Method for conditioning an electrochemical cell unit

Abstract

The invention relates to a method for conditioning an electrochemical cell unit (1) having stacked electrochemical cells (2), comprising the steps: - introducing a primary process fluid into the electrochemical cell unit (1) through a first media port (31) for the primary process fluid, conducting the primary process fluid through channels (13) for the first primary process fluid in a first direction (51a) and discharging the primary process fluid from the electrochemical cell unit (1) through a second media port (41) for the primary process fluid; - introducing a secondary process fluid into the electrochemical cell unit (1) through a first media port (32) for the secondary process fluid, conducting the secondary process fluid through channels (12) for the secondary process fluid in a first direction (51b) and discharging the secondary process fluid from the electrochemical cell unit (1) through a second media port (42) for the secondary process fluid; - carrying out a first conditioning step; - reversing the flow direction of the primary process fluid such that the primary process fluid is introduced into the electrochemical cell unit (1) through the second media port (41) for the primary process fluid, the primary process fluid is conducted through channels (13) for the primary process fluid in a second direction (52a) which is opposite to the first direction (51a) and the primary process fluid is discharged from the electrochemical cell unit (1) through the first media port (31) for the primary process fluid; and - carrying out a second conditioning step.

Description

[0001] Description

[0002] title

[0003] Method for conditioning an electrochemical cell unit

[0004] The present invention relates to a method for conditioning an electrochemical cell unit.

[0005] State of the art

[0006] Fuel cell units as galvanic cells convert continuously supplied fuel and oxidant into electrical energy and water by means of redox reactions at an anode and cathode. Fuel cells are used in a wide variety of stationary and mobile applications, for example in homes without a connection to a power grid or in motor vehicles, in rail transport, aviation, aerospace, and shipping. In fuel cell units, a large number of fuel cells are arranged in a fuel cell stack. Within each fuel cell there is a gas space for the oxidant, i.e. a flow space for the oxidant, such as ambient air with oxygen, to pass through. The gas space for the oxidant is formed by channels on the bipolar plate and by a gas diffusion layer for a cathode.The channels are thus formed by a corresponding channel structure of a bipolar plate, and the oxidant, namely oxygen, passes through the gas diffusion layer to the cathode of the fuel cell. Similarly, a gas space for fuel is provided.

[0007] Electrolysis cell units consisting of stacked electrolysis cells, similar to fuel cell units, are used, for example, for the electrolytic production of hydrogen and oxygen from water. Furthermore, fuel cell units are known that can be operated as reversible fuel cell units and thus as electrolysis cell units. Fuel cell units and electrolysis cell units form electrochemical cell units. Fuel cells and electrolysis cells form electrochemical cells.

[0008] In electrochemical cell units, process fluids are used to convert electrochemical energy into electrical energy and / or electrical energy into electrochemical energy. The process fluids are thus channeled through process fluid channels in a stack of electrochemical cells. During operation, the process fluids are always channeled through the channels in the same flow direction.

[0009] Before commissioning, an electrochemical cell unit is typically conditioned to moisten the membrane and clean or activate the catalysts. The objective of the present invention is to achieve the fastest, most homogeneous, and process-robust conditioning of the electrochemical cells possible.

[0010] Disclosure of the invention

[0011] The method for conditioning an electrochemical cell unit with stacked electrochemical cells comprises the following steps:

[0012] Introducing a primary process fluid through a first media connection for the primary process fluid into the electrochemical cell unit, guiding the primary process fluid through channels for the primary process fluid in a first direction, discharging the primary process fluid from the electrochemical cell unit through a second media connection for the primary process fluid, introducing a secondary process fluid through a first media connection for the secondary process fluid into the electrochemical cell unit, guiding the secondary process fluid through channels for the secondary process fluid in a first direction, discharging the secondary process fluid from the electrochemical cell unit through a second media connection for the secondary process fluid, performing a first conditioning step,

[0013] Reversing the flow direction of the primary process fluid such that the introduction of the primary process fluid into the electrochemical cell unit is carried out through the second media connection for the primary process fluid and the conduction of the primary process fluid through channels for the primary process fluid is carried out in a second direction opposite to the first direction and the discharge of the primary process fluid from the electrochemical cell unit is carried out through the first media connection for the primary process fluid, performing a second conditioning step.

