WO2008057081A1

WO2008057081A1 - Fuel cell systems and methods of operating the same

Info

Publication number: WO2008057081A1
Application number: PCT/US2006/043492
Authority: WO
Inventors: George A. Skinner; Alexander D. Mossman
Original assignee: Bdf Ip Holdings Ltd.; Ballard Power Systems Corporation
Priority date: 2006-11-07
Filing date: 2006-11-07
Publication date: 2008-05-15
Also published as: EP2095454A1; CA2668440A1; CN101548421A; KR20090087039A

Abstract

A fuel cell system comprises a fuel cell stack (10) comprising an anode flow field and a cathode flow field; a fuel supply line (12) for supplying a hydrogen-containing fuel to the anode flow field; a fuel inlet valve (18) in the fuel supply line upstream of the fuel cell stack; and an accumulator (32) comprising an upstream inlet (39) and a downstream outlet, the inlet fluidly connected to the fuel supply line and the outlet (36) fluidly connected to the fuel inlet valve.

Description

FUEL CELL SYSTEMS AND METHODS OF OPERATING THE SAME

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to electrochemical fuel cell systems and methods of operating the same.

Description of the Related Art

Electrochemical fuel cells convert fuel and oxidant into electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly which includes an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The membrane electrode assembly typically comprises a layer of catalyst, usually in the form of finely comminuted platinum that may be supported on a support material, such as carbon or graphite, or unsupported, at each membrane electrode interface to induce the desired electrochemical reaction. In operation, the electrodes are electrically coupled for conducting electrons between the electrodes through an external circuit. Typically, a number of membrane electrode assemblies are electrically coupled in series to form a fuel cell stack having a desired power output.

The membrane electrode assembly is typically interposed between two electrically conductive flow field plates, or separator plates, to form a fuel cell. Such flow field plates comprise flow fields to direct the flow of the fuel and oxidant reactant fluids to the anode and cathode electrodes of the membrane electrode assemblies, respectively, and to remove excess reactant fluids and reaction products, such as water formed during fuel cell operation. It is well-known in the art that with uncontrolled shutdown procedures, air permeates into the anode flow fields. Upon startup, hydrogen is supplied to the anode flow fields, thus forming a hydrogen/air front that moves across the anodes through the anode flow fields and displaces the air in front if it, which is pushed out of the cell. This phenomenon, coupled with air existing in the cathodes, results in elevated cathode potentials and rapid corrosion of the carbonaceous materials in the fuel cell stack, such as the gas diffusion layers and the catalyst support material.

One method of mitigating this problem is described in U.S. Patent No. 6,887,599, which discloses sufficient fast purging of the anode flow field with hydrogen prior to connecting the cells to the load. It is preferred to displace the air within the anode flow field with fuel in less than 1.0 seconds, and preferably less than 0.2 seconds. One method to enable a fast anode purge on startup is to open the fuel flow valve to allow a flow of pressurized hydrogen from the fuel source into the anode flow field. The hydrogen flow pushes the air out of the anode flow field. When substantially all the air has been displaced from the anode flow field, the auxiliary load switch is opened, the air flow valve is opened, and the air blower is turned on.

Most fuel cell systems contain a fuel pressure regulator or similar device to regulate the pressure of the fuel from the fuel supply, which is typically pressurized to very high pressures, so that the fuel is supplied to the fuel cell stack at the optimum pressure. However, such fuel pressure regulators typically restrict the volumetric flow of the fuel, which limits air displacement from the anode flow fields on startup quickly enough to prevent rapid corrosion of the carbonaceous materials.

As a result, there remains a need to develop improved fuel cell systems and methods of operating the same to prevent elevated potentials from occurring in the fuel cell stack on startup. The present invention addresses these issues and provides further related advantages.

BRIEF SUMMARY OF THE INVENTION

Briefly, the present invention relates to electrochemical fuel cell systems, in particular, to fuel cell systems comprising a fuel accumulator upstream of the fuel cell stack, and to methods of operating the same.

