阅读说明：本技术 具有受控的热分布的电解系统 (Electrolysis system with controlled heat distribution ) 是由 S·H·詹森 C·R·格雷夫斯 M·B·莫恩森 于 2020-04-03 设计创作，主要内容包括：本发明涉及包括一个或多个电解电池和为电池供给波动电压的至少一个电力电子单元的系统,并且涉及用于运行一个或多个电解电池的方法,所述方法包括通过至少一个电力电子单元向电解电池提供一个或多个电压波动,使得能够提供同时允许快速响应动态运行、提高的电解效率、增加的寿命和高的杂质耐受性的低成本电解系统。(The present invention relates to a system comprising one or more electrolysis cells and at least one power electronic unit supplying the cells with a fluctuating voltage, and to a method for operating one or more electrolysis cells, the method comprising providing the electrolysis cells with one or more voltage fluctuations by the at least one power electronic unit, such that a low-cost electrolysis system can be provided that simultaneously allows fast-response dynamic operation, improved electrolysis efficiency, increased lifetime and high impurity tolerance.)

1. A system for operating one or more electrolysis cells, the system comprising:

one or more electrolytic cells; and

at least one power electronics unit, wherein the power electronics unit provides one or more voltage fluctuations to the electrolytic cell, wherein the voltage fluctuations are configured such that near-thermal neutral operation at partial load is achieved by matching integrated joule heating generation to integrated reaction heat consumption inside the cell.

2. The system of claim 1, wherein the one or more electrolysis cells are configured to operate above 120 ℃.

3. The system of claim 1 or 2, wherein the one or more electrolysis cells are selected from the group consisting of solid oxide electrolysis cell/solid oxide fuel cell (SOEC/SOFC), molten carbonate electrolysis cell/molten carbonate fuel cell (MCEC/MCFC), high temperature high pressure alkaline electrolysis cell/high temperature high pressure alkaline fuel cell, and ceramic electrolyte proton conducting electrolysis cell/ceramic electrolyte proton conducting fuel cell (PCEC/PCFC).

4. The system of any one of claims 1 to 3, further comprising at least one PID system that controls the voltage fluctuations based on measurements of inlet and outlet temperatures of fluid sent to and from a stack.

5. The system of any one of claims 1 to 4, wherein the voltage fluctuations are configured to effect desorption or dissolution of adsorbed, precipitated, or otherwise formed side reaction compounds in an electrode of the battery.

6. The system of any one of claims 1 to 5, wherein the duration of each voltage fluctuation is in the range of 1 μ s to 1000 s.

7. The system of any one of claims 1 to 6, wherein the power electronics unit comprises a direct current power supply with a Pulse Width Modulated (PWM) motor controller, a bi-directional power supply, or a power supply in combination with an electronic load.

8. The system of any one of claims 1 to 7, wherein the voltage fluctuation ranges between 0.2V and 2.0V.

9. The system of claim 8, wherein the voltage fluctuation ranges between 0.5V and 1.9V.

10. The system of any one of claims 1 to 9, wherein the one or more electrolysis cells are subjected to H₂O and/or CO₂Electrolysis of (2).

11. A method for operating one or more electrolytic cells, the method comprising:

providing one or more voltage fluctuations to the electrolytic cell by at least one power electronics unit,

wherein the voltage fluctuations are configured such that near-thermal neutral operation at partial load is achieved by matching integrated joule heat generation to integrated reactive heat dissipation inside the battery.

12. The method of claim 11, wherein for a portion of the time that the voltage fluctuates, the current in the battery is reversed to operate the battery in a fuel cell mode.

13. The method of any one of claims 11 or 12, wherein the one or more electrolysis cells are subjected to at least CO₂Electrolysis of (2).

Technical Field

The present invention relates to a system comprising one or more electrolytic cells (electrolysis cells) and at least one power electronic unit supplying the cells with a fluctuating voltage, and to a method for operating one or more electrolytic cells, comprising providing the electrolytic cells with one or more voltage fluctuations by the at least one power electronic unit, enabling improved thermal management during operation of the electrolytic cells.

The present invention can improve control of temperature distribution in electrolytic cells and stacks (stacks) by controlling the magnitude and shape of voltage fluctuations. The invention enables isothermal operation under dynamic loading conditions, increasing cell life by reducing thermomechanical stress and by desorbing impurities adsorbed on active sites in the electrodes. In particular for CO₂Electrolysis, the present invention increases the maximum achievable carbon monoxide outlet gas concentration.

