WO2008048270A1

WO2008048270A1 - Hydrogen sensor cell for detecting fuel starvation

Info

Publication number: WO2008048270A1
Application number: PCT/US2006/041267
Authority: WO
Inventors: Michael L. Perry; Robert M. Darling
Original assignee: Utc Fuel Cells, Llc
Priority date: 2006-10-19
Filing date: 2006-10-19
Publication date: 2008-04-24

Abstract

A fuel cell stack includes at least one fuel cell having a fuel inlet for directing a hydrogen fuel to the fuel cell to generate electric current; a sensor cell having an anode flow field, a cathode flow field and an unitized electrode assembly between the anode flow field and the cathode flow field, the anode flow field being communicated with the fuel inlet to receive a portion of fuel from the fuel inlet, the sensor cell being connected in series with the stack to carry the electric current whereby hydrogen from the portion of fuel is evolved into the cathode flow field of the sensor cell; and a sensor communicated with the sensor cell to receive a signal corresponding to content of hydrogen in the sensor cell.

Description

Hydrogen Sensor Cell For Detecting Fuel Starvation

BACKGROUND OF THE DISCLOSURE

[0001] The disclosure relates to fuel cells and, more particularly, to improving operating efficiency of fuel cells while protecting the cells from fuel starvation.

[0002] Fuel cells process reactant streams containing hydrogen and oxygen to generate water and an electric current. Hydrogen-containing fuel and oxidant are fed to the anode and cathode, respectively, of the fuel cell, typically to a stack of fuel cells which forms part of a power plant .

[0003] Operating the cell stack at a high fuel utilization is desirable as high fuel utilization enhances efficiency and reduces the size of the hydrogen tank or fuel source needed. Unfortunately, high fuel utilization also increases the risk of fuel starvation, which is a condition where insufficient hydrogen is present in the anode of one or more fuel cells, and operating under these conditions can permanently damage the fuel cell.

[0004] One solution to the fuel starvation issue has been to operate fuel cells with an internal fuel recycle. This allows a reduced level of internal utilization as compared to external utilization. However, a fuel recycle stream complicates the system design and has an adverse impact upon system cost and reliability.

[0005] It is clear that the need exists for higher operating efficiency with system simplicity.

[0006] It is therefore the primary object of the disclosure to provide a system which allows for high fuel utilization without risk of fuel starvation. Luuυvj Otiier objects and advantages of the present disclosure will appear below.

SUMMARY OF THE DISCLOSURE

[0008] In accordance with the disclosure, the foregoing objects and advantages have been readily attained. [0009] According to the disclosure, a fuel cell stack is provided which comprises at least one fuel cell having a fuel inlet for directing a hydrogen fuel to the fuel cell to generate electric current; a sensor cell having an anode, a cathode and an electrolyte between the anode and the cathode, the anode being communicated with the fuel inlet to. receive a portion of fuel from the fuel inlet, the sensor cell being connected in series with the stack to carry the electric current whereby hydrogen from the portion of fuel is evolved to the cathode of the sensor cell; and a sensor communicated with the sensor cell to receive a signal corresponding to content of hydrogen in the sensor cell. The electrolyte can be any fuel cell electrolyte such as proton-exchange membrane, phosphoric acid, alkaline (KOH) and the like.

[0010] In further accordance with the disclosure, a method is provided for operating a fuel cell power plant, which method comprises operating a fuel cell stack comprising at least one fuel cell having a fuel inlet for directing a hydrogen fuel to the fuel cell to generate electric current, and a sensor cell having an anode, a cathode and an electrolyte between the anode and the cathode, the anode being communicated with the fuel inlet to receive a portion of fuel from the fuel inlet, the sensor cell being connected in series with the stack to carry the electric current whereby hydrogen from the portion of fuel is evolved to the cathode^"" of the sensor cell; and monitoring a parameter corresponding to content of hydrogen in at least one of the anode and the cathode of the sensor cell to determine content of fuel in the sensor cell .