According to one embodiment of the present invention, the fuel cell system is disclosed including a fuel cell stack comprising an anode flow field and a cathode flow field; a fuel supply line for supplying a hydrogen-containing fuel to the anode flow field; a fuel inlet valve in the fuel supply line upstream of the fuel cell stack; and an accumulator comprising an upstream inlet and a downstream outlet, the inlet fluidly connected to the fuel supply line and the outlet fluidly connected to the fuel inlet valve.

According to another embodiment of the present invention, a method of commencing operation of a fuel cell system is disclosed, the fuel cell system including a fuel cell stack, the fuel cell stack comprising an anode flow field and a cathode flow field, wherein at least a portion of the anode flow field comprises air; a fuel supply line for supplying a fuel to the anode flow field; an accumulator comprising an upstream inlet fluidly connected to the fuel supply line, and a downstream outlet; and a fuel inlet valve in the fuel supply line downstream of the downstream outlet in a closed position to substantially isolate the fuel cell stack from the accumulator, wherein the fuel inlet valve is upstream of the fuel cell stack and downstream of the downstream outlet; the method comprising the steps of: supplying a fuel to the accumulator when the fuel inlet valve is closed; at least partially opening the fuel inlet valve to fluidly connect the fuel cell stack to the accumulator when the accumulator is at least partially filled with fuel; and supplying the fuel from the accumulator to the anode flow field.

These and other aspects of the invention will be evident upon review of the following disclosure and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS In the figures, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the figures are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve figure legibility. Further, the particular shapes of the elements, as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the figures.

Figure 1 shows a schematic of a fuel cell system configuration according to one embodiment of the present invention.

Figure 2 shows a schematic of an alternative fuel cell system configuration of the fuel cell system configuration in Figure 1. Figure 3 shows a schematic of another alternative fuel cell system configuration of the fuel cell system configuration in Figure 1.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures associated with fuel cells, fuel cell stacks, and fuel cell systems have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention. Unless the context requires otherwise, throughout the specification and claims which follow, the word "comprise" and variations thereof, such as "comprises" and "comprising" are to be construed in an open, inclusive sense, that is as "including, but not limited to".

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

Figure 1 shows an exemplary fuel cell system 8 comprising a fuel cell stack 10, which comprises a plurality of fuel cells. Each fuel cell typically comprises an anode flow field and a cathode flow field (not shown). Fuel cell system 8 further comprises a fuel supply line 12 fluidly connected to fuel supply 14 for delivering a fuel, such as a hydrogen-containing fuel, to fuel cell stack 10; a fuel pressure regulator 16 in fuel supply line 12 downstream of fuel supply 14; a fuel inlet valve 18, such as a solenoid valve, a pneumatically-driven valve, a pilot operated valve, or a motor driven valve, in fuel supply line 12 upstream of fuel cell stack 10; an oxidant supply 20 for supplying oxidant, such as air, to fuel cell stack 10; a delivery device 22, such as a compressor, blower, fan or the like, for delivering oxidant to fuel cell stack 10, and a controller 24 for controlling at least one operating parameter of the fuel cell stack or fuel cell system. In addition, fuel cell system 8 comprises a coolant loop 26 for allowing the flow of a coolant, such as water, glycol, or mixtures thereof, through fuel cell stack 10 to remove heat from the reactant and product fluids in the anode and cathode flow fields, thereby maintaining fuel cell stack 10 at an optimum temperature during fuel cell operation and preventing damage to the fuel cell components. Furthermore, fuel cell system 8 includes humidification devices 28 and 30 for humidifying the anode and cathode reactant streams, respectively.

Fuel cell system 8 further comprises a fuel purge valve 25 for periodically removing inerts and contaminants from the fuel cell stack. The fuel purge may be time-based (i.e., once every minute) and/or triggered by any detectable fuel cell stack operating parameter detected by sensors in the fuel cell stack or system (not shown), such as (but not limited to) the hydrogen concentration in the anode flow fields, the voltage of at least a portion of the fuel cell stack, and/or the resistance of at least a portion of the fuel cell stack. In some embodiments, the fuel purge valve is a pulse width modulated valve, such as that described in U.S. Provisional Application No. 60/864,722, filed November 7, 2006 and entitled "SYSTEM AND METHOD OF PURGING FUEL CELL STACKS".

