WO2025122560A1

WO2025122560A1 - Anion exchange membrane electrolyzers and revival

Info

Publication number: WO2025122560A1
Application number: PCT/US2024/058379
Authority: WO
Inventors: Jose Andres Zamora ZELEDON
Original assignee: Twelve Benefit Corp
Current assignee: Twelve Benefit Corp
Priority date: 2023-12-06
Filing date: 2024-12-04
Publication date: 2025-06-12
Anticipated expiration: 2026-06-06

Abstract

Methods of reviving electrolytic cells after premature inoperability include returning an applied current density to reset level.

Description

ANION EXCHANGE MEMBRANE ELECTROLYZERS AND REVIVAL

INCORPORATION BY REFERENCE

[0001] A PCT Request Form is filed concurrently with this specification as part of the present application. Each application that the present application claims benefit of or priority to as identified in the concurrently filed PCT Request Form is incorporated by reference herein in its entirety and for all purposes.

BACKGROUND

[0002] Electrolytic reactors can be subject to unforeseen circumstances that can lead to premature cell death. Examples of such circumstances include operating conditions that cause flooding or drying of parts of the cell. Sharp changes in current and voltage, such as those caused by power failures, can lead to cell death.

[0003] Background and contextual descriptions contained herein are provided solely for the purpose of generally presenting the context of the disclosure. Much of this disclosure presents work of the inventors, and simply because such work is described in the background section or presented as context elsewhere herein does not mean that such work is admitted prior art.

SUMMARY

[0004] One aspect of the disclosure relates to a method for reviving a membrane electrode assembly (MEA) electrolyzer for carbon oxide (CO_X) reduction that includes determining that the MEA electrolyzer has been subjected to a condition causing inoperability; in response to determining that the MEA has been subjected to the condition causing inoperability, reducing an applied current density setpoint to a reset level to reduce applied current density to the MEA electrolyzer; allowing the voltage across the MEA electrolyzer to stabilize; and after the voltage stabilizes, gradually increasing the applied current density setpoint.

[0005] In some embodiments, the condition causing inoperability is a non-linear cell voltage increase at constant current. In some embodiments, the cell voltage reaches a maximum of about 5 V to about 10 V. In some embodiments, the method further including maintaining the applied current density setpoint at the reset level until the voltage is returned to an operational level. In some embodiments, the operational level is between 2 V and 3 V, endpoints included. In some embodiments, the reset level is between 0 and 5 mA/cm², endpoints included. [0006] In some embodiments, the method further includes maintaining the applied current density setpoint at the reset level until the MEA electrolyzer is operable. In some embodiments, gradually increasing the applied current density setpoint includes a stepped increase to an operating applied current density. In some embodiments, the applied current density is held at each step until the voltage stabilizes. In some embodiments, the applied current density is held at each step for at least 10 minutes.

[0007] In some embodiments, the MEA electrolyzer includes an anion-conducting polymer electrolyte membrane. In some embodiments, where the MEA electrolyzer is an anion- exchange membrane (AEM)-only MEA electrolyzer. In some embodiments, the anion- conducting polymer membrane includes a styrenic copolymer. In some embodiments, the anion-conducting polymer membrane includes a copolymer of polystyrene and a polymer including a positively charged amine and/or positively charged heterocyclic group.

[0008] Another aspect of the disclosure relates to a system including (a) a carbon oxide electrolyzer including at least one membrane electrode assembly (MEA) including (i) a cathode including a carbon oxide reduction catalyst that promotes reduction of a carbon oxide, (ii) an anode including a catalyst that promotes oxidation, and (iii) a polymer electrolyte membrane (PEM) layer disposed between the cathode and the anode; (b) a power source configured to control electrical current applied to carbon oxide reduction electrolyzer; and (c) one or more controllers configured to cause the system to: determine that the carbon oxide reduction electrolyzer has been subjected to a condition causing inoperability; in response to determining that the carbon oxide reduction electrolyzer has been subjected to the condition causing inoperability, reduce an applied current density setpoint to a reset level to reduce applied current density to the carbon oxide reduction electrolyzer; allow the voltage across the carbon oxide reduction electrolyzer to stabilize; and after the voltage stabilizes, gradually increase the applied current density setpoint.

[0009] In some embodiments, the condition causing inoperability is a non-linear cell voltage increase at constant current. In some embodiments, the one or more controllers are configured to cause the system to maintain the applied current density setpoint at the reset level until the voltage is returned to an operational level. In some embodiments, the operational level is between 2 V to 3 V, endpoints included. In some embodiments, the one or more controllers are configured to cause the system to gradually increase the applied current density by a stepped increase to an operating applied current density. In some embodiments, the one or more controllers are configured to cause the system to hold the applied current density at each step until the voltage stabilizes. In some embodiments, the one or more controllers are configured to cause the system to hold the applied current density at each step for at least 10 minutes.

[0010] In some embodiments, the PEM layer is an anion-conducting polymer electrolyte membrane. In some embodiments, the carbon oxide electrolyzer is an anion-exchange membrane (AEM)-only MEA electrolyzer. In some embodiments, the anion-conducting polymer membrane includes a styrenic copolymer. In some embodiments, the anion- conducting polymer membrane includes a copolymer of polystyrene and a polymer including a positively charged amine and/or positively charged heterocyclic group.

[0011] These and other aspects of the disclosure will be described further herein and with reference to the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figures 1 and 2 are flow diagram showing operations in reviving a membrane electrode assembly (MEA) electrolyzer according to various embodiments.

[0013] Figures 3 A and 3B shows example current density profiles during stages of a revival protocol according to various embodiments.

[0014] Figure 4 depicts a system for controlling the operation of a carbon oxide reduction reactor that includes a cell including an MEA.

[0015] Figures 5 and 6 illustrate example MEAs for use in CO_X reduction.

[0016] Figures 7A-7C depict examples of current setpoints and voltage responses at various stages of cell inoperability and revival or attempted revival according to various embodiments.

DETAILED DESCRIPTION

[0017] Electrolyzers containing polymer-based membrane electrode assemblies (MEAs) are designed to produce products through the electrochemical reduction of reactants at the cathode. For example, carbon oxide electrolyzers containing polymer-based MEAs are designed to produce oxygen at the anode from water and one or more carbon-based compounds through the electrochemical reduction of carbon dioxide or other carbon oxide at the cathode. As used herein, the term carbon oxide includes carbon dioxide (CO2), carbon monoxide (CO), carbonate ions (COv ). bicarbonate ions (HCOf), and any combinations thereof. Water electrolyzer containing polymer-based membrane electrode MEAs are designed to produce hydrogen at the cathode through the electrochemical reduction of water at the cathode. Many of the MEAs described below contain only an anion exchange polymer or multiple anion exchange polymers, optionally provided as a plurality of layers. [0018] Various examples of MEAs and MEA-based carbon oxide electrolyzers are described in the following references: Published PCT Application No. 2017/192788, published November 9, 2017, and titled “REACTOR WITH ADVANCED ARCHITECTURE FOR THE ELECTROCHEMICAL REACTION OF CO₂, CO, AND OTHER CHEMICAL COMPOUNDS,” Published PCT Application No. 2019/144135, published July 25, 2019, and titled “SYSTEM AND METHOD FOR CARBON DIOXIDE REACTOR CONTROL,” and , US Provisional Patent Application No. 62/939,960, filed November 25, 2019, and titled “MEMBRANE ELECTRODE ASSEMBLY FOR COX REDUCTION,” each of which is incorporated herein by reference in its entirety.

[0019] Described herein are electrolytic cells that are capable of being revived after premature cell death. Also described are cell revival procedures and systems for implementing the procedures. While the description below discusses carbon oxide electrolyzers, the electrolytic cells and procedures may be implemented for other electrochemical cells including water electrolyzers.

[0020] An electrolysis system may employ a power supply configured to provide a constant current and/or a constant voltage to a reduction cell. Constant current operation may provide a generally constant rate of products produced at the cathode and the anode. Under some operating conditions, a constant voltage operation may produce a variable amount of product because the current density can change while maintaining a constant voltage. In some implementations, cathode reduction product selectivity may be tuned by varying cell voltage.

[0021] For example, a constant or nominal current density at the cathode of a single electrolyzer cell is about 10 to 2000 mA/cm². In certain embodiments, a constant current density at the cathode of a single electrolyzer cell is about 20 to 600 mA/cm². In these ranges, the current density is defined for a geometrically smooth cathode active surface that does not account for pores or other surface texture.

[0022] In some cases, the current density may affect the selectivity of generated products. Some products may not be generated at low current densities and low cell voltages, and so a higher or lower current density may be chosen to favor or disfavor certain products. For example, a current density above about 200 mA/cm² may promote formation of ethylene over carbon monoxide in an AEM-only cell for CO₂ electrolysis. In some implementations, selectivity for ethylene is promoted (e.g., a majority product) at about 270 to 330 mA/cm² or about 300 mA/cm². Below about 200 mA/cm², CO and H₂ may be the major products.

[0023] Controlled deviations from a constant current and/or voltage may be implemented for various operating modes. Operating modes of a cell may include cell hydration (pre-break-in), break-in, normal operation, planned shut off, and extended shut off or storage. During normal operation, for example, current and/or voltage pulsing can be used to improve performance. In another example, a recovery process may be performed. A recovery process is a process that may be performed after an electrolyzer has been in service, operating under normal conditions, for a period of time such as a few thousand hours. After a recovery process is completed, an electrolyzer may transition back to normal operation. A recovery process may be performed repeatedly over the service life of an electrolyzer or over the life of one or more of its components such as its associated MEA(s), gas diffusion layer(s) (GDL), and flow field(s). For example, a recovery process may be performed every 1,000 to 10,000 hours of service life. In some implementations, a recovery process can include turning off or significantly reducing electrical current, followed by introducing water to the cathode, and resuming normal operation. A return to normal operation can include a current ramp-up or step-up, for example. Recovery processes are described in U.S. Patent Publication No. 2022/0267916, incorporated by reference herein.