Background

In commercial electrolysis systems, the cells are typically stacked with contacts (interconnects) between the cells. The Single Repeat Unit (SRU) of the stack is made up of the battery and contacts. Due to their inherent ability to convert electrical energy into chemical energy, electrolytic systems are considered key technologies for the renewable energy economy.

For example, the CO may be introduced using a high temperature electrolysis cell (such as, for example, a Solid Oxide Electrolysis Cell (SOEC))₂And H₂Conversion of O to CO, H₂And O₂. SOEC typically run at 600 to 900 ℃. The electrolysis reaction is endothermic and therefore requires heat to be supplied to the cell, which can be achieved by preheating the inlet gas (CO)₂And/or H₂O). Alternatively, the electrolytic stack may be thermally neutralOperating at a potential, which represents the SRU voltage at which joule heat (i.e., heat generated by current flowing through internal resistances in the SRU) matches the heat required for the electrolysis reaction, it is possible to minimize the input of energy from or output of energy to the outside and to improve the electrolysis efficiency.

For H₂Electrolysis of O vapor, thermal neutral potential (E)_tn) About 1.3V and for CO₂Electrolysis, which is about 1.5V. In high-temperature electrolytic cells, E_tnSubstantially higher than the Open Circuit Voltage (OCV), i.e. the voltage across the electrolytic cell in the absence of an external current flow. For typical SOEC operating temperatures and gas compositions at ambient pressure, the OCV is slightly below 1V. In such an electrolysis system, the SRU is at E_tnThe operation below typically causes excessive degradation (degradation) due to high electrode overvoltage in the SRU and/or adsorption of impurities at electrochemically active sites in the electrodes. Increasing the SRU voltage beyond E_tn(and thereby further increase the electrolytic current density) will accelerate degradation, reduce conversion efficiency and increase the need for thermal control to dissipate excessive joule heating.

When the SRU voltage and current are changed, the temperature distribution in the stack gradually changes due to changes in reaction and joule heat generation. This introduces thermo-mechanical stresses that may cause a loss of contact at the interface between the layers in the stack (typically between the stack and the bipolar contact plate). Thus, dynamic operation involving repeated changes in SRU voltage and current can reduce lifetime. On the other hand, C.Graves et al, Nature Materials 2015,14,239-^-2In the constant-current electrolysis operation, the voltage is rapidly increased to over 1.3V, and the current is periodically reversed (-1/+ 0.5A-cm)^-2) Resulting in less degradation, more stable battery voltage and extended battery life. However, this beneficial effect was observed in testing of individual cells mounted in an externally heated near isothermal housing. Dynamic testing of larger stacks results in greater temperature fluctuations, which can cause high degradation rates due to thermal stress, masking this beneficial effect ("Solid oxide electrolysis for grid balancing", energyk, item number 2013-1-12013, fig. 27, page 35).

If in OCV and E_tnThe SRU voltage between to operate the high temperature electrolysis cell stack, the gas inside the stack will cool from the inlet to the outlet. One example is shown in fig. 1 and involves CO in a stack with an SRU voltage of 1.228 volts (typical SOEC SRU voltage)₂And (4) electrolyzing. While excess oxygen purges the anode to limit temperature drop inside the stack, the temperature drops significantly from the gas inlet to the gas outlet. As the temperature decreases, the internal resistance increases, thereby decreasing the absolute current density at the stack outlet and causing an uneven current distribution in the stack.

For high temperature CO₂Electrolysis, the reduced outlet temperature lowers the threshold for the onset of carbon deposition. This reduces the maximum carbon monoxide content in the outlet gas that can be obtained without the risk of harmful carbon formation in the electrodes, which translates into an increased cost of gas separation. For CO₂Electrolysis at SRU voltage of E_tnRun with (c) will eliminate the temperature drop from inlet to outlet. Unfortunately, with state of the art batteries at E_tnThe lower run generally results in a high degradation rate.

Local temperature reduction in the stack also causes tensile stress at the contact/cell interface, which can potentially lead to delamination and loss of contact again increasing internal resistance in the cell. The degradation is further accelerated by the reduced uniformity of the current density and the conversion rate of the reaction gas. For smaller stacks, thermomechanical stresses may be limited by using a purge gas, such as an over-blow of air. However, this increases the complexity and cost of the system, particularly if the system is operated at elevated pressures.

Dynamic operation of electrolytic cells is often of particular interest because it provides load-following capabilities, i.e. services that can be sold to grid operators. In addition to this, increased lifetime and improved tolerance to impurities are desired, which may reduce operating costs.

Different approaches have been proposed in the prior art to provide load following capabilities for SOEC systems.

