Upgrading CO2 into acetate on Bi2O3@carbon felt integrated electrode via coupling electrocatalysis with microbial synthesis

Upgrading of atmospheric CO2 into high‐value‐added acetate using renewable electricity via electrocatalysis solely remains a great challenge. Here, inspired by microbial synthesis via biocatalysts, we present a coupled system to produce acetate from CO2 by bridging inorganic electrocatalysis with microbial synthesis through formate intermediates. A 3D Bi2O3@CF integrated electrode with an ice‐sugar gourd shape was fabricated via a straightforward hydrothermal synthesis strategy, wherein Bi2O3 microspheres were decorated on carbon fibers. This ice‐sugar gourd‐shaped architecture endows electrodes with multiple structural advantages, including synergistic contribution, high mass transport capability, high structural stability, and large surface area. Consequently, the resultant Bi2O3@CF exhibited a maximum Faradic efficiency of 92.4% at −1.23 V versus Ag/AgCl for formate generation in 0.5 M KHCO3, exceeding that of Bi2O3/CF prepared using a conventional electrode preparation strategy. Benefiting from the high formate selectivity, unique architecture, and good biocompatibility, the Bi2O3@CF electrode attached with enriched CO2‐fixing electroautotrophs served as a biocathode. As a result, a considerable acetate yield rate of 0.269 ± 0.009 g L−1 day−1 (a total acetate yield of 3.77 ± 0.12 g L−1 during 14‐day operation) was achieved in the electrochemical–microbial system equipped with Bi2O3@CF.


INTRODUCTION
Excessive releases of CO 2 in the atmosphere arising from anthropogenic activities and resultant worldwide environmental damage and climate change have aroused widespread concerns. 1 Upgrading CO 2 into valuable fuels and chemical feedstock is a promising way to realize carbon neutralization. [2][3][4] Electrochemical CO 2 reduction reaction (ECO 2 RR) driven by renewable electricity have been emerged as a promising strategy to fulfill this goal. 5,6 Among diversified products, acetate is an intriguing liquid fuel in a view of considerable market scale and economic values and plays an important role in the chemical industry. [7][8][9] Exciting progress has been achieved in developing active and robust electrocatalysts toward specific product (e.g., formate and CO) by means of composition, morphology and structure engineering, and so on. [10][11][12][13][14][15][16][17] However, selectively producing acetate via ECO 2 RR directly still suffers low selectivity and unwanted side products, which is due to the high energy barrier for C-C bond coupling and the competitive pathways. [7][8][9] Starting from CO 2 and using electrochemical reduction solely to produce acetate is still an opening challenge. [7][8][9] Microbial electrosynthesis (MES) recently emerged as alternative strategy to expand the spectrum of possible products, wherein microorganisms are considered ideal biocatalysts for transforming CO 2 into longer carbon chains. 18,19 Particularly, acetogens, a well-known microbial catalyst in natural environments, are capable of upgrading CO 2 into acetate selectively via the Wood-Ljungdahl (WL) metabolic pathway. [20][21][22] In the MES system, electricity-driven CO 2 conversion to multicarbon molecules is achieved via biocatalysts by uptaking reducing equivalents from the cathode of the electrochemical cell. 23,24 Apart from electrode as an electron source, H 2 and formate mediators can also be served as electron donors for autotrophic microorganisms. 23 Because of some limitations of low solubility, storage, and safety issues, H 2 does not qualify as a broad applicable alternative; however, such limitations do not exist in formate. 25 Moreover, formate is a selective product easily obtained from ECO 2 RR by electrocatalysis. 26,27 Thus, coupling electrocatalysis and microbial synthesis in one compartment to establish a hybrid bioinorganic system is an appealing way for CO 2 conversion, which possesses their respective advantages of both CO 2 fixation ways, including high efficiency and specificity toward acetate by microbial synthesis and high selectivity for formate by electrocatalysis. [28][29][30][31] The in situ generated formate via ECO 2 RR proceeding on the catalyst surface provides reducing equivalent to attached microorganisms for subsequent biotransform of CO 2 to acetate. 23,32 From this perspective, an ideal formate electrocatalyst in the coupled system, which plays a pivotal role in deter-mining the overall performance and acetate production, should simultaneously possess high formate selectivity, good biocompatibility, adequate surface area, and excellent structural stability. High formate selectivity primarily satisfies sufficient energy supply to microorganisms. 33 Good biocompatibility refers to contribution to adhesion toward biofilm as well as no inhibition on metabolism. 34,35 Adequate surface area creates accessible active sites both for electrocatalysis and bacterial affinity. [36][37][38] Structural stability guarantees the stationarity of overall performance in the long-term running process.
