Reactivation process of gas diffusion electrode

Abstract

The present invention relates to a method for reactivation and long-term stable operation of metal-based gas diffusion electrodes (GDEs) and its use in the electrocatalytic conversion of gaseous reactants into reaction products of economic value. By means of the operating process of the invention, such metal-based GDE, and in particular carbon-free metal-based GDE, is particularly useful in the electrochemical conversion of gaseous reactants such as CO2, CO, N2 or O2 to bulk chemicals and fuels such as syngas, formic acid, methanol, ethanol, ethane, ethylene, methane, ammonia, etc. for long-lasting operation (gt; 1000 hours).

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for the reactivation of long-term stable operation of metal-based gas diffusion electrodes (GDEs) for the electrocatalytic conversion of gaseous reactants into reaction products of economic value. By the method of operation of the invention, such metal-based GDEs, and especially carbon-free metal-based GDEs, are particularly useful in the electrochemical conversion of gaseous reactants such as CO2, CO, N2, NO x or O2 into bulk chemicals and fuels such as syngas, formic acid, methanol, ethanol, ethane, ethylene, methane, ammonia, hydroxylamine, hydrogen peroxide, etc. BACKGROUND

[0002] GDEs are a combination of porous structures exhibiting a hydrophilic side and a hydrophobic side. As shown in Figure 1 , they are usually composed of a catalyst layer (CL), a gas diffusion layer (GDL), and optionally a current collector (CC). GDEs allow the direct supply of gases through the hydrophobic side into the liquid medium where the electrochemical process takes place. Due to their low mass transfer resistance, the use of GDEs is promising for electrochemical processes with gaseous reactants as it overcomes the poor mass transfer of gases with limited solubility to the electrode. Due to their high porosity, GDEs have a much larger reactive surface area than their geometric area (projected surface area), which favors process productivity. GDLs are porous media that facilitate the transport of gaseous reactants to the CL and of gaseous products from the CL and are composed of layers of macro / meso / nanopores, with or without a microporous layer (MPL). A large number of GDL parameters have been studied to improve the performance of electrolyzers or fuel cells (Omrani R, Shabani B (2017) Gas diffusion layer modifications and treatments for improving the performance of proton exchange membrane fuel cells and electrolysers: A review. Int J Hydrogen Energy 42:28515-28536. doi: 10.1016 / j.ijhydene.2017.09.132).

[0003] The concept of CO2conversion and utilization (CCU) is considered as a promising alternative for mitigating excess greenhouse gases. Electrochemical CO2reduction (ECR) can become an essential tool for large-scale implementation of CCU when powered by renewable energy. The products (formate) obtained from ECR are critical in chemical feedstocks / building blocks or fuels. To effectively facilitate ECR, gas diffusion electrodes (GDEs) are designed to facilitate a three-phase boundary, which increases active sites for ECR, reducing overall loss of CO2gas and increasing the yield of valuable products, with improved Faradaic efficiency and energy efficiency.

[0004] However, the industrial or large-scale implementation of ECR is currently hindered by moderate current densities, large cell potentials (hence low energy efficiency), and poor stability of electrode activity. For industrial or large-scale implementation, a qualified electrode must meet the following requirements: 1) sufficiently high current density; and 2) significantly high Faradaic efficiency (FE) that is stable to the target product. For example, the industrially relevant production of formic acid requires an electrode with a current density of at least 100 mA / cm 2 and a Faradaic efficiency (FE) of 70% ± 10%. Over time, the electrodes currently used for ECR of CO2are unable to provide stable long-term operation (> 1000 h) for electrocatalytic conversion activity.

[0005] Efforts to obtain such qualified electrodes are based on carbon-based GDEs, where metal catalysts are inked or electrodeposited on the GDE. For example, Agarwal et al. (Patent No. US10273587B2) applied for a patent for a method and manufacturing of Sn-carbon (Sn-C) particles deposited on carbon fibers for ECR. The inventors claim protection for three different methods for preparing Sn-C particles, which are then claimed to have a FE of 95% for formate ions for 100 h of operation.

[0006] In 2017, Zhai et al. (Patent No. US10253420B2) proposed an electrochemical process for ECR. In this patent, the inventors claim protection for a method of depositing tin (Sn) on carbon fiber paper (CFP) to prepare a porous Sn-CFP electrode for ECR activity. However, the long-term activity of this Sn-CFP electrode is hindered due to the fact that when the electrode is operated at a current density of 10 mA / cm 2 , which is less than the proposed commercially appropriate 100 mA / cm 2Graphite deposition on the electrode surface after 20 hours of operation at a current density 10 times lower than the current density at which the Sn-CFP electrode was prepared. Using a combination of deep cathodic polarization (DCP) with anodic polarization (AP), the graphite deposited from the Sn-CFP electrode can be removed, extending the operation time of the cell with Sn-CFP up to 200 hours.

