Method for operating an electrolysis plant, and electrolysis plant

Abstract

The invention relates to a method for operating an electrolysis plant (1, 20) comprising an electrolyzer (11) for generating hydrogen (H2) and oxygen (O2) as product gases. Water is supplied as a reactant and is split into hydrogen (H2) and oxygen (O2) on a proton-conducting membrane (21) made of a fluorine-free polymer (24), said polymer (24) comprising a non-functional polymer material having a functional hydrophilic group, wherein a product gas flow (5) is formed in a phase mixture comprising water (H2O) and a respective product gas, and a product gas flow is fed to a gas separator (3, 13) connected downstream of the electrolyzer (11). The release of an ionic decomposition product of the functional hydrophilic group of the membrane (21) is determined over the operating time, the time curve of the concentration of said decomposition product is determined, and a measurement of the operational degradation of the proton-conducting membrane (21) as a result of a release of the ionic decomposition product of the hydrophilic group is determined. The invention additionally relates to a corresponding electrolysis plant (1, 20) and to a measuring system for carrying out the method.

Description

[0001] Description

[0002] Method for operating an electrolysis plant and electrolysis plant

[0003] The invention relates to a method for operating an electrolysis plant comprising an electrolyzer for producing hydrogen and oxygen as product gases. The invention further relates to an electrolysis plant.

[0004] Hydrogen is currently produced using proton exchange membrane (PEM) electrolysis or alkaline electrolysis, for example. Electrolyzers use electrical energy to produce hydrogen and oxygen from the supplied water.

[0005] An electrolyzer typically comprises a plurality of electrolysis cells arranged adjacent to one another. Water is split into hydrogen and oxygen in the electrolysis cells by means of water electrolysis. In a PEM electrolyzer, for example, deionized water is typically added as a reactant on the anode side and split into hydrogen and oxygen at a proton-permeable membrane (PEM). The water is oxidized to oxygen at the anode. The protons pass through the proton-permeable membrane. Hydrogen is produced on the cathode side. The water is usually pumped from the bottom into the anode and / or cathode compartments.

[0006] In addition to PEM electrolysis, other forms of electrolysis are also known, such as alkaline electrolysis, in which other ions are transported through a membrane.

[0007] For the sake of simplicity, protons will be used as an example in the context of the present invention, for example with the term "proton-permeable" or "proton-conducting". Current research is looking at other types of electrolyzers in which other ions, e.g. hydroxide ions, selectively pass through the membrane and thus enable charge transport for the electrolysis reaction. The general problem on which the invention is based can also be applied to this type of electrolyzer. The protons mentioned here are therefore to be understood as representative. The term "proton-permeable" is therefore to be understood in a much broader sense, namely in the sense of "permeable to a certain type of ion", in particular through a membrane.

[0008] The electrolysis process takes place in the so-called electrolysis stack, composed of several electrolysis cells. Water is introduced as the reactant into the electrolysis stack, which is under DC voltage. After passing through the electrolysis cells, two fluid streams emerge, consisting of water and gas bubbles (oxygen O2 and hydrogen H2). The respective separation of the water and gas phases in the fluid streams takes place in gas separators.

[0009] Hydrogen is an environmentally friendly and sustainable energy source. It has the unique potential to make energy systems, transport, and large parts of the chemical industry climate-friendly and free of carbon dioxide emissions.

[0010] This goal can be achieved if the hydrogen is produced using renewable energy via PEM electrolysis.

[0011] Due to the harsh electrochemical conditions that can always occur during electrolysis, small amounts of H2O2 or OH radicals can form within the electrolyzer, for example in the area of ​​the cells. It is generally known that such species can chemically attack the membrane material of PEM electrolyzers, with fluorides being released as degradation products during such degradation. This is because conventional membranes contain a fluorine component and are made of, for example, PFSA (perfluorosulfic acid). The membrane is a particularly important element for the functionality of the electrolysis cells in PEM electrolysis, so that their service life and the limitations thereof due to degradation effects are given great attention, particularly from an economic point of view.

[0012] An industrially established technology for producing hydrogen is polymer electrolyte membrane water electrolysis (PEMWE), whose name is derived from the electrolyte, the polymer membrane. According to the current state of the art, both the membrane and components of the electrodes are made of perfluorosulfonic acid (PFSA) monomers. However, fluoropolymers are strongly suspected of being hazardous to health because they do not degrade in the environment or in the human body due to their outstanding chemical resistance. In response to these findings, the European Chemicals Agency (ECHA) recently published a proposal for a Europe-wide ban on polyfluoroalkyl substances (PEAS), which include PFSA polymers.The impending ban, but also the announcement by major polymer manufacturers, independently of this, that they will soon stop producing all fluorinated substances, highlights the urgency of developing long-term stable, marketable fluorine-free alternatives in order to enable the production of hydrogen by PEMWE electrolysis with an environmentally friendly alternative in the future.

[0013] As part of the ongoing and continuous development and optimization of PEM electrolyzers, the current challenge is to provide novel membranes that exhibit comparable performance indicators and service lives to PFSA-based membranes while maintaining the required high level of environmental compatibility. Therefore, there is a need, among other things, to develop alternative membrane materials as substitutes for PFSA and ultimately to identify and characterize suitable substitute materials. In addition to numerous other properties, the operational service life of the membrane in the electrolysis cells of an electrolysis plant is increasingly being considered in order to determine the practical suitability and long-term stability of substitute materials during operation.It is therefore important to find a reliable and specific degradation indicator for a membrane made of substitute materials, as well as a measurement method for quantification that is ideally integrated into the operation of the electrolysis plant. Therefore, there is currently an intensive search and research into substitute materials that appear suitable for use in membranes and that preferably do not contain fluorine compounds that can be leached into the process water.

