# Engineering the catalyst interface enables high carbon efficiency in both cation-exchange and bipolar membrane electrolyzers

Huali Wu, Bonito Aristide Karamoko, Wensen Wang, Jiefeng Liu, Eddy Petit, S.M. Li, Chrystelle Salameh, Damien Voiry

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Huali Wu, Bonito Aristide Karamoko, Wensen Wang, Jiefeng Liu, Eddy Petit, et al.. Engineering the catalyst interface enables high carbon efficiency in both cation-exchange and bipolar membrane electrolyzers. Applied Catalysis B: Environmental, 2025, 361, pp.124691. 10.1016/j.apcatb.2024.124691. hal-05084412

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# Engineering the catalyst interface enables high carbon efficiency in both cation-exchange and bipolar membrane electrolyzers

Huali Wu<sup>a,b,*</sup>, Bonito Aristide Karamoko<sup>a</sup>, Wensen Wang<sup>a</sup>, Jiefeng Liu<sup>a</sup>, Eddy Petit<sup>a</sup>, Suming Li<sup>a</sup>, Chrystelle Salameh<sup>a</sup>, Damien Voiry<sup>a,**</sup>

## ARTICLE INFO

Keywords:

CO₂ electroreduction

Carbon-efficiency

Biploar membrane

Cation-exchange membrane

Microenvironment

## ABSTRACT

The formation of (bi)carbonate in alkaline and neutral membrane-electrode assembly (MEA) electrolyzers poses an unsatisfactorily low upper-bound of carbon efficiency. Electrolyzing CO₂ in acidic MEA has been regarded as an effective strategy to prevent carbonate formation and CO₂ loss but poses challenges due to the competitive hydrogen evolution reaction. Here we report the preparation of a hydrogel buffering layer on an Ag-coated gas diffusion electrode to prevent the (bi)carbonate formation and break the theoretical limitation of 50 % SPU in neutral-media electrolyzers. Through precise control of the porosity within the buffering layer, while maintaining superaerophobicity, we found that the hydrogel enhances the mass transfer of regenerated CO₂ at the interface between the buffering layer and the cation-exchange layer. The high energy efficiency of 37 % and carbon utilization of 77 % ±2.4 at a total current density of 375 mA cm⁻² were achieved using an optimal Ag/ buffer electrode when performing acidic MEA electrolysis.

## 1. Introduction

The conversion of CO₂ into renewable fuels and chemical feedstocks through electrochemical processes represents a promising technology for electrifying the chemical industry. The electrochemical reduction of CO₂ (CO₂RR) has made significant progress, drawing inspiration from matured technical features found in fuel cells, such as the gas diffusion electrode (GDE) and membrane-electrode assembly (MEA) architecture. Under optimal neutral and basic conditions, high Faradaic efficiency (FE, >90 %) and a substantial current density (>1 A cm⁻²) have already been reported. This success, notably attributed to catalyst and CO₂ transport optimization, holds great promise for practical applications[1, 2].

While the utilization of anion exchange membranes in the MEA electrolyzer contributes to superior conversion performance, the formation of carbonate during the CO₂RR diminishes the technical feasibility of the process. In the case of the CO₂ to CO reaction, over 50 % of the CO₂ reactant is lost to carbonate (See Supplementary Note 1 in the Supplementary Information). This results in an unsatisfactorily low upper bound for the CO₂ single-pass utilization rate (SPU) and single-

pass conversion rate (SPC) in MEA electrolyzer [3,4]. The single-pass utilization rate (SPU) of CO₂ is defined as the ratio of the reacted CO₂ that is converted to the target product (CO₂RR products) to that of the total CO₂ consumed in the system (This includes the reacted CO₂, the crossover and the production of carbonate)[5]. The CO₂ utilization efficiency is considered independent of flow rate and is not to be confused with the single-pass conversion rate of CO₂ (SPC) within the system for which the reader is referred elsewhere[6,7]. Taking CO₂ to CO product as an example, the C(CO₂, inlet) and C(CO, outlet) represent the volume fraction of CO₂ feeding into the flow electrolyzer and the volume fraction of CO in the gas effluent quantified by the GC during CO₂RR reaction. The SPU and SPC of CO₂ to CO can be expressed as follows,

In alkaline or neutral electrolytes, the viability of CO₂RR technology is limited by poor stability and low SPU and SPC due to the carbonate formation, which not only consumes the reactant but also obstructs the access to the catalyst surface. Although the combination of cation-exchange membrane (CEM) and the acidic electrolyte in MEA theoretically does not produce the carbonates, the acidic local environment at the cathode favors hydrogen evolution reaction (HER) rather than the CO₂RR, resulting in typically low Faradaic efficiency and carbon-

<footnote><sup>a</sup> Institut Européen des Membranes, IEM, UMR 5635, Université Montpellier, ENSCM, CNRS, Montpellier 34000, France</footnote>

<footnote><sup>b</sup> Research Institute of Frontier Science, Southwest Jiaotong University, Chengdu 610031, PR China</footnote>

<footnote>* Corresponding author at: Institut Européen des Membranes, IEM, UMR 5635, Université Montpellier, ENSCM, CNRS, Montpellier 34000, France.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: huali.wu@swjtu.edu.cn (H. Wu), damien.voiry@umontpellier.fr (D. Voiry).</footnote>
efficiency. To improve the performance of CO2RR in acidic electrolyzers, great attention has been paid on regulating the local environment of the cathode in both MEA and flow cell by alkali cations [8–10] and surface modifications [11,12]. However, during periodic operation, the high concentration alkali cations in acidic electrolyte in flow cell can also precipitate and crystallize as alkali salts on the catalyst and/or gas diffusion electrode, limiting the access of CO2 to catalyst. Therefore, developing strategies to mitigate carbonate formation and overcome the associated limitations on single-pass utilization rate (SPU) and single-pass conversion rate (SPC) has become a central focus in advancing CO2 electrolysis technology.

Regulating the surface microenvironment of the cathode to limit the local OH⁻ formation by introducing excess H⁺ is an effective approach to mitigate conversion limitation [10,13–16]. Bipolar membrane (BPM) and cation-exchange membrane (CEM) can provide the excess H⁺ to mitigate the carbonate formation, but the strong acidic microenvironment at the cathode favors the hydrogen evolution reaction (HER) instead of the CO2RR [5,13,14,16,17]. Implementing a strategy that involves incorporating a stationary catholyte layer between the cation-exchange layer (CEL) of a bipolar membrane (BPM) and the cathode has shown promises in addressing cathode acidification and mass-transfer limitations for enhanced selectivity toward CO2RR under acidic conditions (Supplementary Fig. 1) [14,18]. Using such a buffer layer supported on glass fibers, the formation of regenerated CO2 gas bubbles takes place at the interface between the glass fibers and the CEL. The performance however displays a strong dependence to the thickness of the layer as these bubbles tend to be trapped, increasing the internal ohmic resistance of the cell while impeding the mass transfer of the locally produced CO2 back to the cathode [18]. This ultimately results in a high cell voltage and a limited availability of regenerated CO2, decreasing the energy efficient and carbon utilization efficiency.

