Abstract:
A fluid resistance section outside a fuel cell can regulate the reactant flow against the pressure changes in the reaction zones of the electrodes, reducing the fluctuations in reactant flows to the fuel cell electrodes due to dynamic fluctuations in fluid pressure at the fuel cell electrode because of the release of gaseous products. The outside fluidic resistor can have resistance much higher than the resistance of the flow through the electrodes, thus effectively determining the amount of the reactant flow to the electrodes, independent of the electrode areas.

Description:
This application claims priority from U.S. provisional patent application Ser. No. 61/346,887, filed on May 20, 2010, entitled “Flow management in fuel cell configurations”, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to fuel cell systems and, more specifically, to microfluidic fuel cell systems. 
     BACKGROUND OF THE INVENTION 
     A fuel cell is an energy conversion device that consists essentially of two opposing electrodes, an anode and a cathode, ionically connected together via an interposing electrolyte. Unlike a battery, fuel cell reactants are supplied from an external reactant supply source. Fuel cells operate by converting a reactant fuel such as hydrogen or a hydrocarbon (e.g., methanol) to electrical power through an electrochemical process rather than by combustion. It does so by harnessing the electrons released from controlled oxidation-reduction reactions occurring on the surface of a catalyst. A fuel cell can produce electricity continuously so long as proper reactants (i.e., a fuel and an oxidant) are supplied from an outside source. 
     Most conventional micro-scale fuel cell systems include a stack of electrically interconnected electrode pair assemblies (commonly referred to as a fuel cell stack assembly), wherein each electrode pair is configured to receive and react with selected reactants (e.g., methanol and air flowstreams delivered across respective outer electrode surfaces). The interposing electrolyte of most conventional micro-scale liquid-air fuel cell systems (e.g., direct methanol fuel cell (DMFC) systems) generally consist of a solid polymer proton exchange membrane (PEM) (e.g., NAFION). These known micro-scale fuel cell systems all comprise an interconnected series of electrode pair assemblies, wherein each electrode pair utilizes a solid polymer proton exchange membrane as a separator and as a proton (H + ) transfer medium. 
     In contrast, certain liquid-liquid fuel cell systems do not utilize a central PEM. In general, liquid-liquid fuel cell systems typically comprise electrode pairs and related stack assemblies that include a series of micro fluidic flow channels for flowing liquid reactant/electrolyte flowstreams (i.e., electrolytic fuel and oxidant flowstreams referred to herein as anolyte and catholyte flowstreams, respectively) adjacent to and/or through discrete regions of accompanying porous electrode structures. 
     SUMMARY OF THE DESCRIPTION 
     The present invention relates to the management of fluid reactant flows in fuel cells, such as fuel flowstream in PEM fuel cells, or fuel flowstream and/or oxidant flowstream in liquid-liquid fuel cells. In an embodiment, the present invention discloses methods and apparatuses for reducing fluctuations in reactant flows to the fuel cell electrodes, for example, due to dynamic fluctuations in fluid pressure at the fuel cell electrode because of the release of gaseous products. 
     Microchannel structure can improve fuel cell efficiency with increased surface areas, but introducing high pressure fluctuations as the results of the released gaseous products such as CO 2 . High pressure can affect the reactant flow, either blocking or reducing the reactant flow, which then reduces the power generation. In an embodiment, the present invention provides a fluid resistance section outside the electrodes to regulate the reactant flow against the pressure changes in the reaction zones of the electrodes. The outside fluidic resistor can have resistance much higher than the resistance of the flow through the electrodes, thus effectively determining the amount of the reactant flow to the electrodes, independent of the electrode areas. 
     In an embodiment, the present fluidic resistor comprises microchannel sections having fluid resistance or pressure drop that is greater than the fluid resistance or pressure drop across the electrode. The microchannel sections are coupled to the entrance and/or outlet of the fuel cell, regulating the reactant flow to the fuel cell. In an embodiment, the microchannels can be integrated to the fuel cell fluid flow portions, creating an integrated fuel system with improved reactant flows. 
     In an embodiment, the present invention discloses the use of microchannels to control fluid delivery to a multitude of fuel cells arranged in parallel configurations. This allows the use of a single pump to deliver fuel to many cells and flow rates to individual cells which is dominated by channel width and length, making fuel delivery impervious to dynamic changes in pressure of the fuel cell. Microchannels may be made by semiconductor processing, MEMS processing, stamping, etc. that can be integrated easily with the fuel cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary fuel system comprising a flow management according to embodiments of the present invention. 
         FIGS. 2A-2D  illustrate various configurations for connecting a flow management with a fuel cell. 
