METHODS FOR PREVENTION OR REMOVAL OF PRECIPITATED SALTS TO IMPROVE ELECTROCHEMICAL CO2 REDUCTION REACTIONS

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 63/700,400, filed Sep. 27, 2024, which is hereby incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Grant No. DE-EE0009287 awarded by the Department of Energy. The government has certain rights in the invention.

BACKGROUND

(Bi) carbonate salt formation has been widely recognized as one of the primary reasons for the poor operation stability in the electrochemical carbon dioxide reduction reaction (CO₂RR). Accordingly, there exists a need for the prevention and removal of carbonate/bicarbonate salt accumulation within CO₂RR electrolyzers.

This invention was made with support from the Welch Foundation under Grant No. C-2051-20230405.

SUMMARY

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

In one aspect, embodiments disclosed herein relate to a method for removing precipitated salts formed during the electrochemical reduction of CO₂. The method includes providing an electrochemical device including a cathode compartment comprising a cathode endplate and a cathode catalyst; and an anode compartment comprising anolytes. A CO₂ feed stream is supplied to the cathode compartment and reduced at the cathode catalyst to form a carbon species by applying a voltage to the device. Upon applying the voltage to the device, a portion of the anolytes crossover from the anode compartment to the cathode compartment, forming precipitated salts. The method includes reducing accumulation of the precipitated salts in the cathode compartment while reducing the CO₂ feed stream at the cathode catalyst. The CO₂ feed stream may include CO₂ gas and water vapor, such that the precipitated salts are solvated by the water vapor and removed from the cathode compartment as solvated salts. The CO₂ feed stream may include acid vapor. The method may include periodically injecting aqueous acid into the CO₂ feed stream. The cathode endplate may include recessed flow channels coated with a hydrophobic coating to prevent precipitation of the precipitated salts. The hydrophobic coating may be selected from the group consisting of parylene, acrylic resin, epoxy resin, polyethylene, polystyrene, polyvinyl chloride, polytetrafluoroethylene, polydimethylsiloxane, polyester, polyurethane, and combinations thereof. The cathode endplate may include a removable plate with recessed flow channels, where the precipitated salts collect in the recessed flow channels and the removable plate is capable to be removed and cleaned while reducing the CO₂ feed stream at the cathode catalyst. The CO₂ feed stream may be supplied to the cathode compartment at a flow rate ranging from 0.02 to 0.1 sccm·cm²/mA, for example 0.02 to 0.08 sccm·cm²/mA. The electrochemical device further may include an ion exchange membrane. The cathode compartment may include a current collector and a cathode gasket. The anode compartment may include an anode endplate, a current collector, an anode gasket, and an anode catalyst.

In another aspect, embodiments disclosed herein relate to an apparatus for the electrochemical reduction of CO₂. The apparatus includes a cathode compartment comprising a cathode endplate and a cathode catalyst; where the cathode endplate includes a removable plate with recessed flow channels. The apparatus also includes an anode compartment comprising anolytes. The recessed flow channels may be coated with a hydrophobic coating. The hydrophobic coating may be selected from the group consisting of parylene, acrylic resin, epoxy resin, polyethylene, polystyrene, polyvinyl chloride, polytetrafluoroethylene, polydimethylsiloxane, polyester, polyurethane, and combinations thereof. The apparatus may include an ion exchange membrane. The cathode compartment may include a current collector and a cathode gasket. The anode compartment may include an anode endplate, a current collector, an anode gasket, and an anode catalyst.