In this context, we report the construction of a cross-linked polyethyleneimine (PEI) superaerophobic hydrogel buffering layer with controlled microstructure to effectively promote the mass transfer of the in-situ produced CO₂ gas bubbles within the buffering layer while simultaneously achieving a high local cathode pH in an acidic CO₂RR environment. The microstructure of the porous superaerophobic hydrogel can be precisely adjusted by varying the concentration of the PEI polymer. The hydrogel layer is then infiltrated with CsHCO₃ and integrated into a BPM-based membrane-electrode assembly electrolyzer (BPM-MEA). Using Ag as benchmarked catalyst for the CO₂-to-CO conversion, we achieved both high CO₂ single-pass utilization rate (SPU) of 81 %±5.3 and high CO₂ single-pass conversion rate (SPC) of 21.7 % ±3.1 at a CO current density over than 300 mA cm⁻². This performance outperforms the best-reported results of the literature [5,14]. To further demonstrate the feasibility of our approach, we successfully integrated our hydrogel layer into a CEM-based MEA electrolyzer and obtained a CO₂ SPU of 77 %±2.4 by using a pH=0.5 aqueous solution as the anolyte. This value exceeds the SPU limit of 50 % for CO in AEM-based MEA with a neutral KHCO₃ electrolyte, and the incorporation of the CEM effectively suppresses CO₂ crossover to less than 0.5 %. To gain understanding of the role of the hydrogel buffering layer, we further carried COMSOL simulations to examine the behavior of the electrolyzer under dynamic conditions. Our results indicate that appropriate porosity of superaerophobic hydrogel-based buffering layer can facilitate the removal and the mass transfer of the locally produced CO₂ from the interface of buffering layer and CEL to the cathode and minimize the internal ohmic resistance within electrolyzers. We also found that the flow of protons to the cathode is effectively regulated by the superaerophobic hydrogel, which provides a high local cathodic pH, favoring the CO₂RR on the catalyst surface. The use of CsHCO₃ within superaerophobic hydrogel also induces an alkali cation effect (Cs⁺) on catalyst surface and promotes CO₂ regeneration (HCO₃⁻) after accepting the protons from CEM, and thus improves the CO₂ utilization efficiency. Our results demonstrate that the rational design of a superaerophobic hydrogel-based buffering layer within BPM-MEA and CEM-MEA

electrolyzers can overcome the theoretical limit of CO₂ SPU in traditional AEM-MEA electrolyzers and offers practical solution for the development of carbon efficient CO₂RR electrolyzers.

## 2. Experimental

### 2.1. Materials

Silver nanoparticle(<100 nm), Cesium hydroxide monohydrate (CsOH·H₂O, 99 %), cesium bicarbonate (CsHCO₃, 99.9 %), potassium sulfate (K₂SO₄, 99 %), potassium bicarbonate (KHCO₃, 99.7 %), potassium hydroxide (KOH, 99.95 %), potassium chloride (KCl, 99 %), sodium bicarbonate (NaHCO₃, 99.7 %), sodium chloride (NaCl, 99.5 %), sulfuric acid (H₂SO₄, 95 %), hydrochloric acid (HCl, 37 %), silica dioxide-sorbent milled nanofiber (SiO₂, 300–500 nm), polyethyleneimine (PEI) with a number average molecular weight of 10000, (3-aminopropyl)triethoxysilane (APTES, 99 %), Nafion™ solution, isopropanol and absolute ethanol were purchased from Sigma-Aldrich and used as received. Glutaraldehyde (50 wt% in H₂O) was obtained from Alfa Aesar and used as received. XC-2 ionomer was purchased from Dioxide Materials and used as received.

### 2.2. Electrode preparation

Cathodes were prepared by spray coating Ag nanoparticle inks on porous substrates (GDL, Sigracet 22BB) with a loading amount of 1 mg cm⁻². The Ag ink was made by adding Ag nanoparticles (400 mg) to a mixed solution, which contained 8 ml DI water, 8 ml isopropanol, and 70.6 mg Nafion (33.6 µL of 5 wt% #527084, Sigma-Aldrich) corresponding to a concentration of 4.41 g/L. This mixture was sonicated for half hour then spray coated onto a well-cut gas diffusion layer. The anode of iridium oxides deposited Ti mesh were purchased from Shouyiji Trade Xiamen and used as received.

### 2.3. Construction of cross-linked PEI on Ag spray-coated GDL

Before the deposition of cross-linked PEI, the Ag-sprayed GDL was modified with amine groups to improve the adhesion of PEI to the substrates using APTES according to the literatures [19,20]. In brief, 1) the nanofiber of SiO₂ was firstly treated with 1 M KOH for 1 h to graft some hydroxide groups. The KOH-treated SiO₂ was washed by distilled water to remove excess KOH and completely dried. 2) Immersing the well treated SiO₂ nanofiber into 2 wt% APTES solution and spray coated the ink on Ag GDE and incubated for 2 h at room temperature. After that, the Ag GDE with a loading of 0.05 mg/cm² SiO₂-APTES was washed with ethanol and completely dried. 3) Preparing the different of PEI with the concentrations of 0.5 %, 1.5 %, 2.1 % and 3.4 % (v/v) in 37.5 mM NaCl solution. The pH of the PEI solutions was controlled at around 6.0 by using concentrated HCl. 4) Dipping the SiO₂-APTES-coated Ag GDE in different PEI solutions under mild stirring for 1 h and baking them in an oven with the temperature of 120 °C for 10 minutes to better adhere the PEI on the substrate. 5) Cross-linking PEI with 1 wt% glutaraldehyde (in H₂O) for 1 min, 6) Freeze-drying. Finally, the SiO₂ nanofiber supported porous hydrogel were grown on Ag electrodes and ready for further application.