A method relates to SRU voltage at OCV and E_tnIn between operation at constant current. When the SRU voltage is lower than E_tnAt this time, the stack cools until the internal resistance in the SRU is high enough to raise the SRU voltage to E_tn. In this way, the stack will run at a lower power (due to the lower current) than the pre-set point where the current is higher. When higher power is required, the current increases. This will increase the SRU voltage beyond E_tnThis again raises the stack temperature until the internal resistance is low enough to ensure at E_tnThe following operations were carried out. However, this approach is disadvantageous because, when the SRU voltage is changed, the thermal profile in the stack changes, which causes thermo-mechanical stress inside the SRU and damages the weak IC/cell interface. Furthermore, the response time is minutes to hours before reaching steady operation at the new set point.

Another known approach is to operate a multi-stack system in a cascade mode, where the stack is operated at OCV or thermal neutral potential. The number of stacks operating at a thermal neutral voltage divided by the total number of stacks defines the load ratio. This operating strategy requires separate power and gas supply to several stacks, which significantly increases system complexity and cost.

US 2009/0325014 a1 discloses a method in which the concentration of a reactive gas supplied to a cell is varied in response to fluctuations in the current supplied to the cell, thereby varying the current required to reach a thermal neutral potential.

WO 2018/033948 a1 discloses a method of controlling the operation of an electrolysis cell stack for hydrogen production by a temperature control mechanism, with the object of adjusting the voltage (in constant voltage mode) or current (in constant current mode) to a lower target value when a preset upper temperature threshold is exceeded and adjusting the voltage (in constant voltage mode) or current (in constant current mode) to an initial target value when a preset lower temperature threshold is fallen below, and thereby achieving near-thermal neutral operation. US 2009/263681 a1 also discloses a system including means to monitor the temperature of the cell stack and to cycle the input voltage around a thermally neutral voltage. However, these methods essentially require a temperature monitoring system and a control system that ensures that the voltage (or current) is cycled between two thresholds.

Finally, US 2016/0040310a1 describes a hydrogen production system including a capacitor that inputs electric energy from a renewable power source and stores electric power, a pulse voltage generating unit that generates a pulse voltage having a set amplitude and a set cycle period by using the electric power stored in the capacitor, and an electrolytic cell that applies the generated pulse voltage and generates hydrogen by high-temperature steam electrolysis using steam supplied into the electrolytic cell.

However, the proposed method does not satisfactorily solve the problem of providing a low cost electrolysis system that simultaneously achieves fast response dynamic operation, improved electrolysis efficiency, increased lifetime and high impurity tolerance.

In view of the above, there remains a need to provide an electrolysis system and method of operating the same that addresses the above-mentioned problems.

Disclosure of Invention

The present invention solves these objects with the subject matter of the claims as defined herein. Further advantages of the present invention will be further explained in detail in the following sections.

In general, the invention relates to a system for operating one or more electrolysis cells, comprising: one or more electrolysis cells and at least one power electronics unit, wherein the power electronics unit provides one or more voltage fluctuations to the electrolysis cells, wherein the voltage fluctuations are configured such that near-thermal neutral operation at partial load is achieved by matching integrated joule heat generation to integrated reaction heat consumption inside the cell.

In a further embodiment, the present invention provides a method for operating one or more electrolytic cells comprising: providing one or more voltage fluctuations to the electrolytic cell by at least one power electronics unit, wherein the voltage fluctuations are configured such that near-thermal neutral operation at partial load is achieved by matching integrated joule heat generation to integrated reaction heat consumption inside the cell.

The invention achieves dynamic operation of the electrolysis system, increased lifetime, tolerance to impurities and carbon activity in the outlet gas.

Preferred embodiments of systems and associated methods for operating one or more electrolysis cells and other aspects of the invention are described in the following specification and claims.

Drawings

FIG. 1 shows a graph for an over-blown with 5 Xoxygen (i.e., where the cell footprint is 100 cm)²5% CO at the gas inlet and 28% CO, CO at the gas outlet₂Conventional CO as a buffer gas with an SRU voltage of 1.228V and a total current of 50A)₂Simulated OCV, temperature and current density of the electrolysis run curve.

Fig. 2A schematically shows an electrolytic cell.

Fig. 2B shows an electrolysis system comprising two cells stacked in series.

Fig. 3A and 3B show an example of steam electrolysis using a voltage having a sinusoidal waveform fluctuation. The corresponding average voltage is shown as a horizontal line.

FIG. 4 shows that 80% of the time is E_tnAnd 20% of the time is an example of steam electrolysis with fluctuating voltage operation of OCV (bold line). The average voltage is shown as a thin line. E_tnAnd OCV are shown in dashed lines.