[0024] Uncontrolled deviations from constant current and/or voltage can result in detrimental effects and premature cell death. Controlled, but significant, changes can also result in premature cell death. For example, sudden large current step changes (e.g., about 50-300 mA/cm² steps for a 25 cm² CO2 electrolyzer) can result in exponential cell voltage increase until maxing out (e.g., at about 5 to 10 V). Exponential voltage increase while maintaining the current density as constant typically indicates irreversible cell degradation.

Cell revival

[0025] Provided herein are methods of reviving cells after premature cell death due to changing operating conditions. Premature cell death as used herein is distinct from end-of-life cell death that is generally due to material degradation. The methods described herein work with a subset of cells, including certain AEM-only cells.

[0026] Figure 1 shows an example of a method 100 according to certain embodiments. In the method of Figure 1, the method 100 begins by determining in an operation 102 that an MEA electrolyzer has been subject to a condition causing inoperability. Examples of conditions are given above and include uncontrolled deviations from current and/or voltage setpoints. In the operation of commercial or industrial electrolyzers, for example, unmitigated power outages or other power disruptions can lead to inoperability. In some embodiments, a condition causing inoperability can be intentionally performed to determine if the MEA electrolyzer is capable of revival using the methods described herein. Operation 102 can involve determining that the cell is inoperable by measuring or observing an exponential voltage increase. In some embodiments, operation 102 can involve measuring or observing a sharp decline in product selectivity or a sharp change in pressure differential. However, in some embodiments, observing a voltage change is advantageous as it is continuously monitored and can provide near instantaneous signal of premature cell death.

[0027] The method 100 proceeds by adjusting the current setpoint to a reset level in an operation 104. The reset level is much lower than normal operating current density. It may be zero or a non-zero value at which no, or very little, reduction of the reactant occurs. The reset level is material dependent with examples being 0 to 5 mA/cm², endpoints inclusive. In some embodiments, the reset level is non-zero to maintain some control of current flow within the cell.

[0028] In some embodiments, operation 104 is performed promptly after the condition causing inoperability occurs. Leaving a cell in a maxed-out state can lead to degradation of material, so it can be advantageous to perform operation 104 within at most a few minutes of occurrence of the condition causing inoperability. In some embodiments, it may be performed within 60 seconds, 30 seconds, or 5 seconds. And in some embodiments, it may be performed automatically (e.g., within milliseconds) after an indication of cell inoperability and/or an operating condition that can cause cell inoperability is detected. In some embodiments, for example, current density may be lowered at the detection of a threshold voltage increase. An example of such a method is discussed further below with reference to Figure 2.

[0029] Operation 104 can revive the cell. Revival can be indicated, for example, by the voltage returning to a value that may be used during operation (e.g., between about 2 to 3 V for CO2 reduction). Once the voltage has returned to an operational level and/or another indication of cell revival is received, the current density may be slowly increased in an operation 106. A slow increase refers to an increase with small current steps and/or a low rate of increase. For example, current density may be increased at steps of between 5 and 25 mA/cm².

[0030] In some embodiments, each step is held until steady state conditions are reached or are approached. As current is moved from level to level, there is a voltage response. When voltage stabilizes, the current is increased to the next level. A stabilized voltage may exhibit some fluctuation around a particular level. For example, at a set current density, the voltage may be deemed stabilized when it is constant or fluctuates less than a predetermined amount over a period of time. In some examples, the predetermined amount of fluctuation that is indicative of the voltage having stabilized may be less than about 10%, less than about 8%, less than about 6%, less than about 4%, less than about 2%, less than about 0.5%, less than about 0.05%, or less than about 0.005%. The particular period of time may depend on the formulation, and be 5 minutes, 10 minutes, 15 minutes, 20 minutes, 30 minutes, 45 minutes, etc.

[0031] The increase in current can be dynamic (e.g., in response to measuring the voltage and determining it is stabilized) or preset (e.g., based on past experience and/or testing). The duration at each step can be a significant amount of time, for example, between 15-45 minutes. [0032] Shorter or longer durations may be used depending on the particular material formulation. For example, steady state may be reached after 5 minutes or 10 minutes. If a continuous ramp is used, it should be gradual enough to avoid triggering an exponential increase in voltage. A ramped increase with a rate of no more than 7 mA/cm² per hour, or no more than 20 mA/cm², or no more than 100 mA/cm² may be used. Stepped increases may be used to permit more control over the process. In some examples, stepped increases may be executed with uniform increases for each step taken (i.e., the change from level to level is uniform). Alternatively, in various examples, stepped increases may be executed with non- uniform increases for each step taken. For example, when non-uniform step increases are executed, smaller individual steps may be taken when initially increasing the current density from the reset value toward a current density that corresponds with electrochemical generation of desired/target reaction products. In such an example, larger individual steps may be taken as the current density approaches the current density that corresponds with electrochemical generation of desired/target reaction products.

[0033] During operations 104 and 106, reactant flow is generally continued. Additional description of electrical and non-electrical operating parameters during the revival protocol, including during operations 104 and 106, is provided further below.

[0034] In alternate embodiments, the method 100 may be modified such that after operation 102, the voltage is set to 0 volts, if a potentiostat is being used to control the voltage. (In embodiments in which a power source is used, it may be advantageous to lower current as turning the hardware “off’ digitally can allow the cell voltage to go negative as it discharges. This could lead to material damage.)

[0035] In some embodiments, a revival protocol is automatically performed once an operating parameter reaches a threshold value. The threshold value is one that generally indicates premature cell inoperability. Examples of operating parameters include voltage and rate of voltage change.

[0036] Figure 2 show operations in a method 200 of automatically performing a cell revival protocol. The method 200 begins with monitoring cell voltage during operation of the cell in an operation 202. Then at an operation 204, if a voltage-related parameter reaches a threshold value, the current is automatically decreased to a reset level as described above.

[0037] Examples of voltage-related parameters include cell voltage, increase in cell voltage over time, rate of increase in cell voltage over time, and slope or derivative of current density plot-voltage. From a qualitative standpoint, premature cell inoperability may be indicated by the voltage of the cell rapidly increasing (e.g., at more than a rate of 0.1 V/min or 1 V/min) while the current density applied to the cell is either increased or held constant. In various embodiments, as the current density and voltage are being monitored (e.g., by a processor), the slope of the current density-voltage plot may also be monitored (e.g., by employing a processor to plot/monitor the derivative of the current density-voltage plot). In such embodiments, premature cell inoperability may be indicated by the voltage of the cell rapidly increasing (e.g., at a rate of increase over time of more than 0.1 V/min² or 1 V/min²) while the current density applied to the cell is either increased or held constant.

[0038] In some embodiments, the threshold value may be set to be triggered with any deviation from linear change. Also, in an alternate embodiment, operation 204 can be modified to automatically set the voltage to zero as described above.

[0039] Once the voltage returns to an operational level, the current density may be increased in an operation 206. Operation 206 may be performed as described above with respect to operation 106 of Figure 1.

[0040] The revival procedures described above are effective for certain MEA electrolyzers. A description of MEA electrolyzers that are capable of being revived by the procedures is below. In some embodiments, a method may be performed to determine if a particular type of electrolyzer is capable of being revived. In such embodiments, an MEA electrolyzer may be subject to controlled test protocol to deliberately induce premature cell inoperability. A revival procedure as described above may then be performed to determine if the cell can be revived. An example of a controlled test protocol is turning power on and increasing current density by large steps (e.g., at least 50 mA/cm², at least 100 mA/cm² steps, at least 200 mA/cm³, or at least 300 mA/cm² steps) until exponential voltage decay is observed. A controlled test protocol may be implemented, for example, prior to installation in a large-scale industrial setting of electrolyzers having the same build. A system including the electrolyzers may be configured to apply a particular revival protocol in situations in which a test protocol establishes a revival protocol for the particular electrolyzer build. Electrolyzers

[0041] It has been found that certain MEA electrolyzers are capable of revival as described above while others are not. As described above, in some embodiments, a controlled test protocol may be implemented to determine if a cell is capable of being revived.

[0042] Certain electrolyzers that are capable of revival are described below. These electrolyzers are anion-exchange membrane (AEM) electrolyzers characterized by the following:

• A cathode gas diffusion electrode (GDE) including a gas diffusion layer (GDL) and a catalyst/ionomer. The catalyst is in the form of metal nanoparticles and the ionomer is an anion-conducting styrenic copolymer as described below.

• An anion-exchange (AEM) membrane. The AEM is a styrenic copolymer as described below.

• An anode including a porous transport layer or gas diffusion layer.

[0043] Examples of cathode-side GDLs are carbon paper GDLs such as Sigracet 39BB and Toray Carbon Paper 060 with Micro Porous Layer, both of which are available from FuelCell Store. These GDLs are fairly hydrophobic, which may contribute to the cell’s ability to be revived.

[0044] The catalyst can be chosen to favor or disfavor certain products. In some embodiments, gold (Au) and silver (Ag) nanoparticles may be used, for example, to reduce CO2 to produce CO. Other carbon-containing species (CCS) can be produced with example catalysts including copper (Cu), palladium (Pd), and zinc (Zn). Further examples of catalysts are given below. The metal nanoparticle catalysts can be supported on a conductive support. Examples include carbon support particles, such as Ketjenblack EC300J.

[0045] Metal nanoparticle size may vary with examples including particles having diameters between 1 nm and 50 nm or between 5 nm and 30 nm.

[0046] As noted above, in some embodiments, the GDE includes a styrenic anion-conducting ionomer. Examples of such ionomers include Sustainion anion AEMs available from Dioxide Materials.

[0047] Similarly, the membrane between the anode and cathode GDEs is an anion-conducting ionomer such as a Sustainion anion exchange membrane. The ionomer in the GDE may be the same ionomer as the membrane or a different ionomer. Further description of styrenic ionomers is given below.

[0048] In some embodiments, the anode may be a GDE including a carbon GDL, ionomer, and a catalyst. In some embodiments, the ionomer may be a Sustainion ionomer or other styrenic anion-conducting ionomer. In some embodiments, there may not be an ionomer. For example, the anode may be a GDE fabricated by coating iridium oxide on a titanium mesh.

[0049] To fabricate the cathode-side GDE, a catalyst ink including the ionomer, metal particles (supported or unsupported), and solvent (e.g., a water and ethanol mixture) is formulated and sprayed onto the GDL.

