The continuous rise in atmospheric CO₂ concentrations has led to severe environmental issues, including global warming, ocean acidification, and biodiversity loss^[1,2]. Carbon capture and utilization (CCU) technologies have garnered increasing attention due to their capability of reducing CO₂ emissions while simultaneously producing value-added chemicals and fuels^[3-5]. For example, porous materials have been widely studied for CO₂ capture and catalytic conversion, due to their high surface area and tunable structures^[6-8]. Among CCU technologies, electrochemical CO₂ reduction reaction (CO₂RR) is particularly promising, because it can use renewable electricity (e.g., wind and solar power) to convert CO₂ under mild conditions into a variety of valuable products such as CO and formic acid^[9,10]. Currently, most studies on CO₂RR are performed using high-purity CO₂ gas in lab-scale electrolyzers. In practice, industrial flue gas, the main point source of anthropogenic CO₂ emissions, contains 5%-15% CO₂ mixed with N₂, O₂, and other gas impurities. Direct electrolysis of flue gas in gas-fed electrolyzers is challenging because the presence of O₂ severely hinders CO₂RR by triggering the thermodynamically more favorable oxygen reduction reaction (ORR), resulting in reduced selectivity and increased energy loss [Figure 1, Path I]^[11,12]. Moreover, the low CO₂ concentration in the flue gas leads to low reaction efficiency^[13,14].

To overcome these issues, a sequential approach for CO₂ capture and conversion from flue gas has been widely adopted [Figure 1, Path II]^[15-17]. In this process, CO₂ is first absorbed from flue gas using alkaline absorbents such as KOH or amine solutions, forming captured CO₂ solutions enriched with (bi)carbonates or carbamates. This is usually followed by thermal treatment to release CO₂ while simultaneously regenerating the absorbent. The purified CO₂ is compressed and transported for subsequent conversion in a gas-fed electrolyzer^[18]. Although effective at separating CO₂ from flue gas and converting it, this approach involves high energy consumption and capital investment, limiting its economic viability^[19,20]. For instance, capturing CO₂ using a commercial absorbent such as monoethanolamine (MEA) requires up to 4.3 GJ/tCO₂ for MEA regeneration^[21]. Additionally, solvent degradation and equipment corrosion are more likely to occur at elevated temperatures, thereby increasing operational complexity^[22,23].

Electrochemical conversion of captured CO₂ solution has recently emerged as a promising alternative to the conventional sequential CO₂ capture and conversion approach, offering the potential to simplify the overall process and reduce energy consumption [Figure 1, Path III]^[24]. In this strategy, CO₂ capture and conversion occur alternately: CO₂ is captured by alkaline absorbents such as carbonates or MEA to form a captured CO₂ solution, which is then fed into an electrolyzer where CO or other products are electrochemically produced. After the reaction, the regenerated alkaline absorbent can be reused for another round of CO₂ capture and conversion. For instance, Zhang et al. reported the direct electrolysis of captured CO₂ solution (i.e., bicarbonate, obtained by capturing CO₂ in a carbonate solution) in a membrane electrode assembly with a silver foam cathode, achieving a Faraday efficiency for CO (FE_CO) of 60%^[25]. Later, Song et al. developed a Ni-based single-atom catalyst (Ni-SAC) for electrolysis of KHCO₃ solution, achieving a much higher FE_CO exceeding 90%^[26]. In comparison, Leverick et al. attained a relatively low FE_CO of ~19% when employing a simple Ag foil electrode to electrolyze a CO₂-capturing MEA solution^[27].

While path III effectively integrates CO₂ capture and conversion, it relies on pre-captured CO₂. During extended electrolysis, the consumption of captured CO₂ without replenishment decreases its concentration, leading to a drop in product selectivity^[28]. This limitation highlights the need for a concerted CO₂ capture and conversion system operating simultaneously. However, such a strategy has not yet been realized due to the inherent mismatch between the rates of CO₂ capture and electrolysis. This raises a critical question: can the electrolytically regenerated absorbent maintain adequate alkalinity to capture CO₂ effectively and sustain the supply of captured CO₂ species for subsequent conversion cycles at comparable rates^[29,30]? Recent work has demonstrated a coupled system comprising a K₂CO₃ absorption column and an electrolyzer, which achieved a CO₂ capture efficiency of 30% and an FE_CO of 35%, validating the feasibility of such a concerted approach^[31]. Nonetheless, carbonate-based absorbent solutions suffer from slow CO₂ absorption kinetics, hindering their practical applications^[32]. In comparison, MEA exhibits faster reaction kinetics and higher industrial maturity for CO₂ capture, making it a more promising candidate for practical deployment^[33,34].