         FIG. 3  illustrates a schematic configuration for a fuel cell system according to an embodiment of the present invention. 
         FIGS. 4A-4B  illustrate exemplary fluidic resistors having a long winding pattern according to an embodiment of the present invention. 
         FIGS. 5A-5B  illustrate exemplary fluidic resistors having a capillary tube according to an embodiment of the present invention. 
         FIGS. 6A-6C  illustrate exemplary fluidic resistors having porous elements within the flow path according to an embodiment of the present invention. 
         FIGS. 7A-7B  illustrate exemplary fluidic resistors having multiple holes within a blocking element within the flow path according to an embodiment of the present invention.  FIG. 7A  shows a cross section view and  FIG. 7B  shows a perspective view of a fluidic resistor. 
         FIG. 8  illustrates an exemplary integrated fuel cell system with an integrated flow management according to an embodiment of the present invention. 
         FIGS. 9A-9C  illustrate various views of an exemplary serpentine pattern.  FIG. 9A  shows a top view of an exemplary serpentine pattern.  FIG. 9B  shows a cross section view of an exemplary serpentine pattern.  FIG. 9C  shows two flow controller layers. 
         FIG. 10  illustrates an exemplary integrated fuel cell system comprising a plurality of flow managements integrated with a plurality of anode/cathode pairs. 
         FIGS. 11A-11C  illustrates various configurations for flow controller having microchannel layer integrated with a fuel cell stack having 4 electrodes arranged in anode-cathode/anode-cathode configuration. 
         FIGS. 12A-12B  illustrates various configurations for flow controller having microchannel layer integrated with a fuel cell stack having 5 electrodes arranged in anode-cathode/cathode-anode/anode configuration. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to power management in fuel cell systems, and particularly to fuel cell systems employing microchannel structures. In an embodiment, the present invention discloses a fuel cell system having improved operating power characteristics, such as reducing fluctuations in fuel cell power generation, preventing intermittent power interruption, and in general, enabling a fuel cell system to generate a steady and reliable power output. 
     In an embodiment, the present invention discloses a liquid flow management for fuel cell reactants together with integrated fuel cell systems employing liquid flow management. The present liquid flow management controls the reactant flows to the fuel cell electrodes to improve the operation characteristics of fuel cell systems. 
     In an embodiment, the present liquid flow management supplies an improved reactant flow to the electrodes, which is less susceptible to changes in the fuel cell operation behaviors, such as changes in fluidic resistance at the electrode areas due to gaseous formation at the electrode microchannel structures. In an embodiment, the present liquid flow management supplies a steady reactant flow to the fuel cell electrodes to meet a power generation requirement. The term “steady flow” denotes a desirable characteristic of the reactant flow to the electrode, and in the context of the present specification, means a constant flow, a nearly constant flow, a substantially constant flow, an improved constant flow, a flow that has improved constancy characteristics, and in general, a flow that is less susceptible to changes, such as changes in fuel cell operating and environment conditions, changes in fuel cell operation behaviors, changes in fuel cell reaction characteristics, and changes in fuel cell manufacturing processes. 
     In an embodiment, the present liquid flow management employs channel length and pressure drop optimization to reduce or eliminate the dynamic pressure fluctuation in fuel cell reaction chamber, and/or the variations in electrode manufacturing. For example, by supplying a steady reactant flow, the dynamic fluidic resistance fluctuation can be compensated, resulting in steady power generation. 
     The present invention can be used in fuel cell systems that employ liquid reactant flowstreams in separate electrochemical half-cell reactions, such as conventional PEM-based fuel cell systems, together with novel electrode pair assemblies that do not include an interposing solid polymer proton exchange membrane where the liquid anolyte flowstream functions as the interposing electrolyte. 
       FIG. 1  illustrates an exemplary fuel system  10  comprising a flow management according to embodiments of the present invention. The fuel system  10  comprises a flow management  40  supplying a steady anolyte flowstream  14  to a fuel cell. The fuel cell comprises an electrode pair assembly (anode  18  and cathode  20 ), configured to receive and react with a liquid anolyte flowstream  14  and a liquid catholyte flowstream  16 . The electrode pair assembly comprises a porous flow-through anode  18 , a porous flow-by cathode  20  confronting and spaced apart from the anode  18 , and a central plenum  22  interposed between and connected to the anode and the cathode. The electrode pair assembly further comprises a catalyzed separator  31  to prevent the catholyte flowstream  16  from substantially passing through the flow-by cathode  20  and into the central plenum  22 . In an embodiment, the liquid anolyte flowstream comprises a laminarly microfluidic flowing methanol/sulfuric acid solution, and the liquid catholyte flowstream comprises a laminarly microfluidic flowing nitric acid/sulfuric acid solution. 
