•CO2 is captured from dilute sources and converted to synthesis gas using sunlight•PET plastic waste is simultaneously upcycled to glycolic acid during the process•A single encapsulated perovskite solar cell generates the photovoltage to power the process•CO2 is captured with amine/hydroxide, and its reduction is enabled by a molecular catalyst It is becoming likely that a net carbon zero future will rely on the recycling of atmospheric CO2 to produce sustainable fuels and chemicals. Nevertheless, current processes for CO2 utilization use concentrated CO2 streams and are not integrated with the capture of dilute CO2 sources. At the same time, recycling of waste plastics is critical to protect our environment from irreversible damages. We report an integrated photoelectrochemical system that captures CO2 from exhaust stream concentrations and ambient air and directly converts it into synthesis gas (CO + H2, a precursor of industrial liquid fuel production) using sunlight. This process is coupled to and benefits from the concurrent valorization of plastic waste, which is upcycled to a commodity chemical glycolic acid. The overall technology thus produces value-added fuels and chemicals directly from industrially relevant CO2 streams and plastic waste, powered solely by sunlight at ambient temperature and pressure, and is an important step toward a circular economy. Integration of carbon capture with utilization technologies can lead the way to a future net-zero carbon economy. Nevertheless, direct conversion of chemically captured CO2 remains challenging due to its thermodynamic stability. Here, we demonstrate CO2 capture from flue gas or air and its direct conversion into syngas using solar irradiation without any externally applied voltage. The system captures CO2 with an amine/hydroxide solution and photoelectrochemically converts it into syngas (CO:H2 1:2 [captured from concentrated CO2], 1:4 [from simulated flue gas], and 1:30 [from air]) using a perovskite-based photocathode containing an immobilized molecular Co-phthalocyanine catalyst. At the anode, plastic-derived ethylene glycol is oxidized into glycolic acid over a Cu26Pd74 alloy catalyst. The overall process uses flue gas or air as carbon source, discarded plastic waste as an electron donor, and sunlight as the sole energy input. This strategy opens new avenues for future carbon-neutral or even negative solar fuel and waste upcycling technologies. Integration of carbon capture with utilization technologies can lead the way to a future net-zero carbon economy. Nevertheless, direct conversion of chemically captured CO2 remains challenging due to its thermodynamic stability. Here, we demonstrate CO2 capture from flue gas or air and its direct conversion into syngas using solar irradiation without any externally applied voltage. The system captures CO2 with an amine/hydroxide solution and photoelectrochemically converts it into syngas (CO:H2 1:2 [captured from concentrated CO2], 1:4 [from simulated flue gas], and 1:30 [from air]) using a perovskite-based photocathode containing an immobilized molecular Co-phthalocyanine catalyst. At the anode, plastic-derived ethylene glycol is oxidized into glycolic acid over a Cu26Pd74 alloy catalyst. The overall process uses flue gas or air as carbon source, discarded plastic waste as an electron donor, and sunlight as the sole energy input. This strategy opens new avenues for future carbon-neutral or even negative solar fuel and waste upcycling technologies. Mitigation of anthropogenic CO2 accumulation is essential to tackle the current climate change and loss of biodiversity.1Ritchie H. Roser M. Rosado P. CO₂ and greenhouse gas emissions.2020OurWorldInData.orghttps://ourworldindata.org/co2-and-other-greenhouse-gas-emissionsGoogle Scholar Large-scale global efforts are ongoing to develop CO2 conversion technologies for green fuel production.2IEACCUS in Clean Energy Transitions. IEA, 2020https://www.iea.org/reports/ccus-in-clean-energy-transitionsGoogle Scholar Solar-driven CO2 conversion is a promising approach to produce clean fuels and chemicals as it directly utilizes sunlight as the sole energy input.3Creissen C.E. Fontecave M. Solar-driven electrochemical CO2 reduction with heterogeneous catalysts.Adv. Energy Mater. 