Method of running in fuel cell

Abstract

The method of running in a fuel cell of the PEMFC type comprises at least a first running-in phase (P2), followed by an operation (P3) reversing the direction of hydrogen and air flow, followed by a second running-in phase (P4). Each running-in phase comprises a fuel cell stabilization step (4) during which the current density generated by the cell remains constant at a low value for a given period of time and a subsequent oxygen consumption step (5) during which the current density remains constant at a minimum value less than or equal to the low value, and at least partially cutting off the air supply during the oxygen depletion step (5) in order to obtain a cathode stoichiometric coefficient strictly less than 1. When the cell voltage of the fuel cell reaches a predetermined threshold voltage, the oxygen consumption steps of the first running-in phase and the second running-in phase are ended.

Description

Technical Field

[0001] The present invention relates to a method for running-in a fuel cell. Background Art

[0002] A fuel cell is a device used to generate electricity through an electrochemical reaction between a fuel (e.g., hydrogen) and an oxidant (e.g., oxygen contained in air). This invention focuses on solid electrolyte proton exchange membrane fuel cells (PEMFCs), which typically consist of a stack of individual cells, each of which constitutes an electrochemical generator.

[0003] Schematically, each unit cell comprises two separators, also called polar plates, between which a solid electrolyte in the form of a proton exchange membrane is inserted. For example, the proton exchange membrane is made of a sulfonated perfluoropolymer material. Within each cell, each separator defines a reaction compartment with a corresponding membrane. One of the two compartments (called the cathode compartment) houses a cathode part formed by a cathode catalyst layer on the surface of the membrane, while the other compartment (called the anode compartment) houses an anode part 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, commonly referred to as an "MEA".

[0004] For two adjacent cells, a separator from one cell is placed back-to-back with a separator from the other cell. Together, these two separators form a bipolar separator, also known as a bipolar plate. A cooling compartment, in which a cooling fluid such as glycolized water circulates, is typically located between the two separators of the bipolar separator.

[0005] Dihydrogen, air, and coolant are called "working" fluids, which are supplied to the fuel cell during operation. Dihydrogen and air are reactants, while 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] The fuel cell thus has openings for supplying fluid to each reaction compartment and to the fluid between two adjacent cells. Thus, in a widely used design, each bipolar separator supplies fuel on one side to the cell adjacent to that side, and air to support combustion on the other side to the cell adjacent to the other side, with the bipolar separators supplying fuel in parallel.

[0007] Generally speaking, in a unit cell, the cathode compartment is supplied with an oxidant, such as oxygen, most commonly in the form of oxygen-containing air, and the anode compartment is supplied with a fuel, such as dihydrogen.

[0008] Each reaction compartment also typically includes a gas diffusion layer between the bipolar separator and the catalytic layer, which allows good circulation of fuel or oxidant from the separator to the catalytic layer.

[0009] When a fuel cell is operating, an electrochemical reaction generates a potential difference between the two separators in each unit cell. Therefore, the fuel cell includes electrical insulation designed to prevent any electrical contact between two adjacent bipolar separators and between each cell and the external environment, as well as sealing means to prevent leakage of the working fluid, particularly preventing fluid circulating in one reaction compartment from contaminating an adjacent reaction compartment.

[0010] The potential difference between the two separators of each unit cell produces a voltage across each cell, called the “cell voltage.” All cells in a fuel cell are electrically connected in series so that the voltage delivered between the fuel cell terminals is equal to the sum of the cell voltages of all the individual cells.

[0011] When a PEMFC fuel cell is used for the first time after being manufactured and before being put into operation, it requires an activation procedure or a running-in procedure. This running-in procedure aims to improve and stabilize the performance of the fuel cell by changing the physical and chemical characteristics of the unit cells of the fuel cell.

[0012] In particular, the main objectives of the run-in process are to ensure optimal hydration of the membrane, desorption of contaminants present in the anode and cathode catalyst layers, and activation of the anode and cathode catalyst layers, which activation corresponds to morphological changes within the membrane-electrode assembly, in particular changes in the porosity of the catalyst layers and cleaning of the catalyst layers.

[0013] Typically, the fuel cell run-in process occurs on an activation station, where the fuel cell is connected to a load and supplied with reactants. The run-in method involves adjusting the load and the flow rates of the reactants to maintain a predetermined voltage or current profile. During the run-in period, the fuel cell's performance gradually increases until it stabilizes and reaches the activation process's stopping criterion. When this point is reached, the fuel cell is considered to have completed run-in. The stopping criterion can be a certain performance level achieved, particularly in terms of efficiency—for example, expressed as a function of the amount of electrical energy produced per unit mass of hydrogen consumed by the cell, or as the stability of performance between two measurements, or both. Existing stopping criteria simply involve stopping the run-in process after some predetermined time. This type of criterion is the least precise. For example, the stopping criterion is defined as the difference between the voltage at time t and the voltage at time t+1, which must be less than a certain threshold value, at a given constant current density produced by the fuel cell.

[0014] Most commonly, this run-in procedure involves maintaining a constant voltage at the terminals of the fuel cell, or maintaining a constant current density produced by the fuel cell, for a predetermined period of time, then measuring the voltage and current at the terminals of the fuel cell at the end of that period, and repeating the cycle until a stop criterion is reached.

[0015] This run-in process is typically lengthy, lasting up to several hours, and consumes large amounts of hydrogen, particularly several kilograms for fuel cells with a maximum output of 10 kW or more. These two drawbacks mean that the run-in protocol is very expensive, a disadvantage when implemented on an industrial scale. Therefore, it would be desirable to reduce the duration of the run-in process without reducing performance gains or causing damage to the fuel cell.

[0016] Certain techniques are known to accelerate the run-in regime. One of these techniques, known as reducing air over-stoichiometry, involves supplying the fuel cell with reactants having a connected load and then reducing the air supply flow rate so that the fuel cell operates with a cathode stoichiometry coefficient that is always greater than 1 (e.g., between 1 and 1.5), thereby briefly operating the fuel cell with less oxygen than the nominal operating condition. During this phase of reducing air over-stoichiometry, in addition to the air having a reduced oxygen content compared to nominal conditions, the cathode compartment of each unit cell also contains a certain amount of hydrogen, since the fuel cell will locally operate as a proton pump, which results in the formation of hydrogen at the cathode in the oxygen-deficient region. Preferably, during this phase of reducing air over-stoichiometry, the load is controlled so that the cell voltage of each unit cell is low. 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 pollutant oxides on the cathode catalyst layer, thereby cleaning the cathode catalyst layer and increasing the actual active surface area of the cathode catalyst layer. This technology shortens the run-in process.

[0017] However, this technique has the disadvantage of causing uneven fuel cell performance because the reduction in oxygen levels in the air is uneven within the cathode compartment of the unit cell. In fact, this reduction is more pronounced at the air outlet opening than at the air inlet opening. In addition, as practiced in the prior art, reducing air beyond stoichiometric levels can produce fluctuations in the cell voltage, which can lead to damage to the fuel cell. These degradations can include dissolution of platinum, corrosion of the bipolar separator, corrosion of carbon, which is typically a component of the catalytic layer, and / or degradation of the membrane due to the appearance of hot spots. Summary of the Invention

[0018] The present invention specifically addresses these shortcomings and proposes a new method for running-in a fuel cell that is faster, does not damage the fuel cell, and provides better fuel cell performance.

[0019] To this end, the present invention relates to a method for running-in a fuel cell, which comprises a stack of a plurality of cells, each cell comprising a proton exchange membrane arranged between two bipolar plates, each bipolar plate delimiting a reaction compartment with the proton exchange membrane in the cell, each cell therefore comprising a cathode compartment in which a cathode catalyst layer is arranged, an anode compartment in which a cathode catalyst layer is arranged, and an anode compartment in which an anode catalyst layer is arranged.

[0020] The fuel cell includes a dihydrogen inlet for supplying dihydrogen to the anode compartment of each cell and a dihydrogen outlet for discharging dihydrogen from each cell. The fuel cell includes 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 running-in process includes at least the following stages in sequence:

[0022] The first running-in stage includes at least the following steps in sequence:

[0023] Preferably, a fuel cell stabilization step, during which the current density generated by the fuel cell is kept constant at a low value for a predetermined period of time, and

[0024] an oxygen consumption step of the fuel cell, during which the current density generated by the fuel cell is kept constant at a minimum value, which may be less than or equal to the low value, and during which the air supply to the fuel cell via the air inlet is at least partially cut off to drive the fuel cell into the oxygen consumption step, in particular by being advantageously adjusted to obtain a cathode stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9, the oxygen consumption step of the first run-in phase being terminated when the cell voltage of the fuel cell reaches a predetermined threshold voltage;

[0025] An operation to reverse the direction of hydrogen and air flow, wherein the hydrogen inlet and the hydrogen outlet are reversed, and the air inlet and the air outlet are reversed; and

[0026] The second running-in stage includes at least the following steps in sequence:

[0027] Preferably, a fuel cell stabilization step, during which the current density generated by the fuel cell is kept constant at a low value for a predetermined period of time, and

[0028] an oxygen consumption step of the fuel cell, during which the current density generated by the fuel cell is kept constant at a minimum value, which may be lower than or equal to the low value of the second run-in stage, and during which the air supply to the fuel cell via the air inlet is at least partially cut off to reduce the current density generated by the fuel cell, in particular by advantageously being arranged to obtain a cathode stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9, and the oxygen consumption step of the second run-in stage ends when the cell voltage of the fuel cell reaches a predetermined threshold voltage.

[0029] The steps of the first run-in phase are performed until a first stop condition is reached, and the steps of the second run-in phase are performed until a second stop condition is reached.

[0030] Due to the present invention, the oxygen depletion steps in the first and second run-in phases are able to reduce and desorb other impurities on the cathode catalyst layer surface, thereby further cleaning the cathode catalyst layer. These steps are also performed without any significant degradation of the fuel cell, because the air supply is sufficiently reduced to avoid voltage fluctuations due to the gradual reduction in cell voltage, for example, because the cathode stoichiometric coefficient is strictly less than 1, preferably less than or equal to 0.9. Furthermore, degradation is prevented by avoiding cathode depletion at high currents. Furthermore, due to the reversal of the hydrogen and air flow directions between the two run-in phases, consistent fuel cell performance is achieved despite the oxygen depletion steps. Finally, preferably, by connecting the stabilization step and the oxygen depletion step during the run-in phase, the efficiency of the oxygen depletion step is maximized, as the stabilization step enables stable and uniform conditions within the fuel cell, avoiding performance variations during the oxygen depletion step. Furthermore, the stabilization steps in the first and second run-in phases advantageously oxidize impurities on the cathode catalyst layer surface, as maintaining a low current density means the fuel cell operates at a high cell voltage, which favors the oxidation of certain impurities, thereby at least partially cleaning the cathode catalyst layer.

[0031] According to an advantageous but not mandatory aspect of the invention, the running-in process comprises one or more of the following features, used alone or in any technically feasible combination:

[0032] Prior to the oxygen consumption step, the first run-in stage further includes a fuel cell stabilization step, during which the current density generated by the fuel cell remains constant at a low value for a predetermined time period, and the low value of the first run-in stage is greater than a minimum value, and / or prior to the oxygen consumption step, the second run-in stage further includes a fuel cell stabilization step, during which the current density generated by the fuel cell remains constant at a low value for a predetermined time period, and the low value of the second run-in stage is greater than or equal to the minimum value.

[0033] The minimum value of the first running-in stage and the minimum value of the second running-in stage are 0.01A / cm 2 Up to 0.3A / cm 2 between, preferably equal to 0.02A / cm 2 .

[0034] The lowest value in the first running-in stage and the lowest value in the second running-in stage are 0.03A / cm 2 Up to 0.5A / cm 2 between, preferably equal to 0.3A / cm 2 .

[0035] During at least one of said oxygen consumption steps, the air supply to the fuel cell via the air inlet is at least partially shut off by regulation so as to obtain a cathode stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9.

