Method of stopping a fuel cell

Abstract

Method for stopping a fuel cell The present invention relates to a method for stopping a fuel cell connected to a regulation circuit for regulating a temperature (T) of the cell with a heat transfer fluid. The method comprises a stabilization phase (P2), during which: - an air supply to the fuel cell is maintained, - the cell is controlled so that the intensity of a current (I) produced by the cell is maintained substantially equal to a constant target current value (I1), - a reference temperature, corresponding to a relative humidity level in the cathode compartment of each cell of the cell equal to 100%, is calculated on the basis of the current produced, an air flow leaving the cell, and a pressure prevailing in the cathode compartment of each cell, - the regulation circuit is controlled so that the temperature of the heat transfer fluid entering the cell is between 100% and 140% of the reference temperature.Figure for abstract: 3.

Description

Title of the invention: Method for stopping a fuel cell

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

[0002] A fuel cell is a device for generating electricity by electrochemical reaction between a fuel, generally dihydrogen, and an oxidant, generally oxygen contained in the air. We are interested here in fuel cells of the solid electrolyte proton exchange membrane type - also called PEMFC in English -, which usually comprise a stack - called "stack" in English - of several unit cells each constituting an electrochemical generator.

[0003] Schematically, each unit cell comprises two separators, also called polar plates, between which is intercalated a solid electrolyte in the form of a proton exchange membrane. The membrane is made for example from a sulfonated perfluorinated polymer material. Within each cell, each separator delimits with the corresponding membrane a reactive compartment. One of the two compartments, called the cathode compartment, houses a cathode element, formed by a cathode 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 called "AME".

[0004] For two neighboring cells, a separator from one of the two cells is 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 glycolated water circulates, is generally arranged between the two separators of the bipolar separator.

[0005] The dihydrogen, the air and the heat transfer fluid are so-called "operating" fluids, which are supplied to the fuel cell during its operation. The dihydrogen and the air are reactants while the heat transfer fluid does not intervene in the electrochemical reaction. Depending on the operating phases of the fuel cell, the supply of one or more of the operating fluids is done continuously or intermittently.

[0006] The fuel cell thus provides openings for supplying fluids to each of the reactive compartments and 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 ensured by the bipolar separators operating in parallel.

[0007] Generally, in a unit cell, the cathode compartment is supplied with oxygen, most often in the form of an air supply containing oxygen, and the anode compartment is supplied with dihydrogen. Each reactive compartment also generally comprises 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 in operation, 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 each cell, referred to as the "cell voltage." All of the cells in the fuel cell are electrically connected together in series, such that the voltage delivered across the fuel cell is equal to the sum of the cell voltage of all of 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 for example an electric motor and / or a battery belonging to a vehicle. In addition, the DC-DC converter makes it possible to control the fuel cell, by determining the electrical operating point of the fuel cell, that is to say by determining the current supplied by the cell and the voltage at its terminals.

[0010] As is known per se, the temperature within a fuel cell influences the efficiency of the fuel cell. In particular, it is desirable for the temperature of the fuel cell to be sufficiently high to facilitate the electrochemical reaction between dihydrogen and dioxygen. This is why it is known to maintain the fuel cell at a relatively high constant operating temperature during its operation, generally around 80°C.

[0011] However, it has been observed that such an operating temperature, when kept constant during fuel cell shutdown, leads to the acceleration of harmful physicochemical reactions degrading the fuel cell and reducing its lifetime, such as for example the migration of metal nanoparticles into the proton exchange membranes of the cells, the dissolution of the catalyst of the cathodic and anodic catalytic layers, and geometric reorganization mechanisms of the constituent elements of the proton exchange membranes.

[0012] It is therefore desirable to reduce the temperature of the fuel cell when it is shut down. The aim of the invention is to propose a procedure for shutting down a fuel cell fuel during which the temperature of the fuel cell is controlled and optimized to avoid damage to the fuel cell and maximize its lifespan.

[0013] For this purpose, the invention relates to a method for stopping a fuel cell belonging to an electrical system, the electrical system further comprising a regulation circuit for regulating a temperature of the fuel cell with a heat transfer fluid, and a control unit, the fuel cell comprising: - a stack of cells, each cell comprising an anode compartment and a cathode compartment separated by a proton exchange membrane, - a dihydrogen inlet supplying the anode compartment of each cell with dihydrogen, and a dihydrogen outlet evacuating the dihydrogen coming from each cell, - an air inlet supplying air to the cathode compartment of each cell, and an air outlet evacuating air from each cell, - a heat transfer fluid inlet supplying each cell with heat transfer fluid, and a heat transfer fluid outlet discharging the heat transfer fluid from each cell, the heat transfer fluid inlet and the heat transfer fluid outlet being connected to the regulation circuit.