Herein, we report an MEA-based concerted CO₂ capture-conversion system by coupling a CO₂ absorber with a liquid-fed electrolyzer for continuous syngas production from simulated flue gas (containing 15% CO₂, 5% O₂, and 80% N₂) [Figure 1, Path IV]. A bipolar membrane (BPM) is utilized to generate protons, which react with the carbamate/bicarbonate species formed during CO₂ capture in MEA, enabling in situ CO₂ liberation for subsequent electrochemical reduction to CO. Meanwhile, MEA is regenerated and recycled back to capture additional CO₂, forming a concerted system for simultaneous CO₂ capture and electrochemical conversion. During electrolysis, H₂ is also produced from the reduction of protonated MEA generated during CO₂ capture, forming a syngas with H₂/CO = 3.2. In situ electrochemical spectroscopy is performed to elucidate the reaction processes occurring at the electrode surface.

Catalysts are essential for converting CO₂ into valuable products, such as CO, an important feedstock for the chemical industry. Three main types of catalysts have been reported for CO₂-to-CO conversion, including noble metal catalysts (e.g., Au and Ag)^[36-38], molecular metal complex catalysts (e.g., cobalt phthalocyanine supported on carbon black, CoPc/CB)^[39,40], and metal-nitrogen-carbon (M-N-C) catalysts^[41,42]. Noble metal catalysts exhibit high activity but are costly, limiting their practical applications. Molecular catalysts such as CoPc/CB can offer good selectivity but often suffer from limited stability under continuous electrolysis conditions^[28]. In contrast, M-N-C catalysts feature abundant active sites, high activity, and structural robustness, making them promising candidates for CO₂ electroreduction^[43]. In particular, Ni-N-C catalysts with well-defined active Ni-N sites have shown high activity and selectivity for CO₂-to-CO conversion^[44-46]. Therefore, a Ni-N-C single-atom catalyst is used as a model catalyst in this study because of its established CO selectivity and well-understood active site structure. The Ni-N-C is prepared according to previously reported procedures^[35].

A liquid-fed electrolyzer is first fabricated for the electrochemical conversion of CO₂ captured in an MEA solution [Supplementary Figure 1]. The electrolyzer comprises an IrO_x/Ti mesh anode and a carbon paper cathode coated with a Ni-N-C catalyst. Both electrodes are firmly pressed between serpentine flow plates on the anodic and cathodic sides. The active area of the electrode is 1 cm². A 1 M KOH solution is used as the anolyte. A CO₂-capturing 5 M MEA solution, prepared by bubbling CO₂ into the MEA solution until CO₂ capture is completed, is used as the catholyte. NMR spectroscopy shows that the CO₂-capturing 5 M MEA solution primarily contains ammonium, carbamate, and bicarbonate [Supplementary Figure 2].

A BPM is inserted between the anode and cathode to physically separate the two compartments. Under reverse-bias operation, the BPM promotes water dissociation, supplying hydroxide anions (OH^-) to the anode and protons (H⁺) to the cathode. At the anode, OH^- is oxidized at the IrO_x/Ti mesh surface to produce O₂ and H₂O. At the cathode, H⁺ can react with carbamate and bicarbonate species in the CO₂-capturing MEA solution to liberate in situ CO₂, which is then reduced at the Ni-N-C catalyst. Electrochemical performance is evaluated at constant current densities of 50, 100, and 200 mA·cm^-2, with N₂, CO₂, or simulated flue gas continuously sparged into the electrolyte. Gaseous products generated after electrolysis are analyzed using online gas chromatography, confirming that only CO and H₂ are produced. ¹H NMR analysis does not show detectable products in the electrolyte.