     For purposes of illustration and not limitation, the present invention is described herein in the context of a methanol-nitric acid fuel cell system having flow-through anode and flow-by cathode, similar to the fuel cell systems described in application Ser. No. 11/669,895, entitled “Liquid-liquid fuel cell systems having flow-through anodes and flow-by cathodes”, having a common assignee, hereby incorporated by reference. However, other fuel cell systems are possible and within the scope of the present invention, such as other methanol-nitric acid fuel cell system having different configurations, for example, flow-through anode and flow-through cathode fuel cell system of application Ser. No. 10/966,721, entitled “Nitric acid fuel regeneration fuel cell system”, having a common assignee, hereby incorporated by reference. In addition, other liquid fuel and/or liquid oxidant combinations, such as direct methanol liquid/air or methanol/water systems, are also possible and within the scope of the present invention. 
     In an embodiment, the dimensions of the electrode pair assemblies, central plenums  22 , and anolyte/catholyte flow channels (delivery and removal) are generally configured such that the fuel cell system  10  is considered to be a microfluidic device, generally referring to an article of manufacture that has one or more flow channels or plenums with at least one dimension less than about 2 millimeters (2 mm). 
     Thus, typical widths and heights associated with the microfluidic plenums and flow channels generally range from about 10 to about 10,000 μm, preferably from about 50 to about 5,000 μm, and even more preferably from about 100 to about 1,000 μm. In some preferred embodiments, the anode and cathode are confronting and spaced apart a distance of about 50 microns to about 3 millimeters, and more preferably from about 100 microns to about 1 millimeter. As used herein, the term “plenum” means a chamber or compartment such as the spaced apart region between the confronting electrodes disclosed herein, whereas the term “channel” means an enclosed elongated groove or furrow. 
     The anode  18  and cathode  20  generally have one or more discrete porous regions with a plurality of spaced apart flow-through anode pores  19  and flow-by cathode pores  23 . The porous regions can comprise a plurality of acicular or columnar pores (e.g., passageways) that extend through the substrate with average diameter ranging from about 0.2 to about 200 microns. The pores might be microporous (e.g., average pore size &lt;2 nm), mesoporous (e.g., average pore size of 2 nm to 50 nm), or macroporous (e.g., average pore size &gt;50 nm), with regularly spaced apart from one another a distance ranging from about 1 μm to about 20 μm, resulting in aspect ratios greater than 5:1. 
     The illustrated fuel cell is a methanol-nitric acid fuel cell that does not include a conventional interposing solid polymer proton exchange membrane, but employs a liquid anolyte flowstream  14  (having an acidic electrolyte component such as H 2 SO 4  or triflic acid) functioning as the interposing electrolyte. In this configuration, protons (H + ) liberated at the anode  18  are able to migrate through the interposing flowing liquid anolyte flowstream  14  across the central plenum  22  and the catalyzed separation layer  31  to reach and react with oxidant at the opposing cathode  20  to yield reaction products. In the context of a direct methanol-nitric acid fuel cell system, the electrochemical reactions occurring are believed to be essentially as follows:
 
Anode: CH 3 OH+H 2 O→6H + +6 e   − +CO 2   (1)
 
Cathode: 2HNO 3 +6H + +6 e   − →2NO+4H 2 O  (2)
 
Net: CH 3 OH+2HNO 3 →2NO+3H 2 O+CO 2   (3)
 
     The fuel for reacting at the electrodes yields reaction products that include a gaseous component, such as carbon dioxide in an anolyte effluent flowstream or nitric oxide (NO, or nitrogen monoxide). The formation of CO 2  gas inside the fuel cell can result in an increase in the pressure inside the fuel cell and thereby may cause substantial problems such as, e.g., impeding the reactant fluid flow and reducing the power generation of the fuel cell. Under some operating conditions, for example, enough CO 2  can be created at the porous and microchannel anode electrode to create a pressure build-up at the electrode so that the methanol flow can stop, causing the fuel cell to cease functioning. In general, the bubbles are dynamically generated at the electrode and swept to the output of the fuel cell, creating a dynamic pressure fluctuation at the electrodes. This dynamic pressure fluctuation, in turn, creates a dynamic reactant flow fluctuation, for example, the reactant flow to the electrode intermittently turns off due to the pressure build-up in the fuel cell, resulting in a power generation fluctuation at the output of the fuel cell. In addition, variations in porous and microchannel electrodes can result in minor differences in fluidic resistance between parallel cells, creating variations in power generation from fuel cell to fuel cell. 
