Fuel cell break-in procedure

Abstract

This provides a novel break-in process for fuel cells. [Solution] A break-in operation method for a PEMFC-type fuel cell includes at least a first break-in operation stage (P2), followed by an operation to reverse the direction of hydrogen flow and the direction of air flow (P3), and then a second break-in operation stage (P4). Each of the break-in operation stages includes a fuel cell stabilization step (4) in which the current density obtained by the fuel cell during the step is kept at a low and constant value for a given period of time, followed by an oxygen depletion step (5) in which the current density is kept at a minimum value of a low value or less during the step, the supply of air is at least partially cut off during the step, and the cutoff is adjusted so that a cathode stoichiometric coefficient of exactly less than 1 is obtained. The oxygen depletion steps of the first and second break-in operation stages end when the cell voltage of the fuel cell reaches a predetermined threshold voltage.

Description

[Technical Field]

[0001] This invention relates to a method for breaking in a fuel cell. [Background technology]

[0002] A fuel cell is a device that generates electricity through an electrochemical reaction between a fuel, such as dihydrogen, and an oxidizer, such as oxygen contained in the air. The main focus of this specification is on solid electrolyte proton exchange membrane fuel cells (PEMFCs). These PEMFCs generally consist of a stack of several individual cells, each of which constitutes a single electrochemical power generation device.

[0003] In general terms, each single cell comprises two separators, also known as polar plates, and the solid electrolyte exists in the form of a proton exchange membrane inserted between these separators. This membrane is made from, for example, a sulfonated perfluoropolymer material. Within each cell, each separator, together with the corresponding membrane, defines a reaction compartment. Of the two compartments, the compartment known as the cathode compartment houses the cathode element formed by a cathode catalyst layer on the surface of the membrane, and the other compartment known as the anode compartment houses the anode element formed by an anode catalyst layer on the surface of the membrane. The assembly of the membrane, the anode catalyst layer, and the cathode catalyst layer forms a membrane electrode assembly, which is generally called a "MEA".

[0004] For two adjacent cells, the separator of one cell is positioned back-to-back with the separator of the other cell. Together, these two separators form a bipolar separator, also known as a bipolar plate. Generally, a cooling compartment, through which a cooling fluid such as glycol-containing water circulates, is located between the two separators of a bipolar separator.

[0005] Dihydrogen, air, and coolant are the so-called "working" fluids supplied to the fuel cell during operation. Dihydrogen and air are reactants, while the coolant does not participate in the electrochemical reaction. Depending on the operating stage of the fuel cell, one or more working fluids are supplied continuously or intermittently.

[0006] Therefore, the fuel cell has openings for supplying fluid to each of the reaction compartments and to the fluid between two adjacent cells. Thus, in a widely used design, each bipolar separator supplies fuel to the cells adjacent to that side on one side and combustion air to the cells adjacent to that side on the other side, and the bipolar separator supplies fuel in parallel.

[0007] Generally, in a single cell, an oxidizer, such as oxygen, is supplied to the cathode compartment, most commonly in the form of an oxygen-containing air feed, while a fuel, such as dihydrogen, is supplied to the anode compartment.

[0008] Each reaction chamber generally also includes a gas diffusion layer positioned between the bipolar separator and the catalyst layer, which allows for efficient circulation of fuel or oxidizer from the separator to the catalyst layer.

[0009] During operation of a fuel cell, an electrochemical reaction generates a potential difference between two separators within each single cell. Therefore, a fuel cell includes an electrical isolation device designed to prevent electrical contact between two adjacent bipolar separators and between each cell and the external environment, and a sealing device to prevent leakage of the working fluid, in particular to prevent contamination of adjacent reaction compartments by fluid circulating within one reaction compartment.

[0010] The voltage of each cell is generated by the potential difference between the two separators of each individual cell, and this is called the "cell voltage." Since all the cells in a fuel cell are electrically connected in series, the voltage supplied between the fuel cell terminals is the sum of the cell voltages of all the individual cells.

[0011] PEMFC fuel cells require activation and break-in procedures before initial use or operation after manufacturing. These break-in procedures aim to improve and stabilize the fuel cell's output by altering the physicochemical properties of the individual cells.

[0012] In particular, the main objectives of the break-in operation are to ensure optimal humidification of the membrane, to decontaminate contaminants present in the anode and cathode catalyst layers, and to activate the anode and cathode catalyst layers. This activation corresponds to morphological changes within the membrane electrode junction, especially changes in the porosity of the catalyst layer and purification of the catalyst layer.

[0013] Generally, the break-in operation of a fuel cell is performed on an activation bench, during which the fuel cell is connected to a load and supplied with reactants. The break-in method involves adjusting the load and reactant flow rate so that a predetermined voltage or current profile is maintained. During the break-in period, the output of the fuel cell is gradually increased until this output stabilizes and reaches the stop criterion for the activation operation. Once this is reached, the break-in operation of the fuel cell is considered complete. The stop criterion can be the achievement of a predetermined level of output, and in particular, it can be the output in terms of efficiency, expressed in terms of, for example, the electrical energy generated by the cell per unit mass of hydrogen consumed, or the stabilization of the output between two measurements, or both. Existing stop criteria simply involve stopping the break-in process after a predetermined time has elapsed. Criteria of this form are extremely imprecise. For example, the stop criterion is defined as the difference between the voltage at time t and the voltage at time t+1 at a given constant current density obtained by the fuel cell, which must be below a certain threshold.

[0014] In most cases, such a break-in process involves maintaining a constant voltage at the fuel cell terminals or a constant current density from the fuel cell over a predetermined period, and then measuring the voltage and current at the fuel cell terminals at the end of this period. This cycle is repeated until the shutdown criteria are reached.

[0015] Such break-in processes are generally lengthy, taking several hours, and consume a significant amount of hydrogen, especially for fuel cells with a maximum output of 10 kW or more, which can amount to several kilograms of hydrogen. These two drawbacks mean that break-in protocols are very costly, which is a disadvantage when implementing them on an industrial scale. Therefore, it is desirable to shorten the time required for the break-in process without compromising power output or damaging the fuel cell.

[0016] Specific techniques are known for accelerating the break-in protocol. One of these techniques, known as reducing the stoichiometric excess of air, involves supplying reactants to the fuel cell with the load connected, and then reducing the air supply flow rate, thereby briefly operating the fuel cell with a lower oxygen content than the steady-state operating conditions. This is done by operating the fuel cell so that the stoichiometric coefficient on the cathode side is always greater than 1, for example, between 1 and 1.5. In this stage of reducing the stoichiometric excess of air, the cathode compartment of each single cell contains a certain amount of hydrogen in addition to air with a lower oxygen content than the nominal state. This is because operating the fuel cell locally as a proton pump allows hydrogen to form in the oxygen-deficient region of the cathode. Preferably, in this stage of reducing the stoichiometric excess, the load is controlled so that the cell voltage is low for each single cell. The low cell voltage and the presence of hydrogen in the cathode catalyst layer create favorable conditions for the desorption of impurities and the reduction of contaminating oxides located on the cathode catalyst layer. In this way, the cathode catalyst layer is purified, which tends to increase the true active surface area of ​​the cathode catalyst layer. This technique shortens the break-in process.

[0017] However, this technique has the drawback of causing non-uniform fuel cell output because the decrease in oxygen level in the air is not uniform within the cathode compartment of a single cell. In fact, this decrease is more pronounced at the air outlet opening than at the air inlet opening. In addition, reducing the stoichiometric excess of air as practiced in the prior art causes fluctuations in the cell voltage, and these fluctuations are a factor in damaging the fuel cell. Such deterioration can include dissolution of platinum, and / or corrosion of the bipolar separator, and / or corrosion of carbon, which is one of the components of the catalyst layer in general, and / or deterioration of the membrane due to the appearance of hot spots. Summary of the Invention Problems to be Solved by the Invention

[0018] In particular, the present invention addresses the above-mentioned drawbacks, and by the present invention, a new conditioning operation process for a fuel cell is proposed that is faster, does not damage the fuel cell, and provides better fuel cell output. Means for Solving the Problems

[0019] For this purpose, the present invention relates to a conditioning operation process for a fuel cell, wherein the fuel cell comprises a stack of cells, each cell comprises a proton exchange membrane disposed between two bipolar plates, each bipolar plate defines a reaction compartment within the cell together with the proton exchange membrane, and thus each cell comprises a cathode compartment in which a cathode catalyst layer is disposed, an anode compartment in which a cathode catalyst layer is disposed, and an anode compartment in which an anode catalyst layer is disposed.

[0020] The fuel cell comprises a hydrogen inlet for supplying hydrogen to the anode compartment of each cell and a hydrogen outlet for discharging hydrogen from each cell. The fuel cell comprises an air inlet for supplying air to the cathode compartment of each cell and an air outlet for removing air from each cell.

[0021] According to the present invention, the break-in process comprises at least the following steps: - The first break-in phase, which includes at least the following steps: Preferably, a fuel cell stabilization step, comprising the step of maintaining the current density obtained by the fuel cell at a low and constant value for a predetermined period of time during the stabilization step, • An oxygen depletion process for a fuel cell, wherein during the oxygen depletion process, the current density obtained by the fuel cell is kept constant at a minimum value, the minimum value can be less than or equal to the low value, and the fuel cell is driven to the oxygen depletion process by at least partially shutting off the supply of air to the fuel cell through an air inlet during the oxygen depletion process, and the shutoff is particularly advantageously adjusted so that a cathode stoichiometric coefficient of exactly less than 1, preferably 0.9 or less, is obtained, and the oxygen depletion process of the first break-in operation stage ends when the cell voltage of the fuel cell reaches a predetermined threshold voltage, and The first break-in phase shall include the following in this order: - An operation to reverse the direction of hydrogen flow and the direction of air flow, which involves reversing the hydrogen inlet and hydrogen outlet, and also reversing the air inlet and air outlet. - The second break-in phase, which includes at least the following steps: Preferably, a fuel cell stabilization step, comprising the step of maintaining the current density obtained by the fuel cell at a low and constant value for a predetermined period of time during the stabilization step, - A fuel cell oxygen depletion process, wherein during the oxygen depletion process, the current density obtained by the fuel cell is kept constant at a minimum value, the minimum value can be less than or equal to the low value of the second break-in operation stage, the current density obtained by the fuel cell is reduced by at least partially cutting off the supply of air to the fuel cell through the air inlet during the oxygen depletion process, the cutting off is particularly advantageously set so that a cathode stoichiometric coefficient of exactly less than 1, preferably 0.9 or less, is obtained, and the oxygen depletion process of the second break-in operation stage is terminated when the cell voltage of the fuel cell reaches a predetermined threshold voltage, and The second break-in stage shall include the following in this order: It includes them in this order.

[0022] The first break-in phase is carried out until the first stop condition is reached, and the second break-in phase is carried out until the second stop condition is reached.

[0023] According to the present invention, the oxygen depletion process in the first and second break-in stages allows for the reduction and desorption of other impurities on the surface of the cathode catalyst layer, thus further purifying the cathode catalyst layer. These processes are also carried out without causing any significant degradation of the fuel cell. This is because, for example, the cathode stoichiometric coefficient is strictly less than 1, preferably 0.9 or less, which sufficiently reduces the air supply, thereby avoiding voltage fluctuations caused by a gradual decrease in the cell voltage. In addition, degradation is prevented by avoiding depletion at the cathode at high currents. Furthermore, by reversing the direction of hydrogen flow and air flow between the two break-in stages, a consistent fuel cell output is achieved despite the implementation of the oxygen depletion process. Finally, the efficiency of the oxygen depletion process is maximized by preferably linking the stabilization stage and the oxygen depletion process during the break-in stages. This is because the stabilization stage makes it possible to obtain stable and uniform conditions within the fuel cell, thereby avoiding non-uniformity of output during the oxygen depletion process. In addition, the stabilization processes in the first and second break-in phases also favorably enable the oxidation of impurities on the surface of the cathode catalyst layer. This is because maintaining a low current density means operating the fuel cell at a high cell voltage, which promotes the oxidation reaction of certain impurities and, consequently, at least partially, purifies the cathode catalyst layer.

[0024] According to an advantageous but not essential embodiment of the present invention, the break-in process includes one or more of the following features, which are used individually or in any technically feasible combination:

[0025] - The first break-in stage also includes a fuel cell stabilization step before the oxygen depletion step, during which the current density obtained by the fuel cell is kept at a low and constant value for a predetermined period, wherein the low value in the first break-in stage is greater than or equal to the minimum value, and / or the second break-in stage also includes a fuel cell stabilization step before the oxygen depletion step, during which the current density obtained by the fuel cell is kept at a low and constant value for a predetermined period, wherein the low value in the second break-in stage is greater than or equal to the minimum value.

