Electrolysis cell system and method for operating an electrolysis cell system

Abstract

Method for operating an electrolysis cell system with at least one electrolysis cell in which a product gas containing hydrogen (H2) is produced from a feed gas containing water (H2O) by means of electrolysis using electrical energy, comprising the following steps: providing an electrolysis cell system with a feed gas inlet and a product gas outlet; providing a measuring device for recording a measured value (UN,out) representing the partial pressure of oxygen in the product gas; feeding an electrolysis current into the electrolysis cell system; determining an actual feed conversion value (FCist); determining a feed conversion control deviation (FCdelta) between the actual feed conversion value (FCist) and a predefinable target feed conversion value (FCsoll); generating a control signal (S) depending on the feed conversion control deviation (FCdelta);Setting one or more process parameters of the electrolysis cell system depending on the control signal (S).;

Description

[0001] The invention relates to a method for monitoring a conversion rate of an electrolysis cell system in which a product gas comprising hydrogen (H2) is produced from a feed gas comprising water (H2O) by means of electrolysis using electrical energy.

[0002] The method according to the invention is particularly suitable for use in solid oxide electrolyzer cell (SOEC) systems and in reversible solid oxide cell (rSOC) systems in electrolysis operation.

[0003] A solid oxide electrolyzer (SOEC) comprises at least one electrolysis cell, which uses electrical energy to split water (H₂O) into its components, hydrogen and oxygen. The design and function of a solid oxide electrolyzer are similar to those of a solid oxide fuel cell (SOFC), as they are based on the same technology. A key difference is that electrolysis uses water vapor as the feed gas, while fuel cells use oxygen and fuel gas (e.g., hydrogen) as their feed gases.

[0004] A solid oxide electrolyzer consists of several components, each fulfilling different functions. These components generally include an anode support layer, an anode, an electrolyte layer, a cathode, and a cathode support layer. The anode support layer forms the base of the cell and provides mechanical support. It is often made of a porous material such as nickel oxide (NiO). The anode often consists of a mixture of nickel and electrolyte materials. Water, usually in the form of water vapor, is supplied to the anode and diffuses through it. At the anode, the electrooxidation of the water vapor takes place, generating oxygen ions and electrons. The electrolyte layer consists of a solid electrolyte material, such as yttrium-stabilized zirconium dioxide (YSZ). This layer facilitates the transport of oxygen ions (O₂⁻) from the anode to the cathode.The cathode consists of cathode materials such as lanthanum-manganese-cobaltite (LaMnO₃) or lanthanum-ferrite (LaFeO₃). At the cathode, oxygen ions are extracted from the electrolyte and combined with electrons to produce oxygen. The cathode support layer provides structure and stability for the cell. It is structured similarly to the anode support layer.

[0005] A solid oxide electrolyzer utilizes high-temperature operation (typically 700–1000°C) to activate the electrolyte and facilitate ion flow. The water splitting process proceeds as follows: Water vapor (H₂O(g)) is directed to the anode. There, the water vapor decomposes into oxygen ions (O₂⁻) and protons (H⁺). The oxygen ions diffuse through the electrolyte layer to the cathode. The electrolyte layer allows the transport of oxygen ions (O₂⁻) from the anode to the cathode while blocking the flow of electrons. This is achieved by an electrical potential difference between the anode and cathode. At the cathode, the oxygen ions (O₂⁻) combine with electrons supplied by the external circuitry to produce oxygen (O₂).

[0006] Solid oxide electrolysis cells are efficient due to their high operating temperatures and can produce clean hydrogen. They are used in hydrogen production, energy storage, and other industrial processes.

[0007] To operate water electrolysis in a solid oxide electrolysis cell with maximum efficiency, process parameters must be monitored and adjusted. Of particular importance is the so-called "feed conversion rate" (FC) (also known as "current-to-reactant ratio" or "CR") of a cell unit (stack or stack module). Feed conversion refers to the ratio between the electrical current input and the chemical reactants (starting materials) used in the electrolysis cell to produce products such as hydrogen and oxygen. Feed conversion values ​​typically range between 0.1 and 0.9. The conversion rate is a measure of the electrochemical conversion of the gas components contributing to electrolysis (so-called oxygen donors) in the feed gas within the stack relative to the amount of oxygen donors available at the inlet.It can therefore also be interpreted as a relative change in the oxygen partial pressure between the feed gas inlet and the product gas outlet of an electrolysis cell or a stack of fluidically communicating electrolysis cells (stack modules).

