WO2009073452A1

WO2009073452A1 - Recovering performance loss in fuel cells

Info

Publication number: WO2009073452A1
Application number: PCT/US2008/084579
Authority: WO
Inventors: Christopher J. Richards
Original assignee: Bdf Ip Holdings Ltd.; Ballard Material Products Inc.
Priority date: 2007-11-30
Filing date: 2008-11-24
Publication date: 2009-06-11

Abstract

A loss in performance can occur in fuel cell stacks following operation below zero degrees Celsius. The performance loss can be recovered by applying a voltage across the stack with a suitable power supply such that the cathode potential is 1.4V or greater on average than the anode potential for each cell in the stack. These voltages are high enough to electrolyze water at the cathode.

Description

RECOVERING PERFORMANCE LOSS IN FUEL CELLS BACKGROUND OF THE INVENTION

Technical Field

The present invention relates to methods for recovering the performance loss that can occur in a fuel cell stack after the stack has been operated below zero degrees Celsius.

Description of the Related Art

Fuel cells are devices in which fuel and oxidant fluids electrochemically react to generate electricity. A type of fuel cell being developed for various commercial applications, including automotive applications, is the solid polymer electrolyte (SPE) fuel cell which employs a membrane electrode assembly comprising a solid polymer electrolyte or ion exchange membrane disposed between two electrodes. Each electrode comprises an appropriate catalyst, preferably located next to the solid polymer electrolyte. The catalyst may, for example, be a metal black, an alloy, or a supported metal catalyst such as platinum on carbon. The catalyst is typically disposed in a catalyst layer which contains ionomer similar to that used for the solid polymer electrolyte. The electrode may also contain an adjacent fluid diffusion layer (typically a porous, electrically conductive sheet material) that may be employed for purposes of mechanical support and/or reactant distribution. Flow field plates are also typically employed adjacent the fluid diffusion layers in order to distribute reactants to, and to remove reaction byproducts from, the electrodes. For most applications, the output voltage from a single fuel cell is relatively low, and so a plurality of fuel cells are usually stacked together in a series stack in order to deliver higher voltages.

During normal operation of a SPE fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The electrons travel through an external circuit providing useable power and then electrochemically react with protons and oxidant at the cathode catalyst to generate water reaction product. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to react with the oxidant and electrons at the cathode catalyst.

Because the ionic conductivity in typical SPE fuel cell electrolytes increases with hydration level, the fuel cell stacks are usually operated in such a way that the membrane electrolyte is as fully saturated with water as is possible without "flooding" the cells with liquid water ("flooding" refers to a situation where liquid water accumulates and hinders the flow and/or access of gases in the fuel cell).

Operation and storage at below freezing temperatures poses particularly challenging problems for SPE fuel cells. For instance, a significant amount of liquid water may condense in the stack and freeze. The presence of internal ice can result in permanent damage to the stack. Even if such damage is avoided, the presence of ice can still hinder subsequent startup. Further, it has been found that repeated shutdowns at below freezing temperatures can adversely affect fuel cell performance. Stack performance can, however, be recovered with the use of an appropriate drying method, such as the improved method disclosed in US2006/0183005 which reduces damage to the fuel cell and thus extends lifetime. Still, faster and/or simpler alternatives to achieve recovery would be desirable.

For various reasons, power supplies have been employed in the past to impose certain potentials on either the anodes or cathodes in fuel cell stacks. For example, US6399231 discloses that performance recovery can be obtained by briefly reversing fuel cell polarity. Therein, hydrogen is supplied to the anode and hydrogen or inert gas to the cathode while the cathode potential is decreased to less than 0.6V. In US5601936, recovery from anode poisoning is addressed by reversing fuel cell polarity with the usual reactant gases being supplied to the electrodes. In WO01/22515, a cold starting method for a fuel cell stack involves supplying hydrogen to the cathode and electrochemically pumping the hydrogen to the anode using an external power supply (with the cathode potential decreasing as a result). In none of these cases, however, is the power supply raising the cathode potential with respect to the anode.

