Method for operating an electrolyzer to produce hydrogen and oxygen

Abstract

The present invention relates to a method for operating an electrolyzer (10) for producing hydrogen and oxygen using a diaphragm (22) permeable to OH ions separating an anode chamber (14) from a cathode chamber (16), comprising at least the following method steps: a) temporarily operating the cathode chamber (16) dry; b) temporarily diffusing water molecules from the anode chamber (14) into the cathode chamber (16) by means of the diaphragm (22); c) varying a pressure difference (42) between the cathode chamber (16) and the anode chamber (14) by means of a throttle valve (46); and d) adjusting the humidification / wetting of the cathode chamber (16) by adjusting the predetermined pressure difference (42).

Description

[Technical Field]

[0001] The present invention relates to a method for operating an electrolyzer for producing hydrogen and oxygen using a diaphragm permeable to OH ions separating the anode and cathode compartments from each other, and further to the use of said method for operating an electrolyzer or alkaline electrolyzer with temporary water transport through the diaphragm for humidifying / wetting the cathode compartment. [Background technology]

[0002] Electrolyzers typically include a polymer diaphragm permeable to OH ions and two electrodes, usually made of a polymer material, located on opposite sides of the diaphragm. An aqueous electrolyte, e.g., a KOH solution, is fed into the chamber, e.g., on the oxygen-producing or anode side. The side opposite the diaphragm separating the chambers is optionally also circulated with electrolyte to produce hydrogen. Especially during transient operation, localized temperature peaks can weaken the polymer diaphragm material, leading to, among other things, thinning of the diaphragm and subsequent failure due to excessive gas diffusion or hole formation. As originally known from [1], a cathode located on the hydrogen side tends to locally dry out structures near the diaphragm, especially during shutdown. A failure scenario during restart is that the diaphragm has not yet achieved thermal equilibrium, and localized temperature peaks due to the lack of adequate cooling water can lead to diaphragm failure.

[0003] Additionally, dry operated cathodes, due to the lack of water, have a lower heat capacity than cathodes operated in a water flow regime, which causes the cell and stack to cool faster.

[0004] Patent document 1 shows an embodiment in which the cathode side (hydrogen side) is advantageously operated dry, i.e., there is no KOH electrolyte supply, in order to keep costs for drying the hydrogen low, among other things. It proposes using an aqueous alkaline solution, starting from a dry cathode, to produce hydrogen electrolytically. The device comprises two half-cells: an anode half-cell and a cathode half-cell, separated by an anion exchanger diaphragm, the surface of which in contact with the cathode half-cell forms a diaphragm-electrode unit (MEA), with the alkaline solution being present only in the anode half-cell.

[0005] In the case of a dry cathode, or when the cathode chamber is operated dry according to Patent Document 1, there is no water circulation on the cathode side, i.e., on the hydrogen side. When the electrolyzer is shut down, uneven temperatures can form on the diaphragm. The cathode side, i.e., the cathode chamber, is filled only with hydrogen gas and therefore can accumulate less heat than in a cathode chamber filled with water or another medium. Therefore, heating during restart of the electrolyzer can only occur via the anode side, whereas water splitting to OH occurs on the cathode-side diaphragm. Because polymer diaphragms with poor thermal conductivity are used here, local overheating can be expected during fast start-up processes. This is due to the different degrees of electrical contact in the individual areas where water splitting preferentially begins. While the water flow through the diaphragm for OH splitting is not uniform, the catalytic surface operates only to a limited extent, which can lead to local hot spots that can lead to quality degradation in the medium term and, in the short term, to diaphragm damage. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] European Patent No. 2451992B2 [Non-patent literature]

[0007] [Non-Patent Document 1] P. Millet et al., “Cell Failure Mechanisms in PEM Water Electrolyzers”, International Journal of Hydrogen Energy, Vol. 37, No. 22, November 2012, pp. 17478-17487, https: / / doi.org / 10.1016 / j.ijhydene.2012.06.017 Summary of the Invention

