Abstract:
The invention relates to a method ( 40 ) for starting up a fuel cell ( 11 ), wherein hydrogen is introduced into an anode chamber ( 15 ) of the fuel cell ( 11 ), and at the beginning of the start-up process oxygen is present in the anode chamber ( 15 ) of the fuel cell ( 11 ). According to the invention, at the beginning of the hydrogen introduction stage enough hydrogen is introduced to ensure that upon entry into the anode chamber ( 15 ) hydrogen and oxygen are present in no more than a stoichiometric ratio.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method for starting up a fuel cell, wherein hydrogen is introduced into an anode chamber of the fuel cell, and at the beginning of the start-up process oxygen is present in the anode chamber of the fuel cell. The invention also relates to a fuel cell system. 
         [0002]    During the operation of a fuel cell from the prior art, hydrogen and oxygen are electrochemically converted to water with the aid of a catalyst, which contains platinum applied to carbon particles. When starting up a fuel cell, oxygen can initially be present in a cathode chamber and in the anode chamber of the fuel cell. If hydrogen is now introduced into the anode chamber, a sharp hydrogen/oxygen gas front then develops within the anode chamber, wherein the hydrogen previously introduced is situated in a front region of the fuel cell in the anode chamber and oxygen is situated in the entire cathode chamber and in the anode chamber in a rear region of the fuel cell. 
         [0003]    C. A. Reiser et al. describe in “Electrochemical and Solid-State Letters”: 8 (6) A273-A276, 2005 the electrochemical processes of a fuel cell in such a state. In the front region of the fuel cell, the hydrogen of the anode chamber can electrochemically react at an anode to form water and the oxygen of the cathode chamber can electrochemically react at a cathode to form water. Due to the high conductivity of the electrodes, the potentials of the anode and the cathode are approximately constant over the front and the rear region. The low conductivity of the electrolyte in comparison to the electrodes has the effect that a high potential difference is present between the cathode and an electrolyte in the rear, oxygen rich region of the fuel cell. In the rear region of the fuel cell, oxygen is now reduced to water at the anode, whereas carbon of the catalyst is oxidized to carbon dioxide with the aid of water at the cathode. As a result, the catalyst loses its effectiveness and the fuel cell degrades. According to Reiser, this process is referred to as the “reverse current decay” mechanism. 
       SUMMARY OF THE INVENTION 
       [0004]    It is the aim of the present invention to provide a method for starting up a fuel cell and a fuel cell system, which in part overcome the disadvantages of the prior art and particularly in which method and system the fuel cell is degraded less quickly or, respectively, is slowly degraded. 
         [0005]    In order to meet this aim, a method according to the invention is proposed. The aim is furthermore met by a fuel cell system according to the invention. Features and details which are described in connection with the method according to the invention also apply in connection with the fuel cell system according to the invention and vice versa. In so doing, the features mentioned in the claims and in the description can in each case be essential to the invention in isolation or in combination. 
         [0006]    According to the invention, at the beginning of the hydrogen introduction stage enough hydrogen is introduced to ensure that upon entry into the anode chamber hydrogen and oxygen are present in no more than a stoichiometric ratio. 
         [0007]    Hydrogen and oxygen therefore arrive at the anode in no more than a stoichiometric ratio, i.e. no more than a ratio of 2:1. Because the proportion of hydrogen with respect to the proportion of oxygen in a volume flow which enters the anode chamber is present in no more than a stoichiometric ratio at the beginning of the hydrogen introduction stage, the hydrogen completely reacts with the oxygen which is present in the anode chamber and in the anode, whereas the quantity of oxygen in the anode chamber and in the anode decreases. A sharp hydrogen/oxygen gas front is therefore prevented in accordance with the invention. The reaction is catalyzed by the platinum that is present. An electrochemical reaction in which the hydrogen of the anode chamber electrochemically reacts with the oxygen of the cathode chamber is, according to the invention, largely prevented. As a result, a high potential difference between the cathode and the electrolyte is prevented in the rear region of the fuel cell. Thus, the undesirable reaction of the catalyst carrier material: carbon at the cathode hardly occurs in the rear region of the fuel cell. The catalyst remains at least largely or, respectively, substantially intact. The fuel cell degrades less quickly than in the prior art. 