In yet another aspect, embodiments disclosed herein relate to a method for removing precipitated salts formed during the electrochemical reduction of CO₂. The method includes providing the apparatus according to one or more embodiments as described above, supplying a CO₂ feed stream to the cathode compartment, reducing the CO₂ feed stream at the cathode catalyst to form a carbon species by applying a voltage to the device where upon applying the voltage to the device, a portion of the anolytes crossover from the anode compartment to the cathode compartment, forming precipitated salts, and reducing accumulation of precipitated salts in the cathode compartment while reducing the CO₂ feed stream at the cathode catalyst. The CO₂ feed stream may include CO₂ gas and water vapor, such that the precipitated salts are solvated by the water vapor and removed from the cathode compartment as solvated salts. The CO₂ feed stream may include acid vapor. The method may include periodically injecting aqueous acid into the CO₂ feed stream. The apparatus may be stable while reducing the CO₂ feed stream for continuous electrolysis with a current density ranging from 0 to 5000 mA cm⁻². The recessed flow channels of the apparatus may be coated with a hydrophobic coating. The hydrophobic coating may be selected from the group consisting of parylene, acrylic resin, epoxy resin, polyethylene, polystyrene, polyvinyl chloride, polytetrafluoroethylene, polydimethylsiloxane, polyester, polyurethane, and combinations thereof. The precipitated salts may collect in the removable plate with recessed flow channels and the removable plate may be removed and cleaned while reducing the CO₂ feed stream at the cathode catalyst. The CO₂ feed stream may be supplied to the cathode compartment at a flow rate ranging from 0.02 to 0.1 sccm·cm²/mA, for example 0.02 to 0.08 sccm·cm²/mA.

Other aspects and advantages of the claimed subject matter will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an exploded diagram of an electrochemical device according to one or more embodiments of the present invention.

FIG. 2 shows an exploded diagram of the cathode endplate, removable plate with recessed flow channels, and current collector of the device of FIG. 1 according to one or more embodiments of the present invention.

FIG. 3A shows a straight on diagram showing the cathode endplate and the removable plate with recessed flow channels of FIG. 1 according to one or more embodiments of the present invention.

FIG. 3B shows a straight on diagram showing the current collector of FIG. 1 according to one or more embodiments of the present invention.

FIGS. 4A and 4B illustrate a removable plate with recessed flow channels.

FIGS. 5A, 5B, and 5C illustrate a cathode endplate and anode plate corresponding to the removable plate shown in FIGS. 4A-4B.

FIG. 6 shows a plot of an electrochemical cell potential and the corresponding CO and H₂ Faradaic efficiencies with respect to time for a conventional device.

FIG. 7 shows a plot of an inlet CO₂ feed stream pressure reading as a function of time for the conventional device illustrated in FIG. 4.

FIG. 8 shows plots of electrochemical cell potentials for a comparative conventional device (top plot) and exemplary devices with parylene coated flow channels for 3 runs with identical conditions (bottom three plots).

FIG. 9 shows plots of electrochemical cell potentials for a comparative conventional device (top plot) and exemplary devices with acid-humidification using hydrochloric acid (middle plot) or nitric acid (bottom plot).

DETAILED DESCRIPTION

In one aspect, embodiments disclosed herein relate to methods of preventing or removing precipitated carbonate and bicarbonate salts for improved continuous operational stability of electrochemical carbon dioxide reduction by 1) coating the cathode flow channels with hydrophobic materials, 2) flowing a CO₂ feed stream into a bubbler to carry out trace amounts of water vapor and/or acid vapor into the cathode chamber of CO₂RR electrolyzers, and 3) retracting, cleaning, and reinserting removable flow channels back into the electrochemical cell without needing to stop the reduction reaction.

Importantly, the methods and apparatuses disclosed herein are not only suitable for CO₂RR-to-CO and CO₂RR-to-HCOOH, but also serve as a powerful method to improve other electrochemical carbon dioxide reduction reactions and other CO₂RR reactors.

FIG. 1 shows a device (100) according to one or more embodiments of the present invention. The device comprises a cathode endplate (102) with a CO₂ feed stream input (120) and an optional removeable plate (104). Next to the cathode endplate (102) is a decoupled current collector (106), a cathode gasket (108), and cathode catalyst (110). The cathode endplate (102), decoupled current collector (106), cathode gasket (108), and cathode catalyst (110) comprise the cathode compartment (122). An ion exchange membrane (112) separates the cathode compartment (122) from the anode compartment (124). The anode compartment (124) comprises an anode catalyst (114), an anode gasket (116), and an anode endplate (118). The removable plate includes recessed flow channels (126).

