Composite end cell thermal barrier with an electrically conducting layer

A barrier layer for a fuel cell assembly is disclosed, the barrier layer having a thermally insulating layer having a first surface and a second surface, and an electrically conducting layer formed on the first surface of the thermally insulating layer. The thermally insulating layer may include a plurality of apertures formed therethrough, and the electrically conducting layer may be formed on a second surface of the thermally insulating layer and on the walls of the thermally insulating layer forming the apertures.

FIELD OF THE INVENTION

The invention relates to a fuel cell assembly, and more specifically to a fuel cell assembly having a thermally insulating, electrically conducting layer disposed between a terminal plate and a unipolar end plate thereof to mitigate thermal losses from the unipolar end plate and fluid condensation and ice formation in an end fuel cell while maximizing an electric current flowing through the layer.

BACKGROUND OF THE INVENTION

Fuel cell assemblies convert a fuel and an oxidant to electricity. One type of fuel cell power system employs a proton exchange membrane (hereinafter “PEM”) to separate electrodes that facilitate catalytic reaction of fuels (such as hydrogen) and oxidants (such as air or oxygen) to generate electricity. The PEM is typically a solid polymer electrolyte membrane that facilitates transfer of protons from an anode to a cathode in each individual fuel cell normally deployed in a fuel cell power system.

In a typical fuel cell assembly (or stack) within a fuel cell power system, individual fuel cell plates include channels through which various reactants and cooling fluids flow. Fuel cell plates are typically designed with straight or serpentine flow channels. Such flow channels are desirable as they effectively distribute reactants over an active area of an operating fuel cell, thereby maximizing performance and stability. In below-freezing temperatures, water vapor in the fuel cell assembly may condense. Further, the condensate may form ice in the fuel cell assembly. The presence of condensate and ice may affect the performance of the fuel cell assembly.

During typical operating conditions, condensate may also accumulate at the edges of the fuel cell plates adjacent outlet manifolds of the fuel cell assembly, thereby restricting fluid flow from the flow channels to the outlet manifolds. During a starting operation of the fuel cell assembly in below-freezing temperatures, the condensed water in the flow channels of the fuel cell plates and at the edges of the outlet manifolds is in the form of ice which may restrict reactant flow. Similarly, reactant flow maldistribution due to liquid water stagnation during normal operation can result.

Typically, to mitigate the formation of condensation at the outlet manifolds of the fuel cell assembly, the operating temperature of the fuel cell assembly is increased. However, increasing the operating temperature may have a negative impact on ohmic resistance due to increased membrane proton resistance as a result of decreased membrane humidification. Also, decreasing the relative humidity of inlet anode and cathode gas streams may achieve the same effect as increasing the operating temperature of the fuel cell assembly resulting in a negative impact on ohmic resistance due to increased membrane proton resistance. To mitigate thermal losses to the end units of the assembly, a thermally resistive barrier layer may be disposed between the fuel cell stack and the end units. As thermal resistivity increases, electrical conductance typically decreases, thereby generating waste heat in the barrier layer and causing the fuel cell assembly to operate inefficiently. To withstand the elevated temperatures that can arise at high current levels due to this waste heat generation in the barrier layer, the end units must be formed from expensive plastics or other materials able to withstand elevated temperatures, thereby increasing the cost of the fuel cell assembly.

During operation of the fuel cell assembly, waste heat from the fuel cell reaction heats the fuel cell assembly and mitigates water condensation and ice formation in the assembly. However, end plates of the fuel cell assembly tend to have a temperature lower than the temperature of intermediate plates of the fuel cell assembly. The unipolar end plates have a lower temperature due to thermal losses to the environment and thermal losses to terminal plates of the fuel cell assembly adjacent thereto. A difference in the temperature of the fuel cell plates throughout the fuel cell assembly may result in inefficient operation, maldistribution of reactants, condensation of water which may lead to ice formation, and a decreased useful life of the fuel cell assembly.

Typically, to ensure a substantially uniform temperature distribution between the plates in the fuel cell assembly, a heating mechanism is disposed adjacent the unipolar end plates to directly transfer thermal energy thereto. A heating mechanism may also be disposed adjacent the terminal plates to transfer thermal energy thereto. Thermal energy is then transferred from the terminal plates to the unipolar end plates. Alternatively, a resistive heating mechanism adapted to heat the unipolar end plates may be connected in parallel to the fuel cell assembly. If a heating mechanism fails and is in a powered state, the end fuel cells may dry out, thereby leading to an electrical short in the fuel cell assembly. Other methods for heating the unipolar end plates include catalytic heating, and providing a bypass plate disposed between the unipolar end plates and the terminal plates.