### 2.4. Assembly of the hydrogel-derived buffering membrane electrode assembly electrolyzers (HB-MEA)

The prepared porous Ag/hydrogel electrodes were saturated with desirable solution (3 M CsHCO₃, 3 M KHCO₃, 3 M NaHCO₃ and DI water) by drop cast method until the loading of impregnated solution reach to 60 µL/cm². The MEA electrolyzer (Dioxide Materials) was composed of the Ag/hydrogel cathode, a Ti-IrOₓ mesh anode and a membrane. The membranes of Nafion™ 117, Fumasep (FAS-PET-130) and Sustainion X37–60, as well as the gas diffusion layer (GDL, Sigracet 22BB) were
purchased from Fuel Cell Store. The water used in this study was 18 MΩ Milli-Q deionized water (DI water). Nafion membranes were activated before using through a series of procedures (1 h in 80 °C 1 M H₂SO₄, next keeping in 80 °C H₂O₂ for 1 h, then continue activate in 1 M H₂SO₄ for 1 h, finally the activated membranes are stored in DI water). Fumasep bipolar membrane was used as received and stored in 1 M KCl. Sustaininon membranes were activated in 1 M KOH for 48 h and stored in fresh 1 M KOH. When using Fumasep BPM, the membrane was placed with its cation-exchange layer (CEL) facing the cathode side, while the AEL facing the anode of IrO₂ loaded Ti mesh.

## 2.5. Materials characterization

A field emission scanning electron microscope (SEM, TESCAN Mira3) was employed to observe the morphology of samples. Spectroscopic analysis was conducted using a 670/620 Fourier transformed infrared (FT-IR) spectrometer (Agilent, USA) in an ATR mode. The textural properties of the materials were characterized by N₂ sorption. The multipoint Brunauer-Emmett-Teller (BET, Micromeritics ASAP2020) method was employed to calculate the specific surface area, pore-size distribution and pore volumes. The porosity is calculated by using pore volume to divide the material volume. The air contact angles (ACA) were measured by a DSA100 drop shape analyzer (K GmbH, Germany) and a custom-made equipment. In detail, the Ag/hydrogel electrode was carefully placed on the surface of distilled water. Meanwhile, a syringe with a curving needle was applied to create an air bubble under the electrode. Then the air contact angle can be evaluated by the ImageJ software.

## 2.6. Electrochemical in MEA measurements

All electrochemical measurements were carried out at ambient temperature and pressure using a VSP electrochemical station from Bio-Logic Science Instruments equipped with a 5 A booster and FRA32 module. The cell voltages reported in all figures were recorded without iR correction. In all measurements (BPM-MEA, CEM-MEA and AEM-MEA), a flow of 10 ml/min for different anolyte (1 M CsOH for BPM-MEA, pH=1/0.5/0.25 of 0.5 M K₂SO₄ for CEM-MEA, 0.1 M KHCO₃ for AEM-MEA) was fed into the MEA by a peristaltic pump unless otherwise specified. The CO₂ flows at the cathode side are 50 ml/min, 40 ml/min and 20 ml/min for BPM-MEA, CEM-MEA and AEM-MEA with the physical area of cathode of 4 cm², respectively. After three-minutes of initial operation, a full-cell potential of -3.0 V was applied to the electrolyzer and the potential then was gradually increased from -3.0 V with the increments of -0.10 V. The testing system was allowed to stabilize at the specific conditions for > 1500 s before recording the results. All the error bars represent standard deviations based on three measurements.

## 2.7. Quantification of the CO₂RR products

The CO₂ and its related CO₂RR gas products were analyzed by using an automatic sampler connected to the cathode outlet. The gas aliquots were then injected into an online gas chromatograph (Agilent, Micro GC-490) equipped with a TCD detector and Molsieve 5 A column continuously. Hydrogen and argon (99.9999 %) were used as the carrier gases. The gas chromatograph was equipped with a Molecular Sieve 5 A Capillary Column and a packed Carboxen-1000 Column with argon as the carrier gas. For each applied potential, the gas products were collected and analyzed at least 3 times with 110 s intervals. Liquid products were quantified by 1 H NMR spectroscopy (600 Mhz Avance III Bukrer with a cryorobe Prodigy TCI) using deionized water with 0.1 % (w/w) of DSS (Sodium trimethylsilylpropanesulfonate) like internal standard for the quantification of the formate. An 1D sequence water suppression with excitation sculpting with gradients(zgesgp) was used for the acquisition (Number of scan = 32, Delay D1=30 s). Owing to the

liquid product crossover, the FE values of the liquid products were calculated based on the total amount of the products collected on both anode and cathode sides during the same period.

## 2.8. Faradaic Efficiency (FE), CO₂ Single-pass Utilization rate (SPU) and CO₂ Single-pass Conversion rate (SPC), as well as Energy Efficiency (EE) Calculations

The CO₂ Single-pass Utilization rate (SPU) was calculated as follows:

$$CO_2\text{utilization rate}(\text{SPU}, \%) = \frac{C_{(\text{CO, outlet})} \times \text{flow}_{(\text{outlet})}}{C_{(\text{CO2,inlet})} \times \text{flow}_{(\text{inlet})} - C_{(\text{CO2,outlet})} \times \text{flow}_{(\text{outlet})}} \times 100\% \quad (1)$$

The CO₂ Single-pass conversion rate (SPC) was calculated as follows:

$$CO_2\text{conversion rate}(\text{SPC}, \%) = \frac{C_{(\text{CO,outlet})} \times \text{flow}_{(\text{outlet})}}{C_{(\text{CO2,inlet})} \times \text{flow}_{(\text{inlet})} \times 100\%} \quad (2)$$

The Faradaic efficiency (FE) of each gas product was calculated as follows:

$$FE_{\text{gas}} = g_i \times v \times \frac{z_i}{RT} F P_0 \times 1/I_{\text{total}} \times 100\% \quad (3)$$

The Faradaic efficiency (FE) of each liquid product was calculated as follows:

$$FE_{\text{liquid}} = l_i \times z_i / Q_{\text{total}} \times F \times 100\% \quad (4)$$

The full-cell energy efficiencies (EE) were calculated as follows:

$$EE = (1.23 - E_i) \times FE_i / E_{cell} \quad (5)$$

where gᵢ represents the volume fraction of gas product i; v represents the gas flow rate at the outlet in sccm; zᵢ represents the number of electrons required to produce one molecule of product i; Iₜotal represents the total current; lᵢ represents the number of moles of liquid product i; and Qₜotal represents the charge passed while the liquid products are being collected. P₀ = 1.01 × 10⁵ Pa, T = 273.15 K, F = 96,485 C mol⁻¹ and R = 8.314 Jmol⁻¹K⁻¹; Eᵢ represents the thermodynamic potential (versus RHE) for CO₂RR to species i and Eₐcell represents the cell voltage in two-electrode setup.

