Method for shutting down a fuel cell

Abstract

Method for stopping a fuel cell The present invention relates to a method for stopping a fuel cell belonging to an electrical system comprising an electrical load, a DC-DC converter capable of controlling the cell and delivering electrical power produced by the cell to the electrical load, and a dissipative system capable of dissipating electrical power produced by the cell.According to the invention, the method comprises at least, in this order: a discharge phase (P3) during which: the power supply and venting of the battery are cut off, and the DC-DC converter is electrically connected to the battery and controls the battery to maintain a voltage (U) delivered by each cell of the battery substantially equal to a constant discharge voltage (U1); a depolarization phase (P4) during which: the power supply and venting of the battery are cut off, and the dissipative system is electrically connected to the battery and dissipates the electrical power produced by the battery. Figure for the abstract: 3.

Description

Title of the invention: Method for shutting down 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, which is dihydrogen, and an oxidant, which is oxygen, for example, 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 of one of the two cells is located back-to-back with a separator of the other cell. These two separators together form a bipolar separator, also called a bipolar plate.

[0005] Generally, in a unit cell, the cathode compartment is supplied with oxidant, i.e., oxygen, most often in the form of an oxygen-containing air supply, and the anodic compartment is supplied with fuel, i.e., dihydrogen. Each reactive compartment also generally includes a gas diffusion layer, located between the bipolar separator and the catalytic layer, allowing good circulation of oxygen and dihydrogen from the separator to the catalytic layer.

[0006] 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 cell... fuel is equal to the sum of the cell voltage of all the unit cells.

[0007] 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, i.e., by determining the current supplied by the cell and the voltage across its terminals.

[0008] A common problem when using such a fuel cell is that, when the fuel cell is shut down—that is, when the DC-DC converter powered by the fuel cell and the supply of air and hydrogen are stopped—reactants are still present in the cathode and anodic compartments after the fuel cell has stopped, and the electrochemical reaction therefore continues to occur within each cell. The fuel cell thus continues to generate an electrical voltage at its terminals, which is detrimental to its longevity, and remains capable of spontaneously delivering electrical power if its terminals are electrically connected, which is dangerous for a user of the fuel cell.

[0009] Furthermore, in the event of a prolonged shutdown of the fuel cell, oxygen migrates from the cathode compartment to the anode compartment in each cell by diffusion through the membrane. This presence of oxygen in the anode 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 cathode compartment, and more specifically to corrosion of a carbon substrate in the cathode compartment on which a catalyst, such as platinum, rests, which degrades the lifespan 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.

[0010] To avoid these drawbacks, it is known to carry out 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. to depolarize the fuel cell.

[0011] A first known shutdown procedure consists, at the time of the fuel cell shutdown, of disconnecting the fuel cell from the DC-DC converter and connecting it to a dissipative system comprising a resistor, called a "depolarizing resistor," while maintaining the hydrogen supply but having previously cut off the air supply. Thus, the electrical power produced by the fuel cell after its shutdown is rapidly consumed by the resistor, which prevents a continued electrical voltage from being delivered to the fuel cell terminals after its shutdown, and the oxygen present in the cathode compartment in contact with the proton exchange membrane is consumed. However, this method has the disadvantage of leading to a significant energy loss, as all the electrical power produced by the fuel cell after its shutdown is lost as heat.Furthermore, this method corresponds to an uncontrolled and rapid discharge of the fuel cell, which does not allow for the consumption of oxygen present in the cathode compartment away from the membrane. After such a shutdown procedure, oxygen migration towards the anodic compartment is always observed, particularly when the fuel cell shutdown is prolonged, as well as repolarization of the fuel cell, i.e., an increase in the voltage delivered across the fuel cell terminals.

[0012] A second known shutdown procedure comprises two phases. During the first phase, the fuel cell is connected to the DC-DC converter, which consumes the electrical power produced by the fuel cell, generally to power a battery. During this first phase, the DC-DC converter maintains a constant current level from the fuel cell, which rapidly reduces the voltage delivered by the fuel cell and thus quickly depolarizes the fuel cell.When the voltage delivered by the fuel cell becomes too low to be conditioned by the DC-DC converter, a second phase of the second shutdown procedure begins, during which the fuel cell is disconnected from the DC-DC converter and connected to a dissipative system including a resistor. This dissipates the electrical power produced by the fuel cell until the electrochemical reaction stops. This second shutdown procedure has the advantage of recovering a large portion of the electrical power produced by the fuel cell, but it also does not allow for the consumption of oxygen present in the cathode compartment away from the membrane. After such a shutdown procedure, the following is observed. therefore there is always a repolarization of the fuel cell as well as a migration of oxygen towards the anodic compartment.

[0013] The object of the invention is to propose a procedure for stopping a fuel cell which makes it possible to efficiently depopulate the oxygen in the cathode compartment, while depolarizing the fuel cell.

[0014] 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 air to the cathode ray compartment of each cell, and an air outlet removing the air from each cell,

[0015] the electrical system comprising: - an electrical charge, - a DC-DC converter, suitable for electrical connection 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, and - a dissipative system, adapted to be electrically connected to the fuel cell to dissipate electrical power produced by the fuel cell.

[0016] According to the invention, the fuel cell shutdown method comprises at least, in this order: - a discharge phase during which: • the air supply to the fuel cell via the air intake and the air exhaust from the fuel cell via the air outlet are cut off, and • The DC-DC converter is electrically connected to the fuel cell and controls the fuel cell to maintain a voltage delivered by each fuel cell that is substantially equal to a constant discharge voltage value, - a depolarization phase, during 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.

[0017] Furthermore, the discharge phase ends and the depolarization phase begins when the intensity of the current produced by the fuel cell reaches a predefined threshold current value.