[0036] When the cell voltage of the fuel cell reaches a threshold voltage between 0.1 V and 0.4 V, preferably equal to 0.2 V, the oxygen consumption steps of the first run-in phase and the second run-in phase are ended.

[0037] During the oxygen consumption steps of the first run-in phase and the second run-in phase, the air supply to the fuel cell via the air inlet is completely cut off to obtain a cathode stoichiometric coefficient equal to 0. In some embodiments, the cathode stoichiometric coefficient remains equal to 0 throughout the oxygen consumption steps of the first run-in phase and / or the second run-in phase.

[0038] During the oxygen consumption steps of the first run-in phase and the second run-in phase, the air supply to the fuel cell via the air inlet is controlled such that the cell voltage of the fuel cell decreases monotonically.

[0039] The first run-in phase also includes the following steps, which are performed in sequence before the oxygen consumption step:

[0040] a current density increasing step, during which the current density generated by the fuel cell gradually increases from a low value to a high value, the low value being greater than or equal to the minimum value of the first running-in stage,

[0041] a hydration step of the fuel cell, during which the current density produced by the fuel cell is kept constant at the high value of the first run-in phase for a predetermined period of time in order to hydrate the proton exchange membrane, and

[0042] a current density decreasing step, during which the current density generated by the fuel cell decreases from a high value in the first run-in stage to a low value in the first run-in stage;

[0043] The second running-in stage also includes the following steps performed in sequence before the stabilization step and the oxygen consumption step:

[0044] a current density increasing step, during which the current density generated by the fuel cell gradually increases from a low value of the second run-in stage to a high value of the second run-in stage, the low value of the second run-in stage being greater than or equal to a minimum value of the second run-in stage,

[0045] a fuel cell hydration step during which the current density generated by the fuel cell is kept constant at the high value of the second run-in phase for a predetermined period of time to hydrate the proton exchange membrane, and

[0046] a current density decreasing step during which the current density generated by the fuel cell decreases from a high value of the second run-in stage to a low value of the second run-in stage.

[0047] The high value of the first running-in stage and the high value of the second running-in stage are 1.5A / cm 2 Up to 3A / cm 2 between, preferably equal to 1.9A / cm 2 .

[0048] The current density increasing step, the hydration step, the current density decreasing step, the stabilization step, and the oxygen consumption step of the first running-in phase are cyclically performed at least twice. In addition, the first stopping condition is reached when the following conditions occur:

[0049] The difference between the cell voltage of the fuel cell at the end of the hydration step of the last cycle of the first run-in stage and the cell voltage of the fuel cell at the end of the hydration step of the second to last cycle of the first run-in stage is 1 mV to 10 mV, preferably equal to 5 mV, or

[0050] Where appropriate, the cell voltage of the fuel cell at the end of the stabilization step of the last cycle of the first run-in phase differs from the value of the cell voltage of the fuel cell at the end of the stabilization step of the penultimate cycle of the first run-in phase by 1 mV to 10 mV, preferably by 5 mV.

[0051] In addition, the current density increasing step, the hydration step, the current density reducing step, and, where appropriate, the stabilization step and the oxygen consumption step of the second running-in stage are cyclically performed at least twice, and the second stop condition is reached when:

[0052] The cell voltage of the fuel cell at the end of the hydration step of the last cycle of the second run-in stage differs from the value of the cell voltage of the fuel cell at the end of the hydration step of the second to last cycle of the second run-in stage by 1 mV to 10 mV, preferably by 5 mV, or

[0053] Where appropriate, the cell voltage of the fuel cell at the end of the stabilization step of the last cycle of the second run-in phase differs from the value of the cell voltage of the fuel cell at the end of the stabilization step of the penultimate cycle of the second run-in phase by 1 mV to 10 mV, preferably by 5 mV.

[0054] The run-in method also comprises an initialization phase carried out before the first run-in phase, during which the current density produced by the fuel cell is gradually increased from a zero value to a high value of the first run-in phase, and preferably, this initialization phase takes place within a period of time between 15 minutes and 45 minutes, and even more preferably within a period of time equal to 24 minutes.

[0055] The run-in method further comprises a control phase, performed after the second run-in phase, during which the current density produced by the fuel cell is kept at a constant level for a predetermined period of time, preferably for a period of time between 45 minutes and 75 minutes, more preferably for a period of time equal to 60 minutes.

[0056] During the first run-in phase and the second run-in phase, the anode stoichiometric coefficient of the fuel cell is equal to the nominal anode stoichiometric coefficient of the fuel cell, preferably between 1.3 and 2, and even more preferably equal to 1.5. In addition, during the current density increase step, the hydration step, the current density decrease step, and (if appropriate) the stabilization step of the first run-in phase and the second run-in phase, 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 equal to 2.3.

[0057] During the first and second running-in phases, an electrical load in the form of a variable resistor is connected across the fuel cell, causing the fuel cell to generate current. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] An embodiment of a power module, a power system, and an assembly method will be described below based on the principles of the present invention by way of example and with reference to the accompanying drawings, so as to provide a better understanding of the present invention and make other advantages of the present invention more apparent.

[0059] Figure 1 is an enlarged perspective view of a stack of several cells of a fuel cell according to the present invention.

[0060] Figure 2 is a representative curve of the run-in process of the fuel cell according to the present invention.

[0061] Figure 3 Yes Figure 2 The curve of the running-in phase of the running-in process is shown.

[0062] Figure 4 shows the two different stoichiometric conditions, indicating that Figure 3 The two curves (B, C) showing the variation of the cell voltage of the fuel cell during the oxygen consumption step of the shown phase are compared with the curve (A) showing the variation of the cell voltage of the fuel cell during the air exceeding stoichiometric reduction step proposed in the prior art.

[0063] Figure 5 is shown during the oxygen consumption step at the cathode. Figure 1 Diagram of the electrochemical reactions occurring in a fuel cell. Figure 2 The illustrated break-in process is performed during two break-in phases and illustrates operation of the break-in process with the direction of hydrogen and air flow reversed.

[0064] Figure 6 3 is an impedance spectrum analysis diagram of a membrane of a fuel cell, which has been run-in or activated using the run-in method of the present invention.

[0065] Figure 7 is the polarization curve of the fuel cell according to the present invention after running-in or activation using the running-in method of the present invention.

[0066] Figure 8 is a voltammogram centered on the high potential region of the membrane of a fuel cell that has been run-in or activated using the run-in method of the present invention.

[0067] Figure 9 is a voltammogram centered on the low potential region of the membrane of a fuel cell that has been run-in or activated using the run-in method of the present invention. DETAILED DESCRIPTION

[0068] Figure 1A stack of multiple cells 12 of a fuel cell 10 is shown. The fuel cell is designed, for example, to be installed in a vehicle and to generate electricity to power an electric motor to drive the vehicle.

[0069] The fuel cell 10 is of the proton exchange membrane fuel cell type and therefore comprises a stack of said plurality of cells 12. The stack is maintained at Figure 1 These end plates make it possible in particular to keep the stack of cells 12 compressed, that is to say compact, and to supply the stack with fuel (in the example gaseous dihydrogen) and oxidant (in the example gaseous air) and, if necessary, with the circulation of a heat transfer fluid for the cell cooling circuit.

[0070] The present invention will be described more specifically in the context of a common configuration in which each cell 12 includes a membrane-electrode assembly 14 and two bipolar plates 16 disposed on either side of the membrane-electrode assembly. However, the present invention is also applicable to solid electrolyte ion exchange membrane type fuel cells having different configurations.

[0071] It is assumed that for a given fuel cell 12, all cells 12 in the fuel cell are identical to one another and therefore have identical characteristics.

[0072] Figure 1 Also shown is a detail of a cross section of the membrane-electrode assembly 14 of the cell 12 .

[0073] In practice, each bipolar plate 16 is positioned between two cells 12 and is shared by both cells. The first side 16A (called the anode side) supplies dihydrogen to one of the two cells, and the second side 16B (called the cathode side) supplies air to the other of the two cells. In other words, the cell 12 is supplied with dihydrogen by the first bipolar plate 16 and with air by the second bipolar plate. Air contains oxygen, so the cell 12 is supplied with oxygen.

[0074] In the remainder of the specification, the terms oxygen and dioxygen as well as hydrogen and dihydrogen are used interchangeably.

[0075] In this example, each bipolar plate 16 is formed by assembling two half-plates. This assembly forms hydrogen circulation channels on face 16A, air circulation channels on face 16B, and coolant circulation channels between faces 16A and 16B (i.e., within the bipolar plate). This coolant circulation does not participate in the electrochemical reaction of the fuel cell 10, but can control the temperature of the cell 12.

[0076] The membrane-electrode assembly 14 includes two gas diffusion layers 18 disposed on either side of a proton exchange membrane 20, and an anode catalyst layer 22, for example deposited on a first surface of the membrane, and a cathode catalyst layer 24, for example deposited on the other surface of the membrane.

[0077] Thus, in this example, each cell 12 includes, in sequence, 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.

[0078] 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 the membrane. An anode catalyst layer 22 is disposed in the anode compartment, and a cathode catalyst layer 24 is disposed in the cathode compartment.

[0079] Thus, the gas diffusion layers 18, each disposed in their respective anode compartment or cathode compartment, allow fuel and oxidant gas to be transported from the bipolar plate 16 to the anode catalyst layer 22 and the cathode catalyst layer 24, respectively. In practice, the gas diffusion layers are formed of a porous material, such as a nonwoven fabric of carbon fibers, i.e., a carbon fiber fabric in which the fibers are randomly arranged, or porous carbon paper, typically impregnated with a polymer, preferably a hydrophobic polymer, such as a fluoropolymer, for example polytetrafluoroethylene (PTFE), in particular in order to make the fiber surface of the carbon paper more hydrophobic.

[0080] The proton exchange membrane 20 allows hydrogen ions or protons to pass from the anode compartment to the cathode compartment 24 while preventing gases and electrons from circulating between the two compartments. For example, it is made of a sulfurized perfluoropolymer material such as the material sold under the trade name Nafion.

[0081] When the fuel cell 10 is in operation, an oxidation reaction occurs in the anode compartment of each cell 12 at the level of the anode catalyst layer 22. This oxidation reaction involves the catalytic splitting of dihydrogen supplied via the gas diffusion layer 18 into protons and electrons. The protons thus generated pass through the proton exchange membrane 20 until they reach the cathode catalyst layer in the cathode compartment, while the electrons are captured by the anode side 16A of the adjacent bipolar plate 16. These electrons are then conducted to the cathode side 16B of the same bipolar plate, which belongs to the cathode compartment of the adjacent cell 12. Simultaneously, a reduction reaction occurs in the cathode compartment of the cell 12 at the level of the cathode catalyst layer 24. This reduction reaction involves the reaction of oxygen molecules supplied by the air via the gas diffusion layer 18 with protons passing through the membrane 20 and with electrons supplied by the cathode side 16B of the bipolar plate 16 to form water molecules.

[0082] In practice, the catalytic layers 22 and 24 are porous structures formed from three different materials, namely:

[0083] The proton transport material is, for example, the same material as the proton exchange membrane 20, in this case Nafion.

[0084] Materials that transport electrons, such as carbon.

[0085] The material used to catalyze the electrochemical oxidation and reduction reactions, such as platinum, is in the form of particles, preferably spherical, and is deposited on the surface of the material (such as the carbon) during the preparation of the catalytic layer to transport electrons.

[0086] Furthermore, the pores of the catalytic layer allow the reactants (ie, hydrogen and oxygen) to be transported freely within the catalytic layer.

[0087] Within catalytic layers 22 and 24, there are regions where these three materials and pores meet. These regions are called active sites or triple points, and electrochemical reactions occur at these active sites. Regions where not all of the constituent elements of the catalytic layer are present, particularly where platinum is present but Nafion or carbon or reactant access is missing, are called dead zones.

[0088] Catalytic layers 22 and 24 also contain impurities or contaminants, such as residues or additives from the production of the catalytic layers. Furthermore, the platinum particles contained in the catalyst layers typically have an oxide layer on their surface. When this oxide layer becomes too thick, the platinum particles can no longer react with protons and electrons, and thus this thick oxide layer on the surface of the platinum particles can be considered an impurity.