[0026] - The minimum values ​​for the first and second break-in phases are 0.01 A / cm². 2 ~0.3A / cm 2 The current is preferably 0.02 A / cm². 2 That is the case.

[0027] - The lowest values ​​for the first and second break-in phases are 0.03 A / cm². 2 ~0.5A / cm 2 The current is preferably 0.3 A / cm². 2 That is the case.

[0028] - During at least one of the oxygen depletion steps, the supply of air to the fuel cell through the air inlet is at least partially cut off, and this cutoff is adjusted so that a cathode stoichiometric coefficient of exactly less than 1, preferably 0.9 or less, is obtained.

[0029] - When the cell voltage of the fuel cell reaches a threshold voltage of 0.1V to 0.4V, preferably 0.2V, the oxygen depletion process of the first break-in operation stage and the second break-in operation stage is completed.

[0030] - The cathode stoichiometric coefficient is reduced to zero by completely shutting off the supply of air to the fuel cell through the air inlet during the oxygen depletion process of the first and second break-in phases. In some embodiments, the cathode stoichiometric coefficient is maintained at zero throughout the oxygen depletion process of the first and / or second break-in phases.

[0031] - During the oxygen depletion process in the first and second break-in phases, the supply of air to the fuel cell through the air inlet is controlled so that the cell voltage of the fuel cell decreases monotonically.

[0032] - The first break-in phase also includes the following steps, which are performed before the oxygen depletion process: • A current density increasing step, during which the current density obtained by the fuel cell is gradually increased from a low value to a high value, wherein the low value is equal to or greater than the minimum value of the first break-in operation stage. • A fuel cell humidification process, comprising the step of humidifying a proton exchange membrane during the humidification process by maintaining the current density obtained by the fuel cell at a constant high value for a predetermined period of time, corresponding to the first break-in operation stage. • A current density reduction step, wherein during the current density reduction step, the current density obtained by the fuel cell is reduced from the high value of the first break-in operation stage to the low value of the first break-in operation stage. It includes them in this order, Furthermore, the second break-in phase also includes the following steps, which are performed before the stabilization and oxygen depletion processes: • A current density increasing step, wherein during the current density increasing step, the current density obtained by the fuel cell is gradually increased from a low value in the second break-in operation stage to a high value, where the low value in the second break-in operation stage is equal to or greater than the minimum value in the second break-in operation stage. • A fuel cell humidification process, comprising the step of humidifying a proton exchange membrane during the humidification process by maintaining the current density obtained by the fuel cell at a constant high value for a predetermined period of time, corresponding to the second break-in operation stage. · A current density reduction step, during which the current density obtained by the fuel cell is reduced from the high value of the second conditioning operation stage to the low value of the second conditioning operation stage. These are included in this order.

[0033] - The high value of the first conditioning operation stage and the high value of the second conditioning operation stage are 1.5 A / cm 2 ~3 A / cm 2 and preferably 1.9 A / cm 2 is the case.

[0034] - The current density increase step, humidification step, current density reduction step, stabilization step, and oxygen depletion step of the first conditioning operation stage are carried out at least twice in a cycle mode. Also, the following: · When the cell voltage of the fuel cell at the end of the humidification step of the last cycle of the first conditioning operation stage is different from the cell voltage of the fuel cell at the end of the humidification step of the second cycle from the last of the first conditioning operation stage by a value of 1 mV to 10 mV, preferably 5 mV, or · Appropriately, when the cell voltage of the fuel cell at the end of the stabilization step of the last cycle of the first conditioning operation stage is different from the cell voltage of the fuel cell at the end of the stabilization step of the second cycle from the last of the first conditioning operation stage by a value of 1 mV to 10 mV, preferably 5 mV The first stop condition is reached.

[0035] In addition, the current density increase step, humidification step, current density reduction step, and optionally the stabilization step and oxygen depletion step of the second conditioning operation stage are carried out at least twice in a cycle mode, and in that case, the following: · When the cell voltage of the fuel cell at the end of the humidification step of the last cycle of the second conditioning operation stage is different from the cell voltage of the fuel cell at the end of the humidification step of the second cycle from the last of the second conditioning operation stage by a value of 1 mV to 10 mV, preferably 5 mV, or - When the cell voltage of the fuel cell at the end of the stabilization process of the last cycle of the second break-in phase differs from the cell voltage of the fuel cell at the end of the stabilization process of the second-to-last cycle of the second break-in phase by a value of 1mV to 10mV, preferably 5mV. The second stopping condition is reached.

[0036] - The break-in method also includes an initialization step performed before the first break-in step, during which the current density obtained by the fuel cell is gradually increased from zero to the high value of the first break-in step, preferably the initialization step is performed over a period of 15 to 45 minutes, and more preferably over a period of 24 minutes.

[0037] - The break-in method also includes a control stage performed after the second break-in stage, during which the current density obtained by the fuel cell is maintained at a constant level for a predetermined period, preferably 45 to 75 minutes, more preferably 60 minutes.

[0038] - During the first and second break-in periods, the anode stoichiometric coefficient of the fuel cell is equal to the nominal anode stoichiometric coefficient of the fuel cell, preferably 1.3 to 2, and more preferably 1.5. In addition, during the current density increase process, humidification process, current density decrease process, and optionally stabilization process of the first and second break-in periods, the cathode stoichiometric coefficient of the fuel cell is greater than the nominal cathode stoichiometric coefficient of the fuel cell, preferably greater than 2, and more preferably 2.3.

[0039] - During the first and second break-in periods, a variable resistor electrical load is connected to the fuel cell, which forces the fuel cell to generate an electric current.

[0040] Below, embodiments of a power supply module, power supply system, and assembly method, shown as examples, will be described according to their principles with reference to the drawings. This description will further enhance the understanding of the present invention and make other advantages of the present invention more apparent. [Brief explanation of the drawing]

[0041] [Figure 1] This is a perspective view of a stack of several cells of the fuel cell according to the present invention. [Figure 2] This is a typical diagram of the break-in process for a fuel cell according to the present invention. [Figure 3] Figure 2 is a diagram representing one of the break-in stages of the break-in process. [Figure 4] This figure shows two diagrams (B, C) representing the change in cell voltage of a fuel cell during the oxygen depletion process shown in Figure 3 under two different stoichiometric conditions, compared with diagram (A) representing the change in fuel cell voltage during the process of reducing the stoichiometric excess amount of air as proposed in the prior art. [Figure 5] Figure 2 shows the electrochemical reaction that occurs in the fuel cell shown in Figure 1 during the oxygen depletion process at the cathode, which takes place between the two break-in stages of the break-in process shown in Figure 2, and the operation of reversing the direction of hydrogen flow and air flow in the break-in process. [Figure 6] This graph was obtained by impedance spectroscopy of the membrane of a fuel cell according to the present invention that has been broken in or activated using the break-in process of the present invention. [Figure 7] This figure shows the polarization diagram of a fuel cell according to the present invention that has been broken in or activated using the break-in process of the present invention. [Figure 8] This is a voltammogram of the high-potential zone of the membrane of a fuel cell according to the present invention, which has been broken in or activated using the break-in process of the present invention. [Figure 9]This is a voltammogram of the low-potential zone of the membrane of the fuel cell according to the present invention, which has been broken in or activated using the break-in process of the present invention. [Modes for carrying out the invention]

[0042] Figure 1 shows a stack of cells 12 of the fuel cell 10. This fuel cell is designed, for example, to be installed in a vehicle and to generate electricity to power an electric motor, thereby propelling the vehicle.

[0043] The fuel cell 10 is a proton exchange membrane fuel cell and therefore comprises a stack of cells 12. This stack is held between two end plates, which are not shown in Figure 1. In particular, these end plates allow the stack of cells 12 to be kept in a compressed, i.e., sealed state, and to be supplied to the stack with fuel, such as gaseous dihydrogen, an oxidizer, such as gaseous air, and, if necessary, a circulating heat transfer fluid for the cell cooling circuit.

[0044] The present invention will be described more specifically in the context of a general configuration in which each cell 12 comprises a membrane electrode assembly 14 and two bipolar plates 16 arranged on both sides of the membrane electrode assembly. However, the present invention is also applicable to solid electrolyte ion exchange membrane fuel cells with different structures.

[0045] Given a fuel cell 12, it is assumed that all cells 12 within the fuel cell are identical to each other and therefore have the same characteristics.

[0046] Figure 1 also shows a detailed cross-section of the membrane electrode assembly 14 of cell 12.

[0047] In reality, each bipolar plate 16 is positioned between two cells 12, and both cells share this bipolar plate 16. The first surface 16A, called the anode side, supplies dihydrogen to one of the two cells, and the second surface 16B, called the cathode side, supplies air to the other of the two cells. In other words, cell 12 is supplied with dihydrogen by the first bipolar plate 16 and with air by the second bipolar plate. Since air contains oxygen, cell 12 is supplied with oxygen.

[0048] Hereafter, in this specification, the terms oxygen and dioxygen will be used interchangeably, and the terms hydrogen and dihydrogen will also be used interchangeably.

[0049] In this example, each bipolar plate 16 is formed by assembling two half-plates. This assembly has a hydrogen circulation channel on surface 16A, an air circulation channel on surface 16B, and a coolant circulation channel between surfaces 16A and 16B, i.e., inside the bipolar plate. This coolant circulation system does not participate in the electrochemical reaction of the fuel cell 10, but it enables temperature control of the cell 12.

[0050] The membrane electrode assembly 14 comprises two gas diffusion layers 18 positioned on both sides of the proton exchange membrane 20, an anode catalyst layer 22 deposited, for example, on the first surface of the membrane, and a cathode catalyst layer 24 deposited, for example, on the other surface of the membrane.

[0051] Therefore, in this example, each cell 12 is equipped with a bipolar plate 16 that supplies dihydrogen to the cell, a gas diffusion layer 18, an anode catalyst layer 22, a proton exchange membrane 20, a cathode catalyst layer 24, a gas diffusion layer 18, and a bipolar plate 16 that supplies air to the cell, in this order.

[0052] Each cell 12 has an anode compartment formed between a bipolar plate 16 that supplies hydrogen to the cell and a membrane 20, and a cathode compartment formed between the bipolar plate 16 that supplies air to the cell and a membrane. The anode catalyst layer 22 is located within the anode compartment, and the cathode catalyst layer 24 is located within the cathode compartment.

[0053] Therefore, the gas diffusion layers 18 are located within the corresponding anode or cathode compartments, and the gas diffusion layers 18 enable the transport of fuel gas and oxidizer gas from the bipolar plate 16 to the anode catalyst layer 22 and cathode catalyst layer 24. In practice, the gas diffusion layers are formed from porous materials, such as nonwoven carbon fiber textiles, i.e., carbon fiber textiles with randomly arranged fibers, or porous carbon paper, and porous carbon paper is generally impregnated with polymers, preferably hydrophobic polymers, such as polytetrafluoroethylene (PTFE) or other fluoropolymers, in particular to increase the hydrophobicity of the surface of the carbon paper fibers.

[0054] The proton exchange membrane 20 allows hydrogen ions or protons to pass from the anode compartment to the cathode compartment 24, while preventing the circulation of gases and electrons between these two compartments. For example, this proton exchange membrane 20 is made from a sulfurized perfluoropolymer material, such as the material known by the trade name Nafion.

[0055] During operation of the fuel cell 10, an anodic oxidation reaction occurs in the anode catalyst layer 22 within the anode compartment of each cell 12. This oxidation reaction consists of decomposing dihydrogen supplied through the gas diffusion layer 18 into protons and electrons by the catalyst. The protons thus generated pass through the proton exchange membrane 20 to the cathode catalyst layer in the cathode compartment, while the electrons are captured by the anode-side surface 16A of the adjacent bipolar plate 16 and then conducted to the cathode-side surface 16B of the same bipolar plate. This cathode-side surface belongs to the cathode compartment of the adjacent cell 12. Simultaneously, a reduction reaction occurs in the cathode catalyst layer 24 within the cathode compartment of cell 12. This reduction reaction consists of reacting oxygen molecules supplied by air through the gas diffusion layer 18 with protons passing through the membrane 20 and electrons supplied by the cathode side 16B of the bipolar plate 16 to form water molecules.

[0056] In reality, catalyst layers 22 and 24 are porous structures formed from three different materials, namely: - Proton transport material, for example, the same material as the proton exchange membrane 20, in this case Nafion. - Electron transport materials such as carbon, - A material that catalyzes the electrochemical oxidation and reduction reactions described above, such as platinum. This material exists in the form of particles, preferably spherical, and is deposited on the surface of the aforementioned electron transport material, such as the carbon mentioned above, during the production of the catalyst layer.

[0057] Furthermore, the pores in the catalyst layer allow for the free transport of reactants, namely hydrogen and oxygen, within the catalyst layer.

