Abstract:
An electrochemical cell having a membrane electrode assembly comprises a first active area and an opposingly positioned second active area with a flow field support member disposed adjacent to the membrane electrode assembly. Each of the active areas comprise electrodes, and each of the active areas have length to width ratios such that a temperature differential measured across the shortest distance from a center of each of the active areas to an edge of the active areas is less than about 15° C. The flow field support member includes a flow region that aligns with either the first or second active areas of the membrane electrode assembly. A method of cooling an electrochemical cell comprises radiating heat from fins extending from edges of a flow field member of the electrochemical cell. A method of either humidifying a reactant gas fed to an electrochemical cell or heating a reactant gas fed to the electrochemical cell comprises atomizing a liquid product stream of the electrochemical cell and spraying the atomized liquid product stream onto the reactant gas. A method of heating an electrochemical cell comprises passing an electric current through a resistive heating element disposed at a flow field support member of the electrochemical cell.

Description:
BACKGROUND  
         [0001]    This disclosure relates to electrochemical cell systems, and, more particularly, to a design for an electrochemical cell stack in which the flow fields of the stack extend longitudinally between the end plates of the stack and include bipolar plates maintained in compression by side plates. The disclosure furthermore particularly relates to a design in which internally-generated water and heat are utilized for internal humidification of a supplied oxidant and in which heaters are disposed between the bipolar plates to maintain the cell stack at a temperature that minimizes the warm-up time.  
           [0002]    Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. Proton exchange membrane electrolysis cells can function as hydrogen generators by electrolytically decomposing water to produce hydrogen and oxygen gases. Referring to FIG. 1, a section of an anode feed electrolysis cell of the prior art is shown generally at  10  and is hereinafter referred to as “cell 10.” Reactant water  12  is fed into cell  10  at an oxygen electrode (anode)  14  to form oxygen gas  16 , electrons, and hydrogen ions (protons)  15 . The chemical reaction is facilitated by the positive terminal of a power source  18  connected to anode  14  and the negative terminal of power source  18  connected to a hydrogen electrode (cathode)  20 . Oxygen gas  16  and a first portion  22  of the water are discharged from cell  10 , while protons  15  and a second portion  24  of the water migrate across a proton exchange membrane  26  to cathode  20 . At cathode  20 , hydrogen gas  28  is removed, generally through a gas delivery line. The removed hydrogen gas  28  is usable in a myriad of different applications. Second portion  24  of water is also removed from cathode  20 .  
           [0003]    An electrolysis cell system may include a number of individual cells arranged in a stack with reactant water being directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, and each one includes a membrane electrode assembly (MEA) defined by a proton exchange membrane disposed between a cathode and an anode. The cathode, anode, or both may be gas diffusion electrodes that facilitate gas diffusion to the proton exchange membrane. Each membrane electrode assembly is in fluid communication with flow fields adjacent to the membrane electrode assembly, defined by structures configured to facilitate fluid movement and membrane hydration within each individual cell.  
           [0004]    The portion of water discharged from the cathode side of the cell, which is entrained with hydrogen gas, is fed to a phase separator to separate the hydrogen gas from the water, thereby increasing the hydrogen gas yield and the overall efficiency of the cell in general. The removed hydrogen gas may be fed either to a dryer for removal of trace water, to a storage facility, e.g., a cylinder, a tank, or a similar type of containment vessel, or directly to an application for use as a fuel.  
           [0005]    Another type of water electrolysis cell that utilizes the same configuration as is shown in FIG. 1 is a cathode feed cell. In the cathode feed cell, process water is fed on the side of the hydrogen electrode. A portion of the water migrates from the cathode across the membrane to the anode. A power source connected across the anode and the cathode facilitates a chemical reaction that generates hydrogen ions and oxygen gas. Excess process water exits the cell at the cathode side without passing through the membrane.  