2021; 11: 2002652https://doi.org/10.1002/aenm.202002652Crossref Scopus (39) Google Scholar,4Morikawa T. Sato S. Sekizawa K. Suzuki T.M. Arai T. Solar-driven CO2 reduction using a semiconductor/molecule hybrid photosystem: from photocatalysts to a monolithic artificial leaf.Acc. Chem. 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Greenh, Gas Control. 2013; 19: 262-270https://doi.org/10.1016/j.ijggc.2013.09.001Crossref Scopus (26) Google Scholar Here, we report an integrated CO2 capture and solar-driven photoelectrochemical (PEC) utilization process to produce syngas (mixture of CO and H2), a precursor for industrial liquid fuels and chemicals syntheses,25Mahmoudi H. Mahmoudi M. Doustdar O. Jahangiri H. Tsolakis A. Gu S. LechWyszynski M. A review of Fischer Tropsch synthesis process, mechanism, surface chemistry and catalyst formulation.Biofuels Engg. 2017; 2: 11-31https://doi.org/10.1515/bfuel-2017-0002Crossref Google Scholar from concentrated CO2 stream, simulated post-combustion flue gas, and atmospheric air. The process operates by combining CO2-to-fuel reduction with selective oxidation of waste plastic-derived ethylene glycol (EG) to glycolic acid (GA), which has applications in pharmaceutical, food, and textile industries (Figure 1B). The system captures CO2 in an aqueous amine or glycolic hydroxide solution, and the subsequent PEC conversion occurs in a two-compartment, two-electrode reactor equipped with an encapsulated triple cation perovskite (PVK)-based photocathode. Captured CO2 reduction is enabled by an immobilized molecular Co-phthalocyanine catalyst at the photocathode. A bimetallic Cu26Pd74 alloy anode completes the circuit by catalyzing EG oxidation. Replacing anodic water oxidation (ΔG0(H2O/O2) = +237 kJ mol−1) by EG oxidation (ΔG0(EG/GA) ∼ +20 kJ mol−1)26Huq F. Ababneh D. Molecular modelling analysis of the metabolic activation of ethylene glycol.J. Pharmacol. Toxicol. 2006; 2: 54-62Crossref Google Scholar makes the demanding captured CO2 reduction feasible with only sunlight, enabling the system to function even with a single visible-light absorber without any externally applied voltage while simultaneously valorizing waste.27Bhattacharjee S. Rahaman M. Andrei V. Miller M. Rodríguez-Jiménez S. Lam E. Pornrungroj C. Reisner E. Photoelectrochemical CO2-to-fuel conversion with simultaneous plastic reforming.Nat. Synth. 2023; 2: 182-192https://doi.org/10.1038/s44160-022-00196-0Crossref Google Scholar A concentrated CO2 stream (99.995%) was first used to develop and optimize the PEC system. Different amines, including industrially relevant monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), and 1,4-diazabicyclo[2.2.2]octane (DABCO), were used as carbon capture agents in an aqueous medium at ambient temperature. The capture was achieved by purging concentrated CO2 through a 1 M amine solution for 2 h at a flowrate of 30 mL min−1. The solution was further purged with N2 (15 min) to remove any physically dissolved CO2 from the solution (Figure S2). 