FIG. 5 shows CO using square-wave ripple voltages (thick lines) with average cell voltages of 1.18V and 1.32V (thin lines), respectively₂Examples of electrolysis. E_tnAnd OCV are shown in dashed lines.

Fig. 6 is a graph showing OCV, temperature, and current density of a square-wave-shaped fluctuation voltage having an average cell voltage of 1.114V. Aiming at 5 times of oxygen over-blowing (the occupied area of the battery is 100 cm)²5% CO at the gas inlet and 28% CO, CO at the gas outlet₂Cell voltage 74% time was 1.3V and 26% time was 0.7V for buffer gas, and total current was 53A), showing the temperature profile.

Fig. 7 is a graph showing OCV, temperature, and current density under the conditions of fig. 6 except that oxygen over-blowing is omitted.

Fig. 8 is a graph showing OCV, temperature, and current density calculated for example 3.

FIG. 9 shows AC/DC H₂Cell area specific resistance, current and voltage as a function of time during O electrolysis.

FIG. 10 shows H₂Comparison of area specific resistance for DC and AC/DC tests in O electrolysis.

Fig. 11 depicts OCV, temperature and current density curves calculated for example 4.

FIG. 12 shows CO₂Comparison of area specific resistance and gas cleaning effectiveness of DC and AC/DC tests in electrolysis.

Detailed Description

For a more complete understanding of the present invention, reference is now made to the following descriptions of exemplary embodiments thereof.

In a first embodiment, the present invention relates to a system for operating one or more electrolytic cells, comprising: one or more electrolysis cells, and at least one power electronics unit, wherein the power electronics unit provides one or more voltage fluctuations to the electrolysis cells, wherein the voltage fluctuations are configured such that near-thermal neutral operation at partial load is achieved by matching integrated joule heat generation to integrated reaction heat consumption inside the battery.

In practice, practical electrolysis systems are usually operated under conditions that are neither completely isothermal nor completely adiabatic. As used herein, the term "near-thermal neutral operation" refers to electrolytic operation in which the absolute value of the difference between integrated joule heat generation and integrated reaction heat consumption (each integrated over a period of time exceeding 3600 seconds) is less than the absolute value of integrated heat consumption or the absolute value of integrated heat generation or both.

In a preferred embodiment, "near-thermal neutral operation" is understood to be an electric-thermal equilibrium operation that uses electricity (joule) heat to balance the heat of reaction required and can be distinguished from conventional thermal equilibrium operation that uses the heat capacity of excess air flow to limit temperature variations in the electrolysis cells and the stack.

A schematic of an exemplary electrolysis cell configured for the electrolysis of different kinds of reactant materials into desired reaction products is shown in fig. 2 a. The battery 10 includes a first electrode 11 and a second electrode 13, and an electrolyte 12 disposed between the first electrode 11 and the second electrode 13. During the electrolysis operation, power is supplied to the cell 10 (i.e. across the first and second electrodes 11, 13) by the power electronics unit 16 and the reactant 14 passing through the first electrode 11 is separated into ions 15 of the second reaction product 15a and a mixture 15b of the first reaction product and unreacted reactant. The ions 15 pass through the electrolyte 12 and form a second reaction product 15a at the second electrode 13. According to the invention, the power electronics unit 16 provides one or more voltage fluctuations to the electrolytic cell. In a preferred embodiment, the system according to the invention further comprises at least one proportional-integral-derivative (PID) system 17, said system 17 controlling the voltage fluctuations based on measurements of the inlet and outlet temperatures of the fluid (gas or liquid) sent to or from the electrolysis cell or stack or by measuring the temperature directly in the cell compartment (not shown), wherein each temperature measurement can be performed by means of a temperature detection device 18. The PID controller is configured to continuously calculate an error value as the difference between a desired temperature set point and the measured temperature, so that the connected power electronic unit can apply a correction of voltage fluctuations based on a proportional term, an integral term and a differential term.

There is no particular limitation on the reactant materials. In a preferred embodiment, one or more electrolysis cells are subjected to H₂O、CO₂By electrolysis of or H₂O and CO₂Co-electrolysis of (1).