## 2.9. COMSOL one-dimensional modelling

The electrochemical reaction model was performed by COMSOL Multiphysics version 6.1. This simulation was built by following the typical modelling simulations[4,14,21,22]. The local pH and different species concentrations were simulated based on different porosities (7 %, 10 %, 32 %, 38 %, and 60 %) according to PEI concentrations (0.5 %, 1.5 %, 2.1 %, and 3.4 %). Catholytes of CsHCO₃ was used in the simulation. All the chemical reactions between species were considered in this one-dimensional modelling. The simulation (Supplementary Fig. 2) included a 50 µm thick gas diffusion layer (GDL), a 100 nm thick Ag catalyst (CL), a 75 µm catholyte region (CsHCO₃ immersed hydrogel) indicated above, and a cation exchange layer (CEL) boundary from bipolar membrane and cation-exchange membrane (Nafion 117). Constant concentration (Dirichlet) boundary conditions were used. Specifically, a constant concentration 37.8 mM of CO₂ was assumed within the GDL layer, as this region is in direct contact with the input CO₂ flow and thus assumed to be at equilibrium with gas phase CO₂ over this region for the purposes of the simulation. The solution was converged when a relative tolerance of 0.001 was reached.

The BPM was interpreted as a boundary with a constant species concentration (1 M H₃O⁺ at the CEL surface)[23–25], because it was assumed to generate protons as the dominant ionic charge carrier at a
constant rate under a constant potential difference of 4.1 V. A user-controlled mesh is employed in the COMSOL simulation. Edge type of mesh is used for GDL, CL, catholytes, respectively. Specifically, the mesh distribution is predefined with an interval of 500 nm for GDL and catholytes, and an interval of 5 nm for CL. Since the production of formate is trace amount in the whole reaction, here we considered the main product CO. Two different electrode reactions were considered at the cathode catalyst layer in this simulation. Specifically, the hydrogen evolution reaction and the main reaction of CO₂ reducing to CO occurred at the cathode catalyst layer. In BPM-based MEA, the catalyst layer is immersed in a catholyte. Thus, the simulation considers no gas-phase transport in the catalyst layer. The carbonate equilibrium reactions, corresponding catholyte buffer reactions, and a water dissociation reaction were considered in the catholyte region. The electrochemical reaction rates of the specific products were determined from experimental results.

The electrochemical reactions at cathode catalyst layer:

$$CO_2 + H_2O + 2e^- = CO + 2OH^-$$

$$2H_2O + 2e^- = H_2 + 2OH^-$$

The heterogeneous electrochemical reaction rates are determined by the following equations:

$$r_{CO_2} = (\varepsilon \times I_{total} \times FE_{CO}) / (n_{co} \times F \times L_{catalyst}) \quad (6)$$

$$r_{OH^-} = (\varepsilon \times I_{total}) / (n_{OH^-} \times F \times L_{catalyst}) \quad (7)$$

Where $I_{total}$ refers to the total current density of all reaction occurred at the cathode catalyst layer. $FE_{CO}$ refers to Faradaic efficiency of CO according to experimental results. $F$ refers to Faraday's constant. $n_{CO}$ refers to the transferred charges from CO₂ to CO. $I_{total}$ refers the total current density. $\varepsilon$ refers to the catalyst porosity based on the experimental results in Fig. 2a. $L_{catalyst}$ refers to the cathode catalyst length. Since the reaction mostly conducted on the surface of catalyst, the $L_{catalyst}$ is replaced by a specific surface area of catalyst of $8.3 \times 10^{-7}$ (L/m). The chemical reactions at the catholyte region and the corresponding forward $k_f$ rate constants and reverse $k_r$ rate constants taken from the literature [26] (see Table S1).

By following the literatures [13,14,27], both the Transport of Diluted Species physics model and the calculation method of Nernst-Planck equations governed the species diffusion were applied in our simulations. Migration was ignored for simplicity as the experiments were performed in the concentrated electrolyte. The ion species transport is thus calculated by solving the equations below.

$$\partial c_i / \partial t + \nabla \cdot J_i = R_i \quad (8)$$

$$J_i = - D_i \nabla c_i - n_i u_{m,i} F c_i \nabla V \quad (9)$$

$$\nabla \cdot D = \rho_v \quad (10)$$

$$E = - \nabla \cdot V \quad (11)$$

$$D_i = \varepsilon_p / \tau_{F,i} \times D_F \quad (12)$$

$$\tau_{F,i} = \varepsilon_p^{-1/3} \quad (13)$$

Where $J_i$ refers to the molar flux, and $c_i$ refers to the concentration of specific ions in the heterogeneous electrode reactions for CO₂ reduction that were modelled at the cathode catalyst layer. $u_{m,i}$ refers to migration rate, $R_i$ refers to the rates of the homogeneous reactions indicated above. $\rho_v$ refers to space charge density, $V$ and $E$ refer to potential and electric field intensity, respectively. $D$ is diffusion coefficient. Diffusion coefficient ($D_i$) of species is determined by the Millington and Quirk model. $\varepsilon_p$ refers to the porosity coefficient. $\tau_{F,i}$ refers to tortuosity coefficient. The porosity value of 60 % was used for the cathode catalyst, while the porosity value for buffering layer is varied from 7 % to 38 % according

to the experimental results. The species diffusion coefficients are listed in Table S2 [14,24,25,28,29].

Henry's law and sets of Sechenov equation are applied to calculate the CO₂ concentration. The same manner as literatures [13,14,27] was used when estimating the concentration of CO₂ in electrolytes, which depends on temperature and pressure. The Sechenov coefficients are listed in Table S3 [30].

### 3. Results and discussion

#### 3.1. Catalyst design and characterization

We used benchmarked commercial Ag nanoparticles (NPs) as CO₂RR catalyst to produce CO. The pristine cathode was prepared by spray-coating Ag NPs (<100 nm) on gas diffusion layer (GDL) with a loading amount of 1 mg cm⁻² for CO₂RR. We integrated it in a BPM-MEA cell by compressing the pristine Ag-GDL onto the CEL side of BPM, a low Faradaic efficiency (FE) of only 26 % for CO was obtained at a reversed bias of -3.2 V using a 1 M CsOH anolyte (Supplementary Figs. 3a and b). For comparison, a 72 %±1.6 FEₐ₀ was achieved at a forward bias of +3.4 V with the same anolyte (Supplementary Fig. 3c). The low selectivity of CO originates from the high local concentration of protons at the CEL side from the dissociation of water in BPM through a reversed bias, which favors the hydrogen evolution reaction (HER) instead of CO₂RR. While achieving 72 %±1.6 Faradaic Efficiency for CO (FEₐ₀) at a forward bias of +3.4 V is possible in a BPM-based MEA electrolyzer, the positioning of the AEL next to the cathode introduces the same carbonate-related issue, significantly restricting carbon efficiency, similar to AEM-based MEA configurations. Introducing a glass fiber-derived buffering layer into BPM-MEA cells has been proved effective to mitigate the local concentration of proton and improve the CO₂RR performance under reversed bias [30]. We however found that locally produced CO₂ microbubbles accumulate on the side of glass fiber-derived buffering layer facing BPM, which slows the CO₂ mass transfer and increases the inner resistance of electrolyzer resulting in an increased reaction voltages and a decreased current density (Supplementary Fig. 4).