Also, during operation of the fuel cell assembly, electrical current generated by the fuel cell stack is collected in each electrically conductive fuel cell. The current is transmitted through the stacks, via the fuel cell plates, to terminal plates at either end of the fuel cell stack. The terminal plates are in electrical communication with a current collecting body, such as a bus bar, for example. The current collecting body is in electrical communication with a stack interface unit (SIU) or other electrical components of the fuel cell power system. High temperatures in the stack end units will cause heat to flow with the electrical current to the SIU and/or other electrical components, thereby resulting in increased temperatures in the SIU and/or other electrical components which may result in component failure or requiring costly components that can operate at elevated temperatures.

It would be desirable to produce a fuel cell assembly having a thermally insulating, electrically conducting layer disposed between a terminal plate and a unipolar end plate thereof to mitigate thermal losses from the unipolar end plate and fluid condensation and ice formation on the unipolar end plate while maximizing an electric current flowing through the layer.

SUMMARY OF THE INVENTION

Concordant and congruous with the present invention, a fuel cell assembly having a thermally insulating, electrically conducting layer disposed between a terminal plate and a unipolar end plate thereof to mitigate thermal losses from the unipolar end plate, and fluid condensation and ice formation on the unipolar end plate while maximizing an electric current flowing through the layer, has surprisingly been discovered.

In one embodiment, a barrier layer for a fuel cell assembly comprises a thermally insulating layer having a first surface and a second surface; and an electrically conducting layer formed on the first surface of said thermally insulating layer.

In another embodiment, a barrier layer for a fuel cell assembly comprises a thermally insulating layer having a first surface, a second surface, and a plurality of apertures formed therethrough; and an electrically conducting layer formed on the first surface of said thermally insulating layer, the second surface of said thermally insulating layer, and the portions of said thermally insulating layer forming the plurality of apertures.

In another embodiment, a fuel cell assembly comprises a plurality of fuel cells arranged in a stack; a first terminal plate disposed at a first end of the stack of said fuel cells; a second terminal plate disposed at a second end of the stack of said fuel cells; and a barrier layer having an electrically conducting layer formed on a first surface of a thermally insulating layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1depicts a fuel cell assembly10having a two fuel cell stack. The fuel cell assembly10is a proton exchange membrane (REM) fuel cell assembly. Each of the two fuel cells includes a unitized electrode assembly (UEA)12. The UEAs12are separated from each other by an electrically conductive bipolar plate14. The UEAs12have anode and cathode diffusion media (DM)34, an anode62, a cathode64, and an electrolyte membrane60. For simplicity, a fuel cell assembly10with a two-cell fuel cell stack (i.e. one bipolar plate) is illustrated and described inFIG. 1, it being understood that a typical fuel cell assembly has many more such fuel cells and bipolar plates.

The UEAs12and bipolar plate14are stacked together between a pair of terminal plates16,18and a pair of unipolar end plates20,22. The unipolar end plate20, both working faces of the bipolar plate14, and the unipolar end plate22include respective active areas24,26,28,30. The active areas24,26,28,30typically contain flow fields for distributing gaseous reactants such as hydrogen gas and air over the anode62and the cathode64, respectively, of the UEAs12.

The bipolar plate14is typically formed by a conventional process for shaping sheet metal such as stamping, machining, molding, or photo etching through a photolithographic mask, for example. In one embodiment, the bipolar plate14is formed from unipolar plates which are then joined by any conventional process such as welding or adhesion. It should be further understood that the bipolar plate14may also be formed from a composite material. In one particular embodiment, the bipolar plate14is formed from a graphite or graphite-filled polymer. Gas-permeable diffusion media34are disposed adjacent both sides of the bipolar plate14. The unipolar end plates20,22are also disposed adjacent the diffusion media34. In the embodiment shown inFIGS. 1 and 2, a barrier layer32is disposed between the unipolar end plate22and the terminal plate18.

