[0001] 1. Field of the Invention
[0002] The present invention relates to fuel cells and their operation.
[0003] 2. Description of the Related Art
[0004] A fuel cell generates electric power through reaction of a fuel gas fed to a fuel electrode and an oxygen-containing gas fed to an oxygen electrode. As the fuel gas, hydrogen supplied from a hydrogen cylinder or a reformed gas obtained by reforming a city gas to enrich the hydrogen content are used. As the oxygen-containing gas, air is generally fed with a compressor or a blower. An electrode of a fuel cell is generally made of an electroconductive carbon having a noble metal carried on the surface thereof. In a fuel cell using a polyelectrolyte, a fuel gas containing hydrogen electrochemically reacts with an oxidizing agent gas such as oxygen-containing air, thereby simultaneously generating an electric power and heat.
[0005] A catalyst used on the electrode of the fuel cell is gradually oxidized on the surface thereof upon being exposed to an oxidative atmosphere, and adsorbs contaminants in the air and contaminants leaked from the apparatus on the surface of the catalyst. The reaction efficiency of the catalyst is lowered thereby, and thus the generated voltage is lowered with the lapse of time. In order to solve the problem, it has been proposed that in the shutdown period of the fuel cell, an inert gas, such as a nitrogen gas, is charged to prevent oxidation of the electrodes, and the fed gas is fed through a filter to decrease the amount of contaminants in the gas. However, these measures cannot restore the voltage having been once lowered although the lowering of the generated voltage can be suppressed to prolong the service life. Furthermore, a fuel cell has such a nature that the generated voltage thereof is eventually lowered despite of the effect of prolonging the service life.
[0006] In the case where the gas is fed through a filter, it is necessary to exchange the filter on a regular schedule to cause such a problem of consuming labor and cost for exchanging the filter. Furthermore, additional energy is necessary in the compressor or the blower corresponding to the pressure loss of the filter.
[0007] In the case where the cathode is at a high potential, due to the event that the fuel cell holds a very high voltage exceeding 0.9 V, which is close to the open circuit state, it is already understood that problems such as elution of the Pt catalyst of the cathode and reduction in reaction area of the Pt catalyst due to sintering (enlargement of Pt particles) occur.
[0008] Similarly, in the case where the fuel cell holds a very high voltage, which is close to the open circuit state, there occurs a problem that the polyelectrolyte decomposes. It is considered that such problems are caused by the following reasons.
[0009] An open circuit voltage of the fuel cell utilizing hydrogen and oxygen as reaction seeds is theoretically considered to be 1.23 V. However, the actual open circuit voltage is a mixed potential of impurities in the respective electrodes, i.e., an anode and a cathode, and adsorption seeds and is from about 0.93 V to 1.1 V. Also, the open circuit voltage is lowered from the theoretical value due to the event that hydrogen and oxygen are slightly diffused in the polyelectrolyte membrane. Assuming that no dissolution of impurities such as radical metal seeds occurs, the potential of the anode is greatly influenced by the adsorption seeds of the cathode and is considered to become a mixed potential of chemical reactions expressed by the following reaction equations 1 to 5 as described in H. Wroblowa, et al.,
Reaction equation 1 1.23 V O Reaction equation 2 1.11 V PtO Reaction equation 3 0.98 V Pt(OH) Reaction equation 4 0.88 V PtO + 2H Reaction equation 5 0.68 V O
[0010] In order to avoid the foregoing problems caused by the event that the fuel cell becomes in the state of open circuit voltage, there have hitherto been proposed some operation methods of fuel cell system.
[0011] For example, there is proposed an operation method of a fuel cell system in which an electric power consumption measure for consuming an electric power is provided within the fuel cell system individually from an external load, and the fuel cell and the electric power consumption measure are connected during a period of time until the fuel cell and the external load are connected after the fuel cell starts power generation, whereby the electric power formed in the fuel cell is consumed by the electric power consumption measure, thereby avoiding that the fuel cell approaches the state of open circuit voltage (for example, see JP-A-5-251101).
[0012] Also, there is proposed an operation method of a fuel cell system in which a discharge measure for suppressing an open circuit voltage is provided within the fuel cell system, thereby avoiding the event that the fuel cell becomes in the state of open circuit voltage (for example, see JP-A-8-222258).
[0013] According to these operation methods of fuel cell systems, it is possible to avoid the foregoing elution of the Pt catalyst of the cathode and reduction in reaction area of the catalyst due to sintering. Also, it is possible to avoid the event that the polyelectrolyte is decomposed due to the formation of radicals.
[0014] However, in the case of the foregoing operation method of a fuel cell system by purging with an inert gas such as nitrogen, there arises a problem that not only a gas cylinder of the inert gas is necessary, leading to enlargement in size of the fuel cell system, but also the maintenance such as exchange of the gas cylinder is expensive, leading to an increase in costs.