           [0006]    A typical fuel cell also utilizes the same general configuration as is shown in FIG. 1. Hydrogen gas is introduced to the hydrogen electrode (the anode in the fuel cell), while oxygen, or an oxygen-containing gas such as air, is introduced to the oxygen electrode (the cathode in the fuel cell). The hydrogen gas for fuel cell operation can originate from a pure hydrogen source, a hydrocarbon, methanol, or any other source that supplies hydrogen at a purity level suitable for fuel cell operation. Hydrogen gas electrochemically reacts at the anode to produce protons and electrons, the electrons flow from the anode through an electrically connected external load, and the protons migrate through the membrane to the cathode. At the cathode, the protons and electrons react with oxygen to form water.  
           [0007]    Conventional electrochemical cell systems generally include one or more individual cells arranged in a stack, with the working fluids directed through the cells via input and output conduits formed within the stack structure. A typical fuel cell system that can be utilized with other cells and incorporated into a stack structure is shown at  30  with reference to FIG. 2. Cell system  30  comprises an MEA defined by a proton exchange membrane  32  having a first electrode (e.g., an anode)  34  and a second electrode (e.g., a cathode)  36  disposed on opposing sides thereof. Regions proximate to and bounded on at least one side by anode  34  and cathode  36  respectively define flow fields  38 ,  40 . On the anode side of the MEA, a flow field support member  42  may be disposed adjacent to anode  34  to facilitate membrane hydration and/or fluid movement to the membrane. Flow field support member  42  is retained within flow field  38  by a frame  44  and a cell separator plate  48 . A gasket  46  is optionally positioned between frame  44  and cell separator plate  48  to effectively seal flow field  38 . On the cathode side of the MEA, a flow field support member  50  may be disposed adjacent to cathode  36  to further facilitate membrane hydration and/or fluid movement to the membrane.  
           [0008]    A pressure pad  52  is typically disposed between flow field support member  50  and a cell separator plate  54 . Pressure pads may be disposed on either or both sides of membrane  32  and may be positioned within either or both of the flow fields of cell system  30  in place of either or both of the flow field support members. One or more pressure plates  60  may optionally be disposed adjacent to pressure pad  52  to distribute the pressure exerted on pressure pad  52  and increase the pressure within the cell environment. Flow field support member  50  and pressure pad  52  (as well as optional pressure plates  60 ) are retained within flow field  40  by a frame  56  and cell separator plate  54 . A gasket  58  is optionally positioned between frame  56  and cell separator plate  54  to effectively seal flow field  40 . The cell components, particularly frames  44 ,  56 , cell separator plates  48 ,  54 , and gaskets  46 ,  58 , are formed with the suitable manifolds or other conduits to facilitate fluid communication through cell system  30 .  
           [0009]    Heat generated at each cell during the operation of an electrochemical cell system is generally dissipated via “cooler plates” positioned at various locations in the electrochemical stack. The cooler plates generally comprise metal flow tubes arranged in serpentine configurations. The tubes are also typically embedded within a plate structure. A coolant is made to flow through the flow tubes to absorb the heat produced by the reaction of the reactant chemicals. The flow rate of the coolant stream may be adjusted to provide for the controlled removal of heat from the system. While in most systems the coolant of choice is a liquid, some systems utilize reactant oxygen as the coolant.  
           [0010]    Electrochemical cell systems such as that shown in FIG. 2, as well as others of similar constructions, are generally maintained in compression by tie rods, springs, or hydraulic pistons. The springs may be twisted wire springs or ribbon springs that extend between the plates of the cell system to urge the plates together to retain the membrane electrode assemblies. Other devices for maintaining the compression of the cell system include compression bands that circumscribe the plates of the system and the interposed cell assemblies of the stack.  
           [0011]    Most cell systems operate more efficiently when wet. In order to wet the relevant parts of the cells, the reactant gases may be humidified via liquid water injected into a flow channel of a fluid flow plate of a cell assembly. In the humidification of the reactant gases, however, significant amounts of heat may be required to saturate the gases at a temperature close to the operating temperature of the fuel cell. Furthermore, temperature variations within the reactant gas manifolds and plate channels can lead to condensation of the vapor and poor distribution of the reactant gases. Pockets of condensate may accumulate within the plate channels, thereby inhibiting the free flow of the reactant gases. Moreover, because the vapor distribution is unpredictable, drying of the membrane may occur despite the introduction of water vapor into the gas streams at the inlet of the fluid manifold.  
