EP3757253A1 - Electrolysis system and method for storing electric energy using the electrolysis system - Google Patents

Abstract

The invention relates to an electrolysis system and a method for storing electrical energy by means of the electrolysis system. An electrolysis system with at least one electrolysis cell is first provided, the electrolysis cell having a cathode space, a cathode being arranged in the cathode space and the cathode being designed as a gas diffusion electrode. The electrolytic cell comprises a supply line and a discharge line for each electrolyte. The electrolysis system also includes at least one first conductivity measuring device mounted in one of the downcomers. A first conductivity or a quantity dependent on the conductivity is measured with the first conductivity measuring device. A gas breakthrough point through the gas diffusion electrode is then determined by evaluating the first conductivity or a variable dependent on the conductivity in relation to a reference value in an evaluation device.

Description

The invention relates to an electrolysis system and a method for storing electrical energy by means of the electrolysis system.

The demand for electricity fluctuates strongly over the course of the day. Electricity generation also fluctuates over the course of the day as the share of electricity from renewable sources increases. In order to be able to compensate for an oversupply of electricity in times with a lot of sun and strong wind when there is low demand for electricity, controllable power plants or storage facilities are required to store this energy.

One of the solutions currently being considered is the conversion of electrical energy into products of value that can serve in particular as platform chemicals, in particular ethene, methane, ethane, or synthesis gas, which includes carbon monoxide and hydrogen. Electrolysis is a possible technique for converting electrical energy into valuable products.

The electrolysis of water to hydrogen and oxygen is a method known in the art. However, the electrolysis of carbon dioxide to form valuable substances, in particular to carbon monoxide, has been researched for several years and efforts are being made to develop an electrochemical system that can produce a corresponding amount of carbon dioxide of economic interest. Currently, around 80% of the world's energy needs are covered by burning fossil fuels, the combustion processes of which cause global emissions of around 34,000 million tons of carbon dioxide into the atmosphere every year. Carbon dioxide is one of the so-called greenhouse gases, their negative effects on the atmosphere and the climate will be discussed. Utilization of this carbon dioxide is therefore desirable.

One possible cell design of a carbon dioxide electrolyser comprises an anode compartment and a cathode compartment. A gas diffusion electrode is arranged as a cathode in the cathode space. A gas diffusion electrode is a porous structure that separates a gas phase, which typically comprises the starting material carbon dioxide, and a liquid phase from one another. The liquid phase typically consists of an aqueous salt solution, also called an electrolyte. In order to operate this type of carbon dioxide electrolyzer efficiently, a defined differential pressure should be set across the gas diffusion electrode. This differential pressure should be chosen so that the pores of the gas diffusion electrode are essentially filled with the gas phase. This operating point of the gas diffusion electrode is close to the breakthrough point, English "bubble point". This breakthrough point denotes the operating point at which the gas begins to be pressed through the porous structure of the gas diffusion electrode into the electrolyte space. Where the gas displaces the conductive electrolyte in this way, the conductivity averaged over the electrolyte gap decreases, so that a higher operating voltage is required for the same current: The efficiency of the electrolysis process decreases. Disadvantageously, the gas diffusion electrode and other cell components can be damaged, since in particular the current escapes into bubble-free electrolyte areas and higher current densities then occur there, which disadvantageously accelerate the aging process. Exceeding the break-through point should therefore be avoided.

It is advantageous to operate the carbon dioxide electrolyzer as close as possible to the breakthrough point. Exceeding the break-through point must be avoided. The optimum differential pressure across the gas diffusion electrode depends on a number of factors. By temporal effects, in particular by swelling of the gas diffusion electrode, the breakthrough point will change over time. The optimum differential pressure across the gas diffusion electrode therefore also changes disadvantageously. In addition, the differential pressure across the gas diffusion electrode is influenced by hydrostatic and dynamic effects, which means that locally different differential pressures can exist on the active surface of a gas diffusion electrode. The differential pressure between the gas and the electrolyte space, which is measured at a certain point in the cell or in the corresponding supply lines, is therefore disadvantageously only very imprecisely recognizing the breakout point.

It is therefore the object of the present invention to provide a carbon dioxide electrolyzer and a method for operating a carbon dioxide electrolyzer which achieves reliable detection of the through-hole point and thus enables the carbon dioxide electrolyzer to be implemented as efficiently as possible at the breakthrough point.

The object is achieved with an electrolysis system according to claim 1 and a method for operating an electrolysis system according to claim 10.

