Patent Publication Number: US-2007122668-A1

Title: Fuel cell system and method of starting it

Description:
TECHNICAL FIELD  
      The present invention relates to a fuel cell system including a fuel cell which has electrodes containing catalysts supported on carbon catalyst carriers, particularly to a control for preventing deterioration of catalysts and catalyst carrier at start-up and shutdown of the fuel cell system.  
     BACKGROUND ART  
      A fuel cell is an electrochemical device to convert chemical energy of fuel gas such as hydrogen gas and oxidizer gas containing oxygen supplied thereto, directly to electric energy which is extracted from electrodes provided on both sides of an electrolyte thereof. A Polymer Electrolyte Fuel Cell (PEFC) can operate at low temperature and be easily handled, because of the intrinsic nature of the material of a solid polymer electrolyte membrane used therein, and is therefore particularly suitable for vehicular power application. A fuel cell vehicle carries a hydrogen storage device, such as a high-pressure hydrogen tank, a liquid hydrogen tank, or a hydrogen storage alloy tank, and a fuel cell to which hydrogen gas is supplied from the hydrogen storage device to react with air. Electric energy produced by the reaction is extracted from the fuel cell to drive a motor connected to driving wheels. The fuel cell vehicle is thus an ultimate clean vehicle, which discharges only water.  
      Generally, a cell as a component of PEFC consists of a membrane electrode assembly (MEA) which consists of a polymer electrolyte membrane and electrode catalyst layers provided on both sides thereof, and a pair of separators sandwiching the MEA. As disclosed in Japanese patent application laid-open publication No. 2002-373674, the electrode catalyst layer includes platinum catalysts and carbon catalyst carrier. In some cases, platinum fine particles are applied on the surface of the electrolyte membrane to form the electrode catalyst layer. Since the platinum is expensive, generally the platinum fine particles are applied on the surface of carbon catalyst carrier.  
      In PEFC, electrode reactions take place between hydrogen gas supplied to an anode (fuel electrode) and air (or oxygen) supplied to a cathode (oxidizer electrode), as expressed by formulas below, whereby electricity is generated: 
      Anode: H 2 →2H + +2e−  (1) 
      Cathode: 2H + +2e−+(1/2) O 2 →H 2 O  (2) 
     DESCLOSURE OF INVENTION  
      However, in the above-mentioned fuel cell, when the system is started/shutdown, or while the system is kept stopped, carbon corrosion/poisoning takes place, in which carbon reacts with water in an electrode catalyst layer on a cathode side surface of the electrolyte membrane, whereby the electrolyte membrane and the electrode catalyst are deteriorated.  
      The carbon corrosion/poisoning will be explained in detail with reference to  FIG. 1A  and  FIG. 1B .  FIG. 1A  shows reactions of the carbon corrosion/poisoning in a cell at start-up/shutdown of the fuel cell. Conditions under which the reaction takes place at start-up/shutdown of the fuel cell system are listed in the left column of the table of  FIG. 1B .  
      While the fuel cell system is kept stopped, air enters into the anode of the fuel cell. This creates a mixture of oxygen and hydrogen in the anode.  
      Specifically, when the fuel cell system is stopped, air remains in the cathode of the fuel cell and hydrogen gas remains in the anode thereof. If the fuel cell system is kept stopped, air enters into the anode of the fuel cell. This entering air and the remaining hydrogen gas are mixed in the anode, creating a mixture of oxygen and hydrogen therein. After a long stoppage of the system, the hydrogen gas will be blown out of the anode of the fuel cell by the entering air, and the anode will be filled with air. When starting the supply of the hydrogen gas to start up the system, the hydrogen gas to be supplied is mixed with the air in the anode, creating another situation of mixture of oxygen and hydrogen in the anode.  