[0058] Within catalyst layers 22 and 24, there are regions where these three materials come into contact with pores. These regions are called active sites or three-phase interface points, and electrochemical reactions occur at these active sites. There are regions where not all components of the catalyst layer are present; in particular, zones where platinum is present but Nafion, carbon, or reactants cannot reach are called dead zones.

[0059] Catalyst layers 22 and 24 also contain impurities or contaminants, which are, for example, residues or additives derived from the manufacture of the catalyst layers. Furthermore, the platinum particles contained in the catalyst layers typically have an oxide layer on their surface. If this oxide layer becomes too thick, the platinum particles can no longer react with protons and electrons, and therefore, such a thick oxide layer on the surface of the platinum particles can be considered to be an impurity.

[0060] In practice, the membrane electrode assembly 14 of cell 12 is positioned in an opening formed within a support plate 25, which is inserted between two bipolar plates 16. The support plate 25 can be fabricated, for example, in the form of one or two polymer film layers with a thickness of 50 to 200 micrometers. The polymer film is made, for example, from polyethylene terephthalate, also known as PET, or polyethylene naphthalate, also known as PEN. Advantageously, to ensure sealing between the membrane electrode assembly and the bipolar plates 16 in the stack 12, the membrane electrode assembly is provided with two sealing portions 26. These sealing portions 26 are located around the membrane electrode assembly, positioned between the gas diffusion layer 18 and the catalyst layers 22, 24, and extend to the support plate 25.

[0061] The fuel cell 10 includes a dihydrogen inlet 28 that supplies dihydrogen to each cell and a dihydrogen outlet 30 that discharges dihydrogen from each cell.

[0062] The fuel cell is equipped with an air inlet 32 ​​that supplies air to each cell and an air outlet 34 that discharges air from each cell.

[0063] The air outlet 34 and hydrogen outlet 30 can also be used to discharge water produced by the fuel cell.

[0064] The fuel cell includes a coolant inlet 36 that supplies coolant to each cell and a coolant outlet 38 that discharges coolant from each cell.

[0065] In this example, as shown in Figure 1, the inlets 28, 32, 26 and outlets 30, 34, 38 are formed by openings in the bipolar plate 16 and the support plate 25. In addition, these inlets and outlets are connected to openings in the end plate, which are connected to hydrogen, air, and refrigerant supply circuits, such as flexible or rigid piping. Alternatively, these inlets and outlets are formed by conduits arranged around the stack of fuel cell cells and the bipolar plate.

[0066] As can be seen in Figure 1, the bipolar plate 16 has two homogenization zones 40 and 41 and an active zone 42 on its cathode-side surface 16B. The first homogenization zone 40 connects the air inlet 32 ​​to the active zone, and the second homogenization zone 41 connects the active zone to the air outlet 34.

[0067] The active zone 42 is provided with channels 44 throughout its entire surface, which run straight through the active zone, and each of these channels 44 connects the homogenization zone 40 to the homogenization zone 41. Thus, the channels 44 enable the airflow to be transported along the entire length of the cathode section.

[0068] Here, channel 44 is represented as a straight line. In variant forms not shown, channel 44 may have different shapes, such as wavy, meandering, or dashed.

[0069] The homogenization zones 40 and 41 connect the air inlet and air outlet to the active zone 42, and the homogenization zones 40 and 41 enable the distribution of air to all channels 44 across the entire width of the active zone.

[0070] On the anode side 16A, the bipolar plate 16 has the same structure as the cathode side 16B, namely, it has two homogenization zones and an active zone containing channels. On the anode side, the homogenization zones connect the dihydrogen inlet and dihydrogen outlet to the active zone, and the homogenization zones enable the distribution of hydrogen to all channels across the entire width of the active zone.

[0071] In this example, the bipolar plate 16 and the support plate 25 are rectangular in shape. Although not essential in this example, as can be seen in Figure 1, the dihydrogen inlet 28 and dihydrogen outlet 30 are positioned diagonally to each other, and the air inlet 32 ​​and air outlet 34 are also positioned diagonally to each other, which allows for a more uniform distribution of the reactive gas in the active zone 42 of the bipolar plate.

[0072] Preferably, each cell 12 of the fuel cell 10 is 150 cm². 2 ~500cm 2 It has an active surface which coincides with the surface of the active zone 42 of the bipolar plate 16. Alternatively, this active surface may be smaller or larger.

[0073] For both the anode and cathode compartments, the stoichiometric coefficient is defined as the ratio of the injected reactant flow rate to the minimum flow rate of reactant required to supply the electrochemical reaction that provides the current density required by the fuel cell for the electrical load. Therefore, if the stoichiometric coefficient is 1, all reactant supplied to the anode or cathode compartment, i.e., dihydrogen or oxygen from air, is consumed by the electrochemical reaction occurring in the cathode catalyst layer of that compartment. If the stoichiometric coefficient is 2, twice the required amount of reactant is supplied.

[0074] The anode and cathode compartments may have different stoichiometric coefficients.

[0075] Therefore, when the stoichiometric coefficient exceeds 1, the current density obtained by the fuel cell does not increase, but the amount of reactant consumed by the fuel cell increases. This is because the excess reactant supplied to the fuel cell is lost, either completely or partially, without being consumed by the electrochemical reaction. However, it should be noted that, generally speaking, a recirculation device is provided to limit hydrogen loss, at least with respect to the hydrogen supplied to the anode. Regarding the supply to the cathode, an air recirculation unit is not necessarily provided. Furthermore, when the stoichiometric coefficient, especially in the cathode section, exceeds 1, the flow rate increases, and consequently, the pressure loss increases, which affects the output of the fuel cell and the break-in process described later.

[0076] The anodic stoichiometric coefficient is the stoichiometric coefficient of the anode compartment, and the cathode stoichiometric coefficient is the stoichiometric coefficient of the cathode compartment.

[0077] Theoretically, if the anode and cathode stoichiometric coefficients are 1, it is assumed that the supply to the fuel cell is sufficient, and that the diffusion of hydrogen and air within the anode and cathode compartments occurs instantaneously and without loss. However, in practice, when fuel cells are used, for example, in vehicles, it is known that anode and cathode stoichiometric coefficients greater than 1 are used to ensure a good supply to the fuel cell; that is, more hydrogen and air are injected into the anode and cathode compartments at a rate greater than the rate consumed. This type of operation of a fuel cell can be called operation in a stoichiometric excess state in the anode and cathode compartments. Such an excess supply of reactants makes it possible to consider leakage of hydrogen and air within the fuel cell 10, and such an excess supply of reactants is particularly necessary to ensure proper operation of the fuel cell during the transient phase of operation, especially when the current density obtained by the fuel cell is increasing. Furthermore, without supercharging, and considering the time it takes for the reactants to be transported through the gas diffusion layer 18 into the catalyst layers 22 and 24, a localized shortage of reactants is observed, which reduces the output of the fuel cell. In this way, by promoting the supply of reactants, a constant and sufficient supply of reactants in the catalyst layers is also ensured.

[0078] In a method that is publicly known, nominal values ​​for the anodic and cathode stoichiometric coefficients are also specified for each fuel cell, and these nominal values ​​correspond to the values ​​of the anodic and cathode stoichiometric coefficients used during normal operation of the fuel cell, for example, when the fuel cell is used in a vehicle. For a typical nominal value of fuel cell 10, the anodic stoichiometric coefficient is 1.5 and the cathode stoichiometric coefficient is 1.8, which corresponds to operation in a stoichiometric excess state.

[0079] Furthermore, the stoichiometric coefficients defined above are theoretically established and only consider the flow rate of reactants actually injected into the fuel cell and consumed by the electrochemical reaction. In reality, the supply circuits that provide dihydrogen and air to the fuel cell may experience leakage and loss of reactants. Therefore, if the stoichiometric coefficient defined above is 1, the actual flow rate of reactants delivered by the corresponding reactant supply circuit is slightly greater than the minimum flow rate of reactants required to supply the electrochemical reaction that provides the current density required by the fuel cell for the electrical load. Next, the total stoichiometric coefficient is defined. This total stoichiometric coefficient incorporates the leakage and loss from the corresponding reactant supply circuit into the flow rate of injected reactants. For example, if the stoichiometric coefficient defined above is 1, the total stoichiometric coefficient is 1.1.

[0080] Next, the break-in process for the fuel cell 10 will be explained using Figures 2 to 5. This break-in process is intended to be performed after the assembly of the fuel cell 10 and before it is put into operation, and is designed to improve the output of the fuel cell.

[0081] The three main objectives of this break-in process are: - Increase in the number of active sites in catalyst layers 22 and 24, - Desorption and removal of impurities present within and on the surface of the catalyst layer, - Decrease in the electrical resistance of the proton exchange membrane 20 due to humidification.

[0082] These objectives improve the output of the fuel cell. In particular, when the current density obtained by the fuel cell is the same, each of these three main objectives can increase the cell voltage generated by each cell 12 of the fuel cell, thereby improving the output of the fuel cell. For simplicity, the cell voltage generated by each fuel cell 12 will be referred to as the "fuel cell voltage" or "cell voltage".

[0083] For example, the implementation of the break-in process described in the present invention with respect to a fuel cell can be estimated by observing specific characteristics of the cell. For instance, the uniformity of activation on the surface of the proton exchange membrane 20 can be analyzed by sampling one cell, several cells, or even small sections of the membrane located near the air and / or dihydrogen inlets and outlets of each cell. If the characteristics are identical for the analyzed sections of the membrane, the activation is uniform between the inlet and outlet, particularly between the air inlet and air outlet, thus demonstrating that the operation of reversing the direction of dihydrogen flow and the direction of air flow was used for the break-in of the membrane.

[0084] In particular, the following characteristics can be analyzed: - Humidification rate of proton exchange membrane: For example, the humidification rate of a proton exchange membrane can be measured using membrane impedance spectroscopy. An example of such measurement is shown in Figure 6. This is also known as a Nyquist diagram. This is a graphical representation of the real and imaginary parts of the battery impedance over a certain frequency range, in this example, 0.1 Hz to 10 kHz. Alternatively, the humidification rate of the proton exchange membrane can also be obtained by analyzing the cell polarization diagram. An example of a polarization diagram is shown in Figure 7. This diagram shows the average cell voltage of the fuel cell against the current density applied at its terminals.

[0085] - Changes in impurity desorption: For example, impurity desorption can be measured by cyclic voltammetry. An example of such measurement is shown in Figure 8. This diagram shows the current response when the voltage of the fuel cell is swept. For example, to estimate the removal of impurities by the break-in process, platinum oxide desorption can be measured by cyclic voltammetry. This platinum oxide desorption can be observed, for example, in the high-voltage region of the voltammogram, i.e., in the voltage range of 0.6V to 1.2V.

[0086] - The change in porosity of the catalyst layer can be evaluated, for example, by calculating the area of ​​the hydrogen adsorption / desorption zone at low potentials in a voltammogram performed for a fuel cell, i.e., in the voltage range of 0 to 0.4 V. An example of such a voltammogram is shown in Figure 9.

[0087] Advantageously, this break-in process can also be performed on fuel cells that are already in use, thereby eliminating certain reversible losses and partially compensating for the power reduction that occurs with the aging of fuel cells.

[0088] During the break-in process, the fuel cell 10 is mounted on an activation bench, where it is connected to an electrical load and supplied with reactants, i.e., air containing dihydrogen and oxygen, and a coolant. The electrical load requires current, which must be supplied by the fuel cell. The voltage at the terminals of the fuel cell 10 and the power generated by the fuel cell correspond to the voltage supplied to the electrical load and the power consumed by the electrical load, respectively, and are measured using non-representative sensors. The cell voltage of the fuel cell can be estimated by dividing the terminal voltage of the fuel cell 10 by the number of cells 12 in the fuel cell.

[0089] In addition, the electrical load connected to the fuel cell 10 can be likened to a system combining a resistor and a power converter, which can be controlled to impose current generation on the fuel cell. In other words, the electrical load can be likened to a variable resistor that can be controlled to select the amount of current obtained from the fuel cell.

[0090] In a method known to the public, each fuel cell has a correlation between the voltage generated at its terminals, and therefore the cell voltage generated by each cell, and the resulting current. Thus, in the presence of a sufficient amount of reactant, the voltage at the terminals of each cell, and by extension the terminals of the fuel cell, is correlated with the current produced by the fuel cell, and this correlation depends on the physicochemical properties of the fuel cell. This correlation is typically represented by the polarization diagram of the fuel cell. Therefore, by supplying the current produced by the fuel cell to a given resistance of a controllable load, the voltage at the terminals of the fuel cell, and by extension the cell voltage of the fuel cell, can be determined. This cell voltage depends on the characteristics of the fuel cell, more specifically, the polarization diagram of the fuel cell.