The electrolysis system according to the invention for carbon dioxide electrolysis comprises at least one electrolysis cell, one electrolysis cell including a cathode compartment. A cathode is arranged in the cathode compartment. The cathode is designed as a gas diffusion electrode. The electrolytic cell comprises at least a first supply line and a first discharge line for guiding a catholyte. The electrolysis system comprises at least one first conductivity measuring device, which is arranged in the first derivative. The conductivity measuring device is suitable for measuring the conductivity or a quantity dependent on the conductivity and for generating a measurement signal. The electrolysis system further comprises an evaluation device for determining a breakdown point through the gas diffusion electrode based on the first measurement signal of the first conductivity measuring device in relation to a reference value.

The method according to the invention for operating an electrolysis system for carbon dioxide electrolysis initially comprises the provision of an electrolysis system. The electrolysis system comprises at least one electrolysis cell. An electrolysis cell includes a cathode compartment. A cathode is arranged in the cathode compartment. The cathode is designed as a gas diffusion electrode. The cathode comprises at least a first supply line and a first discharge line for guiding a catholyte. The electrolysis system comprises at least one first conductivity measuring device, which is arranged in the first derivation, for measuring the conductivity or a quantity dependent on the conductivity and for generating a measurement signal. The electrolysis system also includes an evaluation device for determining a gas breakthrough point through the gas diffusion electrode based on the measurement signal of the first conductivity measurement device in relation to a reference value. The conductivity or a proportional variable dependent on the conductivity is measured by means of the first conductivity measuring device and a first measurement signal is generated. A gas breakthrough point through the gas diffusion electrode is determined based on the measurement signal of the conductivity measuring device in relation to a reference value in.

It has been shown that the conductivity in the first derivative of the electrolysis cell, that is to say the catholyte discharge, changes depending on the operating point, in particular the breakdown point of the gas diffusion electrode. If the breakthrough point, or "bubble point", is exceeded, gas bubbles enter the electrolyte. The electrolyte with the gas bubbles leaves the carbon dioxide electrolyser and flows through the conductivity measuring device. Due to the gas bubbles in the electrolyte, the conductivity of the electrolyte is reduced. It is thus advantageously possible to use the breakdown point of the gas diffusion electrode can be reliably determined on the basis of a drop in conductivity.

In an advantageous embodiment and development of the invention, a conductivity or a voltage drop is used as the measurement signal. The breakdown point via the gas diffusion electrode can advantageously be reliably determined both via a direct conductivity measurement in the electrolyte, in particular in the catholyte, and via a quantity dependent on the conductivity, namely a voltage drop in the electrolyte.

In a further advantageous embodiment and development of the invention, the reference value is measured with the first conductivity measuring device before the first measurement signal. Advantageously, only one measurement setup in an electrolysis cell is required to determine the breakout point. In the evaluation device, the measurement signals are stored, in particular on a data carrier, and the break-through point is evaluated on the basis of several time-shifted measurement signals. The earlier measurement is expediently carried out before a breakthrough through the gas diffusion electrode, that is to say before a significant amount of gas is present in the discharge line in the electrolyte. It is particularly useful to measure the conductivity or a variable that is dependent on the conductivity, in particular a voltage drop, at the first conductivity measuring device as soon as the electrolysis cell is started up. If the conductivity becomes lower, a breakdown point through the gas diffusion electrode can be deduced.

The evaluation and thus determination of the breakthrough point can in particular be based on the following relationships:
The electrolyte volume flow that is introduced into an electrolysis cell essentially free of gas bubbles, in particular the catholyte input flow, is J _L1 . A liquid electrolyte volume flow, which leaves the electrolysis cell, in particular the cathode compartment, is J _L2 if the gas diffusion electrode is operated below the bubble point. The conductivity of the liquid catholyte in the supply line or main supply line is σ _L1 . The conductivity of the liquid electrolyte in the discharge or main discharge is σ _L2 . The liquid catholyte input stream and the liquid catholyte output stream are essentially the same. There are only minor deviations due to a slight change in the electrolyte composition due to the cell reaction and a slight increase in temperature. Therefore, the following assumptions can be made: $$J_{\mathrm{L.}2}\approx J_{\mathrm{L.}1}{\mathrm{and}\sigma }_{\mathrm{L.}2}\approx {\sigma }_{\mathrm{L.}1}$$

The catholyte discharge from the electrolytic cell and the main discharge from the electrolyte distributor contain only a few bubbles at an operating point below the gas breakdown point through the gas diffusion electrode. The few bubbles result from the release of small amounts of reaction on the catholyte side of the gas diffusion electrode. The gas volume flow that is carried along in the catholyte in the form of a two-phase flow is J _G0 . Equation 2 approximately describes the conductivity σ ₂ in a volume element after the electrolytic cell, taking this gas component into account. Here, σ ₂ corresponds to the conductivity before a gas breakthrough through the gas diffusion electrode. Based on the assumption from equation 1, the total conductivity σ _{2 can} also be approximately described by means of the conductivity of the electrolyte upstream of the electrolytic cell σ _L1 and the electrolyte volume _flow J _L1 . $${\sigma }_2=\frac{{\sigma }_{\mathrm{L.}2}{J}_{\mathrm{L.}2}}{J_{\mathrm{L.}2}+J_{G0}}\approx \frac{{\sigma }_{\mathrm{L.}1}{J}_{\mathrm{L.}1}}{J_{\mathrm{L.}1}+J_{G0}}$$

