FIELD OF THE INVENTION

The present invention relates to an electrolysis system for production of hydrogen gas in which the voltage between the electrodes is reversed periodically for regenerating the electrodes.

BACKGROUND OF THE INVENTION

As discussed in US8308919, electrolysis systems experience a decrease in efficiency due to build-up of contamination on the surface of the electrodes, reducing electrical conductivity and, correspondingly, the rate of gas production. Periodic reversal of polarity, where the voltage on the electrodes is reversed for a time, can be used for regenerating the electrodes. For example, periodical reverse of polarity 3-30% of the time in an electrolyser is disclosed in US4578160A for recovery of concentrated caustic alkali. W02013/078004A1 discloses reversal of polarity <1% of the operational time in an electrolyser for regeneration of a diamond electrode. During the regeneration time, hydrogen and oxygen gas are produced on the opposite side of the membrane as compared to regular operation. Thus, oxygen is introduced into the side where normally hydrogen is produced and accumulated in storage containers, and hydrogen is produced on the side which is normally used for oxygen production. During a time in the order of 20 minutes, as in US8308919, a substantial volume of gas is produced. In order to separate oxygen and hydrogen into the correct container during times with reversed voltage, US8308919 discloses switching the discharge by a gas directional control valve.

Valves for selection of correct exit passage from the electrode chambers, depending on regular operation or regeneration periods with reverse voltage on zinc-plated aluminium electrodes for reverse zinc migration, are also disclosed in US10167561. In this system, the electrode chambers have a collection hood with oxygen outlet and hydrogen outlet at opposite ends, where the hood can be tilted in one or the other way in order to facilitate the collection of oxygen or hydrogen into the respective valve-operated exit passages. Although these systems provide technical solutions that prevent the accumulation of explosive gas mixtures of hydrogen and oxygen in the storage tanks, they are based on mechanical valves or other contraptions, which, on the one hand, make the system more complex and, on the other hand, imply a risk of dangerous gas mixtures in case of malfunction for the mechanical parts, which is a disadvantage.

It would be desirable to increase the safety for electrolysis systems also with respect to this problem.

DESCRIPTION / SUMMARY OF THE INVENTION

It is therefore an objective of the invention to provide an improvement in the art. In particular, it is an objective to provide an increased safety in electrolysis systems that are applying periodical polarity reversal for electrode activation and/or regeneration, especially with respect to preventing explosive gas mixtures in the system. It is a further objective to provide simple technical solutions for the problem. These objectives and further advantages are achieved with a method of operation of electrolysis systems as set forth below and in the claims.

The electrolysis system as described herein is used for splitting water into hydrogen gas, H2, and oxygen gas, 02, and comprises a first electrode chamber with a first electrode and a second electrode chamber with a second electrode. The first and second chambers are separated by a membrane and contain an electrolyte.

During a time T1 of regular gas production, a first voltage of a first polarity is applied to the electrodes. During regular operation, oxygen gas 02 is produced in the first chamber on a first side of the membrane, and hydrogen gas H2 is produced in the second electrode chamber on a second side of the membrane. Accordingly, during regular 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. Periodically for regeneration of the electrodes, a second voltage of opposite polarity, relatively to the first voltage, is applied to the electrodes and across the membrane for a time T2 in order to regenerate the electrodes and increase the efficiency after a drop thereof. The use of opposite polarity is herein called reverse polarity.

In this connection, the following is pointed out in addition. The electrodes used in electrolysis systems may require activation at first use, such first use being here defined as the actual first use of the electrodes after commissioning or the first use of the electrodes after an operational pause having duration above a certain level where the electrodes return to an inactivated stage. The inventor has observed that periodic polarity reversal is a useful method for electrode activation. Accordingly, polarity reversal is not only useful for regeneration after a period of regular operation, but polarity reversal, typically multiple times at shorter timescale than regeneration, is useful for activation of the electrodes.

