Method of operating an electrochemical cell, control system and electrochemical cell system

Abstract

The invention relates to a method of operating an electrochemical cell, the method comprising determining a cell voltage, comparing the cell voltage with a minimum cell voltage corresponding to an end-of-life state of the cell, wherein the minimum cell voltage is updated over the lifetime of the cell depending on an area specific resistance of the cell. The invention also relates to a control system for controlling operation of an electrochemical cell, to an electrochemical cell system, and to computer- implemented methods of determining an end-of-life state of an electrochemical cell.

Description

[0001] MAU / JMR

[0002] Title: Method of operating an electrochemical cell, control system and electrochemical cell system

[0003] Specification

[0004] The present invention relates to methods of operating an electrochemical cell, in particular a solid oxide fuel cell (SOFC), to a control system for controlling operation of an electrochemical cell, to an electrochemical cell system comprising a control system and an electrochemical cell, and to methods for determining an end-of-life state of an electrochemical cell.

[0005] Fuel cells are examples of electrochemical cells. Fuel cells are energy conversion devices that allow for conversion of fuel to electricity. For this, the fuel cells typically comprise electrochemically active layers including a fuel electrode (anode), an electrolyte and an air electrode (cathode).

[0006] The present invention specifically relates to solid oxide cells (SOCs), in particular solid oxide fuel cells (SOFCs). Such solid oxide fuel cells are based upon a solid oxide electrolyte that conducts negative oxygen ions from a cathode to an anode located on opposite sides of the electrolyte. For this, a fuel, or reformed fuel, contacts the anode (fuel electrode) and an oxidant, such as air or an oxygen rich fluid, contacts the cathode (air electrode). Examples of such SOFCs are known, e.g., from WO 2020 / 126486 Al or WO 2022 / 175679 A2.

[0007] The anode of a SOFC is typically based on a cermet, often of Ni and yttria stabilised zirconia (Ni / YSZ). During cell operation, oxidation of the anode shall be avoided as Ni is active only in the reduced state, not in the oxidised state as NiO. Moreover, re-oxidation of the anode after activation will result in volume expansion of the anode leading to cracks in the electrolyte and a concomitant loss of power or failure.

[0008] Therefore, conventional technology may comprise means to protect the anode from Nickel oxidation. For example, US10622646B2 suggests a SOFC with a voltage monitoring device and an anode protection controller in which the anode protection controller decreases the current drawn from the stack by a predetermined amount whenever the voltage drops below a predetermined voltage threshold and decreases a fuel flowrate provided to the anode recirculation loop based on the decreased current. It is further known to monitor the cell voltage of the electrochemical cell during cell operation and stop cell operation if the cell voltage drops below a certain minimum cell voltage representing a threshold for anode oxidation. The minimum cell voltage is typically set to a fixed value determined by theoretical considerations based on the electrochemical Ni / NiO-redoxpotential at certain operating conditions.

[0009] It is an object of the present invention to improve operation of an electrochemical cell.

[0010] According to a first aspect, there is provided a method of operating an electrochemical cell (hereinafter also referred to as 'cell' only), preferably a fuel cell, preferably a solid oxide fuel cell (SOFC). Preferably, the electrochemical cell comprises a Nickel-containing electrode, preferably anode. The method comprises determining, preferably measuring, a cell voltage and comparing the cell voltage with a minimum cell voltage. The minimum cell voltage represents an end-of-life state of the cell. In particular, the minimum cell voltage represents a voltage below which deterioration of the cell, in particular oxidation of an anode of the electrochemical cell, is to be expected. The minimum cell voltage is updated over the lifetime of the cell, in particular during cell operation. That is, the minimum cell voltage the cell voltage is compared with may change over time. The minimum cell voltage is dependent on an area specific resistance of the cell. Preferably, the minimum cell voltage is updated depending on an area specific resistance of the cell.

