The invention generally relates to using a mixture of reducing agent and an oxidizing agent at a given electrode to generate an internal reaction, called “chemical shorting herein,” to control electrode corrosion during the startup or shutdown of a fuel cell.
A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. There are many different types of fuel cells, such as a solid oxide fuel cell (SOFC), a molten carbonate fuel cell, a phosphoric acid fuel cell, a methanol fuel cell, an ethanol fuel cell and a proton exchange membrane (PEM) fuel cell.
As a more specific example, a PEM fuel cell includes a PEM membrane, which permits only protons to pass between an anode and a cathode of the fuel cell. A typical PEM fuel cell may employ polysulfonic-acid-based ionomers and operate in the 50° Celsius (C.) to 75° temperature range. Another type of PEM fuel cell may employ a phosphoric-acid-based polybenzimidazole (PBI) membrane that operates in the 150° to 200° C. temperature range.
At the anode of the PEM fuel cell, diatomic hydrogen (a fuel) ionizes to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:
H2→2H++2e− at the anode of the cell, and Equation 1
O2+4H++4e−→2H2O at the cathode of the cell. Equation 2
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Catalyzed electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM.
In an embodiment of the invention, a technique that is usable with a fuel cell includes providing a fuel and oxidant mixture to a reactant chamber of the fuel cell to regulate an electrode potential of the fuel cell during startup or shutdown of the fuel cell.
In another embodiment of the invention, a fuel cell system includes a fuel cell stack and a control subsystem. The control subsystem provides a fuel and oxidant mixture to a reactant chamber of the fuel cell stack to regulate electrode potentials of fuel cells of the stack during startup or shutdown.
Advantages and other features of the invention will become apparent from the following drawing, description and claims.
Referring to
The fuel cell system 10 and load 50 may be mobile, such as in embodiments of the invention where the load 50 is the power plant of a vehicle that incorporates the system 10. Thus, the fuel cell system 10 may be part of an automobile, truck, forklift, airplane, etc., in accordance with some embodiments of the invention. It is noted, however, that the fuel cell system 10 and load 50 may be stationary, such as embodiments of the invention in which the load 50 is a telecommunication system or a residential load. The fuel cell system 10 may, in accordance with other embodiments of the invention, be part of a portable, handheld electronic device, such as a mobile computer or cellular telephone (as non-limiting examples). More specifically, the fuel cell system 10 may, for example, be part of a charging device of the computer or cellular telephone. Thus, many variations and applications of the fuel cell system 10 beyond those listed herein are contemplated and are within the scope of the appended claims.
The fuel cell stack 20 contains flow plates that provide a structure for communicating fuel and oxidant flows to membrane electrode assemblies (MEAs) of the fuel cells. More specifically, flow plates of the fuel cell stack 20 contain flow channels, which communicate a fuel flow and collectively form the anode chamber of the stack 20; and flow plates of the fuel cell stack 20 contain flow channels, which communicate an oxidant-containing air flow and collectively form a cathode chamber of the stack 20.
When a valve 18 (a solenoid valve, for example) is open, the anode inlet of the fuel cell stack 20 receives an incoming fuel flow that is provided by a fuel source 16 (a reformer, a hydrogen tank, cartridge of direct methanol, ethanol, etc.). The incoming fuel flow is routed through the flow channels of the anode chamber and produces an anode exhaust flow that exits the fuel cell stack 20 at the stack's anode exhaust outlet. The cathode inlet of the fuel cell stack 20 receives an incoming air flow from an air source 12 (an air blower, for example). The incoming air flow is routed through the flow channels of the cathode chamber and produces a cathode exhaust flow that exits the fuel cell stack 20 at the stack's cathode exhaust outlet. During the normal, steady state operation of the fuel cell stack 20, fuel and oxidant are consumed from the fuel and air flows in electrochemical reactions, which are described in Eqs. 1 and 2 above. These electrochemical reactions, in turn, produce electrical power that are converted into the appropriate form (i.e., into the appropriate DC level, AC level, etc.) by a power conditioning subsystem 48 of the fuel cell system 10 before being provided to the load 50.