      When the above-described mixtures exist in the anode, and in a region with higher hydrogen concentration, the hydrogen reacts as expressed by formula (3):
 
H 2 →2H + +2e−  (3)
 
      Proton (H + ) thus produced transfers from the anode, crossing over the electrolyte membrane, to the cathode where the proton reacts with the oxygen as expressed by formula (4) to form water:
 
O 2 +4H + +4e−→H 2 O  (4)
 
      This reaction requires electron (e−). However, when an external circuit connected to the fuel cell is not closed, the electron freed at the anode cannot transfer to the cathode through the external circuit. Therefore, the water present in the cathode reacts with the catalyst carrier carbon on the electrolyte membrane as expressed by formula (5), whereby carbon dioxide, proton, and electron are produced. The electron thus produced is used for water producing reaction in the cathode (formula (4)).
 
C+2H 2 O→CO 2 +4H + +4e−  (5)
 
      By the reaction of formula (5), the carbon on the electrolyte membrane is captured, and the electrolyte membrane is deteriorated.  
      In a region of the anode with air present therein, the oxygen in the air, the proton produced by the reaction of formula (5) and transferred from the cathode, and the electron generated by the reaction of formula (3) are reacted with one another as expressed by formula (4), to form water.  
      As an open end voltage of the fuel cell increases, the electrons moves more easily in the fuel cell, and the reactions expressed by formulas (3) to (5) are accelerated. Therefore, the carbon corrosion of the electrolyte membrane becomes severe.  
      Reaction conditions of corrosion of the platinum catalyst carrier carbon on the electrolyte membrane at shutdown and stoppage of the fuel cell system, will be summarized as follows: air (oxygen) remains in the cathode; hydrogen gas remains in the anode and air (oxygen) enters into the anode from outside; the produced power is not used (power extraction is stopped) and the high open end voltage (see left column of  FIG. 1B ).  
      Reaction conditions of the carbon corrosion at start-up of the fuel cell system will be summarized as follows: air (oxygen) enters into the anode from outside; hydrogen gas is supplied to the anode and mixed with the air (oxygen) in the anode; power extraction is stopped until the anode is filled with the hydrogen gas; and the high open end voltage (see left column of  FIG. 1B ).  
      The corrosion of the catalyst carrier carbon of the electrolyte membrane affects I-V characteristics of the fuel cell. Specifically, a fuel cell with a catalyst carrier carbon corroded has lower output voltage at an output current than a fuel cell in normal condition, and electric power generated thereby becomes low.  
      One of measures for preventing the deterioration of the electrolyte membrane and catalyst is to connect temporarily at the start-up of the system to the fuel cell, an auxiliary circuit for consuming power and letting the current flow. Specifically, at start-up of the fuel cell system, the auxiliary circuit having a resistor, etc., is temporarily connected to the fuel cell, thereby preventing surge increase in cell voltage. Thereafter, when the current flowing in the auxiliary circuit reaches a predetermined level, or when the load voltage of the auxiliary circuit drops to a predetermined level, the electrical connection is switched from the auxiliary circuit to a main load circuit.  
      However, this method requires long time to get the load voltage of the auxiliary circuit lowered, whereby time for the start-up of the fuel cell system becomes long.  
      Moreover, a fuel cell is easily deteriorated when starting power generation with low hydrogen concentration in the anode thereof.  
      The present invention was made in the light of the problems. An object of the present invention is to provide a fuel cell system capable of preventing the catalyst deterioration of a fuel cell thereof and reducing the system start-up time, specifically, by reducing the feed rate of fuel gas to prevent an overvoltage, and after that, increasing the feed rate of the fuel gas to complete gas replacement in the anode in a short period of time.  
      An aspect of the present invention is a fuel cell system, comprising: a fuel gas supply start command unit for commanding start of a fuel gas supply to a fuel cell of the fuel cell system; an operational status detector for detecting an operational status of the fuel cell; a deterioration preventing control unit for performing a control for preventing deterioration of the fuel cell based on output of the operational status detector and output of the fuel gas supply start command unit; and a fuel gas feed rate control unit for controlling fuel gas feed rate according to the output of the fuel gas supply start command unit and the control of the deterioration preventing control unit, wherein the control for preventing deterioration of the fuel cell is performed at start-up of the fuel cell system, wherein the fuel gas supply is started according to the output of the fuel gas supply start command unit, and after the control for preventing deterioration of the fuel cell is started, the fuel gas feed rate is increased by the fuel gas feed rate control unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be described with reference to the accompanying drawings wherein:  
       FIG. 1A  is a schematic view illustrating reactions in a fuel cell at start-up/shutdown;  
       FIG. 1B  is a table showing reaction conditions of carbon corrosion/poisoning at start-up/shutdown/stoppage of the fuel cell, and countermeasures against it;  
       FIG. 2  is a control block diagram of a fuel cell system according to a first embodiment of the present invention;  
       FIG. 3  is a system block diagram of the fuel cell system according to the first embodiment of the present invention.  