[0050] Once fabricated the GDE is pressed against an activated AEM, such as a Sustanion AEM. The anode is then pressed against the AEM.

[0051] The ionomer in the cathode GDE and the membrane may be a styrenic anion- conducting ionomer. In certain embodiments, the ionomer comprises or is a copolymer of polystyrene and polymer comprising a positively charged amine and/or positively charged heterocyclic group. In certain embodiments, the copolymer is a copolymer of polystyrene and a poly(vinylbenzyl-R), where R is or includes a positively charged amine and/or heterocyclic group. As noted above, certain such copolymers are sold as Sustainion anion exchange membranes, the ionomers are not limited to Sustainion anion exchange membranes. In some such embodiments, R is optionally substituted imidazolium:

[0052] Examples of positively charged amines and/or heterocycles can include optionally substituted ammonium, tetramethylammonium, tetraethylammonium, methylamine, dimethylamine, trimethylamine, triethylamine, ethylamine, pyridinium, piperidinium, pyrrolidinium, pyrazolium, imidazolium, quinolinium, isoquinolinium, acridinium, phenanthridinium, pyridazinium, pyrimidinium, pyrazinium, phenazinium, and morpholinium. Any of these may be optionally substituted. [0053] Further description of AEM-only MEAs is provided below with reference to Figure 5 and 6.

Electrical Conditions

[0054] An electrolysis system as described herein may employ a power supply configured to provide a constant current and/or a constant voltage to a reduction cell. Constant current operation may provide a generally constant rate of products produced at the cathode and the anode. Under some operating conditions, a constant voltage operation may produce a variable amount of product because the current density can change while maintaining a constant voltage. In some implementations, cathode reduction product selectivity may be tuned by varying cell voltage.

[0055] In certain embodiments, a constant or nominal current density at the cathode of a single electrolyzer cell is about 10 to 2,000 mA/cm². In certain embodiments, a constant current density at the cathode of a single electrolyzer cell is about 20 to 600 mA/cm². In these ranges, the current density is defined for a geometrically smooth cathode active surface that does not account for pores or other surface texture.

[0056] In some cases, the current density may affect the selectivity of generated products. Some products may not be generated at low current densities and low cell voltages, and so a higher or lower current density may be chosen to favor or disfavor certain products. For example, a current density above about 200 mA/cm² may promote formation of ethylene over carbon monoxide in an AEM-only configuration. In some implementations, selectivity for methane and/or ethylene is promoted (e.g., a majority product) at about 270 to 330 mA/cm² or about 300 mA/cm². Below about 200 mA/cm², CO and Fh may be the major products.

[0057] In some implementations, a power supply for an electrolyzer is configured to adjust current by stepping the cell current up and/or down, ramping the current to a cell up and/or down, and/or pulsing the current to a cell. In some implementations, a power supply for an electrolyzer is configured to adjust voltage by stepping the cell voltage up and/or down, ramping the cell voltage up and/or down, and/or pulsing the cell voltage.

[0058] In certain embodiments, the electrolyzer controller is configured to temporarily apply a positive current (i.e., temporarily run the cathode as an anode and vice versa). This may deplate (or otherwise oxidize away) impurities such as transition metals that might plate onto the cathode during operation. As an example, such impurities may originate in the anode water. Reversing the current may remove carbon oxide reduction product intermediates that may foul a cathode catalyst. [0059] PCT Patent Application No. PCT/US2019/067169, filed December 18, 2019, and titled “ELECTROLYZER AND METHOD OF USE,” (published as WO 2020/132064) describes embodiments involving controlling the electrical conditions of a carbon oxide electrolysis cell and is incorporated herein by reference in its entirety. PCT Patent Application No. PCT/US2022/070797 (published as WO 2022/183190), filed February 23, 2022 and titled “RECOVERY PROCEDURE FOR CARBON OXIDE ELECTROLYZERS,” also describes embodiments involving controlling the electrical conditions of a carbon oxide electrolysis cell and is incorporated herein by reference in its entirety.

[0060] To perform a cell revival procedure, the power supply may be configured to reduce current density to a reset level and/or reset the voltage to zero as described above. It should be noted that in some embodiments, a potentiostat may be used to control electrical conditions to the cell. A potentiostat may be used instead of a DC power supply especially in laboratory or other non-industrial settings.

Current Density and Profiles

Context and Stages of Operation

[0061] In some embodiments, the current applied to the MEA has a non-constant profile. The current profile can differ according to the operating mode, as described further below. Operating modes may include hydration (pre -break-in), break-in, normal operation, planned shut off, extended shut off or storage, revival protocol, or any combination thereof. Other cell operation parameters that may be adjusted during these operating modes — sometimes related to adjustments in the current — include (a) cathode gas composition, flow rate, and pressure, (b) anode water composition and flow rate, (c) temperature, or (d) any combination thereof. In some embodiments, voltage is controlled.

[0062] Applied current may be paused or pulsed during operation of the cell. Current pausing may also be referred to as off/on cycling, with the current turned off and then on one or more times. In some embodiments, the applied current is reduced to zero (i.e., turned off) during a current pause. In some embodiments, a current pause reduces the current to a non-zero level.

[0063] In some embodiments, before applying any current to the cell, the MEA goes through a hydration step. This may involve starting the reactant flows and optionally heating the cell (or stack) so that steady state can be reached before applying current. In some implementations, prior to assembling the stack or cell, the MEAs are soaked in water to begin hydrating the MEA. For a carbon dioxide electrolyzer, after assembly, the anode water and cathode CO2 flows and pressures are set. Flowing dry or humidified CO2 may be beneficial in this step, even if dry CO2 is used as an input during longer term operation. The anode outlet may be observed to confirm that there are no bubbles exiting the outlet. If there are, it indicates significant CO2 crossover (from a pinhole in the membrane) or a leak in the hardware. If the desired operating temperature is higher than ambient, then the cell may be heated to the desired temperature after starting the anode water flow. During this step, the MEA continues to hydrate at the desired temperature.

[0064] The break-in period refers to procedures applied to a MEA or stack for the first time until the operating conditions and performance match the desired, long-term setup. In some embodiments, the first time an MEA is used, a procedure that differs from typical operation may be employed. An MEA that has not been operated before may not be fully hydrated or changes in the structure may occur due to the temperature increase during operation. In some embodiments, the current is ramped up from a lower value to a higher value in a series of steps instead of jumping straight to the desired operational value. A gradual, linear ramp-up may also be used. The number of intermediate steps in a multi-step ramp up may be 1, 2, 3, 4, 5, or 6, for example. The duration at each step may be the same or differ.

[0065] In embodiments in which the operating temperature is reached before break-in (e.g., during a hydration period), the temperature may be held constant at this temperature. In other embodiments, the temperature may be ramped up during the break-in procedure.

[0066] Cycling the stack off and on during normal operation may be useful to maintain performance over extended periods of time. Examples of performance enhancement include increasing the current efficiency of the electrolyzer, increasing the voltage efficiency of the electrolyzer, providing a single pass conversion (less frequent pulsing increases the electrolyzer’s overall conversion/utilization), increasing the lifetime of the electrolyzer’s MEA, increasing the lifetime of other cell components such as the gas diffusion layer (GDL), and increasing selectivity for certain reactions.

[0067] In some embodiments, a current profile or current pause schedule is such that, the current-on period is significantly greater than the pauses periods. During current pauses, the cell voltage may be held at any of various values. In some cases, during a current pause, the anode and cathode are shorted (e.g., through the power supply or by connecting the electrodes with metal or other conductors) in which case the cell voltage is at or near 0 volts. In some cases, during a current pause, the anode and cathode are allowed to float and the cell’s voltage is its open circuit voltage under the prevailing conditions, e.g., between 0.8V- 1.4V, 0.8V-1.2V, or 0.9V-1.1V. According to various embodiments, the flow to the cathode and/or anode may be stopped or allowed to continue during a current pause. [0068] As described above, when a cell revival protocol is performed, the current density may be immediately dropped to the reset level, followed by a gradual increase once the voltage stabilizes. Figure 3 A shows an example of a current density profile as the current density (J) is dropped to the reset level. Figure 3B shows examples of current density profiles for a stepped or continuous ramp return to an operating level. Further examples of current density profiles and voltage responses are discussed below with respect to Figures 7A-7C.

[0069] In addition to cell revival, from time to time, depending on the use of the electrolysis system, planned shutoffs may be performed in which the system is shut off for a brief period and then turned back on. Examples of reasons for planned shutoffs include maintenance of some part of the system (e.g., changing filters on anode water recycle loop, replacing a flow controller, or testing a temperature sensor), a planned power outage, and a pause in a downstream process using products of CO_X reduction. Planned shutoffs may have relatively short shutoff periods lasting from, e.g., a few minutes to a few days.

[0070] At times it may be desirable for the system or stack to be shut off for an extended period. For example, a holiday shut down of the facility, movement of the system to a new facility, or interruption in CO_X supply. During this time, it is expected that the system could be completely disconnected from external inputs. Gases or aqueous solutions different than those used during normal operation could be sealed into the anode or cathode in this case. The start-up procedure after the extended shutoff or storage period can be the same as the break-in procedure described above.

Parameter Values

[0071] For context and in accordance with some embodiments, normal operation of a carbon oxide reduction cell may be performed at a voltage of about 0 to 10V (electrolytic), and/or at a cathode current density of about 0 to 2000mA/cm² (electrolytic). A cell may have normal open circuit voltage (resting voltage) in the range of about 0 to 2.5V. Note that unless otherwise specified herein, all current and voltages having positive values are provided for an electrolytic cell (i.e., cathodic current flows at the positive electrode, which is where carbon oxide is reduced).

[0072] The following parameters may characterize electrical pulsing during normal operation. Unless otherwise specified, the parameters may be implemented by controlling current and/or voltage. Note that if the electrolyzer operates under current control, applied current pulses will have corresponding voltage pulses, which may have different profiles than the current pulses. Similarly, if the electrolyzer operates under voltage control, applied voltage pulses may have corresponding, but different, current pulses. Magnitude and duration of pulses or pauses

[0073] Current pulsing may be performed using a current density cycle where a high current density is about 100 to 2000 mA/cm² or about 200 to 600mA/cm². A high current density state may be held for about 30 minutes to 1000 minutes, with each such state separated by a reduction in current or a pause. According to various embodiments, the current is paused at relatively frequent intervals (e.g., less than about 10 hours, or less than about 2 hours), or at relatively infrequent intervals (e.g., about 10 hours or more). The reduced current between the pulses may have a current density from about 1 to 100 mA/cm² and may be held for a period of time of about 0.5 seconds to 60 minutes. The cycle may be repeated for the duration of normal operation. Note that the low current density pauses may have a reverse direction; e.g., a positive (oxidizing) current at the cathode.

