Abstract:
A controller controls a switch bank which can shunt a voltage source in parallel with one or more fuel cells in a stack. By controlling the voltage of the voltage source, the current through the fuel cells is directly controlled. By increasing the anode potential of the fuel cell through control of the voltage source, poisons deposited on the electrocatalyst are removed, thereby rejuvenating the fuel cells. Fuel cells in a stack can be treated in turn, causing a reduction of the effects of electrocatalyst poison on stack performance.

Description:
This application is a continuation of U.S. application Ser. No. 09/706,858 filed Jan. 7, 2000 U.S. Pat. No. 6,339,313 which is a continuation-in-part of U.S. Ser. No. 08/887,844 filed Jul. 3, 1997 now U.S. Pat. No. 6,239,579 and which relates to U.S. Ser. No. 60/021,185 filed Jul. 5, 1996. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to fuel cells and more specifically, to methods and devices which can manage the operational health of fuel cell stacks. 
     BACKGROUND OF THE INVENTION 
     The past few decades have seen an explosion of interest in environmental matters. One consequence of this has been the beginning of a movement away from fossil fuel based energy sources with their attendant effects on pollution. One seemingly viable alternative to such traditional energy sources, especially for automobiles, is the electrochemical fuel cell. 
     Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to produce electric power and reaction products. Such cells can operate using various reactants—the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol. The oxidant may be substantially pure oxygen or a dilute stream such as air containing oxygen. 
     One drawback to current fuel cells is the degradation in a cell&#39;s power output over time. Impurities, either from the fuel stream or generated from within the fuel cell as intermediate species during fuel cell reactions, may be adsorbed or deposited on the surface of the anode electrocatalyst. This blocks portions of the electrocatalyst and prevents these portions from inducing the desired electrochemical fuel oxidation reaction. Such impurities are known as electrocatalyst “poisons” and their effect on electrochemical fuel cells is known as “electrocatalyst poisoning”. Such “poisoning” reduces fuel cell performance by reducing the voltage output from the cell for that cell&#39;s current density. The deposit of electrocatalyst poisons may be cumulative—over time, even minute concentrations of poisons in a fuel stream may result in a degree of electrocatalyst poisoning which hampers fuel cell performance. 
     The sources of such poisons, as mentioned above, are legion. Reformate streams derived from hydrocarbons or oxygenated hydrocarbons typically contain a high concentration of hydrogen fuel but also typically contain electrocatalyst poisons such as carbon monoxide. Because of such a presence, the fuel stream may be pre-treated prior to its direction to the fuel cell. Pre-treatment methods may employ catalytic or other methods to convert carbon monoxide to carbon dioxide. Unfortunately, pre-treatment methods cannot efficiently remove all of the carbon monoxide. Even trace amounts such as 10 ppm can eventually result in electrocatalyst poisoning. 
     Fuel cell components and other fluid streams in the fuel cell may also be a source of impurities. As an example, fuel cell separator plates are commonly made from graphite. Organic impurities in graphite may leech out and poison the electrocatalyst. Other poisons may be generated by the reaction of substances in the reactant streams with the fuel cell component materials. A further possible source of poison is from intermediate products in the oxidation process. For cells which use complex fuels such as methanol, this is particularly important. 
     A few methods have been developed which attempt to overcome the electrocatalyst poisoning issue. The anode may be purged with an inert gas such as nitrogen. However, this method involves suspending power generation by the fuel cell. Another approach is that of introducing a “clean” fuel stream containing no carbon dioxide or other poisons to a poisoned fuel cell anode. Where the adsorption is reversible, an equilibrium process results in some rejuvenation of the electrocatalyst. However, such a method is not effective against irreversibly adsorbed poisons. Furthermore, the recovery of the anode electrocatalyst by such an equilibrium process can be very slow, during which time the fuel cell is unable to operate at full capacity. 
     Yet another approach is to continuously introduce a low concentration of oxygen into the fuel stream upstream of the fuel cell, as disclosed by Gottesfeld in U.S. Pat. No. 4,910,099. Unfortunately, this approach has its own drawbacks, such as parasitic losses from oxygen bleed, undesirable localized exothermic reactions at the anode, and dilution of the fuel stream. 
