Cutting off the upstream and downstream costs for CO₂ electroreduction by upcycling fermentation emissions into ethanol

Abstract

Electrochemical reduction of CO₂ (CO₂RR), when powered by renewables, opens up a new avenue to mitigate the greenhouse gas while producing value sustainably. Nevertheless, this technology has been largely limited by the high costs of the upstream CO₂ feed and downstream product separation. Here we report a hybrid bio-electrochemical system, integrating yeast fermentation with CO₂RR in one single cell, that upcycles the fermentation-emitted CO₂ into ethanol. We engineer a CuO–Ag tandem electrocatalyst with rationally designed CuO–Ag interfaces that pose minimal impact on the yeast, while efficiently converting CO₂ into ethanol against side reactions, such as hydrogen evolution and glucose reduction. We showcase the win–win model enabled by this hybrid system—the CO₂RR cost can be cut by 17.8% because the fermentation process provides a free, high-purity CO₂ source and free ethanol distillation and in return, the CO₂RR reduces the CO₂ emissions of fermentation and increases the final ethanol product concentration. This proof-of-concept procedure sheds light on a tempting possibility for a cost-effective CO₂ value chain.

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Xu Lu

Dr Xu Lu obtained his BS and PhD degrees from the Department of Mechanical Engineering, University of Hong Kong in 2012 and 2017, respectively. He was then trained as a postdoctoral fellow in the Energy Sciences Institute, Yale University. Dr Lu joined King Abdullah University of Science and Technology (KAUST) as an Assistant Professor in Mechanical Engineering in March 2021. He established the Low-carbon Energy Conversion and Storage (LECS) Laboratory, which focuses on electrochemical upcycling of industrial high-pressure CO2. So far, the LECS Laboratory has published original research articles in Nature Communications (3), Journal of the American Chemical Society, Angewandte Chemie, Chemical Engineering Journal, Journal of Energy Chemistry, etc., and generated two US provisional patents. The LECS Laboratory is also developing kilowatt-scale electrolyzers with industrial partners such as ACWA Power and Saudi Aramco.

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Introduction

Renewable-driven production of value-added chemicals via electrochemical reduction of CO₂ (CO₂RR) holds promise to simultaneously benefit environmental sustainability and realize a low carbon footprint in manufacturing industry.^1–3 To date, the field has demonstrated high-yield and efficient production of multiple CO₂RR products, including carbon monoxide (CO), formic acid/formate (HCOOH or HCOO⁻), methane (CH₄), methanol (CH₃OH), ethylene (C₂H₄), acetic acid/acetate (CH₃COOH or CH₃COO⁻), and ethanol (CH₃CH₂OH).^4–6 Converting CO₂ to ethanol is of particular importance because ethanol is a high-energy-density fuel additive that can facilitate cleaner combustion compared to conventional gasoline, such as the E85 ethanol fuel.^7–9 Ethanol also serves as the key chemical in pharmaceutical and cosmetic industries.^10,11 According to a report by GlobeNewswire, ethanol consumption in the European Union was estimated at approximately 4.8 million tons in 2021.¹²

Despite major advances, CO₂RR technology has been greatly challenged by its costly upstream and downstream processes.^13–15 On the one hand, the upstream feed of CO₂RR should be high-purity CO₂, due to the substantial energy and capital expenses required for direct electroreduction of impure CO₂.^16–18 The prevailing way to generate pure CO₂ gas is to capture CO₂ from point sources or air. Recent studies have described the possibility of integrating CO₂ capture with CO₂RR.^19–21 However, for now at least, CO₂ capture and recycling still incurs a high cost, which accounts to 9.3% of the capital expenditure (CapEx) and 8.3% of the operating expense (OpEx).^22–24 On the other hand, residue CO₂ and the electrolyte solution hold a large portion of the CO₂RR effluent, and this necessitates downstream separation of products, which is another constraint of CO₂RR. Taking ethanol as an example again, the commonly used distillation apparatus not only leads to significant energy compensation, but also jeopardizes the cost effectiveness of the process—contributing approximately 3.9% CapEx and 2.3% OpEx of the overall CO₂RR process.^13,24,25