[0091] Furthermore, it should be noted that the current obtained from the fuel cell 10 depends on the active surface of each cell 12, and therefore the obtained current depends on the geometry of the fuel cell. For this reason, it is preferable to refer to the current density obtained from the fuel cell, so as to avoid having to consider the geometry of the fuel cell. This current density is equal to the current obtained from the fuel cell divided by the active surface area of ​​each cell, and is calculated as A / cm². 2 It is expressed in units of . In this application, it is assumed that all cells 12 of the fuel cell are electrically connected in series, and therefore the current density is the same for all cells 12 of the fuel cell. Within a given cell, the current density is not necessarily uniform across the entire active surface of the cell. Therefore, for a given cell, the current density considered by the applicants is the average current density of the active surface of the cell. Accordingly, this application uses the expression "current density" to refer to the current density obtained by the fuel cell.

[0092] Hereafter, for the sake of brevity, this specification assumes that the voltage at the terminals of the battery is equal to the sum of the voltages at the terminals of each cell in the stack, and that the voltage at the terminals of each cell, i.e., the cell voltage, is the same for all cells 12 in the stack. For each cell 12, the voltage at the terminals of the cell, i.e., the cell voltage, also corresponds to the difference between the potential in the cathode section, also called the "cathode potential," and the potential in the anode section, also called the "anode potential." In the cell assembly 12, the first cell in the stack is assumed to have an absolute anode potential of 0, i.e., 0V relative to ground, where abnormal anode potentials resulting from abnormal operating conditions at the anode are ignored, and such abnormal operating conditions are not implemented in the context of this invention. Therefore, in each subsequent cell, the absolute potential of each anode section is considered to be equal to the sum of the cell voltages of the preceding cells in the stack. Thus, as is common in the field of fuel cells, the potential of the cathode section is defined for a given cell as the potential relative to the anode potential of that cell. Therefore, the cathode voltage of a given cell is considered to be equal to the voltage at the terminals of this cell and is expressed in volts, where any abnormal anode voltage of this cell is ignored. This abnormal anode voltage is caused by abnormal operating conditions at the anode, and such abnormal operating conditions are not used in the context of this invention.

[0093] The improvement in the output of the fuel cell 10 required during the break-in process essentially lies in the increase in the cell voltage for a given current density and for a given load resistance. In other words, this improvement in output involves correcting the correlation between voltage and current density, which is typically represented by the polarization diagram of the fuel cell cells. At the end of the break-in period, the cell voltage for a given current density is higher than the cell voltage for the same current density at the beginning of the break-in period. In other words, the break-in period causes the polarization diagram to "increase" across the entire current range of the fuel cell.

[0094] As shown in Figure 2, the break-in process consists of five stages or operations, namely, - Any initialization stage P1, - The first break-in period P2, - Operation P3, which reverses the direction of hydrogen flow and the direction of air flow, and which will hereafter be referred to as the "reversal operation" in this specification, - The second break-in phase P4, - Any control stage P5 and These are included, and they will be carried out in this order.

[0095] Figure 2 shows the change in current density over time throughout the entire break-in process.

[0096] At the start of the initialization phase P1, the fuel cell 10 is not yet operational. In this case, its output is limited, and in particular, abrupt changes in current density can lead to fuel cell damage. In fact, a rapid increase in current density in a fuel cell immediately after assembly leads to a decrease in cell voltage, resulting in overheating of the fuel cell.

[0097] During the initialization phase P1, the current density gradually increases from zero to a high value. This high value is preferably 1.5 A / cm². 2 ~3A / cm 2 More preferably 1.9 A / cm² 2 In this example, the highest value is 1.9 A / cm². 2 The initialization stage P1 is performed over a long period of time, for example, 15 to 45 minutes, preferably 24 minutes.

[0098] The initialization phase P1 ensures that the initial start-up of the fuel cell 10 is performed gradually and without damaging the cells 12.

[0099] The first break-in phase P2 includes several consecutive steps carried out in a cyclical manner. The three cycles of the first break-in phase P2 are shown in detail in Figure 3.

[0100] Figure 3 shows the change in current density over time throughout the three cycles of the first break-in stage P2 in the upper diagram, and the change in cell voltage over time in the lower diagram.

[0101] It should be noted that the current density values ​​shown in Figure 3 are obtained from representative values ​​given as an example, whereas the cell voltage values ​​are index values ​​intended to present the transition profile of the cell voltage. This is because, as the output of the fuel cell 10 is improved by the break-in protocol, these values ​​actually change during the first break-in stage P2 and throughout the break-in process. The cell voltage is determined by the current density and efficiency of the fuel cell.

[0102] Each cycle of the first break-in phase P2 consists of five steps, namely, - An arbitrary current density increase step 1, - Optional humidification step 2, - An optional current density reduction step 3, - Optional stabilization step 4, - Oxygen depletion process 5 and These are included, and they will be carried out in this order.

[0103] In the embodiment shown in Figure 2, the first break-in period P2 includes five cycles. In practice, the first break-in period P2 may include a number of cycles other than five, as it terminates when the first stop condition is reached. In particular, the first break-in period P2 may include a single cycle.

[0104] The current density increase step 1 involves increasing the current density from a low value to a high value, preferably 1.5 A / cm². 2 ~3A / cm 2 In this example, the current is 1.9 A / cm². 2 This involves increasing the value of [the current density]. This increase in current density can be achieved, for example, by appropriately controlling the charge.

[0105] This increase is rapid compared to the time required for the increase in the initialization stage P1. Preferably, this process is carried out over a period of 2 to 120 seconds, for example, 20 to 60 seconds.

[0106] In practice, the duration of the current density increase process 1 is designed to be as short as possible without risking damage to the fuel cell 10. In reality, the duration of process 1 is long enough to avoid a sudden voltage drop, which could lead to overheating of the fuel cell and ultimately damage to cell 12. Therefore, this time depends on the fuel cell's output and response time. Fortunately, the duration of the current density increase process 1 is not constant from cycle to cycle, but decreases as the fuel cell's output increases. The order of this duration is typically tens of seconds to several minutes. In the examples shown in Figures 2 and 3, process 1 takes 30 seconds.

[0107] It should be noted that the first cycle of the first break-in phase P2 does not represent any of the other cycles of the first break-in phase, because the current density increase process 1 is not performed in the other cycles. At the end of the initialization phase P1, the current density is already high. In other words, in this first cycle, the initialization phase P1 functions as the current density increase process 1.

[0108] Humidification step 2 consists of requiring the fuel cell 10 to produce a high current density, which is a high value, for a predetermined period of time. The high current density value is preferably selected so that reduction conditions are created in the cathode compartment. These reduction conditions are typically obtained when the cathode potential of the cell is 0.5V (volts) or less. Therefore, during this step, the charging of the battery is controlled to produce a high current density, and according to the law of correlation between the resulting current density and the cell voltage at the cell terminals, the cathode potential of the cell is relatively low, typically 0.5V or less. The duration of humidification step 2 is preferably 30 seconds to 10 minutes. In this example, humidification step 2 takes 5 minutes.

[0109] The cathode potential of the cells during the humidification process is said to be relatively low during normal operation of the fuel cell 10 compared to the cathode potential of the cells observed during use after the break-in period, and in this case, the cathode potential of the cells is generally 0.6V to 0.7V.

[0110] Generally speaking, the anode potential of a cell is assumed to be 0V, and therefore, the cathode potential of a cell is approximately equal to the cell voltage.

[0111] Since the degree of electrochemical reactions occurring in the anode catalyst layer 22 and the cathode catalyst layer 24 is proportional to the current density, a high current density generates a significant amount of water in the cathode section of cell 12. This significant amount of water humidifies the proton exchange membrane 20 through contact between the water module and the membrane.

[0112] Furthermore, the generation of this significant amount of water makes it possible to efficiently remove impurities desorbed during the oxygen depletion process 5 of the preceding cycle from the cathode section in the second cycle onward. This process will be described later. These impurities are transported by water molecules formed in the cathode catalyst layer and then discharged through the gas diffusion layer 18.

[0113] In addition, the significant amount of water generated makes it possible to remove the hydrogen produced in the cathode during the oxygen depletion process from the cathode compartment in the second cycle and beyond. The generation of hydrogen in the cathode will be described later.

[0114] The high current density also causes a temperature rise in the fuel cell cell 12. This current density and high temperature promote an increase in the porosity of the catalyst layers 22 and 24, thereby increasing the number of active sites in the catalyst layers.

[0115] Finally, due to the high current density, the cell voltage is relatively low during humidification step 2, and therefore the potential of the cathode compartment of each cell 12 decreases. This potential is typically below 0.5V. In comparison, the cathode potential during humidification step 2 is lower than the cathode potential of the fuel cell when it is operating normally, i.e., when it is running in a vehicle, where the cathode potential is typically in the range of 0.6-0.7V. This low potential allows for a reducing state in which certain impurities and platinum oxides present in the cathode catalyst layer 24 can be reduced. As a result of these reduction reactions, impurities are desorbed, platinum oxides are reduced to platinum particles on the one hand, and impurities are desorbed on the other. This increases the amount of non-oxide platinum available in the cathode catalyst layer, and thus the number of active sites, while simultaneously decreasing the number of impurities in the cathode catalyst layer. These reduced impurities are also expelled from the anode compartment by water molecules.

[0116] The current density reduction step 3 consists of rapidly decreasing the current density from a high value to a low value. This decrease in current density causes the cell voltage to rise. This step is performed as quickly as possible in order to optimize the time required for the break-in process by rapidly reducing the current density required by the electrical load. This is because this rapid decrease in current density and rapid increase in cell voltage do not pose any risk of damage to cell 12.

[0117] The low current density value is preferably 0.03 A / cm², as shown in this example. 2 ~0.5A / cm 2 And preferably, further 0.3 A / cm 2 That is the case.

[0118] Stabilization step 4 consists of requiring the fuel cell 10 to produce a low current density, which is a low value, for a predetermined period of time. During this step, the cell voltage is relatively high because the current density is low.

[0119] The primary objective of this process is to stabilize the cell's operating conditions, specifically the temperature, humidity, and pressure within the anode and cathode compartments, as well as the distribution of reactive gases within these compartments. Therefore, while this stabilization step 4 is certainly advantageous, it is not essential if the cell's operating conditions may have already been stabilized beforehand.

[0120] Furthermore, the stabilization step 4 provides favorable conditions for the oxidation of specific impurities in the cathode catalyst layer 24 due to the high cell voltage, thereby enabling the desorption of these impurities.

[0121] Stabilization step 4 preferably takes 1 to 5 minutes. In this example, stabilization step 4 takes 3 minutes. In practice, this time substantially depends on the dimensions of the cell 12, more specifically the film electrode assembly 14, with larger cell dimensions requiring longer stabilization of the cell operating conditions.

[0122] Based on the stabilization conditions obtained at the end of stabilization step 4, oxygen depletion step 5 consists of requiring a constant current to be obtained from the fuel cell 10, where the current density is at its minimum, and the supply of air to the fuel cell is partially or completely cut off, so that in all cases a cathode stoichiometric coefficient of exactly less than 1, preferably 0.9 or less, is obtained.

[0123] The minimum current density value is less than or equal to a low value. Preferably, the minimum value is not zero, but 0.3 A / cm². 2 The following applies. More preferably, the minimum value is 0.01 A / cm². 2 ~0.3A / cm 2 In this example, the minimum value is 0.02 A / cm². 2 That is the case.

[0124] In practice, the air inlet 32 ​​is adjusted at the start of the oxygen depletion process 5, and this adjustment is carried out by at least partially reducing the supply of air so that, for example, a cathode stoichiometric coefficient of exactly less than 1, preferably 0.9 or less, can be obtained. More preferably, the air inlet 32 ​​is adjusted so that the supply of air is completely shut off, for example by a valve, so that the supply of air to the cathode compartment is restricted or even completely stopped immediately at the start of the process. Thus, throughout the entire process, all the oxygen already present in the cathode compartment when the air supply is shut off is gradually consumed by the reduction reaction occurring in the cathode catalyst layer 24, i.e., the reaction that consumes oxygen to produce water molecules, and if the air inlet 32 ​​is adjusted so that a cathode stoichiometric coefficient of exactly less than 1 can be obtained without complete shutoff, then all the oxygen that continues to be supplied by the reduction in the air supply is also consumed. This is because sufficient oxygen is not regenerated in the cathode compartment, and the reason for this is that not enough air is regenerated compared to the amount required for the reaction. This is especially true when the cathode stoichiometric coefficient is strictly less than 1. A cathode stoichiometric coefficient of strictly less than 1 means that oxygen is consumed faster than it can be regenerated. Therefore, the amount of oxygen in the cathode compartment decreases throughout the oxygen depletion process 5, thereby reducing the number of electrochemical reactions occurring in the cathode catalyst layer 24. As a result of this decrease, the power available to the fuel cell 10 gradually decreases, causing the cell voltage to gradually drop to its minimum level, because the current density is determined by the electrical load. Therefore, the cell voltage gradually decreases by completely or partially cutting off the air supply to the fuel cell while keeping the cathode stoichiometric coefficient strictly less than 1, as defined above.