       FIG. 4A  is a time chart illustrating change in feed rate of hydrogen gas at start-up of a fuel cell system of a comparative example;  
       FIG. 4B  is a time chart illustrating change in a fuel cell voltage at start-up of the fuel cell system of the comparative example;  
       FIG. 4C  is a time chart illustrating status of deterioration preventing control at start-up of the fuel cell system of the comparative example;  
       FIG. 4D  is a time chart illustrating change in amount of oxygen in the cathode at start-up of the fuel cell system of the comparative example;  
       FIG. 4E  is a time chart illustrating change in hydrogen replacement rate in the anode at start-up of the fuel cell system of the comparative example;  
       FIG. 5A  is a time chart illustrating change in feed rate of hydrogen gas at start-up of the fuel cell system of the first embodiment;  
       FIG. 5B  is a time chart illustrating change in a fuel cell voltage at start-up of the fuel cell system of the first embodiment;  
       FIG. 5C  is a time chart illustrating status of deterioration preventing control at start-up of the fuel cell system of the first embodiment;  
       FIG. 5D  is a time chart illustrating change in amount of oxygen in the cathode at start-up of the fuel cell system of the first embodiment;  
       FIG. 5E  is a time chart illustrating change in hydrogen replacement rate in the anode at start-up of the fuel cell system of the first embodiment;  
       FIG. 6  is a general flow chart illustrating a start-up control sequence of the fuel cell system according to the first embodiment;  
       FIG. 7  is a flow chart illustrating hydrogen feed rate increase determination processing according to the first embodiment;  
       FIG. 8  is a flow chart illustrating hydrogen feed rate increase determination processing according to the second embodiment; and  
       FIG. 9  is a flow chart illustrating hydrogen feed rate increase determination processing according to the third embodiment. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      The preferred embodiments of the present invention will be explained with reference to the drawings. Each of the embodiments as will be explained hereunder is a fuel cell system suitable for a fuel cell vehicle.  
     FIRST EMBODIMENT  
      As shown in  FIG. 2 , a fuel cell system according to a first embodiment of the present invention comprises:  
      a fuel gas supply start command unit  101  for commanding start of fuel gas supply to a fuel cell of the fuel cell system;  
      an operational status detector  102  for detecting the operational status of the fuel cell;  
      a deterioration preventing control unit  103  for performing a control for preventing deterioration of the fuel cell based on output from the fuel gas supply start command unit  101  and output from the operational status detector  102 ; and  
      a fuel gas feed rate control unit  104  for controlling fuel gas feed rate according to the output of the fuel gas supply start command unit  101  and the control of the deterioration preventing control unit  103 .  
      In the fuel cell system according to a first embodiment, the operational status detector  102  of  FIG. 2  is realized as a voltage sensor  21  for detecting the voltage of a fuel cell  1  of  FIG. 3 , and the fuel gas supply start command unit  101  and the deterioration preventing control unit  103  and the fuel gas feed rate control unit  104  of  FIG. 2  are realized as a part of a controller  30  for controlling operation of the entire fuel cell system of  FIG. 3 .  
      The controller  30  is a microprocessor having CPU, ROMs which store control programs and parameters, RAMs as working storage memories, and an input/output interface.  
      In  FIG. 3 , the fuel cell (fuel cell main body)  1  is, but not limited to, an internal humidifying type and has an anode  1   a,  a cathode  1   b,  an electrolyte membrane  1   c,  porous separators  1   d  and  1   e,  flow passages of pure water  1   f  and  1   g  through which pure water for humidifying reaction gas passes, a flow passage of coolant  1   i,  and a separator  1   h  separating the flow passage of pure water  1   g  and the flow passage of coolant  1   i.    