     Wilkinson et al in U.S. Pat. No. 6,096,448 discloses periodic fuel starvation of the anode to increase the anode potential. This oxidizes and removes electrocatalyst poisons. Wilkinson describes three methods of accomplishing this fuel starvation: momentary interruption of the fuel supply by closing valves both upstream and downstream of the fuel supply, periodically introducing pulses of fuel free fluid into the fuel supply, and momentarily increasing the electrical load on the cell without increasing the fuel supply. With each of these methods, the anode potential rises because of fuel depletion at the anode. Unfortunately, none of these methods allow direct control of the anode potential. Furthermore, treatment is applied on a stack basis and hence necessarily causes disruption of stack performance. 
     From the above, there is therefore a need for devices and methods which address the issue of electrocatalyst poisoning while avoiding the problems associated with the efforts described above. 
     SUMMARY OF THE INVENTION 
     The present invention provides methods and devices which alleviate the effects of electrocatalyst poisoning while avoiding the problems encountered by the prior art. In one aspect, a controller controls a switch bank which can shunt a voltage source in parallel with each fuel cell in a stack. By controlling the voltage of the voltage source, the current through the fuel cell is directly controlled. By increasing the anode potential of the fuel cell through control of the voltage source, poisons deposited on the electrocatalyst are removed, thereby rejuvenating the fuel cell. Each fuel cell in a stack can be treated in turn, causing a reduction of the effects of electrocatalyst poison on stack performance. 
     In a first aspect the present invention provides a device for removing catalyst poisons in one or more individual fuel cells in a stack while said stack is in use, said stack having plurality of fuel cells coupled to each other in series, said device comprising: 
     a switch bank coupled to fuel cells in the stack; 
     a controller controlling said switch bank; 
     measuring means to measure a voltage, said measuring means being coupled to said switch bank and to the controller; and 
     a variable voltage source coupled to the switch bank and to the controller, wherein 
     said switch bank can couple the variable voltage source in parallel with one or more of the plurality of fuel cells in said stack; 
     said switch bank can couple said measuring means to one or more fuel cells in the stack such that said measuring means can measure the voltage across said one or more fuel cells; 
     said controller controls a voltage setting of said variable voltage source and said one or more fuel cells in said stack are coupled in parallel with said variable voltage source. 
     In another aspect the present invention provides a method of reducing catalyst poisons in a stack of fuel cells while said stack is in use, said method comprising: 
     a) choosing one or more fuel cells for which catalyst poisons are to be reduced; 
     b) determining a voltage across said one or more fuel cells; 
     c) setting a voltage setting for a variable voltage source, said voltage setting being different from said voltage across said one or more fuel cells; and 
     d) momentarily coupling said variable voltage source to be in parallel with said one or more cells. 
     In a further aspect the present invention provides a device for a stack having multiple fuel cells in the event that at least one fuel cell in said stack is unable to produce its required power output, said device comprising: 
     a switch bank coupled to the fuel cell stack; 
     a controller controlling said switch bank; 
     measuring means to measure a voltage, said measuring means being coupled to said switch bank and to the controller; and 
     a variable voltage source coupled to the switch bank and to the controller, wherein 
     said switch bank can couple the variable voltage source in parallel with at least one of the plurality of fuel cells in said stack; 
     said switch bank can couple said measuring means to one or more fuel cell in the stack such that said measuring means can measure a voltage across said at least one fuel cell; 
     said controller controls a voltage setting of said variable voltage source and one or more fuel cells in said stack are coupled in parallel with said variable voltage source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention may be obtained by reading the detailed description of the invention below, in conjunction with the following drawings, in which: 
     FIG. 1 is a block diagram of a management system for fuel cells according to the invention. 
     FIG. 2 is a detailed circuit diagram of the management system illustrated in FIG.  1 . 
     FIG. 3 is a flow chart detailing the steps of the method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a block diagram of a fuel cell manager according to the invention is illustrated. A fuel cell stack  10  is coupled to a switch bank  20 . The switch bank is coupled to a voltage sensing device  30  and a variable voltage source  40 . The switch bank  20 , voltage sensing device  30 , and variable voltage source  40  are all coupled to a controller  50 . The controller  50  controls the switches within the switch bank  20  through line  35 . The controller also controls the voltage setting of the variable voltage source  40  through line  37 . The controller also receives the output of the voltage sensing device  30  through line  45 . 
