Method for running in a fuel cell

Abstract

The invention relates to a method for running in a PEM fuel cell, the method comprising at least a first running-in phase (P2), followed by an operation of reversing the direction of the dihydrogen and air streams (P3), and then a second running-in phase (P4). The running-in phases each comprise a step (4) of stabilizing the fuel cell in which the current density produced by the cell is kept constant at a low value for a given duration, followed by an oxygen depletion step (5), during which the current density is kept constant at a minimum value, lower than or equal to the low value, and during which the air supply is at least partially interrupted, being adjusted so as to obtain a cathode stoichiometry coefficient of strictly less than 1. The oxygen depletion steps of the first and second running-in phases end when the cell voltage of the fuel cell reaches a predefined threshold voltage.

Description

[0001] TITLE: Fuel cell running-in process

[0002] The present invention relates to a method of running in a fuel cell.

[0003] A fuel cell is a device for generating electricity by electrochemical reaction between a fuel, for example dihydrogen, and an oxidizer, for example 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 of several unit cells, each constituting an electrochemical generator.

[0004] Schematically, each unit cell comprises two separators, also called polar plates, between which is inserted a solid electrolyte 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 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 "MEA".

[0005] For two neighboring cells, a separator from one of the two cells is placed back to back with a separator from the other cell. These two separators together form a bipolar separator, also called a bipolar plate. A cooling compartment, in which a cooling fluid such as glycolated water circulates, is generally arranged between the two separators of the bipolar separator.

[0006] Hydrogen, air, and coolant are so-called "operating" fluids that are supplied to the fuel cell during operation. Hydrogen and air are reactants, while the coolant is not involved in the electrochemical reaction. Depending on the operating phases of the fuel cell, one or more of the operating fluids are supplied continuously or intermittently.

[0007] The fuel cell thus provides openings to supply 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 fuel supply to the cell adjacent to this side and on the other side the oxidant supply to the cell adjacent to this other side, the supplies provided by the bipolar separators being done in parallel.

[0008] Generally, in a unit cell, the cathode compartment is supplied with oxidant, for example oxygen, most often in the form of an air supply containing oxygen, and the anode compartment is supplied with fuel, for example dihydrogen.

[0009] Each reactive compartment also generally includes a gas diffusion layer, located between the bipolar separator and the catalytic layer, allowing good circulation of the fuel or oxidizer from the separator to the catalytic layer.

[0010] When the fuel cell is in operation, the electrochemical reaction creates an electrical potential difference between the two separators of each unit cell. The fuel cell thus comprises an electrical insulation device, designed to prevent any electrical contact between two neighboring bipolar separators and between each cell and the external environment, as well as a sealing device to prevent leaks of operating fluids, in particular to prevent the fluid circulating in a reactive compartment from contaminating a neighboring reactive compartment.

[0011] 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 cells in the fuel cell stack are electrically connected together in series, so the voltage delivered across the fuel cell is equal to the sum of the cell voltage of all the unit cells.

[0012] A PEMFC fuel cell requires an activation procedure, or break-in procedure, upon its first use after manufacturing and before commissioning. This break-in procedure aims to increase and stabilize the performance of the fuel cell by modifying the physicochemical properties of the fuel cell unit cells.

[0013] In particular, the main objectives of the running-in procedure are to ensure optimal hydration of the membrane, to desorb pollutants present in the anodic and cathodic catalytic layers, and to activate the anodic and cathodic catalytic layers, this activation corresponding to morphological changes within the membrane-electrode assembly, and in particular to an evolution of the porosity of the catalytic layers and to a cleaning of the catalytic layers.

[0014] Typically, the fuel cell run-in procedure takes place on an activation bench, where the fuel cell is connected to a load and supplied with reactants. The run-in process involves adapting the load and reactant flow rates to maintain a predetermined voltage or current profile. During run-in, the fuel cell's performance gradually increases until it stabilizes and reaches a criterion for stopping the activation procedure. When this criterion is reached, the fuel cell is considered to be run-in. The stopping criterion may be a certain level of performance achieved, in particular in terms of efficiency expressed, for example, as a function of the electrical energy delivered by the cell per mass unit of hydrogen consumed, or a stabilization of performance between two measurements, or both. An existing stopping criterion simply consists of stopping run-in after a certain predetermined duration.This type of criterion is the least precise of all. For example, the stopping criterion is defined as a difference between the voltage at a time t and the voltage at a time t+1, at a given constant current density produced by the fuel cell, which must be less than a certain threshold.

[0015] Most commonly, such a running-in process involves maintaining a constant voltage across the fuel cell, or maintaining a constant current density produced by the fuel cell, for a predetermined period of time, and then measuring the voltage and current across the fuel cell at the end of this period. This cycle is then repeated until the shutdown criterion is reached.

[0016] Such a running-in process is generally long, lasting several hours, and consumes a significant amount of hydrogen, including several kilograms of hydrogen for a fuel cell with a maximum power of 10 kW or more. These two drawbacks result in a high cost of running the running-in protocol, which is a disadvantage for its implementation on an industrial scale. It is therefore desirable to reduce the duration of the running-in processes, without reducing the performance gain or causing degradation of the fuel cell.

[0017] Thus, certain techniques are known to accelerate the duration of the running-in protocol. One of these techniques, called air overstoichiometry reduction, consists of supplying the fuel cell with reactants with a connected load, then reducing the air inlet flow rate, so as to briefly operate the fuel cell with a reduced quantity of oxygen compared to the nominal operating conditions, this by operating the fuel cell with a cathodic stoichiometry coefficient always greater than 1, but for example between 1 and 1.5.During this phase of reduction of the over-stoichiometry in air, the cathode compartment of each unit cell comprises, in addition to air which has a reduced oxygen level compared to the nominal conditions, a certain level of hydrogen because the fuel cell will locally operate as a proton pump which leads to the formation of hydrogen at the cathode in the zones where there is a lack of oxygen. Preferably, during this phase of reduction of the over-stoichiometry, the charge is controlled so that, for each unit cell, the cell voltage is low. The low cell voltage and the presence of hydrogen at the cathode catalytic layer form favorable conditions for the desorption of impurities and the reduction of polluting oxides located on the cathode catalytic layer, which is thus cleaned, which tends to increase the actually active surface of the cathode catalytic layer.Thanks to this technique, the duration of the running-in process is reduced.

[0018] However, this technique has the disadvantage of resulting in non-uniform performances of the fuel cell, because the reduction of the oxygen level in the air is not uniform within the cathode compartment of the unit cells. Indeed, this reduction is more pronounced at the air outlet opening than at the air inlet opening. In addition, the reduction of the air over-stoichiometry, as practiced in the prior art, generates fluctuations in the cell voltage, which causes degradation of the fuel cell. These degradations can include the dissolution of the platinum, and / or the corrosion of the bipolar separator, and / or the corrosion of the carbon which is generally one of the components of the catalytic layer, and / or a degradation of the membrane due to the appearance of excess heat points.

[0019] It is these drawbacks that the invention more particularly intends to remedy, by proposing a new method for running in a fuel cell which is carried out more quickly, which does not degrade the fuel cell and which makes it possible to obtain better performance from the fuel cell.

[0020] To this end, the invention relates to a method for running in a fuel cell, the fuel cell comprising a stack of cells, each cell comprising a proton exchange membrane arranged between two bipolar plates, each bipolar plate delimiting, in said cell, with the proton exchange membrane a reactive compartment, each cell thus comprising a cathode compartment in which a cathode catalytic layer is arranged, and an anode compartment, in which an anode catalytic layer is arranged.

[0021] The fuel cell comprises a hydrogen inlet supplying the anode compartment of each cell with hydrogen, and a hydrogen outlet discharging the hydrogen from each cell. The fuel cell comprises an air inlet supplying the cathode compartment of each cell with air, and an air outlet discharging the air from each cell. According to the invention, the running-in method comprises at least, in this order, the following phases:

[0022] - a first running-in phase, comprising at least, in this order, the following steps: o preferably, a fuel cell stabilization step, during which the current density produced by the fuel cell is kept constant at a low value for a predetermined duration, and o a fuel cell oxygen depletion step, during which the current density produced by the fuel cell is kept constant at a minimum value, where appropriate less than or equal to the low value, and during which the air supply to the fuel cell via the air inlet is at least partially cut off so as to cause a progressive reduction in the cell voltage, in particular by being advantageously adjusted so as to obtain a cathodic stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9,the oxygen depletion step of the first running-in phase ending when the cell voltage of the fuel cell reaches a predefined threshold voltage, an operation of reversing the direction of the flows of dihydrogen and air, in which the dihydrogen inlet and the dihydrogen outlet are reversed and in which the air inlet and the air outlet are reversed, and,

[0023] - a second running-in phase, comprising at least, in this order, the following steps: o preferably, a fuel cell stabilization step, during which the current density produced by the fuel cell is kept constant at a low value for a predetermined duration, and o a fuel cell oxygen depletion step, during which the current density produced by the fuel cell is kept constant at a minimum value, where appropriate less than or equal to the low value of the second running-in phase, and during which the air supply to the fuel cell via the air inlet is at least partially cut off so as to cause a progressive reduction in the cell voltage, in particular by being advantageously adjusted so as to obtain a cathodic stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9,the oxygen depletion step of the second run-in phase ending when the cell voltage of the fuel cell reaches a predefined threshold voltage.,

[0024] The steps of the first running-in phase are carried out until a first stopping condition is reached, and the steps of the second running-in phase are carried out until a second stopping condition is reached.

[0025] By virtue of the invention, the oxygen depletion steps of the first running-in phase and the second running-in phase enable the reduction and desorption of other impurities on the surface of the cathodic catalytic layer, which is thus further cleaned. These steps are also carried out without significant degradation of the fuel cell, since the air supply is sufficiently reduced to avoid voltage fluctuations due to the gradual reduction of the cell voltage, for example due to the fact that the cathodic stoichiometric coefficient is strictly less than 1, preferably less than or equal to 0.9. In addition, degradation is avoided by avoiding high-current cathodic depletions.Furthermore, thanks to the operation of reversing the direction of the hydrogen and air flows carried out between the two running-in phases, homogeneous fuel cell performances are obtained despite the presence of the oxygen depletion steps. Finally, preferentially, thanks to the sequence of the stabilization step and the oxygen depletion step during the running-in phases, the efficiency of the oxygen depletion steps is maximized, because the stabilization steps make it possible to obtain stable and homogeneous conditions within the fuel cell, avoiding generating non-homogeneities in performance during the oxygen depletion steps.Furthermore, the stabilization steps of the first running-in phase and the second running-in phase advantageously allow the oxidization of impurities on the surface of the cathodic catalytic layer, because the current density maintained at a low value implies that the fuel cell operates with a high cell voltage, which promotes the oxidation reactions of certain impurities, thus at least partially cleaning the cathodic catalytic layer.

[0026] According to advantageous, but not mandatory, aspects of the invention, the lapping method incorporates one or more of the following characteristics, taken in isolation or in any technically admissible combination:

[0027] - The first running-in phase further comprises, prior to the oxygen depletion step, a fuel cell stabilization step, during which the current density produced by the fuel cell is kept constant at a low value for a predetermined duration, the low value of the first running-in phase being greater than or equal to the minimum value and / or the second running-in phase further comprises, prior to the oxygen depletion step, a fuel cell stabilization step, during which the current density produced by the fuel cell is kept constant at a low value for a predetermined duration, the low value of the second running-in phase being greater than or equal to the minimum value.

[0028] - The minimum value of the first running-in phase and the minimum value of the second running-in phase are between 0.01 A / cm 2 and 0.3 A / cm2 , preferably equal to 0.02 A / cm 2 .

[0029] - The low value of the first running-in phase and the low value of the second running-in phase are between 0.03 A / cm 2 and 0.5 A / cm 2 , preferably equal to 0.3 A / cm 2 .

[0030] - During at least one of said oxygen depletion steps, the air supply to the fuel cell via the air inlet is at least partially cut off while being adjusted so as to obtain a cathodic stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9.