For inorganic electrocatalysts, bismuth-based materials and their derivants attracted widespread interests in the field of ECO 2 RR research due to their unique catalytic merits, low toxicity, low cost, and high stability. [39][40][41][42][43][44] Generally, a series of bismuth-based electrocatalysts usually exhibit layered structures with thickness around several atomic layers. 45,46 Nevertheless, for preparing working electrode, the electrocatalysts are usually loaded on carbon substrate via the traditional slurry-coating method with the usage of binders. 47 This fabrication process completely differs from that of an integrated electrode. In this case, on the one hand, the usage of binder would decrease the conductivity and enhance the contact resistance. 48,49 On the other hand, the layered structure is unavoidably stacked and aggregated, which leads to the decreased accessible surface area accompanied by numerous active sites being covering up. Besides, the catalyst falling off from the substrate is a common occurrence during the operation. 50 These problems would become aggravated in hybrid bioinorganic system 36 ; decreased electron transportation efficiency reduces energy supplementary for microorganisms, whereas the fragile architectures weaken the affinity toward biofilm formation, thereby ultimately affect the acetate production.
Herein, we fabricated an ice-sugar gourd-shaped Bi 2 O 3 @CF integrated electrode via a straightforward hydrothermal synthesis strategy, in which fibers in carbon felt (CF) are surrounded by Bi 2 O 3 hierarchical microspheres. This one-step in situ growth strategy discarded the usage of binders, simplified the electrode preparation process, and preserved the overall architecture stability. The formed three-dimensional (3D) self-supported electrode with ice-sugar gourd shape not only facilitates electron transport but also affords a large specific area for the contact between electrocatalyst with microorganism. By the virtue of unique features, the Bi 2 O 3 @CF electrode achieved a formate Faradaic efficiency (FE) of 92.4% at −1.23 V versus Ag/AgCl with a partial current density of 2.87 mA cm −2 for formate generation, exceeding that of Bi 2 O 3 /CF prepared using a conventional electrode preparation method. Additionally, by employing Bi 2 O 3 @CF as an electrocatalyst and mixed microbial flora as a F I G U R E 1 Schematic illustration for the fabrication of Bi 2 O 3 @CF integrated electrode via a straightforward hydrothermal synthesis strategy.
biocatalyst, we fabricated an electrochemical-microbial coupled system for CO 2 conversion through the in situ formed formate feeding electroautotrophs. Fine biocompatibility of Bi 2 O 3 @CF reflected in a dense and efficient biofilm attached on its surface, leading to substantially enriched acetogen. As a result, the volumetric acetate production rate was 0.269 ± 0.009 g L −1 day −1 , and an average acetate titer of 3.77 ± 0.12 g L −1 within 14 days under −1.3 V versus Ag/AgCl was achieved over Bi 2 O 3 @CF.