[0011] The proposed method allows for reliable and safe operation of an electrochemical cell with an extended lifetime of the cell. This is achieved dynamically adjusting the 'end-of-life threshold' of the cell, represented by the minimum cell voltage, over the lifetime of the cell. Specifically, the inventors have found that the magnitude of the minimum cell voltage below which unwanted anode oxidation occurs changes, in particular decreases, over lifetime of the cell due to changes in the cell, in particular due to degradation effects of the electrode and electrolyte layers, and that those changes are reflected by the area specific resistance of the cell. Thus, by dynamically adjusting the minimum cell voltage depending on the area specific resistance of the cell, the minimum cell voltage better reflects the actual state of the cell at a certain specific point in time than it is the case for the prior art cells that are operated based on a constant theoretical minimum cell voltage. Advantageously, this allows for safe operation of the cell even for longer operating times than the prior art cells. The proposed method also allows for a precise indication of an end-of-life state of an electrochemical cell. Thus, the method is also a method of determining an end-of-life state of the electrochemical cell. Preferably, the electrochemical cell is a solid oxide fuel cell. Preferably, the solid oxide fuel cell comprises electrochemically active layers (also referred to as cell chemistry layers) comprising a fuel electrode (anode in fuel cell mode), an electrolyte and an air electrode (cathode in fuel cell mode). Preferably, the electrochemically active layers are deposited (e.g., as thin coatings / films) on and supported by a metal support plate (e.g., foil). Preferably, the electrochemically active layers comprise an anode layer deposited onto the support plate (preferably onto the porous region), an electrolyte layer deposited over the anode layer, and a cathode layer deposited over the electrolyte layer. However, in some cell arrangements that order may be reversed (such that the cathode layer is closest to the support plate). The fuel electrode, electrolyte and air electrode may each be formed of one or more layers to optimise operation.

[0012] Preferably, the fuel electrode (anode) is formed from a Nickel-containing material. More preferably, the fuel electrode (anode) is formed from a Nickel-containing cermet, in particular Gadolinia-doped Ceria-Ni cermet or YSZ-Ni cermet. Preferably, the electrolyte is formed from a material selected from the group consisting of: Yttria-stabilized zirconia (YSZ), Gadolinia-doped Ceria (GDC), Cerium Gadolinium Oxide (CGO), Scandia-stabilised Zirconia (ScSZ), and Lanthanum Strontium Gallium Magnesium Oxide (LSGM). Preferably, the air electrode (cathode) is formed from a material selected from the group consisting of: lanthanum cobalt nickel oxide (LCN), lanthanum strontium manganese oxide (LSM), lanthanum strontium cobalt iron oxide (LSCF), barium strontium cobalt iron oxide (BSCF), samarium strontium cobalt oxide (SSC) and praseodymium strontium cobalt oxide (PSC).

[0013] An example of an electrochemical cell, in particular SOFC, comprises: an interconnector structure, in particular interconnector plate, preferably formed from metal or metal alloy; a support structure, in particular support plate, preferably formed from metal or metal alloy, said interconnector structure and said support structure overlying one another and enclosing a cell volume, in particular fuel volume, therebetween; electrochemically active layers provided over a porous region of the support structure, said electrochemically active layers comprising o a fuel electrode layer, preferably formed from a Nickel-containing cermet, in particular from a Gadolinia-doped ceria-nickel cermet or YSZ-Ni-cermet, o an oxidant electrode layer, preferably formed from lanthanum cobalt nickel oxide (LCN), lanthanum strontium manganese oxide (LSM), lanthanum strontium cobalt iron oxide (LSCF), barium strontium cobalt iron oxide (BSCF), samarium strontium cobalt oxide (SSC) or praseodymium strontium cobalt oxide (PSC), and o an electrolyte layer, preferably formed from Yttria-stabilized zirconia (YSZ), Gadolinia- doped Ceria (GDC), Cerium Gadolinium Oxide (CGO), Scandia-stabilised Zirconia (ScSZ), or Lanthanum Strontium Gallium Magnesium Oxide (LSGM), said electrolyte layer being sandwiched between the fuel electrode layer and the oxidant electrode layer, wherein the fuel electrode layer is disposed on the support structure and is fluidical ly connected to the cell volume via said porous region.