Bio-ethanol, produced from biomass through yeast fermentation, is the world's most popular ethanol supply, with a market size of $46.18 billion in 2022.^26,27 However, a typical bio-fermentation process produces bio-ethanol via reaction in an anaerobic environment:

where significant amounts of CO₂ are emitted as a side product.²⁸ Different from fermentation in an aerobic reaction environment, where oxygen plays a part as a gas reactant, the emitted CO₂ in bio-ethanol fermentation has been proposed for use in food industry applications as a result of its high purity.²⁹ The global bio-ethanol yield in 2022 was 140 billion liters.³⁰ That means that approximately 106 million tons of CO₂ were emitted, leading to adverse environmental impacts and a heavy carbon penalty. It is estimated that the bio-ethanol manufacturing industry has been investing over $1.67–3.32 billion per year on the treatment of fermentation emissions.³¹ However, fermentation CO₂ emission exhibits ultra-high purity (Fig. 1a),³² and is thereby a perfect CO₂RR input feed. Meanwhile, CO₂ from ambient air or other point sources, such as vehicle exhaust, oil refineries, steam reforming, ammonia production, iron and steel casting, and cement manufacture, usually come with significant amounts of impurities. In addition, the fermentation process consists of an ethanol distillation step, which could well serve the purpose of product separation for CO₂RR, if carefully designed.

Fig. 1 Schematics of TEA and the proposed hybrid system. (a) CO₂ purity (wt%) of various point sources and ambient air. (b) Schematic illustration of the fermentation–CO₂RR hybrid system. Bio-fermentation emits CO₂, which is upcycled into ethanol via in situ electrochemical conversion. This ethanol produced by the CO₂RR is then distilled together with the fermentation-generated bio-ethanol without extra cost. (c) Techno-economic analysis (TEA) of the proposed bio-electrochemical system vs. the conventional CO₂RR process for the production of ethanol. Our system saves on the costs of (i) acquiring high-purity CO₂ feed and (ii) separating the final product, thereby cutting the CO₂RR cost by at least 17.8%.

Here we propose a fermentation–CO₂RR hybrid that electrochemically converts high-purity fermentation-generated CO₂ into ethanol, which is then distilled together with the fermentation-produced bio-ethanol without extra cost. We tailor a CuO–Ag tandem catalyst with CuO–Ag interfaces that pose minimal influence on yeast activity while maintaining a reasonable faradaic efficiency (FE) for CO₂-to-ethanol conversion. In situ Raman studies show that the role of the CO intermediate on the CuO–Ag surface accelerates the C–C coupling process, reversibly suppressing side reactions such as hydrogen evolution and glucose reduction. Techno-economic analysis (TEA) indicates that our system not only cuts off the upstream and downstream expenses of CO₂RR (that is, around 17.8% of the total CO₂RR cost), but also benefits the fermentation by reducing CO₂ emissions. This proof-of-concept system provides the possibility to address major bottlenecks for both CO₂RR and fermentation.

Results

Pathway design

CO₂RR suffers from high costs of the (i) upstream CO₂ feed and (ii) downstream product separation, making it challenging for commercialization. We therefore propose a fermentation–CO₂RR hybrid for CO₂RR to take a free ride on the bio-fermentation process, which affords the possibility of cutting the two major costs of CO₂RR, by supplying high-purity CO₂ and providing free ethanol distillation. The proposed bio-electrochemical system (Fig. 1b) realizes a greener production of ethanol via a three-step pathway as below.

(i) Biomass fermentation as catalyzed by yeast, where ethanol is generated with high-purity CO₂ emission:

(ii) CO₂RR powered by renewables, where the CO₂ emission is upcycled into ethanol:

(iii) Ethanol distillation for the delivery of final product.

To confirm the cost effectiveness of the fermentation–CO₂RR hybrid system, we performed TEA for CO₂ electroreduction to ethanol, assuming a base electricity cost of 0.02 $ per kW h, in line with the target set by US Department of Energy (DOE) for the year 2030.³³ TEA revealed that the CO₂ feed and the ethanol distillation accounted for 9.1% and 8.7% of the total CO₂RR cost with an operating current density of 200 mA cm⁻² and ethanol FE of 50%—that is, 17.8% of the expense can be saved by our proposed system compared to a stand-alone CO₂RR device (Fig. 1c).