[0015] Also, in the foregoing operation method of a fuel cell system by purging with water or a moistened inert gas, since the temperature of the fuel cell is lowered at the time of stopping the power generation of the fuel cell, dew condensation occurs inside the fuel cell, and the volume is reduced. Accordingly, since the inside of the fuel cell becomes in the state of negative pressure, there arises problems that oxygen enters from the outside and that the polyelectrolyte membrane breaks, leading to potential occurrence of a short circuit of the electrodes.
[0016] Also, in the case where the cell is subjected to power generation in the state of stopping feeding of the oxidizing agent gas to consume oxygen of the cathode, and the anode is then purged with an inert gas, there is a problem in that since the Pt catalyst of the cathode is oxidized with oxygen remaining without being consumed and air incorporated due to diffusion and leakage, the cathode is degraded. Moreover, there arises a problem that since oxygen is forcedly consumed by power generation, the potential of the cathode is not uniform, and the activation state of the cathode varies every time when the power generation of the fuel cell is stopped, leading to scattering of the cell voltage at the time of start.
[0017] In addition, in the case of the foregoing operation method of a fuel cell system by avoiding the matter that the fuel cell becomes in the state of open circuit voltage, the fuel cell is always in the state of power generation. However, in the case of a fuel cell system for household use using a raw material gas such as town gas containing methane as the major component, in order to suppress the heating and lighting expenses, it is desired to control the motions of the fuel cell so as to stop the power generation in a time period where the electric consumption is small and carry out the power generation in a time period where the electric consumption is large. For example, according to the DSS (Daily Start-up & Shut-down) wherein the power generation is carried out in the daytime and is stopped in the middle of the night, it is possible to avoid an increase in the heating and lighting expenses. Therefore, it is desired to control the fuel cell so as to repeat the power generation state and the non-power generation state, and an operation method of a fuel cell system capable of avoiding the matter that the fuel cell approaches open circuit voltage even when repeat the power generation state and the non-power generation state, is desirable.
[0018] All references, patents and priority documents, particularly Japanese Patent Application 2002-317794 filed Oct. 31, 2002, referred to herein, are hereby incorporated by reference for the entirety of their disclosure for all purposes.
[0019] The invention solves the aforementioned problems associated with the prior art, and an object thereof is to provide a method for operating a fuel cell for maintaining a high generated voltage for a long period of time by carrying out a restoring operation for restoring the generated voltage upon decreasing the generated voltage of the fuel cell.
[0020] One aspect of the invention relates to a method for operating a fuel cell comprising an electrolyte, an anode and a cathode sandwiching the electrolyte, and one pair of separator plates each having a gas flow path for feeding and discharging a fuel gas to the anode and for feeding and discharging an oxygen-containing gas to the cathode, the method comprising a step of carrying out a restoring operation by decreasing a voltage of the cathode, upon decreasing a voltage of the fuel cell to a threshold voltage or lower, or upon lapsing a prescribed period of time from a preceding restoring operation.
[0021] Another aspect of the invention relates to a method for operating a fuel cell comprising a plurality of cells each containing an electrolyte, an anode and a cathode sandwiching the electrolyte, and one pair of separator plates each having a gas flow path for feeding and discharging a fuel gas to the anode and for feeding and discharging an oxygen-containing gas to the cathode, the method comprising steps of carrying out a restoring operation by decreasing a voltage of the cathode of at least one of the plurality of cells, and after restoring a voltage of the cells, sequentially carrying out a restoring operation for the remaining cells.
[0022] Another aspect of the invention relates to a fuel cell system comprising a stack of cells each containing an electrolyte, an anode and a cathode sandwiching the electrolyte, and one pair of separator plates each having a gas flow path for feeding and discharging a fuel gas to the anode and for feeding and discharging an oxygen-containing gas to the cathode, the fuel cell system further comprising a voltage detecting device for detecting a voltage of the cells or the stack, and a controlling device for controlling the feed of the oxygen-containing gas to the cells or the stack based on the voltage detected by the voltage detecting device.
[0023] Another aspect of the invention relates to a fuel cell system comprising a stack of cells each containing an electrolyte, an anode and a cathode sandwiching the electrolyte, and one pair of separator plates each having a gas flow path for feeding and discharging a fuel gas to the anode and for feeding and discharging an oxygen-containing gas to the cathode, the fuel cell system further comprising a voltage detecting device for detecting a voltage of the cells or the stack, a feeding means for feeding water to the cells or the stack, and a controlling device for controlling the feeding means based on the voltage detected by the voltage detecting device.
[0024] Another aspect of the invention relates to a fuel cell system comprising a stack of cells each containing an electrolyte, one pair of electrodes sandwiching the electrolyte, and one pair of separator plates each having a gas flow path for feeding and discharging a fuel gas to the anode and for feeding and discharging an oxygen-containing gas to the cathode, the fuel cell system further comprising a voltage detecting device for detecting a voltage of the cells or the stack, a feeding means for feeding an inert gas, a hydrocarbon gas or a reducing agent to the cells or the stack instead of the oxygen-containing gas, and a controlling device for controlling the feeding means based on the voltage detected by the voltage detecting device.