         [0008]    The fact that, at the beginning of the hydrogen introduction stage, hydrogen and oxygen are present in no more than a stoichiometric ratio upon entry into the anode chamber means that the mole fraction of hydrogen upon entry into the anode chamber at the beginning of the hydrogen introduction stage is no more than twice as high as the mole fraction of oxygen in the anode chamber prior to starting up the fuel cell. If, e.g., the anode chamber is filled with air prior to starting up the fuel cell and the mole fraction of oxygen in the anode chamber is therefore approximately 0.21, the mole fraction of hydrogen then lies at no more than 0.42 upon entry into the anode chamber at the beginning of the hydrogen introduction stage. “At the beginning of the hydrogen introduction stage” refers here to a time range in which the oxygen has not yet reacted in the anode chamber. 
         [0009]    At the beginning of the hydrogen introduction stage, the mole fraction of the hydrogen preferably lies below the mole fraction of the oxygen upon entry into the anode chamber. That means the ratio of hydrogen to oxygen upon entry into the anode chamber at the beginning of the hydrogen introduction stage lies below a 1:1 ratio. Oxygen is therefore present in excess. It has been shown here that the degradation was able to be better prevented as a result of such a small quantity of hydrogen. The mole fraction of the hydrogen lies preferably between 30% and 70%, very preferably between 40% and 65%, of the mole fraction of oxygen upon entry into the anode chamber at the beginning of the hydrogen introduction stage. If air is present in the anode chamber prior to starting up the fuel cell, the mole fraction of the hydrogen then lies preferably between 0.06 and 0.15, very preferably between 0.09 and 0.13 upon entry into the anode chamber at the beginning of the hydrogen introduction stage. In the especially preferred value range, a degradation of the fuel cell was able to be reduced by values between 73% and  49 % without greatly delaying the start-up of the electrical power output of the fuel cell. The method according to the invention can be concluded within ten seconds, preferably within five seconds and especially preferably within three seconds. 
         [0010]    The ratios between hydrogen and oxygen mentioned above are in each case based on the entry into the anode chamber. The ratios are particularly based on a volume segment of an anode inlet stream prior to entry into the fuel cell. The ratio of hydrogen to oxygen changes by means of the reactions within the fuel cell; thus enabling another ratio of hydrogen to oxygen to prevail in an anode outlet stream. 
         [0011]    The anode chamber is disposed above an anode, at which the hydrogen reacts. The anode chamber can be designed as a gas channel and particularly contain a gas diffusion layer by way of design. The anode catalyst layer can contain platinum as the catalyst. The anode of the fuel cell is used to oxidize the hydrogen introduced into the anode chamber. The fuel cell also has a cathode for reducing oxygen. In this way, air containing oxygen can be introduced into a cathode chamber, whereat the cathode is situated, by means of a compressor. An electrolyte, by way of example a solid state electrolyte, e.g., a membrane, can be situated between the anode and the cathode. An inlet valve can be disposed between a hydrogen tank and the anode chamber for metering a hydrogen supply. A supply line is preferably situated between the inlet valve and the anode chamber. The supply line can contain oxygen, in particular air, prior to starting up the fuel cell. The absolute amount of hydrogen which is introduced per unit of time into the anode chamber and/or the supply line can be set by means of the position of the inlet valve. 