[0008] According to the invention, a method for operating an electrolyzer for producing hydrogen and oxygen using a diaphragm that is permeable to OH ions and separates the anode and cathode compartments from each other comprises the following method steps: a) temporarily operating the cathode chamber dry; b) temporarily diffusing water molecules from the anode chamber into the cathode chamber by means of the diaphragm; c) varying the pressure difference between the cathode and anode chambers by means of a throttle valve; d) adjusting the humidification / wetting of the cathode chamber by adjusting a predetermined differential pressure; A method is proposed which includes:

[0009] The solution proposed according to the invention allows the collection of water from the anode compartment into the cathode compartment, also called drainage, to be controlled by deliberately influencing the pressure difference between the anode and cathode compartments, so that the temporarily dry-operated part of the cathode compartment, in particular the area around the diaphragm, is regularly and sufficiently humidified, thereby providing a uniform temperature level and preventing premature damage over the life of the diaphragm material of the diaphragm separating the anode and cathode compartments.

[0010] In the method proposed according to the invention, in an advantageous embodiment, the anode and cathode compartments are separated from each other by a diaphragm, which is preferably implemented as a diaphragm made of a polymer material, and which allows for controlled diffusion of preferably fully demineralized water.

[0011] In an advantageous further configuration of the method proposed according to the invention, fully demineralized water, preferably with a conductivity of <0.1 μS, is used for the controlled water diffusion from the anode compartment to the cathode compartment.

[0012] Alternatively, in the method proposed according to the invention, for the case of operating alkaline electrolytic cells, a KOH solution may be used as the liquid medium instead of fully demineralized water.

[0013] The method proposed according to the invention advantageously comprises a step of reducing the pressure p K When the temperature decreases, we propose to increase the molar transport ratio H2O / H2, which is called the mole fraction, i.e., it represents the diffusive movement of moles of water molecules to moles of hydrogen molecules.

[0014] In the method proposed according to the invention, the pressure in the cathode chamber p K is varied in the second range by correspondingly adjusting the throttle valve, resulting in pressure fluctuations.

[0015] In a further advantageous embodiment of the idea underlying the present invention, the pressure p K Fluctuations in H2O and the resulting pressure peaks in the cathode chamber affect the molar transport ratio H2O / H2.

[0016] In the method proposed according to the invention, the temperature in the cathode chamber can be influenced via water transport into the cathode chamber.

[0017] Furthermore, the invention relates to the use of the method for operating an electrolytic cell or alkaline electrolytic cell with temporary water transport by a diaphragm for humidifying / wetting the cathode compartment. [Effects of the Invention]

[0018] The solution proposed according to the present invention allows only one side of a PEM or AEM electrolysis cell or electrolysis stack, particularly the cathode side, to be temporarily dry during operation, thereby reducing the costs of drying hydrogen and the energy required for this. On the other hand, the solution proposed according to the present invention allows the cathode side to be temporarily at least partially flooded or humidified, thereby enabling faster load changes, particularly when increasing hydrogen production, and preventing rapid, uneven cooling of the electrolysis cell or electrolysis stack. Advantageously, the present invention allows for flooding or humidification, preferably on the cathode side, thereby enabling targeted control of hydrogen production, achieved by targeted metering of fully demineralized water, preferably with a conductivity of <0.1 μS. The solution proposed according to the present invention allows for controllable water diffusion from the anode side to the cathode side via a diaphragm. This control, i.e., control of water diffusion from the anode chamber to the cathode chamber, is achieved by changing the pressure difference between the cathode and anode chambers.

[0019] The solution proposed according to the invention advantageously utilizes the fact that, during ion transport through the polymer diaphragm separating the anode and cathode compartments, water molecules are also entrained from the anode compartment to the cathode compartment. This "drainage" depends on the pressure difference between the cathode and anode sides of the electrolysis cell. By varying the pressure difference on the cathode side relative to the anode side, the water transport into the cathode compartment and thus the humidification or wetting of the cathode can be adjusted.

[0020] When the pressure in the cathode chamber is varied over time, a pressure peak occurs on the cathode side through fluctuations in the second range, and this pressure peak can affect the molar transport ratio H2O / H2.

