Abstract:
A fuel cell assembly including a fuel reforming unit for reforming a fuel supply for a series of fuel cells constituting a fuel cell stack. The reformed fuel supply is routed first to the anode of the fuel cell most adjacent the reforming unit, and thereafter to a manifold external to the stack. The manifold intakes that portion of the reformed fuel supply not fully exhausted after passing through the first anode and feeds such reformed fuel to successive fuel cells in series, thus providing staged fuel supply throughout the stack and optimal fuel utilization in producing electricity. The reforming unit includes a series of baffles for directing the reformed fuel supply to the first anode and to the manifold to maximize utilization of fuel consumed by cells in the stack. Also, cooling occurring as a result of the endothermic reaction occurring in the reforming unit is captured and spread optimally throughout the stack to achieve optimal temperature gradients throughout the stack, thus enabling optimal operation of and increased life of the stack.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a non-provisional application based on provisional application Ser. No. 61/158,712, filed Mar. 9, 2009, the entire disclosure of which is hereby incorporated by reference 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to fuel cells arranged in a fuel cell stack and, in particular, to a fuel cell stack design and method configured to enhance overall fuel utilization and control temperature distribution in the stack and thereby provide an increased service life for the stack. 
     A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by a member which serves itself to conduct electrically charged ions or is adapted to hold an electrolyte which conducts electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate separating the cells. 
     Before undergoing the electrochemical reaction in the fuel cell, hydrocarbon fuels such as methane, coal gas, etc. are typically reformed to produce hydrogen for use in the anode of the fuel cell. In internally reforming fuel cells, a steam reforming catalyst is placed within the fuel cell stack to allow direct use of hydrocarbon fuels without the need for expensive and complex reforming equipment. In addition, the endothermic reforming reaction can be used advantageously to help cool the fuel cell stack. 
     Internally reforming fuel cells employ direct internal reforming and indirect internal reforming. Direct internal reforming is accomplished by placing the reforming catalyst within the active anode compartment. Direct internal reforming thus directly exposes the catalyst to the electrolyte of the fuel cell, which can lead to deactivation of the catalyst and an eventual degradation of the fuel cell&#39;s performance. Improvements have been made to the direct internal reforming technique to reduce electrolyte contamination, but these improvements are accompanied by high costs due to the complexity of the fuel cell design, special materials requirements and a reduction in the effectiveness of the reforming catalyst. 
     The second reforming technique, indirect internal reforming, is accomplished by placing the reforming catalyst in an isolated chamber within the fuel cell stack and routing the reformed gas from this chamber into the anode compartment of the fuel cell. With this technique, the need for separate ducting systems raises the cost of the fuel cell stack and also makes the system susceptible to fuel leaks. 
     The current state of the art uses a hybrid assembly in which the fuel cell stack has both direct and indirect internal reforming and in which external manifolds are used for enclosing and directing the flow of fuel and oxidant gases into the stack. U.S. Pat. No. 6,200,696 and U.S. Patent Application Publication No. 2006/0123705, assigned to the same assignee hereof, disclose examples of such hybrid assemblies. As disclosed in the &#39;696 patent and the 2006/0123705 publication, the hybrid assembly includes one or more fuel reformers for indirect internal reforming of input fuel gas, which receive the input fuel gas and convey it in a U-shaped path while reforming the fuel therein. The assembly of the &#39;696 patent and the 2006/0123705 publication also includes a fuel-turn manifold for redirecting reformed gas outputted by the indirect internal reformers to the anode compartments for further reforming through direct internal reforming and electrochemical conversion. In these assemblies, both the U-shaped flow path in the reformer and the flow through the anode compartments of the fuel cells is in cross-flow, or perpendicular to, the oxidant gas passing through the stack. 
     Due to the nature of the fuel flow within the fuel reformers, such hybrid assemblies are sometimes susceptible to non-uniformity in their current density distribution and to temperature gradients near the gas exits of the stack. These effects occur as the stack ages and as the catalyst within the stack plates, the Direct Internal Reforming (DIR) catalyst, is deactivated over the course of the service life of the stack. As a result, thermal instability within the stack may occur and may cause non-optimized fuel utilization in the production of electricity. This is especially true given the maximum allowable temperature at which the stack is designed to operate. 
     It is therefore an object of the present invention to further improve fuel cell stack design and methodology so as to create a fuel flow arrangement which increases the fuel conversion efficiency of the stack. 
     It is also an object of the present invention to provide a fuel cell stack design and methodology which promotes cooling so as to realize a more uniform temperature distribution, thus increasing the overall efficiency of the fuel cell operation and electricity production and extending the operating life of the stack. 
     SUMMARY OF THE INVENTION 
     The above and other objects are realized in a reformer for use in a fuel cell system comprising an enclosure including an inlet port and an outlet port, and a plate assembly supporting reforming catalyst disposed within the enclosure, wherein the outlet port is configured to abut a fuel inlet port of a fuel cell assembly adjacent to the reformer, when the reformer is assembled into the fuel cell system, so that at least a first portion of the fuel reformed by the reformer is supplied directly from the outlet port of the reformer to the inlet port of the fuel cell assembly. 
     In some embodiments, the reformer is configured to supply all of the fuel reformed thereby to the inlet of the fuel cell assembly adjacent the reformer, while in other embodiments the reformer comprises a further outlet port for outputting a second portion of the fuel reformed by the reformer to the fuel cell manifold when the reformer is assembled into the fuel cell system. The plate assembly of the reformer includes a plurality of sections, including an inlet section communicating with the inlet port, an outlet section communicating with the outlet port and a central section disposed between the inlet section and the outlet section, and the plate assembly further includes a plurality of baffles for directing the fuel flow through the plate assembly. The central section of the plate assembly may include a plurality of zones, each of which communicates with the inlet section and with the outlet section and a plurality of baffles for directing the flow of fuel into each of the zones. The loading density of the reforming catalyst supported by the plate assembly is varied so that the inlet section has a first loading density, the central section has a second loading density which is greater than the first loading density, and the outlet section has a third loading density which is smaller than or equal to the second loading density. 
     A fuel cell system that includes the reformer is also disclosed. The fuel cell system comprises a plurality of fuel cell assemblies and at least one reformer, forming a fuel cell stack, with the plurality of fuel cell assemblies including at least one reformer-associated assembly and one or more non-reformer-associated assemblies. Each of the reformer-associated assemblies is adjacent to and associated with a reformer. Each reformer is configured to receive fuel through an inlet port and to output at least a first portion of fuel reformed in the reformer through an outlet port directly to the reformer-associated assembly associated with the reformer, and each reformer-associated assembly is configured to output partially spent fuel for use in one or more non-reformer-associated assemblies. In some embodiments, the fuel cell stack includes a fuel inlet face, a fuel outlet face, an oxidant inlet face and an oxidant outlet face and comprises a plurality of manifolds including at least a fuel inlet manifold that sealingly encloses the fuel inlet face of the stack. In such embodiments, each reformer-associated assembly outputs partially spent fuel into the fuel inlet manifold and the fuel inlet manifold is configured to direct the partially spent fuel to the non-reformer-associated assemblies. In some embodiments the reformer-associated assembly includes no reforming catalyst, while the non-reformer-associated assembly supports reforming catalyst for directly reforming the partially spent fuel. A method of operating the fuel cell system that includes at least one reformer and a plurality of fuel cell assemblies is also described. 