      Hydrogen gas is supplied to the anode  1   a  from a hydrogen tank  2  through a hydrogen tank main valve  3 , a pressure reducing valve  301 , and a hydrogen supplying valve  4 . Pressure of the hydrogen tank  2  is reduced to a predetermined intermediate pressure by the pressure reducing valve  301 , and thereafter, pressure of the hydrogen gas is regulated by the hydrogen supplying valve  4  to a desired hydrogen pressure, and the regulated hydrogen gas is supplied to the anode  1   a.    
      The fuel cell system is controlled by the controller  30  which performs air pressure control for the cathode  1   b,  hydrogen pressure control for the anode  1   a,  pure water collecting control for collecting pure water to a pure water tank  13  at shutdown of the fuel cell under a low temperature environment, and cathode oxygen consumption control for controlling oxygen consumption in the cathode at start-up of the fuel cell.  
      A coolant temperature control unit  24  receives command from the controller  30 , and controls a coolant pump  15 , three-way valves  16 , and a radiator fan  18 , so that a fuel cell temperature T 1  detected by a temperature sensor  19  provided at a coolant outlet of the fuel cell  1 , is adjusted to be a desired temperature.  
      An ejector  5  and a hydrogen circulating pump  8  are fuel gas circulating devices for re-circulating fuel gas to the anode  1   a.  The gas to be supplied to the anode is a mixture of new hydrogen gas supplied through the hydrogen supplying valve  4  and unused hydrogen gas discharged from the anode  1   a.  The hydrogen circulating pump  8  works to cover a range of hydrogen flow rate out of working range of the ejector  5 .  
      The hydrogen pressure at the anode  1   a  is controlled by the controller  30  which performs a feed back control over pressure P 1  detected by a pressure sensor  6   a,  driving the hydrogen supplying valve  4 . By controlling the hydrogen pressure to be constant, the hydrogen gas used in the fuel cell  1  is automatically compensated.  
      A purge valve  7  is provided between the anode  1   a  and a dilution blower  9 . The purge valve  7  opens in the cases (a) to (c): (a) Discharging nitrogen accumulated in a fuel gas system to ensure circulation of hydrogen. (b) Blowing water accumulated in a gas passage to recover cell voltage. (c) Performing a cathode oxygen consumption control at start-up or shutdown of the fuel cell system, in which hydrogen gas is supplied only to the anode  1   a  to consume oxygen in the cathode  1   b,  and replacing gas in the fuel gas system with hydrogen gas to prevent deterioration of the fuel cell.  
      The dilution blower  9  dilutes a gas containing hydrogen discharged from the purge valve  7  with air, reduces the hydrogen concentration thereof to below a noncombustible range, and discharges the diluted gas outside the system.  
      Air is fed to the cathode  1   b  by a compressor  10 . Air pressure P 2  at the cathode  1   b  is detected by a pressure sensor  6   b  provided at cathode inlet side. The controller  30  controls air pressure of the cathode to a desired value, performing feedback control over the air pressure P 2  detected by the pressure sensor  6   b  and driving an air pressure regulating valve  11 .  
      Humidifying pure water in the pure water passages  1   f  and  1   g  is supplied from the pure water tank  13  by a pure water pump  12 . Air pressure, hydrogen pressure, and pure water pressure are determined, taking power generating efficiency and water balance into consideration, and adjusted to a predetermined pressure so that strains are not generated in the electrolyte membrane  1   c  and the separators  1   d  and  1   e.  Some water in the pure water passages  1   f  and  1   g  passes through the porous separators  1   d  and  1   e,  to humidify the hydrogen gas in the anode and the air in the cathode, respectively. Unused pure water returns to the pure water tank  13  through the pure water shut valve  14   d.    
      If the fuel cell system is stopped with pure water remained in the pure water passages  1   f  and  1   g,  expansion of the pure water by freezing occurs at the temperature below freezing point, and in this case, the fuel cell  1  is possibly damaged. Therefore, when the system is stopped, the pure water is collected to the pure water tank  13 . The controller  30  sends the air pressure, which is normally applied to the cathode  1   b  by the compressor  10 , to the pure water passages  1   f  and  1   g  and pure water piping, blows the pure water therein and returns the pure water to the pure water tank  13 . The pure water tank  13  has an improved structure and can be used even if the pure water is frozen in the inside.  
