Method for shutting down a fuel cell and electrical system including a fuel cell

Abstract

Method for shutting down a fuel cell and electrical system comprising a fuel cell. The present invention relates to a method for shutting down a fuel cell comprising a drying phase (P1), a time-delay phase (P2), and a depolarization phase (P3). During the drying and time-delay phases, the fuel cell is supplied with air, produces a current density (I1) lower than its idle current density (IR), and the electrical power produced by the fuel cell is converted into work by an electrical load. During the drying phase, the moisture level (λ) of the proton exchange membrane (20) of each cell is decreased until it reaches a predetermined target value (λ1). During the time-delay phase, the moisture level of the membrane is maintained substantially equal to the predetermined target value.During depolarization, the fuel cell's air supply is cut off, and the fuel cell is connected to a dissipative system that dissipates the electrical power produced by the fuel cell. Figure for the abstract: 4.

Description

Title of the invention: Method for shutting down a fuel cell and electrical system comprising a fuel cell

[0001] The present invention relates to a method for stopping a fuel cell.

[0002] A fuel cell is a device that generates electricity through an electrochemical reaction between a fuel, usually dihydrogen, and an oxidant, usually oxygen from the air. This discussion focuses on proton exchange membrane fuel cells with a solid electrolyte – also known as PEMFCs – which typically comprise a stack of several unit cells, each constituting an electrochemical generator.

[0003] Schematically, each unit cell comprises two separators, also called polar plates, between which a solid electrolyte is interposed in the form of a proton exchange membrane. The membrane is made, for example, of a sulfonated perfluorinated polymer material. Within each cell, each separator, together with the corresponding membrane, delimits a reactive compartment. One of the two compartments, called the cathodic compartment, houses a cathodic element, formed by a cathodic catalytic layer located on the surface of the membrane, while the other compartment, called the anodic compartment, houses an anodic element, formed by an anodic catalytic layer located on the surface of the membrane. The assembly of the membrane and the anodic and cathodic catalytic layers forms a membrane-electrode assembly, generally referred to as an "MEA".

[0004] For two adjacent cells, a separator from one of the two cells is placed back-to-back with a separator from the other cell. These two separators together form a bipolar separator, also called a bipolar plate. A cooling compartment, in which a heat transfer fluid such as glycol water circulates, is generally provided between the two separators of the bipolar separator.

[0005] Dihydrogen, air, and the heat transfer fluid are called "operating fluids," which are supplied to the fuel cell during its operation. Dihydrogen and air are reactants, while the heat transfer fluid does not participate in the electrochemical reaction. Depending on the operating phases of the fuel cell, the supply of one or more of the operating fluids is continuous or intermittent.

[0006] The fuel cell thus provides openings to supply fluids to each of the reactive compartments and to the fluids between two neighboring cells. Thus, in a widely used design, each bipolar separator ensures on one side the supply of dihydrogen to the cell adjacent to this side and on the other side the supply of oxygen to the cell adjacent to this other side, the supplies provided by the bipolar separators being done in parallel.

[0007] Generally, in a unit cell, the cathode compartment is supplied with oxygen, most often in the form of an oxygen-containing air supply, and the anodic compartment is supplied with dihydrogen. Each reactive compartment also generally includes a gas diffusion layer, located between the bipolar separator and the catalytic layer, allowing good circulation of dihydrogen or oxygen from the separator to the catalytic layer.

[0008] When the fuel cell is operating, the electrochemical reaction creates an electrical potential difference between the two separators of each unit cell. The electrical potential difference between the two separators of each unit cell creates a voltage across the terminals of each cell, referred to as the "cell voltage." All the cells of the fuel cell are electrically connected in series, so that the voltage delivered across the terminals of the fuel cell is equal to the sum of the cell voltages of all the unit cells.

[0009] Generally, such a fuel cell is electrically connected to a DC-DC converter, which conditions and delivers the electrical power produced by the fuel cell to an electrical load, such as an electric motor and / or a vehicle battery. Furthermore, the DC-DC converter allows the fuel cell to be controlled by determining its electrical operating point, that is, by determining the current supplied by the cell and the voltage across its terminals.

[0010] A common problem when using such a fuel cell is that, when the fuel cell is stopped, i.e. when the DC-DC converter powered by the fuel cell is stopped and the supply of air and dihydrogen is stopped, reactants are still present in the cathode compartment and in the anode compartment after the fuel cell has stopped, and the electrochemical reaction therefore continues to occur within each cell.

[0011] Furthermore, in the event of a prolonged shutdown of the fuel cell, oxygen migrates from the cathode compartment to the anodic compartment in each cell by diffusion through the membrane. This presence of oxygen in the anodic compartment during the shutdown leads, when the fuel cell is refueled with hydrogen at the beginning of its restart, to corrosion of the gas diffusion layer of this cathodic compartment, and more specifically, corrosion of a carbon substrate in the cathodic compartment. on which a catalyst, such as platinum, rests, which degrades the longevity and performance of the fuel cell. In practice, the presence of oxygen in the anodic compartment when the fuel cell is stopped leads, when pure hydrogen is introduced when the fuel cell is restarted, to the appearance of two zones in the anodic compartment, a first containing oxygen and a second containing hydrogen, these two zones being delimited by a front, which moves towards the outlet of the anodic compartment as hydrogen is introduced, traversing the entire active surface of the proton exchange membrane, which leads to the appearance of a high electrical potential at the cathode, activating the catalytic corrosion of the carbon substrate in the presence of water and forming carbon dioxide.

[0012] To avoid these drawbacks, it is known to perform a shutdown procedure aimed at reducing the amount of oxygen present in the cathode compartment and at reducing the voltage delivered to the terminals of the fuel cell, i.e., depolarizing the fuel cell. These operations take place at the end of the fuel cell shutdown procedure and occur during a phase generally referred to as the "depolarization phase" of the fuel cell. Furthermore, it is known to disconnect the fuel cell from the DC-DC converter and connect it to a dissipative system comprising a resistor, called a "depolarization resistor," while maintaining the hydrogen supply but having previously cut off the air supply.Thus, the electrical power produced by the fuel cell after it is switched off is quickly consumed by the resistor, which avoids continuing to deliver an electrical voltage to the terminals of the fuel cell after it is switched off, and the oxygen present in the cathode compartment in contact with the proton exchange membrane is consumed.

[0013] Furthermore, it is also common practice to dry the fuel cell during its shutdown procedure, that is, to remove residual liquid water and reduce its water content at the proton exchange membrane. Drying the fuel cell thus corresponds more specifically to drying the proton exchange membrane. Such drying makes it possible, in particular, in the event of subsequent startup of the fuel cell in sub-zero temperatures, to prevent the formation of ice plugs on the active surface of the cells, which would degrade the fuel cell's performance and reduce its lifespan. Without drying, these ice plugs can originate either from residual liquid water or from the initial quantities of water produced during the fuel cell's startup, before it reaches a sufficient temperature to prevent the water produced from freezing.

[0014] Generally, such drying takes place at the beginning of the fuel cell shutdown procedure. During such drying, it is important to sufficiently reduce the level humidity of the proton exchange membrane to prevent the appearance of ice plugs, without reducing this humidity level too much, because a proton exchange membrane with too low a humidity level is likely to degrade and its performance will decrease.

[0015] During the shutdown procedure of a fuel cell, it is also desirable to be able to perform diagnostics.

[0016] One difficulty with known shutdown procedures is that the sequence of the drying phase followed by the depolarization phase does not allow, or complicates, the performance of certain control actions or diagnostics during the shutdown procedure, such as hydrogen leak tests, which must be carried out before the depolarization phase. Indeed, the duration of such diagnostics or control actions is sometimes longer than the duration of the drying phase, making it difficult to synchronize the drying phase with these diagnostics or control actions. Furthermore, it is generally undesirable to extend the drying time to allow for these diagnostics or control actions, as extending the drying time generally leads to an excessive reduction in the moisture level of the proton exchange membrane, potentially damaging it.

[0017] The object of the invention is to propose a shutdown procedure including a drying phase and a depolarization phase, but whose duration is easily adaptable to allow for diagnostics or control actions, without negatively affecting the humidity level of the proton exchange membrane.

[0018] To this end, the invention relates to a method for stopping a fuel cell belonging to an electrical system, the fuel cell comprising: - a stack of cells, each cell comprising an anodic compartment and a cathodic compartment separated by a proton exchange membrane, - a dihydrogen inlet supplying dihydrogen to the anodic compartment of each cell, and a dihydrogen outlet removing the dihydrogen from each cell, - an air inlet supplying the cathode compartment of each cell with air, and an air outlet removing the air from each cell.

[0019] In addition, the electrical system comprises: - an electrical consumer, adapted to be electrically connected to the fuel cell to provide work from electrical power produced by the fuel cell, and - a dissipative system, adapted to be electrically connected to the fuel cell to dissipate electrical power produced by the fuel cell.

[0020] The method for shutting down the fuel cell comprises: - a drying phase, in which: • the humidity level of the proton exchange membrane of each cell is decreased until it reaches a predetermined target value, • an air supply to the fuel cell via the air inlet and an exhaust of air from the fuel cell via the air outlet are maintained, • the fuel cell is controlled so that a current density produced by the fuel cell is maintained above OA / cm2 and below an idle current density corresponding to a current density produced by the fuel cell when the fuel cell is operating at idle, and • The electrical consumer is electrically connected to the fuel cell and provides work from electrical power produced by the fuel cell. - a depolarization phase, in which: • The air supply to the fuel cell via the air intake and the exhaust of air from the fuel cell via the air outlet are cut off, and • The dissipative system is electrically connected to the fuel cell and dissipates the electrical power produced by the fuel cell.

[0021] According to the invention, the fuel cell shutdown method further comprises: - a time-delay phase, implemented after the drying phase and before the depolarization phase, the time-delay phase beginning when the humidity level reaches the predetermined target value, in which: • The humidity level of the proton exchange membrane of each cell is maintained approximately equal to the predetermined target value, • The supply of air to the fuel cell via the air inlet and the exhaust of air from the fuel cell via the air outlet are maintained, • the fuel cell is controlled so that the current density produced by the fuel cell is maintained above OA / cm2 and below the idle current density, and • The electrical consumer is electrically connected to the fuel cell and provides work from electrical power produced by the fuel cell.

[0022] and the timing phase ends and the depolarization phase begins when a predefined stopping condition is reached.

[0023] Thanks to the invention, the delay phase makes it possible to postpone the depolarization phase without altering the moisture level of the proton exchange membrane reached at the end of the drying phase. The delay phase thus allows for diagnostics or control actions, for example, a leak test of the hydrogen supply to the fuel cell, without requiring complex synchronization between such diagnostics or control actions and the drying phase.

[0024] According to other advantageous aspects of the invention, the stopping method comprises one or more of the following features, taken individually or in all technically possible combinations:

[0025] - During the waiting phase, the moisture level of the membrane The proton exchanger of each cell is maintained substantially equal to the predetermined target value by regulating the flow rate of the fuel cell air supply through the air inlet.

