Method for operating electrolysis system with periodic polarity reversal

Abstract

The method 5 of operation of an electrolysis system with periodic polarity reversal in order to activate and / or regenerate electrodes (4, 5) in an electrolysis system (1), the polarity between the electrodes (4, 5) is periodically reversed, which results in the production of hydrogen gas in the oxygen gas in the anode chamber (2). In order to prevent the occurrence of explosive gas mixtures in the system, the duration of the activation and / or regeneration period 10 is limited to less than 2% of the duration of the normal production period.

Description

Technical Field

[0001] The present invention relates to an electrolysis system for producing hydrogen in which the voltage between electrodes is periodically reversed to regenerate the electrodes. Background Art

[0002] As described in US8308919, the efficiency of the electrolysis system can be reduced due to the accumulation of contaminants on the electrode surface, thereby reducing the conductivity and thus the gas production rate. Periodic reversal of polarity, that is, the voltage on the electrode is temporarily reversed, can be used to regenerate the electrode. For example, US4578160A discloses periodic polarity reversal of 3-30% of the time in the electrolyzer for recovering concentrated alkaline alkali. WO2013 / 078004A1 discloses polarity reversal for regenerating diamond electrodes in an electrolyzer less than 1% of the operating time. Compared with conventional operation, during regeneration, hydrogen and oxygen are produced on the other side of the membrane. In this way, oxygen is introduced into the side where hydrogen is usually produced and gathered in a storage container, while hydrogen is produced on the side where oxygen is usually produced. As in the case of US8308919, a large amount of gas is produced over a period of about 20 minutes. In order to separate oxygen and hydrogen into the correct container at the time of reverse voltage, US8308919 discloses switching the discharge by a gas directional control valve.

[0003] US10167561 also discloses a valve for selecting the correct outlet channel of the electrode chamber, which valve depends on the normal operation or regeneration cycle, and applies a reverse voltage to the galvanized aluminum electrode to achieve reverse zinc migration. In this system, the electrode chamber has a collection hood with oxygen outlets and hydrogen outlets at both ends of the collection hood, and the collection hood can be tilted in one direction or the other to facilitate the collection of oxygen or hydrogen into the corresponding valve-operated outlet channel.

[0004] Although these systems offer a technical solution to prevent the accumulation of explosive mixtures of hydrogen and oxygen in tanks, they are based on mechanical valves or other devices, which on the one hand makes the system more complex and on the other hand presents a risk of creating a dangerous mixture if a mechanical component fails, which is a disadvantage.

[0005] To address this problem, it is desirable to improve the safety of the electrolysis system. Summary of the invention

[0006] The object of the present invention is therefore to provide an improvement in the art. In particular, the object is to provide enhanced safety in electrolysis systems applying periodic polarity reversal for electrode activation and / or regeneration, in particular to prevent the occurrence of explosive gas mixtures in the system. Another object is to provide a simple technical solution to this problem. These objects and other advantages are achieved by a method for operating an electrolysis system as described below and in the claims.

[0007] The electrolysis system described herein is used to decompose water into hydrogen H2 and oxygen O2, and includes a first electrode chamber having a first electrode and a second electrode chamber having a second electrode. The first and second chambers are separated by a membrane and contain an electrolyte.

[0008] During the time T1 of regular gas production, a first voltage of a first polarity is applied to the electrodes. In normal operation, oxygen O2 is produced in the first electrode chamber on the first side of the membrane, and hydrogen H2 is produced in the second electrode chamber on the second side of the membrane. Therefore, in normal operation, the first electrode is an anode and the first electrode chamber is an anode chamber, and the second electrode is a cathode and the second electrode chamber is a cathode chamber.

[0009] To periodically regenerate the electrodes, a second voltage of opposite polarity relative to the first voltage is applied to the electrodes and across the membrane for a period of time T2 to regenerate the electrodes and improve efficiency after the electrode voltage drops. The use of opposite polarity is referred to herein as reverse polarity.

[0010] In this regard, the following points should also be pointed out. Electrodes used in an electrolysis system may need to be activated on first use, which is defined here as the actual first use of the electrode after commissioning, or the first use of the electrode after an operating pause lasting more than a certain level, during which the electrode returns to an inactive stage. The inventors have found that periodic polarity reversal is a useful method for electrode activation. Therefore, polarity reversal is not only useful for regeneration after a period of normal operation, but polarity reversal, which is usually performed multiple times on a shorter time scale than regeneration, is useful for activation of the electrode.