     The above and other objects are also realized in a fuel cell stack having fuel cell assemblies stacked one after the other in a stacking direction and each including an anode part and a cathode part separated by an electrolyte receiving part and stacked in the stacking direction and one or more reforming units interspersed within the stack each between an associated anode compartment and an associated cathode compartment of fuel cell assemblies which follow one another in the stacking direction, each reforming unit and the associated anode compartment being configured such that reformed fuel gas from the reformer is supplied directly to the associated anode compartment where the reformed fuel gas undergoes partial electrochemical conversion in the fuel cell assembly containing the associated anode compartment and each associated anode compartment being further configured such as to make available to the anode compartment of other fuel cell assemblies the part of the reformed fuel gas that does not undergo electrochemical conversion in the fuel cell assembly containing the associated anode part. 
     In some of the embodiments of the invention, each reformer has an output port in a surface of the reformer in the stacking direction and each associated anode compartment has an input port in a surface of the anode compartment in the stacking direction which communicates with the reformer output port. In certain of these embodiments, an output port of each associated anode compartment is at a fuel inlet face of the fuel cell stack and the input ports of the anode compartments other than the associated anode compartments are also at the fuel inlet face of the stack. In some of these embodiments, a manifold abuts the fuel inlet face of the stack so that reformed fuel gas from the output ports of the associated anode compartments is conveyed by the manifold to the input ports of the anode compartments other than the associated anode compartments. 
     Additionally, in certain embodiments, the stack has a fuel outlet face opposite the fuel inlet face and the output ports of the anode compartments other that the associated anode compartments are at this fuel outlet face. In these embodiments, the input port of the reformers can be at the fuel inlet face of the stack, the output port of the reformers can have a first part which runs in a first direction running between the fuel inlet and fuel outlet faces of the stack and optionally a second part adjacent the second face of the stack that runs transverse to the first direction. Additionally, in these embodiments, the input ports of the associated anode compartments can likewise run in the first direction running between the fuel inlet and fuel outlet faces of the stack. 
     Also, in some of these embodiments, the associated anode compartments contain no or a little amount (less than 50 g) of catalyst for reforming fuel gas, while the anode compartments other than the associated anode compartments contain larger amounts (greater than 400 g) of catalyst for reforming fuel gas. 
     In certain embodiments, the output port of the reformer can have a part which runs along the length of the reformer and a part which runs along the width of the reformer. In some embodiments, the reformer can have an additional output port at a face of the stack. 
     Additionally, in certain embodiments, the reformers and cathode and anode compartments are configured such that flow of gas through the reformers is counter to the flow of oxidant gas through the cathode compartments, while the flow of gas through the associated anode compartments is co-flow with the flow of gas through the anode compartments and the flow of gas through the anode compartments other that the associated anode compartments is transverse or cross to the flow of gas through the cathode compartments. In other embodiments, the reformers and cathode and anode compartments are configured such that flow of gas through the reformers is counter to the flow of oxidant gas through the cathode compartments, while the flow of gas through the associated anode compartments is co-flow with the flow of gas through the cathode compartments and the flow of gas through the anode compartments other that the associated anode compartments is counter to the flow of gas through the cathode compartments. 
     Also, disclosed are particular configurations of the reformer and fuel cell assemblies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows an exploded view of a cross-flow fuel cell stack for an externally manifolded mixed flow field according to the present invention. 
         FIG. 2  shows a schematic plan view of the layout of the reforming unit used in the fuel cell stack of  FIG. 1 . 
         FIG. 2A  shows a schematic plan view of reforming catalyst loading in the reforming unit of  FIG. 2 . 
         FIG. 3  shows the fuel flow path in the anode adjacent the reforming unit of  FIG. 2 . 
         FIG. 4  shows the fuel gas flow path from the reforming unit to the anode adjacent thereto, and the cross-flow of fuel gas exhaust and oxidant gas with respect to successive anodes within the stack. 
         FIG. 5  shows an alternative flow field in which fuel gas exhaust is distributed among successive anodes in a Z-pattern in which fuel flow is initially in co-flow with the oxidant gas and thereafter in counter-flow with the oxidant gas across such anode or anodes. 
         FIG. 6  shows a schematic plan view of the layout of the anode plate design enabling the flow field of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a fuel cell assembly  10 , including a fuel cell stack  12  comprising a plurality of cell assemblies  16  stacked one after the other in a stacking direction of the stack  12 . In the illustrative embodiment shown in  FIG. 1 , the cell assemblies  16  are stacked one on top of the other so as to form the fuel cell stack  12 . The fuel cell assembly  10  also includes one or more reforming units, or reformers  30 , for internally reforming hydrocarbon fuel and for supplying reformed fuel to the fuel cells. In the illustrative embodiment shown in  FIG. 1 , only one reformer  30  is shown. However, in typical fuel cell assemblies, a plurality of reformers  30  is provided at predetermined intervals throughout the stack, e.g. one reformer  30  for every 1 to 6 cell assemblies  16 , so that each reformer  30  supplies reformed fuel to its respective group of assemblies. The number of the reformers  30  provided in each stack  12  is dependent on the size of the stack  12 . As described in more detail below, the cell assemblies  16  include one or more cell assemblies  16 A, each of which is adjacent to, or associated with, a respective reforming unit  30  (hereinafter “reformer-associated assembly  16 A”), and one or more other cell assemblies  16 B not associated with a reformer  30 . It is contemplated that each reformer  30  in the assembly will service a reformer-associated cell assembly  16 A and at least five cell assemblies  16 B not associated with the reformer. The cell assemblies  16  are separated adjacent cell assemblies and/or from adjacent one or more reformers by separator plates (not shown). 
     As shown in  FIG. 1 , each cell assembly  16  includes an electrolyte matrix  18  sandwiched between an anode electrode  20  and a cathode electrode  26 . The electrolyte matrix  18  is adapted to store an electrolyte therein, such as carbonate electrolyte, to conduct electrically charged ions between the electrodes  20 ,  26 . Each assembly  16  further comprises an anode current collector  22  associated with and abutting the anode electrode  20 . In particular, in  FIG. 1 , the anode electrode  20  has two opposing surfaces, wherein one of the opposing surfaces abuts, or faces, the electrolyte matrix and the other of the opposing surfaces abuts, or faces, the anode current collector  22 . The anode current collector  22  includes a plurality of corrugations  24 , which face the anode electrode  20  and which form together with the surface of the anode electrode  20  a plurality of fuel gas channels  32  through which the fuel gas passes. In certain cell assemblies  16 , a reforming catalyst is placed in the fuel gas channels  32  of all or some of the anode current collectors  22  in the fuel cell stack, so that the fuel gas is further reformed by the reforming catalyst as it passes through the gas channels  32  by direct internal reforming. 