During operation with reverse polarity, voltage and electrical current are reversed as compared to regular operation, and hydrogen gas H2 is produced on the first side of the membrane and in the first electrode chamber and oxygen gas 02 is produced on the second side of the membrane and in the second electrode chamber. This leads to gas mixtures on the electrode chambers, which implies a risk for explosion if not proper precautions are taken. In the following, some safety aspects are discussed for preventing production of dangerous gas mixtures when alternating between such regular operation and periods with reverse polarity, be it for regeneration or for electrode activation.

While a first volume of VI of oxygen gas 02 is produced in the first chamber on the first side of the membrane during regular operation, where the first chamber functions an anode chamber, a second volume of V2 of hydrogen gas H2 is produced in the first chamber during operation with reverse polarity. By limiting the production of the second volume of V2 of hydrogen gas to be no more than 4% of the first volume VI of oxygen gas produced by regular operation, the resulting gas mixture of hydrogen gas and oxygen gas to comprise no more than 4% hydrogen gas. For a collection of the oxygen gas from the regular operation in an oxygen gas tank, the gas mixture in the tank would contain no more than 4% H2. This is important, as the ratio interval of 4% to this range are regarded as safe.

For example, a volume VI of oxygen gas is produced in the first chamber during a period of regular operation, and a volume V2 of hydrogen gas is produced in the first chamber during an immediate subsequent period of operation with reverse polarity, wherein the volume of V2 is no more than 4% of the volume VI of oxygen gas produced during the preceding period of regular operation.

Optionally, a fixed first volume of VI of oxygen gas is produced in each of multiple subsequent periods of regular operation, and a fixed second volume V2 of hydrogen gas of no more than 4% of VI is produced in each period of regenerative operation in between each two of subsequent periods of regular operation. Thus, the system alternates between a period of regular operation and a period of regenerative operation with equal volumes produced in the respective periods.

Hydrogen gas is produced at twice the rate as compared to oxygen gas, seeing that each pair of water molecules splits into two H2 molecules and only one 02 molecule.

Accordingly, the 4% upper limit for the hydrogen gas production in the first chamber during the periods with reverse polarity relatively to the production of oxygen gas in the first chamber during the regular operation translates into 2% of oxygen gas produced in the second chamber during the periods with reverse polarity, when relating it to an equal amounts of gas. For example, equal amounts of oxygen and hydrogen are accumulated in the electrode chambers on either side of the membrane during regular production.

Optionally, for the regular production periods having a first time duration T1 and the periods with reverse polarity having a second time duration T2, an alternate switching of the voltage from regular to reverse polarity of the electrodes after T1 and back again after T2 can be done with T2 being no more than 2% of T1 in order to remain in the safe range with respect to gas mixtures in the gas collection tanks. In particular, the accumulation of hydrogen in a potential oxygen collection tank and the accumulation oxygen in a hydrogen collection tank will remain outside the critical range. Although, the 2% rule for the time ratio T2:T1 is sufficient for staying outside the critical range, it is in many embodiments preferred to keep the time duration T2 to no more than 1% of the regular operation time Tl, as this yields an increased safety margin.

In the prior art, regeneration times in the order of 20 minutes or even more have been disclosed. However, the inventor has investigated this problem in detail for alkaline electrolysis systems, especially with KOH or NaOH based electrolytes and stainless steel electrodes. At temperatures in the range of 60-90°C, such electrolytes are chemically very aggressive, and stainless-steel surfaces are corroded with corrosion products depositing on the electrodes. However, reversed voltage is highly efficient, and a reverse time T2 of 3 seconds or less has been found sufficient for activation and/or regeneration.

Although the minimum activation and/or regeneration time Tmin depends on the system configuration, the electrolyte and the operation parameters, in particular the electrode area and the electrical current, a general rule has been found that activation and/or regeneration times of significantly less than 1 second are typically not efficient for cleaning and activating and/or regenerating the electrodes. The activation and/or regeneration effect has to be sufficiently long for the effect to manifest itself. Accordingly for alkaline systems, it is advantageous if T2 > Tmin, where Tmin depends on the system but is usually around 1 second, and it may be up to or even above 3 seconds.

