Monitoring of an electrochemical system via electrochemical impedance spectroscopy and method thereof

Abstract

The disclosure relates to an electrochemical system comprising: an EIS output unit configured to provide an output signal; a plurality of electrochemical units; and one or more power supplies. Each respective electrochemical unit is separately electrically connected to the one or more power supplies via first and second paths. The first path electrically connects the respective electrochemical unit and the one or more power supplies via the output unit for supplying the output signal to the respective electrochemical unit. The second path electrically connects the respective electrochemical unit and the one or more power supplies separately from the output unit. Each of the first and second paths is separately switchable between: a coupled state in which the respective electrochemical unit and the one or more power supplies are electrically coupled; and a decoupled state in which the respective electrochemical unit and the one or more power supplies are electrically decoupled.

Description

[0001] MONITORING OF AN ELECTROCHEMICAL SYSTEM VIA ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY AND METHOD THEREOF

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an electrochemical system, and to a method of operating an electrochemical system. Moreover, the disclosure relates to a method of performing quality control of solid oxide stacks.

[0004] BACKGROUND OF THE INVENTION

[0005] Electrochemical systems comprising multiple separate electrochemical units, such as solid oxide cell (SOC) systems, have gained significant traction in recent years as promising technologies for energy conversion and storage. These systems are employed in a range of applications, including power generation, energy storage, and fuel synthesis, owing to their high efficiency and versatility in handling various fuel sources. To maintain optimal performance and operational longevity, electrochemical units require continuous monitoring and control to ensure that each unit functions within desired operational parameters.

[0006] In the state of the art, one prevalent approach to monitoring performance involves the use of electrochemical impedance spectroscopy (EIS). EIS is a powerful diagnostic technique capable of providing valuable insights into the health and status of electrochemical cells by characterising their impedance spectra. This information enables the detection of degradation phenomena such as electrolyte deterioration, electrode delamination, and gas leakage. Most conventional EIS units are suitable for diagnosing a single electrochemical unit, such as a single solid oxide cell stack. Thus, precise monitoring of each unit's performance to reduce the risk of unexpected failures in conventional electrochemical systems relies on a dedicated EIS unit for each individual electrochemical unit.

[0007] Despite their technical merits, such conventional systems suffer from significant economic and practical drawbacks, particularly in relation to expenses associated with installing, maintaining, and operating EIS units in large-scale systems comprising numerous electrochemical units. Moreover, the physical footprint and complexity of integrating such diagnostic equipment into already intricate electrochemical assemblies further exacerbate these challenges. Consequently, operators are often faced with the dilemma of either incurring high monitoring costs or accepting increased risks of system degradation and failure due to insufficient surveillance. It is therefore an objective of the present invention to provide electrochemical systems and methods which can be operated efficiently and installed inexpensively while maintaining reliable prevention of system failure.

[0008] SUMMARY OF THE INVENTION

[0009] On the above background, it is an object of preferred embodiments of the present disclosure to provide electrochemical systems and methods of operating electrochemical systems, including methods of performing quality control, which ensure efficient operation and inexpensive installation while maintaining reliable prevention of system failure.

[0010] A first aspect of the present disclosure relates to an electrochemical system comprising : an electrochemical impedance spectroscopy output unit configured to provide an impedance spectroscopy output signal; a plurality of electrochemical units; and one or more power supplies, wherein each respective electrochemical unit of the plurality of electrochemical units is separately electrically connected to the one or more power supplies via a first path and a second path, wherein the first path electrically connects the respective electrochemical unit and the one or more power supplies via the output unit for supplying the output signal to the respective electrochemical unit, wherein the second path electrically connects the respective electrochemical unit and the one or more power supplies separately from the output unit, wherein each of the first path and the second path is separately switchable between: a coupled state in which the respective electrochemical unit and the one or more power supplies are electrically coupled; and a decoupled state in which the respective electrochemical unit and the one or more power supplies are electrically decoupled. Such a system can facilitate supplying an output signal from the EIS output unit to any single electrochemical unit of the plurality of electrochemical units while the electrochemical unit which receives the output signal remains powered by a power supply via the first path and while the remaining electrochemical units are decoupled from the EIS output unit and receives power from their respective power supplies via the respective seconds paths. Accordingly, the system disclosed herein allows one single EIS output unit to be utilized in a system comprising a plurality of electrochemical units, with the output unit being capable of sequentially probing separate units of the plurality of electrochemical units. In turn, this allows the realization of electrochemical systems which can be installed inexpensively and operated efficiently while maintaining reliable prevention of failure of the electrochemical units.

[0011] Switching of paths between coupled and decoupled states may generally be facilitated by any type of switches capable of electrically coupling and decoupling a conducting path between components. Examples of such a switch are electromechanical devices with movable electrical contacts, and electronic switches, such as solid-state switches. Within the scope of the present disclosure, switching between coupled and decoupled states may generally be performed manually, for example by an operator, fully automatically, for example controlled by a processing unit configured to regularly and systematically switch paths between coupled and decoupled states, or partly automatically, for example where a human operator initiates a switching procedure in which a processing unit systematically switches paths between coupled and decoupled states. A processing unit may for example be a programmable logic circuit.

[0012] The coupled state may also be referred to as a conducting state, and the decoupled state may also be referred to as a non-conducting state.

[0013] In the context of the present disclosure, a path electrically connecting components separately from, e.g., the output unit may be understood as that path being able to conduct power or current between these components independently / separately from the output signal of the output unit while the output unit provides / generates the output signal (for example via another path).

[0014] According to examples of the present disclosure, the system comprises a processing unit configured to switch each of the first path and the second path between the coupled state and the decoupled state.

[0015] Switching of the paths between the coupled and the decoupled state can generally be controlled by a processing unit. Such an implementation can facilitate control of, e.g., multiple switches, and thereby of whether respective paths are in a coupled or decoupled state. The processor may further be used to implement restrictions and procedures relating to combinations of paths which can be coupled or decoupled, and sequences according to which paths are switched between coupled and decoupled states.