[0014] According to the invention, the method for stopping the fuel cell comprises a stabilization phase, during which: - an air supply to the fuel cell via the air inlet is maintained, - the fuel cell is controlled so that the intensity of a current produced by the fuel cell is maintained substantially equal to a constant target current value, - the control unit calculates a reference temperature corresponding to a relative humidity level in the cathode compartment of each cell equal to 100%, on the basis of the current produced by the fuel cell, a gas flow rate at the air outlet, and a pressure prevailing in the cathode compartment of each cell, and - the control circuit is controlled so that the temperature of the heat transfer fluid at the heat transfer fluid inlet of the fuel cell is between 100% and 140% of the reference temperature, the temperature of the heat transfer fluid and the reference temperature being expressed in degrees Celsius.

[0015] By means of the invention, the temperature of the fuel cell is minimized while being maintained above the condensation temperature of the water in the com cathode compartment of each cell. Thus, harmful physicochemical reactions are minimized, without risking the condensation of water vapor within the fuel cell.

[0016] According to other advantageous aspects of the invention, the stopping method comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0017] - The method further comprises, after the stabilization phase, a stopping phase, during which the air supply to the fuel cell through the air inlet is cut off, and during which the fuel cell is controlled so that the intensity of the current produced by the fuel cell decreases until it reaches a zero current value.

[0018] - During the shutdown phase, the control unit updates the temperature of reference, based on the evolution of the current produced by the fuel cell, the gas flow rate at the air outlet, and the pressure prevailing in the cathode compartment, and the regulation circuit is controlled to decrease the temperature of the heat transfer fluid at the heat transfer fluid inlet of the fuel cell, while maintaining the temperature of the heat transfer fluid at the heat transfer fluid inlet of the fuel cell between 100% and 140% of the reference temperature.

[0019] - The electrical system comprises an electrical load; 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; and a dissipative system, adapted to be electrically connected to the fuel cell to dissipate electrical power produced by the fuel cell. In addition, the shutdown phase comprises a discharge phase, during which the DC-DC converter is electrically connected to the fuel cell and delivers the electrical power produced by the fuel cell to the electrical load, then a depolarization phase, during which the dissipative system is electrically connected to the fuel cell and dissipates the electrical power produced by the fuel cell.

[0020] - During the discharge phase, the DC-DC converter controls the battery to fuel to maintain a voltage delivered by each cell of the fuel cell substantially equal to a constant discharge voltage value.

[0021] - During the stabilization phase, a power produced by the fuel cell fuel is between 10% and 25% of the maximum power produced by the fuel cell.

[0022] - The reference temperature Tref is calculated from the following equation:

[0023] y _____C__. y 1 reS Q-in^ExlQs) +

[0024] in which: • ln(PPExlO5) is the natural logarithm of the partial pressure of water within the cathode compartment of each cell of the fuel cell, • Ci and C3 are two constants expressed in degrees Kelvin; and • C2 is a dimensionless constant.

[0025] - the partial pressure of water vapor PPE within the cathode compartment of each cell of the fuel cell is calculated from the following equation:

[0026] ppE = px ........

[0027] in which: • P is the pressure within the cathode compartment of each cell of the fuel cell; • I is the current produced by the fuel cell; • Qout_cathode is the gas flow rate at the air outlet; and • C4 is a constant.

[0028] - The constant C4 is equal to: c — N x , where: 4 2 xn xexN. f emt ri • N is the number of cells in the fuel cell; • Meau is the molar mass of water; • skin is the density of water vapor within the cathode compartment of each cell; • e is the elementary charge of a proton; and • Na is Avogadro's number.

[0029] The invention will appear more clearly on reading the description which follows, given solely by way of non-limiting example, and made with reference to the drawings in which:

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

[0031] [Fig.2] [Fig.2] is a schematic representation of an electrical system including the fuel cell of [Fig.l].

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

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

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

[0035] [Fig.l] 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 supplying 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 supplying an electric motor ensuring the propulsion of 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.l]. These end plates make it possible in particular to keep the stack of cells 12 compressed, that is to say tight, and to supply the stack with dihydrogen in gaseous form, and with air in gaseous form, and, where appropriate, the circulation of a heat transfer fluid for a cell cooling circuit.

[0038] The invention will be more particularly described in the context of a current 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 fuel cells of the ion exchange membrane type with solid electrolyte having different constructions.

[0039] It is considered 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.l], a detail of a section of the membrane-electrode assembly 14 of a cell 12 is also shown.

[0041] In practice, each bipolar plate 16 is arranged between two cells 12 and is common to these two cells. A first face 16A, called the anode side face, supplies one of the two cells with hydrogen, and a second face 16B, called the cathode side face, supplies the other of the two cells with air. In other words, a cell 12 is supplied with hydrogen 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, the cell 12 is thus supplied with oxygen.

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

[0043] In the example, each bipolar plate 16 is formed from the assembly of two superimposed polar plates, this assembly forming on the face 16A channels for circulation of dihydrogen, on the face 16B channels for circulation of air, and between the faces 16A and 16B, that is to say inside the bipolar plate, therefore between the two polar plates, channels for circulation of heat transfer fluid. The circulation of this heat transfer fluid does not play a direct role in the electrochemical reactions of the fuel cell 10, but 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, there is an anode compartment, formed between the bipolar plate 16 supplying the cell with dihydrogen and the membrane 20, and a cathode compartment, formed between the bipolar plate 16 supplying the cell with air and the membrane. Thus, the anode catalytic layer 22 is arranged in the anode compartment and the cathode catalytic layer 24 is arranged in the cathode compartment.

