CO2 electroreduction to multicarbon products from carbonate capture liquid

Abstract

•The electrochemical upgrade of CO2 captured in the carbonate form to C2+ products•Studying key metrics (local pH and reactant concentration) in reactive capture•Interposer and catalyst joint design enables the electrosynthesis of 56 wt % C2H4•Reactive capture obviates the need for energy to regenerate lost/unreacted CO2 Cradle-to-gate carbon-negative technologies, including direct air capture (DAC), have shown promise in mitigating CO2 emissions. However, these emerging technologies to capture CO2 from the air rely on a thermal swing to release concentrated CO2, and today this comes at a high energy cost. The ensuing step in gas-phase CO2 electrolysis requires additional energy. Furthermore, this step suffers from incomplete CO2 conversion. This leads to a high cost to regenerate/separate (otherwise-lost/unreacted) CO2. To tackle these challenges, a scheme known as reactive capture has been proposed: the integrated systems for capture-and-upgrade of CO2 to valuable products. This approach is the direct conversion of chemisorbed CO2 into value-added products. The benefits of reactive capture are (1) to avoid the energy-intensive and carbon-positive steps associated with concentrating CO2 and (2) to enable ∼0% reactant losses. This obviates the need for energy to regenerate/separate lost/unreacted reactants. Alkali hydroxide systems capture CO2 as carbonate; however, generating a pure CO2 stream requires significant energy input, typically from thermal cycling to 900°C. What is more, the subsequent valorization of gas-phase CO2 into products presents additional energy requirements and system complexities, including managing the formation of (bi)carbonate in an electrolyte and separating unreacted CO2 downstream. Here, we report the direct electrochemical conversion of CO2, captured in the form of carbonate, into multicarbon (C2+) products. Using an interposer and a Cu/CoPc-CNTs electrocatalyst, we achieve 47% C2+ Faradaic efficiency at 300 mA cm−2 and a full cell voltage of 4.1 V. We report 56 wt % of C2H4 and no detectable C1 gas in the product gas stream: CO, CH4, and CO2 combined total below 0.9 wt % (0.1 vol %). This approach obviates the need for energy to regenerate lost CO2, an issue seen in prior CO2-to-C2+ reports. Alkali hydroxide systems capture CO2 as carbonate; however, generating a pure CO2 stream requires significant energy input, typically from thermal cycling to 900°C. What is more, the subsequent valorization of gas-phase CO2 into products presents additional energy requirements and system complexities, including managing the formation of (bi)carbonate in an electrolyte and separating unreacted CO2 downstream. Here, we report the direct electrochemical conversion of CO2, captured in the form of carbonate, into multicarbon (C2+) products. Using an interposer and a Cu/CoPc-CNTs electrocatalyst, we achieve 47% C2+ Faradaic efficiency at 300 mA cm−2 and a full cell voltage of 4.1 V. We report 56 wt % of C2H4 and no detectable C1 gas in the product gas stream: CO, CH4, and CO2 combined total below 0.9 wt % (0.1 vol %). This approach obviates the need for energy to regenerate lost CO2, an issue seen in prior CO2-to-C2+ reports. CO2 capture from air and oceans, when combined with an upgrade into chemicals that serve as precursors to long-lived materials, offers to contribute carbon-negative (cradle-to-gate) solutions that offset difficult-to-abate emissions on the path to net-zero emissions.1Brethomé F.M. Williams N.J. Seipp C.A. Kidder M.K. Custelcean R. Direct air capture of CO2 via aqueous-phase absorption and crystalline-phase release using concentrated solar power.Nat. Energy. 