[0007] Well-known electrocatalysts for the conversion of CO2 to formic acid include Sn, Bi, In and alloys of these metals. However, these metal-based GDEs do not show promising results in long-term operation (1000 h), as required by industrial set-ups (Van Daele, K., De Mot, B., Pupo, M., Daems, N., Pant, D., Kortlever, R. and Breugelmans, T., 2021. Sn-based electrocatalyst stability: a crucial piece to the puzzle for the electrochemical CO2 reduction toward formic acid. ACS Energy Letters, 6(12), pp.4317-4327). For example, in the case of Sn-based electrodes, deactivation or degradation of the electrode is observed, which can be related to changes in the surface and local environment of the electrode during the electrolysis process. Furthermore, the method of preparation of the GDE and the cell configuration also significantly affect the stability of the electrode for long-term operation by increasing the total cell resistance, the way the gases are supplied, depending on whether the cell is an H-cell, a flow cell or a zero-gap configuration (Liniker de Sousa, Nieck E. Benes, and Guido Mul, ACS ES&T Engineering 2022 2 (11), 2034-2042 and Kevin Fernández-Caso, Guillermo Diaz-Sainz, Manuel Alvarez-Guerra, and Angel Irabien ACS Energy Letters 2023 8(4), 1992-2024).

[0008] The long-term operation issues of Sn-GDEs have been discussed several times. The main solutions reported include catalyst modification by adding dopants or incorporating bimetallic systems. For example, in 2006, Oloman and Li (Patent No. US20080223727A1) invented a process for ECR, including a reactor-type design that employed Sn / Sn-based alloys in the form of granules as the working electrode.

[0009] The above-mentioned requirement is about the configuration / design of the reactor, including the electrode in the form of Sn-doped granules containing Sn / Sn-Zn alloys. However, even with these Sn granules as the cathode, the FE of formate decreased from 60% to 45% in a 4 h operation. The cathode activity was restored by combining two methods: 1) treating the Sn granules with 11 wt% nitric acid chemically; 2) polarity reversal, in which 1 A was applied for 5 minutes.

[0010] Although this catalyst modification and cathode restoration method achieved some encouraging results in short-term experiments (1-100 hours), it is not suitable for industrial applications because it would require the electrodes to be replaced after a short period of time and would therefore negatively impact the economic / profitability of the process. Moreover, this method did not make much progress in addressing the original problem of long-term operation of Sn-based electrodes (Al-Tamreh, S.A., Ibrahim, M.H., El - Naas, M.H., Vaes, J., Pant, D., Benamor, A. and Amhamed, A., 2021. Electroreduction of carbon dioxide into formate: A comprehensive review. ChemElectroChem, 8(17), pp.3207-3220). Furthermore, due to the electrode integrity (mechanical strength), maintaining uniform current distribution, and leak-proof characteristics of the GDE, the requirement for electrodes with larger surfaces is not a preferred option (Perry, S.C., de León, C.P. and Walsh, F.C., 2020. The Design, Performance and Continuing Development of Electrochemical Reactors for Clean Electrosynthesis. Journal of The Electrochemical Society, 167(15), p.155525).

[0011] Despite the progress made, ensuring the operational stability of the system, especially of the electrocatalyst which is often degraded or poisoned, remains a major challenge. Thus, for example, in the electrochemical reduction of CO2 to obtain formic acid or other organic compounds, the activity of some electrocatalysts (e.g. Pd) for the conversion of CO2 can decrease significantly after a short period of time, due to the adsorption of the poisoning by-product CO, which clogs the active sites of the electrocatalyst.

[0012] For example, the poisoning of Pd catalysts by CO is well known in the prior art, but most of the work involves this catalyst for attempts at CO2 reduction to work at very low overpotential, and thus at very low current density, to solve the problem. However, this approach is not useful in industrial conditions, since the production efficiency would be very low, and it has been found that attempts to increase the current density by applying a greater overpotential result in the generation of H2. In US patent publication US2021 / 115576, the poisoning of Pd catalysts is solved by operating the electrochemical cell in the following modes: a direct mode for CO2 reduction; and a second, transformation mode, which lasts for a time sufficient to carry out desorption and subsequent removal of the by-product species generated and adsorbed on the surface of the cathode material in the direct EC operating mode. Nonetheless, the method disclosed in US2021 / 115576 still does not prevent the degradation of the catalyst and the achievement of long-term operation at high FE.