We rationally designed series of polyethyleneimine (PEI) hydrogel layers varying the concentrations of PEI and using glutaraldehyde as crosslinker (Fig. 1a and Supplementary Fig. 5). Commercially available SiO₂ nanofibers were treated with 1 M KOH and used as a building framework of the hydrogel on the Ag-GDL. Branched PEI with its inherent hydrophilicity and abundant amine groups was selected for fabricating the hydrogel. By taking advantage of the Schiff-base condensation reaction mechanism, PEI-derived hydrogel was obtained by cross-linking PEI with glutaraldehyde-grafted SiO₂ nanofibers on Ag electrodes, followed by freeze-drying. We hypothesized that PEI hydrogel-derived buffering layer on Ag electrode can exhibit super-aerophobicity and therefore improve the mass transport of regenerated CO₂ [20]. To explore how the structure and aerophobicity of PEI hydrogel-derived buffering layer affect the mass transfer of the regenerated CO₂ microbubbles, we controlled the concentration of PEI (called hereafter, XX% PEI, where XX refers to the concentration of PEI) when cross-linking with amino groups adhered on the SiO₂-Ag electrode.

The concentration of PEI was varied from 0.5 % to 3.4 % while maintaining the concentration of glutaraldehyde constant at 1 wt%. The PEI-derived hydrogels on Ag electrodes present obvious porous network, which is independent of the underlying substrate, i.e. the Ag coated GDL substrate (Fig. 1b to d and Supplementary Figs. 5a to e). We found that PEI can form a gel and constructs a porous structure at a concentration of >0.5 %. Using ATR-FTIR, we confirmed the presence and the modulation of the intensity of the peaks for N-H (3408 and 1640 cm⁻¹), C-H (1422 cm⁻¹), and C-N (1387 cm⁻¹) for the different PEI-derived hydrogels [31,32]. The Si-O-Si peak at 1060 cm⁻¹ [32] from the SiO₂ framework also indicate the successful fabrication of PEI-derived hydrogels on Ag-GDE when the concentration of PEI >0.5 %
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** The schematic shows a method of engineering the catalyst interface to improve the carbon efficiency in the BPM/CEM-MEA configuration (a); the surface morphology (b) and the cross-section morphology (c), as well as the related EDX mapping (d) of 2.1 % PEI constructed buffering layer on Ag GDE. The air-contact angle of pristine Ag GDE and 2.1 % PEI-loaded Ag GDE (e).</figcaption></figure>

(Supplementary Fig. 5 f). After freeze-drying, the PEI hydrogel (i.e., 1.5 % PEI, 2.1 % PEI, and 3.4 % PEI) exhibited a porous network structure with a large surface area of 55 m² g⁻¹ for 2.1 % PEI (Supplementary Fig. 6), which provides the host space to encapsulate the CsHCO₃ buffer. We found that a concentration of 2.1 % PEI displayed a more uniform porous network structure compared to the other concentrations: 0.5 %, 1.5 % and 3.4 %.

To explore the surface properties of the hydrogel layer, we assessed the gas contact angles of the different modified Ag GDE. Our results indicate that the aerophobicity of pristine Ag electrode can be substantially increased by tuning the porosity of the different PEI hydrogels. Specifically, the use of a 2.1 % PEI hydrogel leads to a super-aerophobicity with an air contact angle of 117°, higher than both 1.5 % PEI and 3.4 % PEI at 91° and 112°, respectively (Fig. 1e and Supplementary Fig. 7). We found that controlling the concentration of PEI is key to promote effective cross-linking with amine groups while avoid agglomeration (Fig. 2a). Collectively, our investigations suggest that hydrophilicity and porous structure of PEI play a critical role in implementing super-aerophobicity to the buffering layer. This, in turn,

optimizes the buffer layer's performance for effectively managing the removal of CO₂ from the membrane interface and facilitating gas transport to the active sites.

## 3.2. Assessment of porosity-controllable buffering layer in a BPM-based MEA

We first evaluated the ohmic resistance of the hydrogel for the same thickness of buffering layer but different PEI concentrations under an open circuit potential. We found that the stronger of the aerophobicity ability, the lower the internal resistance and 2.1 % PEI buffering layer exhibits the smallest ohmic resistance among others (Fig. 2b). This highlights an apparent correlation between the porosity – and thus the loading with CsHCO₃ – and the ohmic losses of the system. Subsequently, we explored the performance of Ag electrodes modified by different PEI hydrogel buffer layers in a BPM-based MEA electrolyzer (Fig. 2, and Supplementary Figs. 8 and 9). We observed that the MEA cell with the 2.1 % PEI hydrogel displays the highest FEO of 80 %±3.8 and a specific current density (jCO) of 310 mA cm⁻² at –4.1 V in all
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** The effect of the concentration of PEI on aerophobicity and porosity of the PEI hydrogels (a); the resistance of different loadings of PEI in Ag/buffer (b); the relation between the Faradaic efficiency and the different loadings of PEI in Ag/buffer (c); the CO specific current density of different Ag/buffer electrodes with different loadings of PEI at different operational voltages (d); The dependence of carbon efficiency and Faradaic efficiency on CO₂ flow rate(e) and a comprehensive comparison of different Ag/buffer electrodes for different loadings of PEI in BPM-based MEA cells (f).</figcaption></figure>

