Patent Publication Number: US-6218035-B1

Title: Proton exchange membrane fuel cell power system

Description:
RELATED PATENT DATA 
     This application is a Continuation of U.S. patent application Ser. No. 08/979,853, filed Nov. 20, 1997, and titled “A Proton Exchange Membrane Fuel Cell Power System,” and which is now U.S. Pat. No. 6,030,718, the disclosure of which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a proton exchange membrane (PEM) fuel cell power system, and more specifically to a power system which includes a plurality of discrete fuel cell modules producing respective voltages, and wherein the discrete fuel cell modules are self humidifying, have an electrical efficiency of at least about 40%, and offer plant reliability, ease-of-maintenance, and reduced capital costs not possible heretofore. 
     2. Description of the Prior Art 
     The fuel cell was developed in England more than 150 years ago by Sir William Grove in 1839. The inventor called it a “gaseous battery” at the time to distinguish the fuel cell from another invention of his, the electric storage battery. The fuel cell is an electrochemical device which reacts hydrogen and oxygen which is usually supplied from the air, to produce electricity and water. With prior processing, a wide range of fuels, including natural gas and coal-derived synthetic fuels can be converted to electric power. The basic process is highly efficient, and for those fuel cells fueled directly by hydrogen, pollution free. Further, since fuel cells can be assembled into stacks, of varying sizes, power systems have been developed to produce a wide range of output levels and thus satisfy numerous kinds of end-use applications. 
     Heretofore, fuel cells have been used as alternative power sources in earth and space applications. Examples of this use are unattended communications repeaters, navigational aids, space vehicles, and weather and oceanographic stations, to name but a few. 
     Although the basic process is highly efficient and pollution free, a commercially feasible power system utilizing this same technology has remained elusive. For example, hydrogen-fueled fuel cell power plants based on Proton Exchange Membrane (PEM) Fuel Cells are pollution free, clean, quiet on site, and have few moving parts. Further, they have a theoretical efficiency of up to about 80%. This contrasts sharply with conventional combustion technologies such as combustion turbines, which convert at most 50% of the energy from combusting fuel into electricity and in smaller generation capacities, are uneconomical and significantly less efficient. 
     Although the fundamental electrochemical processes involved in all fuel cells are well understood, engineering solutions have proved elusive for making certain fuel cell types reliable and for other types, economical. In the case of PEM fuel cells, reliability has not been the driving concern to date, but rather the installed cost per watt of generation capacity has. In order to lower the PEM fuel cost per watt, much attention has been placed on increasing power output. Historically this has resulted in additional, sophisticated balance-of-plant systems necessary to optimize and maintain high PEM fuel cell power outputs. A consequence of highly complex balance-of-plant systems is they do not readily scale down to low (single residence) generation capacity plants. Consequently installed cost, efficiency, reliability and maintenance expenses all are adversely effected in low generation applications. 
     As earlier noted, a fuel cell produces an electromotive force by reacting fuel and oxygen at respective electrode interfaces which share a common electrolyte. In the case of a PEM fuel cell, hydrogen gas is introduced at a first electrode where it reacts electrochemically in the presence of a catalyst to produce electrons and protons. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Further, the protons pass through a membrane of solid, polymerized electrolyte (a proton exchange membrane or PEM) to the second electrode. Simultaneously, an oxidant, such as oxygen gas, (or air), is introduced to the second electrode where the oxidant reacts electrochemically in the presence of the catalyst and is combined with the electrons from the electrical circuit and the protons (having come across the proton exchange membrane) thus forming water and completing the electrical circuit. The fuel-side electrode is designated the anode and the oxygen-side electrode is identified as the cathode. The external electric circuit conveys electrical current and can thus extract electrical power from the cell. The overall PEM fuel cell reaction produces electrical energy which is the sum of the separate half cell reactions occurring in the fuel cell less its internal losses. 
     Since a single PEM fuel cell produces a useful voltage of only about 0.45 to about 0.7 volts D.C. under a load, practical PEM fuel cell plants have been built from multiple cells stacked together such that they are electrically connected in series. In order to reduce the number of parts and to minimize costs, rigid supporting/conducting separator plates often fabricated from graphite or special metals have been utilized. This is often described as bipolar construction. More specifically, in these bipolar plates one side of the plate services the anode, and the other the cathode. Such an assembly of electrodes, membranes, and the bipolar plates are referred to as a stack. Practical stacks have heretofore consisted of twenty or more cells in order to produce the direct current voltages necessary for efficient inverting to alternating current. 
     The economic advantages of designs based on stacks which utilize bipolar plates are compelling. However, this design has various disadvantages which have detracted from its usefulness. For example, if the voltage of a single cell in a stack declines significantly or fails, the entire stack, which is held together in compression with tie bolts, must be taken out of service, disassembled, and repaired. In traditional fuel cell stack designs, the fuel and oxidant are directed by means of internal manifolds to the electrodes. Cooling for the stack is provided either by the reactants, natural convection, radiation, and possibly supplemental cooling channels and/or cooling plates. Also included in the prior art stack designs are current collectors, cell-to-cell seals, insulation, piping, and various instrumentation for use in monitoring cell performance. The fuel cell stack, housing, and associated hardware make up the operational fuel cell plant. As will be apparent, such prior art designs are unduly large, cumbersome, and quite heavy. Certainly, any commercially useful PEM fuel cell designed in accordance with the prior art could not be manipulated by hand because of these characteristics. 
     It is well known that PEM fuel cells can operate at higher power output levels when supplemental humidification is made available to the proton exchange membrane (electrolyte). Humidification lowers the resistance of proton exchange membranes to proton flow. Supplemental water can be introduced into the hydrogen or oxygen streams or more directly to the proton exchange membrane by means of the physical phenomena of wicking. The focus of investigation in recent years has been to develop Membrane/Electrode Assemblies (MEAs) with increasingly improved power output when running without supplemental humidification (self-humidified). Being able to run an MEA when it is self-humidified is advantageous because it decreases the complexity of the balance-of-plant and its attendant costs. However, self-humidification heretofore has resulted in fuel cells running at lower current densities, and thus, in turn, has resulted in more of these assemblies being required in order to generate a given amount of power. This places added importance on reducing the cost of the supporting structures, such as the bipolar plates, in conventional designs. 
     Accordingly, a proton exchange membrane fuel cell power system which achieves the benefits to be derived from the aforementioned technology but which avoids the detriments individually associated therewith, is the subject matter of the present invention. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is to provide a proton exchange membrane fuel cell power system having a plurality of discrete PEM fuel cell modules with individual membrane electrode diffusion assemblies, the PEM fuel cell modules further having individual force application assemblies for applying a given force to the membrane electrode diffusion assemblies. Further, the PEM fuel cell modules of the present invention can be easily manipulated by hand. 
     Another aspect of the present invention is to provide a PEM fuel cell module which, in operation, produces a given amount of heat energy, and wherein the same PEM fuel cell module has a cathode air flow which removes a preponderance of the heat energy generated by the PEM fuel cell module. 
     Another aspect of the present invention is to provide a proton exchange membrane fuel cell power system wherein each of the discrete PEM fuel cell modules has opposing membrane electrode diffusion assemblies having a cumulative active area of at least about 60 square centimeters, and wherein each of the discrete fuel cell modules produce a current density of at least about 350 mA per square centimeter of active area at a nominal voltage of about 0.5 volts D.C.; and a power output of at least about 10.5 watts. 
     Still a further aspect of the present invention relates to a proton exchange membrane fuel cell power system which includes an enclosure defining a cavity; and a subrack mounted in the cavity and supporting the plurality of discrete proton exchange membrane fuel cell modules. 
     Another aspect of the present invention relates to a proton exchange membrane fuel cell power system which comprises: 
     a hydrogen distribution frame defining discrete cavities, and wherein individual membrane electrode diffusion assemblies are sealably mounted in each of the cavities, the membrane electrode diffusion assemblies each having opposite anode and cathode sides; and 
     a pair of current collectors received in each of the cavities, the individual current collectors positioned in ohmic electrical contact with the respective anode and cathode sides of each of the membrane electrode diffusion assemblies. 
     A further aspect of the present invention relates to a proton exchange membrane fuel cell power system comprising: 
     a cathode cover which partially occludes the respective cavities of the hydrogen distribution frame, the respective cathode covers individually releasably cooperating with each other, and with the hydrogen distribution frame; and 
     a pressure transfer assembly received in each of the cavities and applying a given force to the current collectors and the membrane electrode diffusion assembly, and wherein the cathode cover is disposed in force transmitting relation relative to the pressure transfer assembly. 
     A further aspect of the present invention relates to a proton exchange membrane fuel cell power system which includes a membrane electrode diffusion assembly comprising: 
     a solid proton conducting electrolyte membrane which has opposite anode and cathode sides; 
     individual catalytic anode and cathode electrodes disposed in ionic contact with the anode and cathode sides of the electrolyte membrane; and 
     a diffusion layer borne on each of the anode and cathode electrodes and which is electrically conductive and has a given porosity. 
     Still another aspect of the present invention is to provide a proton exchange membrane fuel cell power system having a solid electrolyte membrane which comprises crosslinked polymeric chains having sulfonic acid groups, and wherein the crosslinked polymeric chains comprise methacrylates. 
     Moreover, another aspect of the present invention relates to a proton exchange membrane fuel cell power system which includes current collectors which have at least about 70% open area. 
     These and other aspects of the present invention will be discussed in further detail hereinafter. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings serve to explain the principles of the present invention. 
     FIG. 1 is a perspective, front elevation view of a proton exchange membrane fuel cell power system of the present invention and showing some underlying structures in phantom lines. 
     FIG. 2 is a perspective view of a subrack employed with the present invention. 
     FIG. 3 is a fragmentary, transverse, vertical sectional view taken from a position along line  3 — 3  of FIG.  2 . 
     FIG. 4 is a second, fragmentary, transverse, vertical sectional view taken from a position along line  3 — 3  of FIG.  2 . 
     FIG. 5 is a perspective view of a portion of the subrack. 
     FIG. 6 is transverse, vertical, sectional view taken from a position along line  6 — 6  of FIG.  2 . 
     FIG. 7A is a transverse, vertical, sectional view taken through an air mixing valve of the present invention. 
     FIG. 7B is a transverse, vertical, sectional view taken through an air mixing valve of the present invention, and showing the valve in a second position. 
     FIG. 8 is a longitudinal, horizontal, sectional view taken from a position along line  8 — 8  of FIG.  2 . 
     FIG. 9 is a perspective, exploded, side elevation view of a proton exchange membrane fuel cell module utilized with the present invention, and the accompanying portion of the subrack which mates with same. 
     FIG. 10 is a side elevation view of a hydrogen distribution frame utilized with the proton exchange membrane fuel cell module of the present invention. 
     FIG. 11 is a perspective, side elevation view of a proton exchange membrane fuel cell module utilized with the present invention. 
     FIG. 12 is a partial, exploded, perspective view of one form of the PEM fuel cell module of the present invention. 
     FIG. 13 is a partial, greatly enlarged, perspective, exploded view of a portion of the PEM fuel cell module shown in FIG.  12 . 
     FIG. 14 is a partial, exploded, perspective view of one form of the PEM fuel cell module of the present invention. 
     FIG. 15 is a partial, greatly enlarged, perspective, exploded view of a portion of the PEM fuel module shown in FIG.  14 . 
     FIG. 16 is a partial, exploded, perspective view of one form of the PEM fuel module of the present invention. 
     FIG. 17 is a partial, greatly enlarged, perspective, exploded view of a portion of the PEM fuel cell module shown in FIG.  16 . 
     FIG. 18 is a partial, exploded, perspective view of one form of the PEM fuel cell module of the present invention. 
     FIG. 19 is a partial, greatly enlarged, perspective exploded view of a portion of the PEM fuel cell module shown in FIG.  18 . 
     FIG. 20 is a perspective view of a pressure plate which is utilized in one form of PEM fuel cell module of the present invention. 
     FIG. 21 is an end view of the pressure plate shown in FIG.  20 . 
     FIG. 22 is a fragmentary, transverse, vertical sectional view taken through a cathode cover of the present invention and showing one form thereof. 
     FIG. 23 is a fragmentary, transverse, vertical sectional view taken through a cathode cover of the present invention and showing an alternative form thereof. 
     FIG. 24 is a fragmentary, transverse, vertical sectional view taken through a cathode cover of the present invention and showing an alternative form thereof. 
     FIG. 25 is a fragmentary, transverse, vertical sectional view taken through a cathode cover of the present invention and showing an alternative form thereof. 
     FIG. 26 is a greatly simplified, exploded view of a membrane electrode diffusion assembly of the present invention. 
     FIG. 27 is a greatly simplified, exploded view of an alternate form of the membrane electrode diffusion assembly of the present invention. 