[0047] The gas diffusion layers 18, which are therefore each arranged in their respective anode or cathode compartment, allow the transport of fuel and oxidant gases, i.e. dihydrogen and oxygen, from the bipolar plates 16 to the anode 22 and cathode 24 catalytic layers. In practice, the gas diffusion layers are formed from a porous material, such as for example a non-woven carbon fiber textile, i.e. a carbon fiber textile 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 fibers of the carbon paper more hydrophobic.

[0048] The proton exchange membrane 20 allows the passage of hydrogen ions, or protons, from the anode compartment to the cathode 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 in operation, within each cell 12, an oxidation reaction occurs in the anode compartment, at the level of the anode 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 cathode catalytic layer in the cathode compartment, while the electrons are captured by the anode side face 16A of the adjacent bipolar plate 16 and then conducted towards the cathode side face 16B of this same bipolar plate, this cathode side face belonging to the cathode compartment of the adjacent cell 12. At the same time, a reduction reaction occurs in the cathode compartment of the cell 12, at the level of the cathode catalytic layer 24. This reduction reaction consists of reacting the oxygen molecules supplied by the air, through the gas diffusion layer 18, with the protons passing through the proton exchange membrane 20 as well as with the electrons supplied by the cathode side face 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 formed from 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 to transport electrons, for example carbon. - A material for catalyzing the electrochemical oxidation and reduction reactions described above, for example platinum. This material is present in the form of particles, preferably spherical, which are for example deposited on the surface of said material to transport electrons, for example the carbon mentioned above, during the manufacture of the catalytic layers.

[0051] Furthermore, the pores of the catalytic layers allow the free transport of reactants, i.e. dihydrogen and oxygen, within 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 all the constituent elements of the catalytic layers are not present, more particularly areas where there is platinum, but where Nafion, Aquivion or carbon or access for the reactants is missing, are called dead zones.

[0053] The catalytic layers 22 and 24 also contain impurities, or pollutants, which are, for example, residues or additives resulting from the manufacture of the catalytic layers. In addition, the platinum particles contained in the catalytic layers generally have a layer of oxides on their surface. When this layer of oxides becomes too thick, the platinum particles can no longer react with the protons and electrons and such a thick layer of oxides on the surface of the platinum particles can therefore be considered impurities.

[0054] The membrane-electrode assembly 14 of a cell 12 is in practice arranged in an opening made in a support plate 25, the support plate 25 being interposed between two bipolar plates 16. The support plate 25 can be produced 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 designated by the acronym PET, or of polyethylene naphthalate, also designated by the acronym PEN. Advantageously, to ensure sealing between the membrane-electrode assembly and the bipolar plates 16 in the stack 12, the membrane-electrode assembly comprises 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 comprises a dihydrogen inlet 28 supplying each cell with dihydrogen and a dihydrogen outlet 30 evacuating the dihydrogen coming from each cell.

[0056] The fuel cell comprises an air inlet 32 supplying each cell with air and an air outlet 34 evacuating the air coming from each cell, this air being, during operation of the cell, depleted in oxygen.

[0057] The air outlet 34 also makes it possible to evacuate the water produced by the fuel cell contained in the cathode compartments of each cell in the form of water vapor.

[0058] The fuel cell comprises a heat transfer fluid inlet 36 supplying each cell with heat transfer fluid and a heat transfer fluid outlet 38 discharging the heat transfer fluid from each cell.

[0059] In the example, as visible in [Fig.l], the inlets 28, 32 and 26 as well as the outlets 30, 34 and 38 are formed by openings provided in the bipolar plates 16 and the support plates 25, these openings, forming, by the stacking of the cells, distribution galleries of the stack, with a distribution gallery corresponding to each of these inlets and outlets. In addition, these inlets and outlets are connected to openings provided in one of the terminal plates, themselves connected to circuits for supplying dihydrogen, air and heat transfer fluid, such as flexible or rigid pipes. As a variant, these inlets and outlets are formed by conduits provided around the stack of cells and bipolar plates of the fuel cell.

[0060] As visible in [Fig.l], on its cathode side face 16B, a bipolar plate 16 comprises 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 pass through the active zone from one side to the other, each connecting the homogenization zone 40 to the homogenization zone 41. The channels 44 thus make it possible to conduct the air flow over the entire extent of the cathode compartment.

[0062] Here, the channels 44 are shown as being straight. In a non-shown variant of the invention, 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 inlets and outlets to the active zone 42 and make it possible to distribute the air over the entire width of the active zone, towards all of the channels 44.

[0064] At the anode side face 16A, a bipolar plate 16 has the same structure as at the cathode side face 16B, namely two homogenization zones and an active zone comprising channels. On the anode side face, the homogenization zones connect the dihydrogen inlet and outlet to the active zone and make it possible to distribute the dihydrogen over the entire width of the active zone, towards all of the channels.