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] A detailed description of preferred embodiments of the present disclosure follows, with reference to the attached drawings, wherein:

[0012] Figure 1 schematically illustrates a system according to the disclosure;

[0013] Figure 2 further schematically illustrates a manifold setup of a system according to the disclosure;

[0014] Figure 2A illustrates a 2 -pass embodiment of a system according to the disclosure;

[0015] Figure 3 schematically illustrates anode side flow passages for a sensor cell according to one embodiment of the disclosure;

[0016] Figure 3A illustrates an alternate anode side flow passage configuration for a sensor cell according to the disclosure;

[0017] Figure 4 schematically illustrates a cathode side flow configuration for a sensor cell according to one embodiment of the disclosure;

[0018] Figure 4A illustrates an alternate cathode side flow passage configuration for a sensor cell according to the disclosure;

[0019] Figure 4B illustrates a further alternate cathode side flow passage configuration for a sensor cell according to the disclosure; and [^"00^'2O] Figure 5 schematically illustrates a system according to the disclosure with a battery that is used to power loads when low current density needs are detected.

DETAILED DESCRIPTION

[0021] The disclosure relates to detection of fuel concentration in the fuel inlet of a fuel cell and to operation of the fuel cell at high fuel utilizations. According to the disclosure, at least one cell of a fuel cell stack is operated as a hydrogen sensor cell, or in "hydrogen pump" mode, and this cell is much more sensitive than other "normal" cells of the stack, especially to fluctuation in hydrogen content in the anode. [0022] Figure 1 shows a fuel cell stack 10 which includes a plurality of fuel cells 12 (only one illustrated in Figure 1) and a sensor cell 14. Fuel cells 12 have an anode flow field 16, a cathode flow field 18, and a unitized electrode assembly 20 that contains an anode and cathode catalyst layer, anode and cathode gas diffusion layers adjacent the anode and cathode catalyst layers and an electrolyte or membrane disposed between the anode and cathode catalyst layers opposite the gas diffusion layers. Hydrogen or a hydrogen-containing fuel is fed through a fuel inlet 22 to anode flow field 16 of fuel cells 12 while oxidant is fed through an oxidant inlet 24 to cathode flow field 18 of fuel cells 12 to generate electric current across stack 10. Exhaust exits anode flow field 16 through anode exhaust or exit 26, while exhaust exits cathode flow field 18 through cathode exhaust or exit 28. [0023] Sensor cell 14 also has an anode flow field 30 and a cathode flow field 32 as well as a unitized electrode assembly 34 similar in construction to unitized electrode assembly 20 and positioned between anode flow field 30 and cathode flow field 32. Unlike fuel cells 12, sensor cell 14 is operated by being fed only fuel to anode flow field 30, and substantially no oxidant to cathode flow field 32. When operated in this manner, stack current passing through sensor cell 14 drives hydrogen across the membrane of the unitized electrode assembly 34 so that pure hydrogen is evolved at cathode flow field 32. This operation of sensor cell 14 is sometimes referred to as operating as a "hydrogen pump". A difference in potential can be detected and/or measured between hydrogen concentration of the fuel in anode flow field 30 as compared to the substantially pure hydrogen in cathode flow field 32. According to the disclosure, this potential, or some other parameter related to content or concentration of hydrogen in either or both of anode flow field 30 and cathode flow field 32, is measured and concentration and availability of fuel in the anode flow field 30 is determined.

[0024] Operation of sensor cell 14 as a hydrogen pump makes the cell far more sensitive to changes in concentration and availability of fuel in the cell, and thus this cell can advantageously be used to detect fuel conditions, most importantly to detect dangerously low amounts of fuel being provided to the cell and/or low fuel concentration in the fuel inlet, and to correct for such conditions before any permanent damage is done to the other cells of the stack. Sensor cell 14 is also capable of detecting contaminants in the fuel . This advantageously allows the stack to be run at high fuel utilizations, preferably at least about 80%, more preferably at least about 90%, and ideally as high as 97%, without requiring fuel recycle. [00T5^".^" According to the disclosure, sensor cell 14 receives fuel at anode flow field 30 through a sensor cell fuel inlet 36 which is communicated with fuel inlet 22 of fuel cells 12. This inlet 36 can simply be part of the fuel manifold that feeds all the cells. No oxidant, and alternatively a small amount of fuel, is fed to cathode flow field 32, and current from the stack serves to drive hydrogen across the membrane of unitized electrode assembly • 34 as described above.