     FIG. 28 is a top plan view of a current collector employed in the PEM fuel cell module of the present invention. 
     FIG. 29 is a greatly enlarged perspective view of a pressure transfer assembly which is utilized with the present invention. 
     FIG. 30 is a greatly simplified, schematic view of the control assembly of the present invention. 
     FIG. 31 is a greatly simplified schematic view of a heat exchanger which is employed with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The proton exchange membrane (PEM) fuel cell power system of the present invention is generally indicated by the numeral  10  in FIG.  1 . As shown therein, the PEM fuel cell power system includes an enclosure which is generally indicated by the numeral  11 , and which sits on the surface of the earth or other supporting surface  12 . Enclosure  11  has left and right sidewalls  13  and  14 , respectively, and front and rear surfaces  15  and  20 , respectively. The enclosure has a top surface  21  which is joined to the left and right sidewalls; and front and rear surfaces respectively. First and second apertures  22  and  23  respectively are formed in the front surface  15 . Further, a pair of doors which are generally designated by the numeral  24 , are hingedly mounted on the front surface  15  and are operable to occlude the respective apertures  22  and  23 . The enclosure  11 , described above, defines a cavity  25  of given dimensions. 
     Occluding the aperture  23  are a plurality of subracks which are generally indicated by the numeral  30 . The subracks are individually mounted in the cavity  25 , and are operable to support a plurality of discrete PEM fuel cell modules in a given orientation in the cavity  25 . The PEM fuel cell modules will be discussed in greater detail hereinafter. Referring now to FIG. 2, each subrack  30  has a main body  31  for supporting the PEM fuel cell modules. The main body includes supporting flanges  32 , which are attached by suitable fasteners to the enclosure  11 . The subrack  30  has a forward edge  33  and a rearward edge  34 . Further, the main body has top and bottom portions  35  and  40 , respectively. A rear wall  41  (FIGS. 5 and 6) joins the top and bottom portions together at the rearward edge  34 . As best seen in FIG. 2, a plurality of apertures  42  and  43  are formed in the top and bottom portions  35  and  40 , respectively. Further, elongated channels  44  and  45  are formed in the respective top and bottom portions  35  and  40 , respectively. As best understood by reference to FIGS. 3,  4 , and  5 , the main body  31  is made up of a number of discrete mirror image portions  31 A, which when joined together, form the main body. This is further seen in FIG.  8 . These mirror image portions are fabricated from a moldable, dielectric substrate. As best seen by reference to FIGS. 5 and 6, a D.C. (direct current) bus  50  is affixed on the rear wall  41  of the subrack  30 . A repeating pattern of 8 pairs of conductive contacts  51  are attached on the rear wall  41 . Further, first and second valves  52  and  53  are appropriately positioned relative to the 8 pairs of conductive contacts  51 . As best seen in FIG. 6, the respective first and second valves  52  and  53  extend through the rear wall  41  and are coupled in fluid flowing relation relative to first and second conduits  54  and  55 , respectively. Referring now to FIG. 8, the first conduit  54  constitutes a hydrogen distribution assembly which is coupled in fluid flowing relation with a source of hydrogen  60  (FIG.  1 ). Further, a valve assembly  61  is coupled in fluid meter relation relative to the source of hydrogen  60  and the first conduit  54 . The second conduit  55  exhausts to ambient, or may be coupled in fluid flowing relation with other systems such as a hydrogen recovery and recycling system or alternatively a chemical reformer which produces a supply of hydrogen for use by the power system  10 . In this regard, the hydrogen recovery and recycling system would recover or recapture unreacted hydrogen which has previously passed through the individual PEM fuel cell modules. This system, in summary, would separate the unreacted hydrogen from other contaminants (water, nitrogen, etc.) and return it to the power system  10 . In the alternative, a chemical reformer may be utilized for the purpose described above, and the unreacted hydrogen would be returned to the chemical reformer where it would again be delivered to the individual PEM fuel cell modules, as will be described in further detail below. 
     Referring now to FIG. 6, the PEM fuel cell power system  10  of the present invention further includes an air distribution assembly  70  which is received in the enclosure  11  and which is coupled in fluid flowing relation relative to the subrack  30 . The air distribution assembly  70  includes an air plenum which is generally indicated by the numeral  71 . The air plenum  71  has a first intake end  72 , and a second exhaust end  73 . The exhaust end  73  is positioned intermediate the top and bottom portions  35  and  40  and delivers air to each of the PEM fuel cell modules supported on the subrack  30 . Further, the intake end  72  is positioned in fluid flowing relation relative to the top and bottom portions  35  and  40  of the subrack  30 . 
     An air movement ventilation assembly  74  comprising a direct current fan  75  or equivalent substitute is operably coupled to the plenum  71 . The variably adjustable speed fan  75  moves air from the intake end  72  to the exhaust end  73  of the plenum  71 . Referring now to FIGS. 6,  7 A and  7 B, an air mixing valve  80  is operably coupled with the air movement assembly  74 , and the intake end  72  of the plenum  71 . The air mixing valve  80  includes an outer tube  81  which has formed therein a pair of apertures  82  which communicate in fluid flowing relation with the air plenum  71 . Still further, the air mixing valve  80  includes an inner tube  83  which is telescopingly received internally of, and substantially concentrically disposed relative to, the outer tube  81 . The inner tube  83  is selectively rotatable relative to the outer tube  81 . A pair of apertures  84  are formed in the inner tube and provides a convenient means by which the exhaust end  73  may be selectively coupled with the intake end  72  of the air plenum  71 . Still further, it should be understood that the inner tube is connected in fluid flowing relation with ambient air which comes from outside of the plenum  71 . As illustrated most clearly by references to FIG. 2, an actuator, or motor  85 , is disposed in force transmitting relation relative to the air mixing valve  80  and more specifically, to the inner tube  83  thereof. The actuator  85 , when energized, moves the air mixing valve  80  and more specifically, the second tube along a given course of travel  90  between a first position  91 , as seen in FIG. 7B, to a second open position  92 , which is seen in FIG.  7 A. The movement of the air mixing valve  80  along this course of travel  90  facilitates the selective mixing of outside air with the air which has previously passed through the respective PEM fuel cell modules and which has become heated and humidified by way of the chemical reaction taking place within each of the proton exchange membrane fuel cell modules. 
     As best appreciated by a study of FIG. 30, temperature sensors  93  are positioned near the exhaust end  73  of the air plenum  71  for sensing the temperature of the air entering the discrete PEM fuel cell modules and near the plenum intake end  72 . The temperature sensors  93  sense the temperature of the air mixture which comprises outside ambient air, and the air which has just passed through each of the discrete proton exchange membrane fuel cell modules. Still further, and as best seen in FIG. 30, a control assembly  250  is electrically coupled with the temperature sensors  93 , and the actuator  85 . The control assembly selectively energizes the actuator  85  to move the air mixing valve  80  along the course of travel  90  to control the temperature of the air delivered at the exhaust end  73  of the air plenum  71 . As should be understood, the air movement assembly  74  has a speed of operation which is variably adjustable. In this regard, the control assembly is electrically coupled in controlling relation relative to the air movement assembly  74 , temperature sensors  93 , and the air mixing valve  80  to vary or otherwise optimize the performance characteristics of the Proton Exchange Membrane (PEM) fuel cell modules under assorted operational conditions. This relationship is illustrated most accurately by a study of FIG.  30 . 
     Referring now to FIG. 9, a plurality of discrete PEM fuel cell modules are generally indicated by the numeral  100 , and are releasably supported on the subrack  30 . The description which follows relates to a single PEM fuel cell module  100 , it being understood that each of the PEM fuel cell modules are substantially identical in construction, and are light in weight and can be readily manipulated or moved about by hand. 
     A discrete PEM fuel cell module  100  is best illustrated by reference to FIGS. 9 and 11 respectively. Referring now to FIG. 10, each PEM fuel cell module  100  includes a hydrogen distribution frame which is generally indicated by the numeral  110 . The hydrogen distribution frame  110  is fabricated from a substrate which has a flexural modulus of less than about 500,000 pounds per square inch, and a compressive strength of less than about 20,000 pounds per square inch. As such, any number of suitable or equivalent thermoplastic materials can be utilized. The hydrogen distribution frame  110  includes a main body  111  as seen in FIG.  10 . The main body has a first end  112 , and an opposite second end  113 . Further, the main body is defined by a peripheral edge  114 . Positioned in a given location along the peripheral edge is a handle  115  which facilitates the convenient manual manipulation of the PEM fuel cell module  100 . An elongated guide member or spine  116  is located on the first and second ends  112  and  113  respectively. Each spine  116  is operable to be matingly received in, or cooperate with, the respective elongated channels  44  and  45  which are formed in the top and bottom portions  35  and  40  of the subrack  30  (FIG.  9 ). As should be understood, the alignment and mating receipt of the individual spines  116  in the respective channels allows the individual PEM fuel cell modules  100  to be slidably received and positioned in predetermined spaced relation, one to the other, on the subrack  30 . Such is seen most clearly by reference to FIG.  2 . When received on the subrack  30 , the exhaust end  73  of the air plenum  71  is received between two adjacent PEM fuel cell modules  100 . 
     As seen in FIG. 10, the main body  111  defines a plurality of substantially opposed cavities  120 . These cavities are designated as first, second, third, and fourth cavities  121 ,  122 ,  123 , and  124  respectively. Still further, and referring again to FIG. 10, a plurality of apertures  125  are formed in given locations in the main body  111  and are operable to receive fasteners which will be discussed in further detail hereinafter. The main body  111  further defines a pair of passageways designated generally by the numeral  130 . The pair of passageways include a first passage  131  which permits the delivery of hydrogen gas from the source of same  60 , to each of the cavities  121 - 124 ; and a second passageway  132  which facilitates the removal of impurities, water and unreacted hydrogen gas from each of the cavities  121 - 124 . A linking passageway  133  operably couples each of the first and second cavities  121 , and  122 , and the third and fourth cavities  123  and  124  in fluid flowing relation one to the other, such that hydrogen gas delivered by means of the first passageway  131  may find its way into each of the cavities  121 - 124 . Each of the cavities  121  through  124  are substantially identical in their overall dimensions and shape. Still further, each cavity has a recessed area  134  having a given surface area and depth. Positioned in the recessed area  134  and extending substantially normally outwardly therefrom are a plurality of small projections  135 . The function of these individual projections will be discussed in greater detail below. As best seen in FIG. 10, the first and second passageways  131  and  132  are connected in fluid flowing relation relative to each of the recessed areas  134 . Referring still to FIG. 10, the peripheral edge  114  of the main body  111  is discontinuous. In particular, the peripheral edge  114  defines a number of gaps or openings  136 . Referring now to FIG. 11, each passageway  131  and  132  has a terminal end  137  which has a given outside diametral dimension. The terminal end  137  of each passageway  130  is operable to matingly interfit in fluid flowing relation relative to the first and second valves  52  and  53  respectively. 
     Referring now to FIGS. 12,  13 ,  26 , and  27 , sealably mounted within the respective cavities  121  through  124  respectively is a membrane electrode diffusion assembly  150  which is generally indicated by the numeral  150 . The membrane electrode diffusion assembly  150  has a main body or solid electrolyte membrane  151  which has a peripheral edge  152  which is sealably mounted to the hydrogen distribution frame  110 . The membrane electrode diffusion assembly  150  has an anode side  153 , and an opposite cathode side  154 . The anode side  153  is held in spaced relation relative to hydrogen distribution frame  110  which forms the respective cavities  121 - 124  by the plurality of projections  135  (FIG.  10 ). This special relationship ensures that hydrogen delivered to the respective cavities  121 - 124  reaches all parts of the anode side of the membrane electrode diffusion assembly  150 . Electrodes  160 , comprising catalytic anode and cathode electrodes  161  and  162  are formed on the main body  151 . These individual anode and cathode electrodes  161  and  162  are disposed in ionic contact therewith. Still further, a noncatalytic electrically conductive diffusion layer  170  is affixed on the anode and cathode electrodes  160  and has a given porosity. As best illustrated in FIG. 26, the noncatalytic electrically conductive diffusion layer  170  has a first diffusion layer  171  which is positioned in ohmic electrical contact with each of the electrodes  161  and  162  respectively, and a second diffusion layer  172  which is positioned in ohmic electrical contact with the underlying first diffusion layer. As best seen in FIG. 27, a second form of the membrane electrode diffusion assembly  150  is shown and wherein a third diffusion layer  173  is provided. In this form, the third layer is affixed to the main body  151  prior to affixing the first and second diffusion layers thereto. In this regard, a number of commercially available membrane electrode assemblies are fabricated which have a preexisting proprietary diffusion layer attached to same, the composition of which is unknown to the inventors. 