According to one or more embodiments of the present invention, the device is an electrochemical reactor such as a membrane electrode assembly (MEA) reactor, a flow cell reactor, a solid electrolyte reactor, or another reactor that utilizes MEA technology. In one or more embodiments, the device is a reactor for any electrochemical reduction reaction that includes trace amounts of CO₂, which may result in salt formation. In one or more embodiments, the precipitated salts are carbonate and/or bicarbonate salts formed during the electrochemical reaction. In one or more embodiments, the cathode flow channels, also termed fluid flow channels, flow channels, or recessed flow channels, of the device are serpentine flow channels, parallel flow channels, or interdigitated flow channels. The flow channels may channel flow of gas, liquid, or a combination thereof. In one or more embodiments, the recessed flow channels of the device are coated with hydrophobic materials to enhance salt removal, prevent salt accumulation, and protect the channels from acid vapor corrosion.

According to one or more embodiments of the present invention, the cathode catalyst is designed for converting CO₂ to CO. In one or more embodiments, the cathode catalyst is a catalyst for electrochemical carbon dioxide reduction reactions (CO₂RRs) such as CO₂-to-C₂H₄, CO₂-to-HCOOH, and CO₂-to-CH₃COOH. Any variations of CO₂RR can be utilized as long as they maintain performance standards. While CO₂-to-CO conversion is exemplified, this method is adaptable to various reactions relating CO₂ capture and utilization relating with salt formation.

According to one or more embodiments of the present invention, when the CO₂ feed stream is flowed through a bubbler to carry out trace amounts of water vapor and/or acid vapor into the cathode chamber of the device, the CO₂ feed stream comprises CO₂ gas, water and/or acid vapor. Without being bound by any particular theory, increasing the humidity of the CO₂ feed stream may dissolve any precipitated salt and extend the stability of the device. In one or more embodiments, the bubbler is heated, with the temperature ranging from 30-500° C. In one or more embodiments, the bubbler includes a humidity control system configured to monitor, change, and/or maintain the humidity of the CO₂ feed stream. The humidity control system may also monitor, change, and/or maintain the bubbler temperature. The humidity control system may include a humidity sensor. When the humidity sensor indicates that the humidity is sufficient to dissolve the salt or maintain extended stability, the humidity control system may adjust the heating of the bubbler.

According to one or more embodiments of the present invention, when the CO₂ feed stream comprises CO₂ gas, water vapor, and acid vapor, the acid vapor is a high vapor pressure acid such as hydrochloric acid (HCl), formic acid (HCOOH), nitric acid (HNO₃), acetic acid (CH₃COOH), propionic acid (CH₃CH₂COOH), and propanoic acid (CH₃CH₂COOH), or combinations thereof. The high vapor pressure acid may include mixtures of at least one high vapor pressure acid and one or more non-high vapor pressure acids. In one or more embodiments, the high vapor pressure acid and/or the acid mixtures are heated to produce the acid vapor.

According to one or more embodiments of the present invention, aqueous acid is periodically injected into the CO₂ feed stream prior to reaching the cathode compartment. The aqueous acid is a liquid mixture of water and at least one acid. The CO₂ feed stream may comprise dry CO₂ gas, CO₂ gas and water vapor, CO₂ gas and acid vapor, or combinations thereof. The CO₂ feed stream may be a gas, liquid, or combination thereof.

According to one or more embodiments of the present invention, the CO₂ feed stream flow rate is fixed, setting the input CO₂ feed stream flow rate to 0.02-0.1 sccm-cm²/mA, for example 2-10 sccm/cm²_{active area} at an operational current density of 100 mA/cm². Without being bound by any particular theory, the CO₂ feed stream flow rate also impacts the salt formation process in CO₂RR reactors. Flowing CO₂ gas at rates higher than 0.02-0.1 sccm·cm²/mA increases salt formation, thereby shortening the stability of CO₂RR reactors.

According to one or more embodiments of the present invention, when the cathode endplate comprises recessed flow channels coated with a hydrophobic coating, the hydrophobic coating is a hydrophobic coating material such as hydrophobic parylene, acrylic resin, epoxy resin, polyethylene, polystyrene, polyvinyl chloride, polytetrafluoroethylene, polydimethylsiloxane, polyester and polyurethane.