Fuel pressure regulator 16 is used to regulate the pressure of the fuel from fuel supply 14 so that the fuel is supplied to fuel cell stack 10 at the optimum pressure. As mentioned earlier, fuel is typically pressurized to high pressures at the fuel supply, such as about 3000 PSIG (approximately 200 barg) when the fuel is supplied from a hydrogen tank. However, since the oxidant is typically supplied at much lower pressures, it is not desirable to supply fuel at such high pressures because it creates a high pressure differential across the fuel cell and may damage the fuel cell components. Thus, fuel pressure regulators are used to decrease the pressure of the fuel from the fuel supply to a more desirable pressure.

In addition, fuel cell system 8 comprises an accumulator 32 in fuel supply line 12 that is fluidly connected to fuel supply 14 through upstream inlet 34, and fluidly connected to fuel cell stack 10 through downstream outlet 36, to allow for a greater volumetric flow of fuel through the anode flow fields of fuel cell stack 10. For example, during shutdown, fuel inlet valve 18 is closed and accumulator 32 is at least partially filled with pressurized hydrogen supplied through regulator 16 from fuel supply 14. When fuel cell system 8 is started up, fuel inlet valve 18 is opened so that the large volume of fuel in accumulator 32 can be pushed through the anode flow fields of fuel cell stack 10 as quickly as possible by the pressurized fuel supplied through regulator 16 from fuel supply 14. Since accumulator 32 is downstream of regulator 16, accumulator 32 allows for a greater volumetric flow rate of fuel to be pushed through the anode flow fields of fuel cell stack 10 than existing fuel cell systems that do not use an accumulator upstream of the fuel cell stack. In some embodiments, accumulator 32 and regulator 16 are configured such that they allow the replacement of at least one volume of the total volume of the anode flow fields in fuel cell stack 10 in less than or equal to about 1.0 second, for example, in less than or equal to about 0.2 second. The volume of accumulator 32 may be any suitable volume and may depend on the operating conditions of the fuel cell stack. For example, the volume of accumulator 32 may be at least the same as the total volume of the anode flow fields in fuel cell stack 10 and, in some cases, may be at least double the total volume of the anode flow fields in fuel cell stack 10. In addition, accumulator 32 may be any shape, for example, in the shape of a cube or cylinder.

In some embodiments, fuel cell system 8 may further comprise a fuel recirculation loop 38 for recirculating at least a portion of the exhausted fuel from fuel cell stack outlet 40, such as that shown in Figure 2. The exhausted fuel, exhausted through fuel purge valve 25, typically contains a small amount of unused fuel, balance inerts and water vapour. Recirculating at least a portion of the exhausted fuel allows for humidification of the incoming fuel and, thus, may eliminate the use of humidification device 28 for the fuel stream. Recirculation loop 38 may comprise a recirculating device 42, such as a pump, blower, ejector, or the like, to help recirculate the fluids therein. Again, accumulator 32 in fuel supply line 12 is fluidly connected to fuel supply 14 through upstream inlet 34 and fluidly connected to fuel cell stack 10 through downstream outlet 36. In other embodiments, fuel cell system 8 is an air-cooled, low-pressure fuel cell system, such as that shown in Figure 3. Fuel cell stack 10 comprises a plurality of fuel cells that utilize combined oxidant-coolant flow fields to allow for relatively high stoichiometries of air flow (e.g., 100) at ambient pressure through the fuel cells, thereby eliminating the need for additional coolant flow fields in the fuel cells or compressors to compress the air oxidant to an elevated pressure. Since the air is supplied at ambient pressure, oxidant supply 20 may be the ambient environment. In addition, the fuel cells may utilize relatively dense gas diffusion layers so that no additional humidification device is necessary to humidify the fuel or air reactants. Examples of such fuel cells are described in U.S. Patent No. 6,451,470 and published U.S. Patent Appl. No. 2004/0253504. In this embodiment, fuel cell stack 10 can be operated in a dead-ended mode of operation to enhance fuel utilization and efficiency. For example, fuel purge valve 25 may be closed during operation, and opened periodically to purge any inerts and excess water and water vapour that build up in the anode flow fields. In this fuel cell system configuration, the accumulator allows a greater volumetric flow of fuel through the anode flow fields of the fuel cell stack on startup, thereby minimizing elevated potentials in the fuel cell stack.