2018; 3: 553-559https://doi.org/10.1038/s41560-018-0150-zCrossref Scopus (99) Google Scholar,2Kätelhön A. Meys R. Deutz S. Suh S. Bardow A. Climate change mitigation potential of carbon capture and utilization in the chemical industry.Proc. Natl. Acad. Sci. USA. 2019; 116: 11187-11194https://doi.org/10.1073/pnas.1821029116Crossref PubMed Scopus (276) Google Scholar,3Socolow R. Desmond M. Aines R. Blackstock J. Bolland O. Kaarsberg T. Lewis N. Mazzotti M. Pfeffer A. Sawyer K. Direct air capture of CO2 with chemicals: a technology assessment for the APS panel on public affairs. American Physical Society, 2011http://www.aps.org/policy/reports/assessments/upload/dac2011.pdfGoogle Scholar Reactive capture systems unite CO2 capture with CO2 upgrade/utilization into more valuable chemicals. Much progress has been made electrochemically generating CO from captured CO2, on the path to fuels and chemicals via syngas processes.4Lee G. Li Y.C. Kim J.-Y. Peng T. Nam D.-H. Sedighian Rasouli A. Li F. Luo M. Ip A.H. Joo Y.-C. et al.Electrochemical upgrade of CO2 from amine capture solution.Nat. Energy. 2021; 6: 46-53https://doi.org/10.1038/s41560-020-00735-zCrossref Scopus (78) Google Scholar,5Li Y.C. Lee G. Yuan T. Wang Y. Nam D.-H. Wang Z. García de Arquer F.P. Lum Y. Dinh C.-T. Voznyy O. et al.CO2 electroreduction from carbonate electrolyte.ACS Energy Lett. 2019; 4: 1427-1431https://doi.org/10.1021/acsenergylett.9b00975Crossref Scopus (93) Google Scholar,6Li T. Lees E.W. Goldman M. Salvatore D.A. Weekes D.M. Berlinguette C.P. Electrolytic conversion of bicarbonate into CO in a flow cell.Joule. 2019; 3: 1487-1497https://doi.org/10.1016/j.joule.2019.05.021Abstract Full Text Full Text PDF Scopus (109) Google Scholar,7Zhang Z. Lees E.W. Habibzadeh F. Salvatore D.A. Ren S. Simpson G.L. Wheeler D.G. Liu A. Berlinguette C.P. Porous metal electrodes enable efficient electrolysis of carbon capture solutions.Energy Environ. Sci. 2022; 15: 705-713https://doi.org/10.1039/D1EE02608ACrossref Google Scholar,8Xiao Y.C. Gabardo C.M. Liu S. Lee G. Zhao Y. O’Brien C.P. Miao R.K. Xu Y. Edwards J.P. Fan M. Direct carbonate electrolysis into pure syngas.EES Catal. 2023; 1: 54-61Crossref Google Scholar In thermochemistry, reactive capture has proceeded to methane, methanol, and formate.9Kar S. Goeppert A. Prakash G.K.S. Integrated CO2 capture and conversion to formate and methanol: connecting two threads.Acc. Chem. Res. 2019; 52: 2892-2903https://doi.org/10.1021/acs.accounts.9b00324Crossref PubMed Scopus (148) Google Scholar,10Kothandaraman J. Saavedra Lopez J. Jiang Y. Walter E.D. Burton S.D. Dagle R.A. Heldebrant D.J. Integrated capture and conversion of CO2 to methane using a water-lean, post-combustion CO2 capture solvent.ChemSusChem. 2021; 14: 4812-4819https://doi.org/10.1002/cssc.202101590Crossref Scopus (9) Google Scholar,11Sen R. Goeppert A. Kar S. Prakash G.K.S. Hydroxide based integrated CO2 capture from air and conversion to methanol.J. Am. Chem. Soc. 2020; 142: 4544-4549https://doi.org/10.1021/jacs.9b12711Crossref PubMed Scopus (106) Google Scholar C2 and higher products (C2+) represent a large global market: ethylene and ethanol lie in the range of ∼US$230B and ∼US$160B, respectively,12GlobeNewswire. PrecedenceResearchEthanol market size worth around USD 155.6 billion by 2030. t.https://www.globenewswire.com/news-release/2021/01/18/2160198/0/en/Ethanol-Market-Size-Worth-Around-USD-155-6-Billion-by-2030.html#:∼:text=The%20global%20ethanol%20market%20size,5.2%25%20from%202021%20to%202030Date: 2021Google Scholar,13Polaris Market ResearchEthylene market size worth $230.7 billion by 2029|CAGR: 5.5%.https://www.polarismarketresearch.com/press-releases/ethylene-marketDate: 