If the cell 10 is a Solid Oxide Electrolysis Cell (SOEC) designed to electrolyze water, the first electrode 11 represents a hydrogen electrode, the second electrode 13 may be referred to as an oxygen electrode, and the reactant 14 will be a high temperature vapor. The catalyst in the hydrogen electrode promotes separation of the vapor 14 into oxygen ions 15 and a mixture 15b of hydrogen gas and unreacted vapor. As the oxygen ions 15 pass through the electrolyte 10, oxygen gas 15a will be formed at the catalyst in the oxygen electrode 13. However, it should be understood that fig. 2a is used to schematically illustrate an exemplary electrolysis system, and the present invention is not limited to such a configuration (or SOEC) and may include other layers (e.g., membranes or separators) and components, so long as the electrolysis cell is capable of electrolytic operation. In embodiments, the electrolysis cell may be constituted by a fuel cell (reversible fuel cell) operating in electrolysis mode. Furthermore, the electrolytic cell is not limited to a flat stack configuration (flat stack configuration) but may also incorporate other designs including, for example, a cylindrical configuration. Fig. 2b shows an electrolysis stack 20 consisting of two electrolysis cells 10. Additional cells may also be added to the stack 20 depending on the electrolysis production requirements.

Unless otherwise indicated, the term "electrolysis cell(s)" as used herein will be understood to include both a single cell, such as cell 10 shown in fig. 2a, and a stack of more than two cells, such as stack 20 shown in fig. 2 b.

It should be noted that details regarding the materials and construction techniques of electrolytic cells are well known to the skilled artisan and will not be described herein.

Generally, low temperature electrolysis and high temperature electrolysis can be distinguished. For example, in high temperature electrolysis, one or more electrolysis cells are configured to operate at temperatures in excess of 120 ℃, e.g., 200 ℃ to 1100 ℃ or 650 ℃ to 1000 ℃. Although not limited thereto, the electrolysis cell or cells for high-temperature electrolysis are preferably selected from the group consisting of solid oxide electrolysis cell/solid oxide fuel cell (SOEC/SOFC), molten carbonate electrolysis cell/molten carbonate fuel cell (MCEC/MCFC), high-temperature high-pressure alkaline electrolysis cell/high-temperature high-pressure alkaline fuel cell, and ceramic electrolyte proton-conducting electrolysis cell/ceramic electrolyte proton-conducting fuel cell (PCEC/PCFC).

From the above description, it will be understood that the term "electrolysis cell" as used herein also includes fuel cells, i.e. cells which are operated only in fuel cell mode. Thus, the systems and methods of the present invention may also be used to extend the life of fuel cells and fuel cell stacks.

In the electrolysis mode, the electrolysis reaction is generally endothermic, i.e. the heat of reaction is negative. Joule heating due to the necessary overpotential and current is positive in both the fuel cell mode and the electrolysis mode. In high temperature electrolysis, it is desirable to be below E for optimum performance_tnOperating voltage ofNear thermal neutral operation below. For OCV and E_tnIn between, near thermal neutral operation requires heat supply during the electrolysis process. In such systems, heat supply is further required to reduce tensile stress at the contact/cell interface, potentially leading to delamination and loss of contact, poor performance, and degradation. Conventionally, heat is supplied, for example, by using heated purge gas or active heating means. In contrast, in the present invention, joule heating is balanced with the heat of reaction (plus heat loss to the environment) by supplying one or more voltage fluctuations to the electrolytic cell via one or more power electronics units. Thus, the electrolysis system can be operated near thermally neutral without the need for an external heat source.

By adjusting the voltage variation, joule heating can be set to balance the reaction heat or slightly exceed the reaction heat. This enables near-thermal neutral operation or at OCV and E_tnWhile also allowing the outlet gas temperature to be the same as or slightly higher than the inlet gas temperature. Thus, operation with voltage variation enables improved control of the thermal profile in the SRU. The optimized average (integrated) SRU voltage and thermal profile can be used to increase the carbon activity of the carbon-containing gas inside the stack without risking the formation of harmful carbon in the electrodes. For CO₂Electrolysis, which translates into higher outlet gas CO concentrations, which again translates into reduced gas separation costs.

As used herein, the term "voltage fluctuation" refers to a predetermined change in the battery voltage, which may be applied in the form of a periodic voltage change that recurs at predefined intervals. From the viewpoint of reducing the mechanical tension, the duration of each voltage fluctuation is preferably set to a range of 1 μ s to 100 s. By operating the electrolysis system accordingly, the duration of each fluctuation is so short that temperature variations in the fluid (e.g., gas) and in the cells and stack are negligible. In this way, the accumulation of mechanical tension at weak interfaces in the stack can be avoided, so that an increased lifetime can be achieved by reversible operation. In a further preferred embodiment, the frequency of the voltage fluctuations is in the range of 10mHz to 100 kHz.

In a preferred embodiment, the voltage fluctuation ranges between 0.2V and 2.0V, particularly preferably between 0.5V and 1.9V.