Following the construction of the liquid-fed electrolyzer, we first evaluate the effect of the gas atmosphere on the electrochemical conversion efficiency of the CO₂-capturing MEA solutions. Under continuous pure CO₂ sparging, the FE_CO is 34%, 32%, and 15% at 50, 100, and 200 mA·cm^-2, respectively, with the remaining FE attributed to the hydrogen evolution reaction (HER) [Supplementary Figure 3A]. When the atmosphere is switched to 15% CO₂/85% N₂ or pure N₂, comparable FE_CO and cell voltages are observed [Supplementary Figure 3]. This result indicates that even under limited or no CO₂ feed, the chemically captured CO₂ in MEA solution can be effectively utilized for electrochemical CO production.

The conversion efficiencies of CO₂-capturing MEA using a BPM and an AEM are also compared. Because no H⁺ is produced at the AEM and no CO₂ is liberated, replacing the BPM with an AEM leads to a decrease in CO production, despite the lower cell voltage with an AEM [Supplementary Figure 4]. This result further confirms the critical role of H⁺ generated at the BPM in reacting with carbamate/bicarbonate to liberate in situ CO₂, which is essential for subsequent conversion to produce CO.

Given that practical CO₂ sources often contain high concentrations of O₂ ranging from 2% to 12%^[47], the effect of O₂ on the electrochemical conversion of CO₂-capturing MEA is investigated under simulated flue gas conditions, with the CO₂ concentration fixed at 15% and the O₂ adjusted using N₂. For comparison, a gas-fed electrolyzer, structurally identical to the liquid-fed system except for the use of an AEM and the addition of a microporous layer on the carbon paper cathode, is also examined [Supplementary Figure 5]. In the gas-fed electrolyzer, the linear sweep voltammetry shows that the onset potential for reduction current shifts positively upon the introduction of 5% O₂ into the CO₂/N₂ stream (15% CO₂/80% N₂/5% O₂), indicating substantial activity of Ni-N-C for O₂ reduction [Supplementary Figure 6]. Moreover, neither CO nor H₂ product is detected during the electrolysis of 15% CO₂/80% N₂/5% O₂ at current densities of 50 and 100 mA·cm^-2, in stark contrast to the electrolysis of 15% CO₂/85% N₂ (FE_CO = 81% at 50 mA·cm^-2 and 63% at 100 mA·cm^-2), suggesting nearly all electrons are diverted to the competing ORR when 5% O₂ is present in the gas-fed electrolyzer [Figure 2A and Supplementary Figure 7]. Correspondingly, the cell voltages recorded in the presence of O₂ are lower than those recorded under O₂-free conditions. At an increased current density of 200 mA·cm^-2, 45% $$ \mathrm{FE}_{\mathrm{H}_2} $$ and 1% FE_CO are detected, though ORR remains the dominant reaction.

In contrast, the cell voltage for the reduction current observed in CO₂-capturing MEA solution under a gas mixture of 15% CO₂/80% N₂/5% O₂ is quite similar to that under 15% CO₂/85% N₂ [Supplementary Figure 8]. The FE_CO is 31%, 29%, and 16% at 50, 100, and 200 mA·cm^-2, respectively, which are similar to those recorded under O₂-free conditions [Figure 2B and Supplementary Figure 9]. Notably, no obvious decrease in FE_CO is observed when the O₂ concentration increases to 10%, 20%, or even 100% [Supplementary Figure 10]. To further quantify the oxygen effect, the dissolved oxygen concentration in the electrolyte is measured under 15% CO₂/85% N₂, 15% CO₂/80% N₂/5% O₂, and 100% O₂ conditions, with each gas continuously bubbled into the solution for 30 min [Supplementary Figure 11]. The dissolved oxygen concentration is 0.003 mM under 15% CO₂/85% N₂, 0.060 mM under 15% CO₂/80% N₂/5% O₂, and 0.492 mM under 100% O₂ after 30 min. Despite the large differences in dissolved oxygen concentration, the FE_CO in the liquid-fed system remains comparable across all tested conditions. This indicates that O₂ in the flue gas stream has negligible influence on the electrochemical conversion of CO₂-capturing MEA solution, making the system oxygen-tolerant.