     In an embodiment, the present flow management  40  manages the anode flowstream  14  to reduce and/or eliminate the dynamic pressure fluctuation in fuel cell systems and/or the variations in electrode manufacturing. For example, the flow management  40  can supply a steady flow to the anode, regardless of the dynamic pressure build-up due to the bubble formation at microchannel structures. 
     The anode flowstream  14  passing through the anode electrode  18  can exhibit a fluidic resistance R 1 , characterizing the fluid flow rate Qv through the anode area given a pressure difference ΔP (R 1 =ΔP/Qv). Under operations, the resistance R 1  is dynamic, comprising an intrinsic resistance and a variable resistance. The intrinsic resistance is primarily a function of geometry, such as the porous and microchannel structures. For example, the electrodes can be coated with a bubble repelling surface, or a hydrophilic microporous layer to assist in transporting gaseous products out of the fuel cell electrode areas, reducing the fluidic resistance. 
     The variable resistance is typically a function of the fuel cell operating behavior, such as the reaction of the anolyte at the anode surface. For example, the resistance can fluctuate, increasing significantly from the intrinsic resistance value when gaseous products are formed and blocking the fluid pathway and dropping back to the intrinsic level after the bubbles are swept away to the outlet. This dynamic resistance fluctuation causes fluctuation in reactant flow rate, resulting in fluctuation in power generation of the fuel cell. In worst case scenarios, the fuel can turn off intermittently when the reactant flow rate drops below a minimum level. 
     In an embodiment, the flow management  40  comprises a flow controller designed to provide a steady flow Qv to the fuel cell electrode, regardless of the dynamic fluctuation in fuel cell resistance. In addition, the flow management  40  can supply a constant flow Qv to achieve consistent power outputs regardless of variations in fuel cell manufacturing processes. For example, the flow controller can comprise a constant flow source for supplying a constant flow, a feedback mechanism to maintain a constant flow, e.g., increasing an input pressure when the flow reduces and reducing the input pressure when the flow returns to its nominal value. Alternatively, the flow controller can comprise a damping component, effectively reducing the dependence of the flow on the fuel cell pressure. 
     In an embodiment, the flow controller can comprise active components, such as a variable fluidic resistance component, a feedback mechanism, monitoring the flow and adjusting a series resistance or input pressure to ensure a constant flow. Alternatively, the flow controller can comprise passive components, such as a constant fluidic resistance to improve the constancy of the reactant flow, or to reduce the susceptibility of the reactant flow to the pressure fluctuation. 
     In an embodiment, the flow controller comprises a constant fluidic resistor with resistance sufficient to overcome the fluctuations in fuel cell chamber pressure. The constant resistance provides ease of fabrication and simplicity, which can offer high fuel cell reliability. For example, the constant resistance can be similar, higher or much higher than the resistance of the fuel cell electrode. With a constant resistance connected in series with the fuel cell, the reactant flow Qv through the electrode of the fuel cell is ΔP/(R 2 +R 1 ), with ΔP being the total pressure drop across the flow controller (ΔP 2 ) and across the fuel cell (ΔP 1 ), and R 1  and R 2  being the fluidic resistance of the fuel cell and the flow controller, respectively. Thus for a similar resistance configuration (e.g., R 2 ˜R 1 ), the flow controller can reduce the fluctuation in fuel cell flow by a factor of two. 
     In an embodiment, with the resistance of the flow controller much higher than the resistance of the fuel cell (R 2 &gt;&gt;R 1 ), the reactant flow can be estimated as ΔP/R 2 , which is independent of the fuel cell, thus providing a substantially constant flow to the fuel cell, regardless of changes in operating conditions, reaction mechanism or manufacturing variations. In an embodiment, the resistance of the flow controller is 2 to 5 times larger, 5 to 10 times larger, or even 10-100 times larger than the resistance of the fuel cell. In an embodiment, the resistance of the flow controller is between 10 to 30 times the fluidic resistance of the fuel cell. To maintain a desired reactant flow with the addition of the flow controller series resistors, the pressure drop across the system increases proportionally, with higher controller resistance requiring higher pressure drop. Thus, in an embodiment, the controller resistance is chosen to suit an optimum pressure drop of the fuel cell system. In an embodiment, the pressure drop for the fuel cell system is 5-15 psi, for fuel cell electrode configurations that exhibit 0-1 psi pressure drop. 