     Referring now to FIG. 26, the membrane electrode diffusion assembly  150  and more specifically, the first diffusion layer  171  which is affixed thereto comprises a coating of particulate carbon suspended in a binding resin. Further, the second diffusion layer  172  comprises preferably a porous hydrophobic carbon backing layer. With respect to the binding resin, it is substantially hydrophobic and is selected from the group consisting essentially of perfluorinated hydrocarbons or a substitute equivalent. Further, the first diffusion layer  171  has about 20% to about 90% by weight of particulate carbon. With respect to the second diffusion layer  172 , it is selected from the group consisting essentially of carbon cloth, carbon paper, or carbon sponge or a substitute equivalent which has been rendered hydrophobic. In the preferred form of the invention, the first diffusion layer  171  is a composite coating formed of successive layers of the first diffusion layer, each of the successive layers having a given hydrophobicity. Additionally, the first diffusion layer  171  has a hydrophobic gradient. This gradient can be altered by adjusting the hydrophobicity of the successive layers that form the composite coating. Depending upon the desired performance parameters of the membrane electrode diffusion assembly  150  that is employed, the successive layers closest to the second diffusion layer  172  may be the least hydrophobic of all the successive layers, or the most hydrophobic. To affix the first and second diffusion layers  171  and  172  to the underlying anode and cathode electrodes  161  and  162 , a thermoplastic binding agent can be utilized and which is selected from the group consisting essentially of polyethylene or wax, or a substitute equivalent. Still further, these same layers may be attached by pressure and heat. In the preferred form of the invention, the individual anode and cathode electrodes  161  and  162  comprise particulate carbon; a catalyst such as platinum or the like; and a crosslinked copolymer incorporating sulfonic acid groups. 
     The method of forming the first and second diffusion layers  171  and  172 , as described above, is discussed in the paragraphs which follow. The method of forming a diffusion layer  170  for use with a membrane electrode diffusion assembly  150  comprises as a first step, providing a carbon backing layer constituting a second diffusion layer  172 . The carbon backing layer is selected from the group consisting essentially of carbon cloth, carbon paper, or carbon sponge. The subsequent steps in the method comprises applying a hydrophobic polymer to the carbon backing layer constituting the second diffusion layer  172 ; and sintering the carbon backing layer constituting the second diffusion layer at a temperature greater than the melting point of the hydrophobic polymer. As discussed above, the hydrophobic polymer is selected from the group consisting essentially of perfluorinated hydrocarbons or a substitute equivalent. Still further, in the method as described, the sintering step takes place at a temperature of about 275 degrees to about 365 degrees C. The preferred method of forming the diffusion layer  170  for use with the membrane electrode diffusion assembly  150  comprises providing a porous carbon backing layer constituting the second diffusion layer  172 ; and applying a porous coating comprising a slurry of particulate carbon, a binding resin and a delivery fluid which is applied on the porous carbon backing layer. The porous carbon backing layer constituting the second diffusion layer  172  is the same as was described above. Further, the binding resin is hydrophobic and may include perfluorinated hydrocarbons. The porous coating comprises at least about 20% to about 90% by weight of the is particulate carbon. The delivery fluid utilized to form the slurry of particulate carbon and the binding resin comprises water, and a compatible surfactant. In this regard, the delivery fluid consists essentially of about 95% to about 99% by weight of water; and about 1% to about 5% by weight of the compatible surfactant. The surfactant is selected from the group consisting essentially of ammonium hydroxide and/or alcohol. In the examples which follow, the delivery fluid utilized consists of a solution of 2-butoxyethanol and ammonium hydroxide as the surfactants. A solution such as this may be commercially secured. In the present instance, the inventors used a commercially available cleaner with properties such as “Windex”. Windex is the registered trademark of S. C. Johnson and Sons. After the delivery of the slurry which includes the binding resin and particulate carbon, the method further comprises removing the delivery fluid thereby leaving behind or depositing the particulate carbon and the binding resin on the porous carbon backing layer constituting the first diffusion layer  171 . The delivery fluid is removed by applying heat energy to same which facilitates the evaporation of the delivery fluid. 
     In another alternative form of the invention, the binding resin and porous carbon coating slurry as described above, may be applied in successive coats thereby creating a hydrophobic gradient. This hydrophobic gradient, as earlier discussed, may be adjusted by altering the hydrophobicity of each of the successive coats. To achieve this adjustable hydrophobic gradient, binding resins are selected from the group consisting essentially of hydrophobic and hydrophilic binding resins or a substitute equivalent. As should be appreciated, the given hydrophobicity of each coat forming the composite first diffusion layer  171  may be adjusted by providing a predetermined mixture of the hydrophobic and hydrophilic binding resins in the resulting slurry or by altering the proportional relationship of the components. As was discussed above, each of the coatings of the composite first diffusion layer  171  which are applied closest to the porous carbon backing layer may be the most hydrophilic or the least hydrophilic depending upon the performance characteristics desired for the membrane electrode diffusion assembly  150 . Still further, the method may include a sintering step whereby the resulting diffusion layer  170  is sintered at a temperature effective to melt the binding resin and create a substantially homogeneous surface. In addition to the foregoing, the method further comprises, after the sintering step, applying a predetermined pattern of pressure of a given value to the diffusion layer  171 , and which is effective to vary the porosity of the resulting diffusion layer  170 . As was discussed above, the diffusion layer  170  may be attached to the underlying catalytic anode and cathode electrodes  162  and  163  respectively by utilizing a thermoplastic binding emulsion which is selected from the group consisting essentially of polyethylene or wax or alternatively by utilizing heat and pressure. 
     The diffusion layer  170 , described above, is useful in managing the water which is generated as a result of the chemical reaction taking place in each of the PEM fuel cell modules  100 . In this regard, the inventors have discovered that the diffusion layer  170  allows sufficient water to escape from the cathode side of the membrane electrode diffusion assembly  150  such that the PEM fuel cell module  100  does not “flood out” with water thereby inactivating same. On the other hand, the hydrophobic gradient, as described above, facilitates the retention of sufficient moisture such that the PEM fuel cell module  100  becomes self-humidifying, that is, sufficient water is retained in the membrane electrode diffusion assembly  150  such that it achieves substantially the maximum current density possible without the addition of extra moisture or humidification from outside of the PEM fuel cell module  100 . Still further, the air distribution assembly  70  and air mixing valve  80  provides a convenient means by which outside ambient air may be added to air which has previously passed through each of s the PEM fuel cell modules  100  thereby maintaining the PEM fuel cell modules  100  in a properly humidified state. As should be understood, this mixing of air effectively removes water from the cathode side of membrane electrode diffusion assembly  150 . Additionally, the same mixing of air effectively removes heat energy which is a by-product of the chemical reaction taking place in each of the PEM fuel cell modules  100  and thus maintains the PEM fuel cell modules at a stable temperature. In this regard, the air delivered at the exhaust end  73  of the air plenum  71  constitutes the cathode air flow, and in the present invention  10  a novel feature of the power system  10  is that a preponderance of the heat energy produced by each of the PEM cell modules  100  is removed from same by this cathode air flow. 
     Examples of forming the diffusion layer  170  on an underlying main body  151  of the membrane electrode diffusion assembly  150  is set forth below. 
     The examples set forth hereinafter relate to the fabrication of the diffusion layer  170  as seen in FIGS. 26 and 27, respectively. 
     GENERAL TEST PROCEDURES 
     A hydrogen/air fuel cell test fixture was fabricated from a two-part stainless steel fixture which encloses a 4 cm×4 cm proton conductivity membrane electrode diffusion assembly (MEDA) for testing. The hydrogen side of the block (anode) defines a cavity which contains a flat, perforated ceramic plate. Pressure conditions effective to affix top of this plate is a matching perforated platinum coated nickel current collector. Hydrogen gas passes into the anode half of the stainless steel fixture, through the holes in the ceramic plate and the associated current collector. The hydrogen is thus able to reach the anode of the MEDA, which is placed on top of the anode current collector. 
     The proton-conducting MEDA, which is purchased from the W.L. Gore Company under the trade designation Primea 6000 Series is larger than the electrodes which are affixed to same, the MEDA, having dimensions of about 5 cm×5 cm. This allows for the placement of a sealing gasket around the periphery of the electrodes when the stainless steel test fixture is bolted together. 
     The cathode side of the test fixture also defines a cavity which matingly receives a perforated ceramic plate and current collector. However, the stainless steel fixture does not press the current collector against the MEDA directly. Instead, five screws are mounted on the test fixture cathode side. These screws press against a perforated metal pressure plate. The plate has apertures which are substantially coaxially aligned with the holes formed in the ceramic plate. These screws further press against the ceramic plate and the current collector. By threadably advancing the screws, the current collector contact pressure relative to the MEDA can be selectively adjusted after the stainless steel test fixture has been bolted together. 
     A supply of air is provided at the cathode, by means of several holes which have been machined into the stainless steel fixture between the aforementioned pressure screws. This allows air to travel past the screws and the perforated steel pressure plate, ceramic plate, and current collector to the cathode side of the MEDA. 
     The test MEDAs were placed over the cathode of the test fixure along with a sealing gasket. The test fixture is then bolted together. The pressure screws are then threadably advanced until sufficient force has been generated at the current collector cathode/anode interfaces for good electrical contact. Once this has been accomplished, the hydrogen gas is supplied at a pressure of about 5 PSI. The anode side of the MEDA is then purged of any air. A supply of fresh air is then supplied to the cathode side of the MDEA by means of a fan or the like. The supply of air had a dewpoint of 15 degrees Celsius. 
     Electrical performance is tested by loading the fuel cell with a variety of resistors. Since the resistor values are known, the current can be computed by examining the voltage across the resistors. MEDAs are initially short-circuited to condition them, and then are allowed to stabilize at a given load of usually about 0.6 volts. When a steady-state power output has been obtained, the data is gathered. 
     For comparative testing, the diffusion layers  170  which are affixed on the cathode and anode sides of MEDA are often dissimilar. A PEM fuel cell&#39;s electrical performance is largely unaffected when the configuration of the anode diffusion layer is changed. However, the diffusion layer placed on the cathode side of the MEA, on the other hand, has a significant impact on the electrical performance of the PEM fuel cell because water production, and evaporation of same, must occur on the cathode side of the MEDA. As earlier noted, the fabrication method described above includes subjecting the diffusion layer  170  to given temperature and pressure conditions effective to affix the diffusion layer  170  to the underlying MEA thus forming the MEDA. In this regard, the same pressure is applied to the cathode and anode sides of the MEDA. The most accurate comparison between two different diffusion layers made by the foregoing methods is done by using a single MEDA. In this regard, comparative testing between dissimilar layers is done by simply flipping the MEDA over in the test fixture, thus reversing the anode and cathode sides of the MEDA. Therefore, the same MEDA is tested with different diffusion layers acting as the cathode. 
     EXAMPLE 1 
     A sheet of carbon paper (Toray TGP-H-060) was dipped into a solution of 2:3 Teflon-120 (Dupont) and deionized water for several minutes. Teflon-120 is a hydrophobic polymer comprising polytetrafloroethylene. Teflon-120 is a trade designation of the E.I. Dupont Company. After removal from the solution the carbon paper was allowed to dry in a horizontal position on top of an open cell foam sponge. The carbon paper was then placed in an air-filled sintering oven (360° C.) for 3-5 minutes. This rendered the carbon paper hydrophobic. 
     Diffusion Layer Side “A”. 
     A solution comprising water, and a compatible surfactant was then prepared. In this regard, 200 ml of a commercially available cleaner “Windex” was mixed with 4.2 g Vulcan XC-72R (Cabot) particulate carbon powder. The mixture was sonicated for 90 seconds at a power of about 200 watts, using a stir-bar to agitate the mixture during sonication and create a slurry. After sonication, 1 ml of a hydrophobic polymer, Teflon-30 (Dupont), was added. The slurry was then sprayed onto the carbon paper with an air brush using multiple passes. Once the carbon paper was wetted with the solution, it was placed on a hot plate to evaporate the solution comprising the water and surfactant. The spraying/drying process was then repeated. As will be appreciated, the process of evaporation deposited the particulate carbon and associated binding resin until a final (dried) added weight of 6.4 mg/cm 2  had been achieved. Finally, the coated carbon paper was loaded into the air-filled sintering oven at 360° C. for 3-5 minutes. This sintering melted the binding resin and created a substantially homogeneous surface. 
     Diffusion Layer Side “B”. 
     Side “B” was fabricated in approximately the same manner as side “A” described above. However, side “B” was placed into a press for 10 seconds and was subjected to three tons of force. An irregular surface was placed on top of the diffusion layer side “B” prior to subjecting it to pressure. In the present test a 150-grit sandpaper with an aluminum foil spacer sheet was utilized. The spray-on side faced the sandpaper/foil during the pressing stress. 
     MEA Fabrication: 
     The diffusion layers were affixed to a commercially available membrane electrode diffusion assembly such as that which may be secured from the W.L. Gore Company under the trade designation Primea Series 6000. This was done by placing the assembly in a hot press. The MEDA was hot pressed several times using successively higher pressures. Data was taken between successive presses. 
     Results: 
     Each sample was loaded into the test fixture described above. Temperatures were measured at the diffusion layer surface and were within a range of +/−2° C. These temperatures were controlled by varying the cathode air flow. Values are current density as expressed in mA/cm 2  at 0.6 volts. Sides “A” and “B” as noted below refer to that side acting as the cathode. 
     