2021Google Scholar in contrast with the C1 chemicals (CO and formic acid) generated by reactive capture to date, whose combined values are below US$12B.14Polaris Market ResearchFeed acidifiers market size worth $2.03 billion by 2026|CAGR: 4.5%.https://www.polarismarketresearch.com/press-releases/feed-acidifiers-marketDate: 2017Google Scholar,15Polaris Market ResearchCarbon dioxide market size worth $9.91 billion by 2029|CAGR: 4.1%.https://www.polarismarketresearch.com/press-releases/carbon-dioxide-co2-marketDate: 2021Google Scholar Yet, to date, it is mainly C1 products that have been produced in reactive capture systems of both electrochemical and thermochemical types. Direct air capture (DAC) using alkali hydroxide captures CO2 as carbonate and generates a pure/concentrated gas-phase CO2 stream via thermal swing at ∼900°C.16Keith D.W. Holmes G. St. Angelo D. Heidel K. A process for capturing CO2 from the atmosphere.Joule. 2018; 2: 1573-1594https://doi.org/10.1016/j.joule.2018.05.006Abstract Full Text Full Text PDF Scopus (662) Google Scholar The subsequent valorization of gas-phase CO2 into value-added products introduces further energy losses and system complexity. This approach involves introducing CO2 in the gas phase for electrolysis. In contradistinction, reactive capture takes the carbon source from carbonate species, bypassing CO2 concentrating steps. In prior reports of reactive capture from carbonate, Li et al. demonstrated pure syngas production with Faradaic efficiency (FE) of ∼30% CO and ∼70% H2, and with a Cu electrocatalyst, ∼14% C2 FE was observed.5Li Y.C. Lee G. Yuan T. Wang Y. Nam D.-H. Wang Z. García de Arquer F.P. Lum Y. Dinh C.-T. Voznyy O. et al.CO2 electroreduction from carbonate electrolyte.ACS Energy Lett. 2019; 4: 1427-1431https://doi.org/10.1021/acsenergylett.9b00975Crossref Scopus (93) Google Scholar The authors reported no appreciable loss of CO2 during carbonate electrolysis. Such prior studies offer a path to avoid the energy-intensive steps associated with concentrating CO2 and regenerating lost CO2; however, until now, the selectivity toward more valuable CO2-derived products has been limited compared with the diversity of products available in electrochemical CO2 reduction reaction (CO2RR) systems. Such electrochemical CO2RR systems, although they have achieved impressive increases in performance,17García de Arquer F.P.G.d. Dinh C.T. Ozden A. Wicks J. McCallum C. Kirmani A.R. Nam D.H. Gabardo C. Seifitokaldani A. Wang X. et al.CO2 electrolysis to multicarbon products at activities greater than 1 A cm−2.Science. 2020; 367: 661-666https://doi.org/10.1126/science.aay4217Crossref PubMed Scopus (570) Google Scholar,18Li F. Thevenon A. Rosas-Hernández A. Wang Z. Li Y. Gabardo C.M. Ozden A. Dinh C.T. Li J. Wang Y. et al.Molecular tuning of CO2-to-ethylene conversion.Nature. 2020; 577: 509-513https://doi.org/10.1038/s41586-019-1782-2Crossref PubMed Scopus (446) Google Scholar suffer—in the case of alkaline and neutral CO2RR—from low CO2 utilization (the fraction of input CO2 converted into desired products) that is ≤25% in the case of C2 production due to carbonate formation in locally alkaline conditions.19Rabinowitz J.A. Kanan M.W. The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem.Nat. Commun. 2020; 11: 5231Crossref PubMed Scopus (179) Google Scholar The low CO2 utilization leads to a high cost to regenerate (otherwise-lost/emitted) CO2.19Rabinowitz J.A. Kanan M.W. The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem.Nat. Commun. 