[0014] The invention is therefore based on the object of providing a method by which the service life of a fluorine-free membrane in an electrolysis system can be reliably determined. A further object of the invention is to provide an electrolysis system that enables improved operation in terms of environmental compatibility and reliable membrane service life monitoring.

[0015] The object directed to a method is achieved according to the invention by a method for operating an electrolysis plant comprising an electrolyzer for producing hydrogen and oxygen as product gases, wherein water is supplied as a reactant and is split into hydrogen and oxygen on a proton-conducting membrane made of a fluorine-free polymer, wherein the polymer has a non-functional polymer material with a functional hydrophilic group, wherein a product gas stream is formed in a phase mixture comprising water and a respective product gas, and wherein a product gas stream is fed to a gas separator downstream of the electrolyzer, and in which the release of an ionic degradation product of the functional hydrophilic group of the membrane is determined over the operating time, wherein the time course of its concentration is determined,whereby a measure of the operational degradation of the proton-conducting membrane is determined as a result of the release of the ionic degradation product of the hydrophilic group.

[0016] The invention is based on the realization that determining the release rate of membrane degradation products can, in principle, be a well-suited approach and reliable indicator for monitoring membrane degradation. However, the planned move away from the previously used fluorine-containing polymer creates considerable challenges for suitable detection of polymer membrane degradation, since dilute hydrofluoric acid (HF) or highly selective fluorides as degradation products of a PFSA-based membrane are completely eliminated. In this case, the selectivity and reliability of the method are therefore already based on the precise selection and characterization of a suitable fluorine-free polymer and its functional groups with regard to their suitability as a degradation indicator.

[0017] The invention has surprisingly found that a fluorine-free polymer made from a non-functional fluorine-free polymer material with a functional hydrophilic group is particularly suitable. Hydrocarbons are particularly suitable as a fluorine-free polymer for the membrane material because they are characterized by a fluorine-free chemical structure and have advantageous gas permeation behavior. In addition, they are characterized by high thermal stability and low production costs while at the same time having high ionic conductivity due to the functional group. The mechanism of the selected degradation indicator is therefore based on the targeted and primary consideration of the release of ionic degradation products of the membrane which originate from the hydrophilic group.In this case, the combination of the selected and thus specified fluorine-free polymer and the specifically adapted measuring method has proven to be particularly advantageous and reliable. Difficulties in determining the concentration of the ionic degradation products directly using simple means and with sufficient reliability, particularly in a complex electrolysis plant during operation, can be overcome by the "in-situ" measurement over the operating time proposed here. The integrated release rate of the ionic degradation product is determined over time as an absolute or relative value of the degradation. For this purpose, the temporal progression of the concentration during operation of the electrolysis plant is advantageously determined. The concept of an "in-situ" measurement makes information about the material flows or streams present due to operation available in a simple way for determining the temporal progression of the release rate.Volume flows are available, such as the supplied reactant flow of water, as well as the product flows in the phase mixture of water and product gas. This creates a phase separation in the gas separator, so that the liquid phase, i.e. water, leads to a changing fill level in the gas separator, which can be balanced and evaluated. At the same time, the release of the ionic degradation product, i.e. its concentration in the material flow, is recorded over time using suitable sensors for concentration measurement.

[0018] The invention therefore creates a very simple and robust method for determining the released concentrations of the degradation products, in particular the ions formed from the hydrophilic group in solution, in an electrolysis system with a membrane made of a fluorine-free polymer. This makes it possible to draw conclusions about the degradation of the fluorine-free membrane and its remaining operational service life. This is made possible by the fact that the described "in-situ" method provides additional information that allows, and is specifically used here, to balance the volume flows fed into and leaving the electrolysis system. This concept enables temporal integration of the release rate of the ions and the determination of a relative or absolute degree of degradation of the fluorine-free membrane. This makes it possible to easily and reliably determine the service life of the membrane in an electrolysis system.

[0019] The process is very advantageous compared to previously known solutions, which generally required the use of absolute volume flows that could not be quantified with sufficient precision. Volume flow measurements entail significant financial expenditure for acquisition and maintenance. Furthermore, existing ion exchangers and possibly other components of the electrolysis plant, which reduce the fluoride loading of the aqueous medium and can therefore falsify the result, had to be taken into account. These disadvantages are overcome with the process of the invention. The release rate of the fluorine-free membrane in an electrolysis plant depends on a number of factors, for example the operating management (current density, temperature), possible impurities that promote the conversion of H2O2 into OH radicals, and also the service life.It would not be possible, at a reasonable cost, to quantify the release rate for the selected indicator substance for the entire lifetime of an electrolysis plant in order to use these values ​​to draw conclusions about the lifetime of the electrolysis plant if one wanted to do this using the known methods.

[0020] The "in-situ" method of the invention provides a remedy and closes the need for a practical solution at a reasonable cost by combining a balancing process for a fluorine-free membrane with a concentration measurement of ionic degradation products of the hydrophilic group that have gone into solution over the operating time.

[0021] The present invention makes it possible, in general terms, to determine a release rate for degradation products of membrane material from a fluorine-free membrane and, if present, ionomer for the catalyst bond to the membrane. This release rate can be quantified during operation, and a statement is obtained about the expected service life of the membrane. The present method can, for example, be based on sulfate in solution in process water as a degradation product of sulfonic acid groups, which are frequently used in proton-conducting membranes. However, it is possible to use proton-conducting membranes with other functional hydrophilic groups, such as nitro or carboxy groups, in which case the selectively detected degradation products can be nitrate or bicarbonate. These degradation products can also be quantified for a fluorine-free membrane using the method.

[0022] In a particularly preferred embodiment of the process, the ion concentration of the degradation product is determined by measuring the specific conductivity and / or the pH value of the water in the electrolysis plant.