RESULTS AND DISCUSSION
The Bi 2 O 3 @CF integrated electrode was fabricated via a straightforward hydrothermal synthesis strategy (see details in the Supporting Information section (Experimental Methods)) with CF immersed into solution containing Bi precursor, as schematically shown in Figure 1. There were no binders utilized in the procedure, unlike the conventional electrode preparation method. CF was selected as a support due to its flexibility, conductivity, and porosity, offering an open and 3D architecture. Nucleation and growth of Bi 2 O 3 on CF was achieved eventually through one-step hydrothermal treatment. X-ray powder diffraction (XRD) analysis confirmed the phase of Bi 2 O 3 in the resultant catalyst. Figure   To observe morphology, we carried out a scanning electron microscope (SEM). For bare CF, as shown in Figure S1, the carbon fibers have diameters of ∼14 μm with uniform width and length up to several millimeters, showing a high aspect ratio. After the hydrothermal reaction, Bi 2 O 3 microspheres grew uniformly on the carbon fiber backbone, whereas carbon fibers maintained their original skele-ton structure, and no dispersed microspheres were found ( Figure 2B,C). Notably, these hierarchical microspheres on the scale of ∼50 μm were assembled from cross-linked plates densely ( Figure 2E). More interestingly, unlike common architectures with nanosheets arrayed on the substrate, carbon fibers pass through flower-liked Bi 2 O 3 microspheres, forming ice-sugar gourd-shaped architectures, as displayed in Figure 2C,D. SEM image from crosssectional view in Figure 2F could further demonstrate the unique architecture. SEM-energy-dispersive X-ray spectroscopy (EDS) elemental mappings ( Figure 2G) show Bi and O distribute uniformly in the microspheres, decorating on the carbon fibers. Overall, the aforementioned characterizations confirm the unique architecture of this integrated electrode with a hybrid 3D structure in which carbon skeleton is wrapped Bi 2 O 3 microspheres. This integrated electrode may afford an ideal surface beneficial for mass transport, microorganism adherence, and microbial cell growth.
To investigate the microstructure of the coated catalyst, we removed it ultrasonically from CF and carried out transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Figure 2H,I reveals a nanosheet-liked structure, which may be attributed to the collapse of microspheres due to the ultrasonic treatment, leading to fragments. HRTEM image and the corresponding fast Fourier transform pattern in Figure 2J suggest an interplanar spacing of lattice fringe is 0.32 nm, corresponding to (2 0 1) planes, indicating good crystalline of the Bi 2 O 3 crystals. Further, the attached Bi/O atomic ratio determined from EDS analysis is 39.1:60.9 ( Figure S2 Moreover, we also synthesized an unsupported Bi 2 O 3 sample as a control using a similar method in the absence of a CF substrate. XRD evidenced the successful synthesis of Bi 2 O 3 ( Figure 2A). As shown in Figure S3A,B, the resultant Bi 2 O 3 sample is a mixture of many flower-like microspheres and scattered plates. The 3D microspheres consist of hierarchically organized nanosheets, exhibiting a rough topography ( Figure S3C). Lattice analyses reveal the (2 0 1) plane of Bi 2 O 3 , demonstrating its crystal structure ( Figure S3D). Elemental mapping and EDS spectrum suggest the uniform spatial distribution of Bi and O elements with a proper ratio ( Figure S3E,F). Furthermore, for comparison, we also fabricated an electrode (denoted as Bi 2 O 3 /CF) by adopting the conventional approach of dropping this Bi 2 O 3 catalyst ink onto CF. After loading, however, Bi 2 O 3 flower-like microspheres disappeared and turned into small debris in Bi 2 O 3 /CF electrode ( Figure S4). Of note that these small debris filled up the interspace of carbon fibers due to the usage of Nafion as a binder ( Figure  S4B). Obviously, it is difficult for conventional approach to maintain the initial structure of Bi 2 O 3 catalysts, completely different from in situ growth approach. As such, this in situ growth strategy enhances the structural stability, simplifies the fussy procedure of catalyst loading, and avoids the risk of catalyst structure being dilapidated.