[0014] The primary process fluid is advantageously the oxidant or the fuel. The secondary process fluid is advantageously the other corresponding fluid. Channels can also be understood as pores in a porous layer, so that the fluid flows through the pores essentially in one direction between two media connections (i.e., inlet and outlet).

[0015] A conditioning step differs from normal operation of the cell unit in at least one additional measure, namely to moisten the membrane and / or clean or activate the catalysts. The conditioning step can also be integrated into the normal operation of the cell unit. Typically, however, the conditioning process is performed before commissioning of the cell unit, i.e., at the end of the cell unit manufacturing process; however, the conditioning process can also be performed during maintenance of the cell unit.

[0016] Advantageously, the conditioning process with two conditioning steps, which are carried out with opposing flow directions (at least for one of the process fluids), enables the membrane and electrodes of the electrochemical cells to operate very homogeneously across the active area of ​​the cell. This results in an improvement in the proton conductivity of the membrane and electrode layers, an improvement in the activity of the catalysts, and an improvement in the electrical conductivities. According to the invention, these improvements are now also more evenly distributed across the active area of ​​the electrochemical cell.

[0017] The performance of the individual cells required for commissioning the cell unit is achieved comparatively quickly using the proposed methods. The conditioning method according to the invention conditioned the active surface more evenly and more quickly.

[0018] The efficiency of the conditioning process is strongly influenced by parameters within the cell stack, such as temperature, but also humidity and the concentration of the process fluids. Humidity and concentrations, in particular, vary along a channel between the media supply connection and the media discharge connection. This initially results in uneven conditioning in the electrochemical cell; reversing the flow direction before the second conditioning step then ensures homogenization of the conditioning processes. The thermal, electrical, and / or chemical load during the conditioning process is thus significantly more homogeneous across the active area than with a conditioning process without reversing the flow direction. As a result, uneven aging of the electrochemical cells is even reduced to a certain extent during operation of the electrochemical cell unit, or even prevented.

[0019] In advantageous developments of the method, the flow direction of the secondary process fluid is also reversed, so that the introduction of the secondary process fluid into the electrochemical cell unit is carried out through the second media connection for the secondary process fluid, the conduction of the secondary process fluid through channels for the secondary process fluid is carried out in a second direction opposite to the first direction, and the discharge of the secondary process fluid from the electrochemical cell unit is carried out through the first media connection for the secondary process fluid. The reversal of the flow direction for the secondary process fluid preferably takes place before carrying out the second conditioning step or before carrying out a third conditioning step. This means that the flow directions for the primary and secondary process fluid are switched either simultaneously or successively.

[0020] In a supplementary embodiment, the following steps are carried out: introducing a tertiary process fluid into the electrochemical cell unit through a first media connection for the tertiary process fluid; directing the tertiary process fluid through channels for the tertiary process fluid in a first direction; discharging the tertiary process fluid from the electrochemical cell unit through a second media connection for the tertiary process fluid. The tertiary process fluid is preferably a coolant.

[0021] Preferably, the flow direction of the tertiary process fluid is now reversed, so that the tertiary process fluid is introduced into the electrochemical cell unit through the second media connection for the tertiary process fluid, the tertiary process fluid is conducted through channels for the tertiary process fluid in a second direction opposite to the first direction, and the tertiary process fluid is discharged from the electrochemical cell unit through the first media connection for the tertiary process fluid. The reversal of the flow direction for the tertiary process fluid occurs either before the second conditioning step, before the third conditioning step, before a fourth conditioning step, or before a further conditioning step, i.e., either simultaneously with the reversal of the flow directions of the primary and / or secondary process fluid or successively thereafter.This results in even better uniformity of conditioning across the active area.