2020; 11: 5231Crossref PubMed Scopus (179) Google Scholar,20Alerte T. Edwards J.P. Gabardo C.M. O’Brien C.P. Gaona A. Wicks J. Obradović A. Sarkar A. Jaffer S.A. MacLean H.L. et al.Downstream of the CO2 electrolyzer: assessing the energy intensity of product separation.ACS Energy Lett. 2021; 6: 4405-4412https://doi.org/10.1021/acsenergylett.1c02263Crossref Scopus (20) Google Scholar Enticingly, recent studies have overcome this CO2 utilization limit via local CO2 regeneration21Huang J.E. Li F. Ozden A. Sedighian Rasouli A. García de Arquer F.P. Liu S. Zhang S. Luo M. Wang X. Lum Y. et al.CO2 electrolysis to multicarbon products in strong acid.Science. 2021; 372: 1074-1078https://doi.org/10.1126/science.abg6582Crossref PubMed Scopus (262) Google Scholar,22O’Brien C.P. Miao R.K. Liu S. Xu Y. Lee G. Robb A. Huang J.E. Xie K. Bertens K. Gabardo C.M. et al.Single pass CO2 conversion exceeding 85% in the electrosynthesis of multicarbon products via local CO2 regeneration.ACS Energy Lett. 2021; 6: 2952-2959https://doi.org/10.1021/acsenergylett.1c01122Crossref Scopus (80) Google Scholar,23Zhao Y. Hao L. Ozden A. Liu S. Miao R.K. Ou P. Alkayyali T. Zhang S. Ning J. Liang Y. et al.Conversion of CO2 to multicarbon products in strong acid by controlling the catalyst microenvironment.Nat. Synth. 2023; https://doi.org/10.1038/s44160-022-00234-xCrossref Google Scholar; however, until now, the product gas stream has still been diluted by unreacted CO2 and gas-phase CO2-derived products such as carbon monoxide and methane.22O’Brien C.P. Miao R.K. Liu S. Xu Y. Lee G. Robb A. Huang J.E. Xie K. Bertens K. Gabardo C.M. et al.Single pass CO2 conversion exceeding 85% in the electrosynthesis of multicarbon products via local CO2 regeneration.ACS Energy Lett. 2021; 6: 2952-2959https://doi.org/10.1021/acsenergylett.1c01122Crossref Scopus (80) Google Scholar,24Gabardo C.M. O’Brien C.P. Edwards J.P. McCallum C. Xu Y. Dinh C.-T. Li J. Sargent E.H. Sinton D. Continuous carbon dioxide electroreduction to concentrated multi-carbon products using a membrane electrode assembly.Joule. 2019; 3: 2777-2791https://doi.org/10.1016/j.joule.2019.07.021Abstract Full Text Full Text PDF Scopus (228) Google Scholar,25Kim C. Bui J.C. Luo X. Cooper J.K. Kusoglu A. Weber A.Z. Bell A.T. Tailored catalyst microenvironments for CO2 electroreduction to multicarbon products on copper using bilayer ionomer coatings.Nat. Energy. 2021; 6: 1026-1034https://doi.org/10.1038/s41560-021-00920-8Crossref Scopus (92) Google Scholar Even in systems that achieved >76% CO2 utilization via local CO2 regeneration, unreacted CO2 remains >56 wt % (Table 1).Table 1Energy analysis of different systems: alkaline CO2RR, neutral CO2RR, acidic CO2RR, and carbonate electrolysisSystemAlkaline CO2RRNeutral CO2RRAcidic CO2RRCarbonate electrolysisFull cell voltage (V)2.43.93.44.1C2H4 selectivity (%)70662434Current density (mA cm−2)150315200300Energy efficiency (%)3419810CO2 utilization (%)51176100aNo detectable CO2 gas in the cathodic/anodic tail gas.C2H4 concentration at the outlet (wt %)481956CO2 concentration at the outlet (wt %)9388560Energy cost (GJ/tonne of C2H4)Upstream generation2828283Electrolysis142244586499Product separation11555182Anode separation05700Carbonate generation198000Total483384631504a No detectable CO2 gas in the cathodic/anodic tail gas. Open table in a new tab Since CO2 separation is an energy-intensive process (2–4.4 GJ/tonne of CO2),16Keith D.W. Holmes G. St. Angelo D. Heidel K. A process for capturing CO2 from the atmosphere.Joule. 2018; 2: 1573-1594https://doi.org/10.1016/j.joule.2018.05.006Abstract Full Text Full Text PDF Scopus (662) Google Scholar,26Idem R. Supap T. Shi H. Gelowitz D. Ball M. Campbell C. Tontiwachwuthikul P. Practical experience in post-combustion CO2 capture using reactive solvents in large pilot and demonstration plants.Int. J. Greenhouse Gas Control. 