To realize this proof-of-concept system, we first need to validate the CO₂ output from fermentation. We used 0.1 M glucose solution, as is commonly used in literature, as the fermentation feedstock for yeast to generate ethanol.^34,35 Despite the fact that liquids such as glycerol and organic acids may exist as side products, CO₂ was the only gas product from anaerobic bio-ethanol fermentation, as analyzed by gas chromatography (GC)—that is, almost 100% CO₂ purity (Fig. 2a).³⁶ After 25 h of fermentation, CO₂ became saturated, and the saturation state lasted for at least 500 h (Fig. 2b). This indicated the capability of the fermentation to supply sufficient CO₂ to CO₂RR over a prolonged time. The pH value of the fermentation broth remained stable at 5.83 (Fig. 2c), which provides a weakly acidic environment that is beneficial to a high-carbon utilization efficiency during CO₂RR because the otherwise neutral or alkaline environment at the cathode–electrolyte interface may lead to carbonate formation.^37–39 These observations authenticated the idea of feeding the fermentation CO₂ emissions to the CO₂RR, and motivated us to conduct the subsequent electrochemical experiments.

Fig. 2 Fermentation emissions and broth condition. (a) Gas chromatography of fermentation-emitted CO₂. CO₂ is the only gas product. (b) Amount of fermentation-emitted CO₂ and (c) pH value of the fermentation broth over time.

CuO–Ag tandem catalyst for the fermentation–CO₂RR hybrid

Catalyst design strategies for CO₂RR to ethanol have been extensively studied, such as surface control, oxide modulation, oxophilicity engineering, etc.^40–42 Recently, tandem catalysts, especially Cu–Ag tandems, have demonstrated the capability of sequentially catalyzing CO₂-to-CO and CO-to-C₂ at reaction rates approaching industrial relevance.^43,44 Other homogeneously alloyed Cu-based bimetallic nanoparticles, as well as segmented tandem electrodes for increased CO coverage, can also enhance the CO₂-to-ethanol conversion.^45,46 For instance, CoPc@HC/Cu tandem electrode in acid CO₂RR exhibited a C₂₊ FE of 90% and single-pass CO₂ conversion efficiency of 76%.⁴⁷ We therefore sought to explore the possibility of designing a CuO–Ag tandem catalyst for our system (Fig. 3a). In brief, an Ag layer was first deposited on a carbon paper, followed by shadow mask-based Cu deposition to create 40 CuO–Ag interfaces in a 0.5 cm × 2 cm area. Of note, interface numbers higher than 40 were not prepared due to instrumentation constraints. Then, the electrode underwent galvanostatic anodic oxidation to form the final CuO–Ag tandem (Fig. 3b and S1†). The CuO–Ag interfaces were confirmed by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS). Cu(111) and Ag(111) facets were delineated (Fig. 3c). EELS revealed the interface structure at the nanoscale (Fig. 3d), while EDS uncovered the CuO–Ag at microscale (Fig. S2†). X-ray diffraction (XRD) patterns of the Cu segment of the as-prepared tandem electrode showed that CuO(110), CuO(111), and CuO (022) peaks increased during electrochemical oxidation, while those of Cu(111) and Cu(200) decreased over time (Fig. 3e), which implies the formation of CuO.⁴⁸ This was confirmed using Raman spectroscopy, which showed the characteristic peaks of CuO at 282, 330, and 616 cm⁻¹. Consistent with the literature, CuO exhibited a nanoplate morphology, while Ag appeared as nanoparticles (Fig. 3g and S3†).⁴⁹

Fig. 3 CuO–Ag tandem catalyst. Schematic of (a) the CuO–Ag tandem electrode synthesis and (b) the CuO–Ag interfaces. (c) HR-TEM and (d) EELS elemental mapping of the CuO–Ag interface. (e) XRD patterns of CuO–Ag over the electrochemical oxidation time. (f) Raman spectroscopy and (g) SEM of the CuO segment after oxidation.