Regenerative fuel cells are also known in the art. These fuel cells operate in two modes, generating electricity like regular fuel cells in fuel cell mode, and regenerating the hydrogen and oxygen reactants consumed in regeneration mode. This is accomplished by supplying power to the fuel cell and electrolysing water that was produced during fuel cell mode operation. The regeneration process takes place over a significant period of time and is performed to recreate reactants for subsequent operation in fuel cell mode.

BRIEF SUMMARY OF THE INVENTION

It has been discovered that the performance lost in a fuel cell stack following operation below zero degrees Celsius can be recovered by subjecting the stack briefly to high potentials (i.e., potentials capable of electrolysing water). A fuel cell stack comprises at least one fuel cell, but typically comprises a plurality of fuel cells in a series stack. Each fuel cell in the stack comprises an electrolyte, a cathode, and an anode. The method of the invention comprises connecting a power supply across the fuel cell stack, and applying a voltage across the fuel cell stack with the power supply such that the cathode potential is 1.4V or greater, and preferably 1.8V or greater, on average than the anode potential for each cell in the stack.

The method is for use with fuel cells that are not regenerative fuel cells (electrolysis is routinely performed in such cells), and is particularly suitable for use with solid polymer electrolyte fuel cells.

In the method, the voltage can be applied by the power supply with hydrogen present at each anode. The anode potentials are thus kept close to that of dynamic hydrogen electrodes. The cathodes, however, briefly experiences much greater than normal potentials. The voltage is applied briefly such that the performance recovery benefit is achieved without significant adverse effect to the fuel cells. The voltage application period may be quite brief, for instance less than one minute and even less than five seconds. Further, it can be advantageous to apply the voltage with the stack at a reduced temperature, i.e. less than ambient.

During operation below zero degrees C, a subzero operation amount of coulombs is passed through the fuel cell stack. During the voltage applying step, a recovery amount of coulombs is passed through the fuel cell stack in the opposite direction to the coulombs passed during subzero operation. Typically, the recovery amount of coulombs is less than or equal to the subzero amount of coulombs. In practical embodiments for example, the subzero amount can be greater than about 6000 coulombs, while the recovery amount can be less than about 6000 coulombs.

These and other aspects of the invention will be evident in view of the attached figures and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram showing the apparatus and configuration used in the Examples.

Figure 2 shows the average cell voltage and the stack current plotted against time for the third test carried out on the SPE fuel cell stack in the Examples.

DETAILED DESCRIPTION

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".

The method of the invention is used to recover the performance lost in a fuel cell stack after it has been operated at subzero temperatures (below zero degrees Celsius). The method is particularly effective for solid polymer electrolyte fuel cell stacks. In such stacks, subzero operation can result in a significant drop in the output voltage of each cell that is not recovered simply by operating again at elevated temperature. The inventive method involves applying a voltage briefly across the stack with a power supply such that the cathode potential is 1.4V or greater on average than the anode potential for each cell in the stack.

Figure 1 is a schematic diagram showing a typical arrangement of apparatus and configuration involved. During use, solid polymer electrolyte fuel cell stack 1 is electrically connected to load 2 (e.g., an electric motor or, in the case of the experiments in the Examples below, a load bank). If recovery is required after subzero operation, stack 1 is disconnected from load 2 via switch 4 and instead is electrically connected across power supply 3 as shown. Power supply 3 is set to at least force the stack to a potential of 1.4 V or greater per cell. Hydrogen is preferably present at the cell anodes in order to keep the absolute anode potentials from drifting from that of the dynamic hydrogen electrode. (This can be accomplished simply by shutting down the stack with hydrogen present at the anodes or instead by continually supplying hydrogen to the stack during the recovery.)

Unlike certain prior art methods, this invention involves raising the cathode potentials with respect to the anode potentials. The voltages involved can cause corrosion of cathode cell components and thus negatively affect the durability of the stack. Therefore, application of the power supply voltage should be brief and the amount of charge passed should not be excessive. At voltages on the order of 1.8 V/cell, it has been found that application times of less than a minute, and even less than 5 seconds can result in substantial performance recovery {e.g., >90% voltage lost). To further reduce any potential damage resulting from use of the method, it may be advantageous to carry it out at lower temperatures (i.e., below ambient).