[0025] Another aspect of the invention relates to a fuel cell system comprising a stack of cells each containing an electrolyte, an anode and a cathode sandwiching the electrolyte, and one pair of separator plates each having a gas flow path for feeding and discharging a fuel gas to the anode and for feeding and discharging an oxygen-containing gas to the cathode, the fuel cell system further comprising a voltage detecting device for detecting a voltage of the cells or the stack, an electric current adjusting device for increasing and decreasing an electric current applied to the cells or the stack, and a controlling device for controlling the electric current adjusting device based on the voltage detected by the voltage detecting device.
[0026] Another aspect of the invention relates to a method of operating a fuel cell system which is provided with a fuel cell having at least one cell provided with an electrolyte, an anode and a cathode each having a platinum based metallic catalyst, the electrolyte being put between the anode and the cathode, and a pair of separator plates having gas passages for feeding a fuel gas into the anode and feeding an oxidizing agent gas into the cathode, respectively, and which switches connection and disconnection between the fuel cell and a load, wherein; feeding of the oxidizing agent gas into the cathode and feeding of the fuel gas into the anode are continued until a prescribed period of time elapses after disconnection between the fuel cell and the load, and thereafter, feeding of each of the oxidizing agent gas and the fuel gas is stopped, thereby controlling the operation such that the time when the cell of the fuel cell has a voltage of about 0.9 V or more is less than about 10 minutes after either gas is stopped.
[0027] Another aspect of the invention relates to a method of operating a fuel cell system which is provided with a fuel cell having at least one cell provided with an electrolyte, an anode and a cathode each having a platinum based metallic catalyst, the electrolyte being put between the anode and the cathode, and a pair of separator plates having gas passages for feeding a fuel gas into the anode and feeding an oxidizing agent gas into the cathode, respectively and which switches connection and disconnection between the fuel cell and a load, wherein; before disconnection between the fuel cell and the load, at least one of feeding of the oxidizing agent gas into the cathode and feeding of the fuel gas into the anode is stopped, and thereafter, disconnection between the fuel cell and the load is carried out.
[0028] Another aspect of the invention relates to a fuel cell system which is provided with a fuel cell having at least one cell provided with an electrolyte, an anode and a cathode each having a platinum based metallic catalyst, the electrolyte being put between the anode and the cathode, and a pair of separator plates having gas passages for feeding a fuel gas into the anode and feeding an oxidizing agent gas into the cathode, respectively and a control unit of controlling switch of connection and disconnection between the fuel cell and a load, wherein; the control unit is constructed such that feeding of the oxidizing agent gas into the cathode and feeding of the fuel gas into the anode are continued until a prescribed period of time elapses after disconnection between the fuel cell and the load, and thereafter, feeding of each of the oxidizing agent gas and the fuel gas is stopped, thereby controlling the time when the cell of the fuel cell has a voltage of about 0.9 V or more to fall within about 10 minutes after either gas is stopped.
[0029] Another aspect of the invention relates to a fuel cell system which is provided with a fuel cell having at least one cell provided with an electrolyte, an anode and a cathode each having a platinum based metallic catalyst, the electrolyte being put between the anode and the cathode, and a pair of separator plates having gas passages for feeding a fuel gas into the anode and feeding an oxidizing agent gas into the cathode, respectively and a control unit of controlling switch of connection and disconnection between the fuel cell and a load, wherein the control unit is constructed such that before disconnection between the fuel cell and the load, at least one of feeding of the oxidizing agent gas into the cathode and feeding of the fuel gas into the anode is stopped, and thereafter, disconnection between the fuel cell and the load is carried out.
[0030] Another aspect of the invention relates to a method of operating a fuel cell system provided with a fuel cell having at least one cell provided with an electrolyte, an anode and a cathode each having a platinum based metallic catalyst, the electrolyte being put between the anode and the cathode, and a pair of separator plates having gas passages for feeding a fuel gas into the anode and feeding an oxidizing agent gas into the cathode, respectively, wherein; when the fuel cell stops power generation, the cathode is controlled so as to have a voltage with respect to a standard hydrogen electrode within the range of from about 0.6 V to about 0.8 V.
[0031] Another aspect of the invention relates to a fuel cell system provided with a fuel cell having at least one cell provided with an electrolyte, an anode and a cathode each having a platinum based metallic catalyst, the electrolyte being put between the anode and the cathode, and a pair of separator plates having gas passages for feeding a fuel gas into the anode and feeding an oxidizing agent gas into the cathode, respectively and a control unit for controlling feeding of the fuel gas into the anode and feeding of the oxidizing agent gas into the cathode, wherein an external electric source of applying a prescribed voltage between the anode and the cathode is provided; and the control unit is constructed such that when the fuel cell stops power generation, in the state of exposing the anode to the fuel gas, feeding of the oxidizing agent gas into the cathode is stopped, and the external electric source are controlled so as to apply a prescribed voltage between the anode and the cathode, thereby controlling the cathode so as to have a voltage against a standard hydrogen electrode within the range of from about 0.6 V to about 0.8 V.