[0014] As used herein the term "cell voltage" refers to the cell voltage produced by the electrochemical cell during operation.

[0015] As used herein the term "area specific resistance" is used as is common in the field of fuel cells. In particular, it represents the electrical resistance offered by the cell per unit of active area (typically measured in Ohm*cm2).

[0016] As used herein the term "electrochemical cell" may refer to a single cell unit, e.g., a single fuel cell. The "electrochemical cell" may also comprise a plurality of cells in a stack. Thus, the term "electrochemical cell" may refer to a cell stack comprising a plurality of cells. In embodiments comprising a plurality of cells in a stack, the cell voltage may be determined from a measured stack voltage. In embodiments comprising a plurality of cells in a stack, the area specific resistance of the cell may be an average area specific resistance of the stack.

[0017] The minimum cell voltage may be updated continuously during cell operation. The minimum cell voltage may be updated at selected time intervals during cell operation. The minimum cell voltage may be updated continuously over a first time period of the lifetime of the cell and at selected time interval over a second time period of the lifetime of the cell.

[0018] Advantageously, the method further comprises indicating an end-of-life state of the cell if the cell voltage is below the minimum cell voltage. Thus, the method may comprise a step of indicating an end- of-life state of the cell if the following condition is met: UCeii (t) < Umin(t), wherein UCeii(t) is the cell voltage at time t and Umin(t) is the minimum cell voltage at time t.

[0019] Indicating the end-of-life state may comprise various processes. For example, indicating the end-of-life state of the cell may comprise outputting a signal indicative of having reached the end-of-life state of the cell. Alternatively or additionally, indicating the end-of-life state of the cell may comprise changing or stopping cell operation. Thus, the method may comprise a step of changing or stopping cell operation if the cell voltage is below the minimum cell voltage. Changing cell operation may comprise reducing the current (and thus reducing cell power), preferably such that the cell voltage is kept at Umin or above. Advantageously, this ensures safe operation of the cell.

[0020] The minimum cell voltage may be updated based on a predetermined function representing the minimum cell voltage over time ( Umin(t)). Thus, updating the minimum cell voltage may comprise replacing a currently valid minimum cell voltage with an updated cell voltage based on a predetermined function representing the minimum cell voltage over time ( Umin(t)). In particular, updating the minimum cell voltage may be performed based on minimum cell voltage data representing the minimum cell voltage over time. The minimum cell voltage data may be derived from modelling a cell behaviour. The minimum cell voltage data may be stored in a non-transitory storage medium of a control system for controlling operation of the electrochemical cell.

[0021] Advantageously, the minimum cell voltage is updated depending on an area specific resistance of the cell determined, preferably measured. Preferably, updating the minimum cell voltage comprises determining, in particular measuring, an area specific resistance of the cell. As such, the area specific resistance may actually be measured during cell operation. This allows precise determination of the minimum cell voltage. In particular, it enables system-specific degradation effects to be taken into account. Determining the area specific resistance may be performed continuously or at selected time intervals over the lifetime of the cell.

[0022] In some embodiments, updating the minimum cell voltage comprises: determining, preferably measuring, an area specific resistance of the cell; determining an updated minimum cell voltage based on said area specific resistance; setting the updated minimum cell voltage as the minimum cell voltage. Thus, updating the minimum cell voltage may comprise replacing the currently valid minimum cell voltage with an updated minimum cell voltage depending on, preferably calculated based on, a measured area specific resistance of the cell.

[0023] The area specific resistance may be determined continuously. Alternatively, the area specific resistance of the cell may be determined at selected time intervals.

[0024] Preferably, updating the minimum cell voltage, in particular determining the updated minimum cell voltage, further comprises determining, preferably measuring, the current flowing through the cell.