In a preferred embodiment of the invention, the power electronics unit comprises a direct current power supply with a Pulse Width Modulated (PWM) motor controller, a bidirectional power supply or a power supply in combination with an electronic load (e-load).

In a preferred embodiment, the voltage fluctuations are configured to achieve evaporation, desorption or dissolution of the side reaction compounds adsorbed, precipitated or otherwise formed in the electrodes of the battery, for example by increasing their oxidation state (oxidation) or decreasing their oxidation state (reduction), which leads to reduced degradation, more stable battery voltage and extended battery life. While not so limited, as long as their formation is reversible, such side reaction compounds may be undesirable intermediates or originate from impurities in the reactants (e.g., hydroxides formed from alkaline earth metals, hydrocarbons, sulfur-based compounds, formaldehyde, ammonium formate, halogenated compounds) or from electrolytic cell materials (e.g., silicon-based impurities from glass components). For example, desorption or dissolution of the secondary reaction compounds may be achieved by periodic variation of the cell voltage to switch the electrochemical cell between electrolysis cell mode operation and fuel cell mode operation.

Ni migration is known to be one of the major degradation mechanisms in Ni/YSZ-electrodes for conventional DC electrolysis, in H₂O electrolysis and CO₂This mechanism is observed in electrolysis. Impurities such as sulfur are strongly bonded to the Ni surface, which is known to accelerate Ni particle coarsening. Thus, desorption of sulfur (and other) impurities may hinder Ni migration.

Without being bound by theory, it is speculated that the reduction in the degradation rate may be related to some extent to desorption of impurities adsorbed at the electrochemically active sites during cathodic polarization of the Ni/YSZ electrode. As an example thereof, mention may be made of the formation of SiO at the active site during cathodic polarization of Ni/YSZ electrodes₂. This is expected via a method as previously described by a. hauch et al, J Electrochem soc.2007; 154(7) A619-A26 by the reaction Si (OH)₄(g)->H₂O(g)+SiO₂(l) This occurs. During the brief anodic polarization, H can be formed₂O and may be SiO₂And (4) desorbing. The shape of the voltage fluctuations may in principle be of any type. However, voltage fluctuations including sinusoidal and/or square waveform voltage fluctuation curves are preferred. A mixture between sinusoidal and square voltage fluctuations is particularly preferred to minimize peak voltages and to minimize false operating conditions associated with induction phenomena. The calculation of near-thermal neutral operating conditions using sinusoidal and/or square-wave voltage fluctuation curves will be described with the following example.

In a second embodiment, the present invention relates to a method for operating one or more electrolytic cells, comprising: providing one or more voltage fluctuations to the electrolytic cell by at least one power electronics unit, wherein the voltage fluctuations are configured such that near-thermal neutral operation at partial load is achieved by matching integrated joule heat generation to integrated reaction heat consumption inside the cell.

In a preferred embodiment, the current in the cell is reversed for a portion of the voltage fluctuation to operate the cell in the fuel cell mode. Advantageously, this process may reduce damage to the electrode microstructure and/or enable desorption or dissolution of side reaction compounds adsorbed, precipitated or otherwise formed in the electrodes of the battery. Furthermore, not all of the product from the integrated electrochemical reaction that remains inside the cell is converted back to the reactant during a portion of the time that the current is reversed. Thus, in contrast to conventional (DC voltage) operation, there is no need to change the fluid (e.g. gas) composition.

In solid oxide fuel cells, Cr poisoning (poisoning) is known to limit SOFC air-electrode life, and therefore alumina coated steel tubes are typically required for air supply. Cr poisoning is predicted to pass through gaseous CrO₂(OH)₂Reaction with solid strontium oxide (SrO) present at reaction sites in the strontium-rich air electrode, resulting in SrCrO₄And H₂And forming O. By operating in accordance with the invention during fuel cell operationThe invention can realize SrCrO by applying voltage fluctuation₄Desorption of (3).

It will be understood that the preferred features of the first embodiment may be freely combined with the second embodiment in any combination, except combinations where at least some of the features are mutually exclusive.

Examples

Example 1: sine wave shaped voltage fluctuation

In general, the voltage fluctuation supplied to the electrolytic cell can be regarded as a DC voltage (AC/DC voltage) superimposed with a smaller AC voltage.

DC voltage U superimposed with AC sinusoidal voltage₁Can be written as

U(t)＝U₀sin(ωt)+U₁(equation 1)

The heat of reaction can be written as

Herein, U denotes a cell voltage, E is a Nernst voltage, R is a cell resistance, and K is a coulombic ratio heat of reactionFor electrolysis, the heat of reaction is negative.