Figure 2C illustrates the electrolysis process in the gas-fed and liquid-fed electrolyzers in the presence of O₂. In the gas-fed electrolyzer, the ORR dominates over CO₂ reduction due to its more favorable thermodynamics and comparable concentrations to CO₂, thereby suppressing CO production, which aligns with previous studies^[48,49]. However, the impact of O₂ is negligible in the liquid-fed electrolyzer. In aqueous media, the solubility of O₂ is only 0.0012 M (1 atm, 298 K), much lower than that of CO₂ (0.034 M, 1 atm, 298 K)^[50,51]. The inherently low O₂ solubility limits its concentration at the cathode, allowing CO₂ electroreduction to proceed without complications arising from O₂ reduction, regardless of the O₂ concentrations in the gas phase. Additionally, H⁺ generated by the BPM reacts with carbamate/bicarbonate species to release CO₂ in situ, ensuring a high local CO₂ concentration at the electrode surface for electroreduction. These results highlight the large potential of the concerted pathway for capture and conversion of CO₂ from flue gas.

After validating the direct conversion of CO₂-capturing MEA solution, we establish a concerted CO₂ capture and conversion system by coupling a simple CO₂ absorber with an electrolyzer designed for CO₂-capturing MEA [Figure 3A]. To enhance both CO₂ capture efficiency and CO production rate, the active area of the Ni-N-C electrode is increased to 4 cm². A 2 M MEA solution is selected as the capture medium because it provides an optimal trade-off, achieving a higher FE_CO than 1 M MEA and a comparable FE_CO to 5 M MEA while operating at a lower cell voltage [Supplementary Figure 12]. The capture of CO₂ from simulated flue gas (15% CO₂/80% N₂/5% O₂) is performed by continuously bubbling the flue gas into the MEA solution at 50 sccm for 12 h. The completion of CO₂ capture is confirmed when the outlet CO₂ concentration approaches the inlet level (~15%), and the pH decreases from 12.0 to 8.3 [Figure 3B]. The final pH is slightly higher than that for capture of pure CO₂ (pH 7.7), which is consistent with Henry’s Law because of the lower partial pressure of CO₂ in flue gas. The CO₂-capturing MEA solution is subsequently electrolyzed at a constant current density of 50 mA·cm^-2, with a flue gas flow rate of 5 sccm and a solution flow of 200 mL/min. Meanwhile, the regenerated MEA solution is circulated throughout the system, enabling concerted capture and electrochemical conversion of CO₂.

The system achieves a maximum FE_CO of 26% after 2 h of operation at 50 mA·cm^-2 [Figure 3C]. The FE_CO then gradually declines to 14% over the next 22 h of operation, with a cumulative production of 401 mL CO and 1,531 mL H₂, forming a syngas with H₂/CO = 3.8 [Figure 3D]. Meanwhile, the concentration of CO₂ in the flue gas after CO₂ capture ([CO₂]_outlet) decreases markedly during the first 12 h and changes only slightly thereafter [Figure 3B]. Correspondingly, the CO₂ capture efficiency increases during the first 12 h, then becomes relatively stable over the subsequent 12 h, and ultimately reaches a maximum of 64% [Figure 3E]. These observations indicate that MEA is gradually regenerated as the 15% CO₂-capturing MEA solution is electrochemically converted, enabling the solution to capture increasing amounts of CO₂ from the flue gas. After approximately 12 h, the system enters a dynamic equilibrium where the CO₂ capture rate matches the electrochemical conversion rate, accomplishing concerted CO₂ capture and conversion.

To investigate the influence of reaction rate on the concerted CO₂ capture and conversion system, the electrolysis is then carried out at a higher current density of 100 mA·cm^-2 [Supplementary Figure 13]. However, a lower FE_CO of 20% is observed due to the intensified HER. Nevertheless, a higher CO₂ capture efficiency of 73% is achieved compared to electrolysis at 50 mA·cm^-2, which can be attributed to accelerated regeneration of MEA driven by the enhanced consumption of reactive carbon species at the higher electrolysis current density. This result shows that increasing the current density of electrolysis can improve CO₂ capture efficiency, though it may reduce the efficiency of the subsequent conversion step. Therefore, a trade-off exists between the efficiencies of CO₂ capture and electrochemical conversion.