[0032] One aspect of the present invention relates to a fuel cell system comprising at least one anode comprising a gas diffusion layer and a catalyst, the anode connected to a fuel gas control unit controlling a flow of an fuel gas; at least one cathode comprising a gas diffusion layer and a catalyst, the cathode connected to an oxidizing gas control unit controlling a flow of an oxidizing gas; an electrolyte membrane disposed between the anode and the cathode; a load; at least one cell voltage detection unit; at least one external electric source capable applying a current to control the voltage of the cathode; and a control unit receiving information from and/or capable of controlling the fuel gas control unit, oxidizing gas control unit, cell voltage detection unit, load and the external electric source.
[0033] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair, comprising; starting fuel cell power generation; and stopping fuel cell power generation, by stopping flow of an oxidizing gas; maintaining flow of a fuel gas to avoid degradation of the anode; applying current from an external voltage source to maintain a voltage between the anode and/or the cathode of about 0.6V to about 0.8V; and decreasing flow of a fuel gas.
[0034] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair, comprising; starting fuel cell power generation; stopping fuel cell power generation by stopping the flow of a fuel gas or an oxidizing gas to the cell, while applying a current from an external voltage source such that the voltage between the anode and cathode does not decrease to more than a threshold level and decreasing a flow of a fuel gas.
[0035] Another aspect of the invention relates to a fuel cell system comprising at least one anode comprising a gas diffusion layer and a catalyst, the anode connected to a fuel gas control unit controlling a flow of a fuel gas; at least one cathode comprising a gas diffusion layer and a catalyst, the cathode connected to an oxidizing gas control unit controlling a flow of an oxidizing gas; an electrolyte membrane disposed between the anode and the cathode; a load; at least one cell voltage detection unit; a purging gas control unit controlling a flow of a purging gas to purge the anode; and a control unit receiving information from and/or capable of controlling the fuel gas control unit, oxidizing gas control unit, cell voltage detection unit, load, and the purging gas control unit.
[0036] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair comprising; starting fuel cell power generation; disconnection of a load; stopping fuel cell power generation, by; stopping a flow of an oxidizing gas and a flow of a fuel gas after a prescribed period of time following the disconnection of the load; and purging the anode with a flow of a purging gas after the prescribed period of time has elapsed.
[0037] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair comprising starting fuel cell power generation; disconnection of a load; stopping fuel cell power generation, by; gradually reducing a flow of an oxidizing gas after a prescribed period of time following the disconnection of the load, until the flow of the oxidizing gas has stopped; gradually reducing a flow of a fuel gas after the stopping the flow of oxidizing gas; and purging the anode with a flow of a purging gas after the flow of the fuel gas has stopped.
[0038] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair comprising starting fuel cell power generation; disconnection of a load; stopping fuel cell power generation, by; gradually reducing a flow of a fuel gas after a prescribed period of time following the disconnection of the load, until the flow of the fuel gas has stopped; gradually reducing a flow of a oxidizing gas after the stopping the flow of fuel gas; and purging the anode with a flow of a purging gas after the flow of the fuel gas has stopped.
[0039] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair comprising starting fuel cell power generation; stopping fuel cell power generation, by decreasing flow of an oxidizing gas and decreasing flow of an fuel gas a prescribed period of time before disconnection of a load; purging the anode with a flow of a purging gas after the flow of the fuel gas has stopped.
[0040] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair comprising starting fuel cell power generation; stopping fuel cell power generation, by; decreasing flow of an oxidizing gas a prescribed period of time before disconnection of a load; disconnecting the load; decreasing a flow of a fuel gas; and purging the anode with a flow of a purging gas after the flow of the fuel gas has stopped.
[0041] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair comprising starting fuel cell power generation; stopping fuel cell power generation, by; continuing a flow of an oxidizing gas for a prescribed first period of time following a disconnection of a load; then decreasing the flow of the oxidizing gas; decreasing a flow of a fuel gas a second prescribed period of time prior to the disconnection of the load; and purging the anode with a flow of purging gas after the flow of the fuel gas has stopped.
[0042] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair comprising starting fuel cell power generation; stopping fuel cell power generation, by continuing a flow of an oxidizing gas for a prescribed first period of time following a disconnection of a load; then decreasing the flow of the oxidizing gas; decreasing a flow of a fuel gas a second prescribed period of time prior to the disconnection of the load; and purging the anode with a flow of purging gas after the flow of the fuel gas has stopped.