[0016] According to examples of the present disclosure, the processing unit is configured to: switch the first path between the respective electrochemical unit and the one or more power supplies from the decoupled state to the coupled state while the second path between the same respective electrochemical unit and the one or more power supplies is in the coupled state; switch the first path between the respective electrochemical unit and the one or more power supplies from the coupled state to the decoupled state while the second path between the same respective electrochemical unit and the one or more power supplies is in the coupled state; switch the second path between the respective electrochemical unit and the one or more power supplies from the coupled state to the decoupled state while the first path between the same respective electrochemical unit and the one or more power supplies is in the coupled state; switch the second path between the respective electrochemical unit and the one or more power supplies from the decoupled state to the coupled state while the first path between the same respective electrochemical unit and the one or more power supplies is in the coupled state; or any combination thereof.

[0017] By switching according to any of the sequences exemplified above, the respective electrochemical unit is temporarily powered via both the first path and the second path. That is, both paths are temporarily in a coupled state.

[0018] A respective electrochemical unit may generally be powered via the first path and / or via the second path. Powering through the first path is relevant when an output signal is to be provided from the output unit to the electrochemical unit, and powering through the second path is relevant otherwise, particularly if another electrochemical unit is to receive an output signal. Switching between the states in which a respective electrochemical unit is powered via the first path and the second path generally requires switching one path from the coupled state to the decoupled state and the other path from the decoupled state to the coupled state. By performing these switches in a certain order, particularly by ensuring that both paths are temporarily in the coupled state, for example before switching one of the paths to a decoupled state, degradation of the electrical components in the system, such as switches, electrochemical stacks, output unit, and power supplies, may be reduced. Generally, a system according to the first aspect may facilitate such operation, for example without necessarily having a processing unit.

[0019] According to examples of the present disclosure, the processing unit is configured to control the output unit to provide the output signal while the first path is in a coupled state and the second path is in a decoupled state.

[0020] Such control ensures that the respective electrochemical unit receives the output signal from the output unit, which thereby facilitates efficient and reliable probing of the respective electrochemical unit.

[0021] According to examples of the present disclosure, the processing unit is configured to control each of the first paths between the plurality of electrochemical units and the one or more power supplies such that at most one of these first paths is in the coupled state.

[0022] While one first path is in the coupled state, the other first paths may then in the decoupled state. Furthermore, while this one first path is in the coupled state, the second path electrically connecting the same electrochemical unit and power supply as that one first path may be in the decoupled state, while remaining second paths may be in the coupled state.

[0023] Such control ensures that the just one respective electrochemical unit is coupled to the output unit, which thereby facilitates efficient and reliable probing of the respective electrochemical unit.

[0024] According to examples of the present disclosure, the system comprises one or more impedance spectroscopy recording units configured to measure an impedance spectroscopy measurement signal when the output signal is provided to the respective electrochemical unit.

[0025] According to examples of the present disclosure, the one or more recording units comprises a common recording unit electrically connected several electrochemical units of the plurality of electrochemical units. According to examples of the present disclosure, the one or more recording units is a plurality of recording units, wherein each respective recoding unit of the plurality of recording units is separately electrically connected to a respective electrochemical unit of the plurality of electrochemical units.

[0026] According to examples of the present disclosure, the one or more recording units are configured to measure the measurement signal from different sub-groups of the respective electrochemical unit.

[0027] Impedance spectroscopy may be performed using an EIS output unit and one or more complementary recording units. As an example, the EIS output unit may supply an alternating current to an electrochemical unit, while a recording unit measures the resulting alternating voltage induced in that electrochemical unit.

[0028] To further improve cost-efficiency of electrochemical systems, it may be advantageous to have a provide a single common recording unit configured to measure a response from several electrochemical units. Such a topology may for example involve a recording unit having several probes, such that it can be coupled to several electrochemical units simultaneously, or a recording unit in combination with a switching arrangement configured to selectively couple the recording unit to a relevant electrochemical unit.

[0029] Considering, as an example, solid oxide cell stacks as electrochemical units, it can both be relevant to have many individual stacks, while each stack also has many individual cells which can be desirable to probe as sub-groups of the stack. A sub-group may, for example consist of between 1 and 20 solid oxide cells, for example between 2 and 15, such as between 3 and 10. A single recording unit probing multiple electrochemical units, and potentially probing different sub-groups of each electrochemical unit is in principle possible, but may necessitate highly cumbersome cabling and switching arrangements. Therefore, it can be advantageous to have several recording units to provide a simple straightforward topology, even though more recording units may be required. Furthermore, despite the provision of several recording units, cost-efficiency of the system can be maintained since recording units are generally not necessarily as expensive as an output unit.

[0030] According to examples of the present disclosure, the one or more power supplies are a plurality of power supplies, wherein each respective electrochemical unit of the plurality of electrochemical units is separately electrically connected to a respective power supply of the plurality of power supplies via the first path and the second path, wherein the first path electrically connects the respective electrochemical unit and the respective power supply via the output unit for supplying the output signal to the respective electrochemical unit, wherein the second path electrically connects the respective electrochemical unit and the respective power supply separately from the output unit, wherein each of the first path and the second path is separately switchable between: the coupled state in which the respective electrochemical unit and the respective power supply are electrically coupled; and the decoupled state in which the respective electrochemical unit and the respective power supply are electrically decoupled.

[0031] By using such a topology of components, each electrochemical unit can be powered by a separate power supply, while allowing facilitation of sequential probing of separate units of the plurality of electrochemical units.

[0032] According to examples of the present disclosure, the first path and the second path are at least partly parallel paths between the respective power supply and the respective electrochemical unit.

[0033] According to examples of the present disclosure, the electrochemical system is a solid oxide cell system, wherein the plurality of electrochemical units is a plurality of solid oxide cell stacks.

[0034] Solid oxide cells (SOC) are efficient electrochemical devices for, in example, H2 / H2O and / or CO / CO2redox reactions. Such cells can be operated reversely either in electrolysis (SOEC) or fuel cell (SOFC) mode which facilitates this technology for power-to-gas-to-power application in renewable energy storage systems.

[0035] To increase throughout, many cells are often stacked in SOC stacks, where the cells of the SOC stack receive power from a power supply in series.

[0036] The systems and methods described within the present disclosure are particularly advantageous to utilize for solid oxide cell stacks, since such stacks are highly complex systems involving many components which can give rise to a plethora of different types of faults, where each stack generally requires to be examined in isolation from other stacks. Additionally, solid oxide cell stacks presently undergo significant technological developments, which further imposes a need for efficient quality and performance control. Thereby, solid oxide cell stacks which are electrochemical units for which the provision of efficient quality control and prevention of failures is particularly advantageous.