To gain molecular-level insight into the electrode environment during this concerted process, we employed in situ ATR-FTIR spectroscopy to monitor the electrolysis of a 2 M MEA solution pre-saturated with the simulated flue gas (15% CO₂/80% N₂/5% O₂). As shown in Figure 4, during potentiostatic electrolysis, a distinct decrease in the CO₂ characteristic peak (2,362 cm^-1) was observed, indicating CO₂ consumption at the electrode surface^[52,53]. Key solution species were identified by peaks at 1,567, 1,495, 1,390, and 1,330 cm^-1, which were attributed to RNHCOO^-[53]. The peak at 1,222 cm^-1 corresponded to HCO₃^-[54], consistent with the NMR results [Supplementary Figure 2]. The presence of MEA-H⁺ (1,632-1,651 cm^-1) was also confirmed, though this peak may overlap with water bending vibrations in the same range^[52,55]. Notably, weak features associated with adsorbed CO intermediates (CO_L at ~1,944 cm^-1 and CO_B at ~1,795 cm^-1) were detected, offering insight into the surface reaction pathway on the Ni-N-C catalyst^[56]. This in situ analysis supports the proposed mechanism, where H⁺ from the BPM facilitates MEA regeneration and the release of CO₂ from carbamate/bicarbonate pools, which is then reduced to CO at the Ni-N-C working electrode.

Building on this understanding of the electrode microenvironment, we sought to further enhance the CO₂ conversion efficiency by modifying the electrode structure. We incorporated PTFE into the Ni-N-C catalyst to increase the hydrophobicity of the electrode surface, a feature that has been shown to suppress HER^[57]. Using the same procedure as for the Ni-N-C electrode, 100 µL of 2 wt% PTFE solution was added to the catalyst ink to fabricate the Ni-N-C-PTFE electrode. CO₂ capture and electrolysis were then carried out simultaneously by applying a constant current of 50 mA·cm^-2 in 15% CO₂-capturing MEA solution [Figure 5 and Supplementary Figure 14]. As shown in Figure 5A, the Ni-N-C-PTFE electrode exhibits a FE_CO of up to 32% within the first 2 h, surpassing the unmodified Ni-N-C electrode (FE_CO = 26%). Over the following 22 h, however, the FE_CO gradually declines, eventually reaching the same level in both cases. The corresponding CO₂ capture efficiency remains stable at ~63%, consistent with that observed for the unmodified Ni-N-C cathode [Supplementary Figure 14B]. Encouragingly, the higher initial CO production rate with the PTFE-modified electrode results in a cumulative output of 432 mL CO and 1,381 mL H₂, forming a syngas with a H₂/CO = 3.2, lower than 3.8 for the unmodified electrode. The resulting syngas composition is suitable for Fischer-Tropsch synthesis to produce value-added liquid fuels, such as light olefins^[58].

We also performed electrolysis of 15% CO₂-capturing MEA under a N₂ flow to examine the role of continuous CO₂ supply. At the same current density (50 mA·cm^-2), the [CO₂]_outlet is lower in the absence of CO₂ feed than with the supply of 15% CO₂ [Supplementary Figure 15A]. Consequently, FE_CO remains lower under N₂ flow than under 15% CO₂ throughout the 24 h of electrolysis. Notably, FE_CO declines to only 5% at the end of 24 h of electrolysis under N₂ flow [Supplementary Figure 15B]. As a result, 269 mL CO and 1,685 mL H₂ are produced with a H₂/CO = 6.3 [Supplementary Figure 15C]. Additionally, the solution pH rises from 8.3 to 9.3 [Supplementary Figure 15D], compared with only a slight increase to 8.6 under 15% CO₂/80% N₂/5% O₂ supply. This is primarily because MEA regenerated during electrolysis cannot recapture CO₂ and therefore accumulates in the absence of CO₂ feeding. The marked differences between electrolysis experiments using flue gas and N₂ streams confirm that considerable CO is produced directly through the capture and electrochemical conversion of CO₂ in flue gas.