     As shown in  FIG. 1 , each cathode current collector  26  also has a plurality of corrugations  24 A which define, together with the associated cathode electrode  28  that abuts the cathode current collector, a plurality of oxidant gas channels  32 A through which the oxidant gas passes. Oxidant gas inlet ports  34  are formed at one end of the oxidant gas channels  32 A and are situated on a first face  1 A of the stack, and oxidant gas exhaust ports  36 , or oxidant outlet ports, are formed at the other end of the oxidant gas channels  32 A, and are situated on a second face  1 B of the stack  12 , opposing the first face  1 A. In this way, oxidant gas is supplied to each assembly  16  through the oxidant gas inlet ports  34  and carried through the oxidant gas channels  32 A for use in the respective fuel cell cathode electrode  26 . Spent oxidant gas is then outputted from each assembly  16  through the oxidant gas exhaust ports  36 . 
     In the anode current collectors  22  of the assemblies  16 B, other than the reformer-associated assemblies  16 A, fuel gas inlet ports  38   a  are formed at one end of the fuel gas channels  32  and are situated on a third face  1 C of the stack  12  and fuel gas exhaust ports  40 , or fuel gas outlet ports, are formed at the other end of the fuel gas channels  32  on a fourth face  1 D of the stack  12 , opposing the third face  1 C. In this way, fuel is supplied to each assembly  16 B through the fuel gas inlet ports  38   a  and carried through the fuel gas channels  32  for use in the respective fuel cell anode electrode  20 . Spent fuel is then outputted from each assembly  16 B through the fuel gas exhaust ports  40 . 
     In the fuel cell assembly  10  shown in  FIG. 1 , each reformer  30  comprises a fuel inlet  42  located on the same side as the oxidant gas exhaust ports, i.e., on the second face  1 B of the stack, and an outlet  44  through which fuel gas, comprising reformed or partially reformed hydrocarbon fuel, is emitted after having been reformed in the reformer  30 . As shown, fuel is supplied to the fuel inlet  42  of each reformer  30  via a fuel supply feed  46 . An example of a fuel supply feed  46  and a reformer fuel delivery system for supplying fuel to the reformers in the stack is disclosed in U.S. Pat. No. 6,200,696, assigned to the same assignee herein and incorporated herein by reference. As shown in  FIG. 1 , the fuel supply feed  46  extends along and adjacent to a side of the reformer  30  on the third stack face  1 C. 
     The fuel cell assembly  10  includes a plurality of manifolds enclosing second, third and fourth stack faces  1 B- 1 D. As shown, a fuel-turn manifold  48  sealingly encloses the third stack face  1 C, the fuel supply feed  46  and the adjacent side of the reformer  30 . The fuel-turn manifold  48  prevents loss of fuel during its delivery to the one or more reformers  30  and receives reformed or partially reformed fuel outputted from the one or more reformers  30  and from each reformer-associated assembly  16 A. The fuel-turn manifold  48  also directs the reformed or partially reformed fuel to the fuel gas inlet ports  38   a  of the assemblies  16 B that are not adjacent to, or associated with, the reformer  30 , as described in more detail herein below. The fuel-turn manifold  48  comprises an internal feed tube and supply header (not shown) for distribution of fuel to each of the reformers  30  throughout the stack  12 . Manifolds  50  and  52  enclose second and fourth stack faces  1 B and  1 D, respectively, receive exhausted oxidant and fuel gases, respectively, leaving the stack  12 . 
     As can be seen in  FIG. 1 , fuel enters the reformer  30  from the fuel supply feed  46  through the fuel inlet  42 , which is located on the same side of the stack  1 B as the oxidant gas exhaust ports, and flows across the reformer  30  in a counter-flow direction relative to the oxidant gas flowing through the assemblies  16  of the stack  12 . That is, the oxidant gas flows through each cell assembly  16  of the stack  12  in a first direction, shown as direction of arrow “G” in  FIGS. 4 and 5 , while the fuel gas flows through the reformer  30  in a second direction, which is opposite to the first direction, shown as direction of arrows “A” in  FIG. 1 . The amount of fuel flow provided to the reformer  30  from the fuel supply feed  46  is in excess of the fuel amount consumed in the electrochemical reactions of the stack so as to achieve stable operation of the stack and sufficient production of electricity by the stack. In particular, the amount of fuel provided to the reformer  30  is typically 20-30% greater than the amount consumed by the electrochemical reactions in the stack. 
     In certain embodiments, the fuel flowing through the reformer  30  is divided into two portions, with a first portion of the fuel flowing in the direction of the arrows “A” and a second portion being directed to the fuel-turn manifold  48 , as shown by arrow “B” in  FIG. 1 . The second portion of the fuel is output from the reformer outlet  44   a  located on the third side  1 C of the stack and is received by the fuel turn manifold  48  which directs the fuel to the cell assemblies  16 B which are not associated with the reformer  30 . The first portion of the fuel flows toward the reformer outlet  44   b  located along the first side  1 A of the stack corresponding to the oxidant inlet side, and is output from the reformer outlet  44   b  directly into an inlet port  54  of an associated or adjacent anode current collector  22   a  of the reformer-associated cell assembly  16 A. The flow of the first portion of the fuel from the reformer outlet  44   b  to the inlet port  54  of the associated anode current collector  22   a  is shown by arrows “C” in  FIG. 1 . 
     In certain embodiments, the reformer  30  and the reformer-associated cell assembly  16 A are separated by a separator plate, which includes one or more openings corresponding to and aligned with the reformer outlet  44   b  and the inlet port  54  of the anode current collector  22   a . In addition, in some embodiments, the reformer outlet  44   b  is formed as a plurality of openings in a wall of the reformer that abuts the anode current collector  22   a  and the inlet port  54  is formed as a plurality of openings corresponding to the reformer outlet  44   b  openings in a wall of the anode current collector  22   a  that abuts the reformer  30 . 
     The ratio of the fuel flow amounts between the first and second portions of fuel is based on thermal management requirements of the stack  12  and also on the pressure drop across the associated anode current collector  22   a . In particular, for improved thermal management and gas mixing, it is desirable that all or substantially all of the fuel flow is directed from the reformer  30  directly to the associated anode current collector  22   a  as the first portion of the fuel. However, pressure drop in the associated anode current collector  22  should be minimized in order to keep the differential pressure between the anode and the cathode sides in the reformer-associated cell assembly  16 A within 7″. As a result, if the pressure drop in the associated anode current collector  22  is too high, the amount of fuel flow as the second portion of the fuel to the fuel turn manifold  48  is increased so as to reduce the pressure drop in the associated anode current collector  22 . 
     As discussed in more detail herein below, reformed or partially reformed first portion of the fuel flows unobstructed through the associated anode current collector  22   a  which is free of reforming catalyst or stores only a small amount of reforming catalyst therein. In addition, the associated anode current collector  22   a  does not include any baffles or has only a few baffles so as to allow the fuel to flow through the current collector unobstructed. The first portion of the fuel undergoes an electrochemical reaction in the reformer-associated cell assembly  16 A and exits the associated anode current collector  22   a  through an outlet port  38  into the fuel turn manifold  48 . In the fuel turn manifold  48 , the first portion of the fuel output from the outlet port  38  is mixed with the second portion of the reformed or partially reformed fuel from the reformer  30 , and is then directed by the fuel turn manifold  48  to the other cell assemblies  16 B. 
