Patent Application
20100291448
The present application claims priority from Japanese Patent application serial No. 2009-117443, filed on May 14, 2009, the content of which is hereby incorporated by reference into this application.
1. Field of the Invention
The present invention relates to a fuel cell system using an electric power generated by a fuel cell.
2. Description of Related Art
A fuel cell is an electrochemical device for directly converting the energy of a fuel into electric energy by an electrochemical reaction. The fuel cell is broadly classified according to a charge carrier to be used into a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, a polymer electrolyte fuel cell (hereinafter abbreviated as “PEFC”), and an alkaline fuel cell. The PEFC among these full cells is expected to be extensively applied, for example, to a stationary power supply and a backup power supply as well as a mobile power supply because the PEFC is capable of generating electricity at a high current density and operating at a comparatively low temperature.
Several tens μm to hundred and several tens μm thick proton exchange membrane is used as an electrolyte membrane for the MEA 33. In general, the ion exchange membrane has a structure in which a side chain with sulfonic acid group (sulfo group) is coupled with perfluorocarbon forming a main chain.
The electrode of the MEA 33 is formed by binding a platinum catalyst formed by dispersing platinum fine particles with several to several tens Å diameter on a surface of a carbon particle with several to several hundreds nm diameter as a carrier with a polymer having the same proton conductivity as the electrolyte membrane as a binder. The electrode is generally several μm to several tens μm in thickness. The reason why carbon is used as the carrier of the catalyst is that carbon is high in electron conductivity and chemical stability. Platinum is used in the micronized state to increase the surface area of the electrode and to increase the reaction rate of electrochemical reaction.
The GDL 32 is formed of a conductive porous material. The GDL 32 plays a role in quickly supplying fuel and oxidizer gas which are supplied from the GDL 32 and the separator 30 and contribute to electrochemical reaction to an active spot of the electrode being a reaction field and discharging the gas after the reaction with products. In general, the GDL 32 is made of a woven fabric or an unwoven fabric using a carbon fiber as a material.
The separator 30 includes an anode channel 36 and a cathode channel 37 supplying an anode gas (fuel gas) 34 and a cathode gas (oxidizer gas) 35 to the electrode through the GDL 32 respectively while separating both the gases supplied to unit cells adjacent to each other. The separator 30 is formed of a material having a high electric conductivity and a high corrosion resistance against a corrosive atmosphere in the cell because the separator 30 needs to cause electric current to flow into itself to capture the electric current produced by power generation. The separator 30 is formed of a carbon material or a metallic material subjected to an anti-corrosion process, for example.
The PEFC shown in
The PEFC operates at a comparatively low temperature as low as 70° C. to 90° C. For this reason, it is easier to start and stop the PEFC than other high-temperature fuel cells. A fuel cell for automobiles has been developed on the assumption that the number of starting and stopping the fuel cell is several thousands or more.
Component materials of a fuel cell are deteriorated with the start and the stop of the fuel cell system. For example, in a state where a cathode potential is reduced to lower the cell voltage and the system is stopped, oxidizer gas is supplied to the fuel cell to acquire electric power to produce a difference in potential between the upstream and the downstream of the cathode electrode surface. The difference in potential causes current to flow into the cathode electrode surface, and thereby a so-called local cell is formed. If a starting process forming the local cell is repeated, the carbon carrier of the cathode is gradually corroded and disappears, platinum fine particles on the surface of the carbon carrier aggregate together to decrease the surface area of the platinum fine particles, and thereby a problem of deterioration of cell characteristics occurs.
Patent Literature 1 (Japanese Patent Application Laid-Open No. 2007-287674), for example, discloses a fuel cell system restricting an upper limit of a cell voltage, an in-plane distribution of the cell voltage, and an increase rate of a unit-cell voltage at the time of supplying the oxidizer and minimizing the corrosion of an electrode material in order to solve the above problem.
A conventional fuel cell system takes much time from start to the completion of start. Even if the cell voltage is controlled as described above at the time of supplying the oxidizer, the local cell is substantially formed to progress the deterioration of the fuel cell.
To solve the above problem, the present invention has its object to provide a fuel cell system capable of minimizing the formation of a local cell even if the start and the stop are repeated, thereby substantially minimizing a decrease in the performance of the cell.