       
         
           
               
               
               
               
            
               
                   
                   
               
               
                   
                 36° C. 
                 45° C. 
                 53° C. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Press Method 
                 A 
                 B 
                 A 
                 B 
                 A 
                 B 
               
               
                   
               
               
                 3.5 tons, 190° C., 20 sec 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 4 tons, 190° C., 20 sec 
                 318 
                 319 
                 293 
                 325 
                 277 
                 316 
               
               
                 4.5 tons, 180° C., 30 sec 
                 372 
                 363 
                 350 
                 353 
                 322 
                 322 
               
               
                 5 tons, 180° C., 30 sec 
                 399 
                 371 
                 363 
                 374 
                 319 
                 361 
               
               
                 5.5 tons, 180° C., 30 sec 
                 338 
                 363 
                 375 
                 394 
                 — 
                 — 
               
               
                 6 tons, 170° C., 40 sec 
                 269 
                 250 
                 356 
                 380 
                 363 
                 354 
               
               
                   
               
            
           
         
       
     
     Conclusions: The patterned-press (side “B”) yields slightly better or equal performance at 45° C. and 53° C. at 8 of the 9 comparative data points. Good performance is also obtained with side “A”. 
     EXAMPLE 2 
     A sheet of carbon paper (Toray TGP-H-060) was dipped into a solution of 2:3 Teflon-120 (Dupont) and deionized water solution for several minutes. As earlier noted, Teflon-120 is a trade designation of E.I. Dupont Company. After removal from the solution the carbon paper was allowed to dry in a horizontal position on top of an open cell foam sponge. The carbon paper was then placed in an air-filled sintering oven (360° C.) for 3-5minutes. The heat energy melted the polytetrafluoroethylene (Teflon-120) thus making a substantially homogeneous surface. This sintering rendered the carbon paper hydrophobic. 
     Diffusion layer side “A”: 
     A slurry was then prepared utilizing water and a compatible surfactant such as ammonia or the like. In the present example the slurry was prepared by mixing 200 ml of a commercially available cleaner “Windex” with 4.2 g Vulcan XC-72R (Cabot) particulate carbon powder. The slurry was then sonicated for 90 seconds at a power of about 200 watts, using a stir-bar to agitate the slurry during sonication. After sonication, 1 ml of hydrophobic polymer solution Teflon-30 (Dupont) was added. The slurry was then sprayed onto the carbon paper with an air brush using multiple passes as was described earlier. Once the carbon paper had been wetted with the slurry, it was placed on a hot plate to evaporate the solution of water and surfactant. The spraying/drying process was then repeated until a final (dried) added weight of 6.4 mg/cm 2  had been achieved. Side “B” was subsequently placed into a press for 10 seconds and subjected to three tons of force. As with the first example, an irregular surface was utilized between the press and the sprayed on layers. In this example, a 150-grit sandpaper with an aluminum foil spacer sheet was employed. The spray-on side faced the sandpaper/foil combination during pressing. 
     Diffusion layer side “B”: 
     A similar slurry was prepared by mixing 200 ml of “Windex” with 4.2 g Vulcan XC-72R (Cabot) carbon powder. The mixture was sonicated for 90 seconds at a power of about 200 watts, using a stir-bar to agitate the mixture during sonication. After sonication, a 1.2 ml solution of Teflon-30 (Dupont) was added to the slurry. The mixture was then sprayed onto the carbon paper with an air brush using multiple passes. Once the carbon paper had been wetted with the solution, it was placed on a hot plate to evaporate the water and surfactant solution (Windex). The spraying/drying process was then repeated until a final (dried) added weight of 4.91 mg/cm 2  had been achieved. 
     A second slurry was then prepared by mixing 200 ml of “Windex” with 4.2 g Vulcan XC-72R (Cabot) carbon powder. The slurry was sonicated for 90 seconds at a power of about 200 watts, using a stir-bar to agitate the mixture during sonication. After sonication, 0.5 ml of Teflon-30 (Dupont) was added. The slurry was then sprayed onto the previously coated carbon paper with an air brush using multiple passes. Once the carbon paper had been wetted with the slurry, it was placed on a hot plate to evaporate the water and surfactant and thus deposit the hydrophobic polymer and the particulate carbon. The spraying/drying process was then repeated until a final (dried) added weight of 1.58 mg/cm 2  had been achieved. The total weight of the spray-on layers was 6.5 mg/cm 2 . Side “B” was placed into the press for 10 seconds at 3 tons of force underneath an irregular surface (150-grit sandpaper with an aluminum foil spacer sheet). The spray-on side faced the sandpaper/foil combination during pressing. 
     MEA Fabrication: 
     The diffusion layers were affixed to a commercially available MEDA such as what was described in Example 1, above. This was done by placing the assembly in the hot press. The MEDA was hot pressed several times using successively higher pressures. Data was taken between successive presses. 
     Results: 
     Each sample was loaded into the test fixture as described earlier. Temperatures as noted below were measured at the diffusion layer surface and are within a tolerance of +/−2° C. The values which are set forth are expressed in mA/cm 2  at 0.6 volts. Sides “A” and “B” refer to that side acting as the cathode. For this sample, as identified, hot pressing involved two identical steps at the same pressure and rotating the MEDA 180 degrees between each pressing. 
     