[0011] During reverse polarity operation, the voltage and current are reversed compared to normal operation, and hydrogen H2 is produced on the first side of the membrane and in the first electrode compartment, and oxygen O2 is produced on the second side of the membrane and in the second electrode compartment. This can lead to mixed gases in the electrode compartments, which pose a risk of explosion if appropriate precautions are not taken. Some safety aspects are discussed below to prevent the generation of dangerous gas mixtures when alternating between such normal operation and reverse polarity cycles, either for regeneration or electrode activation.

[0012] During normal operation, a first volume V1 of oxygen O2 is produced in a first chamber on a first side of the membrane, wherein the first chamber functions as an anode chamber, while during operation with the opposite polarity, a second volume V2 of hydrogen H2 is produced in the first chamber. By limiting the production of hydrogen in the second volume V2 to no more than 4% of the first volume V1 of oxygen produced in normal operation, the resulting gas mixture of hydrogen and oxygen contains no more than 4% of hydrogen. For oxygen collected from oxygen tanks for normal operation, the content of H2 in the gas mixture in the tank does not exceed 4%. This is important because there is a risk of explosion in the range of 4% to 94% of H2 in the H2+O2 mixture, and gas mixtures outside this range are considered safe.

[0013] For example, during normal operation, a volume V1 of oxygen is produced in the first chamber, and during a subsequent operation with opposite polarity, a volume V2 of hydrogen is produced in the first chamber, wherein the volume V2 does not exceed 4% of the volume V1 of oxygen produced during the previous normal operation.

[0014] Optionally, in a plurality of subsequent normal operation cycles, a fixed first volume V1 of oxygen is produced in each cycle, and in each regeneration operation cycle between each two subsequent normal operation cycles, a fixed second volume V2 of hydrogen is produced that is no greater than 4% of V1. Thus, the system alternates between normal operation periods and regeneration operation periods, with each cycle producing equal production.

[0015] Hydrogen is produced twice as fast as oxygen because each pair of water molecules splits into two H2 molecules and one 02 molecule.

[0016] Thus, the upper limit of 4% of hydrogen produced in the first chamber during reverse polarity relative to the oxygen produced in the first chamber during normal operation corresponds to an upper limit of 2% of oxygen produced in the second chamber during reverse polarity when related to equal amounts of gas. For example, during normal production, equal amounts of oxygen and hydrogen will accumulate in the electrode chambers on both sides of the membrane.

[0017] Optionally, for a normal production cycle with a first duration T1 and a reverse polarity cycle with a second duration T2, the voltage may be switched alternately from the normal polarity of the electrodes to the reverse polarity after T1 and back again after T2, wherein T2 does not exceed 2% of T1, to keep the gas mixture in the gas collection tank within a safe range. In particular, potential hydrogen accumulation in the oxygen collection tank and oxygen accumulation in the hydrogen collection tank will be kept outside the critical range.

[0018] Although the 2% rule for the time ratio T2:T1 is sufficient to stay outside the critical range, in many embodiments it is preferred to keep the time duration T2 to no more than 1% of the normal operating time T1 as this creates an increased safety margin.

[0019] In the prior art, regeneration times of about 20 minutes or even longer have been disclosed. However, the inventors have studied this problem in detail for alkaline electrolysis systems, in particular for electrolytes based on KOH or NaOH and stainless steel electrodes. In the temperature range of 60-90°C, the chemical aggressiveness of such electrolytes is very strong, the stainless steel surface will corrode, and the corrosion products will be deposited on the electrodes. However, the reverse voltage efficiency is very high, and it has been found that a reverse time T2 of 3 seconds or less is sufficient to achieve activation and / or regeneration.

[0020] Although the minimum activation and / or regeneration time Tmin depends on the system configuration, electrolyte and operating parameters, in particular the electrode area and current, a general rule has been found: activation and / or regeneration times significantly less than 1 second are generally ineffective for cleaning and activating and / or regenerating the electrodes. The activation and / or regeneration effect must be long enough to be apparent. Therefore, for alkaline systems, it is advantageous if T2>Tmin, where Tmin depends on the system but is typically around 1 second and may be as high as or even exceed 3 seconds.

[0021] During activation and / or regeneration, the anode becomes the cathode and the cathode becomes the anode due to the opposite polarity. The electrode and its chamber (i.e., the anode and the anode chamber) that produce oxygen during normal operation are referred to herein as the first electrode and the first electrode chamber. Similarly, the electrode that acts as a cathode during normal operation and the electrode that acts as a cathode chamber and produces hydrogen and its chamber are referred to as the second electrode and the second electrode chamber, respectively.