[0037] Other examples of electrochemical units which can be utilized within the scope of the present disclosure are batteries such as alkaline batteries, redox flow batteries, or lithium-ion batteries, and electrolysis units such as proton exchange membrane electrolysis units, alkaline water electrolysis units, and anion exchange membrane electrolysis units.

[0038] A second aspect of the present disclosure relates to a method of operating an electrochemical system, the method comprising the steps of: coupling an electrochemical impedance spectroscopy output unit to a first electrochemical unit; providing an impedance spectroscopy output signal from the output unit to the first electrochemical unit; decoupling the output unit from the first electrochemical unit; coupling the output unit to a second electrochemical unit; and providing an impedance spectroscopy output signal from the output unit to the second electrochemical unit.

[0039] Methods according to the second aspect may generally provide the same or similar advantages as those provided by electrochemical systems according to the first aspect. The coupling and decoupling of the output unit to the first and second electrochemical units may be facilitated by first and seconds paths, for example controlled by a processing unit. The electrochemical units may be operated by a common power supply, or by separate respective power supplies.

[0040] The method of operating an electrochemical system according to the second aspect may be a part of a quality control process.

[0041] According to examples of the present disclosure, a power supply is coupled to the first electrochemical unit via the output unit while the output unit is coupled to the first electrochemical unit, wherein the method comprises a step a coupling the power supply to the first electrochemical unit separately from the output unit prior to the step of decoupling the output unit from the first electrochemical unit.

[0042] This step may further be performed after the step of providing the impedance spectroscopy output signal from the output unit to the first electrochemical unit. The power supply may also be referred to as a first power supply.

[0043] According to examples of the present disclosure, a power supply is coupled to the second electrochemical unit via the output unit while the output unit is coupled to the second electrochemical unit, wherein the method comprises a step of coupling the power supply to the second electrochemical unit separately from the output unit prior to the step of coupling the output unit to the second electrochemical unit.

[0044] The power supply may optionally be the same power supply as one which is coupled to the first electrochemical unit. Alternatively, the power supply may also be referred to as a second power supply, for example such that a first power supply is coupled to the first electrochemical unit and the second power supply is coupled to the second electrochemical unit.

[0045] According to examples of the present disclosure, the method comprises a step of decoupling a power supply to the second electrochemical unit separately from the output unit after the step of coupling the output unit to the second electrochemical unit.

[0046] This power supply may be the second power supply. This step may further be performed prior to the step of providing the impedance spectroscopy output signal from the output unit to the second electrochemical unit.

[0047] According to examples of the present disclosure, the method is performed on an electrochemical system according to any example according to the first aspect, wherein the plurality of electrochemical units comprises the first electrochemical unit and the second electrochemical unit.

[0048] Any of the optional additional method steps exemplified above under the second aspect may ensure that a respective electrochemical unit is temporarily powered through two paths, both separately from and via the output unit, when switching to and from a state in which the output unit is coupled to that electrochemical unit. Thereby, degradation of the electrical components in the system, such as switches, electrochemical stacks, output unit, and power supplies, may be reduced.

[0049] A third aspect of the present disclosure relates to a method of performing quality control (QC) of solid oxide cell stacks, the method comprising: a. Operating the solid oxide cell stacks in electrolysis mode under a fuel side QC atmosphere comprising hydrogen and steam at a QC current density; b. Recording an electrochemical impedance (EIS) spectrum of the stack during such operation, wherein the device used for measuring EIS is shared between the stacks when performing the quality control, and then c. Comparing the obtained spectrum with a QC reference.

[0050] This third aspect according to the present disclosure may have any of the same or similar advantages as the first and second aspects according to the present disclosure. The method according to the third aspect may be performed in a system according to any example under the first aspect.

[0051] The device used for measuring EIS can be an EIS recording unit as described herein.

[0052] The fuel side QC atmosphere is meant to refer to the atmosphere in the fuel side inlet during operation at the operation temperature. During operation it is generally preferred in steam electrolysis that there is a slightly reducing atmosphere on the fuel side to retain the nickel catalyst in reduced form.

[0053] According to examples of the present disclosure, a single power supply is shared between the solid oxide cell stacks.

[0054] According to examples of the present disclosure, the solid oxide cell stacks which are operable at full current load, wherein the QC current density is of below 50% of the full current load, for example below 40%, such as below 35, 30, 25, 20, 15, or 10% of the full current load. An advantage of operating at low current densities is that a single power supply may be shared between two to six stacks or more during the quality control procedure. Another advantage of operating at low current densities is that the device used for measuring EIS may be shared between two to six stacks or more during the quality control procedure.

[0055] A further advantage of operating at low current densities in the context of a quality control is that it is quicker to perform the quality control procedure, while the procedure accurately and reliably predicts faults likely to occur in a stack during full current load even though the test procedure only involved operating the stack at a lower current load.

[0056] In the present context, the term "full current load" of a stack is meant to refer to the current load which the stack is designed to accommodate during normal operation in an electrolyser plant. The full current load can thereby also or alternatively be referred to as an operational current (density). In the context of solid oxide electrolysis, currents are often described in terms of current density. As an example, a full current load can be a current density in a range from 0.65 A / cm2to 4 A / cm2, such as in a range from 0.70 A / cm2to 3 A / cm2, 0.75 A / cm2to 2 A / cm2, for example in a range from 0.80 A / cm2to 1.5 A / cm2. As an example, a SOC stack is designed to be operated at a current density of 1.0 A / cm2. Operation of such a solid oxide cell stack at a current density below 50% of the full current load thereby corresponds to operation at a current density below 0.50 A / cm2. In this context, relative currents provided in terms of percent of a full current load within the present disclosure can alternatively be expressed in terms of absolute currents calculated assuming a full current load of 1.0 A / cm2. For example, the QC current density of QC1 expressed in terms of full current load in the further below can also be expressed as 0.05 to 0.15 A / cm2. This applies to all relative currents expressed herein.

[0057] According to examples of the present disclosure, the solid oxide cell stacks are two to six solid oxide cell stacks.

[0058] The device for measuring EIS should preferably be able to provide reliable results in the frequency range of 0.1 - 40.000 Hz, or at least 0.1- 20.000 Hz.

