Chem-bio interface design for rapid conversion of CO2 to bioplastics in an integrated system

Abstract

•CO2 conversion via EMC2 intermediate•Soluble C2 intermediates play critical roles in the efficient integration•Systematic design enables continuous and rapid conversion of CO2 to bioproducts•EMC2 produces PHA with substantially improved productivity and product chain length The research uniquely addresses two daunting challenges our generation faces: global climate change and plastic waste accumulation. We demonstrated an efficient route for converting CO2 to biodegradable plastics via a systematic design of electrocatalysis, a chemical-biological interface, and microbes. The integration of chemical and biological conversions must overcome the intermediate incompatibility, the harsh chemical catalysis conditions, and the inefficient mass, energy, and electron transfers. This study overcomes these critical challenges by exploiting the soluble two-carbon molecules as intermediates from electrocatalytic CO2 reduction to bioconversion. These intermediates are better carriers for electrons and energy, facilitate mass transfer, and can readily enter primary metabolism as building blocks for bioproduction. Moreover, the systematic design achieved integrated, continuous, and rapid microbial biomass and PHA production from CO2 with record-level productivity. Integrating catalytic CO2 reduction with bioconversion could substantially advance carbon capture and utilization and mitigate climate change. However, the state-of-the-arts are limited by inefficient electron and mass transfers, unfavorable metabolic kinetics, and inadequate molecular building blocks. We overcome these barriers with the systematic design of electrocatalysis, a chemical-biological (chem-bio) interface, and microorganisms to enable efficient electro-microbial conversion with C2 (EMC2) intermediates. The soluble C2 intermediates can facilitate rapid mass transfer, readily enter primary metabolism, have less toxicity, carry more energy and electrons, and serve as better molecular building blocks for many microorganisms. The multi-tier chem-bio interface design delivered the EMC2 system to achieve 6 and 8 times increase of microbial biomass productivity compared to C1 intermediate and hydrogen-driven routes, respectively. The multi-module synthetic biology design produced medium-chain-length polyhydroxyalkanoates (PHAs), biodegradable polymers, representing much higher productivity and molecular chain length than the platforms based on C1 intermediates, hydrogen, or electrons. Integrating catalytic CO2 reduction with bioconversion could substantially advance carbon capture and utilization and mitigate climate change. However, the state-of-the-arts are limited by inefficient electron and mass transfers, unfavorable metabolic kinetics, and inadequate molecular building blocks. We overcome these barriers with the systematic design of electrocatalysis, a chemical-biological (chem-bio) interface, and microorganisms to enable efficient electro-microbial conversion with C2 (EMC2) intermediates. The soluble C2 intermediates can facilitate rapid mass transfer, readily enter primary metabolism, have less toxicity, carry more energy and electrons, and serve as better molecular building blocks for many microorganisms. The multi-tier chem-bio interface design delivered the EMC2 system to achieve 6 and 8 times increase of microbial biomass productivity compared to C1 intermediate and hydrogen-driven routes, respectively. The multi-module synthetic biology design produced medium-chain-length polyhydroxyalkanoates (PHAs), biodegradable polymers, representing much higher productivity and molecular chain length than the platforms based on C1 intermediates, hydrogen, or electrons. The synthesis of fuels, chemicals, and materials from CO2 is fundamental to human society.1Ort D.R. Merchant S.S. Alric J. Barkan A. Blankenship R.E. Bock R. Croce R. Hanson M.R. Hibberd J.M. Long S.P. et al.Redesigning photosynthesis to sustainably meet global food and bioenergy demand.Proc. Natl. Acad. Sci. USA. 2015; 112: 8529-8536https://doi.org/10.1073/pnas.1424031112Crossref PubMed Scopus (515) Google Scholar In the biosphere, plants, and algae convert CO2 through photosynthesis to macromolecules and diverse chemicals to sustain their own growth and feed other heterotrophic organisms. However, facing the increasing CO2 concentration caused by human activities, alternative CO2 conversion platforms were developed to mitigate the global climate change and manufacture valuable industrial products.2Hepburn C. Adlen E. Beddington J. Carter E.A. Fuss S. Mac Dowell N. Minx J.C. Smith P. Williams C.K. 