[0089] In practice, the membrane-electrode assembly 14 of the cell 12 is arranged in an opening formed in a support plate 25, which is interposed between the two bipolar plates 16. The support plate 25 can be made in the form of a single or double polymer film, for example, with a thickness of between 50 and 200 micrometers. The polymer film is made, for example, of polyethylene terephthalate (also known as PET) or polyethylene naphthalate (also known as PEN). Advantageously, in order to ensure the seal between the membrane-electrode assembly and the bipolar plates 16 in the stack 12, the membrane-electrode assembly includes two seals 26, which are located at the periphery of the membrane-electrode assembly, arranged between the gas diffusion layer 18 and the catalytic layer 22 and between the gas diffusion layer 18 and the catalytic layer 24, and extend to the support plate 25.

[0090] The fuel cell 10 includes a dihydrogen inlet 28 for supplying dihydrogen to each cell and a dihydrogen outlet 30 for discharging dihydrogen from each cell.

[0091] The fuel cell includes an air inlet 32 that supplies air to each cell and an air outlet 34 that exhausts air from each cell.

[0092] The air outlet 34 and hydrogen outlet 30 also allow for the drainage of water produced by the fuel cell.

[0093] The fuel cell includes a coolant inlet 36 that supplies coolant to each cell and a coolant outlet 38 that exhausts coolant from each cell.

[0094] In this example, if Figure 1 As shown, inlets 28, 32, and 26 and outlets 30, 34, and 38 are formed by openings in the bipolar plate 16 and the support plate 25. Alternatively, these inlets and outlets are connected to openings in the end plates, which in turn are connected to hydrogen supply circuits, air supply circuits, and refrigerant supply circuits, such as flexible or rigid pipes. Alternatively, these inlets and outlets are formed by conduits provided around the stack of multiple cells and the bipolar plates of the fuel cell.

[0095] like Figure 1 As shown, on its cathode side 16B, the bipolar plate 16 has two homogenizing regions 40, 41 and an active region 42. The first homogenizing region 40 connects the air inlet 32 to the active region, and the second homogenizing region 41 connects the active region to the air outlet 34.

[0096] The active area 42 has channels 44 over its entire surface, which pass right through it, each channel 44 connecting the homogenizing area 40 to the homogenizing area 41. The channels 44 thus enable the air flow to be conducted over the entire length of the cathode compartment.

[0097] Here, the channel 44 is shown as being straight. In a variant not shown, the channel 44 has a different shape, for example a wave shape, a serpentine shape or a broken line shape.

[0098] Homogenizing zones 40 and 41 connect the air inlet and the air outlet, respectively, to the active zone 42 and enable the air to be distributed to all channels 44 across the entire width of the active zone.

[0099] On the anode side 16A, the bipolar plate 16 has the same structure as on the cathode side 16B, namely two homogenizing regions and an active region containing channels. On the anode side, the homogenizing region connects the two hydrogen inlets and outlets to the active region, respectively, and enables hydrogen to be distributed to all channels across the entire width of the active region.

[0100] In this example, the bipolar plate 16 and the support plate 25 are rectangular in shape. In this example, but not necessarily, and as Figure 1 As can be seen in FIG, the two hydrogen inlets 28 and the two hydrogen outlets 30 are positioned diagonally relative to each other, and the air inlet 32 and the air outlet 34 are also positioned diagonally relative to each other, which allows for a more uniform reactive gas distribution across the active area 42 of the bipolar plate.

[0101] Preferably, each cell 12 of the fuel cell 10 has an active surface, corresponding to the surface of the active area 42 of the bipolar plate 16, with a size of 150 cm2 Up to 500cm 2 Alternatively, the active surface can be smaller or larger.

[0102] For each of the anode and cathode compartments, the stoichiometric coefficient is defined as the ratio between the flow rate of the reactants injected and the minimum flow rate of the reactants required to drive the electrochemical reaction that delivers the current density required for the electrical load generated by the fuel cell. Thus, for a stoichiometric coefficient of 1, the total amount of reactant, dihydrogen or oxygen from the air supplied to the anode or cathode compartment is consumed by the electrochemical reaction occurring at the level of the cathode catalytic layer of that compartment. For a stoichiometric coefficient of 2, twice as much reactant is supplied as required.

[0103] The anode compartment and the cathode compartment may also have different stoichiometric coefficients.

[0104] Therefore, a stoichiometric coefficient greater than 1 does not lead to an increase in the current density generated by the fuel cell, but does lead to an increase in the amount of reactants consumed by the fuel cell, since excess reactants supplied to the fuel cell are not consumed by the electrochemical reaction and are usually lost in whole or in part. However, it should be noted that a recirculation device is usually provided for at least the hydrogen supplied to the anode to limit the loss of hydrogen. When it comes to supplying the cathode, air recirculation is not always provided. In addition, a stoichiometric coefficient greater than 1, especially in the cathode compartment, leads to higher flow rates and therefore higher pressure losses, which affects the performance of the fuel cell and the run-in process described below.

[0105] The anode stoichiometric coefficient is the stoichiometric coefficient of the anode compartment, and the cathode stoichiometric coefficient is the stoichiometric coefficient of the cathode compartment.

[0106] In theory, assuming that the diffusion of hydrogen and air within the anode and cathode compartments occurs instantaneously and without losses, anode and cathode stoichiometric coefficients equal to 1 are sufficient to power the fuel cell. However, in practice, when using the fuel cell, for example in a vehicle, it is known to use anode and cathode stoichiometric coefficients greater than 1, i.e., injecting hydrogen and air into the anode and cathode compartments at flow rates greater than the flow rate consumed to ensure a good supply to the fuel cell. This fuel cell operation is sometimes referred to as superstoichiometric operation in the anode and cathode compartments. This oversupply of reactants is particularly important to account for leakage of hydrogen and air from the fuel cell 10 and is necessary to ensure proper operation of the fuel cell during transient operation, particularly when the current density generated by the fuel cell is increasing. Furthermore, without pressurization and given the transport time of the reactants through the gas diffusion layer 18 and into the catalytic layers 22 and 24, localized reactant shortages can be observed, leading to reduced fuel cell performance. In this way, increasing the reactant supply also ensures that there is always an adequate supply of reactants in the catalyst layers.

[0107] In a manner known per se, nominal values for the anode and cathode stoichiometric coefficients are also defined for each fuel cell, corresponding to the values of the anode and cathode stoichiometric coefficients used when the fuel cell is operating normally (such as when the fuel cell is used in a vehicle). Typical nominal values for the fuel cell 10, corresponding to superstoichiometric operation, are an anode stoichiometric coefficient of 1.5 and a cathode stoichiometric coefficient of 1.8.

[0108] In addition, the stoichiometric coefficient defined above is determined in theory, taking into account only the flow rate of the reactants actually injected into the fuel cell and consumed by the electrochemical reaction. In fact, the supply loop for supplying hydrogen and air to the fuel cell may leak and lose reactants. Therefore, for a stoichiometric coefficient equal to 1 as defined above, the actual flow rate of the reactants transported by the corresponding reactant feed loop is slightly greater than the minimum flow rate of the reactants required for the electrochemical reaction, which supplies the current density required for the electrical load generated by the fuel cell. The total stoichiometric coefficient is then defined, and the leakage and loss from the corresponding reactant feed loop are integrated into the reactant flow rate injected. For example, for a stoichiometric coefficient equal to 1 as defined above, the total stoichiometric coefficient is equal to 1.1.

[0109] refer to Figures 2 to 5 , a break-in process will now be described for the fuel cell 10. This break-in process is intended to be performed after the fuel cell 10 has been assembled and before it is put into use, and is used to improve the performance of the fuel cell.

[0110] The three main purposes of this run-in process are:

[0111] The increase in the number of active sites in the catalytic layers 22 and 24,

[0112] desorption and removal of impurities present in the catalytic layer and on the surface of the catalytic layer, and

[0113] The hydration of the proton exchange membrane 20 results in a reduction in membrane resistance.

[0114] These objectives improve fuel cell performance. Each of these three primary objectives increases the cell voltage delivered by each fuel cell 12 at the same current density produced by the fuel cell, thereby improving fuel cell performance. For simplicity in this specification, the cell voltage delivered by each fuel cell 12 is referred to as "fuel cell voltage" or "cell voltage."

[0115] For example, by observing certain characteristics of the fuel cell, it is possible to assess the performance of the run-in process described herein on the fuel cell. For example, the uniformity of activation across the surface of the proton exchange membrane 20 can be analyzed by sampling small sections of the membrane at the air and / or dihydrogen inlets and outlets located near the cells (e.g., several cells or even each cell). If the characteristics of these analyzed sections of the membrane are identical, then the activation is uniform between the inlet and outlet, particularly between the air inlet and outlet, thus demonstrating that reversing the flow directions of the dihydrogen and air flows has had an effect on the membrane run-in.

[0116] In particular, the following characteristics can be analyzed:

[0117] - Proton exchange membrane hydration rate: For example, the hydration rate of a proton exchange membrane can be measured using membrane impedance spectroscopy. Figure 6 An example of such a measurement, also known as a Nyquist plot, is shown in Figure 1. This is a graphical representation of the real and imaginary parts of the cell's impedance over a frequency range, in this case from 0.1 Hz to 10 kHz. Furthermore, the hydration rate of the proton exchange membrane is obtained by analyzing the cell's polarization curve. Figure 7 An example of a polarization curve is shown in . This curve represents the average cell voltage of a fuel cell as a function of the current density applied to its terminals.

[0118] -Impurity desorption evolution: Impurity desorption can be measured, for example, by cyclic voltammetry. Figure 8 An example of such a measurement is shown in . This curve represents the current response of the fuel cell as the voltage is swept. For example, to assess the removal of impurities during the run-in process, the desorption of platinum oxide can be measured using cyclic voltammetry. This desorption of platinum oxide can be observed in the high-potential region of the voltammogram, for example, in the voltage range between 0.6 V and 1.2 V.

[0119] - the evolution of the porosity of the catalytic layer, which can be determined, for example, by calculating the area of the hydrogen adsorption / desorption regions at low potentials on a voltammogram performed on a fuel cell, the voltage range being between 0V and 0.4V. Figure 9 An example of such a voltammogram is shown in .

[0120] Advantageously, this run-in procedure can also be performed on already used fuel cells in order to eliminate certain reversible losses and thus partially compensate for the performance loss associated with fuel cell aging.

[0121] During the running-in process, the fuel cell 10 is mounted on an activation platform where it is connected to an electrical load and supplied with reactants (i.e., dihydrogen and air including oxygen, and a coolant). The charge is applied to the current that must be generated by the fuel cell. A non-representative sensor is used to measure the voltage at the terminals of the fuel cell 10 and the power delivered by the fuel cell (corresponding to the voltage delivered to the electrical load and the electrical power consumed by the electrical load, respectively). The cell voltage of the fuel cell can then be derived by dividing the terminal voltage of the fuel cell 10 by the number of cells 12 in the fuel cell.

[0122] Furthermore, the electrical load connected to the fuel cell 10 can be likened to a system that combines a resistor with a power electronic converter that is controllable to cause the fuel cell to generate current. In other words, the electrical load can be likened to a variable resistor that can be controlled to select the amount of current generated by the fuel cell.

[0123] In a manner known per se, there is a certain relationship between the voltage output by each fuel cell at its terminals (i.e., the cell voltage output by each cell) and the current generated. Thus, in the presence of a sufficient number of reactants, the voltage at each cell terminal (i.e., the voltage at the terminals of the fuel cell) is a function of the current generated by the fuel cell, a function that depends on the physicochemical characteristics of the fuel cell. This function is generally represented by the polarization curve of the fuel cell. Thus, for a given resistance of the controllable load, the current generated by the fuel cell is applied, which determines the voltage at the terminals of the fuel cell, and therefore the cell voltage of the fuel cell, which depends on the characteristics of the fuel cell, and more specifically on the polarization curve of the fuel cell.