[0059] The inventors have found, that when the device for measuring EIS includes a debiasing unit to ensure that the transformer of the device operates efficiently and safely by preventing saturation and allowing for real-time adjustments based on the current flowing through the system to improve the efficiency of the transformer, then reliable results can be obtained in the frequency range of 0.1 - 40.000 Hz, or at least 0.1 - 20.000 Hz. To be more specific, a preferred device for measuring EIS on an electrochemical unit, such as an SOC stack, comprises

[0060] - a transformer with at least one primary winding and two secondary windings, where:

[0061] - the primary winding of the transformer connects in series a direct-current (DC) power supply with the electrochemical unit and the DC power supply generates a DC current across the primary winding to power the electrochemical unit;

[0062] - the first secondary winding is connected in series to a variable DC voltage generator suitable for generating a DC de-biasing current across the first secondary winding to reverse a DC flux bias generated in the transformer by the DC current flowing across the primary winding;

[0063] - the second secondary winding is connected in series to a variable alternating-current (AC) voltage generator suitable for generating at least one electrochemical unit analysis signal; and the device further comprises

[0064] - at least one sensor suitable for being connected to the electrochemical unit to measure the impedance response of the electrochemical unit;

[0065] - a controller connected to one or more of the at least one sensor, the direct-current (DC) power supply, the variable DC voltage generator, and the variable alternating-current (AC) voltage generator, the controller being suitable for receiving a signal from the at least one sensor and using this information to determine the impedance properties of the electrochemical unit.

[0066] In particular this type of device for measuring EIS provides reliable data in the high frequency range of 10.000-40.000 Hz which is useful for reliably testing for loss of contact within and between the repeat units of the electrochemical unit as well as the conditioning and the reduction of the electrochemical unit. This type of device is described in e.g. WO 21 / 003577.

[0067] An advantage of such a device is that the first secondary winding producing the de-biasing current extends the frequency range where the apparatus produces reliable results.

[0068] Accordingly, a system comprising an arrangement for continuously operating a solid oxide cell stack in electrolysis mode and a device for measuring EIS may be envisaged, wherein : the arrangement comprises means for adjusting the DC current applied to the stack and means for adjusting the fuel side feed to the stack the device comprises a de-biasing unit

[0069] The de-biasing unit functions to reverse a DC flux bias generated in a transformer of the device by a DC current flowing in the transformer.

[0070] BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Embodiments of the invention will now be further described by reference to the accompanying drawings, in which:

[0072] Fig. 1 illustrates an electrochemical system according to the present disclosure,

[0073] Fig. 2 illustrates another electrochemical system according to the present disclosure,

[0074] Figs. 3a-c illustrate switching an output unit from being decoupled to being coupled to an electrochemical unit,

[0075] Figs. 4a-b illustrate various arrangements of recording units, and

[0076] Fig. 5 illustrates method steps according to examples of the present disclosure.

[0077] DETAILED DESCRIPTION

[0078] Fig. 1 illustrates an electrochemical system 1 according to the present disclosure.

[0079] The system 1 comprises a plurality power supplies 4a, 4b and a plurality of electrochemical units 3a, 3b. In the present illustration, the plurality of power supplies comprises a first power supply 4a and a second power supply 4b, and the plurality of electrochemical units comprises a first electrochemical unit 3a and a second electrochemical unit 3b. As indicated in the illustration by the vertical ellipses, systems according to the present disclosure may comprise additional power supplies and electrochemical units.

[0080] Preferably, a respective power supply of the plurality of power supplies is configured to power a respective electrochemical unit of the plurality of electrochemical units such that the plurality of power supplies power the plurality of electrochemical units. In the present example, the first power supply 4a is configured to power the first electrochemical unit 3a and the second power supply 4b is configured to power the second electrochemical unit 3b.

[0081] The electrochemical system 1 further comprises an electrochemical impedance spectroscopy output unit 2. This output unit 2 is configured to provide an impedance spectroscopy output signal in the form of an alternating current with varying frequency which can be supplied in addition to a direct current. The output unit 2 is integrated in the system so as to be capable of providing said output signal separately to the electrochemical units 3a, 3b.

[0082] Each respective electrochemical unit 3a, 3b is separately connected to a respective power supply 4a, 4b via a first path 5a, 5b and a second path 6a, 6b for powering the respective electrochemical unit. Thus, each respective electrochemical unit 3a, 3b is associated with a respective first path and a respective second path. More specifically, the first electrochemical unit 3a is electrically connected to the first power supply 4a via a first path 5a and a second path 6a such that the first power supply 4a can supply power to the first electrochemical unit 3a via either of these two paths 5a, 6a. Similarly, the second electrochemical unit 3b is electrically connected to the second power supply 4b via a first path 5b and a second path 6b such that the second power supply 4b can supply power to the second electrochemical unit 3b via either of these two paths 5b, 6b.

[0083] The first path of each electrochemical unit electrically connects that respective electrochemical unit to a power supply via the output unit 2. In contrast, the second path of each electrochemical unit electrically connects that respective electrochemical unit to a power supply separately from the output unit 2.

[0084] In practice, in this example, the first path and the second path electrically connecting a respective power supply to a respective electrochemical unit have a common section proximate to the respective power supply. From the common section, the first path and the second path branch out, and the first path runs through the output unit 2 before the first and the second path join in a common section proximate to the respective electrochemical unit. Each of the first paths 5a, 5b thereby run though the output unit 2. In this example, the first paths 5a, 5b join in a common section prior to running though the output unit 2, and the first paths then branch out again after the output unit 2.

[0085] Each of the paths connecting respective electrochemical units 3a, 3b with the respective power supplies 4a, 4b is separately switchable between a coupled state in which the respective power supply and the respective electrochemical unit are electrically coupled, and a decoupled state in which the respective power supply and the respective electrochemical unit are electrically decoupled. In the present example, the respective paths comprise respective switches which can be used to facilitate switching between the coupled and the decoupled state.

[0086] Generally, the output unit 2 may further have one or more electrical connections and corresponding switches (not illustrated) to the electrical connection running from the respective electrochemical unit back to the power supply (from right to left in the illustrations of the present disclosure), for example to receive the alternating current after passing through the electrochemical unit. Alternating current and direct current can be separated, for example by one or more filters of the output unit.

[0087] The first and second paths, which are switchable between coupled and decoupled states, can be used to provide the output signal of the output unit 2 selectively to any of the electrochemical units 3a, 3b.