[0031] - The oxygen depletion steps of the first running-in phase and the second running-in phase end when the cell voltage of the fuel cell reaches a threshold voltage of between 0.1 V and 0.4 V, preferably equal to 0.2 V.

[0032] - During the oxygen depletion steps of the first running-in phase and the second running-in phase, the air supply to the fuel cell via the air inlet is completely cut off, so as to obtain a cathodic stoichiometric coefficient equal to 0. In certain embodiments, the cathodic stoichiometric coefficient will be maintained equal to 0 during the entirety of the oxygen depletion step of the first running-in phase and / or the second running-in phase.

[0033] - During the oxygen depletion stages of the first running-in phase and the second running-in phase, the air supply to the fuel cell through the air inlet is controlled so that the cell voltage of the fuel cell is monotonically decreasing.

[0034] - The first running-in phase also comprises the following steps, in this order and carried out before the oxygen depletion step: o a current density increase step, during which the current density produced by the fuel cell is gradually increased from a low value to a high value, the low value being greater than or equal to the minimum value of the first running-in phase o a fuel cell hydration step, during which the current density produced by the fuel cell is kept constant at the high value of the first running-in phase for a predetermined duration, so as to hydrate the proton exchange membrane, and o a current density drop step, during which the current density produced by the fuel cell is lowered from the high value of the first running-in phase to the low value of the first running-in phase,and the second running-in phase also comprises the following steps, in this order and carried out before the stabilization and oxygen depletion steps: o a current density increase step, during which the current density produced by the fuel cell is gradually increased from a low value to a high value of the second running-in phase, the low value of the second running-in phase being greater than or equal to the minimum value of the second running-in phase o a fuel cell hydration step, during which the current density produced by the fuel cell is kept constant at the high value of the second running-in phase for a predetermined duration, so as to hydrate the proton exchange membrane, and o a current density drop step,during which the current density produced by the fuel cell is reduced from the high value of the second running-in phase to the low value of the second running-in phase.,

[0035] - The high value of the first running-in phase and the high value of the second running-in phase are between 1.5 A / cm 2 and 3 A / cm 2 , preferably equal to 1.9 A / cm 2 .

[0036] - The steps of increasing current density, hydration, decreasing current density, if necessary stabilization and oxygen depletion of the first running-in phase are carried out at least twice in a cyclical manner.In addition, the first stopping condition is reached when: o the cell voltage of the fuel cell at the end of the hydration step of the last cycle of the first running-in phase differs from the cell voltage of the fuel cell at the end of the hydration step of the penultimate cycle of the first running-in phase by a value between 1 mV and 10 mV, preferably equal to 5 mV, or o where appropriate, the cell voltage of the fuel cell at the end of the stabilization step of the last cycle of the first running-in phase differs from the cell voltage of the fuel cell at the end of the stabilization step of the penultimate cycle of the first running-in phase by a value between 1 mV and 10 mV, preferably equal to 5 mV.

[0037] Furthermore, the steps of increasing current density, hydration, dropping current density, where appropriate stabilization and oxygen depletion of the second running-in phase are carried out at least twice cyclically, and the second stopping condition is reached when: o the cell voltage of the fuel cell at the end of the hydration step of the last cycle of the second running-in phase differs from the cell voltage of the fuel cell at the end of the hydration step of the penultimate cycle of the second running-in phase by a value between 1 mV and 10 mV, preferably equal to 5 mV, or o where appropriate,the cell voltage of the fuel cell at the end of the stabilization step of the last cycle of the second running-in phase differs from the cell voltage of the fuel cell at the end of the stabilization step of the penultimate cycle of the second running-in phase by a value between 1 mV and 10 mV, preferably equal to 5 mV.,

[0038] - The running-in method further comprises an initialization phase, carried out before the first running-in phase, during which the current density produced by the fuel cell is gradually increased from a zero value to the high value of the first running-in phase, and, preferably, the initialization phase is carried out over a period of between 15 minutes and 45 minutes, more preferably over a period of 24 minutes.

[0039] - The running-in method further comprises a control phase, carried out after the second running-in phase, during which the current density produced by the fuel cell is maintained at a constant level for a predetermined period, preferably for a period of between 45 minutes and 75 minutes, more preferably for a period equal to 60 minutes.

[0040] - During the first running-in phase and the second running-in phase, an anodic stoichiometric coefficient of the fuel cell is equal to a nominal anodic stoichiometric coefficient of the fuel cell, preferably between 1.3 and 2, more preferably equal to 1.5. In addition, during the current density increase, hydration, current density drop and, where appropriate, stabilization steps of the first running-in phase and the second running-in phase, a cathodic stoichiometric coefficient of the fuel cell is greater than a nominal cathodic stoichiometric coefficient of the fuel cell, preferably greater than 2, more preferably equal to 2.3.

[0041] - During the first running-in phase and during the second running-in phase, an electrical load of variable resistance is connected to the terminals of the fuel cell, the electrical load imposing a production of current on the fuel cell.

[0042] The invention will be better understood and other advantages thereof will appear more clearly in the light of the following description of an embodiment of an electrical power supply module, of an electrical power supply installation and of an assembly method, in accordance with its principle, given solely by way of example and with reference to the drawings in which:

[0043] [Fig. 1] Figure 1 is an exploded perspective view of a stack of a few cells of a fuel cell according to the invention.

[0044] [Fig. 2] Figure 2 is a curve representative of a fuel cell running-in process, the process being in accordance with the invention.

[0045] [Fig. 3] Figure 3 is a representative curve of a running-in phase of the running-in process illustrated in Figure 2.

[0046] [Fig. 4] Figure 4 shows two representative curves (B, C) of a cell voltage evolution of the fuel cell during an oxygen depletion step of the phase illustrated in Figure 3, under two distinct stoichiometric conditions, in comparison with (A) a representative curve of a cell voltage evolution of the fuel cell during a step of reducing the over-stoichiometry in air as proposed in the prior art.

[0047] [Fig. 5] Figure 5 is a diagram illustrating the electrochemical reactions occurring in the fuel cell of Figure 1 during oxygen depletion steps at the cathode, carried out during two running-in phases of the running-in process illustrated in Figure 2, and illustrating an operation of reversing the direction of the hydrogen and air flows of the running-in process.

[0048] [Fig. 6] Figure 6 is a graph resulting from an impedance spectroscopy of a membrane of a fuel cell according to the invention which has been run-in, or activated, using the running-in method of the invention.

[0049] [Fig. 7] Figure 7 is a polarization curve of a fuel cell according to the invention which has been run-in, or activated, using the running-in method of the invention.

[0050] [Fig. 8] Figure 8 is a voltammogram centered on the high potential zone of a membrane of a fuel cell according to the invention which has been run-in, or activated, using the running-in method of the invention.

[0051] [Fig. 9] Figure 9 is a voltammogram centered on the low potential zone of a membrane of a fuel cell according to the invention which has been run-in, or activated, using the running-in method of the invention.

[0052] Figure 1 illustrates a stack of cells 12 for a fuel cell 10. This fuel cell is for example intended to be installed in a vehicle and to produce electricity supplying an electric motor ensuring the propulsion of the vehicle.

[0053] 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, that is to say tight, and to supply the stack with fuel, in the example dihydrogen in gaseous form, and with oxidant, in the example air in gaseous form, and, where appropriate, the circulation of a heat transfer fluid for a cell cooling circuit.

[0054] The invention will be more particularly described 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 fuel cells of the ion exchange membrane type with solid electrolyte having different constructions.

[0055] It is considered that, for a given fuel cell 12, all the cells 12 of the fuel cell are identical to each other, therefore having identical characteristics.

[0056] In Figure 1, a detail of a section of the membrane-electrode assembly 14 of a cell 12 is also shown.

[0057] 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 contains oxygen, the cell 12 is thus supplied with oxygen.

[0058] In the remainder of the description, the terms oxygen and dioxygen, as well as the terms hydrogen and dihydrogen, are used interchangeably.

[0059] In the example, each bipolar plate 16 is formed from the assembly of two half-plates, this assembly forming on the face 16A channels for the circulation of dihydrogen, on the face 16B channels for the circulation of air, and between the faces 16A and 16B, that is to say inside the bipolar plate, channels for the circulation of refrigerant. The circulation of this refrigerant plays no role in the electrochemical reactions of the fuel cell 10, but makes it possible to control the temperature of the cells 12.

[0060] 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.

[0061] 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.

[0062] In each cell 12, there is an anode compartment, formed between the bipolar plate 16 supplying the cell with hydrogen 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.

[0063] 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 from the bipolar plates 16 to the anode 22 and cathode 24 catalytic layers. In practice, the gas diffusion layers are formed of a porous material, such as for example a non-woven carbon fiber textile, that is to say 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.

[0064] 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 perfluorinated sulfur polymer material, such as a material known under the trade name Nation.

[0065] 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 to 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 membrane 20 as well as with the electrons supplied by the cathode side face 16B of the bipolar plate 16, to form water molecules.

[0066] In practice, the catalytic layers 22 and 24 are porous structures formed from three different materials, namely:

[0067] A material to transport protons, for example the same material as the proton exchange membrane 20, here from the Nation.

[0068] A material for transporting electrons, for example carbon.

[0069] 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.

[0070] Furthermore, the pores of the catalytic layers allow the free transport of reactants, i.e., dihydrogen and oxygen, within the catalytic layers.

[0071] Within catalytic layers 22 and 24, there are regions where these three materials, along with the pores, meet. These regions are called active sites, or triple points, and electrochemical reactions occur at these active sites. Regions where all the building blocks of the catalytic layers are not present, specifically areas where platinum is present but N, carbon, or access for reactants is lacking, are called dead zones.

[0072] 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.

[0073] 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.

[0074] The fuel cell 10 comprises a hydrogen inlet 28 supplying each cell with hydrogen and a hydrogen outlet 30 discharging the hydrogen from each cell.

[0075] The fuel cell comprises an air inlet 32 ​​supplying each cell with air and an air outlet 34 discharging air from each cell.

[0076] The air outlets 34 and hydrogen outlets 30 also allow the water produced by the fuel cell to be evacuated.

[0077] The fuel cell includes a refrigerant inlet 36 supplying refrigerant to each cell and a refrigerant outlet 38 discharging refrigerant from each cell.

[0078] In the example, as visible in Figure 1, 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. Furthermore, these inlets and these outlets are connected to openings provided in the terminal plates, themselves connected to hydrogen, air and refrigerant supply circuits, such as flexible or rigid pipes. Alternatively, these inlets and outlets are formed by conduits provided around the stack of cells and bipolar plates of the fuel cell.

[0079] As seen in Figure 1, 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.

[0080] 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.

[0081] 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.

[0082] Thus, the homogenization zones 40 and 41 connect the air inlets and outlets to the active zone 42 and allow the air to be distributed over the entire width of the active zone, towards all of the channels 44.

[0083] 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 hydrogen inlet and outlet to the active zone and make it possible to distribute the hydrogen over the entire width of the active zone, towards all of the channels.

[0084] 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. 1, 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 areas 42 of the bipolar plates.

[0085] 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 cm 2 and 500 cm 2 Alternatively, this active surface area may be smaller, or much larger.

[0086] For each of the anode and cathode compartments, a stoichiometric coefficient is defined as the ratio between the injected reactant flow rate and the minimum reactant flow rate required to power the electrochemical reactions providing the current density produced by the fuel cell required by the electrical load. Thus, for a stoichiometric coefficient of 1, all of the reactant, dihydrogen or oxygen from the air, supplied to an anode or cathode compartment is consumed by the electrochemical reactions occurring at the catalytic layer of this compartment. For a stoichiometric coefficient of 2, twice as much reactant as necessary is supplied.

[0087] The anodic and cathodic compartments can also have different stoichiometric coefficients.