When the breakthrough point is reached, a gas flow J _{GE, which} increases rapidly with the differential pressure across the gas diffusion electrode, occurs from the reactant gas space through the gas diffusion electrode into the catholyte and leads there accordingly to a strong increase in the gas volume flow towards (J _G0 + J _GE ). The conductivity σ ₂ 'corresponds to the conductivity after a gas breakthrough through the gas diffusion electrode. The conductivity decreases according to equation 3. $${\mathrm{<figure-callout\; id="2"\; label="\sigma "\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\sigma </figure-callout>}}_2^{ʹ}=\frac{{\sigma }_{\mathrm{L.}2}{J}_{\mathrm{L.}2}}{J_{\mathrm{L.}2}+J_{G0}+J_{\mathit{GE}}}\approx \frac{{\sigma }_{\mathrm{L.}1}{J}_{\mathrm{L.}1}}{J_{\mathrm{L.}1}+J_{G0}+J_{\mathit{GE}}}$$

Since J _GE >> J _G0 occurs quickly in the event of a gas breakthrough, the following approximation applies: $$\frac{{\mathrm{<figure-callout\; id="2"\; label="\sigma "\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\sigma </figure-callout>}}_2^{ʹ}}{{\sigma }_1}=\frac{J_{\mathrm{L.}1}}{J_{\mathrm{L.}1}+J_{\mathit{GE}}}$$

The gas breakthrough through the gas diffusion electrode can be measured by reducing the conductivity or by reducing a voltage drop, measured in a first measuring resistor in the first derivative, i.e. by indirectly measuring the conductivity in the catholyte discharge by the factor specified in equation 5. This value of the conductivity σ ₂ 'can be recorded several times in succession. A drop in the value indicates a gas breakthrough through the gas diffusion electrode. $${\mathrm{<figure-callout\; id="2"\; label="\sigma "\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\sigma </figure-callout>}}_2^{ʹ}=\left(\frac{J_{\mathrm{L.}1}+J_{G0}}{J_{\mathrm{L.}1}+{\mathrm{<figure-callout\; id="2"\; label="J\; GE\; \sigma "\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">J</figure-callout>}}_{\mathit{GE}}},\right){\sigma }_{}{}_2$$

In this case, only one measuring device is advantageously used which records measured values several times in succession and outputs a measuring signal.

In a further advantageous embodiment and development of the invention, the reference value is measured by means of a second conductivity measuring device which is arranged in a feed line of the electrolytic cell. In other words, the electrolysis system now comprises two conductivity measuring devices. The first conductivity measuring device measures the conductivity or one that is dependent on the conductivity Size after the electrolysis cell in a first discharge, in particular in a catholyte discharge. The second conductivity measuring device measures the conductivity or a quantity dependent on the conductivity upstream of the electrolytic cell, that is to say in a feed line to the electrolytic cell. The measured value of the second conductivity measuring device then represents the reference value for an evaluation of the breakthrough point.

The conductivity in the catholyte discharge can advantageously be described with the approximation described in equation 4. With this approximation, a reference value can be determined without an earlier measurement. A conductivity or a voltage drop at the electrolyte feed is measured as a reference value. This allows conclusions to be drawn about the gas volume flow J _GE in the discharge of the electrolysis cell and thus the breakdown point can be determined. The operating point of the gas diffusion electrode can thus advantageously be determined in real time, since no previous measurements are necessary.

In a further advantageous embodiment and development of the invention, the electrolysis system comprises a control device for controlling a differential pressure across the gas diffusion electrode as a function of the measurement signal from the conductivity measurement device. In other words, this means that in the event that the evaluation shows that a gas breakthrough point has been exceeded, the differential pressure across the gas diffusion electrode is reduced by means of the control device. The control system thus advantageously enables the electrolysis cell to be operated as close as possible to the gas breakthrough point and, in the event of a gas breakthrough, to reduce the pressure difference such that the operating point of the gas diffusion electrode remains below the gas breakthrough point.

In a further advantageous embodiment and development of the invention, the first conductivity measuring device has conductivity sensors or resistance measuring sections the first derivation and / or in the first supply line. Alternatively or additionally, the electrolysis system has a second supply line into the electrolysis cell and a second discharge line from the electrolysis cell for carrying an anolyte. A second conductivity measuring device then also has conductivity sensors or resistance measuring sections in the second supply line and / or the second discharge line. These are advantageously robust measuring units that require little maintenance and measure reliably.