During activation and/or regeneration, the anode changes to be a cathode due to the reverse polarity, and the cathode changes to become an anode. The electrode and its chamber which are an anode and an anode chamber and produce oxygen gas during regular operation are called herein the first electrode and the first electrode chamber. Similarly, the electrode and its chamber which is a cathode and a cathode chamber and produces hydrogen gas during regular operation are called the second electrode and the second electrode chamber.

As oxygen gas is produced at half the rate of the H2 production, the 2% production limit for H2 in the oxygen gas produced in the first electrode chamber translates to a volume of 1% of oxygen gas produced in the second electrode chamber relatively the volume of hydrogen gas produced in the second electrode chamber during regular production. This implies for the cathode side of the system that the hydrogen gas collection tank contains no more than 1% of oxygen gas if the criterion is applied that no more than 4% of hydrogen gas is produced in the gas mix of H2+O2 on the anode side. Having in mind that the critical range for oxygen gas in the H2+O2 gas mixture is above 6%, it follows that the collection of hydrogen gas in the oxygen gas tank is the most restrictive one with respect to determining the duration of the period with reverse polarity relatively to the duration of the regular operation.

During close study of the problem of improving safety in electrolysis systems, the inventor also discovered that accumulation of explosion-critical gas mixtures in the electrode chambers is a real, although often overlooked problem. Although prior art systems, as discussed above, take into account separation of the produced gases into the correct storage tank during activation and/or regeneration periods with reversed polarity, it seems that is has not been thoroughly considered that reverse voltage also leads to accumulation of potentially dangerous mixtures of hydrogen and oxygen gas inside the electrode chambers themselves. A separation of gasses at the exit of the electrode chambers, as in the prior art, does not solve this problem. While a minor portion of hydrogen is filled into the oxygen gas in the second electrode chamber, and a minor portion of oxygen is filled into the hydrogen gas in the second electrode chamber, the system may pass a range where the ratio between oxygen and hydrogen involves a potential risk for fire and explosion. Accordingly, it would be desirable to increase the safety for electrolysis systems also with respect to this problem. In particular, an objective is seen in providing increased safety in electrolysis systems with respect to preventing explosive gas mixtures in the gas-collecting electrode chambers during periods with reversed polarity for electrode activation and/or regeneration.

For electrode chambers that during periods of reversed polarity accumulate gas mixtures of H2+O2 with a ratio that could become critical for the system, further considerations apply as to the ratio between the respective production volumes as well as the time T1 of regular operation and the time T2 for reversed polarity.

In the following, the parameter VO expresses a maximum volume of oxygen gas that can be contained in the first electrode chamber during regular production. In order to avoid producing critical gas mixtures in the first electrode chamber, the second volume V2 of hydrogen gas added during regenerative operation should be no more than 4% of the volume VO. In this case, not only is the relative amount of hydrogen gas in the 02 tank no more than 4% but also inside the first electrode chamber.

The criterion that the second volume V2 of hydrogen gas during regenerative operation should be no more than 4% of the volume VO does not necessarily imply that, during regular operation, only a volume of oxygen gas of VO is produced. The produced volume VI of oxygen, and the correspondingly twice as large produced volume of hydrogen gas, during the regular production can be by far higher. However, in order to avoid the critical range inside the first electrode chamber itself, the parameter VO is the decisive one. For example, a production of V1=10 VO of oxygen gas during a period of regular operation would lead to 9 VO of oxygen gas thereof released into the 02 accumulation tank and only the volume of VO of the produced oxygen gas would remain in the first electrode chamber. When the polarity is, then, reversed for the activation and/or regeneration period, hydrogen gas is added to the volume of VO of 02 gas in the first electrode chamber, which is why the volume VO is decisive, independently of the regular production volume VI being manifold larger than VO.