[0025] Preferably, the minimum cell voltage at a time t ( Umin(t)) is determined using the following expression:

[0026] Umin(t) = UN- ASR(t)*j(t)

[0027] According to a second aspect, there is provided a method of operating an electrochemical cell, preferably a solid oxide fuel cell (SOFC), the method comprising: determining a cell voltage; determining a current flowing through the cell;

[0028] (optionally) determining an area specific resistance of the cell determining a minimum cell voltage corresponding to an end-of-life state of the cell, said minimum cell voltage being determined using the following expression: Umin(t) = UN- ASR(t)*j(t); comparing the cell voltage with the minimum cell voltage;

[0029] (optionally) indicating an end-of-life state of the cell, preferably stopping cell operation, if the cell voltage is below the minimum cell voltage.

[0030] The following advantages and optional features apply to the method of the first aspect and to the method of the second aspect respectively:

[0031] In the formula Umin(t) = UN- ASR(t)*j(t) as used herein:

[0032] 1. UNis a constant. UNmay vary from 0.5 to IV. In one example, UNis around 0.78V.

[0033] 2. j is the current flowing through the cell at time t.

[0034] 3. ASR is a resistance value or a function representing the area specific resistance of the cell at time t. UNmay be dependent on the operating temperature and the gas composition at fuel electrode (anode) side and / or air electrode (cathode) side, particularly the oxygen partial pressure on air electrode side. Preferably, UNrepresents the redoxpotential UredoxOf a fuel electrode material, more preferably the Ni / NiO redoxpotential, in particular at a given operating temperature and at a given gas composition at air electrode (cathode) side. In addition, UNmay include a correction factor UCOrr- Specifically, UNmay be calculated as follows: UN= Uredox + UCOrr- The value UCOrr preferably is a correction factor that accounts for concentration differences of the fuel within the cell. If the electrochemical cell comprises a plurality of cells in a stack, the value UCOrr may additionally account for concentration differences of the fuel within the cell stack.

[0035] The ASR value may correspond to the actual area specific resistance of the cell at time t (e.g., if determined continuously). The ASR value may also differ from the actual area specific resistance of the cell at time t (e.g., if determined at selected time intervals only). The ASR value may be a resistance value derived from the actual area specific resistance of the cell. In some embodiments, the ASR value may represent the area specific resistance of the cell corrected, in particular reduced, by a correction factor accounting for concentration gradients of the fuel in the cell, in particular due to diffusion barriers in the flow path between a fuel supply of the cell unit and the electrochemically active layers. Thus, ASR(t) may be calculated as follows: ASR(t) = (area specific resistance of the cell (t) - ASRCOrrj, wherein ASRcorr is a correction factor that particularly accounts for concentration gradients of the fuel in the cell. For example, in embodiments in which the electrochemically active layers are provided over a porous region of a support structure (see above), said porous region may form a diffusion barrier for fuel to flow from the cell volume to the electrochemically active layers. In such embodiments, ASRCOrr may be a correction factor accounting for a concentration gradient of the fuel that arises due said diffusion barrier.

[0036] Preferably, the ASR is updated, in particular at least at selected time intervals, over the lifetime of the cell.

[0037] The ASR may be updated based on a predetermined function representing the ASR value of the cell over time. For example, ASR data representing the ASR over time may be stored in a non-transitory storage medium of a control system for controlling operation of the electrochemical cell. Preferably, however, the ASR is a measurement-based value. More preferably, the ASR is determined by determining, preferably measuring, the area specific resistance of the cell.

[0038] Preferably, updating the ASR comprises determining, preferably measuring, the area specific resistance of the cell. Preferably, updating the ASR comprises determining, preferably measuring, the area specific resistance of the cell and setting the determined area specific resistance of the cell as updated ASR.

[0039] The area specific resistance of the cell may be determined continuously.

[0040] The area specific resistance of the cell may be determined at selected time intervals. In such embodiments, the ASR value may be set to, preferably kept constant at, the last determined area specific resistance. That is to say, in the time interval between two consecutive determinations of the area specific resistance, the ASR is set to the last determined area specific resistance, optionally corrected by a correction factor ASRCOrr- In other words, the ASR value remains constant until a new area specific resistance is determined. ASR(t) may be a step function.

[0041] Alternatively, the ASR may be extrapolated based on previously determined area specific resistance values.