Since the composition of flue gas varies in real-world scenarios, we then examine the influence of CO₂ concentration in simulated flue gas on the efficiency of CO₂ capture and subsequent conversion. To this end, the CO₂ concentration is varied from 15% to 5% by diluting with N₂ and O₂. The resulting gas mixtures are continuously bubbled into 2 M MEA solutions at the same flow rate (50 sccm) to perform CO₂ capture. Due to the lower CO₂ concentration, CO₂ capture requires an additional 10 h for flue gas containing 5% CO₂ compared with its 15% CO₂ counterpart, as evidenced by measurements of the CO₂ concentration in the outlet gas and the pH of the MEA solution [Figure 6A and Supplementary Figure 16]. Notably, a higher steady-state pH is observed in the system supplied with 5% CO₂, suggesting that a lower CO₂ partial pressure reduces CO₂ loading at the end of CO₂ capture.

Similarly, concerted capture and electrochemical conversion of CO₂ from flue gas containing 5% CO₂ are performed using the Ni-N-C-PTFE electrode under the same conditions used for flue gas containing 15% CO₂. As shown in Figure 6B and C, the FE_CO recorded in 5% CO₂-capturing MEA solution is consistently lower than that in a 15% CO₂-capturing MEA solution, resulting in the production of 254 mL CO and 1,495 mL H₂. This decrease in FE_CO can be attributed to the reduced partial pressure of CO₂, which lowers the CO₂ loading in MEA and thus reduces the concentration of reactive carbon species available for electrochemical conversion. Nonetheless, the full-cell voltages recorded in 5% CO₂ and 15% CO₂-capturing MEA solutions are comparable, despite marked differences in CO₂ loading and product distribution [Supplementary Figure 17]. This is due to the increased HER activity at lower CO₂ concentrations. Notably, the CO₂ capture efficiency increases from 67% when supplied with 15% CO₂ flue gas to 88% when supplied with 5% CO₂ flue gas [Figure 6D]. Additionally, for both CO₂ concentrations, the solution pH increases during electrolysis as a result of MEA regeneration associated with CO₂ conversion and HER [Figure 6A]. The rate of pH increase in the system with 5% CO₂ is faster than that with 15% CO₂, because the CO₂ capture rate is lower than the MEA regeneration rate under low-concentration CO₂.

Building on the above parameter optimization, we determined that CO₂ capture from 15% CO₂ by 2 M MEA and electrolysis at 50 mA·cm^-2 with the Ni-N-C-PTFE electrode provides a favorable balance between CO₂ capture efficiency and CO selectivity. To further elucidate the origin of this balance, we analyze the dynamic relationship between CO₂ capture and electrolysis by calculating CO₂ capture rate and CO production rate (the latter corresponding to the MEA regeneration rate), enabling a quantitative evaluation of their relationship during operation.

The reactions involved in CO₂ capture and electrolysis processes are presented in Supplementary Equations 1-14 in Supplementary Figure 18. As shown in Supplementary Figure 19, the CO₂ capture rate is very low initially. This is because the starting solution is a 15% CO₂-capturing MEA solution, in which MEA predominantly exists as MEA-H⁺, carbamate, and HCO₃^- [Supplementary Equations 1-3], leaving negligible free MEA available for additional CO₂ absorption. Upon electrolysis, several steps occur simultaneously. Water splitting occurs at the BPM of the membrane electrode assembly, generating H⁺ and OH^- [Supplementary Equation 4]. In an ideal case, all H⁺ react with carbamate and bicarbonate species to release CO₂ and partially regenerate MEA [Supplementary Equations 5 and 6]. The CO₂ is then reduced to CO at the cathode, generating CO and OH^- [Supplementary Equation 7]. The OH^- can react with MEA-H⁺ [Supplementary Equation 8], regenerating MEA completely. At the anode, the OH^- from water splitting is oxidized to generate O₂ [Supplementary Equation 12]. Overall, the net reaction for CO₂ capture and conversion is CO₂ splitting [Supplementary Equation 13], which simultaneously depletes the captured CO₂ species and regenerates MEA [Supplementary Equations 5-8]. However, the electrolysis system is very complicated. Some H⁺ from the BPM and MEA-H⁺ itself will be reduced directly to generate H₂ [Supplementary Equations 10 and 11], competing with CO₂ release and reduction steps. However, the overall reaction for H₂ evolution will not regenerate MEA [Supplementary Equation 14]. In addition, some H⁺ will react with OH^- generated during CO₂ reduction, neutralizing the additionally generated OH^- in the cathodic chamber [Supplementary Equation 9]. Therefore, the MEA regeneration rate would in principle be the same as the CO production rate.