         [0012]    The fuel cell system can contain a recirculation line in order to reintroduce the anode outlet stream into the anode chamber. The reintroduction is referred to as recirculation. The recirculated anode outlet stream can be mixed with the hydrogen stream from the hydrogen tank upstream of the anode chamber. The recirculated anode outlet stream and the hydrogen stream from the hydrogen tank form in this case the anode inlet stream. The supply line can contain a mixing point, at which the recirculation line ends. A recirculation means for the reintroduction of the anode outlet stream can be designed as a fan. The recirculation means can be disposed in the recirculation line. 
         [0013]    An open-loop and/or closed-loop control unit is provided in the fuel system. The open-loop and/or closed-loop control unit can, e.g., control the inlet valve and/or the recirculation means. 
         [0014]    The ratio of hydrogen to oxygen which is supposed to be present at the beginning of the hydrogen introduction stage is referred to as the desired ratio. A maximum absolute amount of hydrogen which is introduced per unit of time into the anode chamber is set as the absolute amount of hydrogen which can be introduced into the anode chamber per unit of time in order to obtain the desired ratio. This maximum amount is not exceeded during the entire start-up of the fuel cell. The inlet valve is particularly set at the beginning of the hydrogen introduction stage in such a way that the desired ratio is achieved. The inlet valve is particularly not further opened as long as oxygen is present in the anode chamber. 
         [0015]    During the start-up process, the mole fraction of the oxygen in the anode chamber drops due to the reaction of the hydrogen with the oxygen of the anode chamber. It is thus conceivable that, in the course of the start-up process, the ratio of hydrogen to oxygen moves above the desired ratio if the absolute amount of hydrogen which is introduced per unit of time into the anode chamber remains constant during the start-up process or drops only slightly. This course of the start-up process is included in the concept of the invention. 
         [0016]    The introduction of hydrogen preferably remains constant during the start-up process until oxygen is no longer present in the anode chamber. That means the absolute amount of hydrogen which is introduced per unit of time into the anode chamber remains constant until oxygen is no longer present on the anode side. Particularly the position of the inlet valve is not changed here or rather remains unchanged. 
         [0017]    Oxygen is preferably first introduced into the cathode chamber if there is no longer oxygen present on the anode side. In particular, a compressor is only switched on if oxygen is no longer present on the anode side. It has been proven experimentally that, prior to introducing oxygen into the cathode chamber, the membrane is not damaged by a pressure difference to be expected between the anode and the cathode chamber. 
         [0018]    A period of time which is required to allow the oxygen to completely react on the anode side, is referred to below as the reaction time period. The reaction time period can particularly be between 0.1 milliseconds and ten seconds, preferably between 1 and 3 seconds. The reaction time period is ended if there is no longer oxygen present on the anode side. The term “no longer oxygen” means that the amount of oxygen on the anode side corresponds to the amount of oxygen during a normal operation of the fuel cell. The term “on the anode side” includes here a total volume from which the oxygen has to be removed. The total volume is the volume from which oxygen can move into the anode chamber by means of the anode inlet stream. The total volume can, e.g., include the anode chamber, the anode and the recirculation and supply lines. 
         [0019]    The reaction time period can be estimated and particularly be established as a predefined value in the method. The predefined value can, for example, be stored for the reaction time period in the open-loop and/or closed-loop control system. The reaction time period can alternatively be determined from the total amount of hydrogen that was introduced into the anode chamber and the known total volume at the known oxygen mole fraction at the beginning of the method. The total volume can be deposited in the open-loop and/or closed loop control unit. The determination of the reaction time period can be carried out in said open-loop and/or closed-loop control unit. The oxygen mole fraction can in this case be deposited as a predefined value or be measured. 
         [0020]    The steps of the method described above can therefore mean that the introduction of hydrogen remains constant during the start-up process until, according to estimated measures, oxygen is no longer present on the anode side and/or that oxygen is first introduced into the cathode chamber and/or electric power is first extracted if, according to estimated measures, oxygen is no longer present on the anode side. 
