Deriving an efficient and stable microenvironment for a CO2 MEA electrolyzer by reverse osmosis

doi:10.21203/rs.3.rs-3562766/v1

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Deriving an efficient and stable microenvironment for a CO₂ MEA electrolyzer by reverse osmosis

https://doi.org/10.21203/rs.3.rs-3562766/v1

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In a membrane electrode assembly (MEA) electrolyzer based on a cation exchange membrane, achieving an efficient and stable CO₂ reduction reaction (CO₂RR) is challenging because the transport of protons, cations, and electroosmotic water from the anode changes the balance of ions. Herein, we derived a microenvironment for stable and efficient CO₂RR performance using two strategies. First, mixture of carbon and anion- exchange ionomer buffer layers is used to hold cations while managing water in local alkaline media. The second strategy involves pressurizing only the cathode side, resulting in a high local CO₂ concentration and enhanced the reverse osmosis phenomenon. The synergistic effects of these two strategies create an efficient microenvironment by managing water and cations, leading to stable and efficient CO₂RR operation. Our approach to utilize reverse osmosis to balance cations and water is viable for industrial applications because pressurized CO₂ and MEA systems are efficient processes that can be commercialized.

Physical sciences/Chemistry/Electrochemistry/Electrocatalysis

Physical sciences/Chemistry/Catalysis/Electrocatalysis

The electrochemical CO₂ reduction reaction (CO₂RR) is a promising technology for recycling CO₂ into value-added chemicals, while utilizing renewable electricity.^1–5 To achieve practical-scale electrochemical CO₂RR, the design of devices and systems has been focused. A membrane electrode assembly (MEA) is a promising configurations among various cells that uses a gas diffusion electrode (GDE),^6–9 which can achieve a high current density by decreasing the ohmic resistance with a zero-gap electrode configuration.^10,11

The local reaction microenvironment plays a crucial role in determining the performance of the CO₂RR in a MEA configuration. Anion exchange membrane (AEM)-based MEAs exhibit high selectivity and Faradaic efficiency (FE) owing to their alkaline reaction environments. However, the crossover of CO₂ via bicarbonate ion transfer through the membrane reduces the single-pass conversion (SPC) of CO₂.^12,13 To suppress CO₂ crossover and achieve a high SPC, cation exchange membrane (CEM)-based CO₂RR systems have been suggested.^14–16 Although the CEM-based MEA system effectively suppress the crossover of CO₂, the FE of the CO₂RR is significantly lower because the local acidic environment close to the CEM derived from protons accelerates the hydrogen evolution reaction (HER).

Local pH is the key factor that increases the CO₂RR activity by creating alkaline media during the reaction. In the CO₂RR, water dissociation occurs to supply protons, generating hydroxides at the cathode. Therefore, a strategy to prevent direct contact between the CEM and cathode by changing the structure of the MEA has been proposed, as listed in Table S1. In this approach, buffer structures, such as bipolar membranes or anion exchange ionomer (AEI) layers, have been adapted between the CEM and cathode.^15–18 An AEI is suitable as a buffer layer owing to its high conductivity, high-CO₂ solubility, and ability to hinder proton transfer.¹⁹ In a buffer structure, a neutralization boundary layer (NBL) inevitably forms between the acidic CEM and the alkaline cathode under CO₂RR conditions. Thus, minimizing the NBL of the buffer plays a key role in increasing local pH at the cathode. The NBL in the buffer layer is affected by the distribution of cations and water at the cathode. Alkali cations play an essential role in stabilizing the intermediates of the CO₂RR and enabling the derivation of alkaline media.²⁰ Water, the proton source of the CO₂RR, determines the concentration of ions; however, excess water washes out ions and induces flooding, degrading the CO₂RR activity.²¹ In CEM-based MEAs, water crosses proportional with cations owing to electroosmosis, causing difficulty in the management of water and cations at the cathode. Therefore, strategies for balancing cations and water can be an issue in CEM-based CO₂ electrolyzers.