           [0012]    If the cell system forms part of a back-up power application, rapid startup of the system is generally desired in order to minimize or eliminate any discontinuities in power generation. Because the startup of a cell is a function of its temperature, the cell is generally maintained at or near the operating temperature of the cell even when the cell is non-operational. In order to maintain the cell at such a temperature, a return coolant stream from the electrolysis unit of the cell stack may be directed through or around the cell stack to transfer heat removed from the electrolysis unit to the cell stack. However, because there may be periods of time when both the electrolysis unit and the fuel cell are idle, maintaining the cell stack at a temperature sufficient for rapid startup of the system may not be possible with the return coolant stream.  
           [0013]    While existing electrolysis cell systems are suitable for their intended purposes, there still remains a need for improvements. Some of the improvements needed include apparatuses and methods of effectively dissipating heat from the cell stack and, in particular, allowing for the maximum dissipation of heat from the edges of the cell stack, maintaining the cell stack components in compression, internally humidifying the reactant gases, and maintaining the cell stack at a temperature that minimizes the warm-up time of the cell system.  
         BRIEF SUMMARY  
         [0014]    Disclosed herein is an electrochemical cell having a membrane electrode assembly comprising a first active area and an opposingly positioned second active area with a flow field support member disposed adjacent to the membrane electrode assembly. Each of the active areas comprise electrodes, and each of the active areas have length to width ratios such that a temperature differential measured across the shortest distance from a center of each of the active areas to an edge of the active areas is less than about 15° C., and more preferably less than about 10° C. The flow field support member includes a flow region that aligns with either the first or second active areas of the membrane electrode assembly.  
           [0015]    A method of cooling an electrochemical cell comprises radiating heat from fins extending from edges of a flow field member of the electrochemical cell. A method of either humidifying a reactant gas fed to an electrochemical cell or heating a reactant gas fed to the electrochemical cell comprises atomizing a liquid product stream of the electrochemical cell and spraying the atomized liquid product stream onto the reactant gas. A method of heating an electrochemical cell comprises passing an electric current through a resistive heating element disposed at a flow field support member of the electrochemical cell.  
           [0016]    The above discussed and other features will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    Referring now to the drawings wherein like elements are numbered alike in several Figures:  
         [0018]    [0018]FIG. 1 is a schematic representation of an anode feed electrolysis cell of the prior art;  
         [0019]    [0019]FIG. 2 is a schematic representation of a cell system of the prior art;  
         [0020]    [0020]FIG. 3 is an exploded perspective view of a cell stack;  
         [0021]    [0021]FIG. 4 is a plan view of an MEA;  
         [0022]    [0022]FIG. 5 is a plan view of a cathode face of a bipolar plate;  
         [0023]    [0023]FIG. 6 is a cross sectional view of a bipolar plate;  
         [0024]    [0024]FIG. 7 is a sectional view of an atomizer;  
         [0025]    [0025]FIG. 8 is a perspective view of an atomizer that functions as an alignment pin; and  
         [0026]    [0026]FIG. 9 is a perspective view of a heating element disposed on a bipolar plate. 
     
    
     DETAILED DESCRIPTION  
       [0027]    Disclosed herein is an electrochemical cell that may be operated as either a fuel cell or an electrolysis cell. Also disclosed are methods for operating the electrochemical cell. While the discussion below is directed to a fuel cell, it should be understood that anode feed electrolysis cells, cathode feed electrolysis cells, and regenerative fuel cells are also within the scope of the embodiments disclosed.  
         [0028]    A cell stack is formed of a plurality of individual cells and includes flow field support members that are preferably in the form of bipolar plates. The plates are elongated and maintained in compression by being urged together in a direction normal to the face of the plate at the top of the stack. The flow regions disposed on the faces of each plate comprise channels that extend in the lengthwise dimension between openings at opposing ends of the plates. Heat is generally dissipated from each plate via fins that extend along the edges thereof. A coolant (e.g., air) is preferably made to flow along the fins to effect the removal of the heat. By-product liquid (e.g., water in a fuel cell that employs hydrogen and oxygen gases as the reactants) is recycled within the cell stack and utilized to humidify at least one of the inlet reactant gases. In order to minimize startup up time of the cell subsequent to prolonged periods of downtime in a cold environment, heaters are preferably disposed intermediate the plates to maintain the temperature of the cell at or near its typical operating temperature.  