[0074] In some embodiments, the current pause period durations are significantly less than the current-on periods for high throughput. For example, the current-on periods may be at least twice, at least 3 times, at least 5 times, at least 10 times, at least 20 times, at least 50 times, at least 100 times, or at least 500 times greater than the current pause periods. In certain embodiments, the periodic pulsing/pulsing has a duty cycle of about 0.2-1.

[0075] As mentioned, cell voltage may be controlled to effect pulsing or pausing. As an example, voltage pulsing is implemented using cycle in which a high voltage state ranges from about 2.7 to 3.9V. In these or other examples, a low voltage state ranges from about 1.5to 2.7V. In some examples, the high voltage is held for about 30 minutes to 1000 minutes and/or the low voltage is held for about 5 minutes to 100 minutes. Such cycles may be repeated for the duration of normal operation. In certain embodiments, the periodic pulsing/pulsing has a duty cycle of about 0.2-1.

[0076] In certain embodiments, current pulsing helps remove liquid water from the cathode. The lowered current density may decrease the water being transported to the cathode. The operating current density may be about 200 to 600 mA/cm² for the majority of operating time, ranging from, e.g., about 65% to 95% of the total time. The paused current density is set to lower, e.g., from about 1 to 100 mA/cm², correspondingly, for a small portion of the total time, from, e.g., about 5% to 35%.

Stepped and ramping changes

[0077] Step changes or ramps (rising and falling) may be utilized during an initial break-in protocol, or a transition protocol between different current densities during pulsing, or before and after planned shutdown. Step changes may include 2 to 10 steps (e.g., about 2 to 5 steps). In some embodiments, step magnitudes are about 50 to 300 mA/cm². In some embodiments, step durations are about 1 minute to 300 minutes (e.g., about 30 to 150 minutes or about 60 to 120 minutes). A ramping protocol could include raising or dropping to the target current within about 1 second to 200 minutes. In some implementations, the ramps are linear.

[0078] In some embodiments, periods in which electrical pulsing or pausing occur are punctuated by periods when no pulsing or pausing occurs. Such alternating periods of pulsing/pausing and no pulsing/pausing may occur during normal operation, break-in, planned shutdowns, etc. Periods when no pulsing occurs may be employed as a second step break-in protocol before normal operations. As an example, a constant medium current density ranging from about 200 to 400mA/cm² may be applied for about 50 to 100 hours before pulsing protocol starts.

[0079] Pulses may have a reverse cell current (or polarity) in which the cathode temporarily operates at oxidative currents and voltages. A reverse potential pulse may be in the range of about 0 to -3.5V with a corresponding current density in the range of -10 to 0 mA/cm². The reverse pulse may have a duration of about 0 to 60 minutes. The reverse pulses may be implemented with the same frequency and/or other parameters as described herein for forward electrical pulsing. In some embodiments, reverse electrical pulses are interleaved with forward electrical pulses.

[0080] Some relevant values of pausing or pulsing parameters are provided in US Patent Application Publication No. 2020/0220185, filed December 18, 2019, which is incorporated herein by reference in its entirety.

[0081] The following parameters may characterize a planned shut down cycle. A shut-down cycle could be arranged every 100 to 10,000 hours of operation, the ‘off’ current status could be at absolute zero current (OCV mode) or at the minimal current status (short mode).

Non-electrical Parameter Pulsing

Context

[0082] Electrical current is not the only reactor condition that may be pulsed or paused. Examples of other reactor conditions that may be pulsed or paused include gas flow rate to the cathode, gas pressure to the cathode, cell temperature, and water flow to the anode. Nonelectrical parameter pulsing may be performed in synchronization with electrical pulsing, or may be performed independently of the electrical pulsing, if used. In some implementations, COx flow rate, electrical parameters, cell temperature, and COx pressure are pulsed independently or all together or in different combinations.

[0083] The mechanisms and effects of non-electrical parameter pausing or pulsing may overlap with those for electrical parameter pausing or pulsing. In certain embodiments, the mechanisms implicate “water management,” which may improve COx mass transfer. Water management can involve clearing water out of flow fields, gas diffusion layers, catalyst layers (the pores as mentioned above), and/or the MEAs. In certain embodiments, water management clears unwanted intermediates in liquid form. In certain embodiments, water management clears potential salt blockage when lowering gas flow.

Non-electrical Parameters Pulsing Ranges

[0084] The following are non-limiting examples of non-electrical parameter values that be used in pulsing or pausing embodiments.

Pressure magnitude of pulse

[0085] A reactor gas pressure may have a normal operating setpoint ranging from about 90 to 150 psi that is maintained for an operating period ranging from about minutes to hundreds of hours.

[0086] A reactor’ s gas pressure may, during a pulse or pause, have a lower gas pressure ranging from about 0 to 70 psi that is maintained for a period of time ranging from, e.g., about a few minutes to an hour, with or without applying current.

[0087] Such a cycle may repeat a number of times, e.g., at least about 5 times or at least about 10 times, during normal operation.

Duration of pulses

[0088] An electrolytic reactor may operate with cathode gas pressure at a normal (high) level for an operating period ranging from about 30 minutes to 1000 hours.

[0089] The reactor may operate at a lower cathode gas pressure for a period of time ranging from, e.g., about 5 minutes to 60 minutes, with or without applying current.

[0090] As an example, a carbon oxide reduction cell is operated at about 90 psi for about 45 minutes, then at about 0 psi (gauge) for about 5 minutes. Pulsing from normal operation 0 to 70psi has been found to help with water management.

Volumetric flow rate variation during pulsing

[0091] A gas flow rate to the cathode of an electrolytic reactor may have a normal operating setpoint ranging from, e.g., about 2 to 80 seem (for a cathode planar surface area of 1 cm², scalable) for a duration of about 30 minutes to 1000 hours. In some embodiments, the reactor gas flow rate increases to a higher flow rate ranging from, e.g., about 12 to 120 seem (for a cathode planar surface area of 1 cm², scalable). In some embodiments, the reactor gas flow rate decreases to a lower flow rate such as, e.g., about 0.4 to 4 seem (for a cathode planar surface area of 1 cm², scalable). The period of gas flow rate deviation (higher or lower than the normal operating setpoint) may be shorter than the period of normal gas flow rate. For example, the deviation gas flow rate may range from about 0.1 second to 12 hours, with or without current applying. As with other parameter variations, the reactor gas flow rate cycle may repeat multiple times.

[0092] In one example, a gas flow rate cycle includes a carbon oxide flow rate setpoint of about 1000 seem, which is maintained for about 45 minutes. In the example, the carbon oxide flow rate then increases to about 2000 seem for about 5 minutes. This cycle repeats over normal operation.

Temperature pulsing

[0093] In certain embodiments, a carbon oxide reduction electrolytic cell has a temperature that varies during normal operation. In some cases, the normal operating temperature is about 30-70C and a lower pause or pulse temperature is about 20-40C. In some cases, the normal operating temperature is maintained for about 1 to 100 days and the lower temperature is maintained for about 1 hour to 1 day.

[0094] As an example, a carbon oxide reduction electrolyzer may employ temperature variations as follows. The electrolyzer is operated at about 50 C for about 10 days, and then operated at about 30 C for about 1 day. This cycle may be repeated multiple times during normal operation of the electrolyzer. Adjusting the cell temperature may improve catalyst selectivity and change polymer electrolyte properties such as the water uptake and chemical transport rate, thereby promoting effective water management.

Ramp rate of pulses (rising and falling; linear and/or stepped)

[0095] Any of the gas pressure pulses, gas flow rate pulses, or temperature pulses may be realized by step changes or ramping.

Cell Recovery, Cell Protection, and Cell Revival

[0096] Electrical and non-electrical parameters during normal operation are discussed above. Deviations from normal operation may be performed in various contexts. One is cell revival, described above. Another is a recovery operation, which can include temporarily deviating from normal operating conditions to flow water or other liquid to the cathode and/or to flow a gas to the cathode under non-standard conditions. It has been found that flowing water to the cathode and/or flowing a gas (e.g., a gas other than the normal carbon oxide reactant) to cathode can facilitate a recovery in performance of a carbon oxide electrolyzer. Still further is a protective mode that may be invoked to mitigate damage to a cell.

Recovery and Protection

[0097] A recovery process may be performed after a carbon oxide electrolyzer has been in service, operating under normal conditions, for a period of time such as a few thousand hours. After a recovery process is completed, an electrolyzer may transition back to normal operation. A recovery process may be performed repeatedly over the service life of an electrolyzer or over the life of one or more of its components such as its associated MEA(s), gas diffusion layer(s) (GDL), and flow field(s). For example, a recovery process may be performed every 1,000 to 10,000 hours of service life. In one example, a recovery process includes the following sequence: pause electrical current to the electrolyzer, then flow water over the cathode, and then restart flow of electrical current to the electrolyzer. In another example, a recovery process includes the following sequence: pause electrical current to the electrolyzer, then flow gas over the cathode, then flow water over the cathode, then again flow gas over the cathode, and finally restart normal operation by flowing electrical current through the cell. Included below are a few further examples of recovery sequences.

[0098] In some examples, a recovery operation comprises contacting the cathode with water while no current flows to the cathode. In some implementations, a relatively small amount of current flows while water is present in the cathode. In some cases, this current flows in the reverse direction (anodic at the carbon oxide reduction cathode). As an example, no more than about 1 mA/cm² of current flows to the cathode in the reverse direction while water is present. In some examples, during a portion of the recovery process, water flows over the cathode, rather than quiescently contacting the cathode.