[0042] The area specific resistance of the cell may be determined in various ways. The area specific resistance may be determined at normal cell operation. Alternatively, the area specific resistance may be determined in a specific measuring cycle, preferably with normal operation of the cell being interrupted during said measuring cycle.

[0043] In some exemplary embodiments, determining the area specific resistance of the electrochemical cell comprises a current-voltage measurement. Specifically, determining the area specific resistance of the electrochemical cell may comprise: setting a first current ji through the electrochemical cell and measuring the resulting voltage Ui, setting a second current j2through the electrochemical cell and measuring the resulting voltage U2, wherein the second current j2is different to the first current j

[0044] The area specific resistance of the electrochemical cell may then be determined as the ratio of change in voltage to change in current, optionally corrected by a correction factor accounting for concentration gradients within the cell. Preferably, the first and second currents are below a maximum current level, said maximum current level preferably being indicative of the begin of gas transport influences on the I- V characteristic of the cell. Preferably, the maximum current is the current at the transition point from a linear regime of the l-V-characteristic curve to a non-linear regime (mass transport or concentration regime). Preferably, the second current is lower than the first current.

[0045] In some exemplary embodiments, determining the area specific resistance of the electrochemical cell comprises: determining an l-V-characteristic (current-voltage-characteristic) of the electrochemical cell, wherein the area specific resistance of the electrochemical cell is determined as the ratio of change in voltage to change in current in a linear region (ohmic region) of the l-V-characteristic, optionally corrected by a correction factor accounting for concentration gradients within the cell. Preferably, the I- V-characteristic is determined by performing an l-V-sweep, preferably within a time span of 3 to 30 seconds.

[0046] Alternatively or additionally to the proposed current-voltage-measurements, the area specific resistance may be inferred from alternating current impedance spectroscopy ('ACIS '), also called electrochemical impedance spectroscopy 'EIS').

[0047] Preferably, other operating conditions of the electrochemical cell are unchanged during the area specific resistance determination, in particular during current-voltage-measurements. Said unchanged operating conditions preferably comprise temperature. Preferably, the area specific resistance determination is started at a predefined temperature and at predefined cell conditions, in particular current load and fuel utilisation.

[0048] Preferably, the area specific resistance is determined at a fuel utilisation of 30-50%. As used herein the term "fuel utilisation" refers to the relation between the amount of fuel gas reacting in the cell and the amount of fuel gas supplied.

[0049] According to a third aspect, there is provided a control system, preferably control unit, for controlling the operation of an electrochemical cell, the control system being configured and arranged to perform the method as described above. The control system may comprise a non-transitory storage device having control data stored thereon, said control data when executed, in particular by a computer of the control system, causing the control system to carry out the method described above.

[0050] As used herein a "system" may refer to a device or an assembly of several devices, which are in particular connected to form a device or integrated into a higher-level device.

[0051] The control system may comprise a device for determining a cell voltage. The control system may comprise a device for determining an area specific resistance of the cell. For example, the control system may comprise a device for performing current-voltage-measurements and / or a device for performing alternating current impedance spectroscopy. The control system may comprise a device for determining a current flowing through the cell. The control system may comprise a device, preferably including a processor and associated non-transitory memory, for determining a minimum cell voltage corresponding to an end-of-life state, preferably according to the expression Umin (t) = UN- ASR(t)*j(t).

[0052] According to a fourth aspect, there is provided an electrochemical cell system comprising: at least one electrochemical cell, preferably comprising a plurality of cells in a stack; and a control system adapted to control the operation of the at least one electrochemical cell, preferably a control system according to the third aspect.

[0053] According to a fifth aspect, there is provided a computer-implemented method of determining an end- of life state of an electrochemical cell, comprising: receiving cell voltage data representing a cell voltage of the cell at a time t, preferably acquired by a device for determining a cell voltage of the cell; receiving minimum cell voltage data representing a minimum cell voltage corresponding to an end-of-life state of the cell at time t; comparing the cell voltage data with the minimum cell voltage data; wherein receiving the minimum cell voltage data comprises receiving ASR data, preferably acquired by a device for determining an area specific resistance of the cell, representing an ASR value at time t, said ASR value representing an area specific resistance of the cell. Preferably, receiving minimum cell voltage data further comprises: receiving current data representing a current j flowing through the cell at time t, preferably acquired by a device for determining the current flowing through the cell.