[0065] In the example, the bipolar plates 16 and the support plates 25 are rectangular in shape. In the example, but not necessarily, and as can be seen in [Fig.l], the hydrogen inlet 28 and the hydrogen 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 makes it possible to obtain a more homogeneous distribution of the reactive gases over the active zones 42 of the bipolar plates.

[0066] Preferably, each cell 12 of the fuel cell 10 has an active surface, corresponding to the surface of the active zone 42 of a bipolar plate 16, between 150 cm2 and 500 cm2. Alternatively, this active surface may be smaller, or even larger.

[0067] A polarization curve of the fuel cell 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 polarization curve of the fuel cell 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 surface 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] [Fig.2] illustrates an electrical system 50, which includes the fuel cell 10. In the example, the electrical system 50 is part of a motor vehicle with an electric propulsion engine.

[0070] The vehicle 50 comprises a hydrogen supply system 52, making it possible to supply hydrogen to the dihydrogen inlet 28, and making it possible to collect the dihydrogen evacuated from the dihydrogen outlet 30. For this purpose, the hydrogen supply system 52 comprises 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 comprises a recirculation circuit which connects the dihydrogen outlet 30 to the dihydrogen inlet 28, and which comprises means for recirculating the dihydrogen, such as a motorized pump or a passive pump, for example a Venturi type ejector.

[0071] The vehicle 50 comprises an air supply system 54, making it possible to supply oxygen to the air inlet 32, and making it possible to collect the air evacuated from the air outlet 34. For this purpose, the air supply system 54 comprises for example a compressor adapted to take air from outside the vehicle 50 and a circulation circuit. This air supply circuit is provided with at least two cut-off valves, one corresponding to the air inlet and the other to the air outlet.When these cut-off valves are closed, they isolate, with respect to the outside air, a volume of fuel cell cathode air which notably includes the volumes of the cathode compartments of each cell 12 of the fuel cell, as well as notably the volume of the air inlet and air outlet distribution galleries of the stack, plus possibly the volumes of pipework between each of these cut-off valves and that of the air inlet and the air outlet which corresponds, plus possibly the volume of an air humidification device belonging to the air supply system 54 and not described here.

[0072] The vehicle 50 comprises a temperature regulation circuit 56, in particular a cooling circuit, making it possible to supply heat transfer fluid to the heat transfer fluid inlet 36 of the fuel cell 10, and to collect the heat transfer fluid discharged from the heat transfer fluid outlet 38. For this purpose, the regulation circuit 56 comprises, for example, a heat transfer fluid reservoir, a circulation pump and a thermodynamic circuit making it possible to regulate the temperature of the heat transfer fluid. The heat transfer fluid used is, for example, glycolated water. The regulation circuit 56 therefore makes it possible to regulate the temperature of the fuel cell.

[0073] The vehicle 50 comprises a DC-DC converter 58, an electric motor 60 and a battery 62. By DC-DC, it is meant that the converter converts a direct current into another direct current. The DC-DC converter 58 is electrically connected to the fuel cell 10, conditions an electrical power produced by the fuel cell and delivers this conditioned electrical power to the electric motor 60 and / or to the battery 62. In practice, the DC-DC converter 58 conditions the electrical power produced by the fuel cell in particular by modifying the voltage delivered by the fuel cell, to deliver a voltage adapted to the electric motor 60 and to the battery 62.

[0074] Furthermore, the DC-DC converter 58 makes it possible to control the fuel cell 10, by imposing a given operating point on the fuel cell. For 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 polarization curve.

[0075] The battery 62 is further connected to the electric motor 60, and makes it possible to supply the electric motor with electrical energy and / or to store 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 in the vehicle, such as for example 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 comprises, for example, a controlled switch, arranged 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 comprises a dissipative system 64, which is electrically connected to the fuel cell 10. The dissipative system 64 makes it possible to dissipate an electrical power produced by the fuel cell. The dissipative system 64 comprises, for example, a resistor, which dissipates the electrical power produced by the fuel cell in the form of heat.

[0079] The dissipative system 64 is disconnectable from the fuel cell 10. For this purpose, the dissipative system comprises, 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.

[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 to the dissipative system is particularly useful for establishing a transient regime during which the fuel cell fuel cell is disconnected from the DC-DC converter and connected to the dissipative system, or during 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 avoid the fuel cell being connected neither to the DC-DC converter nor to the dissipative system at the time of switching between the DC-DC converter and the dissipative system, which could generate an overvoltage of the fuel cell.

[0082] The vehicle 50 comprises a set of sensors 66 and a control unit 68. The set of sensors 66 is connected on the one hand to the fuel cell 10 and on the other hand to the control unit 68. The control unit 68 is further connected to the DC-DC converter and to the electric motor 60.