We then retrofitted a conventional fermentation system into an H-type hybrid cell that is compatible with CO₂RR—the cathode compartment contained 0.1 M glucose (a typical fermentation broth) and the anode compartment was filled with 0.5 M KHCO₃ aqueous solution (Fig. 4a). Nevertheless, the fermentation broth cannot provide the necessary electrochemical reaction conditions—the low electrolyte concentration resulted in sluggish adsorption of OH⁻ on the catalyst surface, and increased both the charge transfer resistance (R_ct) and the electrical double layer (EDL) thickness.^50,51 Consequently, the CO₂RR current density was limited below 8.85 mA cm⁻² at cathode potentials as negative as −1.6 V vs. the reversible hydrogen electrode (RHE) (Fig. 4b). To accommodate a reasonably high CO₂RR reactivity, we attempted to add the most used cation, K⁺, into the fermentation broth, such as K₂CO₃, KCl, and KHCO₃.^52–54 At a cathode potential of −1.6 V vs. RHE, the current density reached −181, −152, and −161 mA cm⁻² in 0.5 M K₂CO₃, 0.5 M KCl, and 0.5 M KHCO₃ glucose solutions, respectively, which is significantly higher than those in 0.1 M glucose, as suggested by linear sweep voltammetry (LSV) curves (Fig. 4b). However, the addition of carbonate and bicarbonate anions resulted in an adverse impact on fermentation by tuning the pH towards alkaline, which was unfavorable for yeast activity,⁵⁵ whereas adding KCl maintained the fermentation broth as a weak acid to facilitate the fermentation process (Fig. S4†).⁵⁶ This was further confirmed by weighing the fermentation CO₂ emissions, where 0.5 M KCl merely changed the amount of yeast-generated CO₂, while in the case of K₂CO₃ and KHCO₃ obvious degradation of yeast activity was shown (Fig. 4c).⁵⁷ Therefore, we selected 0.1 M glucose aqueous solution (0.5 M KCl) as the catholyte for the fermentation–CO₂RR hybrid cell.

Fig. 4 CO₂RR performance in the fermentation–CO₂RR hybrid cell. (a) Photo of the fermentation–CO₂RR hybrid cell. (b) LSV curves of the CuO–Ag catalyst in different solutions. (c) Amount of fermentation CO₂ emissions in various glucose solutions. (d) Ethanol FEs and partial current densities with different CuO–Ag interface numbers (0, 10, 20, 30, and 40) in 0.1 M glucose (0.5 M KCl).

Rationally, more CuO–Ag interfaces in a fixed area should benefit the CO₂-to-CO and CO-to-ethanol tandem reaction.^58,59 To confirm this, we compared CuO–Ag interface numbers of 0 (i.e. CuO), 10, 20, 30, and 40. It was observed that increasing the interface number resulted in higher ethanol selectivity; at −0.87 V vs. RHE, the ethanol FE was improved from 14.4% on CuO to 27.7% on CuO–Ag (20), and eventually reached 42.7% on CuO–Ag (40). Similar phenomena hold for the ethanol productivity; at −1.27 V vs. RHE, the partial current density of ethanol was −8.4 mA cm⁻² on CuO, whereas it gradually increased to −19.6 mA cm⁻² on CuO–Ag (40) under the same conditions (Fig. 4d).

Understanding the enhanced ethanol production for proof-of-concept

To gain insights into the improved ethanol yield at higher CuO–Ag interface numbers, in situ studies and density functional theory (DFT) calculations were conducted. Time-resolved X-ray diffractograms (Fig. S5†) revealed that CuO was reduced to Cu(111) and Cu(200) under CO₂RR condition, whereas the diffraction peaks of Ag barely changed, which is consistent with the X-ray photoelectron spectroscopy (XPS) results (Fig. S6†). This indicated that metallic Cu and Ag served as the dominant active sites during electrolysis, and prepared us to probe the key species distributions on the catalyst surface using in situ Raman spectroscopy. As mentioned above, the conversion of CO₂ to ethanol on a tandem catalyst proceeded via two steps: (i) CO formation, as catalyzed by Ag (Fig. S7 and S8†) and (ii) C–C coupling, as catalyzed by Cu. Three regions in the Raman spectra were associated with the CO intermediates on the surface at negative potentials—the band at around 280 cm⁻¹ referred to the Cu–CO frustrated rotation mode,^58,60,61 the band at 360 cm⁻¹ was related to the Cu–CO stretch mode,^60,62 and the band at approximately 2075 cm⁻¹ can be ascribed to the C≡O stretch of the top-bound CO (Fig. 5a).^63,64 Compared to CuO, the band of the Cu–CO stretch was blue-shifted on the CuO–Ag interface at −1.3, −1.5, and −1.7 V vs. RHE (Fig. 5b and S9†), indicating a stronger binding of CO to the CuO–Ag interface.^65–67 The enhanced CO binding implied an enriched CO environment, which would promote the subsequent C–C coupling step, thereby resulting in a higher ethanol productivity.⁶⁸ DFT calculations confirm that the interface of Cu–Ag is more effective in ethanol generation (Fig. S10†). Compared to bare Cu, the free energy difference on the Cu–Ag interface is more gradual. For instance, the energy difference for adsorption of the second CO₂ is lower on the Cu–Ag interface (0.27 eV vs. 0.79 eV). Additionally, for C–C coupling, the Cu–Ag interface exhibits lower energy (1.09 eV vs. 1.46 eV).