2015; 40: 6-25https://doi.org/10.1016/j.ijggc.2015.06.005Crossref Scopus (98) Google Scholar,27Lin Y.-J. Rochelle G.T. Approaching a reversible stripping process for CO2 capture.Chem. Eng. J. 2016; 283: 1033-1043https://doi.org/10.1016/j.cej.2015.08.086Crossref Scopus (52) Google Scholar unreacted CO2 significantly increases overall system energy requirements. Eliminating CO2 at the downstream could lead to a lower cost of purification demand.20Alerte T. Edwards J.P. Gabardo C.M. O’Brien C.P. Gaona A. Wicks J. Obradović A. Sarkar A. Jaffer S.A. MacLean H.L. et al.Downstream of the CO2 electrolyzer: assessing the energy intensity of product separation.ACS Energy Lett. 2021; 6: 4405-4412https://doi.org/10.1021/acsenergylett.1c02263Crossref Scopus (20) Google Scholar,28Greenblatt J.B. Miller D.J. Ager J.W. Houle F.A. Sharp I.D. The technical and energetic challenges of separating (photo)electrochemical carbon dioxide reduction products.Joule. 2018; 2: 381-420https://doi.org/10.1016/j.joule.2018.01.014Abstract Full Text Full Text PDF Scopus (113) Google Scholar Here, we pursue C2+ products from carbonate solution—a liquid used in DAC—in an electrochemical reactive capture system. Among the striking results is a negligible presence (sub 1%) of CO2 and C1 gas products such as CO and CH4 in the electrolyzer outlet, a finding promising for the minimization of product separation costs. In prior reports of reactive capture from carbonate,5Li Y.C. Lee G. Yuan T. Wang Y. Nam D.-H. Wang Z. García de Arquer F.P. Lum Y. Dinh C.-T. Voznyy O. et al.CO2 electroreduction from carbonate electrolyte.ACS Energy Lett. 2019; 4: 1427-1431https://doi.org/10.1021/acsenergylett.9b00975Crossref Scopus (93) Google Scholar in situ CO2 is regenerated via an acid/base reaction between carbonate and protons. Protons come from the cation-exchange layer (CEL) of a bipolar membrane (BPM) under reverse bias. The in situ CO2 is converted to CO2-derived products at the surface of a Cu electrocatalyst, with C2 (e.g., to ethylene and ethanol) selectivity totaling below 14% (Figure S1). We used modeling of chemical species generation, consumption, and diffusion to seek an explanation of why C2+ productivity is low in prior reactive capture studies and to identify system architectures to increase it (Figure 1B; Notes S1 and S2). The modeling results show that the spacing between the CEL of BPM and the electrocatalyst influences species concentrations in the reactive capture system (Figures S2–S6). The concentrations of CO32−, in situ CO2(g), and CO2(aq) vary in the spacing where the local pH changes, and the distance of spacing is the most significant descriptor for the concentration of reactant, in situ CO2(g). In a prior study,5Li Y.C. Lee G. Yuan T. Wang Y. Nam D.-H. Wang Z. García de Arquer F.P. Lum Y. Dinh C.-T. Voznyy O. et al.CO2 electroreduction from carbonate electrolyte.ACS Energy Lett. 2019; 4: 1427-1431https://doi.org/10.1021/acsenergylett.9b00975Crossref Scopus (93) Google Scholar at a CEL:catalyst spacing of ∼60 μm (Figures 1F, 1G, and S7), the volume fraction of CO2(g) ([CO2(g)]) at the plane of the catalyst was found to be ∼2 vol % with balanced gases of H2 and C2H4 at current densities of 200–350 mA cm−2 (Figure 1C)—yet [CO2(g)] is required to rise above 4 vol % at the catalyst to reach a meaningful conversion rate of C2+ partial current densities of 100+ mA cm−2 (Notes S3 and S4; Figure S8). We studied these effects further, noting that if the CEL and the catalyst are closely spaced (Figures 1D and 1E), the local pH at the CEL goes only as low as pH 10, and CO32− and OH− diffusion neutralize the acidic CEL surface,29Lees E.W. Bui J.C. Song D. Weber A.Z. Berlinguette C.P. Continuum model to