[0099] Deviation from normal operating procedure may be implemented during a protection mode. A protection mode may be used to protect the electrolyzer from detrimental effects of some unanticipated event such as loss of power to the electrolyzer. A carbon oxide electrolyzer may be placed in a protection mode when an unexpected event is determined to be occurring or likely to occur soon. If unmitigated, such unexpected events could damage the electrolyzer or infrastructure supporting the electrolyzer. In some implementations, any of the operations, or any combination of such operations, described herein for performing recovery may also be employed for the protection of a carbon oxide electrolyzer.

[0100] In some embodiments, an electrolyzer and/or associated control system implements a protection mode by (a) determining that an unexpected and potentially detrimental event is occurring or will likely occur in the future and such unexpected event will, if unmitigated, likely damage or degrade the carbon oxide electrolyzer; and (b) performing one or more protective operations on the carbon oxide electrolyzer that reduce the likelihood that the electrolyzer will be damaged or degraded if the unexpected event continues to occur or does in fact occur in the future. [0101] Examples of unexpected events that may trigger the protective operations include sudden decrease or loss of an input material such as anolyte or carbon oxide (e.g., CO2) gas decrease or loss of heating or cooling, and loss of power to the electrolyzer. A substantial decrease or loss of input material may require adjusting the power to the electrolyzer to produce open circuit voltage or no current. Loss of power to the electrolyzer may cause the electrolyzer to discharge from operating voltage to an uncontrolled voltage, such as open circuit voltage or zero voltage either rapidly or gradually.

[0102] Examples of protective operations to mitigate the impact of the unexpected event include applying a relatively low current density to the electrolyzer, transitioning the electrolyzer voltage to open circuit voltage and reducing or ramping down the current applied to the electrolyzer. Any of these protective operations may be applied for a limited time such as only while the unexpected event continues to occur or until the likelihood of such event occurring is substantially reduced.

[0103] In some embodiments, the protective operation reduces electrolyzer current density to a relatively small (in comparison to normal operation) forward current density of about 1-50 mA/cm² or about 5-25 mA/cm² (e.g., about 10 mA/cm²), or about 0.3% to 20% of the current density in normal operating conditions.

[0104] In some embodiments, the protective operation ramps down current to the electrolyzer. A ramp may have any form or slope. In some cases, the average ramp rate from full current (normal operation) to a final current is about 0.1 to 1 mA/cm² per minute, or about 1 to 10 mA/cm² per minute. In some cases, the ramping is stepped. The number of steps, the time duration of the steps, and the magnitude of the current density changes of the steps may vary. As an example, a ramp may have about 2 to 50 steps, or about 5 to 30 steps. As a further example, the duration of the steps may be about 1 to 100 seconds, or about 5 to 50 seconds. As a further example, the current magnitude of the steps may be about 0.1 to 10 mA/cm² or about 0.5 to 5 mA/cm².

[0105] In one example, a step profile reduces current density to an electrolyzer from a normal operating value (e.g., about 300 mA/cm² to 2 A/cm) via a sequence of steps, each having a much smaller value (e.g., about 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 mA/cm²) and each having a defined duration (e.g., about 30 seconds each), and then sets the final current output to maintain the electrolyzer at open circuit for about 5-10 minutes.

[0106] In some embodiments, an electrolyzer returns from recovery or protection mode to normal operating conditions via a current ramp. Such a return ramp may have any of the characteristics just identified for ramping current down but in the opposite direction, i.e., from lower current density to higher current density.

Process Parameters associated with Revival

[0107] Operations and process parameters associated with a cell revival protocol are discussed below. Revival is often performed on an MEA electrolyzer that had been very recently operating normally as described below.

Normal operation

[0108] During normal operation, an electrolyzer may be continuously run to generate desired products. In an industrial setting, such products may undergo one or more downstream processing operations to generate useful chemicals. Normal operation may include a set of normal operating conditions as described elsewhere herein. Such conditions may include (a) normal reactant gas flow, which may be characterized by normal levels of a reactant gas pressure and flow rate or flow velocity at the cathode, (b) a reactant gas composition, (c) a set temperature or temperature profile, (d) an electrical current or voltage magnitude, optionally with a non-constant waveform, or (e) any combination thereof. In some embodiments, during normal operation, the electrical current or voltage has a pulsed or paused profile in which the current magnitude at the electrolyzer is periodically temporarily decreased or increased.

[0109] Normal operation may comprise converting a carbon oxide in the reactant gas to a carbon-containing product. In some embodiments, the carbon oxide is CO2 and/or CO and the carbon-containing reduction product comprises CO, a hydrocarbon, and/or an organic oxygencontaining compound. Typically, during normal operation, liquid (e.g., water) is not introduced to the cathode via the carbon oxide inlet or other source outside the MEA. However, liquid in the form of mist or droplets may, during normal operation, contact the cathode along with the inlet gas.

[0110] As indicated above, recovery processes may be performed throughout the lifetime of the electrolyzer. An electrolyzer may be placed into protection mode to mitigate the effect of an unexpected events. Details of operating conditions for these modes are described in PCT Patent Application No. PCT/US2022/070797, incorporated by reference above.

Test mode

[0111] While revival may be performed on an electrolyzer that had been operating normally (until an unexpected event occurred to cause inoperability) in some embodiments, in others it may be performed on an electrolyzer operating in a test mode.

[0112] As described above, in some embodiments, cell revival is performed after a cell is deliberately rendered inoperable to determine if the cell is capable of revival. The operating mode prior to revival may be referred to as a test mode, in which the electrolyzer is operating but with one or more parameters adjusted to cause inoperability.

[0113] In some embodiments, a test mode is used not to deliberately render the cell inoperable but to determine optimal operating parameters, perform single or multi-variable operational testing, etc. In such embodiments, an event that unexpectedly causes inoperability may occur. As an example, current density may be increased significantly. In other examples, sudden and/or significant increases in pressure, flow rate, temperature, or relative humidity may be used.

Revival - electrical current reduction or stoppage

[0114] In certain embodiments, the revival protocol stops the flow of electrical current to the electrolyzer or reduces the magnitude of the current density. In some examples, a reduced current density at the cathode has a magnitude of at most about 10 mA/cm² of planar cathode surface area. While reducing the current density can be performed as a continuous or stepped ramp, it may be done in a single step to quickly reach the reset level.

Duration of current at reset level

[0115] In some embodiments, the duration of time that current is maintained at a reset level in a revival protocol is at least long enough for voltage to return to a normal operating range. According to various implementations, this can range from 1 to 45 minutes. This will depend on the material formulation and how quickly the current was reduced to the reset level.

Short Circuit

[0116] In some embodiments, a power source for powering a carbon oxide electrolyzer is short circuited during revival. A short circuit may occur when the electronic resistance is not large enough in the circuit to impede current flow between the anode and cathode. In such cases, the potential or potentials of the anode and cathode equalize; in other words, the cell voltage is 0 volts. When shorted, the electrolyzer discharges from a normal operating state or from open circuit voltage. During shorting of the electrolyzer, the cell voltage transitions to a level below the open circuit voltage.

Types of gases flowed to the cathode

[0117] In some implementations of a revival protocol, a gas flows to the cathode for a period of time after the electrical current is stopped or reduced. Sometimes this gas is referred to herein as a “revival gas.” In many embodiments, the revival gas has the same composition as the carbon oxide reactant that flows during normal operation, optionally at a different pressure and/or flow rate than employed in normal operation. For example, the gas flowed during normal operation and during the revival process contains carbon dioxide or carbon monoxide at a defined concentration. This allows the cell to reach steady state under operating conditions. [0118] In some cases, a revival gas that flows to the cathode during a revival protocol has composition that is different from that of the reactant gas. In some cases, compared to the reactant gas, the revival gas has a lower concentration of carbon oxide reactant. In some cases, the revival process gas contains an inert gas that is not present in (or is present at a different concentration in) the normal process gas. Examples of inert gases include noble gases (e.g., Ar, He, or Kr) or nitrogen. In some cases, the recovery process gas is or contains air. In some cases, the recovery gas contains an oxidative gas such as oxygen. In some cases, the oxidative gas is simply air, which may contain about 21% oxygen. In other cases, the oxidative gas is oxygen or a different oxidizer that is provided apart from air. For example, oxygen produced at the electrolyzer anode, during normal operation, may be used as an oxidative recovery gas. In some implementations, the revival gas is humidified. In some embodiments, component gases include carbon dioxide, air, water, an inert gas, or any combination thereof.

[0119] In some examples, the revival gas is 100% or pure reactant gas. In some examples, the revival gas is 100% or pure inert gas. In some examples, the revival gas comprises a reactant gas and an inert gas in any ratio. In some examples, the revival gas comprises an oxidative gas and an inert gas in any ratio. In some examples, the revival gas is a humidified gas having water vapor present in a concentration of about 0-2% by volume. In some cases, a humidified gas comprises a reactant gas, an inert gas, an oxidative gas, or any combination thereof.

Gas pressure at cathode

[0120] In some embodiments, after current stoppage or reduction, the pressure of a revival gas flowing to the cathode may be at a level up to normal operating pressure of the electrolyzer cell. In some embodiments, after current stoppage or reduction, the cathode gas back pressure is reduced to, e.g., 0 psig. Cathode gas back pressure may be controlled by a pressure regulator located downstream from the cathode in the gas flow path. After reducing the cathode gas back pressure, the revival gas may be present and optionally flowing under a pressure of about 0- 600 psig, or about 0-400 psig, or about 0-50 psig.

Gas flow rate going through the cathode

[0121] In certain embodiments in which a revival gas flows after reducing or stopping the electrical current, the gas may be flowed at a rate of about 0 to 50 sccm/cm² of planar cathode surface area, or about 10 to 30 sccm/cm² of planar cathode surface area. Note that the flow rate values presented herein are provided on a per surface area of cathode (e.g., per cm² of planar cathode surface). As a single example, the gas flow rate may be about 500 seem for an electrolyzer having 25cm² of cathode surface area. The gas flow rate may scale linearly or non-linearly with surface area of the cathode. The flow rate values presented here may be instantaneous flow rates or average flow rates.

Ramping up the current

[0122] Once the voltage stabilizes at the reset level, the current density is slowly returned to an operating value. The number of steps, the time duration of the steps, and the magnitude of the current density changes of the steps may vary. As an example, a ramp may have about 2 to 50 steps, or about 5 to 30 steps. As a further example, the current density may be increased at steps of between 0.1 mA/cm² and 25 mA/cm² or 5 mA/cm² and 25 mA/cm².