[0018] Thanks to the discharge phase of the shutdown method of the invention, a greater quantity of oxygen present in the cathode compartment is consumed compared to prior art shutdown procedures. Thus, a thicker nitrogen skin than in the prior art forms in contact with the membrane in the cathode compartment, hindering the diffusion of oxygen through the membrane to the anodic compartment. The shutdown method of the invention therefore makes it possible to keep the fuel cell shut down for an extended period without risking corrosion of the gas diffusion layer of the cathode compartment when the fuel cell is restarted. Furthermore, the depolarization phase ensures efficient depolarization of the fuel cell, preventing any power generation after the fuel cell is shut down and slowing down the repolarization of the fuel cell.

[0019] 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:

[0020] - The predefined threshold current value is between 2 A and 10 A.

[0021] - The discharge voltage value is between 0.75 V / cell and 0.82 V / cell.

[0022] - During the discharge phase, the DC-DC converter conditions and delivers the electrical power produced by the fuel cell to the electrical load.

[0023] - The DC-DC converter maintains the voltage delivered by the fuel cell substantially equal to the discharge voltage value by controlling the current produced by the fuel cell, so as to decrease the intensity of the current produced by the fuel cell throughout the discharge phase.

[0024] - Supplying the fuel cell with dihydrogen via the inlet of Dihydrogen is decreased throughout the discharge phase and the depolarization phase, until it is cut off at the end of the depolarization phase.

[0025] - The duration of the discharge phase is greater than 3 seconds.

[0026] - The discharge phase is preceded by a stabilization phase during in which the voltage delivered by each cell of the fuel cell is maintained substantially equal to the discharge voltage value, and in which the intensity of the current produced by the fuel cell is maintained substantially equal to a stabilization current value.

[0027] - The dissipative system includes a resistor configured to dissipate a maximum power less than 2%, preferably less than 1%, of the maximum electrical power produced by the fuel cell.

[0028] - The resistance of the dissipative system has a resistance equal to the value of discharge voltage multiplied by the number of fuel cell cells, divided by the predefined threshold current value.

[0029] 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:

[0030] [Fig-1] The [Fig. 1] is an exploded perspective view of a stack of some cells of a fuel cell according to the invention.

[0031] [Fig.2] The [Fig.2] is a schematic representation of an electrical system comprising the fuel cell of the [Fig.1].

[0032] [Fig.3] The [Fig.3] is a representative timing diagram of a fuel cell shutdown procedure of the [Fig.1].

[0033] [Fig.4] The [Fig.4] is a graph representing several polarization curves of the fuel cell of the [Fig.1], during a phase of the shutdown procedure of the [Fig.3],

[0034] [Fig.5] The [Fig.5] is a graph representing several polarization curves of the fuel cell of the [Fig.1], during another phase of the shutdown procedure of the [Fig.3].

[0035] 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.

[0036] The fuel cell 10 is, for example, intended to be installed in a vehicle and to produce electricity powering an electric motor providing propulsion for the vehicle.

[0037] 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 end plates, which are not shown in [Fig. 1]. These end plates make it possible in particular to keep the stack of cells 12 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.

[0038] 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. However, the invention is also applicable in the context of of solid electrolyte ion exchange membrane fuel cells exhibiting different constructions.

[0039] It is assumed that, for a given fuel cell 10, all the cells 12 of the fuel cell are identical to each other, therefore having identical characteristics.

[0040] In [Fig.1], a detail of a cross-section of the membrane-electrode assembly 14 of a cell 12 is also shown.

[0041] In practice, each bipolar plate 16 is positioned 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 with air by a second bipolar plate. Since air essentially contains a mixture of nitrogen and oxygen, the cell 12 is thus supplied with oxygen.

[0042] In the following description, the terms oxygen and dioxygen, as well as hydrogen and dihydrogen, are used interchangeably.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] In each cell 12, an anodic compartment is distinguished, 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 the anodic compartment and the cathodic catalytic layer 24 is arranged in the cathodic compartment.

[0047] 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.

[0048] 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 circulation of gases and electrons between these two compartments. It is, for example, made of a sulfonated perfluorinated polymer material, such as a material known under the trade name Nafion or a material known under the trade name Aquivion.

[0049] 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 the same bipolar plate, this cathodic side belonging to the cathodic compartment of the adjacent cell 12. At the same time, a reduction reaction occurs in the cathodic compartment of the cell 12, at the level of the cathodic catalytic layer 24.This reduction reaction consists of reacting dioxygen molecules supplied by the air, through the gas diffusion layer 18, with protons passing through the proton exchange membrane 20 and with electrons supplied by the cathodic side 16B of the bipolar plate 16, to form water molecules, in the form of water vapor.

[0050] 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 manufacture of the catalytic layers.

[0051] In addition, the pores of the catalytic layers allow the free transport of reactants, i.e. dihydrogen and oxygen, inside the catalytic layers.

[0052] 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.

[0053] 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.

[0054] 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 seals 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] In the example shown in [Fig. 1], the inlets 28, 32, and 26, 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.

[0060] 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.

[0061] 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.

[0062] Here, the channels 44 are shown to be straight. In an alternative embodiment of the invention not shown, the channels 44 have another shape, for example a wavy, serpentine, or broken line shape.

[0063] 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.

[0064] 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 across the entire width of the active zone, towards all the channels.

[0065] 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.

[0066] 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.

[0067] 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 is noted that the current I produced by the fuel cell is equal to the current density produced by the fuel cell multiplied by the sum of the active area of ​​all the cells 12, 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.

[0068] Preferably, the fuel cell comprises between 60 and 500 cells 12. In the example, the fuel cell comprises 400 cells.

[0069] 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 propulsion motor vehicle.

[0070] 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.