define the chemistry and mass transfer in a bicarbonate electrolyzer.ACS Energy Lett. 2022; 7: 834-842https://doi.org/10.1021/acsenergylett.1c02522Crossref Scopus (17) Google Scholar,30Kas R. Yang K. Yewale G.P. Crow A. Burdyny T. Smith W.A. Modeling the local environment within porous electrode during electrochemical reduction of bicarbonate.Ind. Eng. Chem. Res. 2022; 61: 10461-10473https://doi.org/10.1021/acs.iecr.2c00352Crossref Scopus (7) Google Scholar leading to no in situ CO2(g) generation at applied current densities of 200–350 mA cm−2 (Figure S9). By contrast, when we varied the CEL:catalyst spacing over the range 100–300 μm (Figures 1H–1K), we noted the opportunity to achieve the desired conditions of low pH (<4) at the CEL for in situ CO2(g) generation and [CO2(g)] > 4 vol % at the catalyst layer (CL) to trigger CO2RR toward C2+ production. At the CL, the pH is above 13 since hydroxide ions are produced from CO2RR. This high local pH accelerated the C–C coupling needed for C2+ to dominate over C1.31Dinh C.T. Burdyny T. Kibria M.G. Seifitokaldani A. Gabardo C.M. García de Arquer F.P. Kiani A. Edwards J.P. De Luna P. Bushuyev O.S. et al.CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface.Science. 2018; 360: 783-787https://doi.org/10.1126/science.aas9100Crossref PubMed Scopus (1228) Google Scholar,32Varela A.S. Kroschel M. Reier T. Strasser P. Controlling the selectivity of CO2 electroreduction on copper: the effect of the electrolyte concentration and the importance of the local pH.Cat. Today. 2016; 260: 8-13https://doi.org/10.1016/j.cattod.2015.06.009Crossref Scopus (348) Google Scholar The pH gradient was measured using pH-sensitive dyes (Note S5). We observed a progressive pH increase from pH ∼2 at the CEL surface to ∼12 at the edge of the interposer in the system designed for the experimental study of pH. For the spacing range of 130–270 μm, optimal conditions, including [CO2(g)] > 4 vol % and the desired local pH, were achieved at the current density range 250–350 mA cm−2, a regime of applied interest.33Jouny M. Luc W. Jiao F. General techno-economic analysis of CO2 electrolysis systems.Ind. Eng. Chem. Res. 2018; 57: 2165-2177https://doi.org/10.1021/acs.iecr.7b03514Crossref Scopus (687) Google Scholar,34Shin H. Hansen K.U. Jiao F. Techno-economic assessment of low-temperature carbon dioxide electrolysis.Nat. Sustain. 2021; 4: 911-919https://doi.org/10.1038/s41893-021-00739-xCrossref Scopus (111) Google Scholar In the case wherein the amount of carbonate is limited due to a small spacing, such as <130 μm, there is no increase in the in situ CO2(g) concentration at higher current densities (Figure S3). However, the current density influences the rate of proton diffusion through the CEL: more protons diffuse at higher current densities and in situ CO2(g) generally increases (Figure 1C). Increasing the spacing to >130 μm promotes carbonate-rich conditions, which provide more opportunities for protons to react with carbonate (Figures S4 and S5). However, [CO2(g)] decreases at a spacing >540 μm due to an increased possibility of in situ CO2 capture over long distances in the layer (Figure S6). We then turned to the experimental implementation of these concepts (Figure 1A). We needed an approach to construct a well-defined spacing—in effect, a stand-off—between the CEL and electrocatalyst. We used a hydrophilic membrane as an interposer and explored different interposer material compositions (Note S6; Figures S10–S12). We observed that C2+ FE was improved in a higher porosity system. We account for these observations via faster diffusion of species which enabled a higher concentration of CO2(g) at the CL. In light of these findings, we focused