As best shown inFIG. 2, the barrier layer32is formed from an insulating layer66having a first surface68and a second surface70. The barrier layer32has plurality of apertures72formed therethrough. The apertures72are formed in linear rows, but the apertures72may be randomly formed in the barrier layer32or the apertures72may be formed in another pattern, as desired. An electrically conducting layer74is formed on each of the first surface68, the second surface70, and the portions of the barrier layer32forming the apertures72. The electrically conducting layer74may be formed on any one or more of the first surface68, the second surface70, and the walls of the barrier layer32forming the apertures72, as desired. In the embodiment shown, the insulating layer66is a carbon foam, but the insulating layer66may be formed from any thermally insulating material such as a plastic, a syntactic foam, and a ceramic. The insulating layer66may also be micro-truss structure formed from a plastic, as described in further detail hereinbelow and shown inFIGS. 5 and 6. The electrically conducting layer74is typically formed from a nickel-based metal. More favorable results have been obtained with a barrier layer32having a nickel-based electrically conducting layer74having a thickness from about 0.5 microns to about 2 microns. More favorable results have been obtained from a nickel-based electrically conducting layer74having a thickness of about 0.5 microns. The electrically conducting layer74may also be formed from a chromium-based metal, such as a chrome, having a thickness from about 10 microns to about 17 microns. It is understood that the electrically conducting layer74may be formed from any metal or other conducting material and may have any thickness as desired and as required based on the size and operating conditions of the fuel cell assembly10. The electrically conducting layer74may be applied to the insulating layer66using an electroless plating process, an electroplating process, a brushing process, a spraying process, a dipping process, and the like. It is understood that a second barrier layer (not shown) may be disposed between the unipolar end plate20and the terminal plate16, as desired.

The bipolar plate14, unipolar end plates20,22, and the UEAs12each include a cathode supply aperture36and a cathode exhaust aperture38, a coolant supply aperture40and a coolant exhaust aperture42, and an anode supply aperture44and an anode exhaust aperture46. Supply manifolds and exhaust manifolds of the fuel cell assembly10are formed by an alignment of the respective apertures36,38,40,42,44,46in the bipolar plate14, unipolar end plates20,22, and the UEAs12. The hydrogen gas is supplied to an anode supply manifold via an anode inlet conduit48. The air is supplied to a cathode supply manifold of the fuel cell assembly10via a cathode inlet conduit50. An anode outlet conduit52and a cathode outlet conduit54are also provided for an anode exhaust manifold and a cathode exhaust manifold, respectively. A coolant inlet conduit56is provided for supplying liquid coolant to a coolant supply manifold. A coolant outlet conduit58is provided for removing coolant from a coolant exhaust manifold. It should be understood that the configurations of the various inlet conduits48,50,56and outlet conduits52,54,58inFIG. 1are for the purpose of illustration, and other configurations may be chosen as desired.

UEAs12for use in the fuel cell assembly10may include a plurality of components. As shown inFIG. 2, the UEA12includes the electrolyte membrane60, the anode62, the cathode64, and the diffusion media34. The components of the UEA12are assembled during production of the UEA12and affixed to one another by any conventional process such as hot pressing, for example. An adhesive may be used between individual components, as desired. For clarity, the diffusion media34and electrolyte membrane60inFIG. 1have been linearly displaced to more clearly show the electrolyte membrane60.

The anode62and the cathode64of the fuel cell assembly10may be disposed on the electrolyte membrane60and/or the diffusion media34. The electrode may be formed by a catalyst ink applied to the components by any conventional process such as spraying, dipping, brushing, roller transfer, slot die coating, gravure coating, Meyer rod coating, decal transfer, and printing, for example. Either the anode62or the cathode64may be referred to as an electrode.

The electrolyte membrane60may be a membrane layer formed from an ionomer. The ionomer perfluorosulfonic acid (PFSA) such as sold under the trademark Nafion® NRE211, is a typical ionomer well known in the art for use as the electrolyte membrane60of a fuel cell. The electrolyte membrane60is disposed between the anode62and the cathode64.

Generally, during operation of a fuel cell power system, a stream of hydrogen is fed into the anode side of the fuel cell assembly10. Concurrently, a stream of oxygen is fed into the cathode side of the fuel cell assembly10. On the anode side, the hydrogen in the hydrogen stream is catalytically split into protons and electrons. The oxidation half-cell reaction is represented by: H2⇄2H++2e−. In a PEM fuel cell, the protons permeate through the membrane to the cathode side. The electrons travel along an external load circuit to the cathode side creating the current of electricity of the fuel cell assembly10. On the cathode side, the oxygen in the oxidant stream combines with the protons permeating through the membrane and the electrons from the external circuit to form water molecules. This reduction half-cell reaction is represented by: 4H++4e−+O2⇄2H2O. Anode exhaust from the anode side is typically recirculated through the system to maintain high anode conversion to electricity and low hydrogen emissions.