[0054] Preferably, receiving minimum cell voltage data further comprises: calculating the minimum cell voltage based on the ASR data and the current data, preferably using the following formula:

[0055] Umin(t) = UN- ASR(t)*j(t).

[0056] Preferably, the method according to the fifth aspect further comprises outputting an end-of-life signal, if the cell voltage is below the minimum cell voltage. The end-of-life signal may cause the electrochemical cell to stop operation.

[0057] According to a sixth aspect, there is provided a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according the fifth aspect.

[0058] According to a seventh aspect, there is provided a non-transitory storage medium having stored thereon the computer program according to the sixth aspect.

[0059] Further embodiments are derivable from the following description and the drawings.

[0060] In the drawings:

[0061] Figure 1 shows a cross section through an exemplary electrochemical cell in the form of a metal- supported solid oxide fuel cell;

[0062] Figure 2 shows a typical l-V-characteristic of a fuel cell;

[0063] Figure 3 shows a flow diagram showing a method of operating an electrochemical cell.

[0064] Figure 1 shows an exemplary embodiment of an electrochemical cell unit 10 in the form of a metal- supported solid oxide fuel cell. The invention, however, is not limited to this type of electrochemical cell The cell 10 comprises an interconnector structure 12 and a support structure 14 in a cross-sectional view. In the described examples the interconnector structure 12 and the support structure 14 are plate like. Other configurations are possible. The interconnector structure 12 and the support structure 14 may also be referred to as interconnector plate 12 and support plate 14. The support structure 14 and the interconnector structure 12 overlay one another.

[0065] A cell volume 16 is delimited and enclosed by the support structure 14 and the interconnector structure 12. Preferably the support structure 14 is attached to the interconnector structure 12. In order to supply fluid to the cell volume 16, the cell unit 10 may comprise one or more fluid ports (not shown), e.g., in the form of through-holes.

[0066] Preferably the support structure 14 and the interconnector structure 12 are metallic and are connected by welding. Other materials and other methods of connection are possible in the sense of the invention at hand.

[0067] Electrochemically active layers 18 (also referred to as cell chemistry layers) are applied (e.g., coated or deposited) on the support structure 14. The support structure 14 is located between the cell chemistry layers 16 and the interconnector structure 12.

[0068] The electrochemically active layers 18 comprise a fuel electrode layer 20, an oxidant electrode layer 22 and an electrolyte layer 24 sandwiched between the fuel electrode layer 20 and the oxidant electrode layer 22.

[0069] The fuel electrode layer 20 is applied on the support structure 14 (i.e., is an innermost layer of electrochemically active layers, in other words a layer of the electrochemically active layers that is nearest to the support structure), the oxidant electrode layer 22 is an outermost layer of the electrochemically active layers (i.e., a layer of the electrochemically active layers that is furthest from the support structure 14). The electrolyte layer 24 is located between the fuel electrode layer 20 and oxidant electrode layer 22 and separates the fuel electrode layer 20 and oxidant electrode layer 22.

[0070] Exemplarily, the fuel electrode layer 20 is formed from a Gadolinia-doped ceria-nickel cermet or YSZ-Ni- cermet. Exemplarily, the electrolyte layer 24 is formed from Yttria-stabilized zirconia (YSZ). In other examples, the electrolyte layer 24 may be formed from Gadolinia-doped Ceria (GDC), Cerium Gadolinium Oxide (CGO), Scandia-stabilised Zirconia (ScSZ), or Lanthanum Strontium Gallium Magnesium Oxide (LSGM). Exemplarily, the oxidant (air) electrode layer 22 is formed from lanthanum cobalt nickel oxide (LCN). In other examples, the oxidant (air) electrode layer 22 may be formed from lanthanum strontium manganese oxide (LSM), lanthanum strontium cobalt iron oxide (LSCF), barium strontium cobalt iron oxide (BSCF), samarium strontium cobalt oxide (SSC), or praseodymium strontium cobalt oxide (PSC).