In a further advantageous embodiment and development of the invention, the first conductivity measuring device comprises a first control electrode for receiving stray currents and a first measuring resistor for measuring a first voltage drop. The at least one first control electrode is attached in the first derivative and the first measuring resistor is electrically connected to the first control electrode. In other words, the first measuring resistor is fitted in the electrical line through which the stray current leaves the catholyte discharge or the anolyte discharge. The first measuring resistor can advantageously be used to measure a voltage drop which is dependent on the conductivity.

Control electrodes are arranged in electrolysis systems, in particular with at least two electrolysis cells, which are connected to one another as a stack, for receiving stray currents. The individual electrolysis cells are connected to supply lines and discharge lines for the supply of electrolyte. In order to make the pipe connections efficient, the individual feed lines are in turn connected in parallel to a common feed line, a main feed line. The leads are connected in parallel to a common lead, a main lead. The first and / or second conductivity measuring device can also be arranged in this main discharge line or main supply line. In other words, derivation and supply are each a generic term that also includes the Terms used here include main derivation or main supply line.

By adding a first measuring resistor to these control electrodes already present in an electrolysis system, these control electrodes can also be used to analyze the gas breakdown point in addition to recording stray currents. It is advantageously not necessary to introduce additional sensors in the supply line and / or discharge line of the electrolysis system.

In a further advantageous embodiment and development of the invention, the electrolysis system comprises at least two control electrodes, the second conductivity measuring device having a second control electrode which is arranged in the feed line of the electrolysis cell. The second control electrode is advantageously electrically connected to a second measuring resistor for measuring a second voltage drop across it. The second control electrode is arranged in the feed line to the electrolytic cell, that is to say in front of the electrolytic cell in the direction of flow. A second voltage drop is measured at the second measuring resistor. This second voltage drop is used as a reference value. Since the second measuring resistor is arranged upstream of the electrolytic cell in the direction of flow, it can be assumed that this voltage drop corresponds to the voltage drop at which there is essentially no gas comprising carbon dioxide in the electrolyte. The operating point of the gas diffusion electrode can thus advantageously be recorded in real time. If the breakthrough point is exceeded, a quick adjustment of the pressure difference across the gas diffusion electrode is thus advantageously possible.

In a further advantageous embodiment and development of the invention, the first and the second control electrode are connected to the same potential via the first and second measuring resistor. In other words, that means between the control electrode and the potential point of the measuring resistor is arranged.

In a further advantageous embodiment and development of the invention, the first and the second control electrode are grounded via the respective first measuring resistor or the second measuring resistor. In other words, that means that the measuring resistor is arranged between the control electrode and the grounding point. This structure advantageously prevents corrosion of parts of the electrolysis system that are in contact with the electrolyte, since the electrodes ground the electrolyte.

In a further advantageous embodiment and development of the invention, the cathode of the electrolysis cell is electrically connected to a negative pole of a voltage source and the negative pole is grounded. All control electrodes thus act cathodically. This has the advantage that a large number of materials can be used for the control electrode. In particular, reduction-stable metals, in particular silver, can be used.

In a further advantageous embodiment and development of the invention, the electrolysis cell comprises an anode compartment and the cathode compartment. The anode compartment is separated from the cathode compartment by a membrane. Both a diaphragm and a classic membrane are referred to here as a membrane. The job of this membrane is to separate gases and conduct ions to apply an electric current. Different products, in particular gases, are advantageously produced separately from one another in the anode compartment and the cathode compartment. These can then also leave the electrolyser separately, which advantageously makes separating the products superfluous. Since separation processes have a high energy requirement, energy is also advantageously saved. That makes the electrolyzer energy efficient.

In a further advantageous embodiment and development of the invention, the first and / or second control electrode is arranged along the electrolyte line at least 10 cm, particularly preferably at least 50 cm, from the electrolytic cell. This has the advantage that major electrical losses due to high grounding currents are avoided. In particular, the proportion of the electrical resistance of the electrolyte becomes greater in relation to the total measured resistance, the total resistance being essentially composed of the resistance of the pipeline, the measuring resistor and the electrical resistance. As a result, the change in the voltage drop at the breakdown point increases the further away the measuring point is from the electrolytic cell. In other words, the measurement quality is advantageously better if the control electrode is at least 10 cm, particularly preferably at least 50 cm, from the electrolytic cell.