According to one or more embodiments of the present invention, the cathode endplate includes a removeable plate with recessed flow channels as shown in FIG. 1 and more closely in FIG. 2 and FIG. 3 and in FIG. 4 and FIG. 5. FIG. 2 is an exploded view that shows the cathode endplate (202) with a CO₂ feed stream input (208) and a product output (210), a removeable plate (204) with recessed flow channels (226), and a decoupled current collector (206). FIG. 3 is a side view showing the cathode endplate (302) with CO₂ feed stream input (308) and a product output (310), and a removable plate (304) with recessed flow channels (326). FIG. 4A is a side view and FIG. 4B a top view of a slider (428) that includes a removable plate (404), an optional first end portion (430A) and an optional second end portion (430B). FIGS. 5A and 5B show cathode endplate (502) with CO₂ feed stream input (508) and a product output (510). FIG. 5C shows corresponding anode endplate (518). FIG. 4A shows that the removable plate (404) includes first recessed flow channels (426A) and second recessed flow channels (426B). In one or more embodiments, the removable recessed flow channels (326) or one of first recessed flow channels (426A) and second recessed flow channels (426B) are retracted while the electrochemical cell is still operational, allowing removal of the salt deposits, and reinserted back into the electrochemical cell. This ensures the smooth removal of the salt deposits to extend the operational stability of the CO₂ reduction cell. In one or more embodiments, the presence of the pair of recessed flow channels (426A) and second recessed flow channels (426B) in removable plate (404) allows continued operation by removal of salt deposits from one of recessed flow channels (426A) and second recessed flow channels (426B) while the other of recessed flow channels (426A) and second recessed flow channels (426B) are engaged in operation. The optional first end portion (430A) and optional second end portion (430B) when present assist slider (428) to be secured to cathode endplate (502) when removable plate (404) is moved so that either recessed flow channels (426A) or second recessed flow channels (426B) is engaged in operation. It will be understood that when optional first end portion (430A) and an optional second end portion (430B) are included removable plate (404) is removable in the sense that removable plate (404) is slidable to permit one of recessed flow channels (426A) and second recessed flow channels (426B) to be removed from operation in order to remove salt deposits and to other of recessed flow channels (426A) and second recessed flow channels (426B) to be engaged in operation of the device (100).

According to one or more embodiments of the present invention, the device comprising removable recessed flow channels can be used for any electrochemical reduction reaction that includes trace amounts of CO₂, which may result in salt formation, especially at higher operational currents. Such reduction reactions include, but are not limited to, nitrite reduction reactions, nitrate reduction reactions, O₂ reduction reactions, CO reduction reactions, and N₂ reduction reactions.

According to one or more embodiments of the present invention, a design of removable cathode flow channels, also termed removable gas flow channels, removable recessed flow channels or removeable flow channels, may include single or multiple serpentine flow channels as well as parallel or interdigitated channels. Without being bound by any particular theory, during the CO₂ reduction process, the inlet CO₂ feed stream reacts with the locally generated hydroxide ions, forming carbonate, which further reacts with the mobile cations in the system, resulting in the formation of salt deposits that block the flow channels and in the failure of the electrolyzer. The process herein introduces the idea of retracting the removable flow channels while the electrochemical cell is still operational, removing the salt deposits, and reinserting the removable flow channels back into the electrochemical cell (FIG. 2 and FIG. 3). This ensures the smooth removal of the salt deposits to extend the operational stability of the CO₂ reduction cell.

According to one or more embodiments of the present invention, without being bound by any particular theory, the inventors disclose a correlation between the measured pressure of the CO₂ feed stream and the salt formation pressure while relating both to the CO₂ reduction selectivity towards the desired product. When the CO₂ feed stream includes CO₂ gas, the input CO₂ gas pressure reading can be used as a proxy for salt accumulation in the recessed flow channels.

According to one or more embodiments, an apparatus for the electrochemical reduction of CO₂ is disclosed. In one or more embodiments, the apparatus comprises a cathode compartment and an anode compartment. In one or more embodiments, the cathode compartment comprises a cathode endplate and a cathode catalyst, where the cathode endplate comprises a removable plate with recessed flow channels. In one or more embodiments, the recessed flow channels are coated with a hydrophobic coating. In one or more embodiments, the hydrophobic coating is selected from the group consisting of parylene, acrylic resin, epoxy resin, polyethylene, polystyrene, polyvinyl chloride, polytetrafluoroethylene, polydimethylsiloxane, polyester, polyurethane, and combinations thereof. In one or more embodiments, the anode compartment comprises anolytes. The apparatus may include an ion exchange membrane. The cathode compartment may include a current collector and a cathode gasket. The anode compartment may include an anode endplate, a current collector an anode gasket, and an anode catalyst