In this study, we created an efficient and stable microenvironment for a CEM-based CO₂ electrolyzer by reverse osmosis using two strategies: modification of the cathode structure and pressurization of CO₂. In the first strategy, an electrode structure modified using an AEI and carbon black (C) suppresses the HER by neutralizing protons and holding alkali cations. In the second strategy, pressurized CO₂ enhances not only CO₂ adsorption on the cathode to overcome the mass transfer limitation of CO₂ but also the reverse osmosis phenomenon for maintaining sufficient alkali cation concentration by water management. Furthermore, these two strategies have a synergistic effect in accelerating the reverse osmosis phenomenon, establishing the optimal reaction environment for the CO₂RR.

Effect of Electrode structure on CO₂RR performance

As a first strategy to overcome the low CO₂RR FE of CEM-based MEA cells by modifying the cathode structure, we prepared four types of cathodes: Ag black, two layers of Ag black and AEI buffer (Ag/AEI), two layers of Ag black and AEI-carbon buffer (Ag/AEI-C), and a mixture of Ag, AEI, and carbon (Ag-AEI-C), as shown in Fig. 1a. Carbon is widely used as a catalyst material because of its large surface area, high porosity, and hydrophobicity.^22,23 The amounts of AEI and C in the buffer layer were maintained at an AEI/Ag ratio of 2.5, and a C/Ag ratio of 0.5, based on weight. The cross-sectional scanning electron microscopy (SEM) image of each electrode demonstrates that the structure of the electrodes was congruent with the design by the spraying method (Fig. S1).

To determine the effect of the electrode structure, the CO₂RR performance of the manufactured electrodes was examined in a CEM-based zero-gap electrolyzer under ambient pressure. The cell voltage increases with electrode thickness because the ion transport resistance (generally, the membrane resistance in MEAs) is significantly influenced by the distance of the ion pathway (Fig. 1b). For the Ag black electrode, the Faraday efficiency of CO production (FE_CO) was approximately 20% over whole range of applied current density, and the maximum partial current density of CO (j_CO) is only 37.9 mA cm^− 2 at 200 mA cm^− 2 (Fig. 1c and Fig. S2a). These results indicated an acidic reaction environment for the CO₂RR in the CEM-based MEA electrolyzer.

For the Ag/AEI electrode, a high FE_CO (80.4%) was achieved at 50 mA cm^− 2. However, as the applied current density increased, the FE_CO sharply decreased to below 40%. In contrast, in presence of an AEI-C layer, a relatively high FE_CO was achieved until 150 mA cm^− 2 with FE_CO values of 59.2% and 61.5% at 50 and 100 mA cm^− 2, respectively. The AEI and AEI-C buffer layers have similar thicknesses, indicating that the activity of the CO₂RR is influenced not only by the buffer layer thickness but also by the properties of the buffer layer. Interestingly, the Ag-AEI-C electrode without a buffer layer exhibits a higher CO₂RR performance than the Ag/AEI electrode at current densities higher than 100 mA cm^− 2, suggesting that the properties of the electrode strongly influence the local reaction environmental for the CO₂RR.

Cation and water distribution of different electrode structures

The local reaction environment of the MEA is significantly affected by the distribution of cations and water at the cathode. To gain insight into the cation and water distribution under CO₂RR conditions, ex situ SEM-energy-dispersive X-ray spectroscopy (EDS) and in situ/operando micro-computed tomography (micro-CT) analyses were performed. The ex situ cross-sectional SEM-EDS images of each electrode were obtained after chronopotentiometry experiments under 100 mA cm^− 2 for 30 min (Fig. 2a). The EDS mapping represents the cation distribution in each electrode structure. The alkali cations exist close to the Ag catalyst because the charged cathode surface has a high cation density. However, the number of cations differed for each electrode. The K/Ag ratio of each electrode increased in the order Ag/AEI (0.08) ≈ Ag black (0.09) < Ag-AEI-C (0.28) < < Ag/AEI-C (1.29). When C was present in the electrode, the K⁺/Ag ratio increased, indicating that carbon can preserve more alkali cations than AEI under CO₂RR conditions.