         [0021]    The open-loop and/or closed-loop control unit sets the desired ratio. The position of the inlet valve that is required to obtain the desired ratio can particularly be predefined as a fixed value in the open-loop and/or closed-loop control unit. A pressure can alternatively be measured upstream of the anode chamber and the open-loop and/or closed-loop control unit can set the inlet valve such that a certain change in pressure is achieved upstream of the anode chamber. The pressure or the pressure differential can be deposited in said open-loop and/or closed-loop control unit as a fixed value. Said open-loop and/or closed-loop control unit can alternatively calculate the required change in pressure, from which the desired ratio results, from the pressure prior to the opening of the inlet valve and the proportion of oxygen which—measured or estimated—is situated in the anode chamber. It is also alternatively conceivable that the hydrogen and/or oxygen concentration is measured prior to entry into the anode chamber and said open-loop and/or closed-loop control unit sets the intake valve on the basis of these values. 
         [0022]    Hydrogen can preferably be introduced such that the hydrogen and the oxygen are in the desired ratio in the supply line. Hydrogen and oxygen can thus be at least partially, preferably totally, mixed. In this way, the desired ratio can be achieved upon entry into the anode chamber. According to the invention, provision is particularly made at the beginning of the hydrogen introduction stage for enough hydrogen to be introduced to ensure that hydrogen and oxygen are present in no more than a stoichiometric ratio or in another desired ratio in the supply line. 
         [0023]    In a preferred manner, the anode outlet stream is reintroduced into the anode chamber during the start-up process. The anode outlet stream is especially completely reintroduced into the anode chamber. The recirculation enables the complete reaction of the oxygen in the anode chamber as well as in the recirculation line. The desired ratio at the beginning of the hydrogen introduction stage can be achieved in the anode inlet stream, in particular at the mixing point. In particular, the anode outlet stream is reintroduced to the anode chamber at full speed. The maximum recirculation speed provided for the fuel cell system is referred to as full speed. 
         [0024]    It is conceivable that an outlet valve is already or is closed prior to the introduction of the hydrogen into the anode chamber. It may be the case that the closed position is controlled prior to the introduction of the hydrogen. In addition, a recirculation means can be switched on for the reintroduction of the anode outlet stream into the anode chamber prior to introducing hydrogen into the anode chamber, in particular after closing the outlet valve. 
         [0025]    Oxygen is present in the anode chamber particularly after a lengthy stoppage of the fuel cell. For example, an oxygen mole fraction arises in a cathode chamber open to the ambient environment quickly after the fuel cell has stopped which is equal to that of said ambient environment, namely as rule 0.21. The oxygen moves by means of diffusion into the anode chamber, where the oxygen reacts with the hydrogen. In the case of the fuel cell being stopped for longer periods of time, the hydrogen completely reacts; and the oxygen of the anode chamber and the oxygen of the cathode chamber are in equilibrium. As a rule, an oxygen mole fraction of 0.21 and a pressure of 1 atm are present in the anode chamber after a lengthy stoppage. The method can be attuned accordingly. The values, which are deposited in the open-loop and/or closed-loop control unit, for a position of the inlet valve, for a mole fraction of the oxygen, for a reaction time period and/or for a pressure to be set upstream of the anode chamber can be based on the aforementioned values. 
         [0026]    In order to prevent an unnecessary time delay when starting up the fuel cell, provision can be made for the steps of the method to only then be carried out if it is to be expected that the hydrogen in the anode chamber has completely reacted. To this end, a query can be made whether a predefined period of time since stopping the fuel cell has been exceeded. The predefined period of time can, e.g., be fixed between three to five hours. When using the method in a fuel cell vehicle, the predefined steps of the method can alternatively be carried out each time the ignition key is inserted, in particular early on in the process. 
         [0027]    The method may end up with the oxygen being introduced into the cathode chamber and a current circuit of the fuel cell being closed. The position of the inlet valve can only be set according to the required electrical output of the fuel cell. 