13C nuclear magnetic resonance (13C-NMR) spectra of the post-capture solutions revealed that under these conditions, MEA and DEA captured 0.75 ± 0.07 and 0.77 ± 0.10 mol CO2 per mol of amine, respectively, as a combination of bicarbonate and carbamate species.28García-Abuín A. Gómez-Díaz D. López A.B. Navaza J.M. Rumbo A. NMR characterization of carbon dioxide chemical absorption with monoethanolamine, diethanolamine, and triethanolamine.Ind. Eng. Chem. Res. 2013; 52: 13432-13438https://doi.org/10.1021/ie4010496Crossref Scopus (49) Google Scholar In contrast, the tertiary amines TEA and DABCO captured CO2 only as bicarbonate salts (0.60 ± 0.05 and 0.85 ± 0.08 mol CO2 per mol amine, respectively, after 2 h; Scheme 1; Equations 1 and 2; Figure S3). Other than amines, aqueous and organic solutions of sodium hydroxide (NaOH), an industrially applied well-known CO2 scrubber, were also used for efficient CO2 capture (Scheme 1, Equation 3).29Kar S. Goeppert A. Galvan V. Chowdhury R. Olah J. Prakash G.K.S. A carbon-neutral CO2 capture, conversion, and utilization cycle with low-temperature regeneration of sodium hydroxide.J. Am. Chem. Soc. 2018; 140: 16873-16876https://doi.org/10.1021/jacs.8b09325Crossref PubMed Scopus (59) Google Scholar When concentrated CO2 was purged through an aqueous 1 M NaOH solution (2 h), quantitative formation of NaHCO3 was observed.30Chowdhury F.A. Goto K. Yamada H. Matsuzaki Y. A screening study of alcohol solvents for alkanolamine-based CO2 capture.Int. J. Greenh. Gas Control. 2020; 99: 103081https://doi.org/10.1016/j.ijggc.2020.103081Crossref Scopus (16) Google Scholar,31Sen R. Koch C.J. Goeppert A. Prakash G.K.S. Tertiary amine-ethylene glycol based tandem CO2 capture and hydrogenation to methanol: direct utilization of post-combustion CO2.ChemSusChem. 2020; 13: 6318-6322https://doi.org/10.1002/cssc.202002285Crossref Scopus (19) Google Scholar Similarly, NaOH (1 M) dissolved in an organic EG medium was also efficient in CO2 capture, and 0.96 ± 0.02 mol CO2 per mol NaOH was chemically captured after 2 h as sodium glycol carbonate upon CO2 purging (Scheme 1, Equation 4; Figure S4). The chemically captured CO2 solutions thus obtained were directly used in subsequent captured CO2 conversion studies. The conversion of chemically captured CO2 in different media was first studied electrochemically to obtain insights about the required potentials. A tetramine substituted cobalt(II) phthalocyanine molecular catalyst (CoPcNH2, Figure 1B) was used for the captured CO2 reduction reaction (c-CO2RR) due to its ability to form syngas at low overpotentials.32Wu Y. Jiang Z. Lu X. Liang Y. Wang H. Domino electroreduction of CO2 to methanol on a molecular catalyst.Nature. 2019; 575: 639-642https://doi.org/10.1038/s41586-019-1760-8Crossref PubMed Scopus (481) Google Scholar Although several metallic electrode catalysts have been previously investigated for electrochemical c-CO2RR, a well-defined molecular complex has not yet been reported for this process. CoPcNH2 was synthesized from a tetranitro precursor by sodium sulfide-mediated reduction (see experimental procedures; Figure S5).32Wu Y. Jiang Z. Lu X. Liang Y. Wang H. Domino electroreduction of CO2 to methanol on a molecular catalyst.Nature. 2019; 575: 639-642https://doi.org/10.1038/s41586-019-1760-8Crossref PubMed Scopus (481) Google Scholar Electrodes were prepared by first immobilizing CoPcNH2 on multi-walled carbon nanotubes (MWCNTs) through π-π stacking, followed by drop-casting the composite on a graphite foil substrate (CoPcNH2@MWCNT; Figures S6).33Zhang X. Wu Z. Zhang X. Li L. Li Y. Xu H. Li X. Yu X. Zhang Z. Liang Y. et al.Highly selective and active CO2 reduction electrocatalysts based on cobalt phthalocyanine/carbon nanotube hybrid structures.Nat. Commun. 2017; 8: 14675https://doi.org/10.1038/ncomms14675Crossref PubMed Scopus (548) Google Scholar The amount of immobilized molecular catalyst on the electrode surface was determined to be 14.8 ± 1.8 nmol cm−2 by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis, and its homogeneous distribution was confirmed by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) mapping (Figures S7 and S8). A cross-sectional SEM image showed a ∼1.3 μm thick CoPcNH2@MWCNT catalyst layer deposited over the graphite foil substrate (Figure S9). Electrochemical reduction of aqueous captured CO2 solutions was then explored in a two-compartment, three-electrode configuration with the fabricated CoPcNH2@MWCNT electrode as