[0013] The object of the present invention is to solve the above-mentioned problems in the development of an operating method that combines the intrinsic characteristics of the electrolyte used with the changes that occur at the electrode / electrolyte interface due to the induction of a change in polarity at the electrode. The process has been demonstrated with carbon-free Sn-GDEs developed previously by the applicant (the details of which are described in WO2022013042A1, incorporated by reference herein), and with bismuth (Bi) GDEs, and is being extended to alloys such as Sn-Bi, Ag-Cu, etc. As will be apparent from the examples below, in the method of the present invention, the combination of controlling the pH of the electrolyte with the application of pulses with current density and pulse periods related to the surface area and catalyst load, achieves the in situ regeneration of the catalytic surface, more specifically, by oxidizing the catalyst to its oxide form, which is key to restoring selectivity. More importantly, our method guarantees a longer-term stabilization, with a time scale of 1 hour to 24 hours. SUMMARY

[0014] This invention relates to a method for operating an electrochemical cell comprising a metal-based gaseous electrode (GDE) as a working electrode, a counter electrode, and an electrolyte, wherein gaseous reactants are electrochemically converted into bulk chemicals at the working electrode. The method is characterized by maintaining the pH of the electrolyte within a useful range of two units from its initial value; and applying inverse electrical pulses at fixed intervals to the metal-based GDE working electrode, which is the electrode where the electrochemical reaction of interest occurs.

[0015] According to the embodiments described herein, in an implementation, a metal-based GDE working electrode is used to reduce gaseous reactants into bulk chemicals, such as the electrochemical reduction of CO2 to formic acid; in this case, the GDE working electrode is the cathode, and the reverse electrical pulse will be the anode pulse.

[0016] Therefore, in embodiments, the present invention provides a method for operating an electrochemical battery comprising a metal-based GDE working electrode as a cathode, an anode, and an electrolyte, wherein gaseous reactants are electrocatalytically converted into bulk chemicals at the working electrode. The method is characterized by maintaining the pH value and conductivity of the electrolyte within a range of 2 units from their initial values; and applying anodic pulses to the working electrode at fixed intervals.

[0017] In an embodiment of the method according to the invention, the electrochemical cell includes an anode compartment and a cathode compartment, and the method is characterized by: maintaining the pH value of the cathode electrolyte within a range of 2 units from its initial value; and applying reverse electrical pulses, particularly anode pulses, to the GDE working electrode at fixed intervals.

[0018] In an embodiment of the method according to the invention, the anodic pulse applied to the working electrode (GDE) consists of a current of 50-200 mA cm⁻¹. -2 The current is preferably applied for at least 30 seconds; in particular, at least 100 mA cm⁻¹. -2 The current is preferably applied for at least 1 minute.

[0019] In an embodiment of the method according to the invention, the anode pulse consists of a charge of 1.5-10 coulombs (cm) applied to the working electrode (GDE). -2 The charge; in particular, at least 6 C cm. -2 The charge.

[0020] In the method according to any of the foregoing embodiments, the anode pulse is applied at intervals selected to maintain a sufficiently high current density for the target product and a stable and significantly high Faradaic efficiency (FE). In one embodiment, the pulse is applied at intervals that maintain or restore the Faradaic efficiency (FE) for the target product to a window range of 20% of its initial value before the pre-pulsing. In another embodiment, the anode pulse is applied to the cathode at fixed intervals, such as every hour, every 2 hours, every 3 hours, every 4 hours, every 5 hours, or longer. In yet another embodiment, the anode pulse is applied to the cathode at fixed intervals, starting from 6 hours, particularly from 12 hours, more particularly from 24 hours, and even more particularly every 48 hours.

[0021] In the method according to any of the foregoing embodiments, the gaseous reactants are selected from CO2, CO, N2, and NO. x Or O2; specifically, CO2.

[0022] In any of the methods described above, the bulk chemicals are selected from syngas, formic acid, methanol, ethanol, ethane, ethylene, methane, ammonia, hydroxylamine, hydrogen peroxide, etc.