One method of starting up a fuel cell stack and system comprising an accumulator upstream of the fuel cell stack is described herein. With reference to Figure 1, during shutdown, fuel inlet valve 18 is closed and fuel pressure regulator 16 is opened so that accumulator 32 is at least partially filled with pressurized hydrogen. On shutdown, at least a portion of the anode flow fields of fuel cell stack 12 will typically contain air that migrates from the cathode flow fields to the anode flow fields or through minor leaks in the fuel cell system. When controller 24 receives a signal that fuel cell operation is initiated, fuel inlet valve 18 is opened and hydrogen in accumulator 32 is pushed through the anode flow fields of fuel cell stack 10 to remove any air in the anode flow fields as quickly as possible, for example, in less than or equal to about 1.0 seconds, and more preferably, in less than or equal to about 0.2 seconds. In fuel cell systems that contain a fuel purge valve downstream of the stack, the fuel purge valve may be at least partially open or fully open to allow for a faster purge through the anode flow fields. Accumulator 32 may also help with transient conditions where a sudden increase in fuel flow is required, such as during a periodic fuel purge in a dead-ended mode of operation. During such a fuel purge, a high fuel flow rate is desirable to increase the efficiency of the purge by increasing the pressure drop across the anode flow fields, thereby removing inerts and water more quickly as reducing the amount of excess fuel purged. The required flow rate of fuel is typically higher than that required during regular operation and, in some cases, the desired flow rate may be about 10 times greater than the flow rate during regular operation. In this situation, the extra fuel in the accumulator increases the pressure drop in the anode flow fields for a longer period of time to minimize the purge duration. As a result, when the fuel purge valve is actuated, the accumulator can supply fuel while maintaining a higher pressure drop in the anode flow fields until the fuel supply catches up to the increased demand or until the stack has returned to normal operation (i.e., returned to a dead-ended mode of operation). The purge duration will depend on the size of the accumulator and the size of the purge valve.

In some embodiments, the accumulator may be placed upstream of the pressure regulator if the fuel pressure regulator is large enough to allow for a sufficient volumetric flow therethrough without significantly restricting fuel flow. In this case, the fuel pressure regulator may replace the fuel inlet valve, thereby eliminating a component and simplifying the fuel cell system configuration. In other embodiments, accumulator 26 may act as a dampener for the fuel supply to filter out pressure spikes and fluctuations that can potentially damage the stack.

The following examples are provided for the purpose of illustration, not limitation.

EXAMPLES Fuel Cell Stack Configuration:

Two 10-cell combined air-coolant fuel cell stacks were assembled with low porosity gas diffusion layers as described in published Canadian Patent No. 2,489,043. Fuel Cell System Configuration:

Two fuel cell systems were assembled with the fuel cell stacks described above. The first fuel cell system was configured as described in Figure 3. The second fuel cell system was similarly configured, with a 0.42-litre volume accumulator between the fuel regulator and the fuel inlet valve. In both fuel cell systems, a fan was used to deliver air as the oxidant to the fuel cell stack. The operating temperature of the fuel cell stack was maintained by adjusting the speed of the fan that delivered cooling air to the fuel cell stack.

Test Procedure: Each of the fuel cell systems were operated under the following on-off cycling procedure:

Startup: 1) Start flowing air at ambient pressure at 30 degrees Celsius through the cathode flow fields at 200 slpm for 10 seconds. 2) Apply fuel to the anode flow fields at 350 mbar and immediately open purge valve for 3 seconds.