[0088] A stoichiometric coefficient greater than 1 therefore does not lead to an increase in the current density produced by the fuel cell, but does lead to an increase in the quantity of reactant consumed by the fuel cell, since the reactant supplied to the fuel cell in excess, which is not consumed by the electrochemical reaction, is generally totally or partially lost. It should be noted, however, that a recirculation device is generally provided, at least for the hydrogen supplied to the anode, to limit the loss of hydrogen. As for the cathode supply, air recirculation is not always provided. In addition, a stoichiometric coefficient greater than 1, particularly in the cathode compartment, leads to a higher flow rate, therefore higher pressure drops, which impacts the performance of the fuel cell and the running-in process described below.

[0089] The anodic stoichiometric coefficient is the stoichiometric coefficient of the anodic compartment and the cathodic stoichiometric coefficient is the stoichiometric coefficient of the cathodic compartment.

[0090] In theory, anodic and cathodic stoichiometric coefficients equal to 1 are sufficient to power the fuel cell, assuming that the diffusion of hydrogen and air within the anodic and cathodic compartments takes place instantaneously and without losses. However, in practice, under fuel cell operating conditions, for example within a vehicle, it is known to use anodic and cathodic stoichiometric coefficients greater than 1, that is to say to inject into the anodic and cathodic compartments a flow rate of hydrogen and air greater than the flow rate consumed, in order to guarantee good power supply to the fuel cell.This operation of the fuel cell is sometimes referred to as overstoichiometric operation in the anode and cathode compartments. This overfeeding of reactants makes it possible in particular to take into account possible leaks of dihydrogen and air in the fuel cell 10, and is in particular necessary to ensure proper operation of the fuel cell during transient phases of operation of the fuel cell, in particular when the current density produced by the fuel cell is increasing. Furthermore, without overfeeding and taking into account the transport time of the reactants through the gas diffusion layers 18 and inside the catalytic layers 22, 24, local shortages of reactants may be observed, leading to a reduction in the performance of the fuel cell.Thus, the supercharging of reactants also makes it possible to constantly ensure a sufficient presence of reactants at the level of the catalytic layers.

[0091] In a manner known per se, nominal values ​​of anodic and cathodic stoichiometric coefficients are also defined for each fuel cell, which correspond to a value of the anodic stoichiometric coefficient and to a value of the cathodic stoichiometric coefficient used when the fuel cell is in normal operation, such as for example when the fuel cell is used in a vehicle. Typical nominal values ​​of the fuel cell 10 are an anodic stoichiometric coefficient equal to 1.5 and a cathodic stoichiometric coefficient equal to 1.8, thus corresponding to operations in over-stoichiometry.

[0092] Furthermore, the stoichiometric coefficients defined above are established theoretically, considering only the flow rates of reactants actually injected into the fuel cell and consumed by the electrochemical reactions. In reality, the supply circuits supplying the fuel cell with dihydrogen and air may have leaks and losses of reactants. Thus, for a stoichiometric coefficient as defined above equal to 1, the actual flow rate of reactant delivered by the supply circuit of the corresponding reactant is slightly higher than the minimum flow rate of reactant necessary to supply the electrochemical reactions providing the current density produced by the fuel cell required by the electrical load. A global stoichiometric coefficient is then defined, which integrates, in the flow rate of reactant injected, the leaks and losses of the supply circuit of the corresponding reactant.For example, for a stoichiometric coefficient as defined above equal to 1, the overall stoichiometric coefficient is equal to 1.1.

[0093] A method of running in the fuel cell 10 is now described with reference to FIGS. 2 to 5. This running-in method is intended to be carried out after assembly of the fuel cell 10 and before its commissioning and is intended to improve the performance of the fuel cell.

[0094] The three main objectives of this lapping process are: increasing the number of active sites of the catalytic layers 22 and 24, desorption and removal of impurities present in and on the surface of the catalytic layers, and hydration of the proton exchange membrane 20, resulting in a reduction in the electrical resistance of the membrane.

[0095] These objectives make it possible to increase the performance of the fuel cell. Among other things, each of these three main objectives makes it possible to increase the cell voltage delivered by each cell 12 of the fuel cell at equal current density produced by the fuel cell, which improves the performance of the fuel cell. For simplification in the present description, the cell voltage delivered by each cell 12 of the fuel cell is designated by the expression “cell voltage of the fuel cell” or by the expression “cell voltage”.

[0096] It is for example possible to estimate the implementation of the running-in method which is the subject of the present invention on a cell of a fuel cell by observing certain characteristics of said cell. Thus, the homogeneity of the activation on the surface of the proton exchange membrane 20 can be analyzed by sampling small sections of said membrane located close to the air and / or dihydrogen inlet and outlet, from one cell, from several cells or even from each cell. If the characteristics are identical for the membrane sections analyzed, then the activation is uniform between the inlet and the outlet, in particular between the air inlet and the air outlet, thus proving that the operation of reversing the direction of flow of the dihydrogen and air flows was used for running-in said membrane.

[0097] In particular, the following characteristics can be analyzed:

[0098] - Hydration rate of the proton exchange membrane: for example, the hydration rate of the proton exchange membrane is obtained with a possible measurement by membrane impedance spectroscopy. An example of such a measurement is illustrated in Figure 6, also called a Nyquist plot. This is a graphical representation of the real and imaginary part of the cell impedance over a frequency range, here from 0.1 Hz to 10 kHz. Alternatively, or in addition, the hydration rate of the proton exchange membrane is obtained by analyzing a polarization curve of the cell. An example of a polarization curve is given in Figure 7. This curve represents the average cell voltage of a fuel cell, as a function of the current density imposed on its terminals.

[0099] - Evolution of impurity desorption: for example, a measurement of this impurity desorption is possible by cyclic voltammetry. An example of such a measurement is illustrated in Figure 8. This curve represents the current response of a fuel cell, when it is swept in voltage. Thus, to estimate the evacuation of impurities carried out using the running-in method of the invention, the desorption of platinum oxide can be observed on a measurement carried out by cyclic voltammetry. This desorption of platinum oxide is for example observable on the high potential zone of the voltamgram, that is to say on the voltage range between 0.6V and 1.2V.

[0100] - Evolution of porosity of the catalytic layer, this evolution can, for example, be determined by calculating the area of ​​the hydrogen adsorption / desorption zone at low potential on a voltammogram carried out on the fuel cell, i.e. on the voltage range of the voltammogram between 0 and 0.4V. An example of such a voltammogram is illustrated in figure 9.

[0101] Advantageously, this running-in process can also be implemented on a fuel cell that has already been used, in order to eliminate certain reversible losses and thus partially compensate for the loss of performance linked to the aging of the fuel cell.

[0102] During the running-in process, the fuel cell 10 is mounted on an activation bench, on which the fuel cell is connected to an electrical load and supplied with reactants, i.e., dihydrogen and air comprising oxygen, as well as refrigerant. The electrical load imposes a current to be produced by the fuel cell. The voltage across the fuel cell 10 and the power delivered by the fuel cell, corresponding respectively to the voltage delivered to the electrical load and the electrical power consumed by the electrical load, are measured using sensors not shown. The cell voltage of the fuel cell can then be deduced by dividing the voltage across the fuel cell 10 by the number of cells 12 of the fuel cell.

[0103] Furthermore, the electrical load connected to the fuel cell 10 can be likened to a system associating a resistor with a power electronics converter, which is controllable so that a current production is imposed on the fuel cell. In other words, the electrical load is likened to a controllable variable resistor so that the quantity of current produced by the fuel cell can be chosen.

[0104] In a manner known per se, each fuel cell exhibits a relationship between the voltage delivered to its terminals, hence the cell voltage delivered by each of its cells, and the current produced, such that, in the presence of a sufficient quantity of reactants, the voltage at the terminals of each cell, hence also the voltage at the terminals of the fuel cell, is a function of the current produced by the fuel cell, this function being dependent on the physicochemical characteristics of the fuel cell. This function is typically represented by the polarization curve of a cell of the fuel cell.Thus, for a given resistance of the controllable load, the current produced by the fuel cell is imposed, which makes it possible to determine the voltage at the terminals of the fuel cell, therefore the cell voltage of the cell, which depends on the characteristics of the fuel cell, and more particularly on the polarization curve of the cells of the fuel cell.

[0105] Furthermore, it should be noted that the current produced by the fuel cell 10 depends on the active surface area of ​​each cell 12, so that the current produced depends on the geometry of the fuel cell. Thus, to overcome the geometric considerations of the fuel cell, reference is preferably made to the current density produced by the fuel cell, this current density being equal to the current produced by the fuel cell divided by the active surface area of ​​each cell, and expressed in A / cm 2. In the present application, it is assumed that the current density is identical for all the cells 12 of the fuel cell, since they are electrically connected in series. Within a given cell, the current density is not necessarily homogeneous over the entire extent of the active surface of the cell. For a given cell, the current density considered is therefore the average current density over the active surface of the cell. In the application, reference is therefore made to the current density produced by the fuel cell using the expression “current density”.

[0106] In the remainder of the description, for the sake of brevity, it is considered that the voltage across the terminals of the battery is equal to the sum of the voltages across each of the cells in the stack, further considering that the voltage across each of the cells, i.e. the cell voltage, is identical for all the cells 12 in the stack. For each cell 12, the voltage across the terminals of the cell, i.e. the cell voltage, also corresponds to the difference between, on the one hand, the electrical potential at the cathode compartment, also referred to as the “cathode potential”, and on the other hand, the electrical potential at the anode compartment, also referred to as the “anode potential”.In the cell assembly 12, it is considered that a first cell of the stack has an absolute zero anode potential, therefore equal to 0 V with respect to the ground, neglecting a possible abnormal anode potential which would be due to abnormal operating conditions at the anode, and which are not implemented within the scope of the invention. For each subsequent cell, the absolute potential of each anode compartment is therefore considered to be equal to the sum of the cell voltages of the preceding cells in the stack. Consequently, as usual in the field of fuel cells, an electrical potential of a cathode compartment is defined, for a given cell, as a potential relative to the anode potential of the cell in question.We will therefore consider that the cathodic potential of a given cell is therefore equal to the voltage at the terminals of this cell, and is expressed in Volts, neglecting any possible abnormal anodic potential, for this cell, which would be due to abnormal operating conditions at the anode, which are not implemented within the framework of the invention.

[0107] The improvement in the performance of the fuel cell 10 sought during the running-in process essentially consists of an increase in the cell voltage, for a given current density and for a given load resistance. In other words, this improvement in performance consists of a modification of the function linking the voltage to the current density typically represented by the polarization curve of the cells of the fuel cell. Thus, at the end of running-in, the cell voltage corresponding to a given current density is higher than the cell voltage corresponding to this same current density at the start of running-in. In other words, running-in causes a “rise” in the polarization curve, this over the entire current range of the fuel cell.

[0108] As seen in Figure 2, the running-in process includes five phases or operations, executed in this order: an initialization phase P1, which is optional,

[0109] - a first running-in phase P2, an operation of reversing the direction of the flows of dihydrogen and air P3, also referred to as “inversion operation” in the remainder of the description,

[0110] - a second P4 running-in phase, and

[0111] - a P5 control phase, which is optional.

[0112] Figure 2 shows, throughout the running-in process, the evolution of the current density as a function of time.

[0113] At the start of the initialization phase P1, the fuel cell 10 has never been put into operation. Its performance is then limited, and in particular, any rapid variation in the current density would lead to damage to the fuel cell. In practice, a sudden increase in the current density on a fuel cell that has just been assembled involves a drop in cell voltage, leading in particular to overheating of the fuel cell.

[0114] Thus, during the initialization phase P1, the current density is gradually increased, from a zero value to a high value. This high value is preferably between 1.5 A / cm 2 and 3 A / cm 2 , preferably still equal to 1.9 A / cm 2 In the example, the high value is equal to 1.9 A / cm 2The initialization phase P1 is carried out over a long period, for example between 15 minutes and 45 minutes, preferably equal to 24 minutes. The initialization phase P1 ensures that the first start-up of the fuel cell 10 is carried out gradually and without degradation of the cells 12.

[0115] The first running-in phase P2 comprises several successive steps, executed cyclically. Three cycles of the first running-in phase P2 are shown in detail in Figure 3.