[0023] In any of the methods described above, the electrolyte is selected from KHCO3, NaHCO3, K2CO3, Na2CO3, KCl, NaCl, K2HPO4, KH2PO4, Na2HPO4, NaH2PO4, H2SO4, HClO4, H3PO4, K2SO4, K3PO4, Na2SO4, NaClO4, Na3PO4, KOH, and NaOH; in a specific embodiment, the electrolyte is selected from KHCO3 (0.5-2 M), H2SO4 (0.05-0.5 M), and K2SO4 (0.5-1 M).

[0024] In any of the methods described above, the pH of the electrolyte is maintained near its initial pH (weakly alkaline, particularly from about 7 to about 9 or acidic, particularly from about 0 to about 5). Attached Figure Description

[0025] Figure 1 Experimental setup for long-term testing of metal-based GDEs.

[0026] Figure 2Formate / formic acid, CO, and H2 FE were measured using Sn-GDE during a 1000-hour operation. Solid blocks are obtained from GC measurements, and black dots represent the FE of formate calculated from HPLC measurements of formate concentration in the cathodic electrolyte.

[0027] Figure 3 The pH value of the catholyte solution throughout the experimental period. Four bottles of 0.5 MkHCO3 were used during the experiment, and the changes in the catholyte are indicated by dashed lines to simulate continuous single-pass operation.

[0028] Figure 4 Anode pulses used for electrode reactivation.

[0029] Figure 5 Current density plot of Sn-GDE applied in a flow cell for operation exceeding 1000 h.

[0030] Figure 6 Schematic diagram of battery operation in flow-through mode. 1. Gas chamber outlet - 2. Cathode electrolyte outlet - 3. Cathode electrolyte chamber - 4. Cathode electrolyte inlet - 5. GDE - 6. Gas inlet - 7. Gas chamber.

[0031] Figure 7 The working electrode potential (E) of Sn-GDE during 1000 h of operation. WE (Changes).

[0032] Figure 8 Formate / formic acid, CO, and H2 FE during a 320 h operation using Bi-GDE. Solid blocks are obtained from GC measurements, and black dots represent the FE of formate calculated from HPLC measurements of formate concentration in the cathodic electrolyte.

[0033] Figure 9 Current during the pulse ( Figure 9 A) and its effect on the Faradaic efficiency of formic acid electrochemically reduced by CO2 using Bi-GDE ( Figure 9 Figure B).

[0034] Figure 10 A schematic diagram of an H-type battery.

[0035] Figure 11 The effect of anodic pulse on tin-foil electrode.

[0036] Figure 12 During 780 hours of operation using Sn GDE in an acidic catholyte, the formate / formic acid, CO, and H2 FE values ​​are shown. The vertical dashed line represents changes in the catholyte reservoir. Detailed Implementation

[0037] This invention includes a stable ECR process using a metal-based gas diffusion electrode (GDE), specifically for the electrochemical conversion of CO2, such as, for example, the electrochemical conversion to formate / formic acid. Electrochemical CO2 reduction (ECR) using metal-based GDEs such as Bi / Sn-GDE, In, Zn, etc., as working electrodes is well known. However, and as mentioned above, these electrodes are prone to failure for long-term stable operation (≥1000 h) with respect to ECR activity, which hinders their industrial application. For an industrially suitable ECR catalyst, CO2 should be converted into bulk chemicals such as formic acid, methanol, ethanol, ethane, ethylene, methane, etc., using commercially viable current densities, with stable and significantly high energy efficiency and yield. Failure to meet these requirements can have serious economic implications, thus necessitating a solution. In this invention, we have developed a process incorporating a metal-based GDE to improve ECR performance.

[0038] In this invention, we address the pressing and critical issue of long-term stability of electrocatalysts used for CO2 reduction and invent a method for reactivating metal-based GDEs after long-term operation (≥1000 h). This invention provides a method (process) and apparatus for increasing the lifetime and stability of metal-based GDEs to enhance the Faradaic efficiency (FE) with respect to ECR activity when used in electrochemical cells. This is also crucial for achieving an overall energy-efficient process. The process combines the inherent properties of the electrolyte used with changes due to polarity shifts at the induced electrode to achieve the aforementioned requirements and has been found to be particularly useful for carbon-free metal-based GDEs, such as the carbon-free Sn-GDE disclosed in the applicant's earlier patent application WO2022013042A1.

[0039] Therefore, in one aspect of the invention, a carbon-free metal-based GDE is used; in particular, a carbon-free Sn-GDE; and even more particularly, a carbon-free Sn-GDE electrode prepared by the method disclosed in patent number WO2022013042A1 is used.