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Rep. 2018; 1: 61-68https://doi.org/10.1016/j.biteb.2018.02.007Crossref Scopus (15) Google Scholar the direct electrocatalytic CO2 reduction to C2+ products allows us to maximize the benefits of electrocatalytic step for electron carrying, improve the system efficiency, and drastically increase bioproduction rate. Despite the potential, such integration between electrocatalysis and microbial conversion has not been achieved before. No study has established the scientific concept of such a new system, demonstrated feasibility, or overcame the challenges in the chem-bio interface to achieve integrated and continuous production. In this study, we have designed and implemented a novel route (route 4 in Figure 1A), namely electro-microbial conversion with C2 (EMC2) intermediates. EMC2 achieved seamless and systematic integration of electrocatalysis and bioconversion at chemical, cellular, and process levels, enabling continuous production. With this system, we have demonstrated the efficient electro-microbial CO2 conversion using a broadly adopted industrial microorganism, Pseudomonas putida. The innovative EMC2 platform achieved significantly improved cell biomass growth and bioproduct productivity as compared to other state-of-the-art routes (routes 1, 2, 5, and 6) and photosynthetic systems. The study highlighted the broad applicability of the EMC2 system in carbon fixation and conversion to macromolecule products. Even though acetate and ethanol are much better electron carriers and carbon intermediates than C1 compounds (Figure 1A), an efficient EMC2 system requires a four-tier design to achieve the integration of electrochemical CO2 reduction reaction (CO2RR) and the microbial conversion (Figure 1B). The first tier focuses on electrocatalysis, where the design and selection of electrolyzer, catalysts, and electrolytes need to ensure the efficient electrocatalytic CO2RR to C2 products under biocompatible conditions. The second tier is the design of a chem-bio interface to make sure that microbial conversion does not interfere with CO2RR (Figure 1B). The third tier is the design of an integrated EMC2 system to achieve efficient mass transfer of C2 products between electrocatalysis and fermentation (Figure 1B). The fourth tier is the microbial design to efficiently channel the C2 compounds to the tricarboxylic acid (TCA) cycle and to enhance the carbon flux toward target bioproducts (Figure 1B). The four-tier design will achieve complete and efficient electrotrophic microbial conversion of CO2, where ethanol and acetate will carry the electrons and carbon from electrocatalytic CO2RR to drive bioconversion. As the starting point to convert CO2 to C2 intermediates, electrocatalysis design is critical to generate sufficient C2 products for microbial conversion under bio-amicable conditions. These constraints require the integrated design and selection of electrolyzer, electrolytes, and catalysts (Figure 2A). First, a flow electrolyzer equipped with gas diffusion electrodes (GDEs) was selected for C2 production, considering the potentially high yield of C2 products (Figure 2A). We found that flow cells with GDE setting can achieve much higher current density and C2 product content than the conventional electrocatalysis with H-cell, as the CO2 feeding to GDE is not limited.21Overa S. Ko B.H. Zhao Y. Jiao F. Electrochemical approaches for CO2 conversion to chemicals: a journey toward practical applications.Acc. Chem. Res. 2022; 55: 638-648https://doi.org/10.1021/acs.accounts.1c00674Crossref PubMed Scopus (16) Google Scholar,28Lees E.W. Mowbray B.A.W. Parlane F.G.L. Berlinguette C.P. Gas diffusion electrodes and membranes for CO2 reduction electrolysers.Nat. Rev. Mater. 2022; 7: 55-64https://doi.org/10.1038/s41578-021-00356-2Crossref Scopus (48) Google Scholar Second, the selection of electrolytes is critical. When the flow electrolyzer is used for CO2RR, the whole chem-bio system will operate in a cascade mode with salt solution flows between the electrocatalysis and bioconversion. The same salt solution will serve as the catholyte for electrocatalysis and the media to support microbial growth. The electrolyte thus needs to be suited for both electrocatalysis and cell cultivation. Frequently used CO2RR electrolytes such as extreme alkaline solutions are not amenable to microorganism growth. Considering that the microbes need a neutral and stable pH environment, we investigated the CO2RR reaction in two solutions. The first one is the bicarbonate solution that contained mixed NaHCO3 and KHCO3 with Na/K ratio suitable for microbial growth and also was proven effective for CO2RR.29Ren S. Joulié D. Salvatore D. Torbensen K. Wang M. Robert M. Berlinguette C.P. Molecular electrocatalysts can mediate fast, selective CO2 reduction in a flow cell.Science. 