[0125] Here, the cathode stoichiometric coefficients considered are not equivalent to the total cathode stoichiometric coefficients; that is, they consider the air flow rate actually consumed by each cell at the inlet to the cathode compartment, without considering any leakage upstream of the cell that may occur, for example, in an activated bench.

[0126] Oxygen depletion processes in which the cathode stoichiometric coefficient is strictly less than 1 are further defined as "global starvation" processes.

[0127] Furthermore, the oxygen depletion step 5 can also be designed to gradually cut off the air supply until the air supply is completely or partially cut off. On the other hand, if such gradual cutoff of the air supply is implemented, the oxygen depletion step 5 preferably begins with an air inlet 32 ​​set to obtain a cathode stoichiometric coefficient of exactly less than 1, and then, during step 5, the air inlet 32 ​​is adjusted to gradually decrease the cathode stoichiometric coefficient. Thus, even with the gradual cutoff of the air supply, the cathode stoichiometric coefficient is always exactly less than 1. Preferably, when transitioning from the stabilization step 4 to the oxygen depletion step 5, the cathode stoichiometric coefficient changes very clearly from greater than 1, for example, 1.8 or more, or even 2 or more, to exactly less than 1, preferably 0.9 or less. This change in the cathode stoichiometric coefficient occurs stepwise, but is almost instantaneous compared to the cell dynamics. In other words, the change in the cathode stoichiometric coefficient occurs almost instantaneously compared to the cell reaction. In other words, the change in the cathode stoichiometric coefficient is not gradual but abrupt. It should be noted that a cathode stoichiometric coefficient that is significantly larger than 1, for example, 1.8 or greater, can be kept constant at this value during steps 1-4.

[0128] The gradual decrease in cell voltage during oxygen depletion process 5 is best illustrated in Figure 4. In Figure 4, the gradual decrease in cell voltage during this oxygen depletion process 5 when the air supply is completely cut off, i.e., when the cathode stoichiometric coefficient is 0 or substantially 0, is schematically represented by Diagram B.

[0129] As can be seen from diagram B in Figure 4, the cell voltage decreases monotonically during oxygen depletion process 5; that is, the cell voltage decreases continuously without any short-term increases or slight amplitude increases. In addition, the rate of decrease in cell voltage increases monotonically; that is, the cell voltage decreases more and more rapidly as the oxygen depletion process progresses. This cell voltage profile is caused by the gradual decrease in the amount of oxygen available in the anode section, and this gradual decrease causes the cell voltage to drop sharply.

[0130] Furthermore, in Figure 4, Diagram A shows the cell voltage profile that occurs when the stoichiometric excess of air is reduced, which is achieved by a known technique for reducing the stoichiometric excess of air, where the cathode stoichiometric coefficient is greater than or equal to 1 and less than the nominal cathode coefficient. Therefore, the reduction of the stoichiometric excess of air is achieved by setting the cathode stoichiometric coefficient to a value between the nominal cathode stoichiometric coefficient and 1, and by reducing the air supply flow rate compared to the nominal flow rate. This reduction of the stoichiometric excess of air can also be called a reduction of the stoichiometric excess of oxygen, or a reduction of the stoichiometric excess of cathode. As can be seen from Diagram A, such a reduction in air supply, i.e., a reduction in the stoichiometric excess of air, causes the cell voltage to decrease stepwise on average, although this stepwise decrease is accompanied by significant fluctuations in the cell voltage. These fluctuations can cause damage to the fuel cell, such as the dissolution of platinum and / or corrosion of the bipolar separator and / or corrosion of carbon, which is one of the components of the catalyst layer, and / or deterioration of the membrane due to the appearance of overheating areas.

[0131] Theoretically, when the stoichiometric excess of air is reduced, the stoichiometric coefficient is greater than or equal to 1, so the fuel cell has enough oxygen to produce the desired current density at a stable operating point, that is, enough oxygen to produce a stable voltage at the fuel cell terminals. This stable operating point is determined by the polarization diagram of the fuel cell 12, depending on the generation of the desired current density. However, in practice, when the cathode stoichiometric coefficient is lower than the nominal cathode stoichiometric coefficient, and even closer to 1, a local deficiency of oxygen molecules is observed at the active site of the cathode catalyst layer. In order to reach the active site, oxygen molecules must pass through the gas diffusion layer 18 and then enter the cathode catalyst layer 24, which requires a certain amount of travel time. Therefore, the inventors observed that when an oxygen molecule reaches an active site, it is immediately consumed in a reduction reaction, and this reduction reaction generates an attractive force that guides other oxygen molecules toward the active site. However, since there is no excess supply of oxygen, these other oxygen molecules do not reach the active site until a certain period has elapsed, during which time a deficiency of reactants is observed, meaning that no reduction reaction occurs at the active site during this period. For each active site, there are alternating periods in which a reduction reaction occurs and periods in which no reduction reaction occurs. This alternation at the level of cell 12 and at the level of fuel cell 10 results in fluctuations in the cell voltage as shown in Figure A. In practice, selecting a nominal stoichiometric coefficient greater than 1, for example 1.8, also helps to avoid such fluctuations, because the presence of an excess of oxygen ensures a continuous supply of oxygen to the active site.

[0132] Furthermore, Figure A shows the gradual decrease in cell voltage after this fluctuation has been averaged. This gradual decrease, associated with the reduction in the stoichiometric excess of air, is caused by reversible and / or irreversible degradation occurring within the fuel cell. These degradations are caused by cell voltage fluctuations, particularly by localized deficiencies in the humidification of the proton exchange membrane 20 in each cell 12, which in turn are caused by repeated localized deficiencies in the reduction reaction at the active sites. In addition, the cell voltage fluctuations result in fluctuations in the efficiency of the fuel cell 10, which in turn causes fluctuations in the fuel cell temperature. These fluctuations cause the fuel cell to deviate from its optimal operating parameters, which in turn leads to observed degradation. Moreover, it can be seen that the average cell voltage observed in Figure A tends to decrease and then stabilizes at a value above 0V. In practice, as the cathode stoichiometric coefficient approaches 1 while remaining greater than 1, the average cell voltage observed in diagram A, averaged over time intervals containing multiple fluctuations, tends to converge more asymptotically. This asymptotic value decreases as the cathode stoichiometric coefficient approaches 1 while remaining greater than 1, but it never becomes zero. Simultaneously, as the cathode stoichiometric coefficient approaches 1 while remaining greater than 1, the cell voltage fluctuates more significantly around this average value, and the amplitude of these fluctuations increases as the cathode stoichiometric coefficient approaches 1 while remaining greater than 1. Finally, as the cathode stoichiometric coefficient approaches 1 while remaining greater than 1, the minimum cell voltage observed when diagram A fluctuates tends to approach 0V.

[0133] It should be noted that this fluctuation phenomenon is not observed in Diagram B when the air supply is completely cut off. This is because, since no new oxygen molecules are supplied to the cathode compartment, the attractive force generated at each active site cannot draw new oxygen molecules toward the active site. Therefore, in Diagram B, the cell voltage gradually decreases as each active site consumes more oxygen molecules.

[0134] Furthermore, in Figure 4, diagram C schematically shows the decrease in cell voltage observed when the air supply is partially cut off and the cathode stoichiometric coefficient is strictly between 0 and 1. In the example of diagram C, the cathode stoichiometric coefficient is 0.5. As a result of partially cutting off the air supply, a partial supply of air is maintained, but this is insufficient to maintain the cell voltage. In reality, the oxygen molecules supplied by the air inlet 32 ​​are consumed more and more rapidly as the oxygen present in the cathode compartment at the start of process 5 is consumed. As a result, oxygen molecules are consumed at active sites closer to the air inlet 32 ​​and therefore cannot reach active sites further away from the air inlet. Consequently, there is an increasingly large no-current zone for each cell 12. In reality, when the air supply is partially cut off, the decrease in cell voltage takes longer than when it is completely cut off. In other words, the decrease in cell voltage is slower because a partial air supply is maintained. In addition, fluctuations with amplitudes smaller than those observed in diagram A may also be observed. Because these fluctuations are not very significant, their effects cause little to no damage to the fuel cell.

[0135] Therefore, in order to avoid or significantly limit cell voltage fluctuations and associated damage, it is particularly advantageous to completely shut off the air supply, as done in the present invention, or to at least partially shut off the air supply to a degree sufficient to keep the cathode stoichiometric coefficient strictly less than 1, preferably 0.9 or less. Thus, by using a cathode stoichiometric coefficient strictly less than 1, preferably 0.9 or less, during the oxygen depletion process 5, and even more so if the air supply is completely shut off, damage to the fuel cell is prevented.

[0136] The diagrams in Figure 4 show the trends in cell voltage when different cathode stoichiometric coefficients are applied. However, it should be noted that these diagrams are not derived from actual data and are therefore provided solely to illustrate the phenomena described above. In particular, diagrams A, B, and C in Figure 4 do not represent actual cell voltage values.

[0137] The oxygen depletion process 5 terminates as soon as the cell voltage reaches a predetermined threshold voltage. In practice, this threshold voltage is 0.1V to 0.4V. In this example, this threshold voltage is 0.2V. This minimum voltage threshold is selected to prevent damage to the cells 12 of the fuel cell 10. If the cell voltage is too low, the cells are generally damaged, for example, by the formation of excessive amounts of hydrogen peroxide, which is harmful to cells 12. By selecting to stop the oxygen depletion process when this threshold voltage is reached, these degradations are avoided.

[0138] In practice, the time required for oxygen depletion step 5 is 1 to 120 seconds, preferably 30 to 60 seconds. In this example, this time is approximately 45 seconds. Furthermore, this time tends to vary from cycle to cycle.

[0139] In addition, the time required for oxygen depletion process 5 is particularly dependent on the current density during this process. The higher the current density, the shorter the time required for oxygen depletion process.

[0140] As shown in Figure 5, during the oxygen depletion process 5, there is a gradient in the amount of oxygen available in the cathode section, with more oxygen available at the air inlet 32 ​​than at the air outlet 34.

[0141] In reality, during the oxygen depletion process 5, oxygen consumption is relatively uniform within the cathode compartment from the air inlet 32 ​​to the air outlet 34. However, when the fuel cell 10 is operating normally with a stoichiometric coefficient strictly greater than 1, such as during the stabilization process 4, an oxygen concentration gradient is always observed, with the oxygen concentration higher at the air inlet than at the air outlet. This gradient is caused by the gradual consumption of oxygen due to electrochemical reactions occurring in the layers of the cathode catalyst layer 24 as air moves through the cathode compartment in the direction of the airflow between the air inlet and the air outlet. During normal operation of the fuel cell, although this oxygen concentration gradient exists, there is a sufficient amount of oxygen to ensure that the electrochemical reactions proceed, including in the active zone closest to the air outlet, so this gradient does not affect the operation of the fuel cell. When the air supply is partially or completely cut off, at the start of the oxygen depletion process 5, the supplied oxygen is no longer sufficient to replenish the consumed oxygen, and a decrease in oxygen concentration is observed. Therefore, although this decrease is relatively uniform within the cathode section, the oxygen concentration reaches zero more rapidly at the air outlet 34 than at the air inlet 32 ​​due to the oxygen concentration gradient observed before the start of the oxygen depletion process.

[0142] Therefore, during the oxygen depletion process 5, zero oxygen concentration exists near the air outlet 34, but a non-zero oxygen concentration exists near the air inlet 32.

[0143] Therefore, in the half of the cathode compartment closest to the air inlet 32, labeled "C" in Figure 5, a normal reduction reaction occurs, in which available oxygen is consumed and water molecules are formed.

[0144] On the other hand, in the half of the cathode compartment closest to the air outlet 34, labeled "D" in Figure 5, i.e., where the oxygen concentration is lowest or zero, an electrochemical reaction is observed. In this electrochemical reaction, protons that have passed through the membrane 20 and electrons coming from the cathode-side surface 16B of the adjacent bipolar plate 16 are consumed to form dihydrogen molecules. This is because, in the absence of oxygen molecules, the reduction reaction that normally occurs in the cathode compartment cannot take place, and electrons and protons react together to form dihydrogen. This phenomenon is known as the "proton pump."