[0043] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair; starting fuel cell power generation; stopping fuel cell power generation by; disconnecting a load; decreasing a flow of an oxidizing gas; applying a current from an external voltage source to maintain a voltage between the anode and the cathode; increasing the fuel cell temperature; and restarting fuel cell power generation by increasing a flow of an oxidizing gas and removing the current.
[0044] Another aspect of the invention relates to a fuel cell system comprising at least one anode comprising a gas diffusion layer and a catalyst, the anode connected to a fuel gas control unit controlling a flow of a fuel gas; at least one cathode comprising a gas diffusion layer and a catalyst, the cathode connected to an oxidizing gas control unit controlling a flow of an oxidizing gas; an electrolyte membrane disposed between the anode and the cathode; a load; at least one cell voltage detection unit; at least one temperature sensing unit; at least one external electric source capable applying a current to control the voltage of the cathode; and a control unit receiving information from and/or capable of controlling the fuel gas control unit, oxidizing gas control unit, cell voltage detection unit, temperature sensing unit, load and the external electric source.
[0045] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair comprising starting fuel cell power generation; stopping fuel cell power generation, by decreasing a flow of an oxidizing gas; applying an external current from an external electric source capable control the voltage of the cathode; determining a temperature of the pair; and decreasing a flow of a fuel gas and purging the pair with air if the temperature of the pair is falls below a threshold temperature.
[0046] Another aspect of the invention relates to a method of operating a fuel cell system comprising at least one anode and cathode pair comprising starting fuel cell power generation; disconnecting a load; stopping fuel cell power generation, by decreasing a flow of an oxidizing gas; applying an external current from an external electric source capable control the voltage of the cathode; determining a temperature of the pair; and decreasing a flow of a fuel gas and purging the pair with air if the temperature of the pair is falls below a threshold temperature; increasing flow of oxidizing gas and the flow of fuel gas; and starting fuel cell power generation.
[0047] Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. The present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail, in order not to unnecessarily obscure the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.
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[0083] A fuel cell is constituted essentially with an electrolyte membrane and electrode disposed on both side thereof. The electrode for a fuel cell is constituted with a gas diffusion layer for feeding a reaction gas and a catalyst layer for actually effecting a chemical reaction. A noble metal catalyst carried on carbon is used as the catalyst layer.
[0084] A fuel cell generates electric power by reacting a fuel gas fed to a fuel electrode and an oxygen-containing gas fed to an oxygen electrode. As the oxygen-containing gas, air is generally fed with a compressor or a blower. However, air contains a nitrogen oxide and a sulfur oxide, which deteriorate the electric power generation reaction. Furthermore, members constituting the apparatus leak organic substances, such as a solvent.
[0085] These contaminants are gradually accumulated on the surface of the catalyst during the operation of the fuel cell and, as a result deteriorate the generated voltage. Most parts of the contaminants can be decomposed or removed by changing the electric potential on the surface of the catalyst. The accumulation of contaminants may occur on both the fuel electrode and the oxygen electrode, but the electric potential of the fuel electrode receives less influence of the accumulation of contaminants due to the small overvoltage thereof. Therefore, the deterioration of the generated voltage occurring on the operation of the fuel cell is mainly ascribed to the accumulation of contaminants on the oxygen electrode. Preferably, a noble metal, such as platinum, is used as the catalyst. Pt is generally hard to be oxidized, but because a fuel cell using a polymer electrolyte is in a strongly acidic atmosphere, the surface of the catalyst is oxidized in the case where the electric potential of the oxygen electrode is high in the fuel cell. The surface of platinum is oxidized in the case where the electric potential of the fuel electrode is 0.7 V or more with respect to the standard hydrogen electrode in a pH range of from 1 to 2. When the surface of the catalyst is oxidized, the rate of the redox reaction of oxygen is decreased, and thus the generated voltage is lowered. Furthermore, since the oxide has a large adsorption power to the contaminants, it promotes the accumulation of contaminants to accelerate lowering of the generated voltage.
[0086] In order to remove the accumulation of contaminants and the oxidation on the surface of the catalyst to restore the generated voltage, it is effective to carry out the restoring operation for lowering the electric potential of the oxygen electrode.
[0087] The term “voltage” as used herein and as commonly used in the art refers to the difference in electrical charge between two points in a circuit expressed in volts or the rate at which energy is drawn from a source that produces a flow of electricity in a circuit; expressed in volts. The terms electric potential, electromotive force, emf, potential, potential difference, potential drop are all used interchangeably herein.
[0088] An embodiment of the constitution of a fuel cell enabling the restoring operation according to the invention is described with reference to
[0089] A fuel cell
[0090] A fuel cell system that is suitable for carrying out the restoring operation will be described.
[0091] In a preferred embodiment, as described in the foregoing, the fuel cell system comprising a stack of cells has a voltage detecting means for detecting a voltage of the cells or the stack, and a controlling means for controlling the feed of the oxygen-containing gas to the cells or the stack based on the voltage detected by the voltage detecting means.