      A pure water shut valve  14   d  is a shut-off valve which prevents gas leakage into the pure water pipe line. When the hydrogen gas is supplied to the anode  1   a,  with no pure water in the pure water passages  1   f  and  1   g  at start-up or shutdown of the fuel cell system, the hydrogen leakage into the pure water piping can be prevented by closing a pure water collecting valve  14   b  and the pure water shut valve  14   d.    
      The coolant is supplied to a coolant passage  1   i  in the fuel cell  1  by the coolant pump  15 . Three-way valves  16  switches the passage of the coolant, guides the coolant to either of a radiator  17  or a radiator bypass, or to both of them in parallel. The radiator fan  18  forcibly sends air to the radiator  17  to cool the coolant, when the coolant is not sufficiently cooled by natural airflow at traveling. The coolant temperature control unit  24  adjusts the temperature of the coolant by performing feedback of the temperature of the coolant detected by the temperature sensor  19  and driving the three-way valves  16  and the radiator fan  18 .  
      A power manager  20  extracts electric power from the fuel cell  1  and supplies the extracted power to a load device such as a vehicle driving motor (not shown).  
      In the control for preventing deterioration of the fuel cell performed at start-up or shutdown of the fuel cell system, the controller  30  extracts electric power from the fuel cell to consume oxygen of the cathode according to the fuel cell voltage CV and the elapsed time detected by the voltage sensor  21 .  
      Next, explanation will be given to the control in the fuel cell system of the first embodiment at start-up, with reference to the flow charts of  FIG. 6  and  FIG. 7 .  FIG. 6  is a general flow chart of the control by the controller  30  at start-up of the fuel cell system in the first embodiment, and  FIG. 7  is a flow chart of the determination of hydrogen gas flow rate increase.  
      As a condition of the system before the control of the flow chart of  FIG. 6  is started, the main valve  3  of the hydrogen tank is closed, the compressor  10  is stopped, and hydrogen and air are not supplied to the fuel cell  1  yet.  
      In  FIG. 6 , first, in step S 10 , the fuel gas supply start command unit  101  determines to start a hydrogen gas supply based on signals from various vehicular devices such as a key switch  302 , sends a signal for adjustment of the hydrogen supply pressure, such as a setting pressure for idling of the system, to the hydrogen supply valve  4 , and sends a signal for opening the main valve  3  of the hydrogen tank  2 , whereby the hydrogen gas is started to be supplied to the anode  1   a  of the fuel cell  1  from the hydrogen tank  2 . Next, in step S 12 , a cell group voltage or a total voltage CV 1  of the fuel cell  1  is detected by the voltage sensor  21  (operational status detector  102 ), and the detected voltage is read in the sequence controller  30 .  
      In step S 14 , based on the detected voltage of step S 12 , it is determined whether or not the deterioration preventing control is started. In the determination, the detected voltage CV 1  and a predetermined value Vp are compared, and if the detected voltage is equal to or greater than the predetermined value Vp, the process is advanced to step S 16 , and the deterioration preventing control is started. The predetermined value Vp is called a deterioration preventing control start threshold value.  
      Here, if the voltage sensor  21  detects voltages of a plurality of cell groups of the fuel cell  1 , the maximum value of the detected voltages is defined as the detected voltage CV 1 , and this voltage CV 1  and the predetermined value Vp are compared.  
      The predetermined value Vp to be compared, is set to be smaller (Vp&lt;Vd) than a deterioration threshold value Vd, which is a voltage at which the deterioration of the fuel cell  1  is caused, and which is previously obtained by an experiment, etc. When the detected voltage CV 1  is below the predetermined value Vp in step S 14 , the process is returned to step S 12 .  