[0021] Such a fast pressure change can be achieved, for example, by means of an electrically controllable throttle valve, advantageously located downstream of the water separator and on the cathode side of the electrolysis cell or stack.

[0022] Furthermore, the solution proposed according to the invention makes it possible to influence the temperature level on the cathode side by adjusting the water transport from the anode side to the cathode side.

[0023] The embodiments of the invention will be explained in more detail on the basis of the drawings and the following description. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 shows an electrolytic cell with a cathode chamber operated dry and an anode chamber with a pumped circulation path. [Figure 2] 1 is a graph plotting the relationship between the molar ratio of water to hydrogen and the differential pressure between the anode chamber and the cathode chamber. DETAILED DESCRIPTION OF THE INVENTION

[0025] In the following description of embodiments of the present invention, identical or similar components are provided with the same reference numerals, and individual repeated descriptions of these components are omitted. The figures only show the subject matter of the present invention in a schematic manner.

[0026] 1 shows an electrolysis cell 10 equipped with an electrolysis stack 12. The electrolysis cell 10 comprises an anode chamber 14 and a cathode chamber 16, which are separated from each other by a diaphragm 22 that is permeable to OH ions. The diaphragm 22 is preferably configured as a polymer diaphragm 23. In the cathode chamber 16, which is temporarily operated dry, gaseous hydrogen is discharged in a hydrogen discharge section 20 into a second water separator 30.

[0027] In the anode chamber 14 of the electrolyzer 10, an oxygen outlet 18 discharges the gaseous oxygen formed in the anode chamber 14 into a first water separator 26, which is part of a pump circuit 24. The pump circuit 24, into which the first water separator 26 is integrated, further comprises a pump 28, via which the water circulating in the pump circuit 24 reaches the anode chamber 14 again in a water feed 32. The opening of the water feed 32 is located in the bottom 34 of the anode chamber 14. The pump circuit 24 is operated substantially at a pressure level of approximately 1 bar in order to reduce the necessary sealing costs.

[0028] 1, downstream of the second water separator 30 on the cathode side is a throttle valve 46. The throttle valve 46 is preferably located at a mounting position 48 at the outlet of the second water separator 30, downstream of the cathode chamber 16.

[0029] In the case where the electrolyzer 10 is operated with water according to the diagram in FIG. 1, fully demineralized water with a conductivity of <0.1 μS is preferably used. On the other hand, the electrolyzer 10 according to FIG. 1 can also be operated as an alkaline electrolyzer 10. In the latter case, instead of fully demineralized water, a KOH solution is used as the circulating liquid medium in the anode chamber 14. The electrolysis stack 12 shown in FIG. 1 comprises an anode chamber 14 for producing oxygen and a cathode chamber 16 for producing hydrogen. The anode chamber 14 is part of a pump circuit 24 with a first water separator 26, so that oxygen can be separated and water can be transported again into the anode chamber 14 via a pump 28. On the cathode side, hydrogen is formed in the cathode chamber 16 and is also guided through a water separator, in this case a second water separator 30, so that the hydrogen obtained is as dry as possible. Regulating the pressure on the hydrogen side via a throttle valve 46 to e.g. 30 bar reduces the compression energy costs of the downstream compression unit for cases where the hydrogen is intended to be stored under pressure, which is usually the case due to the low molar weight of hydrogen.