Cathode exhaust from the cathode side is exhausted to atmosphere. A control module (not shown) regulates the conditions of the hydrogen stream, oxygen stream, and exhaust streams by operating various control valves (not shown), and compressors (not shown) in response to signals from pressure sensors (not shown) and electrical power sensors (not shown) connected to the fuel cell assembly10. One exemplary exhaust system is disclosed in commonly-owned U.S. Pat. No. 7,235,318 for FUEL CELL SYSTEM BACK-PRESSURE CONTROL WITH A DISCRETE VALVE, hereby incorporated herein by reference in its entirety.

When the fuel cell assembly10is in operation, the barrier layer32mitigates a loss of thermal energy from the unipolar end plate22to the environment, and from the unipolar end plate22to the terminal plate18and to a lower end unit (not shown) of the fuel cell assembly10. Minimizing the temperature of the lower end unit mitigates the need for separate cooling systems and other thermal energy management systems in other components of a fuel cell power system incorporating the fuel cell assembly10, such as a stack interface unit (SIU). By mitigating the need for separate cooling systems for other components of the fuel cell power system, the cost and complexity of the fuel cell power system is minimized. Furthermore, by minimizing the thermal energy transferred to the barrier layer32and subsequently transferred to the lower end unit, the required thermal energy tolerance of the lower end unit is minimized. By minimizing the required thermal energy tolerance of the lower end unit, specialty plastics are not required for the manufacturing thereof and the cost to manufacture the lower end unit is minimized.

Because the thermal energy of the unipolar end plate22is conserved by the insulating layer66of the barrier layer32, a temperature of the unipolar end plate22is maximized during all operational modes, especially during a start-up operation of the fuel cell assembly10in below-freezing temperatures. By maximizing the temperature of the unipolar end plate22during typical operation, liquid water formed from condensed water vapor in the channels of the unipolar end plate22is minimized. Similarly, because condensation is minimized, the formation of ice in the channels of the unipolar end plate22in below-freezing temperatures is also minimized, thereby facilitating efficient below-freezing start-up of the fuel cell assembly10. Additionally, by maximizing the temperature of the unipolar end plate22during start-up of the fuel cell system in below-freezing temperatures, formation of liquid water or ice within the anode62and the cathode64is minimized, thereby facilitating efficient cold start-up of the fuel cell assembly10. An undesired increase in the thermal energy generation due to electrical resistance of the barrier layer32may be compensated for by an amount of the coolant flowing through the fuel cell assembly10. By conserving the thermal energy of the unipolar end plate22, a heating mechanism is not required to heat the unipolar end plate22, thereby minimizing the complexity and cost of the fuel cell assembly10.

The electrically conducting layer74provides an electrically conducting material that maximizes the flow of electrical current from the plates14,20,22of the fuel cell assembly10to the terminal plate18. Because the portion of the electrically conducting layer74formed on the first surface68is connected to the portion of the electrically conducting layer74formed on the second surface70by the portion of the electrically conducting layer74formed on the portions of the insulating layer66forming the apertures72, the electrically conducting layer74forms a continuous path for the flow of electrical current across the thickness of the barrier layer32. By having a continuous connection of the portions of the electrically conducting layer74, the electrical conductivity of the barrier layer32and the flow of electrical current therethrough is maximized. By maximizing the flow of electrical current from the fuel cell assembly10through the barrier layer32to the terminal plate18, the power output of the fuel cell power system is also maximized. This is of particular importance during peak usage of the fuel cell power system incorporating the fuel cell assembly10.

FIGS. 3 and 4illustrate a barrier layer76according to another embodiment of the invention. The barrier layer76is similar to the barrier layer32except as described hereinbelow. Like the barrier layer32, the barrier layer76may be used in the fuel cell assembly10ofFIG. 1disposed between terminal plate18and the unipolar end plate22. It is understood that a second barrier layer (not shown) similar to the barrier layer32or the barrier layer76may be disposed between the unipolar end plate20and the terminal plate16of the fuel cell assembly10, as desired.