[0116] Figure 3 shows, throughout the three cycles of the first running-in phase P2, on the top curve, the evolution of the current density as a function of time, and, on the bottom curve, the evolution of the cell voltage as a function of time.

[0117] It should be noted that the current density values ​​indicated in Figure 3 are derived from representative values ​​given as an example, while the cell voltage values ​​are indicative values, intended to present the profile of the evolution of the cell voltage, because these values ​​vary in practice during the first running-in phase P2 and during the running-in process, as the performance of the fuel cell 10 improves as a result of the running-in protocol. Thus, the cell voltage results in particular from the current density and the efficiency of the fuel cell.

[0118] Each cycle of the first running-in phase P2 comprises five steps, executed in this order: an optional current density increase step 1; an optional hydration step 2;

[0119] - an optional current density drop step 3;

[0120] - an optional stabilization step 4 and an oxygen depletion step 5.

[0121] In the example of Figure 2, the first running-in phase P2 comprises five cycles. In practice, the first running-in phase P2 ends when a first stopping condition is reached, and may therefore comprise a number of cycles other than five. In particular, the first running-in phase P2 may comprise a single cycle.

[0122] Current density increase step 1 consists of increasing the current density from a low value to the high value, which is preferably between 1.5 A / cm 2 and 3 A / cm 2 , and in the example equal to 1.9 A / cm 2 This increase in current density is achieved, for example, by adequately controlling the electrical charge.

[0123] This increase is rapid, in comparison with the duration of the increase carried out in the initialization phase P1. Preferably, this step is carried out in a duration of between 2 seconds and 120 seconds, for example between 20 seconds and 60 seconds. The duration of the current density increase step 1 is in practice intended to be as short as possible without risking degrading the fuel cell 10. In practice, the duration of step 1 is sufficiently long to avoid any sudden voltage drop that could lead to overheating of the fuel cell and therefore degradation of the cells 12. This duration therefore depends on the performance and response time of the fuel cell. Advantageously, the duration of the current density increase step 1 is not constant from one cycle to another, but is reduced as the performance of the fuel cell increases.The order of magnitude of this duration is from a few tens of seconds to a few minutes. In the example in Figures 2 and 3, step 1 has a duration of 30 seconds.

[0124] It should be noted that the first cycle of the first running-in phase P2 is not representative of the other cycles of the first running-in phase, because the current density increase step 1 is not executed there. Indeed, at the end of the initialization phase P1, the current density is already equal to the high value. In other words, for this first cycle, the initialization phase P1 plays the role of the current density increase step 1.

[0125] The hydration step 2 consists of requesting the fuel cell 10 to produce a high current density, equal to the high value, for a predetermined duration. The high current density value is preferably chosen so as to create, in the cathode compartment, reducing conditions. These reducing conditions are typically obtained when the cathode potential of the cells is less than or equal to 0.5 V - Volt -. Thus, during this step, the charging of the cell is controlled such that the current density is high, and, according to the relationship between the current density produced and the cell voltage at the terminals of a cell, the cathode potential of the cells is relatively low, typically less than or equal to 0.5 V. The duration of the hydration step 2 is preferably between 30 seconds and 10 minutes. In the example, the hydration step 2 lasts 5 minutes.

[0126] The cathodic potential of the cells, during the hydration step, is said to be relatively low in comparison with the cathodic potential of the cells observed during normal operation of the fuel cell 10, during its use subsequent to its running-in, where the cathodic potential of the cells is generally between 0.6V and 0.7V.

[0127] Generally, the anodic potential of the cells is considered to be equal to 0V and the cathodic potential of the cells is thus substantially equal to the cell voltage. The high current density results in a significant production of water in the cathodic compartment of the cells 12, because the amount of electrochemical reactions occurring in the anodic 22 and cathodic 24 catalytic layers is proportional to the current density. This significant production of water results in the hydration of the proton exchange membrane 20, by contact between the water modules and the membrane.

[0128] Furthermore, from the second cycle, this significant production of water makes it possible to efficiently remove from the cathode compartment the impurities desorbed during the oxygen depletion step 5 of the previous cycle, this step being described below. Indeed, these impurities are transported by the water molecules formed at the level of the cathode catalytic layer and then removed through the gas diffusion layer 18.

[0129] In addition, from the second cycle, this significant production of water also makes it possible to evacuate from the cathode compartment the hydrogen produced at the cathode during the oxygen depletion stage, this production of hydrogen at the cathode being described below.

[0130] Furthermore, the high current density also causes the temperature of the fuel cell cells 12 to rise. This current density and high temperature promote an increase in the porosity of the catalytic layers 22 and 24, which increases the number of active sites of the catalytic layers.

[0131] Finally, during hydration step 2, the cell voltage is relatively low, since the current density is high, so the electrical potential of the cathode compartment of each cell 12 decreases. This potential is typically less than or equal to 0.5V. For comparison, the cathode potential during hydration step 2 is lower than the cathode potential of the fuel cell when the fuel cell is in normal service, i.e., operating for example within a vehicle, which is typically in the range of 0.6 to 0.7V. This low potential makes it possible to obtain reduction conditions that make it possible to reduce certain impurities located in the cathode catalytic layer 24, as well as platinum oxides.Through these reduction reactions, impurities are desorbed and platinum oxides are reduced to platinum particles on the one hand, and to desorbed impurities on the other, thus increasing the amount of unoxidized platinum available in the cathodic catalytic layer, and therefore the number of active sites, while decreasing the number of impurities in the cathodic catalytic layer. These reduced impurities are also removed from the anodic compartment by water molecules.

[0132] The current density drop step 3 consists of rapidly decreasing the current density from the high value to the low value. This decrease in current density results in an increase in the cell voltage. This step is carried out as quickly as possible in order to optimize the duration of the running-in process, by rapidly decreasing the current density required by the electrical load, because this rapid decrease in current density and this rapid increase in cell voltage does not entail any risk of degradation of the cells 12.

[0133] The low current density value is preferably 0.03 A / cm 2 and 0.5 A / cm 2 , preferably still equal to 0.3 A / cm 2 , as in the example.

[0134] The stabilization step 4 involves instructing the fuel cell 10 to produce a low current density, equal to the low value, for a predetermined duration. During this step, since the current density is low, then the cell voltage is relatively high.

[0135] The main objective of this step is to stabilize the operating conditions of the cell, that is to say to stabilize the temperature, the humidity level and the pressure prevailing in the anode and cathode compartments, as well as the distribution of the reactive gases within these compartments. It is therefore understood that this stabilization step 4 is, although preferential, but not obligatory, insofar as the operating conditions of the cell may already have been stabilized previously.

[0136] The stabilization step 4 also makes it possible, thanks to the high cell voltage, to obtain conditions favorable to the oxidation of certain impurities of the cathodic catalytic layer 24, making it possible to desorb these impurities.

[0137] The stabilization step 4 preferably has a duration of between 1 minute and 5 minutes. In the example, the stabilization step 4 lasts 3 minutes. In practice, this duration depends essentially on the dimensions of the cells 12, and more particularly of the membrane-electrode assemblies 14, because the larger the dimensions of the cells, the longer the stabilization of the operating conditions of the battery.

[0138] The oxygen depletion step 5 consists, from the stable conditions obtained at the end of the stabilization step 4, in requesting a constant current production from the fuel cell 10, with a current density equal to a minimum value, then in partially or completely closing the air supply to the fuel cell to obtain, in all cases, a cathodic stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9.

[0139] The minimum current density value is less than or equal to the low value. Preferably, the minimum value is a non-zero value and less than or equal to 0.3 A / cm 2 . More preferably, the minimum value is between 0.01 A / cm 2 and 0.3 A / cm 2 In the example, the minimum value is equal to 0.02 A / cm 2. In practice, at the start of the oxygen depletion step 5, the air inlet 32 ​​is adjusted so as to at least partially reduce the air supply, for example so as to obtain a cathodic stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9. Even more preferably, the air inlet 32 ​​is adjusted so as to completely cut off the air supply, for example using a valve, so that from the start of the step, the air supply to the cathodic compartment is limited, or even completely stopped.Throughout this step, the reduction reaction occurring at the cathodic catalytic layer 24, which consumes oxygen to produce water molecules, therefore gradually consumes all the oxygen already present in the cathodic compartment at the time of closing the air supply, and in cases where the air inlet 32 ​​is adjusted so as to obtain a cathodic stoichiometric coefficient strictly less than 1 without being entirely cut off, also consumes all the oxygen which continues to be supplied by the reduced air supply. Indeed, the oxygen is not renewed in sufficient quantity within the cathodic compartment, since the air is not renewed in sufficient quantity compared to the quantity which would be necessary for the reaction, in particular when the cathodic stoichiometric coefficient is strictly less than 1, which implies a faster consumption of oxygen than its renewal.The amount of oxygen in the cathode compartment therefore decreases throughout the oxygen depletion step 5, which leads to a reduction in the number of electrochemical reactions occurring at the cathode catalytic layer 24. Due to this reduction, the power that can be supplied by the fuel cell 10 gradually decreases, which results in a progressive drop in the cell voltage, since the current density is imposed by the electrical load, to a level equal to the minimum value. Thus, the complete or partial cutoff of the air supply to the fuel cell, as defined above, advantageously with a cathode stoichiometric coefficient strictly less than 1, results in a progressive decrease in the cell voltage.

[0140] Here, the cathodic stoichiometric coefficient considered does not correspond to the overall cathodic stoichiometric coefficient, i.e. it takes into account the air flow at the inlet of the cathodic compartment of each cell and actually consumed by the battery, i.e. without taking into account possible leaks upstream of the battery, for example which could occur on the activation bench.

[0141] An oxygen depletion step during which the cathodic stoichiometric coefficient is strictly less than 1 is further referred to as a “global depletion” step. Furthermore, the oxygen depletion step 5 may also be provided so as to obtain a progressive cutoff of the air supply, until a total or partial cutoff is reached. Furthermore, if such a progressive cutoff of the air supply is implemented, then the oxygen depletion step 5 preferably begins with the air inlet 32 ​​adjusted so as to obtain a cathodic stoichiometric coefficient strictly less than 1, then, during step 5, the air inlet 32 ​​is adjusted so as to progressively decrease the cathodic stoichiometric coefficient.Thus, even when the air supply is cut off gradually, the cathodic stoichiometric coefficient is, at all times, strictly less than 1. Preferably, when moving from the stabilization step 4 to the oxygen depletion step 5, one therefore moves from a cathodic stoichiometric coefficient very clearly greater than 1, for example greater than or equal to 1.8, or even greater than or equal to 2, to a cathodic stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9. This modification of the cathodic stoichiometric coefficient is carried out in a step, that is to say almost instantaneously with respect to the dynamics of the cell, that is to say with respect to its reaction to changes in the cathodic stoichiometric coefficient. In other words, the modification of the cathodic stoichiometric coefficient is not gradual, but abrupt.It should be noted that the cathodic stoichiometric coefficient very clearly greater than 1, for example greater than or equal to 1.8, can be maintained constantly at this value during steps 1 to 4.

[0142] The progressive decrease in cell voltage, during the oxygen depletion step 5, is better seen in Figure 4, where it is represented schematically by curve B, for a complete cut-off of the air supply, i.e. for a cathodic stoichiometric coefficient equal to 0, or substantially equal to 0.

[0143] As seen in curve B of Figure 4, during the oxygen depletion step 5, the cell voltage is monotonically decreasing, i.e., the cell voltage decreases continuously, without any increase, even of small duration or amplitude. Furthermore, the rate of decay of the cell voltage is monotonically increasing, i.e., the cell voltage decreases faster and faster as the oxygen depletion step progresses. This cell voltage profile is caused by the progressive decrease in the amount of oxygen available in the anode compartment, which results in an increasingly rapid drop in cell voltage.