2019; 365: 367-369https://doi.org/10.1126/science.aax4608Crossref PubMed Scopus (362) Google Scholar The second one is a “basal” solution mainly composed of phosphate buffer (NaH2PO4 + K2HPO4 + NaCl), which is widely used in fermentation, but not electrocatalysis. We have maintained both solutions at pH close to 7 to allow bioconversion integration, and the total ion concentrations in both solutions are kept the same to minimize the differences of resistance. To evaluate bioconversion compatibility, we first compared the microbial growth in the two electrolytes using ethanol as the primary carbon source. Although the pH of the bicarbonate solution was 7.2 (with CO2 and air bubbling), the cell growth of P. putida remained to be minimal after 72 h. While in the basal solution, the P. putida had shown significant growth within 24 h (Figure 2B). We therefore prioritized the basal solution as the electrolyte and compared the catalytic performances in both bicarbonate solution and basal solution to identify the best catalysts. Third, the design and configuration of catalysts are critical for C2 productivity, especially in biologically compatible conditions. We first investigated several typical CO2RR catalysts including CuO, Cu2O, and Cu–B (copper reduced by NaBH4), yet none of them achieved appreciable ethanol or C2 productivity when the basal solution was used as the electrolyte. These catalysts showed higher production of hydrogen, formate, and other C1 compounds (Figure S1). It is likely that these catalysts will undergo reduction and surface reconstruction in the electrocatalysis process. The strongly coordinating phosphate anions in the electrolyte could interact substantially with copper species in the catalysts and alternate their surface properties, which favor the generation of C1 products such as formate as well as hydrogen.30Zhao J. Sun L. Canepa S. Sun H. Yesibolati M.N. Sherburne M. Xu R. Sritharan T. Loo J.S.C. Ager III, J.W. et al.Phosphate tuned copper electrodeposition and promoted formic acid selectivity for carbon dioxide reduction.J. Mater. Chem. A. 2017; 5: 11905-11916https://doi.org/10.1039/C7TA01871ACrossref Google Scholar Based on these discoveries, we hypothesized that the metallic copper-based catalysts could maintain better catalytic properties under phosphate electrolyte, as metallic copper has more stable surface properties than the oxide-based or chemically pre-reduced copper catalysts.31Nitopi S. Bertheussen E. Scott S.B. 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Two Cu-based GDEs were designed by sputtering Cu on porous polytetrafluoroethylene film (PTFE film, the derived GDE catalyst is denoted as Cu/PTFE) and carbon paper (Sigracet 28 BC, the derived GDE catalyst is denoted as Cu/28BC), respectively. Scanning electronic microscope (SEM) images confirmed that the Cu layers were uniformly coated around the GDLs (Figures 2C and 2D). Although their morphologies vary due to the original substrates, powder X-ray diffraction (XRD) analysis showed that the ratios of Cu (111) to Cu (100) facets were the same on the two copper GDEs (Figure 2E), proving that the sputtering method is reliable and repeatable for fabricating catalytic electrodes. We then evaluated the CO2RR performances of these cathode designs in biocompatible phosphate electrolytes (basal solution) and electrocatalysis-favorable bicarbonate electrolytes. Figure 2F shows the FE for CO2RR products in the two electrolytes at the current densities ranging from 100 to 200 mA cm−2, which are comparable to industrial relevant current densities. In both electrolytes, Cu-based catalytic GDEs showed significant CO2 conversion to various C2 products. The main soluble C2 product was ethanol, while a small amount of acetate and 1-propanol was co-produced. Compared to the bicarbonate solution (left group, Figure 2F), the reaction in phosphate-based solution showed a higher FE of ethylene production but a lower concentration of the soluble C2 (middle group, Figure 2F). However, the total FE for soluble C2 products still reached around 15%, indicating that CO2RR in phosphate electrolytes could produce sufficient C2 intermediates for subsequent fermentation. Considering that P. putida grows much better in the basal solution (Figure 2B), the basal solution was selected as the primary electrolyte for CO2RR. With the select