[0071] 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 outlet distribution galleries of the stack, plus possibly the volumes of piping between each of these shut-off valves and the corresponding air inlet and outlet valve, plus possibly the volume of an air humidification device belonging to the air supply system 54 and not described here.

[0072] 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.

[0073] The vehicle 50 comprises a DC-DC converter 58, an electric motor 60, and a battery 62. DC-DC means that the converter converts one direct current into another direct current. The DC-DC converter 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.

[0074] 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 this end, 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.

[0075] 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 loads electrical features not shown in the vehicle, such as an entertainment system or a heating system.

[0076] 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.

[0077] 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.

[0078] 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. The dissipative system 64 includes, for example, a resistor, which dissipates the electrical power produced by the fuel cell as heat.

[0079] The dissipative system 64 is disconnectable from the fuel cell 10. For this purpose, the dissipative system includes, for example, a controlled switch, located between the dissipative system and the fuel cell. In other words, the fuel cell can be isolated from the dissipative system.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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 I, expressed in Amperes (A), produced by the fuel cell; the temperature T of the heat transfer fluid at the heat transfer fluid inlet 36 of the fuel cell, expressed in degrees Celsius (°C); the mass flow rate of dihydrogen QH2 supplied by the hydrogen supply system 52 to each cell 12 of the fuel cell, expressed in grams per second per cell (g / s / cell); the mass flow rate of air Qair 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)), and the pressure P prevailing within the cathode compartment of each cell, more precisely at the air inlet 32 ​​of the cathode compartment of each cell, for example expressed in absolute bar (barA).

[0084] 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.

[0085] A method for stopping the fuel cell 10, for example implemented when stopping the vehicle 50, is now described with reference to Figures 3 to 5.

[0086] The shutdown process is preferably implemented by the control unit 68.

[0087] Before the shutdown process begins, the fuel cell 10 is in its normal operating phase, denoted PF. For example, the vehicle 50 is moving and the fuel cell is producing the electrical power necessary to propel the electric motor 60.

[0088] 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. The three main objectives of this shutdown procedure are: - reducing the voltage across the fuel cell to zero, i.e., depolarizing the fuel cell, - achieving a sustained depolarization of the fuel cell over time, and - the formation of a "nitrogen skin", in contact with the proton exchange membrane 20 within the cathodic compartment, at maximum thickness, i.e. the consumption of a maximum amount of dioxygen within the cathodic compartment.

[0089] The shutdown procedure begins at a time t0, when a shutdown command for the fuel cell 10 is received by the control unit 68. This shutdown command is issued, for example, when a shutdown of the vehicle 50 is desired. Preferably, before the At the start of the shutdown procedure, a check is performed by the control unit 68 to verify whether the requested shutdown is a normal shutdown or an emergency shutdown, and the shutdown procedure is only executed when the requested shutdown is a normal shutdown. Indeed, in the event of an emergency shutdown, it is preferable to shut down the fuel cell 10 as quickly as possible, regardless of any potential damage that may occur to the fuel cell during such an emergency shutdown.

[0090] At time t0, the characteristic quantities of the operation of the fuel cell 10 are: - U = Uo, with Uo preferably between 0.82 V / cell and 0.65 V / cell, for example equal to 0.67 V / cell; - I = Iq, with Io preferably between 30 A and 400 A, for example equal to 350 A; - Qh 2 = Qh2 o, with Qh 2 o preferably between 3.13x 10'4 g / s / cell and 4.18x10'4 g / s / cell, for example equal to 3.66x10'4g / s / cell; - Qair = Qair o, with Qair. 0 preferably between 0.053 g / s / cell, and 0.256 g / s / cell, for example equal to 0.228 g / s / cell; - T = To, with To preferably between 50°C and 95°C, for example equal to 80°C.

[0091] Thus, the electrical power produced by the fuel cell during the PF operating phase is, in the example of a stack of 400 cells, between 9.84 kW and 104 kW, for example equal to 93.80 kW. In the example, the maximum power of the fuel cell is therefore equal to 104 kW.

[0092] In practice, the maximum power of the fuel cell 10 tends to decrease over the life of the fuel cell, due to wear and tear and the appearance of defects.

[0093] 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.

[0094] During the shutdown process, the flow rate QH2 is regulated by the hydrogen supply system 52, the flow rate Qair is regulated by the air supply system 54, the temperature T is regulated by the control circuit 56 and the voltage U and the current I are regulated by the DC-DC converter 58, based on commands issued by the control unit 68.

[0095] The shutdown process begins with a speed reduction phase PB which lasts until a time tb. During the speed reduction phase, the fuel cell 10 is connected to the DC-DC converter 58, which drives the fuel cell so as to decrease the power generated by the fuel cell 10, while raising the voltage U of each cell 12 to a discharge voltage value.

[0096] At time tb, the characteristic quantities of the operation of the fuel cell 10 are: - U = Ui, with Ui corresponding to the discharge voltage value, preferably between 0.82 V / cell and 0.75 V / cell, for example equal to 0.8 V / cell; - I = h, with h preferably between 30 A and 80 A, for example equal to 70 A; - Qh 2 = Qh2 i, with Qh 2 i preferably between 3.13x 10'4 g / s / cell and 8.36x10'4 g / s / cell, for example equal to 7.31 x10'4 g / s / cell; - Qair = Qair i, with Qair. i preferably between 0.053 g / s / cell and 0.089 g / s / cell, for example equal to 0.077 g / s / cell; - T = Ti, with Ti preferably between 50°C and 65°C, for example equal to 60°C.

[0097] The discharge voltage value Ui is advantageously chosen to correspond to a maximum cell voltage that does not cause a risk of dissolution of the platinum contained in the catalytic layers 22 and 24, which would reduce the performance of the fuel cell.