[0040] The device used typically refers to an electrochemical cell, which includes: a metal-based GDE working electrode as the cathode, an anode (particularly an inert anode, such as an inert Pt anode), and an electrolyte; thereby, gas is supplied to the GDE using the electrolyte, and the electrolyte permeates to the opposite side of the GDE; the cell is used to allow current to flow externally through a load circuit connecting the anode and cathode. As is known to those skilled in the art, in electrochemical cells including a GDE, the anode compartment and the cathode compartment are typically separated by an ion-conducting membrane (such as an anion exchange membrane (AEM), a cation exchange membrane (CEM), or a bipolar membrane (BPM)), wherein each of the anode compartment and the cathode compartment respectively contains an anolyte solution and a catholyte solution.

[0041] The method according to the invention is a method of operating such a device in the electrochemical conversion of gaseous reactants (such as CO2, CO, N2, or O2) into bulk chemicals and fuels (such as syngas, formic acid, methanol, ethanol, ethane, ethylene, methane, ammonia, etc.); characterized by maintaining the pH value of the electrolyte within a range of 2 units of its initial value, and applying anodic pulses to the GDE working electrode (i.e., the cathode) at fixed intervals. In the example where the electrochemical cell includes an anode compartment and a cathode compartment, the method is characterized by maintaining the pH value of the cathode electrolyte within a range of 2 units of its initial value, and applying anodic pulses to the GDE working electrode (i.e., the cathode) at fixed intervals.

[0042] As is evident from the examples below, the electrocatalytic instability and observed deactivation / degradation of metal-based GDEs may be related to the inherent characteristics of the electrode and its local environment (e.g., K) during the electrolysis process. + HCOO - CO3 2- The concentration of these ions is related to changes in the concentration of other substances. For example, in the case of electrochemical CO2 reduction (ECR), the reduction in formate ion production observed at the GDE cathode is directly related to a decrease in the pH of the catholyte, which favors the competitive hydrogen evolution reaction (HER). Controlling the pH of the catholyte and maintaining it within 2 units (especially 1 unit) of its initial value—the pH of the fresh catholyte at the start of the electrochemical process—counts to counteract such reactions.

[0043] These operating conditions themselves prevent the Faraday efficiency (FE) from dropping below 40%, but are insufficient to restore the FE to near its initial value, i.e., the FE at the start of the electrochemical process. Faraday efficiency (FE) describes the selectivity of an electrochemical process for a specific target product and is defined as the amount of product collected relative to the amount that can be produced from the total charge passing through, expressed as a fraction or percentage.

[0044] By combining pH control of the cathode electrolyte with an anodic pulse at the cathode, the FE yield of the process is restored to its initial value. Note that in the context of this invention, "anodic pulse" is intended to refer to a reverse current or potential pulse applied to the working electrode, i.e., the electrode on which the reaction of interest occurs in the electrochemical reaction system. Depending on whether the reaction at the electrode is a reduction or oxidation reaction, the working electrode is referred to as either a cathode or an anode. In the example of this invention, the GDE working electrode is a cathode, but the invention is not limited thereto. The pulse is reversed relative to the operating charge at the working electrode in the electrochemical conversion of gaseous reactants into bulk chemicals. For example, during ECR, a negative potential (reduction current) is applied to the GDE working electrode, and the anodic pulse will consist of a positive potential (oxidation current) applied to the GDE working electrode.

[0045] In an embodiment of the method according to the invention, the anode pulse consists of a current applied to the GDE working electrode (cathode) of 50-200 mA cm⁻¹. -2 The current is preferably applied for at least 30 seconds; in particular, at least 100 mAcm. -2 The current is preferably applied for at least 1 minute. In an embodiment of the method according to the invention, the anode pulse consists of the following current applied to the GDE working electrode (cathode): 50-200 mA cm⁻¹ -2 A constant current is preferably applied for at least 30 seconds; in particular, at least 100 mA cm⁻¹. -2 A constant current is preferably applied for at least 1 minute.

[0046] In an embodiment of the method according to the invention, the anode pulse consists of a charge of 1.5-10 coulombs (C) cm applied to the cathode. -2 The positive charge; in particular, at least 6 C cm applied to the cathode. -2 The charge. In an embodiment of the method according to the invention, the anode pulse consists of the following charge: 1.5-10 coulombs (C) cm applied to the cathode. -2 A constant charge; specifically, at least 6 C cm applied to the cathode. -2 A constant charge.