In a further advantageous embodiment and development of the invention, the conductivity measuring electrodes are designed as pipe sections in the feed line and / or discharge line of the electrolytic cell. The pipe sections have in particular an inlet or outlet diameter. In particular, the length of the pipe sections corresponds at most to the length of a few pipe diameters of the pipe in which they are arranged. A wall thickness of the pipe sections can be 0.1 mm to a few mm. With these wall thicknesses, mechanical stability and service life are advantageously guaranteed even in the event of a weak corrosive attack.

In a further advantageous embodiment and development of the invention, the evaluation takes place in such a way that the measured conductivity values are detected over time. The values can be stored in a data storage device and evaluated in a data evaluation device. In particular, a limit value can be set from which Voltage drop a gas breakdown point (bubble point) has been reached.

In a further advantageous embodiment and development of the invention, the conductivity is evaluated with the reference value, that is, an earlier conductivity or a conductivity measured in the feed line of the electrolysis cell, using a ratio of conductivity and the reference value. It is possible to specify a fixed ratio of the two variables to one another as a limit value. It is also possible to dynamically adapt the limit value for the ratio value to the electrolysis cell and, in particular, to also consider aging effects.

In a further advantageous embodiment and development of the invention, the electrolysis system includes an electrolyte conditioning system. The electrolyte conditioning system provides electrolytes for carbon dioxide electrolysis. Furthermore, after the carbon dioxide electrolysis, the electrolytes can be fed back into the electrolyte conditioning system, where they are regenerated. In particular, this means that they are cleaned and cooled and / or a pH value is adjusted.

• Figur 1 ein Elektrolysesystem mit zwei Elektrolysezellen und Leitfähigkeitsmessvorrichtungen;
• Figur 2 ein Elektrolysesystem mit zwei Elektrolysezellen, Steuerelektroden und Messwiderständen;
• Figur 3 ein elektrisches Ersatzschaltbild eines Ausschnitts des Elektrolysesystems;
• Figur 4 einen ersten Verlauf eines Spannungsabfalls in dem ersten Messwiderstand über der Zeit und einen Differenzdruck über der Zeit;
• Figur 5 einen zweiten Verlauf eines Spannungsabfalls in dem ersten Messwiderstand über der Zeit und einen Differenzdruck über der Zeit.

Further features, properties and advantages of the present invention emerge from the following description with reference to the accompanying figures. They show schematically:

• Figure 1 an electrolysis system with two electrolysis cells and conductivity measuring devices;
• Figure 2 an electrolysis system with two electrolysis cells, control electrodes and measuring resistors;
• Figure 3 an electrical equivalent circuit diagram of a section of the electrolysis system;
• Figure 4 a first profile of a voltage drop in the first measuring resistor over time and a differential pressure over time;
• Figure 5 a second profile of a voltage drop in the first measuring resistor over time and a differential pressure over time.

Figure 1 shows an electrolysis system 1 with two electrolysis cells 2. Typically, significantly more than two electrolysis cells 2, in particular 50 to 100 electrolysis cells 2, are arranged in an electrolysis stack and thus in an electrolysis system 1. This is done in Figure 1 indicated by dots. For the sake of clarity, however, only two electrolysis cells 2 are shown in the figure.

An electrolytic cell 2 comprises a separating membrane 14 which divides the electrolytic cell 2 into an anode compartment 3 and a cathode compartment. This separating membrane 14 can also be designed as a diaphragm. A cathode 6, in this example a gas diffusion electrode, is arranged in the cathode compartment. The cathode 6 divides the cathode compartment into a first cathode compartment 4 and a second cathode compartment 5. An anode 7 is arranged in the anode compartment 3. Appropriately, both the cathode 6 and the anode 7 are electrically connected.

A gas comprising essentially carbon dioxide 10 is fed into the second cathode compartment 5. This is converted to valuable substances, in particular to carbon monoxide, at the cathode 6. The gas comprising carbon monoxide 11 leaves the second cathode compartment 5. A liquid catholyte 12 is fed into the first cathode compartment 4. The catholyte 12 is fed into the electrolytic cells 2 in parallel from a main feed line 50 of an electrolyte distributor. From this main feed line 50, feed lines 51 lead into the first cathode compartment 4 of the respective electrolysis cell 2. A liquid anolyte 13 is fed into the anode compartment 3.

This is also fed in parallel into the individual electrolysis cells 2 via a main feed line 50. In turn, a feed line leads from the main feed line 50 to the individual electrolysis cells 2.

The catholyte in turn leaves the electrolysis cell 2 via a discharge line 52. The discharge line 52 leads into a main discharge line 53. The discharge lines 52 from the first cathode compartment 4 of the electrolysis cells 2 are connected in parallel to this main discharge line 53. The anolyte also leaves the anode compartment 3 via a discharge line 52 and is led to a main discharge line 53. The leads 52 are in turn connected in parallel to this main lead 53.