      In step S 16 , the deterioration preventing control is started so as to prevent deterioration of the fuel cell. The deterioration preventing control unit  103  performs the deterioration preventing control, in which supply of the hydrogen gas to the anode  1   a  is continued while supply of air to the cathode  1   b  is stopped, and a command is sent out to the power manager  20  to extract the electric power from the fuel cell  1  for consuming the oxygen in the cathode.  
      Extraction of the electric power (current) from the fuel cell  1  in the deterioration preventing control in step S 16  may be realized by the power manager  20  as described above, which is a load device at the time of normal power generation, or by a method connecting resistors or the like, which is separately prepared, to the fuel cell  1 .  
      Next, it is determined in step S 18  whether or not the hydrogen gas flow rate to be supplied to the anode la is increased. In the step S 20 , a determination result of the step S 18  is judged.  
      Determination of the increase in the hydrogen gas flow rate in step S 18  will be explained later with reference to  FIG. 7 .  
      When it is determined in step S 20  that the hydrogen gas flow rate is not increased, the process is returned to step Sl 8 .  
      When it is determined in step S 20  that the hydrogen gas flow rate is increased, the process is advanced to step S 22 .  
      In step S 22 , the flow rate of hydrogen gas supplied to the anode  1   a  is increased by increasing the hydrogen supply pressure, the command to increase the hydrogen supply pressure is sent out to the hydrogen supply valve  4 .  
      Increase in the flow rate of the hydrogen gas in step S 22  may be realized by increasing a target pressure value of the hydrogen gas supplied from the hydrogen supply valve  4 , or may be realized by increasing an opening of the purge valve  7  for discharging the hydrogen gas.  
      In addition, a plurality of valves (at least a valve for low flow rate and a valve for high flow rate) which respectively have openings different in size and are different in flow rate at the time of opening the valve, are provided at the anode outlet, and the valve for use may be switched from the valve for low flow rate to the valve for high flow rate.  
      Next, the hydrogen gas replacement rate in the anode  1   a  is determined in step S 24 . In the step S 26 , it is determined whether or not the anode gas replacement is ended.  
      When it is determined in step S 26  that the hydrogen replacement of the anode  1   a  is not ended, the process is returned to step S 24 . When it is determined in step S 26  that the hydrogen replacement of the anode  1   a  is ended, the process is advanced to step S 28 , and the deterioration preventing control is ended. Then, in step S 30 , normal power generation is started and air and hydrogen gas required for the power generation is supplied to the fuel cell and the start-up control is ended.  
       FIG. 7  is a flow chart showing procedures of the determination of hydrogen gas flow rate increase in step S 18  of  FIG. 6 . In this embodiment, when the deterioration preventing control is started, the hydrogen gas flow rate is increased at the same time. In step S 40 , it is determined that the hydrogen gas flow rate is unconditionally increased, and the process is returned to main routine.  
     SECOND EMBODIMENT  
      Next, explanation will be given to a start-up control of the fuel cell system according to a second embodiment of the present invention, with reference to the flow chart of  FIG. 8 . The structure of the fuel cell system of the second embodiment is the same as the structure of the first embodiment shown in  FIG. 2  and  FIG. 3 . The general flow chart of  FIG. 6  is the same as that of the first embodiment, and therefore only  FIG. 8  will be explained.  
       FIG. 8  shows procedures in step S 18  of  FIG. 6 . In this embodiment, after the deterioration preventing control is started, and if it is determined in step S 18  that the oxygen of the cathode is consumed (cathode oxygen consumption determination unit), the hydrogen gas flow rate is increased.  
      In step S 50  of  FIG. 8 , an oxygen consumption parameter for determining the consumption of the oxygen of the cathode is detected. In Step S 52 , it is determined whether or not the oxygen of the cathode is consumed based on the detected oxygen consumption parameter.  
      If it is determined in step S 52  that the oxygen of the cathode is consumed, and the hydrogen gas flow rate is increased in step S 54 , and the process is returned to the main routine.  
      If it is determined in step S 52  that the oxygen of the cathode is not consumed, the process is returned to the main routine, skipping the step of determining increase in the hydrogen gas flow rate.  
      The oxygen consumption parameter detected by step S 50  may be the maximum value of the voltages of the plurality of cell groups each of which consists of a plurality of cells of the fuel cell  1 , or may be the total voltage of the fuel cell.  