     The absence of reforming catalyst in the associated anode current collector  22   a  or the reduced catalyst loading in the associated current collector  22   a  enables endothermic cooling from the reforming reaction in the reformer  30  to be transferred to the cell assemblies  16 B not associated with the reformer, and, in particular, to the cell assemblies  16 B which are located further away from the reformer  30  and which need additional cooling. The reduced or no catalyst loading in the associated anode current collector  22   a  also allows the reformer  30  to achieve a high reforming rate, without reducing direct internal reforming within the assemblies  16 B not associated with the reformer, and thus without reducing the cooling resulting from the direct internal reforming in those assemblies  16 B. Further, the absence of catalyst or reduced catalyst loading in the associated anode current collector lowers the pressure drop across the reformer-associated cell assembly  16 A and results in a decreased pressure differential between the anode and cathode sides of the assembly. 
     As shown in  FIG. 1 , reformed or partially reformed fuel received in the fuel-turn manifold  48 , comprising a mixture of the second portion of the fuel from the reformer  30  and the first portion of the fuel partially spent in and output from the reformer-associated cell assembly  16 A, is directed to the cell assemblies  16 B not associated with the reformer. In particular, fuel from the fuel-turn manifold  48  enters the fuel inlet ports  38   a  of the cell assemblies  16 B and flows through the fuel gas channels  32  of the respective cell assemblies  16 B where the fuel undergoes an electrochemical reaction in the anode electrode to produce electricity. The fuel flows through the gas channels  32  in a general direction of fuel gas exhaust ports  40 , in a cross-flow configuration with respect to the flow of oxidant flow. In particular, the flow of fuel through the anode side of each assembly  16 B is perpendicular to the flow of oxidant gas through the cathode side of the assembly  16 B. Such cross-flow configuration accomplishes uniform flow of fuel to each fuel cell and results in a low cost and simple design of the cell assembly  16 B. The cross-flow configuration of the anode side of the assembly  16 B is described in more detail below with reference to  FIG. 4 . In certain embodiments, the flow of fuel through each assembly  16 B has a Z-pattern flow configuration, which is described in more detail below with reference to  FIG. 5 . 
     As mentioned herein above, the fuel flowing through the gas channels  32  of the cell assemblies  16 B is also directly internally reformed by the reforming catalyst stored in the channels  32 . The direct internal reforming of fuel within each assembly  16 B produces cooling within the assembly  16 B. As described in more detail below, the reforming catalyst may be loaded within the channels  32  at varying loading densities so as to achieve greater or smaller amounts of cooling in predetermined areas of the respective assembly  16 B and to accomplish a desired thermal profile of the stack. 
     As shown in  FIG. 1 , spent fuel, after undergoing the electrochemical reaction in the anode of the cell assembly  16 B, is output from the fuel gas exhaust ports  40  of the anode current collector  22  into the anode exhaust stack manifold  52 . Spent fuel received in the stack manifold  52  may then be exhausted out of the fuel cell assembly  10 . In certain embodiments, all or a portion of the spent fuel may be recycled for further use in the fuel cell assembly  10 . Also, in some embodiments, spent fuel may be further processed so as to extract water therefrom for humidifying fuel input into the assembly, before recycling the remaining spent fuel to the assembly  10  or exhausting it from the assembly  10 . 
     An illustrative configuration of a reformer  30  that can be used in the fuel cell assembly  10  of  FIG. 1  is shown in more detail in  FIG. 2 . The reformer  30  shown is rectangular in shape and has dimensions corresponding to the dimensions of the fuel cell stack&#39;s  12  cross-section. The corners of the reformer are labeled A through D and correspond to the respective corners of the fuel cell stack  12 . Corner A of the reformer is adjacent the fuel inlet of the reformer and corresponds to the corner of the fuel cell stack that is adjacent the fuel inlet and oxidant outlet faces. Corner B of the reformer is adjacent the fuel gas outlet of the reformer and corresponds to the corner of the fuel cell stack  12  which is adjacent the fuel inlet and oxidant inlet faces of the stack  12 . Corner C of the reformer is also adjacent the fuel gas outlet of the reformer and corresponds to the stack corner which is adjacent the fuel outlet and oxidant inlet faces of the stack  12 , while corner D of the reformer corresponds to the fuel cell stack corner adjacent the oxidant outlet face and fuel outlet faces of the stack. The reformer sidewall extending between corners A and B of the reformer faces the third face  1 C of the stack  12  and is enclosed by the fuel-turn manifold  48 . Reformer sidewall extending between corners B and C faces the first stack face  1 A corresponding to the oxidant inlet side of the stack, while reformer sidewall extending between corners C and D faces the fourth stack face  1 D and is enclosed by the manifold  52 . Finally, reformer sidewall that extends between corners D and A of the reformer faces the second stack face  1 B, corresponding to the oxidant outlet face of the stack, and is enclosed by the manifold  50 . 
     Referring to  FIG. 2 , the reformer  30  comprises a plurality of sections, including a fuel inlet section, labeled as Section A, and reforming sections, labeled as Section B and Section C. As discussed in more detail below, a plurality of baffles, labeled as Baffle 1-6, are provided in the reformer to define the Sections A-C and to guide the fuel through these sections to achieve a desired fuel flow and distribution through the reformer. 
     As shown in  FIG. 2 , fuel gas enters the reformer  30  through the fuel gas inlet  42  and flows along the inlet Section A so as to be laterally distributed along Section A. From Section A, the fuel flows into and through Section B, which includes a plurality of zones 1-4. The first zone of Section B, labeled as “Zone 1,” is located furthest away from the reformer inlet  42 , while the fourth zone of Section B, labeled as “Zone 4,” is located adjacent to the reformer inlet  42 . The second and third zones of Section B, labeled as “Zone 2” and “Zone 3,” are located in the central portion of Section B, between the first and fourth zones. 
     As shown in  FIG. 2 , a plurality of baffles 1-3 are provided to separate the inlet Section A from the reforming Section B of the reformer and to guide the flow of fuel from the inlet Section A to the four zones of Section B. In particular, Baffle 1 is provided at the inlet of the fourth zone, Zone 4, to achieve a desired flow restriction of the fuel from the inlet Section A to the fourth Zone 4. Similarly, Baffle 2 is provided at the inlet of the third zone, Zone 3, and Baffle 3 is provided at the inlet of the second zone, Zone 2, to restrict the flow of fuel from the inlet Section A to the third and second zones, respectively. Baffles 1-3 also ensure that the fuel flowing through the inlet Section A is distributed throughout the inlet section and into each of the zones of the reforming Section B. In particular, Baffles 1-3 are calibrated to have flow resistances from Section A to each zone in Section B so as to achieve a desired flow distribution of fuel through each zone of Section B. In addition, as shown in  FIG. 2 , zone 1 is free of baffling to ensure that the flow of fuel through the reformer  30  is constant, particularly if Baffles 1-3 are non-optimized. 