[0026] - During the timing phase, the flow rate of the fuel cell air supply The fuel cell air supply through the air inlet is increased when the humidity level of the proton exchange membrane of each cell tends to increase, and the fuel cell air supply flow rate through the air inlet is decreased when the humidity level of the proton exchange membrane of each cell tends to decrease.

[0027] - The electrical system further comprises a control unit. During the During the timing phase, the control unit determines a quantity representative of the humidity level of the proton exchange membrane of each cell, said representative quantity being obtained from a measurement of a temperature prevailing within the cathode compartment of each cell of the fuel cell and a measurement of a condensation temperature of the water vapor contained in the air supplying the fuel cell at the air inlet or in the air exhausted from the fuel cell at the air outlet, and / or from a measurement of a temperature prevailing within the cathode compartment of each cell of the fuel cell and a measurement of the impedance of the proton exchange membrane, and regulates the flow rate of the air supply to the fuel cell through the air inlet according to the determined representative quantity.

[0028] - During the timing phase, the control unit regulates the flow rate of the supply of air to the fuel cell via the air intake so as to vary the condensation temperature of the water vapor contained in the air supplying the fuel cell at the air inlet or in the air exhausted from the fuel cell at the air outlet, depending on the temperature prevailing within the cathode compartment of each fuel cell, to maintain the humidity level of the proton exchange membrane of each cell substantially equal to the predetermined target value.

[0029] - The idle current density is between 0.05 A / cm2 and 0.2 A / cm2, of preference equal to 0.1 A / cm2.

[0030] - During the timing phase, the fuel cell is controlled so that the current density produced by the fuel cell shall be maintained greater than OA / cm2 and less than 0.1 A / cm2, preferably less than 0.08A / cm2, preferably even less than 0.04A / cm2.

[0031] - During the drying phase, the fuel cell is controlled so that the current density produced by the fuel cell shall be maintained greater than OA / cm2 and less than 0.1 A / cm2, preferably less than 0.08A / cm2, preferably even less than 0.04A / cm2.

[0032] - During the drying phase and during the time-delay phase, the battery fuel is controlled so that the current density produced by the fuel cell is maintained substantially equal to a constant target current density value.

[0033] - The predefined stopping condition is reached when a duration of the phase of time delay reaches a predefined duration and in which the predefined duration of the time delay phase is preferably greater than 120 seconds.

[0034] - The electrical system further includes a regulation circuit for regulating The fuel cell temperature is controlled by a heat transfer fluid. The fuel cell further comprises a heat transfer fluid inlet supplying each cell with heat transfer fluid, and a heat transfer fluid outlet removing the heat transfer fluid from each cell. Both the inlet and outlet are connected to the control circuit. During the drying phase, the control circuit is activated to lower the temperature of the heat transfer fluid at the fuel cell outlet to a predetermined temperature. During the cool-down phase, the control circuit is activated to lower the temperature of the heat transfer fluid at the fuel cell outlet from the predetermined temperature to a second predetermined temperature.

[0035] - The predefined stopping condition is reached when the fluid temperature heat transfer fluid at the outlet of the fuel cell reaches the second predetermined temperature.

[0036] - During the depolarization phase, the regulatory circuit is controlled to that the temperature of the heat transfer fluid at the outlet of the fuel cell decreases from the second predetermined temperature to a third predetermined temperature.

[0037] - The electrical consumer includes an electrical load, for example a electric motor and / or a battery, and, optionally, a DC-DC converter, adapted to be electrically connected to the fuel cell, to control the fuel cell and to condition and deliver electrical power produced by the fuel cell to the electrical load.

[0038] According to another aspect, the invention also relates to an electrical system comprising a fuel cell, an electrical consumer adapted to be electrically connected to the fuel cell to provide work from electrical power produced by the fuel cell, a dissipative system adapted to be electrically connected to the fuel cell to dissipate electrical power produced by the fuel cell, and a control unit.The fuel cell comprises: a stack of cells, each cell having an anodic compartment and a cathodic compartment separated by a proton exchange membrane; a hydrogen inlet supplying the anodic compartment of each cell with hydrogen, and a hydrogen outlet removing the hydrogen from each cell; an air inlet supplying the cathodic compartment of each cell with air, and an air outlet removing the air from each cell. Furthermore, the control unit is configured to implement the shutdown procedure described above.

[0039] This electrical system induces the same advantages as those mentioned above regarding the stopping method of the invention.

[0040] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0041] [Fig. 1] The [Fig. 1] is an exploded perspective view of a stack of some cells of a fuel cell belonging to an electrical system according to the invention.

[0042] [Fig.2] The [Fig.2] is a schematic representation of an electrical system comprising the fuel cell of the [Fig.1], the electrical system being in accordance with the invention.

[0043] [Fig.3] The [Fig.3] is a schematic representation of a temperature control circuit belonging to the electrical system of the [Fig.2].

[0044] [Fig.4] The [Fig.4] is a representative timing diagram of a fuel cell shutdown procedure of the [Fig.1].

[0045] [Fig.5] The [Fig.5] is a schematic representation of a control loop implemented during a phase of the shutdown procedure of the [Fig.4].

[0046] Fig. 1 illustrates a stack of cells 12 for a fuel cell 10. This fuel cell is intended to be installed in an electrical system and to produce electricity powering an electrical load of the electrical system.

[0047] The fuel cell 10 is for example intended to be installed in a vehicle and to produce electricity powering an electric motor, directly or indirectly via a battery, providing propulsion for the vehicle.

[0048] The fuel cell 10 is of the proton exchange membrane fuel cell type and therefore comprises said stack of cells 12. This stack is held between two terminal plates 13A, 13B, which are not shown in [Fig. 1] but are visible in [Fig. 3]. These terminal plates notably allow the stack of cells 12 to be kept compressed, i.e. tightly packed, and to supply the stack with dihydrogen in gaseous form, and with air in gaseous form, and, where applicable, the circulation of a heat transfer fluid for a cell cooling circuit.

[0049] The invention will be described more particularly in the context of a common construction in which each cell 12 comprises a membrane-electrode assembly 14 and two bipolar plates 16, arranged on either side of the membrane-electrode assembly. The bipolar plates 16 are also called polar separators. However, the invention is also applicable in the context of solid electrolyte ion-exchange membrane fuel cells having different constructions.

[0050] It is assumed that, for a given fuel cell 10, all the cells 12 of the fuel cell are identical to each other, and therefore have identical characteristics. Furthermore, for the sake of simplicity, it is also assumed that the physicochemical properties of all the cells 12 and the characteristic quantities representing the state and operation of all the cells 12 are identical, that is to say, homogeneous. Thus, in what follows, referring to a characteristic of one cell 12 is equivalent to referring to that same characteristic for all the cells 12.

[0051] In [Fig.1], a detail of a cross-section of the membrane-electrode assembly 14 of a cell 12 is also shown.

[0052] In practice, each bipolar plate 16 is arranged between two cells 12 and is common to both cells. A first face 16A, called the anode face, supplies one of the two cells with dihydrogen, and a second face 16B, called the cathode face, supplies the other of the two cells with air. In other words, a cell 12 is supplied with dihydrogen by a first bipolar plate 16 and is supplied with air by a second bipolar plate. Since the air essentially contains a mixture of nitrogen and oxygen, cell 12 is thus supplied with oxygen.

[0053] In the following description, the terms oxygen and dioxygen, as well as hydrogen and dihydrogen, are used interchangeably.

[0054] In the example, each bipolar plate 16 is formed by the assembly of two superimposed polar plates. This assembly forms hydrogen circulation channels on face 16A, air circulation channels on face 16B, and between faces 16A and 16B, that is, inside the bipolar plate, therefore between the two polar plates, heat transfer fluid circulation channels. The circulation of this heat transfer fluid does not play a direct role in the electrochemical reactions of the fuel cell 10, but it allows the temperature of the cells 12, and therefore of the fuel cell, to be controlled.

[0055] The membrane-electrode assembly 14 comprises two gas diffusion layers 18 arranged on either side of a proton exchange membrane 20, as well as an anodic catalytic layer 22, which is for example deposited on a first surface of the membrane, and a cathodic catalytic layer 24, which is for example deposited on the other surface of the membrane.

[0056] Thus, in the example, each cell 12 comprises, in this order, a bipolar plate 16 supplying the cell with dihydrogen, a gas diffusion layer 18, an anodic catalytic layer 22, a proton exchange membrane 20, a cathodic catalytic layer 24, a gas diffusion layer 18 and a bipolar plate 16 supplying the cell with air.

[0057] Each cell 12 is divided into an anodic compartment, formed between the bipolar plate 16 supplying the cell with dihydrogen and the membrane 20, and a cathodic compartment, formed between the bipolar plate 16 supplying the cell with air and the membrane. Thus, the anodic catalytic layer 22 is arranged in the anodic compartment and the cathodic catalytic layer 24 is arranged in the cathodic compartment.

[0058] The gas diffusion layers 18, which are therefore each arranged in their respective anodic or cathodic compartment, allow the transport of the fuel and oxidizing gases, i.e. dihydrogen and oxygen, from the bipolar plates 16 to the catalytic anodic 22 and cathodic 24 layers. In practice, the gas diffusion layers are formed of a porous material, such as for example a non-woven textile of carbon fibers, i.e. a textile of carbon fibers whose fibers are randomly arranged, or a porous carbon paper, generally impregnated with a polymer, preferably a hydrophobic polymer, for example a fluoropolymer such as polytetrafluoroethylene (PTFE), in particular with the aim of making the surface of the carbon paper fibers more hydrophobic.

[0059] The proton exchange membrane 20 allows the passage of hydrogen ions, or protons, from the anodic compartment to the cathodic compartment 24, while preventing the flow of gases and electrons between these two compartments. It is, for example, made of a sulfonated perfluorinated polymer material, such as a material known by the trade name Nafion or a material known by the trade name Aquivion. In practice, the proton exchange membrane 20 comprises a polymer matrix, or polymer main chains, for example, of polytetrafluoroethylene (PTFE), to which sulfonic acid groups (-SO3H) are attached. In the case of Nafion, the proton exchange membrane 20 comprises PTFE main chains with perfluorinated side chains terminated by the sulfonic acid groups. Furthermore, the sulfonic acid groups allow the conduction of protons through the proton exchange membrane.More precisely, a sulfonic group is said to be "neutral" when it consists of a sulfur atom bonded to three oxygen atoms, one of which is bonded to a hydrogen atom, thus having the formula -SO3H, and a sulfonic group is said to be "charged" when it has undergone dissociation, that is to say, it has released a proton H+ and corresponds to a sulfonate ion with the formula -SO3. In particular, a sulfonic group becomes charged in the presence of water, then releasing a proton H+ which is free to move across the proton exchange membrane 20 towards another charged sulfonic group, the charged sulfonic groups being able to temporarily accept and then release protons H+ during their movement.The movement of H+ protons between charged sulfonic groups occurs primarily via the Grotthuss mechanism, through water molecules occupying the free spaces formed in the polymer matrix, i.e., the free spaces formed between the main polymer chains. Thus, the hydration of the proton exchange polymer membrane improves its proton conductivity, firstly by allowing the sulfonic groups to be charged, and secondly by allowing the movement of protons between the charged sulfonic sites.