[0083] The set of sensors 66 comprises several sensors making it possible to measure several quantities characteristic 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 level of the air inlet 32 of the cathode compartment of each cell, for example expressed in absolute bar (barA).;

[0084] The control unit 68 makes it possible to control the vehicle 50, in particular by controlling the DC-DC converter 58 and the electric motor 60, in particular as a function of the data obtained by the set of sensors 66 and as a function of control signals, coming for example from a driver of the vehicle.

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

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

[0087] Before the start of the shutdown method, the fuel cell 10 is in the normal operating phase, denoted PF. For example, the vehicle 50 is in motion and the fuel cell produces the electrical power necessary for propelling the electric motor 60.

[0088] The method for stopping the fuel cell 10 is intended to be carried out at each normal shutdown of the fuel cell and aims to preserve the lifetime of the fuel cell by avoiding degradation of its performance. The three objectives The main features of this shutdown process are: - reducing the voltage across the fuel cell to zero, i.e. depolarizing the fuel cell, - obtaining a depolarization of the fuel cell that lasts over time, and - the formation of a “nitrogen skin”, in contact with the proton exchange membrane 20 within the cathode compartment, at maximum thickness, i.e. the consumption of a maximum quantity of oxygen within the cathode compartment.

[0089] The stopping method begins at a time t0, when a stop instruction for the fuel cell 10 is received by the control unit 68. This stop instruction is for example issued when a stop of the vehicle 50 is desired. Preferably, before the start of the stopping method, a check is carried out by the control unit 68 to verify whether the requested stop is a normal stop, or an emergency stop, and the stopping method is only executed when the requested stop is a normal stop. Indeed, in the event of an emergency stop, it is preferable to stop the fuel cell 10 as quickly as possible, without taking into account any possible damage that may occur in the fuel cell during such an emergency stop.

[0090] At time to, 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 = Io, with Io preferably between 30 A and 400 A, for example equal to 350 A; - Qh2 = Qh2 o, with Qi 12 o preferably between 3.13x10 4 g / s / cell and 4.18x10 4 g / s / cell, for example equal to 3.66x10 4 g / 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 operating phase PF 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 thus 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 its wear and the appearance of defects.

[0093] For simplification, the pressure P prevailing within the air inlet 32 of the compartment cathode pressure 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 regulation circuit 56 and the voltage U and the current I are regulated by the DC-DC converter 58, on the basis of commands issued by the control unit 68.

[0095] The stopping process begins with a phase of reduction in speed Pb which lasts until a time tb. During the phase of reduction in speed, the fuel cell 10 is connected to the DC-DC converter 58, which controls the fuel cell so as to reduce the power generated by the fuel cell 10, while bringing the voltage U of each cell 12 up 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 L preferably between 30 A and 80 A, for example equal to 70 A; - Qh2 = Qh2 i, with Qh2 i preferably between 3.13x10 4 g / s / cell and 8.36xl0 4 g / s / cell, for example equal to 7.31xl0 4 g / s / cell; - Qair = Qair i, with Qair l 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 which does not entail any 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 of 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 power reduction phase Pi 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 speed reduction phase Pi ends as soon as the temperature T of the heat transfer fluid at the heat transfer fluid inlet 36 of the fuel cell reaches the value Th. In other words, the condition for stopping the speed reduction phase is reaching the temperature Ti which is lower 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] For this, the control unit 68 calculates a reference temperature Tref, expressed in degrees Celsius, for which the relative humidity level in the cathode compartment of each cell 12 is equal to 100%. This calculation is notably carried out on the basis of the current I produced by the fuel cell, the gas flow rate Q cathode_output at the air outlet 34, and the pressure P prevailing within the cathode compartment of the cells 12.

[0103] More precisely, 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 = ^water^mass 2xexNA

[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 1.602 176 634 x 10 19 Coulombs; and - Na is Avogadro's number, equal to 6.022 140 76 x 1023 mol *.

[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 (1 / s) using the following equation:

[0108] ~ *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 / 1.

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

[0112] PPE = px ^Mpeur-'a“

[0113] [Equation 3]

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

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

[0116] T = , r C^PPE*}®') 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 to obtain a value of Tref expressed in degrees Celsius; and - ln(PPExlO5) 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:

[0120] ppE = px . — .wrt!e_i:a ihode

[0121] [Equation 5]

[0122] in which C4 is a constant equal to: = ? N x

[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, more preferably between 100% and 110% of Tref.

[0125] The shutdown process continues from time ti to time t2 by 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 intensity 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 heat transfer fluid inlet 36 of the fuel cell to stabilize at the value Tp. Thus, at the end of the stabilization phase P2, the temperature of the heat transfer fluid at the heat transfer fluid inlet 36 of the fuel cell, 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 the cells 12.

[0127] During the down-regulation phases Pi and stabilization P2, the air and hydrogen supplies to the fuel cell are maintained. Furthermore, 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 method continues with a discharge phase P3, starting at time t2, during which the air supply to the fuel cell 10 via the air inlet 32 is cut off. More precisely, the volume of cathode air from the cell is isolated by closing the cut-off valves corresponding respectively to the air inlet and the air outlet of the 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 constant voltage.