         [0028]    In a preferred manner, provision can be made for the following order of the steps of the method: a check can, e.g., initially be made whether a predefined period of time has been exceeded since the fuel cell was last stopped. A check is subsequently made whether the outlet valve is closed and the recirculation line is open. The recirculation means is then switched on, in particular at maximum capacity. The inlet valve is opened to such an extent that the desired ratio of hydrogen to oxygen arises in the anode inlet stream, in particular at the mixing point. If air is present in the anode chamber prior to the start-up process, the mole fraction of the hydrogen in the anode inlet stream is no more than 0.42 at the beginning of the hydrogen introduction stage. The mole fraction of the hydrogen in the anode inlet stream is preferably less than 0.21 at the beginning of the hydrogen introduction stage. In a particularly preferred manner, the mole fraction lies between 0.09 and 0.13. The position of the inlet valve is preferably held constant during the reaction time period. If the reaction time period is exceeded, the open-loop and/or closed-loop control unit can ensure that a current circuit comprising a load and the fuel cell is closed, that oxygen is fed to the cathode, particularly that a compressor is switched on and that the introduction of hydrogen is controlled according to the required electrical output of the fuel cell. The starting up of the fuel cell is ended so that a normal operation of the fuel cell occurs. 
         [0029]    The aim of the present invention is also met by a fuel cell system comprising a fuel cell, which has an anode chamber and a cathode chamber, a hydrogen tank, an inlet valve for metering an introduction of hydrogen into an anode chamber and an open-loop and/or closed-loop control unit. The open-loop and/or closed-loop control unit is designed in such a way that, at the beginning of a start-up process, the inlet valve is only opened to such an extent that enough hydrogen is introduced to ensure that upon entry into the anode chamber hydrogen and oxygen are present in no more than a stoichiometric ratio. The fuel cell system can have a recirculation means for reintroducing an anode outlet stream into the anode chamber, wherein the open-loop and/or closed-loop control unit switches on the recirculation means at maximum capacity prior to the opening of the inlet valve. A method previously described is particularly deposited in the open-loop and/or closed-loop control unit. 
         [0030]    The fuel cell can relate to a polymer electrolyte membrane (PEM) fuel cell. Instead of a single fuel cell, a stack of fuel cells can also be used. In this case, the chamber at the anodes of all of the fuel cells is referred to as the anode chamber. The fuel cell can be disposed in a fuel cell vehicle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    Further measures which improve the invention ensue from the following description with regard to the exemplary embodiments of the invention, which are schematically depicted in the drawings. All of the features and/or advantages, including constructive details, of spatial arrangement and steps of the method, which arise from the claims, the description or the drawings, can be essential to the invention in isolation as well as in a variety of combinations. In the drawings: 
           [0032]      FIG. 1  shows a fuel cell system according to the invention; and 
           [0033]      FIG. 2  shows a method according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    A fuel cell system  10  according to the invention is depicted in  FIG. 1 . The fuel cell system  10  comprises a fuel cell stack, of which only one fuel cell  11  is exemplarily depicted in  FIG. 1 . The fuel cells  11  of the fuel cell stack are designed as PEM fuel cells. Each fuel cell  11  has an anode  12 , a cathode  13  and a membrane  14  as an electrolyte lying between said anode and cathode. During the operation of the fuel cell system  10 , hydrogen is introduced via an anode chamber of the anode  12  and air containing oxygen via a cathode chamber  16  of the cathode  13 . The hydrogen and the oxygen are electrochemically converted to water during the operation of the fuel cells  11 , wherein an electrical output is generated. 