[0023] The release rate is determined on the basis that the primary and intended degradation product contains a salt of an acid or forms this salt in an aqueous solution, for example dilute sulfates as a degradation product of the sulfonic acid group in the process water of the electrolysis plant. Typically, the concentration of ions introduced into the process water is very low, with a pH value above 4 and often even above 5. In this range, for example, released sulfuric acid is practically completely dissociated. Although the amount of sulfuric acid released is comparatively small, sulfuric acid dissociates into two protons and sulfate anions. Since the aqueous medium is also to be regarded as very dilute, the ionic substances contribute very closely to the specific conductivity of the solution in a linear manner, with the protons of the sulfuric acid contributing by far the largest part to the conductivity.This is due to the so-called Grotthuss mechanism, which causes the specific conductivity of the protons to be exceptionally high. This effect means that impurities in the concentrations typically found in PEM electrolysis systems lead to a significantly smaller increase in the specific conductivity.

[0024] Accordingly, it has been advantageously shown that the method according to the invention is surprisingly robust against impurities, although it uses a relatively simple and rather unspecific measuring method, in particular a conductivity measurement.

[0025] Preferably, the concentration of the ionic degradation product, in particular an ion concentration of the degradation product, in the electrolysis system is therefore determined via the specific conductivity, since a clear correlation was found for the reasons stated above and it is therefore a good measure of the concentration of the anions formed from the hydrophilic group in solution. In this respect, robust and inexpensive conductivity sensors can be used, which is advantageous for implementation in an electrolysis system. Alternatively, pH sensors can also be used. However, since these are more complex to construct and require more frequent maintenance or calibration, conductivity sensors are the preferred design over pH sensors.Another potential disadvantage of pH sensors compared to conductivity sensors is that typical sensor types can release potassium chloride (KCl) during operation, thus contaminating the water from the electrolysis process. In principle, however, a pH measurement, or a combination of a conductivity measurement and a pH measurement, can also be performed within the framework of the method described here.

[0026] In a particularly preferred embodiment, a release rate of the degradation product is determined, whereby a temporal change in the fill level in the gas separator is determined, which is used to quantify the volume flows, from which a measure of the temporally accumulated degradation due to ion release is determined. In this way, the balancing of the material flows via a temporal change in fill levels in the gas separator, which is also proposed and required here in addition to the "in-situ" measurement of the concentration of the fluorine-free ionic degradation products, can be carried out particularly advantageously and easily during operation. Existing components and equipment of an electrolysis plant can be used for this, namely the gas separator. Additional complex equipment is not required.

[0027] Preferably, the fill level in the gas separator is controlled over time between a predetermined maximum fill level and a predetermined minimum fill level, with respective operating phases being set with an increasing fill level and with a decreasing fill level.

[0028] The method proposes the observation of a temporal, i.e., dynamic, change or control of the fill level. The fill level or level of the water in the gas separator is particularly easy to evaluate, and temporal changes can be specifically induced, recorded, and the temporal progression recorded for the measurement process as needed. By monitoring the fill level over time and specifically influencing it, a balancing process is particularly simple, allowing for a reliable measure of membrane degradation along with the recording of the fluoride release rate.

[0029] With further preference, the volume flows of water pumped through the membrane, hereinafter referred to as transfer water, and of water discarded from the electrolysis system are quantified separately. For precise balancing of the water throughput in the electrolysis system, the volume flows are advantageously used. In addition to a preferred balancing of fill levels in the gas separator, the transfer water can also be determined alternatively, for example, via the electrolysis current strength. In PEM electrolysis, the transfer water flows from the anode compartment (oxygen side) through the membrane into the cathode compartment (hydrogen side) and is roughly proportional to the current strength.

[0030] Preferably, when a certain threshold of the specific conductivity is exceeded and / or a threshold of the pH is undershot, a portion of the water in the gas separator is drained and discarded. The targeted discarding of a definable volume of water enables a flushing process in the water-carrying system of the electrolysis plant, particularly in the gas separator, which leads to a cleaning of the water system by the controlled drainage of ion-laden water with a correspondingly high specific conductivity. This phase is advantageously also used as a measurement phase for balancing.

[0031] The drainage of water is preferably interrupted when the minimum filling level is reached, with demineralized water being added during the interruption and the gas separator being filled up until the maximum filling level is reached again.

[0032] In a particularly preferred embodiment of the method, water is alternately drained during the operating phase with a decreasing fill level and refilled during the operating phase with a rising fill level until a predetermined minimum specific conductivity is reached. In this way, a rinsing procedure is carried out iteratively by alternately and controlled draining and refilling of water, while the correspondingly decreasing specific conductivity is observed and recorded over time until a constant, minimum specific conductivity is reached, i.e. a plateau formation or "saturation" is observed with little or no further changes in the specific conductivity in the process water and thus a correlated reduced or lowest ionic concentration of the respective indicator substance formed from the hydrophilic group of the hydrocarbon membrane.In a further preferred embodiment, a temperature measurement is carried out, by means of which a correction of the determined value of the ion concentration is carried out, so that a temperature effect which distorts the determined value is compensated for on the basis of the measurement.

[0033] When calculating the ion concentration of the degradation products from the measured specific conductivity values, a correction or compensation can usually be performed if necessary. This correction term is temperature-dependent, since the ionic conductivity of anions and also of protons in aqueous solution is temperature-dependent, which is why the temperature is also advantageously measured "in situ." By applying a temperature-dependent correction term to the measured conductivity value, a very precise and reliable determination of the conductivity is advantageously achieved, and its temporal progression can be accurately traced and recorded.

[0034] In a particularly preferred embodiment of the method, the polymer material comprises a fluorine-free hydrocarbon polymer having a sulfonic acid group as a functional group, wherein the concentration of sulfate ions is determined as a degradation product.