[0144] Also shown in Figure 4 is curve A, the cell voltage profile that would occur in the event of air overstoichiometry reduction with a cathodic stoichiometric coefficient greater than or equal to 1 and less than the nominal cathodic coefficient, as is achieved in known air overstoichiometry reduction techniques. Air overstoichiometry reduction therefore consists of setting the cathodic stoichiometric coefficient to a value between the nominal cathodic stoichiometric coefficient and 1, and is achieved by reducing the air supply flow rate, relative to the nominal flow rate. This air overstoichiometry reduction may also be referred to as oxygen overstoichiometry reduction, or cathodic overstoichiometry reduction.As can be seen from curve A, such a reduction in the air supply, i.e. a reduction in the air overstoichiometry, leads to a progressive decrease in the cell voltage, on average, but with significant fluctuations in this cell voltage. This fluctuation causes degradation of the fuel cell, such as for example the dissolution of platinum, and / or corrosion of the bipolar separator, and / or corrosion of the carbon which is generally one of the components of the catalytic layer, and / or degradation of the membrane due to the appearance of excess heat points.

[0145] In theory, in the case of a reduction in the over-stoichiometry in air, the fuel cell has enough oxygen to achieve the desired current density production at a stable operating point, i.e. by delivering a stable voltage to the terminals of the fuel cell, since the stoichiometric coefficient is greater than or equal to 1. This stable operating point would be determined, depending on the desired current density production, by the polarization curve of the cells 12 of the fuel cell. However, in practice, when the cathodic stoichiometric coefficient is lower than the nominal cathodic stoichiometric coefficient, and a fortiori, close to 1, then local shortages of oxygen molecules are observed at the active sites of the cathodic catalytic layer.Indeed, to reach the active sites, the oxygen molecules must cross the gas diffusion layer 18 and then penetrate into the cathodic catalytic layer 24, which requires a certain travel time. Thus, it is observed that when a oxygen molecule reaches an active site, it is immediately consumed in a reduction reaction, which generates a suction driving other oxygen molecules towards the active site, but that, due to the absence of oxygen oversupply, these other oxygen molecules only reach the active site after a certain period of time, during which a lack of reactant is then observed, that is to say that, during this period, no reduction reaction takes place at the active site. For each active site, an alternation of periods where reduction reactions occur, and periods where no reduction reaction takes place, is then observed.At the level of a cell 12, and at the level of the fuel cell 10, this alternation causes fluctuations in the cell voltage, as shown on curve A. In practice, choosing a nominal stoichiometric coefficient greater than 1, for example equal to 1.8, also helps to avoid these fluctuations, because the overabundance of oxygen ensures a permanent supply of oxygen to the active sites.

[0146] Furthermore, a progressive decrease in the cell voltage is observed on curve A, once the fluctuations have been averaged. This progressive decrease as the over-stoichiometry in air is reduced is caused by reversible and / or irreversible degradations occurring in the fuel cell. These degradations are a consequence of the cell voltage fluctuations, and are notably caused by local lacks of hydration of the proton exchange membrane 20 of each cell 12, this lack of hydration itself being caused by repeated lacks of reduction reactions, locally at the active sites. Furthermore, due to the cell voltage fluctuations occurring, a fluctuation in the efficiency of the fuel cell 10 is observed and, consequently, a fluctuation in the temperature of the fuel cell.These fluctuations move the fuel cell away from its optimal operating parameters, which also generates the observed degradations. Furthermore, we note that the average cell voltage observed on curve A tends to decrease and then stabilize at a value greater than 0V. In practice, the closer the cathodic stoichiometric coefficient is to 1, being greater than 1, then the more the average cell voltage observed on curve A, averaged over a time interval covering several fluctuations, tends to stabilize at an asymptotic value which decreases when the cathodic stoichiometric coefficient approaches 1 while remaining greater than 1, without however becoming zero.At the same time, the closer the cathodic stoichiometric coefficient is to 1, being greater than 1, then the more the cell voltage will fluctuate, around this average value, with a fluctuation amplitude which increases when the cathodic stoichiometric coefficient approaches 1 while remaining greater than 1. Finally, the closer the cathodic stoichiometric coefficient is to 1, being greater than 1, then the more the minimum cell voltage observed during a fluctuation of curve A tends to approach 0V.

[0147] It should be noted that this fluctuation phenomenon is not observed on curve B when the air supply is completely cut off, because, since no new oxygen molecules are supplied to the cathode compartment, the suction generated at each active site is not able to draw new oxygen molecules to the active site. On curve B, the cell voltage therefore decreases progressively, as each active site consumes oxygen molecules. In Figure 4, curve C also schematically represents the decrease in cell voltage observed in the case of a partial cut off of the air supply, with a cathodic stoichiometric coefficient strictly between 0 and 1. In the example of curve C, the cathodic stoichiometric coefficient is equal to 0.5.Due to the partial cutoff of the air supply, a partial supply of air is maintained, but this supply is insufficient to maintain the cell voltage. In practice, the oxygen molecules supplied by the air inlet 32 ​​are consumed more and more quickly, as the oxygen present in the cathode compartment at the start of step 5 is consumed. Thus, the oxygen molecules are consumed by active sites closer and closer to the air inlet 32 ​​and therefore do not have the possibility of reaching active sites further away from the air inlet. Therefore, for each cell 12, an increasingly significant zone without current production is observed. In practice, in the case of a partial cutoff of the air supply, the decrease in cell voltage is longer than in the case of a total cutoff.In other words, the decrease in cell voltage is slowed down, due to the partial air supply that is maintained. Furthermore, fluctuations may also be observed, of a lower amplitude than the fluctuations observed on curve A. These fluctuations being less marked, their impact does not cause, or only slightly, damage to the fuel cell.

[0148] Thus, it is particularly advantageous to completely cut off the air supply, or at least to partially cut it off to a sufficient extent so that the cathodic stoichiometric coefficient is strictly less than 1, preferably less than or equal to 0.9, as is done in the invention, in order to avoid or greatly limit cell voltage fluctuations and associated degradations. By implementing, during the oxygen depletion step 5, a cathodic stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9, even more so in the event of a total cutoff of the air supply, degradations of the fuel cell are avoided.

[0149] It should be noted that the curves in Figure 4 illustrate the trends in cell voltage evolution when different cathodic stoichiometric coefficients are applied, but these curves are not derived from real data and are therefore provided only to illustrate the phenomena described above. In particular, curves A, B and C in Figure 4 do not represent real cell voltage values.

[0150] The oxygen depletion step 5 ends as soon as the cell voltage reaches a predefined threshold voltage. This threshold voltage is in practice between 0.1 V and 0.4 V. In the example, this threshold voltage is equal to 0.2 V. This minimum voltage threshold is chosen so as to avoid damage to the cells 12 of the fuel cell 10. Indeed, a cell voltage that is too low generally leads to damage to the cells, such as for example the formation of too large a quantity of hydrogen peroxide harmful to the cells 12. Thanks to the choice to stop the oxygen depletion step when this threshold voltage is reached, this damage is avoided.

[0151] In practice, the duration of the oxygen depletion step 5 is between 1 and 120 seconds, more preferably between 30 and 60 seconds. In the example, this duration is approximately 45 seconds. Furthermore, this duration tends to vary from one cycle to another.

[0152] Furthermore, the duration of the oxygen depletion step 5 depends in particular on the current density during this step. Thus, the higher the current density, the shorter the duration of the oxygen depletion step.

[0153] During the oxygen depletion step 5, as shown in FIG. 5, a gradient in the amount of oxygen available in the cathode compartment is observed, with a greater amount of oxygen available at the air inlet 32 ​​than at the air outlet 34.

[0154] In practice, during the oxygen depletion step 5, the oxygen consumption is relatively homogeneous within the cathode compartment, from the air inlet 32 ​​to the air outlet 34. However, during normal operation of the fuel cell 10, with a stoichiometric coefficient strictly greater than 1, such as for example during the stabilization step 4, an oxygen concentration gradient is always observed, with a higher oxygen concentration at the air inlet than at the air outlet. This gradient is caused by the progressive consumption of oxygen by the electrochemical reactions occurring at the cathode catalytic layer 24 as the air progresses in the cathode compartment, in the direction of air flow between the air inlet and the air outlet.In normal operation of the fuel cell, despite this oxygen concentration gradient, there is a sufficient quantity of oxygen to ensure the electrochemical reactions including in the active zone closest to the air outlet, and this gradient therefore has no impact on the operation of the fuel cell. At the moment when the air supply is partially or completely cut off, at the start of the oxygen depletion step 5, then the oxygen supplied is no longer sufficient to replace the oxygen consumed, and a decrease in the oxygen concentration is then observed. This decrease is thus relatively homogeneous within the cathode compartment, but due to the oxygen concentration gradient observed before the start of the oxygen depletion step, then the oxygen concentration will reach a zero value more quickly at the air outlet 34 than at the air inlet 32.We therefore observe, during the oxygen depletion step 5, a zero oxygen concentration near the air outlet 34 but a non-zero oxygen concentration near the air inlet 32.

[0155] Thus, in the half of the cathode compartment closest to the air inlet 32, denoted "C" in Figure 5, normal reduction reactions occur, consuming the available oxygen to form water molecules.

[0156] On the contrary, in the half of the cathode compartment closest to the air outlet 34, denoted "D" in FIG. 5, that is to say where the oxygen concentration is the lowest or even zero, the presence of electrochemical reactions is observed consuming the protons having crossed the membrane 20 and the electrons coming from the cathode-side face 16B of the adjacent bipolar plate 16 to form dihydrogen molecules. Indeed, due to the lack of oxygen molecules, the reduction reactions normally occurring in the cathode compartment cannot occur, allowing the electrons and protons to react together to form dihydrogen. This phenomenon is known as a "proton pump" phenomenon.

[0157] The presence of dihydrogen in the cathode compartment and the low cell voltage generate strong reducing conditions in the D half of the cathode compartment, making it possible to reduce certain impurities located in the cathode catalytic layer 24, in this half of the compartment, as well as platinum oxides. Thanks to these reduction reactions, the impurities are desorbed and the platinum oxides reduced to platinum particles, on the one hand, and to desorbed impurities on the other hand, thus increasing the quantity of unoxidized platinum available in the cathode catalytic layer, and therefore the number of active sites, while decreasing the number of impurities in the cathode catalytic layer.

[0158] In practice, the reducing conditions obtained during the oxygen depletion step 5 are stronger than the reducing conditions obtained during the hydration step 2, thus making it possible to desorb impurities from the cathodic catalytic layer more efficiently, in particular, for this first running-in phase P2, in half D of the cathodic compartment. In particular, step 5 makes it possible to desorb impurities that cannot be desorbed during step 2 due to the lack of sufficiently strong reducing conditions. These desorbed impurities are mostly removed from the cathodic compartment during steps 1 and 2.

[0159] In addition, the oxygen depletion step 5 also allows the proton exchange membrane 20 to be hydrated. Indeed, the absence of oxygen in the cathode compartment, in particular in the half of the compartment closest to the air outlet 34, leads to an increase in the molar fraction of water at the interface between the membrane and the cathode catalytic layer, and the absence of air flow in the cathode compartment results in an absence of water evacuation. Thus, the quantity of water in the presence of the membrane increases, allowing its hydration. More precisely, when the oxygen contained in the air is consumed without being renewed, or by being renewed less quickly than it is consumed, then the molecular fraction of water becomes more important in the composition of the mixture of air and water present in the cathode compartment.

[0160] Furthermore, the absence of air supply, while the supply of dihydrogen is maintained, leads to an increase in the difference between the pressure prevailing in the anode compartment and the pressure prevailing in the cathode compartment. This pressure difference leads to the opening of new pores, or channels, in the anodic 22 and cathodic 24 catalytic layers, and the enlargement of the already existing pores.

[0161] Advantageously, in order to further force the opening of pores in the anodic and cathodic catalytic layers, and in order to obtain stronger reducing conditions, it is possible to further increase the difference between the pressure prevailing in the anodic compartment and the pressure prevailing in the cathodic compartment, for example up to a value equal to 500 millibars, by adjusting the pressure setpoints of the test bench, that is to say by adjusting the hydrogen pressure in the anodic compartment and the air pressure in the cathodic compartment. Indeed, it is possible to control the hydrogen and air pressure independently of the hydrogen and air flow rate, by controlling the hydrogen 28 and air 32 inlets as well as the hydrogen 30 and air 34 outlets.