[0008] It has been shown that exceeding a certain feed conversion value, for example 0.9, can lead to soot formation in individual cells of the electrolysis system. Furthermore, exceeding a feed conversion value of 0.9 poses a risk of feed gas depletion in the electrolysis cell or stack. Soot formation and feed gas depletion can each lead to total system failure, or failure of individual cells or a cell stack, during subsequent operation. Therefore, feed conversion serves as a control variable, among other things, for regulating electrical current and throughput.

[0009] To prevent soot formation or depletion of the electrolysis cells in the cell system, feed conversion is monitored during the electrolysis process. Monitoring feed conversion using conventional methods requires a significant amount of sensory equipment. Both the quantity and composition of the feedstock supplied to the electrolysis cells must be precisely known. Furthermore, the temperature and electrolysis current for the electrolysis cell or stack must be measured accurately. This necessitates a complex measurement system and the evaluation of numerous, inherently error-prone measurements, which can lead to measurement inaccuracies of + / - 10% in determining the feed conversion. Regular calibration of the measuring instruments is also required to maintain these inaccuracies over time.

[0010] The invention is based on the objective of providing a method and a system of the type mentioned above that allows improved monitoring of feed conversion.

[0011] The problem is solved by a method according to claim 1. Advantageous embodiments are specified in the dependent claims.

[0012] A fundamental aspect of the present invention is to model a differential voltage on the electrolysis current at a defined, low frequency, for example in the range of 1 Hz to 0.1 Hz. Due to the relationship between the feed conversion (FC) and the current amplitude shown, this leads to a fluctuation in the conversion rate at approximately the same capacitance.

[0013] Normally, the O₂ content in the product gas is too low to be detected by a lambda sensor. By modulating the electrolysis current, signals with a similar frequency can be searched for in the lambda sensor signals. This evaluation is preferably performed using Furier analysis. Signals with a frequency component corresponding to the modulation frequency of the electrolysis current can then be analyzed and provide an indication of the oxygen content in the product gas and thus of the risk of soot formation. Since the relationship between the Nernst voltage output by a lambda sensor and the feed conversion is non-linear, the sensitivity of the lambda sensor is significantly greater than the sensitivity of the applied current.

[0014] Another fundamental aspect of the present invention is the monitoring of feed conversion based on the acquisition of only two measured variables. Firstly, the electrolysis current must be measured in order to control it with high accuracy. Secondly, the oxygen content of the produced product gas must be determined. This can be done, for example, using a lambda probe, which measures the Nernst voltage.

[0015] Using the Nernst voltage, the oxygen partial pressure, representing the oxygen content, can be determined in the product gas supplied at the outlet of the electrolysis cell. However, the feed conversion cannot be calculated from the oxygen partial pressure alone, as the oxygen partial pressure in the feed gas supplied to the electrolysis cell and the absolute amount of gas supplied to the electrolysis cell or cell stack (module / stack) are unknown. This problem can be circumvented by slightly modulating the electrolysis current, by a few percent of the nominal current, e.g., D I / I = 1,5 %. The feed conversion is directly proportional to the electrolysis current, so the feed conversion varies by the same amount, e.g., + / - 1.5%. The following relationship is therefore valid. D FC / FC = Δ I / I .However, the Nernst voltage is non-linearly dependent on the feed conversion. This relationship can be derived from the Nernst equation, which includes the gas concentrations that depend on the feed conversion.

[0016] This effect can be exploited. For example, with a relatively low feed conversion, a small change in feed conversion causes only a small change in the Nernst voltage in the product gas, whereas with a high feed conversion, the Nernst voltage reacts much more strongly to changes in feed conversion.

[0017] The precise measurement of the Nernst voltage using a lambda probe is a known technique. If the voltage becomes too noisy due to a fluctuating gas supply, the electrolysis current can oscillate at a defined, low frequency (e.g., in the range of 1 Hz to 0.1 Hz). The measured Nernst voltage can then be analyzed at this frequency using a Fourier filter to suppress the noise.

[0018] By selecting suitable current amplitudes and limits for the voltage response, exceeding a critical limit can be prevented.