According to one or more embodiments, a method for removing precipitated salts formed during the electrochemical reduction of CO₂ is disclosed. In one or more embodiments, the method comprises providing an electrochemical device. In one or more embodiments, the electrochemical device comprises a cathode compartment and an anode compartment. In one or more embodiments, the cathode compartment comprises a cathode endplate and a cathode catalyst. In one or more embodiments, the anode compartment comprises anolytes. In one or more embodiments, the method further comprises supplying a CO₂ feed stream to the cathode compartment, reducing the CO₂ feed stream at the cathode catalyst to form a carbon species by applying a voltage to the device, where upon applying the voltage to the device, a portion of the anolytes crossover from the anode compartment to the cathode compartment, forming precipitated salts, and reducing accumulation of precipitated salts in the cathode compartment while reducing the CO₂ feed stream at the cathode catalyst. According to one or more embodiments, the CO₂ feed stream comprises CO₂ gas and water vapor, where the precipitated salts are solvated by the water vapor and removed from the cathode compartment as solvated salts. The CO₂ feed stream may further comprise acid vapor. The method may include periodically injecting aqueous acid into the CO₂ feed stream. According to one or more embodiments, the cathode endplate comprises recessed flow channels coated with a hydrophobic coating to prevent precipitation of the precipitated salts. In one or more embodiments, the hydrophobic coating is selected from the group consisting of parylene, acrylic resin, epoxy resin, polyethylene, polystyrene, polyvinyl chloride, polytetrafluoroethylene, polydimethylsiloxane, polyester, polyurethane, and combinations thereof. According to one or more embodiments, the cathode endplate comprises a removable plate with recessed flow channels, where the precipitated salts collect in the recessed flow channels and the removable plate may be removed and cleaned while reducing the CO₂ feed stream at the cathode catalyst. In one or more embodiments, the CO₂ feed stream is supplied to the cathode compartment at a flow rate ranging from 0.02 to 0.1 sccm·cm²/mA. The electrochemical device may include an ion exchange membrane. The cathode compartment may include a current collector and a cathode gasket. The anode compartment may include an anode endplate, a current collector an anode gasket, and an anode catalyst.

According to one or more embodiments, a method for removing precipitated salts formed during the electrochemical reduction of CO₂ using the apparatus described above is disclosed. In one or more embodiments, the method comprises supplying a CO₂ feed stream to the cathode compartment, reducing the CO₂ feed stream at the cathode catalyst to form a carbon species by applying a voltage to the device, where upon applying the voltage to the device, a portion of the anolytes crossover from the anode compartment to the cathode compartment, forming precipitated salts, and reducing accumulation of precipitated salts in the cathode compartment while reducing the CO₂ feed stream at the cathode catalyst. In one or more embodiments, the CO₂ feed stream comprises CO₂ gas and water vapor, where the precipitated salts are solvated by the water vapor and removed from the cathode compartment as solvated salts. The CO₂ feed stream may further comprise acid vapor. The method may include periodically injecting aqueous acid into the CO₂ feed stream. In one or more embodiments, when the CO₂ feed stream comprises CO₂ gas, water vapor, and acid vapor, the device is stable while reducing the CO₂ feed stream for continuous electrolysis with current densities ranging from 0 to 5000 mA cm⁻². In one or more embodiments, the recessed flow channels are coated with a hydrophobic coating. In one or more embodiments, when the recessed flow channels are coated with a hydrophobic coating, the stability of the device is extended through removal of the droplets before forming salt precipitation compared to the devices without coating hydrophobic materials. In one or more embodiments, the hydrophobic coating is selected from the group consisting of parylene, acrylic resin, epoxy resin, polyethylene, polystyrene, polyvinyl chloride, polytetrafluoroethylene, polydimethylsiloxane, polyester, polyurethane, and combinations thereof. In one or more embodiments, the precipitated salts collect in the removable plate with recessed flow channels and the removable plate may be removed and cleaned while reducing the CO₂ feed stream at the cathode catalyst. In one or more embodiments, the CO₂ feed stream is supplied to the cathode compartment at a flow rate ranging from 0.02 to 0.1 sccm·cm²/mA. The methods of preventing or removing precipitated carbonate and bicarbonate salts for improved continuous operational stability of electrochemical carbon dioxide reduction disclosed herein can be applied individually or in any combination.