[0124] It should also be noted that the current produced by the fuel cell 10 depends on the active surface of each cell 12 and therefore on the geometry of the fuel cell. Therefore, to disregard the geometry of the fuel cell, it is best to refer to the current density produced by the fuel cell, which is equal to the current produced by the fuel cell divided by the active surface area of each cell, and is expressed in A / cm 2In the present invention, it is assumed that the current density of all cells 12 of the fuel cell is the same because the cells are electrically connected in series. 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 we consider is the average current density across the active surface of the cell. Therefore, the present invention uses the term "current density" to refer to the current density generated by the fuel cell.

[0125] In the remainder of the description, for the sake of simplicity, it is assumed that the voltage at the terminals of a 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 battery terminals (i.e., the cell voltage) also corresponds to the difference between the potential at the cathode compartment (also referred to as the "cathode potential") and the potential at the anode compartment (also referred to as the "anode potential"). In a battery assembly 12, the first cell in the stack is assumed to have an absolute anode potential of zero, i.e., equal to 0V relative to ground, while ignoring any abnormal anode potentials due to abnormal operating conditions at the anode, which do not exist in the context of the present invention. Therefore, at each subsequent cell, the absolute potential of each anode compartment is considered to be equal to the sum of the cell voltages of the preceding cells in the stack. Therefore, as is common practice in the fuel cell field, for a given cell, the potential of the cathode compartment is defined as the potential relative to the anode potential of the cell in question. Therefore, the cathode potential of a given cell is considered to be equal to the voltage at the terminals of the cell and is expressed in volts, without taking into account, in the context of the present invention, any abnormal anode potential of the cell due to abnormal operating conditions at the anode.

[0126] The improvement in fuel cell 10 performance sought during the run-in process primarily involves increasing the cell voltage at a given current density and a given load resistance. In other words, this performance improvement involves changing the function relating voltage and current density, which is typically represented by the polarization curve of the fuel cell unit. At the end of the run-in period, the cell voltage corresponding to a given current density is higher than the cell voltage corresponding to the same current density at the beginning of the run-in period. In other words, run-in causes the polarization curve to "rise" over the entire current range of the fuel cell.

[0127] like Figure 2 As shown, the running-in process consists of five stages or operations performed in the following order:

[0128] Initialization phase P1, which is optional,

[0129] Initial running-in phase P2,

[0130] Operation P3 for reversing the direction of hydrogen and air flow, also referred to as "reverse operation" in the rest of the description,

[0131] Second run-in phase P4, and

[0132] Optional control stage P5.

[0133] Figure 2 The evolution of the current density as a function of time during the entire running-in process is shown.

[0134] At the beginning of initialization phase P1, fuel cell 10 is not yet operational. Its performance is limited; in particular, any rapid change in current density can damage the fuel cell. In practice, a sudden increase in current density in a newly assembled fuel cell causes a drop in cell voltage, which can lead to overheating.

[0135] During the initialization phase P1, the current density is gradually increased from zero to a high value. The high value is preferably 1.5 A / cm 2 2 to 3A / cm 2 between 1.9A / cm and 2.5A / cm 2 In this example, the high value is equal to 1.9A / cm 2 The initialization phase P1 is performed over a long period of time (for example between 15 and 45 minutes, preferably 24 minutes).

[0136] The initialization phase P1 ensures that the fuel cell 10 starts up gently for the first time without damaging the cell 12 .

[0137] The first running-in phase P2 consists of a number of consecutive steps that are performed cyclically. Figure 3 The three cycles of the first running-in phase P2 are shown in detail in FIG.

[0138] Figure 3 The upper curve showing the current density as a function of time and the lower curve showing the battery voltage as a function of time are shown in three cycles of the first run-in phase P2 .

[0139] It should be noted that in practice, during the first run-in phase P2 and during the run-in process, since the current density values and cell voltage values change as the performance of the fuel cell 10 improves through the run-in protocol, Figure 3 The current density values shown in are taken from representative values given by way of example, while the cell voltage values are indicative values intended to present a graph of the cell voltage evolution. The cell voltage is a result of the current density and efficiency of the fuel cell.

[0140] Each cycle of the first running-in phase P2 consists of five steps, performed in the following order:

[0141] Optional current density increase step 1;

[0142] Optional hydration step 2;

[0143] Optional current density reduction step 3;

[0144] optional stabilization step 4, and

[0145] Oxygen consumption step 5.

[0146] exist Figure 2 In the example shown, the first running-in phase P2 includes five cycles. In practice, the first running-in phase P2 ends when the first stop condition is reached and can therefore include several cycles other than five cycles. In particular, the first running-in phase P2 can include a single cycle.

[0147] The step of increasing the current density 1 comprises increasing the current density from a low value to a high value, which is preferably between 1.5 A / cm 2 Up to 3A / cm 2 and in this example the high value is equal to 1.9 A / cm 2 This increase in current density is achieved, for example, by suitable control of the charge.

[0148] This increase is rapid compared to the increase during the P1 initialization phase.Preferably, this step is performed for 2 seconds to 120 seconds, such as 20 seconds to 60 seconds.

[0149] In practice, the duration of the current density increase step 1 is designed to be as short as possible without risking damage to the fuel cell 10. In practice, the duration of step 1 is long enough to avoid any sudden drop in voltage, which could cause the fuel cell to overheat and thus damage the cell 12. Therefore, this time depends on the performance and response time of the fuel cell. Advantageously, the duration of the current density increase step 1 is not constant from one cycle to the next, but decreases as the performance of the fuel cell increases. This duration is of the order of tens of seconds to several minutes. Figure 2 and Figure 3 In the example shown, step 1 lasts 30 seconds.

[0150] It should be noted that the first cycle of the first run-in phase P2 does not represent the other cycles of the first run-in phase, because the current density increase step 1 is not performed during the other cycles. At the end of the initialization phase P1, the current density is already equal to a high value. In other words, for this first cycle, the initialization phase P1 acts as the current density increase step 1.

[0151] Hydration step 2 comprises causing the fuel cell 10 to generate a high current density equal to a high value for a predetermined period of time. The high current density value is preferably selected to create reducing conditions in the cathode compartment. These reducing conditions are typically achieved when the cathode potential of the battery is less than or equal to 0.5V (volts). Therefore, during this step, the battery charge is controlled so that the current density is high and, based on the relationship law between the current density generated and the cell voltage at the terminals of the battery, the cathode potential of the battery is relatively low, typically less than or equal to 0.5V. The duration of hydration step 2 is preferably between 30 seconds and 10 minutes. In this embodiment, hydration step 2 lasts for 5 minutes.

[0152] The cathode potential of the cell during the hydration step is believed to be relatively low compared to the cathode potential of the cell observed during normal operation of the fuel cell 10 and during use after run-in of the fuel cell 10, which is typically between 0.6V and 0.7V.

[0153] Generally speaking, the anode potential of a battery is assumed to be equal to 0V, so the cathode potential of the battery is approximately equal to the battery voltage.

[0154] Since the amount of electrochemical reaction occurring in the anode catalyst layer 22 and the cathode catalyst layer 24 is proportional to the current density, high current density results in a large amount of water generation in the cathode compartment of the cell 12. This large amount of water generation results in hydration of the proton exchange membrane 20 through contact between the water molecules and the membrane.

[0155] Furthermore, starting from the second cycle, this large amount of water production effectively removes from the cathode compartment the impurities desorbed during the oxygen consumption step 5 of the previous cycle, which is described below. These impurities are transported by water molecules formed in the cathode catalyst layer and then discharged through the gas diffusion layer 18.

[0156] Furthermore, starting from the second cycle, this substantial production of water also enables the removal from the cathode compartment of hydrogen produced at the cathode during the oxygen consumption step, such production of hydrogen at the cathode being described below.

[0157] The high current density also causes the temperature of the fuel cell 12 to increase. The current density and high temperature promote an increase in the porosity of the catalytic layers 22 and 24, which increases the number of active sites in the catalytic layers.

[0158] Finally, during the hydration step 2, the cell voltage is relatively low because the current density is high, so the potential of the cathode compartment of each cell 12 is reduced. This potential is typically less than or equal to 0.5V. By comparison, the cathode potential during the hydration step 2 is lower than the cathode potential of the fuel cell when the fuel cell is in normal use (i.e., in operation, such as in a vehicle), which is typically in the range of 0.6V to 0.7V. This low potential can achieve reducing conditions that reduce certain impurities and platinum oxide located in the cathode catalyst layer 24. As a result of these reduction reactions, impurities are desorbed and the platinum oxide is reduced to platinum particles on the one hand and to desorbed impurities on the other hand. This increases the amount of non-oxidized platinum available in the cathode catalyst layer and therefore increases the number of active sites while reducing the number of impurities in the cathode catalyst layer. These reduced impurities are also discharged from the anode compartment via water molecules.

[0159] Current density reduction step 3 involves rapidly reducing the current density from a high value to a low value. This reduction in current density results in an increase in the battery voltage. This step is performed as quickly as possible to optimize the duration of the run-in process by rapidly reducing the current density required for the electrical load, as this rapid reduction in current density and rapid increase in battery voltage does not pose any risk of damaging the battery 12.

[0160] The low current density value is preferably 0.03A / cm 2 Up to 0.5A / cm 2 between, as in the example, preferably even equal to 0.3 A / cm 2 .

[0161] The stabilization step 4 comprises causing the fuel cell 10 to generate a low current density equal to a low value for a predetermined period of time. During this step, the cell voltage is relatively high due to the low current density.

[0162] The main purpose of this step is to stabilize the operating conditions of the cell (i.e., to stabilize the temperature, humidity and pressure in the anode and cathode compartments, as well as the distribution of the reactive gases in these compartments). It will therefore be understood that this stabilization step 4 is of course preferred, but not essential, since the operating conditions of the cell may have already been stabilized previously.

[0163] Due to the high cell voltage, the stabilization step 4 also makes it possible to obtain conditions favorable for the oxidation of certain impurities in the cathode catalytic layer 24, enabling the desorption of these impurities.

[0164] Stabilization step 4 preferably lasts from 1 minute to 5 minutes. In this embodiment, stabilization step 4 lasts for 3 minutes. In practice, this time depends substantially on the size of the battery 12, and more specifically on the size of the membrane-electrode assembly 14, because the larger the battery size, the longer it takes to stabilize the operating conditions of the battery.

[0165] Based on the stable conditions obtained at the end of the stabilization step 4, the oxygen consumption step 5 consists in requesting a constant current generation from the fuel cell 10, with a current density equal to a minimum value, and then partially or completely cutting off the air supply to the fuel cell, so as to obtain in all cases a cathode stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9.

[0166] The minimum current density value is less than or equal to the low value. Preferably, the minimum value is non-zero and less than or equal to 0.3A / cm 2 Even more preferably, the minimum value is 0.01A / cm 2 Up to 0.3A / cm 2 In this example, the minimum value is 0.02A / cm 2 .

[0167] In practice, at the start of the oxygen consumption step 5, the air inlet 32 is adjusted to at least partially reduce the air supply, for example, to achieve a cathode stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9. Even more preferably, the air inlet 32 is adjusted to completely shut off the air supply, for example by means of a valve, so that once the step begins, the air supply to the cathode compartment is limited or even completely stopped. Throughout this step, the reduction reaction occurring at the level of the cathode catalytic layer 24 consumes oxygen to produce water molecules, thereby gradually consuming all the oxygen already present in the cathode compartment when the air supply was shut off, and, without completely shutting off the air inlet 32 and adjusting the air inlet 32 to achieve a cathode stoichiometric coefficient strictly less than 1, also consumes all the oxygen that continues to be provided by the reduced air supply. This is because there is not enough oxygen being replaced in the cathode compartment because not enough air is being replaced compared to the amount required for the reaction, especially when the cathode stoichiometric coefficient is strictly less than 1, which means that the oxygen is consumed faster than it is replaced. Therefore, throughout the oxygen consumption step 5, the amount of oxygen in the cathode compartment is reduced, which leads to a reduction in the number of electrochemical reactions occurring at the level of the cathode catalytic layer 24. As the current density applied by the electrical load gradually decreases, the power that the fuel cell 10 can provide gradually decreases, causing the cell voltage to gradually decrease until it reaches a minimum value. Therefore, as defined above, a complete or partial shutoff of the air supply to the fuel cell (more advantageously when the cathode stoichiometric coefficient is strictly less than 1) results in a gradual decrease in the cell voltage.