[0060] When the fuel cell 10 is operating, within each cell 12, an oxidation reaction occurs in the anodic compartment, at the level of the anodic catalytic layer 22. This oxidation reaction consists of catalytically splitting the dihydrogen supplied through the gas diffusion layer 18 into protons and electrons. The protons thus produced pass through the proton exchange membrane 20 until they reach the cathodic catalytic layer in the cathodic compartment, while the electrons are captured by the anode side 16A of the adjacent bipolar plate 16 and then conducted to the cathodic side 16B of this same bipolar plate, this cathodic side belonging to the cathodic compartment of the adjacent cell 12. At the same time, a reduction reaction takes place in the cathodic compartment of cell 12, at the level of the cathodic catalytic layer 24. This reduction reaction consists of reacting the dioxygen molecules supplied by the air, through the gas diffusion layer 18, with the protons crossing the proton exchange membrane 20 as well as with the electrons supplied by the cathodic side face 16B of the bipolar plate 16, to form water molecules, in the form of water vapor.

[0061] In practice, the catalytic layers 22 and 24 are porous structures made of three different materials, namely: - A material to transport protons, for example the same material as the proton exchange membrane 20, here Nafion or Aquivion. - A material for transporting electrons, for example carbon. - A material to catalyze the electrochemical oxidation and reduction reactions described above, for example platinum. This material is present in the form of particles, preferably spherical, which are deposited, for example, on the surface of said material to transport electrons, for example the carbon mentioned above, during the fabrication of the catalytic layers.

[0062] In addition, the pores of the catalytic layers allow the free transport of reactants, i.e. dihydrogen and oxygen, inside the catalytic layers.

[0063] Within the catalytic layers 22 and 24, there are regions where these three materials and the pores meet. These regions are called active sites, or triple points, and the electrochemical reactions occur at these active sites. Regions where not all the constituent elements of the catalytic layers are present, particularly areas where platinum is present but Nafion, Aquivion, or carbon is lacking, or where access for the reactants is insufficient, are called dead zones.

[0064] The catalytic layers 22 and 24 also contain impurities, or contaminants, which are, for example, residues or additives from the manufacture of the catalytic layers. Furthermore, the platinum particles contained in the catalytic layers generally have an oxide layer on their surface. When this oxide layer becomes too thick, the platinum particles can no longer react with protons and electrons, and such a thick oxide layer on the surface of the platinum particles can therefore be considered an impurity.

[0065] The membrane-electrode assembly 14 of a cell 12 is in practice arranged in an opening formed in a support plate 25, the support plate 25 being interposed between two bipolar plates 16. The support plate 25 can be made in the form of one or two layers of polymer film with a thickness of, for example, between 50 and 200 microns. The polymer film is, for example made of polyethylene terephthalate, also known by the acronym PET, or of polyethylene naphthalate, also known by the acronym PEN. Advantageously, to ensure the seal between the membrane-electrode assembly and the bipolar plates 16 in the stack 12, the membrane-electrode assembly includes two reinforcements 26, located at the periphery of the membrane-electrode assembly, arranged between the gas diffusion layers 18 and the catalytic layers 22, 24, and extending to the support plate 25.

[0066] The fuel cell 10 includes a dihydrogen inlet 28 supplying each cell with dihydrogen and a dihydrogen outlet 30 removing the dihydrogen from each cell.

[0067] The fuel cell includes an air inlet 32 ​​supplying each cell with air and an air outlet 34 removing the air from each cell, this air being, in operation of the cell, depleted in oxygen.

[0068] The air outlet 34 also allows the water produced by the fuel cell contained in the cathode compartments of each cell to be evacuated in the form of water vapor.

[0069] The fuel cell includes a heat transfer fluid inlet 36 supplying each cell with heat transfer fluid and a heat transfer fluid outlet 38 removing the heat transfer fluid from each cell.

[0070] In the example shown in [Fig. 1], the inlets 28, 32, and 36, as well as the outlets 30, 34, and 38, are formed by openings in the bipolar plates 16 and the support plates 25. These openings, through the stacking of the cells, form distribution channels for the stack, with one distribution channel corresponding to each of these inlets and outlets. Furthermore, these inlets and outlets are connected to openings in one of the terminal plates, which are themselves connected to supply circuits for hydrogen, air, and heat transfer fluid, such as flexible or rigid piping. Alternatively, these inlets and outlets are formed by conduits around the stack of cells and bipolar plates of the fuel cell.

[0071] As seen in [Fig.1], on its cathode side 16B, a bipolar plate 16 has two homogenization zones 40, 41 and an active zone 42. A first homogenization zone 40 connects the air inlet 32 ​​to the active zone and a second homogenization zone 41 connects the active zone to the air outlet 34.

[0072] The active zone 42 has channels 44 over its entire surface, which cross the active zone from one side to the other, each connecting the homogenization zone 40 to the homogenization zone 4L. The channels 44 thus allow the airflow to be conducted over the entire extent of the cathode compartment.

[0073] Here, the channels 44 are shown to be straight. In an alternative embodiment of the invention, the channels 44 have a different shape, for example a wavy, serpentine, or broken line shape.

[0074] Thus, the homogenization zones 40 and 41 connect the air inlet and outlet to the active zone 42 and allow the air to be distributed over the entire width of the active zone, towards all the channels 44.

[0075] On the anode side 16A, a bipolar plate 16 has the same structure as on the cathode side 16B, namely two homogenization zones and an active zone containing channels. On the anode side, the homogenization zones connect the hydrogen inlet and outlet to the active zone and allow the hydrogen to be distributed over the entire width of the active zone, towards all the channels.

[0076] In the example, the bipolar plates 16 and the support plates 25 are rectangular in shape. In the example, but not obligatorily, and as seen in [Fig. 1], the dihydrogen inlet 28 and the dihydrogen outlet 30 are located diagonally to each other, and the air inlet 32 ​​and the air outlet 34 are also located diagonally to each other, which allows for a more homogeneous distribution of the reactive gases over the active areas 42 of the bipolar plates.

[0077] Preferably, each cell 12 of the fuel cell 10 has an active area, corresponding to the area of ​​the active zone 42 of a bipolar plate 16, of between 150 cm2 and 500 cm2. Alternatively, this active area may be smaller, or much larger.

[0078] A fuel cell polarization curve 10 is defined as the curve representing the voltage delivered by each of the cells 12 of the fuel cell as a function of the current produced by the fuel cell. The fuel cell polarization curve varies, in particular, as a function of the partial pressure of oxygen within the cathode of each cell. It should be noted that the current produced by the fuel cell is equal to the current density produced by the fuel cell multiplied by the active area of ​​a cell, and that the voltage delivered by the fuel cell is equal to the voltage U delivered by each of the cells 12 multiplied by the number N of cells. In practice, since the cells 12 are electrically connected in series, the current produced by the fuel cell does not depend on the number of cells.In the following description, reference is made to the current density produced by the fuel cell 10, denoted I and expressed in A / cm2, rather than to the intensity of the current produced by the fuel cell.

[0079] Preferably, the fuel cell comprises between 60 and 500 cells 12. In the example, the fuel cell comprises 400 cells.

[0080] Figure 2 illustrates an electrical system 50, which includes the fuel cell 10. In the example, the electrical system 50 is part of an electric motor vehicle. In other words, in the example, the electrical system 50 is a vehicle.

[0081] The vehicle 50 includes a hydrogen supply system 52, for supplying hydrogen to the dihydrogen inlet 28, and for collecting the dihydrogen discharged from the dihydrogen outlet 30. For this purpose, the hydrogen supply system 52 includes, for example, a dihydrogen tank, an intake circuit comprising a valve system to allow the circulation of the dihydrogen, and a dihydrogen pressure regulator. Preferably, the hydrogen supply system 52 further includes a recirculation circuit that connects the dihydrogen outlet 30 to the dihydrogen inlet 28, and which includes means for recirculating the dihydrogen, such as a motorized pump or a passive pump, for example, a Venturi-type ejector.

[0082] The vehicle 50 includes an air supply system 54, which supplies oxygen to the air inlet 32 ​​and collects the exhaust air from the air outlet 34. For this purpose, the air supply system 54 includes, for example, a compressor adapted to draw air from outside the vehicle 50 and a circulation circuit. This air supply circuit is equipped with at least two shut-off valves, one corresponding to the air inlet and the other to the air outlet.When these shut-off valves are closed, they isolate, from the outside air, a volume of fuel cell cathode air which includes in particular the volumes of the cathode compartments of each cell 12 of the fuel cell, as well as in particular the volume of the air inlet and air outlet distribution galleries of the stack, plus possibly the piping volumes between each of these shut-off valves and the corresponding air inlet and air outlet valve, plus possibly the volume of an air humidification device belonging to the air supply system 54.

[0083] Advantageously, but optionally, the vehicle 50 includes a humidifier 55, connected to the air supply system 54. The humidifier 55 allows for the upward regulation of the humidity level of the air supplying each cell 12 of the fuel cell 10 via the air inlet 32. Thus, thanks to the humidifier 55, the humidity level of the air supplying the cathode compartment of each cell 12 is adaptable, being able to be increased, but not decreased, as the humidifier 55 does not allow the air supplying each cell 12 to dry out. For example, the humidifier 55 is a passive humidifier, which collects the water produced by the fuel cell during its operation and reinjects this water into the air supplying the fuel cell via the air inlet 32. Generally, such a humidifier 55 does not cannot be isolated from the air supply system 54 and is therefore permanently in operation.

[0084] The vehicle 50 includes a temperature control circuit 56, specifically a cooling circuit, for supplying heat transfer fluid to the heat transfer fluid inlet 36 of the fuel cell 10 and for collecting the heat transfer fluid discharged from the heat transfer fluid outlet 38. For this purpose, the control circuit 56 includes, for example, a heat transfer fluid reservoir, a circulation pump, and a thermodynamic circuit for regulating the temperature of the heat transfer fluid. The heat transfer fluid used is, for example, glycol water. The control circuit 56 thus allows the temperature of the fuel cell to be regulated.

[0085] An advantageous, but optional, embodiment of the control circuit 56 is illustrated in [Fig. 3]. In this embodiment, the control circuit 56 comprises a heat exchanger 57A and a fan 57B. The heat transfer fluid circulates in the heat exchanger 57A so as to allow the transfer of heat from the heat transfer fluid to the atmosphere, thereby cooling the heat transfer fluid. The heat exchanger 57A is thus part of the thermodynamic circuit for regulating the temperature of the heat transfer fluid. The fan 57B forces air circulation over the heat exchanger 57A, thereby facilitating the transfer of heat from the heat transfer fluid to the atmosphere and thus accelerating the cooling of the heat transfer fluid.