Without being bound by theory, it is believed that during subzero operation, water reaction product is produced at the cathodes and may be unable to migrate into the adjacent frozen gas diffusion layers. It then may become trapped and is eventually forced into small pores in the cathode catalyst. Once the stack has been raised to normal operating temperature, these pores are not easily drained. The high cell voltages applied using the power supply are enough to electrolyze water at the cell cathodes. This would consume water at the cathode catalysts and evolve gaseous oxygen. Any oxygen gas bubbles formed may then assist in clearing water blocked pores. Thus, a recovery in performance may be expected when applying any voltage capable of electrolysis (e.g., 1.4V or greater per cell). However, the recovery method would be expected to work faster at higher voltages (e.g., 1.8V or greater per cell).

During subzero operation of the stack, a certain amount of coulombs is passed (a subzero operation amount) and this corresponds to a certain amount of water product formed. When performing the recovery method of the invention, a certain amount of coulombs is passed in the opposite direction (a recovery amount) during the voltage applying step and this may correspond to a certain amount of water electrolyzed. It is expected therefore that the recovery amount of coulombs required to achieve a significant performance recovery will be equal to or less than the subzero operation amount of coulombs. This has been demonstrated in the Examples below. Further, the recovery amount can be significantly less than the subzero amount of coulombs while still achieving a substantial performance recovery. In an automotive size stack, a typical subzero amount might be of order of 6,000 coulombs or more and the corresponding recovery amount 6,000 coulombs or much less. The following examples are provided to illustrate certain aspects and embodiments of the invention but should not be construed as limiting in any way.

EXAMPLES

In the following tests, a 10 cell Mk1100 solid polymer electrolyte fuel cell stack (designed for automotive applications) was connected up to a 5kW load bank and a 1 OkW power supply. When using the method of the invention, the apparatus was configured as indicated in Figure 1.

The stack was assembled and initially discharged through the load bank using pure hydrogen and air reactants at 63°C and 440A discharge rate. Next, the stack performance was checked at 25°C and was found to be about 63OmV average output per cell. The load bank was then disconnected and the stack purged as disclosed in US2006/0121322 in preparation for freezing. The stack was then cooled in a freezer to -15°C and 6000 coulombs were discharged through the stack at 3OA discharge rate while at -15°C. The stack was then warmed to 25°C, operated as before, and the performance was checked. The average cell voltage was now only about 510 mV indicating a loss of about 120 mV/cell.

The stack was then subjected to a series of unrelated "hydrogen pumping" experiments in which hydrogen was electrochemically pumped from the cell anodes to the cathodes to see what effect this had on performance loss. (This was accomplished by shutting off the air supply to the cathodes but continuing the flow of hydrogen to the anodes while forcing the cells into voltage reversal using the power supply with the polarity reversed to that shown in Figure 1.) These tests also involved repeating operation at -15°C. In total, about 39,000 coulombs of charge was passed through the stack at -15°C. The hydrogen pumping experiments had little effect on the performance loss. In fact, on checking performance again at 25°C, the average cell voltage was slightly worse at about 500 mV/cell. By analyzing the average cell voltage during the subzero operation, it was calculated that about 1 ,800 coulombs of charge was presumably consumed in rehydrating the ionomer electrolytes which had been dried by the pre-freezing purging step. The other 5,700 coulombs presumably was consumed in generating other water that remained at the cell cathodes. (These estimates were based on the following. The "dry" electrolytes cause the cell impedances to be slightly higher and thus there is an initial higher voltage drop observed during subzero operation. With continued operation, the cell voltages rise with time briefly until the electrolytes have rehydrated. Prior to this point of maximum voltage, the charge is presumed to be rehydrating the electrolytes. After this point, the charge is presumed to be generating other water.)