[0026] Exhaust from anode flow field 30, if any, can be fed to anode exhaust 26 of fuel cell 12, although other alternatives are discussed below. Sensor cell 14 is advantageously run at as high a fuel utilization as possible so as to minimize wasted fuel and, thus, anode flow field 30 can even be provided with restricted or no exhaust outlet if desired. More detail of preferred anode and cathode sensor cell flow field structures is provided below.

[0027] Exhaust from cathode flow field 32 is substantially pure hydrogen, and this hydrogen can advantageously be further used in accordance with the disclosure. Thus, this exhaust is not merely vented to exhaust out of the stack. According to the disclosure, this fuel exhaust can be recycled back to the fuel inlet 22 through an exhaust line 33 as shown in Figure 1. The exhaust from cathode flow field 32 can be fed through an exhaust line 33 back to fuel inlet 22 at a point upstream or downstream from the branch from fuel inlet 22 which feeds sensor cell 14. In this way, fuel used in sensor cell 14 is not wasted and is used in a manner which does not adversely affect the function of sensor cell 14. Even if this hydrogen is fed to an inlet which also feeds the sensor cell 14, as shown in Figure 1, the pure hydrogen is available to feed all of the anodes in the stack.

[0028] Still referring to Figure 1, a water loop 38 can be provided for feeding water for cooling and/or humidification to fuel cells 12 and, if desired, to sensor cell 14. This water can be fed to a water transport plate 40 as shown, which advantageously conveys water as needed to fuel cells 12. As shown, a water transport plate 42 can also be provided in sensor cell 14. Sensor cell 14 is less likely to need cooling, but water transport plate 42 can still be useful for managing water in the sensor cell 14. [0029] In accordance with the disclosure, sensor cell 14 is advantageously more sensitive than the rest of stack 10 to variations in the concentration and/or availability of hydrogen in fuel inlet 22.

[0030] For example, if fuel in fuel inlet 22 suffers a dilution, or if the electrical load on stack 10 increases substantially, the hydrogen in the fuel will be split substantially equally between the inlets of all the different fuel cells 12 as well as the inlet to sensor cell 14. While this would eventually affect all cells 12, 14, it will be detected by cell 14 more readily due to operation of cell 14 in a hydrogen pump mode. Additionally, cell 14 will not be permanently damaged by local fuel starvation as would other cells 12. When local starvation occurs, the current distribution will shift in cell 14 and this will be easily detected due to higher local current density causing higher ohmic losses, which are readily detected in a hydrogen pump cell. [0031] In order to monitor sensor cell 14, a sensor 44 can be provided for determining a difference between the anode and cathode sides of sensor cell 14. Figure 1 shows this as a simple voltage meter, tout it should be appreciated that a wide variety of parameters can be measured, and a further wide variety of instruments can be used to make such measurements, well within the scope of the present disclosure. Further, a control unit 45 can be communicated with sensor 44 and used to issue local or transmitted warnings to operators of the stack, and/or to take control actions such as increase fuel flow, shut down the stack, decrease load, or any other suitable step which can be taken to address potential fuel starvation conditions. The actual components of control unit 45 are well known to a person of skill in the art, and could comprise one or more sensors coupled with a processor such as a desk top computer, workstation, "on board" processing and the like, programmed with suitable instructions to cause the proper warning and/or control steps to be taken depending upon information received from sensor 44.

[0032] In accordance with the disclosure, it is desired for sensor cell 14 to be as sensitive as possible, and certainly more sensitive than the fuel cells 12 of the stack, so that fuel shortage is detected well prior to any damage fuel starvation can cause to the other cells. In this regard, since the potential of the hydrogen sensor cell consists of just the polarization of the hydrogen reaction (both reduction and oxidation) and the resistance of the cell, and since these are very small relative to the cathode reaction (oxygen reduction reaction) , the sensor cell is very sensitive to any change in either the polarization or resistance, and reduction in hydrogen concentration in anode flow field 30 or changes in local current densities due to local fuel starvation will thereby increase the potential across sensor cell 14. [0033] As set rorth above, it is desired to make sensor cell 14 as sensitive as possible so that warning can be given and suitable action taken before any permanent damage is done to the fuel cells of the stack, or before gross fuel starvation occurs in sensor cell 14 since gross fuel starvation can also result in permanent damage to sensor cell 14. Gross fuel starvation is defined herein as a state of a fuel cell where less hydrogen is supplied to the cell than is electrochemically required to carry the current of the cell.