[0130] Given that at time t2, that is to say at the time of cutting off the air supply, a quantity of air containing oxygen is present in the cathode compartment of each cell 12, generally in the volume of cathode air of the cell, and forms a stock of oxygen, the reduction reactions of 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 discharge phase P3, the oxygen contained in the cathode compartment is therefore progressively consumed by the reduction reaction forming water molecules. Thus, the oxygen content in the quantity of air contained in the cathode compartment of each cell 12 progressively decreases during the discharge phase P3.

[0132] This reduction in the oxygen concentration modifies 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, because the lower the number of electrochemical reactions producing electrons in the cells 12.

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

[0134] At time t2, the polarization curve of the fuel cell corresponds to the curve PA, 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 Ib

[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 Ui, the current I produced by the fuel cell is increasingly weak.

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

[0137] In summary, during the discharge phase P3, cutting off the air supply and maintaining a constant voltage U by controlling the DC-DC converter results in a reduction 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 Ub, the DC-DC converter 58 controls the current I produced by the fuel cell, so as to reduce the intensity of the current produced by the fuel cell throughout the discharge phase P3, by following the evolution of the polarization curve of the fuel cell.

[0139] Furthermore, during the discharge phase P3, the temperature T of the heat transfer fluid at the heat transfer fluid inlet 36 is controlled to be regularly updated, so as to reflect the evolution of the current I produced by the fuel cell. For this purpose, the reference temperature Tref is updated regularly on the basis of the evolution of the current I, which results in a decrease in the temperature of the heat transfer fluid at the heat transfer fluid inlet 36 of the fuel cell, and thus a reduction 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, more preferably between 100% and 110% of Tref, throughout the duration of the 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 overconsumption of hydrogen.

[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 impose 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, that is to say 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; - Qh2 = Qh2 3, with Qh2 3 preferably between 2.089x105 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 2.089x105 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 lower than the value of T2.

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

[0145] In practice, since the air inlets and outlets 32, 34 are cut off, the quantity of air included in the cathode compartment of each cell is static. Furthermore, the consumption of oxygen within this quantity of air is not uniformly localized: it only occurs in contact with the proton exchange membrane 20. The quantity of air being static, the different gases composing the air, that is to say essentially oxygen and nitrogen, can only mix by diffusion. Thus, the consumption of oxygen localized at the level of the proton exchange membrane 20 and the absence of air movement result in an oxygen concentration gradient, which is minimal in contact with the proton exchange membrane 20, and is greater in contact with the bipolar plate delimiting the cathode compartment of each cell and in the rest of the cathode air volume of the cell. This oxygen concentration gradient results in the formation of a layer of air having a higher nitrogen concentration than the ambient air, located in contact with the proton exchange membrane 20, which is also called a “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 of the fuel cell 10, which depends on a resistance value R of the resistance of the dissipative system. Thus, the voltage U delivered by each cell 12 is equal to the product of the resistance R by 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 piloted, 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 Ub multiplied by the number N of cells 12 of the fuel cell, divided by the predefined threshold current value I3.

[0151] During the depolarization phase P4, 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 quantity of air contained in the cathode compartment of each cell 12, in particular near the proton exchange membrane, progressively decreases during the depolarization phase P4.

[0152] This reduction in the oxygen concentration modifies the electrochemical properties of the fuel cell, and in particular 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, because the lower the number of electrochemical reactions producing electrons in the cells 12.

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

[0154] At time t3, that is to say at the end of the discharge phase P3 and at the start of the depolarization phase P4, the polarization curve of the fuel cell corresponds to the curve PE, and it is observed that for a voltage U delivered by each cell 12 equal to Ui, 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 weak.

[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 visible 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 reduction is observed both in the current I produced by the fuel cell and in the voltage U delivered by each cell 12.

[0158] Thus, for the PF polarization curve, a voltage UF and a current IF lower than Ui and I3 are produced; for the PG polarization curve, a voltage UG and a current IG lower than UF and IF are produced; for the PH polarization curve, a voltage UH and a current IH lower than UG and IG are produced; and for the Pb polarization curve a voltage U! 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 cause a reduction 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. For this purpose, the reference temperature Tref is updated regularly on the basis of the evolution of the current I, which results in 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, more preferably between 100% and 110% of Tref, throughout the duration of the depolarization phase P4. Advantageously, when the temperature T is controlled during the depolarization phase P4, this control is maintained until the temperature T becomes equal to the ambient temperature surrounding the vehicle 50. In other words, this control makes it possible to control the decrease in the temperature T until it reaches the ambient temperature.

[0161] It is also noted that during the depolarization phase P4, the hydrogen flow rate Q H2 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 overfeeding of the anode compartment of each cell 12, thus making it possible to avoid 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 zero value, corresponding to the current I 4. 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 completed. The discharge phase P3 and the depolarization phase P4 together form a shutdown phase of the shutdown process.