     Referring to FIG. 2, a more detailed diagram of FIG. 1 is illustrated, showing some of the details of the switch bank  20  and the fuel cell stack  10 . The fuel cell stack  10  has fuel cells  15 A,  15 B,  15 C, and  15 D in series with one another. Also coupled to the fuel cells is a load  16 . Load  16  is coupled at a first end to fuel cell  15 A and at a second end to fuel cell  15 D. While a load  16  is generally not considered nor required to be illustrated as part of a fuel cell stack, it has been included with the stack for simplicity. Also illustrated in FIG. 2 as being within the fuel cell stack  10  are connection points  17 - 1 ,  17 - 2 ,  17 - 3 ,  17 - 4 , and  17 - 5 . Connection point  17 - 1  is located between the first end of load  16  and cell  15 A. Connection point  17 - 2  is located between cells  15 A and  15 B. Connection point  17 - 3  is located between cells  15 B and  15 C. Connection point  17 - 4  is located between cells  15 C and  15 D. Connection point  17 - 5  is between the second end of load  16  and cell  15 D. 
     Within the switch bank  20  are lines  55  and  57  along with switches  60 A,  60 B,  70 A,  70 B,  80 A,  80 B,  90 A,  90 B,  100 A, and  100 B. As can be seen, switch  60 A is between connection point  17 - 1  and line  55 . Switch  70 A is between connection point  17 - 2  and line  55  while switch  80 A is between connection point  17 - 3  and line  55 . Switch  90 A is between connection point  17 - 4  and line  55 . Switch  60 B is between connection point  17 - 2  and line  57  while switch  70 B is between connection point  17 - 3  and line  57 . Switch  80 B is between connection point  17 - 4  and line  57  while switch  90 B is between connection point  17 - 5  and line  57 . The voltage sensing device  30  is coupled to line  55  at one lead and to line  57  at another. Switch  100 A is between a lead of the voltage sensing device  30  and a lead of the variable voltage source. Switch  100 B is between another lead of the voltage sensing device and another lead of the variable voltage source. It should be noted that while a 4 cell stack is illustrated in FIG. 2, the design can be extend to include as many fuel cells as required. It should also be clear that the addition of fuel cells to the stack would require additional switches in the switch bank to couple the additional fuel cells to the lines  55  and  57 . 
     As can be seen, closing switches  60 A and  60 B and leaving all other switches open effectively couples the voltage sensing device across cell  15 A. Similarly, closing switches  70 A and  70 B while leaving all other switches open accomplishes the same result for cell  15 B. Thus, switches  60 A and  60 B are associated with cell  15 A, switches  70 A and  70 B are associated with cell  15 B, switches  80 A and  80 B are associated with cell  15 C, and switches  90 A and  90 B are associated with cell  15 D. Associated with the variable voltage source  40  are switches  100 A and  100 B. If the switches associated with a specified cell or cells are closed and the switches associated with the voltage source are also closed, the voltage source is effectively shunted in parallel with the specified cell or cells. 
     By closing only the switches associated with a source or cells, the voltage sensing device  30  can measure the voltage across such cells or source. 
     The system works by determining the voltage across one or more of the fuel cells. If this voltage does not meet a threshold, then the cells are determined to require rejuvenation. A proper voltage setting for the variable voltage source is determined and then the variable voltage source is shunted to be in parallel with the cells in question. If the voltage setting is properly chosen, then the poisons deposited on the electrocatalyst within the fuel cells are oxidized and the cell is rejuvenated. 
     On a theoretical level, the system works by controlling the flow of current through either the variable voltage source, the cell in question, or both. If the voltage source is shunted in parallel to the cell, the voltage setting (V v ) for the voltage source is crucial. If the voltage source voltage is equal to the cell voltage (V c ), then no current will flow through the voltage source. If the voltage source voltage is equal to the open circuit voltage of the cell, then no current will flow through the cell and all of the current will flow through the variable voltage source. In general, if the voltage source voltage is less than the voltage of the cell, then current through the cell will be increased with some current flowing backward through the variable voltage source. Therefore, if the voltage source is shunted in parallel to the cell in question, then cell is rejuvenated by the oxidation of the electrocatalyst poisons already deposited. 