[0162] The end of the oxygen depletion step 5 marks the end of a cycle of the first running-in phase P2.

[0163] At the end of a cycle of the first running-in phase P2, it is checked whether a first stopping condition is reached. If this is the case, the first running-in phase P2 ends and the running-in process continues with the reversal operation P3.

[0164] Otherwise, the first running-in phase P2 continues with a new cycle performing steps 1 to 5 again. The air supply to the fuel cell 10 is then restored, which ends the oxygen depletion step and allows a new cycle to start. Furthermore, and as can be seen in FIG. 3, when a new cycle of the first running-in phase P2 must be performed, then the oxygen depletion step 5 comprises an increase in the current density from the minimum value to the low value. This increase in current density coincides with the restoration of the air supply and occurs just before the transition to the new cycle.Preferably, the first stopping condition is reached when: the cell voltage at the end of the hydration step 2 of the current cycle differs from the cell voltage at the end of the hydration step 2 of the previous cycle by a value between 1 mV (millivolt) and 10 mV, preferably equal to 5 mV, or preferably, when the first running-in phase P2 comprises the stabilization step 4, the cell voltage at the end of the stabilization step 4 of the current cycle differs from the cell voltage at the end of the stabilization step 4 of the previous cycle by a value between 1 mV and 10 mV, preferably equal to 5 mV. This stopping condition therefore consists of a condition of stability of the cell voltage.

[0165] Advantageously, the first stopping condition is performed by comparing voltage measurements, namely cell voltage measurements or voltage measurements at the terminals of the fuel cell, at the end of the hydration step 2, rather than at the end of the stabilization step 4, because the cell voltage measurement or the voltage measurement at the terminals of the fuel cell is more accurate when the current flow rate is higher. Indeed, when the current flow rate is lower, the losses observed in the fuel cell 10 are more difficult to identify and result in a less accurate cell voltage measurement, or a voltage measurement at the terminals of the fuel cell.Measuring the cell voltage of a battery can be done by measuring the voltage across an individual cell, preferably by averaging the voltage measured across several cells, or by dividing the voltage across the fuel cell by the number of cells in the cell.

[0166] In practice, other stopping conditions may be used. For example, a stopping condition may correspond to the achievement of a minimum performance criterion, or may be chosen as the completion of a certain predetermined number of cycles.

[0167] When a stopping condition is not based on a performance comparison between two consecutive cycles, it is thus possible for the first running-in phase P2 to comprise only one cycle. It is also possible not to define a specific stopping condition and to choose, prior to starting the running-in process, to execute only one cycle in the first running-in phase P2.

[0168] In the example in Figure 2, the first stopping condition is reached after five cycles of steps 1 to 5.

[0169] During the inversion operation P3, the electrical load is controlled so as not to request the production of any current by the fuel cell 10, and the fuel cell is not supplied with reactants, i.e. dihydrogen and oxygen. In other words, during this phase, the fuel cell is not in operation. Thus, at the end of the last cycle of phase P2, the fuel cell is stopped as soon as the cell voltage reaches the predefined threshold voltage, with however the possibility of maintaining, if necessary, a circulation of cooling fluid through the fuel cell in order to control its temperature.

[0170] The inversion operation P3 consists of reversing, that is to say permuting, on the one hand the dihydrogen inlet 28 and the dihydrogen outlet 30, and, on the other hand, the air inlet 32 ​​and the air outlet 34. In other words, during the inversion operation P3, the direction of circulation of the reactants in the fuel cell 10 is reversed. Thus, the direction of circulation of the reactants in the channels 44 of the anode side 16A and cathode side 16B faces of the bipolar plates 16 is reversed.

[0171] In practice, to achieve this inversion of inputs and outputs, several methods can be used.

[0172] A first method is to manually disconnect the pipes connected to the hydrogen and air supply circuits and then reconnect these pipes by reversing the inlets and outlets. Preferably, when this method is used, a purge of the cells 12 of the fuel cell 10 is carried out before disconnecting the pipes with a neutral gas, preferably with nitrogen, so as to prevent any chemical reaction at the level of the catalytic layers 22 and 24, then a purge of the anode compartments with hydrogen and a purge of the cathode compartments with oxygen is carried out before restarting the fuel cell, to ensure the proper supply of reactants to the fuel cell.

[0173] Another method is to connect the inlets 28, 32 and outlets 30, 34 to the hydrogen and air supply circuits using four-way valves, making it possible to swap the hydrogen inlet 28 with the hydrogen outlet 30 and to swap the air inlet 32 ​​with the air outlet 34 without having to resort to disconnecting and reconnecting the pipes. This method using a four-way valve is advantageous because it prevents any infiltration of gases other than the reactive gases into the anode and cathode compartments. In addition, swapping with a four-way valve avoids having to purge the circuits.

[0174] Once the inversion operation P3 has been carried out, the fuel cell 10 is restarted and the second running-in phase P4 begins.

[0175] The second running-in phase P4 comprises several successive steps, executed cyclically, which are preferably identical to the steps of the first running-in phase P2. Thus, each cycle of the second running-in phase P4 preferably comprises a current density increase step 1, a hydration step 2, a current density drop step 3, a stabilization step 4, and an oxygen depletion step 5.

[0176] These steps being identical to those of the first running-in phase P2, they are not detailed further in the rest of the description.

[0177] Alternatively, the cycles of the second running-in phase P4 differ from the cycles of the first running-in phase P2 in that they do not include the same steps among steps 1 to 5. For example, each cycle of the first running-in phase P2 includes steps 1, 2, 3, 4 and 5 and each cycle of the second running-in phase P4 includes steps 1, 2, 3 and 5 and thus does not include a stabilization step 4. According to another example, each cycle of the first running-in phase P2 includes steps 1, 2, 3 and 5 and thus does not include a stabilization step 4 and each cycle of the second running-in phase P4 includes steps 1, 2, 3, 4 and 5. According to another example, each cycle of the first running-in phase P2 includes steps 4 and 5 and each cycle of the second running-in phase P4 includes only step 5 and thus does not include a stabilization step 4. stabilization 4.According to another example, each cycle of the first running-in phase P2 comprises only step 5 and thus does not comprise a stabilization step 4 and each cycle of the second running-in phase P4 comprises steps 4 and 5.

[0178] The second running-in phase P4 therefore provides the same advantages as the first running-in phase P2, that is to say that it makes it possible, thanks to the alternation of steps 1 to 5, to hydrate the membrane 20, to increase the number of active sites of the catalytic layers 22 and 24 in particular by modifying their porosity, to reduce, oxidize and desorb certain impurities, and to evacuate these impurities from the catalytic layers.

[0179] It is particularly advantageous to chain the first running-in phase P2, the inversion operation P3 and the second running-in phase P4, because this chain makes it possible to obtain uniform running-in over the entire surface of the membrane-electrode assemblies 14.

[0180] Indeed, at the end of the first running-in phase P2, the cathodic catalytic layer 24 is not uniformly run-in, since during the oxygen depletion steps 5 a gradient in the quantity of available oxygen is observed between halves C and D of the cathodic compartment leading to the appearance of reducing conditions only in half D of the cathodic compartment, as explained previously. The oxygen depletion steps 5 of the first running-in phase P2 therefore mainly allow impurities to be desorbed in half D of the cathodic compartment. However, since the second running-in phase P4 is carried out after the inversion operation P3, it is observed that the gradient in the quantity of available oxygen observed during the oxygen depletion steps 5 of the second running-in phase is reversed, in comparison with the gradient of available oxygen observed during steps 5 of the first running-in phase P2.

[0181] In other words, the impurity desorption imbalance observed during the first running-in phase is also observed during the second running-in phase, but in an inverse manner, i.e., during the first running-in phase, impurity desorption takes place mainly in a first physical half of the cathode compartment, whereas during the second running-in phase, impurity desorption takes place mainly in the second physical half of the cathode compartment.

[0182] Thus, during the second running-in phase P4, the oxygen depletion steps 5 mainly allow the desorption of impurities in half C of the cathode compartment.

[0183] The sequence of phase P2, operation P3 and phase P4 therefore makes it possible to obtain uniform desorption of impurities over the entire surface of the cathodic catalytic layer 24, and therefore a uniform improvement in the performance of the membrane-electrode assembly 14.

[0184] Furthermore, this sequence is also particularly advantageous for optimizing the duration of the running-in protocol, by making it possible to obtain a fully and homogeneously run-in cell over the entire surface of the cathodic catalytic layer 24 more quickly than with known running-in protocols.

[0185] At the end of a cycle of the second running-in phase P4, it is checked whether a second stopping condition is reached. If this is the case, the second running-in phase P4 ends and, in certain embodiments, the running-in process continues, for example with the control phase P5.

[0186] Otherwise, the second running-in phase P4 continues with a new cycle carrying out steps 1 to 5 again, with the same direction of circulation of the reactive gases as during the first cycle of this second running-in phase P4. The air supply to the fuel cell 10 is then restored, which ends the oxygen depletion step and allows the start of a new cycle.

[0187] Preferably, the second stopping condition is reached when: the cell voltage at the end of hydration step 2 of the current cycle differs from the cell voltage at the end of hydration step 2 of the previous cycle by a value between 1 mV (millivolt) and 10 mV, preferably equal to 5 mV, or the cell voltage at the end of stabilization step 4 of the current cycle differs from the cell voltage at the end of stabilization step 4 of the previous cycle by a value between 1 mV and 10 mV, preferably equal to 5 mV.

[0188] This stopping condition therefore consists of a cell voltage stability condition.

[0189] Advantageously, the second stopping condition is performed by comparing cell voltage measurements or voltage measurements at the terminals of the fuel cell, at the end of the hydration step 2, rather than at the end of the stabilization step 4, because the cell voltage measurement or the voltage measurement at the terminals of the fuel cell is more accurate when the current flow rate is higher. Indeed, when the current flow rate is lower, any losses observed in the fuel cell 10 are more difficult to identify and result in a less accurate cell voltage measurement or voltage measurement at the terminals of the fuel cell.

[0190] In practice, other stopping conditions may be used. For example, a stopping condition may correspond to the achievement of a minimum performance criterion, or may be chosen as the completion of a certain predetermined number of cycles.

[0191] When a stopping condition is not based on a performance comparison between two consecutive cycles, it is thus possible for the second running-in phase P4 to comprise only one cycle. It is also possible not to define a specific stopping condition and to choose, prior to starting the running-in process, to execute only one cycle in the first running-in phase P2.

[0192] Preferably, as in the example, the second stopping condition is identical to the first stopping condition.

[0193] Preferably, as in the example, the second running-in phase P4 comprises the same number of cycles as the first running-in phase P2.

[0194] In the example in Figure 2, the first stop condition is reached after five cycles of steps 1 to 5.

[0195] During the optional control phase P5, the electrical load imposes a constant current density on the fuel cell 10 for a predetermined duration, and the evolution of the cell voltage of the cell is observed. This optional control phase P5 makes it possible to verify whether the cell performance is indeed entirely stable. If it is observed that the cell voltage of the cell remains stable throughout the control phase P5, then the correct execution of the running-in protocol is confirmed and the fuel cell is ready to be put into service.

[0196] The duration of the optional control phase P5 is preferably between 45 minutes and 75 minutes. In the example, this duration is equal to 60 minutes. The current density during the control phase P5 is preferably between 0.3 A / cm 2 and 1.9 A / cm 2 In the example, the current density is equal to 1 A / cm 2 . Furthermore, in the example, the control phase P5 lasts 60 minutes. The duration of the control phase may be different. It is preferably longer than 10 minutes, for example between 10 and 100 minutes.

[0197] In addition, measuring the cell voltage during the control phase makes it possible to measure the performance gain of the fuel cell 10 provided by the running-in protocol.