     As also shown in  FIG. 2 , Baffles 4-6 are provided in the reformer  30  to separate the respective zones of Section B and to straighten and guide the flow of fuel through each zone of Section B. In particular, Baffle 4 is provided between Zone 4 and Zone 3, Baffle 5 is provided between Zone 3 and Zone 2, and Baffle 6 is provided between Zone 2 and Zone 1. In certain embodiments, Baffles 4-6 may extend into Section C of the reformer so as to further guide the flow of fuel to achieve a desired fuel flow through the reformer. 
     The Baffles 1-6 used in the reformer may have various constructions. In certain embodiments some or all of the baffles are formed from one or more of: rods inserted into the corrugations of the reformer  30 , porous structured materials inserted into or between the corrugations of the reformer  30  or sheet metal folded at the edge to form mechanical baffles. The materials from which the baffles 1-6 are formed have be able to withstand the high temperatures in the fuel cell stack. For example, ceramic rope is a suitable porous structured material for forming one or more of Baffles 1-6. 
     In addition, the configuration and arrangement of the baffles in the reformer is not limited to the one shown in  FIG. 2 . In particular, since the optimum thermal management in the fuel cell stack  12  is best achieved by routing substantially all of fuel gas flow from the reformer  30  to the reformer-associated cell assembly  16 A, the configuration of the baffles may be varied to achieve such routing. 
     As shown in  FIG. 2  and as mentioned herein above, the reformer includes an outlet  44   b  through which fuel gas flows to the associated or adjacent anode  20  and current collector  22  of a reformer-associated cell assembly  16 B. The outlet  44   b  is formed in the top portion of the enclosure of the reformer that abuts the associated current collector  22 . In the embodiment shown in  FIG. 2 , the outlet  44   b  is L-shaped and extends along, or adjacent to, the wall of the reformer between corners B and C and partially along, or adjacent to, the wall between corners C and D of the reformer. In particular, the reformer outlet  44   b  extends from corner C in the direction of corner D over end portions of Sections C and B of the reformer, without reaching the inlet Section A of the reformer. In other embodiments, the reformer outlet  44   b  extends only between corners B and C of the reformer or only between corners C and D of the reformer. 
     In the illustrative embodiment shown in  FIG. 2 , the reformer also includes a second outlet  44   a , which outputs the second portion of the fuel into the fuel-turn manifold  48 . As discussed above with respect to  FIG. 1 , the second portion of the reformed or partially reformed fuel is output from the reformer&#39;s second outlet  44   a  into the fuel-turn manifold  48  and thereafter supplied to the cell assemblies  16 B not associated with the reformer. The first portion of the reformed or partially reformed fuel is output from the reformer&#39;s outlet  44   b  to the anode current collector  22  of the reformer-associated cell assembly  16 A. However, in other embodiments, all of the fuel flowing through the reformer  30  is outputted to the reformer-associated cell assembly  16 A through the outlet  44   b , and in such other embodiments, the reformer  30  does not include the second outlet  44   a  shown in  FIG. 2 . 
     The reformer  30  shown in  FIG. 2  includes reforming catalyst disposed in the corrugations of the reformer to promote the reforming of the fuel. The loading density of the reforming catalyst in the reformer, and in particular in the different Zones and Sections of the reformer, may be varied for improved thermal management in the stack and to achieve the desired temperature distributions in the reformer. In particular, greater loading density of the reforming catalyst can be provided in the areas of the reformer where additional cooling is required, and smaller loading density of the reforming catalyst is provided in areas of the reformer which do not require as much cooling. In addition, gradual loading density variations are preferred so as to obtain smooth thermal transitions in the reformer.  FIG. 2A  shows an illustrative catalyst loading configuration which can be used in the reformer  30  of  FIG. 2 . 
     As shown in  FIG. 2A , the reforming catalyst loading densities are varied not only between the different sections of the reformer, but also within each section of the reformer. The loading density of the reforming catalyst disposed in the inlet section A of the reformer is lower than the loading density in the other sections. As shown, the initial catalyst loading density in the inlet section A of the reformer near the inlet  42  is 1/64, i.e. 1 catalyst unit or pellet for every 64 corrugations. The loading density in the inlet section then increases to 1/48 and thereafter to 1/16 as the fuel travels along the length of the inlet section A. 
     In the illustrative embodiment shown in  FIG. 2A , the catalyst loading density in Section B of the reformer, and in particular, in each of the Zones 1-4 is greater than the catalyst loading density in the inlet section A. In addition, the catalyst loading density in Zone 1 is greater than the catalyst loading in Zones 1-4, the catalyst loading density in Zone 2 is smaller than in Zone 1 but greater than in Zones 3-4, and the catalyst loading density in Zones 3 and 4 is smaller than in Zones 1 and 2. 
     In particular, the catalyst loading density in Zone 1 is 1/12, i.e. 1 catalyst unit or pellet for every 12 corrugations, in an area adjacent to the inlet section A and to the outlet  56  of the reformer, and thereafter gradually increases to 1/5 loading density. A portion of Zone 1 that extends from the inlet section A to Section C of the reformer and which is adjacent to Zone 2 has increased catalyst loading at 1/2 loading density. 
     The catalyst loading density in Zone 2 is 1/48, i.e. 1 catalyst unit or pellet for every 48 corrugations, in an area adjacent to the inlet section A and to Zone 1 of the reformer, and thereafter gradually increases to 1/8 loading density and to 1/2 loading density in the direction from the inlet section A to Section C of the reformer. Additionally, a portion of Zone 2 which extends from the inlet section A to Section C of the reformer and which is adjacent to Zone 3 has an increased catalyst loading density of 1/2. 
     The catalyst loading density in Zone 3 is 1/48 in an area of Zone 3 adjacent to the inlet section A and to Zone 2 of the reformer, and thereafter gradually increases to 1/16 loading density and 1/2 loading density in the direction from the inlet section A to Section C of the reformer. In addition, a portion of Zone 3 which extends from the inlet section A to Section C of the reformer and which is adjacent to Zone 4 has an increased catalyst loading density of 1/2. Similarly, the catalyst loading density in the area of Zone 4 that is adjacent to the inlet section A and to Zone 3 of the reformer is 1/48, and thereafter increases to 1/16 and to 1/2 loading density in the direction from Section A to Section C of the reformer. 
     In the outlet Section C of the reformer, the catalyst loading density is 1/2 in the area adjacent to Zone 4 and a portion of Zone 3 of the reformer, thereafter gradually decreasing to a loading density of 1/3 in the area adjacent to a portion of Zone 3 and a portion of Zone 2, and to a loading density of 1/16 in the area adjacent to a portion of Zone 2 and a portion of Zone 1. The catalyst loading density is gradually reduced to 0 in the outlet area near corner C of the stack. 
     The catalyst loading configuration shown in  FIG. 2A  achieves a temperature distribution which provides more cooling in the central area of the reformer as well as in the area of the reformer near the fuel outlet side of the stack. The configuration of  FIG. 2A  also reduces temperature gradients in the reformer-associated cell assembly  16 A. It is understood, however, that the catalyst loading configuration shown in  FIG. 2A  is illustrative and can be modified depending on the configuration of the fuel cell stack and so as to achieve other temperature distributions to provide more cooling to other areas of the stack. 