To achieve the above problem, a fuel cell system according to the present invention includes a fuel cell; a hydrogen supply unit for supplying hydrogen to an anode channel of the fuel cell; an air-amount adjusting unit for adjusting the amount of air supplied to a cathode channel of the fuel cell and discharged from the cathode channel; an external load connected to the fuel cell; and a control unit for controlling an operation of the hydrogen supply unit, the air-amount adjusting unit, and the external load, in which the control unit controls the fuel cell system such that the cathode channel of the fuel cell is filled with air whose pressure is not higher than a saturated vapor pressure at the time of starting the fuel cell system, thereafter, the air is continuously led through the cathode channel of the fuel cell, and a load is drawn from the fuel cell.
A preferred embodiment of the present invention is described below with reference to the accompanied drawings.
As shown in
In
An outlet of the anode channel 36 of the fuel cell 1 is connected to a hydrogen discharge line 3 for discharging hydrogen. The hydrogen discharge line 3 is provided with a hydrogen outlet valve (anode outlet valve) 8. An outlet of the cathode channel 37 of the fuel cell 1 is connected to an air discharge line 2 for discharging air. The air discharge line 2 is provided with an air outlet valve (cathode outlet valve) 7. The hydrogen supply line 14 is provided with a hydrogen circulation system 6 for re-circulating unused hydrogen from the hydrogen discharge line 3. The fuel cell 1 is further provided with a voltage sensor 13 for measuring an average cell voltage of the fuel cell stack. A power collection terminal of the fuel cell 1 is connected to an external load 12 for delivering electric power generated in the fuel cell 1 to the outside.
Pressure and voltage measured by the anode pressure sensor 10, the cathode pressure sensor 11, and the voltage sensor 13 are input to the control unit 9. The control unit 9 controls the drive of the hydrogen supply source 20, the anode pressure regulation valve 5, the air blower 21, the cathode pressure regulation valve 4, the hydrogen outlet valve 8, the air outlet valve 7, the anode circulation system 6 (these are collectively referred to as auxiliary equipments), and the external load 12. The control unit 9 controls the drive of the auxiliary equipments to switch among an external load start mode in which the voltage generated by the fuel cell 1 is started to be supplied to the external load 12; a power generation mode in which the voltage generated by the fuel cell 1 is constantly supplied to the external load 12; and an external load stop mode in which the voltage of the fuel cell 1 is reduced to decrease the supply of voltage to the external load 12.
A method for stopping the fuel cell system is described below.
As shown in
A method for starting the fuel cell system according to the present embodiment is described below.
As shown in
If the cell voltage is not greater than the first threshold voltage (YES in step S44), in step S45, pressure in the cathode channel 37 is increased to a predetermined pressure by the cathode pressure regulation valve 4. When pressure in the cathode channel 37 is increased to the predetermined pressure, in step S46, the control unit 9 determines whether the average cell voltage is not greater than a predetermined threshold voltage (taken as a second threshold voltage). If the average cell voltage is greater than the predetermined threshold voltage (NO in step S46), the processing proceeds to a second voltage reduction routine and resumes the start of the fuel cell system in step S41 through the second voltage reduction routine. The second voltage reduction routine is described later in
When pressure in the cathode channel 37 is increased to the predetermined pressure, if the average cell voltage is not greater than the second threshold voltage (YES in step S46), in step S47, the control unit 9 operates the auxiliary equipment on the anode side such as the hydrogen supply source 20, the anode pressure regulation valve 5, the anode circulation system 6, and the hydrogen outlet valve 8 to supply hydrogen to the fuel cell 1. Thereafter, in step S48, the air outlet valve 7 is opened automatically or by receiving an output fetch signal to eject air, continuously circulating air in the cathode channel 37. In step S49, the control unit 9 shifts the external load 12 to the start mode and controls the current value output from the fuel cell 1 so that the average cell voltage can be kept within a predetermined range between a [mV] or more (700 mV, for example) and b [mV] or less (800 mV, for example). The average cell voltage is taken as a [mV] or more to confirm that fuel gas and oxidizer gas are sufficiently supplied to the electrodes on the anode and the cathode side respectively. On the other hand, the average cell voltage is taken as b [mV] or less to prevent platinum in catalyst from melting at a high potential. In step S50, the control unit 9 determines whether the average cell voltage is within the above predetermined range. If the average cell voltage is within the predetermined range (YES in step S50), in step S51, the start of the fuel cell system is completed, and the processing proceeds to the power generation mode according to the request from the external load 12.