     In an embodiment, the reactant flow through the electrodes is between 0.05 to 100 ml/min, resulting in a pressure drop across the electrode of typically less than 0.4 psi with microchannel and porous electrodes. Thus the fluidic resistance is typically between roughly 4×10 −3  to 4 psi/mlpm. A typical anolyte flowstream generally has a flow rate ranging from about 0.3 ml/min to about 3 ml/min, thus for a 1 ml per minute flow rate, the fluidic resistance is about 0.4 psi/mlpm. In the present description, the fluidic resistance can also be characterized by the pressure difference ΔP, since these values are proportional to each other, with a factor of the fluid flow Qv. 
     The flow management  40  is shown to be fluidly connected in series at the inlet of the anode flowstream  14  in the fuel cell system  10 . However, the invention is not so limited, and the flow management can be connected at the outlet of the fuel cell. In addition, the flow management can be connected to the cathode flowstream, or connected to both anode and cathode flow streams.  FIGS. 2A-2D  illustrate various configurations for connecting a flow management  250  with a fuel cell  260 , including connecting a flow controller  210 A in series with the inlet of an electrode  230  (e.g., an anode electrode) to provide a steady flow stream  235  ( FIG. 2A ), connecting a flow controller  210 C in series with the outlet of an electrode  230  to provide a steady flow stream  235  ( FIG. 2B ), connecting a flow controller  210 B in series with the inlet of another electrode  220  (e.g., a cathode electrode) to provide a steady flow stream  225  ( FIG. 2C ), or connecting flow controllers  210 A/ 210 B in series with the inlets of both electrodes  220 / 230  respectively to provide both steady flow streams  225 / 235  ( FIG. 2D ). Other configuration variations are within the scope of the present invention, for example, connecting two flow controllers to the outlets of the anode/cathode, or connecting a flow controller to the outlet of the cathode  220 . Alternatively, the flow management can be connected in parallel with the fuel cell, creating a bypass flow for the reactant to compensate for the increase in fuel cell resistance. The outlets of the flow management and the fuel cell can be connected or separated, for example, returning the un-reacted reactant flow at the outlet of the flow controller to the fuel reservoir and directing the reacted reactant flow at the outlet of the fuel cell to the waste container. 
     For multiple fuel cell configurations, each electrode can have its own flow controller, multiple electrodes (having the same reactant) can share a flow controller, or some electrodes do not require any flow controller.  FIG. 3  illustrates a schematic configuration for a fuel cell system  300  according to an embodiment of the present invention, comprising multiple fuel cells  320 A- 320 D and a flow management system  330 . The flow management system  330  comprises multiple flow controllers  310 A- 310 D connected to the fuel cells  320 A- 320 D, with each flow controller controlling a fuel cell in series at the inlet. Alternatively, the flow controllers can be connected at the outlets of the fuel cells, or one flow controller can control two or more fuel cells. As shown, a flow controller, e.g., one of the pluralities of flow controllers  310 A- 310 D, is connected to one fuel cell, e.g., one of the pluralities of fuel cells  320 A- 320 D. This configuration can represent any of the configurations shown in  FIG. 2 , meaning the flow controller  310 A can control the anode flowstream, the cathode flowstream, or both anode and cathode flowstreams, at the inlets or outlets of the fuel cells. 
     In an embodiment, the fluidic resistor can be fabricated through various restriction configurations in the fluid transport paths. Examples of the fluidic resistors includes long winding pattern, capillary tube, or porous restrictor elements such as porous materials, wicking fiber, packed bed filler materials, porous dense filter papers. 
       FIGS. 4A-4B  illustrate exemplary fluidic resistors having a long winding pattern according to an embodiment of the present invention. A fuel cell  100  comprises electrodes  110  and  120 , each with reactant flow streams  114  and  124 , respectively. A flow controller  120 / 122  regulates reactant flow  124  to the fuel cell  100 . As shown, the flow controller  120 / 122  regulates the inlet reactant flow to electrode  120 , however, alternative configurations exist, such as the configuration described above. 
     The flow controller  120  and  122  comprises a fluidic resistor having a serpentine pattern  128  and a circular wrap-around pattern  126 . The fluidic resistance is a function of the length of the long winding resistor. The cross section of the fluid flow can be similar to the inlet of the fuel cell, or can be larger or smaller, to provide a desired fluidic resistance to the flow controller  120 . 
       FIGS. 5A-5B  illustrate exemplary fluidic resistors having a capillary tube according to an embodiment of the present invention. Capillary resistors may be inserted into a fluidic manifold with single inlet and outlet to deliver liquid to individual cells. Also, a bank of fluidic resistors can be integrated into a manifold with single fluid inputs and outputs. The capillary tubes can be coated with a surface tension reduction coating to reduce clogging failure due to bubbles affected by high surface tension within narrow tubes and shut off access to fluid flow. 