[0088] In case no output signal is to be provided from the output unit 2 to any of the electrochemical units 3a, 3b, then the system 1 can be operated in a state in which each of the first paths 5a, 5b is in a decoupled state while each of the second paths 6a, 6b is in a coupled state. Thereby, in the present example, the first power supply 4a can supply power to the first electrochemical unit 3a and the second power supply 4b can supply power to the second electrochemical unit 3b while the output unit 2 is entirely decoupled from the power supplies 4a, 4b and the electrochemical units 3a, 3b.

[0089] In case an output signal is to be provided to the first electrochemical unit 3a, then the first path 5a between the first power supply 4a and the first electrochemical unit 3a can be switched from the decoupled state to the coupled state, while the second path 6a between these can be switched from the coupled state to the decoupled state. Subsequently, an output signal can be provided from the output unit 2 to the first electrochemical unit 3a. Next, the relevant second path 6a can be switched back from the decoupled state to the coupled state and the first path 5a can be switched from the coupled state to the decoupled state. Through such a procedure, an output signal can be provided from the output unit 2 separately to the first electrochemical unit 3a, while the second electrochemical unit 3b is decoupled from the output unit 2 and the output signal. In case an output signal is to be provided to the second electrochemical unit 3b, the procedure outlined above for the first and second paths 5a, 6a between the first power supply 4a and the first electrochemical unit 4a can be reproduced for the first and second paths 5b, 6b between the second power supply 4b and the second electrochemical unit 4b.

[0090] The concept exemplified above can straightforwardly be extended to involve any number of pairs of respective power supplies and respective electrochemical units by having each pair being separately electrically connected by a switchable first path and a switchable second path as explained above.

[0091] Fig. 2 illustrates another electrochemical system 1 according to the present disclosure.

[0092] In comparison with the system illustrated in Fig. 1, the electrochemical system 1 illustrated in Fig. 2 has a single power supply 4 electrically connected to several separate electrochemical units 3a, 3b.

[0093] Each of the electrochemical units has associated therewith a respective first path and a respective second path for powering the electrochemical units. More specifically, the first electrochemical unit 3a is electrically connected to the power supply 4 via a first path 5a and a second path 6a such that the power supply 4 can supply power to the first electrochemical unit 3a via either of these two paths 5a, 6a. Similarly, the second electrochemical unit 3b is electrically connected to the same power supply 4 via a first path 5b and a second path 6b such that the power supply 4 can supply power to the second electrochemical unit 3b via either of these two paths 5b, 6b. The first path of each electrochemical unit electrically connects that respective electrochemical unit to the power supply 4 via the output unit 2, and in contrast, the second path of each electrochemical unit electrically connects that respective electrochemical unit to the power supply 4 separately from the output unit 2.

[0094] In practice, in this example, the first paths and seconds paths have a common section proximate to the power supply 4. Then, these paths branch out to the paths associated with the first electrochemical unit 5a, 6a and the paths associated with the second electrochemical unit 5b, 6b, respectively. From this point forward, the topology of the paths corresponds to the topology of the paths in Fig. 1.

[0095] As for the system illustrated in Fig. 1, the first and second paths in Fig. 2 switchable between coupled and decoupled states can be used to provide the output signal of the output unit 2 selectively to any of the electrochemical units 3a, 3b.

[0096] In case no output signal is to be provided from the output unit to the first electrochemical unit 3a while that electrochemical unit 3a is powered, then the second path 6a associated with the first electrochemical unit 3a can be in the coupled state while the remaining paths 5a, 5b, 6b are in the decoupled state. Similarly, the second path associated with the second electrochemical unit 3b can be in the coupled state in case that electrochemical unit is to be powered. In case an output signal is to be provided to the first electrochemical unit 3a while it is powered, then the first path 5a between the power supply 4 and the first electrochemical unit 3a can be switched from the decoupled state to the coupled state, while the second path 6a between these can be switched from the coupled state to the decoupled state. Subsequently, an output signal can be provided from the output unit 2 to the first electrochemical unit 3a. Next, the relevant second path 6a can be switched back from the decoupled state to the coupled state and the first path 5a can be switched from the coupled state to the decoupled state. A corresponding procedure can be used to provide an output signal to the second electrochemical unit 3b.

[0097] The concept exemplified in Fig. 2 can straightforwardly be extended to involve any number of respective electrochemical units by having such electrochemical units being separately electrically connected to the power supply 4 by a switchable first path and a switchable second path as explained above.

[0098] The system topology illustrated in Fig. 2, where one power supply is electrically connected to several electrochemical units, can for example be relevant in the context of quality control of electrochemical units, such as SOC stacks. In such situations, it can be relevant to sequentially power electrochemical units separately, and / or to provide power to several electrochemical units at a low current compared to actual operation. However, note that aspects disclosed herein are not limited to such uses.

[0099] Figs. 3a-c illustrate switching an output unit 2 from being decoupled to being coupled to an electrochemical unit 3.

[0100] For the sake of simplicity, only a single electrochemical unit 3 is illustrated in these figures. However, the system 1 may comprise any number of electrochemical units, for example as explained in relation to Figs. 1-2.

[0101] To switch the system 1 from a state in which the output unit 2 is decoupled from the electrochemical unit 3 to a state in which the output unit 2 is coupled to the electrochemical unit 3, the system 1 may go from the state illustrated in Fig. 3a, through the state illustrated in Fig. 3b, and into the state illustrated in Fig. 3c. Correspondingly, to switch the system 1 from a state in which the output unit 2 is coupled to the electrochemical unit 3 into a state in which the output unit 2 is decoupled from the electrochemical unit 3, the system 1 may go from the state illustrated in Fig. 3c, through the state illustrated in Fig. 3b, and into the state illustrated in Fig. 3a. The system 1 illustrated in Figs. 3a-c comprises a processing unit 9, which is configured to switch the first path 5 and the second path 6 between the coupled state and the decoupled state. Thereby, the system can be switched to and from a state in which the output unit 2 is coupled to the electrochemical unit 3.

[0102] In this particular example, the switching is facilitated by switches 7a-b, 8. More specifically, the first path 5 comprises a first switch 7a located between the power supply 4 and the output unit 2 for separately coupling and decoupling the power supply 4 to the output unit 2. Moreover, the first path comprises a second switch 7b located between the output unit 2 and the electrochemical unit 3 for separately coupling and decoupling the output unit 2 to the electrochemical unit 3. The first and second switches 7a-b of the first path 5 are located separately from the second path 6. The second path 6 comprises a switch 8 located between the power supply 4 and the electrochemical unit 3 for separately coupling and decoupling the power supply 4 and the electrochemical unit 3. The switch 8 of the second path 6 is located separately from the first path 5.