Observation of the water distribution in the electrode during the CO₂RR is difficult using the ex situ technique. Thus, micro-CT was employed to trace water in the MEA CO₂RR electrolyzer. Each material exhibits a different transmittance to X-rays, and the amount of water crossover from the anode to the cathode can be distinguished in a 2D image. The 3D CT image was obtained by repeating the 2D analysis with rotation of the CT cell and Fourier transform-recombination of the 2D image.²⁴ A customized micro-CT device was manufactured to implement the MEA CO₂ electrolyzer for in situ/operando micro-CT analysis (Fig. 2b and Fig. S3). A micro-CT cell has the same configuration as a typical MEA electrolyzer but is constructed with an acrylic body to allow X-ray transmission. The acrylic body with a screw head was sealed with a screw cap and an acrylic pillar to press the MEA electrolyzer. The in situ/operando micro-CT images of the three-dimensional structure of the Ag/AEI and Ag/AEI-C electrodes were obtained at a current density of 100 mA cm^− 2 (Fig. 2c and d). Before carrying out the CO₂RR reaction, CEM, C, AEI, Ag layers and a gas diffusion layer (GDL) were observed (Fig. S4a and b). During the CO₂RR, water transported from the anode was observed at the cathode side. In the case of Ag/AEI, almost half of the back GDL (gray) was wet (6.97 v/v% based on the total GDL volume; Fig. 2c) from transported water (blue), whereas in the case of Ag/AEI-C, only a few drops of water (0.05 v/v% based on the total GDL volume; Fig. 2d) were detected in the back GDL. The specific XY-plane of the GDL also represented a fully wetted Ag/AEI electrode (Fig. S4c). These results suggest that carbon enables the management of both cations and water in the CEM MEA electrolyzer.

The impact of electrode structure

Based on the CO₂RR performance, cation and water distributions, and previous literature, we delineated the effect of the electrode structure on the CO₂RR. In the case of Ag black (Fig. 3a), the CEM surface has abundant transferred protons, and the Ag catalyst layer is close, leading to an acidic environment for the CO₂RR. Furthermore, H₂O that transfer via electroosmosis diminished the concentration of alkali cations. This leads to poor CO₂RR performance, which was mentioned as one of the disadvantages of CEM-based CO₂RR electrolyzers in the Introduction. When the AEI buffer layer was placed over Ag black (Ag/AEI), the CEM surface and Ag catalyst layer were separated by the AEI buffer layer. In this buffer, the neutralization and regeneration of CO₂ occur via protons from the CEM and bicarbonate ions from the Ag catalyst layer (Fig. 3b). The AEI buffer structure derives local alkaline media for Ag/AEI electrode, resulting in high selectivity for the CO₂RR at a current density of 50 mA cm^− 2. However, the CO₂RR performance of Ag/AEI degrades with increasing current density. Based on the ex situ SEM and in situ/operando micro-CT results, the Ag/AEI electrode possessed a small amount of alkali cations and abundant water during the reaction. The amount of water transferred via electroosmosis increases with current density. The hydrophilicity of the AEI buffer prevents water from draining during the CO₂RR, leading to flooding. The large amount of water in the AEI layer reduced the cation concentration and washed out the cations from the cathode. Alkali cations stabilize the intermediates of the CO₂RR and derive an alkaline environment via water dissociation.²⁰ Therefore, the NBL increased with increasing current density, and the local alkaline environment of the Ag/AEI electrode was converted to acidic media, leading to low selectivity for the CO₂RR.

In the Ag/AEI-C electrode, with only carbon added to the Ag/AEI electrode, a high FE_CO for the CO₂RR was maintained in the high-current density region. These results suggest that carbon enhances the effects of the buffer layer in CEM-based MEA electrolyzers. The properties of carbon black are advantageous for holding large amounts of alkali cations and mixing the ion flux from the CEM and cathode (Fig. 3c). Furthermore, the flooding phenomenon disappeared in the Ag/AEI-C electrode according to the in situ/operando micro-CT results. This suggests that the hydrophobicity of C facilitates the management of electroosmotic water (Fig. S5). The characteristics of Ag/AEI-C accelerated the neutralization and regeneration of CO₂ in the buffer layer, reducing the thickness of the NBL and deriving a local alkaline environment for the cathode. This derived reaction environment enhanced the CO₂RR performance of the Ag/AEI-C electrode in the high-current density region.