       
         
           
               
               
               
               
            
               
                   
                   
               
               
                   
                 36° C. 
                 45° C. 
                 53° C. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Press Method 
                 A 
                 B 
                 A 
                 B 
                 A 
                 B 
               
               
                   
               
               
                 2 × 3.5 tons, 170° C., 40 s 
                 238 
                 219 
                 318 
                 219 
                 277 
                 244 
               
               
                 2 × 4 tons, 170° C., 40 s 
                 263 
                 263 
                 331 
                 356 
                 331 
                 325 
               
               
                 2 × 4.5 tons, 170° C., 40 s 
                 206 
                 206 
                 338 
                 344 
                 325 
                 356 
               
               
                 2 × 5 tons, 170° C., 40 s 
                 219 
                 219 
                 325 
                 338 
                 325 
                 356 
               
               
                 2 × 5.5 tons, 170° C., 40 s 
                 188 
                 213 
                 263 
                 269 
                 244 
                 256 
               
               
                   
               
            
           
         
       
     
     The results demonstrate that the reverse gradient samples, when subjected to greater than 3.5 tons pressure, produces current densities which are equal to or better than the non-gradient samples in 11 of 12 comparative tests. 
     EXAMPLE 3 
     A sheet of carbon paper (Toray TGP-H-090) was dipped into a solution of 4:9 Teflon-120 (Dupont) and deionized water for several minutes. After removal from the solution the carbon paper was allowed to dry in a horizontal position on top of an open cell foam sponge. The carbon paper was then placed in an air-filled sintering oven (360° C.) for 3-5 minutes. This rendered the carbon paper hydrophobic. 
     Diffusion layer side “A”: 
     A slurry was then prepared by mixing 200 ml of “Windex” with 4.2 grams of Vulcan XC-72R (Cabot) carbon powder. This is identical to the previous examples. The slurry was then sonicated for 90 seconds at a power of about 200 watts, using a stir-bar to agitate the mixture during sonication. After sonication, a 1.2 ml solution of Teflon-30 (Dupont) was added to the slurry. The slurry was then sprayed onto the carbon paper with an air brush using multiple passes. Once the carbon paper had been wetted with the solution, it was placed on a hot plate to evaporate the water and surfactant solution (Windex) and thus deposit the hydrophobic polymer and particular carbon. The spraying/drying process was then repeated until a final (dried) added weight of 1.0 mg/cm 2  had been achieved. 
     A second slurry was then prepared by mixing 200 ml of “Windex” with 4.2 grams of Vulcan XC-72R (Cabot) carbon powder. The slurry was then sonicated for 90 seconds at a power of about 200 watts, using a stir-bar to agitate the mixture during sonication. After sonication, 0.5 ml of a solution of Teflon-30 (Dupont) was added. The slurry was then sprayed onto the previously coated carbon paper with an air brush using multiple passes. Once the carbon paper had been wetted with the slurry, it was placed on a hot plate to evaporate the water and surfactant solution (Windex), and thereby deposit the hydrophobic polymer and particulate carbon. The spraying/drying process was repeated until a final (dried) added weight of 0.5 mg/cm 2  had been achieved. The total weight of the spray-on layers is approximately 1.5 mg/cm 2 . 
     Diffusion layer side “B”: 
     Side “B” was prepared exactly the same as side “A”. However, side “B” was placed into a press for 10 seconds and subjected to three tons of force underneath an irregular surface (150-grit sandpaper with an aluminum foil spacer sheet). The spray-on side faced the sandpaper/foil combination during pressing. 
     MEA Fabrication: 
     The diffusion layers were affixed to a commercially available MEDA as was discussed in Example 1, above. This was done by placing the MEDA in a hot press. The MEDA was hot pressed several times using successively higher pressures. Data was taken between successive presses. 
     Results: 
     Each sample was loaded into the aforementioned test fixture. The temperature as noted below was measured at the diffusion layer surface and is within a tolerance of +/−2° C. The values below are expressed in mA/cm 2  at 0.6 volts. Sides “A” and “B” refer to that side acting as the cathode. For this sample, hot pressing usually involved two identical steps at the same pressure. The MEDA was rotated about 180 degrees between each pressing. 
     
       
         
           
               
               
               
               
            
               
                   
                   
               
               
                   
                 36° C. 
                 45° C. 
                 53° C. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Press Method 
                 A 
                 B 
                 A 
                 B 
                 A 
                 B 
               
               
                   
               
               
                 2 × 3.5 tons, 170° C., 40 s 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 2 × 4 tons, 170° C., 40 s 
                 363 
                 316 
                 369 
                 338 
                 350 
                 319 
               
               
                 2 × 4.5 tons, 170° C., 40 s 
                 363 
                 — 
                 363 
                 — 
                 325 
                 — 
               
               
                 2 × 5 tons, 170° C., 40 s 
                 413 
                 — 
                 416 
                 — 
                 400 
                 — 
               
               
                 4 × 5.5 tons, 170° C., 30 s 
                 388 
                 — 
                 419 
                 — 
                 413 
                 — 
               
               
                   
               
            
           
         
       
     
