316 ,  318 , respectively, are comprised of a number of individual fuel cell elements  322 . The number of fuel cell elements  322  can be varied among the fuel cell stacks  316 ,  318 , respectively. Conduit  316   a  can be provided to provide fluid communication between fuel cell stack  316  and fuel cell stack  318 . 
     Fuel cell stack  316  is provided with an oxidant gas inlet  324  (e.g., an stack cathode inlet) operable to receive an amount of oxidant gas, and fuel cell stack  318  is provided with an exhaust outlet  326  (e.g., a stack cathode outlet) operable to exhaust an amount of exhaust gas. Fuel gas inlet  324  is in fluid communication with at least one compressor  328 . Compressor  328  can also be operably associated and/or in fluid communication with another compressor  330 . Compressors  328 ,  330 , respectively, can be powered by motor  332 . 
     Compressors  328 ,  330 , respectively, can be in fluid communication with a flow meter  334 , via conduit  334   a , which is in fluid communication with an oxidant gas storage/supply source  336 , e.g., via conduit  336   a . By way of a non-limiting example, the oxidant gas can be comprised of air, e.g., ambient air. 
     In this embodiment, the compressors  328 ,  330 , respectively, and any associated components as previously described, comprise the humidification/gas recirculation system  312 . However, instead of employing pumps, ejectors and/or injectors, the humidification/gas recirculation system  312  employs the compressors  328 ,  330 , respectively to accomplish the recirculation function. 
     Compressors  328 ,  330 , respectively, are in fluid communication, either directly or indirectly, with any of the fuel cell stacks,  316 ,  318 , respectively, e.g. through conduits  316   b ,  318   b , respectively. A recycle flow control valve  340  can be disposed in conduit  316   c  and flow control valves  342 ,  344 , respectively, can be disposed in conduits  316   b ,  318   b , respectively. In this manner a direct input  346 ,  348 , respectively, can be provided for fuel cell stacks  316 ,  318 , respectively, from humidification/gas recirculation system  312 , and more specifically compressors  328 ,  330 , respectively. Additionally, a pressure control valve  350  can be provided that is in fluid communication with exhaust outlet  326 , e.g., via conduit  350   a.    
     Because the exhaust gas exiting the fuel cell system  310  is typically higher in moisture than the oxidant gas being introduced into the fuel cell system  310 , the present invention employs the humidification/gas recirculation system  312 , and more specifically the compressors,  328 ,  330 , respectively, to provide a method for reintroducing the relatively moist and/or humid exhaust gas (e.g., from conduit  316   c ) back into the first fuel stack  316  after it has exiting the first fuel cell stack  316 . However, it should be appreciated that the exhaust gas can be re-circulated after it has exited the second fuel cell stack  318 . 
     Due to the fact that the exhaust gas leaves the first fuel stack  316  (and subsequent fuel cell stacks) at a lower pressure than the incoming oxidant gas, humidification/gas recirculation system  312 , and more specifically the compressors,  328 ,  330 , respectively, are employed to mix both of the gas streams (i.e., the exhaust gas and the oxidant gas). The compressors,  328 ,  330 , respectively, are operable to increase the pressure of the re-circulated exhaust gas to the required value. In this manner, the relatively dry incoming fuel gas can be mixed with at least a portion of the relatively moist and/or humid exhaust gas of either the first, second and/or third fuel cell stacks  316 ,  318 ,  320 , respectively. Once mixed, the gas mixture (i.e., the relatively moist and/or humid exhaust gas and the relatively dry oxidant gas) can then be introduced back into the fuel cell system  310 , and more specifically, either of the first and/or second fuel cell stacks  316 ,  318 , respectively. 
     Some of the benefits of the present invention include, without limitation: (1) humidified inlet gas for all stages of the cascaded stack, which is expected to increase performance and durability; (2) cost reduction by eliminating the need for expensive external humidification devices (e.g., a water vapor transfer unit); (3) reducing the number of components in the fuel cell system, which reduces cost, system size and controls effort; and (4) reduced start-stop degradation by using a diluted gas supply. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.