[0145] The presence of dihydrogen in the cathode compartment and a low cell voltage create strong reduction conditions within half D of the cathode compartment, allowing for the reduction of specific impurities and platinum oxides located within the cathode catalyst layer 24 in this half of the compartment. As a result of these reduction reactions, impurities are desorbed, with platinum oxides being reduced to platinum particles on the one hand and impurities being desorbed on the other. This increases the amount of non-oxide platinum available in the cathode catalyst layer, and consequently the number of active sites, while simultaneously decreasing the number of impurities in the cathode catalyst layer.

[0146] In practice, the reduction conditions obtained during the oxygen depletion step 5 are stronger than those obtained during the humidification step 2, and therefore impurities can be more effectively desorbed from the cathode catalyst layer, particularly in half of the cathode section D during this first break-in stage P2. In particular, the desorption of impurities that cannot be desorbed during step 2 due to insufficient reduction conditions is made possible by step 5. Most of these desorbed impurities are removed from the cathode section during steps 1 and 2.

[0147] The oxygen depletion process 5 also humidifies the proton exchange membrane 20. In fact, the absence of oxygen in the cathode compartment, particularly in the half of the compartment closest to the air outlet 34, increases the fraction of water molecules at the interface between the membrane and the cathode catalyst layer, and the absence of airflow in the cathode compartment prevents water from being discharged. This increases the amount of water in the membrane, enabling humidification of the membrane. More precisely, if the oxygen in the air is consumed without being regenerated, or if it is regenerated but this regeneration is slower than consumption, the fraction of water molecules in the air-water mixture composition present in the cathode compartment becomes larger.

[0148] In addition, the lack of air supply while the supply of dihydrogen is maintained increases the pressure difference between the anode and cathode compartments. This pressure difference causes new pores or channels to open in the anode catalyst layer 22 and the cathode catalyst layer 24, and expands existing pores.

[0149] Advantageously, in order to further force the pores in the anode and cathode catalyst layers to open and to obtain stronger reduction conditions, the difference between the pressure in the anode and cathode compartments can be further increased to, for example, 500 millibars by adjusting the pressure setting point of the test bench, i.e., by adjusting the dihydrogen pressure in the cathode compartment. The hydrogen and air pressures can be controlled independently of the hydrogen and air flow rates by controlling the hydrogen inlet 28 and air inlet 32, and the hydrogen outlet 30 and air outlet 34.

[0150] The completion of oxygen depletion process 5 marks the end of the cycle in the first break-in phase P2.

[0151] At the end of the cycle in the first break-in phase P2, it is determined whether the first stop condition has been met. If the first stop condition has been met, the first break-in phase P2 ends, and the break-in process proceeds to the reverse operation P3.

[0152] If the first stop condition is not met, the first break-in phase P2 proceeds to a new cycle, in which steps 1 to 5 are performed again. Subsequently, the supply of air to the fuel cell 10 is restored, thereby ending the oxygen depletion process and allowing a new cycle to begin. Furthermore, as can be seen from Figure 3, when a new cycle of the first break-in phase P2 is performed, the oxygen depletion process 5 includes an increase in current density from minimum to low. This increase in current density occurs simultaneously with the restoration of air supply and takes place immediately before the switch to a new cycle.

[0153] Preferably, the following: - If the cell voltage at the end of humidification step 2 of the current cycle differs from the cell voltage at the end of humidification step 2 of the preceding cycle by a value of 1mV to 10mV, preferably 5mV, - Preferably, the first break-in stage P2 includes a stabilization stage 4, and the cell voltage at the end of the stabilization stage 4 of the current cycle differs from the cell voltage at the end of the stabilization stage 4 of the preceding cycle by a value of 1mV to 10mV, preferably 5mV. The first stopping condition is reached.

[0154] Therefore, this stopping condition is the cell voltage stabilization condition.

[0155] Advantageously, the first stopping condition is implemented not at the end of the stabilization process 4, but at the end of the humidification process 2, by comparing voltage measurements, i.e., cell voltage measurements or voltage measurements at the fuel cell terminals. This is because cell voltage measurements or voltage measurements at the fuel cell terminals are more accurate as the amount of current flowing increases. As the amount of current flowing decreases, the losses observed in the fuel cell 10 become more difficult to identify, thereby reducing the accuracy of cell voltage measurements or voltage measurements at the fuel cell terminals. The cell voltage of the fuel cell can be measured by measuring the voltage at individual cells, preferably by averaging the voltages measured at several cells, or by dividing the voltage at the fuel cell terminals by the number of cells in the fuel cell stack.

[0156] In practice, other conditions can be used. For example, the stop condition could correspond to reaching a minimum output standard, or it could be selected as the completion of a certain number of predetermined cycles.

[0157] Therefore, if the stopping conditions are not based on a comparison of the output between two consecutive cycles, the first break-in stage P2 may include a single cycle. It is also possible to choose not to specify specific stopping conditions and to perform only one cycle in the first break-in stage P2 before starting the break-in process.

[0158] In the example shown in Figure 2, the first stop condition is reached after 5 cycles of processes 1 to 5.

[0159] During the reversal operation P3, the electrical load is controlled so that the fuel cell 10 does not need to generate current, and the fuel cell is not supplied with reactants, namely dihydrogen and oxygen. In other words, the fuel cell is not operating during this stage. Therefore, at the end of the last cycle of stage P2, the fuel cell is stopped as soon as the cell voltage reaches a predetermined threshold voltage, although it is still possible to maintain the flow of coolant through the fuel cell to control the temperature of the fuel cell if necessary.

[0160] The reversal operation P3 consists of reversing, or exchanging, the dihydrogen inlet 28 and dihydrogen outlet 30 on the one hand, and the air inlet 32 ​​and air outlet 34 on the other hand. In other words, during the reversal operation P3, the direction of the flow of reactants in the fuel cell 10 is reversed. This reverses the flow direction of reactants in the channels 44 on the anode 16A side and the cathode 16B side of the bipolar plate 16.

[0161] In practice, it is possible to reverse the inlet and outlet using several methods.

[0162] The first method involves manually disconnecting the piping connected to the hydrogen and air supply circuits, and then reconnecting these pipes by reversing the inlet and outlet. Preferably, when this method is used, any chemical reactions in the catalyst layers 22 and 24 are prevented by purging the cells 12 of the fuel cell 10 with a neutral gas, preferably nitrogen, before disconnecting the piping, and then, before restarting the fuel cell, the accurate supply of reactants to the fuel cell is ensured by purging dihydrogen into the anode section and oxygen into the cathode section.

[0163] Another method involves using four-way valves to connect inlets 28 and 32 and outlets 30 and 34 to the dihydrogen and air supply circuits, thereby allowing the dihydrogen inlet 28 to be swapped with the dihydrogen outlet 30 and the air inlet 32 ​​with the air outlet 34 without disconnecting and reconnecting the piping. This method using four-way valves is advantageous because the valves prevent gases other than reactive gases from entering the anode and cathode compartments. Furthermore, switching with four-way valves eliminates the need to purge the circuit.

[0164] When the reversal operation P3 is performed, the fuel cell 10 is restarted and the second break-in operation phase P4 begins.

[0165] The second break-in stage P4 includes several consecutive steps. These steps are carried out in a cyclical manner and are preferably the same as the steps of the first break-in stage P2. Therefore, each cycle of the second break-in stage P4 preferably includes a current density increase step 1, a humidification step 2, a current density decrease step 3, a stabilization step 4, and an oxygen depletion step 5.

[0166] These steps are identical to those in the first break-in stage P2, and therefore will not be described in further detail below.

[0167] Alternatively, the cycle of the second break-in stage P4 differs from the cycle of the first break-in stage P2 in that the cycle does not include the same step between steps 1 to 5. For example, each cycle of the first break-in stage P2 includes steps 1, 2, 3, 4, and 5, and each cycle of the second break-in stage P4 includes steps 1, 2, 3, and 5, and therefore does not include stabilization step 4. In another example, each cycle of the first break-in stage P2 includes steps 1, 2, 3, and 5, and therefore does not include stabilization step 4, and each cycle of the second break-in stage P4 includes steps 1, 2, 3, 4, and 5. In yet another example, each cycle of the first break-in stage P2 includes steps 4 and 5, and each cycle of the second break-in stage P4 includes only step 5, and therefore does not include stabilization step 4. In another example, each cycle of the first break-in stage P2 includes only step 5 and therefore does not include stabilization step 4, while each cycle of the second break-in stage P4 includes steps 4 and 5.

[0168] Therefore, the second break-in stage P4 offers the same advantages as the first break-in stage P2, namely, by alternating steps 1 to 5, it becomes possible to humidify the film 20, increase the number of active sites in the catalyst layers 22 and 24, particularly by changing their porosity, reduce, oxidize and desorb specific impurities, and remove these impurities from the catalyst layers.

[0169] It is particularly advantageous to carry out the first break-in operation stage P2, the reverse operation P3, and the second break-in operation stage P4 in sequence, because this sequence makes it possible to obtain a uniform break-in operation across the entire surface of the film electrode assembly 14.

[0170] At the end of the first break-in stage P2, the cathode catalyst layer 24 is not uniformly broken in. This is because, during the oxygen depletion process 5, there is a gradient in the amount of oxygen available between half of the cathode section, C and D, which results in reduction conditions occurring only in half of the cathode section D, as explained earlier. Therefore, the oxygen depletion process 5 of the first break-in stage P2 is mainly effective in removing impurities from half of the cathode section D.

[0171] However, since the second break-in stage P4 is performed after the reversal operation P3, the gradient of available oxygen observed during the oxygen depletion process 5 of the second break-in stage is reversed compared to the gradient of available oxygen observed during process 5 of the first break-in stage P2.

[0172] In other words, the imbalance in impurity desorption observed during the first break-in phase is also observed during the second break-in phase, but in reverse: during the first break-in phase, impurity desorption mainly occurs in the first physical half of the cathode compartment, while during the second break-in phase, impurity desorption mainly occurs in the second physical half of the cathode compartment.

[0173] Therefore, during the second break-in phase P4, the oxygen depletion process 5 makes it possible to desorb impurities mainly from half of the cathode section C.

[0174] Therefore, the sequence of steps P2, P3, and P4 allows for the uniform desorption of impurities across the entire surface of the cathode catalyst layer 24, thereby uniformly improving the output of the film electrode assembly 14.

[0175] In addition, this sequence is particularly advantageous for optimizing the time required for the break-in protocol, as it enables complete and uniform cell break-in across the entire surface of the cathode catalyst layer 24 and is faster than known break-in protocols.

[0176] At the end of the cycle in the second break-in phase P4, it is determined whether the second stop condition has been met. If the second stop condition has been met, the second break-in phase P4 ends, and in some embodiments, the break-in process proceeds to the control phase P5.

[0177] If the second stop condition is not met, the second break-in phase P4 proceeds to a new cycle, in which steps 1 to 5 are performed again, with the reactive gas flow in the same direction as during the first cycle of this second break-in phase P4. Subsequently, the supply of air to the fuel cell 10 is restored, thereby ending the oxygen depletion process and enabling the start of a new cycle.

[0178] Preferably, the following: - If the cell voltage at the end of humidification step 2 of the current cycle differs from the cell voltage at the end of humidification step 2 of the preceding cycle by a value of 1mV to 10mV, preferably 5mV, - When the cell voltage at the end of the current cycle's stabilization step 4 differs from the cell voltage at the end of the preceding cycle's stabilization step 4 by a value of 1mV to 10mV, preferably 5mV. The second stopping condition is reached.

[0179] Therefore, this stopping condition is the cell voltage stabilization condition.

[0180] Advantageously, the second termination condition is implemented not at the end of stabilization step 4, but at the end of humidification step 2, by comparing the cell voltage measurement or the voltage measurement at the fuel cell terminals. This is because the cell voltage measurement or the voltage measurement at the fuel cell terminals becomes more accurate as the amount of current flowing increases. As the amount of current flowing decreases, the losses observed in the fuel cell 10 become more difficult to identify, which reduces the accuracy of the cell voltage measurement or the voltage measurement at the fuel cell terminals.

[0181] In practice, other conditions can be used. For example, the stop condition could correspond to reaching a minimum output standard, or it could be selected as the completion of a certain number of predetermined cycles.

[0182] Therefore, if the stopping conditions are not based on a comparison of the output between two consecutive cycles, the second break-in stage P4 may include a single cycle. It is also possible to choose not to specify specific stopping conditions and to perform only one cycle in the first break-in stage P2 before starting the break-in process.

[0183] Preferably, as in this example, the second stopping condition is the same as the first stopping condition.

[0184] Preferably, as in this example, the second break-in stage P4 includes the same number of cycles as the first break-in stage P2.

[0185] In the example shown in Figure 2, the first stop condition is reached after 5 cycles of processes 1 to 5.