For example, a characteristic time constant TO can be determined for production of the volume VO of oxygen gas during regular operation. Using this for more general terms, regular operation with a fixed constant production rate for a duration of N times TO would produce a volume of gas of N times VO, of which VO would remain in the electrode chamber, and (N-l) times VO would be collected in the pipe system and the tank or, alternatively, be released into the environment or delivered to use. In the subsequent operation with reverse polarity, the duration T2 should not be longer than 2% of TO according to the above discussion of avoiding the critical range of hydrogen in the H2+O2 gas mixture.

For example, the first electrode chamber may not take up more oxygen gas than what is produced during T0=100 seconds in a period of regular operation, which may have a longer duration T1>TO. The above applied 2% rule would yield T2=2 seconds, which is above the Tmin=l second as minimum activation and/or regeneration time for the electrodes. Optionally, in order to increase the activation and/or regeneration time for the electrodes without risking that the ratio between oxygen and hydrogen involves a potential risk for fire and explosion, the opposite directed electrical current applied to the electrodes during periods of activation and/or regeneration may be selected lower than the regular current during regular operation.

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

The current may advantageously be adjusted to a level which ensures both that the production of hydrogen in the first electrode chamber is always less than the critical level, and that the reverse polarity has sufficient duration to achieve the desired regenerative effect.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the drawing, where FIG. 1 illustrates an electrolysis system for producing hydrogen gas;

FIG. 2 illustrates the effect of electrode activation by reversal of the polarity;

FIG. 3 illustrates the effect of electrode regeneration by reversal of the polarity;

FIG. 4A illustrates an electrolysis system for producing hydrogen gas according to the invention,

FIG. 4B illustrates the system with a regular polarity and

FIG. 4C illustrates the system with revers polarity.

DETAILED DESCRIPTION / PREFERRED EMBODIMENT

FIG. l is a simplified sketch of an electrolysis system 1 for splitting water into hydrogen gas, H2, and oxygen gas, 02. The system 1 comprises a first electrode chamber 2 with a first electrode 4 and a second electrode chamber 3 with a second electrode 5. The electrodes 4, 5 are provided on opposite sides of a membrane 6. The first electrode chamber 2 and the second electrode chamber 3 contain electrolyte 7. For example, the electrolyte is as alkaline water-based electrolyte, optionally KOH or NaOH solutions. Typically, the electrolyte has a temperature above ambient temperatures, for example in the range of 60-90°C. As an option, the system is pressurised above ambient pressure.

Electrical power 8 is supplied to the electrodes 4, 5. The electrical current through the electrodes 4, 5 is used for splitting water in the electrolyte 7 into 02 and H2. During regular operation, 02 is produced in the first electrode chamber 2, and the first electrode 4 functions as an anode, in which case the first electrode chamber 2 is an anode chamber. During regular operation, H2 is produced in the second electrode chamber 3, and the second electrode 5 functions as a cathode, in which case the second electrode chamber 3 is a cathode chamber. The gases, respectively, exit the corresponding electrode chambers 2, 3 into conduits 10 and 11, which guide the gas into an oxygen container 12 and a hydrogen container 13, respectively.

By a functional connection 14, the electrical connection system 9 is connected to a control system 18 comprising a programmable computer 16 and an electrical switch controller 15 for reversing the voltage and current direction for the electrical power 8. A concrete example of means for polarity reversal is given below.

FIG. 2 and 3, respectively, illustrate the effects of electrode improvement in activation mode or in regeneration periods by reversal of the polarity. In the assessment of the effect of the reversal of polarity it is useful to observe that the efficiency of an electrolysis system for production of hydrogen gas can be calculated with the expression Eta = Uo/U, where Eta is the efficiency, Uo is a reference voltage which has the value of 1.48 V, and U is the actual voltage. FIG. 2A shows a time history of the electrode voltage at a fixed current. Consequently, a lower voltage is advantageous for the efficiency of an electrolysis system for production of hydrogen gas.