[0034] Figure 2 shows the normal inlet and exit manifolds of a fuel cell stack according to the disclosure, and shows a cell 12 having fuel inlet and exit manifolds 46, 48 respectively, as well as oxidant inlet and exit manifolds 50, 52 respectively. Coolant inlets 54 and outlets 56 can also be provided, substantially as shown. With this usual arrangement of manifolds or other flow structures, fuel, oxidant and coolant are all circulated through the manifolds which are communicated as desired with the flow fields of the electrodes and the water transport plates as, appropriate .

[0035] Flow fields across cell 12 would typically be a series of flow channels or passages from the inlet manifold to the outlet manifold, and these specific structure are not illustrated in Figure 2.

[0036] In connection with sensor cell 14, however, a different structure of flow channels is desired. Specific points of interest in this regard are that the sensor cell needs little or no provision for anode exhaust and/or cathode inlet, and high fuel utilizations are desirable. High fuel utilizations are made more likely by reducing the flow rate of reactant through anode flow field 30. Thus, according to the disclosure, flow passages in anode flow field 30 can be made smaller or otherwise less conducive to flow so that less fuel is made available to anode flow field 30 of sensor cell 14 than to all other anode flow fields 16 of other cells 12. Thus, according to the disclosure, anode flow field 30 preferably has a greater resistance to fuel flow at normal fuel cell operating conditions than anode flow fields 16 of fuel cells 12. Figure 3 shows a flow configuration for anode flow field 30 having flow passages 31 which are connected between fuel inlet manifold 46 and fuel exit manifold 48. Flow passages 31 are connected to these manifolds only at one portion of the flow inlet and outlet manifolds. This serves to restrict fuel flow to anode flow field 30 and, thereby, to increase the fuel utilization as desired. Further, the outlet channels shown in Figure 3 are themselves optional, and these channels can be dead ended such that the only fuel exit is restricted flow through the gas diffusion layer of the cell and into the exit manifold 48. [0037] It should be appreciated that the reason for increasing flow resistance to anode flow field 30 of sensor cell 14 is that such increased resistance helps to make sure that of all cells in the stack, sensor cell 14 will receive the lowest fuel flow rate. This helps to increase the function of sensor cell 14 as a warning or "canary" cell which gives warning of low hydrogen content in the stack before actual harm is done to the other cells. It should also be appreciated that since sensor cell 14 is evolving pure hydrogen to the cathode side, even if this cell experiences local hydrogen starvation which could permanently damage a fuel cell, the presence of hydrogen on both electrodes of the sensor cell serves to reduce the livelihood of permanent damage to sensor cell 14, thereby further improving the robustness of sensor cell 14 as a warning or "canary" cell.

[0038] Figure 4 shows a preferred flow field for cathode flow field 32 of sensor cell 14. As shown, no oxidant inlet from manifold 50 is needed in this structure. Instead, flow passages 33 are connected to fuel inlet manifold 46 at an outlet 62 which in this embodiment is on the same side as the inlets of other cells, so that routing this pure hydrogen back to the fuel inlet for cells 12, 14 is facilitated.

[0039] In accordance with a further embodiment of the disclosure, it has been found that high fuel utilization is much easier to maintain at high current densities. Under such circumstances, distribution of fuel among the various cells is most uniform, thus reducing the likelihood of a single fuel starved cell when all others and even sensor cell 14 are not affected. At low current densities, reduced levels of fuel utilization should be used. As an alternative, Figure 5 schematically illustrates a system 64 wherein a stack 10 is one source of power to a load 66, and a battery 68 is also provided. A controller 70 monitors these components as schematically illustrated by the dashed arrows in Figure 5, and switches load 66 to battery 68 when current densities are low, that is, when they are below a current density threshold. In this way, operation of stack 10 at low loads is avoided, for example operation may be avoided at loads less than a threshold of current density of about 200 mA/cm² of active area, preferably less than about 100 mA/cm² of active area. The increased risk of local or isolated fuel starvation due to operation at low current densities is also avoided. When the stack is operating at a high current density, battery 68 can advantageously be charged for the next needed use, preferably from stack 10. The actual components of such a monitoring and switching system are known to a person of skill in the art and details of such components form no part of the present disclosure.