[0198] For example, on a fuel cell formed by a stack of cells 12 with a useful surface area of ​​each membrane-electrode assembly 14 equal to 250 cm 2, the performance gain obtained thanks to the running-in protocol on the fuel cell voltage is between 10% and 50%, for example equal to 30%, when the electrical load imposes a current density of 1 A / cm 2 to the fuel cell, for a total duration of the running-in protocol of between 20 minutes and 300 minutes. The performance gain is preferably measured by comparing the quantity of electrical energy produced by the fuel cell 10 per kilogram of dihydrogen consumed by the electrochemical reactions before and after execution of the running-in protocol, the electrical energy produced by the fuel cell being for example measured in Wh.

[0199] In a manner known per se, it can be considered in practice that the current density delivered by the fuel cell is imposed and controlled by the electrical load connected to the terminals of the fuel cell 10, and therefore does not depend on the quantity of reactants present in the anode and cathode compartments, as long as this quantity is sufficient, as determined by Faraday's laws, to power the electrochemical reactions occurring in the anode 22 and cathode 24 catalytic layers, that is to say as long as the flow rate of reactant gases supplied to the dihydrogen 28 and air 32 inlets is sufficient. In other words, a surplus of dihydrogen and air does not lead to an increase in the current density. Conversely, a lack of dihydrogen and / or air, therefore oxygen, leads to a drop in cell voltage, as in the example of the oxygen depletion step 5 of the first and second running-in phases P2 and P4.

[0200] In practice, Faraday's laws state that the current supplied by an electrochemical reaction is a direct measure of the rate of the electrochemical reaction, i.e., the molar flow rate of reactant consumed. Thus, the current supplied / by an electrochemical reaction is equal to: i = nx F xv With / the current in Amperes, n the number of electrons transferred by the electrochemical reaction, F ia Faraday's constant, approximately equal to 96,485 C / mol (coulombs per mole), and v the reaction rate, expressed in mol / s (moles per second), also referred to as the molar flow rate consumed, corresponding to the rate of consumption of the reactants.

[0201] Thus, as long as a sufficient molar flow rate of reactants is ensured at the active sites, the reaction rate v, and therefore the molar flow rate of reactants consumed, depends on the current supplied by the electrochemical reactions occurring at the active sites, and does not vary in the presence of excess reactants.

[0202] Advantageously, throughout the running-in phases P2 and P4, the anodic stoichiometric coefficient is constant and equal to the nominal anodic stoichiometric coefficient of the fuel cell 10, preferably between 1.3 and 1.5, more preferably equal to 1.5. This relatively low value makes it possible to limit the consumption of dihydrogen during the running-in process, and thus to reduce the cost of carrying out the running-in process.

[0203] Advantageously, throughout the running-in phases P2 and P4, the cathodic stoichiometric coefficient is constant and greater than the nominal cathodic stoichiometric coefficient of the fuel cell 10, except during the oxygen depletion phases 5 where the cathodic stoichiometric coefficient is strictly less than 1, preferably less than or equal to 0.9, more preferably equal to 0. Preferably, throughout the running-in phases P2 and P4, except during the oxygen depletion phases 5, the cathodic stoichiometric coefficient is greater than 2, for a fuel cell having a humidity level of between 30% and 80% at the cathodic compartment. In the example, the cathodic stoichiometric coefficient is equal to 2.3.

[0204] This relatively high cathodic stoichiometric coefficient makes it possible to reduce the duration of the running-in protocol. Indeed, the high flow rate of oxygen, and even more so of air, in the cathodic compartment leads to a greater circulation of gas in the pores of the cathodic catalytic layer, which makes it possible to enlarge the already existing pores, or channels, and to open new channels in the cathodic catalytic layer, thus leading to an increase in the number of active sites of the cathodic catalytic layer. In addition, this high flow rate promotes the evacuation of impurities desorbed from the cathodic catalytic layer 24 throughout the running-in process. This evacuation of impurities is particularly effective during the hydration steps 2 of the running-in phases P2 and P4.Furthermore, this relatively high cathodic stoichiometric coefficient, which therefore imposes a relatively high flow rate at the cathode, makes it possible to improve the response time of the fuel cell, and thus to reduce the duration of the current density increase step 1 of the running-in phases P2 and P4.

[0205] Compared with already known lapping methods, the lapping method of the invention has several advantages.

[0206] First of all, the running-in method is particularly effective in increasing the performance of the fuel cell 10 without generating degradations likely to reduce the lifetime of the fuel cell. In particular, the alternation between oxygen depletion steps 5, and lower cell voltage steps, i.e. the hydration steps 2, promotes the desorption of impurities and pollutants present in the catalytic layers 22 and 24, by successively reducing and oxidizing a maximum of impurities. The method also makes it possible to use all the means for improving the performance of the fuel cell 10, namely the hydration of the proton exchange membrane 20, the increase in the number of active sites of the catalytic layers and the desorption and removal of impurities and pollutants present in and on the surface of the catalytic layers.Degradation of the fuel cell is notably avoided thanks to the stabilization steps 4, which make it possible to chain the hydration steps 2 and oxygen depletion steps 5 without generating negative operating conditions for the fuel cell, and, thanks to the total or partial cut-off of air, therefore of oxygen, as defined above carried out during the oxygen depletion steps, making it possible to avoid cell voltage fluctuations that are harmful to the fuel cell.

[0207] Then, the lapping process makes it possible to achieve uniform performance over the entire surface of the membrane-electrode assemblies 14, in particular thanks to the inversion operation P3 which makes it possible to eliminate the lack of homogeneity, in particular those caused by the oxygen depletion steps 5, but also those which are generally present in a fuel cell. Indeed, the inversion is also beneficial for the hydration step 2, and for the stabilization step 4. Indeed, given that an oxygen concentration gradient always exists in the cathode compartment, as explained above, carrying out the stabilization step 4 before and after the inversion operation P3 also makes it possible to obtain more uniform membrane hydration and more uniform oxidation / reduction of impurities.

[0208] In addition, the running time of the lapping process is advantageously short, thanks to a sequence of effective steps for reducing and then oxidizing the impurities of the catalytic layers, and in particular thanks to the sequence of the hydration 2 and oxygen depletion 5 steps. In addition, the consumption of dihydrogen is controlled throughout the lapping process, in particular thanks to the relatively low anodic stoichiometric coefficient and the short total time spent in high current phases, which consume more reagents. The lapping process is thus economical in dihydrogen, which is advantageous since dihydrogen is a generally expensive gas.

[0209] The running-in protocol promotes the achievement of a catalytic layer morphology that is unattainable with a protocol without oxygen depletion or gas direction reversal. The catalytic layer and membrane morphology obtained with this process promotes the achievement of cell efficiency and durability levels that are unattainable with protocols without oxygen depletion or gas direction reversal. This favorable morphology corresponds in particular to a larger total active platinum surface area, and is promoted mainly by the oxygen depletion step 5 at the cathode.

[0210] Finally, the running-in process is particularly simple to set up. It does not require any specific additional components, as it is sufficient to apply a current cycle and control the arrival of the reactants, particularly during the oxygen depletion steps and during the P3 inversion operation, to carry out the entire running-in process. It is therefore economical to set up.

[0211] In the present description, the phenomena for improving the performance of the cathodic catalytic layer 24 have mainly been explained, by increasing the number of active sites, by increasing its porosity and by desorption of impurities, whereas the improvement of the performance of the anodic catalytic layer 22 has been little discussed. In practice, it is known that it is easier to achieve good performance of the anodic catalytic layer than of the cathodic catalytic layer, for several reasons. First of all, since the anode compartment is generally supplied with pure gas, in particular dihydrogen, it is less subject to pollutants than the cathodic compartment, which is generally supplied with air which may contain numerous pollutants.Furthermore, the cathodic catalytic layer is thicker and comprises more platinum particles than the anodic catalytic layer, making the increase in the number of active sites and the desorption of impurities longer. Finally, oxygen tends to be deposited on the cathodic catalytic layer, forming platinum oxides, which must be reduced during the running-in process, whereas such a phenomenon does not occur at the level of the anodic catalytic layer. Thus, the performance gain objectives are more easily achieved for the anodic catalytic layer than for the cathodic catalytic layer, which implies that, when the cathodic catalytic layer has reached a satisfactory level of performance, the same is necessarily true for the anodic catalytic layer.

[0212] It should be noted that the concepts of high value, low value and minimum value, used in the present description with reference to the current density, are arbitrary, and are understood only relative to each other, within the framework of the running-in method described herein, and without connection with the actual values ​​of current density corresponding in a particular application. Thus, in the present description, the high value is strictly greater than the low value, and the minimum value is less than or equal to the low value. The high and minimum values ​​are therefore defined relative to the low value. Alternatively, the low value is designated as the first predetermined value, the minimum value is designated as the second predetermined value and the high value is designated as the third predetermined value.

[0213] In a non-shown variant of the invention, the high, low and minimum values ​​of the first running-in phase P2 differ from the high, low and minimum values ​​of the second running-in phase P4. For example, the high, low and minimum values ​​of the second running-in phase P4 are greater or lower than the high, low and minimum values ​​of the first running-in phase P2, in order to take into account the evolution of the performance of the fuel cell 10 during the running-in process.Such an evolution of these current density values ​​can be determined to allow the same cathodic potential levels to be reached, for the different stages and / or the different phases, in order to generate desired levels of oxidation, in particular for the low current density values, and in order to generate desired levels of reduction, in particular for the high current density values, if possible identical throughout the activation cycles, this depending on the evolution of the characteristics of the cells of the battery during activation.

[0214] In a variant of the invention that is not shown, the running-in method does not include the initialization phase P1. In such a variant, the fuel cell 10 is for example initialized at the end of its assembly, before its installation on the activation bench. It is also possible not to provide an initialization phase P1, and to provide that the first cycle of the first running-in phase P2 includes a current density increase step 1, which is preferably long enough not to cause damage to the fuel cell.

[0215] In a variant of the invention that is not shown, the running-in method does not include the control phase P5. In such a variant, the stability of the cell voltage may be evaluated by another means, or may not be evaluated before commissioning the fuel cell. In a variant of the invention that is not shown, the running-in phases P2 and P4 do not include steps 1, 2 and 3. In such a variant, the running-in phases therefore only include the stabilization 4 and oxygen depletion 5 steps. This variant has the advantage of minimizing the amount of hydrogen consumed by the running-in method, because the fuel cell is never constrained by the electrical load to a high current density resulting in a significant consumption of reactants. However, in this variant, the running-in method takes longer. In particular, the running-in phases P2 and P4 include a greater number of cycles.Indeed, without the hydration steps 2, the hydration of the proton exchange membrane 20 is still obtained thanks to the oxygen depletion steps 5, but requires a longer running-in time.

[0216] In a variant of the invention that is not shown, the tightness of the stack of cells 12 obtained by means of the end plates is controlled throughout the running-in protocol, and adjusted so as to maintain a constant tightness of the stack of cells, making it possible to take into account possible variations in the dimensions of the cells that may occur during running-in. Indeed, the hydration of the proton exchange membrane 20 and the physicochemical modification of the catalytic layers 22 and 24 is likely to cause variations in the thickness of these elements during the running-in process.

[0217] In a variant of the invention not shown, during the hydration steps 2 of the running-in phases P2 and P4, the temperature within the cells 12 is increased to 100°C, either during all of these steps, or for a short period during these steps, making it possible to avoid possible re-adsorption of impurities and pollutants in the catalytic layers 22 and 24. By increasing the relative humidity of the reactive gases, this increase in temperature makes it possible to avoid flooding of the proton exchange membrane.

[0218] In the example, the predefined threshold voltage is constant throughout the running-in process, i.e. it is identical for all the oxygen depletion steps 5 of the running-in phases P2 and P4. In a non-represented variant of the invention, slight variations in the predefined threshold voltage may be provided during the running-in process, for example by being slightly greater or lower during the second running-in phase P4 than during the first running-in phase P2. However, these variations are sufficiently small so that the predefined threshold voltage of the first running-in phase P2 is substantially equal to the predefined threshold voltage of the second running-in phase P4.