     The flow controller  130  and  132  comprises a fluidic resistor having a straight capillary tube  138  and a circular wrap-around capillary tube  136 , respectively. Pressure drop or fluidic resistance may be calculated via Bernoulli&#39;s equation where ΔP is a function of capillary diameter, fluid velocity, fluid length, and viscosity. Compatible capillary resistor materials may include, but are not limited to, PVDF, HDPE, Teflon, fluorine-silicone based polymers (e.g., Hubtron). 
       FIGS. 6A-6C  illustrate exemplary fluidic resistors having porous elements within the flow path according to an embodiment of the present invention. In an embodiment, the porous elements can be porous plugs disposed within a tube, a porous disk or plate on the path of the liquid flow or integrated immediately near the fuel cell, wicking fiber such as SiO 2  glass wool, packed bed filler materials, porous dense filter paper(s) or other phyllic or wicking material stuffed inside the flow path. In general, porous elements may be inserted into a body with single inlet and outlet to deliver reactant liquid to individual cells. The porous body can provide reduced surface tension for incoming fluid, by their hydrophilic nature, such that bubbles do not effect operation of fluidic resistors. In addition, the porous resistor can use phyllic or other wicking materials which can reduce or eliminate surface tension failure. This is opposed to capillary resistors where bubbles can be affected by high surface tension within narrow tubes. A porous resistor embodiment may also include a plate with micro machined pores that can be integrated with reactant flow of a fuel cell to provide fluidic resistance. Compatible resistor materials may include SiO 2 , Ta, Carbon, W, PVDF, HDPE, and Teflon. 
     The flow controller  160  comprises a fluidic resistor having a porous element  164 ,  166  or  168 . The porous element provides fluidic resistance to the incoming liquid flow  124  with the resistance being a function of the diameter, length, and pore characteristics of the porous element. The porous element can be in the form of porous cylinder  166 , installed in a tube, to which a number of porous cylinders can be added or removed to change the fluidic resistance of the flow controller  160 . The porous element can be in the form of porous disk or plate  168 . 
       FIGS. 7A-7B  illustrate exemplary fluidic resistors having multiple holes within a blocking element within the flow path according to an embodiment of the present invention. The flow controller  170  comprises a fluidic resistor  174  having a plurality of pass-through holes  175 , which size and density can determine the fluidic resistance of an incoming flow  171 . 
     The above description illustrates various embodiments of the present fluidic resistors for providing steady reactant flow in a flow controller attached to a fuel cell. Other fluidic resistor configurations are also within the scope of the present invention, which is to provide a fluidic resistance to the flow to the fuel cell. The dimensions of the fluidic resistors are designed to restrict a liquid flow, and/or to provide a steady flow to the fuel cell. The materials of the fluidic resistors are selected to be compatible with the fuel cell reactant fluids, such as resistance to nitric acid or sulfuric acid. The configurations of the fluidic resistors are designed to provide ease of fabrication and reproducibility. 
     In an embodiment, the present invention discloses an integrated fuel cell system for delivery of liquid fuels through use of microscaled encapsulated fluidic ports. The integrated fuel cell system comprises a flow controller integrated directly with an electrode of the fuel cell where the resistance of the flow controller can be managed through the use of integrated microchannels adjacent to the fuel cell. For example, each fuel cell comprises an anode/cathode pair and a flow controller having fluidic layers used to redirect either the catholyte, the anolyte, or both catholyte and anolyte. In an embodiment, the fuel cell system only comprises a flow management for the cathode reactant flow. In another embodiment, the fuel cell system only comprises flow management for the anode reactant flow. In yet another embodiment, the fuel cell system comprises flow management for both cathode and anode reactant flows, either by two separate fluidic layers (where each layer delivers one reactant flowstream) or by one fluidic layer (where the layer comprises two separate microchannel structures with each microchannel structure delivering one reactant flowstream). 