[0040] Figure 2A shows an alternative embodiment of the present disclosure wherein stack 10 has a 2 -pass fuel flow configuration for fuel cells 12. Figure 2A shows the same air and coolant flow patterns as the embodiment shown in Figure 2. However, fuel inlet 46 and fuel exit 48 are located, in this embodiment, on the same side of the stack. A fuel turn manifold 47 is provided for directing fuel back through fuel cells 12 for a second pass. Fuel in this embodiment flows through cells 12 along the path of the arrows in Figure 2A.

[0041] Figure 3A illustrates an alternative embodiment of fuel flow configuration for anode flow field 30 of sensor cell 14 in connection with the 2 -pass embodiment of Figure 2A. As shown in Figure 3A, fuel flowing through sensor cell 14 is made in a single pass which outlets to fuel turn manifold 47. Thus, fuel not evolved into cathode flow field 32 is, in this embodiment, fed to manifold 47 for the second pass through cells 12, that is, along the flow path of Figure 2A from turn manifold 47 to fuel exit manifold 48.

[0042] Figure 4A illustrates an alternative embodiment cathode flow field 32 of sensor cell 14, which is well suited to use with the two pass configuration of Figure 2A. As shown, flow passages 33 are not communicated with air manifolds 50, 52, and are connected to feed evolved hydrogen through outlet 62a to fuel inlet manifold 46 in similar manner to the flow pattern illustrated in Figure 4. Alternatively, Figure 4B shows a further embodiment of the flow field for cathode flow field 32 of sensor cell 14 wherein passages 33 are communicated through outlet 62b to feed evolved hydrogen to fuel turn manifold 47. [0043] It should be appreciated that the various flow schemes illustrated in the drawings are schematic in nature, and the actual connections may be configured differently than shown in the simplified illustration of the drawings. Alterations to the exact connections are considered to be within the broad scope of the present disclosure.

[0044] The discussion above contains discussion of the desire to maintain high fuel utilizations. Specifically, the present disclosure allows operation at very high fuel utilizations by providing very sensitive detection of any fuel starvation. In accordance with the present disclosure, it should be appreciated that control of fuel utilization can be done through many different approaches. Typically, this control can be accomplished by increasing or decreasing the resistance to flow of fuel through the fuel cells. This can be accomplished in a number of ways. Figure 1 schematically illustrates one method, and shows a control valve 51 positioned along exhaust outlet 26. Opening and closing of valve 51 directly influences the resistance to flow of reactants through the fuel cells, and thereby increases or decreases fuel utilization throughout the stack. This valve 51 can advantageously be controlled in accordance with the present disclosure and based upon measurements from the sensor cell 14 so as to maintain a high fuel cell utilization as desired. This control valve 51 can also be operated in accordance with the control aspect of the present disclosure to take other actions as needed, for example, to temporarily reduce fuel utilization when sensor cell 14 indicates a potential fuel starvation condition.

[0045] It should be appreciated that the system and method of the present disclosure advantageously provide for detection of fuel starvation before harm is done to the fuel cells, thereby allowing for operation at high fuel utilizations with or without fuel recycle. This advantageously increases the efficiency of the system and also reduces the complexity and cost of the stack, for example by simplifying the manifold structures and the like .

[0046] While the present disclosure has been described in the context of specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.

Claims

We claim:

1. A fuel cell stack, comprising: at least one fuel cell having a fuel inlet for directing a hydrogen fuel to the fuel cell to generate electric current; a sensor cell having an anode flow field, a cathode flow field and a unitized electrode assembly between the anode flow field and the cathode flow field, the anode flow field being communicated with the fuel inlet to receive a portion of fuel from the fuel inlet, the sensor cell being connected in series with the stack to carry the electric current whereby hydrogen from the portion of fuel is evolved to the cathode flow field of the sensor cell; and a sensor communicated with the sensor cell to receive a signal corresponding to content of hydrogen in at least one of the anode flow field and the cathode flow field of the sensor cell.

2. The apparatus of claim 1, wherein the sensor is communicated with the sensor cell to receive a signal corresponding to the voltage of the sensor cell.