[0022] In a further development of the conditioning method, the channels for the primary and secondary process fluids are swapped, so that the primary process fluid is introduced into a media connection for the secondary process fluid, and the secondary process fluid is introduced into a media connection for the primary process fluid, followed by a third, fourth, and / or further conditioning step. Therefore, the oxidant and fuel are preferably fed to the opposite sides of the electrochemical cell. In the case of a fuel cell, hydrogen is applied to the cathode and / or air to the anode for at least one of the conditioning steps, for example, to remove contamination from the anode catalyst layer.

[0023] In a supplementary embodiment, the electrochemical cell unit is a fuel cell unit as a fuel cell stack for converting electrochemical energy into electrical energy and / or an electrolysis cell unit for converting electrical energy into electrochemical energy. In the case of a fuel cell unit, the primary process fluid is preferably the oxidant and the secondary process fluid is the fuel.

[0024] The bipolar plates are expediently designed as separator plates and an electrical insulation layer, in particular a proton exchange membrane, is arranged between each anode and each cathode.

[0025] Short description of the drawings

[0026] In the following, exemplary embodiments of the invention are described in more detail with reference to the accompanying drawings. They show:

[0027] Fig. 1 shows a longitudinal section through electrochemical cells designed as fuel cells, with only the essential areas shown,

[0028] Fig. 2 is a perspective, schematic view of a bipolar plate, showing only the essential areas.

[0029] Figure 1 shows the basic structure of an electrochemical cell 2 designed as a fuel cell. The principle of fuel cells 2 is that electrical energy or electrical current is generated by means of an electrochemical reaction. Hydrogen as a gaseous fuel is fed to an anode 7, which forms the negative pole. A gaseous oxidizing agent, namely air with oxygen, is fed to a cathode 8; in other words, the oxygen in the air provides the necessary gaseous oxidizing agent. Reduction (electron absorption) takes place at the cathode 8. Oxidation (electron loss) takes place at the anode 7.

[0030] A fuel cell unit 1 is a series connection of several fuel cells 2 and is often also referred to as a fuel cell stack 1.

[0031] The fuel cell 2 comprises a membrane 5, which is arranged between the anode 7 and the cathode 8. The anode 7 and the cathode 8 are layered or disc-shaped. The membrane 5 functions as an electrolyte, catalyst carrier and separator for the reaction gases. The membrane 5 also functions as an electrical insulator and prevents an electrical short circuit between the anode 7 and cathode 8. The membrane 5 is essentially impermeable to the reaction gases oxygen O2 and hydrogen H2, i.e. it blocks the flow of oxygen and hydrogen between a gas space 31 at the anode 7 containing hydrogen as fuel and the gas space 32 at the cathode 8 containing air or oxygen as oxidizing agent. The proton conductivity of the membrane 5 increases with increasing temperature and increasing water content.

[0032] On both sides of the membrane 5, facing the gas spaces 31, 32, lie the electrodes 7, 8, serving as the anode 7 and cathode 8. A unit comprising the membrane 5 and the electrodes 7, 8 is referred to as a membrane electrode assembly (MEA). The electrodes 7, 8 comprise, for example, platinum-containing carbon particles.

[0033] A gas diffusion layer 9 (GDL) lies on the anode 7 and the cathode 8. The gas diffusion layer 9 on the anode 7 evenly distributes the fuel from channels 12 for fuel to the catalysts on the anode 7. The gas diffusion layer 9 on the cathode 8 evenly distributes the oxidant from channels 13 for oxidant to the catalysts on the cathode 8. The GDL 9, in particular on the cathode 8, also draws off reaction water in the opposite direction to the flow direction of the reaction gases, i.e. in a direction from the cathode 8 to the channels 13. Furthermore, the GDL 9 keeps the membrane 5 moist and conducts the current. The GDL 9 is constructed, for example, from a hydrophobicized carbon paper as a carrier and substrate layer and a bonded carbon powder layer as a microporous layer.