Furthermore, we also employed X-ray photoelectron spectra (XPS) to study the chemical states of elements in the electrodes. XPS survey spectra of Bi 2 O 3 @CF and Bi 2 O 3 ( Figure S5A) confirm the existence of Bi, O, and C. The core level XPS spectrum of C 1s for Bi 2 O 3 @CF ( Figure S5B) can be deconvoluted into five peaks at 284.56, 285.30, 286.55, 287.49, and 289.0 eV, assigned to C-C, C-N, C-O, C=O, and π-π*, respectively. According to the O 1s core level XPS spectra of pure Bi 2 O 3 in Figure 3A, the fitted peak at 529.9 eV is ascribed to lattice oxygen bounded to metal, whereas the other one at 531.7 eV corresponds to defected oxygen. For Bi 2 O 3 @CF, the other peak appeared at 533.3 eV belongs to chemically adsorbed oxygen. Apparently, the relative area of defected oxygen in the O element of Bi 2 O 3 @CF is higher than that of pure Bi 2 O 3 , which means there are more oxygen vacancies in Bi 2 O 3 @CF. Analyses of high-resolution XPS spectra of Bi for Bi 2 O 3 @CF show two fitted peaks at the bonding energy of 164.4 and 159.1 eV ( Figure 3B), which can be indexed to Bi 4f 7/2 and Bi 4f 5/2 for Bi 3+ , demonstrating the existence of a single-oxidation state of the Bi 3+ state within Bi 2 O 3 . Besides, there are downshifts by 0.1 eV in comparison with Bi 4f in Bi 2 O 3 , suggesting a higher electron density of Bi atoms in Bi 2 O 3 @CF. This phenomenon can be attributed to higher oxygen vacancy concentrations in Bi 2 O 3 @CF, which is consistent with the XPS observations of O1s. Previous studies have reported that a lower oxidation degree of Bi in Bi 2 O 3 with more undercoordinated Bi sites can facilitate the formate production from CO 2 via electrocatalysis. 51 Roman spectrum of Bi 2 O 3 in Figure 3C displays one peak at 90.4 cm −1 corresponding to the A 1g vibration of Bi in Bi 2 O 3 , and the peaks at 120.7 and 310.9 cm −1 are ascribed to the Bi-O stretches. 52 For Bi 2 O 3 @CF, the additional two peaks appear at 1350.0 and 1578.4 cm −1 , which can be assigned to the D and G bands of the CF, respectively, further confirming the successful growth of Bi 2 O 3 on CF.
We also examined CO 2 adsorption capacities of Bi 2 O 3 @CF, Bi 2 O 3 /CF, and CF. CO 2 , acting as an important carbon feedstock for formate mediates and acetate end product, proceeds via the adsorption at the hybrid bioinorganic cathode in the hybrid system. Thus, it is of great significance to supply adequate CO 2 to electrocatalyst and microorganism, which not only influence the concentration of formate but also determine the final production of acetate. 53 Figure 3D shows CO 2 adsorption isotherms, suggesting an improved CO 2 adsorption ability of Bi 2 O 3 @CF in comparison with Bi 2 O 3 /CF and CF. Large surface area in Bi 2 O 3 @CF and high-distribution Bi 2 O 3 microspheres accommodates for accessibility of CO 2 molecules and improves the conversion efficiency. Overall, from the perspectives of both the choice of electrocatalytic material and the design of overall architecture, we consider that Bi 2 O 3 @CF meets the requirements of biocathode appropriate for the ECO 2 RR-biosynthetic coupled system. First, we explored the electrocatalytic properties for ECO 2 RR on the Bi 2 O 3 @CF integrated electrode. ECO 2 RR was conducted in a typical H-typed cell equipped with a three-electrode system operated under CO 2 -saturated 0.5 M KHCO 3 environments. For comparison, Bi 2 O 3 /CF was assessed under the identical conditions. Linear sweep voltammetry curves were recorded in the CO 2 -and Arpurged electrolyte ranging from −0.83 to −1.60 V versus Ag/AgCl electrode. As depicted in Figure 4A, in CO 2 environment, a remarkable current-voltage response on Bi 2 O 3 @CF was observed. The current density reached up to 58.99 mA cm −2 at −1.60 V, much larger than that found for Bi 2 O 3 /CF (37.52 mA cm −2 ) under the same potential. In addition, Bi 2 O 3 @CF exhibits an onset potential of around −1.13 V; by comparison, onset potential is around −1.25 V for Bi 2 O 3 /CF. The more positive onset potential reflects enhanced CO 2 RR kinetics on Bi 2 O 3 @CF relative to Bi 2 O 3 /CF. Afterward, by using the chronoamperometry technique, the activity and selectivity were further elevated. After electrolysis in the potential window of −1.23 to −1.53 V, formate was found to be the only liquid product, which was quantitatively analyzed via 1 H nuclear magnetic resonance. Online gas chromatography identified H 2 and CO