      In the case that the oxygen consumption parameter is defined as the maximum value of the cell group voltages or the total voltage of the fuel cell, it is determined in step S 52  that the oxygen in the cathode is consumed by an amount equal to or greater than a predetermined amount, if the maximum value of the cell group voltages or the total voltage of the fuel cell falls below the predetermined oxygen consumption determining threshold value Vc ( FIG. 5B ).  
      Moreover, if oxygen in the air of the cathode is consumed, the hydrogen transferred from the anode to the cathode by crossing over the electrolyte membrane  1   c  cannot react with the oxygen. A hydrogen detection sensor is provided downstream the air pressure regulating valve  11 , and by this sensor, if the hydrogen is detected in the air passage, signals from the hydrogen detection sensor may be defined as the oxygen consumption parameter.  
      In addition, a current sensor is provided to detect an output current of the fuel cell  1 , and the amount of oxygen consumed can be estimated from an integral current value calculated from the detected current. In this case, an amount of oxygen need to be consumed in the cathode is calculated from volume and pressure of the air system.  
      In addition, the time elapsed from start of extracting electric power for preventing deterioration is measured, and the time thus obtained may be defined as the oxygen consumption parameter. These methods may be used solely or in combination with the others.  
      In the case that it is determined that oxygen of the cathode is consumed if the fuel gas is detected in the cathode air passage, it is possible to detect complete consumption of the oxygen.  
      In the case that it is determined that oxygen of the cathode is consumed, when the predetermined time has elapsed since the deterioration preventing control is started, the construction of control software can be simple.  
       FIGS. 4A  to  4 D are time charts as comparative examples, showing the start-up control of the fuel cell, in which the hydrogen gas flow rate is set to be a low flow rate Q 1  from the start of supply to the completion of the hydrogen replacement in the anode.  
      When the supply of the fuel gas (hydrogen gas) is started (time t 0 ) to the fuel cell at a predetermined flow rate Q 1  (or pressure) and the cell group voltage or the total voltage exceeds the deterioration preventing control start threshold value Vp, the deterioration preventing control is started (time t 1 ). Accordingly, power generation is started, and the oxygen amount in the cathode starts decreasing. Since the hydrogen gas flow rate is suppressed to be a low flow rate Q 1 , so that the voltage of the fuel cell is held below the predetermined deterioration threshold value Vd, long time is required from start of the supply to complete the hydrogen replacement in the anode (time t 3 ). Therefore, the process cannot advance to the next process, and long time is required for starting the system.  
      In the second embodiment, as shown in  FIGS. 5A  to  5 D, when the supply of the fuel gas (hydrogen) to the fuel cell is started (time t 0 ) at the predetermined flow rate Q 1  (or pressure), and the cell group voltage or the total voltage of the fuel cell exceeds the deterioration preventing control start threshold value Vp, the deterioration preventing control is started (time t 1 ). Thereafter, when the oxygen amount in the cathode lowers to an upper limit value q below which the deterioration of the fuel cell can be avoided, the flow rate of the hydrogen gas is increased to a predetermined flow rate Q 2 . Thus, when the flow rate of the hydrogen gas supplied to the anode is increased, the time from the start of supply of the hydrogen gas to completion of the hydrogen replacement in the anode (time t 3 ′&lt;time t 3 ) can be shortened, and the start-up time of the system can also be shortened without deterioration of the fuel cell.  
     THIRD EMBODIMENT  
      Next, explanation will be given to the control at start-up in the fuel cell system according to a third embodiment of the present invention, with reference to the flow chart of  FIG. 9 . The structure of the fuel cell system of the third embodiment is the same as the structure of the first embodiment as shown in  FIG. 2  and  FIG. 3 .  
      In this embodiment, the controller  30  of  FIG. 3  serves as a cathode gas supply start command unit for commanding the start of the air (cathode gas) supply, and also serves as a deterioration possibility determination unit for determining the possibility of the deterioration of the fuel cell based on output of the operational status detector  102 .  
