Abstract:
The invention relates to a fluid delivery head for an electrochemical cell, for example a fuel cell. The delivery head combines the inlets and outlets of the lines for delivering the fluids, notably hydrogen and oxygen. The hydrogen delivery line feeds an active part of the fuel cell and includes a cavity in communication with a discharge pipe via a solenoid valve. The invention performs several functions with a minimum of elements and with reduced bulk.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International patent application PCT/EP2009/057385, filed on Jun. 15, 2009, which claims priority to foreign French patent application No. FR 08 03637, filed on Jun. 27, 2008, the disclosures of which are incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a fluid delivery head for an electrochemical cell. The invention also relates to a fuel cell equipped with such a fluid delivery head. 
     BACKGROUND OF THE INVENTION 
     Electrochemical cells are energy conversion devices. These devices are generally classified as a function of the direction of the energy conversion. Devices that produce chemical energy from electrical energy are referred to as electrolytic cells, whereas devices that produce electrical energy from chemical energy are referred to as fuel cells or batteries. 
     A fuel cell enables the production of electricity by means of two coupled chemical reactions: the oxidation of a reductive fuel on a first electrode, known as the anode, and the reduction of an oxidizing agent on a second electrode, known as the cathode. At the present time, hydrogen is commonly used as combustible and atmospheric oxygen is used as oxidizing agent. 
     A fuel cell finds particular usefulness in the field of transport, which, hitherto, has essentially used fossil energy mainly derived from petroleum. The use of this energy produces a large amount of carbon dioxide that contributes toward increasing the greenhouse effect on the planet. Other pollutants, such as particles or nitrogen oxides, are also produced by the use of petroleum-based fuels. 
     The main advantage of using a fuel cell using hydrogen and oxygen as feed gases is that the only product of the chemical oxidation and reduction reactions is water. 
     Among the various types of fuel cell that may be distinguished is the proton exchange membrane fuel cell, also known as a polymer electrolyte membrane fuel cell. Such a cell is formed from an elemental cell or a stack of elemental cells intercalated between a terminal plate forming the anode and a terminal plate forming the cathode. 
       FIG. 1  diagrammatically shows a proton exchange membrane fuel cell comprising a single elemental cell  1  intercalated between an anode  2  and a cathode  3 . The elemental cell  1  comprises a polymer electrolyte membrane  4 , known as the membrane  4 , intercalated between two active layers  5   a  and  5   b , for example of porous carbon. Each active layer  5   a ,  5   b  is in contact with a diffusion layer  6   a  or  6   b , respectively, for example a paper or carbon fabric substrate. The diffusion layers  6   a  and  6   b  allow diffusion of the feed gases originating from delivery lines  7   a  and  7   b  to the active layers  5   a  and  5   b , respectively. The delivery lines  7   a  and  7   b  are, for example, partly housed in bipolar plates  8   a  and  8   b . In this figure, the bipolar plates  8   a  and  8   b  are directly in contact with the anode  2  and the cathode  3 . Needless to say, in the case of a stack of elemental cells  1 , a bipolar plate  8   b  of a first elemental cell comes into contact with a bipolar plate  8   a  of a second elemental cell, and so on. 
     In a proton exchange membrane fuel cell using hydrogen and oxygen as feed gases, the hydrogen is introduced in gaseous form at the anode  2 , for example via the delivery line  7   a , while the oxygen is introduced, also in gaseous form, at the cathode  3 , for example via the delivery line  7   b . In the presence of a catalyst, for instance platinum contained in the active layer  5   a , the hydrogen releases electrons e −  according to the following oxidation reaction:
 
H 2 →2H + +2 e   − 
 
     The electrons e −  released into the active layer  5   a  join the active layer  5   b  via an electrical circuit  10  using the electrical energy produced by the fuel cell, and the protons H + , released during this first reaction, migrate to the active layer  5   b  by crossing the membrane  4 . At the active layer  5   b , the protons H +  combine with oxygen O 2  and with the electrons e − , again in the presence of a catalyst, according to the following reduction reaction:
 
2H + +½O 2 +2 e   − →H 2 O
 
     Overall, the following redox reaction takes place:
 