       FIG. 8  illustrates a portion of an exemplary integrated fuel cell system  400  with integrated flow management according to an embodiment of the present invention. The integrated fuel cell system  400  shows one electrode layer  410  (e.g., a cathode electrode  18 ) and a flow controller layer  420  integrated together. The other half of the fuel cell, e.g., the anode electrode, is not shown. As shown, the flow controller  420  comprises a serpentine pattern  430 , directing the entrance reactant flow  450  to a tortuous path  440 . Also shown is an optional outer cap layer  460  capping the serpentine pattern, and an inner cap layer  27  which is the cap layer for the fuel cell electrode chamber. The cap layers are optional, for example, the outer cap layer  460  can be accomplished by a cap layer of the adjacent fuel cell. In an embodiment, the dimensions and configurations of the flow controller  420  are designed to achieve a desired fluid resistance, matching the resistance of the fuel cell electrode  410  to achieve an improved flow through the electrode. For example, the flow controller is designed for managing fluid flow, including channel length optimization to provide acceptable pressure drops and flow rates with the fluidic resistance of the flow controller being a function of fluid channel path length, height, and width. In an embodiment, the resistance of the flow controller pattern is about 10 to 30 times the resistance of the electrode, characterized by the geometry and material of the fuel cell. 
     In an embodiment, the flow controller layer can have embossed flow channels, which create a preferential flow path upon which the fluid will travel before reaching the electrode portion. For example, the flow channel can be in a serpentine pattern, spiral pattern or a lattice layout. In addition, obstruction members can be incorporated, extending from the channel walls into the flow channels, partially blocking the channels to restrict the flow of reactant gas into the channels, increasing the resistance of the flow controller to a desired value. The channels can have rectangular shape or rounded shape. The channel walls can be angled relative to one another to provide a taper in the channels rather than a relatively uniform channel width. It should be understood that many other geometric patterns may be formed as flow channels in the flow controller layer, while remaining within the scope of the present invention. The invention may also be used where a stack or other assembly containing more than one fuel cell is connected. 
       FIGS. 9A and 9B  illustrate a top view and a cross section, respectively, of an exemplary serpentine pattern  520  for the flow controller layer  530  where the flow channels can compensate for the buildup of pressure in the fuel cell electrode. A serpentine pattern  520  is formed throughout the flow controller layer  530  to create a channel for the reactant flow, which enters  550  at one end and exits  510  at another end. The flow controller layer can be connected to an inlet of an electrode chamber to provide regulated flow  510  to the electrode chamber. Alternatively, the flow controller layer can be connected to an outlet of an electrode chamber to accept the flow  550  from the electrode chamber. 
     Cap layers  560 A and  560 B are shown for capping the serpentine pattern  520  and confining the reactant flow  540 . The cap layers are optional and the serpentine pattern  520  can use the cap layers of the electrode chambers instead of providing additional layers. 
       FIG. 9C  illustrates two exemplary flow controller layers according to embodiments of the present invention. The reactant runs a serpentine pattern within the flow controller layer, and enters and exits the layer through connection holes. 
     In an embodiment, the design of the individual layers for microfluidic channel integration includes consideration of reliability, manufacturability, materials compatibility, size, cell performance and cost with improved manufacturing processes such as tighter control of serpentine geometry, separate features to reduce leak paths, repetition, etc. Maintaining a minimal form factor is also a desirable consideration in designing fuel cells having integrated flow controllers. 
     In addition, in an embodiment, the fuel cell and the flow controller chambers are sealed hermetically in order to allow safe and comfortable use, transportation and storage of the fuel cell in any orientation. For example, components are manufactured by stamping core components and sealing them with automated epoxy dispense and screen printing techniques followed by pressing the components. In addition integrated microchannels may be manufactured with standard MEMS fabrication techniques such as surface and bulk micromachining techniques (i.e. masking, fusion bonding or other, deposition, and etching by dry or wet techniques). Other methods can be used, such as a compression molding process or machining process. The materials are selected to address highly corrosive (acidic) environments, such as Polyvinylidene Fluoride, (PVDF) and Pelseal 2112™ (a fluorinated epoxy). 
       FIG. 10  illustrates a portion of an exemplary integrated fuel cell system  600  comprising a plurality of flow management configurations integrated with a plurality of anode/cathode pairs, arranged in an alternating anode/cathode/anode/cathode/ . . . configuration. The illustrated portion shows a flow management microchannel layer  1050  sandwiched between a cathode frame  620 /cathode  610  and an anode frame  680 /anode  690 . In addition to the electrodes, the fuel cell system also comprises fluid chambers  630 / 670  and encapsulant layers  640 / 660  for the cathode  610  and anode  690 , respectively. The layers also comprise multiple ports  625 / 685  for anolyte and catholyte inlets and outlets. In this configuration, the microchannel layer  650  employs the encapsulant layers  640 / 660  of the cathode and anode reaction chambers for capping the microchannel layer  650 . 