     Referring now back to  FIG. 1 , fuel gas leaving the reformer  30  through the outlet  44   b  enters the inlet  54  of the associated current collector  22  of the reformer-associated cell assembly  16 A. In the associated current collector, fuel flow has a co-flow configuration relative to the oxidant gas flow, i.e. parallel to the oxidant gas flow, in certain areas of the associated anode current collector, and a cross-flow configuration relative to the oxidant gas flow, i.e. perpendicular to the oxidant gas flow, in other areas of the associated anode current collector. The co-flow configuration of the fuel flow in the anode current collector is shown by arrows “D” in  FIG. 1 , while the cross-flow configuration of the fuel flow in the anode current collector is shown by arrows “E” in  FIG. 1 . 
       FIG. 3  shows in more detail the anode current collector  22  of the reformer-associated cell assembly  16 A of  FIG. 1  and the flow path of fuel gas through the current collector  22 . As shown, the anode current collector  22  includes an inlet  54 , the relative location and shape of which corresponds to the location and shape of the outlet  44   b  of the reformer. The fuel flows through the channels  32  in the anode current collector  22  and the direction of the fuel flow through the current collector  22  is illustrated by arrows “E”, which include portions “e 1 ” and “e 2 ”. Portions “e 1 ” of the arrows “E” represent the flow of fuel which is substantially parallel to the flow of oxidant gas through the fuel cell stack, i.e. having a co-flow configuration. Portions “e 2 ” of the arrows “E” represent the direction of the flow of fuel which is perpendicular to the flow of oxidant gas through the fuel cell stack, i.e. having a cross-flow configuration. 
     As shown, the flow of fuel through the anode current collector of the reformer-associated assembly  16 A starts in the same direction as the flow of oxidant gas through the stack, and then changes direction so that the fuel flows in a direction that is perpendicular to the flow of oxidant gas toward the outlet of the anode current collector  38 . Fuel gas exits the channels  32  of the anode current collector substantially uninhibited through the outlet  38 , shown by the arrows “E” and is outputted into the fuel-turn manifold  48 . 
     Fuel gas is not completely reacted during the electrochemical reaction in the associated or adjacent anode  20  of the reformer-associated assembly. Fuel gas exhaust leaving the anode current collector  22  of the reformer-associated assembly  16 A and collected in the fuel-turn manifold  48  is then distributed to the other cell assemblies  16 B not associated with the reformer. In this way, unreacted fuel in the fuel gas exhaust of the reformer-associated assembly  16 A is electrochemically reacted in the other cell assemblies  16 B to produce electricity. 
     As discussed herein above with respect to  FIG. 1 , in certain embodiments, the flow of fuel through the anode side of the cell assemblies  16 B not associated with the reformer  30  has a cross-flow configuration relative to the flow of oxidant fuel through these assemblies.  FIG. 4  shows the flow of fuel in such embodiments, including the flow of fuel through the reformer  30 , through the fuel cell  58  of the reformer-associated assembly  16 A and through the next cell assembly  16 B not associated with the reformer. The flow of fuel through the next cell assembly  16 B is exemplary of the flow of fuel through the other cell assemblies in series with cell  58 . 
     As shown in  FIGS. 1 and 4 , the flow of fuel gas through the reformer  30  is labeled by the arrow “A” and the continued flow of fuel from the reformer  30  to the fuel gas inlet  54  of the reformer-associated cell assembly  16 A is labeled by the arrow “C.” As also shown, the flow of fuel passing through the anode  20  and anode current collector  22  of the reformer-associated cell assembly  16 A is indicated by arrows “E”, which show the fuel flowing first in a co-flow configuration with respect to the oxidant gas and thereafter changing to the cross-flow configuration relative to the flow of oxidant gas. The co-flow and cross-flow configuration of the fuel flow through the reformer-associated assembly  16 A is shown by the relationship between arrows “E” and “G”, wherein the arrow “G” represents the direction of the oxidant flow. As discussed above, fuel leaving the reformer-associated assembly  16 A is outputted to the fuel-turn manifold  48 , which directs the fuel to the fuel inlets of the other assemblies  16 B not associated with the reformer  30 . 
     The direction of the flow of fuel from the fuel-turn manifold  48  through the anode side of the successive assemblies  16 B not associated with the reformer is shown by arrows “H” in  FIG. 4 . As can be seen in  FIG. 4 , the fuel gas flows through the anodes and anode current collectors of the successive assemblies  16 B in a cross-flow configuration relative to the direction of the flow of oxidant gas. This cross-flow configuration is demonstrated by the arrows “H” and “G” which show the flow of fuel and oxidant, respectively, through the assembly. The cross-flow arrangement of the flow fields through the assemblies  16  of the stack as shown in  FIG. 4  ensures uniform distribution of the fuel to the successive cell assemblies  16 B in the stack  12 , while maintaining low cost and simple design of the stack. 
     As discussed above, in certain embodiments, the cell assemblies  16 B not associated with the reformer have a Z-pattern flow configuration for the flow of fuel through the anode side is the assemblies  16 B.  FIG. 5  shows the flow of fuel in such embodiments, including the flow of fuel through the reformer  30 , through the fuel cell  58  of the reformer-associated assembly  16 A and through the next cell assembly  16 B not associated with the reformer. The flow of fuel through the next cell assembly  16 B is exemplary of the flow of fuel through the other cell assemblies in series with cell  58 . 
     As shown in  FIG. 5 , the flow of fuel through the reformer  30  and through the fuel cell  58  of the reformer-associated cell assembly  16 A is the same, or substantially the same, as the flow of fuel through the reformer and the reformer-associated cell assembly of  FIG. 4 . As in  FIG. 4 , the fuel leaving the reformer-associated cell assembly  16 A is outputted to the fuel-turn manifold  48  which directs the fuel to the next or successive cell assemblies  16 B not associated with the reformer. 
     In  FIG. 5 , the Z-pattern flow path of fuel through the anode  20  and anode current collector  22  of the successive cell assemblies  16 B is shown by arrows “I” and “J.” As shown, the arrow “I” shows the direction of the fuel flow in a counter-flow configuration relative to the oxidant gas flow, labeled by arrow “G,” while the arrow “J” shows the direction of fuel flow in a cross-flow configuration relative to the flow of oxidant gas. In the Z-pattern flow configuration shown in  FIG. 5 , the fuel flow path combines the counter-flow direction of the fuel flow and the cross-flow direction of the fuel flow relative to the direction of the oxidant gas flow, so that the fuel flowing through each of the successive assemblies  16 B flows in a direction counter to the direction of oxidant flow over a portion of its path and in a direction substantially perpendicular to the direction of the oxidant flow over the other portion of its flow path. As shown in  FIG. 5 , some of the fuel flowing the assembly  16 B first has a counter-flow configuration and thereafter has a cross-flow configuration relative to the oxidant gas flow, while another portion of the fuel flowing through the assembly has a cross-flow configuration followed by the counter-flow configuration. 