[0047] In an embodiment of the method according to the invention, the anode pulse consists of a current applied to the cathode of 50-200 mA cm⁻¹. -2 The current is preferably applied for at least 30 seconds; in particular, at least 100 mA cm⁻¹.-2 The current is preferably applied for at least 1 minute, particularly up to about 10 minutes, more particularly from about 4 to 8 minutes; wherein the anodic pulse is applied at fixed intervals, the fixed intervals being to begin after 6 hours, particularly after 12 hours, more particularly after 24 hours, and even more particularly every 48 hours. In an embodiment of the method according to the invention, the anodic pulse consists of the following current applied to the cathode: 50-200 mA cm⁻¹ -2 A constant current is preferably applied for at least 30 seconds; in particular, at least 100 mA cm⁻¹. -2 A constant current is preferably applied for at least 1 minute, particularly up to about 10 minutes, more particularly from about 4 to 8 minutes, wherein the anode pulse is applied at fixed intervals, the fixed intervals being to begin after 6 hours, particularly after 12 hours, more particularly after 24 hours, and even more particularly every 48 hours.

[0048] In an embodiment of the method according to the invention, the anode pulse consists of a charge of 1.5-10 coulombs (C) cm applied to the cathode. -2 The charge; in particular, at least 6 C cm applied to the cathode. -2 The charge, wherein the anodic pulse is applied at fixed intervals, starting after 6 hours, particularly after 12 hours, more particularly after 24 hours, and even more particularly every 48 hours. In an embodiment of the method according to the invention, the anodic pulse consists of a charge of 1.5-10 coulombs (C) cm⁻¹ applied to the cathode. -2 A constant charge; in particular, at least 6 C cm applied to the cathode. -2 A constant charge, wherein an anode pulse is applied at fixed intervals, starting after 6 hours, particularly after 12 hours, more particularly after 24 hours, and even more particularly every 48 hours.

[0049] Alternatively, the anodic pulse can be expressed as the magnitude of electrical charge, i.e., 6 C cm applied to the cathode. -2The charge. Referring to the example, it has been observed that a charge of 15–60 C is insufficient to increase FE to its initial value. While theoretically sufficient to alter the polarity near the primarily affected double-layer charging area, the recovery of FE obtained with such a small charge (15–60 C) does not have a lasting effect on the electrode's selectivity with respect to ECR. Only a 10%–15% increase in FE was observed, which decreased to its pre-pulse value within 24–48 hours. In contrast, at charges between 240 C and 360 C, FE increased by 20%–30%, which had a lasting effect on the electrode's ECR selectivity. It is speculated that such high charge values ​​are sufficient to oxidize the metal catalyst present in the GDE, for example, from Sn metal to SnO2, and this also removes impurities deposited on the electrode during long-term processes, and also functions as a cleaning method for the electrode. For example, when Sn-GDE experiences a cathode potential after an anode pulse, as disclosed herein, it is reduced back to metallic tin, making the electrode more porous, increasing the three-phase interface, and increasing the FE of the formate.

[0050] Maintaining the cathodic electrolyte pH and conductivity (Condition I) in combination with anodic pulses (Condition II) helps restore the electrode's initial selectivity for ECR and allows the electrochemical cell to operate for at least 1000 hours and beyond. Failure to meet these conditions (Condition I and Condition II) at any time may result in a decrease in the electrode's selectivity for ECR.

[0051] In summary, by combining Condition I and Condition II, metal-based GDEs, particularly Bi-GDEs or Sn-GDEs, can be reactivated to their initial form / activity. Condition I is maintaining the pH of the electrolyte within 2 units of its initial pH, more particularly maintaining the pH of the electrolyte (and even more particularly the cathodic electrolyte) at a weakly alkaline pH (from about 7 to about 9). Condition II is employing anodic pulses at fixed intervals of 48 hours for several minutes (4-8 minutes) to prolong the activity of Sn-GDEs for ECR. In embodiments, Condition II is represented as an anodic pulse with a charge of at least 120 coulombs (C); particularly an anodic pulse with a charge of at least 240 C; more particularly an anodic pulse with a charge of up to about 600 C; and even more particularly an anodic pulse with a charge from about 240 C to 360 C and between 240 C and 360 C.