[0168] Here, the cathode stoichiometric coefficient considered does not correspond to the overall cathode stoichiometric coefficient, i.e. the air flow rate at the inlet of the cathode compartment of each cell and actually consumed by the cell is taken into account, i.e. any leakage upstream of the cell that may occur, for example, on the activation station is not taken into account.

[0169] Oxygen consumption steps with cathode stoichiometric coefficients strictly less than 1 are further defined as “global starvation” steps.

[0170] Alternatively, oxygen depletion step 5 can be designed to gradually shut off the air supply until the air is completely or partially shut off. On the other hand, if such a gradual shutoff of the air supply is implemented, oxygen depletion step 5 preferably begins by setting air inlet 32 to achieve a cathode stoichiometric coefficient strictly less than 1, and then, during step 5, adjusting air inlet 32 to gradually reduce the cathode stoichiometric coefficient. Thus, even when the air supply is gradually shut off, the cathode stoichiometric coefficient remains strictly less than 1. Preferably, when entering oxygen depletion step 5 from stabilization step 4, the cathode stoichiometric coefficient changes from a value significantly greater than 1, such as greater than or equal to 1.8, or even greater than or equal to 2, to a value strictly less than 1, preferably less than or equal to 0.9. This change in the cathode stoichiometric coefficient occurs gradually, i.e., almost instantaneously with respect to the cell's kinetics and its reaction to the change in the cathode stoichiometric coefficient. In other words, the change in the cathode stoichiometric coefficient is abrupt rather than gradual. It should be noted that the cathode stoichiometric coefficient, which is significantly greater than 1, such as greater than or equal to 1.8, can be maintained constantly at this value during steps 1 to 4.

[0171] from Figure 4 It can be clearly seen that in the oxygen consumption step 5, the battery voltage gradually decreases. Curve B in the figure schematically represents the situation where the air supply is completely cut off, that is, the cathode stoichiometric coefficient is equal to 0 or substantially equal to 0.

[0172] from Figure 4 As can be seen from curve B in FIG, during oxygen consumption step 5, the cell voltage decreases monotonically, i.e., the cell voltage decreases continuously, even if the duration is short or the amplitude is small, without any increase. In addition, the rate of decrease in the cell voltage increases monotonically, i.e., the cell voltage decreases more and more rapidly as the oxygen consumption step progresses. This cell voltage profile is caused by the gradual decrease in the amount of available oxygen in the anode compartment, which results in an increasingly rapid decrease in the cell voltage.

[0173] As is known from the art of reducing air above stoichiometric, Figure 4A graph of the cell voltage when the air over stoichiometry is reduced when the cathode stoichiometric coefficient is greater than or equal to 1 but less than the nominal cathode coefficient is also shown by curve A. Therefore, reducing the air over stoichiometry involves setting the cathode stoichiometric coefficient to a value between the nominal cathode stoichiometric coefficient and 1, and is achieved by reducing the air supply flow rate compared to the nominal flow rate. This reduction in air over stoichiometry can also be referred to as a reduction in oxygen over stoichiometry, or as a reduction in cathode over stoichiometry. As can be seen from curve A, this reduction in air supply, i.e., a reduction in air over stoichiometry, results in an average gradual decrease in the cell voltage, but the cell voltage fluctuates significantly. Such fluctuations lead to damage to the fuel cell, such as dissolution of platinum, and / or corrosion of the bipolar separator, and / or corrosion of carbon, which is typically one of the components of the catalytic layer, and / or degradation of the membrane due to the occurrence of hot spots.

[0174] In theory, because the stoichiometric coefficient is greater than or equal to 1, if the air is reduced beyond the stoichiometric amount, the fuel cell has sufficient oxygen to produce the desired current density at a stable operating point, i.e., to deliver a stable voltage at the fuel cell terminals. This stable operating point, as a function of the desired current density produced, is determined by the polarization curve of the fuel cell 12. However, in practice, when the cathode stoichiometric coefficient is lower than the nominal cathode stoichiometric coefficient and approaches 1, a local shortage of oxygen molecules is observed at the active sites of the cathode catalyst layer. To reach the active sites, the oxygen molecules must pass through the gas diffusion layer 18 and then penetrate the cathode catalyst layer 24, which requires a certain amount of travel time. Therefore, we observe that when an oxygen molecule reaches the active site, it is immediately consumed in the reduction reaction, which creates an attractive force that draws other oxygen molecules to the active site. However, due to the absence of an oversupply of oxygen, these other oxygen molecules only reach the active site after a certain period of time, during which a shortage of reactants is observed, i.e., no reduction reaction occurs at the active site during this period of time. For each active site, the time when the reduction reaction occurs and the time when the reduction reaction does not occur alternate. This alternation results in fluctuations in the cell voltage at the level of the cell 12 and at the level of the fuel cell 10, as shown by curve A. In practice, the choice of a nominal stoichiometric coefficient greater than 1 (e.g. equal to 1.8) also helps to avoid these fluctuations, since the excess oxygen ensures a continuous supply of oxygen to the active sites.

[0175] Furthermore, curve A shows that the cell voltage gradually decreases once the fluctuations have averaged out. This gradual decrease as air exceeds the stoichiometric value is caused by reversible and / or irreversible degradation occurring in the fuel cell. This degradation is a result of cell voltage fluctuations, and is particularly caused by a localized lack of hydration in the proton exchange membrane 20 of each cell 12, which itself is caused by a repeated lack of reduction reactions at the local active sites. Furthermore, the resulting cell voltage fluctuations lead to fluctuations in the efficiency of the fuel cell 10 and, therefore, in the fuel cell temperature. These fluctuations cause the fuel cell to deviate from its optimal operating parameters, which also leads to the observed degradation. It can also be seen that the average cell voltage observed on curve A tends to decrease and then stabilize at a value above 0 V. In fact, the closer the cathode stoichiometric coefficient approaches 1 (greater than 1), the more the average cell voltage observed on curve A (averaged over a time interval covering several fluctuations) tends to stabilize at an asymptotic value that decreases as the cathode stoichiometric coefficient approaches 1, but remains greater than 1 and does not reach zero. At the same time, the closer the cathode stoichiometric coefficient is to 1 (and greater than 1), the more the battery voltage fluctuates around the average value, and the amplitude of the fluctuation increases as the cathode stoichiometric coefficient approaches 1 and remains greater than 1. Finally, the closer the cathode stoichiometric coefficient is to 1 (greater than 1), the closer the minimum battery voltage observed when the A curve fluctuates is to 0V.

[0176] It should be noted that when the air supply is completely cut off, this fluctuation phenomenon is not observed on curve B. Because no new dioxygen molecules are supplied to the cathode compartment, the suction force generated at each active site cannot pull new oxygen molecules toward the active site. Therefore, on curve B, as each active site consumes more oxygen molecules, the cell voltage gradually decreases.

[0177] Figure 4Curve C also schematically illustrates the reduction in cell voltage observed with a partial air supply cutoff, where the cathode stoichiometric coefficient is strictly between 0 and 1. In the example of Curve C, the cathode stoichiometric coefficient is 0.5. Due to the partial air supply cutoff, a partial air supply is maintained, but this is insufficient to maintain the cell voltage. In practice, as the oxygen present in the cathode compartment at the start of step 5 is consumed, the oxygen molecules supplied by air inlet 32 are consumed at an increasingly rapid rate. As a result, the oxygen molecules are consumed by active sites closer and closer to air inlet 32, leaving them with less opportunity to reach active sites further away. Consequently, the dead zone becomes increasingly larger for each cell 12. In fact, with a partial air supply cutoff, the cell voltage decreases over a longer period of time than with a complete air supply cutoff. In other words, because the partial air supply is maintained, the decrease in cell voltage is slowed. Fluctuations of smaller magnitude than those observed in Curve A can also be observed. Because these fluctuations are less pronounced, their effects cause little or no damage to the fuel cell.

[0178] Therefore, it is particularly advantageous to completely cut off the air supply, or at least partially cut it off to an extent sufficient to make the cathode stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9, as is done in the present invention, in order to avoid or greatly limit cell voltage fluctuations and related damage. Due to the use of a cathode stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9, during the oxygen consumption step 5, this is especially true in the case of a complete cutoff of the air supply, which prevents damage to the fuel cell.

[0179] It should be noted that Figure 4 The curves in FIG. 5 show the trends of the cell voltage when different cathode stoichiometric coefficients are applied, but these curves are not derived from real data and are therefore provided only to illustrate the phenomena described above. In particular, Figure 4 The curves A, B and C in FIG do not represent actual battery voltage values.

[0180] Once the cell voltage reaches a predetermined threshold voltage, the oxygen consumption step 5 ends. In practice, this threshold voltage is between 0.1 V and 0.4 V. In this example, this threshold voltage is equal to 0.2 V. This minimum voltage threshold is selected to prevent damage to the cells 12 of the fuel cell 10. Excessively low cell voltages typically result in cell damage, such as excessive formation of hydrogen peroxide gas, which is harmful to the cells 12. By choosing to stop the oxygen consumption step when this threshold voltage is reached, these degradations are avoided.

[0181] In practice, the duration of the oxygen consumption step 5 is between 1 second and 120 seconds, preferably between 30 seconds and 60 seconds. In this example, the duration is about 45 seconds. In addition, the duration is often different in different cycles.

[0182] Furthermore, the duration of the oxygen consumption step 5 depends in particular on the current density during this step. The higher the current density, the shorter the duration of the oxygen consumption step.

[0183] During the oxygen consumption step 5, as Figure 5 As shown, there is a gradient in the amount of oxygen available in the cathode compartment, with more oxygen available at the air inlet 32 than at the air outlet 34 .

[0184] In practice, during the oxygen consumption step 5, oxygen consumption within the cathode compartment is relatively uniform from the air inlet 32 to the air outlet 34. However, during normal operation of the fuel cell 10, the stoichiometric coefficient is strictly greater than 1, and for example, during the stabilization step 4, an oxygen concentration gradient is always observed, with the oxygen concentration at the air inlet being greater than the oxygen concentration at the air outlet. This gradient is caused by the gradual consumption of oxygen by electrochemical reactions occurring at the level of the cathode catalytic layer 24 as air passes through the cathode compartment in the direction of the air flow between the air inlet and the air outlet. During normal operation of the fuel cell, despite this oxygen concentration gradient, sufficient oxygen is present (including in the active area closest to the air outlet) to ensure electrochemical reactions, so this gradient has no effect on the operation of the fuel cell. When the air supply is partially or completely cut off, at the start of the oxygen consumption step 5, the supplied oxygen is no longer sufficient to replace the consumed oxygen, and a decrease in oxygen concentration is observed. Therefore, this decrease is relatively uniform within the cathode compartment, but due to the oxygen concentration gradient observed before the start of the oxygen consumption step, the oxygen concentration will reach zero more quickly at the air outlet 34 than at the air inlet 32.

[0185] Thus, during the oxygen consumption step 5 , there is a zero oxygen concentration near the air outlet 34 , but a non-zero oxygen concentration near the air inlet 32 .

[0186] Therefore, in the half of the cathode compartment closest to the air inlet 32, Figure 5 Marked as "C", a normal reduction reaction occurs, consuming the available oxygen to form water molecules.

[0187] On the other hand, in the half of the cathode compartment closest to the air outlet 34, Figure 5 In the figure marked "D," i.e., when the oxygen concentration is minimal or zero, an electrochemical reaction is observed, consuming protons that have passed through membrane 20 and electrons from cathode side 16B of adjacent bipolar plate 16 to form dihydrogen molecules. This is because the reduction reaction that normally occurs in the cathode compartment cannot occur due to the lack of oxygen molecules, allowing electrons and protons to react together to form dihydrogen. This phenomenon is called "proton pumping."