the working electrode, a Pt mesh as the counter electrode, and an Ag/AgCl (saturated NaCl) electrode as the reference electrode. The catholyte was chemically captured CO2 solutions of different amines (pH 7.8–8.3), whereas the anolyte was 0.1 M potassium sulfate (K2SO4, pH 7.6). The compartments were separated by a bipolar membrane. Cyclic voltammetry (CV) scans in MEA, DEA, and TEA (1 M) with the captured CO2 showed similar onset potentials around −0.40 to −0.45 V vs. the reversible hydrogen electrode (RHE; Figure 2A). Controlled potential electrolysis (CPE) at different potentials (1 h) with TEA captured CO2 showed selective syngas (CO and H2) formation as product (Figures 2B and S10). CO and H2 formation started at −0.5 V vs. RHE, and an optimum CO faradaic efficiency (FECO) was obtained at −0.7 V vs. RHE (46.2% ± 2.0%, Figure 2B). CPE with the primary and secondary amine (MEA and DEA) captured CO2 solutions at this potential resulted in a lower FECO of 10.2% ± 1.7% and 16.5% ± 1.5%, respectively, but the overall FE for syngas formation remained >95% (Figure 2C). Beside CoPcNH2, other substituted or unsubstituted cobalt phthalocyanine catalysts are also active in c-CO2RR, although with slightly reduced activities (Figure S11). Control experiments without an amine did not capture CO2 and, consequently, produced no CO during CPE, confirming the role of amine in the CCU process (Figure 2C). Similarly, a control experiment with MWCNT fabricated on a graphite substrate without CoPcNH2 catalyst produced only H2, highlighting the role of the molecular catalyst in c-CO2RR (Figure S11). Electrochemical reduction of aqueous NaOH captured CO2 species (NaHCO3 and sodium carbonate [Na2CO3]) with a CoPcNH2@MWCNT electrode at −0.7 V vs. RHE showed low FECO (<3%, Figure S12) due to overwhelming H2 formation. To suppress H2 formation, electrochemical c-CO2RR in a non-aqueous solvent (EG) was explored. Here, the chemically captured CO2 in glycolic NaOH solution (as sodium glycolate carbonate, Scheme 1, Equation 4) was directly used, after adding tetrabutylammonium tetrafluoroborate (TBABF4, 0.15 M) as a supporting electrolyte and 20% v/v acetonitrile (MeCN) as a co-solvent to ensure a homogeneous solution. CV scans with CoPcNH2@MWCNT catalyst in this medium showed an onset potential of −1.65 V vs. Fc/Fc+ (Figure 2D). Subsequent CPE studies (for 10 h) revealed an optimum CO production at −1.85 V vs. Fc/Fc+ with 19.0% ± 1.4% FECO (Figures 2E and S13), with overall syngas FE of around 75%. Isotopic-labeling experiments with captured carbon-13C dioxide (13CO2) in both the aqueous (TEA/H2O) and non-aqueous (NaOH/EG) medium showed only labeled 13CO as a reduction product (gas-phase FTIR analysis of the headspace), confirming that CO was derived from captured CO2 (Figure 2F). The ICP-OES analysis of the post-electrolysis CoPcNH2@MWCNT electrode provided a 12.4 ± 0.9 nmol cm−2 catalyst loading indicating minimal catalyst leaching during experiment (Figure S14). From the electrochemical studies, it is evident that the ideal potential range for c-CO2RR with a CoPcNH2@MWCNT catalyst is −0.5 to −0.7 V vs. RHE in the TEA/H2O medium and around −1.85 V vs. Fc/Fc+ in the NaOH/EG medium. To obtain the required potential by solar irradiation, a state-of-the art triple cation lead halide PVK photovoltaic was employed due to its high open circuit voltage (VOC ∼ 1.1 V) and the ability to absorb over a broad-range of the solar spectrum (360–750 nm).34Andrei V. Hoye R.L.Z. Crespo-Quesada M. Bajada M. Ahmad S. De Volder M. Friend R. Reisner E. Scalable triple cation mixed halide perovskite–BiVO4 tandems for bias-free water splitting.Adv. Energy Mater. 2018; 8: 1801403https://doi.org/10.1002/aenm.201801403Crossref Scopus (103) Google Scholar,35Andrei V. Reuillard B. Reisner E. Bias-free solar syngas production by integrating a molecular cobalt catalyst with perovskite–BiVO4 tandems.Nat. Mater. 