[0186] During any control phase P5, the electrical load imposes a constant current density on the fuel cell 10 for a predetermined period, and the cell voltage changes are observed. This optional P5 control phase is used to confirm that the battery output is completely stable. If the cell voltage is observed to remain stable throughout the entire P5 control phase, it is confirmed that the break-in protocol has been correctly implemented and the fuel cell is ready to begin operation.

[0187] The duration of any P5 control stage is preferably 45 to 75 minutes. In this example, this duration is 60 minutes. The current density during control stage P5 is preferably 0.3 A / cm². 2 ~1.9A / cm 2 In this example, the current density is 1 A / cm². 2Furthermore, in this example, the P5 control stage takes 60 minutes. The time required for each control stage may vary. Preferably, the time required for each control stage is longer than 10 minutes, for example, 10 to 100 minutes.

[0188] In addition, by measuring the cell voltage during the test phase, the power increase of the fuel cell 10 provided by the break-in protocol can be measured.

[0189] For example, if the effective surface area of ​​each membrane electrode assembly 14 is 250 cm² 2 In a fuel cell formed by a stack of cells 12, the electrical load is 1 A / cm² to the fuel cell for a total break-in period of 20 to 300 minutes. 2 When a current density is applied, the power increase obtained by the break-in protocol for the cell voltage of the fuel cell is 10% to 50%, for example, 30%. The power increase is preferably measured by comparing the amount of electrical energy generated by the fuel cell 10 per kilogram of dihydrogen consumed by the electrochemical reaction before and after the break-in protocol is implemented, and the electrical energy generated by the fuel cell is measured, for example, in Wh units.

[0190] In a method that is known in itself, the current density generated by the fuel cell is actually imposed and controlled by the electrical load connected to the terminals of the fuel cell 10, and therefore, based on Faraday's law, this current density can be considered independent of the amount of reactants supplied by the anode and cathode compartments, as long as the amount of reactants supplied by the anode and cathode compartments is sufficient to drive the electrochemical reactions occurring in the anode and cathode compartments in the anode catalyst layer 22 and the cathode catalyst layer 24, that is, as long as the flow rate of the reactive gas supplied to the dihydrogen inlet 28 and the air inlet 32 ​​is sufficient. In other words, the current density does not increase even if there is a surplus of hydrogen and air. Conversely, if hydrogen and / or air, and therefore oxygen, are deficient, the cell voltage decreases, as in the example of the oxygen depletion process 5 in the first and second break-in stages P2 and P4.

[0191] In practice, Faraday's law stipulates that the current supplied by an electrochemical reaction is a direct indicator of the rate of the electrochemical reaction, that is, the number of moles of reactant consumed per unit time. Therefore, the current i supplied by the electrochemical reaction is: i = n × F × ν Here, i is the electric current in amperes, n is the number of electrons moved by the electrochemical reaction, F is the Faraday constant, which is approximately equal to 96,485 C / mol (coulombs / mol), and ν is the reaction rate expressed in mol / s (moles per second), which is also known as the number of moles consumed per unit time and corresponds to the rate at which the reactants are consumed.

[0192] Therefore, as long as a sufficient molar flow rate of the reactants is ensured at the active site, the reaction rate ν, and thus the molar flow rate of the reactants consumed, depends on the current supplied by the electrochemical reaction occurring at the active site and does not change if there is a surplus of reactants.

[0193] Advantageously, throughout the break-in phases P2 and P4, the anodic stoichiometric coefficient remains constant, equal to the nominal anodic stoichiometric coefficient of the fuel cell 10, preferably 1.3 to 1.5, and more preferably 1.5. This relatively low value limits the consumption of dihydrogen during the break-in process, thereby reducing the cost of implementing the break-in process.

[0194] Advantageously, throughout the break-in phases P2 and P4, the cathode stoichiometric coefficient remains constant and is greater than the nominal cathode stoichiometric coefficient of the fuel cell 10, except during the oxygen depletion phase 5 when the cathode stoichiometric coefficient is strictly less than 1, preferably less than 0.9, and more preferably 0. Preferably, throughout the break-in phases P2 and P4, except during the oxygen depletion phase 5, the cathode stoichiometric coefficient is greater than 2 for fuel cells where the humidity level in the cathode compartment is 30% to 80%. In this example, the cathode stoichiometric coefficient is 2.3.

[0195] This relatively high cathode stoichiometric coefficient shortens the time required for the break-in protocol. The high flow rate of oxygen, and even air, within the cathode compartment increases the gas flow rate within the pores of the cathode catalyst layer, thereby expanding existing pores or channels and opening new channels within the cathode catalyst layer, thus increasing the number of active sites within the cathode catalyst layer. This high flow rate also assists in the removal of impurities desorbed from the cathode catalyst layer 24 throughout the entire break-in process. This removal of impurities is particularly effective during the humidification phase 2 of break-in phases P2 and P4. Therefore, this relatively high cathode stoichiometric coefficient also imposes a relatively high flow rate in the cathode, thereby improving the response time of the fuel cell and thus shortening the time required for the current density increase phase 1 of break-in phases P2 and P4.

[0196] Compared to the break-in process of the previous process, the break-in process of the present invention has many advantages.

[0197] Firstly, the break-in process is particularly effective in improving the output of the fuel cell 10 without causing damage that could shorten the lifespan of the fuel cell. In particular, by alternating between the oxygen depletion process 5 and the low cell voltage process, i.e., the humidification process 2, the desorption of impurities and contaminants present in the catalyst layers 22 and 24 is promoted, and it becomes possible to continuously reduce and oxidize the maximum number of impurities. This process also makes it possible to use all means to improve the output of the fuel cell 10, namely, humidification of the proton exchange membrane 20, increasing the number of active sites in the catalyst layer, and desorption and removal of impurities and contaminants present in and on the surface of the catalyst layer. Damage to the fuel cell is prevented in particular by the stabilization process 4, which makes it possible to continue with the humidification process 2 and the oxygen depletion process 5 without creating unfavorable operating conditions for the fuel cell, and it is also possible to avoid cell voltage fluctuations that are harmful to the fuel cell by the whole or partial cutoff of air, and therefore oxygen, as defined above, which is carried out during the oxygen depletion process.

[0198] Next, the break-in process makes it possible to achieve uniform output across the entire surface of the membrane electrode assembly 14, particularly through the reversal operation P3, which eliminates the lack of uniformity. This lack of uniformity is specifically caused by the oxygen depletion process 5, which is commonly seen in fuel cells. Reversal is also beneficial for the humidification process 2 and the stabilization process 4. In fact, as mentioned above, considering that an oxygen concentration gradient still exists within the cathode compartment, performing the stabilization process 4 before and after the reversal operation P3 also makes it possible to obtain more uniform humidification of the membrane and more uniform oxidation / reduction of impurities.

[0199] Furthermore, the break-in period of the break-in process is advantageously short. This is due to the sequence of steps that are effective in reducing and then oxidizing impurities in the catalyst layer, particularly the sequence of steps 2 (humidification) and 5 (oxygen depletion).

[0200] In addition, dihydrogen consumption is kept under control throughout the entire break-in process. This is particularly due to the relatively low anodic stoichiometric coefficient and the short overall time spent in the high-current phase, which consumes more reactants. This means that less dihydrogen is used in the break-in process, which is advantageous considering that dihydrogen is generally an expensive gas.

[0201] The break-in protocol helps achieve catalyst layer morphologies that are not possible with protocols that do not involve oxygen depletion or gas inversion. The catalyst layer and film morphologies obtained through this process enable levels of cell efficiency and durability that are not possible with protocols that do not involve oxygen depletion or gas inversion. This preferred morphology corresponds in particular to a larger total active surface area of ​​platinum, which is mainly promoted by the oxygen depletion step 5 at the cathode.

[0202] Finally, the break-in method is particularly easy to set up. The break-in process requires no special additional components. This is because the entire break-in process can be carried out by applying an electric cycle to control the arrival of the reactants, especially during the oxygen depletion process and the reverse operation P3. Therefore, implementation is economical.

[0203] This specification has primarily discussed how increasing the number of active sites, increasing its porosity, and desorbing impurities can improve the output of the cathode catalyst layer 24. However, it has hardly discussed the improvement of the output of the anode catalyst layer 22. In practice, for several reasons, it is known that achieving good output from the anode catalyst layer is easier than achieving good output from the cathode catalyst layer. First, the anode section is generally supplied with pure gas, especially dihydrogen, resulting in less exposure to contaminants than the cathode section, which is supplied with air that can generally contain many contaminants. Furthermore, the cathode catalyst layer is thicker and contains more platinum particles than the anode catalyst layer, so increasing the number of active sites and desorbing impurities takes longer. Finally, oxygen tends to deposit on the cathode catalyst layer to form platinum oxides, which need to be reduced during the break-in process, whereas this does not occur in the anode catalyst layer. Thus, the target output improvement is easier to achieve for the anode catalyst layer than for the cathode catalyst layer. This means that if the cathode catalyst layer achieves a satisfactory level of output, the same will inevitably apply to the anode catalyst layer.

[0204] It should be noted that the concepts of high, low, and minimum current density used herein are for convenience only and have meaning only in relation to each other within the context of the break-in process described herein, regardless of the corresponding actual current density values ​​in a particular application. Herein, the high value is strictly greater than the low value, and the minimum value is less than or equal to the low value. Therefore, the high and minimum values ​​are defined in relation to the low value. Alternatively, the low value may be designated as the first predetermined value, the minimum value as the second predetermined value, and the high value as the third predetermined value.

[0205] In one variant of the present invention not shown, the high, low, and minimum values ​​of the first break-in stage P2 are different from those of the second break-in stage P4. For example, the high, low, and minimum values ​​of the second break-in stage P4 are higher or lower than those of the first break-in stage P2 to account for changes in the output of the fuel cell 10 during the break-in process. Such changes in these current density values ​​can be determined to allow the same cathode potential level to be reached for different processes and / or different stages. This is particularly to obtain a desired oxidation level at low current density values ​​and a desired reduction level at high current density values, and, if possible, to ensure that the level remains the same throughout the entire activation cycle in accordance with the transition of the fuel cell's characteristics during activation.

[0206] In one variant of the present invention not presented, the break-in process does not include the initialization step P1. In such a variant, the fuel cell 10 is initialized, for example, at the end of its assembly, before being placed on the activation bench. It is also possible to omit the initialization step P1 and have the first cycle of the first break-in step P2 include the current density increase step 1, but it is preferable to make this current density increase step 1 sufficiently long so as to prevent damage to the fuel cell.

[0207] In one variant of the present invention not presented, the break-in process does not include control step P5. In such a variant, the stability of the cell voltage can be evaluated by other means, or may not be evaluated until the fuel cell is operational.

[0208] In one variant of the present invention not presented, the break-in stages P2 and P4 do not include steps 1, 2, and 3. Therefore, in this variant, the break-in stage includes only the stabilization 4 and oxygen depletion 5 steps. This variant has the advantage that the amount of dihydrogen consumed by the break-in process is minimized because the fuel cell is not constrained by an electrical load to a high current density that would lead to high consumption of reactants. However, in this variant, the break-in process takes a relatively long time. In particular, the break-in stages P2 and P4 include a larger number of cycles. Although humidification of the proton exchange membrane 20 is still achieved by the oxygen depletion step 5 even without the humidification step 2, a longer break-in time is required.

[0209] In one variant of the present invention not presented, the airtightness of the cell stack obtained by the end plate can be monitored throughout the break-in process and adjusted to maintain a constant airtightness of the cell stack, thereby allowing for potential variations in cell dimensions during the break-in period. Humidification of the proton exchange membrane 20 and physicochemical modifications of the catalyst layers 22 and 24 may cause variations in the thickness of these components during the break-in process.

[0210] In one variant of the present invention not presented, the temperature inside cell 12 can be raised to 100°C during the humidification stage 2 of break-in stages P2 and P4, either throughout these stages or for a short period between these stages, thereby preventing the re-adsorption of impurities and contaminants in catalyst layers 22 and 24. By increasing the relative humidity of the reactive gas, this temperature increase prevents flooding of the proton exchange membrane.

[0211] In this example, the predetermined threshold voltage remains constant throughout the entire break-in process, i.e., it is identical for all oxygen depletion steps 5 of break-in stages P2 and P4. In one non-representative variant of the present invention, slight variations in the predetermined threshold voltage can be provided during the break-in process, which can be done, for example, by making it slightly higher or lower during the second break-in stage P4 than during the first break-in stage P2. However, these variations are sufficiently small that the predetermined threshold voltage in the first break-in stage P2 is substantially equal to the predetermined threshold voltage in the second break-in stage P4.