For the exemplary fuel cell system 10 that is depicted in
Not all of the anode exhaust may be re-circulated back to the anode inlet. For example, as depicted in
Inert gases may accumulate in the anode chamber over time. If allowed to reach a significant concentration, the inert gases may adversely affect the performance of the fuel cell stack 20. Therefore, in accordance with some embodiments of the invention, the fuel cell system 10 includes a purge flow path that is intermittently opened to purge the inert gases from the anode chamber. As shown in
As depicted in
In general, the controller 40 controls the overall operation of the fuel cell system 10, including the system's operation during the startup of the fuel cell stack 20; the system's normal, or steady state operation, outside of the startup and shutdown operations; and the system's operation during shutdown of the fuel cell stack 20. Furthermore, as described herein, the controller 40 is part of a control subsystem that controls the fuel cell system 10 to inhibit carbon corrosion during the fuel cell stack's startup and shutdown.
Among its other features, the fuel cell system 10 may include a coolant subsystem (not shown in
For purposes of promoting the electrochemical reactions inside the fuel cell stack 20, each membrane electrode assembly of the fuel cell stack 20 has a platinum catalyst that is supported by high surface area carbon. Although during the normal steady state operation of the fuel cell stack 20, the carbon remains in its ideal solid phase, the dynamics associated with the startup and shutdown of the fuel cell stack 20 may undesirably promote carbon corrosion. Therefore, if not for the features of the fuel cell system 10 that are described herein, these dynamics cause the membrane electrode assembly carbon to change from its solid phase to a gaseous phase (i.e., the dynamics tend to convert the carbon into carbon dioxide (CO2)). In addition, the platinum can also oxidize to Pt2+ and erode out of usable active area. The result of this conversion is that the electrode and the catalyst support of the membrane electrode assemblies may, if not for the features of the fuel cell system 10 described herein, significantly corrode to a degree that causes failure of the fuel cell stack 20.
The above-described carbon corrosion on the cathode side during the startup and shutdown of the fuel cell stack 20 is the result of the front of fuel and oxidant in the stack's anode chamber. The air and fuel front may be present in the anode chamber due to one of the following scenarios. At the beginning of the startup of the fuel cell stack 20, air is generally present in the anode chamber. A fuel-air front develops inside the anode chamber when the fuel is initially communicated to the anode chamber at the beginning of the startup phase. Although the incoming fuel displaces the air, the fuel and oxidant react at the fuel-air front and more specifically, react at the platinum sites at the stack's anode electrodes. The potentials generated at the electrode interfaces provide energy to sustain these reactions at the anode electrodes, and thus, the potentials act as a power supply to produce corresponding reactions at the stack's cathode electrodes. These cathode reactions convert the carbon support at the cathode electrodes into gas (carbon dioxide, for example). A similar effect occurs during the shutdown of the fuel cell stack 20, as a fuel-air boundary forms in the anode chamber after the stack's shutdown due to air leaking/diffusing into the anode chamber. In some cases even at steady state low power operation, recent studies have demonstrated carbon corrosion on the cathode electrode, due to continuous flux of oxygen from the cathode side, but constrained flow of hydrogen to the corresponding catalyst site through the channels and the gas diffusion layer. This in turn, produces the same effect as a front and carbon corrodes at a constant rate, if the blockage at the anode chamber is not removed.
The degree to which carbon corrosion occurs during the stack's startup and shutdown cycle directly impacts the lifetime of the fuel cell stack 20. It is noted that the expected lifetime of the fuel cell stack 20 may need to be relatively high, depending on the stack's application. For example, for embodiments of the invention in which the fuel cell system 10 is part of the power plant for a vehicle, the vehicle (and thus, the fuel cell stack 20) may be started up and shut down several times each day, which may translate into over 18,250 cold (5 cycles per day) startup and hot shutdown cycles during the expected lifetime of the fuel cell stack 20. The fuel cell stack 20 therefore cannot be subject to significant carbon corrosion during each startup and shutdown cycle.