[0052] Example The experimental setup mentioned in Example 1 was used ( Figure 1), including an electrochemical cell for electrochemical CO2 reduction (ECR) containing Sn-GDE as disclosed in WO2022013042A1, at 100 mA / cm 2 At the given current density, it was initially stable for 200 h. The stability of the electrode is represented by the Faradaic efficiency (FE) of formate ions generated by the ECR. After 500 h of operation, the FE of formate ions decreased from 75% to 25%. Figure 2 The sustained decrease in formate ions (FE) may be directly related to the decrease in the pH of the cathode electrolyte. A decrease in pH favors HER (the reaction that competes with ECR). Figure 3 To inhibit HER and promote ECR, the electrolyte was replaced with fresh 0.5 M KHCO3 to restore the pH to its initial value of 7.5, causing the formate FE to increase from 25% to 40%. Furthermore, this change in FE was stable if the pH of the catholyte was 7.0 ± 0.5 and the conductivity was ≥ 45 ± 5 mS / cm. Subsequently, an anolyte pulse was applied to the cathode, such as... Figure 4 As shown, this is done to restore the electrode activity to its initial level.

[0053] An anode pulse (I = +1 A, j = +100 mA / cm) is introduced every 48 hours of battery operation. 2 The anode pulse is in the form of a constant current pulse of anode current that lasts for several minutes (i.e., the anode pulse is applied in the form of a constant current for a given period of time).

[0054] As is evident from Examples 2 and 3, similar results can be obtained using other metal-based GDEs.

[0055] Example 1 - Sn-GDE in a Flow Cell All electrochemical experiments were performed using a VSPBioLogic potentiostat with a VMP3 BioLogic current booster. ElectroCell® microflow cells were employed, with an effective active area of ​​10 cm² for the working electrode. 2 All experiments were conducted in constant current mode with a current density of 100 mA / cm². 2 ( Figure 5 Before each electrochemical experiment, the cathode electrolyte (0.5 M KHCO3) was saturated with CO2 and incubated overnight (6 h–8 h). BPC GO was used at the cell outlet. ®A flow meter is used to monitor the gas flow rate from the battery outlet. To maintain identical reaction conditions, the application of current (I) is delayed until a stable signal of outlet CO2 is recorded by the BPC GO® flow meter for 20–30 minutes. This ensures identical reaction conditions in terms of gas mass balance, pH, and cathode electrolyte conductivity before the experiment begins. The battery is designed to operate in flow-through mode. Figure 6 ), having a 100cm diameter as the working electrode 2 A geometrically large, carbon-free Sn-GDE (as disclosed in patent number WO2022013042A1) was used as the size-stabilized anode (DSA) and the Ag / AgCl electrode as the reference electrode (RE). The pH and conductivity of the bulk cathode electrolyte were monitored throughout these experiments. Changes in the WE potential were also monitored throughout the experiments. Figure 7 ).

[0056] The experimental mode was a single partial pass, which was achieved by circulating the electrolyte from a large feed reservoir (5 L), inversely to the electrolyte volume (calculated for both the anode and cathode). The cathode electrolyte was periodically replenished to maintain the electrolyte pH and conductivity within the ranges of 7.8–6.5 and 45–80 mS / cm, respectively.

[0057] Cathode electrolyte: 0.5 M KHCO3, 5 L (replace regularly), pre-saturated with CO2. Anodic electrolyte: 2 M KOH, 2 L (never refreshed during 1000 h of operation). Electrolyte flow rate: 50-60 mL / min CO2 flow rate: 30 mL / min Membrane: Fumasep BPM (Fumasep FBM-PK) Operating mode: Flow mode — blocks the battery's gas outlet and forces the gas through the electrolyte ( Figure 6 ).

[0058] The experiment was designed in constant current mode to avoid the potential influence of external parameters such as electrolyte recirculation, flow rate, and electrode flooding.

[0059] The products of this electrochemical reaction were analyzed by gas chromatography (GC) (for gaseous products such as CO and H2) and high performance liquid chromatography (HPLC) (for liquid products such as formate ions).

[0060] A constant current is applied across the electrochemical cell to obtain 100 mA / cm 2The current density (commercially appropriate current density). During operation, monitor the pH and conductivity of the bulk cathode electrolyte. Figure 2 Liquid samples were periodically collected from the bulk cathode electrolyte and their formate ions were analyzed by HPLC. The experimental setup was connected to a headspace GC for gas analysis.