H 2 +½O 2 →H 2 O
 
     For a better energy yield, the oxidation and reduction reactions must take place within a certain temperature and pressure range. To ensure this adequate operating temperature, a heat-exchange fluid maintained at a temperature within this temperature range circulates in a pipe passing around or through the elemental cells  1 . 
     The functioning of a fuel cell requires numerous fluid exchanges with devices peripheral to the fuel cell. In particular, the heat-exchange fluid requires passage through a device for maintaining its temperature. Similarly, the delivery lines  7   a  and  7   b  need to be connected to hydrogen and oxygen feed circuits. For the sake of reducing the bulk, the fluid delivery lines may open into the same component, known as the fluid delivery head or, more simply, the delivery head. 
       FIGS. 2   a  and  2   b  represent an example of a delivery head  21  in front view and cross-sectional view, respectively. 
     The delivery head  21  comprises an inlet connector  22   a  connected to the heat-exchange fluid pipe  23  at an inlet orifice  24   a . The pipe  23  comprises a pipe portion  23   a  integrated into the delivery head  21 . The pipe  23  extends inside or around the elemental cell(s)  1  and ends with a pipe portion  23   b , which is, for example, integrated into the delivery head  21 . This pipe portion  23   b  comprises an outlet orifice  24   b  that can receive an outlet connector, not shown. The delivery head  21  also comprises an inlet orifice  26   a  and an outlet orifice  26   b  for connecting the delivery line  7   a  to an external circuit such as a hydrogen feed circuit, and also an inlet orifice  28   a  and an outlet orifice  28   b  for connecting the delivery line  7   b  to an external circuit such as an oxygen feed circuit. 
     For correct functioning of the fuel cell, the membrane  4  must contain water in order to allow the transfer of protons H +  from the active layer  5   a  on the anode  2  side to the active layer  5   b  on the cathode  3  side. The membrane  4  is water-permeable. Consequently, a transfer of water takes place from the cathode  3  to the anode  2  via a diffusion mechanism due to the difference in water concentration on each side of the membrane  4 . This water diffusion mechanism cohabits with a diffusion of other species such as nitrogen. Due to the temperature of the fuel cell, the water exiting the delivery line  7   a  is essentially present in gaseous form. In order to optimize the yield of the cell and to increase its service life, this water and the other species such as nitrogen are reinjected into the inlet of the delivery line  7   a  with the hydrogen. The circulation of hydrogen and the reinjection of water and nitrogen may be ensured by a circuit external to the fuel cell, comprising, for example, a pump or an ejector  30 , shown in  FIGS. 2   a  and  2   b . The amount of water present in liquid form must, however, be precisely controlled. The reason for this is that an excessive amount of water in the elemental cell  1  prevents the feeding of hydrogen and oxygen to the active layers  5   a  and  5   b , resulting in a voltage inversion at the terminals of the fuel cell and thus to electrolysis of water. In certain cases, for example in the case of a strong current demand, the fuel cell may be destroyed. In order to limit the risks of injection of water in liquid form at the inlet of the delivery line  7   a , it is possible to place a conventional phase separator in the hydrogen feed circuit upstream of the delivery line  7   a . However, such a phase separator is generally bulky. This bulk is an inconvenience for portable use. Moreover, a phase separator placed upstream of the delivery line  7   a  cannot control the amount of water present in liquid form within the elemental cell  1  due to the absence of control of the condensation of water in the pipe connecting the separator to the inlet orifice  26   a.    
     SUMMARY OF THE INVENTION 
     One aim of the invention is notably to overcome all or some of the abovementioned drawbacks by proposing a sparingly bulky device that can control the amount of water inside the elemental cells  1 . To this end, one subject of the invention is a fluid delivery head comprising a line for delivering a fluid to an active part of an electrochemical cell, characterized in that the delivery line comprises a cavity in communication with a discharge pipe via a solenoid valve. A subject of the invention is also a fuel cell comprising an elemental cell in which takes place a redox reaction that can generate an electrical current between two electrodes of the fuel cell, characterized in that it comprises a delivery head according to the invention. 
     An advantage of the invention is notably that it can control the amount of water present in liquid form in the fuel cell with a minimum of bulk. The invention also minimizes the length of the hydrogen feed circuit and, consequently, limits the condensation. The water in gaseous form is thus preserved and is reinjected into the inlet of the hydrogen delivery line, ensuring an increase in the service life of the fuel cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other advantages will emerge on reading the detailed description of an embodiment given as an example, the description being made with regard to the attached drawings, which show: 
         FIG. 1 , a proton exchange membrane fuel cell and its operating principle; 
         FIGS. 2   a  and  2   b , an example of a fluid delivery head for a fuel cell; 
         FIGS. 3 and 4 , a part of the fluid delivery head concerned by the invention. 
     
    
    