As shown in
As shown in
The action of the present embodiment is described below.
The fuel cell system is stopped or the average cell voltage is reduced in the method described in
However, even if air is supplied to the cathode channel 37 at a low flow rate with the air outlet valve 7 closed to increase the pressure therein, the cathode potential can be caused to remain lowered. In the present exemplary embodiment, the air outlet valve 7 is opened with the pressure in the cathode channel 37 increased to discharge the air with which the cathode channel 37 is filled outside the fuel cell at a burst, thereby enabling almost uniformly removing the reaction product water covering the catalyst of the cathode from the electrode surface.
In this state, since the catalyst is in contact with air almost at the same time on the cathode surface, the cathode potential can be increased substantially equally on the surface. For this reason, the local cell can be prevented from being formed even at the time of starting to allow minimizing the disappearance of the carbon carrier caused by the formation of the local cell.
More specifically, the fuel cell system of the present embodiment enables the cathode potential to be increased momentarily and uniformly by leading air filled in the cathode channel 37 continuously after filling the cathode channel 37 with the air, or by opening the air outlet valve 7 and discharging the filled air in the cathode channel 37 at a burst after filling the cathode channel 37 with air with the air outlet valve 7 closed to increase the pressure of the air, particularly. For this reason, the formation of the local cell on the cathode can be minimized to reduce the deterioration of the membrane electrode assembly (carbon carrier) 33.
The fuel cell system of the present embodiment can prevent platinum in the catalyst from dissolving at a high potential by drawing a load from the fuel cell and setting the maximum value of the average cell voltage to 800 mV or less after leading the air. Dissolving platinum causes re-precipitating platinum thereafter, coarsening the particle size thereof, reducing the surface area of the particle of platinum, and degrading the cell properties. In other words, the fuel cell system of the present embodiment can prevent the cell characteristics from being degraded.
According to the fuel cell system of the present embodiment, even if air pressure is lowered for some reason, it is possible to return quickly the air pressure to a predetermined value by controlling the amount of air supplied to the cathode channel 37 based on an information from the pressure sensors 10 and 11 and the voltage sensor 13 attached to the fuel cell 1 when the cathode channel 37 of the fuel cell 1 is filled with air whose pressure is not higher than a saturated vapor pressure through the air supply line 15. This also eliminates a risk of breaking down gas seal configuration of the cell caused when the air pressure is increased.
If the average cell voltage is increased while the fuel cell system is being started, the local cell may be formed.
However, it is possible to lower the voltage speedily and to start the system again by shifting the fuel cell system to the first and the second voltage reduction routine described in
An example of the present invention is described with reference to a comparative example.
In the fuel cell system described in
As shown in
A method for stopping the fuel cell system of the comparative example is similar to that described in
As shown in
As is the case with the above example of the present invention, the comparative example was measured. Specifically, an initial average cell voltage was first confirmed at a current density of 0.5 A/cm2 and then the stop and the start mode were repetitively executed 5000 times, thereafter a load of a current density of 0.5 A/cm2 was applied to measure the average cell voltage. As a result, the range of the decrease in the average cell voltage from the initial average cell voltage obtained after the stop and the start were repeated 5000 times was 80 mV.
The reason why the range of the decrease in the average cell voltage of the example of the present invention is smaller than that of the comparative example is that the cell voltage is lowered by the corrosion of the carbon carrier caused by the reaction of local cell of the cathode at the time of starting in the comparative example, on the other hand, deterioration of the carbon carrier hardly occurs in the example of the present invention since the distribution of voltage on the cathode surface is made substantially uniform at the time of starting not to form a local cell.
According to the fuel cell system of the present embodiment, the method for starting the fuel cell system whereby to minimize the reaction of the local cell of the cathode is used to substantially reduce degradation in performance of the fuel cell system even if the start and stop of the system are frequently repeated.
The present invention is applicable to not only the above embodiments but other various examples.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-117443 | May 2009 | JP | national |