FIG. 2 shows experimental data of early activation of the electrodes in an alkaline electrolysis system for production of hydrogen gas. The experiment was conducted at constant current. The polarity was reversed for 3 seconds every 10 minutes. It is seen that the cell voltage, which is the voltage difference between the anode and the cathode, is gradually reduced as a function of the repeated polarity reversal in the electrode activation period extending over 7 hours. At the beginning of the experiment, the temperature of the electrolyte is about 17 °C, as shown by the flat curve in the drawing and related to the scale on the right vertical axis. During the first four hours the temperature of the electrolyte increases to about 22 °C and subsequently remains stable at this level. The cell voltage at a given current is a function of the temperature of the electrolyte, declining about 5 mV per °C. Therefore, part of the voltage reduction in the first four hours of the experiment is attributable to the temperature increase. However, the cell voltage continues to reduce in the period from 3 to 7 hours, although, the temperature has stabilized to a constant value at 3 hours. This is demonstrating the activation effect of the polarity reversal.

FIG. 3 shows regeneration of previously activated electrodes, which are in regular operation in an alkaline electrolysis system for production of hydrogen gas. The polarity is reversed for 3 seconds after several hours of operation at regular polarity. As illustrated, the time between polarity reversal is 16 hours. Also in this case, it is observed that the voltage is significantly reduced by the polarity reversal, and the voltage does not revert to the previous before the regeneration level even several hours after the single polarity reversal. During this experiment the temperature is almost constant, and therefore, the voltage reduction is not attributable to any temperature increase.

It should be noted that these experiments illustrated in FIG. 2 and FIG. 3 were carried out at ambient temperature. In commercial applications electrolysis is normally carried out at temperatures in the range of 60-90 °C.

FIG. 4a is a simplified sketch of an electrolysis system 1 according to the invention for splitting water into hydrogen gas, H2, and oxygen gas, 02. The system has the same basic constituent parts as the system in FIG. 1, however, showing as example additionally two sets of contactors 21, 22, 23, 24 as part of the electrical connection 9 as means for polarity reversal. The first set of contactors 21 and 22 is used for connection of the electrical power 8 to the electrodes 4 and 5 during periods of regular operation. The second set of contactors 23 and 24 is used for connection of the electrical power 8 in to the electrodes 4 and 5 during periods of operation at reversed polarity. FIG. 4B illustrates operation at regular polarity. The first set of contactors 21 and 22 has been closed and connects the power 8 to the electrodes 4 and 5 such that the left electrode 4, which is the anode in regular operation, has a positive voltage relative to the right electrode 5, which is the cathode during regular operation. During regular operation, H2 is produced in the second electrode chamber 3 on the right side of the membrane 6, and 02 is produced in the first electrode chamber 2 on the left side of the membrane 6.

FIG. 4C illustrates operation with reversed polarity, for example in regenerative mode. While the first set of contactors 21, 22 is open, the seconds set of contactors 23 and 24 has been closed and connect the power 8 in reverse polarity mode to the electrodes 4 and 5 so the left electrode 4 has a negative voltage relative to the right electrode 5.

The regenerative opposite voltage and opposite current may optionally be at the same level as during regular operation, although, this is not necessary and may advantageously be adjusted for increased activation and/or regeneration efficiency or for adjustment of the time for activation and/or regeneration. Such adjustment can be necessary in order to prevent dangerous gas mixtures during regenerative operation, which is discussed in more detail in the following.

During regenerative operation, the first electrode 4 on the first side of the membrane 6, which functions as anode during regular operation, temporarily acts as a cathode, and H2 is produced in the first electrode chamber 2. This leads to a mix of H2 and the 02 in the first electrode chamber 2. Similar considerations applies on second side of the membrane 6 where the second electrode 5, which functions as cathode during regular operation, temporarily acts as an anode, and 02 is produced in the second electrode chamber 3. This leads to a mix of H2 and 02 gas in the second electrode chamber 3. The ratios between the gases added during the regenerative periods should be small in order to avoid risk for explosion. A ratio interval of 4% to 94% between H2 and 02 is known to imply an increased risk for fire and explosion, but conditions are regarded safe outside this range. Accordingly, there should be no more than 4% by volume of hydrogen gas added to the oxygen gas on the anode side of the system and no more than 6% oxygen gas added to the hydrogen gas on the cathode side of the system. In production, each two water molecules split into two hydrogen gas molecules H2 but only one oxygen gas molecule 02. Accordingly, the production rate of hydrogen gas is twice as high as the rate for production of oxygen gas. Accordingly, during a production of a volume of 2- VI of hydrogen gas, H2 in the second electrode chamber 3, there is produced a volume of VI oxygen gas, 02 in the first electrode chamber 2.