3. The apparatus of claim 1, wherein the at least one fuel cell comprises a plurality of fuel cells each having an anode flow field and a cathode flow field and a unitized electrode assembly positioned between the anode flow field and the cathode flow field, and wherein the anode flow field of the sensor cell has a greater resistance to gas flow than anode flow fields of the plurality of fuel cells.

4. The apparatus of claim 1, further comprising a control system communicated with the sensor and programmed to provide at least one of an alarm and instructions for corrective action upon detecting the presence of low fuel content within the sensor cell .

5. The apparatus of claim 4 , wherein the control system is adapted to operate the stack at a high fuel utilization when current density through the stack is equal to or greater than a threshold current density, and to operate the stack at a reduced fuel utilization when current density through the stack is less than the threshold current density, wherein the reduced fuel utilization is less than the high fuel utilization.

6. The apparatus of claim 5, further comprising an additional power source, and wherein the control system is adapted to stop operation of the stack and switch loads to the additional power source when current density is less than the threshold current density.

7. The apparatus of claim 1, wherein the sensor cell has an anode flow field which has a greater resistance to flow than anode flow fields of the at least one fuel cell .

8. The apparatus of claim 1, wherein the sensor cell has a cathode exhaust which is communicated back to the fuel inlet.

9. The apparatus of claim 1, wherein the sensor cell is at an end of the stack.

10. The apparatus of claim 1, wherein the at least one fuel cell has a fuel turn manifold communicated to feed fuel to the at least one fuel cell for at least a second pass.

11. The apparatus of claim 10, wherein the sensor cell has an anode exhaust which is connected to the fuel turn manifold.

12. The apparatus of claim 10, wherein the sensor cell has a cathode exhaust which is connected to the fuel inlet.

13. The apparatus of claim 10, wherein the sensor cell has a cathode exhaust which is connected to the fuel turn manifold.

14. A method for operating a fuel cell power plant, comprising: operating a fuel cell stack comprising at least one fuel cell having a fuel inlet for directing a hydrogen fuel to the fuel cell to generate electric current, and a sensor cell having an anode flow field, a cathode flow field and a unitized electrode assembly between the anode flow field and the cathode flow field, the anode flow field being communicated with the fuel inlet to receive a portion of fuel from the fuel inlet, the sensor cell being connected in series with the stack to carry the electric current whereby hydrogen from the portion of fuel is evolved to the cathode flow field of the sensor cell; and monitoring a parameter corresponding to content of hydrogen in at least one of the anode flow field and the cathode flow field of the sensor cell to determine content of fuel in the sensor cell .

15. The method of claim 14, wherein the at least one fuel cell comprises a plurality of fuel cells each having an anode flow field and a cathode flow field and a unitized electrode assembly positioned between the anode flow field and the cathode flow field, and wherein the anode flow field of the sensor cell has a greater resistance to gas flow than anode flow fields of anodes of the plurality of fuel cells.

16. The method of claim 14, further comprising providing at least one of an alarm and instructions for corrective action upon detecting the low fuel content in the sensor cell.

17. The method of claim 14, further comprising a control system communicated with the sensor and programmed to provide at least one of an alarm and instructions for corrective action upon detecting the low fuel content in the sensor cell.

18. The method of claim 14, further comprising operating the stack at a high fuel utilization when current density through the stack is equal to or greater than a threshold current density and operating the stack at a reduced fuel utilization when current density through the stack is less than the threshold current density, wherein the reduced fuel utilization is less than high fuel utilization.

19. The method of claim 14, further comprising stopping operation of the stack and switching loads to an additional power source when current density is less than a threshold value .

20. The method of claim 14, wherein the sensor cell has a cathode exhaust which is communicated back to the fuel inlet.

21. The method of claim 14, wherein the sensor cell is at an end of the stack.

22. The method of claim 14, wherein the at least one fuel cell has a fuel turn manifold communicated to feed fuel to the at least one fuel cell for at least a second pass.

23. The method of claim 22, wherein the sensor cell has an anode exhaust which is connected to the fuel turn manifold.

24. The method of claim 22, wherein the sensor cell has a cathode exhaust which is connected to the fuel inlet.

25. The method of claim 22, wherein the sensor cell has a cathode exhaust which is connected to the fuel turn manifold.