Fig. 5 Enhanced ethanol production and proof-of-concept demonstration. (a) In situ Raman spectra acquired on CuO–Ag (40) under different potentials. (b) Comparison of in situ Raman spectra on CuO–Ag (40) and CuO catalysts at −1.3, −1.5, and −1.7 V vs. RHE. (c) FEs and current densities toward CO₂RR products, H₂, and sorbitol. (d) Prolonged operation of the CO₂RR–fermentation hybrid at −0.87 V vs. RHE. Pt foil was used as the anode. 0.1 M glucose (0.5 M KCl) solution was the electrolyte, and the CuO–Ag interface number was 40 unless otherwise stated.

To further rationalize our choice of a high CuO–Ag interface number, we examined the side reactions that occurred simultaneously with CO₂RR. In a typical CO₂RR cell, the hydrogen evolution reaction (HER) is known to be the major competing reaction,^69–73 and it was slightly impacted by the CuO–Ag interface number because the active sites for the HER were essentially unchanged. In our CO₂RR–fermentation hybrid system, however, another competing reaction existed, namely, the glucose reduction reaction (GRR), where glucose is electrochemically reduced to sorbitol.⁷⁴ Compared to the HER, the GRR may pose a greater threat to CO₂RR because both Cu and Ag have been reported to be active GRR catalysts.⁷⁵ Strikingly, our CuO–Ag (40) catalyst can effectively suppress the GRR: the GRR FE was 2.68% at −1.27 V vs. RHE, and this number even lower at 2.37% at −0.87 V vs. RHE (Fig. 5c). By stark contrast, the GRR FE increased to 2.91%, 3.51%, 3.71%, and 4.86% when the CuO–Ag interface number was 30, 20, 10, and 0, respectively (Fig. S11†). These observations verified our tandem catalyst design strategy, and, to the best of our knowledge, developing a CO₂RR catalyst that can simultaneously suppress the GRR has never been reported. Finally, we sought to stably operate the proof-of-concept system. Our CO₂RR–fermentation hybrid exhibited a maximal ethanol FE of 42.7% at −0.87 V vs. RHE, with an overall current density of 27 mA cm⁻² (Fig. 5c). The partial current density towards ethanol reached 20 mA cm⁻², with an FE of 18.9% at −1.27 V vs. RHE. The current density and ethanol FE were well retained over the course of 12 h of chronoamperometric operation at −0.87 V vs. RHE (Fig. 5d). Ethanol was detected as the predominant liquid product (Fig. S12†).

Conclusions

In summary, we have demonstrated a compelling bio-electrochemical system, combining yeast fermentation and CO₂RR, to upcycle the fermentation-emitted CO₂ into ethanol. Taking a free ride from the high-purity CO₂ emissions and ethanol distillation during fermentation, the CO₂RR–fermentation hybrid cuts the total CO₂RR cost by 17.8%. A CuO–Ag tandem electrocatalyst with minimal impact on yeast was rationally designed, efficiently converting CO₂ to ethanol while suppressing side reactions, such as hydrogen evolution and glucose reduction. These results illustrate the possibility of a cost-effective CO₂ value chain for ethanol production. Further improvements are expected by optimizing the ethanol efficiency and yield through rational tandem electrode design, yeast selection, and system integration.

Author contributions

X. L. supervised the project. X. L. and R. S. conceived the idea. R. S. synthesized and characterized the catalysts, and conducted electrochemical measurements. J. Z. made the DFT calculations. R. S. performed the fermentation experiments. R. S. and J. Z. carried out in situ Raman spectroscopy. R. S. conducted the TEA calculation. R. S. and X. L. wrote the manuscript. All authors discussed the results and assisted with the manuscript preparation.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was financially supported by the Baseline Fund (BAS/1/1413-01-01) to X. L. from King Abdullah University of Science and Technology (KAUST).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ta07558c

‡ These authors contributed equally.

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