[0098] In the example in the figures, the value Ui is greater than the value Uo.

[0099] The electrical power produced by the fuel cell at the end of the Pi deceleration phase is, in the example of a stack of 400 cells, between 10.4 kW and 26 kW, i.e. between 10% and 25% of the maximum power of the fuel cell.

[0100] In practice, the Pi phase of deceleration ends as soon as the temperature T of the heat transfer fluid at the inlet of the fuel cell reaches the value Tp. In other words, the condition for stopping the phase of deceleration is the attainment of the temperature Ti which is less than the value To.

[0101] The temperature value Ti corresponds to a calculated target value, which is chosen to be a minimum temperature while being higher than the condensation point of the water vapor present in the cathode compartment of each cell 12.

[0102] To this end, the control unit 68 calculates a reference temperature Tref, expressed in degrees Celsius, for which the relative humidity in the cathode compartment of each cell 12 is equal to 100%. This calculation is notably performed on the basis of the current I produced by the fuel cell, the gas flow rate Qexit_cathode at the air outlet 34, and the pressure P prevailing within the cathode compartment of the cells 12.

[0103] More specifically, the reference temperature Tref is obtained in several calculation steps. First, the control unit 68 calculates the mass flow rate of water produced by the fuel cell, expressed in grams per second (g / s), using the following equation:

[0104] q _ watermassic XexN^

[0105] [Equation 1]

[0106] in which: - I is the intensity produced by the fuel cell 10; - N is the number of cells in the fuel cell; - Meau is the molar mass of water, equal to 18 g / mol; - e is the elementary charge of a proton, equal to 1.602176634 x 10⁻¹⁹ Coulombs; and - Na is Avogadro's number, equal to 6.02214076 x 1023 mol-1.

[0107] From the mass flow rate of water produced by the fuel cell, the control unit 68 calculates the flow rate of water vapor produced by the fuel cell, expressed in liters per second (l / s) using the following equation:

[0108] ~iMU_music ^water vapor — Skin

[0109] [Equation 2]

[0110] in which: - skin is the density of water vapor, at the temperature and pressure conditions prevailing within the cathode compartment of cells 12. In the context of the shutdown process, skin is considered constant, and approximately equal to 0.59 g / l.

[0111] Next, the control unit 68 calculates the partial pressure of water vapor within the cathode compartment of the cells 12, denoted PPE and expressed in absolute bar, from the following equation:

[0112] nn„ n Qvapeur water PPE = P x 75---=— ^5:oftie_c(aiwde

[0113] [Equation 3]

[0114] in which: - P is the pressure prevailing within the cathode compartment of cells 12, expressed in absolute bar; and - Qsortie_cathode is the flow rate of all gases exiting the cathode compartment of cells 12, measured at the air outlet 34 and expressed in liters per second.

[0115] Next, the reference temperature Tref is calculated from the following equation:

[0116] T <\ r ref Cr\^PPE* 105) + C 3

[0117] [Equation 4]

[0118] in which: - Ci is a constant expressed in degrees Kelvin, preferably equal to 3816.44 K; - C2 is a dimensionless constant, preferably equal to 23.1961; - C3 is a constant expressed in degrees Kelvin, preferably equal to - 226.86 K, allowing in particular for obtaining a Tref value expressed in degrees Celsius; and ln(PPEx105) corresponds to the natural logarithm of the partial pressure of water vapor PPE, this partial pressure of water vapor being expressed in absolute bar.

[0119] Furthermore, based on equations 1, 2 and 3, the partial pressure of the vapor can be expressed according to a simplified equation: [0i20] ppE = Px^__

[0121] [Equation 5]

[0122] in which C4 is a constant equal to ; sy _ ^eau 4~ 2xPe^exNA

[0123] Thus, thanks to the above equations, the control unit 68 is able to calculate the reference temperature Tref on the basis of the current produced by the fuel cell, the gas flow rate at the air outlet 34, and the pressure prevailing in the cathode compartment of each cell.

[0124] The temperature value Ti is then defined from the calculation of the reference value Tref. Advantageously, Ti is defined as being between 100% and 140% of Tref, preferably between 100% and 125% of Tref, and even more preferably between 100% and 110% of Tref.

[0125] The shutdown process continues from time ti until time t2 with a stabilization phase P2. During this stabilization phase, the characteristic quantities of the operation of the fuel cell 10, in particular the flow rates QH2 and Qair, the voltage U, the current I and the temperature T, are kept constant.

[0126] The stabilization phase P2 allows, in particular, the temperature T of the heat transfer fluid at the inlet of the fuel cell 36 to stabilize at the value Tb. Thus, at the end of the stabilization phase P2, the temperature of the heat transfer fluid at the inlet of the fuel cell 36, and consequently the temperature within each cell 12 of the fuel cell, are stabilized at a value close to the minimum temperature that can be reached without condensing the water vapor generated in the cathode compartment of cells 12.

[0127] During the Pi deceleration and P2 stabilization phases, the air and hydrogen supplies to the fuel cell are maintained. In addition, during these phases, the fuel cell 10 is connected to the DC-DC converter 58 and the power generated by the fuel cell is thus conditioned by the DC-DC converter and delivered to an electrical load, preferably to the battery 62.

[0128] The shutdown process continues with a discharge phase P3, beginning at time t2, during which the air supply to the fuel cell 10 via the air inlet 32 ​​is cut off. More precisely, the cathode air volume of the fuel cell is isolated by closing the shut-off valves corresponding to the air inlet and outlet of the fuel cell. During the discharge phase P3, the hydrogen supply to the fuel cell via the dihydrogen inlet 28 is maintained. Thus, during the discharge phase P3, the fuel cell 10 is no longer supplied with air but remains supplied with hydrogen.