were minor products, which together accounted for the rest part of reduced products. Hydrogen evolution reaction was found to be significantly suppressed over the Bi 2 O 3 catalyst. FEs depending on applied potentials give the product distribution, as presented in Figure 4B and Figure S6. For Bi 2 O 3 @CF integrated electrode, FE for formate reached an optimal value of 92.4% at −1.23 V with a partial current density j formate of 2.87 mA cm −2 ( Figure 4C). As the working potential increased, the selectivity for formate maintained above 80%, and the current density for formate production dramatically increased in the entire potential region. At an applied potential of −1.53 V, Bi 2 O 3 @CF exhibited the j formate up to 18.37 mA cm −2 , whereas considerably lower j formate was measured for Bi 2 O 3 /CF. Of note that Bi 2 O 3 @CF electrode showed a merely slight increase in formate selectivity when compared with Bi 2 O 3 /CF electrode ( Figure S6); however, it delivered much higher j formate in the whole process, indicative of its high intrinsic CO 2 activity. We then assessed the amount of formate produced in the catholyte. Figure S7 shows formate yield on Bi 2 O 3 @CF is 2.47 mg h −1 at −1.23 V, and the yield increases to 15.77 mg h −1 at −1.53 V, outperforming those values of Bi 2 O 3 /CF at the same potentials. As expected, the formate yield of Bi 2 O 3 @CF and Bi 2 O 3 /CF followed a similar trend to those of their current densities in all the examined potentials. Such an improvement of electrocatalytic performance for CO 2 -to-formate over Bi 2 O 3 @CF was highly associated with its special structure. Moreover, high formate yield might provide sufficient electron donors for microorganism.
In addition, electrochemcial active surface area (ECSA) is an important metric to elevate the performance of electrocatalyst, which is determined from double-layer capacitance. In the non-faradaic region, we plotted Δj at 0.35 V against scan rate ( Figure S8). As shown in Figure 4D, the C dl value for the Bi 2 O 3 @CF electrode is 1640 μF cm −2 , which was approximately 10 times higher than that of Bi 2 O 3 /CF (160 μF cm −2 ), suggesting more accessible active sites preferentially exposed in the integrated electrode. The significant enhancement in ECSA is because the onedimensional carbon fibers could hold the monodisperse spheres and prevent them from aggregating. In short, the much higher values in terms of current density, formate yield, and ECSA observed on Bi 2 O 3 @CF demonstrate the intrinsic ECO 2 RR of the Bi 2 O 3 @CF integrated electrode is remarkable and superior to the Bi 2 O 3 /CF electrode. We also performed the electrochemical impedance spectroscopy to measure the charge transfer resistance (R ct ). Nyquist plots in Figure 4E reveal that R ct for Bi 2 O 3 @CF is 15.03 Ω, versus about 16.92 Ω for Bi 2 O 3 /CF. The smaller R ct signifies accelerated charge transfer kinetics in Bi 2 O 3 @CF, verifying in situ growth of Bi 2 O 3 on CF could decrease the resistance between catalyst and substrate. Besides, Tafel represents the relationship between the current density and overpotential, which reflects the intrinsic ECO 2 RR properties. Tafel plots ( Figure 4F) offer a slope of 271.56 and 274.52 mV dec −1 for Bi 2 O 3 @CF and Bi 2 O 3 /CF, respectively. According to the above results, ECO 2 RR was found highly more favorable on Bi 2 O 3 @CF compared with Bi 2 O 3 /CF.
Motivated by the high formate selectivity and yield, we further employed Bi 2 O 3 @CF as the cathode to electrochemically produce formate to fabricate an electrocatalysis-microbial synthesis coupled system. Figure 5A shows the schematic of the two-compartment electrochemical reactor separated by a proton exchange membrane (PEM). The PEM can prevent the diffusion of oxygen generated in anodic chamber into cathodic chamber because oxygen can inhibit the activity of anaerobic microorganisms. The bacteria from a mixed culture were anaerobic, which were obtained by training the planktonic cells in a long-running reactor of our laboratory. Inoculating microorganisms into the cathodic chamber to form a biofilm on the cathode, electrochemicalmicrobial CO 2 reduction via Bi 2 O 3 @CF-bacteria hybrid electro-biocatalyst was carried out. In this process, formate ions generated from CO 2 are a vital intermediate, acting as electron donors of attached electroautotrophic microorganisms for subsequent CO 2 conversion to acetate.