[0103] In Fig. 3a, the first path 5 is in a decoupled state and the second path 6 is in a coupled state. In practice, in this example, the first switch 7a and the second switch 7b of the first path 5 are open, and the switch 8 of the second path 6 is closed. Thereby, the electrochemical unit 3 is powered by the power supply 4 via the second path 6. The output unit 2 is decoupled from the electrochemical unit 3.

[0104] In Fig. 3b, both the first path 5 and the second path 6 are in a coupled state. In practice, in this example, the first switch 7a of the first path 5, the second switch 7b of the first path 5, and the switch 6 of the second path 8 are all closed. Thereby, the electrochemical unit 3 is powered by the power supply 4 via either or both of the two paths 5, 6.

[0105] In Fig. 3c, the first path 5 is in a coupled state and the second path 6 is in a decoupled state. In practice, in this example, the first switch 7a and the second switch 7b of the first path 5 are closed, and the switch 8 of the second path 6 is open. Thereby, the electrochemical unit 3 is powered by the power supply 4 via the first path 5 running via the output unit 2. This in turn allows the output unit 2 to efficiently provide an output signal to the electrochemical unit.

[0106] Thereby, by going through the states illustrated in and explained in relation to Figs. 3a-c, an output unit 2 can be brought from being decoupled to being coupled to an electrochemical unit and vice versa. In systems comprising several electrochemical units, the same procedure as exemplified here may be utilized to couple and decouple the output unit 2 to any other electrochemical unit by switching the first and second paths associated with such other electrochemical unit accordingly.

[0107] Figs. 4a-b illustrate various arrangements of recording units lOa-b, 10.

[0108] The system 1 illustrated in Fig. 4a is similar to the one illustrated in Fig. 1, with the addition that each respective electrochemical unit 3a-b has associated therewith a respective recording unit lOa-b. A respective recording unit lOa-b is electrically connected to a respective electrochemical unit 3a-b for measuring an impedance spectroscopy measurement signal when an output signal is provided to the respective electrochemical unit 3a-b. Typically, the output signal of the output unit 2 is an alternating current, and the resulting impedance spectroscopy measurement signal is an alternating voltage. These are then correlated to provide electrochemical impedance spectroscopy.

[0109] Typically, a recording unit has at least two connections to an electrochemical unit, such that a voltage across these two connections can be measured. In case a measurement signal is to be measured from different sub-groups of an electrochemical unit, then the recording unit can have more than two connections to the electrochemical unit. This is also indicated in Fig. 4a by ellipses between the respective electrochemical units 3a-b and the respective recording units lOa-b. This can for example be relevant in case SOC stacks are used as electrochemical units, where each sub-group of the respective SOC stack comprises one or more solid oxide cells.

[0110] In contrast to the system illustrated in Fig. 4a, the system illustrated in Fig. 4b comprises a common recording unit 10. This common recording unit is coupled to several electrochemical units 3a-b via a switching arrangement 11.

[0111] For the sake of simplicity, the electrical connection between the electrochemical units 3a-b, the switching arrangement 11, and the recording unit 10 are illustrated by single lines, but these single lines can represent any number of electrical connections, for example for measuring an impedance spectroscopy measurement signal across several sub-groups of a single electrochemical unit.

[0112] The switching arrangement 11 is configured to electrically couple the recording unit 10 with a respective electrochemical unit selectively, such that the recording unit 10 can measure the measurement signal from the respective electrochemical unit.

[0113] For example, while the output unit 2 provides an output signal to a first electrochemical unit 3a, the switching arrangement 11 electrically couples the recording unit 10 with the first electrochemical unit 3a, and while the output unit 2 provides an output signal to a second electrochemical unit 3b, the switching arrangement 11 electrically couples the recording unit 10 with the second electrochemical unit.

[0114] Considering, as an example, a system having N electrochemical units, with each electrochemical unit having M connections across which voltages are to be recorded, then the switching arrangement 11 can be implemented with N x M switches selectively coupling the recording unit 10 with the electrochemical units.

[0115] The switching arrangement may optionally be controlled by a processing unit, for example such that the configuration of the switching arrangement 11 is coordinated with the states of the first and second paths, preferably such that the recording unit and the output unit are coupled to the same electrochemical unit.

[0116] Recording units may be communicatively connected to the output unit, for example such that the measurement signal can be synchronized with the output signal.

[0117] As previously, the principles outlined in relation to Figs. 4a-b can be extended to any number of electrochemical units and power supplies, including systems with one power supply and with more than two power supplies.

[0118] Fig. 5 illustrates method steps Sl-12 according to examples of the present disclosure. The method is a method of operating an electrochemical system, such as one of the electrochemical systems disclosed herein.

[0119] In a first step SI, a power supply is coupled to a first electrochemical unit separately from an electrochemical impedance spectroscopy output unit. The output unit is configured to supply an impedance spectroscopy output signal. The power supply can then supply power to the first electrochemical unit separately from the output unit.

[0120] In another step S2, the output unit is coupled to the first electrochemical unit. Preferably, in this step, the output unit is coupled to the first electrochemical unit by coupling the power supply to the first electrochemical unit via a path through the output unit. Thereby, the power supply can supply power to the first electrochemical unit via this path through the output unit.

[0121] In another step S3, the power supply is decoupled from the first electrochemical unit separately from the output unit. Preferably, the power supply is then coupled to the first electrochemical unit via the path through the output unit. This step S3 may thereby correspond to a reversal of the step labelled SI.

[0122] In another step S4, an output signal is provided from the output unit to the first electrochemical unit.

[0123] In another step S5, the power supply is coupled to the first electrochemical unit separately from the output unit. This step S5 may thereby be substantially similar to the step labelled SI, but is performed at a later stage of the method.

[0124] In another step S6, the output unit is decoupled from the first electrochemical unit. This step S6 may thereby correspond to a reversal of the step labelled S2.

[0125] The power supply supplying power to the first electrochemical unit, via a path through the output unit or separately from the output unit, may be referred to as a first power supply.

[0126] In another step S7, a power supply is coupled to a second electrochemical unit separately from the output unit. This power supply can then supply power to the second electrochemical unit separately from the output unit. Further, this power supply may be the same power supply as the one supplying power to the first electrochemical unit, or it may be another power supply referred to as a second power supply.