In order to spatially concentrate the interfaces with the environment, all electrolyte feed and discharge lines leave the stack advantageously on one side. This is in Figure 1 not visible for reasons of clarity. Usually control electrodes made of silver are used on a negative potential. They are then inserted into the electrolyte supply lines and leads on the negative stack side. The polarity of the control electrodes can generally be selected independently of the polarity of the respective stack end. It is also possible to place the electrolyte-side stack connection in the middle of the stack. Then the interfaces of the electrolysis system to the environment are distributed over the stack surface.

In this example, a first conductivity measuring device 15 is arranged in each of the main discharge lines 53. Second conductivity measuring devices 16 are arranged in each of the main supply lines 50. The first and second conductivity measuring devices 15, 16 are connected to an evaluation device 60 via data transmission lines 61.

The first and the second conductivity measuring device 15, 16 measure the conductivity in the electrolyte and generate a measurement signal. The measurement signal is transmitted to an evaluation device 60 via the data transmission line 61. The Evaluation device evaluates the ratio of the conductivity measured at the first conductivity measuring device 15 in the catholyte after the electrolytic cell 2 to the conductivity measured at the second conductivity measuring device 16 in the catholyte upstream of the electrolytic cell. As soon as the ratio becomes smaller due to the reduced conductivity of the gas bubbles of a gas breakthrough through the gas diffusion electrode 6, the evaluation device outputs the information that a breakthrough point of the gas diffusion electrode 6 has been exceeded.

Figure 2 shows a second example of an electrolysis system with two electrolysis cells and control electrodes with measuring resistors. The structure of the electrolysis cells corresponds to the structure of the electrolysis cells of the first example from FIG Figure 1 .

In this example, control electrodes are arranged as first and second conductivity measuring devices in the main leads 50 and the main leads 53. These control electrodes are intended to keep corrosive stray currents in the aqueous electrolyte, that is to say anolyte 13 and catholyte 12, away from the surroundings of the electrolyte stack. The cells are typically connected electrically in series to form a stack, in particular a bipolar stack. The stray currents that occur are diverted from the electrolyte lines via the control electrodes and thus do not reach the system parts connected to the electrolysis cell via the electrolytes.

In this example, the control electrodes are designed as metallic pipe sections. These are only electronically connected to the negative end of the stack via the measuring resistors. The electrolytes can be conducted through these metallic pipe sections. The metallic pipe sections in particular have the circumference of the pipeline in which they are attached. They are connected to plastic pipes with the help of screw connections. The control electrodes are advantageous such an easy to manufacture geometry. Furthermore, there is an electronic connection to the negative end of the stack or the cathode only via the measuring resistors.

In this exemplary embodiment, the electrolysis system 1 comprises four control electrodes. A first control electrode 20 is arranged in the main discharge line 53 of the catholyte 12. A second control electrode 21 is arranged in the main supply line 50 of the catholyte 12. A third control electrode 22 is arranged in the main discharge line 53 of the anolyte 13. A fourth control electrode 23 is arranged in the main supply line 50 of the anolyte 13.

The control electrodes are set to a defined potential, in this example the potential of the negative end of the stack, which also forms the grounding point. A measuring resistor 24, 25, 26, 27, English shunt, is arranged between the control electrodes 20, 21, 22, 23 and the grounding point 31. Each of these measuring resistors is used to measure a voltage drop, which in turn can be used to determine the stray currents flowing off via the measuring resistor (shunt).

The diameter and length of the pipelines are to be selected depending on the resistances of the pipeline R _R , the control electrode R _M and the measuring resistor R _M. The resistance of the pipelines R _R = L / (σA) L: pipe length, A: cross-sectional area) between the connection on the stack with the potential U _HV and the control electrode must be so high that the current I _SE = U flowing through it to the control electrode _HV / (R _E + R _M + R _R ) generates a voltage U on this control electrode and the measuring resistor that should not significantly exceed one volt: 1V> U = I _SE (R _E + R _M ). In this way, earthed, electrically conductive system parts connected to the electrolyte side are protected from the corrosive effect of the stray current that could not be discharged via the control electrode. This means that R _E and R _M must be negligible compared to R _R.

Typical main distribution voltages are, depending on the design of the cell stack, in the range from 10 V to 100 V, from which R _R / (R _E + R _M )> 10 follows. Ground currents in the range from 1 A to 10 A can be tolerated. A measuring resistor R _M of 10 mQ is sufficient to generate easily measurable signals> 10 mV for the conductivity measurement, which on the other hand are so small that they do not negatively affect the quality of the grounding of the electrolyte. Technically sensible pipe lengths are therefore in a range from 0.5 m to 3 m. Larger electrical losses (> 0.1% of the nominal load) due to Joule heat release and gas development on the control electrode due to the earth currents are then excluded.