[0086] In addition, the control circuit 56 also includes a three-way valve 57C, allowing the heat transfer fluid from the heat transfer fluid outlet 38 to the heat exchanger 57A to be selectively distributed, or directly to the heat transfer fluid inlet 36 without going through the heat exchanger 57A, or to distribute the heat transfer fluid from the heat transfer fluid outlet 38 partly directly to the heat transfer fluid inlet 36 and partly to the heat exchanger 57A.

[0087] In addition, a pump 57D allows the circulation of the heat transfer fluid from the heat exchanger 57A and / or directly from the three-way valve 57C, to the heat transfer fluid inlet 36.

[0088] In general, the vehicle 50 includes an electrical consumer adapted to be electrically connected to the fuel cell 10 to provide work from electrical power produced by the fuel cell, in particular mechanical, electrical or electrochemical work, in particular work usable by the vehicle 50.

[0089] In the example, the electrical consumer comprises a DC-DC converter 58, an electric motor 60, and a battery 62. By DC-DC, it is understood that the converter converts one direct current into another direct current. The DC- DC 58 is electrically connected to the fuel cell 10, conditions the electrical power produced by the fuel cell, and delivers this conditioned electrical power to the electric motor 60 and / or the battery 62. In practice, the DC-DC converter 58 conditions the electrical power produced by the fuel cell, notably by modifying the voltage delivered by the fuel cell, to provide a voltage suitable for the electric motor 60 and the battery 62. The DC-DC converter 58 thus performs electrical work. The electric motor 60 and the battery 62 therefore correspond to electrical loads of the vehicle 50, and the DC-DC converter conditions and delivers the electrical power produced by the fuel cell 10 to these electrical loads.

[0090] Furthermore, the DC-DC converter 58 allows the fuel cell 10 to be controlled by imposing a given operating point on the fuel cell. To do this, the DC-DC converter imposes an operating voltage and / or current on the fuel cell 10, which imposes the operating point of the fuel cell 10 on its bias curve.

[0091] The battery 62 is further connected to the electric motor 60, and allows the electric motor to be supplied with electrical energy and / or to be stored electrical energy produced by the electric motor, for example during a regenerative braking phase of the vehicle 50. The battery 62 can also be connected to other electrical loads not shown of the vehicle, such as for example an entertainment system or a heating system.

[0092] Thus, the electrical power produced by the fuel cell 10 is conditioned by the DC-DC converter 58, then used by the electric motor 60 and / or stored by the battery 62.

[0093] Furthermore, the DC-DC converter 58 can be disconnected from the fuel cell 10. For this purpose, the DC-DC converter includes, for example, a controlled switch located between the DC-DC converter and the fuel cell. In other words, the fuel cell can be isolated from the DC-DC converter.

[0094] The vehicle 50 includes a dissipative system 64, which is electrically connected to the fuel cell 10. The dissipative system 64 dissipates electrical power produced by the fuel cell, in particular by heat transfer, notably by the Joule effect. Thus, the dissipative system 64 does no work, that is to say, no work usable by the electrical system 50. The dissipative system 64 includes, for example, a resistor, which dissipates the electrical power produced by the fuel cell in the form of heat.

[0095] The dissipative system 64 is disconnectable from the fuel cell 10. For this purpose, the dissipative system includes, for example, a controlled switch, arranged between the dissipative system and the fuel cell. In other words, the fuel cell can be isolated from the dissipative system.

[0096] Preferably, in normal operation of the fuel cell 10, the fuel cell is connected either to the DC-DC converter 58 or to the dissipative system 64.

[0097] In practice, the DC-DC converter 58 and the dissipative system 64 can also be simultaneously electrically connected to the fuel cell 10. Such a simultaneous connection of the fuel cell to the DC-DC converter and the dissipative system is particularly useful for establishing a transient regime in which the fuel cell is disconnected from the DC-DC converter and connected to the dissipative system, or in which the fuel cell is disconnected from the dissipative system and connected to the DC-DC converter. This transient regime makes it possible, in particular, to prevent the fuel cell from being connected to either the DC-DC converter or the dissipative system at the moment of switching between the DC-DC converter and the dissipative system, which could generate an overvoltage in the fuel cell.

[0098] The vehicle 50 includes a set of sensors 66 and a control unit 68. The set of sensors 66 is connected on one side to the fuel cell 10 and on the other side to the control unit 68. The control unit 68 is further connected to the DC-DC converter and the electric motor 60.

[0099] The sensor assembly 66 comprises several sensors for measuring several characteristic parameters of the operation of the fuel cell 10, and in particular: - the voltage U delivered by each of the cells 12 of the fuel cell, expressed in Volts per cell (V / cell); - the current density I, expressed in Amperes per square centimeter (A / cm2), produced by the fuel cell; - the temperature T of the heat transfer fluid at the outlet of the heat transfer fluid 38 of the fuel cell, expressed in degrees Celsius (°C), this temperature being considered as corresponding to the temperature prevailing within the cathode compartment of each cell 12 of the fuel cell; - the mass flow rate of air Q02 supplied by the air supply system 54 to each cell 12 of the fuel cell, expressed in grams per second per cell (g / s / cell); - the pressure P prevailing within the cathodic compartment of each cell, more precisely at the level of the air inlet 32 ​​of the cathodic compartment of each cell for example expressed in absolute bar (barA); - a humidity level of the air supplied by the air supply system 54 to the cells 12 of the fuel cell at the outlet of the humidifier 55; - the high-frequency impedance Z of the proton exchange membrane 20; and - the condensation temperature Tref of water vapor, also called dew point and expressed in degrees Celsius (°C), this measurement being preferably carried out at the level of the air inlet 32 ​​but can also be carried out at the level of the air outlet 34.

[0100] In practice, the sensor assembly 66 preferably includes a sensor capable of directly providing the dew point value Tref, through a combination of a relative humidity sensor, a temperature sensor and a pressure sensor.

[0101] In addition, the control unit 68 is able to determine in real time the mass flow rate of dihydrogen QH2 consumed by the fuel cell, expressed in grams per second per cell (g / s / cell), from the current density I.

[0102] The sensor assembly 66 includes in particular a sensor 66A measuring the temperature of the heat transfer fluid at the heat transfer fluid inlet 36, and a sensor 66B measuring the temperature of the heat transfer fluid at the heat transfer fluid outlet 38, these sensors being shown in [Fig.3].

[0103] The control unit 68 allows the vehicle 50 to be controlled, in particular by controlling the DC-DC converter 58 and the electric motor 60, in particular according to the data obtained by the sensor set 66 and according to control signals, for example from a driver of the vehicle.

[0104] A method for shutting down the fuel cell 10 is now described with reference to Figures 4 and 5. In a first example, the fuel cell shutdown method is implemented when the vehicle 50 comes to a complete stop, before a short or long period of stopping. "Complete stop" of the vehicle is preferably defined as the moment when the driver decides to stop the vehicle after completing the desired journey, for example, by removing the key from the vehicle or pressing a stop button. In another example, the fuel cell shutdown method is implemented without the vehicle 50 coming to a stop, for example, when the battery 62 is sufficiently charged and the fuel cell 10 does not need to provide additional electrical power to propel the vehicle. In this latter case, the shutdown is generally not initiated by the driver but managed automatically by the vehicle.In other words, the fuel cell shutdown process can be implemented in a vehicle hybridization strategy 50.

[0105] The stopping procedure is preferably implemented by the control unit 68.

[0106] Before the start of the shutdown process, the fuel cell 10 is in normal operating phase, denoted PF. For example, the vehicle 50 is running and the fuel cell produces the electrical power necessary to propel the electric motor 60 and / or to recharge the battery 62.

[0107] An idle current density, denoted IR, is defined as the minimum current density produced by the fuel cell 10 during idle operation. The idle current density IR is known to be between 0.05 A / cm² and 0.2 A / cm², generally equal to 0.1 A / cm². In other words, during the normal operating phase PF of the fuel cell, the current density I cannot be less than the idle current density IR. In practice, the idle current intensity IR corresponds to the minimum current density produced by the fuel cell 10 when it is operating but delivering minimal electrical power to the DC-DC converter 58.

[0108] The fuel cell shutdown procedure 10 is intended to be carried out at each normal shutdown of the fuel cell and aims to preserve the fuel cell's lifespan by preventing performance degradation. "Normal shutdown" preferably means a shutdown in one of the two cases mentioned above, excluding an emergency shutdown, in which, for safety reasons, it is necessary to shut down the fuel cell as quickly as possible, without regard to preserving its lifespan. The main objective of the shutdown procedure is to reduce the voltage across the fuel cell to a value close to zero, i.e., to depolarize the fuel cell. The voltage across the fuel cell is considered to be close to zero when it is less than or equal to 200 mV, preferably less than or equal to 100 mV.In addition, the fuel cell shutdown method 10 according to the invention has many other advantages: . - Dry the fuel cell, i.e. remove the liquid water residues and reduce its water content at the level of the proton exchange membrane 20, to avoid the appearance of ice plugs at the level of the proton exchange membrane in case of subsequent start-up of the fuel cell in negative temperatures. - Lower the temperature within the fuel cell, to slow down the harmful physicochemical reactions that degrade the fuel cell and reduce its lifespan. - Achieve these objectives while allowing for diagnostics or control actions to be carried out during fuel cell shutdown, such as, for example, a leak test of the hydrogen supply to the fuel cell. fuel, without requiring complex synchronization with the drying phase. - To achieve all of these objectives within a reasonable timeframe, in order to facilitate the implementation of the shutdown process.

[0109] The timing diagram in [Fig. 4] illustrates the shutdown procedure of the fuel cell 10. More specifically, the changes in five parameters representative of the operation of the fuel cell 10 during the shutdown procedure are illustrated in [Fig. 4]. These five parameters are: - The humidity of the proton exchange membrane 20 of each cell 12, denoted X and expressed as the number of water molecules contained by the proton exchange membrane 20 of each cell 12 divided by the number of charged sulfonic acid sites in the proton exchange membrane 20 of each cell, i.e., expressed as H2O / SO3'. From a theoretical point of view, experimental results suggest that the value of X can vary between 0 H2O / SO3' for a completely dehydrated membrane, and 22 H2O / SO3', for a membrane fully saturated with water. - The mass flow rate of air Q02 supplied by the air supply system 54 to each cell 12 of the fuel cell, expressed in grams per second per cell (g / s / cell), which corresponds to the flow rate of the air supply to the fuel cell 10 through the air inlet 32. - The temperature T of the heat transfer fluid at the outlet of the fuel cell 10, expressed in degrees Celsius (°C). For simplicity, this temperature T is considered to be equal to, or approximately equal to, the temperature within the cathode compartment of each cell 12 of the fuel cell. In practice, the heat exchange between the heat transfer fluid and the fuel cell 10 means that the temperature within the cathode compartment of each cell 12 is approximately equal to the temperature T of the heat transfer fluid at the outlet of the fuel cell 38, so that measuring the temperature T is equivalent to measuring the temperature within the cathode compartment of each cell 12. - The current density I produced by the fuel cell 10, expressed in A / cm2. - The voltage U delivered by each of the 12 cells of the fuel cell, expressed in Volts per cell (V / cell).