[0127] In another step S8, the output unit is coupled to the second electrochemical unit. Preferably, in this step, the output unit is coupled to the second electrochemical unit by coupling the power supply to the second electrochemical unit via a path through the output unit. Thereby, the power supply can supply power to the second electrochemical unit via this path through the output unit.

[0128] In another step S9, the power supply is decoupled from the second electrochemical unit separately from the output unit. Preferably, the power supply is then coupled to the second electrochemical unit via the path through the output unit. This step S9 may thereby correspond to a reversal of the step labelled S7.

[0129] In another step S10, an output signal is provided from the output unit to the second electrochemical unit. In another step Sil, the power supply is coupled to the second electrochemical unit separately from the output unit. This step Sil may thereby be substantially similar to the step labelled S7, but is performed at a later stage of the method.

[0130] In another step S12, the output unit is decoupled from the second electrochemical unit. This step S12 may thereby correspond to a reversal of the step labelled S8.

[0131] In the following, electrochemical impedance spectroscopy (EIS) is briefly reviewed in the context of the present disclosure.

[0132] Electrochemical impedance spectroscopy (EIS) is a well-known technique to study electrochemical systems. This technique is a tool to investigate the dielectric and transport properties of materials, investigate interfaces and mechanisms, or electrochemical reactions and to explore the properties of, e.g., porous electrodes. As an example, an EIS measurement can study the response of a system subjected to, e.g., a sinusoidal current, in a frequency range of, e.g., 0.1Hz to 40 kHz. This sinusoidal current can be applied on top of a direct current (DC) as well as under open circuit voltage (OCV). The amplitude of the applied sinusoidal current is usually small, generally a small percentage of the DC in a galvanostatic EIS measurement (except for OCV, where the QC current density is approximately 0 A / cm2). Such a sinusoidal or alternating current is also referred to herein as an output signal. The voltage response of the system to the sinusoidal current is then measured. The impedance response Z can then be calculated from the perturbation current and the voltage response by using the generalized Ohm's law. The EIS data can be plotted in the complex plane showing the real part of the impedance in the abscissa and the imaginary part of the impedance in the ordinate, referred to as a Nyquist plot. The imaginary part can be presented with the opposite sign, to bring it into the first quadrant. Notably, when performing an EIS measurement, two currents are typically needed, the amplitude of the sinusoidal current (EIS output signal) and the direct current (power from a power supply). I.e. adding an EIS output signal having an amplitude of 5 A on top of a direct current of 100 A from a power supply, provides a total current to an electrolysis unit oscillating in a range from 95 A to 105 A.

[0133] An EIS output signal, such as a sinusoidal current, can be supplied by an EIS output unit. The response of an electrochemical unit can then be measured, for example by an impedance spectroscopy recording unit. Such a recording unit may be separate from the output unit, or the recording unit and the output unit may be arranged together in an integrated EIS apparatus comprising the EIS output unit and one or more recording units. The plurality of electrochemical units can each be connected to a respective recording unit, or they can be connected to a common recording unit. The alternating current supplied by an output unit and the resulting alternating voltage recorded by a recording unit can then be correlated to provide an impedance spectroscopy.

[0134] Within the scope of the present disclosure, the output signal of the EIS output unit may have any amplitude, and the output signal may be supplied while any magnitude of direct current is provided to the electrochemical unit. Preferably, but not exclusively, the direct current supplied to the relevant electrochemical unit while the output signal provided is a non-zero (non-OCV) current.

[0135] Generally, within the scope of the present disclosure, electrochemical impedance spectroscopy is not limited to any particular type of use or application. As examples, EIS of electrochemical units can be used for quality control of electrochemical units prior to or as part of integrating the electrochemical units in an electrochemical plant. As further examples within the scope of the present disclosure, EIS can be applied to electrochemical units in operation in an electrochemical plant as ongoing diagnostic analysis, as quality assurance, as predictive maintenance, as data acquisition for post-analysis, as input for control of the electrochemical units, or as a combination thereof.

[0136] Moreover, the EIS and its use in, e.g., quality control or diagnosis may be performed manually, automatically, or based on a combination of manual and automatic processing of an EIS spectrum.

[0137] Various adaptations of the electrochemical system and the measurement setup for acquiring an impedance spectroscopy measurement signal may be performed to ensure sufficient data quality. Such adaptations may, for example, be of relevance for systems in which the electrochemical units are solid oxide cell stacks comprising more than just a few cells, for cells having a large cell area, and for measurement at high frequencies, such as above 10 or 20 kHz. As an example, the disclosed adaptations may be of particular relevance if the solid oxide cell stack comprises at least 15 cells, for example at least 20 cells such as at least 25 cells. As an additional example, such adaptations may be of relevance if the obtained spectrum is acquired separately for at least five sub-groups of solid oxide cell stack. Moreover, such adaptations may, for example, be of relevance if each cell of the solid oxide cell stack has an active surface area of at least 100 cm2, for example of at least 200 cm2, such as of at least 300 cm2.

[0138] Typically, a solid oxide cell stack may have a central axis extending perpendicularly to the cells of the stack, and the solid oxide cell stack is operated by supplying current via a first conductor in contact with a first end of the solid oxide cell stack and by discharging current via a second conductor in contact with a second end of the solid oxide cell stack, the first end of the solid oxide cell stack being opposite to the second end of the solid oxide cell stack.

[0139] To improve data quality, particularly for stacks comprising more than just a few cells, for cells having a large cell area, and for measurement at high frequencies, one or more of the following adaptations may be implemented in any combination:

[0140] - the first conductor is parallel to the central axis at a portion of the first conductor proximate to the solid oxide cell stack; and / or the second conductor is parallel to the central axis at a portion of the first conductor proximate to the solid oxide cell stack.

[0141] - a contact point of said first conductor with the solid oxide cell stack is at the central axis; and / or a contact point of said second conductor with the solid oxide cell stack is at the central axis.

[0142] - the electrochemical impedance spectrum is recorded via voltage contacts of the solid oxide cell stack, the respective voltage contacts providing electrical contact to respective cells of the stack, wherein the voltage contacts are collectively arranged in parallel to the central axis.

[0143] - neighbouring wires connected to said respective voltage contacts are pairwise twisted into twisted wire pairs, wherein neighbouring twisted wire pairs are pairwise twisted into twisted wire bundles.