         [0029]    Referring to FIG. 3, one exemplary embodiment of an electrochemical cell stack is shown at  70  and is hereinafter referred to as “cell stack 70.” Cell stack  70  typically includes a plurality of cells employed as part of the cell system. When cell stack  70  is utilized as a fuel cell, power outputs are dependent upon the number of cells. Typically, the outputs are about 0.4 volts to about 1 volt, with current densities being about 0.1 A/ft 2  (amperes per square foot) to about 10,000 A/ft 2 . When used as an electrolysis cell, voltage inputs are generally 1.48 volts to about 3.0 volts, with current densities of about 50 A/ft 2  to about 4,000 A/ft 2 . Current densities exceeding 10,000 A/ft 2  may also be obtained depending upon the fuel cell dimensions and configuration. The number of cells within the stack and the dimensions of the individual cells is scalable to the cell power output and/or gas output requirements.  
         [0030]    Cell stack  70  comprises a plurality of cells  72  and a compression device  74  configured to maintain plurality of cells  72  in compression with each other. The cells are maintained in alignment with each other via alignment pins  111 . Each cell comprises an MEA  76  defined by a proton exchange membrane having electrodes (an anode and a cathode) disposed at opposing sides thereof. Flow field support members are disposed between each MEA  76 . The flow field support members are typically bipolar plates  78  having channeled surfaces formed in the opposing sides of each plate. Bipolar plates  78  are generally fabricated of carbon composite materials, e.g., about 70% to about 90% graphite and about 10% to about 30% plastic, wherein the plastic may be a polypropylene, a polyethylene, a polyvinylester, a phenolic, polyvinylidene difluoride, combinations of the foregoing materials, and the like. Other types of flow field support members, for example, screen packs, may be utilized. Compression device  74  comprises one or more resilient members  80  configured to urge a plate (e.g., a base plate  82 ) against the MEA/bipolar plate assemblies and against a support plate (e.g., a manifold  84 ).  
         [0031]    Referring to FIG. 4, MEA  76  is shown. MEA  76  comprises a proton exchange membrane  86  and the electrodes (anode  88  and cathode  90 ) disposed at opposing sides of proton exchange membrane  86 . Both anode  88  and cathode  90  are positioned on the surface of proton exchange membrane  86  within the active areas, which are defined as the areas of a fuel cell at which the chemical reaction of hydrogen and oxygen take place to produce the desired flow of electrons and the by-product water. As shown, the active areas of MEA  76  are substantially longer in a dimension l than they are wide in a dimension w to define a cell of the improved configuration. The amount that MEA  76  is longer than wide is such that a temperature differential measured across the shortest distance from the center of the active area of MEA  76  to the edge of the active area of MEA  76  is less than about 15° C., and more preferably less than about 10° C. In particular, the length to width ratio of the active areas of MEA  76  (and thus each cell of the cell system) is preferably greater than or equal to about 4:1.  
         [0032]    Membrane  86  comprises electrolytes that are preferably solids under the operating conditions of the electrochemical cell. Useful materials from which membrane  86  can be fabricated include proton conducting ionomers and ion exchange resins. Useful proton conducting ionomers include complexes comprising an alkali metal salt, an alkali earth metal salt, a protonic acid, a protonic acid salt, or the like, as well as combinations of the foregoing materials. Counter-ions useful in the above salts include halogen ion, perchloric ion, thiocyanate ion, trifluoromethane sulfonic ion, borofluoric ion, and the like, as well as combinations of the foregoing materials. Representative examples of such salts include, but are not limited to, lithium fluoride, sodium iodide, lithium iodide, lithium perchlorate, sodium thiocyanate, lithium trifluoromethane sulfonate, lithium borofluoride, lithium hexafluorophosphate, phosphoric acid, sulfuric acid, trifluoromethane sulfonic acid, and the like, as well as combinations of the foregoing materials. The alkali metal salt, alkali earth metal salt, protonic acid, or protonic acid salt is complexed with one or more polar polymers such as a polyether, polyester, or polyimide, or with a network or cross-linked polymer containing the above polar polymer or combination of polymers as a segment. Useful polyethers include polyoxyalkylenes, such as polyethylene glycol, polyethylene glycol monoether, and polyethylene glycol diether; copolymers of at least one of these polyethers, such as poly(oxyethylene-co-oxypropylene) glycol, poly(oxyethylene-co-oxypropylene) glycol monoether, and poly(oxyethylene-co-oxypropylene) glycol diether; condensation products of ethylenediamine with the above polyoxyalkylenes; and esters, such as phosphoric acid esters, aliphatic carboxylic acid esters or aromatic carboxylic acid esters of the above polyoxyalkylenes. Copolymers of, e.g., polyethylene glycol with dialkylsiloxanes, maleic anhydride, or polyethylene glycol monoethyl ether with methacrylic acid, are known in the art to exhibit sufficient ionic conductivity to be useful.  