A series of experiments using Bi 2 O 3 @CF, along with Bi 2 O 3 /CF and CF as references, were afterward conducted in parallel under the same conditions and environment for 2 weeks except for the operating potentials. Under a long-term operation, during each 24-h period, acetate concentration accumulated in the reactor was measured (Figure 5B). At a constant potential −1.3 V versus Ag/AgCl in the coupled system equipped with Bi 2 O 3 @CF, on day 14 of the operation, acetate production achieved 3.77 ± 0.12 g L −1 (Figure 5B), and the corresponding volumetric acetate production rate was 0.269 ± 0.009 g L −1 day −1 . In contrast, Figure 5B reveals an acetate production of 1.90 ± 0.13, 2.54 ± 0.15, and 3.28 ± 0.06 g L −1 under the potentials of −1.0, −1.1, and −1.2 V versus Ag/AgCl, respectively. By comparing the acetate production at variously given potentials on Bi 2 O 3 @CF hybrid biocathode, it showed the highest acetate yield at −1.3 V. This finding represents adjusting applied potential affects the formate selectivity and thus determines the amount of formed acetate.
To explore the effect of the substrate on the performance of the electrosynthesis of acetate, we also carried out a control experiment without loading electrocatalyst. We evaluated acetate production on a bare CF; only an acetate of 2.39 ± 0.04 g L −1 could be detected under the same potential of −1.3 V after 14 days ( Figure 5C). It should be noted that the CO 2 -to-acetate conversion occurred yet in the absence of Bi 2 O 3 electrocatalyst. This can be attributed to CO 2 conversion by biocatalyst via H 2 mediated electron transport way, which occurs unavoidably in the coupled system at this potential. Furthermore, when Bi 2 O 3 /CF was used in this coupled system, the acetate titer under the same condition is 2.91 ± 0.06 g L −1 , and the yield rate was 0.208 ± 0.004 g L −1 day −1 (Figure 5C). By comparing the acetate production at −1.3 V among these three biocathodes, Bi 2 O 3 @CF showed the highest acetate yield, which revealed that the formate-mediated electron transport was the dominated pathway, and the integrated electrode could in situ provide formate.
During the MES process, the current value represents the biotransformation rate. Figure 5D describes the current-time response at different fixed potentials. In the coupled system using Bi 2 O 3 @CF, as the potential shifted more negatively to −1.3 V, the current raised up to 30.50 ± 0.50 mA ( Figure S9). The increase in current against given potential was along with an improvement in acetate concentration. Besides, on day 14 of the experiment, the current density was 11.45 ± 0.95 mA, much higher than 7.75 ± 0.20 mA for Bi 2 O 3 /CF and 6.35 ± 0.55 mA for bare CF. Figure 5E shows OD 600 values recorded along the running time. OD 600 is the optical density measured at a wavelength of 600 nm, which can be used to estimate the concentration of planktonic cells in a cathodic chamber and reflects the microorganism growth situation. 54 With Bi 2 O 3 in situ loading on the CF substrate, OD 600 values exhibit a rising trend along with the bias potential and running time, as shown in Figure 5E. Under −1.3 V versus Ag/AgCl, OD 600 values reached up to 0.142 ± 0.003 on the 14th day, much larger than that for experimental groups of Bi 2 O 3 /CF and CF, suggesting that bacterial reproduction during the continuous operation. Additionally, in the case of Bi 2 O 3 @CF, among various potentials applied, the OD 600 values at −1.3 V are the highest, well matching the acetate production ( Figure S10).