[0129] During the discharge phase P3, the fuel cell 10 is electrically connected to the DC-DC converter 58, and the DC-DC converter controls the fuel cell to maintain the voltage U delivered by each cell 12 equal to the discharge voltage value Up. In other words, the voltage U remains constant during the discharge phase P3: the fuel cell is controlled at a constant voltage.

[0130] Since at time t2, i.e., when the air supply is cut off, a quantity of oxygen-containing air is present in the cathode compartment of each cell 12, and generally in the fuel cell's cathode air volume, forming an oxygen reservoir, the reduction reactions in the cathode compartment of each cell continue to occur. The fuel cell is therefore still able to deliver a voltage and produce a current. The electrical power thus produced by the fuel cell is conditioned by the DC-DC converter and then delivered by the DC-DC converter to the battery 62.

[0131] During the P3 discharge phase, the oxygen contained in the cathode compartment is therefore 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 gradually decreases during the P3 discharge phase.

[0132] This decrease in oxygen concentration alters the electrochemical properties of the fuel cell. In particular, it modifies the polarization curve of the fuel cell. Indeed, the lower the oxygen concentration in the cathode compartment of each cell, the lower the current I produced for a given voltage U is low, because the lower the number of electrochemical reactions producing electrons in the cells 12.

[0133] This phenomenon is illustrated in [Fig.4], on which are represented several polarization curves of the fuel cell 10, each being representative of the fuel cell at an instant of the discharge phase P3.

[0134] At time t2, the fuel cell bias curve corresponds to the PA curve, and it is observed that for a voltage U delivered by each cell 12 equal to Ui, the current I produced by the fuel cell is equal to Ip

[0135] As the oxygen present in the cathode compartment of each cell is consumed, the polarization curve of the fuel cell 10 is modified so that for a voltage U delivered by each cell 12 equal to Ub the current I produced by the fuel cell is increasingly lower.

[0136] In the example, simplified polarization curves PB, Pc, Pd, and PE are shown in [Fig. 4], and correspond to the polarization curve of the fuel cell 10 taken at four different times after t2. The PE polarization curve is representative of the operation of the fuel cell 10 at time t3. Thus, for the PB polarization curve, a current IB lower than L is produced; for the Pc polarization curve, a current Ic lower than IB is produced; for the PD polarization curve, a current ID lower than Ic is produced; and for the PE polarization curve, a current I3 lower than ID is produced.

[0137] In summary, during the discharge phase P3, the interruption of the air supply and the maintenance of a constant voltage U by means of the control of the DC-DC converter result in a decrease in the intensity of the current I produced by the fuel cell.

[0138] Advantageously, to maintain the voltage U equal to the discharge voltage value Ui, the DC-DC converter 58 controls the current I produced by the fuel cell, so as to decrease the intensity of the current produced by the fuel cell throughout the discharge phase P3, following the evolution of the bias curve of the fuel cell.

[0139] Furthermore, during the discharge phase P3, the temperature T of the heat transfer fluid at the inlet 36 is controlled to be regularly updated so as to reflect the evolution of the current I produced by the fuel cell. To this end, the reference temperature Tref is regularly updated based on the evolution of the current I, which leads to a decrease in the temperature of the heat transfer fluid at the inlet 36 of the fuel cell, and thus a decrease in the temperature of the cells 12 of the fuel cell. Advantageously, the temperature T is regulated so as to be maintained between 100% and 140% of Tref, preferably between 100% and 125% of Tref, and even more preferably between 100% and 110% of Tref, throughout the entire discharge phase P3. In other words, updating the reference temperature Tref results in an update of the temperature T.

[0140] It is also noted that during the discharge phase P3, the hydrogen flow rate Qh2 supplied by the hydrogen supply system 52 to the fuel cell gradually decreases. In practice, this flow rate is adjusted so as not to limit the current production of the fuel cell while avoiding excessive hydrogen consumption.

[0141] The discharge phase P3 ends at time t3, when the intensity of the current I produced by the fuel cell reaches a predefined threshold current value, corresponding to the current I3. Preferably, the current I3 is between 2 A and 10 A.

[0142] Preferably, the current I3 corresponds to a minimum current value that can be conditioned by the DC-DC converter. In other words, the DC-DC converter is not able to force the production of a current lower than I3 on ​​the fuel cell, while maintaining a cell voltage U equal to Ui.

[0143] At the end of the discharge phase P3, i.e. at time t3, the characteristic quantities of the operation of the fuel cell 10 are: U = Uj; - I = I3, the value I3 being less than the value I2; - Qh 2 = Qh2 3, with Qh 2 3 preferably between 2.089x10'5 g / s / cell, for a current I3 equal to 2 A, and 1.04x10'4 g / s / cell, for a current I3 equal to 10 A, for example equal to 2.089x10'5 g / s / cell, for a current I3 equal to 2 A; - Qair = 0 g / s / cell; and - T = T3, with T3 preferably between 15°C and 60°C, for example equal to 15°C, the value of T3 being less than the value of T2.

[0144] Furthermore, at the end of the P3 discharge phase, the oxygen concentration within the cathode compartment of each cell 12 is reduced, since the amount of air contained in the cathode compartment of each cell is not renewed.

[0145] In practice, since the air inlets and outlets 32, 34 are blocked, the amount of air in the cathode compartment of each cell is static. Furthermore, oxygen consumption within this amount of air is not uniformly localized: it occurs only at the contact with the proton exchange membrane 20. Because the amount of air is static, the different gases composing air, that is, essentially oxygen and nitrogen, can only mix by diffusion. Thus, the localized oxygen consumption at the proton exchange membrane 20 and the absence of air movement result in an oxygen concentration gradient, which is minimal at the contact with the proton exchange membrane 20 and greater at the contact with the bipolar plate delimiting the compartment cathodic of each cell and in the rest of the cathodic air volume of the stack. This oxygen concentration gradient leads to the formation of a layer of air with a higher nitrogen concentration than the ambient air, located in contact with the proton exchange membrane 20, which is also called the "nitrogen skin".