[0022] Since the rate of oxygen generation is half that of hydrogen generation, the generation limit of 2% of hydrogen in the oxygen generated in the first electrode chamber is equivalent to 1% of the volume of oxygen generated in the second electrode chamber. Hydrogen generated in the second electrode chamber during normal production. This means that, for the cathode side of the system, if the standard of not more than 4% of hydrogen generated in the H2+O2 gas mixture on the anode side is adopted, then the hydrogen collection tank contains no more than 1% of oxygen. Considering that the critical range of oxygen in the H2+O2 gas mixture is above 6%, the collection of hydrogen in the oxygen tank is the most stringent in determining the duration of the reverse polarity period relative to the duration of normal operation.

[0023] While carefully studying the problem of improving the safety of the electrolysis system, the inventors also found that the accumulation of explosion-critical gas mixtures in the electrode chamber is a real but often overlooked problem. Although, as mentioned above, the prior art system considers separating the generated gas with opposite polarity into the correct storage tank during activation and / or regeneration, it does not seem to have thoroughly considered that the reverse voltage will also cause a potentially dangerous hydrogen and oxygen mixture to accumulate inside the electrode chamber. In the prior art, gas separation at the outlet of the electrode chamber does not solve this problem. When the oxygen in the second electrode chamber is filled with a small amount of hydrogen and the hydrogen in the second electrode chamber is filled with a small amount of oxygen, the system may exceed the ratio range between oxygen and hydrogen, thereby posing a potential risk of fire and explosion. Therefore, in view of this problem, it is also desirable to improve the safety of the electrolysis system. Specifically, the purpose is to improve the safety of the electrolysis system to prevent the formation of an explosive gas mixture in the gas collection electrode chamber during electrode activation and / or regeneration polarity reversal.

[0024] For the electrode compartments that accumulate the H2+O2 gas mixture during the polarity reversal, the ratio may be critical to the system, so further consideration needs to be given to the ratio between the respective production amounts and the time T1 for normal operation and the time T2 for polarity reversal.

[0025] The following parameter VO represents the maximum volume of oxygen that the first electrode chamber can accommodate during normal production. In order to avoid the generation of critical mixed gas in the first electrode chamber, the second volume V2 of hydrogen added during the regeneration operation should not be greater than 4% of the volume VO. In this case, not only the relative amount of hydrogen in the O2 tank is not greater than 4%, but also the relative amount of hydrogen in the first electrode chamber is not greater than 4%.

[0026] The criterion that the second volume V2 of hydrogen during regeneration operation should not exceed 4% of the volume VO does not necessarily mean that only a volume of VO of oxygen is produced during normal operation. During normal production, both the production volume V1 of oxygen and the production volume of hydrogen are correspondingly much higher. However, in order to avoid critical ranges inside the first electrode chamber itself, the parameter VO is decisive. For example, the production of V1=10VO of oxygen during normal operation will result in 9VO of this oxygen being released into the O2 storage tank, while only a volume of VO of the produced oxygen will remain in the first electrode chamber. Then, when the polarity is reversed during activation and / or regeneration, hydrogen is added to the VO volume of O2 gas in the first electrode chamber, which is why the volume VO is decisive, regardless of the fact that the normal production volume V1 is several times greater than VO.

[0027] For example, a characteristic time constant TO of the oxygen production VO during normal operation can be determined. In more general terms, a normal operation at a fixed constant production rate will produce a gas volume of N times VO over a duration of N times TO, of which VO will remain in the electrode chamber and (Nl) times VO will be collected in the piping system and tanks, or released into the environment or transported for use. In the subsequent reverse polarity operation, the duration T2 should not be longer than 2% of TO in accordance with the above discussion to avoid a critical range of hydrogen in the H2+O2 mixture.

[0028] For example, the first electrode compartment must not absorb more oxygen than is produced during T0 = 100 seconds during normal operation, which may be of duration T1 > T0. The 2% rule applied above would yield T2 = 2 seconds, higher than Tmin = 1 second, as the minimum activation and / or regeneration time of the electrode.

[0029] Optionally, in order to increase the activation and / or regeneration time of the electrodes without risking the potential risk of fire and explosion with respect to the ratio between oxygen and hydrogen, the current in the opposite direction applied to the electrodes during activation and / or regeneration can be chosen to be lower than the conventional current during conventional operation.