[0110] As mentioned earlier, for the sake of simplicity, we consider that these parameters are, at every moment, identical for all cells 12, that is to say that they do not vary from one cell to another.

[0111] Furthermore, the mass flow rate of hydrogen QH2 consumed by the fuel cell is not an important parameter in the context of the process of the invention. Thus, the values ​​of the mass flow rate of hydrogen QH2 are not specified. Generally, it is understood that the mass flow rate of hydrogen QH2 results from the current produced by the fuel cell 10. In addition, the hydrogen supply system 52 regulates a hydrogen pressure at the dihydrogen inlet 28 in order to ensure the presence of hydrogen in the cells 12.

[0112] For simplification, the pressure P prevailing within the air inlet 32 ​​of the cathode compartment of each cell of the fuel cell 10 is considered constant throughout the shutdown process, and is for example equal to 1 bar absolute.

[0113] Generally, during the shutdown process, the flow rate Q02 is regulated by the air supply system 54, the temperature T is regulated by the control circuit 56 and the current density I is regulated by the DC-DC converter 58, based on commands issued by the control unit 68, and data provided by the sensor set 66. In addition, the voltage U results from the current I and the bias curve of the fuel cell.

[0114] Furthermore, during the shutdown process, a quantity representative of the humidity level X of the proton exchange membrane 20 of each cell 12 is determined by the control unit 68, preferably from a measurement of the condensation temperature Tref of the water vapor at the air inlet 32 ​​or at the air outlet 34, and / or from a high-frequency impedance measurement Z of the proton exchange membrane, these measurements being provided by the sensor assembly 66, as detailed previously. Alternatively, other methods for determining the humidity level X are conceivable.

[0115] The condensation temperature Tref of water vapor, also called dew point, is related to the humidity level X because the relative humidity q> prevailing within the cathode compartment of each cell 12 depends on the dew point and the temperature T and because the humidity level X is dependent on the relative humidity q>, in a way known per se.

[0116] In practice, the direct relationship between relative humidity q> and humidity level X is well known and depends on the physicochemical properties of each proton exchange membrane; it is provided in the form of a nomogram by the manufacturer of each proton exchange membrane. In practice, the higher the relative humidity q>, the higher the humidity level X of the proton exchange membrane 20.

[0117] Thus, the dew point Tref forms a quantity representative of the humidity level X, because knowing the evolution of the dew point and the temperature T allows us to deduce the evolution of the humidity level X.

[0118] For the implementation of the method of the invention, it is not necessary to be interested in the relative humidity q>, the measurements of the dew point Tref and the temperature T, or of the impedance Z and the temperature T, being sufficient to study the evolution of the humidity level X.

[0119] Alternatively, or in addition, the quantity representing the humidity level X determined by the control unit 68 is a high-frequency impedance measurement Z of the fuel cell.

[0120] Indeed, the proton conductivity of the proton exchange membrane 20 increases with its humidity level X, so that the humidity level X is directly dependent on the impedance Z and the temperature T. Thus, determining the impedance Z and the temperature T allows us to obtain a quantity representative of the humidity level X.

[0121] The impedance measurement frequency that most accurately represents the moisture content X of the membrane is generally between 1 and 5 kHz. The impedance Z is measured using a generator that injects an alternating current with a frequency between 1 and 5 kHz across the terminals of the fuel cell. This current is on the order of a few amperes, for example, between 1 A and 10 A. The resulting alternating component of the voltage U across the fuel cell is then measured, and the impedance is deduced from this. This measurement is generally performed on the entire fuel cell. Since the physicochemical properties of all the cells are identical, and in particular the moisture content X of their membranes, the measurement can be performed on a portion of the fuel cell, or even on a single cell.

[0122] In practice, it is not necessary to know the value of the humidity level X within the framework of the invention, to determine a quantity which is representative of it, such as the impedance or the dew point at the cathode inlet, and to know the temperature T, being sufficient to implement the method of the invention.

[0123] However, calibration is necessary to determine a precise relationship between the representative quantity and the humidity level X, so that the humidity level X can be regulated to an appropriate level, neither too dry nor too humid. Furthermore, since the temperature T modifies the relationship between the humidity level X and the dew point Tref or the impedance Z, it is necessary to take into account the temperature T in addition to the dew point Tref or the impedance Z to evaluate the humidity level X.

[0124] Preferably, to determine the relationship between the representative quantity, whether it be the dew point Tref or the impedance Z, a calibration phase is implemented. During this calibration phase, an experimental design is established, in which Numerous drying and cold start tests of the fuel cell 10 are carried out, varying on the one hand the temperature T and on the other hand the quantity representing the humidity level X, in the example the dew point Tref or the impedance Z. From this experimental design, it is possible to identify, for each temperature T, the value of the quantity representing the humidity level X leading to the best behavior of the fuel cell during its start-up, and therefore to identify an ideal value of the representative quantity, corresponding to an ideal value of the humidity level X.

[0125] In other words, the target value of the humidity level X used in the process according to the invention is determined experimentally.

[0126] Thus, in the description of the shutdown process, reference is made to the humidity level X for simplicity, but it is understood that the humidity level X is not directly used to implement the shutdown process. In practice, the quantity representing the humidity level X is used to implement the shutdown process, the relationship between this representative quantity, such as the dew point Tref or the impedance Z, the temperature T, and the humidity level X being known, in this case determined experimentally.

[0127] The stopping process begins at a time t0, when a stop command for the fuel cell 10 is received by the control unit 68. This stop command is issued for example when a stop of the vehicle 50 is desired or when the battery 62 is sufficiently charged and it is not necessary for the fuel cell 10 to provide additional electrical power to propel the vehicle.

[0128] Preferably, before the shutdown procedure begins, the control unit 68 performs a check to verify whether the requested shutdown is a normal shutdown or an emergency shutdown, and the shutdown procedure is executed only when the requested shutdown is a normal shutdown. Indeed, in the event of an emergency shutdown, it is generally preferable to shut down the fuel cell 10 as quickly as possible for safety reasons, regardless of any potential damage that may occur to the fuel cell during such an emergency shutdown. An emergency shutdown is desirable, for example, in the event of an accident involving the vehicle 50.

[0129] Just before time t0, the characteristic quantities of the operation of the fuel cell 10 are: — X = Xq, - QO2 = QO2 o, with QO2.0 preferably between 0.00358 g / s / cell, and 0.358 g / s / cell, for example, equal to 0.0358 g / s / cell. In practice, the mass flow rate of air to be supplied to the fuel cell depends on the current produced by the fuel cell. The range of values ​​indicated allows the fuel cell to fuel, during normal operation, to produce a current between 10A and 1000A. - T = To, with To preferably between 50°C and 95°C, for example equal to 80°C. - I = lo, with Io preferably between 0.1 A / cm2 and 2.5 A / cm2, for example equal to 0.5 A / cm2. In practice, the current density Io is greater than or equal to the idle current density IR, since the fuel cell 10 is in operation until time t0. - U = Uo, with Uo preferably between 0.5 V / cell and 0.85 V / cell, for example equal to 0.7 V / cell.

[0130] Furthermore, the mass flow rate of hydrogen QH2 consumed by the fuel cell is non-zero and is essentially proportional to the current produced.

[0131] The maximum electrical power produced by the fuel cell during the PF operating phase is, in the example of a stack of 400 cells, between 5 kW and 150 kW, for example equal to 20 kW.

[0132] In practice, the maximum power of the fuel cell 10 tends to decrease over the life of the fuel cell, due to its degradation and / or contamination.

[0133] The shutdown process begins, at time t0, with a cooling phase which, in the example, includes a drying phase Pi that ends at time 0 and then a timing phase P2 that starts at time 0 and ends at time t2. After the timing phase P2, the shutdown process continues and ends with a depolarization phase P3, which starts at time t2 and ends at time t3, at the end of which the fuel cell 10 is shut down.

[0134] During the drying phase Pi and the time-delay phase P2, the fuel cell 10 is connected to the DC-DC converter 58 and disconnected from the dissipative system 64, so that the electrical power produced by the fuel cell performs work that is utilized by the electrical system 50, for example by charging the battery 62. Conversely, during the depolarization phase P3, the fuel cell 10 is connected to the dissipative system 64 and disconnected from the DC-DC converter, so that the power produced by the fuel cell 10 is dissipated by the resistance of the dissipative system 64, in the form of heat, notably by Joule heating. Thus, during the depolarization phase P3, the fuel cell 10 performs no work.

[0135] At the beginning of the Pb drying phase, i.e. at time t0, the following characteristic parameters of the operation of the fuel cell 10 are modified: - The mass flow rate of air Q02 is increased up to Q02. b with Q02 i preferably between 0.02 g / s / cell, and 0.2 g / s / cell, for example equal to 0.1 g / s / cell. - The current density I is reduced to h, with L strictly greater than 0 A / cm2 and strictly less than the idle current density IR. Preferably, L is strictly greater than 0 A / cm2 and strictly less than 0.1 A / cm2, preferably even less than 0.08 A / cm2, for example less than 0.04 A / cm2. - U = Uo, with Uo preferably between 0.8 V / cell and 0.90 V / cell, for example equal to 0.85 V / cell.

[0136] Furthermore, the mass flow rate of hydrogen QH2 is maintained non-zero.

[0137] In the example, the evolutions of mass flow rate of air Q02, current density I and voltage U at time t0 are illustrated as being instantaneous, by simplification.

[0138] In a non-shown variant of the invention, the shutdown method comprises a speed reduction phase, implemented after the operating phase PF and before the drying phase Pb, during which the mass flow rate of air Q02 is progressively increased from Q02.0 to Q02_i, the current density I is progressively decreased from Io to L, and the voltage U is progressively increased from Uo to Ub.

[0139] The drying phase Pi lasts from time t0 to time tb. During the drying phase, the humidity level X of the proton exchange membrane 20 of each cell 12 is lowered from the value Xo to a predetermined target value Xi lower than Xo. In other words, the proton exchange membrane 20 is dried.

[0140] In practice, the predetermined target value Xi is determined experimentally, as detailed above. In other words, the predetermined target value Xi corresponds to a predetermined target value of the representative quantity, therefore to a predetermined target value of dew point or impedance.

[0141] It is understood that, in practice, the evolution of the humidity level X is not controlled during the drying phase PH but that the evolution of the quantity representing the humidity level X is controlled.