     The following steps are thus taken by the system in rejuvenating one or more cells in a stack (as illustrated in FIG.  3 ): 
     a) choose the cells (step  110 ) 
     b) determine the cells voltage (step  120 ) 
     c) determine if the cells voltage meets a threshold (step  130 ) 
     d) if the cells voltage meets a threshold, then return to step a) to choose other cells (step  140 ) 
     e) if the cells voltage does not meet a threshold, then the cells require rejuvenation and the following steps are executed (step  130 ): 
     e1) determine a voltage setting for the variable voltage source (step  150 ) 
     e2) momentarily shunt the variable voltage source in parallel with the cells being rejuvenated (step  160 ) 
     e3) determine if the cells voltage meets the threshold (step  180 ) 
     e4) if the cells voltage meets the threshold, then the cells are rejuvenated and other cells need to be chosen (step  180 ) 
     e5) if the cells voltage does not meet the threshold then steps e2) to e5) are repeated. 
     It should be noted that the shunting of the variable voltage source is momentary. This is done to be able to properly control the increments by which the cell in question is being rejuvenated. It should also be noted that different voltage/current pulses could be used in the rejuvenation process by merely controlling the opening and closing of the switches  101 A and  100 B and by controlling the voltage setting of the voltage source. There is, in fact, some evidence that a short duration reversal of the cell potential is beneficial to the cell. This can be implemented again by merely controlling the voltage setting of the variable voltage source. 
     Concerning the threshold value, this value will have to be determined for the specific type of fuel cell and the amount of cell health required by the user. 
     The programming and control of the system is, as noted above, contained in the controller. Thus, by properly programming the controller, periodic tests of the cell or cells can be carried out automatically. Any cells which are lagging in performance to neighbours can be rejuvenated to an acceptable level. This enhances the performance of the stack as a whole. 
     A further benefit of the above system is that the system can insure against a cell failure causing stack failure. If a specific cell fails, the variable voltage source can be used as an interim or permanent replacement to the failed cell. By shunting the voltage source across a failed cell and setting the voltage of the variable voltage source to be that of a healthy cell, the stack will continue to work at a proper level. Catastrophic stack failure is therefore averted, allowing qualified personnel to arrive and replace the failed cell or the stack as a whole. 
     The above benefit is more advantageous than previous attempts at shunting current around defective fuel cells. U.S. Pat. No. 6,096,449 describes using a diode to shunt the current around a defective cell. This use of a diode and the lack of a replacement or supplement power source causes the stack to not only lose the power of the defective cell but also the voltage required to turn on the diode. For a small stack, this power loss can be a significant portion of the amount of power the stack can deliver. With the present invention, there is no voltage change as the variable power source acts as the replacement or supplement to the defective cell. 
     As a further benefit, the variable voltage source need not only replace the defective cell. If a weak cell is incapable of being rejuvenated, perhaps because of age of some other factor, the variable voltage sourced can supplement the power from the weak cell. This can be done by, again, controlling the voltage setting of the variable voltage source prior to shunting it in parallel with the weak cell. 
     Regarding implementation, the switch bank can be either solid state or regular switches. The controller is ideally a programmable microcontroller drawing a minimum of power while the voltage sensing device can be a voltmeter or other device which can measure voltage and transmit its readings to the controller. The variable voltage source can be implemented by using a battery with suitable circuitry to control its output voltage. 
     As an alternative, the variable voltage source can be powered by the fuel cell itself. The fuel cell output can be fed into a DC—DC converter at with the converter&#39;s output being used to power the variable voltage source. 
     For optimum performance, the system should continuously monitor the temperature of each cell along with the output voltage. This continuous monitoring would be advantageous as it would serve as a warning if any cell were about to malfunction. Ideally, the controller would keep a track history of not only the temperature but the output voltage of each cell as well. Then, based on these measurements, the health of each cell can be determined and whichever cell has the lowest health based on a user&#39;s programmed criteria, can be given the proper attention by the system. If a cell, based on its readings, were considered to be unhealthy, the system would then switch to that cell and perform the rejuvenation process on that cell. This whole process including continuous monitoring, selection of a particular cell, and rejuvenation of cells, is performed while the stack continues to service its load. 
     It should be noted that to monitor the temperature of a cell, a suitable means must be used. This can take the form of a temperature sensor attached to each cell with the sensor being coupled to a temperature determining device from which the controller receives temperature readings. 
     For continuous voltage readings of a cell&#39;s output voltage, the system can periodically switch to one or more cells to perform voltage measurements. Given the ease at which voltage readings can be made, this sampling of cells voltage can be carried out as frequently as required by a user. 
     A person understanding the above-described invention may now conceive of alternative designs, using the, principles described herein. All such designs which fall within the scope of the claims appended hereto are considered to be part of the present invention.