[0092] In another preferred embodiment, the fuel cell system has a voltage detecting means for detecting a voltage of the cells or the stack, a feeding means for feeding an inert gas, a hydrocarbon gas, a reducing agent or water instead of the oxygen-containing gas to the cells or the stack, and a controlling means for controlling the feeding means based on the voltage detected by the voltage detecting means.
[0093] In still another preferred embodiment, the fuel cell system has a voltage detecting means for detecting a voltage of the cells or the stack, an electric current adjusting means for increasing and decreasing an electric current applied to the cells or the stack, and a controlling means for controlling the electric current adjusting means based on the voltage detected by the voltage detecting means.
[0094]
[0095] While the controlling device in this embodiment controls a resistance value of a resistor, it is possible that a relay or a transistor is used instead of the resistor, and the voltage of the cell to be restored is forcedly lowered.
[0096]
[0097] In a fuel cell using a polymer electrolyte, the cell voltage upon normal operation under no load is about 0.95 V, and the cell voltage upon operation with a load is lowered to 0.8 to 0.6 V. The electric potential of the fuel electrode is substantially equal to the electric potential of the standard hydrogen electrode in the case where a hydrogen-containing gas is used as the fuel gas. Furthermore, the electric potential of the oxygen electrode (with respect to the fuel electrode) is substantially equal to the cell voltage owing to the low overvoltage of the fuel electrode. Accordingly, the electric potential of the oxygen electrode can be comprehended by detecting the cell voltage to find completion of the restoring operation. The threshold value of the cell voltage, which is an indication for carrying out the restoring operation of the invention, is preferably 95% of the aforementioned initial voltage. In the case where the threshold value is too high, it is complicated since the restoring operation should be frequently carried out. In the case where the threshold value is too low, on the other hand, there is such a possibility that the electric generation efficiency is lowered, and sufficient restoration cannot be attained.
[0098] The electric potential where the restoring operation is carried out may be less than 0.7 V (with respect to the fuel electrode) in the case where restoration is attained by reducing the oxidized and deteriorated catalyst. In particular, it is also effective that the fuel cell is electrically shorted out for several tens seconds. In the case where deterioration due to adsorbed contaminants is restored by reduction and desorption, it is preferably 0.4 V or less (with respect to the fuel electrode). Both the deterioration due to oxidation of the catalyst and the deterioration due to adsorption of contaminants can be resolved by setting the restoring electric potential at 0.4 V (with respect to the fuel electrode).
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[0100] On the other hand, reduction of oxidized Pt on the cathode reaches a peak at a potential in the vicinity of 0.7 V against SHE and similarly proceeds up to a potential in the vicinity of 0.5 V against SHE.
[0101] In the case of the usual stationary type, the operation voltage of the fuel cell is in the vicinity of 0.7 to 0.75 V. Here, in the case where the fuel cell carries out power generation, the potential of the anode is closed to that of SHE, and hence, the potential of the cathode is approximately equal to the operation voltage of the fuel cell. Accordingly, as is also clear by referring to the cyclic voltammogram of
[0102] Whe the current is stopped while feeding a fuel gas and an oxidizing agent gas into the fuel cell, the potential of the cathode rises to about 1 V, and oxidation proceeds inside Pt, whereby the catalytic activity is lowered. On the other hand, for the sake of recovering the activity of the oxidized Pt catalyst, when the potential of the cathode is held at a low potential, the surface of Pt is reduced, whereby the catalytic activity is recovered.
[0103] However, as described previously, when oxidation and reduction are repeated on the surface of Pt, fluctuation occurs on the surface of Pt, and expansion and contraction of the surface, rearrangement of atoms, and the like occur. As a result, the catalytic activity is gradually lowered.
[0104] Accordingly, even in the case where the fuel cell repeats the power generation state and the non-power generation state, it is necessary to prevent repetition of oxidation and reduction on the surface of Pt of the cathode.
[0105] The restoring operation may be carried out simultaneously to all the cells constituting a stack, or in alternative, it may be carried out for the respective cells one-by-one or for a part of the cells, and then sequentially carried out for other cells. In the case where the restoring operation is carried out simultaneously for all the cells, detection of the voltage of the entire stack can be substituted for the detection of the cell voltage. In the case where the restoring operation is carried out for the respective cells one-by-one, such an advantage can be obtained that the restoring operation can be carried out more certainly, although a complicated constitution of the stack is required for detecting the voltages of the respective cells.
[0106] Examples of the restoring operation include the following methods. (1) Electric power generation is carried out under such a state that the feeding amount of oxygen is lowered to consume oxygen, (2) a hydrocarbon gas, an inert gas or water is fed for replacing oxygen, (3) a reducing agent is fed, and (4) the load of the fuel cell is increased. These methods will be described in more detail below.