[0035] This design provides a meaningful indicator of membrane degradation. With comparatively little effort, a specific sulfate release rate can be determined with high temporal resolution for such a fluorine-free membrane. This makes it possible to determine the amount of sulfate released seamlessly over the entire service life of an electrolysis plant and thus to draw conclusions about the service life of a fluorine-free hydrocarbon membrane due to degraded functional groups. The choice of a hydrophilic functional group in the fluorine-free polymer has proven particularly useful here. The fact that this approach works is surprising given that the sulfonic acid groups are typically present in a relatively small proportion within the polymer membrane. The largest part of the membrane mass, typically well over 90%, consists of non-functional fluorine-free hydrocarbon polymers.This selected chemical composition of the membrane represents a significant difference to fluorine-based polymer membranes. Here, the degradation of the non-functional polymer inevitably leads to very high fluoride release, which, due to electroneutrality, results in a stoichiometric release of protons, which in turn can be easily quantified using conductivity. However, this effect can neither be exploited nor easily transferred to fluorine-free hydrocarbon membranes due to the lack of fluorine. In this respect, the selection of a suitable hydrophilic functional group is of particular importance, as is the mechanism of salt formation in the solution as a degradation indicator that this enables.

[0036] In a further preferred embodiment of the method, the polymer material comprises a fluorine-free hydrocarbon polymer with optionally a nitro group or a carboxy group as a hydrophilic functional group, wherein optionally the concentration of nitrate or carbonate is determined as a degradation product.

[0037] It has been shown that nitrates and carbonates are very good indicator salts for degradation when the membrane has a corresponding composition with regard to the hydrophilic functional group. In organic chemistry, the nitro group is the functional NO2 group that is bonded to the organic radical R via the nitrogen atom. The carboxy group, the older but still used name carboxyl group or carboxylic acid group, is in chemistry the functional group -COCH of carboxylic acids. The name is formally derived from the combination of the two elements contained, carbonyl group and hydroxy group. The carboxy group is often represented in structural formulas by a doubly bonded oxygen atom and a singly bonded hydroxy group. Due to the electronegativity difference between oxygen and carbon, the carbon atom carries a positive partial charge.Therefore, the carboxyl group is easily attacked by nucleophiles. The hydroxy unit of the carboxyl group is relatively acidic, and the proton is easily donated to a corresponding partner, resulting in increased acidity compared to alcohols.

[0038] However, it is important that the functional groups of ion-specific membranes are hydrophilic in order to perform their function. Their selectivity ultimately results from the localization of charges. Accordingly, an increased degradation of the functional groups is to be expected compared to the non-functional and less hydrophilic part of the membrane. This leads, on the one hand, to reliable detection of the degradation products based on the charge and, on the other hand, to reliable determination of the lifetime of the fluorine-free membrane, since these functional groups are essential for its function. Furthermore, degradation products of, for example, proton-conducting membranes lead to dissolved species with a negative charge, i.e. anions such as sulfate.As a result, an effect comparable to that achieved with released fluoride from fluorine-containing membranes can be specifically induced: The dissolved anions lead to an increased proton concentration in the solution. This, in turn, enables reliable detection and quantification. This effect is neither limited to sulfate as a degradation product of sulfonic acid groups nor to proton-conducting membranes. Rather, it is advantageously possible to profitably apply the method of the invention, with appropriate adaptations, to ion-selective, fluorine-free membranes.

[0039] The object directed to an electrolysis plant is achieved according to the invention by an electrolysis plant with an electrolyzer for generating hydrogen and oxygen as product gases, comprising a proton-conducting membrane which is formed from a fluorine-free polymer which has a non-functional polymer material with a functional hydrophilic group, and with a gas separator connected downstream of the electrolyzer, and further comprising a measuring device for determining the concentration of a degradation product of the functional hydrophilic group and a fill level control device by means of which the release of the degradation product can be determined over the operating time, wherein the temporal profile of the concentration can be determined, so that a measure of the operational degradation of the proton-conducting membrane as a result of a release of the degradation product can be determined.

[0040] This provides an electrolysis plant with a fluorine-free membrane which enables improved operation in terms of environmental compatibility and reliable lifetime monitoring of the membrane. For this purpose, the membrane is formed from a fluorine-free polymer which comprises a non-functional polymer with a functional, hydrophilic group. The measuring device makes it possible to determine the ion concentration of a degradation product of the hydrophilic group in the process water “in situ” during operation and over an observation period. As a result, the electrolysis plant is particularly designed to carry out the method according to the invention described above, so that a measure of the operational degradation of the fluorine-free proton-conducting membrane as a result of release of the degradation product can be determined.

[0041] The measuring device preferably comprises a conductivity sensor arranged at a location with high pressure during operation of the system, in particular at a geodetically lowest location. This prevents falsification of measurement results when measuring conductivity during operation of the electrolysis system, since degassing of dissolved components in the water is to be expected at lower pressures. Alternatively or additionally, it is also preferable to arrange the conductivity sensors on the pressure side of pumps.

[0042] Furthermore, the measuring device preferably has a pressure sensor and a temperature sensor, so that the gas humidity in the product gas can be determined via saturation calculations.

[0043] Furthermore, the measuring device preferably has a flow sensor so that the volume flows can be determined.

[0044] In a particularly preferred embodiment of the electrolysis plant, the polymer material comprises a fluorine-free hydrocarbon polymer having a sulfonic acid group as a functional group, wherein the measuring device is designed to determine the concentration of sulfate ions as a degradation product.

[0045] In a further preferred embodiment of the electrolysis plant, the polymer material comprises a fluorine-free hydrocarbon polymer with optionally a nitro group or a carboxy group as a functional group, wherein the measuring device is designed to optionally determine the concentration of nitrate or carbonate as a degradation product.