[0164] At this time 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, that is to say at time t4, the characteristic quantities of the operation of the fuel cell 10 are: U = 0 V / cell; I = 0A; Qh2 = 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 depolarization phase P4, the oxygen concentration within the cathode compartment of each cell 12 is reduced, since the quantity of air included in the cathode 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 20. This absence of mixing also prevents the consumption of the oxygen located in the rest of the cathode air volume of the cell, such as in the pipes between the cut-off valves and the cells 12.

[0168] Thus, at the end of the depolarization phase P4, 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 a zero voltage at the terminals of each cell 12, and therefore to a 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 thereof.

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

[0172] The shutdown method of the invention also makes it possible to minimize the migration of oxygen towards the anode compartment of each cell by diffusion through the proton exchange membrane 20 during a prolonged shutdown, 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, in comparison with the shutdown methods known from the prior art. This maximization of the thickness of the nitrogen skin is obtained thanks to the discharge phase P3, 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 down the drop in the current produced by the fuel cell, and therefore to extend the duration of the discharge phase P3.

[0174] Thus, in the example, the discharge phase P3 has a duration, between the times t2 and t3, greater than 3 seconds, preferably still 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 has been cut off for a duration of less than 0.5 seconds.

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

[0176] Furthermore, thanks to the effectiveness of the discharge phase P3 in depopulating the cathode compartment of each cell 12 with oxygen, the duration of the depolarization phase P4 is reduced, and the power dissipated by the resistance of the dissipative system 64 is also reduced. This makes it possible to limit the energy losses of the fuel cell 10, because all the power generated by the fuel cell during the discharge phase P3 is recovered by the battery 62. This also makes it easier to manufacture the dissipative system 64, because the resistance can be sized to dissipate a lower 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 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 greater than 10% of the maximum power of the fuel cell.

[0177] In summary, the discharge phase P3 makes it possible to reduce the stock of oxygen contained in the cathode compartment of each cell 12 in preparation for the depolarization phase P4, which makes the depolarization phase simpler to implement and which makes the depolarization of the fuel cell more durable, 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 Ub 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 a 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 the water vapor. Indeed, the presence of condensation in the cells 12 is thus avoided. In addition, keeping the temperature as low as possible makes it possible to slow down any harmful electrochemical reactions that may occur in the cells 12, since temperature is a factor influencing the speed of the electrochemical reactions. This also makes it possible to optimize the efficiency of the fuel cell, by increasing the humidity level of the air contained in the cathode compartment of each cell, which humidifies the heat exchange membrane. protons 20 and thus decreases its resistance, facilitating the conduction of protons through the proton exchange membrane and the conduction of electrons. In addition, the drop in temperature during the discharge phase P3 also results in a decrease in the catalytic activity of the cathode catalytic layer 24, which causes the cell voltage U to drop for a given current level I. Regulating the current to maintain a constant voltage results in a lower current being controlled at low temperatures than if this operation would have been carried out at higher temperatures, prolonging oxygen consumption to a maximum extent, and thus increasing the duration of the discharge phase.

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

[0181] The shutdown method begins with the phase of decreasing the speed Pi to stabilize the quantities characteristic of the operation of the fuel cell. This phase of decreasing the speed is optional, in particular when the shutdown method is started while the quantities characteristic of the operation of the fuel cell are already stabilized, and in particular when U = Up

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

[0183] In a variant of the invention not shown, 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 variant of the invention not shown, during the discharge P3 and depolarization P4 phases, 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, in which the discharge voltage value Up is located. By approximation, when the voltage U is within this voltage range, it is then said that the voltage U is substantially equal to the discharge voltage value Up.

[0185] In a variant of the invention not shown, the stopping 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 duration has elapsed.

[0186] In a variant of the invention not shown, during the discharge phase P3, the DC-DC converter 58 controls the fuel cell 10 by commanding the fuel cell to deliver a constant voltage value, rather than commanding a reduction in the current produced by the fuel cell.

[0187] In a variant of the invention not shown, during the discharge P3 and depolarization P4 phases, the temperature T of the heat transfer fluid at the heat transfer fluid inlet 36 of the fuel cell is not regulated as a function of 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 Ti, or else equal to another arbitrarily chosen temperature value.

[0188] In a variant of the invention not shown, the method for stopping the fuel cell comprises a stopping phase different from that described above comprising the discharge phase P3 and the depolarization phase P4, while comprising a drop in intensity I. For example, the stopping 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 downwards 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 comprises 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 downward by the DC-DC converter, either continuously or in stages, while the voltage U delivered by each cell 12 is controlled to decrease in stages; then a depolarization phase during which the fuel cell is connected to the dissipative system 64. According to another example, the shutdown phase comprises only a depolarization phase during which the fuel cell is connected to the dissipative system 64.In such a variant, during the fuel cell shutdown phase, the temperature T of the heat transfer fluid at the heat transfer fluid inlet 36 of the fuel cell is regulated in the same way as in the shutdown method described above, i.e. the temperature T is regulated to decrease as a function of the change in 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 one embodiment or variation in the foregoing may be implemented for the other embodiments and variations described above, as long as technically feasible.