Biofilm thickness reflects the surface attachment of microorganisms, which is an important indicator to evaluate the biocompatibility of electrodes. Thus, we utilized confocal laser scanning microscopy (CLSM) to qualify the 3D structures of biofilms and measured the thickness of biofilm on Bi 2 O 3 @CF, Bi 2 O 3 /CF, and bare CF after running for 14 days of MES experiments. As displayed in Figure 6A, a biofilm coverage on Bi 2 O 3 @CF was F I G U R E 5 (A) Schematic diagram of designing electrochemical-microbial coupled system to reduce CO 2 for the synthesis of acetate; (B) acetate production in the coupled system using Bi 2 O 3 @CF as catalyst an applied bias of −1.0, −1.1, −1.2, and −1.3 V versus Ag/AgCl; acetate production (C), i-t curves (D), and OD 600 value (E) in the coupled system using Bi 2 O 3 @CF, Bi 2 O 3 /CF, and carbon felt (CF) as cathodic electrocatalyst in 14 days at an applied potential of −1.3 V. estimated with 600 μm in z direction from CLSM observation, whereas the biofilm thickness on Bi 2 O 3 /CF and bare CF was 549 and 330 μm, respectively. In general, CLSM analysis visualized demonstrated an increase in biofilm coverage level on Bi 2 O 3 @CF, and the thickening of biofilm suggests cell attachment. Specifically, Bi 2 O 3 @CF were covered by continuous green fluorescence, indicative of the presence of a large amount of living bacteria after experiment yet. 55 By contrast, in the case of Bi 2 O 3 /CF and bare CF, CLSM images ( Figure 6B,C) exhibit a large area of yellow fluorescence, accompanied with some fractions of red fluorescence and pale green fluorescence. The yellow signal originated from the overlapping of green and red fluorescence. 55,56 Especially, CLSM observation revealed that abundant cells died within 2 weeks of operation in the absence of Bi 2 O 3 . These results indicated that Bi 2 O 3 @CF is more favorable for microbial attachment.
To investigate microorganism growth on the surface, the morphology of biofilm at the end of experiments was also studied by SEM. No obvious changes in the morphologies of the electrocatalyst microspheres and fiber substrate were observed after the reaction, except for heavy bacteria loading ( Figure S11). We found that Bi 2 O 3 @CF maintained the original ice-sugar gourd-shaped architecture, substantiating its excellent structural stability. Figure 6D shows a much rougher and denser biofilm on Bi 2 O 3 @CF, whereas relatively smooth and thin biofilms on Bi 2 O 3 /CF ( Figure 6E) and bare CF ( Figure 6F), suggesting contrasting roughness of these three electrodes. Bi 2 O 3 @CF allows for the efficiently enrichment of microorganisms for biofilm formation. A good accordance was achieved between SEM observation and CLSM analysis aforesaid.
In addition, we also quantified the total protein content that is highly associated with biomass growth. By comparing the protein content in Bi 2 O 3 @CF biocathode operated under various potentials for 2 weeks, we found that the protein content at −1.3 V is 86 ± 1 mg L −1 , which is the highest ( Figure S12). After operating at −1.3 V for 2 weeks, Bi 2 O 3 /CF and bare CF exhibited a protein amount of 65 ± 3 and 58 ± 2 mg L −1 , respectively, both lower than that for Bi 2 O 3 @CF ( Figure 6G). Therefore, in view of the above analysis from OD 600 values, CLSM images, SEM images, and protein contents, our result verified a good biocompatibility of Bi 2 O 3 @CF. The unique architecture should be the main reason for the good biocompatibility, leading to sufficient biocatalysts on the surface of the electrode in the hybrid electrochemical biosystem.