[0144] In the following, a list of further embodiments within the scope of the present disclosure is provided.

[0145] List of numbered embodiments:

[0146] 1. A method of performing quality control (QC) of solid oxide cell stacks, the method comprising : a. Operating the solid oxide cell stacks in electrolysis mode under a fuel side QC atmosphere comprising hydrogen and steam at a QC current density; b. Recording an electrochemical impedance (EIS) spectrum of the stack during such operation, wherein the device used for measuring EIS is shared between the stacks when performing the quality control, and then c. Comparing the obtained spectrum with a QC reference.

[0147] 2. A method according to embodiment 1, wherein a single power supply is shared between the solid oxide cell stacks.

[0148] 3. A method according to any of embodiments 1-2, wherein the solid oxide cell stacks which are operable at full current load, wherein the QC current density is of below 50% of the full current load, for example below 40%, such as below 35, 30, 25, 20, 15, or 10% of the full current load.

[0149] 4. A method according to any of embodiments 1-3, wherein the solid oxide cell stacks are two to six solid oxide cell stacks.

[0150] List of figure references:

[0151] 1 electrochemical system

[0152] 2 EIS output unit

[0153] 3 electrochemical unit

[0154] 4 power supply

[0155] 5 first path

[0156] 6 second path

[0157] 7 switch of first path

[0158] 8 switch of second path

[0159] 9 processing unit

[0160] 10 recording unit

[0161] 11 switching arrangement

[0162] Sl-12 method steps

Claims

CLAIMS1. An electrochemical system comprising: an electrochemical impedance spectroscopy output unit configured to provide an impedance spectroscopy output signal; a plurality of electrochemical units; and one or more power supplies, wherein each respective electrochemical unit of the plurality of electrochemical units is separately electrically connected to the one or more power supplies via a first path and a second path, wherein the first path electrically connects the respective electrochemical unit and the one or more power supplies via the output unit for supplying the output signal to the respective electrochemical unit, wherein the second path electrically connects the respective electrochemical unit and the one or more power supplies separately from the output unit, wherein each of the first path and the second path is separately switchable between: a coupled state in which the respective electrochemical unit and the one or more power supplies are electrically coupled; and a decoupled state in which the respective electrochemical unit and the one or more power supplies are electrically decoupled.

2. An electrochemical system according to claim 1, wherein the system comprises a processing unit configured to switch each of the first path and the second path between the coupled state and the decoupled state.

3. An electrochemical system according to claim 2, wherein the processing unit is configured to: switch the first path between the respective electrochemical unit and the one or more power supplies from the decoupled state to the coupled state while thesecond path between the same respective electrochemical unit and the one or more power supplies is in the coupled state; switch the first path between the respective electrochemical unit and the one or more power supplies from the coupled state to the decoupled state while the second path between the same respective electrochemical unit and the one or more power supplies is in the coupled state; switch the second path between the respective electrochemical unit and the one or more power supplies from the coupled state to the decoupled state while the first path between the same respective electrochemical unit and the one or more power supplies is in the coupled state; switch the second path between the respective electrochemical unit and the one or more power supplies from the decoupled state to the coupled state while the first path between the same respective electrochemical unit and the one or more power supplies is in the coupled state; or any combination thereof.

4. An electrochemical system according to any of claims 2-3, wherein the processing unit is configured to control the output unit to provide the output signal while the first path is in a coupled state and the second path is in a decoupled state.

5. An electrochemical system according to any of claims 2-4, wherein the processing unit is configured to control each of the first paths between the plurality of electrochemical units and the one or more power supplies such that at most one of these first paths is in the coupled state.

6. An electrochemical system according to any of the preceding claims, wherein the system comprises one or more impedance spectroscopy recording units configured to measure an impedance spectroscopy measurement signal when the output signal is provided to the respective electrochemical unit.

7. An electrochemical system according to claim 6, wherein the one or more recording units comprises a common recording unit electrically connected to several electrochemical units of the plurality of electrochemical units.

8. An electrochemical system according to claim 6, wherein the one or more recording units is a plurality of recording units, wherein each respective recoding unit of the plurality of recording units is separately electrically connected to a respective electrochemical unit of the plurality of electrochemical units.

9. An electrochemical system according to any of the preceding claims, wherein the one or more power supplies are a plurality of power supplies, wherein each respective electrochemical unit of the plurality of electrochemical units is separately electrically connected to a respective power supply of the plurality of power supplies via the first path and the second path, wherein the first path electrically connects the respective electrochemical unit and the respective power supply via the output unit for supplying the output signal to the respective electrochemical unit, wherein the second path electrically connects the respective electrochemical unit and the respective power supply separately from the output unit, wherein each of the first path and the second path is separately switchable between: the coupled state in which the respective electrochemical unit and the respective power supply are electrically coupled; and the decoupled state in which the respective electrochemical unit and the respective power supply are electrically decoupled.

10. An electrochemical system according to any of the preceding claims, wherein the electrochemical system is a solid oxide cell system, wherein the plurality of electrochemical units is a plurality of solid oxide cell stacks.

11. A method of operating an electrochemical system, the method comprising the steps of: coupling an electrochemical impedance spectroscopy output unit to a first electrochemical unit; providing an impedance spectroscopy output signal from the output unit to the first electrochemical unit; decoupling the output unit from the first electrochemical unit;coupling the output unit to a second electrochemical unit; and providing an impedance spectroscopy output signal from the output unit to the second electrochemical unit.

12. A method according to claim 11, wherein a power supply is coupled to the first electrochemical unit via the output unit while the output unit is coupled to the first electrochemical unit, wherein the method comprises a step a coupling the power supply to the first electrochemical unit separately from the output unit prior to the step of decoupling the output unit from the first electrochemical unit.

13. A method according to any of claims 11-12, wherein a power supply is coupled to the second electrochemical unit via the output unit while the output unit is coupled to the second electrochemical unit, wherein the method comprises a step of coupling the power supply to the second electrochemical unit separately from the output unit prior to the step of coupling the output unit to the second electrochemical unit.

14. A method according to any of claims 11-13, wherein the method comprises a step of decoupling a power supply to the second electrochemical unit separately from the output unit after the step of coupling the output unit to the second electrochemical unit.

15. A method according to any of claims 11-14, wherein the method is performed on an electrochemical system according to any of claims 1-10, wherein the plurality of electrochemical units comprises the first electrochemical unit and the second electrochemical unit.

Images