[0061] Example 2 - Bi-GDE in a Flow Cell Using similar experimental conditions to those used in the ECR experiment with Sn-GDE (Example 1), FE was produced from formic acid starting with ECR using BiGDE (a Bi powder-based GDE prepared in exactly the same manner as Sn GDE described in WO2022013042A1). Figure 8 The results are shown in the figure. Notably, after the FE to formic acid decreased from an initial 95% to about 70% at 260 h, the FE to formic acid was significantly recovered by applying pulses (an anodic pulse was applied every 12 h starting at 280 h). Figure 9 The effect of the pulsed method is directly illustrated. After the first pulse (1000 mA, 2 min), the FE to formic acid increased from 70% to approximately 87%, and then began to level off over time. However, by using periodic pulses (1000 mA, 1 min every 12 hours), the FE to formic acid could be maintained within a window of 85%–90%, making a stable and controllable process for the electrochemical production of formic acid from CO2 promising.

[0062] Example 3 - Sn-foil in H-type battery Using an H-type cell structure to understand the effect of anode pulses on the Sn plate ( Figure 10 ).

[0063] Details of the H-type battery structure: Anodic electrolyte: 0.1 M KHCO3, 8 mL Cathode electrolyte: 0.1 M KHCO3, 8 mL CO2 removed from solution: 10 mL / min Membrane: AEM (FAA-3-50) A potentiostatic test was performed using a fixed potential of -1.0 V relative to RHE for 30 min, and its activity for ECR in formate production was monitored. Prior to electrocatalysis at -1 V relative to RHE, an increase in FE of formate ions from 60% to 90% was observed under a sustained anodic pulse of 0.1 V for 30 min. Figure 11 ).

[0064] Example 4 - Sn-GDE (acidic medium) in a flow cell The same experimental setup as that used in the ECR experiment using Sn-GDE (Example 1) was employed, using the following electrolytes as mentioned below.

[0065] Cathode electrolyte: 0.25 M K₂SO₄, 5 L, acidified with H₂SO₄, pH=3 (updated periodically), pre-saturated with CO₂. Anodic electrolyte: 2 M KOH, 2 L (never refreshed during 1000 h of operation). Electrolyte flow rate: 50-60 mL / min CO2 flow rate: 30 mL / min Membrane: Fumasep BPM (Fumasep FBM-PK) exist Figure 12 The regeneration of Sn-GDE in an acidic electrolyte is demonstrated. Similar to the alkaline environment, the FE produced from formic acid using ECR starts with a high FE (>80%), but decreases to 20% earlier than under alkaline conditions after 200 h. Notably, the FE to formic acid was recovered by pulse (anodic pulses applied every 12 h starting from 240 h), although not to the same extent as in the alkaline medium, but the trend was similar and consistent, indicating the applicability of the proposed process in both alkaline and acidic environments.

Claims

1. A method of operating an electrochemical cell to electrochemically convert gaseous reactants into bulk chemicals at a working electrode, the method comprising using a metal-based GDE working electrode as a cathode, an anode, and an electrolyte, the method being characterized by maintaining the pH of the electrolyte within a range of 2 units of its initial value, and applying anodic pulses to the working electrode at regular intervals.

2. The method according to claim 1, wherein the electrolyte is a cathodic electrolyte present in the cathode compartment of the electrochemical cell, and the method is characterized in that: the pH value of the cathodic electrolyte is maintained within 2 units of its initial value, and an anodic pulse is applied to the working electrode at fixed intervals.

3. The method according to claim 1 or 2, wherein the anode pulse consists of the following current applied to the working electrode: 50-200 mA cm -2 The current is preferably applied for at least 30 seconds; in particular, at least 100 mA cm⁻¹. -2 The current is preferably applied for at least 1 minute.

4. The method according to claim 1 or 2, wherein the anode pulse is composed of 1.5-10 coulombs / cm applied to the cathode. -2 The charge composition; specifically, consisting of at least 6 coulombs cm⁻¹ applied to the working electrode. -2 The composition of charge.

5. The method according to any one of the preceding claims, wherein the anode pulse is applied at fixed intervals ranging from 1 hour to 48 hours.

6. The method according to any one of the preceding claims, wherein the gaseous reactant is selected from CO2, CO, N2, NO. x Or O2; specifically, CO2.

7. The method according to any one of the preceding claims, wherein the bulk chemical is selected from syngas, formic acid, methanol, ethanol, ethane, ethylene, methane, ammonia, hydrogen, hydrogen peroxide, etc.

8. The method according to any one of the preceding claims, wherein in the case of an alkaline electrolyte, the pH value is maintained at about 7 to about 9, and in the case of an acidic electrolyte, the pH value is maintained at about 3 to about 5.

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