Effect of pressurized CO₂ on the CO₂RR

To assess the second strategy to promote the CO₂RR activity of CEM-based MEA cells by pressurized CO₂, the CO₂RR performance of the Ag/AEI and Ag/AEI-C electrodes was evaluated in a CEM-based MEA electrolyzer at pressures of 2–6 atm (Fig. 4a and b, Fig. S6 and S7). For both electrodes, the performance of CO₂RR increased as the CO₂ pressure increased, indicating that pressurized CO₂ enhanced the kinetics of the CO₂RR, which is in good accordance with previous research.^25–30 However, when the current density increased, the trend of FE_CO improvement with pressurized CO₂ was different for each electrode. Under ambient pressure (1 atm), the FE_CO values of the Ag/AEI and Ag/AEI-C electrodes decreased with increasing current density. The FE_CO of the Ag/AEI electrode showed a similar trend, even when the CO₂ pressure was increased to 6 atm. Under 6 atm, the Ag/AEI electrode achieved a maximum j_CO of 115.9 mA cm^− 2 (Fig. S6b). In contrast, the FE_CO of the Ag/AEI-C electrode increased up to 200 mA cm^− 2 and was maintained until 350 mA cm^− 2. The combination of the Ag/AEI-C electrode and pressurization system achieved a maximum j_CO of 222.8 mA cm^− 2 (Fig. S7b). As the current density increased, the amount of transferred protons and electroosmotic water increased, boosting the HER. However, the local pH of the cathode increases with current density.³¹ The performance of the buffer layer in preserving the local pH was fixed according to the current density. However, the effect of the AEI-C buffer was enhanced with pressurized CO₂, demonstrating that the AEI-C buffer layer and CO₂ pressure have a synergistic effect on increasing the local pH of the cathode for the CO₂RR.

To investigate the stability of developed electrode with pressurized CO₂ CEM-based MEA electrolyzer, we conduct a chronopotentiometry test at constant current density of 100 mA cm^− 2 for 12 h (Fig. 4c). It is difficult to achieve stable operation of the CEM-based MEA electrolyzer because the reaction environment is unstable owing to the cations and electroosmotic water passing through the membrane.³² These phenomena can result in the flooding of the electrode, washing out of cations, and blocking of CO₂ flow plate by salt precipitation, acting as an obstacle for the CO₂RR. Under 1 atm, the Ag/AEI electrode exhibited poor stability, whereas the Ag/AEI-C electrode exhibited relatively stable CO selectivity for 6 h. This suggests that the cation-holding ability of the AEI-C buffer layer enhances the stability of the CEM-based MEA electrolyzer. Under 6 atm CO₂, the stability of both electrodes is improved, and the Ag/AEI-C electrodes show no degradation after 12 h. These results indicate that pressurized CO₂ not only provides a high FE but also good stability to the CEM-based MEA electrolyzer. Moreover, the synergistic effect of the AEI-C buffer layer and CO₂ pressure maintain an effective reaction environment for the CO₂RR. To increase the alkali cation concentration, the CO₂RR was conducted with a 0.1 M KHCO₃ anolyte. The initial CO selectivity increased to approximately 94.5%; however, the CO₂ flow plate was blocked at approximately 4 h because of salt formation, and the CO selectivity drastically decreased to 63.1% (Fig. S8b). We expected that an excessive amount of K⁺ passing through the membrane would block the CO₂ flow plate and cause flooding by accelerating the HER (Fig. S8c).

SPC is a major advantage of CEM-based electrolyzers. Thus, we conducted CO₂RR under 6 atm with a different flow rate of 10–50 mL min^− 1 to measure the SPC of pressurized CO₂RR (Fig. 4d). In an AEM, the amount of crossover CO₂ is proportional to the current density. The CEM blocks the electrochemical crossover of CO₂; however, a small amount of CO₂ physically crosses at a pressure of 6 atm (Fig. S9a). When the CO₂ flow rate decrease to 10 mL min^− 1, maximum SPC and outlet syngas concentration of 51.6% and 53.3%, respectively, were obtained (Fig. 4d and Fig. S9b).