Claims

Claims

1. A method of stopping a fuel cell (10) belonging to an electrical system (50), the electrical system further comprising a regulation circuit (56) for regulating a temperature (T) of the fuel cell (10) with a heat transfer fluid, and a control unit (68), the fuel cell comprising: - a stack of cells (12), each cell comprising an anode compartment and a cathode compartment separated by a proton exchange membrane (20), - a dihydrogen inlet (28) supplying the anode compartment of each cell (12) with dihydrogen, and a dihydrogen outlet (30) evacuating the dihydrogen coming from each cell, - an air inlet (32) supplying the cathode compartment of each cell (12) with air, and an air outlet (34) evacuating the air coming from each cell, - a heat transfer fluid inlet (36) supplying each cell (12) with heat transfer fluid, and a heat transfer fluid outlet (38) discharging the heat transfer fluid coming from each cell, the heat transfer fluid inlet and the heat transfer fluid outlet being connected to the regulation circuit (56), characterized in that the method of stopping the fuel cell (10) comprises a stabilization phase (P2), during which: - an air supply to the fuel cell (10) via the air inlet (32) is maintained, - the fuel cell is controlled so that the intensity of a current (I) produced by the fuel cell is maintained substantially equal to a constant target current value (R), - the control unit (68) calculates a reference temperature (Tref) corresponding to a relative humidity level in the cathode compartment of each cell (12) equal to 100%, on the basis of the current (I) produced by the fuel cell, a gas flow rate (Qout_cathode) at the air outlet (34), and a pressure (P) prevailing in the cathode compartment of each cell, and - the regulation circuit (56) is controlled so that the temperature (T) of the heat transfer fluid at the heat transfer fluid inlet (36) of the fuel cell is between 100% and 140% of the reference temperature (Tref), the temperature (T) of the heat transfer fluid and the reference temperature (Tref) being expressed in degrees Celsius.

2. Method according to claim 1, further comprising, after the stabilization phase (P2), a shutdown phase (P3, P4), during which: - the air supply to the fuel cell (10) via the air inlet (32) is cut off, - the fuel cell is controlled so that the intensity of the current (I) produced by the fuel cell decreases until it reaches a zero current value (I4).

3. Method according to claim 2, wherein, during the shutdown phase (P3, P4): - the control unit (68) updates the reference temperature (Tref), on the basis of the evolution of the current (I) produced by the fuel cell, the gas flow rate (Qoutie_cathode) at the air outlet (34), and the pressure (P) prevailing in the cathode compartment, and - the regulation circuit (56) is controlled to decrease the temperature (T) of the heat transfer fluid at the heat transfer fluid inlet (36) of the fuel cell, while maintaining the temperature of the heat transfer fluid at the heat transfer fluid inlet of the fuel cell between 100% and 140% of the reference temperature (Tref).

4. Method according to one of claims 2 and 3, in which the electrical system (50) comprises: - an electrical load (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 load electrical (60, 62), and - a dissipative system (64), adapted to be electrically connected to the fuel cell (10) to dissipate an electrical power produced by the fuel cell, and in which the shutdown phase (P3, P4) comprises: - a discharge phase (P3), during which the DC-DC converter (58) is electrically connected to the fuel cell (10) and delivers the electrical power produced by the fuel cell to the electrical load (60, 62), then - a depolarization phase (P4), during which the dissipative system (64) is electrically connected to the fuel cell (10) and dissipates the electrical power produced by the fuel cell.

5. Method according to claim 4, in which, during the discharge phase (P3), the DC-DC converter (58) controls the fuel cell to maintain a voltage (U) delivered by each cell (12) of the fuel cell substantially equal to a constant discharge voltage value (Ul).

6. Method according to one of claims 1 to 5, in which, during the stabilization phase (P2), a power produced by the fuel cell (10) is between 10% and 25% of a maximum power produced by the fuel cell.

7. Method according to one of claims 1 to 6, in which the reference temperature Tref is calculated from the following equation (equation 4): T . — 1__L c ret Crln^PExlO5) in which: - In(PPE) is the natural logarithm of the partial pressure of water within the cathode compartment of each cell (12) of the fuel cell (10), - Ci and C3 are two constants expressed in degrees Kelvin; and - C2 is a dimensionless constant.

8. Method according to claim 7, in which the partial pressure of water vapor PPE within the cathode compartment of each cell (12) of the fuel cell (10) is calculated from the following equation (equation 5): PPE = P^-^-^— in which: - P is the pressure within the cathode compartment of each cell (12) of the fuel cell (10); - I is the current produced by the fuel cell; - Qoutie_cathode is the gas flow rate at the air outlet (34); and - C4 is a constant.

9. Method according to claim 8, in which the constant C4 is equal to: f — N x , where: ^4 2xp xexN. ' water zi - N is the number of cells (12) of the fuel cell (10); - Mwater is the molar mass of water; - skin is the density of water vapor within the cathode compartment of each cell; - e is the elementary charge of a proton; and - Na is Avogadro's number.

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