Method for operating an electrolysis plant, electrolysis plant

Abstract

The invention relates to a method for operating an electrolysis system (1), comprising at least one stack (2) having a plurality of electrolysis cells, with an anode (3) and a cathode (4). During normal operation of the electrolysis system (1), water is supplied to the anode (3) via a water circuit (5) with an integrated pump (6), which water is split into hydrogen and oxygen in the at least one stack (2) by electrolysis, and the hydrogen produced by electrolysis is discharged via a cathode outlet (9) of the stack (2) and a media line (7) connected thereto. According to the invention, when the electrolysis system (1) is switched off, a reduced stack current is maintained, and with the aid of the stack current and a cell-side recombination catalyst (10), oxygen present on the anode side is recombined with hydrogen, which diffuses from the cathode side to the anode side, to form water. The invention further relates to an electrolysis plant (1) which is suitable for carrying out the method or can be operated according to the method.

Description

[0001] The invention relates to a method for operating an electrolysis plant. Furthermore, the invention relates to an electrolysis plant that is suitable for carrying out the method or can be operated according to the method.

[0002] The preferred field of application of the invention is the production of hydrogen by electrolysis. State of the art

[0003] Electrolysis is a chemical process in which electrical current is used to break down a chemical compound. This allows hydrogen and oxygen to be extracted from water.

[0004] To carry out the electrolysis, at least one electrolysis cell is required, for example a PEM (“Proton Exchange Membrane”) electrolysis cell, an AEL (“Liquid Alkaline Electrolysis”) electrolysis cell, a solid oxide electrolysis cell or SOEC (“Solid Oxide Electrolyzer Cell”) or an AEM (“Anion Exchange Membrane”) electrolysis cell, whereby the electrolysis cells mentioned differ in the type of ionic conductivity.

[0005] Typically, a large number of identical electrolysis cells are stacked and connected to form a cell stack, the so-called stack. This requires additional components, in particular those for power and water supply, as well as for gas processing. The power supply components usually include a rectifier for converting AC to DC and a DC / DC converter for adjusting the DC voltage. The water supply components include at least one water circuit and a pump. The gas processing components include gas-liquid separators to separate the product gases hydrogen and oxygen from water. Furthermore, means for controlling and / or regulating the system are required.

[0006] Against the backdrop of decarbonization, hydrogen is becoming increasingly important as an energy carrier. The current goal is to produce green hydrogen, i.e., hydrogen with very low CO 2-footprint, through electrolysis on a large industrial scale. Renewable energy sources such as wind and solar energy are required to produce green hydrogen. Since these fluctuate, electrical energy is not always available, and the electrolysis plant must be shut down temporarily. This negatively impacts the service life of the electrolysis plant for the reasons listed below.

[0007] When the system is shut down, the water supply to the anode side is interrupted. Due to the higher pressure on the cathode side, hydrogen diffuses from the cathode to the anode side through the not entirely gas-tight membranes of the electrolysis cells. As a result, the cell voltage drops steadily from the usual 1.5 V to 2.0 V during operation to as low as 0 V. At the same time, the electrochemical potential at the anode catalyst drops from over 1.3 V to as low as 0 V. A potential drop to below 1 V is accompanied by a reduction in the anode catalyst, which is in an oxidized state during normal operation. The reduction and re-oxidation when electrolysis operation is resumed leads to a significant acceleration of the aging of the anode catalyst, which decreases the catalyst activity. As a result, the electrolysis cell voltage increases for a given current density and the electrolysis efficiency decreases accordingly.Frequent shutdown and restart can therefore reduce the service life of an electrolysis plant to an uneconomical level.

[0008] The present invention is concerned with the task of minimizing or completely eliminating the disadvantages that occur when an electrolysis plant is shut down.