       FIG. 9  is a general flow chart for explaining the control of the fuel cell system of this embodiment at start-up.  
      For control steps executing the same processing as that of control steps in the general flow chart ( FIG. 6 ) of the first embodiment, the same reference signs and numerals are used, the overlapping explanations thereof are omitted, and only difference in the general flow chart between this embodiment and the first embodiment will be explained.  
      In this embodiment, the hydrogen supply pressure is increased to increase the hydrogen gas flow rate to the anode  1   a.  The command of increasing the hydrogen supply pressure is sent out to the hydrogen supply valve  4  in step S 22   a  that follows step S 20  where the hydrogen gas flow rate is determined to be increased. Further, in step S 22   a,  the compressor  10  is started to supply air to the cathode  1   b.    
      In this embodiment, based on a determination result of the increase in hydrogen gas flow rate in step S 18 , it is determined that there is less possibility of the deterioration of the fuel cell, and the air supply to the cathode  1   b  is allowed.  
      Increase in the flow rate of the hydrogen gas in step S 22   a  may be realized, similarly to the first embodiment, by increasing a target pressure value of the hydrogen gas supplied through the hydrogen supply valve  4 , or may be realized by increasing an opening of the purge valve  7  for discharging the hydrogen gas.  
      In addition, a plurality of valves (at least a valve for low flow rate and a valve for high flow rate) which respectively have openings different in size and are different in flow rate at the time of opening the valve, are provided at the anode outlet, and the valve for use may be switched from the valve for low flow rate to the valve for high flow rate.  
      In this embodiment, based on a determination result of the increase in hydrogen gas flow rate in step S 18 , it is determined that there is less possibility of the deterioration of the fuel cell, and the air supply to the cathode  1   b  is allowed. Accordingly, the start-up of the fuel cell system can be shortened by starting the air supply to the cathode  1   b  before completing the hydrogen replacement of the anode  1   a.    
      Note that it is not necessary to start the air supply to the cathode  1   b  at the time when the flow rate of the hydrogen gas is increased. If it is previously obtained by an experiment etc. a time required for the hydrogen gas in the anode to be dispersed in a range where the deterioration of the fuel cell can be avoided, timing of starting the air supply to the cathode  1   b  can be determined based on the elapsed time from start of the hydrogen gas supply or the increase in the flow rate of the hydrogen gas.  
      In addition, in the case that priming the pure water pump  12  is performed by sending the compressed air, which is supposed to be sent to the cathode  1   b,  to the pure water tank  13 , an additional time is required for starting the fuel cell system. In this embodiment, since the compressor  10  is started before completing the hydrogen replacement, the time required for starting the fuel cell system is further shortened.  
      The present disclosure relates to subject matters contained in Japanese Patent Application No. 2003-396795, filed on Nov. 27, 2003, and Japanese Patent Application No. 2004-090115, filed on Mar. 25, 2004, the disclosure of which are expressly incorporated herein by reference in their entirety.  
      The preferred embodiments described herein are illustrative and not restrictive, and the invention may be practiced or embodied in other ways without departing from the spirit or essential character thereof. The scope of the invention being indicated by the claims, and all variations which come within the meaning of claims are intended to be embraced herein.  
     INDUSTRIAL APPLICABILITY  
      In a fuel cell system according to the present invention, at start-up thereof, hydrogen gas supply to a fuel cell  1  is first started, and when the voltage of the fuel cell  1  detected by a voltage sensor  21  reaches a predetermined value, a deterioration preventing control is started in which power is extracted from the fuel cell  1  while the hydrogen gas supply to anode  1   a  is continued and air supply to the cathode  1   b  is stopped. Then, when it is determined that oxygen in the cathode  1   b  is consumed, flow rate of the hydrogen gas supplied to the anode  1   a  is increased.  
      According to the fuel cell system, since the flow rate of the hydrogen gas is increased after the deterioration preventing control is started, gas in the anode can be quickly replaced with hydrogen gas without causing deterioration of the fuel cell. Also, the fuel cell system can be applied to a technique for shortening start-up time while preventing corrosion/poisoning of a catalyst carrier carbon on the electrolyte membrane at start-up of the fuel cell system.