[0071] The cell volume 16 (providing an enclosed inner fluid volume / cell space of the cell 10) serves to supply operating fluid (e.g., fuel in case of fuel cells) to the electrochemically active layers 18. For this, the support structure 14 comprises a porous region 26 below the electrochemically active layers 18 such that fluid (fuel) may pass through the porous region 26 from the fluid volume (cell volume 16) to the electrochemical layers 18, specifically the fuel electrode layer 20. The porous region 26 may comprise a number of (e.g., laser-drilled) through holes 30.

[0072] During normal cell operation, fuel is supplied to the fuel electrode layer 20 via the cell volume 16 and the porous region, and oxidant, in particular air, is supplied to the oxidant electrode 22.

[0073] Typically, multiple of such cells 10 are stacked upon one another along a stacking direction 32 to form a cell stack.

[0074] Figure 2 shows a typical current-voltage characteristic (l-V-curve) of a fuel cell. As can be seen from Fig. 2, the l-V-curve comprises a linear region (ohmic regime) in between a non-linear activation region at lower current values and a non-linear concentration (or mass transport) region at higher current values. As will be discussed below, the area specific resistance of the cell 10 may be determined from said linear (ohmic) region.

[0075] Figure 3 is a flow diagram showing a method of operating an electrochemical cell. The operating method starts, preferably once the stack is in an operating condition, i.e. once a startup procedure has finished.

[0076] At the start, it may be assumed that Umin= UN, or ASR and the current are set to a default value when calculating Umin.

[0077] At step 100, Umin is updated. This is performed by determining the ASR and updating Umin by the formula Umin = UN- ASR*j. Step 100 may include sub-steps (not shown) of measuring the area specific resistance of the cell and the current as described above. Alternatively, step 100 may include estimating the ASR and / or the current based on previous measurements. It should be appreciated that a mixture of methods may be used, for example, interspacing measurements with predictions, or performing a certain number of measurements before switching to predicting. A learning approach may be used where measurements are used to improve a prediction algorithm. At block 102, the voltage of the cell Uceii is measured. Steps 100 and 102 may occur concurrently if the measurements can be performed independently.

[0078] The measured cell voltage UCeii value is then compared to the most recent value of Umin at step 104. If Uceii > Umin, the process returns to step 100, after an optional pause shown by step 106. If Uceii < Umin, the process stops at step 108 by indicating an end-of-life state. As set out above, indicating an end-of-life state may comprise outputting a signal indicative of having reached the end-of-life state of the cell and / or changing or stopping cell operation.

Claims

Claims1. A method of operating an electrochemical cell; the method comprising: determining a cell voltage; comparing the cell voltage with a minimum cell voltage corresponding to an end-of-life state of the cell; wherein the minimum cell voltage is updated over the lifetime of the cell depending on an area specific resistance of the cell.

2. The method according to claim 1, further comprising indicating an end-of-life state of the cel I, preferably changing or stopping cell operation, if the cell voltage is below the minimum cell voltage.

3. The method according to claim 1 or 1, wherein updating the minimum cell voltage comprises determining, preferably measuring, an area specific resistance of the cell.

4. The method according to any one of the preceding claims, wherein the minimum cell voltage is dependent on the current flowing through the cell, wherein updating the minimum cell voltage comprises determining, preferably measuring, the current flowing through the cell.

5. The method according to any one of the preceding claims, wherein the minimum cell voltage (Umin) at a time t is determined using the following expression:Umin(t) = UN- ASR(t)*j(t) where UNis a constant, preferably between 0.5 and IV; j is the current flowing through the cell at time t;ASR is a resistance value representing the area specific resistance of the cell at time t.