     The data above further confirms the novelty of the diffusion layer  170  and associated membrane electrode diffusion assembly construction  150 . 
     EXAMPLE 4 
     Diffusion Layer Side “A” 
     A sheet of carbon paper (Toray TGP-H-060) was dipped into a solution of 4:9 Teflon-120 (Dupont) and deionized water for several minutes. After removal from the solution the carbon paper was allowed to dry in a horizontal position on top of an open cell foam sponge. The carbon paper was then placed in an air-filled sintering oven (360° C.) for 3-5minutes. This sintering rendered the carbon paper hydrophobic. 
     A slurry was then prepared utilizing water and a compatible surfactant such as ammonia or the like. In the present example the slurry was prepared by mixing 200 ml of a commercially available cleaner “Windex” with 4.2 g Vulcan XC-72R (Cabot) carbon powder. The slurry was sonicated for 90 seconds at a power of about 200 watts, using a stir-bar to agitate the slurry during sonication. After sonication, 1.2 ml of hydrophobic polymer solution Teflon-30 (Dupont) was added. The mixture was then sprayed onto the carbon paper with an air brush using multiple passes as was described earlier. Once the carbon paper had been wetted with the slurry, it was placed on a hot plate to evaporate the solution of water and surfactant. The spraying/drying process was then repeated until a final (dried) added weight of 4.2 mg/cm 2  had been achieved. 
     A second slurry was then prepared by mixing 200 ml Windex with 4.2 g Vulcan XC-72R (Cabot) carbon powder. The slurry was sonicated for 90 seconds at a power of about 200 watts, using a stir-bar to agitate the slurry during sonication. After sonication, 0.5 ml of Teflon-30 (Dupont) was added. The slurry was then sprayed onto the previously coated carbon paper with an air brush using multiple passes. Once the carbon paper had been wetted with the slurry, it was placed on a hot plate to evaporate the water and surfactant. The spraying/drying process was then repeated until a final (dried) added weight of 1.6 mg/cm 2  had been achieved. The total weight of the spray-on layers was 5.8 mg/cm 2 . 
     Diffusion Layer Side “B” 
     A sheet of carbon paper (Toray TGP-H-090) was dipped into a solution of 4:9 Teflon-120 (Dupont):DI water for several minutes. After removal from the solution the carbon paper was allowed to dry in a horizontal position on top of an open cell foam sponge. The carbon paper was then placed in an air-filled sintering oven (360° C.) for 3-5 minutes. This rendered the carbon paper hydrophobic. 
     A slurry was then prepared utilizing water and a compatible surfactant such as ammonia or the like. The slurry was prepared by mixing 200 ml of a commercially available cleaner “Windex” with 4.2 g Vulcan XC-72R (Cabot) carbon powder. The slurry was sonicated for 90 seconds at a power of about 200 watts, using a stir-bar to agitate the slurry during sonication. After sonication, 1.2 ml of hydrophobic polymer solution Teflon-30 (Dupont) was added. The mixture was then sprayed onto the carbon paper with an air brush using multiple passes. Once the carbon paper had been wetted with the slurry, it was placed on a hot plate to evaporate the solution of water and surfactant. The spraying/drying process was then repeated until a final (dried) added weight of 1.0 mg/cm 2  had been achieved. 
     A second slurry was then prepared by mixing 200 ml Windex with 4.2 g Vulcan XC-72R (Cabot) carbon powder. The slurry was sonicated for 90 seconds at a power of about 200 watts, using a stir-bar to agitate the slurry during sonication. After sonication, 0.5 ml of Teflon-30 (Dupont) was added. The slurry was then sprayed onto the previously coated carbon paper with an air brush using multiple passes. Once the carbon paper had been wetted with the slurry, it was placed on a hot plate to evaporate the water and surfactant. The spraying/drying process was then repeated until a final (dried) added weight of 0.5 mg/cm 2  had been achieved. The total weight of the spray-on layers was 1.5 mg/cm 2 . 
     MEDA Fabrication and Testing 
     The respective diffusion layers were affixed to a commercially available membrane electrode assembly, such as that which may be secured from the W.L. Gore Company under the trade designation Primea Series 6000. This was done by placing the assembly in a hot press. The 60 cm 2  MEDA was hot pressed once for 4 minutes at 150° C. at a pressure of 32 tons, and then re-pressed for an additional minute at 150° C. at a pressure of 37 tons. The process was repeated to fabricate four MEDAs. 
     Results 
     A PEM fuel cell module  100  was fabricated using the four MEDAs. The fuel cell module  100  was configured as shown in FIGS. 10,  11 , and  14 , except that the ceramic plate  205  was deleted, and the pressure transfer assembly  203  directly contacted the cathode current collector  192 . The fuel cell module  100  was tested by inserting it into a test stand similar to that illustrated in FIG. 5, and using a small fan to pass approximately 12 cubic feet per minute of air through the fuel cell module. The hydrogen feed pressure was set to about 8 psi. At 2.004 volts (approximately 0.5 volts per MEDA), a current of 24.0 amperes was measured using a calibrated DC current transducer, which yielded a current density of 400 mA/cm 2  and a PEM fuel cell module power of 48.096 watts. 
     The main body  151  of the membrane electrode diffusion assembly  150 , as earlier discussed, comprises an electrolyte membrane having substantially linear crosslinked polymeric chains incorporating sulfonic acid groups. In particular, the crosslinked polymeric chains are formed from monomeric units which are selected from the group consisting essentially of poly (ethylene glycol) methacrylate, poly (propylene glycol) methacrylate, poly (ethylene glycol) ethyl ether methacrylate, and poly (propylene glycol) methyl ether methacrylate, hydroxpropyl methacrylate, 2-hydroxyethyl methacrylate, the acrylate analogs, and 4-hydroxybutyl acrylate. Linear copolymers composed of similar monomeric units, but synthesized without crosslinking agents will be described below. 
     Still further, the sulfonic acid groups are selected from the group consisting essentially of 3-alkoxy-2-hydroxy-1-propanesulfonic acid, 4-styrenesulfonic acid, vinylsulfonic acid, 3-sulfopropyl methacrylate, 3-sulfopropylacrylate and fluorinated derivatives thereof. Additionally, the crosslinking agent utilized to crosslink the copolymers is selected from the group consisting essentially of ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, glycerol dimethacrylate, diallyloxyacetic acid, and allylmethacrylate. 
     In the preferred form of this invention, the membrane electrode diffusion assembly  150  comprises: 
     about 35% to about 50% by molar concentration of a methacrylate monomer; 
     about 30% to about 50% by molar concentration of an acrylate monomer; 
     about 25% to about 45% by molar concentration of a sulfonic acid; and 
     about 5% to about 20% by molar concentration of a compatible crosslinking agent. 
     In the preferred form of the invention as described above, the electrolyte membrane, or main body  151 , which is incorporated into the membrane electrode diffusion assembly  150 , has a glass transition temperature of at least about 110 degrees C., and has a preferred thickness of about 0.2 millimeter. Additionally, this electrolyte membrane  151  must be substantially stable in the presence of water, and operational at temperatures of less than about 80 degrees C. The electrolyte membrane  151 , as noted above, may further comprise a compatible plasticizer which is selected from the group consisting essentially of phthalate esters. In still another form of the invention, the electrolyte membrane  151  includes a porous supporting matrix which is made integral with the electrolyte membrane  151 , and which provides mechanical strength to same. In this regard, the porous supporting matrix does not reactively produce hydrogen ions and is dielectric. Further, the porous supporting matrix is substantially inert and has a porosity of about 30% to about 80% and has a given proton conductivity which is proportional to the porosity of the supporting matrix. An acceptable porous supporting matrix may be selected from the group consisting essentially of grafted hydrophilic polyethylenes. 
     In its most preferred form, the electrolyte membrane  151  of the present invention comprises at least about 10% to about 50% by molar concentration of a copolymer which has monomeric units which are selected from the group consisting essentially of poly (ethylene glycol) methacrylate, poly (propylene glycol) methacrylate, poly (ethylene glycol) ethyl ether methacrylate, poly (propylene glycol) methyl ether methacrylate, hydroxypropyl methacrylate, 2-hydroxyethyl methacrylate, acrylate analogs and 4-hydroxybutyl acrylate; 
     at least about 25% to about 45% by molar concentration of an acid selected from the group consisting essentially of 3-alkoxy-2-hydroxy-1-propanesulfonic acid, 4-styrenesulfonic acid, vinylsulfonic acid, 3-sulfopropyl methacrylate, 3-sulfopropylacrylate and fluorinated derivatives thereof; 
     at least about 5% to about 20% by molar concentration of a crosslinking agent selected from the group consisting essentially of ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, glycerol dimethacrylate, diallyloxyacetic acid, and allylmethacrylate; 
     a compatible plasticizer; and 
     a support matrix having a given minimum porosity, and which is dielectric. 
     As discussed above, one example of a suitable electrolyte membrane  151  may be secured from the W.L. Gore Company under the trade designation Primea Series 6000 MEA. 
     Representative examples which concern the synthesis of the electrolyte membrane  151  are set forth below. 
     EXAMPLE 1 
     10.56 mL of a 15.78% w/v aqueous solution (8 mmol) of 3-sulfopropyl methacrylate was first concentrated so as to yield a final reaction mixture with a water content of 16.3% v/v. Poly(propylene glycol) methacrylate (2.9600 g, 8 mmol), hydroxypropyl methacrylate (0.14422 g, 1 mmol), and glycerol dimethacrylate (0.4565 g, 2 mmol) were added to, and well mixed with the concentrated acid solution. The mixture was cooled to 4 degrees C., and then cold ethylene glycol divinyl ether (0.2283 g, 2 mmol), and ammonium persulfate (0.5052 g, 2.2 mmol) dissolved in 0.72 mL of water were added. After thorough mixing, the reaction mixture was de-aerated and applied onto grafted polyethylene (E15012) that was previously rendered hydrophilic. Photochemical polymerization was achieved under UV light for 10 minutes. 
     EXAMPLE 2 
     A 15.78% w/v aqueous solution of 3-sulfopropyl methacrylate (7.92 mL, 6 mmol) was concentrated so as to obtain a final reaction mixture with 17.7% v/v water, and poly (propylene glycol) meth-acrylate (5.1800 g, 14 mmol) was then added and well mixed. Benzoyl peroxide (0.4844 g, 2 mmol) and 1,1-azobis(1-cyclohexanecarbonitrile) (0.4884 g, 2 mmol) were dissolved in acetone (4 mL) and added. The reaction mixture was then de-aerated under vacuum and substantially all of the acetone was removed. Thermal polymerization was effected at 71-74 degrees C. for 90 minutes. After cooling to room temperature overnight, the product crystallized into bundles of needle-shaped crystals. Similar linear polymers were also synthesized by using hydroxypropyl methacrylate (14 mmol) and poly(ethylene glycol) methacrylate (14 mmol), in place of the poly (propylene glycol) methacrylate. 
     EXAMPLE 3 
     A 35% aqueous solution of 3-allyloxy-2-hydroxy-1-propanesulfonic acid (21.60 mL, 40 mmol) was concentrated so as to yield a final reaction mixture containing 120.9% v/v water. Poly(propylene glycol) methacrylate (11.1000 g, 30 mmol), hydroxpropyl methacrylate (2.8834 g, 20 mmol), and diethylene glycol dimethacrylate (2.4227 g, 10 mmol) were added, and well mixed with the acid. The initiator ammonium persulfate (1.1410 g, 5 mmol) was dissolved in the proper amount of water and added. After thorough mixing, the reaction mixture was de-aerated, and either thermally polymerized in a mold at 75 degrees C. for 90 minutes or photochemically polymerized under UV light for 10 minutes using grafted and hydrophilized polyethylene as a support material. Poly(ethylene glycol) methacrylate was also used as a substitute for poly(propylene glycol) methacrylate, and the crosslinked mixture consisting of glycerol dimethacrylate (or diallyloxyacetic acid) and ethylene glycol divinyl ether was also used as a substitute for diethylene glycol dimethacrylate. 
     Each electrolyte membrane synthesized from the examples, above, were tested and were found to yield the performance characteristics as earlier discussed. 
     As seen in FIGS. 12-19 and  28 , the proton exchange membrane fuel cell power system  10  of the present invention further includes a pair of current collectors  190  which are received in each of the respective cavities  121  through  124 , respectively. The current collectors for identification have been given the numerals  191  and  192 , respectively. The current collectors  190  are individually disposed in juxtaposed ohmic electrical contact with the opposite anode and cathode sides  153  and  154  of each membrane electrode diffusion assembly  150 . As best seen in FIG. 28, each current collector  190  has a main body  193  which has a plurality of apertures or open areas formed therein  194 . In this regard, the main body has a given surface area of which, at least about 70% is open area. A conductive member  195  extends outwardly from the main body and is operable to extend through one of the openings or gaps  136  which are formed in the hydrogen distribution frame  110 . Such is seen in FIG.  11 . Each conductive member  195  is received between and thus electrically coupled within one of the 8 pairs of conductive contacts  51  which are mounted on the rear wall  41  of the subrack  30 . This is illustrated most clearly by reference to FIGS. 3 and 4. 