Joule heating can be written as:

for thermally neutral operation, it is necessary:

fig. 3A and 3B show exemplary sinusoidal undulations operating at 50 Hz. The graph in fig. 3A represents a + symbol, and the graph in fig. 3B represents a-symbol in the last row of equation 4. The corresponding average voltage is shown as a horizontal line.

Example 2: square wave shaped voltage fluctuation

If a square voltage variation is used, the following expression applies for near-thermal neutral operation:

here,. DELTA.U₁＝(U₁-E) and Δ U₂＝(U₂-E). In equation 5, assume that the voltage is only at U₁And U₂Switch between the two to make the total time T ═ T₁+T₂。

Average voltage U_avGiven as follows:

U_av＝T₁·U₁+T₂·U₂ (1)

suppose E ═ 1V, K ═ 0.3V for steam electrolysis, and for CO₂Electrolysis, K ═ 0.5, for the sine function, U can be obtained from equation 4₁And U₀A first approximation of the relationship between. Using the same assumptions for E and K, for a square voltage change, U is obtained from equation 5₁、U₂And T₁The relationship between them.

The voltage variation may be of any shape. Sine wave shapes and square wave shapes are provided for mathematical simplicity. An important aspect is the integral joule heat balance integral heat of reaction. The smooth curve is better than the square waveform curve to minimize stray-inductance (stray-inductance) and eddy currents in the SRU.

FIG. 4 shows a schematic representation for vapor (H)₂O) electrolytic example with square voltage fluctuations. Here, the voltage is at OCV and E_tnTo fluctuate. At these voltages. The cell operates near thermal neutral.

FIG. 5 shows thermally neutral CO₂An example of square voltage fluctuation during electrolysis. Herein, when a cell voltage lower than the OCV is applied, the square-wave cell voltage fluctuation causes switching between the electrolysis mode and the fuel cell mode, which promotes desorption or dissolution of the side reaction compounds formed in the cell electrodes. Further, the heat generated during operation in the fuel cell mode (i.e., the time the cell voltage is below 20% of the OCV) balances the heat consumed during operation in the electrolysis mode (i.e., the time the cell voltage exceeds 80% of the OCV). The average cell voltages were 1.18V and 1.32V (thin line), respectively. Both examples shown in fig. 4 and 5 operate at 1 kHz.

In a further experiment shown in fig. 6, in addition to applying the voltage fluctuation according to fig. 4 (the fluctuation period of 74% was 1.3V and the fluctuation period of 26% was 0.7V; the integrated current was 53A), according to fig. 1 (i.e., the occupied area of the battery was 100 cm)²5% CO at the gas inlet and 27.8% CO, CO at the gas outlet₂5 x oxygen over-blow) as a buffer gas to calculate the OCV/temperature/current density curve. Fig. 6 shows that, contrary to conventional electrolysis operation (see fig. 1), a flat temperature profile is achieved inside the stack and the current density variation between gas inlet and gas outlet is kept to a minimum.

Fig. 7 shows OCV, temperature, and current density under the conditions of fig. 6, except that oxygen over-blowing is omitted. A larger temperature change is observed compared to fig. 6. However, the current density can be maintained at a similar steady level.

The present invention is shown to achieve near-thermal neutral operation of electrolytic cells and stacks by controlling the magnitude and shape of voltage fluctuations. By reducing the thermo-mechanical stress in the stack, the lifetime of the electrolytic cell and the stack can be further increased.

Thus, a low cost electrolysis system may be provided that simultaneously achieves fast response dynamic operation, improved electrolysis efficiency, increased lifetime and high impurity tolerance, as well as improved conditions for CO production.

_{2 2}Example 3: H/HO electrolysis test

For H₂/H₂The electrolysis system for O electrolysis is set as follows: an asymmetric square wave function is set at the function generator (Wavetek Model 145). The resulting signal is amplified by a bipolar power supply (Kepco BOP 20-20D) and transmitted to a test device according to C.Graves et al, Nature Materials 2015,14, 239-. An oscilloscope (Philips PM 3384) was used to monitor the battery voltage and the signal from the function generator. In AC/DC mode, the electrolytic cell was operated at 30Hz, fluctuating between 1.27V (90% of the time) and 0.8V (10% of the time), i.e., 90% duty cycle, giving an average voltage of 1.22V. For comparison, electrolysis of electricityThe cell also runs in DC mode for H₂The operating conditions for the O electrolysis test are shown in table 1 below. For both electrolysis tests, the gas flow to the negative electrode was 24l/H H₂:H₂O (ratio 0.5:0.5) and an inlet temperature of 700 ℃.