6. A method of operating an electrochemical cell; the method comprising: determining a cell voltage; determining a current flowing through the cell; determining a minimum cell voltage corresponding to an end-of-life state of the cell, said minimum cell voltage being determined using the following expression:Umin(t) = UN- ASR(t)*j(t) where UNis a constant, preferably between 0.5 and IV;j is the current flowing through the cell at time t,ASR is a resistance value representing the area specific resistance of the cell at time t. comparing the cell voltage with the minimum cell voltage; indicating an end-of-life state of the cel I, preferably stopping cell operation, if the cell voltage is below the minimum cell voltage.

7. The method according to the preceding claim, further comprising determining, preferably at selected time intervals, the area specific resistance of the cell.

8. The method according to any one of claims 5 to 7, wherein the ASR is determined by determining, preferably measuring, the area specific resistance of the cell.

9. The method according to any one of claims 5 to 8, wherein the ASR is updated, preferably at selected time intervals, over the lifetime of the cell.

10. The method according to the preceding claim, wherein updating the ASR comprises: determining, preferably measuring, the area specific resistance of the cell; setting said determined area specific resistance of the cell as new ASR.

11. The method according to any one of claims 5 to 10, wherein the area specific resistance of the cell is determined at selected time intervals, with the ASR being set to the last determined area specific resistance or being extrapolated based on previously determined area specific resistance values.

12. The method according to any one of the preceding claims, wherein determining the area specific resistance of the electrochemical cell comprises: setting a first current ji through the electrochemical cell and measuring the resulting voltage Ui, setting a second current jz through the electrochemical cell and measuring the resulting voltage U2, wherein the second current jz is different to the first current ji; and the area specific resistance of the electrochemical cell is determined from the ratio of change in voltage to change in current.

13. The method according to the preceding claim, wherein the first and second currents are below a maximum current level.

14. The method according to any of claims 12 or 13, wherein other operating conditions of the electrochemical cell are unchanged during determination of the area specific resistance.

15. The method according to the preceding claim, wherein the unchanged operating conditions comprise temperature and fuel utilization.

16. The method according to any one of the preceding claims, wherein the electrochemical cell comprises a plurality of cells in a stack, wherein the cell voltage is determined from a measured stack voltage and / or the area specific resistance of the cell is an average area specific resistance of the stack.

17. The method according to any one of the preceding claims, wherein the area specific resistance is inferred from alternating current impedance spectroscopy (ACIS)18. A control system for controlling the operation of an electrochemical cell, the control system being configured and arranged to perform the method according to any one of the preceding claims.

19. The control system according to the preceding claim, comprising: a device for determining a cell voltage; a device for determining an area specific resistance of the cell; a device for determining a minimum cell voltage corresponding to an end-of-life state; wherein the minimum cell voltage is dependent on the area specific resistance of the cell.

20. An electrochemical cell system comprising: at least one electrochemical cell; and a control system according to claim 18 or claim 19 adapted to control the operation of the at least one electrochemical cell.

21. A computer-implemented method of determining an end-of life state of an electrochemical cel I, comprising: receiving cell voltage data representing a cell voltage of the cel I, preferably acquired by a device for determining a cell voltage of the cell; receiving minimum cell voltage data representing a minimum cell voltage corresponding to an end-of-life state of the cell; comparing the cell voltage data with the minimum cell voltage data; wherein receiving the minimum cell voltage data comprises receiving ASR data, preferably acquired by a device for determining an area specific resistance of the cell, said ASR data representing an ASR value, said ASR value representing an area specific resistance of the cell.

22. The method according to the preceding claim, wherein receiving minimum cell voltage data further comprises: receiving current data representing a current j flowing through the cell [at time t], preferably acquired by a device for determining the current flowing through the cell; calculating the minimum cell voltage based on the ASR data and the current data, preferably using the following formula:Umin(t) = UN- ASR(t)*j(t) where UNis a constant, preferably between 0.5 and IV j is the current flowing through the cell at time t,ASR is a resistance value representing the area specific resistance of the cell at time t.

23. The method according to claim 21 or 22, further comprising outputting an end-of-life signal, preferably stopping cell operation, if the cell voltage is below the minimum cell voltage.

24. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to any one of claims 21 to 23.

25. A non-transitory storage medium having stored thereon the computer program according to the preceding claim.

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