[0019] A suitable lambda sensor could either be purchased and integrated into the exhaust system, or a custom-designed lambda sensor integrated into the stack could be used. This is because each SOC repeater effectively acts as a lambda sensor. A cell can be inserted as a zero level below the stack, which is not in the current path but only measures the open-circuit voltage of the exhaust gas. A sketch of this is included below. This would further reduce sensor costs and allow for individual monitoring of each stack tower to ensure reliable Co-SOEC operation even at higher FC (fluid flow) levels. However, this requires a homogeneous and well-defined feed gas distribution across the tower, as well as a slight overpressure on the O2 side to prevent potential leaks from leading to a loss of H2O and CO2. Both of these requirements are incorporated into the pSyTower.

[0020] The operating method according to the invention offers, among other things, the following advantages: Compared to the state of the art, significantly fewer measuring devices are needed for monitoring feed conversion and controlling the electrolysis process. This results in cost savings. Measuring devices for current and voltage require less maintenance than those used for gas and flow measurements. The achievable measurement accuracy is higher than with the state of the art and less prone to errors, allowing the electrolysis cell system to operate at higher feed conversion rates with a reduced risk of soot buildup or cell depletion. This increases system efficiency and lowers operating costs.

[0021] The following describes exemplary embodiments of the invention with reference to figures, which are intended to illustrate the invention and are not to be considered limiting. They are therefore possible embodiments or variants.

[0022] They show: Fig. 1 a schematic representation of the operating principle of an electrolysis cell system according to the invention, Fig. 2 a schematic representation of an electrolysis cell module, Fig. 3 a diagram showing the assignment of Nernst voltage values ​​to feed conversion values, and Fig. 4 a diagram with an example calculation in which a typical feed gas composition is varied within foreseeable limits.

[0023] Figure 1Figure 1 illustrates the basic operating principle of an electrolysis cell system according to the invention. The electrolysis cell system shown in principle comprises at least one electrolysis cell 10, wherein the electrolysis cell 10 can be a solid oxide electrolyzer cell (SOEC) or a reversible solid oxide cell (rSOC) that can be operated in an electrolysis mode. Preferably, the electrolysis cell system comprises at least one so-called stack with several electrolysis cells 10 arranged in a stack-like manner. With regard to the operating principle, the electrolysis cell 10 can also be considered a stack module with several individual cells. The electrolysis cell 10 comprises an oxygen electrode (anode) 11 and a hydrogen electrode (cathode) 12.The electrolysis cell 10 is supplied with, for example, air as a purge medium 13 on the anode side and a feed gas 15 on the cathode side. Through an electrochemical reaction using electrical energy, a hydrogen (H₂)-containing product gas is generated as product gas 16, while oxygen-enriched purge medium 14 is released on the anode side. The product gas may contain other components, such as carbon monoxide (CO). The feed gas 15 contains water (H₂O) as water vapor. Furthermore, the feed gas 15 may contain carbon dioxide, natural gas, or other hydrocarbons; in particular, the feed gas 15 may contain CH₄, CO₂, CO, and / or H₂ in addition to water vapor.

[0024] An additional amount of hydrogen to compensate for cell degradation or to cover short-term increased demand can be generated by supplying hydrocarbons (CmHn)18 to the solid oxide cell system. The hydrocarbons, together with a feed gas19 comprising water vapor or a mixture comprising water vapor and carbon dioxide, which is provided, for example, in a high-temperature co-electrolysis unit, can be pre-reformed (partially converted) or fully reformed in an external reformer17 (optional, shown with a dashed line). For this purpose, heat QR from the electrolysis cell10 can be coupled into the reformer17.

[0025] Alternatively or additionally to the use of an external reformer 17, internal reforming is possible, in which hydrocarbons, preferably methane, are directly converted into H₂ and CO at the catalytically active hydrogen electrode. Heat QE from the electrochemical conversion is removed by the endothermic reforming. Alternatively, if hydrocarbons are added, the process can also be carried out without an external reformer 17. The feed gas can then be fed directly to the electrolysis cell 10 together with the hydrocarbons (C₂M₆H₆N₂) and reformed there.

[0026] The application of external or internal hydrocarbon reforming is particularly advantageous when hydrogen / product gas is to be provided and waste heat is available due to ohmic losses in the electrolysis cell 10 or in the stack consisting of several cells. This is the case when the electrolysis cell degrades and its ohmic resistance increases.