[0107] In a preferred embodiment of the invention, the restoring operation contains such an operation that electric power generation is continued while the feeding amount of the oxygen-containing gas on the oxygen electrode is decreased, and after lowering the cell voltage to the restoring voltage of the oxygen electrode (with respect to the fuel electrode), the feeding amount of the oxygen-containing gas is then increased.
[0108] In another preferred embodiment, the restoring operation contains such an operation that the electric power generation is continued while the feed of the oxygen-containing gas is terminated, and after lowering the cell voltage to the restoring voltage of the oxygen electrode (with respect to the fuel electrode), feed of the oxygen-containing gas is restarted.
[0109] In still another preferred embodiment, the restoring operation contains such an operation that an inert gas or a hydrocarbon gas is fed to the oxygen electrode, and after lowering the cell voltage to the restoring voltage of the oxygen electrode (with respect to the fuel electrode), feed of the oxygen-containing gas is restarted.
[0110] In a further preferred embodiment, the restoring operation contains such an operation that water is fed to the oxygen electrode, and after lowering the cell voltage to the restoring voltage of the oxygen electrode (with respect to the fuel electrode), feed of water is terminated. During the restoring operation of this method, feed of the oxygen-containing gas may be continued.
[0111] In a still further preferred embodiment, the restoring operation contains such an operation that an inert gas, a hydrocarbon gas or a reducing agent is fed to the oxygen electrode instead of the oxygen-containing gas, i.e., feed of the oxygen-containing gas is terminated, and after lowering the cell voltage to the restoring voltage of the oxygen electrode (with respect to the fuel electrode), feed of the oxygen-containing gas is restarted.
[0112] In a still further preferred embodiment, the restoring operation contains such an operation that the load of the fuel cell is increased, and after lowering the cell voltage to the restoring voltage of the oxygen electrode (with respect to the fuel electrode), the load is decreased.
[0113] In the method of the restoring operation by decreasing the feeding amount of oxygen, oxygen deficit occurs in a logical sense in the case where the utilization ratio of oxygen, i.e., four times the number of electrons flowing in the cell per the number of oxygen molecules fed to the cell, exceeds 100%, whereby the potential of the oxygen electrode is lowered. However, in an actual situation, even in the case where the utilization ratio is less than 100%, the potential of the oxygen electrode is lowered due to nonuniformity of gas feed and inhibition of gas diffusion to enable the restoring operation. The utilization ratio enabling the restoring operation is typically 70% or more while it varies depending on the constitution of the gas flow path and the constitution of the gas diffusion layer. The utilization ratio can be increased by decreasing the feeding amount of oxygen, and the same effect can be obtained by increasing the load to increase the electric current flowing in the cell. In the case where the utilization ratio of oxygen is increased by increasing the electric current, the feeding amount of hydrogen should be increased in an amount corresponding to the electric current, so as to prevent the utilization ratio of hydrogen from being increased.
[0114] In the method of the restoring operation by feeding a hydrocarbon gas, an inert gas or water for replacing oxygen, the oxygen partial pressure is decreased to lower the electric potential of the oxygen electrode.
[0115] Examples of the hydrocarbon gas include a city gas desulfurized with a desulfurizer, a propane gas and a butane gas.
[0116] Examples of the inert gas include nitrogen, argon and carbon dioxide.
[0117] Water used herein may be in a vapor state or in a liquid state.
[0118] In the method of the restoring operation by feeding a reducing agent, the oxygen partial pressure is lowered by reacting oxygen with the reducing agent to lower the electric potential of the oxygen electrode. Furthermore, the deteriorated catalyst is reduced with the reducing agent to decompose contaminants. Examples of the reducing agent include a hydrogen gas, a sodium borohydride aqueous solution and hydrazine.
[0119] In the method of the restoring operation by increasing the load of the fuel cell, the electric current flowing in the cell is temporarily increased to lower the cell voltage, whereby lowering the electric potential of the oxygen electrode. In a typical situation, while depending on the constitution of the cell and the constitution of the electrode, when the electric current is increased to 0.4 A per 1 cm
[0120] The restoring operation having been described in the foregoing is carried out in such a state that a load is applied to the cell. However, although the efficiency is reduced, such an operation can also be employed that an inert gas, a hydrocarbon gas, water or a reducing agent is fed to the oxygen electrode in a state where the electric power generation is terminated, i.e., the load is detached, to effect the restoring operation of lowering the electric potential of the oxygen electrode, and then the operation of the fuel cell is terminated.
[0121] The method of operating the fuel cell having at least one cell provided with an electrolyte, an anode and a cathode each having a platinum based metallic catalyst, the electrolyte being placed between the anode and the cathode, and a pair of separator plates having formed thereon gas passages for feeding a fuel gas into the anode and feeding an oxidizing agent gas into the cathode, respectively, wherein when the fuel cell stops power generation, the cathode is controlled so as to have a potential against a standard hydrogen electrode within the range of 0.6 V to 0.8 V.