[0123] In some embodiments, each step is held until steady state conditions are reached or are approached. The period of time that each step is held can be fairly significant, for example, 15-45 minutes. Shorter or longer durations may be used depending on the particular material formulation. If a continuous ramp is used, it should be gradual enough to avoid triggering an exponential increase. A ramped increase with a rate of no more than 7 mA/cm² per hour, or no more than 20 mA/cm² per hour, or no more than 100 mA/cm² per hour may be used. Stepped increases may be used to allow reaching steady state at each step.

Recycle

[0124] As noted above, in many embodiments, the revival protocol includes flowing a reactant gas to the electrolyzer. Process conditions may generally be the same as during operation, in some embodiments, with the exception of the applied current density and/or voltage. Some reactant may be converted to one or more products and some may be unreacted. Unreacted reactant may be recycled in some embodiments.

System Embodiments

[0125] Figure 4 depicts a system 401 for controlling the operation of a carbon oxide reduction reactor 403 that may include a cell comprising a MEA such as any one or more of those described herein. The reactor may contain multiple cells or MEAs arranged in a stack. System 401 includes an anode subsystem that interfaces with an anode of reduction reactor 403 and a cathode subsystem that interfaces with a cathode of reduction reactor 403. System 401 is an example of a system that may be used with, or to implement, any of the methods or operating conditions described above.

[0126] As depicted, the cathode subsystem includes a carbon oxide source 409 configured to provide a feed stream of carbon oxide to the cathode of reduction reactor 403, which, during operation, may generate an output stream that includes product(s) of a reduction reaction at the cathode. The product stream may also include unreacted carbon oxide and/or hydrogen. See 408.

[0127] The carbon oxide source 409 is coupled to a carbon oxide flow controller 413 configured to control the volumetric or mass flow rate of carbon oxide to reduction reactor 403. One or more other components may be disposed on a flow path from flow carbon oxide source 409 to the cathode of reduction reactor 403. For example, an optional humidifier 404 may be provided on the path and configured to humidify the carbon oxide feed stream. Humidified carbon oxide may moisten one or more polymer layers of an MEA and thereby avoid drying such layers. Another component that may be disposed on the flow path is a purge gas inlet coupled to a purge gas source 417. In certain embodiments, purge gas source 417 is configured to provide purge gas during periods when current is paused to the cell(s) of reduction reactor 403. In some implementations, flowing a purge gas over an MEA cathode facilitates recovery of catalyst activity and/or selectivity. This may be due, at least in part, to flushing certain reaction intermediates off catalyst active sites and/or remove water from the cathode. Examples of purge gases include carbon dioxide, carbon monoxide, hydrogen, nitrogen, argon, helium, oxygen, and mixtures of any two or more of these.

[0128] During operation, the output stream from the cathode flows via a conduit 407 that connects to a backpressure controller 415 configured to maintain pressure at the cathode side of the cell within a defined range (e.g., about 50 to 800 psig, depending on the system configuration). The output stream may provide the reaction products 408 to one or more components (not shown) for separation and/or concentration.

[0129] In certain embodiments, the cathode subsystem is configured to controllably recycle unreacted carbon oxide from the outlet stream back to the cathode of reduction reactor 403. In some implementations, the output stream is processed to remove reduction product(s) and/or hydrogen before recycling the carbon oxide. Depending upon the MEA configuration and operating parameters, the reduction product(s) may be carbon monoxide, hydrogen, hydrocarbons such as methane and/or ethylene, oxygen-containing organic compounds such as formic acid, acetic acid, and any combinations thereof. In certain embodiments, one or more components, not shown, for removing water from the product stream are disposed downstream from the cathode outlet. Examples of such components include a phase separator configured to remove liquid water from the product gas stream and/or a condenser configured to cool the product stream gas and thereby provide a dry gas to, e.g., a downstream process when needed. In some implementations, recycled carbon oxide may mix with fresh carbon oxide from source 409 upstream of the cathode. [0130] As depicted in Figure 4, an anode subsystem is configured to provide an anode feed stream to an anode side of the carbon oxide reduction reactor 403. In certain embodiments, the anode subsystem includes an anode water source, not shown, configured to provide fresh anode water to a recirculation loop that includes an anode water reservoir 419 and an anode water flow controller 411. The anode water flow controller 411 is configured to control the flow rate of anode water to or from the anode of reduction reactor 403. In the depicted embodiment, the anode water recirculation loop is coupled to components for adjusting the composition of the anode water. These may include a water reservoir 421 and/or an anode water additives source 423. Water reservoir 421 is configured to supply water having a composition that is different from that in anode water reservoir 419 (and circulating in the anode water recirculation loop). In one example, the water in water reservoir 421 is pure water that can dilute solutes or other components in the circulating anode water. Pure water may be conventional deionized water or even ultrapure water having a resistivity of, e.g., at least about 15 MOhm-cm or over 18.0 MOhm-cm. Anode water additives source 423 is configured to supply solutes such as salts and/or other components to the circulating anode water.

[0131] During operation, the anode subsystem may provide water or other reactant to the anode of reactor 403, where it at least partially reacts to produce an oxidation product such as oxygen. The product along with unreacted anode feed material is provided in a reduction reactor outlet stream. Not shown in Figure 4 is an optional separation component that may be provided on the path of the anode outlet stream and configured to concentrate or separate the oxidation product from the anode product stream.

[0132] Other control features may be included in system 401. For example, a temperature controller may be configured to heat and/or cool the carbon oxide reduction reactor 403 at appropriate points during its operation. In the depicted embodiment, a temperature controller 405 is configured to heat and/or cool anode water provided to the anode water recirculation loop. For example, the temperature controller 405 may include or be coupled to a heater and/or cooler that may heat or cool water in anode water reservoir 419 and/or water in reservoir 421. In some embodiments, system 401 includes a temperature controller configured to directly heat and/or cool a component other than an anode water component. Examples of such other components in the cell or stack and the carbon oxide flowing to the cathode.

[0133] Depending upon the phase of the electrochemical operation, including whether current is paused to carbon oxide reduction reactor 403, certain components of system 401 may operate to control non-electrical operations. For example, system 401 may be configured to adjust the flow rate of carbon oxide to the cathode and/or the flow rate of anode feed material to the anode of reactor 403. Components that may be controlled for this purpose may include carbon oxide flow controller 413 and anode water controller 411.

[0134] In addition, depending upon the phase of the electrochemical operation including whether current is paused, certain components of system 401 may operate to control the composition of the carbon oxide feed stream and/or the anode feed stream. For example, water reservoir 421 and/or anode water additives source 423 may be controlled to adjust the composition of the anode feed stream. In some cases, additives source 423 may be configured to adjust the concentration of one or more solutes such as one or more salts in an aqueous anode feed stream.

[0135] In some cases, a temperature controller, such as controller 405, is configured to adjust the temperature of one or more components of system 401 based on a phase of operation. For example, the temperature of cell 403 may be increased or decreased during break-in, a current pause in normal operation, and/or storage.

[0136] In some embodiments, a carbon oxide electrolytic reduction system is configured to facilitate removal of a reduction cell from other system components. This may be useful with the cell needs to be removed for storage, maintenance, refurbishment, etc. In the depicted embodiments, isolation valves 425a and 425b are configured to block fluidic communication of cell 403 to a source of carbon oxide to the cathode and backpressure controller 415, respectively. Additionally, isolation valves 425c and 425d are configured to block fluidic communication of cell 403 to anode water inlet and outlet, respectively.

[0137] The carbon oxide reduction reactor 403 may also operate under the control of one or more electrical power sources and associated controllers. See block 433. Electrical power source and controller 433 may be programmed or otherwise configured to control current supplied to and/or to control voltage applied to the electrodes in reduction reactor 403. The current and/or voltage may be controlled to execute the current schedules and/or current profiles described elsewhere herein. For example, electrical power source and controller 433 may be configured to periodically pause current applied to the anode and/or cathode of reduction reactor 403. Any of the current profiles described herein may be programmed into power source and controller 433.

[0138] In certain embodiments, electric power source and controller 433 performs some but not all the operations necessary to implement desired current schedules and/or profiles in the carbon oxide reduction reactor 403. A system operator or other responsible individual may act in conjunction with electrical power source and controller 433 to fully define the schedules and/or profiles of current applied to reduction reactor 403. For example, an operator may institute one or more current pauses outside the set of current pauses programmed into power source and controller 433.

[0139] In certain embodiments, the electrical power source and controller acts in concert with one or more other controllers or control mechanisms associated with other components of system 401. For example, electrical power source and controller 433 may act in concert with controllers for controlling the delivery of carbon oxide to the cathode, the delivery of anode water to the anode, the addition of pure water or additives to the anode water, and any combination of these features. In some implementations, one or more controllers are configured to control or operate in concert to control any combination of the following functions: applying current and/or voltage to reduction cell 403, controlling backpressure (e.g., via backpressure controller 415), supplying purge gas (e.g., using purge gas component 417), delivering carbon oxide (e.g., via carbon oxide flow controller 413), humidifying carbon oxide in a cathode feed stream (e.g., via humidifier 404), flow of anode water to and/or from the anode (e.g., via anode water flow controller 411), and anode water composition (e.g., via anode water source 405, pure water reservoir 421, and/or anode water additives component 423).

[0140] In the depicted embodiment, a voltage monitoring system 434 is employed to determine the voltage across an anode and cathode of an MEA cell or across any two electrodes of a cell stack, e.g., determining the voltage across all cells in a multi-cell stack. The voltage determined in this way can be used to control the cell voltage during a current pause, inform the duration of a pause, etc. In certain embodiments, voltage monitoring system 434 is configured to work in concert with power supply 433 to cause reduction cell 403 to remain within a specified voltage range. For example, power supply 433 may be configured to apply current and/or voltage to the electrodes of reduction cell 403 in a way that maintains the cell voltage within a specified range during a current pause. If, for example, during a current pause, the cell’s open circuit voltage deviates from a defined range (as determined by voltage monitoring system 434), power supply may be configured to apply current or voltage to the electrodes to maintain the cell voltage within the specified range.