EXAMPLES

Example 1—Overview of Examples

By observing the salt formation process via operando characterization tools and carefully quantifying the salt precipitation against different device operation conditions, the present inventors found that found that liquid droplets carrying cations and carbonate/bicarbonate ions were observed to migrate from the catalyst/membrane interface towards the backside of the gas diffusion electrode (GDE), which was revealed to be driven by interfacial gas evolution and CO₂ flow. The liquid droplets were eventually dried out to form carbonate/bicarbonate salt precipitates that could block the flow channels in CO₂RR electrolyzers.

In a conventional CO₂RR electrolysis setup, the CO₂ feed stream was first humidified via a water bubbler then fed into the cathode chamber to avoid dehydration of the anion exchange membrane (AEM). To understand the device stability under a continuous CO₂RR electrolysis operation in a membrane electrode assembly (MEA) reactor, the current density or cell voltage could be fixed while recording the products Faradaic efficiencies (FEs). While the cell voltage and FEs remained stable over the initial dozens of hours, catastrophic degradation could happen, signaled by the mass flow meter (MFC) showing increased pressure and fewer CO₂ bubbles in the water bubbler. The reason for this sudden degradation of conventional MEA reactors was revealed when the cell was disassembled. A significant amount of carbonate/bicarbonate salt precipitation was observed inside the flow channels of the endplate as well as on the GDE backside, completely blocking the CO₂ flow.

Based on this observation, firstly, the present inventors coated the surface of the cathode flow channels with hydrophobic parylene layers to facilitate the removal of salt droplets, resulting in a CO₂RR stability extension from ˜100 hours to over 500 hours under 200 mA cm⁻² current density.

Additionally, the present inventors further demonstrated an acid-humidified CO₂ feed stream input method that can effectively prevent or remove the formed salt for improved continuous operational stability. By flowing CO₂ gas into an acid bubbler to carry out trace amount of acid vapor into a CO₂RR-to-CO (carbon monoxide) electrolyzer, the present inventors observed a dramatic decrease in salt precipitation inside the flow channels, leading to a high stability of at least 2,000-hour continuous electrolysis under 100 mA cm⁻² current density while maintaining >90% CO Faradaic efficiency (FE). The acid-humidification method was demonstrated to be effective in a 100-cm² scaled-up CO₂ electrolyzer to achieve an impressive stability milestone of 3,000 hours without compromising the CO FE, while the case of conventional water-humidified CO₂ could only reach a stability of ˜80 hours.

Moreover, the present inventors found a correlation between the measured reactant CO₂ pressure and the salt formation pressure while relating both to the CO₂ reduction selectivity towards the desired product. The present inventors found that the input CO₂ gas pressure reading can be used as a proxy for salt accumulation in the flow channels. Finally, the present inventors designed removable flow channels and introduced the idea of retracting the removeable flow channels while the electrochemical cell is still operational, removing the salt deposits, and reinserting the removable flow channels back into the electrochemical cell. This ensures the smooth removal of the salt deposits to extend the operational stability of the CO₂ reduction cell.

Example 2—Pressure Change with Salt Build-Up

This example illustrates correlation between pressure change and salt build-up in a conventional MEA reactor. This example further illustrates stability of ˜80 hours in a conventional MEA reactor. In order to relate salt formation to measurable process parameters, the inventors investigated the correlation between the measured reactant CO₂ pressure and the salt formation pressure while relating both to the CO₂ reduction selectivity towards the desired product. The inventors showed that the input CO₂ gas pressure reading can be used as a proxy for salt accumulation in the flow channels. Using the reactant CO₂ stream pressure as a proxy for salt formation for a silver-based 100 cm² CO₂-to-CO electrolyzer, FIG. 6 shows a graph 600 of the cell potential and the corresponding CO and H₂ Faradaic efficiencies with respect to time. FIG. 7 shows a graph 700 of the inlet CO₂ feed stream pressure reading as a function of time. The drop in CO Faradaic efficiency and the increase in H₂ Faradaic efficiencies are accompanied by an increase in the input CO₂ gas pressure reading due to the salt build-up in the flow channels and the GDE backside. As shown in FIG. 6 and FIG. 7, the drop in the CO selectivity and voltage fluctuations were accompanied by an increase in the input CO₂ stream pressure from ˜15 PSIA at the beginning of the test to more than ˜18 PSIA towards the end of the test. Upon disassembly and visual observation of salt build up in the flow channels and the GDE backside, it was concluded that this pressure increase was because of salt accumulation in the flow channels. FIGS. 6 and 7 also demonstrate a stability of ˜80 hours.