The ratio between the hydrogen gas volume V2 produced in the first electrode chamber 2 during regenerative operation and the oxygen gas volume VI produced in the first electrode chamber 2 during regular operation should not exceed 4%. In mathematical terms V2/V1 < 0.04.

This is identical to V2/(2 V1) < 0.02. Thus, relatively to a hydrogen gas production of 2 VI in the second electrode chamber 3 during regular operation the 4% upper safety limit relatively to the produced oxygen on the anode side translates into a 2% upper limit relatively to the hydrogen gas produced on the anode side. This is a relevant number here to mention in case that the switch between regular operation and regenerative operation is regulated by predetermined time durations, which will be explained in more detail below.

During a period of activation by polarity reversal, as illustrated in FIG. 2, and/or during polarity reversal for regeneration, as illustrated in FIG. 3, a volume V2 of hydrogen gas is accumulated in the oxygen tank 12, and a volume of 0.5 V2 of oxygen is accumulated in the hydrogen tank 11, due to each pair of water molecules splitting into two H2 molecules but only one 02 molecule. For risk assessment in the hydrogen tank 11, the volume of 0.5 V2 oxygen gas from the activation and/or regeneration period has to be compared with the production of 2 VI of hydrogen gas from the regular operation, so that the mixture entering the H2 tank 11 contains only 0.5 V2 oxygen gas and 2 VI hydrogen gas, which yields a ratio O2:H2 = (O.5 V2)/(2 V1) which when inserting the expression V2/Vl=0.04 results in a gas ratio O2:H2 of 0.5 0.04/2=1%, which is the gas ratio that is actually obtained in the H2 tank 11 when the safety requirements of V2/Vl<0.04 are observed for the 02 tank 12. Thus, there remain at least 99% H2 and no more than 1% 02 gas in the gas mixture in the H2 tank 11 despite activation and/or regeneration, if the safety range is observed for the 02 tank 12. Accordingly, the addition of oxygen gas to the hydrogen gas on the cathode side is not the most critical aspect due to the activation and/or regeneration. The hydrogen gas addition to the oxygen gas on the anode side during activation and/or regeneration is the most critical aspect, and the H2 tank 11 is implicitly safe if the safety range is observed for the 02 tank 12.

Accordingly, the 2% volume limit for the hydrogen gas production by regenerative operation relatively to the hydrogen gas production during regular operation is a proper limit for safety for both tanks 11, 12. If this limit is observed, no more than 1% of 02 would be accumulated in the H2 tank 11 and no more than 4% of H2 would be accumulated in the 02 tank 12.

As an option, the electrolyser system 1 is producing a fixed hydrogen gas volume of 2- VI and a fixed oxygen gas volume of VI in each of multiple subsequent periods of regular operation, where the regular periods are intermitted by activation and/or regeneration periods between each two of the regular periods. Additionally, the electrolyser system 1 is producing a fixed hydrogen gas volume of V2 and a fixed oxygen gas volume of 0.5 V2 in each period of regenerative operation following each one of these multiple periods of regular operation.

For example, the electrolyser system 1 is producing gas at a constant rate during regular operation and at the same rate during regenerative operation. In this case, the volumeratios translate equivalently into time-ratios, where the time length T2 of the period for regenerative operation should be no longer than 2% of the time length T1 of the period for regular operation in order to produce 2% hydrogen gas in the activation and/or regeneration period relatively to the produced volume 2 VI of hydrogen gas in the period of regular operation and, equivalently, 4% hydrogen gas relatively to the volume VI of produced oxygen gas in the period of regular operation, the latter being the critical limit on the cathode side of the membrane 6, which is on the left side of the membrane 6 in the drawing.