[0034] A bipolar plate 10 rests on the GDL 9. The electrically conductive bipolar plate 10 serves as a current collector, for water drainage and for conducting the reaction gases as process fluids through the channel structures 12, 13, 14, and for dissipating the waste heat, which occurs particularly during the exothermic electrochemical reaction at the cathode 8. To dissipate the waste heat, channels 14 for conducting a liquid or gaseous coolant as a process fluid are incorporated into the bipolar plate 10. The channel structure in the gas space for fuel is formed by channels 12. The channel structure in the gas space 32 for oxidizing agent is formed by channels 13. The material used for the bipolar plates 10 includes, for example, metal, conductive plastics, composite materials, and / or graphite.

[0035] In a fuel cell unit 1, several fuel cells 2 are arranged in alignment and stacked.

[0036] An electrolysis cell unit 1 is fundamentally constructed in a very similar manner and could therefore also be described by Fig. 1. However, for electrochemical cells 2 designed as electrolysis cells, it is common for their bipolar plates 10 not to have channel structures 14 for coolant, but rather the cooling function is provided by the process water, i.e., via the channel structures 12 on the anode 7 side.

[0037] Figure 2 shows a perspective view of a bipolar plate 10 in a schematic representation. In the center of the bipolar plate 10 is the preferably rectangular active region 21 with the schematically indicated channel structures 12, 13. In the embodiment of Figure 2, the bipolar plate 10 has three media connections 31, 32, 33 on its narrow left end face for the supply of the media fuel, oxidant, and coolant, and opposite it on its narrow right end face three media connections 41, 42, 43 for the removal of the media fuel, oxidant, and coolant. Between the media connections 31, 32, 33, 41, 42, 43 and the active area 21, the so-called distribution area 22 is formed, which serves for the distribution (supply side) or the collection (discharge side) of the media from the comparatively narrow media connections 31, 32, 33, 41, 42, 43 to the comparatively wide active area 21.

[0038] In the embodiment of Figure 2, the three media fuel, oxidant and coolant flow during operation of the electrochemical cell 2 from left to right in a first direction 51, i.e. from the media supply connections 31, 32, 33 to the media discharge connections 41, 42, 43.

[0039] In the bipolar plate 10 shown in Figure 2, the primary process fluid is supplied as the oxidant through the media connection 31, the secondary process fluid as the fuel through the media connection 32, and the tertiary process fluid as the coolant through the media connection 33 to the channels 13, 12, 14 in the fuel cell unit 1. Thus, the end region of the bipolar plate 10 shown on the left in Figure 2 forms an inlet region for the primary, secondary, and tertiary process fluids. Alternatively, the channels 12, 13, in particular, can also be formed by open-pore structures and arise in a quasi-chaotic manner.

[0040] The primary process fluid as the oxidant is discharged from the channels 13, 12, 14 through the media connection 41, the secondary process fluid as the fuel is discharged through the media connection 42, and the tertiary process fluid as the coolant is discharged through the media connection 43. Thus, the end region of the bipolar plate 10 shown on the right in Figure 2 forms an outlet region for the primary, secondary, and tertiary process fluids.

[0041] During normal operation of the fuel cell unit, all three process fluids flow in a first direction 51. In alternative embodiments, the process fluids can also flow in different directions 51a, 51b, 51c; for example, the first process fluid can flow in one direction 51a and the second process fluid in the opposite direction 51b. In further alternative embodiments, the electrochemical cell unit 1 can also have only two process fluids; in this case, the channels 14 for the coolant are usually omitted. Before commissioning an electrochemical cell unit 1, it is connected to a test bench and conditioned through various steps in order to achieve the so-called beginning-of-life performance of the electrochemical cell unit 1 or the electrochemical cells 2 installed therein.

[0042] During the conditioning steps, the ionomer of membrane 5 and electrodes 7, 8 is primarily moistened. This also purifies the catalysts embedded in electrodes 7, 8, particularly removing contaminants. This increases the activation of the catalysts, for example, by removing process agents used in production, such as solvents.