[0146] At time t3, a depolarization phase P4 begins. Furthermore, at time t3, the DC-DC converter 58 is disconnected from the fuel cell 10, and the dissipative system 64 is connected to the fuel cell.

[0147] Thus, during the depolarization phase P4, the power produced by the fuel cell 10 is dissipated by the resistance of the dissipative system 64, in the form of heat.

[0148] In practice, 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 P4, the fuel cell 10 is controlled by the resistance R of the dissipative system 64.

[0149] This operation of the fuel cell 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.

[0150] Advantageously, the value of the resistance R is chosen to be equal to the discharge voltage value Ui, multiplied by the number N of cells 12 of the fuel cell, divided by the predefined threshold current value I3.

[0151] During the P4 depolarization phase, the oxygen contained in the cathode compartment 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 P4 depolarization phase.

[0152] This decrease in oxygen concentration alters 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 number of electrochemical reactions producing electrons in the cells 12 is lower.

[0153] This phenomenon is illustrated in [Fig. 5], on which several polarization curves of the fuel cell 10 are shown, each being representative of the fuel cell at a moment in the depolarization phase P4, and on which is also represented a line representing the resistance R.

[0154] At time t3, that is to say at the end of the discharge phase P3 and at the beginning of the depolarization phase P4, the polarization curve of the fuel cell corresponds to the PE curve, and it is observed that for a voltage U delivered by each cell 12 equal to Ub the intensity of the current I produced by the fuel cell is equal to I3.

[0155] As the oxygen present in the cathode compartment of each cell is consumed, the polarization curve of the fuel cell 10 is modified so that for a given voltage U delivered by each cell 12, the intensity of the current I produced by the fuel cell is increasingly lower.

[0156] In the example, polarization curves PF, PG, PH and P! are also represented in a simplified manner in [Fig.5], and correspond to the polarization curve of the fuel cell 10 taken at four different times after time t3.

[0157] As can be seen in Figures 3 and 5, 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 I produced by the fuel cell and the voltage U delivered by each cell 12.

[0158] Thus, for the PF bias curve, a voltage UF and a current IF lower than Ui and I3 are produced; for the PG bias curve, a voltage UG and a current IG lower than UF and IF are produced; for the PH bias curve, a voltage UH and a current IH lower than UG and IG are produced; and for the Pb bias curve, a voltage Ui and a current h lower than UH and IH are produced.

[0159] In summary, during the depolarization phase P4, the cutting off of the air supply and the connection of the fuel cell to the resistive circuit 64 result in a decrease in the voltage U delivered by each cell 12 and in the current I produced by the fuel cell.

[0160] Preferably, during the depolarization phase P4, the temperature T is controlled to be regularly updated so as to reflect the evolution of the current I produced by the fuel cell. To this end, the reference temperature Tref is regularly updated based on the evolution of the current I, which leads to a decrease in the temperature of the heat transfer fluid at the heat transfer fluid inlet 36, and therefore a decrease in the temperature of the fuel cell. Advantageously, the temperature T is maintained between 100% and 140% of Tref, preferably between 100% and 125% of Tref, and even more preferably between 100% and 110% of Tref, throughout the entire duration of the depolarization phase P4. Advantageously, when the temperature T is controlled during the depolarization phase P4, this control is maintained until this that the temperature T becomes equal to the ambient temperature surrounding the vehicle 50. In other words, this control allows the decrease of the temperature T to be controlled until the ambient temperature is reached.

[0161] It is also noted that during the depolarization phase P4, the hydrogen flow rate Qh2 supplied by the hydrogen supply system 52 to the fuel cell gradually decreases. In practice, this flow rate is adjusted so as not to limit the current production of the fuel cell while avoiding oversupply of the anodic compartment of each cell 12, thus preventing overconsumption of hydrogen.

[0162] The depolarization phase P4 ends at time t4, when the intensity of the current I produced by the fuel cell 10 reaches a value of zero, corresponding to the current I4. In other words, the depolarization phase P4 ends when no more electrochemical reactions producing electrons occur within the cells 12 of the fuel cell.

[0163] At time t4, the shutdown process of the fuel cell 10 is complete. The discharge phase P3 and the depolarization phase P4 together form a shutdown phase of the shutdown process.

[0164] At this instant t4, the supply of dihydrogen is then cut off, to isolate the hydrogen supply system 52 from the fuel cell 10 until the fuel cell is restarted.

[0165] At the end of the depolarization phase P4, i.e. at time t4, the characteristic quantities of the operation of the fuel cell 10 are: U = 0 V / cell; I = 0A; Qh 2 = 0 g / s / cell; - Qair = 0 g / s / cell; and - T = T4, with T4 preferably between 15°C and 60°C, for example equal to 15°C.

[0166] At the end of the P4 depolarization phase, the oxygen concentration within the cathodic compartment of each cell 12 is decreased, since the amount of air contained in the cathodic compartment of each cell is not renewed, without however reaching a zero concentration.

[0167] Indeed, the absence of mixing of the air contained in the cathode compartment of each cell prevents the consumption of all the oxygen in the cathode compartment, and in particular the oxygen located near the bipolar plate delimiting this compartment, that is to say, at a distance from the proton exchange membrane. This absence of mixing also prevents the consumption of oxygen located in the remainder of the stack cathode air volume, such as for example in the pipes between the shut-off valves and the cells 12.