[0030] The rate of production of hydrogen and oxygen is proportional to the current, and by reducing the current, the amount of hydrogen produced during the polarity reversal time T2 is proportionally reduced.

[0031] The current may advantageously be adjusted to a level to ensure that the hydrogen production in the first electrode compartment remains below a critical level and that the reverse polarity is of sufficient duration to achieve the desired regeneration effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present invention will be described in more detail with reference to the accompanying drawings, in which

[0033] FIG1 shows an electrolysis system for producing hydrogen;

[0034] Figure 2 shows the effect of activating the electrodes by reversing their polarity;

[0035] FIG3 shows the effect of electrode regeneration by polarity reversal;

[0036] FIG. 4A shows an electrolysis system for producing hydrogen according to the present invention,

[0037] FIG4B shows a system with regular polarity and

[0038] FIG. 4C shows a system with reverse polarity. DETAILED DESCRIPTION

[0039] 1 is a simplified schematic diagram of an electrolysis system 1 for decomposing water into hydrogen H2 and oxygen O2. The system 1 comprises a first electrode chamber 2 having a first electrode 4 and a second electrode chamber 3 having a second electrode 5. The electrodes 4, 5 are arranged on opposite sides of a membrane 6. The first electrode chamber 2 and the second electrode chamber 3 contain an electrolyte 7.

[0040] For example, the electrolyte is an alkaline water-based electrolyte, optionally a KOH or NaOH solution. Typically, the temperature of the electrolyte is higher than ambient temperature, for example in the range of 60-90° C. As an option, the system pressure is higher than ambient pressure.

[0041] Electricity 8 is supplied to electrodes 4 and 5. The current passing through electrodes 4 and 5 is used to decompose water in electrolyte 7 into O2 and H2. During normal operation, O2 is generated in the first electrode chamber 2, and the first electrode 4 acts as an anode, and the first electrode chamber 2 is an anode chamber. During normal operation, H2 is generated in the second electrode chamber 3, and the second electrode 5 acts as a cathode, and the second electrode chamber 3 is a cathode chamber. Gases leave from the corresponding electrode chambers 2 and 3 and enter conduits 10 and 11, respectively, and conduits 10 and 11 guide the gases to oxygen container 12 and hydrogen container 13, respectively.

[0042] Through the functional connection 14, the electrical connection system 9 is connected to a control system 18, which includes a programmable computer 16 and an electrical switch controller 15 for reversing the voltage and current direction of the power supply 8. A specific example of the polarity reversal method is given below.

[0043] Figures 2 and 3 show the effect of electrode modification by reversing polarity in activation mode or during regeneration, respectively.

[0044] When evaluating the impact of polarity reversal, it is worth noting that the efficiency of the electrolysis system to produce hydrogen can be calculated using the expression Eta = Uo / U, where Eta is the efficiency, Uo is the reference voltage, which is 1.48V, and U is the actual voltage. Figure 2A shows the time variation curve of the electrode voltage under a fixed current. Therefore, a lower voltage is beneficial to improve the efficiency of the electrolysis system to produce hydrogen.

[0045] Figure 2 shows experimental data for early activation of electrodes to produce hydrogen in an alkaline electrolysis system. The experiment was conducted at a constant current. The polarity was reversed for 3 seconds every 10 minutes. It can be seen that the cell voltage (i.e., the voltage difference between the anode and the cathode) gradually decreases with repeated polarity reversals during the electrode activation period of more than 7 hours. At the beginning of the experiment, the temperature of the electrolyte was about 17°C, as shown by the flat curve in the figure and related to the scale on the right vertical axis. Within the first four hours, the temperature of the electrolyte rises to about 22°C and then remains stable at this level. The cell voltage at a given current is a function of the electrolyte temperature, and it drops by about 5mV for every 1°C increase. Therefore, part of the reason for the voltage drop within four hours after the start of the experiment is the increase in temperature. However, although the temperature has stabilized to a constant value within 3 hours, the cell voltage continues to decrease from 3 to 7 hours. This proves the activation effect of polarity reversal.

[0046] Figure 3 shows the regeneration of a previously activated electrode that was operating normally in an alkaline electrolysis system for the production of hydrogen. After operating for several hours at normal polarity, the polarity was reversed for 3 seconds. As shown in the figure, the time between polarity reversals was 16 hours. In this case, it can also be observed that the voltage is significantly reduced by the polarity reversal and that the voltage does not return to the level before the regeneration level even after several hours of a single polarity reversal. The temperature was almost constant during this experiment, so the voltage reduction was independent of the temperature increase.