The impact of pressurized CO₂

To determine the effect of CO₂ pressurization on the local environment, a pressurized CO₂RR was conducted using a CEM-based MEA cell with different alkali cations (K⁺ and Cs⁺). The cation effect is widely known to be a crucial factor in the CO₂RR.^20,33,34 According to theoretical research, large alkali cations, such as Cs⁺, possess a small hydration shell, forming a thin electrical double layer with a strong electric field.³⁵ This leads to a high local CO₂ concentration, stabilization of the intermediates, and increased accumulation of cations, and reduced HER activity. In Fig. 5a and Fig. S10a, FE_CO and j_co increased with Cs⁺ compared with K⁺ under 1 atm, which is in good agreement with previous research.^17,20,34 In the case of K⁺, more than 50% of the FE_CO was obtained up to 150 mA cm^− 2, but HER became dominant reaction when the applied current density exceeds 150 mA cm^− 2. In the case of Cs⁺, 50% of the FE_CO was obtained at 250 mA cm^− 2. Interestingly, no difference in the CO₂RR activity between K⁺ and Cs⁺ was observed at 6 atm CO₂ (Fig. 5b). Under 6 atm of CO₂, FE_CO increased from approximately 60% at 50 mA cm^− 2 to approximately 80% at 150–200 mA cm^− 2, and as the applied current density increased, FE_CO decreased, and finally, HER became dominant at 400 mA cm^− 2 regardless of the cation. These results show that the CO₂RR activity in the CEM-based MEAs was almost independent of the cation effects under highly pressurized CO₂. The cell voltage was reduced by pressurized CO₂, indicating that pressurized CO₂ improves the kinetics of the CO₂RR, regardless of the alkali cation. The cell voltage increased slightly in the case of Cs⁺ because Cs⁺ has a higher resistance than K⁺ to penetrate the membrane (Fig. 5c).³⁶

The cation effect in CEM-based MEAs under highly pressurized CO₂ provides insight into the cation effects in MEA devices and the effects of CO₂ pressure. The cation effect is typically studied in CO₂ saturated electrolytes at low current densities. MEA electrolyzers with a GDE have been developed for the mass transfer of CO₂ and exhibit a large current density. The impact of the cation effect on MEA electrolyzers differ from that of theoretical research. Among the cation size effects in the CO₂RR, the local concentration of CO₂ with a pH buffer controlled by the hydrated cation size in the outer Helmholtz plane is associated with the CO₂ pressure. Therefore, we expected that cation effects would mainly contribute to the local concentration of CO₂ to increase the CO₂RR performance in MEA electrolyzers. Under sufficiently high CO₂ pressures, the pressurization effect enhanced the pH buffer with local CO₂ concentration beyond the cation size effect, resulting in a high FE_CO in the high-current density region (Fig. 5d).

Despite the pressure effect derived from the cation experiment, the explanation for the synergistic effect of the AEI-C buffer layer and CO₂ pressure is unclear. A high local CO₂ concentration by pressurization would be applied regardless of the buffer layer, indicating that CO₂ pressure can affect the cation and water distribution in the buffer layer. To unveil the origin of this synergistic effect, ex situ SEM-EDS analysis of the Ag/AEI and Ag/AEI-C electrodes was performed at a pressure of 6 atm. In the case of Ag/AEI-C, the preservation of K⁺ (K⁺/Ag ratio of 0.254) was higher than that of Ag/AEI (K⁺/Ag ratio of 0.023) under 6 atm CO₂, which is similar to that under ambient pressure (Fig. S11). The amount of cations significantly decreased under a pressure of 6 atm compared to that under ambient pressure. We propose that reverse osmosis is key to explaining this synergistic effect (Fig. 5e). Because a MEA is a system that separates anode and cathode using a membrane, osmotic pressure can be applied.³⁷ CO₂ pressure was applied only to the cathode, and reverse osmosis occurred when water moved from the cathode to the anode. To prove this hypothesis, water crossover for the Ag/AEI and Ag/AEI-C electrodes during the CO₂RR was measured in a CEM-based MEA electrolyzer. The CO₂ pressurization decreased water crossover from 3.83 g (1 atm) to 1.55 g (6 atm) and from 5.46 g (1 atm) to 0.95 g (6 atm) for Ag/AEI and Ag/AEI-C electrode, respectively (Table S2). This supports the reverse osmosis phenomenon in the MEA electrolyzer. The AEI-C buffer layer acts as a reservoir for cations, further increasing the cation concentration of the buffer layer by reverse osmotic water under high CO₂ pressure, minimizing the NBL and creating a local alkaline environment for the Ag catalyst. The osmotic pressure is proportional to the ion concentration and the electrode structure determines the amount of preserved cations, indicating that the CO₂ pressure can govern the amount of cations and water at the cathode side of a MEA electrolyzer. This can be the origin of the synergistic effects of the AEI-C buffer layer and CO₂ pressure that enhance the CO₂RR activity. Furthermore, the movement of reverse osmotic water can offset the movement of electroosmotic water, suggesting a solution to the water and ion balance issues in CEM-based MEAs. This leads to a stable CO₂RR operation under high CO₂ pressures.