[0009] To achieve this object, the method having the features of claim 1 and the electrochemical electrolysis system having the features of claim 7 are proposed. Advantageous developments of the invention can be found in the respective subclaims. Disclosure of the invention

[0010] A method is proposed for operating an electrolysis system, comprising at least one stack having a plurality of electrolysis cells with an anode and a cathode. During normal operation of the electrolysis system, water is supplied to the anode via a water circuit with an integrated pump, which water is split into hydrogen and oxygen by electrolysis in the at least one stack. The hydrogen generated by electrolysis is discharged via a cathode outlet of the stack and a media line connected thereto. According to the invention, a reduced stack current is maintained when the electrolysis system is switched off, and with the aid of the stack current and a cell-side recombination catalyst, oxygen present on the anode side is recombined with hydrogen, which diffuses from the cathode side to the anode side, to form water.

[0011] The recombination of oxygen and hydrogen when the electrolysis system is shut down reduces the amount of hydrogen on the anode side that has diffused from the cathode side to the anode side. The disadvantages described above therefore do not occur, or only to a significantly reduced extent. This is because the conversion of hydrogen achieved by recombination – combined with the maintenance of a reduced stack current – ​​prevents the anode-side potential from falling below 1 V. The stack current is the current that flows between the cathode and the anode during operation of the electrolysis system.

[0012] When switching off the electrolysis plant, the current density caused by the stack current is preferably kept to ≤ 10 mA / cm 2reduced. The resulting cell voltages are then below the usual 1.5 V to 2.0 V, for example, 1.4 V or 1.3 V. The anode-side potential is then certainly more than 1 V.

[0013] Furthermore, by controlling the current density, the amount of oxygen produced on the anode side is preferably adjusted in a stoichiometric ratio to the amount of hydrogen diffusing from the cathode side to the anode side. In this way, the entire amount of hydrogen on the anode side can be converted by recombination with oxygen. Ideally, the amount of oxygen (amount of substance in moles) is exactly half the amount of hydrogen (amount of substance in moles). The amount of hydrogen diffusing from the cathode side to the anode side is preferably estimated.

[0014] The hydrogen permeation flow through the membrane from the cathode to the anode can be stored in a control unit, for example, as a map. Since the H2 permeation flow for a given membrane essentially depends on the hydrogen pressure at the cathode and the temperature, particularly the temperature of the water at the stack inlet, the permeation flow can be stored in the control unit in the form of a table.

[0015] Furthermore, when shutting down the electrolysis system, the anode-side water supply and / or discharge is preferably blocked by closing at least one shutoff valve. This measure retains the oxygen produced in the at least one stack so that it is available for recombination with hydrogen. Since oxygen and hydrogen recombine to form water, the supply of water via the water circuit is no longer necessary. This means that the pump integrated into the water circuit can be shut off, thereby reducing the energy consumption of the electrolysis system.

[0016] To block the water supply and / or discharge, a single shut-off valve can be closed, which is integrated into the water circuit either upstream or downstream of the at least one stack. The shut-off valve can also be integrated in the region of a gas line branching off from the water circuit, in which a gas-liquid separator is arranged. Preferably, at least two shut-off valves are provided, which are furthermore preferably integrated into the water circuit close to the stack, namely a first shut-off valve on the inlet side and a second shut-off valve on the outlet side with respect to the at least one stack. The oxygen present in the at least one stack is then trapped in the stack.

[0017] Furthermore, when shutting down the electrolysis system, the medium line connected to the cathode outlet is preferably blocked by closing a valve integrated into the medium line. By closing the valve, the cathode of the at least one stack is separated from the medium line. This results in the cathodic pressure being blocked and decreasing over time due to leaks. With the decrease in cathodic pressure, the diffusion of hydrogen from the cathode side to the anode side also decreases. Accordingly, less hydrogen has to be recombined or converted on the anode side.

[0018] In a further development of the invention, it is proposed that when the electrolysis system is shut down, the cathode-side pressure in the at least one stack is actively reduced by releasing hydrogen from the cathode. The desired pressure drop can thus be achieved more quickly. Preferably, the pressure is actively reduced to ambient pressure, so that the diffusion of hydrogen comes to a complete halt.

[0019] The electrolysis plant proposed to solve the above-mentioned problem also includes: - at least one stack comprising a plurality of electrolysis cells with an anode and a cathode, - a water circuit with an integrated pump to supply the anode with water and - a media line connected to a cathode outlet of the at least one stack.

[0020] According to the invention, the electrolysis cells each have a membrane which, in addition to an anode-side catalyst and a cathode-side catalyst, comprises a recombination catalyst.