electrodes. Conversely 0.5 % PEI-loaded Ag gives the worst performance with a low FE<sub>CO</sub> of 44 %±4.8 and *j*<sub>CO</sub> of 72 mA cm<sup>-2</sup> (Fig. 2c and d). We also found that the Ag electrode with 2.1 % PEI hydrogel buffering layer exhibits a high CO<sub>2</sub> single-pass utilization rate of 81 %±5.3 at a high current density of 375 mA cm<sup>-2</sup> with a CO<sub>2</sub> flow of 50 ml/min (Fig. 2e), higher than the theoretical limitation of 50 % in AEM based MEA electrolyzer in neutral media[13,14]. 3.4 % PEI also exhibited significantly enhanced aerophobicity but a slightly lower porosity than 2.1 % PEI: 32 % compared to 38 %, respectively. We anticipated that the lower porosity induces a difficult transport of the regenerated CO<sub>2</sub> microbubbles, resulting in an accumulation of CO<sub>2</sub> near the membrane interface. Indeed, we found that 3.4 % PEI exhibited a decreased current density and an increased cell voltage. From our observations, we can conclude that higher concentrations of PEI form a dense network of polymer chains in the hydrogel, resulting in a decreased porosity[20], which increases the resistance for the regenerated CO<sub>2</sub> microbubbles transfer. The detrimental effect of the high PEI concentration can be also detected from the low SPU at 68 %±3.7 in the case of 3.4 % PEI compared to 2.1 % PEI (Supplementary Fig. 10). These results suggest that the microstructure of 2.1 % PEI hydrogel not only minimizes the internal resistance thanks to the optimal encapsulation of CsHCO<sub>3</sub> but also promotes the transfer of regenerated CO<sub>2</sub> from the interface of buffer layer and CEL to catalyst. It results in the best electrochemical performance compared to all different concentrations of PEI (Fig. 2f). Collectively, these results indicate that the superaerophobicity and porosity of PEI-derived hydrogel plays a critical role in effectively transporting the regenerated CO<sub>2</sub> microbubbles to the catalyst surface while regulating the concentration of protons near catalyst. Next, we investigated the role of the cations on the CO<sub>2</sub>RR properties of the PEI buffer layer by replacing the CsHCO<sub>3</sub> with KHCO<sub>3</sub>, NaHCO<sub>3</sub> and DI water. The Faradaic efficiency and specific current density of CO display an obvious increases with the increased diameter of alkali cations

(Na⁺<K⁺<Cs⁺), which is consistent with previous reports[33] (Supplementary Fig. 11).

### 3.3. Buffering ability in an acid-fed CEM-based MEA

The 2.1 % PEI hydrogel layer demonstrated remarkable performance compared to the previously reported BPM-based electrolyzers in terms of CO Faradaic efficiency, current density, and carbon efficiency[5,30]. Nevertheless, the large reaction voltages exceeding 4.1 V significantly compromise energy efficiency (EE), while escalating electrical power consumption (EPC, kWh N<sup>-1</sup>m<sup>-3</sup>). These factors are crucial parameters for assessing the practical feasibility of CO<sub>2</sub>RR technology. To generalize our strategy and improve the energy efficiency of the reaction, we then replaced BPM with CEM.

To assess the performance of the 2.1 % PEI buffering layer in acid-fed CEM-MEA electrolyzers, we first analyzed and compared the CO₂ reaction activity of pristine Ag-GDL without buffering layer in a pH=0.5H₂SO₄ anolyte. We found that the linear sweeping voltammetry (LSV) curves overlapped with N₂ and CO₂ feeds, suggesting that hydrogen evolution reaction (HER) dominates the reaction throughout the whole voltage range (Fig. 3a). However, when using 2.1 % PEI between the Ag catalyst and CEM (Ag/buffer-H₂SO₄-CO₂), a plateau from -2.6 V to -3.4 V is observed when CO₂ is supplied to the cell. These results highlight that the PEI hydrogel layer not only provides effective diffusion pathways for the regenerated CO₂ microbubbles, but also shields the Ag surface from high proton flux by locally constructing an electrical double layer through concentrated alkali cation-Cs⁺ [23,33, 34]. The accumulation of Cs⁺ cations from the buffering layer at the outer Helmholtz plane (OHP) [23,33,34] reduces the access of H⁺ to the catalyst surface; thereby increasing the availability of the active sites for CO₂RR. The presence of Cs⁺ at the outer OHP, characterized by a higher hydration energy compared to Li⁺/Na⁺, contributes to enhance the
<figure><img src="image_5.png" /><figcaption>Fig. 3. The linear sweeping voltammetry (LSV) curves of different samples under N₂ and CO₂ flowing (a); The specific current density of CO in different pH solutions (b); The relationship between FE<sub>CO</sub>, the pH value of anolyte and the ratio of protons to alkaline cations (c); The dependence of carbon efficiency and Faradaic efficiency on CO₂ flow rate (d); A long-term operation stability and the corresponding single-pass utilization rate (SPU) and CO FEs in an acid-fed CEM-based MEA with Ag/buffer cathode under 300 mA cm⁻² (e).</figcaption></figure>

interfacial electric field. Simultaneously, it reinforces coordination with the adsorbed CO₂ intermediate while maintaining a high concentration at the OHP, promoting favorable CO selectivity (Supplementary Fig. 12).

We also compared the CO₂RR properties of the Ag/buffer-H₂SO₄-CO₂ and Ag/buffer-H₂SO₄/K₂SO₄-N₂ systems at pH = 5, and we observed that the incorporation of K₂SO₄ into the anolyte further enhances the CO₂RR activity (Ag/buffer-H₂SO₄/K₂SO₄-CO₂) at on onset potential as low as ≈ -2.6 V. This suggests a synergistic effect arising from the alkaline cations Cs⁺ from the buffering layer and K⁺ from the anolyte in a strongly acidic medium[33] (Fig. 3a). As the H⁺/K⁺ in the anolyte competes to transport through CEM to cathode, we prepared different acidic solutions (H₂SO₄+0.5 M K₂SO₄) with same K⁺ concentration but different anolyte pH values (0.25, 0.5 and 1) to further clarify the ability of buffering layer on regulating the local protons concentration in CEM-MEA configurations. We found that Ag/buffer can reach a high CO Faradaic efficiency of 92 %±1.9 at -3.3 V and specific current density of 275 mA cm⁻² for CO at -3.4 V and pH=0.5 (Fig. 3b and Supplementary Fig. 13b). This value is found to be larger than for pH=0.25 and 1, which exhibit 87 %±2.5 and 80 %±1.8 FE for CO, respectively (Supplementary Figs. 13a and c). Surprisingly, this result differs from previous observations, which showed that a higher H⁺ concentration facilitates the HER. Our results unambiguously prove that the high interfacial H⁺ flux is still fully consumed by the buffering layer (Fig. 3c). Additionally, the alkali ions (Cs⁺ from buffering layer and K⁺ from anolyte) at the OHP synergistically exhibit a cationic effect, effectively preventing proton diffusion to the catalyst surface.