[0142] During the Pb drying phase, the current density I is maintained above 0 A / cm² and strictly below the idle current density IR, i.e., strictly below 0.1 A / cm², preferably even below 0.08 A / cm², for example below 0.04 A / cm². Preferably, the current density I is maintained substantially equal to Ib as illustrated in [Fig. 4]. By substantially equal to h, we mean here that the current density I is maintained equal to L to within ±10%, preferably within ±5%.

[0143] Furthermore, during the drying phase Pb the mass flow rate of air Q02 is maintained constant, equal to Q021. The voltage U is represented constant for simplification, but in practice can change during the drying phase Pb in particular decrease, as a consequence of the decrease in the humidity level X, because the drying of the proton exchange membrane of the cells 12 leads to a decrease in proton conduction and therefore to a decrease in voltage.

[0144] In practice, the current density value I is imposed by the DC-DC converter, which controls the fuel cell 10 according to the instructions issued by the control unit 68, and the voltage value U is derived in particular from the current density I and the bias curve of the fuel cell.

[0145] Advantageously, the humidity level X is decreased from X0 to Xi by increasing the mass flow rate of air Q02 supplied by the air supply system 54 to each cell 12 of the fuel cell, from Q02 o to Q02 b. Indeed, by increasing the air flow rate, the humidity level within the cathode compartment of each cell 12 is decreased, which leads to a decrease in the humidity level X of the proton exchange membrane 20. More precisely, by renewing the air in the cathode compartment of each cell more rapidly, the water produced by the operation of the fuel cell 10 is removed more rapidly, thus decreasing the relative humidity q> within the cathode compartment of each cell 12 and, consequently, the humidity level X of the membrane.

[0146] It is noted in the example of [Fig.4] that the mass flow rate of air Q021 is greater than the mass flow rate of air Q02 o of the operating phase PF: thus, it is understood that, during the drying phase Pb the fuel cell 10 is over-ventilated, so as to accelerate the drying of the proton exchange membrane.

[0147] In practice, the drying phase Pi ends and the timing phase P2 begins when the humidity level X of the proton exchange membrane 20 reaches the value Xb at time tb

[0148] Advantageously, during the drying phase Pb, the control unit 68 controls the regulation circuit 56 so as to lower the temperature T from the temperature To to a predetermined temperature Th, with Ti preferably between 60°C and 95°C, for example, 63°C. In practice, the predetermined temperature Ti is determined during the experimental design used to characterize the relationship between the humidity level X and its representative quantity and corresponds to a temperature at which the drying of the membrane during the drying phase Pi has been calibrated.

[0149] Preferably, to reach the predetermined temperature T1 more quickly, the three-way valve 57C is operated so that all of the heat transfer fluid coming from the heat transfer fluid outlet 38 is redirected to the heat exchanger 57A, so as to optimize the cooling of the heat transfer fluid through the action of the heat exchanger 57A and the fan 57B, the latter being then in operation.

[0150] It is noted that, in the example, the predetermined temperature Ti is reached before time tb. Thus, as soon as the predetermined temperature Tb is reached, the control circuit 56 is activated so as to maintain the temperature T equal to the predetermined temperature Tb.

[0151] Preferably, when the predetermined temperature Ti is reached, the three-way valve 57C is operated so that all of the heat transfer fluid from the heat transfer fluid outlet 38 is redirected to the heat transfer fluid inlet 36, in this example via the pump 57D, without passing through the heat exchanger 57A. Furthermore, after the predetermined temperature Tp is reached, the fan 57B is kept running to cool the heat exchanger 57A itself, since the heat transfer fluid does not pass through the heat exchanger. In practice, the heat exchanger 57A generally has significant thermal inertia, for example, through a volume of water contained within the heat exchanger.

[0152] Alternatively, if the predetermined temperature Ti is not reached when the humidity level X of the proton exchange membrane 20 reaches the value Xb, then the drying phase Pi still ends at time 0 when the humidity level X reaches the value Xi, and the cooling of the fuel cell 10 continues during the delay phase P2, to reach the predetermined temperature Th.

[0153] Thanks to the drying phase Pb, the humidity level X of the proton exchange membrane 20 is reduced, i.e., the membrane is dried, to a level that ensures a safe restart of the fuel cell, including when the fuel cell is restarted in sub-zero ambient temperatures. The fuel cell's lifespan is thus improved.In particular, with the humidity level X reduced, the proton exchange membrane 20 is able to absorb the first water molecules produced during the restart of the fuel cell, thus preventing these water molecules from freezing and clogging the channels 44 of the active zone or the pores of the proton exchange membrane.

[0154] The timing phase P2 starts at time ti and ends at time t2.

[0155] During the P2 time delay phase, the humidity level X of the proton exchange membrane and the voltage U are maintained substantially constant and equal to the values ​​at time tb, i.e., respectively equal to Xi and Up

[0156] Furthermore, during the timing phase P2, the current density I is maintained above 0 A / cm² and strictly below the idle current density IR, i.e., strictly below 0.1 A / cm², preferably even below 0.08 A / cm², for example below 0.04 A / cm². Preferably, the current density I is maintained substantially equal to Ib as illustrated in [Fig. 4]. By substantially equal to Ib, we mean here that the current density I is maintained equal to F to within ±10%, preferably ±5%. In an unrepresented variant of the invention, during phase P2, the current I differs from the value Ib by being kept constant at a value greater than or less than Ib or by varying during phase P2 while always being kept greater than 0 A / cm2 and strictly less than the idle current density IR, i.e. strictly less than 0.1 A / cm2 in the example.

[0157] Furthermore, during the P2 timing phase, the mass flow rate of air Q02 supplied by the air supply system 54 and the mass flow rate of hydrogen QH2 supplied by the hydrogen supply system 52 are maintained above zero.

[0158] Thus, the delay phase makes it possible to postpone the start of the depolarization phase P3 without altering the humidity level X of the proton exchange membrane reached at the end of the drying phase. The delay phase therefore allows for diagnostics or control actions, for example a leak test of the hydrogen supply to the fuel cell, without requiring complex synchronization between such diagnostics or control actions and the drying phase.

[0159] Advantageously, during the P2 timeout phase, the humidity level X of the proton exchange membrane 20 of each cell is maintained substantially equal to the predetermined target value Xi by regulating the flow rate of the fuel cell air supply 10 through the air inlet 32, i.e. the mass flow rate of air Q02. Indeed, as detailed previously, a change in the mass flow rate of air Q02 results in a change in the humidity level X, by changing the relative humidity q>. Therefore, during the P2 timing phase, the mass flow rate of air Q02 is not kept constant, but varies continuously as regulated by the control unit 68. This variation is shown in [Fig.4] by a non-linear evolution of the mass flow rate of air Q02, which does not represent a real evolution of the mass flow rate of air and is shown only for illustrative purposes.

[0160] More specifically, the mass flow rate of air Q02 is increased when the humidity level X of the proton exchange membrane 20 of each cell 12 increases and the mass flow rate of air Q02 is decreased when the humidity level X decreases.

[0161] In practice, to maintain a constant humidity level X, the control unit 68 controls the quantity representative of the humidity level X, such as the condensation temperature Tref and / or the impedance measurement Z of the heat exchanger membrane. protons 20, as described above, then regulates the mass flow rate of air Q02 according to the determined representative quantity.

[0162] Thus, the control unit 68 maintains the humidity level X constant by means of a control loop, an example of which is illustrated in [Fig. 5]. In real time, or at a given periodicity, the control unit 68 determines the representative quantity of the humidity level X, i.e. the condensation temperature Tref and / or the impedance measurement Z, then compares the determined representative quantity with a target value of the representative quantity corresponding to the predetermined target value Xi and, in the event of a difference between these two values, modifies the air flow rate Q02 supplied by the air supply system 54.

[0163] Ideally, the humidity level X is kept constant during the timeout phase P2. In practice, variations in the humidity level X are generally observed during the timeout phase P2, for example, due to the inertia of the control loop implemented by the control unit 68. Preferably, considering that the humidity level X of the proton exchange membrane 20 of each cell is kept substantially equal to the predetermined target value Xi is equivalent to considering that the humidity level X remains equal to the predetermined target value Xi to within ±30%, preferably to within ±10%, and preferably even to within ±5%. Since the humidity level X is not directly measured or determined during the shutdown process, it is understood that keeping the humidity level X substantially constant is equivalent to keeping the representative quantity substantially constant, at least for a given temperature T.In any case, the data obtained experimentally makes it possible to know the range of value of the representative quantity which allows the humidity level X to be maintained substantially equal to the predetermined target value Xb.

[0164] Preferably, during the time delay phase P2, the temperature T is decreased from the predetermined temperature Ti to a predetermined temperature T2, with T2 preferably between 20°C and 40°C, for example equal to 30°C. This decrease in temperature T is achieved by controlling the control circuit 56. If, at the beginning of the time delay phase P2, the predetermined temperature Tj has not been reached, then the temperature T is first decreased to the predetermined temperature Ti and then to the predetermined temperature T2.

[0165] Preferably, to reach the predetermined temperature T2 more quickly, the three-way valve 57C is operated so that all of the heat transfer fluid from the heat transfer fluid outlet 38 is redirected to the heat exchanger 57A, so as to optimize the cooling of the heat transfer fluid by means of the action of the heat exchanger 57A and the fan 57B, the latter being in operation at that time.

[0166] Particularly advantageously, thanks to the cooling of the heat exchanger 57A after reaching the predetermined temperature Ti during the drying phase Pb, the reduction of the temperature T at the beginning of the timing phase P2 is more efficient, because the heat transfer fluid is more efficiently cooled when it comes into contact with the previously cooled heat exchanger 57A.

[0167] Due to the decrease in temperature T during the timing phase P2, there is a tendency for the mass flow rate of air Q02 to increase during this phase, because the decrease in temperature tends to increase the relative humidity within the cathode compartment of each cell 12, and thus the humidity level X of the proton exchange membrane 20, which requires an increase in the ventilation of the fuel cell to oppose this increase in the humidity level X and keep the humidity level X constant.

[0168] In practice, during the P2 delay phase, the air mass flow rate Q02 is preferably maintained between a high value corresponding to the air mass flow rate Q021 and a low value corresponding to an air mass flow rate Q022, with Q022 preferably between 0.02 g / s / cell and 0.2 g / s / cell, for example equal to 0.02 g / s / cell. In the example, the air mass flow rate Q02 is maintained between Qm2 and Q02o-

[0169] Thus, advantageously, the drying phase Pi and the time-delay phase P2 together form a cooling phase, during which the temperature T of the fuel cell is reduced from the initial temperature To to the predetermined temperature T2. Thanks to this cooling, the harmful physicochemical reactions that can degrade the fuel cell and that can occur during the subsequent shutdown process are slowed down, thereby improving the fuel cell's lifespan. Furthermore, it is particularly advantageous for the cooling to take place simultaneously with the drying phase Pb on the one hand, and with the time-delay phase P2 on the other, because the duration of the shutdown process is then reduced, compared to a process in which the cooling would not take place simultaneously with the drying and / or the time-delay.