[0212] A particularly advantageous embodiment of the present invention corresponds to a break-in process for a fuel cell 10, wherein the fuel cell 10 comprises a stack of cells 12, each cell comprising a proton exchange membrane 20 disposed between two bipolar plates 16, each bipolar plate 16 defining a reaction compartment together with the proton exchange membrane 20 within the cell, and thus each cell comprises a cathode compartment in which a cathode catalyst layer 24 is disposed and an anode compartment in which a cathode catalyst layer 22 is disposed.

[0213] The fuel cell includes a dihydrogen inlet 28 that supplies dihydrogen to the anode section of each cell, and a dihydrogen outlet 30 that discharges dihydrogen from each cell. The fuel cell also includes an air inlet 32 ​​that supplies air to the cathode section of each cell 12, and an air outlet 34 that discharges air from each cell 12.

[0214] The break-in process according to this particularly convenient embodiment of the present invention comprises at least the following steps: - The first break-in stage P2, comprising at least the following steps: • A stabilization step 4 for the fuel cell 10, comprising the step of maintaining the current density obtained by the fuel cell at a low and constant value for a predetermined period of time during the stabilization step 4. • A fuel cell oxygen depletion step 5, wherein during the oxygen depletion step 5, the current density obtained by the fuel cell is kept constant at a minimum value, the minimum value being less than or equal to the low value, and during the oxygen depletion step 5, the supply of air to the fuel cell through the air inlet 32 ​​is at least partially cut off, the cutoff is set so that a cathode stoichiometric coefficient of exactly less than 1, preferably 0.9 or less is obtained, and the oxygen depletion step of the first break-in operation stage ends when the cell voltage of the fuel cell 10 reaches a predetermined threshold voltage, and The first break-in stage P2 includes the following in this order: - Operation P3 to reverse the direction of hydrogen flow and the direction of air flow, reversing the dihydrogen inlet 28 and dihydrogen outlet 30, and reversing the air inlet 32 ​​and air outlet 34, - The second break-in phase P4, comprising at least the following steps: • A stabilization step 4 for the fuel cell 10, comprising the step of maintaining the current density obtained by the fuel cell at a low and constant value for a predetermined period of time during the stabilization step 4. • A fuel cell oxygen depletion step 5, wherein during the oxygen depletion step 5, the current density obtained by the fuel cell is kept constant at a minimum value, the minimum value being less than or equal to the low value of the second break-in operation stage, the supply of air to the fuel cell through the air inlet 32 ​​is at least partially cut off during the oxygen depletion step 5, the cutoff is adjusted so that a cathode stoichiometric coefficient of exactly less than 1, preferably 0.9 or less is obtained, and the oxygen depletion step of the second break-in operation stage ends when the cell voltage of the fuel cell 10 reaches a predetermined threshold voltage. The second break-in stage P4 includes the following in this order: It includes them in this order.

[0215] Furthermore, each step of the first break-in phase P2 is carried out until the first stop condition is reached, and the steps of the second break-in phase P4 are carried out until the second stop condition is reached.

[0216] Any feature described for one embodiment or variant described above can be implemented for the other embodiments and variants described above, insofar as it is technically feasible.

Claims

1. A method for breaking in a fuel cell (10), wherein the fuel cell comprises a stack of cells (12), each cell comprising a proton exchange membrane (20) disposed between two bipolar plates (16), each bipolar plate (16) together with the proton exchange membrane (20) defining a reaction section within the cell, and thus each cell comprises a cathode section in which a cathode catalyst layer (24) is disposed, and an anode section in which an anode catalyst layer (22) is disposed. The fuel cell comprises a dihydrogen inlet (28) for supplying hydrogen to the anode section of each cell, and a dihydrogen outlet (30) for discharging hydrogen from each cell. In a method for breaking in a fuel cell (10), the fuel cell (10) is provided with an air inlet (32) for supplying air to the cathode section of each cell (12) and an air outlet (34) for discharging air from each cell (12). The aforementioned break-in process comprises at least the following steps: - A first break-in operation stage (P2), the first break-in operation stage (P2) includes at least one oxygen depletion step (5) of the fuel cell, during which the current density obtained by the fuel cell is kept constant at a minimum value, and during the oxygen depletion step (5), the cell voltage is gradually reduced by at least partially cutting off the supply of air to the fuel cell through the air inlet (32), and the oxygen depletion step of the first break-in operation stage ends when the cell voltage of the fuel cell (10) reaches a predetermined threshold voltage, - An operation (P3) to reverse the direction of hydrogen flow and the direction of air flow, wherein the dihydrogen inlet (28) and the dihydrogen outlet (30) are reversed, and the air inlet (32) and the air outlet (34) are reversed, - A second break-in period (P4), the second break-in period (P4) includes at least one oxygen depletion step (5) of the fuel cell, during which the current density obtained by the fuel cell is kept constant at a minimum value, and during the oxygen depletion step (5), the cell voltage is gradually reduced by at least partially cutting off the supply of air to the fuel cell through the air inlet (32), the oxygen depletion step of the second break-in period ends when the cell voltage of the fuel cell (10) reaches a predetermined threshold voltage. It includes them in this order, The at least one step of the first break-in stage (P2) is performed until the first stop condition is reached. A method for breaking in a fuel cell (10), characterized in that at least one of the steps of the second break-in stage (P4) is performed until the second stop condition is reached.

2. A method for breaking in a fuel cell (10) according to claim 1, wherein the first break-in stage (P2) also includes a stabilization stage (4) of the fuel cell (10) before the oxygen depletion stage (5), during which the current density obtained by the fuel cell is kept at a low and constant value for a predetermined period, where the low value in the first break-in stage is greater than or equal to the minimum value, and / or the second break-in stage (P4) also includes a stabilization stage (4) of the fuel cell (10) before the oxygen depletion stage (5), during which the current density obtained by the fuel cell is kept at a low and constant value for a predetermined period, where the low value in the second break-in stage is greater than or equal to the minimum value.

3. The minimum value for the first break-in stage and the minimum value for the second break-in stage are 0.01 A / cm². ２ ~0.3 A / cm ２ The preferred current is 0.02 A / cm². ２ A method for breaking in a fuel cell (10) according to claim 1 or 2.

4. The low value in the first break-in stage (P2) and the low value in the second break-in stage (P4) are 0.03 A / cm². ２ ~0.5 A / cm ２ Preferably 0.3 A / cm ２ A method for breaking in a fuel cell (10) according to claims 2 and 3, which are considered in combination.

5. A method for breaking in a fuel cell (10) according to any one of claims 1 to 4, wherein during the at least one oxygen depletion step (5), the supply of air to the fuel cell through the air inlet (32) is at least partially cut off, and the cutoff is adjusted so that a cathode stoichiometric coefficient of exactly less than 1, preferably 0.9 or less, is obtained.

6. A method for breaking in a fuel cell (10) according to any one of claims 1 to 5, wherein the oxygen depletion step (5) of the first break-in stage (P2) and the second break-in stage (P4) is terminated when the cell voltage of the fuel cell (10) reaches a threshold voltage of 0.1V to 0.4V, preferably 0.2V.

7. A method for breaking in a fuel cell (10) according to any one of claims 1 to 6, wherein a cathode stoichiometric coefficient of 0 is obtained by completely shutting off the supply of air to the fuel cell (10) through the air inlet (32) between the oxygen depletion step (5) of the first break-in stage (P2) and the second break-in stage (P4).

8. A method for breaking in a fuel cell (10) according to any one of claims 1 to 7, wherein the supply of air to the fuel cell (10) through the air inlet (32) is controlled between the oxygen depletion step (5) of the first break-in stage (P2) and the second break-in stage (P4), and the control is performed such that the cell voltage of the fuel cell (10) decreases monotonically.

9. - The first break-in stage (P2) is further performed before the oxygen depletion process (5), which includes the following steps: - A current density increase step (1), during which the current density obtained by the fuel cell (10) is gradually increased from a low value to a high value, wherein the low value is equal to or greater than the minimum value of the first break-in operation stage, - A humidification step (2) of the fuel cell (10), wherein during the humidification step (2), the proton exchange membrane (20) is humidified by maintaining the current density obtained by the fuel cell at the high value of the first break-in operation stage for a predetermined period of time, - A current density reduction step (3), wherein during the current density reduction step (3), the current density obtained by the fuel cell is reduced from the high value in the first break-in operation stage to the low value in the first break-in operation stage. It includes them in this order, - The second break-in stage (P4) is further performed before the stabilization step (4) and the oxygen depletion step (5), which include the following steps: - A current density increase step (1), wherein during the current density increase step (1), the current density obtained by the fuel cell (10) is gradually increased from the low value of the second break-in operation stage to the high value, where the low value of the second break-in operation stage is equal to or greater than the minimum value of the second break-in operation stage, - A humidification step (2) of the fuel cell (10), wherein during the humidification step (2), the proton exchange membrane (20) is humidified by maintaining the current density obtained by the fuel cell at the high value of the second break-in operation stage for a predetermined period of time, - Current density reduction step (3), wherein during the current density reduction step (3), the current density obtained by the fuel cell is reduced from the high value in the second break-in operation stage to the low value in the second break-in operation stage. A method for breaking in a fuel cell (10) according to any one of claims 1 to 8, comprising the following in this order.

10. The high values of the first acclimation operation stage (P2) and the high values of the second acclimation operation stage (P4) are 1.5 A / cm ２ to 3 A / cm ２ , preferably 1.9 A / cm ２ The acclimation operation method of the fuel cell according to claim 9, which is as described above.

11. - The current density increase step (1), the humidification step (2), the current density decrease step (3), the stabilization step (4), and the oxygen depletion step (5) of the first break-in operation stage (P2) are performed at least twice in a cycle. - below: - When the cell voltage of the fuel cell (10) at the end of the humidification step (2) in the last cycle of the first break-in phase differs from the cell voltage of the fuel cell (10) at the end of the humidification step (2) in the second to last cycle of the first break-in phase by a value of 1 mV to 10 mV, preferably 5 mV, or - If appropriate, the cell voltage of the fuel cell (10) at the end of the stabilization step (4) of the last cycle of the first break-in phase differs from the cell voltage of the fuel cell (10) at the end of the stabilization step (4) of the second to last cycle of the first break-in phase by a value of 1 mV to 10 mV, preferably 5 mV. When the first stop condition is reached, - The current density increase step (1), the humidification step (2), the current density decrease step (3), the stabilization step (4), and the oxygen depletion step (5) of the second break-in operation stage (P4) are performed at least twice in a cycle. - below: - When the cell voltage of the fuel cell (10) at the end of the humidification step (2) in the last cycle of the second break-in phase differs from the cell voltage of the fuel cell (10) at the end of the humidification step (2) in the second to last cycle of the second break-in phase by a value of 1 mV to 10 mV, preferably 5 mV, or - If appropriate, the cell voltage of the fuel cell (10) at the end of the stabilization step (4) of the last cycle of the second break-in phase differs from the cell voltage of the fuel cell (10) at the end of the stabilization step (4) of the second to last cycle of the second break-in phase by a value of 1 mV to 10 mV, preferably 5 mV. A method for breaking in a fuel cell (10) according to any one of claims 9 or 10, wherein the second stop condition is reached.

12. The method for breaking in the fuel cell (10) according to any one of claims 9 to 11, further comprising an initialization step (P1) performed before the first break-in step (P2), wherein during the initialization step (P1), the current density obtained by the fuel cell (10) is gradually increased from zero to the high value of the first break-in step, and preferably the initialization step (P1) is performed over a period of 15 to 45 minutes, more preferably over a period of 24 minutes.

13. The method for breaking in the fuel cell (10) according to any one of claims 9 to 12, further comprising a control step (P5) performed after the second break-in step (P4), wherein during the control step (P5), the current density obtained by the fuel cell (10) is maintained at a constant level for a predetermined period, preferably 45 to 75 minutes, more preferably 60 minutes.

14. - During the first break-in period (P2) and the second break-in period (P4), the anode stoichiometric coefficient of the fuel cell (10) is equal to the nominal anode stoichiometric coefficient of the fuel cell, preferably 1.3 to 2, and more preferably 1.

5. - A method for breaking in a fuel cell (10) according to any one of claims 9 to 13, wherein during the current density increase step (1), the humidification step (2), the current density decrease step (3), and optionally the stabilization step (4) of the first break-in stage (P2) and the second break-in stage (P4), the cathode stoichiometric coefficient of the fuel cell is greater than the nominal cathode stoichiometric coefficient of the fuel cell, preferably greater than 2, and more preferably 2.

3.

15. A method for breaking in a fuel cell (10) according to any one of claims 1 to 14, wherein during the first break-in period (P2) and the second break-in period (P4), a variable resistor electrical load is connected to the terminals of the fuel cell (10), and the electrical load imposes the generation of current on the fuel cell.

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