The above model yields a safe activation and/or regeneration scheme without the necessity of valve systems that direct the gases altematingly in dependence on the period being regular or regenerative. This implies a simpler system at lower production costs and lower maintenance costs as well as reduced risk for mechanical failure. As mentioned above, there is a risk for 02 accumulation in the H2 tank 11. The critical criterion for the H2 tank 11 is that no more than 6% 02 is accumulated together with the H2. Using the results from the discussion above, this would imply that the volume of oxygen gas produced during activation and/or regeneration is no more than 6% of the volume of hydrogen gas produced during regular 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 and thus lead to a higher local concentration in the lower part of the tank. Oxygen may be removed therefrom by a valve system. However, by keeping the 2% rule above, not only there is safety for the 02 tank 12 but also for the H2 tank 11.

Having discussed the risk in the H2 tank 11 and the optional 02 tank 12, attention is now given to explosion risk in the electrode chambers 11, 12, which is discussed in the following.

With reference to FIG. 1, it is recalled that

- VI is the volume of oxygen gas 02 produced by the anode 4 during a time interval of T1 of regular production,

- 2 V1 is the volume of hydrogen gas H2 produced by the cathode 5 during the time interval T1 of regular production,

- V2 is the volume of oxygen gas 02 produced during the time interval T2 of activation and/or regeneration where the voltage is reversed, and

- 0.5 V2 the volume of hydrogen gas H2 produced during the time interval during activation and/or regeneration.

These volume figures may also be applied for the maximum amount of gas that may be accumulated in the electrode chambers 2, 3 without causing risk for explosion. During regular operation, a certain volume of oxygen gas is accumulated in the first electrode chamber 2 prior to the oxygen gas leaving the first electrode chamber 2 through the conduit 10, for example into atmosphere or into an oxygen tank 12 for accumulation. During regenerative operation, hydrogen gas is added to the oxygen gas that remains in the first electrode chamber 2, and oxygen gas is added to the hydrogen gas remaining in the second electrode chamber 3. Although, the risk for explosion is slightly lower in the electrode chambers 2, 3, as compared to the tanks 11, 12, it is not negligible. In the following, attention is given to the gas mixtures produced inside the electrode chambers 2, 3.

For the risk assessment in the first electrode chamber 2, the term VO is in the following used for the volume of oxygen gas that can be maximum accumulated in the first electrode chamber 2 during regular operation.

As an example, a production of V1=10 VO of oxygen gas during a period of regular operation would lead to 9 VO of oxygen gas thereof released out of the first electrode chamber and into atmosphere or into the 02 accumulation tank 12. Only the volume of VO of the produced oxygen gas would remain in the first electrode chamber 2 during this period of regular operation. When the polarity is, then, reversed for the activation and/or regeneration period, hydrogen gas is added to oxygen gas remaining in the first electrode chamber 2.

Following the discussion above, the maximum allowable volume V2 of hydrogen in this first electrode chamber 2 is 4% hydrogen gas relative to the volume VO of oxygen gas in the first electrode chamber 2, which now turns into a gas mixture by the addition of hydrogen gas. Accordingly, for high safety in the first electrode chamber 2, it is important to determine the maximum amount VO of oxygen gas that can be accumulated in the first electrode chamber 2 during regular operation, and correspondingly limiting the volume V2 of hydrogen gas produced in the first electrode chamber 2 during regenerative operation with reverse voltage to no more than 4% of VO. This sets an upper limit for the volume V2 of hydrogen gas production during an activation and/or regeneration period, irrespective of the actual oxygen gas volume VI produced during regular operation being much larger than VO. For clarification, reference is made to the above example with the produced volume of oxygen gas being V1=10 VO.

Determining the volume VO depends on the parameters of the overall setup and operational parameters, including dimensions, type and conditions for the electrolyte, and electrical current.