In this example, the control electrodes are not arranged in the immediate vicinity of the electrolysis cells 2, but at a distance of at least 10 cm, particularly preferably from 50 cm to 2 m, from the electrolysis cell 2. The line filled with electrolyte acts as an electrical resistor, which prevents excessive stray current.

In this example, the voltage source is connected to the control electrodes in such a way that the negative pole of the voltage source is grounded. All control electrodes 20 to 23 thus act cathodically. A larger selection of material is advantageously available for the control electrodes. Reduction-stable metals, in particular silver in this example, are particularly preferably used.

Figure 3 shows an electrical equivalent circuit diagram of a section of the electrolysis system 1 based on a derivative 52 of an anolyte. The electrolysis stack 33 comprises several electrolysis cells 2 with a cell voltage U _Z. These are electrically connected in series. The electrolysis stack 33 is grounded on the negative side. This avoids an expensive bipolar power supply. The electrical equivalent circuit diagram of the anolyte supply line looks accordingly, that of the two catholyte leading lines 51, 52 differs from this in that the line resistance R _S at the positive The end is omitted and instead is added at the negative end.

The connection between the main discharge line 53 of the electrolyte stack 3 and the electrolyte conditioning system 17 has the stack connection resistance R _P. The current resulting from the stray currents would flow via this resistor to the first electronically conductive and grounded surface of the electrolyte conditioning system 17, which is in contact with the electrolyte in the lines. In order to prevent corrosion at this point, a grounded control electrode 22 is attached before entry into the electrolyte conditioning system 17, which protects the following components.

The control electrode 22 is connected to a third measuring resistor 26. A voltage drop 102 is measured here.

Figure 4 shows the course of the voltage drop over time at the first control electrode 20 measured with the first measuring resistor 24. Time 100 is plotted on the x-axis. The differential pressure 101 over the gas diffusion electrode is plotted on the left y-axis. The voltage drop 102, which was measured at the first measuring resistor 24, is plotted on the right y-axis. The differential pressure is increased over time. The voltage drop across the first measuring resistor of 0.1 Ohm remains almost constant at the beginning. From the differential pressure at which gas bubbles break through the gas diffusion electrode, the voltage drop begins to decrease. On the basis of the first voltage drop 105, from which the voltage drop is no longer constant, a first differential pressure 106 can be read off the pressure drop curve, at which a breakdown point occurs across the gas diffusion electrode in the electrolysis system 1. The falling voltage drop across the first measuring resistor 24 indicates a lower leakage current. This lower stray current is based on an increased proportion of gas in the catholyte. Parameters such as the current strength of the electrolysis and volume flow rates of the media are kept constant in this example. In this example, the voltage drop which is present before the increase in the differential pressure across the gas diffusion electrode at the beginning is used as a reference value 104. A first voltage drop 105 is recognized when the voltage drop is below the reference value. Alternatively, it is possible to use the voltage drop that is measured across the second measuring resistor 25 as a reference value. As soon as the value of the voltage drop across the first measuring resistor 24 deviates from the reference voltage drop, a gas breakdown through the gas diffusion electrode can be concluded. To determine a pressure difference at which the breakthrough takes place, the first voltage drop 105 can be used to infer the first pressure difference 106. The evaluation can in particular take place in an evaluation device with the aid of a computer.

Figure 5 FIG. 10 shows a second example of a voltage drop which was measured across the first measuring resistor 24. Time 100 is again plotted on the x-axis. The differential pressure 101 on the left y-axis and the voltage drop 102 on the right y-axis. In this example, the gas breakthrough point was reached more quickly. Again, however, it can be clearly seen that the initially constant voltage drop 104 drops after a certain time. From this first voltage drop 105, it is then possible in turn to infer the presence of a gas breakdown through the gas diffusion electrode. On the basis of the pressure difference curve, as in Figure 4 shown, the first differential pressure of the breakthrough point 106 can be inferred.