TABLE 1

For the AC/DC test, the current density was about-0.55A/cm²In the case of (1), H₂The O utilization was 30% and the air blow factor was 16. The nernst voltage was calculated relative to air at 941mV and 965mV at inlet and outlet, respectively. The calculated outlet temperature and gas composition was 701 ℃ and 65% H₂+35％H₂And O. The curves of temperature, nernst voltage and current density from inlet to outlet are shown in fig. 8 (for co-flow configuration). FIG. 9 shows AC/DC H₂Cell area specific resistance, current and voltage as a function of time during O electrolysis.

In FIG. 10, the comparison is made for AC/DC and DC H₂Evolution of cell area specific resistance of the O electrolysis test, which shows that the applied voltage fluctuation according to the present invention effectively prevents the increase of cell resistance during cell operation. Note that the area specific resistance measured for the AC/DC test is initially higher than the area specific resistance measured for the DC test. The impedance spectra recorded before and after the AC/DC test indicate that the effect is based on a higher positive resistance when compared to the positive resistance of the reference cell.

₂Example 4: CO/CO Electrolysis test

CO Using the test apparatus described in example 3, with CO₂Three cells were tested in the electrolysis mode.

All cells tested were multilayer-tape cast cells (MTC) with CGO (cerium gadolinium oxide) barrier layers, LSC (strontium lanthanum cobaltate) -CGO oxygen electrodes and LSC contact layers.

The cells were tested under the following conditions:

1) DC test without gas purge: the total flow rate is 18L/h, and the inlet CO is CO₂The ratio is 0.13: 0.87; the average cell current density was 0.31A/cm²Inlet temperature of-695 ℃ and fuel utilization rate of 13.3% (outlet CO: CO)₂The ratio was 0.24:0.76), the total fuel flow rate was 18L/h, and the air flow rate to the oxygen electrode was 50L/h.

2) DC test with gas purging: the inlet flow rate in the case of recirculation of the outlet gas comprising 20% CO was 10.5L/h, the total fuel flow rate was 21L/h and the inlet CO: CO₂The ratio was 0.1:0.9, and the average cell current density was 0.31A/cm²Inlet temperature of-695 ℃ and 11% fuel utilization rate (outlet CO: CO)₂The ratio was 0.20:0.80), and the air flow rate to the oxygen electrode was 50L/h.

3) AC/DC test: the total flow rate is 16L/h, wherein CO is CO₂The ratio is 0.11:0.89, and the average cell current density is-0.3A/cm²Inlet temperature of-695 ℃ and fuel utilization rate of-14.7% (outlet CO: CO)₂The ratio was-0.24: 0.76), the total fuel flow rate was 16L/h and the air flow rate to the oxygen electrode was 140L/h. The gas purifier was shut down after 250h of testing. Operating the cell at 30Hz, fluctuating between 1.30V (-60% of the time) and 0.75V (-40% of the time), i.e. 60% duty cycle, gives an average voltage of 1.09V.

The calculated temperature, current density and nernst voltage curves from the fuel and air inlets to the outlet for the AC/DC test are shown in fig. 11. A counter-current test setup was used for the actual cell test, while a co-current configuration was used for the calculations shown, however, the temperature profile was relatively flat in both cases. The Nernst voltages (OCV) at the inlet and outlet were 877mV and 913mV, respectively. A slight temperature increase from 695 ℃ to 696 ℃ was measured from inlet to outlet.

A comparison of the cell area specific resistance curves for the two DC tests and the AC/DC test respectively is shown in fig. 12. As a time-dependent change in area specific resistance (in m Ω · cm)²·kh^-1Meter) shows that the degradation rate shown in fig. 12 indicates that it is not unusual to operate with purified gasCompared to conventional DC operation, AC/DC operation reduces the degradation rate by a factor of about 3.7. The results show that in the case of CO, CO₂An increase in the area-specific cell resistance during the operation of the cell can also be effectively suppressed in the electrolytic cell, which enables increased cell life, higher efficiency, and stable cell operation.

Numerous other features, modifications and improvements will become apparent to the skilled person once given the above disclosure.

Reference numerals

10: electrolytic cell

11: a first electrode

12: electrolyte solution

13: second electrode

14: reactants

15: ion(s)

15 a: second reaction product

15 b: mixture of first reaction product and unreacted reactant

16: power electronic unit

17: optional PID System

18: temperature detection device

10: and (4) electrolyzing the cell stack.