2020; 19: 189-194https://doi.org/10.1038/s41563-019-0501-6Crossref PubMed Scopus (124) Google Scholar,36Rahaman M. Andrei V. Pornrungroj C. Wright D. Baumberg J.J. Reisner E. Selective CO production from aqueous CO2 using a Cu96In4 catalyst and its integration into a bias-free solar perovskite–BiVO4 tandem device.Energy Environ. Sci. 2020; 13: 3536-3543https://doi.org/10.1039/D0EE01279CCrossref Google Scholar A c-CO2RR photocathode was prepared by interfacing the CoPcNH2@MWCNT catalyst with an integrated PVK photoabsorber using a conducting graphite epoxy (GE) paste (PVK|CoPcNH2@MWCNT, see experimental procedures; Figure S15; Note S1).37Pornrungroj C. Andrei V. Rahaman M. Uswachoke C. Joyce H.J. Wright D.S. Reisner E. Bifunctional perovskite-BiVO4 tandem devices for uninterrupted solar and electrocatalytic water splitting cycles.Adv. Funct. Mater. 2021; 31: 2008182https://doi.org/10.1002/adfm.202008182Crossref Scopus (23) Google Scholar,38Andrei V. Ucoski G.M. Pornrungroj C. Uswachoke C. Wang Q. Achilleos D.S. Kasap H. Sokol K.P. Jagt R.A. Lu H. et al.Floating perovskite-BiVO4 devices for scalable solar fuel production.Nature. 2022; 608: 518-522https://doi.org/10.1038/s41586-022-04978-6Crossref PubMed Scopus (26) Google Scholar The solar-driven c-CO2RR was then explored with the PVK|CoPcNH2@MWCNT photocathode in TEA/H2O and NaOH/EG post-capture solutions, in a two-compartment PEC cell, where a counter oxidation reaction was coupled to c-CO2RR. Preliminary experiments suggested that the fixed VOC provided by the PVK is not sufficient to efficiently drive both c-CO2RR and conventional water oxidation (Figure S16). Hence, the water oxidation was replaced with a less thermodynamically demanding EG oxidation in all PEC experiments, which can be sourced directly from discarded polyethylene terephthalate (PET) plastic waste. A bimetallic Cu26Pd74 alloy electrodeposited on a Ni foam substrate (Ni foam|Cu26Pd74, see experimental procedures) was used as the dark anode (Figures S17 and S18) to facilitate EG oxidation under alkaline conditions.5Bhattacharjee S. Andrei V. Pornrungroj C. Rahaman M. Pichler C.M. Reisner E. Reforming of soluble biomass and plastic derived waste using a bias-free Cu30Pd70.Adv. Funct. Mater. 2022; 32: 2109313https://doi.org/10.1002/adfm.202109313Crossref Scopus (26) Google Scholar Operating conditions of the two-electrode PEC setup without any external bias were determined from the overlap of individual CV curves of the PVK|CoPcNH2@MWCNT photocathode for c-CO2RR (taken under 1 sun, Air Mass 1.5 Global [AM 1.5G] irradiation) and a Ni foam|Cu26Pd74 anode for EG oxidation (taken under “dark” conditions) in three-electrode configurations (Figures S19–S22). The overlap potentials (Voverlap) were 0.52 V vs. RHE and −0.85 V vs. Fc/Fc+ in the TEA/H2O and NaOH/EG media, respectively, with respective overlap current densities of 5.8 and 0.27 mA cm−2 (Figures S20 and S22). Accounting for the open circuit voltage (VOC) of the PVK devices (∼1.05 ± 0.03 V, Figure S23), the potential experienced by the CoPcNH2@MWCNT catalyst in the two-electrode setup without external voltage is around −0.53 V vs. RHE and −1.9 V vs. Fc/Fc+ in the TEA/H2O and NaOH/EG media, respectively (calculated as Voverlap − VOC). These potentials fall within the optimum potential range of c-CO2RR activity of the CoPcNH2@MWCNT catalyst (Figures 2B and 2E). Following the establishment of operating conditions, a two-electrode PEC experiment was set up for solar-driven c-CO2RR coupled to EG oxidation. The catholyte was chemically captured CO2 solution in TEA (similar to Figure 2) and the anolyte was 0.5 M EG in 0.5 M aqueous potassium hydroxide (KOH), with the two compartments being separated by a bipolar membrane. The bipolar membrane introduces an internal chemical bias (∼0.35 V) to the system. CV scans under solar irradiation showed an onset voltage at −0.4 V with j ∼ 4.9 mA cm−2 at zero applied voltage (Figure 