     The microchannel layer  650  can deliver one or two reactant flows to one (either the anode or the cathode) or two electrodes (both anode and cathode). For example, in an embodiment, if the fuel cell system  600  only uses one regulated reactant flow for one electrode chamber (with the other electrode chamber using unregulated reactant flow), then the microchannel layer  650  can comprise one microchannel structure for delivering regulated reactant flow to the electrode requiring regulated flow. 
     In another embodiment, the microchannel layer  650  comprises two separate microchannel structures within one layer, one for transporting the anolyte reactant and one for transporting the catholyte reactant. The anolyte enters a first microchannel structure before reaching the anolyte fluid chamber  680 . Similarly, the catholyte enters a second microchannel structure before reaching the catholyte fluid chamber  630 . 
     In another embodiment, the fuel cell system  600  has two separate microchannel layers (only one layer shown), one for transporting the anolyte and one for transporting the catholyte. 
     In an embodiment, the integrated fuel cell system can have multiple anode/cathode pairs arranged in a different configurations, such as alternating . . . anode/cathode-anode/cathode . . . or alternating . . . cathode/cathode-anode/anode . . . , each with different flow controller configurations. 
       FIGS. 11A-11C  illustrate various configurations for flow controller having microchannel layer integrated with a fuel cell stack having 4 electrodes arranged in anode-cathode/anode-cathode configuration. 
       FIG. 11A  illustrates an exemplary integrated fuel cell stack  700  having a microchannel layer delivering steady catholyte reactant to the cathodes. The catholyte  714  is delivered to the microchannel layers, passing through the serpentine pattern before reaching each cathode  18 . In contrast, the anolyte  16  enters each anode chamber undisturbed. In this configuration, one microchannel layer having one serpentine pattern is used for each fuel cell (each anode-cathode pair). Alternatively, the same configuration can be used to deliver steady anolyte reactant flow to each anode  20 , and deliver un-disturbed catholyte reactant flow to each cathode  18 . 
       FIG. 11B  illustrates an exemplary integrated fuel cell stack  740  having a microchannel layer delivering steady catholyte reactant to the cathodes and steady anolyte reactant to the anodes. The catholyte  714  is delivered to the microchannel layers, passing through a first serpentine pattern before reaching each cathode  18 . Similarly, the anolyte  756  is delivered to the microchannel layers, passing through a second serpentine pattern before reaching each anode  20 . In this configuration, one microchannel layer having two embedded serpentine patterns is used for each fuel cell (each anode-cathode pair). 
       FIG. 11C  illustrates an exemplary integrated fuel cell stack  770  having two microchannel layers with one delivering steady catholyte reactant to the cathodes and the other delivering steady anolyte reactant to the anodes. The catholyte  714  is delivered to a first microchannel layer, passing through a serpentine pattern before reaching each cathode  18 . Similarly, the anolyte  716  is delivered to a second microchannel layer, passing through a serpentine pattern before reaching each anode  20 . In this configuration, one microchannel layer having one serpentine pattern is used for each electrode and two microchannel layers used for each fuel cell (each anode-cathode pair). 
       FIGS. 12A-12B  illustrate various configurations for flow controller having microchannel layer integrated with a fuel cell stack having 5 electrodes arranged in anode-cathode/cathode-anode/anode configuration. 
       FIG. 12A  illustrates an exemplary integrated fuel cell stack  800  having a microchannel layer delivering steady catholyte reactant to the cathodes. The catholyte  814  is delivered to the microchannel layers, passing through the serpentine pattern before reaching two adjacent cathodes  18 . In contrast, the anolyte  16  enters each anode chamber undisturbed. In this configuration, one microchannel layer having one serpentine pattern is used for two adjacent fuel cells (two anode-cathode pairs). Alternatively, the same configuration can be used to deliver steady anolyte reactant flow to two adjacent anodes  20 , and deliver un-disturbed catholyte reactant flow to each cathode  18 . 
       FIG. 12B  illustrates an exemplary integrated fuel cell stack  840  having microchannel layers delivering steady catholyte reactant to the cathodes and steady anolyte reactant to the anodes. The catholyte  814  is delivered to the microchannel layers, passing through the serpentine pattern before reaching two adjacent cathodes  18 . Similarly, the anolyte  816  is delivered to the microchannel layers, passing through the serpentine pattern before reaching two adjacent anodes  18 . In this configuration, two microchannel layers each having one serpentine pattern are used for two adjacent fuel cells (two anode-cathode pairs). 
     While the present invention has been described in the context of the embodiments illustrated and described herein, the invention may be embodied in other specific ways or in other specific forms without departing from its spirit or essential characteristics. Therefore, the described embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.