[0188] The presence of dihydrogen in the cathode compartment and the low cell voltage create strong reducing conditions in the D half of the cathode compartment, which can reduce certain impurities located in the cathode catalyst layer 24 (in this half of the compartment) and reduce oxidized platinum. As a result of these reduction reactions, impurities are desorbed and platinum oxide is reduced to platinum particles on the one hand and to desorbed impurities on the other. This increases the amount of non-oxidized platinum available in the cathode catalyst layer and, therefore, increases the number of active sites while reducing the number of impurities in the cathode catalyst layer.

[0189] In practice, the reducing conditions achieved during oxygen consumption step 5 are more intense than those achieved during hydration step 2, and therefore make it possible to more effectively desorb impurities from the cathode catalytic layer, particularly in the D half of the cathode compartment during this first run-in phase P2. In particular, step 5 allows the desorption of impurities that could not be desorbed during step 2 due to insufficient reducing conditions. During steps 1 and 2, these desorbed impurities are largely removed from the cathode compartment.

[0190] Oxygen consumption step 5 also hydrates proton exchange membrane 20. In practice, the lack of oxygen in the cathode compartment, particularly in the half of the compartment closest to air outlet 34, leads to an increase in the molecular fraction of water at the interface between the membrane and the cathode catalyst layer. Furthermore, the lack of air flow in the cathode compartment results in poor drainage. This increases the amount of water in the membrane, hydrating it. More precisely, when the oxygen contained in the air is consumed without being renewed, or when it is renewed at a rate lower than the consumption rate, the molecular fraction of water in the composition of the air and water mixture present in the cathode compartment becomes greater.

[0191] Furthermore, the absence of air supply while maintaining the dihydrogen supply causes the pressure difference between the anode compartment and the cathode compartment to increase. This pressure difference causes new pores or channels to open in the anode catalyst layer 22 and the cathode catalyst layer 24, and existing pores to expand.

[0192] Advantageously, in order to further force the pores in the anode and cathode catalyst layers to open and to achieve more strongly reducing conditions, the difference between the pressure prevailing in the anode compartment and the pressure prevailing in the cathode compartment can be further increased, for example to a value equal to 500 mbar, by adjusting the pressure set point of the test stand, i.e., by adjusting the hydrogen pressure in the cathode compartment. By controlling the hydrogen inlet 28 and the air inlet 32, as well as the hydrogen outlet 30 and the air outlet 34, the pressures of the hydrogen and air can be controlled independently of their flow rates.

[0193] The end of the oxygen consumption step 5 marks the end of the cycle in the first run-in phase P2.

[0194] At the end of the cycle in the first running-in phase P2, it is checked whether the first stop condition has been reached. If the first stop condition has been reached, the first running-in phase P2 ends and the running-in process continues with the reverse operation P3.

[0195] If the first stop condition is not reached, the first run-in phase P2 continues with a new cycle, executing steps 1 to 5 again. The air supply to the fuel cell 10 is then restored, so that the oxygen consumption step ends and a new cycle can begin. Figure 3 As shown, when a new cycle of the first run-in phase P2 is executed, the oxygen consumption step 5 includes an increase in current density from a minimum value to a low value. This increase in current density coincides with the restoration of the air supply and occurs just before switching to a new cycle.

[0196] Preferably, the first stopping condition is reached when:

[0197] The difference between the battery voltage at the end of the hydration step 2 of the current cycle and the battery voltage at the end of the hydration step 2 of the previous cycle is between 1mV (millivolt) and 10mV, preferably equal to 5mV, otherwise

[0198] Preferably, when the first running-in phase P2 includes a stabilization step 4, the difference between the battery voltage at the end of the stabilization step 4 of the current cycle and the battery voltage at the end of the stabilization step 4 of the previous cycle is between 1 mV and 10 mV, preferably equal to 5 mV.

[0199] Therefore, the stop condition is a battery voltage stability condition.

[0200] Advantageously, the first stop condition is performed by comparing voltage measurements (i.e., cell voltage measurements or fuel cell terminal voltage measurements) at the end of the hydration step 2 rather than at the end of the stabilization step 4, because cell voltage measurements or fuel cell terminal voltage measurements are more accurate when the current flow rate is large. When the current flow rate is low, losses observed in the fuel cell 10 are more difficult to identify and result in inaccurate cell voltage measurements or inaccurate fuel cell terminal voltage measurements. The cell voltage of the fuel cell can be measured by measuring the voltage across a single cell, preferably by averaging the voltages measured across a plurality of cells, or by dividing the voltage at the fuel cell terminals by the number of cells in the fuel cell stack.

[0201] In practice, other conditions may be used. For example, the stopping condition may correspond to reaching a minimum performance standard, or completing a certain number of predetermined cycles.

[0202] When the stop condition is not based on a performance comparison between two consecutive cycles, the first run-in phase P2 may therefore comprise a single cycle. It is also possible not to define a specific stop condition and to choose to perform only one cycle in the first run-in phase P2 before starting the run-in process.

[0203] exist Figure 2 In the example shown, the first stop condition is reached after five cycles of steps 1 to 5 .

[0204] During reverse operation P3, the electrical load is controlled so that the fuel cell 10 does not need to generate any current, and the reactants, namely, dihydrogen and oxygen, are not supplied to the fuel cell. In other words, during this phase, the fuel cell is not operating. Therefore, at the end of the last cycle of phase P2, once the cell voltage reaches a predetermined threshold voltage, the fuel cell is shut down, although coolant flow through the fuel cell may still be maintained to control its temperature, if necessary.

[0205] The reverse operation P3 involves reversing, i.e., swapping, the hydrogen inlet 28 and the hydrogen outlet 30 on the one hand, and the air inlet 32 and the air outlet 34 on the other hand. In other words, during the reverse operation P3, the flow direction of the reactants in the fuel cell 10 is reversed. This reverses the flow direction of the reactants in the channels 44 on the anode side 16A and the cathode side 16B of the bipolar plate 16.

[0206] In practice, several methods can be used to reverse ingress and egress.

[0207] The first method consists in manually disconnecting the pipes connected to the hydrogen and air supply circuits and then reconnecting them by reversing the inlet and outlet. Preferably, when using this method, a purge of the cell 12 of the fuel cell 10 is performed before disconnecting the pipes with a neutral gas (preferably nitrogen) to prevent any chemical reaction at the level of the catalytic layers 22 and 24, and then a purge of the anode compartment with dihydrogen and of the cathode compartment with oxygen is performed before restarting the fuel cell to ensure a correct supply of reactants to the fuel cell.

[0208] Another approach is to use four-way valves to connect inlets 28, 32 and outlets 30, 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 to be swapped with the air outlet 34, without having to disconnect and reconnect the pipes. This approach of using four-way valves is advantageous because it prevents any gas other than the reactive gas from permeating into the anode and cathode compartments. In addition, when switching using four-way valves, there is no need to purge the circuit.

[0209] Once the reverse operation P3 has been performed, the fuel cell 10 is restarted and a second run-in phase P4 begins.

[0210] The second run-in phase P4 comprises a plurality of consecutive steps performed cyclically, which are preferably the same as the steps in the first run-in phase P2. Thus, each cycle of the second run-in phase P4 preferably comprises a current density increase step 1, a hydration step 2, a current density decrease step 3, a stabilization step 4, and an oxygen consumption step 5.

[0211] Since these steps are the same as those in the first running-in phase P2, they will not be described in further detail below.

[0212] Alternatively, the cycles of the second running-in phase P4 differ from the cycles of the first running-in phase P2 in that they do not include the same steps in steps 1 to 5. For example, each cycle of the first running-in phase P2 includes steps 1, 2, 3, 4, and 5, and each cycle of the second running-in phase P4 includes steps 1, 2, 3, and 5, and thus does not include the stabilization step 4. According to another example, each cycle of the first running-in phase P2 includes steps 1, 2, 3, and 5, and thus does not include the stabilization step 4, and each cycle of the second running-in phase P4 includes steps 1, 2, 3, 4, and 5. According to another example, each cycle of the first running-in phase P2 includes steps 4 and 5, and each cycle of the second running-in phase P4 includes only step 5, and thus does not include the stabilization step 4. According to another example, each cycle of the first running-in phase P2 includes only step 5, and thus does not include the stabilization step 4, and each cycle of the second running-in phase P4 includes steps 4 and 5.

[0213] Therefore, the second running-in stage P4 offers the same advantages as the first running-in stage P2, namely, by alternating steps 1 to 5, the membrane 20 can be hydrated to increase the number of active sites in the catalytic layers 22 and 24, in particular by changing their porosity, to reduce, oxidize and desorb certain impurities and remove these impurities from the catalytic layers.

[0214] The sequential execution of the first running-in phase P2 , the reversal operation P3 and the second running-in phase P4 is particularly advantageous because this sequence enables uniform running-in to be achieved over the entire surface of the membrane electrode assembly 14 .

[0215] At the end of the first run-in phase P2, the cathode catalytic layer 24 is not uniformly run-in, because during the oxygen consumption step 5, there is a gradient in the amount of available oxygen between the C and D halves of the cathode compartment, which, as previously explained, results in reducing conditions occurring only in the D half of the cathode compartment. Therefore, the oxygen consumption step 5 of the first run-in phase P2 primarily removes impurities from the D half of the cathode compartment.

[0216] However, because the second run-in phase P4 occurs after the reversal operation P3, it can be seen that the available oxygen gradient observed during the oxygen consumption step 5 of the second run-in phase is reversed compared to the available oxygen gradient observed during step 5 of the first run-in phase P2.

[0217] In other words, the impurity desorption imbalance observed during the first run-in period is also observed during the second run-in period, but in reverse, i.e., during the first run-in period, impurity desorption occurs primarily in the first physical half of the cathode compartment, whereas during the second run-in period, impurity desorption occurs primarily in the second physical half of the cathode compartment.

[0218] Thus, during the second run-in phase P4 , the oxygen consumption step 5 primarily enables the desorption of impurities from the C-half of the cathode compartment.

[0219] Therefore, the sequence of phase P2, operation P3, and phase P4 enables uniform impurity desorption across the entire surface of the cathode catalyst layer 24, thereby uniformly improving the performance of the membrane-electrode assembly 14.

[0220] Furthermore, this sequence is particularly advantageous for optimizing the duration of the run-in protocol by enabling the cell to be fully and evenly operated across the entire surface of the cathode catalyst layer 24 more quickly than known run-in protocols.

[0221] At the end of the cycle in the second running-in phase P4, it is checked whether the second stop condition has been reached. If the second stop condition has been reached, the second running-in phase P4 ends and, in some embodiments, the running-in process continues with, for example, a control phase P5.

[0222] If the second stop condition is not met, the second run-in phase P4 continues with a new cycle of steps 1 to 5, with the reactant gas flow direction being the same as during the first cycle of this second run-in phase P4. The air supply to the fuel cell 10 is then restored, allowing the oxygen consumption step to end and a new cycle to begin.

[0223] Preferably, the second stopping condition is reached when:

[0224] The difference between the battery voltage at the end of the hydration step 2 of the current cycle and the battery voltage at the end of the hydration step 2 of the previous cycle is between 1mV (millivolt) and 10mV, preferably equal to 5mV, otherwise

[0225] The difference between the battery voltage at the end of the stabilization step 4 of the current cycle and the battery voltage at the end of the stabilization step 4 of the previous cycle is between 1 mV and 10 mV, preferably equal to 5 mV.

[0226] Therefore, the stop condition is a battery voltage stability condition.

[0227] Advantageously, the second stop condition is performed by comparing voltage measurements (i.e., cell voltage measurements or voltage measurements at the fuel cell terminals) at the end of the hydration step 2 rather than at the end of the stabilization step 4, because cell voltage measurements or voltage measurements at the fuel cell terminals are more accurate when the current flow is high. When the current flow rate is low, losses observed in the fuel cell 10 are more difficult to identify and result in inaccurate cell voltage measurements or inaccurate voltage measurements at the fuel cell terminals.