         [0033]    Ion-exchange resins useful as proton conducting materials include hydrocarbon- and fluorocarbon-type resins. Hydrocarbon-type ion-exchange resins include phenolic resins, condensation resins such as phenol-formaldehyde, polystyrene, styrene-divinyl benzene copolymers, styrene-butadiene copolymers, styrene-divinylbenzene-vinylchloride terpolymers, and the like, that are imbued with cation-exchange ability by sulfonation, or are imbued with anion-exchange ability by chloromethylation followed by conversion to the corresponding quaternary amine.  
         [0034]    Fluorocarbon-type ion-exchange resins can include hydrates of tetrafluoroethylene-perfluorosulfonyl ethoxyvinyl ether or tetrafluoroethylene-hydroxylated (perfluoro vinyl ether) copolymers. When oxidation and/or acid resistance is desirable, for instance, at the cathode of a fuel cell, fluorocarbon-type resins having sulfonic, carboxylic and/or phosphoric acid functionality are preferred. Fluorocarbon-type resins typically exhibit excellent resistance to oxidation by halogen, strong acids, and bases. One family of fluorocarbon-type resins having sulfonic acid group functionality is NAFION® resins (commercially available from E.I. du Pont de Nemours and Company, Wilmington, Del.).  
         [0035]    Anode  88  and cathode  90  are fabricated from catalyst materials suitable for performing the needed electrochemical reaction (i.e., electrolyzing water to produce hydrogen and oxygen). Suitable materials for anode  88  and cathode  90  include, but are not limited to, platinum, palladium, rhodium, carbon, gold, tantalum, tungsten, ruthenium, iridium, osmium, alloys thereof, and the like, as well as combinations of the foregoing materials. Anode  88  and cathode  90  may be adhesively disposed on membrane  86 , or they may be positioned adjacent to, but in contact with, membrane  86 .  
         [0036]    Referring now to FIG. 5, a bipolar plate  78  is shown. Bipolar plate  78  is preferably substantially longer in a dimension l than it is wide in a dimension w to correspond with an adjacently-positioned MEA. Preferably the length to width ratio of bipolar plate  78  corresponds to the length to width ratio of the MEA and is greater than or equal to about 4:1.  
         [0037]    Bipolar plate  78  is also preferably electrically conductive and disposable at each electrode to facilitate the distribution of the reactant gases to their respective electrodes and the hydration of the MEA. Materials from which bipolar plate  78  can be fabricated commonly include metals, carbon, or carbon composite structures incorporating a polymeric binder. Typical metals that may be used to fabricate bipolar plates include, but are not limited to, niobium, zirconium, tantalum, titanium, carbon steel, stainless steel, nickel, cobalt, and the like, as well as alloys and combinations of the foregoing materials. In alternate embodiments of the cell system, screen packs (not shown) may be utilized in place of either or both bipolar plates in a cell. Screen packs include one or more layers of perforated sheets or a woven mesh formed from metal strands. Typical metals that may be used to fabricate screen packs include, but are not limited to, niobium, zirconium, tantalum, titanium, carbon steel, stainless steel, nickel, cobalt, and the like, as well as alloys and combinations of the foregoing materials.  