[0122] In this way, even when the fuel cell repeats the power generation state and the non-power generation state, it is possible to avoid repetition of oxidation and reduction of Pt of the cathode, whereby degradation of the fuel cell can be prevented.
[0123] In the fuel cell operation method it is preferable that the state of exposing the anode to the fuel gas, feeding of the oxidizing agent gas into the cathode is stopped, and a prescribed voltage is applied between the anode and the cathode using an external electric source, thereby controlling the cathode so as to have a potential against a standard hydrogen electrode within the range of from 0.6 V to 0.8 V.
[0124] By utilizing the external electric source in the state of exposing the anode to the fuel gas, it is possible to easily control the potential of the cathode.
[0125] Also, in the operation method of a fuel cell system according to the foregoing invention, it is preferable that the fuel cell is a fuel cell stack comprising a laminate of plural cells, and in the state of exposing the anode of each cell to the fuel gas, feeding of the oxidizing agent gas into the cathode of each cell is stopped, and a prescribed voltage is applied between the anode and the cathode of each cell using an external electric source, thereby controlling the cathode of each cell so as to have a potential against a standard hydrogen electrode within the range of from 0.6 V to 0.8 V.
[0126] Also, in the operation method of a fuel cell system according to the foregoing invention, it is preferable that after stopping the power generation of the fuel cell until the cell temperature of the fuel cell drops to below about 50° C., the cathode is controlled so as to have a potential against a standard hydrogen electrode within the range of from 0.6 V to 0.8 V.
[0127] Also, in the operation method of a fuel cell system according to the foregoing invention, it is preferable that in the case where the fuel cell stops the power generation, when the cell temperature of the fuel cell drops to below about 50° C., the cathode and the anode are purged with air.
[0128] Also, in the operation method of a fuel cell system according to the foregoing invention, it is preferable that the purge is carried out using dry air.
[0129] Also, the fuel cell system of the invention is preferably a fuel cell system provided with a fuel cell having at least one cell provided with an electrolyte, an anode and a cathode each having a platinum based metallic catalyst, the electrolyte being put between the anode and the cathode, and a pair of separator plates having formed thereon gas passages for feeding a fuel gas into the anode and feeding an oxidizing agent gas into the cathode, respectively, and a control unit for controlling feeding of the fuel gas into the anode and feeding of the oxidizing agent gas into the cathode, wherein an external electric source of applying a prescribed voltage between the anode and the cathode is provided, and the control unit is constructed such that in the case where the fuel cell stops power generation, in the state of exposing the anode to the fuel gas, feeding of the oxidizing agent gas into the cathode is stopped, and the motions of the external electric source are controlled so as to apply a prescribed voltage between the anode and the cathode, thereby controlling the cathode so as to have a potential against a standard hydrogen electrode within the range of from 0.6 V to 0.8 V.
[0130] Also, in the fuel cell system according to the foregoing invention, it is preferable that the fuel cell is a fuel cell stack comprising a laminate of plural cells, and the control unit is constructed such that in the case where the fuel cell stack stops power generation, in the state of exposing the anode of each cell to the fuel gas, feeding of the oxidizing agent gas into the cathode of each cell is stopped, and the motions of the external electric source are controlled so as to apply a prescribed voltage between the anode and the cathode of each cell, thereby controlling the cathode of each cell so as to have a potential against a standard hydrogen electrode within the range of from 0.6 V to 0.8 V.
[0131] Also, in the fuel cell system according to the foregoing invention, it is preferable that the fuel cell system is further provided with a temperature sensor for measuring the cell temperature of the fuel cell; and the control unit judges whether or not the cell temperature of the fuel cell measured by the temperature sensor drops to below about 50° C., and as a result, in the case where the control unit judges that the cell temperature of the fuel cell drops to below about 50° C., the motions of the external electric source are controlled so as to stop the application of a voltage between the anode and the cathode.
[0132] Also, in the fuel cell system according to the foregoing invention, it is preferable that the control unit judges whether or not the cell temperature of the fuel cell measured by the temperature sensor drops to below about 50° C., and as a result, in the case where the control unit judges that the cell temperature of the fuel cell drops to below about 50° C., the cathode and the anode are purged with air.
[0133] In addition, in the fuel cell system according to the foregoing invention, it is preferable that the purge is carried out using dry air.
[0134] In the operation method of the fuel cell system and fuel cell system for carrying out the method according to the invention, even in the case where the fuel cell repeats the power generation state and the non-power generation state, it is possible to avoid degradation of the fuel cell.
[0135] The mode for carrying out the invention will be described below in detail with reference to the drawings.
[0136]
[0137] Incidentally, the construction of the fuel cell stack
[0138] The fuel cell stack
[0139] The anode
[0140] The fuel gas control unit