[0228] In practice, other conditions may be used. For example, the stopping condition may correspond to reaching a minimum performance standard, or completing a certain number of predetermined cycles.

[0229] When the stop condition is not based on a performance comparison between two consecutive cycles, the second running-in phase P4 may therefore comprise a single cycle. It is also possible not to define a specific stop condition and to choose to perform only one cycle in the first running-in phase P2 before starting the running-in process.

[0230] Preferably, as in this example, the second stop condition is identical to the first stop condition.

[0231] Preferably, as in the example, the second running-in phase P4 comprises the same number of cycles as the first running-in phase P2.

[0232] exist Figure 2 In the example shown, the first stop condition is reached after five cycles of steps 1 to 5 .

[0233] During the optional control phase P5, an electrical load applies a constant current density to the fuel cell 10 for a predetermined period of time, and the evolution of the cell voltage is observed. This optional P5 control phase is used to check whether the stack performance is fully stable. If the cell voltage is observed to remain stable throughout the P5 control phase, it is confirmed that the run-in protocol has been correctly executed and the fuel cell is ready for commissioning.

[0234] The duration of the optional P5 control phase is preferably 45 to 75 minutes. In this example, this is 60 minutes.

[0235] The current density during the control phase P5 is preferably 0.3 A / cm 2 Up to 1.9A / cm 2 In this example, the current density is equal to 1A / cm 2 Furthermore, in this example, the P5 control phase lasts for 60 minutes. The duration of the control phase may vary. The duration is preferably longer than 10 minutes, for example between 10 and 100 minutes.

[0236] Additionally, measuring the cell voltage during the test phase enables measurement of the performance gain of the fuel cell 10 provided by the break-in protocol.

[0237] For example, in a fuel cell composed of a stack of multiple cells 12, the effective surface area of each membrane-electrode assembly 14 is equal to 250 cm 2 , when the current density applied to the fuel cell by the electric load is 1A / cm 2 The run-in protocol can provide a performance gain on the cell voltage of the fuel cell of between 10% and 50%, for example, 30%, with a total duration of between 20 minutes and 300 minutes. The performance gain can be best measured by comparing the electrical energy generated per kilogram of hydrogen consumed by the fuel cell 10 through the electrochemical reaction before and after the run-in protocol, for example, the electrical energy generated by the fuel cell is measured in Wh.

[0238] In a known manner, in practice it can be considered that the current density delivered by the fuel cell is imposed and controlled by the electrical load connected to the terminals of the fuel cell 10, so that, as long as the amount of reactants (as determined by Faraday's law) is sufficient to drive the electrochemical reactions occurring in the anode compartment and the cathode compartment, in the anode catalyst layer 22 and the cathode catalyst layer 24, that is, as long as the flow rate of the reactant gases supplied to the inlets of dihydrogen 28 and air 32 is sufficient, the current density does not depend on the amount of reactants provided by the anode compartment and the cathode compartment. In other words, an excess of hydrogen and air does not lead to an increase in the current density. On the contrary, a lack of hydrogen and / or air and therefore a lack of oxygen leads to a drop in the cell voltage, such as in the oxygen consumption step 5 in the first running-in phase P2 and the second running-in phase P4.

[0239] In practice, Faraday's law states that the current supplied by an electrochemical reaction is a direct measure of the speed of the electrochemical reaction, that is, the molar rate of consumption of the reactants. Therefore, the current i supplied by an electrochemical reaction is equal to:

[0240] i=n×F×v

[0241] Where i is the current in amperes; n is the number of electrons transferred in the electrochemical reaction; F is the Faraday constant, approximately equal to 96,485 C / mol (coulombs / mole); and v is the reaction rate in mol / s (mole / second), also known as the molar consumption rate, which is equivalent to the rate at which the reactants are consumed.

[0242] Therefore, as long as there is sufficient molar flow of reactants at the active sites, the reaction rate v and therefore the molar flow rate of consumed reactants depends on the current provided by the electrochemical reaction occurring at the active sites and will not change due to excess reactants.

[0243] Advantageously, throughout the run-in phases P2 and P4, the anode stoichiometry is constant and equal to the nominal anode stoichiometry of the fuel cell 10, preferably between 1.3 and 1.5, and even more preferably equal to 1.5. This relatively low value limits the consumption of dihydrogen during the run-in process, thereby reducing the cost of performing the run-in process.

[0244] Advantageously, throughout the run-in phases P2 and P4, the cathode stoichiometric coefficient is constant and greater than the nominal cathode stoichiometric coefficient of the fuel cell 10, except during the oxygen consumption phase 5, when the cathode stoichiometric coefficient is strictly less than 1, preferably less than 0.9, and more preferably equal to 0. Preferably, throughout the run-in phases P2 and P4, except during the oxygen consumption phase 5, the cathode stoichiometric coefficient is greater than 2 for fuel cells with a humidity level in the cathode compartment between 30% and 80%. In this example, the cathode stoichiometric coefficient is 2.3.

[0245] This relatively high cathode stoichiometric coefficient reduces the duration of the run-in scheme. The high flow rate of oxygen (more precisely, air) in the cathode compartment causes an increase in the gas flow in the pores of the cathode catalyst layer, thereby enlarging the existing pores or channels and opening new channels in the cathode catalyst layer, thereby increasing the number of active sites in the cathode catalyst layer. The high flow rate also helps to remove impurities desorbed from the cathode catalyst layer 24 throughout the run-in process. The removal of such impurities is particularly effective during the hydration step 2 of the run-in phases P2 and P4. In addition, the relatively high cathode stoichiometric coefficient (which therefore applies a relatively high flow rate at the cathode) can improve the response time of the fuel cell and thus reduce the duration of the current density increase step 1 of the run-in phases P2 and P4.

[0246] Compared to known running-in methods, the running-in method according to the invention has many advantages.

[0247] First, the run-in process is particularly effective in improving the performance of the fuel cell 10 without causing damage that could reduce the life of the fuel cell. In particular, the alternation between the oxygen consumption step 5 and the lower cell voltage step (i.e., the hydration step 2) promotes the desorption of impurities and contaminants present in the catalytic layers 22 and 24 by continuously reducing and oxidizing the maximum number of impurities. The method also utilizes all the means for improving the performance of the fuel cell 10, namely, hydration of the proton exchange membrane 20, increasing the number of active sites in the catalytic layer, and desorption and removal of impurities and contaminants present in and on the surface of the catalytic layer. In particular, damage to the fuel cell is prevented by the stabilization step 4, which allows the hydration step 2 and the oxygen consumption step 5 to proceed without creating adverse operating conditions for the fuel cell, while completely or partially shutting off air, i.e., oxygen as defined above, during the oxygen consumption step, thereby avoiding cell voltage fluctuations that are harmful to the fuel cell.

[0248] Secondly, the run-in process enables uniform performance across the entire surface of the membrane-electrode assembly 14, primarily due to the reversal operation P3. This eliminates uniformity deficiencies, particularly those caused by the oxygen depletion step 5, and also eliminates uniformity deficiencies generally found in fuel cells. The reversal also facilitates the hydration step 2 and the stabilization step 4. Indeed, given the continued presence of an oxygen concentration gradient in the cathode compartment, performing the stabilization step 4 before and after the reversal operation P3, as described above, also results in more uniform membrane hydration and impurity oxidation / reduction.

[0249] Furthermore, the running-in time of the running-in process is advantageously shortened due to a series of steps for effectively reducing and then oxidizing impurities in the catalytic layer, in particular due to the sequential execution of the hydration 2 and oxygen consumption 5 steps.

[0250] Furthermore, dihydrogen consumption remains controlled throughout the run-in process, particularly due to the relatively low anode stoichiometry and the short total time spent in the high current phase, which consumes more reactants. This means that the run-in process uses less dihydrogen, which is advantageous given that dihydrogen is typically an expensive gas.

[0251] The run-in protocol facilitates the achievement of a catalyst layer morphology that is not achievable with protocols that do not involve oxygen depletion or gas reversal. The resulting catalyst layer and membrane morphology enables cell efficiency and durability levels unattainable with protocols that do not involve oxygen depletion or gas reversal. This favorable morphology corresponds particularly to a greater total platinum active surface area, primarily driven by the cathode oxygen consumption step 5.

[0252] Finally, the run-in method is particularly easy to set up. It does not require any special additional parts, as the entire run-in process is performed simply by applying a current cycle and controlling the delivery of reactants, particularly during the oxygen consumption step and the reverse operation P3. Therefore, installation is economical.

[0253] In this specification, the primary focus has been on the phenomenon that the performance of the cathode catalyst layer 24 can be improved by increasing the number of active sites, by increasing its porosity, and by desorption of impurities, while improvements in the performance of the anode catalyst layer 22 have been less discussed. In practice, it is well known that achieving good performance in the anode catalyst layer is easier than in the cathode catalyst layer for several reasons. First, because the anode compartment is typically supplied with pure gas, particularly hydrogen, it is less exposed to pollutants than the cathode compartment, which is typically supplied with air that may contain a variety of pollutants. Furthermore, the cathode catalyst layer is thicker and contains more platinum particles than the anode catalyst layer, requiring longer time to increase the number of active sites and desorb impurities. Finally, oxygen tends to deposit on the cathode catalyst layer, forming platinum oxide that needs to be reduced during the run-in process, while this does not occur in the anode catalyst layer. In this way, performance improvement targets for the anode catalyst layer are more easily achieved than those for the cathode catalyst layer, meaning that by the time the cathode catalyst layer has achieved a satisfactory performance level, the anode catalyst layer has reached the same level.

[0254] It should be noted that the concepts of high, low, and minimum values related to current density in this specification are arbitrary and are understood only in conjunction with each other during the run-in process described herein. They have no connection to the actual current density values corresponding to a specific application. Throughout this specification, 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 relative 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.

[0255] In a variant of the present invention (not shown), the high, low, and minimum values of the first run-in phase P2 differ from the high, low, and minimum values of the second run-in phase P4. For example, to account for variations in the performance of the fuel cell 10 during the run-in process, the high, low, and minimum values of the second run-in phase P4 may be higher or lower than the high, low, and minimum values of the first run-in phase P2. This variation in current density values may be determined so that the same cathode potential level is achieved at different steps and / or stages, thereby generating the desired oxidation level, particularly at low current density values, and the desired reduction level, particularly at high current density values, which, if possible, remains the same throughout the activation cycle, depending on the evolution of the fuel cell's characteristics during the activation process.

[0256] In a variant of the present invention (not shown), the run-in process does not include an initialization phase P1. In such a variant, the fuel cell 10 is initialized before being installed on the activation station, for example, at the end of its assembly. Alternatively, the initialization phase P1 may be omitted, and a current density increase step 1 may be provided in the first cycle of the first operating phase P2. This step is preferably long enough to avoid damage to the fuel cell.

[0257] In a variant of the invention not shown, the run-in process does not include the control phase P5. In such a variant, the stability of the cell voltage can be assessed in another way, or it can not be assessed until the fuel cell is put into operation.

[0258] In an unrepresented variation of the present invention, the run-in phases P2 and P4 do not include steps 1, 2, and 3. In this variation, the run-in phase therefore only includes a stabilization step 4 and an oxygen consumption step 5. The advantage of this variation is that the amount of dihydrogen consumed during the run-in process can be minimized because the fuel cell is not limited by the electrical load and produces a high current density, thereby leading to high reactant consumption. However, in this variation, the run-in process takes longer. In particular, the run-in phases P2 and P4 include a greater number of cycles. In the absence of the hydration step 2, the hydration of the proton exchange membrane 20 can also be achieved through the oxygen consumption step 5, but a longer run-in time is required.

[0259] In a non-illustrated variant of the invention, the compactness of the stack of multiple cells 12 achieved by the end plates is monitored throughout the run-in process and adjusted to maintain a constant compactness of the stack of multiple cells, thereby accounting for any cell dimensional changes that may occur during the run-in process. The hydration of the proton exchange membrane 20 and the physicochemical modification of the catalytic layers 22 and 24 can lead to variations in the thickness of these components during the run-in process.