One technique to inhibit carbon corrosion during the startup of a fuel cell stack involves minimizing the residence time in which the fuel-air boundary remains inside the stack's anode chamber. In this technique, a relatively large flow of fuel (a flow that is over ten times the fuel flow during normal operation, for example) is forced into the anode chamber to quickly expel the air from the chamber and minimize the residence time of the damaging front. Another technique that may be used in connection with the shutdown of the fuel cell stack involves a fuel takeover of both the anode and cathode chambers. In this technique, the fuel is introduced into both the cathode and anode chambers of the fuel cell stack when the stack is shut down, and a determination is made of the rate at which leakage occurs from the anode chamber. Thus, additional fuel is supplied to the anode chamber to accommodate the anode chamber leakage to prevent air from entering the anode chamber. Another similar technique to control carbon corrosion includes operating the fuel cell stack as an electrochemical pump when the fuel cell stack is shut down to fill the cathode chamber with fuel. Another technique to control carbon corrosion involves using an external circuit that is connected to the fuel cell stack to lower the electrochemical potentials of the fuel cells' cathode electrodes during the startup and shutdown of the stack.
Significant carbon corrosion may still occur using the above-described techniques, and these techniques may be relatively costly and/or complex to implement. In accordance with embodiments of the invention, which are described herein, the cathode electrodes of the fuel cell stack 20 are “chemically shorted” during the startup and shutdown of the stack 20. In other words, a technique in accordance with embodiments of the invention involves promoting a chemical reaction using a mixture of the fuel and oxidant (the same damaging constituents now not damaging due to the well-formed mixture prior to entering the electrode) inside the fuel cell stack 20 to limit the potential of the stack's cathode electrodes. The lowered potentials, in turn, inhibit the unfavorable reactions at the cathode electrodes, which otherwise corrode the carbon support. [0031 ] More specifically, in accordance with embodiments of the invention described herein, a fuel, such as hydrogen (as a non-limiting example), is introduced into the cathode chamber during the startup or shutdown of the fuel cell stack 20. As a specific example, the volumetric flow rate of the introduced hydrogen and/or an air flow that is mixed with the hydrogen is regulated to keep the fuel-air mixture below the lower flammability limit. In accordance with some embodiments of the invention, the volumetric flow rate of the introduced fuel is four percent or less of the volumetric flow rate of the air flow entering the cathode chamber. The fuel and oxidant (air) react inside the cathode chamber to produce currents that lower the potentials of the cathode electrodes. The chemical shorting inhibits the carbon corrosion by reducing the cathode electrode potentials during startup and shutdown of the fuel cell stack 20.
As a more specific example, in accordance with some embodiments of the invention, existing components of the fuel cell system 10 may be operated to route the fuel-air mixture to the cathode chamber during the startup and shutdown of the fuel cell stack 20. For example, in accordance with some embodiments of the invention, during the startup and shutdown of the fuel cell stack 20, the controller 40 opens the purge valve 34 to communicate a fuel bleed flow from the anode exhaust outlet to the cathode inlet. The flow rate of air into the cathode chamber is adjusted (by adjusting the speed of an air blower of the air source 12, for example) to keep the fuel-air mixture in the cathode chamber below the lower limit of flammability. It is noted that by using existing components of the fuel cell system 10, the cost/complexity of the carbon corrosion control is minimal, as essentially only the software of the controller 40 may be updated with instructions that are executed by the controller 40 for purposes of allowing the chemical shorting. It is noted that in accordance with other embodiments of the invention, however, the fuel cell system may contain components that are dedicated to establishing the flows for the chemical shorting.
Referring to
The controller 40 subsequently opens the purge valve 34 to communicate a fuel bleed flow to the cathode inlet path, pursuant to block 108. Thus, the reducing and oxidizing agents are mixed prior to entering the cathode chamber. It is noted that the incoming fuel pressure is maintained slightly higher than the cathode blower exit pressure to maintain the fuel flow to the cathode chamber.