3A). Accordingly, a stable photocurrent density of 1.1 ± 0.3 mA cm−2 was obtained during subsequent long-term photoelectrolysis without any external voltage under solar irradiation (Figure 3B). After 10 h of photoelectrolysis, syngas was detected at the photocathode with 54.6 ± 9.2 μmol cm−2 CO and 106.6 ± 8.4 μmol cm−2 H2 (FECO 34.1% ± 2.2% and FEH2 70.3% ± 1.8%, Figure 3C). The product formation rates remained steady throughout the experiment (Figure S24). The turnover number of the molecular catalyst for CO formation (TONCO) reached 3,657 ± 591 after this time (Figure 3D). The 1H and 13C-NMR analysis of the post-photoelectrolysis catholyte showed no other reduction product and the unreacted captured CO2 remained in the solution (Figure S25). The high-performance liquid chromatography (HPLC) analysis of the anolyte showed GA as the selective oxidation product in the anode (85.8 ± 16.2 μmol cm−2; FEGA 92.5% ± 5.3%). The above results indicate that our developed integrated Ni foam|Cu26Pd74||PVK|CoPcNH2@MWCNT PEC system can concurrently drive c-CO2RR (in TEA) and EG oxidation under sunlight without any external voltage. To investigate the possibility of employing real-world PET waste as an EG precursor, an experiment was performed where an alkaline pre-treated commercial PET plastic bottle solution was directly used as an anolyte (see experimental procedures). As per HPLC analysis, the pre-treated solution contained ∼0.2 M EG, ∼0.2 M disodium terephthalate as PET decomposition products in 1 M KOH. After 10 h of photoelectrolysis under 1 sun irradiation, comparable yields and FE for H2, CO, and GA formation were observed as with the EG model substrate (Figure S26). This experiment demonstrates the possibility of simultaneous solar-driven c-CO2 reduction and valorization of waste polymers. The Ni foam|Cu26Pd74||PVK|CoPcNH2@MWCNT is also active in the PEC conversion of captured CO2 in the NaOH/EG medium (Note S2). CV scans of CO2 captured in the NaOH/EG electrolyte under solar irradiation showed an onset voltage around −0.4 V (Figure 3E) with j ∼ 0.35 mA cm−2 at zero applied voltage. The steady-state photocurrent density was 0.18 ± 0.07 mA cm−2 over 10 h of photoelectrolysis experiment without external voltage (Figure 3F). After 10 h, 5.2 ± 1.1 μmol cm−2 CO and 16.4 ± 0.6 μmol cm−2 H2 were produced (FECO 18.2% ± 1.1%, FEH2 58.2% ± 4.0%) with total syngas FE ∼ 76% and TONCO of 347 ± 74 (Figures 3G and 3H). The product formation rates remained steady over the experimental time (Figure S27). 1H and 13C-NMR analysis of the post-electrolysis solution showed only unreacted captured CO2 with no other product formation (Figure S28). The amount of GA was 11.7 ± 3.5 μmol cm−2 (FEGA ∼ 86% ± 11%, Figure 3F), as the selectively detected oxidation product at anode by HPLC analysis. Although the current densities and product formation rates in NaOH/EG are sluggish, presumably due to low conductivity of the organic medium, its high CO2 affinity and non-aqueous nature can be beneficial to CO formation, especially when working with ultra-dilute CO2 concentrations, as will be apparent in the later sections. Following the successful development of solar-driven PEC c-CO2RR, its real-world implications were explored by coupling the system with carbon capture from dilute CO2 streams. Post-combustion flue gas from large industrial plants as well as from small point sources like automobile exhausts are a major contributor to global carbon emissions. We therefore investigated the activity of our system for integrated capture and utilization of flue gas derived CO2 into syngas. Industrial flue gas typically contains ∼15% of CO2 and 3%–5% of O2 and N2 gas with some SOx and NOx impurities that can be minimized by wet scrubbing.39Sharif H.M.A. Mahmood N. Wang S. Hussain I. Hou Y.N. Yang L.H. Zhao X. Yang B. Recent advances in hybrid wet scrubbing techniques for NOx and SO2 removal: state of the art and futu