The barrier layer76is formed from an insulating layer78having a first surface80, a second surface82, and a plurality of protuberances86formed on each surface80,82. As best shown inFIG. 4, a protuberance86on the first surface80cooperates with a corresponding protuberance86on the second surfaces82to form a column having a substantially circular cross-sectional shape. The protuberances86may have an ovular, triangular, or rectangular cross-sectional shape, as desired. Each of the columns formed by the plurality of protuberances86has an aperture84formed therethrough. The protuberances86are formed in a linear row, but the protuberances86may be formed in the barrier layer76in a random pattern or in a desired design. Because the barrier layer76includes the columnar protuberances86as opposed to a solid layer, material used to form the barrier76between the protuberances86is minimized, thereby minimizing a thermal mass and a thermal contact area of the barrier layer76.

A electrically conducting layer88is formed on each of the first surface80, the second surface82, and the portions of the insulating layer78forming the apertures84. The electrically conducting layer88may be formed on any one or more of the first surface80, the second surface82, and the portions of the barrier layer76forming the apertures84, as desired. The insulating layer78is typically formed from a thermally insulating plastic material. The electrically conducting layer88is typically formed from a nickel-based metal, such as an electroless nickel, for example. More favorable results have been obtained with a barrier layer76having an electrically conducting nickel layer88having a thickness from about 10 microns to about 50 microns. More favorable results have been obtained by a electrically conducting layer nickel88having a 25 micron thickness. It is desirable to form the electrically conducting layer88from a material which will provide a low contact resistance between the barrier layer76and the terminal plate18, as well as the unipolar plate22. It is understood that the electrically conducting layer88may be formed from any metal or other conducting material and may have any thickness as desired and as required based on the size and operating conditions of the fuel cell assembly10. The electrically conducting layer88may be applied to the insulating layer78using an electroless plating process, an electroplating process, a brushing process, a spraying process, a dipping process, and the like, for example.

FIGS. 5 and 6illustrate a barrier layer90according to another embodiment of the invention. The barrier layer90is similar to the barrier layers32except as described hereinbelow. Like the barrier layer32, the barrier layer90may be used in the fuel cell assembly10ofFIG. 1disposed between terminal plate18and the unipolar end plate22. It is understood that a second barrier layer (not shown) similar to the barrier layer32, the barrier layer76, or the barrier layer90may be disposed between the unipolar end plate20and the terminal plate16of the fuel cell assembly10, as desired.

The barrier layer90is formed from a micro-truss structure92having a first surface94and a second surface96. A plurality of columnar members98cooperates to form the micro-truss structure92. Void space equivalent in function to the apertures72,84is formed between the members98of the micro-truss structure92. The members98are generally columnar and have a substantially circular cross-sectional shape, but the members98may have an ovular, triangular, rectangular, or other cross-sectional shape, as desired. A electrically conducting layer102is formed on an outer surface100of each of the members98. The surfaces94,96of the barrier layer90are adapted to contact the unipolar end plate22and the terminal plate18, respectively. As shown inFIG. 5, the barrier layer90includes four stacked layers of micro-trusses. It is understood that the barrier layer90may include any number of layers of micro-trusses, as desired.

The micro-truss structure92is a thermally insulating layer formed from a thermally insulating plastic material. The plastic material may be a light/radiation curing polymer such as those disclosed in commonly owned U.S. patent application Ser. No. 12/339,308 hereby incorporated herein by reference in its entirety. It is understood that the micro-truss structure92may be formed from any thermally insulating material, as desired. Use of a thermally insulating micro-truss structure92provides the barrier layer90with increased strength and rigidity capable of withstanding compressive forces placed on the fuel cell assembly10during assembly. Furthermore, an amount of material used to build the micro-truss structure92may be minimized compared to sheet-like structures. By minimizing the material to form the micro-truss structure92, a thermal mass and a thermal contact area of the barrier layer90is also minimized.

The electrically conducting layer102is formed from nickel-based metal. More favorable results have been obtained with a barrier layer90having a nickel-based electrically conducting layer102having a thickness from about 2 microns to about 20 microns. More favorable results have been obtained from a nickel-based electrically conducting layer102having a thickness of about 5 microns. The electrically conducting layer102may also be formed from a nickel-chromium alloy or a nickel-cobalt alloy, having a thickness from about 10 microns to about 20 microns. It is understood that the electrically conducting layer102may be formed from any metal or other conducting material and may have any thickness as desired and as required based on the size and operating conditions of the fuel cell assembly10. The electrically conducting layer102may be applied to the micro-truss structure92using an electroless plating process, an electroplating process, a brushing process, a spraying process, a dipping process, and the like.