[0043] In the electrochemical cell unit 1, the process fluids (e.g., the gases and the coolant) flow through the cells 2, in the embodiment of Figure 2 typically from left to right, i.e., in a first direction 51, 51a, 51b, 51c. Therefore, for example, the electrochemical reactions create different states along the length of the channels 12, 13 in the active region 21. Depending on the conditioning protocol and the conditioning conditions used, this can result in uneven conditioning of the cell surface (of membrane 5 and electrodes 7, 8) along the channels 12, 13.

[0044] According to the invention, the conditioning method is now optimized such that, after a first conditioning step of the conditioning protocol, the media supply connections 31, 32, 33 and the media discharge connections 41, 42, 43 of the cathode, anode, and / or coolant are interchanged via a valve circuit, and a second conditioning step is subsequently performed. A conditioning step can, for example, be the application of an electrical load or an electrical load profile over a certain period of time, while simultaneously supplying fuel and oxidant in certain—also variable—mass flows via the media connections 31, 32.

[0045] The conditioning process according to the invention conditioned the surfaces of the membrane 5 and the electrodes 7, 8, particularly the active surfaces 21, very evenly along the channels 12, 13. The humidification of the membrane 5, which is very important for proton transport, was thus carried out very homogeneously, and the contamination on the catalysts was very efficiently removed across the entire active area 21.

[0046] An exemplary conditioning process according to the invention comprises the following process steps:

[0047] Introducing a primary process fluid (e.g., oxidizing agent) through the first media connection 31 for the primary process fluid into the electrochemical cell unit 1, guiding the primary process fluid through channels 13 for the primary process fluid in the first direction 51a, discharging the primary process fluid from the electrochemical cell unit 1 through the second media connection 41 for the primary process fluid, introducing a secondary process fluid (e.g., fuel) through the first media connection 32 for the secondary process fluid into the electrochemical cell unit 1, guiding the secondary process fluid through channels 12 for the secondary process fluid in the first direction 51b, discharging the secondary process fluid from the electrochemical cell unit 1 through the second media connection 42 for the secondary process fluid, performing a first conditioning step (e.g., applying a constant electrical load for a period of 5 minutes),

[0048] Reversing the flow direction of the primary process fluid, so that the introduction of the primary process fluid into the electrochemical cell unit 1 is carried out through the second media connection 41 for the primary process fluid, and the conduction of the primary process fluid through channels 13 for the primary process fluid is carried out in the second direction 52a opposite to the first direction 51a, and the discharge of the primary process fluid from the electrochemical cell unit 1 is carried out through the first media connection 31 for the primary process fluid,

[0049] Perform a second conditioning step (e.g. applying a constant electrical load for a period of 5 minutes).

[0050] The second conditioning step can therefore be the same as the first conditioning step, but at least with a reversed flow direction 51a, 52a. In addition, or for a further conditioning step, the flow direction of the second process fluid can of course also be reversed analogously from a first direction 51b to a second direction 52b.

[0051] In alternative embodiments, the electrochemical cell 2 can also be operated in a countercurrent flow principle, meaning that the first directions 51a, 51b of the first and second process fluids are opposite during normal operation. Even then, the second conditioning step takes place with the opposite flow direction 52a, 52b for at least one of the two process fluids.

[0052] By way of example, a further conditioning method may comprise the following steps: introducing primary, secondary and tertiary process fluid into the electrochemical cell 1. The three process fluids thus flow through the cell 1 in a first direction 51a, 51b, 51c.

[0053] Carrying out a first conditioning step.

[0054] Reversing the flow directions for the primary process fluid (from 51a to 52a) and the secondary process fluid (from 51b to 52b).

[0055] Perform a second conditioning step.

[0056] Reversing the flow direction for the tertiary process fluid (from 51c to 52c).

[0057] Perform a third conditioning step.

[0058] Swapping the channels 12, 13 or media connections 31, 32, 41, 42 for the primary and secondary process fluid.

[0059] Perform a fourth conditioning step.