The method of the invention was then employed to see what effect this had on performance loss. Still at 25°C, a positive voltage was applied across the stack terminals until a level of 1.8V/cell was reached. The power supply current had been ramped to -200A and held for 30 seconds, passing a total of roughly -6,000 coulombs of charge. The performance was markedly improved with the average cell voltage now at 620mV/cell.

Testing was then repeated on the same stack. In this second test, the stack was purged and cooled as before but with 7,000 coulombs worth of charge passed at 3OA discharge rate. Upon checking performance at 25°C, it was found that the stack could not handle 440A load. The average cell voltage was 0 mV/cell at the maximum (limiting) current of 294A. Again, a positive 1.8V/cell voltage was applied across the stack terminals and roughly 2,000 coulombs was passed through the stack. Again, the performance was markedly improved afterwards with the average cell voltage now at 600mV/cell.

A third test was performed on the same stack. This time, the stack was purged and cooled as before but with 6,500 coulombs worth of charge passed at 45A discharge rate. Upon checking performance at 25°C, the average cell voltage was found to be 216mV/cell. Again, a positive 1.8V/cell voltage was applied across the stack terminals. The charge was more accurately measured this time and 1 ,000 coulombs was passed through the stack. Again, the performance was markedly improved with the average cell voltage now at 558mV/cell. Figure 2 shows the average cell voltage and the stack current plotted against time for this third test. Region A shows the initial performance test in which the stack is initially found to provide about 216 mV/cell after subzero operation. Region B shows the voltage and current when the power supply voltage of 1.8V/cell is applied to the stack. And finally Region C shows the performance test afterwards.

A final test was performed on the stack to see if additional charge passed at high voltage would have any additional effect. Here, the stack voltage was briefly raised (for 7 seconds) to 1.8V/cell. There was no significant change noticed on the average cell voltage after this brief exposure.

These examples show that a marked improvement in cell performance can be achieved using the method of the invention. The greatest percentage of voltage recovery was observed when more recovery coulombs were passed {i.e., those coulombs passed during application of the 1.8V/cell voltage to the stack) than were passed during previous subzero operation. However, almost the same voltage recovery could be obtained even when substantially fewer recovery coulombs were passed through the stack than were passed during previous subzero operation. This suggests that the recovery coulombs required to achieve a given result are not proportional to the coulombs passed during subzero operation. The above tests further suggest that most of the voltage recovery effect is obtained during the first seconds when 1.8V is applied per cell.

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

1. A method for recovering performance in a fuel cell stack following operation below zero degrees Celsius, the stack comprising at least one fuel cell, and each fuel cell in the stack comprising an electrolyte, a cathode, and an anode, wherein the method comprises: connecting a power supply across the fuel cell stack; and applying a voltage across the fuel cell stack with the power supply such that the cathode potential is 1.4V or greater on average than the anode potential for each cell in the stack.

2. The method of claim 1 comprising applying the voltage across the fuel cell stack such that the cathode potential is 1.8V or greater on average than the anode potential for each cell in the stack.

3. The method of claim 1 wherein the fuel cell stack is not a regenerative fuel cell stack.

4. The method of claim 1 wherein hydrogen is present at each anode.

5. The method of claim 1 wherein each fuel cell is a solid polymer electrolyte fuel cell.

6. The method of claim 1 wherein the fuel cell stack comprises a plurality of fuel cells in series.

7. The method of claim 1 wherein the voltage is applied for less than one minute.

8. The method of claim 7 wherein the voltage is applied for less than 5 seconds.

9. The method of claim 1 comprising applying the voltage to the stack at a temperature below ambient.

10. The method of claim 1 wherein a subzero operation amount of coulombs is passed through the fuel cell stack during operation below zero degrees C and a recovery amount of coulombs is passed through the fuel cell stack in the opposite direction during the voltage applying step.

11. The method of claim 10 wherein the recovery amount of coulombs is less than or equal to the subzero amount of coulombs.

12. The method of claim 10 wherein the recovery amount is less than about 6000 coulombs.

13. The method of claim 10 wherein the subzero operation amount is greater than about 6000 coulombs.