[0168] Thus, at the end of the P4 depolarization phase, the oxygen concentration within the cathode compartment of each cell 12 is zero in contact with the proton exchange membrane 20, and increases progressively as it approaches the bipolar plate.

[0169] The zero oxygen concentration in contact with the proton exchange membrane 20 leads to zero voltage across each cell 12, and therefore to complete depolarization of the fuel cell 10.

[0170] Preceding the depolarization phase P4 with the discharge phase P3, carried out at constant voltage, is particularly advantageous for improving the stability of the fuel cell 10 in the event of prolonged shutdown of the latter.

[0171] In particular, the shutdown method of the invention makes it possible to minimize the repolarization of the fuel cell during a prolonged shutdown, which may occur when the oxygen initially located in the cathode compartment of each cell 12 away from the proton exchange membrane 20 migrates into the cathode compartment until it is in contact with the proton exchange membrane.

[0172] The stopping method of the invention also makes it possible to minimize the migration of oxygen to the anodic compartment of each cell by diffusion through the proton exchange membrane 20 during a prolonged stop, which may occur for the same reasons.

[0173] Indeed, the shutdown method of the invention makes it possible to obtain a nitrogen skin with maximized thickness, compared to known prior art shutdown methods. This maximization of the nitrogen skin thickness is achieved thanks to the P3 discharge phase, and more particularly thanks to the fact that the discharge phase is carried out with a constant and high voltage Ui, that is to say, thanks to the fact that the discharge phase is carried out without depolarizing the fuel cell. More precisely, cutting off the oxygen supply, while controlling the fuel cell so that the voltage delivered by each cell 12 remains high, makes it possible to slow the decrease in the current produced by the fuel cell, and therefore to lengthen the duration of the P3 discharge phase.

[0174] Thus, in the example, the discharge phase P3 has a duration, between times t2 and t3, greater than 3 seconds, preferably even greater than 10 seconds. In comparison, in known shutdown methods, the fuel cell generally remains connected to a DC-DC converter after the air supply is cut off for a period of less than 0.5 seconds.

[0175] This relatively long duration of the P3 discharge phase allows time for oxygen to diffuse into the cathode compartment of each cell 12 up to the proton exchange membrane 20. This duration therefore allows a greater quantity of oxygen to be consumed than in prior art arresting processes, thus forming a nitrogen skin with an improved thickness.

[0176] Furthermore, thanks to the efficiency of the P3 discharge phase in depleting the cathode compartment of each cell 12 of oxygen, the duration of the P4 depolarization phase is reduced, and the power dissipated by the resistance of the dissipative system 64 is also reduced. This helps to limit the energy losses of the fuel cell 10, since all the power generated by the fuel cell during the P3 discharge phase is recovered by the battery 62. This also simplifies the manufacturing of the dissipative system 64, since the resistance can be sized to dissipate less power than the resistances of known dissipative systems, thus reducing the cost and size of the dissipative system.Thus, thanks to the invention, the resistance of the dissipative system 64 is sized to dissipate a maximum power of less than 2% of the maximum power produced by the fuel cell 10, preferably less than 1%, whereas the resistance of known dissipative systems is generally sized to dissipate a maximum power of more than 10% of the maximum power of the fuel cell.

[0177] In summary, the P3 discharge phase reduces the oxygen stock contained in the cathode compartment of each cell 12 in preparation for the P4 depolarization phase, making the depolarization phase simpler to implement and making the depolarization of the fuel cell more durable, with the repolarization of the fuel cell being delayed.

[0178] Furthermore, choosing the value of the resistance R to be equal to the discharge voltage value Ui, multiplied by the number N of cells 12 of the fuel cell, divided by the predefined threshold current value I3, is particularly advantageous for obtaining a continuous evolution of the voltage during the discharge phases P3 and depolarization P4, without voltage jump during the transition from the discharge phase to the depolarization phase, which limits the degradation of the fuel cell.

[0179] Furthermore, it is particularly advantageous to modify the target value for the temperature T of the heat transfer fluid at the heat transfer fluid inlet 36 of the fuel cell 10 throughout the shutdown process so as to maintain it just above the condensation temperature of water vapor. This prevents condensation from occurring in the cells 12. Moreover, keeping the temperature as low as possible slows down any potentially harmful electrochemical reactions that may occur in the cells 12, because the temperature This is a factor influencing the rate of electrochemical reactions. It also optimizes fuel cell efficiency by increasing the humidity level of the air in the cathode compartment of each cell. This humidifies the proton exchange membrane 20, thus reducing its resistance and facilitating proton conduction across the membrane and electron conduction. Furthermore, the temperature drop during the P3 discharge phase also leads to a decrease in the catalytic activity of the cathode catalytic layer 24, causing the cell voltage U to fall for a given current level I. Regulating the current to maintain a constant voltage results in a lower current being controlled at lower temperatures than if this operation were performed at higher temperatures, thus maximizing oxygen consumption and increasing the duration of the discharge phase.

[0180] The graphics shown in figures 3, 4 and 5 are only used to illustrate the stopping procedure, and, for clarity of the drawings, are not shown to scale.

[0181] The shutdown process begins with a Pi speed reduction phase to stabilize the characteristic parameters of the fuel cell's operation. This speed reduction phase is optional, particularly when the shutdown process is initiated after the characteristic parameters of the fuel cell's operation have already stabilized, and especially when U = Up

[0182] In a non-represented variant of the invention, the resistance of the dissipative system 64 has a different resistance value R, which is not, for example, chosen according to the discharge voltage value Ui or the threshold current value I3.

[0183] In a non-shown variant of the invention, the electrical system 50 has a different architecture. Thus, during phases Pi to P3 of the shutdown procedure, the fuel cell 10 can supply an electrical load other than the battery 62.