The fuel and oxidant are “well mixed” before the corresponding mixture enters the cathode chamber. As can be appreciated by one of skill in the art, such parameters as flow densities and length and size of the conduit leading to the cathode inlet are selected for purposes of ensuring adequate mixing of the fuel and oxidant flows before the mixture enters the cathode chamber. Thus, a fuel and oxidant mixture, instead of a fuel-oxidant front, enters the cathode chamber.
Due to the fuel-air mixture being provided to the fuel cell stack 20, the chemical shorting begins to suppress the cathode potentials and thus, inhibit carbon corrosion.
After the purge valve 34 is opened to communicate the fuel bleed flow to the cathode inlet, the technique 100 includes waiting (block 110) until the cathode potential has been lowered. As specific examples, the waiting may involve using a timer to measure a predetermined delay (five seconds, as a non-limiting example) for the cathode potential to be lowered. As another example, the waiting in block 110 may be based on a cumulative cell voltage threshold that is measured by a cell voltage monitoring circuit (not shown). Thus, many variations are contemplated and are within the scope of the appended claims.
After the delay imposed by block 110, the controller 40 opens the valve 18, pursuant to block 116, to communicate fuel to the anode inlet of the fuel cell stack 20. The communication of the fuel through the anode chamber of the fuel cell stack 20 eventually displaces any air in the anode chamber and removes the fuel-air front. However, during the residence time in which the fuel-air boundary exists, the above-described chemical shorting lowers the potential of the cathode electrodes to inhibit carbon corrosion. After a sufficient time has passed for the fuel-air front to move through the anode chamber, the controller 40 closes the purge valve 34, pursuant to block 120 and proceeds towards steady state operation (block 121).
Referring to
More specifically, during the shutdown, electrical devices of the fuel cell system 10 (such as an air blower of the air source 12, for example) continue to operate. Therefore, energy may be drawn from a battery (not shown in
After the delay, the controller 40 adjusts (increases, for example) the speed of the air blower, pursuant to block 260, to establish the appropriate air flow to ensure that the fuel-air mixture does not exceed the lower flammability limit. The controller 40 then opens (block 262) the purge valve 34 to initiate the fuel bleed flow to the cathode inlet and thus, the chemical shorting begins. The chemical shorting proceeds until a determination is made (diamond 270) by the controller 40 whether the cell voltage is below a certain threshold (a fuel cell voltage between 0 and −0.2 volts, as a non-limiting example). It is noted that this determination may be made by examining an average fuel cell voltage of the stack, a selected fuel cell voltage, the average of a selected group of cell voltages, etc. The controller 40 may monitor the cell voltages through a cell voltage monitoring or scanning circuit, which is not depicted in
Referring also to
It is noted that factors other than the cathode potential affect the rate at which the carbon corrodes. More specifically, during the startup of the fuel cell stack 20, the stack 20 is relatively “cold” and during the shutdown of the stack 20, the stack 20 is relatively “hot.” Therefore, more energy is available to promote carbon corrosion during the stack's shutdown than during its startup. Thus, a given cathode electrode potential reduction may result in significantly less carbon corrosion in the shutdown of the fuel cell stack 20 than in the startup of the stack 20.
Many variations are contemplated and are within the scope of the appended claims. For example, many different fuel cell types and fuel cell system architectures may be used, in accordance with other embodiments of the invention. As another non-limiting example,
As shown in
Experiments were conducted using a miniature test stack of eight fuel cells using no carbon corrosion control, the chemical shorting described herein and conventional carbon corrosion techniques for purposes of evaluating the time the cathode electrode potential remained above 1.2 volts and 1.4 volts. The electrode potential was determined by the use of reference electrode and in our case, a Reversible Hydrogen electrode was attached to the study system. Such a Reversible Hydrogen Electrode may be incorporated into either fuel cell system 10 (
Referring to Table 1, for the scenario in which no carbon corrosion technique was used, the cathode potential remained above 1.2 volts for a mean of 12.073 seconds, and the cathode potential remained above 1.4 volts for a mean of 8.5 seconds. For a carbon corrosion control technique in which fuel was forced at a high rate through the anode chamber for purposes of rapidly removing the fuel-air boundary, the electrode potential remained above 1.2 volts for a mean of 1.460 seconds and remained above 1.4 volts for a mean of 0.880 seconds. These two techniques are to be contrasted to the chemical shorting technique for which the cathode electrode potential remained below 1.2 volts during the stack's startup period.