     The Z-pattern flow path configuration of the fuel is achieved by blocking a portion of the fuel gas inlet port  38  of the anode current collector  22  so as to impede the flow of fuel through the blocked portion of the fuel gas inlet port  38   a  and to direct the fuel to enter the anode current collector  22  only through the open or unblocked portion of the fuel gas inlet port  38   a . As shown in  FIG. 5 , the blocked portion of the fuel gas inlet port  38   a  starts from a corner of the anode current collector  22  adjacent to the oxidant gas inlet ports  34  and the first face of the stack  1 A and extends along the portion of the fuel gas inlet port  38   a  in a direction of the other corner of the anode current collector  22  adjacent to the oxidant gas outlet ports  36  and the second stack face  1 B. In this way, an open fuel inlet portion is formed in the anode current collector  22  which is near the oxidant gas outlet ports  36  of the stack  12 , so that fuel is directed to enter the anode current collector  22  adjacent to the oxidant outlet face  1 B of the stack  12 . 
     As shown, a portion of the fuel outlet port  40  of each assembly  16 B can also be blocked off so as to direct the fuel leaving the anode current collector  22  through the open, or unblocked, portion of the outlet port  40 . In particular, the blocked off portion of the fuel outlet port  40  extends from a corner of the anode current collector  22  adjacent to the oxidant gas outlet ports  36  and the second stack face  1 B in a direction of the other corner of the anode current collector  22  adjacent to the oxidant gas inlet ports  34  and the first stack face  1 A. The open or unblocked portion of the fuel outlet port  40  is located adjacent to the first face of the stack  1 A and the oxidant inlet ports  34 . 
     The blocked off portions of the fuel inlet port and the fuel outlet port are formed by using baffles, wall extensions or any other suitable means for impeding the flow of fuel through the inlet and outlet ports. The blocking of the portions of the fuel inlet and the fuel outlet ports as described above directs the fuel to enter the anode current collector  22  of each assembly  16 B adjacent to, or near, the face of the stack  1 B associated with the oxidant outlet ports  36 , to flow through the anode side of the assembly  16 B in a Z-shaped path and to exit the anode current collector  22  adjacent to, or near, the face of the stack  1 A associated with the oxidant inlet ports  34 . This configuration of the anode current collector  22  combines the cross-flow and counter-flow configurations of the fuel relative to the oxidant gas flow since the fuel is directed to flow in a direction perpendicular to the flow of oxidant gas and also in a direction opposite to that of the flow of oxidant gas in order to get from the open portion of the fuel inlet port  38  to the open portion of the fuel outlet port  40 . 
     In addition, one or more baffles may be used in the anode current collector to direct the flow of fuel in the Z-pattern flow path, and/or the direction of the corrugations in the anode current collector  22  of each assembly  16 B may be configured so as to direct the flow of fuel through the anode current collector in a Z-shaped path. One or more baffles may also be used to control the fuel flow distribution through the anode current collector so as to achieve fuel flow uniformity throughout the anode current collector. In certain embodiments, the baffles and/or the configured direction of the corrugations are used together with the blocked off fuel inlet and outlet port portions to promote the flow of fuel in a Z-shaped path. In other embodiments, the baffles and/or the configured direction of the corrugations may be used without blocking off portions of the fuel inlet and outlet portions to achieve the Z-pattern flow path. 
     As shown in  FIGS. 1 and 5 , the Z-pattern flow path configuration through the anode side of the cell assemblies  16 B realizes the counter-flow configuration of the fuel relative to the oxidant fuel without requiring separate fuel and oxidant gas manifolds to be present on the same sides of the stack. The Z-pattern flow path configuration also results in a substantially lower differentials in pressure gradients along the anode flow channels, and in an improved uniformity of current density throughout the stack  12 . As a result, greater efficiency in the production of electricity and extended service life of the stack  12  can be achieved. 
     Although the Z-pattern flow path configuration shown in  FIG. 5  combines the combination of the cross-flow and counter-flow configurations of the fuel flow relative to the oxidant gas flow, it is understood that the Z-pattern flow configuration may be modified so as to combine the cross-flow and co-flow configurations of the fuel flow relative to the oxidant flow. Such modified Z-pattern flow configuration can be achieved by blocking off a portion of the anode current collector inlet from a corner of the anode current collector adjacent the oxidant gas outlet ports and the second face of the stack and by blocking off a portion of the anode current collector outlet from a corner of the anode current collector adjacent the oxidant gas inlet ports and the first face of the stack. In this way, fuel is allowed to enter the anode side of the cell assembly through the unblocked portion of the anode current collector inlet adjacent to the oxidant gas inlet ports and to flow through the anode side so as to exit through the unblocked portion of the anode current collector outlet adjacent to the oxidant gas outlet ports. 
       FIG. 6  shows an illustrative construction of an anode current collector of one of the cell assemblies  16 B not associated with a reformer, wherein the anode current collector enables the Z-pattern flow configuration discussed above. As shown, the anode current collector includes an inlet  60  through which fuel enters the anode current collector, an inlet section  62  of the current collector, an outlet section  72  of the current collector, a central area divided into a plurality of zones, i.e. zones 1-4, and a plurality of baffles for directing the flow of fuel through the anode current collector. 
     In particular, the inlet  60  of the anode is formed as an unblocked portion of the inlet side of the anode current collector and extends from the corner of the anode current collector adjacent to the oxidant gas outlet ports of the stack. Fuel enters the anode current collector through the inlet  60  in cross-flow configuration relative to the oxidant gas. In the anode current collector, the fuel is first distributed over the inlet section  62  of the anode current collector which extends from the inlet  60  along the length of the side of the current collector adjacent to, or aligned with, the oxidant outlet ports. 
     As shown in  FIG. 6 , the plurality of baffles  64 ,  66 ,  68  and  70  are disposed in the anode current collector for directing the fuel flow from the inlet section  62  to the respective zones of the central section of the anode current collector. In particular, baffles  64 ,  66  and  70  are disposed between the inlet section  62  and Zone 4, Zone 3 and Zone 1, respectively. These baffles  64 ,  66  and  70  provide flow resistance to limit the amount of fuel flowing into each of Zone 4, Zone 3 and Zone 1, respectively, so that the fuel is distributed between the Zones 1-4. The flow resistance of each baffle  64 ,  66  and  70  may be adjusted so as to allow greater or smaller amount of fuel flow from the inlet section to the Zone corresponding to the baffle. In the illustrative embodiment of  FIG. 6 , no baffle is provided between the inlet section  62  and Zone 2 so that the fuel flow from the inlet section  62  into Zone 2 is unobstructed. In addition, baffle  68  extends between Zone 2 and Zone 3 for directing the flow of fuel along Zone 2 and along Zone 3 and preventing the mixing of fuel between Zones 2 and 3. 