[0170] It is noted that, during this cooling phase, the cooling takes place in two stages, first from the initial temperature To to the predetermined temperature Tb then from the predetermined temperature Ti to the predetermined temperature T2.

[0171] In a non-shown variant of the invention, the control circuit 56 is controlled to decrease the temperature T from the initial temperature To to the predetermined temperature T2 during the drying Pi and then the time-delay P2 phases, without The timing of reaching the predetermined temperature T2 or temperature Ti is conditional upon the progress of the drying phase Pi and the time delay phase P2. For example, in such a variant, the drying phase Pi can end when the temperature T is higher than the predetermined temperature Th, which will then only be reached during the time delay phase P2.

[0172] The P2 timing phase ends and the P3 depolarization phase begins when a predefined stopping condition is reached.

[0173] In the example, the predefined stopping condition is reached when the temperature T reaches the predetermined temperature T2. Thus, as soon as the temperature T reaches the predetermined temperature T2, the timing phase P2 ends and the depolarization phase P3 begins.

[0174] The temperature T2 may not be attainable during the delay phase P2, for example, due to an excessively high ambient temperature reducing the efficiency of the cooling system. In such a situation, the predefined shutdown condition is advantageously reached when the duration of the delay phase P2 reaches a predefined duration, which is preferably greater than 60 seconds, and even more preferably between 10 and 180 seconds.

[0175] In an unrepresented variant, the predefined stopping condition is reached when the two preceding criteria are met, i.e. when the duration of the timing phase P2 reaches the predefined duration and when the temperature T reaches the predetermined temperature T2.

[0176] According to another embodiment, the predefined stopping condition is reached when a diagnostic or control action implemented during the P2 delay phase is completed. For example, if the delay phase is used to perform a hydrogen leak test, the stopping condition will be reached when the amount of hydrogen consumed in the leak test sequence reaches a predefined amount. Such a stopping condition can replace, or be added to, the stopping conditions mentioned previously.

[0177] At time t2, to complete the timing phase P2 and start the depolarization phase P3, the control unit 68 disconnects the DC-DC converter 58 from the fuel cell 10 and connects the dissipative system 64 to the fuel cell. Thus, during the depolarization phase P3, the dissipative system 64 is electrically connected to the fuel cell 10 and dissipates the electrical power produced by the fuel cell. Furthermore, since the fuel cell is disconnected from the DC-DC converter during the depolarization phase P3, it does not perform any work, that is, no work usable by the electrical system 50.

[0178] Furthermore, at time t2 and throughout the depolarization phase P3, the mass flow rate of air Q02 is equal to zero, i.e., the air supply system 54 does not supply air to the cells 12 of the fuel cell. In other words, the supply of air to the fuel cell 10 through the air inlet 32 ​​and the exhaust of air from the fuel cell through the air outlet 34 are cut off.

[0179] Thus, at the beginning of the depolarization phase P3, i.e. at time t2, the following characteristic quantities of the operation of the fuel cell 10 are modified: - The mass flow rate of air Q02 is equal to zero. - The current density I is reduced by h to I2, with I2 strictly greater than 0 A / cm2 and strictly less than the idle current density IR. For example, I2 is between 0.001 A / cm2 and 0.02 A / cm2. - The voltage U remains approximately equal to the value Up. However, if the current density I2 is significantly lower than Ib, there follows an increase in the voltage U, depending in particular on the bias curve of the fuel cell.

[0180] Furthermore, the mass flow rate of hydrogen QH2 is maintained non-zero throughout the depolarization phase P3, the hydrogen supply provided by the hydrogen supply system 52 being interrupted only at the end of the depolarization phase P3, at time t3.

[0181] In practice, at time t2, a jump in current density is observed, from L to I2. This jump is caused by connecting the fuel cell 10 to the dissipative system 64 and then disconnecting it from the DC-DC converter. More precisely, the dissipative system 64 imposes an operating point on the fuel cell 10, which depends on a resistance value R of the dissipative system. Thus, the voltage U delivered by each cell 12 is equal to the product of the resistance R and the current I produced by the fuel cell. In other words, during the depolarization phase P3, the operating point of the fuel cell 10 is determined by the resistance R of the dissipative system 64.

[0182] This operation of the fuel cell during the depolarization stage P3 is not controlled or controlled, insofar as it is not possible to intervene in the evolution of the voltage U and the current I, which are solely determined by the resistance R when the fuel cell is sufficiently supplied with hydrogen.

[0183] During the P3 depolarization phase, the oxygen contained in the cathode compartment of each cell 12 continues to be progressively consumed by the reduction reaction forming water molecules. Thus, the oxygen content in the amount of air contained in the cathode compartment of each cell 12, particularly near the proton exchange membrane, gradually decreases during the P3 depolarization phase since the air supply is interrupted.

[0184] This decrease in oxygen concentration modifies the electrochemical properties of the fuel cell, and in particular modifies the fuel cell's polarization curve. Indeed, the lower the oxygen concentration in the cathode compartment of each cell, the lower the current I produced for a given voltage U, because the partial pressure of oxygen in the cathode compartment of the cells 12 decreases. Thus, since the fuel cell 10 is connected to the resistance of the dissipative system 64, which imposes an operating point for which U = R x I, a progressive decrease is observed in both the current density I produced by the fuel cell and the voltage U delivered by each cell 12 during the depolarization phase P3.

[0185] The depolarization phase P3 ends at time t3, when the current density produced I by the fuel cell and the voltage U across the cells 12 become sufficiently low, for example when the voltage across the fuel cell becomes close to zero when it is less than or equal to 200 mV, preferably less than or equal to 100 mV, in other words when the current produced by the fuel cell becomes sufficiently low. At time t3, the fuel cell is then shut down.

[0186] Thanks to the depolarization phase, efficient depolarization of the fuel cell is achieved, preventing any power production after the fuel cell is shut down, slowing down the repolarization of the fuel cell, and promoting a low residual oxygen concentration. The fuel cell's lifespan is thus increased.

[0187] During the P3 depolarization phase, the humidity level X of the proton exchange membrane 20 of each cell 12 remains essentially constant and equal to the predetermined target value Xb, despite the formation of water molecules in the cathode compartment of each cell 12 by electrochemical reaction. Indeed, the quantity of water molecules formed is small due to the reduction in the current density I produced by the fuel cell. Furthermore, the depolarization phase is of relatively short duration, so the impact of this low water production on the humidity level X is small, even negligible.

[0188] Particularly advantageously, during the depolarization phase P3, the control circuit 56 is controlled by the control unit 68 so that the temperature T of the heat transfer fluid at the heat transfer fluid inlet 36 of the fuel cell decreases from the predetermined temperature T2 to a temperature predetermined T3, with T3 preferably between 20°C and 40°C, for example equal to 25°C. In other words, the cooling of the fuel cell continues during the depolarization phase P3.

[0189] In practice, this cooling is forced by the control circuit 56, in particular by operating the fan 57B, so as to accelerate the evacuation to the atmosphere of the calories extracted from the fuel cell at the level of the heat exchanger 57A and thus the cooling of the heat transfer fluid.

[0190] It is particularly advantageous for the cooling of the fuel cell 10 to take place both during the cooling phase, i.e., during the drying phase Pi and then the time-delay phase P2, and during the depolarization phase P3, as this improves the cooling efficiency. Thus, the shutdown process allows for faster and more efficient cooling of the fuel cell, making it easy to implement the process, even when fuel cell shutdowns are frequent and must be carried out quickly.

[0191] Consequently, thanks to the invention, several advantageous operations are carried out in parallel with the cooling of the fuel cell. More specifically, the drying phase Pb, the timing phase P2, and the depolarization phase P3 are performed simultaneously with the cooling, the timing phase P2 also allowing for the implementation of diagnostics or control actions. The shutdown method of the invention thus makes it possible to achieve a large number of objectives in a reduced execution time, facilitating its implementation.

[0192] Furthermore, the method of the invention is also particularly advantageous when the vehicle 50 includes the humidifier 55. Indeed, the humidifier tends to increase the humidity of the air supplying the fuel cell 10, and therefore to increase the humidity level X. In other words, the humidifier has an advantageous function during the normal operation of the fuel cell, by ensuring sufficient humidification of the proton exchange membrane of each cell 12 to allow the proper functioning of the fuel cell 10, but a disadvantageous function during the shutdown process, since shutting down the fuel cell despite a high humidity level X is problematic. The shutdown process according to the invention is thus particularly useful and advantageous, because by decreasing the humidity level X, i.e., by drying the proton exchange membrane of each cell 12, the process also allows the humidifier 55 to dry.In other words, the shutdown method according to the invention has the particularly advantageous consequence of compensating for the adverse effects of the humidifier 55 during the fuel cell shutdown procedure. In other words, the method of the invention allows the fuel cell 10 to be dried. but also, advantageously, the humidifier 55. This is particularly advantageous for facilitating future cold starts, especially in case of frost, of the electrical system 50.

[0193] By way of example, the duration of the stopping process is between 30 seconds and 300 seconds, with the drying phase Pi lasting between 10 seconds and 120 seconds, preferably 30 seconds, the timing phase P2 lasting between 0 seconds and 120 seconds, preferably 60 seconds, and the depolarization phase P3 lasting between 10 seconds and 120 seconds, preferably 60 seconds.

[0194] In practice, the duration of the P2 delay phase can be modified without constraint, since it occurs without impacting the humidity level X. In other words, the duration of the P2 delay phase can be easily and freely adjusted to allow for the proper execution of diagnostics or control actions implemented simultaneously. The duration range indicated above is therefore given as an example and is not limiting. Conversely, it is desirable that the durations of the Pi drying phase and the P3 depolarization phase fall within the ranges indicated above.

[0195] In the example in [Fig. 4], the temperature T reaches the predetermined temperature T3 when the current density I and the voltage U become close to zero, at time t3. This synchronization is not a specific objective of the depolarization phase P3, which is described in this way only for illustrative purposes. The voltage U is considered to become close to zero when it is less than or equal to 200 mV, preferably less than or equal to 100 mV.

[0196] Thus, in the variant not shown, the temperature T reaches the predetermined temperature T3 before time t3, that is, before the current density I and the voltage U become close to zero. In such a variant, once the predetermined temperature T3 is reached, either the control circuit 56 is switched off, the temperature T no longer being regulated, or the control circuit 56 is activated to maintain the temperature T equal to the predetermined temperature T3. In practice, at this stage of the shutdown process, the fuel cell 10 no longer produces heat, or very little heat, so that the temperature T is no longer affected by the operation of the fuel cell.