[0141] The voltage monitoring system 434 and power supply 433 may also act in concert to invoke protection mode or a revival protocol as described above.

[0142] A condition that may trigger protection mode is loss of power to the electrolyzer. Under such a condition, it may be desirable to apply a small current to the electrolyzer while power is otherwise unavailable. To accomplish this, some electrolyzer systems include an uninterruptible power supply (UPS) which may include a power source such as a battery or battery pack having a capacity sufficient to provide at least limited amounts of current to the electrolyzer. As indicated, supplying such current may mitigate problems or potential problems created by unforeseen interruptions such as a power outage.

[0143] In some embodiments, a UPS is directly integrated with a carbon oxide electrolyzer or a group of electrolyzers. Some industrial scale carbon oxide electrolyzer systems may employ a dedicated UPS. Examples of industrial scale electrolyzers include those configured to consume at least about 100 kg of carbon dioxide per day. In some cases, such industrial scale carbon oxide electrolysis systems can operate off the power of about 100 kW or greater.

[0144] If the voltage monitoring system 434 indicates a condition has occurred that results in inoperability of the electrolyzer, a revival protocol may be invoked. In such situations, a controller and power supply 433 and/or UPS may be configured to reduce current to a reset level as described above.

[0145] An electrolytic carbon oxide reduction system such as that depicted in Figure 4 may employ a control system that includes one or more controllers and one or more controllable components such as pumps, sensors, dispensers, valves, and power supplies. Examples of sensors include pressure sensors, temperature sensors, flow sensors, conductivity sensors, voltmeters, ammeters, electrolyte composition sensors including electrochemical instrumentation, chromatography systems, optical sensors such as absorbance measuring tools, and the like. Such sensors may be coupled to inlets and/or outlets of an MEA cell (e.g., in a flow field), in a reservoir for holding anode water, pure water, salt solution, etc., and/or other components of an electrolytic carbon oxide reduction system.

[0146] Among the various functions that may be controlled by one or more controllers are: applying current and/or voltage to a carbon oxide reduction cell, controlling backpressure on an outlet from a cathode on such cell, supplying purge gas to a cathode inlet, delivering carbon oxide to the cathode inlet, humidifying carbon oxide in a cathode feed stream, flowing anode water to and/or from the anode, and controller anode feed composition. Any one or more of these functions may have a dedicated controller for controlling its function alone. Any two or more of these functions may share a controller. In some embodiments, a hierarchy of controllers is employed, with at least one master controller providing instructions to two or more component controllers. For example, a system may comprise a master controller configured to provide high level control instructions to (i) a power supply to a carbon oxide reduction cell, (ii) a cathode feed stream flow controller, and (iii) an anode feed stream flow controller. For example, a programmable logic controller (PLC) may be used to control individual components of the system. [0147] In certain embodiments, a control system is configured to apply current to a carbon oxide reduction cell comprising an MEA in accordance with a current schedule, which may have any of the characteristics described herein. For example, the current schedule may provide periodic pauses in the applied current. In some cases, the control system provides the current pauses with defined profiles such as ramps and/or step changes as described herein.

[0148] In certain embodiments, a control system is configured to control the flow rate of one or more feed streams (e.g., a cathode feed stream such as a carbon oxide flow and an anode feed stream) in concert with a current schedule. For example, the flow of carbon oxide or a purge gas may be turned on, turned off, or otherwise adjusted when current applied to an MEA cell is paused.

[0149] In certain embodiments, a control system may be configured to implement a revival protocol as described herein. Such control system may be configured to pause or reduce current, flow a revival gas, resume normal operation, or any combination thereof. The controller may be configured to control the initiation of a revival sequence, control the duration of any operation in a revival protocol, etc.

[0150] A controller may include any number of processors and/or memory devices. The controller may contain control logic such software or firmware and/or may execute instructions provided from another source. A controller may be integrated with electronics for controlling operation the electrolytic cell before, during, and after reducing a carbon oxide. The controller may control various components or subparts of one or multiple electrolytic carbon oxide reduction systems. The controller, depending on the processing requirements and/or the type of system, may be programmed to control any of the processes disclosed herein, such as delivery of gases, temperature settings (e.g., heating and/or cooling), pressure settings, power settings (e.g., electrical voltage and/or current delivered to electrodes of an MEA cell), liquid flow rate settings, fluid delivery settings, and dosing of purified water and/or salt solution. These controlled processes may be connected to or interfaced with one or more systems that work in concert with the electrolytic carbon oxide reduction system.

[0151] In various embodiments, a controller comprises electronics having various integrated circuits, logic, memory, and/or software that receive instructions, issue instructions, control operations described herein. The integrated circuits may include chips in the form of firmware that store program instructions, digital signal processors (DSPs), chips defined as application specific integrated circuits (ASICs), and/or one or more microprocessors, or microcontrollers that execute program instructions (e.g., software). Program instructions may be instructions communicated to the controller in the form of various individual settings (or program files), defining operational parameters for carrying out a process on one or more components of an electrolytic carbon oxide reduction system. The operational parameters may, in some embodiments, be part of a recipe defined by process engineers to accomplish one or more processing steps during generation of a particular reduction product such as carbon monoxide, hydrocarbons, and/or other organic compounds.

[0152] The controller, in some implementations, may be a part of, or coupled to, a computer that is integrated with, coupled to the system, otherwise networked to the system, or a combination thereof. For example, the controller may utilize instructions stored remotely (e.g., in the “cloud”) and/or execute remotely. The computer may enable remote access to the system to monitor current progress of electrolysis operations, examine a history of past electrolysis operations, examine trends or performance metrics from a plurality of electrolysis operations, to change parameters of current processing, to set processing steps to follow a current processing, or to start a new process. In some examples, a remote computer (e.g. a server) can provide process recipes to a system over a network, which may include a local network or the internet. The remote computer may include a user interface that enables entry or programming of parameters and/or settings, which are then communicated to the system from the remote computer. In some examples, the controller receives instructions in the form of data, which specify parameters for each of the processing steps to be performed during one or more operations.

[0153] The controller may be distributed, such as by comprising one or more discrete controllers that are networked together and working towards a common purpose, such as applying current to an MEA cell and other process controls described herein. An example of a distributed control system for such purposes includes one or more processors on a system for electrolytically reducing a carbon oxide and one or more processors located remotely (such as at the platform level or as part of a remote computer) that combine to control a process.

[0154] Controllers and any of various associated computational elements including processors, memory, instructions, routines, models, or other components are sometimes described or claimed as “configured to” perform a task or tasks. In such contexts, the phrase “configured to” is used to connote structure by indicating that the component includes structure (e.g., stored instructions, circuitry, etc.) that performs a task or tasks during operation. As such, a controller and/or associated component can be said to be configured to perform the task even when the specified component is not necessarily currently operational (e.g., is not on).

[0155] Controllers and other components that are “configured to” perform an operation may be implemented as hardware — for example, circuits, memory storing program instructions executable to implement the operation, etc. Additionally, controllers and other components “configured to” perform an operation may be implemented as hardware that is manipulated by software and/or firmware (e.g., an FPGA or a general-purpose processor executing software) to operate in manner that is capable of performing the recited task(s). Additionally, “configured to” can refer to one or more memories or memory elements storing computer executable instructions for performing the recited task(s). Such memory elements may include memory on a computer chip having processing logic.

MEA Configurations

[0156] In certain embodiments, an MEA has a cathode layer, an anode layer, and a polymer electrolyte membrane (PEM) between the anode layer and the cathode layer. The polymer electrolyte membrane provides ionic communication between the anode layer and the cathode layer, while preventing electronic communication, which would produce a short circuit.

[0157] Examples of MEAs include MEAs that include cation-conducting membranes, MEAs that include cation-conducting membranes, and MEAs that include bipolar membranes. Any MEA that is shown to be capable of revival using a test protocol may be revived according to the description herein, including ones in which the PEM is a cation-exchange membrane or a bipolar membrane. However, it has been found that the revival protocol may work disproportionately for MEAs in which the PEM is an anion-exchange membrane (AEM). Such MEAs are referred to as AEM-only MEAs.

[0158] An anion-exchange membrane (AEM)-only (AEM-only) MEA allows conduction of anions across the MEA. In embodiments in which none of the MEA layers has significant conductivity for cations, hydrogen ions have limited mobility in the MEA. In some implementations, an AEM-only membrane provides a high pH environment (e.g., at least about pH 7) and may facilitate CO2 and/or CO reduction by suppressing the hydrogen evolution parasitic reaction at the cathode. As with other MEA designs, the AEM-only MEA allows ions, notably anions such as hydroxide ions, to move through polymer-electrolyte. The pH may be lower in some embodiments; a pH of 4 or greater may be high enough to suppress hydrogen evolution. The AEM-only MEA also permits electrons to move to and through metal and carbon in catalyst layers. In embodiments, having pores in the anode layer and/or the cathode layer, the AEM-only MEA permits liquids and gas to move through pores.

[0159] In certain embodiments, the AEM-only MEA comprises an anion-exchange polymer electrolyte membrane with an electrocatalyst layer on either side: a cathode and an anode. In some embodiments, one or both electrocatalyst layers also contain anion-exchange polymer- electrolyte. [0160] In certain embodiments, an AEM-only MEA is formed by depositing cathode and anode electrocatalyst layers onto porous conductive supports such as gas diffusion layers to form gas diffusion electrodes (GDEs) and sandwiching an anion-exchange membrane between the gas diffusion electrodes.

[0161] In certain embodiments, an AEM-only MEA is used for CO2 reduction. The use of an anion-exchange polymer electrolyte avoids low pH environment that disfavors CO2 reduction. Further, water is transported away from the cathode catalyst layer when an AEM is used, thereby preventing water build up (flooding) which can block reactant gas transport in the cathode of the cell. In certain embodiments, an AEM-only MEA is employed in CO reduction reactions. Unlike the CO2 reduction reaction, CO reduction does not produce carbonate or bicarbonate anions that could transport to the anode and release valuable reactant.