[0046] Depending on the choice of the hydrophilic functional group of the fluorine-free membrane polymer, use in an electrolysis plant is thus possible, whereby the degradation can be determined "in-situ" by detecting the ionic substance of the functional group, which is largely completely dissociated in solution.

[0047] A further aspect of the invention relates to a measuring system for carrying out the method according to the invention, wherein the measuring system has a measuring device and a fill level control device. The measuring system enables quantification of the lifetime consumption of a fluorine-free polymer membrane due to operational degradation. For this purpose, the measuring system is preferably configured to measure the specific conductivity and volume flows. The necessary balancing is then carried out via the temporal change in fill levels in the gas separators by means of the fill level control device in combination with the measuring device.

[0048] Advantages and advantageous embodiments of the method of the invention are to be regarded as advantages and advantageous embodiments of the electrolysis plant and the measuring system and vice versa.

[0049] Further advantages, features, and details of the invention will become apparent from the following description of preferred embodiments with reference to the drawings. The features and combinations of features mentioned above in the description, as well as the features and combinations of features mentioned below in the description of the figures and / or shown alone in the single figures, can be used not only in the respective combination specified, but also in other combinations or on their own, without departing from the scope of the invention.

[0050] Examples of the invention are explained in more detail with reference to the accompanying drawings. These show schematically and in a highly simplified manner:

[0051] FIG 1 shows the operation of an electrolysis cell for water electrolysis, in particular a PEM electrolysis cell, with a catalyst-coated fluorine-free membrane used therein;

[0052] FIG 2 an electrolysis plant with a circuit on the oxygen side;

[0053] FIG. 3 shows a time profile of a control of the fill level within a gas separator; FIG. 4 shows a further example of a time profile of a control of the fill level within a gas separator;

[0054] FIG 5 an electrolysis plant with two circuits, one circuit on the oxygen side and one circuit on the hydrogen side.

[0055] The same reference symbols have the same meaning in the figures.

[0056] In FIG. 1, an electrolysis cell 25, in particular a PEM electrolysis cell for water electrolysis, is shown in the left-hand section. An important functional element of such a polymer electrolyte membrane electrolysis cell 25 is generally formed by a catalyst-coated membrane 23, also referred to as a catalyst-coated membrane or CCM for short. Such a membrane 23 or CCM is shown in more detail in an illustration to the right of the electrolysis cell 25 in FIG. 1.

[0057] The catalyst-coated membrane 23 has a membrane 21 made of a membrane substrate and coated on both sides with a respective catalyst material 22. The membrane 21 or the polymeric membrane substrate forming the membrane 21 is for this purpose usually coated on both the anode side and the cathode side on two opposite, mutually remote surfaces with a layer of a respective catalyst material 22. The respective electrode reaction of the electrolysis takes place in the region of the layer formed by the respective catalyst material 22. During normal operation, electrons are diverted to the contact or bipolar plates 29 via the respective catalyst material 22 and a support or channel structure, which can be formed by or provide an electrically conductive gas diffusion layer 27, compare the electrolysis cell 25 in the illustration on the left.It can also be seen that demineralized reactant water H20 is usually provided on the anode side and fed to the PEM electrolysis cell 25. The reactant water H20 is decomposed into oxygen O2 and hydrogen H2 during the electrolysis process. The electrolysis products formed as product gases are oxygen O2 at the anode and hydrogen H2 at the cathode. These gases are recovered separately and discharged from the PEM electrolysis cell 25.

[0058] The catalyst-coated membrane 23 is also referred to as a membrane electrode assembly (MEA) or catalyst-coated membrane (CCM) and comprises a membrane 21 made of a polymeric membrane material as a substrate, which is coated with a pasty, viscous catalyst paste as a coating material during the production of a coated membrane 23. The catalyst paste contains the catalyst material 22. The membrane 21 is designed as a fluorine-free polymer 24 and contains a sulfonated hydrocarbon polymer, a so-called hydrocarbon, as the ionomer for proton conduction, or is formed therefrom. Materials selected as the fluorine-free polymer 24 for the membrane 21, for example, are from the sulfonated polyaromatics. Thus, as illustrated in FIG. 1, so-called sPEEK or sPPX can be used for the membrane 21, or the membrane substrate can be formed therefrom.The largest group is the poly(arylene ether) based hydrocarbons, which include sulfonated polyetheretherketones (sPEEK). Another group is sulfonated polyphenylenes (sPPX), which have similar properties. Their major advantage is their high stability against chemical degradation. Ionomers are polymers with ionic groups, which give them the characteristic property of proton conductivity. In the case of PEM water electrolysis, the ionic groups are sulfonic acid groups (-SO3H). These can be randomly distributed or fixedly arranged. In PEM water electrolysis, ionomers are used primarily because of their proton conductivity and simultaneous electrical insulation capacity as membrane material in the membrane 21 and in the electrode applied to the membrane substrate, i.e.inserted into the layer comprising the catalyst material 22, so that here a material adaptation is carried out in particular for the fluorine-free catalyst-coated membrane 23 (MEA or CCM).