[0060] A conditioning step can, for example, be the application of an electrical load over a period of, for example, 5 minutes, or the substoichiometric supply of an electrode with the oxidizing agent over, for example, 3 minutes.

[0061] The flow directions of the process fluids during the conditioning process are therefore reversed before individual conditioning steps, and in certain cases, the process fluids are even swapped. This means that the inlet and outlet areas of the electrochemical cells 1 for individual process fluids are swapped for individual conditioning steps. Over the cumulative time of the entire conditioning process, individual locations on the active surface 21 still experience very similar boundary conditions (particularly thermal, electrical, and chemical), thus ensuring the most homogeneous conditioning possible.

Claims

Claims 1 . A method for conditioning an electrochemical cell unit (1) with stacked electrochemical cells (2) comprising the steps: Introducing a primary process fluid through a first media connection (31) for the primary process fluid into the electrochemical cell unit (1), guiding the primary process fluid through channels (13) for the primary process fluid in a first direction (51a), discharging the primary process fluid from the electrochemical cell unit (1) through a second media connection (41) for the primary process fluid, introducing a secondary process fluid through a first media connection (32) for the secondary process fluid into the electrochemical cell unit (1), guiding the secondary process fluid through channels (12) for the secondary process fluid in a first direction (51b), discharging the secondary process fluid from the electrochemical cell unit (1) through a second media connection (42) for the secondary process fluid, carrying out a first conditioning step, Reversing the flow direction of the primary process fluid so that the introduction of the primary process fluid into the electrochemical cell unit (1) is carried out through the second media connection (41) for the primary process fluid and the conduction of the primary process fluid through channels (13) for the primary process fluid is carried out in a second direction (52a) opposite to the first direction (51a) and the discharge of the primary process fluid from the electrochemical cell unit (1) is carried out through the first media connection (31) for the primary process fluid, carrying out a second conditioning step.

2. Method according to claim 1, characterized in that the flow direction of the secondary process fluid is reversed, such that the introduction of the secondary process fluid into the electrochemical cell unit (1) is carried out through the second media connection (42) for the secondary process fluid and the conduction of the secondary process fluid through channels (12) for the secondary process fluid is carried out in a second direction (52b) opposite to the first direction (51b) and the discharge of the secondary process fluid from the electrochemical cell unit (1) is carried out through the first media connection (32) for the secondary process fluid.

3. Method according to claim 2, characterized in that after the reversal of the flow direction for the secondary process fluid, a third conditioning step is carried out.

4. Method according to one of claims 1 to 3, characterized in that the following steps are carried out: Introducing a tertiary process fluid through a first media connection (33) for the tertiary process fluid into the electrochemical cell unit (1), Conducting the tertiary process fluid through channels (14) for the tertiary process fluid in a first direction (51c), discharging the tertiary process fluid from the electrochemical cell unit (1) through a second media connection (43) for the tertiary process fluid. Carrying out the second, third and / or fourth conditioning step.

5. The method according to claim 4, characterized in that the flow direction of the tertiary process fluid is reversed, so that the introduction of the tertiary process fluid through the second media connection (43) for the tertiary process fluid into the electrochemical cell unit (1) is carried out and the conduction of the tertiary process fluid through channels (14) for the tertiary process fluid in a second direction (52c) opposite to the first direction and the discharge of the tertiary process fluid from the electrochemical cell unit (1) is carried out through the first media connection (33) for the tertiary process fluid, wherein after the reversal of the flow direction for the tertiary process fluid, a further conditioning step is carried out.

6. Method according to one of the preceding claims, characterized in that the channels (12, 13) for the primary and secondary process fluid are exchanged, so that the primary process fluid is introduced into a media connection (32, 42) for the secondary process fluid and the secondary process fluid is introduced into a media connection (31, 41) for the primary process fluid and then a third, fourth and / or further conditioning step is carried out.

7. Method according to one of the preceding claims, characterized in that the electrochemical cell unit (1) is designed as a fuel cell unit and the primary process fluid is oxidizing agent and the secondary process fluid is fuel.

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