[0027] The application of external or internal hydrocarbon reforming is also advantageous when the solid oxide cell system is connected to a synthesis process, e.g., Fischer-Tropsch synthesis. In this process, a hydrocarbon-rich return / recycle gas is produced, which is converted into H₂ and CO in the external reformer 17 or directly in the electrolysis cell 10 using heat from an exothermic electrolysis process. This heat extraction allows the electrolysis to be operated at a power density where otherwise the electrolysis cell or stack would either overheat or the system would require air cooling, thus reducing efficiency.

[0028] The heat supply to the external reformer 17 can be effected in various ways known in the prior art.

[0029] The operating principle and construction of an electrolysis cell system of the type according to the invention are described in more detail in the preceding application EP 3901329 A1. EP 3901329 A1 is incorporated in its entirety into the present application.

[0030] The different colors indicate the feed conversion. The y-axis shows the absolute change in Nernst voltage at the gas outlet in mV when the electrolysis current is modulated by + / - 1.5% around the nominal current (e.g., + / - 1.5 A at 100 A). It can be seen that the feed gas composition has only a minor influence on the change in Nernst voltage. For example, a limit of 10 mV could be set for the change in Nernst voltage so that the feed conversion is always below 86% (light blue dots in the diagram). Pressure and temperature have only a minor influence. The voltage change is primarily defined by the current change, so, as mentioned, it must be controlled very precisely. REFERENCE MARK LIST

[0031] 10 Electrolysis cell 11 Oxygen electrode 12 Hydrogen electrode 13 Air / purge medium 14 Oxygen-enriched air / purge medium 15 Feed gas 16 Product gas 17 Catalytic reactor / reformer 18 Hydrocarbons (C m H n ) 19 Feed gas 20 Power source for electrolysis current 21 Measuring device QR: Heat dissipated during external reforming; QE: Heat consumed during internal reforming

Claims

1. Method for operating an electrolysis cell system with at least one electrolysis cell (10) in which a product gas (16) containing hydrogen (H2) is produced from a feed gas (15) containing water (H2O) by means of electrolysis using electrical energy, comprising the steps of: providing an electrolysis cell system with a feed line (15) and a discharge line (16) for the discharge of product gas (16); providing a measuring device (21) for recording a measured value (U) representing the partial pressure of oxygen in the product gas (16). N,out ); Feeding an electrolysis current into the electrolysis cell system; Determining a feed conversion actual value (FC) ist ); Determining a feed conversion rule difference (FC) delta ) between the feed conversion actual value (FC ist ) and a predefinable target feed conversion value (FC) soll); Generating a control signal (S) depending on the feed conversion control difference (FC) delta ); Setting one or more process parameters of the electrolysis cell system depending on the control signal (S).

2. Method according to claim 1, characterized by the fact that A solid oxide electrolysis cell system (SOEC), in particular a solid oxide cell (rSOC) that can be operated in reverse mode, is used as the electrolysis cell system.

3. Method according to one of claims 1 or 2, characterized by the fact that the electrolysis cell system comprises at least one stack containing several electrolysis cells (10).

4. Method according to claim 3, characterized by the fact that the electrolysis cell system comprises several stacks, several of which form a stack group which are fed with the feed gas (15) from a common feed line and wherein the stacks of the stack group each feed the product gas (16) produced in the stack into a common outlet.

5. Method according to claim 4, characterized by the fact that the measuring device (21) is arranged and designed such that the measured value representing the oxygen partial pressure in the product gas (16) (U) N,out ) is detected at the output of each stack of the stack group, namely upstream before the product gas (16) of the respective stack is fed into the common outlet.

6. Method according to any of the preceding claims, characterized by the fact that The electrolysis current is a process parameter that can be set depending on the control signal (S).

7. Method according to claim 6, characterized by the fact that The electrolysis current is modulated with a modulation amplitude and a modulation frequency.

8. Method according to claim 7, characterized by the fact that a modulation amplitude is selected which lies in a range of 0.1 percent to 10 percent, preferably in the range of 1 percent to 2 percent, of the electrolysis current value to be set.

9. Method according to one of claims 7 or 8, characterized by the fact that a modulation frequency is selected which lies in a range of 0.01 Hz to 10 Hz, preferably in the range of 0.1 Hz to 1 Hz.

10. Method according to any of the preceding claims, characterized by the fact that the measuring device (21) comprises a lambda probe, wherein the measured value (U) acquired by means of the lambda probe N,out ) the Nernst tension, which represents the oxygen partial pressure in the product gas (16).

11. Electrolysis cell system that is set up and configured to carry out a method according to one of the preceding claims.

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