         [0038]    A flow region  92  on a cathode face of bipolar plate  78  is dimensioned to align with the active area (i.e., the cathode) of the adjacently-positioned MEA. When the cell is utilized as a fuel cell, flow region  92  provides for the flow of oxygen to the oxygen side of the MEA and comprises a plurality of channels  96  cut or otherwise formed in the surface of bipolar plate  78  to define pathways by which oxygen can be introduced to the MEA. Flow region  92  further provides for the removal of water from the cathode, which removes latent heat from the cell. Channels  96  extend between openings  98 ,  100  at opposing ends of bipolar plate  78 . Because of the dimensions of bipolar plate  78  and the positioning of openings  98 ,  100 , channels  96  extend substantially linearly between openings  98 ,  100  to effect the substantially unimpeded flow of oxygen to each bipolar plate  78 . When a plurality of bipolar plates  78  and MEAs are stacked to form the cell system, openings  98 ,  100  form a manifold through which oxygen is simultaneously introduced to each bipolar plate  78  and the cathode side of each MEA and through which water is removed from each cathode.  
         [0039]    A flow region (not shown) disposed on the anode face of bipolar plate  78  is configured to be similar to flow region  92  and, when bipolar plate  78  is incorporated into a fuel cell, provides for the flow of hydrogen gas to the anode side of each MEA. The flow region comprises a plurality of channels (not shown) that extend between openings  104 ,  106  at opposing ends of bipolar plate  78 . As with channels  96  on the cathode side of bipolar plate  78 , the channels extend substantially linearly between openings  104 ,  106  to effect the substantially unimpeded flow of hydrogen to the anode sides of each bipolar plate  78 . When a plurality of bipolar plates  78  and MEAs are stacked to form the cell system, openings  104 ,  106  form a manifold through which hydrogen is simultaneously introduced to each bipolar plate  78  and each MEA.  
         [0040]    Operation of a cell stack into which bipolar plate  78  is incorporated is preferably such that air and hydrogen enter the cell at the same end of bipolar plate  78 . In a vertically-oriented cell stack, bipolar plate  78  is arranged such that the openings at one end are at a lower elevation than the openings at the opposing end. Air and hydrogen are fed to the openings at the lower end such that the flow of air through the channels at one side of bipolar plate  78  is co-current with the flow of hydrogen through the channels at the opposing side of bipolar plate  78 . The gases may also be fed to the openings at the upper end of bipolar plate  78  for co-current flow or to both ends for counter-current flow. In either configuration, the flow of air and hydrogen vertically through the channels provides a flow path with no laterally-oriented legs, thereby allowing for the mass transport of water though the cell due to pressure of the gas in the channels as well as gravity.  
         [0041]    In a horizontally-oriented cell stack, on the other hand, water is transported through the cell primarily due to the flow of the gas. Because channels extend laterally across the faces of bipolar plate  78 , gravity typically causes water to accumulate in the channels in the absence of gas flow. Thus, sufficient gas flow through a cell stack having a horizontally-oriented bipolar plate  78  provides for the removal of water from bipolar plate  78  and the cell stack.  
         [0042]    Referring now to FIG. 6, both the anode side and the cathode side of bipolar plate  78  can be seen. Channels  96  on the cathode side of bipolar plate  78  and the channels on the anode side of bipolar plate  78  (shown at  102 ) are typically rectilinear in cross sectional geometry, although other geometries, for example, semi-circular configurations, V-shaped configurations, and the like, are possible. Edges of bipolar plate  78  along the length are also removed to provide cut-out sections  108  that define fins  110  that extend along the length of bipolar plate  78 . When a plurality of bipolar plates  78  are assembled to form the cell stack, fins  110  provide means by which heat can be conducted from the inner regions of bipolar plate  78  (proximate channels  96 ,  102 ) and allowed to radiate to the surrounding environment. Because the length of bipolar plate  78  is substantially longer than wide, fins  110  account for most of the edges of bipolar plate  110 , thereby providing a large surface area from which heat can be removed. Bipolar plates that define the end plates of the cell stack are substantially similar to the plates that comprise the body of the fuel cell stack. Such bipolar plates, however, include channels disposed only at a functional side of plate.  