It is pointed out, however, that the activation and/or regeneration time T2 cannot be made infinitely short, as there is a certain time needed for the activation and/or regeneration to be efficient. The activation and/or regeneration time depends on the materials and design parameters of the system. Advantageously, the design and parameters of the system are such as to allow an activation and/or regeneration time of more than 1 second, for example not less than 3 seconds, while at the same time not producing more hydrogen gas during activation and/or regeneration than 4% of the oxygen gas volume VO in the first electrode chamber 2.

For example, the rate of production of gas during regular operation and during operation with reverse polarity is constant if the current is unchanged. In this case, the time duration T1 for regular operation may be set to produce a volume VI of oxygen gas, where VI > VO. The time duration for producing the volume VO is TO. The time T2 for operation with reverse polarity is then set in the interval of Tmin < T2 < 0.02 TO. Thus, the time T2 for reverse polarity has to be no smaller than the minimum time Tmin necessary for activation and/or regeneration of the electrodes but no larger than 2% of the duration of a period for regular operation.

For clarification, as already discussed above, the 2% limit for the time T2 relatively to TO stems from the fact that the production of VO of oxygen gas takes time TO, during which 2 VO of hydrogen gas is produced, and the 4% rule for the regenerative production of hydrogen gas relatively to the regular production of VO of oxygen gas translates into 2% for the regenerative production of hydrogen gas relatively to the regular production of hydrogen gas.

If a higher safety margin is desired, the 2% limit can advantageously be translated into a limit that is lower, for example a 1% limit.

In comparison, the maximum volume of hydrogen gas contained in the second electrode chamber 3 is VH, which can be determined as well, for example experimentally or by calculation. The volume of produced oxygen gas in the same second electrode chamber 3 during the activation and/or regeneration period should not exceed 6% of VH, due to the critical interval of 4% to 94% H2 in a H2+O2 gas mixture. If it takes a duration of TH to produce VH in the second electrode chamber 3 during regular operation, the volume of oxygen produced in this activation and/or regeneration time period of TH is 0.5 V2, as the production rate of 02 is half the production rate of H2. Accordingly, due to the lower production rate of 02, the time duration T2 for the activation and/or regeneration phase should be no more than 12% of TH, namely twice the 6% limit. Taking also into regard the minimum time duration Tmin for proper activation and/or regeneration, this yields Tmin < T2 < 0.12- TH.

The upper limit for this criterion on the time duration T2 for the period with reverse polarity is higher than for the comparative critical limit with respect to H2 in the gas mixture in the first electrode chamber. However, even if oxygen gas is released into atmosphere and not accumulated in an 02 storage tank, so that the above discussion with respect to H2 accumulation in the 02 storage tank 12 is mood, the risk assessment for the gas collection in the first electrode chamber 2 remains. Thus, also in this case, for risk mitigation in the first electrode chamber 2, and, thus, for the overall system 1, the above discussed 2% rule applies as a proper criterion, which is also a safe limit with respect to the second electrode chamber and the H2 tank.

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 lead to a very short allowable time duration T2. For example, this may be the case where the first electrode chamber 2 is particularly small, leading to a small volume VO of oxygen gas. T2 may then be so short that proper activation and/or regeneration is not achieved.

In order to increase the activation and/or regeneration time T2, the opposite directed electrical current applied to the electrodes during periods of reverse polarity, in particular for activation and/or regeneration, may be selected lower than the regular current during regular operation. The current may advantageously be adjusted to a level which ensures both that the production of hydrogen in the first electrode chamber is always less than the critical level, and that the reverse polarity has sufficient duration to achieve the desired regenerative or activation effect.

It will be understood that the system for polarity reversal in FIG. 4 is by example only and that other arrangements for polarity reversal may be provided. For example, the control system 18 with the computer 16 for regulating the distribution of the electrical power 8 may be constructed so as to be able to provide polarity reversal without the need of the contactors 21, 22, 23 and 24.