Table 1 also depicts the performance of the chemical shorting for the shutdown of the stack in comparison with hydrogen takeover. As shown, for the hydrogen takeover, the electrode potential remained above 1.2 volts for a mean of 13.355 seconds and remained above 1.4 volts for a mean of 10.094 seconds. For the chemical shorting technique, the cathode electrode potential remained above 1.2 volts for a mean of 7.44 seconds and remained above 1.4 volts for a mean of 4.289 seconds.
Graphs 350, 354, 358 and 360 in
As depicted in the graph 350 of
Other embodiments are within the scope of the following claims. For example, in other embodiments of the invention, the chemical shorting may be used to adjust the potential of the anode electrode during the startup and shutdown for purposes of inhibiting carbon corrosion. In this manner, during the startup/shutdown of the fuel cell stack, air may be introduced with fuel into the anode chamber while the volumetric air and fuel flow rates are regulated to comply with the lower limit of flammability. Thus, many variations are contemplated and are within the scope of the appended claims.
There may be a number of side benefits to using the chemical shorting that is described herein. For example, in accordance with some embodiments of the invention, approximately 1200 Watts (W) of thermal energy may be available due to the fuel-oxidant mixture reaction in the cathode chamber at the low fuel flow condition. Significantly higher thermal energy can be obtained with higher fuel flow condition. Therefore, this thermal energy may be used for purposes of quickly warming up the fuel cell stack from a freezing environment and may be used for rapidly thawing the fuel cell stack, in accordance with some embodiments of the invention.
Additionally, the chemical shorting may be used for purposes of humidifying the fuel cells. In this regard, in accordance with some embodiments of the invention, a water flow rate of 0.13 grams per second is available for fuel cell humidification for purposes of humidifying the fuel cell membranes at relatively low flow rates of fuel and oxidant mixtures.
The chemical shorting may also be used for purposes of suppressing the electrochemical potential of the cathode of the fuel cell stack, when needed in special cases, apart from starting and shutting down. As an example, the chemical shorting has the capability to suppress the cathode electrode potentials without electrical current in the external circuit, which makes the chemical shorting technique useful for purposes of a dormancy startup protocol. More specifically, the chemical shorting may be used for purposes of mitigating dormancy related performance losses, by removing any adsorbed species on the platinum electrode. Since it is required to suppress the electrochemical potential of the cathode electrode to clean the electrode surface, the typical method includes generating electrical current and dumping load to the external circuit, namely customer site, and running at high power to reduce the cathode potential. In this regard, typically, the fuel cell stack may be expected to deliver power to a load. However, if the power that is demanded by the external load, namely the customer site, is lower than the power that is required to enable reduction in cathode electrochemical potential to clean the catalyst surface, there may be no other place to dump energy. This creates high reliance on the customer site load and constrains any performance recovery that can be obtained on a timely basis. However the electrochemical potential suppression caused by the chemical shorting only causes heat which can be dumped into the thermal management system. So the chemical shorting gives independence from the customer site bus when needing to recover performance of a fuel cell after a prolonged dormancy. This degree of freedom enables embedded timer based controls that will ascertain the chemical shorting for a pre-set duration after start-up to recover performance losses of the fuel cell stack. During steady state power generation, the purge valve 34 can be kept open to reduce the cathode potential to guarantee corrosion-free operation at low power operation. Many other benefits, advantages and/or other uses of the chemical shorting are contemplated and are within the scope of the appended claims
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.