[0047] It is worth noting that these experiments shown in Figures 2 and 3 were performed at ambient temperature. In commercial applications, electrolysis is typically performed in the temperature range of 60-90°C.

[0048] FIG4a is a simplified sketch of an electrolysis system 1 for decomposing water into hydrogen H2 and oxygen O2 according to the present invention. The system has the same basic components as the system in FIG1 , however, as an example, two sets of contactors 21, 22, 23, 24 are additionally shown as part of the electrical connection 9 as a polarity reversal device. The first set of contactors 21 and 22 is used to connect the power supply 8 to the electrodes 4 and 5 during normal operation. The second set of contactors 23 and 24 is used to connect the power supply 8 to the electrodes 4 and 5 during reverse polarity operation.

[0049] FIG4B shows operation under normal polarity. The first set of contacts 21 and 22 have been closed and the power supply 8 is connected to the electrodes 4 and 5, so that the left electrode 4 (anode during normal operation) has a positive voltage relative to the right electrode 5 (cathode during normal operation). During normal operation, H2 is produced in the second electrode chamber 3 on the right side of the membrane 6, and O2 is produced in the first electrode chamber 2 on the left side of the membrane 6.

[0050] 4C shows operation with reverse polarity, for example in regeneration mode. When the first set of contactors 21, 22 is open, the second set of contactors 23 and 24 is closed and connects the power supply 8 to the electrodes 4 and 5 in reverse polarity mode, so that the left electrode 4 has a negative voltage relative to the right electrode 5.

[0051] The regeneration reverse voltage and reverse current may optionally be at the same level as during normal operation, but this is not required and may advantageously be adjusted to improve activation and / or regeneration efficiency or to adjust the timing of activation and / or regeneration. Such adjustments may be necessary to prevent dangerous gas mixing during regeneration operation, as will be discussed in more detail below.

[0052] During the regeneration operation, the first electrode 4 on the first side of the membrane 6, which acts as an anode during normal operation, temporarily acts as a cathode and produces H2 in the first electrode chamber 2. This results in a mixture of H2 and O2 in the first electrode chamber 2. Similar considerations apply to the second side of the membrane 6, where the second electrode 5, which acts as a cathode during normal operation, temporarily acts as an anode and produces O2 in the second electrode chamber 3. This results in a mixture of H2 and O2 gases in the second electrode chamber 3. In order to avoid explosion hazards, the proportion of gas added during regeneration should be small. It is well known that the risk of fire and explosion increases when the ratio between H2 and O2 is in the range of 4% to 94%, but the situation outside this range is considered safe. Therefore, on the anode side of the system, the volume of hydrogen added to the oxygen should not exceed 4%, and on the cathode side of the system, the volume of oxygen added to the hydrogen should not exceed 6%. During the generation process, every two water molecules split into two hydrogen molecules H2, but only one oxygen molecule O2. Therefore, the production rate of hydrogen is twice that of oxygen. Therefore, while hydrogen H2 with a volume of 2V1 is produced in the second electrode chamber 3, oxygen O2 with a volume of V1 is produced in the first electrode chamber 2.

[0053] The ratio of the amount of hydrogen V2 produced in the first electrode chamber 2 during regeneration operation to the amount of oxygen V1 produced in the first electrode chamber 2 during normal operation should not exceed 4%. In mathematical terms, V2 / V1≤0.04.

[0054] This is the same as V2 / (2V1)≤0.02. Therefore, the 4% upper limit safety limit relative to the oxygen produced on the anode side is converted to a 2% upper limit relative to the hydrogen produced on the anode side, relative to the 2V1 hydrogen production in the second electrode chamber 3 during normal operation. This is a relevant figure to be mentioned here in case the switching between normal operation and regeneration operation is regulated by a predetermined time duration, which will be explained in more detail below.