     The combination of baffles  64 ,  66 ,  68  and  70  as shown in  FIG. 6  results in a Z-pattern flow configuration of the fuel flow through the anode side of the cell assembly  16 B. In particular, the flow of fuel along the inlet section  62  and the outlet section  72  of the anode current collector has a cross-flow configuration relative to the oxidant gas flow, while the flow of fuel along Zones 1-4 of the central section of the anode current collector has a counter-flow configuration relative to the oxidant gas flow. It is understood that in other illustrative embodiments, the baffles  64 ,  66 ,  68  and  70  may be arranged so that the fuel flow along Zones 1-4 of the central section of the anode current collector has a co-flow configuration relative to the oxidant gas flow. 
     In the embodiment shown in  FIG. 6 , the loading of the reforming catalyst in the anode current collector is varied so as to provide a desired flow resistance and a desired amount of reforming in each section of the anode current collector. In particular, the inlet section  62  of the anode current collector has low or no reforming catalyst disposed therein so as to minimize fuel flow resistance. In each of Zones 1-4, catalyst loading density is increased relative to the catalyst loading density in the inlet section  62  so as to increase flow resistance in Zones 1-4 and to achieve flow uniformity through the Zones 1-4. The greater catalyst loading density in Zones 1-4 lowers the gas flow velocity due to the increased flow resistance, and optimizes the electrochemical reaction needed to produce electricity. As a result, most of the direct internal reforming occurs in Zones 1-4 of the central section. As discussed herein below, the reforming catalyst loading density in each of Zones 1-4 may be varied from one Zone to another and throughout each zone. For example, the overall catalyst loading density in one of the Zones may be greater than the catalyst loading density in another zone so as to provide more reforming, and thus more cooling, in the zone with greater catalyst loading density. In addition, the density of the reforming catalyst loading can be varied along each Zone 1-4 so that the catalyst loading density is greatest in the areas of the cell assembly where the most cooling is required. 
     The configuration of the anode current collector shown in  FIG. 6  is compatible with the structure and design of conventional carbonate fuel cell stacks. In particular, in conventional carbonate fuel cell systems, boundary regions, also called wet-seal regions, of each cell assembly are inactive where no electrochemical reaction occurs. U.S. patent application Ser. No. 12/016,564, which is incorporated herein by reference, discloses an example of such fuel cell design, particularly a fuel cell employing a bipolar separator plate that forms the wet seal regions of the fuel cell. The inlet and outlet sections of the anode current collector shown in  FIG. 6 , which are used for distributing fuel throughout the central section of the current collector and for collecting spent fuel gas from the central section, are disposed within the anode side wet seal regions of the cell assembly. By using the inactive wet seal regions for distributing and collecting fuel, the amount of reforming and the location of the central region of the anode current collector where the reforming occurs can be optimized for improved operation of the fuel cell assembly. In addition, pressure drop in the inlet section of the anode current collector is decreased, thus improving the uniformity in the flow of fuel through the central region of the anode current collector. 
     As described herein above, the assembly includes a two-stage supply of fuel to the fuel cell stack  12 , wherein the first stage comprises fuel supply from one or more reformers  30  to a respective reformer-associated cell assembly  58 , and the second stage comprises distribution of partially-reformed fuel from the fuel-turn manifold  48  to each of the remaining fuel cells of the stack  12 . When compared to prior stack designs, the stack shown in  FIG. 1  requires lower fuel flow for powering the stack  12  because the fuel from the first stage is recycled during the second stage. In addition, since the second stage receives and uses partially spent fuel from the first stage, the total amount of fuel flow to the stack may be reduced as compared to the total amount of fuel flow received in conventional stacks. As a result, high fuel utilization, i.e., high efficiency in the production of electricity, by the stack  12  is achieved by the two-stage configuration of the invention. 
     In addition to the two-stage fuel supply described above, the stack  12  shown in  FIG. 1  has improved thermal management, which increases the stack&#39;s service and operating life. The flow path of the fuel through the reformer  30 , as described above, contributes to such improved thermal management by optimizing the endothermic reaction occurring in the reformer  30 . 
     Also, the absence of catalyst or the reduced catalyst loading in reformer-associated cell assemblies  16 A contributes to more stable stack temperature gradients compared to conventional stacks since fuel gas supplied thereto is reformed to a larger extent in the reformer  30 . In particular, since there is no, or a very small amount of, reforming catalyst in the reformer-associated cell assembly  16 A, a larger fraction of the endothermic reforming reaction can be produced by the reformer. Thus, the efficiency of the reformer and of the reforming reaction rate in the reformer are improved. This is particularly important to the performance and service life of the stack because the reforming catalyst in the reformer is not exposed to carbonate electrolyte and is therefore more likely to have stable activity as the stack ages. The improved reforming efficiency in the reformer therefore improves the thermal stability of the stack. 
     Also, the two-stage fuel supply in the assembly minimizes the volatility in temperature gradients that result from catalyst deactivation in the cell assemblies  16 B not associated with the reformer  30  and improves uniformity in the reforming reaction in the reformer. The cell assemblies  16 B not associated with the reformer also benefit from the cooling that results from the cooled fuel gas exhaust supplied from the reformer-associated cell assembly  16 A to the fuel-turn manifold  48  and from the endothermic direct internal reforming reaction within each of the cell assemblies  16 B. In particular, the higher reforming and thus higher cooling rate in the cell assemblies  16 B not associated with the reformer reduces the peak current density within the cell assemblies and makes the current density distribution in the stack more uniform. Uniform current density reduces local high temperatures and results in an enhanced control of temperature gradients from one cell assembly  16 B to another. Greater thermal stability and reduced temperature gradients in the stack result in reduced thermal stresses on the components of the stack and in decreased contact losses between the components of the cell assemblies. 
     Further, the fuel flow field in the reformer-associated cell assembly  16 A causes a shift in current density distribution in the stack which results in an increased temperatures at the oxidant inlet and fuel outlet regions of the stack. The increased temperatures at the oxidant inlet and fuel outlet regions, in turn, increase the reforming activity of the catalyst disposed in the other cell assemblies  16 B not associated with the reformer so as to provide adequate methane conversion. In addition, the shift in the current density in the fuel cell stack of  FIG. 1  and the two-stage fuel delivery described above counteract the tendency to concentrate current density near the anode inlet region of the stack, which is often experienced by conventional stacks with cross-flow configuration. This, in turn, minimizes temperature shifts in the stack, particularly if the fuel utilization rate is increased, thus leading to higher operating efficiency of the stack. Furthermore, since the reformer inlet section is located near the oxidant outlet side of the neighboring cell assemblies, cooling is provided to the oxidant outlet gas, thus reducing thermal management requirements of the stack. For these reasons, the efficiency in the production of electricity by the stack and the service life of the stack are increased. 
     In all cases it is understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which represent applications of the present invention. Numerous and varied other arrangements can be readily devised in accordance with the principles of the present invention without departing from the spirit and scope of the present invention. For example, it is within the contemplation of the present invention to further provide thermal management for the stack by providing additional external means to modulate the fuel temperature even prior to entering the stack. 
     INCORPORATION BY REFERENCE 
     The following patents and published patent applications, assigned to the same assignee herein, are incorporated herein by reference:
     U.S. Pat. No. 6,200,696   U.S. Pat. No. 5,175,062   U.S. Application Publication No. 2006/0123705   U.S. Application Publication No. 2004/0071617