[0021] The proposed electrolysis system is particularly suitable for carrying out the previously described process according to the invention or can be operated according to this process, so that the same advantages can be achieved. In particular, the stoichiometric recombination of hydrogen and oxygen at the recombination catalysts can prevent a potential drop below 1 V on the anode side and thus premature degradation of the anode-side catalysts. The proposed electrolysis system therefore has an increased service life.

[0022] According to a preferred embodiment of the invention, the cell-side recombination catalyst consists of platinum, a platinum alloy, palladium, a palladium alloy or another noble metal that is suitable for catalyzing the recombination of oxygen and hydrogen to water at temperatures between 20°C and 100°C. The recombination catalyst can be designed as a band of nanoparticles that is integrated into the membranes of the electrolysis cells. The position within the membrane is preferably chosen such that under typical pressure conditions in the off state of the electrolysis system, the diffusion fluxes of hydrogen and oxygen to this position are as equal as possible, assuming that the concentrations of both gas species are nearly zero at this point due to complete recombination.

[0023] According to a further preferred embodiment of the invention, at least one shut-off valve is integrated into the water circuit. With the aid of the at least one shut-off valve, the water supply and / or discharge via the water circuit can be blocked when the electrolysis system is shut down. Preferably, at least two shut-off valves are integrated into the water circuit, with a first shut-off valve preferably being arranged on the inlet side and a second shut-off valve on the outlet side with respect to the at least one stack. The at least one stack can thus be completely separated from the water circuit.

[0024] Furthermore, it is proposed that a valve for blocking the media line be integrated into the cathode outlet or into the media line. With the aid of this valve, the cathode can be separated from the media line, preferably from a gas-liquid separator arranged in the media line. The valve integrated into the media line can be a shut-off valve or a directional control valve, in particular a 3-way valve, via which a gas line is connected to the media line. In the directional control valve design, the media line can be blocked in one direction, preferably in the direction of the gas-liquid separator, while the gas line is open. Hydrogen can then be vented from the cathode via the open gas line in order to actively reduce the pressure on the cathode side.

[0025] Furthermore, a pressure sensor is preferably integrated into the cathode outlet or the media line. The pressure sensor can be used to monitor the pressure on the cathode side. For example, it can be detected when the pressure on the cathode side reaches ambient pressure. At this point, hydrogen diffusion ceases, and the gas line initially opened via the directional control valve can also be blocked.

[0026] Advantageously, the proposed electrolysis system comprises a control unit configured to carry out steps of a method according to the invention. The control unit can be used, for example, to regulate the current density when shutting down the electrolysis system. Furthermore, the control unit can be used to control all the valves required to disconnect the at least one stack from the anode-side water circuit and / or the cathode-side media line when shutting down the electrolysis system.

[0027] Preferred embodiments of the invention are described in more detail below with reference to the accompanying drawings, which show: Fig. 1 a schematic representation of an electrolysis plant according to the invention, Fig. 2 a schematic cross section through an electrolysis cell of a stack for an electrolysis plant according to the invention and Fig. 3 a schematic representation of a media line for connecting a stack with a gas-liquid separator in an electrolysis plant according to the invention. Detailed description of the drawings

[0028] The Fig. 1 shows an electrolysis system 1 according to the invention. The electrolysis system 1 serves to produce hydrogen. For this purpose, the electrolysis system 1 comprises a stack 2 having a plurality of electrolysis cells 14 with an anode 3 and a cathode 4. During operation of the system, water is supplied to the anode 3 via a water circuit 5 with an integrated pump 6. In the stack 2 or in the electrolysis cells of the stack 2, the water is then split into hydrogen and oxygen. The hydrogen produced on the cathode side is fed to a gas-liquid separator 8 via a media line 7, which is connected to a cathode outlet 9 of the stack 2. The gas separated from the water is then passed through a droplet separator 21. The water is recirculated and fed back to the stack 2. The oxygen produced on the anode side is discharged together with water from the anode 3 of the stack 2 and fed to a further gas-liquid separator 22.A droplet separator 23 is also connected downstream of the gas-liquid separator 22. The water separated from the oxygen in the gas-liquid separator 22 is also recirculated and fed back to the stack 2. Furthermore, a fish water supply 24 is provided, through which fresh water is fed into the water circuit 5.