[0055] During the polarity reversal activation as shown in FIG2 , and / or during the polarity reversal regeneration as shown in FIG3 , since each pair of water molecules splits into two H2 molecules but only splits into one O2 molecule, a volume of V2 of hydrogen is accumulated in the oxygen tank 12, and a volume of 0.5V2 of oxygen is accumulated in the hydrogen tank 11. In order to conduct a risk assessment of the hydrogen tank 11, the amount of 0.5V2 of oxygen generated during the activation and / or regeneration must be compared with the amount of 2V1 of hydrogen generated during normal operation, so that the mixed gas entering the H2 tank 11 contains only 0.5V2 of oxygen and 2V1 of hydrogen, thereby obtaining the ratio of O2:H2=(0.5V2) / (2V1), and when substituting the expression V2 / Vl=0.04, the gas ratio of O2:H2 is obtained to be 0.50.04 / 2=1%, which is the gas ratio actually obtained in the H2 tank 11 when the O2 tank 12 meets the safety requirement of V2 / Vl≤0.04. Therefore, if the safety margins of the 02 tank 12 are observed, at least 99% of H2 and no more than 1% of 02 gas will remain in the gas mixture in the H2 tank 11 despite activation and / or regeneration. Therefore, the addition of oxygen to the hydrogen on the cathode side is not the most critical aspect due to activation and / or regeneration. The addition of hydrogen to the oxygen on the anode side during activation and / or regeneration is the most critical aspect, and if the safety margins of the 02 tank 12 are observed, the H2 tank 11 is definitely safe.

[0056] Therefore, a 2% volume limit on hydrogen produced during regeneration operation relative to hydrogen production during normal operation is a safe and appropriate limit for both tanks 11, 12. If this limit is followed, no more than 1% of H2 will accumulate in the H2 tank 11 and no more than 4% of H2 will accumulate in the H2 tank 12.

[0057] As an option, the electrolyzer system 1 produces a fixed volume of hydrogen 2V1 and a fixed volume of oxygen V1 in each of a plurality of subsequent normal operation cycles, wherein the normal cycles are separated by activation and / or regeneration cycles between each two normal cycles. In addition, the electrolyzer system 1 produces a fixed volume of hydrogen V2 and a fixed volume of oxygen 0.5V2 in each regeneration operation cycle following each of these plurality of normal operation cycles.

[0058] For example, the electrolyzer system 1 produces gas at a constant rate during normal operation and produces gas at the same rate during regeneration operation. In this case, the volume ratio is equivalently converted into a time ratio, where the time length T2 of the regeneration operation cycle should not be longer than 2% of the time length T1 of the normal operation cycle, so that 2% of hydrogen is produced in the activation and / or regeneration cycle relative to the hydrogen volume 2V1 produced in the normal operation cycle, and equivalently, 4% of hydrogen is produced relative to the oxygen volume V1 produced in the normal operation cycle, the latter being the critical limit on the cathode side of the membrane 6, which is located on the left side of the membrane 6 in the figure.

[0059] The above model produces a safe activation and / or regeneration scheme without the need for a valve system that alternates the routing of gas depending on whether the cycle is regular or regeneration. This means a simpler system, lower production costs, lower maintenance costs and less risk of mechanical failure.

[0060] As mentioned above, there is a risk of O2 accumulation in the H2 tank 11. The key criterion for the H2 tank 11 is that no more than 6% of O2 accumulates together with the H2. Based on the results discussed above, this means that the amount of oxygen produced during activation and / or regeneration does not exceed 6% of the amount of hydrogen produced during normal operation. However, even in this case, a lower safety level is advantageous because oxygen tends to accumulate in the lower part of the tank 11, resulting in a higher local concentration in the lower part of the tank. The oxygen can be removed by a valve system. However, by complying with the above 2% rule, not only the O2 tank 12 is safe, but also the H2 tank 11.

[0061] Having discussed the risks in the H2 tank 11 and the optional O2 tank 12, attention should now be paid to the explosion risks in the electrode chambers 11, 12, which will be discussed below.

[0062] Referring to Figure 1, recall that

[0063] V1 is the volume of oxygen O2 produced by the anode 4 during the normal production time interval T1,

[0064] 2V1 is the volume of hydrogen H2 produced by the cathode 5 during the normal production time interval T1,

[0065] V2 is the amount of oxygen O2 produced during the activation and / or regeneration time interval T2 of voltage reversal, and

[0066] 0.5V2 Volume of hydrogen H2 produced during the time interval during activation and / or regeneration.

[0067] These volume figures also apply to the maximum amount of gas that can accumulate in the electrode chambers 2, 3 without causing an explosion hazard. During normal operation, a certain amount of oxygen accumulates in the first electrode chamber 2 before it leaves the first electrode chamber 2 through the conduit 10 (e.g. into the atmosphere or oxygen tank 12 for accumulation). During the regeneration action, hydrogen is added to the oxygen remaining in the first electrode chamber 2, and oxygen is added to the hydrogen remaining in the second electrode chamber 3. Although the explosion risk in the electrode chambers 2, 3 is slightly lower than in the tanks 11, 12, it is still not negligible. In the following, attention is paid to the gas mixture generated inside the electrode chambers 2, 3.