[0162] Figure 5 illustrates an example construction of a CO_X reduction MEA 501 having a cathode catalyst layer 503, an anode catalyst layer 505, and an anion-conducting PEM 507. In certain embodiments, cathode catalyst layer 503 includes metal catalyst particles (e.g., nanoparticles) that are unsupported or supported on a conductive substrate such as carbon particles. In some implementations, cathode catalyst layer 503 additionally includes an anion- conducting polymer. The metal catalyst particles may catalyze CO_X reduction, particularly at pH greater than a threshold pH, which may be pH 4-7, for example, depending on the catalyst. In certain embodiments, anode catalyst layer 505 includes metal oxide catalyst particles (e.g., nanoparticles) that are unsupported or supported on a conductive substrate such as carbon particles. In some implementations, anode catalyst layer 503 additionally includes an anion- conducting polymer. Examples of metal oxide catalyst particles for anode catalyst layer 405 include iridium oxide, nickel oxide, nickel iron oxide, iridium ruthenium oxide, platinum oxide, and the like. Anion-conducting PEM 507 may include any of various anion-conducting polymers such as, for example, Sustainion by Dioxide Materials, and the like. As illustrated in Figure 5, CO_X such as CO2 gas may be provided to cathode catalyst layer 503. In certain embodiments, the CO2 may be provided via a gas diffusion electrode. At the cathode catalyst layer 503, the CO2 reacts to produce reduction product indicated generically as C_xO_yH_z. Anions produced at the cathode catalyst layer 503 may include hydroxide, carbonate, and/or bicarbonate. These may diffuse, migrate, or otherwise move to the anode catalyst layer 505. At the anode catalyst layer 505, an oxidation reaction may occur such as oxidation of water to produce diatomic oxygen and hydrogen ions. In some applications, the hydrogen ions may react with hydroxide, carbonate, and/or bicarbonate to produce water, carbonic acid, and/or C0₂. Fewer interfaces give lower resistance. In some embodiments, a highly basic environment is maintained for C2 and C3 hydrocarbon synthesis.

[0163] Figure 6 illustrates an example construction of a CO reduction MEA 601 having a cathode catalyst layer 603, an anode catalyst layer 605, and an anion-conducting PEM 607. Overall, the constructions of MEA 601 may be similar to that of MEA 501 in Figure 5. However, the cathode catalyst may be chosen to promote a CO reduction reaction, which means that different reduction catalysts would be used in CO and CO₂ reduction embodiments.

[0164] In some embodiments, an AEM-only MEA may be advantageous for CO reduction. The water uptake number of the AEM material can be selected to help regulate moisture at the catalyst interface, thereby improving CO availability to the catalyst. AEM-only membranes can be favorable for CO reduction due to this reason. In various embodiments, cathode catalyst layer 603 includes metal catalyst particles (e.g., nanoparticles) that are unsupported or supported on a conductive substrate such as carbon particles. In some implementations, cathode catalyst layer 603 additionally includes an anion-conducting polymer. In certain embodiments, anode catalyst layer 605 includes metal oxide catalyst particles (e.g., nanoparticles) that are unsupported or supported on a conductive substrate such as carbon particles. In some implementations, anode catalyst layer 603 additionally includes an anion- conducting polymer. Examples of metal oxide catalyst particles for anode catalyst layer 605 may include those identified for the anode catalyst layer 505 of Figure 5. Anion-conducting PEM 607 may comprise any of various anion-conducting polymer such as, for example, those identified for the PEM 507 of Figure 5. As illustrated in Figure 6, CO gas may be provided to cathode catalyst layer 603. In certain embodiments, the CO may be provided via a gas diffusion electrode. At the cathode catalyst layer 603, the CO reacts to produce reduction product indicated generically as C_xO_yH_z. Anions produced at the cathode catalyst layer 603 may include hydroxide ions. These may diffuse, migrate, or otherwise move to the anode catalyst layer 605. At the anode catalyst layer 605, an oxidation reaction may occur such as oxidation of water to produce diatomic oxygen and hydrogen ions. In some applications, the hydrogen ions may react with hydroxide ions to produce water. While the general configuration of the MEA 601 is similar to that of MEA 501, there are certain differences in the MEAs. First, MEAs may be wetter for CO reduction, helping keep the polymer electrolyte hydrated. Also, for CO2 reduction, a significant amount of CO2 may be transferred to the anode for an AEM- only MEA such as shown in Figure 5. For CO reduction, there is less likely to be significant CO gas crossover. In this case, the reaction environment could be very basic. MEA materials, including the catalyst, may be selected to have good stability in high pH environment. In some embodiments, a thinner membrane may be used for CO reduction than for CO2 reduction.

Examples

[0165] Figures 7A-7C depict examples of current setpoints and voltage responses at various stages of cell inoperability and revival or attempted revival. First, in Figure 7A, the voltage response of an electrochemical cell that enters into a condition of inoperability as a result of the application of current thereto is depicted. More specifically, as the current setpoint is gradually increased, the cell voltage response begins to exhibit a non-linear response that eventually reaches a maximum voltage of the hardware of the electrochemical cell.

[0166] Figures 7B and 7C depict examples of current setpoints and voltage responses for successful and unsuccessful revival protocols. As depicted in Figure 7B, as the current setpoint is gradually increased from a lesser value (e.g., the reset value) toward a greater value (e.g., a target value), the cell voltage responds with a corresponding gradual increase that may be primarily characterized by change that eventually stabilizes. By contrast, as depicted in Figure 7C, as the current setpoint is gradually increased from the lesser value (e.g., the reset value) toward the greater value (e.g., the target value), the cell voltage responds with a non-linear increase that mimics the voltage response in Figure 7A. In various examples of electrochemical cells that do not revive in response to the protocols/methods discussed herein, the cell voltage may reach the maximum value for the hardware prior to attaining the target value of the current setpoint.

Other Embodiments

[0167] Although omitted for conciseness, embodiments of the system and/or method can include every combination and permutation of the various system components and the various method processes, wherein one or more instances of the method and/or processes described herein can be performed asynchronously (e.g., sequentially), concurrently (e.g., in parallel), or in any other suitable order by and/or using one or more instances of the systems, elements, and/or entities described herein.

[0168] As a person skilled in the art will recognize from the previous detailed description and from the figures and claims, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of this invention defined in the following claims.

Claims

CLAIMS What is claimed is:

1. A method for reviving a membrane electrode assembly electrolyzer (MEA) for COx reduction comprising: determining that the MEA electrolyzer has been subjected to a condition causing inoperability; in response to determining that the MEA has been subjected to the condition causing inoperability, reducing an applied current density setpoint to a reset level to reduce applied current density to the MEA electrolyzer; allowing the voltage across the MEA electrolyzer to stabilize; and after the voltage stabilizes, gradually increasing the applied current density setpoint.

2. The method of claim 1, wherein the condition causing inoperability is a nonlinear cell voltage increase at constant current.

3. The method of claim 2, wherein the voltage reaches a maximum of about 5 V to about 10 V.

4. The method of claim 2, further comprising maintaining the applied current density setpoint at the reset level until the voltage is returned to an operational level.

5. The method of claim 4, wherein the operational level is between 2 V and 3 V, endpoints included.

6. The method of claim 1, wherein the reset level is between 0 and 5 mA/cm², endpoints included.

7. The method of claim 1, further comprising maintaining the applied current density setpoint at the reset level until the MEA electrolyzer is operable.

8. The method of claim 1, wherein gradually increasing the applied current density setpoint comprises a stepped increase to an operating applied current density.

9. The method of claim 8, wherein the applied current density is held at each step until the voltage stabilizes.

10. The method of claim 8, wherein the applied current density is held at each step for at least 10 minutes.

11. The method of claim 1, wherein the MEA electrolyzer comprises an anion- conducting polymer electrolyte membrane.

12. The method of claim 11, where the MEA electrolyzer is an anion-exchange membrane (AEM)-only MEA electrolyzer.

13. The method of claim 11, wherein the anion-conducting polymer membrane comprises a styrenic copolymer.

14. The method of claim 11, wherein the anion-conducting polymer membrane comprises a copolymer of polystyrene and a polymer comprising a positively charged amine and/or positively charged heterocyclic group.

15. A system comprising:

(a) a carbon oxide electrolyzer comprising at least one membrane electrode assembly (MEA) comprising (i) a cathode comprising a carbon oxide reduction catalyst that promotes reduction of a carbon oxide, (ii) an anode comprising a catalyst that promotes oxidation, and (iii) a polymer electrolyte membrane (PEM) layer disposed between the cathode and the anode;

(b) a power source configured to control electrical current applied to carbon oxide reduction electrolyzer; and

(c) one or more controllers configured to cause the system to: determine that the carbon oxide reduction electrolyzer has been subjected to a condition causing inoperability; in response to determining that the carbon oxide reduction electrolyzer has been subjected to the condition causing inoperability, reduce an applied current density setpoint to a reset level to reduce applied current density to the carbon oxide reduction electrolyzer; allow the voltage across the carbon oxide reduction electrolyzer to stabilize; and after the voltage stabilizes, gradually increase the applied current density setpoint.

16. The system of claim 15, wherein the condition causing inoperability is a nonlinear cell voltage increase at constant current.

17. The system of claim 16, wherein the one or more controllers are configured to cause the system to maintain the applied current density setpoint at the reset level until the voltage is returned to an operational level.

18. The system of claim 17, wherein the operational level is between 2 V to 3 V, endpoints included.

19. The system of claim 15, wherein the one or more controllers are configured to cause the system to gradually increase the applied current density by a stepped increase to an operating applied current density.

20. The system of claim 19, wherein the one or more controllers are configured to cause the system to hold the applied current density at each step until the voltage stabilizes.

21. The system of claim 20, wherein the one or more controllers are configured to cause the system to hold the applied current density at each step for at least 10 minutes.

22. The system of claim 15, wherein the PEM layer is an anion-conducting polymer electrolyte membrane.

23. The system of claim 15, wherein the carbon oxide electrolyzer is an anion- exchange membrane (AEM)-only ME A electrolyzer.

24. The system of claim 22, wherein the anion-conducting polymer membrane comprises a styrenic copolymer.

25. The system of claim 22, wherein the anion-conducting polymer membrane comprises a copolymer of polystyrene and a polymer comprising a positively charged amine and/or positively charged heterocyclic group.