[0029] The electrical energy required for electrolysis is provided by power electronics 25 connected to the stack 2. The electrolysis process is controlled or regulated by a control unit 20 of the electrolysis system 1.

[0030] In the water cycle 5 of the Fig. Two shut-off valves 11, 12 are integrated into the electrolysis system 1 shown in Figure 1. A first shut-off valve 11 is arranged on the inlet side and a second shut-off valve 12 is arranged on the outlet side with respect to the stack 2. The stack 2 can thus be completely separated from the water circuit 5.

[0031] Furthermore, a valve 13, which is designed as a directional control valve in this case, is integrated into the media line 7. A gas line 18 is connected to the media line 7 via the directional control valve. With the help of the valve 13, designed as a directional control valve, the media line 7 can be blocked in the direction of the gas-liquid separator 8 and the gas line 18 can be opened, for example, to actively reduce the pressure on the cathode side by discharging hydrogen from the cathode 4 of the stack 2 via the gas line 18. A pressure sensor 19 is also integrated into the media line 7 upstream of the valve 13. The pressure on the cathode side can be monitored using the pressure sensor 19.

[0032] As exemplified in the Fig. 2, the electrolysis cells 14 of the stack 2 each have a membrane 15 on which a first catalyst 16 is applied on the anode side and a second catalyst 17 on the cathode side.

[0033] In addition, the membrane 15 has a recombination catalyst 10. Accordingly, when the electrolysis system 1 is shut down, the method according to the invention can be carried out. In the method, a reduced stack current is maintained, and with the aid of the stack current and the recombination catalyst, oxygen present on the anode side is recombined with hydrogen, which diffuses from the cathode side to the anode side, to form water.

[0034] Specifically, the method can consist of the following steps, which are preferably regulated with the aid of the control device 20: a) Reduction of the current density to a value less than or equal to 10 mA / cm 2 , such that the cell voltage is below 1.4 V, preferably 1.3 V, b) Closing the shut-off valves 11, 12 to retain the oxygen produced in the stack 2, c) Blocking the media line 7 in the direction of the gas-liquid separator 8 and releasing the gas line 18 by switching the valve 13 accordingly, d) continuous measurement of the cathode-side pressure using the pressure sensor 19 e) blocking the gas line 18 when the cathode-side pressure is at least approximately equal to the ambient pressure, by switching the valve 13 accordingly, f) Controlling the current density to adjust the amount of oxygen on the anode side, with the aid of which the hydrogen diffused from the cathode side to the anode side can be recombined to form water.

[0035] The amount of oxygen to be adjusted on the anode side depends on the amount of hydrogen that has diffused to the anode side. This amount can be estimated. The estimate is made from the temperature measured on the stack and the hydrogen partial pressure on the cathode 4 calculated from the pressure measurement with pressure sensor 19. The cell voltage is maintained, preferably permanently, at a value of 1.1 V to 1.3 V.

[0036] The steps described result in a stoichiometric recombination of hydrogen and oxygen at the recombination catalyst 10. Since water is again produced in this process, further water supply via pump 6 from the gas-liquid separator 22 or from the fresh water supply 24 is not required. In this way, the energy consumption of the electrolysis system 1 is reduced when switched off. By actively reducing the cathode-side pressure via the gas line 18, low hydrogen permeation rates can be expected, corresponding to a current density of approximately 1 mA / cm 2 This ensures that energy losses during the shutdown period are minimal.

[0037] To restart electrolysis plant 1, the following steps can be carried out: i) Increasing the stack current, ii) Opening the shut-off valves 11, 12 integrated in the water circuit 5, iii) Commissioning of the pump 6 integrated into the water circuit 5, iv) Monitoring the cathode-side pressure using pressure sensor 19 and releasing the media line 7 toward the gas-liquid separator 8 via valve 13 when the cathode-side pressure has reached a predetermined target pressure. The target pressure may correspond to the pressure in the gas-liquid separator 8. Normal operation of the electrolysis plant 1 can then begin.