on a hydrophilic mixed cellulose ester (MCE) interposer, a highly porous medium (material porosity > 84%) with a selection of thicknesses ranging from 130 to 540 μm (Figure S13). We then moved to a cation-exchange membrane (CEM) in the system to transport protons from the anodic oxygen evolution reaction (OER) (Figure 2A)—an improvement that enabled reduced full cell potential compared with a BPM system (Figure 2B). The CEM system may supply more protons to the cathodic side than the BPM system due to the concentration gradient, neutralizing the capture species (OH−). However, we observed a similar product distribution for both the BPM system and CEM system (Figure S14), indicating that excess proton diffusion in the CEM system is negligible when using 0.5 M H2SO4 as an anolyte (Note S7). Experimentally, we first reconfirmed the findings of prior studies that, at ∼60 μm spacing, FE to C2+ resides below 14% at 250 mA cm−2. When we optimized interposer thickness of 130–270 μm, we achieved a much-increased C2+ FE of 40% at 250 mA cm−2 (Figure 2C). When the distance is smaller than 135 μm or larger than 540 μm, a lower rate of C2+ product generation is seen, the result of the limited concentration of in situ CO2(g) (Figure 1C). In all cases, the only C1 product detected was HCOO− with FE below 2%. No CO, CH4, and CO2 were detected for all applied current densities and different concentrations of carbonate electrolyte in the interposer system (Figures 2D and S11). The product distribution, including high C2+ FE and negligible CO FE, was also observed in a simulated carbonate electrolysis system with CO2-depleted conditions (Note S3). The experimental studies suggest that in the carbonate electrolysis system, low [CO2(g)] and slow in situ CO2 flux contribute to steering C–C coupling by achieving locally concentrated CO and the enhanced residence time of CO. In the outlet stream, gaseous C1 products and CO2 were <0.9 wt % (0.1 vol %) based on the detection limit of the gas chromatography (GC): CO for 24 ppm, CH4 for 56 ppm, and CO2 for 1,000 ppm, respectively (experimental procedures). To investigate whether carbonate was the source of carbon in electroreduction, we used 13C labeled CO32−, and the isotope experiment result ruled out any chemical reactions of interposer, cathode, and dissolved CO2 (Figure S15). To test for the possibility of chemical reactions related to the MCE membrane or its possible decomposition products, we compared electrochemical performance in two conditions: (1) carbonate electrolyte and (2) carbonate electrolyte with a dispersed MCE membrane in a PVDF interposer system (Figure S16). In both cases, we observed a C2+ FE of ∼15% at the applied current density of 200 and 300 mA cm−2. We also conducted nuclear magnetic resonance (NMR) analysis to examine the chemical decomposition of the MCE membrane after long-term electrolysis of carbonate. We only detected signals for CO2-derived products, which supports that there is no chemical decomposition or reactions of MCE in the carbonate electrolysis system (Figure S17). We turned to further system tuning toward increased C2+ FE. We posited that a portion of in situ CO2 is converted into CO32− at the catalyst surface due to the highly alkaline conditions.31Dinh C.T. Burdyny T. Kibria M.G. Seifitokaldani A. Gabardo C.M. García de Arquer F.P. Kiani A. Edwards J.P. De Luna P. Bushuyev O.S. et al.CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface.Science. 