We demonstrated an efficient and stable CO₂RR in CEM-base MEA electrolyzers using reverse osmosis. To maximize the reverse osmosis phenomenon, two strategies were used; the first strategy uses an AEI-C buffer layer at cathode to accelerate local pH effects by the neutralization of protons and preserve cations by managing water under CO₂RR. The second strategy involved the application of a pressure difference by applying a high feed CO₂ pressure only on the cathode side. Pressurized CO₂ not only led to a high local CO₂ concentration but also to a reverse osmosis phenomenon that increases the alkali cation concentration at the cathode. The synergistic effects of these two strategies create a stable and efficient microenvironment for the CO₂RR by managing water and cations. As a result, over 70% FE_CO and maximum j_CO values of 222.8 mA cm^− 2 at 300 mA cm^− 2 were achieved. Our study suggests that reverse osmosis is a suitable solution to the ion balance problem in CEM-based CO₂RR MEA electrolyzers.

Electrode preparation

The cathode was prepared with four different structures: Ag black, two layers of Ag and AEI (Sustainion XA-9, dioxide material) (Ag/AEI), two layers of Ag and mixed layers of AEI and C (KB600J) (Ag/AEI-C), and a mixed layer of Ag, AEI, and C (Ag-AEI-C). For Ag black, the Ag ink was prepared by dispersing 20 mg Ag (20–40 nm, Alfa Aesar) and 10 mg AEI in 1 mL ethanol using tip sonication for 20 min, and then sprayed onto a 5 cm² GDL (Sigracet 39BB, Fuel cell store) at 70°C with an Ag loading of 1 mg cm^− 2. To prepare Ag/AEI, the Ag layer was prepared in the same manner as Ag black, and then, an AEI (5 wt% XA-9 in ethanol) layer was sprayed over the Ag layer at 70°C with an AEI/Ag ratio of 2.5 (w/w). For Ag/AEI-C, the Ag layer was prepared in the same manner as for Ag black. The AEI-C ink was prepared by dispersing 50 mg AEI and 10 mg C in 2 mL ethanol via tip sonication for 20 min. The desired amount of AEI-C ink (C/Ag ratio of 0.5–1) was sprayed onto the CEM (Nafion 211, Fuel Cell Store) to avoid mixing Ag and C. For Ag-AEI-C, catalyst ink was prepared by dispersing 20 mg Ag, 60 mg AEI, and 10 mg C in 3 mL ethanol using tip sonication, and then sprayed onto the GDL at 70°C with an Ag loading of 1 mg cm^− 2.

The anode was prepared by spraying an IrO₂ ink onto a Pt-coated Ti mesh. The IrO₂ ink was prepared by dispersing 30 mg IrO₂ (99.99%, Alpha Aesar) with 5 wt% Nafion binder in 1 mL isopropyl alcohol by tip sonication for 20 min. The catalyst ink was sprayed onto a 5 cm² Pt-coated Ti mesh at 70°C with an IrO₂ loading of approximately 1 mg cm^− 2.