Operation: 3) Operate the fuel cell stack in a dead-ended mode of operation for 30 minutes at 350 niA/cm² at 65 degrees Celsius while purging fuel periodically.

Shutdown: 4) Remove load from stack.

5) Turn off fuel supply and oxidant supply.

6) Keep stack shut down for 30 minutes before beginning next cycle (fuel purge valve and fuel inlet valve are closed throughout shut down period).

The above on-off cycling procedure was repeated until the average cell voltage decreased to less than about 580 mV at 350 mA/cm², or when the stack leakage exceeded 20 cc/min.

The first fuel cell stack accumulated 210 cycles before the cycling test was stopped due to the average cell voltage dropping to below 580 mV at 350 mA/cm², and had an average stack voltage degradation rate of about 345 μV/cycle. The integrated current collector showed that the hydrogen/air front purge duration was approximately 1.0 second (i.e., 1.0 second to push all the air out of the anode flow fields on startup). The second fuel cell stack accumulated 563 cycles before the cycling test was stopped due to the stack leakage rate exceeding 20 cc/min, and had an average stack voltage degradation rate of about 150 mV/cycle. The integrated current collector showed that the hydrogen/air front purge duration was approximately 0.2 second (i.e., 0.2 second to push all the air out of the anode flow fields). As a result, the second fuel cell stack (having an accumulator upstream of the fuel cell stack) demonstrated a significant improvement in both the number of on-off cycles and voltage degradation rate over the first fuel cell stack, accumulated more than double the number of cycles of the first fuel cell stack, and exhibited less than half the voltage degradation rate of the first fuel cell stack.

All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.

While particular elements, embodiments, and applications of the present invention have been shown and described, it will be understood that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings.

Claims

CLAIMS What is claimed is:

1. A fuel cell system, comprising: a fuel cell stack having an anode flow field and a cathode flow field; a fuel supply line for supplying a hydrogen-containing fuel to the anode flow field; a fuel inlet valve in the fuel supply line upstream of the fuel cell stack; and an accumulator comprising an upstream inlet and a downstream outlet, the upstream inlet fluidly connected to the fuel supply line and the downstream outlet fluidly connected to the fuel inlet valve.

2. The fuel cell system of claim 1, further comprising a fuel pressure regulator in the fuel supply line.

3. The fuel cell system of claim 2 wherein the fuel pressure regulator is upstream of the accumulator.

4. The fuel cell system of claim 2 wherein the fuel pressure regulator is downstream of the accumulator.

5. The fuel cell system of claim 1 wherein the fuel inlet valve is a solenoid valve.

6. The fuel cell system of claim 1 wherein the anode flow field has a total volume and the volume of the accumulator is at least equal to the volume of the total anode flow field volume.

7. The fuel cell system of claim 6 wherein the volume of the accumulator is at least twice the volume of the total anode flow field volume.

8. A method of commencing operation of a fuel cell system, the fuel ceil system comprising a fuel ceil stack, the fuel cell stack comprising an anode flow field and a cathode flow field, wherein at least a portion of the anode flow field comprises air; a fuel supply line for supplying a fuel to the anode flow field; an accumulator comprising an upstream inlet fluidly connected to the fuel supply line, and a downstream outlet; and a fuel inlet valve in the fuel supply line downstream of the downstream outlet in a closed position to substantially isolate the fuel cell stack from the accumulator, wherein the fuel inlet valve is upstream of the fuel cell stack and downstream of the downstream outlet; the method comprising the steps of: supplying a fuel to the accumulator when the fuel inlet valve is closed; at least partially opening the fuel inlet valve to fluidly connect the fuel cell stack to the accumulator when the accumulator is at least partially filled with fuel; and supplying the fuel from the accumulator to the anode flow field,

9. The method of claim 8 wherein the air in the anode flow field is purged in less than about 1.0 second.

10. The method of claim 8 wherein the air in the anode flow field is purged in less than about 0.2 second.

1 1. The method of claim 8, further comprising the steps of: supplying an oxidant to the cathode flow field of the fuel cell stack; and drawing a load from the fuel cell stack when air in the anode flow field is substantially purged therefrom.