     As a general matter, the current collectors  190  comprise a base substrate forming a main body  193 , and wherein a coating(s) or layer(s) is applied to same and which is effective in maintaining electrical contact with the adjacent membrane electrode diffusion assembly  150 . The main body  193  includes four discrete components. The first component is an electrically conductive substrate which may be capable of surface passivation if exposed to oxygen. Suitable materials which may be used in this discrete component of the main body  193  include current carrying substrates consisting essentially of 3XX Series chromium containing stainless steel or equivalent substitutes. These substrates have a bulk conductivity of about 2.4 IACS, and an overall thickness of about 0.7 to about 3 mm. Additionally, copper or nickel substrates having a bulk conductivity of greater than about 24% IACS, and a thickness of about 0.20 to about 1.3 mm. may be used with equal success. 
     The second component, which may be utilized in forming the main body  193  comprises a protection layer formed in covering relation over the conductive substrate, and which will passivate if inadvertently exposed to oxygen. Suitable materials for this second component include a foil cladding of 3XX Series chromium-containing stainless steel which has a bulk conductivity of about 2.4% IACS, and a thickness of about 0.02 to about 0.15 mm; or a coating or alloy combination consisting essentially of column IVB metal(s) such as tantalum and niobium which form a highly passivated pentoxide if exposed inadvertently to air. This coating or alloy combination has a thickness of about 0.2 to about 2 microns. 
     The third component forming the main body  193  comprises an electrically conductive contact layer which is galvanically cathodic, and oxygen stable. Suitable materials which may be utilized in the contact layer include coatings formed from elements classified in column IVB and which are capable of forming nitrides. Examples of these materials include titanium or zirconium nitride, or an electrically conductive metal oxide such as indium-tin oxide (ITO). An equivalent substitute material includes platinum group metals such as palladium, platinum, rhodium, ruthenium, iridium and osmium. This third component has a thickness of about 0.2 to about 2 microns. 
     The fourth component forming the main body  193  comprises an electrolyte/oxygen exclusion layer. A suitable material for this function includes a graphite-filled electrically conductive adhesive. Such may be synthesized from a two-part epoxy or a silicone rubber. 
     Many combinations of the four components may be fabricated to produce a suitable main body  193 . Each main body  193  will have an electrically conductive substrate. The assorted combinations of the other three components which are used therewith are not specifically set forth below, it being understood that not less than one of the remaining three components, and not more than all three remaining components must be brought together to form a suitable main body  193 . In the preferred embodiment, the inventors have discovered that a main body  193  which is formed from an electrically conductive substrate, such as nickel or copper; a foil cladding comprising 3XX series chromium-containing stainless steel; and a coating formed from column IVB electrically conductive materials which can form nitrides operates with good results. 
     Referring now to FIGS. 12-19 and  22 - 25 , respectively, the proton exchange membrane fuel cell power system  10  of the present invention further includes individual force application assemblies  200  for applying a given force to each of the pair of current collectors  190 , and the membrane electrode diffusion assembly  150  which is sandwiched therebetween. In this regard, the individual force application assemblies are best illustrated by reference to FIGS. 13,  15 ,  17  and  19 , respectively. In the first form of the force application assembly, which is shown in FIG. 12 and 22, the force application assembly comprises a cathode cover  201  which partially occludes the respective cavities of the hydrogen distribution frame  110 . As seen in the drawings, the respective cathode covers  201  individually releasably cooperate with each other and with the hydrogen distribution frame  110 . A biasing assembly which is designated by the numeral  202 , and shown herein as a plurality of metal wave springs cooperates with the cathode cover and is operable to impart force to an adjacent pressure transfer assembly  203  by means of a pressure distribution assembly  204 . Referring now to FIGS. 14,  15 , and  23 , and in a second form of the invention, the pressure transfer assembly  203  transfers the force imparted to it by the cathode covers  201  to an adjoining pressure plate  205 . In this form of the invention, the pressure distribution assembly is eliminated. 
     In a third form of the invention as seen in FIGS. 16 17 , and  24 , the force application assembly  202  comprises a cathode cover  201 , a plurality of wave springs  202 ; and a corrugated pressure plate  232 . In this form of the invention, the pressure transfer assembly  203  is eliminated from the assembly  200 . In yet still another fourth form of the invention as seen in FIG. 18,  19 , and  25 , the force application assembly  200  comprises a cathode cover  201 ; wave springs  202 , and a pressure transfer assembly  203 . In this form of the invention, the pressure plate  205  (of either design) and pressure distribution assembly  204  are absent from the combination. In all the forms of the invention described above, a force of at least about 175 pounds per square inch is realized between the membrane electrode diffusion assembly  150  and the associated pair of current collectors  190 . 
     Referring now to FIG. 11, each cathode cover  201  has a main body  210  which is fabricated from a substrate which has a flexural modulus of at least about 1 million pounds per square inch. This is in contrast to the hydrogen distribution frame  110  which is fabricated from a substrate having a flexural modulus of less than about 500,00 pounds per square inch, and a compressive strength of less than 20,000 pounds per square inch. The main body  210  has an exterior facing surface  211 , and an opposite interior facing surface  212  (FIG.  13 ). Further, the main body has a peripheral edge  213  which has a plurality of apertures  214  formed therein. Each cathode cover nests, or otherwise matingly interfits with one of the respective cavities  121  through  124 , respectively, which are defined by the hydrogen distribution frame  110 . When appropriately nested, the individual apertures  214  are substantially coaxially aligned with the apertures  125  which are formed in the main body  111  of the hydrogen distribution frame  110 . This coaxial alignment permits fasteners  215  to be received therethrough. When tightened, the opposing cathode covers exert a force, by means of the intermediate assemblies, described above, on the membrane electrode diffusion assembly  150  which is effective to establish good electrical contact between the respective current collectors  190  and the adjacent membrane electrode diffusion assembly  150 . Still further, the main body  210  defines in part a third passageway  216 . The third passageway  216  as seen in FIGS. 9 and 11, provides a convenient means by which the cathode air flow which is delivered by the exhaust end  73  of the air plenum  71 , can be delivered to the cathode side  154  of the membrane electrode diffusion assembly  150 . In this regard, the air passageway has a first, or intake end  217  and a second, or exhaust end  218 . As seen in FIG. 9, the exhaust end of each third passageway  216  is located near one of the opposite ends  112  and  113  of the hydrogen distribution frame  110 . As illustrated in FIG. 6, the air which has exited through the exhaust end  218  passes through the apertures  42  and  43  formed in the top and bottom portions  35  and  40  of the subrack  30 . As such, the air passes into the air plenum  71  and may be recycled by means of the air mixing valve  80  as was earlier described. As best illustrated by reference to FIGS. 13 and 22, the interior surface  212  of the cathode cover defines a cavity  219  of given dimensions. The interior surface further defines a plurality of channels  220 . The channels  220  are operable to matingly receive the individual wave springs which constitute the biasing assembly  202 . 
     Referring now to FIG. 29, the pressure transfer assembly  203  has an elongated main body  221  which comprises a central backbone  222 . Additionally, a plurality of legs or members  223  extend or depend from the central backbone  222  and are operable to forcibly engage the pressure plate  205  in one form of the invention (FIGS.  14  and  23 ). Still further, the main body  220  has a first surface  224  and an opposite second surface  225 . A channel  226  is formed in the first surface and matingly interfits or receives one of the metal wave springs constituting the biasing assembly  202  (FIG.  25 ). In an alternative form of the invention, the pressure plate  205  is eliminated, and a pressure distribution member  204  is positioned between the biasing assembly  203  and the first surface  224  of the pressure transfer assembly (FIGS. 12 and  22 ). In this form of the invention, the pressure transfer assembly  204  is fabricated from a resilient substrate such that the individual legs or members will deform under pressure to an amount equal to about 0.001 to 0.004 inches. 
     As noted above, one form of the invention  10  may include a pressure plate  205  (FIGS.  14  and  15 ). In this regard the pressure plate  205 , as illustrated, is a ceramic plate or an equivalent substitute having a main body  230 . The ceramic plate, as shown in FIG. 15, has a plurality of apertures  231  formed therein which allows air to pass therethrough and which has traveled through the third passageway  216  which is formed, in part, in the main body  210  of the cathode cover  201 . The main body  230  of the pressure plate  205  is substantially planar to less than about 0.002 inches. An alternative form of the pressure plate is shown in FIGS. 16 and 17 and is designated by the numeral  232 . This second form of the pressure plate is thicker than the pressure plate  205  which is shown in FIG.  15 . Referring now to FIGS. 20 and 21, the pressure plate  232  defines a given open area therebetween a plurality substantially equally spaced corrugations or undulations  233  which are formed in its surface. These corrugations or undulations define specific channels  234  therebetween through which air can move. When the second form  232  of the pressure plate  205  is employed, the pressure transfer assembly  203  may be eliminated from the assembly as was earlier discussed. The channels, or open area  234  defined by the pressure plate  205 , whether it be in the first form of the pressure plate as shown in FIG. 15, or that shown in FIG. 20, defines, in part, the third passageway  216  which allows air to pass through the cathode cover  201  to the cathode side  154  of the membrane electrode diffusion assembly  150 . Such is best illustrated by reference to FIG.  11 . As earlier discussed, and as seen in FIGS. 12 and 13, one form of the invention  10  utilizes a pressure distribution assembly  204 . When employed, the pressure plate  205  is eliminated and the pressure distribution assembly  204  is positioned between the wave springs which constitute the biasing assembly  202 , and the pressure transfer assembly  203  which were described earlier. In this regard, the if pressure distribution assembly comprises a first substantially noncompressible and flexible substrate  240  (FIG.  22 ). The first non-compressible substrate has a first surface  241  and an opposite second surface  242 . The first surface  241  is in contact with the biasing assembly  202 . Mounted upon the opposite, second surface  242  is a compressible substrate  243 . The compressible substrate has an outwardly facing surface  244  which is in contact with the first surface  224  of the pressure transfer assembly  203 . In operation, as the respective cathode covers and associated biasing assemblies  202  exert force, a certain amount of deflection or bending in the cathode covers may occur. This is shown in the drawings at FIG.  22 . When this event happens, the first surface of the pressure transfer assembly presses against the compressible surface  243  thereby maintaining a substantially constant pressure across the entire surface of the adjacent current collector  190 . 
     The proton exchange membrane fuel cell power system  10  further includes a digital programmable control assembly  250 , as seen in the schematic view of FIG.  30 . The digital programmable control assembly  250  is electrically coupled with each of the discrete PEM fuel cell modules  100  such that they can be monitored with respect to the electrical performance of same. This digital programmable control assembly  250  is further electrically coupled with the air distribution assembly  70 . Still further, the digital programmable control assembly  250  is electrically coupled with the fuel distribution assembly which comprises the source of hydrogen  60 , accompanying valve assembly  61  and associated first conduit  54  which delivers the hydrogen by means of one of the valves  52  to each of the discrete PEM fuel cell modules  100 . 
     Still further, and referring to FIG. 31, the PEM fuel cell power system  10  of the present invention includes a heat exchanger  260  which is operably coupled with the air distribution assembly  70  which delivers air to the individual discrete PEM fuel cell modules  100 . The heat exchanger  260  captures useful thermal energy emitted by the discrete PEM fuel cell modules  100 . Additionally, the power system  10  includes a power conditioning assembly  270  (FIG. 1) comprising an inverter which is electrically coupled with the direct current bus  50  and which receives the direct current electrical energy produced by the individual discrete PEM fuel cell modules  100  and which converts same into suitable alternating current. 