Electrochemical reduction of CO₂

For a typical electrochemical CO₂RR experiment, a custom built 5 cm² MEA-zero gap electrolyzer (CNL Energy, Korea) was used. The CEM was placed between the anode and cathode for the MEA configuration. The MEA was placed between a serpentine-type flow plate at the cathode and a pin-type flow plate at the anode. The flow channel was sealed with a rubber gasket. Humidified gaseous CO₂ was fed to the cathode flow plate at 50 mL min^− 1 and anolyte was fed to the anode flow plate at 150 mL min^− 1. The anolyte was then heated to 40°C. Details of the MEA-zero gap electrolyzer are shown in Fig. 1a. A custom-built station (CNL Energy, Korea) with a potentiostat (Sorensen XG 20–76, Ametek) was used for electrochemical CO₂RR experiments. Electrochemical CO₂RR experiments were conducted by chronopotentiometry at the desired currents. Each chronopotentiometry was stepped for 18 and 54 min under ambient pressure and pressurized CO₂ (2–6 atm), respectively. The pressurized system required a longer gas replacement time in the CO₂RR cell and separator to evaluate the gas product compared with the ambient pressure system. The pressure was controlled using a CO₂ mass flow meter and back pressure regulator. Details of the pressurized electrolysis system are shown in Figure S12. The typical chronopotentiometric operation of the pressurized electrolysis system is shown in Figure S13. The mole fraction of the outlet gas was analyzed using gas chromatography (GC; Agilent 7890A) with a flame ionization detector (FID) and a thermal conductivity detector. A methanizer was installed in front of the FID to enhance the sensitivity of the carbon-based products. The Faraday efficiency of the products (FE_product) was calculated using the following equation:

$${\varvec{F}\varvec{E}}_{\varvec{p}\varvec{r}\varvec{o}\varvec{d}\varvec{u}\varvec{c}\varvec{t}}\left(\%\right)= \frac{{\varvec{i}}_{\varvec{p}\varvec{r}\varvec{o}\varvec{d}\varvec{u}\varvec{c}\varvec{t}}}{{\varvec{i}}_{\varvec{t}\varvec{o}\varvec{t}\varvec{a}\varvec{l}}} \times 100 \% = \frac{{\varvec{x}}_{\varvec{p}\varvec{r}\varvec{o}\varvec{d}\varvec{u}\varvec{c}\varvec{t} }\times \varvec{Q} \times \frac{{\varvec{e}}_{\varvec{p}\varvec{r}\varvec{o}\varvec{d}\varvec{u}\varvec{c}\varvec{t}}\varvec{F}\varvec{p}}{\varvec{R}\varvec{T}}}{{\varvec{i}}_{\varvec{t}\varvec{o}\varvec{t}\varvec{a}\varvec{l}}} \times 100 \% \left(1\right)$$

where i is current (mA), x_product is the mole fraction of product in the outlet gas, Q is the flow rate of outlet gas (mL s^− 1), e_product is the required number of electrons to produce one molecule of product, F is Faraday’s constant (96485 C mol^–1), p is atmospheric pressure (101.325 kPa), R is the ideal gas constant (8.314 J mol K^–1), and T is room temperature (298 K)

The SPC of CO was calculated using the following equation:

$$\varvec{S}\varvec{P}\varvec{C} \left(\%\right)= \frac{{\varvec{Q}}_{\varvec{o}\varvec{u}\varvec{t}\varvec{l}\varvec{e}\varvec{t}} \times {\varvec{x}}_{\varvec{C}\varvec{O}}}{{\varvec{Q}}_{\varvec{i}\varvec{n}\varvec{l}\varvec{e}\varvec{t}}} \times 100 \% \left(2\right)$$

where Q is flow rate (mL s^− 1) and x_co is mole fraction of CO in the outlet gas.

The energy efficiency of CO₂RR was calculated using the following equation:

$$\varvec{E}\varvec{n}\varvec{e}\varvec{r}\varvec{g}\varvec{y} \varvec{e}\varvec{f}\varvec{f}\varvec{i}\varvec{c}\varvec{i}\varvec{e}\varvec{n}\varvec{c}\varvec{y} \left(\%\right)= \frac{\left(1.23\right)-\left(-0.109\right)\left(\varvec{V}\right)}{\varvec{C}\varvec{e}\varvec{l}\varvec{l} \varvec{v}\varvec{o}\varvec{l}\varvec{t}\varvec{a}\varvec{g}\varvec{e} \left(\varvec{V}\right)}\times {\varvec{F}\varvec{E}}_{\varvec{C}\varvec{O}} \left(\%\right) \left(3\right)$$

where 1.23 and − 0.109 V are the thermodynamic potentials for water oxidation and CO₂ reduction to CO, respectively. The cell voltage was measured using a potentiostat. FE_CO was calculated using Eq. (1).