[0068] For the risk assessment in the first electrode chamber 2 , the term VO is used hereinafter to denote the volume of oxygen which can maximally accumulate in the first electrode chamber 2 during normal operation.

[0069] For example, during normal operation, the production of V1 = 10VO oxygen will result in 9VO of the oxygen being released from the first electrode chamber into the atmosphere or into the O2 storage tank 12. During this normal operation, only the VO volume of the oxygen produced will remain in the first electrode chamber 2. Then, when the polarity is reversed for an activation and / or regeneration cycle, hydrogen is added to the remaining oxygen in the first electrode chamber 2.

[0070] In summary, the maximum permissible volume V2 of hydrogen in the first electrode chamber 2 is 4% hydrogen relative to the volume VO of oxygen in the first electrode chamber 2, which is now turned into a mixed gas by the addition of hydrogen. Therefore, for the high safety of the first electrode chamber 2, it is important to determine the maximum amount of oxygen VO that can be accumulated in the first electrode chamber 2 during normal operation, and accordingly limit the amount of hydrogen V2 produced in the first electrode chamber 2 during the reverse voltage regeneration operation to no more than 4% of VO. This sets an upper limit on the amount of hydrogen V2 produced during activation and / or regeneration, regardless of the fact that the actual amount of oxygen V1 produced during normal operation is much greater than VO. For the sake of clarity, refer to the above example, in which the amount of oxygen produced is V1=10VO.

[0071] Determining the volume VO depends on the overall setup and operating parameters, including size, type and condition of electrolyte, and current.

[0072] However, it should be noted that the activation and / or regeneration time T2 cannot be infinitely short, because activation and / or regeneration requires a certain amount of time. The activation and / or regeneration time depends on the material and design parameters of the system. Advantageously, the design and parameters of the system make the activation and / or regeneration time greater than 1 second, for example, not less than 3 seconds, and the hydrogen generated during the activation and / or regeneration period is no more than 4% of the oxygen volume VO in the first electrode chamber 2.

[0073] For example, when the current remains unchanged, the gas production rate during normal operation and reverse polarity operation is constant. In this case, the duration T1 of normal operation can be set to produce a volume of oxygen V1, where V1 ≥ VO. The duration of producing the volume VO is TO. Then, the time T2 of reverse polarity operation is set in the interval of Tmin ≤ T2 ≤ 0.02TO. Therefore, the time T2 of reverse polarity must be not less than the minimum time Tmin required to activate and / or regenerate the electrode, but not more than 2% of the duration of the normal operation cycle.

[0074] To clarify, as mentioned above, the 2% limit on time T2 relative to TO stems from the fact that VO generation from oxygen requires time TO during which 2VO hydrogen is generated, while the 4% rule for hydrogen regeneration generation relative to conventional VO generation from oxygen means that hydrogen regeneration generation is 2% VO relative to conventional hydrogen generation.

[0075] If a higher safety margin is required, the 2% limit can advantageously be converted to a lower limit, such as a 1% limit.

[0076] In contrast, the maximum volume of hydrogen contained in the second electrode chamber 3 is VH, which can also be determined, for example, by experiments or calculations. Since the critical interval of H2 in the H2+O2 gas mixture is 4% to 94%, the amount of oxygen produced in the same second electrode chamber 3 during activation and / or regeneration should not exceed 6% of VH. If, during normal operation, a duration TH is required to generate VH in the second electrode chamber 3, the amount of oxygen generated during this activation and / or regeneration time period TH is 0.5V2, because the generation rate of O2 is half the generation rate of H2. Therefore, due to the lower productivity of O2, the time length T2 of the activation and / or regeneration phase should not exceed 12% of TH, i.e. twice the 6% limit. Taking into account also the minimum time duration Tmin for correct activation and / or regeneration, Tmin is obtained. <T2<0.12TH。

[0077] The upper limit of the duration T2 of the reverse polarity period of this standard is higher than the comparative critical limit of H2 in the gas mixture in the first electrode chamber. However, even if oxygen is released into the atmosphere and does not accumulate in the 02 tank, making the above discussion about H2 accumulation in the 02 tank 12 invalid, the risk assessment for gas collection in the first electrode chamber 2 still exists. Therefore, in this case, in order to reduce the risk in the first electrode chamber 2, and thus for the entire system 1, the 2% rule discussed above applies as an appropriate standard, which is also the safety limit for the second electrode chamber and the H2 tank.