Next, we explored the relationships between CO₂ single-pass utilization rate (SPU), conversion rate (SPC) and the CO₂ inlet flow rates for an optimal pH=0.5 anolyte and a current density of 300 mA cm⁻². Since the largest FE of liquid product (here only formate) in CEM-based MEA electrolyzer is lower than 4 % (Supplementary Fig. 13), both SPU and SPC were evaluated without considering formate production. When flowing CO₂ at a limited value of 1 ml/min, the buffered Ag electrode shows the same values for both the SPC and the SPU at 33.5 %±1.4, which means that there is no unreacted CO₂ in the gas outlet and the CO₂ feed is insufficient to sustain a high selectivity towards CO (Fig. 3d). When increasing the CO₂ flow rate, both CO selectivity and CO₂ SPU

were gradually increase and peaked at 91 %±4.1 and 72 %±2.9 for FE<sub>CO</sub> and SPU respectively for a flow rate of 40 ml/min – higher than in neutral MEA with a AEM membrane (Fig. 3d). Using these optimized parameters, we demonstrated a 50 h CO₂RR stable operation at a current density of 300 mA cm⁻² with a high CO selectivity of ≈ 80 % and a steady-state SPU of ≈ 65 % in an acid-fed MEA with a pH=0.5 anolyte (Fig. 3e).

### 3.4. Numerical simulations of the porosity effect of PEI buffering layer on CO₂ microbubbles mass transfer

Based on our experimental investigations, we hypothesize that the persistent regeneration of CO₂ (aq.) occurs by inserting a CsHCO₃-saturated PEI buffering layer between the Ag cathode and CEL. This regeneration is achieved through the recombination of H⁺ and HCO₃⁻ ions at the membrane interface (I₁) and the reformation of CsHCO₃ at the GDL interface (I₂) through the reaction of CO₂ with OH⁻ produced by CO₂RR (Fig. 4a). The consumption of H⁺ and the regeneration of CO₂ (aq.) is therefore expected to create a pH and a [CO₂] gradient across the buffering layer, where the pH is high near the Ag cathode and low at the CEL side. Conversely, a high concentration of CO₂ (aq.) microbubbles accumulates at the CEL side, while a low [CO₂] is found nears the Ag cathode. It is anticipated that tailoring the porosity of buffering layer to promote the detachment of regenerated CO₂ microbubbles is key for achieving high efficiency in CO₂ mass transfer. To gain further insight into our experimental results, we investigated the role of the porosity of the PEI hydrogel buffering layer. Using a one-dimensional multi-physics model in COMSOL we computed the CO₂ mass transfer in both BPM- and CEM-based MEA electrolyzers for a 75 µm-thick hydrogel layer (Fig. 4a and d).

In BPM-based MEA systems, buffering catholyte layers (e.g., 3 M CsHCO₃) with porosities of 0.60, 0.38, 0.32, 0.10 and 0.07 all induce a high local pH microenvironment (~13) near the catalyst, which benefit for CO₂RR (Fig. 4b and Supplementary Fig. 2). The bigger the porosity, the smaller the influence of high pH on the cathode microenvironment. Our results indicate that under steady-state conditions at -4.1 V, the regenerated CO₂ (aq.) at the interface between the buffering layer and
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Schemes and mechanisms of buffering layer inserted in BPM-MEA configurations (a) and CEM-MEA configurations (d). COMSOL Multiphysics simulations of pH (b) and CO₂ concentration (c) in BPM-MEA configurations with –4.0 V continuous operation, and a comparison of BPM and CEM (e), as well as a comparison of CO₂ concentration in CEM-MEA configurations with –3.5 V continuous operation (f).</figcaption></figure>

CEL is continuously supplied to the cathode participating in CO₂RR and forming a concentration gradient toward the cathode surface (Fig. 4c). The simulations also suggest that under stable operating conditions, the CO₂ (aq.) is first generated at the boundary of buffering layer and CEL via the reaction between HCO₃⁻ and H⁺ coming from the water dissociation in the middle of BPM. The CO₂ then diffuses back to catalyst, thus validating our hypothesis. Although the concentration of regenerated CO₂ (aq.) increases with the decrease of porosity from 0.38 (2.1 % PEI) to 0.1 (1.5 % PEI), the accumulations of the regenerated CO₂ (aq.) at the interface I₁ increases the CO₂ diffusion resistance to catalyst and results in an insufficient CO₂RR reaction. When further decreasing the porosity to 0.07 (0.5 % PEI), the protons cannot be fully recombined with HCO₃⁻ leading to a decreased CO₂ (aq.) concentration while the accumulation of CO₂ at the CEL is more pronounced, translating into a large interfacial resistance. These predictions are in qualitative agreement with our experimental results showing low reaction activity and current density (Fig. 2d). Conversely, 2.1 % PEI buffering layer with a porosity of 0.38 displays sufficient CO₂ regeneration reaction with a stable production of CO₂ within ~ 45 µm buffering thickness and a shorter diffusion distance to catalyst, compared to 3.4 % PEI and 1.5 % PEI with the porosities of 0.32 and 0.10, respectively. Further increase of the porosity to 0.60 maintains a stable formation of CO₂ (aq.) in the PEI layer but is expected to lower the buffering capacity to regulate a large number of protons, which is demonstrated by the rapid consumption rate of HCO₃⁻ and the change of pH (Fig. 4d and Supplementary Fig. 14).

Our results also demonstrated a high local pH microenvironment on the catalyst surface is also induced by employing 2.1 % PEI hydrogel layer in CEM-based MEA models, even though operating with different acidic anolytes (Fig. 4d). The concentration of regenerated CO₂ (aq.) at the interface of buffering layer and CEM is also higher than in BPM-based MEA electrolyzer due to the high flux of protons from the acidic

anolyte (Fig. 4e). Driven by the concentration gradient and potential difference under negative voltage, the local concentration of regenerated CO₂ (aq.) between the membrane boundary and the catalyst surface increases with the decrease of pH value of anolyte, pointing out to a strong buffering capacity of the buffering layer even when flowing with more acidic anolyte in CEM configuration (Fig. 4f and Supplementary Fig. 15).

### 3.5. Carbon efficiencies comparison of optimal Ag/buffer cathode in BPM and CEM, as well as Ag cathode in AEM

Carbon efficiency, including CO₂ single-pass conversion rate (SPC) and single-pass utilization rate (SPU), is an important parameter to evaluate the feasibility of the CO₂RR in industry, as it reflects the costs of downstream product separation and the recovery of the CO₂ feedstock. Currently, neutral media AEM-based MEA electrolyzers display the best catalytic performance in terms of FE and current density, with a CO Faradaic efficiency close to 100 % by using AEM [35–37]. This is mainly attributed to the creation of a strongly alkaline microenvironment at the cathode interface that favors CO₂RR compared to competitive HER. Despite these superior performance metrics, AEM-based MEA configurations suffer from carbonate formation through the reaction of CO₂ with hydroxide ions (2OH⁻ + CO₂ → CO₃²⁻ + H₂O) leading to severe losses of CO₂, which translates to a low theoretically upper bound value of 50 % for the single-pass conversion (SPC) and the single-pass utilization rate (SPU). To go beyond these theoretical limitations, conducting CO₂RR in BPM/CEM-based MEA electrolyzers have been considered, since no (bi)carbonate precipitation and crossover is expected, allowing for higher local CO₂ availability at the catalyst surface. To better assess our strategy, we further evaluated carbon efficiencies of our buffering strategy in BPM/CEM-based MEA and compared them
with the neutral media MEA.