[0260] In a non-representative variant of the invention, during the hydration step 2 of the run-in phases P2 and P4, the temperature inside the cell 12 is raised to 100° C. (either during the entire duration of these steps or for a short period of time during these steps) to avoid the re-adsorption of impurities and pollutants in the catalytic layers 22 and 24. This temperature increase prevents flooding of the proton exchange membrane by increasing the relative humidity of the reactive gases.

[0261] In this example, the predetermined threshold voltage is constant throughout the run-in process, i.e., it is the same for all oxygen consumption steps 5 of the run-in phases P2 and P4. In a non-representative variant of the present invention, the predetermined threshold voltage may be slightly adjusted during the run-in process, e.g., in the second run-in phase P4, the threshold voltage may be slightly higher or lower than in the first run-in phase P2. However, these variations are small enough that the predetermined threshold voltage of the first run-in phase P2 is substantially equal to the predetermined threshold voltage of the second run-in phase P4.

[0262] A particularly advantageous mode of the invention corresponds to a method for running-in a fuel cell 10 comprising a stack of a plurality of cells 12, each cell comprising a proton exchange membrane 20 arranged between two bipolar plates 16, each bipolar plate 16 delimiting a reaction compartment in the cell with the proton exchange membrane 20, each cell therefore comprising a cathode compartment in which a cathode catalyst layer 24 is arranged and an anode compartment in which an anode catalyst layer 22 is arranged.

[0263] The fuel cell includes a dihydrogen inlet 28 for supplying dihydrogen to the anode compartment of each cell and a dihydrogen outlet 30 for exhausting dihydrogen from each cell. The fuel cell includes an air inlet 32 for supplying air to the cathode compartment of each cell 12 and an air outlet 34 for exhausting air from each cell 12.

[0264] The running-in method according to this particularly advantageous mode of the invention comprises at least the following steps in sequence:

[0265] The first running-in phase P2 includes at least the following steps in sequence:

[0266] a stabilization step 4 of the fuel cell 10, during which the current density produced by the fuel cell is kept constant at a low value for a predetermined period of time, and

[0267] an oxygen consumption step 5 of the fuel cell, during which the current density generated by the fuel cell is kept constant at a minimum value, which is less than or equal to the low value, and during which the air supply to the fuel cell via the air inlet 32 is at least partially cut off, so as to obtain a cathode stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9, by such an arrangement. When the cell voltage of the fuel cell 10 reaches a predetermined threshold voltage, the oxygen consumption step of the first run-in stage ends;

[0268] Operation P3, which reverses the direction of hydrogen and air flows, wherein the dihydrogen inlet 28 and the dihydrogen outlet 30 are reversed, and wherein the air inlet 32 and the air outlet 34 are reversed, and

[0269] The second running-in phase P4 includes at least the following steps in sequence:

[0270] a stabilization step 4 of the fuel cell 10, during which the current density produced by the fuel cell is kept constant at a low value for a predetermined period of time, and

[0271] The fuel cell performs an oxygen consumption step 5, during which the current density generated by the fuel cell is kept constant at a minimum value, which is less than or equal to the low value of the second run-in stage, and during which the air supply to the fuel cell via the air inlet 32 is at least partially cut off, so as to obtain a cathode stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.9, by such adjustment. The oxygen consumption step of the second run-in stage ends when the cell voltage of the fuel cell 10 reaches a predetermined threshold voltage.

[0272] Furthermore, the steps of the first running-in phase P2 are performed until a first stop condition is reached, and the steps of the second running-in phase P4 are performed until a second stop condition is reached.

[0273] Where technically feasible, any functions described in one embodiment or variant above may be used in other embodiments and variants above.

Claims

1. A method for running-in a fuel cell (10), said fuel cell comprising a stack of a plurality of cells (12), each of said cells comprising a proton exchange membrane (20) disposed between two bipolar plates (16), each bipolar plate (16) delimiting a reaction compartment in said cell with said proton exchange membrane (20), each of said cells thus comprising a cathode compartment in which a cathode catalyst layer (24) is disposed and an anode compartment in which an anode catalyst layer (22) is disposed, The fuel cell comprises two hydrogen inlets (28) for supplying hydrogen to the anode compartment of each of the cells and two hydrogen outlets (30) for discharging hydrogen from each of the cells, The fuel cell includes an air inlet (32) for supplying air to the cathode compartment of each of the cells (12) and an air outlet (34) for exhausting air from each of the cells (12). It is characterized by: The running-in process includes at least the following stages in sequence: a first run-in phase (P2) comprising at least an oxygen consumption step (5) of the fuel cell, during which the current density generated by the fuel cell is kept constant at a minimum value, and during which the air supply to the fuel cell via the air inlet (32) is at least partially cut off to cause a gradual decrease in the cell voltage, and the oxygen consumption step of the first run-in phase ends when the cell voltage of the fuel cell (10) reaches a predetermined threshold voltage; - an operation (P3) of reversing the direction of the hydrogen and air flows, wherein the dihydrogen inlet (28) and the dihydrogen outlet (30) are reversed, and the air inlet (32) and the air outlet (34) are reversed; as well as a second run-in phase (P4), comprising at least an oxygen consumption step (5) of the fuel cell, during which the current density generated by the fuel cell is kept constant at a minimum value, and during which the air supply to the fuel cell via the air inlet (32) is at least partially cut off to cause a gradual decrease in the cell voltage, the oxygen consumption step of the second run-in phase ending when the cell voltage of the fuel cell (10) reaches a predetermined threshold voltage; wherein at least one step of the first running-in phase (P2) is performed until a first stop condition is reached; Therein, at least one step of the second running-in phase (P4) is performed until a second stop condition is reached.

2. The method for running-in a fuel cell (10) according to claim 1, wherein: Before the oxygen consumption step (5), the first running-in stage (P2) further includes a stabilization step (4) of the fuel cell (10), during which the current density generated by the fuel cell is kept constant at a low value for a predetermined period of time, and the low value of the first running-in stage is greater than or equal to the minimum value; and / or, before the oxygen consumption step (5), the second running-in stage (P4) further includes a stabilization step (4) of the fuel cell (10), during which the current density generated by the fuel cell is kept constant at a low value for a predetermined period of time, and the low value of the second running-in stage is greater than or equal to the minimum value.

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

4. A method for running-in a fuel cell (10) according to a combination of claims 2 and 3, wherein: The low value of the first running-in stage (P2) and the low value of the second running-in stage (P4) are 0.03A / cm 2 Up to 0.5A / cm 2 between, preferably equal to 0.3A / cm 2 .

5. A method for running-in a fuel cell (10) according to any one of the preceding claims, wherein: During at least one of the oxygen consumption steps (5), the air supply to the fuel cell via the air inlet (32) is at least partially shut off by regulating to obtain a cathode stoichiometric coefficient strictly less than 1, preferably less than or equal to 0.

9.

6. A method for running-in a fuel cell (10) according to any one of the preceding claims, wherein: When the cell voltage of the fuel cell (10) reaches a threshold voltage between 0.1V and 0.4V, preferably equal to 0.2V, the oxygen consumption step (5) of the first running-in phase (P2) and the second running-in phase (P4) ends.

7. A method for running-in a fuel cell (10) according to any one of the preceding claims, wherein: During the oxygen consumption step (5) of the first run-in phase (P2) and the second run-in phase (P4), the air supply to the fuel cell (10) via the air inlet (32) is completely cut off to obtain a cathode stoichiometric coefficient equal to 0.

8. A method for running-in a fuel cell (10) according to any one of the preceding claims, wherein: During the oxygen consumption step (5) of the first run-in phase (P2) and the second run-in phase (P4), the air supply to the fuel cell (10) via the air inlet (32) is controlled so that the cell voltage of the fuel cell (10) decreases monotonically.

9. A method for running-in a fuel cell (10) according to any one of the preceding claims, wherein: The first running-in phase (P2) further comprises the following steps, which are carried out in sequence before the oxygen consumption step (5): o a current density increasing step (1), during which the current density generated by the fuel cell (10) is gradually increased from a low value to a high value, the low value being greater than or equal to the minimum value of the first running-in stage, o a hydration step (2) of the fuel cell (10), during which the current density generated by the fuel cell is kept constant at the high value of the first run-in phase for a predetermined period of time in order to hydrate the proton exchange membrane (20), and o a current density decreasing step (3), during which the current density generated by the fuel cell decreases from a high value of the first run-in stage to a low value of the first run-in stage; The second running-in phase (P4) further comprises the following steps, which are performed in sequence before the stabilization step (4) and the oxygen consumption step (5): o a current density increasing step (1), during which the current density generated by the fuel cell (10) gradually increases from a low value of the second running-in stage to a high value of the second running-in stage, the low value of the second running-in stage being greater than or equal to a minimum value of the second running-in stage, o a hydration step (2) of the fuel cell (10), during which the current density generated by the fuel cell is kept constant at the high value of the second run-in phase for a predetermined period of time to hydrate the proton exchange membrane, and o a current density decreasing step (3), during which the current density generated by the fuel cell decreases from the high value of the second run-in stage to the low value of the second run-in stage.

10. The method for running-in a fuel cell according to claim 9, wherein: The high value of the first running-in stage (P2) and the high value of the second running-in stage (P4) are both 1.5A / cm 2 Up to 3A / cm 2 between, preferably equal to 1.9A / cm 2 .

11. The method for running-in a fuel cell (10) according to claim 9 or 10, wherein: - the current density increasing step (1), the hydration step (2), the current density decreasing step (3), the stabilization step (4) and the oxygen consumption step (5) of the first running-in phase (P2) are performed at least twice in a cyclic manner, - The first stopping condition is reached when: o the cell voltage of the fuel cell (10) at the end of the hydration step (2) of the last cycle of the first run-in phase differs from the value of the cell voltage of the fuel cell (10) at the end of the hydration step (2) of the second to last cycle of the first run-in phase by 1 mV to 10 mV, preferably by 5 mV, or o where appropriate, the cell voltage of the fuel cell (10) at the end of the stabilization step (4) of the last cycle of the first run-in phase differs from the value of 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 run-in phase by 1 mV to 10 mV, preferably by 5 mV, - the current density increasing step (1), the hydration step (2), the current density decreasing step (3), the stabilization step (4) and the oxygen consumption step (5) of the second running-in phase (P4) are performed at least twice in a cyclic manner, and - The second stopping condition is reached when: o the cell voltage of the fuel cell (10) at the end of the hydration step (2) of the last cycle of the second run-in phase differs from the value of the cell voltage of the fuel cell (10) at the end of the hydration step (2) of the second to last cycle of the second run-in phase by 1 mV to 10 mV, preferably by 5 mV, or o Where appropriate, the cell voltage of the fuel cell (10) at the end of the stabilization step (4) of the last cycle of the second run-in phase differs from the value of 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 run-in phase by 1 mV to 10 mV, preferably by 5 mV.

12. The method of running-in a fuel cell (10) according to any one of claims 9 to 11, further comprising an initialization phase (P1) performed before the first running-in phase (P2), during which the current density generated by the fuel cell (10) is gradually increased from a zero value to the high value of the first running-in phase, and wherein Preferably, said initialization phase (P1) occurs during a period of time comprised between 15 and 45 minutes, more preferably during a period of time equal to 24 minutes.

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

14. The method for running-in a fuel cell (10) according to any one of claims 9 to 13, wherein: - during the first run-in phase (P2) and the second run-in phase (P4), the anode stoichiometric coefficient of the fuel cell (10) is equal to the nominal anode stoichiometric coefficient of the fuel cell, preferably between 1.3 and 2, more preferably equal to 1.5, and - During the current density increase step (1), the hydration step (2), the current density decrease step (3) and, if appropriate, the stabilization step (4) of the first run-in phase (P2) and the second run-in phase (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, more preferably equal to 2.

3.

15. A method for running-in a fuel cell (10) according to any one of the preceding claims, wherein: During the first running-in phase (P2) and the second running-in phase (P4), an electrical load of a variable resistor is connected to the terminals of the fuel cell (10), the electrical load causing the fuel cell to generate an electric current.

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