[0059] FIG. 2 shows an electrolysis system 1 for the electrolysis of water. The electrolysis system 1 has only one circuit on the oxygen side. This is a simple exemplary embodiment of an electrolysis system 1 for carrying out the invention. The electrolysis system 1 has an electrolyzer 11 and electrolysis cell stack 2 with a large number of electrolysis cells (not shown in detail) stacked in the axial direction. An anodic half-cell and the cathodic half-cell of an electrolysis cell are separated by a membrane 21 (see FIG. 1), not shown in detail here. The membrane material comprises or consists of a fluorine-free polymer 24. This has a non-functional polymer material with a functional hydrophilic group. According to the electrolysis cell 25 shown and described in more detail in FIG. 1, for example, a membrane 21 with a fluorine-free membrane substrate is used.This contains, or is formed from, a sulfonated hydrocarbon polymer, a so-called hydrocarbon, as an ionomer for proton conduction. Materials selected from the sulfonated polyaromatics with a fluorine-free hydrocarbon polymer and a sulfonic acid group as the hydrophilic functional group are used as the fluorine-free polymer 24 for the membrane substrate. However, it is also possible to use a fluorine-free hydrocarbon polymer with either a nitro group or a carboxyl group as the functional group. As a result, a nitrate or a bicarbonate is introduced into the aqueous solution of the process water as a result of the degradation-induced release or dissociation of molecules of the hydrophilic functional group from the membrane polymer. In the case of the sulfonic acid group, sulfate is released into the process water.This simple circuit supplies the electrolysis cell stack 2 with water for the electrolysis reaction, whereby the water also serves to cool the cells. The oxygen produced as the product gas from the electrolysis is fed into the gas separator 3 for oxygen in a phase mixture. In the gas separator 3, phase separation takes place and the gaseous oxygen is separated from the liquid water and removed from the circuit via the oxygen discharge line 6. The circulation pump 4 is provided in the circuit to maintain the water circulation. Used water is compensated for by adding fully demineralized water (DE water) via the supply line 7. Although the water is fully demineralized, any minor impurities could build up in the circuit.To counteract this effect, a solenoid valve 12a is temporarily opened and part of the water is discarded from the circuit via the drain line 8.

[0060] In the exemplary embodiment shown in FIG. 2, there is no circuit on the hydrogen side of the electrolysis plant 1. The hydrogen produced is simply discharged via the discharge line 10 for the product gas hydrogen and is available for further use, for example for compression. A pressure control valve is usually arranged in the discharge line 10 for the product gas hydrogen, although this is not shown in more detail in the exemplary embodiments. This serves to discharge the hydrogen when a certain excess pressure is reached, which is very desirable in most applications for the further processing of the hydrogen. Since liquid water usually accumulates on the hydrogen side during PEM electrolysis, a condensate line 9 is also provided, which opens when a certain amount of water has accumulated in order to discharge this water. This can be implemented, for example, using a float.FIG. 2 indicates that this water is discarded. However, it is also possible for this water to be reused for electrolysis by being returned to the oxygen-side circuit. This return to the process is generally an economically viable approach for large electrolysis plants.

[0061] Furthermore, a conductivity sensor 5a and a conductivity sensor 5b for measuring the specific conductivity are installed in the electrolysis plant 1. These conductivity sensors 5a, 5b are used to draw conclusions about the ion concentrations of the degradation products of the hydrophilic functional group in the water, in this case the concentration of sulfate anions in the process water, via a specific correlation. It is particularly advantageous to install the conductivity sensors 5a, 5b at locations where the pressure in the system is as high as possible, since degassing of dissolved hydrogen or oxygen is particularly low or unlikely here. Gas bubbles would disrupt the precise measurement of conductivity and falsify the result.Therefore, the conductivity sensors are positioned at a geodetically low point of the electrolysis plant 1 in order to specifically exploit hydrostatic pressure advantages and thereby effectively counteract degassing.

[0062] The specific conductivities of the material flows, in particular of the water, are used via the conductivity sensors 5a, 5b to determine the sulfate release of the membrane 21 over the operating time. The temporal progression of the sulfate concentration is determined, with the specific conductivity being determined as a measure of the operational degradation of the proton-permeable membrane 21 as a result of a release of sulfate. The determination of the release rate is therefore based on the fact that the primary and intended degradation product comprises a salt of an acid or forms this salt in aqueous solution, i.e. in the present exemplary embodiment diluted sulfates as a degradation product of the sulfonic acid group, which are introduced into the process water of the electrolysis plant. In addition to the specific conductivities of the material flows, volume flows are quantified and balanced to determine the sulfate release rate.In principle, one could install a volume flow sensor on each outgoing water stream to measure these volume flows. However, this would be very disadvantageous, as it would involve considerable effort in an electrolysis system 1, particularly with regard to the cost of the flow sensors and the calibration effort, while also being relatively error-prone and inaccurate.

[0063] The invention follows a different approach and proposes a very advantageous method in which a temporal change in filling levels within the gas separator 3 is used for the most accurate quantification of the volume flows.

[0064] This is illustrated by way of example in FIG. 3, which shows a schematic representation of the time course of the filling level within the gas separator 3.

[0065] With the help of a level control within the gas separator 3 it is ensured that if the threshold value or lower level L is undershot min demineralized water is refilled via the supply line 7. This is done by opening the solenoid valve 12b and / or starting a feed pump (not shown in detail). A corresponding increase in the fill level in the gas separator 3 is carried out during the increasing phases a. If a defined maximum fill level L maxreached, the refilling is stopped. The fill level decreases in the phases b, which decrease over time, primarily because, on the one hand, water is consumed in the electrolysis reaction by splitting into the product gases hydrogen and oxygen and, on the other hand, water is also transported through the membrane 21, the so-called transfer water, which is transferred from the anodic half-cell to the cathodic half-cell through the membrane 21. The method is advantageously an in-situ method which is carried out during the regular operation of the electrolysis plant 1. Here, the fill level in the gas separator 3 is maintained between the predetermined maximum fill level L max and the predetermined minimum fill level L mincontrolled in time, with respective operating phases being set with an increasing fill level a and a decreasing fill level b. Several cycles with operating phases with an increasing fill level a and a decreasing fill level b can be carried out alternately one after the other, whereby generally, but not necessarily, linear transients of the time course of the fill level are passed through.