Example 3—Hydrophobic Coated Flow Channels

This example demonstrates preventing salt in the cathode chamber by coating the surface of the gas flow channel, which is typically made of stainless steel and is normally hydrophilic, with hydrophobic layers to allow easy flow of KHCO₃ droplets before they are dried out to precipitate salt crystals. To demonstrate this, a conformal parylene coating was employed, with dichlorodi-p-xylylene as the precursor, via chemical vapor deposition (CVD) due to its hydrophobic and conformal characteristics. Contact angle measurements using water droplets revealed that the pristine cathode metal plate exhibited a contact angle of 91°, indicating a relatively weak hydrophobic nature. However, upon the application of a 0.5 g dichlorodi-p-xylylene coating, the contact angle was significantly increased to 123°, effectively transforming the surface of the gas flow channels into a hydrophobic surface. In the parylene-coated system, much more K+ cations (16 μm) were flushed out of the cathode flow channels compared to negligible K+ cations (0.1 μm) flushing out in the pristine electrolyzer without surface coating. FIG. 8 illustrates the stabilities of Ag—NP towards CO₂RR using 0.1 M KHCO₃ anolyte in MEA electrolyzer with pristine cathode flow channels vs parylene-coated cathode flow channels. In a pristine CO₂RR MEA electrolyzer with 0.1 M KHCO₃ anolyte at 200 mA cm-2, the CO FE maintained above 80% for ˜100 hours as shown in graph 800 in FIG. 8. Finally, in the pristine device, the CO₂ gas was blocked due to the salt accumulated on the backside of the GDE and in the flow channels. In a sharp contrast, the parylene-coated device successfully extended the operation stability to over 500 hours while maintaining over 90% of CO FE without totally blocking the flow channels (FIG. 8).

Example 4—Periodic Injection of Aqueous Acid

This example illustrates a simple but highly effective, robust, and scalable method to prevent salt formation inside the cathode chamber for dramatically extended operational stability in CO₂RR electrolysis using acid humidification. By humidifying the reactant CO₂ gas flow using an acid solution with high vapor pressure such as hydrochloric acid (HCl), nitric acid (HNO₃), formic acid (HCOOH) or acetic acid (CH₃COOH), a significant decrease in the salt crystal formation inside the cathode chamber was observed when compared to that of the regular H₂O-humidification. Upon optimizing the chosen acid concentrations, we observed no negative impacts of the acid vapor on the CO₂RR FEs or cell voltages were observed. The present inventors believe that due to the higher solubility of salts with anions from stronger acid than that of carbonic acid, the trace amount of acid vapor carried into the cathode chamber of CO₂RR can effectively prevent bicarbonate salt from precipitating out inside the gas flow channels. Using high vapor pressure acid-humidified CO₂ gas input, an exceptional CO₂RR stability of over 2,000-hour continuous electrolysis was demonstrated under a current density of 100 mA cm-2 with CO FE well maintained at >90%. This stability performance was successfully extended to a scaled-up MEA electrolyzer with an active electrode area of 100 cm² and a CO₂RR current of 10 A (FIG. 9). Graph 900 in FIG. 9 illustrates stability tests of the 100-cm² CO₂RR MEA at 100 mA cm⁻², with 200 sccm of H₂O-humidified CO₂ vs. 0.05 M HCl- and 0.05 M HNO₃-humidified CO. A stability of over 3,000 hours was observed under these conditions for the scaled-up MEA electrolyzer. The excellent salt removal capability of acid-humidification method does not come with a price of severe flooding as in the case of introducing more water vapor or directly injecting liquids, suggesting its non-invasive nature is different from other methods.

Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.