List of reference symbols

1

Electrolysis system

2

Electrolytic cell

3

Anode compartment

4th

first cathode compartment

5

second cathode compartment

6th

cathode

7th

anode

10

carbon dioxide

11

Gas comprising carbon monoxide

12th

Catholyte

13

Anolyte

14th

Separating membrane

15th

first conductivity measuring device

16

second conductivity measuring device

17th

Electrolyte conditioning plant

20th

first control electrode

21st

second control electrode

22nd

third control electrode

23

fourth control electrode

24

first measuring resistor

25th

second measuring resistor

26th

third measuring resistor

27

fourth measuring resistor

30th

DC power source

31

Grounding

33

Stack

40

electrical line

50

Main line

51

Supply line

52

Derivation

53

Main derivative

60

Evaluation device

61

Data transmission line

100

time

101

Differential pressure

102

Voltage drop

104

Reference value

105

first voltage drop

106

first differential pressure

RS

Line resistance

ISN

Stray current

RP

Stack connection resistance

IP

Stack connection current

AROUND

Voltage at the third measuring resistor

UZ

Cell voltage

Claims (15)

Electrolysis system (1) for carbon dioxide electrolysis with: - At least one electrolysis cell, wherein an electrolysis cell (2) comprises a cathode compartment, a cathode (6) being arranged in the cathode compartment and the cathode (6) being designed as a gas diffusion electrode, the electrolysis cell (2) having at least one first supply line (51) and a first drain (52) for guiding a catholyte, - At least one first conductivity measuring device, which is arranged in the first derivative (52), for measuring a conductivity or a quantity dependent on the conductivity and for generating a measurement signal, - An evaluation device (60) for determining a gas breakthrough point through the gas diffusion electrode based on the measurement signal of the first conductivity measuring device in relation to a reference value (104). Electrolysis system (1) according to Claim 1, having a regulating device for regulating a differential pressure across the gas diffusion electrode as a function of the measurement signal from the first conductivity measuring device. Electrolysis system (1) according to one of the preceding claims, wherein the first conductivity measuring device has conductivity sensors or resistance measuring sections in the first supply line and / or in the first discharge line and / or the electrolysis system (1) has a second supply line and a second discharge line for carrying an anolyte and a second conductivity measuring device has conductivity sensors or resistance measuring sections in the second supply line and / or in the second discharge line. Electrolysis system (1) according to one of the preceding claims, wherein the first conductivity measuring device has a first control electrode for receiving stray currents and a comprises first measuring resistor for measuring a first voltage drop, wherein the at least one first control electrode (20) is mounted in the first derivative (52) and wherein the first measuring resistor (24) is electrically connected to the first control electrode (20). Electrolysis system (1) according to claim 4 with at least two control electrodes, wherein the second conductivity measuring device comprises a second control electrode (21) which is attached in the first supply line (51) of the electrolysis cell (2) and is electrically connected to at least one second measuring resistor (25) is. Electrolysis system (1) according to claim 5, wherein the first control electrode (20) via the first measuring resistor (24) and the second control electrode (21) via the second measuring resistor (25) are connected to the same potential, in particular earth potential. Electrolysis system (1) according to one of the preceding claims, wherein the cathode (6) of the electrolysis cell (2) is electrically connected to a negative pole of a voltage source (30) and the negative pole is grounded. Electrolysis system (1) according to one of Claims 4 to 7, the at least one control electrode (20) being arranged along the catholyte discharge line at least 10 cm, in particular at least 50 cm, away from the electrolysis cell (2). Electrolysis system (1) according to one of the preceding claims, wherein the first and / or second conductivity measuring device at least partially as pipe sections in the first and / or second supply line (51) and / or the first and / or second discharge line (52) of the electrolysis cell (2 ) are trained. Method for operating an electrolysis system (1) for carbon dioxide electrolysis with the following steps: - Provision of an electrolysis system (1) with at least one electrolysis cell, wherein an electrolysis cell (2) comprises a cathode space, a cathode (6) being arranged in the cathode space and the cathode (6) being designed as a gas diffusion electrode, the electrolysis cell (2) comprises at least one first supply line (51) and one first discharge line (52) for guiding a catholyte, and with at least one first conductivity measuring device, which is arranged in the first discharge line (52), for measuring the conductivity or a quantity dependent on the conductivity and for generating a measurement signal and with an evaluation device for determining a gas breakthrough point through the gas diffusion electrode based on the measurement signal of the first conductivity measuring device in relation to a reference value (104), - measuring the conductivity or a quantity dependent on the conductivity by means of the first conductivity measuring device and generating a first measurement signal, - Determination of a gas breakthrough point through the gas diffusion electrode by evaluating the first measurement signal in relation to a reference value in the evaluation device. Method according to claim 10, wherein a conductivity or a voltage drop is used as the measurement signal. Method according to one of Claims 10 or 11, the reference value being measured with the first conductivity measuring device before the first measurement signal. Method according to one of Claims 10 to 12, the reference value being measured by means of a second conductivity measuring device which is arranged in a feed line of the electrolysis cell. Method according to one of Claims 12 or 13, the first and / or the second conductivity measuring device having a first control electrode electrically connected to a measuring resistor and a voltage drop across the measuring resistor is measured as a measuring signal. Method according to Claim 11 or 14, the evaluation being carried out in such a way that a ratio of the first measurement signal and the reference value (104) is formed.

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