[0066] To counteract the accumulation of contaminants in the circuit, a certain portion of the water is removed from the circuit. This removal is preferably achieved by a continuous volume flow through the discharge line 8.

[0067] The drop in the fill level in phases b shown in FIG. 3 would then be caused not only by the transfer water and the electrolysis reaction, but also by the discarded water. This would be somewhat disadvantageous for precise measurement and balancing of the volume flows in-situ, since in this case the volume flows through discharge line 8 and through membrane 21 could not be determined separately.

[0068] FIG 4 shows a further example of a time course of a control of the fill level in the gas separator 3, with an improved method and measuring concept compared to FIG 3 with regard to an in-situ determination under precise consideration of the volume flows of discarded water through the discharge line 8 and the transfer water through the membrane 21 of the electrolysis plant 1.

[0069] In FIG. 4, the control concept, which is improved compared to FIG. 3, and the precise balancing for the fill level within the gas separator 3 are explained with reference to FIG. 2 merely as an example, with which the volume flows of the transfer water and the discarded water through the discharge line 9 can be quantified separately.

[0070] The underlying feature of the fill level control in the gas separator 3 shown in FIG 4 is that in phases a and b no water is led out of the circuit via the drain line 9 and drained from the gas separator 3. The solenoid valve 12a therefore closes the drain line 9. The particular advantage of this process is that the falling fill level in the gas separator 3 in phase b can now be attributed solely to the following three contributions: Firstly, the water consumed during the electrolysis reaction through electrochemical splitting. Further, the moisture led out of the process with the product gases and finally the transfer water which passed through the membrane 21 during the electrolysis.

[0071] Since the first two components can be precisely calculated, it is also possible to accurately determine how much water has passed through the membrane 21 as transfer water in a given time. An accumulation of impurities in the circuit is counteracted by temporarily discarding water via discharge line 8, which occurs during phases c in FIG. 4.

[0072] The method is carried out in such a way that the discarding of water via the discharge line 8 is started when a certain threshold value of the specific conductivity of the dissolved sulphate ions is exceeded and when a minimum level L is reached minis briefly interrupted, during which time water is refilled via supply line 7, which corresponds to phases d in FIG 3. If a defined minimum specific conductivity is finally achieved by the alternating rinsing or draining and refilling in phases c and d, the discarding is stopped and the electrolysis operation continues normally without discarding water.

[0073] A major advantage is that the method illustrated in FIG. 4 can be relatively easily transferred and applied to more complex water electrolysis systems. The method and the measuring system are thus largely independent of the specific storage type and can therefore be flexibly adapted.

[0074] For example, FIG 5 shows an electrolysis plant 20 with two circuits and a water recirculation with a water treatment unit 16. The electrolysis plant 20 shown in FIG 5 has two circuits, this time on both the oxygen side and the hydrogen side. The gas separator 3 on the oxygen side is essentially constructed in the same way as the gas separator 3 in FIG 2. It is also operated according to the control principle shown in FIG 4. This more complex example is intended to illustrate that the invention can also be applied to electrolysis plants 20 with two circuits and is easily transferable, the electrolysis plant 20 having a fluorine-free polymer membrane 21.

[0075] It is also possible for water conditioners 16, in particular ion exchangers, to be used in the electrolysis system 20. In this case, it is preferred that the specific conductivities are determined before and after the water conditioner 16. In FIG. 5, however, no additional conductivity sensor is provided directly before the water conditioner 16. In the system concept of the electrolysis system 20 with two circuits, it can be assumed that the specific conductivity in the circuit almost corresponds to the specific conductivity at the inlet of the water conditioner 16 due to a relatively high pumping rate via the circulation pumps 4, 14 and the transfer pump 15. The measuring point after the water conditioner 16 can therefore be omitted without having to fear any disadvantages in the quality of the measurement.In addition to balancing volume flows by level measurements, the present invention is based on measurements of the specific conductivity with conductivity sensors 5a, b, 5c in order to draw conclusions about the existing sulfate concentrations in situ and to determine the degree of degradation of the fluorine-free membrane 21. In principle, pH sensors are also suitable, although these are comparatively expensive and involve greater calibration effort. Therefore, the electrolysis systems shown which are equipped with conductivity sensors 5a, 5b, 5c are particularly advantageous. These can advantageously be placed particularly easily at locations with higher pressures or temperatures. It is particularly advantageous to place the conductivity sensors 5a, 5b, 5c within the circuits.The alternative of providing them at outlets, such as discharge line 8, would mean, in the discontinuous operation described above, that the specific conductivity in the circuit could not be measured in-situ at all times and that additional measurement inaccuracies would have to be feared due to a less constant temperature. Furthermore, it is particularly advantageous to place the conductivity sensors 5a, 5b, 5c at those points in the electrolysis plant 1, 20 where there is a relatively high pressure, since the tendency towards degassing is lower at these points. Gas bubbles would interfere with the measurement. These points are placed, as already shown by way of example in FIGS. 2 and 5, on the pressure sides of the pumps 4, 14 or on the pressure side of a condensate drain, or generally at a relatively low geothermal level due to the hydrostatic pressure.

[0076] The present invention makes it possible to determine the release rate of ionic degradation products of a hydrophilic functional group with high temporal resolution without great additional effort. Only then is it possible to determine the quantity of released ions seamlessly over the entire service life of an electrolysis system 1, 20 with a membrane 21 based on a fluorine-free polymer 24. Only then is it possible to reliably determine the achieved or still achievable service life of a fluorine-free membrane 21 based on a hydrocarbon (hydrocarbon membrane) due to degraded sulfate, nitrate or bicarbonate or released into solution due to degradation, and to accurately predict remaining service lives and available operating times.The determination of the sulfate release rate according to the invention is based on the fact that, surprisingly, during degradation of hydrocarbon membranes, a sufficient amount of dilute sulfuric acid is released from the membrane 21 for reliable detection. This acid is completely dissociated in the process water at the typically prevailing pH values ​​of greater than 4.