         [0043]    Referring back to FIG. 3, the components of the cell stack are maintained in alignment by alignment pins  111  extending from manifold  84 , through holes disposed in the ends of MEAs  76 , bipolar plates  78 , and base plate  82  and to a support plate (e.g., a spring support plate  124 ). MEAs  76  and bipolar plates  78  are held in a stack arrangement by being compressed between base plate  82  and an inner surface of manifold  84 . Side plates  122  extend between spring support plate  124  and manifold  84  to provide rigidity to the stack structure and are attached to spring support plate  124  and manifold  84  via the engagement of tabs  126  with holes  128  disposed proximate the opposing edge surfaces of side plates  122 . Spring support plate  124 , manifold  84 , and side plates  122  are dimensioned such that, when assembled, the resulting structure substantially corresponds in length and width to the cells.  
         [0044]    The compression of plurality of cells  72  is effected in the direction normal to the direction in which the plane of the cells extend by the biasing of base plate  82  against the stack structure, which is in turn biased against manifold  84 . Base plate  82  is removably positioned between bipolar plate  78  and spring support plate  124  so as to be in a parallel planar relationship. The compression is effected by resilient members  80 , which may be, for example, urethane springs mounted on pins  130  extending normally from the facing surfaces of base plate  82  to spring support plate  124 . Other types of resilient members that may be used include, but are not limited to, Belleville washers mounted on pins  130 .  
         [0045]    Space may be defined between side plates  122  and the edges of plurality of cells  72 . The fins protruding longitudinally along the lengths of each bipolar plate  78  generally extend into such space. In order to provide further cooling of the cell stack, an air stream may be passed through the space between the outer edges of the fins and the inner faces of side plates  122  to convectively remove heat generated during the operation of the cell stack.  
         [0046]    Although an electrochemical cell operated as a fuel cell generates water at the cathode, the air introduced to the cathode is preferably humidified to prevent drying of the entrance region of the fuel cell flow field. The amount of water generated at the cathode is generally sufficient to humidify the inlet air and keep it saturated as it passes from the entrance region of the flow field to the cathode. In order to utilize the generated water for humidification of the air, the water is removed from the cathode of each cell and returned to manifold  84 . Once returned to manifold  84 , the water can be sprayed through an atomizer into the incoming air stream to saturate the air.  
         [0047]    Referring now to FIG. 7, the atomizer for the electrochemical cell operated as a fuel cell is shown at  140 . Atomizer  140  comprises a spray nozzle  142  and a water return line  144  disposed in fluid communication with spray nozzle  142 . Water return line  144  receives the water generated at each cathode of the cell from a cathode outlet  146  and feeds it through spray nozzle  142  to humidify the air as it passes through an air inlet  148 . Preferably, the rate of water sprayed is metered and regulated such that a minimum amount of liquid water is removed from the cell. Any excess water that is sprayed, however, may be returned to cathode outlet  146  via an overflow duct  149  that extends between air inlet  148  and cathode outlet  146 .  
         [0048]    In another exemplary embodiment of the cell stack shown in FIG. 8, an atomizer  240  may be configured as an alignment pin located at an edge of manifold  84 . The alignment pin is preferably a rigid structure extending in the direction normal to the stack of bipolar plates  78  to provide a directional assembly of bipolar plates  78 . A water return line  244  is defined by the hollow core of the alignment pin, and a spray nozzle  242  provides a water spray at the open port areas intermediate the cells to humidify the air as it passes from water return line  244  to each cell. Because the alignment pin is preferably rigid, bipolar plates  78  are maintained in alignment within the cell stack.  
         [0049]    Referring to both FIGS. 7 and 8, because the air supplied to the fuel cell is saturated by atomizer  140  at a temperature significantly less than the operating temperature of the fuel cell, the cell is operated within a prescribed envelope defined by temperatures, pressures, and flow rates. For example, in one exemplary embodiment, the fuel cell is operated at about 55° C. to about 85° C., the pressure is maintained up to about 30 pounds per square inch (psi) (206.8 kPa (kilo Pascals)), and the stoichiometric ratio of air to hydrogen is about one to about three. The spraying of the inlet air with atomized water at a temperature of about 55° C. to about 85° C. facilitates the heating of the inlet air.  