[0219] A particularly advantageous embodiment of the invention corresponds to a method for running in a fuel cell 10, the fuel cell comprising a stack of cells 12, each cell comprising a proton exchange membrane 20 arranged between two bipolar plates 16, each bipolar plate 16 delimiting, in said cell, with the proton exchange membrane 20 a reactive compartment, each cell thus comprising a cathode compartment in which a cathode catalytic layer 24 is arranged, and an anode compartment, in which an anode catalytic layer 22 is arranged.

[0220] The fuel cell comprises a hydrogen inlet 28 supplying the anode compartment of each cell with hydrogen, and a hydrogen outlet 30 discharging the hydrogen from each cell. The fuel cell comprises an air inlet 32 ​​supplying the cathode compartment of each cell 12 with air, and an air outlet 34 discharging the air from each cell 12.

[0221] The lapping method according to this particularly advantageous embodiment of the invention comprises at least, in this order, the following steps:

[0222] - a first running-in phase P2, comprising at least, in this order, the following steps: o a stabilization step 4 of the fuel cell 10, during which the current density produced by the fuel cell is kept constant at a low value for a predetermined duration, and o an oxygen depletion step 5 of the fuel cell, during which the current density produced by the fuel cell is kept constant at a minimum value, less than or equal to the low value, and during which the air supply to the fuel cell via the air inlet 32 ​​is at least partially cut off, being adjusted so as to obtain a cathodic stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9, the oxygen depletion step of the first running-in phase ending when the cell voltage of the fuel cell 10 reaches a predefined threshold voltage,an operation of reversing the direction of the flows of dihydrogen and air P3, in which the dihydrogen inlet 28 and the dihydrogen outlet 30 are reversed and in which the air inlet 32 ​​and the air outlet 34 are reversed, and,

[0223] - a second running-in phase P4, comprising at least, in this order, the following steps: o a stabilization step 4 of the fuel cell 10, during which the current density produced by the fuel cell is kept constant at a low value for a predetermined duration, and o an oxygen depletion step 5 of the fuel cell, during which the current density produced by the fuel cell is kept constant at a minimum value, less than or equal to the low value of the second running-in phase, and during which the air supply to the fuel cell via the air inlet 32 ​​is at least partially cut off, being adjusted so as to obtain a cathodic stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9,the oxygen depletion step of the second running-in phase ending when the cell voltage of the fuel cell 10 reaches a predefined threshold voltage.,

[0224] In addition, the steps of the first running-in phase P2 are carried out until a first stopping condition is reached, and the steps of the second running-in phase P4 are carried out until a second stopping condition is reached.

[0225] 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. Method for running in a fuel cell (10), the fuel cell comprising a stack of cells (12), each cell comprising a proton exchange membrane (20) arranged between two bipolar plates (16), each bipolar plate (16) delimiting, in said cell, with the proton exchange membrane (20) a reactive compartment, each cell thus comprising a cathode compartment in which a cathode catalytic layer (24) is arranged, and an anode compartment, in which an anode catalytic layer (22) is arranged, the fuel cell comprising a dihydrogen inlet (28) supplying the anode compartment of each cell with dihydrogen, and a dihydrogen outlet (30) discharging the dihydrogen coming from each cell, the fuel cell comprising an air inlet (32) supplying the cathode compartment of each cell (12) with air, and an air outlet (34) discharging the air coming from each cell (12),characterized in that the running-in process comprises at least, in this order, the following phases:, - a first running-in phase (P2), comprising at least one step of oxygen depletion (5) of the fuel cell, during which the current density produced by the fuel cell is kept constant at a minimum value, and during which the air supply to the fuel cell via the air inlet (32) is at least partially cut off so as to cause a progressive decrease in the cell voltage, the oxygen depletion step of the first running-in phase ending when the cell voltage of the fuel cell (10) reaches a predefined threshold voltage, an operation of reversing the direction of the hydrogen and air flows (P3), in which the hydrogen inlet (28) and the hydrogen outlet (30) are reversed and in which the air inlet (32) and the air outlet (34) are reversed, and - a second running-in phase (P4), comprising at least one step of oxygen depletion (5) of the fuel cell, during which the current density produced by the fuel cell is kept constant at a minimum value, and during which the air supply to the fuel cell via the air inlet (32) is at least partially cut off, so as to cause a progressive reduction in the cell voltage, the oxygen depletion step of the second running-in phase ending when the cell voltage of the fuel cell (10) reaches a predefined threshold voltage, wherein said at least one step of the first running-in phase (P2) is carried out until a first stopping condition is reached, and wherein said at least one step of the second running-in phase (P4) is carried out until a second stopping condition is reached.

2. Method for running in a fuel cell (10) according to claim 1, wherein the first running-in phase (P2) further comprises, prior to the oxygen depletion step (5), a step (4) of stabilizing the fuel cell (10), during which the current density produced by the fuel cell is kept constant at a low value for a predetermined duration, the low value of the first running-in phase being greater than or equal to the minimum value, and / or wherein the second running-in phase (P4) further comprises, prior to the oxygen depletion step (5), a step (4) of stabilizing the fuel cell (10), during which the current density produced by the fuel cell is kept constant at a low value for a predetermined duration, the low value of the second running-in phase being greater than or equal to the minimum value.

3. Method for running in a fuel cell (10) according to any one of the preceding claims, in which the minimum value of the first running-in phase and the minimum value of the second running-in phase are between 0.01 A / cm 2 and 0.3 A / cm 2 , preferably equal to 0.02 A / cm 2 .

4. Method for running in a fuel cell (10) according to claims 2 and 3 considered in combination, in which the low value of the first running-in phase (P2) and the low value of the second running-in phase (P4) are between 0.03 A / cm 2 and 0.5 A / cm 2 , preferably equal to 0.3 A / cm 2 5. Method for running in a fuel cell (10) according to any one of the preceding claims, wherein, during at least one of said oxygen depletion steps (5), the air supply to the fuel cell via the air inlet (32) is at least partially cut off while being adjusted so as to obtain a cathodic stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.

9.

6. Method for running in a fuel cell (10) according to any one of the preceding claims, in which the oxygen depletion steps (5) of the first running-in phase (P2) and the second running-in phase (P4) end when the cell voltage of the fuel cell (10) reaches a threshold voltage between 0.1 V and 0.4 V, preferably equal to 0.2 V.

7. Method for running in a fuel cell (10) according to any one of the preceding claims, wherein during the oxygen depletion steps (5) of the first running-in phase (P2) and the second running-in phase (P4), the air supply to the fuel cell (10) via the air inlet (32) is completely cut off, so as to obtain a cathodic stoichiometric coefficient equal to 0.

8. A method of running-in a fuel cell (10) according to any one of the preceding claims, wherein during the oxygen depletion steps (5) of the first running-in phase (P2) and the second running-in phase (P4), the air supply to the fuel cell (10) through the air inlet (32) is controlled so that the cell voltage of the fuel cell (10) is monotonically decreasing.

9. Method for running in a fuel cell (10) according to any one of the preceding claims, in which: - the first running-in phase (P2) also comprises the following steps, in this order and carried out before the oxygen depletion step (5): o a current density increase step (1), during which the current density produced by the fuel cell (10) is gradually increased from a low value to a high value, the low value being greater than or equal to the minimum value of the first running-in phase, o a fuel cell (10) hydration step (2), during which the current density produced by the fuel cell is kept constant at the high value of the first running-in phase for a predetermined duration, so as to hydrate the proton exchange membrane (20), and o a current density drop step (3),during which the current density produced by the fuel cell is reduced from the high value of the first running-in phase to the low value of the first running-in phase, - the second running-in phase (P4) also includes the following steps, in this order and carried out before the stabilization (4) and oxygen depletion (5) steps: o a current density increase step (1), during which the current density produced by the fuel cell (10) is gradually increased from a low value to a high value of the second running-in phase, the low value of the second running-in phase being greater than or equal to the minimum value of the second running-in phase, o a hydration step (2) of the fuel cell (10), during which the current density produced by the fuel cell is kept constant at the high value of the second running-in phase for a predetermined duration, so as to hydrate the proton exchange membrane (20), and o a current density drop step (3), during which the current density produced by the fuel cell is lowered from the high value of the second running-in phase to the low value of the second running-in phase.

10. Method for running in a fuel cell according to claim 9, in which the high value of the first running-in phase (P2) and the high value of the second running-in phase (P4) are between 1.5 A / cm 2 and 3 A / cm 2 , preferably equal to 1.9 A / cm 2 .

11. A method of running-in a fuel cell (10) according to any one of claims 9 or 10, wherein: the steps of increasing current density (1), hydration (2), dropping current density (3), where appropriate stabilization (4) and oxygen depletion (5) of the first running-in phase (P2) are carried out at least twice cyclically, the first stopping condition is reached when: o the cell voltage of the fuel cell (10) at the end of the hydration step (2) of the last cycle of the first running-in phase differs from the cell voltage of the fuel cell (10) at the end of the hydration step (2) of the penultimate cycle of the first running-in phase by a value between 1 mV and 10 mV, preferably equal to 5 mV, or o where appropriate,the cell voltage of the fuel cell (10) at the end of the stabilization step (4) of the last cycle of the first running-in phase differs from the cell voltage of the fuel cell (10) at the end of the stabilization step (4) of the penultimate cycle of the first phase of, running-in with a value between 1 mV and 10 mV, preferably equal to 5 mV, the steps of increasing current density (1), hydration (2), dropping current density (3), where appropriate stabilization (4) and oxygen depletion (5) of the second running-in phase (P4) are carried out at least twice cyclically, and the second stopping condition is reached when: o the cell voltage of the fuel cell (10) at the end of the hydration step (2) of the last cycle of the second running-in phase differs from the cell voltage of the fuel cell (10) at the end of the hydration step (2) of the penultimate cycle of the second running-in phase by a value between 1 mV and 10 mV, preferably equal to 5 mV, or o where appropriate,the cell voltage of the fuel cell (10) at the end of the stabilization step (4) of the last cycle of the second running-in phase differs from the cell voltage of the fuel cell (10) at the end of the stabilization step (4) of the penultimate cycle of the second running-in phase by a value between 1 mV and 10 mV, preferably equal to 5 mV., 12. Method for running in a fuel cell (10) according to any one of claims 9 to 11, further comprising an initialization phase (P1), carried out before the first running-in phase (P2), during which the current density produced by the fuel cell (10) is progressively increased from a zero value to the high value of the first running-in phase, and in which, preferably, the initialization phase (P1) is carried out over a period of between 15 minutes and 45 minutes, more preferably over a period equal to 24 minutes.

13. Method for running in a fuel cell (10) according to any one of claims 9 to 12, further comprising a control phase (P5), carried out after the second running-in phase (P4), during which the current density produced by the fuel cell (10) is maintained at a constant level for a predetermined duration, preferably for a duration of between 45 minutes and 75 minutes, more preferably for a duration equal to 60 minutes.

14. Method for running in a fuel cell (10) according to any one of claims 9 to 13, in which: - during the first running-in phase (P2) and the second running-in phase (P4), an anodic stoichiometric coefficient of the fuel cell (10) is equal to a nominal anodic stoichiometric coefficient of the fuel cell, preferably between 1.3 and 2, more preferably equal to 1.5, and during the current density increase (1), hydration (2), current density drop (3) and, where appropriate, stabilization (4) steps of the first running-in phase (P2) and the second running-in phase (P4), a cathodic stoichiometric coefficient of the fuel cell is greater than a nominal cathodic stoichiometric coefficient of the fuel cell, preferably greater than 2, more preferably equal to 2.

3.

15. Method for running in a fuel cell (10) according to any one of the preceding claims, in which, during the first running-in phase (P2) and during the second running-in phase (P4), an electrical load of variable resistance is connected to the terminals of the fuel cell (10), the electrical load imposing a production of current on the fuel cell.

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