[0030] To improve the functioning of the electrolyzer 10 proposed according to the invention with fully demineralized water or alkaline with KOH solution, the cathode side, i.e. the cathode chamber 16, is temporarily wetted or humidified (see the diagram in FIG. 2), for example at a specific operating point given by a specific temperature and a specific current density, e.g., 45° C. and 0.8 A / cm 2 . 2 At an operating point of 0.8 A / cm 2 , it is observed that water molecules are entrained from the anode chamber 14 through the diaphragm 22 into the cathode chamber 16 during ion transport by the diaphragm 22, which is provided as a polymer diaphragm 23. At a particular operating point 38, for example, at a temperature of 45° C. and a current of 0.8 A / cm 2 in FIG. 22, i.e., a decrease 50 in the H2O / H2 molar ratio 40 is observed. Water diffusion from the anode chamber 14 to the cathode chamber 16 can be influenced by fluctuations in the pressure difference 42 between the cathode chamber 16 and the anode chamber 14. Therefore, depending on the magnitude of the pressure difference 42, the water transport into the cathode chamber 16 can be influenced and thus the humidification or wetting of the cathode can be adjusted. Therefore, the proposed method of utilizing drainage water from the anode chamber 14 into the cathode chamber 16 allows wetting or humidification of the components of the cathode chamber 16, in particular the diaphragm 22 formed as a polymer diaphragm 23, to be achieved at specific intervals.

[0031] This temporary wetting or humidification of the polymer diaphragm 23 on the cathode side makes it possible to avoid local overheating during the fast start-up process of the electrolyzer 10. By the method proposed according to the invention, i.e. by using water diffusion from the anode compartment 14 into the cathode compartment 16, a temperature equalization or dissociation of OH ions can be achieved, resulting in a homogeneous functioning behavior and preventing local hot spots associated with the occurrence of overheating, which leads to quality degradation in the medium term and diaphragm damage in the short term.

[0032] From the illustration in FIG. 2 it can be seen how an increase in the pressure difference 42 between the cathode and the anode results in a decrease 50 in the molar ratio H2O / H240 or mole fraction of water molecules to hydrogen molecules.

[0033] The solution proposed according to the invention, in particular by means of a throttle valve 46 provided downstream of the cathode chamber 16, allows the cathode pressure p prevailing in the cathode chamber 16 to be reduced. KAdvantageously, time variations in the range of seconds can be achieved. In the case of variations in the second range, pressure peaks occurring on the cathode side can be achieved, which in turn can influence the above-mentioned molar ratio HO / H240. Pressure variations of this kind are achieved by suitable control of, in particular, an electrically controllable throttle valve 46, which is located on the cathode side downstream of the second water separator 30.

[0034] The present invention is not limited to the embodiments described herein and the aspects set forth herein, but rather many variations within the skill of the art are possible within the scope of the claims. [Explanation of symbols]

[0035] 10 Electrolytic cell 14 Anode chamber 16 Cathode chamber 22 diaphragm 40 Molar transport rate 42 Differential pressure between the anode and cathode chambers 46 Throttle valve

Claims

1. A method for operating an electrolyzer (10) for producing hydrogen and oxygen using a diaphragm (22) permeable to OH ions and separating an anode compartment (14) and a cathode compartment (16) from each other, comprising the following method steps: a) temporarily operating the cathode chamber (16) dry; b) temporarily diffusing water molecules from the anode chamber (14) into the cathode chamber (16) by means of the diaphragm (22); c) varying the pressure difference (42) between the cathode chamber (16) and the anode chamber (14) by means of a throttle valve (46); d) adjusting the humidification / wetting of the cathode chamber (16) by adjusting the pressure difference (42); Including, The throttle valve (46) is electrically controllable and is provided downstream of a water separator (30) into which hydrogen discharged from the cathode chamber (16) flows; In the method step c), the differential pressure (42) is varied by controlling the throttle valve (46) to generate a pressure peak in the cathode chamber (16). method.

2. A method as described in claim 1, characterized in that fully demineralized water is diffused in a controlled manner from the anode chamber (14) into the cathode chamber (16).

3. 3. A method according to claim 2, characterized in that for controlled water diffusion, fully demineralized water with a conductivity of less than 0.1 μS / cm is used.

4. 2. The method according to claim 1, characterized in that, when the electrolytic cell (10) is operated alkaline, a KOH solution is used.

5. 2. A method according to claim 1, characterized in that the temperature in the cathode chamber (16) is influenced via water transport into the cathode chamber (16).

6. 5. The method according to claim 1, wherein the electrolytic cell or alkaline electrolytic cell is operated with temporary water transport through a diaphragm for humidifying / wetting the cathode chamber.

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