In BPM-based MEA, the SPU of an optimal Ag/buffer cathode presents a volcano-like distribution in this two-dimensional coordinate $(CO_2$ flow rate and $j_{total})$, where the high SPU area is located at the upper right of this map (Fig. 5a). Namely, high SPU is achieved at a high $CO_2$ flow rate where the CO selectivity is greatly promoted (Fig. 2e). For a constant inlet flow rate, the larger the current density, the higher the SPU. The highest SPU of 81%±5.3 is achieved at a current density of $375 \text{ mA cm}^{-2}$ under an inlet flow of 50 ml/min. Conversely, the SPC of Ag/buffer tested in BPM-based MEA presents a decline trend with an increased flow rate, and the biggest SPC of 39%±1.6 is obtained at a low flow of 1 ml/min, where the HER dominates the reaction and leads to a higher outlet flow than the inlet (Supplementary Fig. 16). Similarly, Ag/buffer in acid-fed CEM-based MEA electrolyzer shows a high SPU of 77%±2.4 at a flow rate of 40 ml/min and a current density of $375 \text{ mA cm}^{-2}$ where it achieves the highest selectivity towards CO, while a SPC of 37%±1.4 is obtained at 1 ml/min of $CO_2$ flow rate (Fig. 5b and Supplementary Fig. 16). For comparison, pure Ag cathode (i.e. without PEI layer) tested in an AEM-based MEA exhibits only a 55% ± 1.9 SPU at a current density of $375 \text{ mA cm}^{-2}$ with an inlet flow of 20 ml/min, despite achieving a Faradaic Efficiency (FE) for CO of 75% ±1.7 (Fig. 5c). We attribute the lower SPU value in AEM-based MEA compared to that in BPM/CEM-based MEA to the severe carbonate formation that consumes significant amount of the $CO_2$ feed and results in a low outlet flow. We also found that without using the hydrogel buffer layer, the AEM-based MEA exhibited an SPC of 46%±1.6 at a current density of $375 \text{ mA cm}^{-2}$ and a flow rate of 1 ml/min. This result is higher than for BPM/CEM-based MEA using an optimal Ag/buffer as cathodes with an SPC of 39%±1.6 and 37%±1.4, respectively. This is attributed to the high $CO_2$RR selectivity in AEM configuration for extremely low $CO_2$ feed (Supplementary Figs. 16 and 17). We then

compared the energy efficiency (EE) measured at different partial current densities (Fig. 5d). An energy efficiency of 37% for CO can be achieved using an acid-fed CEM-based MEA electrolyzers at $j_{CO}$ of 260 mA cm$^{-2}$ thanks to the Ag/buffer cathode. This value is larger than those measured using a neutral media AEM-based MEA with Ag cathode and a BPM-based MEA, with values of 36% and 26% respectively. We finally compared our results on BPM and CEM-based MEA cells with the literature. Our BPM results notably compares favorably with the performance from previous reports with notably a record-high SPC and SPU values [5, 10, 13, 14, 38, 39] (Fig. 5e and f, Supplementary Fig. 18 and Table 4).

## 4. Conclusion

We report an in-situ constructed superaerophilic polymeric hydrogels to improve the carbon efficiency in both BPM/CEM-based MEA configurations. Our strategy allows to precisely tuning the reaction microenvironment between catalyst and membrane during the $CO_2$RR by promoting the mass transfer of regenerated $CO_2$ and regulating the availability of protons to minimize the competitive HER. We found that the buffering capacity of PEI buffering layer in BPM/CEM-based MEA is controlled by tailoring the porosity of the layer through changing the concentration of building block polymers for cross-linking as supported by our numerical simulation results. Compared to neutral AEM-based MEA electrolyzers, acid-fed CEM-based MEA cells can achieve a CO energy efficiency as high as of 37% at a $j_{CO}$ of 260 mA cm$^{-2}$ and a high $CO_2$ single-pass utilization efficiency of 77%±2.4 when using an optimal Ag/buffer cathode. With the same buffer layer, BPM-based MEA cells demonstrated two-fold increase of the $CO_2$ utilization efficiency at 81%±5.3 compared to the neutral $CO_2$RR using AEM membranes. In summary, our work provides a simple and efficient approach for

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** The relationships among flow rate, current density and $CO_2$ utilization efficiency in BPM-based MEA with an optimized Ag/buffer cathode (a), CEM-based MEA with an optimized Ag/buffer cathode (b) and AEM-based MEA electrolyzers with a pure Ag cathode (c). Comparison of energy efficiency on different MEA configurations (d). Comparisons of carbon efficiency in BPM-based MEA(e) and CEM-based MEA(f) electrolyzers [5, 10, 13, 14, 38, 39]. The red balls and red hollow rhombus refer to the SPU and SPC of this work, while the black balls and black hollow rhombus refer to the SPU and SPC of literature measured at 15 ml/min and 1 ml/min of $CO_2$ flow rates. The dashed lines indicate theoretical carbon efficiency for CO in neutral media AEM-based MEA.</figcaption></figure>
engineering the interfaces between the GDL/catalyst and the electrolyte membrane in MEA electrolyzers to better manage CO₂ and H⁺/H₂O mass transport and adsorption.

## Author Contributions

D.V. conceived the idea, designed the experiments. H.W. designed the experiments with D.V., prepared the electrodes and performed the electrochemical measurements. H.W. and D.V. analyzed the data and wrote the manuscript. B.A.K. performed the BET measurements and analyzed the data with H.W. W.W. J. L. and S.L. assisted H.W. with the physical characterization of the samples. E.P. carried out the liquid NMR spectroscopy measurements. C.S. discussed the electrocatalytic performance with H.W. and D.V. All authors discussed the results and assisted during manuscript preparation.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This work is supported by funds from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 804320), the French National Agency (ANR, JCJC program, MONOMEANR-20-CE08-0009) and the Centre National de la Recherche Scientifique under the INSIS-PEPS programme (CNRS-INSIS-PEPS-BIPLAS).

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2024.124691.

## Data availability

I have shared the link to my data at the attached files.

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