Electrode characterization

SEM-EDS (Regulus 8230) was used to analyze the cross-sectional areas of the electrodes and membranes. Prior to SEM analysis, the electrode and membrane were frozen in liquid nitrogen to obtain a sharp cross-sectional area. A contact angle meter (DSA 25, Kruss) was used to determine the water contact angles of the buffer layer-coated membranes. In situ/operando micro-CT was used to analyze the morphology and local environment of the NBL and catalysts. micro-CT was performed at the 6C synchrotron beamline of the Pohang Accelerator Laboratory. A custom-built MEA-zero gap electrolyzer with 1 cm² of the electrode was used for in situ/operando micro-CT. Octopus software (TESCAN) was used to reconstruct the micro-CT images. Avizo software (Thermo Fisher Scientific) was used to modify the reconstructed micro-CT images.

Acknowledgements

This work was supported by institutional program grants from the Korea Institute of Science and Technology (KIST) and Research Project for ‘Carbon Upcycling Project for Platform Chemicals’ of the National Research Foundation (NRF) funded by the Ministry of Science and ICT (grant number: 2022M3J3A1050053) and ‘Carbon to X Project’ (Project No. 2020M3H7A1098229) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT, Republic of Korea. This research was also supported by the National Research Council of Science & Technology (NST) grant by the Korean government (MSIT) (No. CAP21011-100) and National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (NRF-2021R1A6A3A01086623). We are grateful for access to the 6C PAL beamlines to conduct CT. We would also like to thank Dr. In-Kyoung Ahn, Dr. Taekeun Kim and Dr. Taegeun Noh from LG Chem’s Platform Technology Research Center for their help with the CO2 electrolysis measurements.

Author contributions

J.P. and W.H.L. Conceptualized the study. J.P., E.D.K. and Y.J.K. methodized the study. S.K. designed MEA electrolyzer. J.P., H.K. and C.L. performed ex-situ and in-situ/operando analysis and data curation, including SEM and micro-CT analysis. J.P. and E.D.K. performed the experiments of MEA electrolyzer. J.P., J.Y.C. and H.S.O. investigated the study. J.P. and W.H.L. validated the study. J.P. drafted the manuscript. W.H.L. and H.S.O. reviewed and edited manuscript, supervised the project, and acquired the funding. All authors were involved in the writing of the manuscript.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information

Correspondence and requests for materials should be addressed to Hyung-Suk Ohor Woong Hee Lee.

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There is NO Competing Interest.

PressurizedCEMforCO2RRSI231105.docx

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Deriving an efficient and stable microenvironment for a CO₂ MEA electrolyzer by reverse osmosis

Status:

Version 1

Abstract

Figures

Introduction

Results

Effect of Electrode structure on CO₂RR performance

Cation and water distribution of different electrode structures

The impact of electrode structure

Effect of pressurized CO₂ on the CO₂RR

The impact of pressurized CO₂

Conclusion

Methods

Electrode preparation

Electrochemical reduction of CO₂

Electrode characterization

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Deriving an efficient and stable microenvironment for a CO2 MEA electrolyzer by reverse osmosis

Status:

Version 1

Abstract

Figures

Introduction

Results

Effect of Electrode structure on CO2RR performance

Cation and water distribution of different electrode structures

The impact of electrode structure

Effect of pressurized CO2 on the CO2RR

The impact of pressurized CO2

Conclusion

Methods

Electrode preparation

Electrochemical reduction of CO2

Electrode characterization

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Deriving an efficient and stable microenvironment for a CO₂ MEA electrolyzer by reverse osmosis

Effect of Electrode structure on CO₂RR performance

Effect of pressurized CO₂ on the CO₂RR

The impact of pressurized CO₂

Electrochemical reduction of CO₂