[0038] The Fig. 3 shows a media line 7 with a valve 13 and a pressure sensor 19 for a further electrolysis system 1 according to the invention. In contrast to the electrolysis system 1 of Fig.1, the valve 13 is designed as a simple shut-off valve. This means that no gas line 18 is provided through which hydrogen can be released from the cathode 4 for active pressure reduction. Nevertheless, this electrolysis system 1 can also be operated according to a method according to the invention, so that the associated advantages are achieved.

Claims

[1] Method for operating an electrolysis plant (1), comprising at least one stack (2) having a plurality of electrolysis cells with an anode (3) and a cathode (4), wherein, during normal operation of the electrolysis plant (1), water is supplied to the anode (3) via a water circuit (5) with an integrated pump (6), which water is split into hydrogen and oxygen in the at least one stack (2) by electrolysis, and wherein the hydrogen produced by electrolysis is discharged via a cathode outlet (9) of the stack (2) and a media line (7) connected thereto, characterized by that when the electrolysis plant (1) is switched off, a reduced stack current is maintained and, with the aid of the stack current and a cell-side recombination catalyst (10), oxygen present on the anode side is recombined with hydrogen, which diffuses from the cathode side to the anode side, to form water. [2] Method according to claim 1, characterized bythat when the electrolysis plant (1) is switched off, the current density caused by the stack current is ≤ 10 mA / cm 2 is reduced. [3] Method according to claim 2, characterized by that by regulating the current density, the amount of oxygen produced on the anode side is adjusted in a stoichiometric ratio to the amount of hydrogen that diffuses from the cathode side to the anode side, with the amount of hydrogen preferably being estimated. [4] Method according to one of the preceding claims, characterized by that when the electrolysis system (1) is switched off, the anode-side water supply and / or discharge is blocked by closing at least one shut-off valve (11, 12), which is preferably integrated into the water circuit (5). [5] Method according to one of the preceding claims, characterized bythat when the electrolysis system (1) is switched off, the media line (7) connected to the cathode outlet (9) is blocked by closing a valve (13) integrated into the media line (7). [6] Method according to one of the preceding claims, characterized by that when the electrolysis plant (1) is switched off, the cathode-side pressure in the stack (2) is actively reduced by releasing hydrogen from the cathode (4), preferably to ambient pressure. [7] Electrolysis plant (1), comprising - at least one stack (2) comprising a plurality of electrolysis cells with an anode (3) and a cathode (4), - a water circuit (5) with an integrated pump (6) for supplying the anode (3) with water and - a media line (7) connected to a cathode outlet (9) of the at least one stack (2), characterized bythat the electrolysis cells (14) each have a membrane (15) which, in addition to an anode-side catalyst (16) and a cathode-side catalyst (17), comprises a recombination catalyst (10). [8] Electrolysis plant (1) according to claim 7, characterized by that the cell-side recombination catalyst (10) consists of platinum, a platinum alloy, palladium, a palladium alloy or another noble metal which is suitable for catalyzing the recombination of oxygen and hydrogen to water at temperatures between 20°C and 100°C. [9] Electrolysis plant (1) according to claim 7 or 8, characterized by that at least one shut-off valve (11, 12) is integrated into the water circuit (5), wherein preferably a first shut-off valve (1) is arranged on the inlet side and a second shut-off valve (12) is arranged on the outlet side with respect to the at least one stack (2). [10] Electrolysis plant (1) according to one of claims 7 to 9, characterized by that a valve (13) for blocking the media line (7) is integrated into the cathode outlet (9) or into the media line (7). [11] Electrolysis plant (1) according to claim 10, characterized by that the valve (13) integrated into the media line (7) is a shut-off valve or a directional control valve, in particular a 3-way valve, via which a gas line (18) is connected to the media line (7). [12] Electrolysis plant (1) according to one of claims 7 to 11, characterized by that a pressure sensor (19) is integrated into the cathode outlet (9) or into the media line (7). [13] Electrolysis plant (1) according to one of claims 7 to 12, characterized by that the electrolysis plant (1) comprises a control device (20) which is designed to carry out steps of a method according to one of claims 1 to 6.

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