[0078] In some configurations of the first electrode chamber 2, the criterion that the time duration T2 needs to be 2% or less relative to TO may result in a very short allowed time duration T2. ​​This may be the case, for example, if the first electrode chamber 2 is particularly small, resulting in a small volume of oxygen VO. T2 may be too short to achieve a correct activation and / or regeneration.

[0079] In order to increase the activation and / or regeneration time T2, the current in the opposite direction applied to the electrodes during the reverse polarity (especially for activation and / or regeneration) can be selected to be lower than the normal current during normal operation. The current can be advantageously adjusted to a certain level to ensure that the hydrogen production in the first electrode chamber is always below a critical level and that the reverse polarity has a sufficient duration to achieve the desired regeneration or activation effect.

[0080] It should be understood that the polarity reversal system in Figure 4 is only an example, and other polarity reversal arrangements may be provided. For example, a control system 18 with a computer 16 for regulating the distribution of power 8 may be configured to provide polarity reversal without contactors 21, 22, 23 and 24.

Claims

1. A method for operating an electrolysis system (1) for decomposing water into hydrogen H2 and oxygen O2, the electrolysis system (1) comprising a first electrode chamber (2) with a first electrode (4) and a second electrode chamber (3) with a second electrode (5), the electrode chambers (2, 3) containing an electrolyte (7) and separated by a membrane (6); wherein the method comprises alternating between normal operation and reverse polarity operation, wherein during normal operation, oxygen O2 is produced in the first electrode chamber (2) on the first side of the membrane (6) and hydrogen H2 is produced in the second electrode chamber (3) on the second side of the membrane (6), wherein during reverse polarity operation, the voltage and current are reversed compared to normal operation, hydrogen is produced in the first side of the membrane (6) and in the first electrode chamber (2), and oxygen is produced in the second side of the membrane (6) and in the second electrode chamber (3); wherein, The method comprises producing a first volume V1 of oxygen in a first electrode chamber (2) on a first side of a membrane (6) by conventional operation, and producing a second volume V2 of hydrogen in the first electrode chamber (2) by reverse polarity operation, characterised in that the method comprises limiting the production of the second volume V2 of hydrogen to no more than 4% of the first volume V1 of oxygen produced by conventional operation, so that the resulting gas mixture of H2 and O2 contains no more than 4% of H2.

2. The method according to claim 1, characterized in that Therein, for a volume V1 of oxygen produced during normal operation, the method comprises producing a volume V2 of hydrogen during a subsequent operation with opposite polarity, wherein the volume V2 of hydrogen does not exceed 4% of the volume V1 of oxygen produced during the preceding normal operation.

3. The method according to claim 2, characterized in that The method comprises producing a fixed volume of V1 in each of a plurality of subsequent normal operating cycles, and producing a fixed volume of hydrogen V2 not exceeding 4% of V1 in each operating cycle, and having opposite polarity between every two subsequent normal operating cycles.

4. The method according to claim 1, characterized in that: wherein VO is the maximum volume of oxygen that can be contained in the first electrode chamber (2) during normal operation; and wherein the method comprises producing a second volume V2 of hydrogen during operation with reverse polarity, wherein the second volume V2 does not exceed 4% of the volume VO.

5. The method according to claims 1-4, characterized in that: Wherein the method comprises determining a duration TO for producing a volume of oxygen VO during normal operation, and wherein the method comprises reverse polarity operation for a duration T2 not greater than TO.

6. The method according to claim 3, characterized in that wherein the method comprises producing H2 and O2 at a fixed constant rate during normal operation and during reverse polarity operation, wherein the normal operation period has a first duration T1 and the reverse polarity operation period has a second duration T2, and wherein the method comprises alternately switching the voltage to the reverse polarity of the electrodes (4, 5) after T1 and switching back again after T2, wherein T2 does not exceed 2% of T1.

7. The method according to any one of claims 1 to 5, characterized in that Therein the method includes reducing the current applied to the electrode during operation to a polarity that is opposite relative to the current applied to the electrode during normal operation.

8. The method according to claim 7, characterized in that Wherein the current applied to the electrodes during reverse polarity operation is less than 75% of the current applied to the electrodes during normal operation.

9. The method according to any one of the preceding claims, characterized in that The duration T2 of the reverse polarity operation is at least Tmin=3 seconds.

10. The method according to any one of the preceding claims, characterized in that Therein the method comprises providing an alkaline electrolyte to the system and operating the electrolysis at a temperature in the range of 50°C to 90°C.

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