[0197] According to another variant, the temperature T does not decrease to the predetermined temperature T3 during the depolarization phase P3, i.e., before and up to time t3. In such a variant, the depolarization phase P3 nevertheless ends, and the control circuit 56 is stopped, when the current density I reaches a value close to zero, regardless of the temperature T of the heat transfer fluid. Such a situation is encountered, for example, when the temperature of the atmosphere is higher than the predetermined temperature T3, particularly during heat waves, making it impossible for the control circuit 56 to lower the temperature T of the heat transfer fluid to the predetermined temperature T3, since the heat exchanger 57A cannot lower the temperature T below atmospheric temperature. In this variant, the control circuit 56 is nevertheless kept operating during the depolarization phase P3, to lower the temperature T of the heat transfer fluid as much as possible, and thus cool the fuel cell as much as possible.

[0198] In a non-shown embodiment of the invention, during the drying phase Pb, the temperature T is not decreased. Such an embodiment is particularly suitable when the temperature T at time t0 is already equal to, or lower than, the predetermined temperature Ti, for example, when the operating temperature of the fuel cell 10 is particularly low. Such a situation may, for example, arise when the fuel cell is shut down while operating at a temperature below the temperature To and being heated by the control circuit 56. Otherwise, the temperature T is lowered to the predetermined temperature T2 before the start of the depolarization phase P3.In practice, it is necessary to control the regulation circuit 56 to reduce the temperature T to the predetermined temperature T2, if the predetermined temperature T2 is far from an operating temperature of the fuel cell during its PF operating phase.

[0199] In a non-shown embodiment of the invention, the shutdown method further comprises a transition phase, which occurs between the timing phase P2 and the depolarization phase P3, during which the dissipative system 64 is electrically connected to the fuel cell 10 and the DC-DC converter 58 is kept electrically connected to the fuel cell. The DC-DC converter 58 and the dissipative system 64 are electrically connected in parallel to the fuel cell 10. In other words, the fuel cell 10 is simultaneously connected to the DC-DC converter 58 and the dissipative system 64 during the transition phase. The transition phase avoids the need for perfect synchronization of the disconnection of the DC-DC converter 58 and the connection of the dissipative system 64 to the fuel cell 10.

[0200] In an alternative variant not shown, the electrical system 50 does not include a DC-DC converter. In such a variant, the fuel cell 10 is electrically connected to the battery 62 and / or to a variable frequency drive supplying the electric motor 60, which are then electrical loads directly supplied by the fuel cell. Thus, the electrical power produced by the fuel cell 10 is directly transformed into work by the battery 62 and / or by the electric motor 60 via the variator, without prior conditioning by a converter.

[0201] Any feature described for a variant in the foregoing may be implemented for the other variants described above, provided that it is technically feasible.

Claims

1. Demands Method for stopping a fuel cell (10) belonging to an electrical system (50), the fuel cell (10) comprising: - a stack of cells (12), each cell comprising an anodic compartment and a cathodic compartment separated by a proton exchange membrane (20), - a dihydrogen inlet (28) supplying the anodic compartment of each cell (12) with dihydrogen, and a dihydrogen outlet (30) removing the dihydrogen from each cell, - an air inlet (32) supplying air to the cathode compartment of each cell (12), and an air outlet (34) removing the air from each cell, the electrical system (50) further comprising: - an electrical consumer (58, 60, 62), adapted to be electrically connected to the fuel cell (10) to provide work from electrical power produced by the fuel cell, and - a dissipative system (64), adapted to be electrically connected to the fuel cell (10) to dissipate electrical power produced by the fuel cell, in which the fuel cell shutdown method (10) comprises: - a drying phase (Pi), in which: • a humidity level (X) of the proton exchange membrane (20) of each cell is decreased until a predetermined target value (Xi) is reached, • an air supply to the fuel cell (10) via the air inlet (32) and an exhaust of air from the fuel cell via the air outlet (34) are maintained, • The fuel cell is controlled so that a current density (I) produced by the fuel cell is maintained greater than OA / cm2 and less than an idle current density (Ircf) corresponding to a current density (I) produced by the fuel cell when the fuel cell (10) is operating at idle, and • The electrical consumer (58, 60, 62) is electrically connected to the fuel cell (10) and provides work from electrical power produced by the fuel cell, - a depolarization phase (P3), in which: • the air supply to the fuel cell (10) via the air inlet (32) and the exhaust of air from the fuel cell via the air outlet (34) are cut off, and • The dissipative system (64) is electrically connected to the fuel cell (10) and dissipates the electrical power produced by the fuel cell, characterized in that the fuel cell shutdown method (10) further comprises: - a delay phase (P2), implemented after the drying phase (Pi) and before the depolarization phase (P3), the delay phase (P2) beginning when the humidity level (X) reaches the predetermined target value (Xi), in which: • the humidity level (X) of the proton exchange membrane (20) of each cell is maintained substantially equal to the predetermined target value (XJ, • The supply of air to the fuel cell (10) via the air inlet (32) and the exhaust of air from the fuel cell via the air outlet (34) are maintained, • the fuel cell is controlled so that the current density (I) produced by the fuel cell is maintained above OA / cm2 and below the idle current density (Iref), and • the electrical consumer (58, 60, 62) is electrically connected to the fuel cell (10) and provides work from electrical power produced by the fuel cell, and in that the timing phase (P2) ends and the depolarization phase (P3) begins when a predefined stopping condition is reached.

2. A method according to claim 1, wherein, during the timeout phase (P2), the humidity level (X) of the proton exchange membrane (20) of each cell is maintained substantially equal to the predetermined target value (Xi) by regulating a flow rate of the air supply (Q02) of the fuel cell (10) through the air inlet (32).

3. A method according to claim 2, wherein, during the timeout phase (P2), the flow rate of the air supply (Q02) to the fuel cell (10) through the air inlet (32) is increased when the humidity level (X) of the proton exchange membrane (20) of each cell tends to increase and the flow rate of the air supply to the fuel cell through the air inlet is decreased when the humidity level (X) of the proton exchange membrane (20) of each cell tends to decrease.

4. A method according to any one of claims 2-3, wherein the electrical system (50) further comprises a control unit (68) and wherein, during the timing phase (P2), the control unit (68): - determines a representative quantity (Tref, Z) of the humidity level (X) of the proton exchange membrane (20) of each cell (12), said representative quantity being obtained from: • a measurement of a temperature (T) prevailing within the cathode compartment of each cell (12) of the fuel cell (10) and a measurement of a condensation temperature of the water vapor contained in the air supplying the fuel cell (10) at the air inlet (32) or in the air exhausted from the fuel cell at the air outlet (34), • and / or a measurement of a temperature (T) prevailing within the cathode compartment of each cell (12) of the fuel cell (10) and a measurement of impedance (Z) of the proton exchange membrane, and - regulates the flow rate of the air supply (Q02) of the fuel cell (10) through the air inlet (32) as a function of the representative quantity determined.

5. A method according to claim 4, wherein, during the timing phase (P2), the control unit (68) regulates the flow rate of the air supply (Qo2) to the fuel cell (10) through the air inlet (32) so as to vary the condensation temperature (Tref) of the water vapor contained in the air supplying the fuel cell (10) at the air inlet (32) or in the air exhausted from the fuel cell at the air outlet (34) as a function of the temperature (T) prevailing within the cathode compartment of each cell (12) of the fuel cell (10), to maintain the humidity level (X) of the proton exchange membrane (20) of each cell substantially equal to the predetermined target value (XJ).

6. A method according to any one of claims 1-5, wherein the idle current density (Ir ef) is between 0.05 A / cm2 and 0.2 A / cm2, preferably equal to 0.1 A / cm2.

7. A method according to any one of claims 1-6, wherein, during the timing phase (P2), the fuel cell is controlled so that the current density (I) produced by the fuel cell is maintained above OA / cm2 and below 0.1 A / cm2, preferably below 0.08 A / cm2, preferably even below 0.04 A / cm2.

8. A method according to claim 7, wherein, during the drying phase (Pi), the fuel cell is controlled so that the current density (I) produced by the fuel cell is maintained above OA / cm2 and below 0.1 A / cm2, preferably below 0.08 A / cm2, preferably even below 0.04 A / cm2.

9. A method according to any one of claims 1-8, wherein, during the drying phase (P1) and during the time-delay phase (P2), the fuel cell is controlled so that the current density (I) produced by the fuel cell is maintained substantially equal to a constant target current density value (II).

10. A method according to any one of claims 1-9, wherein the predefined stopping condition is reached when a duration of the timing phase reaches a predefined duration and wherein the predefined duration of the timing phase is preferably greater than 120 seconds.

11. A method according to any one of claims 1-10, the electrical system (50) further comprising a control circuit (56) for regulating a temperature (T) of the fuel cell (10) with a heat transfer fluid, the fuel cell (10) further comprising a heat transfer fluid inlet (36) supplying each cell (12) with heat transfer fluid, and a heat transfer fluid outlet (38) removing the heat transfer fluid from each cell, the heat transfer fluid inlet and the heat transfer fluid outlet being connected to the control circuit (56), and wherein: - during the drying phase (Pi), the control circuit (56) is controlled so that the temperature (T) of the heat transfer fluid at the heat transfer fluid outlet (38) of the fuel cell decreases to a first predetermined temperature (Ti),and - during the time-delay phase (P2) the control circuit (56) is controlled so that the temperature (T) of the heat transfer fluid at the heat transfer fluid outlet (38) of the fuel cell decreases from the first predetermined temperature (Ti) to a second predetermined temperature (T2).

12. A method according to claim 11, wherein the predetermined shutdown condition is reached when the temperature (T) of the heat transfer fluid at the heat transfer fluid outlet (36) of the fuel cell (10) reaches the second predetermined temperature (T2).

13. A method according to any one of claims 11-12, wherein, during the depolarization phase (P3), the control circuit (56) is controlled so that the temperature (T) of the heat transfer fluid at the heat transfer fluid outlet (36) of the fuel cell decreases from the second predetermined temperature (T2) to a third predetermined temperature (T3).

14. A method according to any one of claims 1-13, wherein the electrical consumer (58, 60, 62) comprises: - an electrical load (60, 62), for example an electric motor (60) and / or a battery (62), and - optionally, a DC-DC converter (58), adapted to be electrically connected to the fuel cell (10), to control the fuel cell and to condition and deliver electrical power produced by the fuel cell to the electrical load (60, 62).

15. Electrical system (50) comprising: - a fuel cell (10), comprising: • a stack of cells (12), each cell comprising an anodic compartment and a cathodic compartment separated by a proton exchange membrane (20), • a dihydrogen inlet (28) supplying the anodic compartment of each cell (12) with dihydrogen, and a dihydrogen outlet (30) removing the dihydrogen from each cell, • an air inlet (32) supplying air to the cathode compartment of each cell (12), and an air outlet (34) removing the air from each cell, - an electrical consumer (58, 60, 62), adapted to be electrically connected to the fuel cell (10) to provide work from electrical power produced by the fuel cell, - a dissipative system (64), adapted to be electrically connected to the fuel cell (10) to dissipate electrical power produced by the fuel cell, and - a control unit (68), configured to implement the method of stopping any one of claims 1 to 14.

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