[0077] Using this methodology and the measuring system, for example, service measures can be planned very precisely and future requirements for replacement cells can be anticipated, which is very beneficial for the long-term, economical operation of an electrolysis plant 1, 20. According to current standards, a degradation of typically around 10% of the material of the hydrophilic group originally incorporated into the membrane 21 is defined as the technical and economic end of life. It is therefore important to know the ion release rate for the preferred indicator salts in an ion-selective, fluorine-free membrane 21 and to determine its integral over the operating time of the electrolysis plant 1, 20.

Claims

Patent claims 1. A method for operating an electrolysis plant (1, 20) comprising an electrolyzer (11) for generating hydrogen (H2) and oxygen (O2) as product gases, wherein water is supplied as a reactant and is split into hydrogen (H2) and oxygen (O2) on a proton-conducting membrane (21) made of a fluorine-free polymer (24), wherein the polymer (24) comprises a non-functional polymer material with a functional hydrophilic group, wherein a product gas stream (5) is formed in a phase mixture comprising water (H2O) and a respective product gas, and wherein a product gas stream is fed to a gas separator (3, 13) downstream of the electrolyzer (11), and in which the release of an ionic degradation product of the functional hydrophilic group of the membrane (21) is determined over the operating time, wherein the temporal course of its concentration is determined,wherein a measure of the operational degradation of the proton-conducting membrane (21) as a result of a release of the ionic degradation product of the hydrophilic group is determined., 2. The method according to claim 1, wherein the ion concentration of the degradation product is determined by measuring the specific conductivity and / or the pH value of the water in the electrolysis plant.

3. Method according to one of claims 1 or 2, in which a release rate of the degradation product is determined, wherein a temporal change in the fill level in the gas separator (3, 13) is determined, which is used to quantify the volume flows, from which a measure of the temporally accumulated degradation due to the ion release is determined.

4. Method according to claim 3, wherein the filling level in the gas separator (3, 13) is between a predetermined maximum filling level (L max ) and a predetermined minimum filling stand (L min) is controlled in time, whereby respective operating phases are set with an increasing filling level (a, c) and with a decreasing filling level (b, d).

5. Method according to one of the preceding claims, in which the volume flows of the transfer water conveyed through the membrane (21) and of water discarded from the electrolysis plant (1, 20) are quantified separately.

6. Method according to one of the preceding claims, in which, when a certain threshold value of the specific conductivity is exceeded and / or a certain threshold value of the pH is undershot, a part of the water in the gas separator (3, 13) is drained off and discarded.

7. Method according to claim 6, wherein when the minimum fill level (L min) the drainage of water is interrupted, whereby during the interruption fully demineralized water is supplied and the gas separator (3, 13) is refilled until the maximum filling level (L max ) is reached again.

8. The method according to claim 6, wherein alternately draining water in the operating phase with decreasing fill level (b, d) and refilling water in the operating phase with increasing fill level (a, c) is carried out until a predetermined minimum specific conductivity is reached.

9. Method according to one of the preceding claims, in which a temperature measurement is carried out by means of which a correction of the determined value of the ton concentration is carried out, so that a temperature effect which distorts the determined value is compensated for on the basis of the measurement.

10. A method according to any one of the preceding claims, wherein the polymer material comprises a fluorine-free hydrocarbon polymer having a sulfonic acid group as a functional group, wherein the concentration of sulfate ions as a degradation product is determined.

11. A method according to any one of claims 1 to 9, wherein the polymer material comprises a fluorine-free hydrocarbon polymer having optionally a nitro group or a carboxy group as a hydrophilic functional group, wherein optionally the concentration of nitrate or carbonate as a degradation product is determined.

12. Electrolysis plant (1, 20) with an electrolyzer (11) for producing hydrogen (H2) and oxygen (O2) as product gases, comprising a proton-conducting membrane (21) formed from a fluorine-free polymer (24) having a non-functional polymer material with a functional hydrophilic group, and with a gas separator (3, 13) connected downstream of the electrolyzer (11), and further comprising a measuring device for determining the concentration of a degradation product of the functional hydrophilic group and a fill level control device by means of which the release of the degradation product can be determined over the operating time, wherein the temporal course of the concentration can be determined, so that a measure of the operational degradation of the proton-conducting membrane (21) can be determined as a result of the release of the degradation product.

13. Electrolysis plant (1, 20) according to claim 10, wherein the measuring device comprises a conductivity sensor (5a, 5b, 5c) which is arranged at a point of high pressure during operation of the plant, in particular at a geodetically lowest possible point and / or on the pressure side of pumps.

14. Electrolysis plant (1, 20) according to claim 10 or 11, wherein the measuring device comprises a pressure sensor and a temperature sensor, so that the gas humidity in the product gas can be determined via saturation calculations.

15. Electrolysis plant (1, 20) according to one of claims 10 to 12, wherein the measuring device has a flow sensor so that the volume flows can be determined.

16. Electrolysis plant (1, 20) according to one of claims 10 to 13, wherein the polymer material comprises a fluorine-free hydrocarbon polymer having a sulfonic acid group as a functional group, wherein the measuring device is arranged to determine the concentration of sulfate ions as a degradation product.

17. Electrolysis plant (1, 20) according to one of claims 10 to 13, wherein the polymer material comprises a fluorine-free hydrocarbon polymer having optionally a nitro group or a carboxy group as a functional group, wherein the measuring device is arranged to optionally determine the concentration of nitrate or carbonate as a degradation product.

18. Measuring system configured to carry out the method according to one of claims 1 to 11, comprising a measuring device and a fill level control device.

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