         [0035]    In order to provide hydrogen to the fuel cells  11 , the fuel cell system  10  has a hydrogen tank  21  comprising a pressure reducer, which is not depicted, an adjustable inlet valve  22  and a supply line  23  which is attached thereto. The supply line  23  ends at the anode chamber  15 . The hydrogen flows according to the arrows  50  to the anode chamber  15 . A recirculation line  26  is connected to the anode chamber  15  in order to be able to reintroduce hydrogen, which did not react during operation, into the fuel cell  11 . A fan  27  is provided as the recirculation means in the recirculation line  26 , said fan guiding an anode outlet stream according to the arrows  51  back to the supply line  23 . The recirculation line  26  ends at a mixing point  28  in the supply line  23 . A pressure sensor  32  measures the pressure in the supply line  23 . The anode outlet stream is defined as a volume flow which leaves the anode chamber  15 . 
         [0036]    A 3/2 directional control valve  25  is used as the outlet valve, which both facilitates and prevents a recirculation. Via the 3/2 directional control valve  25 , the exhaust can be released through an outlet line  24  into the ambient environment  30 . The air containing oxygen is led from the ambient environment  30  into the cathode chamber  16  by means of a compressor  31  and released to the ambient environment  30  after passing through the fuel cell  11 . 
         [0037]    The fuel cell system  10  has an open-loop and/or closed-loop control unit  29 , which monitors and adjusts the position of the inlet valve  22 , the position of the 3/2 directional control valve  25  and the electrical output of the fan  27  and of the compressor  31 . The open-loop and/or closed-loop control unit  29  receives measured pressure values for the anode inlet stream from a pressure sensor  32 . The following inventive method, which is depicted in  FIG. 2 , is deposited in the open-loop and/or closed-loop control unit  29 . 
         [0038]    In  FIG. 2 , an exemplary embodiment for a method  40  according to the invention is depicted. At the beginning of the method, air is contained in the anode chamber  15 , in the supply line  23  and in the recirculation line  26 . In a first step  41  of the method, a check is made whether a predefined period of time has been exceeded since the fuel cell  11  was last stopped. In a second step  42 , the position of the 3/2 directional control valve  25  is checked. The 3/2 directional control valve should be open to the recirculation line  26  and closed to the outlet line  24 . In a third step  43  of the method, the fan  27  is switched on at maximum capacity. In so doing, the air which is situated in the anode chamber  15 , in the supply line  23 , in the recirculation line  26  and in the outlet line  24  is moved at maximum speed according to the arrows  50 ,  51 . 
         [0039]    In a fourth step  44  of the method, the inlet valve  22  is subsequently brought into a position which was predefined in the open-loop and/or closed-loop control device  29  and by means of which a predefined pressure at the pressure sensor  32  can be measured. In the adjusted position of the inlet valve  22 , the ratio of the hydrogen to oxygen at the mixing point  28  corresponds to the desired ratio, e.g., the mole fraction of the hydrogen can be 0.13 at the mixing point  28 . 
         [0040]    In a fifth step  45  of the method, the period of time that the inlet valve  22  has been opened is detected. In a sixth step  46  of the method, a query is made whether the period of time corresponds to the reaction time period defined in the open-loop and/or closed-loop control unit  29 . If this is not yet the case, which is depicted with “−” in  FIG. 2 , the steps  45  and  46  of the method are then repeated. If the predefined period of time has been achieved, which is depicted with “+” in  FIG. 2 , the compressor  31  is then switched on in a seventh step  47  of the method; the required hydrogen volume flow is introduced into the anode chamber  15  and, if required, an anode inlet pressure is adjusted. A current circuit of the fuel cell  11 , which is not depicted, is subsequently closed. The position of the inlet valve  22  can be adjusted according to the required electrical output of the fuel cells  11 . In the event that less than 0.79 bar of nitrogen is supposed to be present in the anode chamber  15 , the anode outlet stream has to be discharged out of the anode chamber  15  via the outlet valve  25  out of the fuel cell system  10 . 
         [0041]    The method for starting up the fuel cells  11  is thus finished. Particularly the first, second, and/or seventh step  41 ,  42 ,  47  of the method can optionally be omitted.