As the concentration of free MEA increases, the solution progressively regains its capacity to capture CO₂, leading to a continuous increase in the CO₂ capture rate. Around 12 h, a dynamic balance between MEA regeneration by electrolysis and consumption by CO₂ capture is established, and the capture rate approaches a steady state [Supplementary Figure 19].

In contrast, the CO production rate gradually decreases over time. This decline is primarily attributed to the increase in local pH near the cathode. Continuous CO₂ reduction generates OH^- [Supplementary Equation 7], which raises the electrode interfacial alkalinity and shifts the equilibrium away from free CO₂ (toward carbonate/bicarbonate species). This shift makes CO₂ release more difficult, thereby reducing the availability of molecular CO₂ for electroreduction. In addition, prolonged operation may also induce catalyst surface changes and mass transport limitations, further contributing to the decrease in CO production rate.

Notably, the CO₂ capture rate and CO production rate intersect at approximately 4.5 h [Supplementary Figure 19]. This crossover point represents a transient state where CO₂ supply and consumption are momentarily balanced. However, due to the direct reduction of H⁺ from BPM [Supplementary Equation 10], the generated OH^- from CO₂ reduction cannot be immediately neutralized [Supplementary Equation 9], which can capture additional CO₂. This indicates that the capture capacity of the catholyte has not yet been reached. As a result, the capture rate continues to rise beyond this point, while the CO production rate cannot be sustained due to the progressively unfavorable cathodic microenvironment. At a certain point (under the selected CO₂ capture and electrolysis conditions for ~12 h), the balance for CO₂ capture and electrolysis is reached.

These results demonstrate that the rate mismatch originates from the different temporal evolution of CO₂ capture in the bulk solution versus CO₂ conversion at the electrode. Importantly, this balance can be tuned by adjusting both CO₂ supply (inlet CO₂ concentration) and CO₂ utilization (electrolysis current density), enabling optimal conditions under which CO₂ capture efficiency and CO selectivity are simultaneously optimized.

Finally, the MEA-mediated concerted system is compared with both conventional sequential and recently reported carbonate-based concerted systems [Supplementary Table 1]. The comparison involves key metrics such as Faraday efficiency and partial current density for CO production, total current density, stability, CO₂ capture efficiency, and energy consumption (Details of the energy consumption calculation are provided in Supplementary Note 1). We note that the Faraday efficiency and partial current density for CO production of our MEA-mediated system are currently lower than those of conventional sequential systems, yet comparable to recently reported carbonate-based concerted systems. Notably, our system offers an advantage in overall energy consumption compared to the conventional sequential route, as it eliminates the need for energy-intensive upstream CO₂ desorption (e.g., thermal regeneration). Additionally, our system also exhibits a twofold higher CO₂ capture efficiency compared to carbonate-based concerted systems, because of the faster CO₂ absorption kinetics of amines. These results suggest that the MEA-mediated system enables effective CO₂ capture and utilization.

We have developed an MEA-based system for concerted CO₂ capture and electrochemical conversion by combining a simple CO₂ absorber with a liquid-fed electrolyzer. The CO₂-capturing MEA solution is directly utilized as the liquid feedstock for electrochemical conversion, enabling in situ CO₂ release and subsequent syngas production over a Ni single-atom catalyst. The system shows strong tolerance to oxygen and is feasible for concerted CO₂ capture and electrochemical conversion under flue gas conditions. Through optimization of current density, electrode hydrophobicity, and CO₂ concentration, we achieved a high CO₂ capture efficiency of 63% and efficiently produce syngas with H₂/CO = 3.2. This work demonstrates the feasibility of MEA-mediated concerted CO₂ capture-conversion for flue gas upgrading, while further optimizations in amine structure, catalyst stability, electrode design, and system operation are warranted for long-term applications.