         [0050]    If the electrochemical cell is operated as a fuel cell, the cell stack may further comprise a heater to maintain the thermal integrity of the cell stack and its ability to start rapidly with minimum warm-up time when cold. One exemplary embodiment of the heater disposed on a bipolar plate  78  is shown at  150  with reference to FIG. 9. Heater  150  comprises a heater element  152  disposed at the surface of bipolar plate  78  between an edge of bipolar plate  78  and flow region  94 . When the cell stack is assembled, heater element  152  is disposed in contact with an adjacently positioned bipolar plate (not shown). Heater element  152  is disposed in electrical communication with an electrical source (not shown) through leads  154 , which supply electrical current to heater element  152 .  
         [0051]    In one exemplary embodiment of heater  150 , heater element  152  comprises an electrically resistive material deposited onto the surface of bipolar plate  78 . The electrically resistive material may be deposited onto the surface of bipolar plate  78  in any one of or a combination of a variety of manners including, but not limited to, sputtering, chemical vapor deposition, stenciling, screen printing, painting, and the like. Thicker depositions of the electrically resistive material are generally sputtered or deposited using vapor deposition techniques. For example, the metal is formed into a paste, screen printed onto the surface of bipolar plate  78 , and dried. The metal may be combined with a binder, cellulose, and a solvent to make the paste. The paste is preferably applied to the surface of bipolar plate  78 , dried, and sintered onto bipolar plate  78 . Heater element  152  may be deposited in a wave pattern, as is shown, or in any other pattern.  
         [0052]    A plurality of heater elements  152  may be incorporated into the architecture of the electrochemical cell. In particular, heater elements  152  may be disposed at least one bipolar plate in order to provide optimum heating to the cell stack and to minimize the warm-up time prior to starting the fuel cell. Heater elements  152  are generally maintained in electrical communication with each other through a wiring harness.  
         [0053]    The electrochemical stack design described above provides superior functionality of an electrochemical cell. The shape of the flow field facilitates the dissipation of heat from the edges of the flow field members and radiation of the heat outward from the cell. Convective airflow along the edges of the flow field members further enhances the efficiency of the heat removal. By effectively removing the heat, the life of the cell can be prolonged beyond the life of a conventional cell. The design further allows for the removal of latent heat via the water stream from the cathode. Heat removal via a combination of the water stream and airflow enables the electrochemical cell to be operated with fewer plates.  
         [0054]    Because the cell stack includes the stacked plates maintained in alignment by side plates, misalignment of the MEAs with respect to the plates is minimized. Furthermore, because of the dimensions of the plates, a cell stack can be constructed from fewer plates than a fuel cell having the same output and power requirements, which translates into fewer pieces for assembly and service. Compression of the plates is also improved, in that multiple compression devices (e.g., Belleville washers or urethane springs) exerting pressure against a common element (e.g., the base plate) tend to provide more uniform compression over a greater area than conventional compression devices for cell stacks.  
         [0055]    Furthermore, by minimizing the angle at which the channels extend over the bipolar plates, a substantially linear path for the flow of reactant gases and the removal of water is effected. Such a substantially linear path minimizes the probability of fluid holdup in the channels, which also minimizes the amount of phase separation of the humidified reactant gases in the bipolar plates, further contributing to effective heat removal.  
         [0056]    By recycling the water stream from the cathode to humidify at least one of the reactant feed streams, the need for introducing a separate humidification stream is reduced, which, in turn reduces the amount of cell inputs. Use of the water stream as a humidification stream further reduces the by-product output of the cell and promotes a more self-contained and self-sufficient aspect of fuel cell technology.  
         [0057]    Heating of the cell stack by inserting heaters between adjacently-positioned bipolar plates allows for the rapid startup of the fuel cell when the ambient temperature is markedly less than the temperature at which the fuel cell normally operates. Because the time for a resistive heater to reach its rated temperature is generally quick, such heaters are generally superior in performance compared to systems in which coolant return streams are used to provide heat. Because of their ability to be screen printed or otherwise deposited onto surfaces, resistive heaters may be employed in places and in configurations (e.g., disposed between adjacently-positioned bipolar plates) that conventional heaters cannot be employed.  
         [0058]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.