     DETAILED DESCRIPTION 
     For the rest of the description, a fuel cell, for example a proton exchange membrane fuel cell, will be considered using hydrogen and oxygen as feed gases. However, it is possible to use other feed gases without departing from the scope of the invention. Furthermore, the invention equally applies to other types of fuel cell and, in general, to any electrochemical cell. 
       FIGS. 3 and 4  show a cross-sectional view of a part of the fluid delivery head  21 . The delivery head  21  comprises the outlet orifice  26   b  of the delivery line  7   a . The delivery line  7   a  is in communication with an active part of the fuel cell, in this instance the delivery layer  6   a  on the anode  2  side. Consequently, a mixture of fluids  31  containing hydrogen H 2  not consumed by the oxidation reaction, nitrogen N 2  and water in liquid and gaseous form H 2 O liquid+vapor  arrives from the active part of the fuel cell and heads toward the outlet orifice  26   b . A connector  32  connects the delivery line  7   a  to an external circuit such as a hydrogen feed circuit, not shown. According to the invention, the delivery line  7   a  comprises a cavity  34  in communication with a discharge pipe  35  via a solenoid valve  36 . The cavity  34  is made, for example, in the form of the delivery head  21  or, as shown in  FIGS. 3 and 4 , is made by a pipe  38  opening into the delivery line  7   a  via an orifice  39  made in the delivery head  21 . The latter embodiment allows the invention to be applied to delivery heads  21  for which the cavity  34  was not originally planned. The invention allows, during the passage of a liquid in the cavity  34 , the guiding of this liquid in the cavity  34 . When the solenoid valve  36  is closed, the liquid, for example water in liquid form H 2 O liquid , is stored in the cavity  34 , and when the solenoid valve  36  is open, the liquid is discharged to the discharge pipe  35 . 
     According to one particular embodiment, the cavity  34  is formed in the region of the outlet orifice  26   b . This embodiment makes it possible to separate the water present in liquid form H 2 O liquid  from the rest of the fluid mixture  31 , notably hydrogen H 2 , nitrogen N 2  and water H 2 O in gaseous form H 2 O vapor.  The rest of the fluid mixture  31  is referred to as the gaseous mixture  41 . According to this embodiment, during the passage of the fluid mixture  31  in the cavity  34 , the water in liquid form H 2 O liquid  is guided in the cavity  34 , while the gaseous mixture  41  is directed toward the outlet orifice  26   b.    
     According to one particular embodiment, the cavity  34  is formed at a bottom point of the delivery line  7   a . The term “bottom point” means the point in the delivery line  7   a  where a liquid naturally accumulates due to the effect of the Earth&#39;s gravity. This embodiment makes it possible to collect in the cavity  34  all of the water in liquid form H 2 O liquid  present in the delivery line  7   a . Consequently, the amount of liquid water H 2 O liquid  in the delivery line  7   a , and thus in the elemental cell(s)  1 , may be precisely controlled by monitoring the amount of water present in the cavity  34 . 
     According to one particular embodiment, the solenoid valve  36  is connected to means for controlling its opening. These means comprise, for example, an independent computer or a system for management of the functioning of the fuel cell. The means for controlling the opening of the solenoid valve  36  may also comprise a liquid level sensor  42  located in the delivery line  7   a , for example connected to the computer. According to this embodiment, the opening of the solenoid valve  36  may be controlled as a function of a level of liquid in the delivery line  7   a . The solenoid valve  36  is, for example, open when the liquid sensor  42  detects the presence of liquid, and is closed when the liquid sensor  42  does not detect any liquid. 
     In one embodiment, shown in  FIGS. 3 and 4 , the liquid level sensor  42  is located in the cavity  34 , above the solenoid valve  36 . In other words, the liquid level sensor  42  and the solenoid valve  36  are arranged such that a liquid can accumulate by gravity in the cavity  34  before reaching the liquid level sensor  42 . This embodiment makes it possible to conserve a minimum amount of water in liquid form H 2 O liquid  at the bottom of the cavity  34  while at the same time avoiding overflow of the cavity  34 . Thus, the gaseous mixture  41  does not run the risk of being discharged via the discharge pipe  35 , and the liquid water H 2 O liquid  is discharged from the delivery line  7   a.    
     In one embodiment, the means for controlling the opening of the solenoid valve  36  comprise a pressure sensor located in the delivery line  7   a . This pressure sensor may be connected to a computer or to the system for management of the functioning of the fuel cell. The pressure sensor provides information regarding the pressure prevailing inside the delivery line  7   a . This pressure may also be taken into account for the piloting of the opening of the solenoid valve  36 . In particular, the solenoid valve  36  may be opened when the pressure passes above a predetermined threshold. All the liquid water H 2 O liquid  may then be discharged via the discharge pipe  35 . The opening of the solenoid valve  36  also makes it possible to discharge all or some of the fluid mixture  31 , thus reducing the pressure in the delivery line  7   a . This operating phase is shown in  FIG. 4 . The solenoid valve  36  can then be reclosed either when the pressure returns below a certain value, or after a certain time. The combination of the pressure sensor, the solenoid valve  36  and the discharge pipe  35  acts, in point of fact, as an excess-pressure check valve. 
     According to one particular embodiment, the delivery head  21  comprises a tube  44  for discharging gases from the delivery line  7   a . The tube  44  may cross the cavity  34  in order to obtain a compact device. In particular, the discharged gases may be hydrogen H 2 , nitrogen N 2  and water in gaseous form H 2 O vapor  of the fluid mixture  31 . The tube  44  has, for example, an inside diameter of between 0.1 and 0.7 mm. The tube  44  allows periodic or continuous sampling of a certain amount of gases from the delivery line  7   a . This sampling of gases notably limits the increase in concentration of nitrogen N 2  in the hydrogen H 2  feed circuit, thus regulating the ratio between hydrogen H 2  and the other gases present at the inlet of the delivery line  7   a . As visible in  FIGS. 3-4 , the tube  44  is oriented downwardly from the inlet end of the tube. 
     In one embodiment, one end  45  of the tube  44  is located above the liquid level sensor  42 . The gas present in the delivery line  7   a  may thus be sampled without discharging liquid water H 2 O liquid . 
     In summary, the invention can perform several functions with a minimum of elements and with reduced bulk. The invention notably performs a function of discharging liquid water H 2 O liquid , a function as an excess-pressure check valve and a function of regulating the amount of nitrogen N 2 .