[0184] In a non-shown embodiment of the invention, during the discharge phases P3 and depolarization P4, the voltage U produced by each cell 12 of the fuel cell 10 is not controlled by the DC-DC converter 58 to be equal to the discharge voltage value Ub, but is controlled by the DC-DC converter to be within a voltage range from 0.75 V / cell to 0.82 V / cell, within which lies the discharge voltage value Up. By approximation, when the voltage U is within this voltage range, it is said that the voltage U is substantially equal to the discharge voltage value Up.

[0185] In a non-represented variant of the invention, the shutdown method does not switch from the discharge phase P3 to the depolarization phase P4 when the current I produced by the fuel cell 10 reaches the predefined threshold current value I3, but when another condition is reached, for example when a predetermined time has elapsed.

[0186] In a non-represented variant of the invention, during the discharge phase P3, the DC-DC converter 58 drives the fuel cell 10 by commanding the fuel cell to deliver a constant voltage value, rather than commanding a decrease in the current produced by the fuel cell.

[0187] In a non-shown embodiment of the invention, during the discharge phases P3 and depolarization phases P4, the temperature T of the heat transfer fluid at the heat transfer fluid inlet 36 of the fuel cell is not regulated according to the current I produced by the fuel cell, but is regulated according to another strategy. For example, the temperature T is kept constant, preferably equal to the value Tb or equal to another arbitrarily chosen temperature value.

[0188] In a non-shown variant of the invention, the fuel cell shutdown method comprises a shutdown phase different from that described above, comprising the discharge phase P3 and the depolarization phase P4, while also comprising a decrease in current I. For example, the shutdown phase comprises a discharge phase during which the fuel cell is connected to the DC-DC converter 58 and during which the voltage U delivered by each cell 12 and the current I produced by the fuel cell are controlled downward by the DC-DC converter; then a depolarization phase during which the fuel cell is connected to the dissipative system 64.According to another example, the shutdown phase includes a discharge phase during which the fuel cell is connected to the DC-DC converter 58 and during which the current I produced by the fuel cell is controlled downwards by the DC-DC converter, either continuously or in steps, while the voltage U delivered by each cell 12 is controlled to decrease in steps; then a depolarization phase during which the fuel cell is connected to the dissipative system 64. According to another example, the shutdown phase includes only a depolarization phase during which the fuel cell is connected to the dissipative system 64.In such a variant, during the shutdown phase of the fuel cell, the temperature T of the heat transfer fluid at the inlet of the fuel cell is regulated in the same way as in the shutdown process described above, i.e. the temperature T is regulated to decrease according to the evolution of the current produced by the fuel cell, while maintaining the temperature T above the reference temperature Tref, preferably between 100% and 140% of the reference temperature.

[0189] Any feature described for an embodiment or variant in the foregoing may be implemented for the other embodiments and 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) comprising: - an electric charge (60, 62), - 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), and - a dissipative system (64), adapted to be electrically connected to the fuel cell (10) to dissipate electrical power produced by the fuel cell, characterized in that the fuel cell shutdown method (10) comprises at least, in this order: - a discharge phase (P3) during which: • the air supply to the fuel cell (10) via the air inlet (32) and the air exhaust from the fuel cell via the air outlet (34) are cut off, and • The DC-DC converter (58) is electrically connected to the fuel cell (10) and controls the fuel cell to maintain a voltage (U) delivered by each cell (12) of the fuel cell. fuel substantially equal to a constant discharge voltage value (Ui), - a depolarization phase (P4), during which: • 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, and • the dissipative system (64) is electrically connected to the fuel cell (10) and dissipates the electrical power produced by the fuel cell, and in that the discharge phase (P3) ends and the depolarization phase (P4) begins when the intensity of the current (I) produced by the fuel cell reaches a predefined threshold current value (I3).

2. Method according to claim 1, wherein the predefined threshold current value (I3) is between 2 A and 10 A.

3. A method according to any one of claims 1 to 2, wherein the discharge voltage value (Ui) is between 0.75 V / cell and 0.82 V / cell.

4. A method according to any one of claims 1 to 3, wherein, during the discharge phase (P3), the DC-DC converter (58) conditions and delivers the electrical power produced by the fuel cell (10) to the electrical load (60, 62).

5. A method according to any one of claims 1 to 4, wherein the DC-DC converter (58) maintains the voltage (U) delivered by the fuel cell (10) substantially equal to the discharge voltage value (Ui) by controlling the current (I) produced by the fuel cell, so as to decrease the intensity of the current produced by the fuel cell throughout the discharge phase (P3).

6. A method according to any one of claims 1 to 5, wherein a supply of dihydrogen to the fuel cell (10) by the dihydrogen inlet (28) is decreased throughout the discharge phase (P3) and the depolarization phase (P4), until it is cut off at the end of the depolarization phase.

7. A method according to any one of claims 1 to 6, wherein a duration of the discharge phase (P3) is greater than 3 seconds.

8. A method according to any one of claims 1 to 7, wherein the discharge phase (P3) is preceded by a stabilization phase (P2) during which the voltage (U) delivered by each cell (12) of the fuel cell (10) is maintained substantially equal to the discharge voltage value (Ui), and during which the current intensity (I) produced by the fuel cell (10) is maintained substantially equal to a stabilization current value (Ii).

9. A method according to any one of claims 1 to 8, wherein the dissipative system (64) comprises a resistor configured to dissipate a maximum power of less than 2%, preferably less than 1%, of a maximum electrical power produced by the fuel cell.

10. A method according to claim 9, wherein the resistance of the dissipative system (64) has a resistance (R) equal to the discharge voltage value (Ui) multiplied by the number of cells (12) of the fuel cell (10), divided by the predefined threshold current value (I3).

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