2018; 360: 783-787https://doi.org/10.1126/science.aas9100Crossref PubMed Scopus (1228) Google Scholar We, therefore, sought catalyst-design strategies to convert CO2 to CO with faster kinetics at the catalyst surface to preserve the reactant. We used molecularly dispersed cobalt phthalocyanines on carbon nanotubes (CoPCs-CNTs) known to produce CO from CO2 with high turnover frequencies (Figure 3A).35Zhang X. Wang Y. Gu M. Wang M. Zhang Z. Pan W. Jiang Z. Zheng H. Lucero M. Wang H. et al.Molecular engineering of dispersed nickel phthalocyanines on carbon nanotubes for selective CO2 reduction.Nat. Energy. 2020; 5: 684-692https://doi.org/10.1038/s41560-020-0667-9Crossref Scopus (244) Google Scholar,36Li F. Li Y.C. Wang Z. Li J. Nam D.-H. Lum Y. Luo M. Wang X. Ozden A. Hung S.-F. et al.Cooperative CO2-to-ethanol conversion via enriched intermediates at molecule–metal catalyst interfaces.Nat. Catal. 2020; 3: 75-82https://doi.org/10.1038/s41929-019-0383-7Crossref Scopus (271) Google Scholar We fabricated the layer-by-layer catalyst via airbrushing. The CoPC-CNTs layer is uniformly distributed on the Cu layer as shown in the scanning electron microscopy (SEM) image (Figure 3B). X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) confirm the existence of cobalt at the surface (Figures S18 and S19). The product distribution now showed a considerable further improvement: the C2+ total FE now rose to 47% at 300 mA cm−2 (Figure 3C). The C2H4 FE is 34%, resulting in 56 wt % of C2H4 in the product gas stream due to the absence of gaseous C1 products and unreacted CO2. The C2+ alcohols FE is 13% including 12% C2H5OH FE and 1% C3H7OH FE. As shown in Figure 3D, we achieve 140+ mA cm−2 of C2+ partial current density at −4.1 V. We then constructed a prototype that operates both CO2 capture and electrolysis on a continuous basis (Figures 4A and S20). The KOH capture liquid is regenerated during carbonate electrolysis as shown in the chemical balance (Figure 2A; Note S7). We recycled the resultant KOH solution to continuously capture additional CO2, converting it into K2CO3. There are two reservoirs: the absorber is for CO2 capture and the electrolyte reservoir provides the carbonate to the liquid-fed electrolyzer. Two reservoirs and the electrolyzer are connected by peripheral pumps circulating the capture liquid. During the electrolysis of carbonate into C2H4 and C2+ alcohols, generated OH− returns to the absorber. We demonstrated capture-and-electrolysis sustained over 20 h (Figure 4B) at the current density of 200 mA cm−2, with the C2+ FE consistently in the range of 36%–42%. The pH of reservoirs remains 11.8 for the absorber, 11.9 for the electrolyte, and 1.8 for the anolyte (Table S7). We found that after 10 h of operation, performance does show a decline (Figure 4B). We studied the cause, finding that the pore structure of the MCE membrane degrades in alkali solution, producing an increase of full cell voltage and hydrogen evolution recations (HER). It will be important to seek interposer materials that are stable under relevant conditions. In Table 1, we offer an analysis that also estimates energy costs—associated with upstream generation for the gas-phase CO2 and carbonate capture solutions, electrolysis, separation, and carbonate regeneration—in systems including alkaline CO2 electrolysis,31Dinh C.T. Burdyny T. Kibria M.G. Seifitokaldani A. Gabardo C.M. García de Arquer F.P. Kiani A. Edwards J.P. De Luna P. Bushuyev O.S. et al.CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface.Science. 2018; 360: 783-787https://doi.org/10.1126/science.aas9100Crossref PubMed Scopus (1228) Google Scholar neutral CO2 electrolysis in a membrane electrode assembly (MEA),37Ozden A. Li F. García de Arquer F.P. Rosas-Hernández A. Thevenon A. Wang Y. Hung S.-F. Wang X. Chen B. Li J. et al.High-rate and efficient ethylene electrosynthesis using a catalyst/promoter/transport layer.ACS Energy Lett. 2020; 5: 2811-2818http