     OPERATION 
     The operation of the described embodiments of the present invention are believed to be readily apparent and are briefly summarized at this point. 
     In its broadest aspect, the present invention comprises a proton exchange membrane fuel cell power system  10  having a plurality of discrete proton exchange membrane fuel cell modules  100  which are self-humidifying and which individually produce a given amount of heat energy. Further, each of the discrete proton exchange membrane fuel cell modules  100  have a cathode air flow, and a preponderance of the heat energy produced by each of PEM fuel cell modules  100  is removed from same by the cathode air flow. 
     Another aspect of the present invention relates to a proton exchange membrane fuel cell power system  10  for producing electrical power and which comprises a plurality of discrete fuel cell modules  100 , each having at least two membrane electrode diffusion assemblies  150 . Each of the membrane electrode diffusion assemblies  150  have opposite anode  153 , and cathode sides  154 . Additionally, this PEM fuel cell power system  10  includes a pair of current collectors  190  each disposed in juxtaposed ohmic electrical contact with the opposite anode  153  and cathode sides  154  of each of the membrane electrode diffusion assemblies  150 . Further, individual force application assemblies  200  for applying a given force to each of the current collectors and the individual membrane electrode diffusion assemblies are provided. The individual force application assemblies, as earlier noted, may be in several forms. Commonly each form of the force application assemblies has a cathode cover  201 , and a biasing assembly  202 . However, in one form of the invention, a pressure plate  205  may be utilized, and comprises a ceramic plate having a plurality of apertures formed therein. As seen in FIG. 14, a pressure transfer assembly  205  is provided and is effective to transmit force, by way of the pressure plate, to the underlying membrane electrode diffusion assembly  150 . In an alternative form (FIG.  16 ), a second pressure plate  232  may be employed. When used, the pressure transfer assembly  203  may be eliminated from the construction of the PEM fuel cell module  100 . In still another form of the force application assembly  200  (FIG.  12 ), the pressure plate  205  is eliminated and the pressure distribution assembly  204  is utilized to ensure that substantially equal force is applied across the surface area of the adjacent current collector  190 . 
     As presently disclosed, the PEM fuel cell power system  10  and more particularly, the discrete PEM fuel cell modules  100  have an electrical efficiency of at least 40% and are self-humidifying, that is, no additional external humidification must be provided to the hydrogen fuel  60 , or air which is supplied to same. Still further, the membrane electrode diffusion assemblies  150  which are utilized in the present invention have an active area which has a given surface area. It has been determined that the discrete PEM fuel cell modules  100  produce a current density of at least about 350 m.A. per square centimeter of active area at a nominal cell voltage of at least about 0.5 volts D.C. Additionally, the discrete fuel cell modules  100  each have an electrical output of at least about 10.5 watts. 
     The individual proton exchange membrane fuel cell modules  100  are mounted within an enclosure  11  which includes a subrack  30  for supporting same. The enclosure  11  which is utilized with the present proton exchange membrane fuel cell modules  100  further comprises a fuel distribution assembly  52 ,  54  and  60 , for delivering hydrogen to the individual discrete PEM fuel cell modules  100 . An air distribution assembly  70  for delivering air to the individual discrete PEM fuel cell modules  100  is provided, and a direct current output bus  50 , and a power conditioning assembly  270  for receiving and inverting the electrical power produced by each of the discrete PEM fuel cell modules  100  are also received in the enclosure  11 . As earlier discussed, each of the subracks  30  are mounted in the cavity  25  which is defined by the enclosure  11 . The subracks  30  have forward and rearward edges  33 , and  34  and top and bottom portions  35  and  40 , respectively. Each of the discrete PEM fuel cell modules  100  are. operably coupled with the fuel distribution assembly, direct current output bus  50  and power conditioning assembly  270  in the vicinity of the rearward edge  34  of each of the subracks  30  as seen most clearly in FIGS. 3,  4  and  6 . Further, the discrete PEM fuel cell modules  100  are coupled in fluid flowing relation with the air distribution assembly  70  at the top and bottom portions  35  and  40  of each of the subracks  30  and with the air plenum  70  at the exhaust end  73  thereof. 
     Referring to FIG. 6, the air distribution assembly  70  which is utilized in the present device includes a plenum  71  which is made integral with each of the subracks  30 . The plenum has an exhaust end  73  which delivers air to each of the PEM fuel cell modules  100  supported on the subrack  30 , and an intake end  72  which receives both air which has passed through each of the PEM fuel cell modules  100  and air which comes from outside the respective PEM fuel cell modules  100 . Further, the air distribution assembly  70  includes an air movement assembly  74  in the form of a fan  75  which is operably coupled to the plenum  71  and which moves the air in a given direction along the plenum  71  to the individual PEM fuel cell modules  100 . An air mixing valve  80  is borne by the plenum  71 , and controls the mixture of air which is recirculated back to the respective PEM fuel cell modules  100 . 
     As described earlier in greater detail, the individual discrete PEM fuel cell modules  100  include a hydrogen distribution frame  110  defining discrete cavities  120 , and wherein the respective membrane electrode diffusion assemblies  150  are individually sealably mounted in each of the cavities  120 . In the preferred form of the invention, the hydrogen distribution frame  110  is oriented between the individual membrane electrode diffusion assemblies  150 . As best seen in FIG. 10, the hydrogen distribution frame  110  comprises multiple pairs of discretely opposed cavities  121 - 124 . 
     The hydrogen distribution frame  110  permits the delivery of hydrogen gas to each of the cavities  121 - 124 . In this regard, the hydrogen distribution frame  110  defines a first passageway  131  which permits the delivery of hydrogen gas to each of the cavities  121 - 124  which are defined by the hydrogen distribution frame  110  and to the anode side  153  of the membrane electrode diffusion assembly  150 . Still further, the hydrogen distribution frame  110  includes a second passageway  132  which facilitates the removal of impurities, water, and unreacted hydrogen from each of the cavities  121 - 124 . As noted earlier, each of the cathode covers  201  and the respective force application assemblies  200  define a third passageway  216  which permits delivery of air to each of the cavities  121 - 124 , and to the cathode side  154  of each of the respective membrane electrode diffusion assemblies  150 . Hydrogen gas is supplied by means of the first passageway  131  to each of the cavities  121 - 124  of the hydrogen distribution frame  110  at a pressure of about 1 PSIG to about 10 PSIG; and air is supplied at above ambient pressure by the air distribution assembly  70 . 
     Also as discussed above, the source of hydrogen  60  is illustrated herein as a pressurized container of same which is received in the enclosure  11  (FIG.  1 ). However, it is anticipated that other means will be employed for supplying a suitable quantity of hydrogen to the hydrogen distribution assembly  110 . In this regard, a chemical or fuel reformer could be utilized and enclosed within or outside of the enclosure  11  and which would, by chemical reaction, produce a suitable quantity of hydrogen. The chemical reformer would be coupled with a supply of hydrogen rich fluid such as natural gas, ammonia, or similar fluids. The chemical reformer would then, by means of a chemical reaction, strip away the hydrogen component of the hydrogen rich fluid for delivery to the hydrogen distribution assembly. The remaining reformer by-products would then be exhausted to ambient (assuming these by-products did not produce a heath, environmental or other hazard), or would be captured for appropriate disposal, or recycling. 
     The membrane electrode diffusion assembly  150  which is employed with the power system  10  of the present invention includes, as a general matter, a solid proton conducting electrolyte membrane  151  which has opposite anode and cathode sides  153  and  154 ; individual catalytic anode and cathode electrodes  161  and  162  which are disposed in ionic contact with the respective anode and cathode sides  153  and  154  of the electrolyte membrane  151 ; and a diffusion layer  170  borne on each of the anode and cathode electrodes  161  and  162  and which is electrically conductive and has a given porosity. With respect to the diffusion layer  170 , in the preferred embodiment of the present invention  10 , the diffusion layer  170  comprises a first diffusion layer  171  borne on the individual anode and cathode electrodes  161  and  162  and which is positioned in ohmic electrical contact therewith. The first diffusion layer  171  is electrically conductive and has a given pore size. Additionally, a second diffusion layer  172  is borne on the first layer  171  and is positioned in ohmic electrical contact with the underlying first diffusion layer  171 . The second diffusion layer  172  is electrically conductive and has a given pore size which is greater than the given pore size of the first diffusion layer  171 . 
     In its broadest aspect the present invention  10  includes an electrolyte membrane  151  which comprises crosslinked polymeric chains incorporating sulfonic acid groups. More specifically, the electrolyte membrane  151  has at least a 20% molar concentration of sulfonic acid. The diffusion layer  170  which is employed with the membrane electrode diffusion assembly  150  of the present invention is deposited by means of a given method which was described earlier, and is not repeated herein. 
     In the present invention  10 , the individual anode and cathode electrodes  161  and  162  in their broadest aspect, include particulate carbon; a catalyst; a binding resin; and a crosslinked copolymer incorporating sulfonic acid groups. In addition to the foregoing, the power system  10  further includes a pair of current collectors  190  which, in their broadest aspect, include a base substrate which is electrically conductive and is capable of surface passivation if exposed to oxygen; and a contact layer which is electrically conductive, galvanically cathodic and oxygen stable. Still further, the pair of current collectors  190  have a thickness of about 0.1 millimeters to about 1.3 millimeters and the contact layer has a thickness of about 0.2 microns to about 2 microns. In addition to the foregoing, the base substrate  190  has a given surface area of which at least 70% is open area. 
     The power system  10  includes a digital programmable control assembly  250  for monitoring the performance of the individual proton exchange membrane fuel cell modules  100 , and other parameters of operation such as the flow rate of hydrogen  60  to the individual discrete PEM fuel cell modules  100 , the heat output of each of the proton exchange membrane fuel cell modules  100 , and the operation of the air distribution assembly  70  which mixes both outside air and air which has previously passed through the individual proton exchange membrane fuel cell modules  100 . The air mixing valve  80  is effective in controlling the temperature of the air which is delivered to each of the proton exchange membrane fuel cell modules  100 , as well as the relative humidity. In this fashion, the preponderance of heat energy generated by each of the PEM fuel cell modules  100  is effectively removed from same and either exhausted to ambient, or captured for other uses. The control assembly  250  is operable therefore to effectively optimize the operational conditions of the individually discrete PEM fuel cell modules  100  such that maximum current densities and efficiencies can be realized. 
     Some of the most significant advantages of the present invention  10  is that it is modularized, simple, efficient in operation and easy to maintain. For example, in the event that a particular PEM fuel cell module  100  becomes inoperable, the disabled PEM fuel cell module  100  can be quickly removed, by hand, from the subrack  30  and replaced with an operational module without interrupting the operation of the power system  10 . This is a significant advancement in the art when considering the prior art teachings which show that a defective PEM fuel cell (manufactured as a stack) would require total disassembly of same while repairs were undertaken. 
     The present power system  10  has numerous other advantages over the prior art techniques and teachings, including the elimination of many of the balance-of-plant subassemblies typically utilized with such devices. Yet further, in view of the self-humidifying nature of the present proton exchange membrane fuel cell modules  100 , other control measures have been simplified or otherwise eliminated thereby increasing the performance capabilities of same while simultaneously reducing the costs to generate a given amount of electrical power. 
     In compliance with the statute, the invention has been described in language more or less specific as to structural or methodical features. It is to be understood, however, that the invention is not limited to specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.