We further evaluated the abundance of microbial communities in the cathodic biofilm of these three biocathodes by high-throughput sequencing. Figure 6H and Figure  S13 illustrate the diversity and abundance analysis of microbial communities. At the phylum level ( Figure S13), the dominant abundances in the microbial communities were Epsilonbateraeota, Firmicutes, Proteobacteria, and Bacteroidetes, which are all common microorganisms in MES. At the genus level ( Figure 6H), the relative abundance of Acetobacterium, Arcobacter, Sulfurospirillum, and Bacteroides accounted for the majority in most groups. By comparing with Bi 2 O 3 /CF and bare CF, the highest proportion of Acetobacterium was observed on Bi 2 O 3 @CF in the entire range of potentials. Particularly, the abundance of Acetobacterium is 50.9% and 49.7% at an applied bias of −1.2 and −1.3 V, respectively, which were the top two values among all the experimental groups. This phenomenon was in accordance with the acetate yield. It is well known that Acetobacterium has been regarded to be the main acetogen in the microbial community, which produce acetate through the WL metabolic pathway. 21,22 In this pathway of CO 2 fixation, in the presence of acetogens and electron donors, CO 2 is reduced and joined to acetyl-CoA, which is then turned into end product acetate via acetyl phosphate. 21,22 Consequently, the enrichment of Acetobacterium in microbial community is responsible for the high acetate concentration. However, a decreased abundance of Arcobacter in the cathodic microbial community was recognized on Bi 2 O 3 @CF, generally lower than that on CF and Bi 2 O 3 /CF, which was a completely opposite behavior of what has been observed on Acetobacterium. Arcobacter is recognized as an exoelectrogen that can transfer electrons through the electrode. 57 Low abundance of Arcobacter implied conversion of CO 2 to acetate via formate-related metabolic pathways became the dominant mode of electron transport. Sulfurospirillum is a facultative anaerobic microorganism that can consume oxygen. 58 Bacteroides, an obligately anaerobic fermentative bacterium, is a common microorganism in microbial synthesis. The variation in abundance of microbial communities well explained the higher acetate concentration over Bi 2 O 3 @CF. Taken together, the abundance of formate-electron carriers obtained via the electrocatalysis process, accelerated the conversion toward acetate, and led to more acetogen enriched in the biofilm on the surface of Bi 2 O 3 @CF to digest formate. By optimizing the microbial community, the biocatalytic activity was thereby boosted.

CONCLUSION
In summary, a 3D Bi 2 O 3 @CF integrated electrode with an ice-sugar gourd-shape was fabricated via a straightforward hydrothermal synthesis strategy, wherein Bi 2 O 3 contributed to high selectivity toward formate, and CF ensured the accelerated electron transfer and overall structural stability. This integrated electrode displayed remarkable ECO 2 RR activity with an FE for formate production, delivering an FE of 92.4% at −1.23 V versus Ag/AgCl in 0.5 M KHCO 3 . We further established an electrochemicalmicrobial coupled system equipped with Bi 2 O 3 @CF as an inorganic electrocatalyst and enriched CO 2 -fixing acetogen community as biocatalyst, wherein in situ yielded formate via electrocatalysis serves as energy and carbon source for microorganism. Impressively, due to the unique architectures and good biocompatibility, Bi 2 O 3 @CF cathode provides an ideal surface favorable microorganism attachment and accumulation, leading to an acetate production of 3.77 ± 0.12 g L −1 within 14 days in the coupled system. By building a bridge to link electrocatalysis and microbial catalysis, an increased throughput of CO 2 conversion is achievable with a direct utilization of formate. Overall, this work is expected to give a direction of the material design of a formate-mediated electrochemicalmicrobial system for sustainable CO 2 conversion into high-value-added chemicals as expected.

4.1.1
In situ fabrication of Bi 2 O 3 @CF electrode Typically, a piece of carbon felt (CF) (2 cm × 2 cm × 0.5 cm, length × width × thickness) substrate was treated with suc-cessive ultrasonication in acetone, ethanol and de-ionized water to remove impurities on the surface. In a typical procedure of preparing Bi 2 O 3 @CF integrated electrode, 0.97g of Bi(NO 3 ) 3 ⋅5H 2 O was dissolved in a mixed solution of 28 mL glycol and 14 mL ethanol with magnetic stirring until the solution became transparent. Subsequently, the mixed solution was transferred into a Teflon-lined stainless-steel autoclave (100 mL); whereafter, the bare CF was immersed into the solution. The autoclave was maintained at 160 • C for 8 h, and then was allowed to cool down to room temperature naturally. Afterward, the coated CF was taken out from the solution, and was washed with deionized water for several times to remove any ionic residue and dried at 80 • C in air.

Electrochemical CO 2 reduction measurement
Electrochemical CO 2 reduction measurements on Bi 2 O 3 @CF and Bi 2 O 3 /CF were carried out in a twocompartment H-type cell using a three-electrode set-up, which contains graphite rod counter electrode, Ag/AgCl (3.5 M KCl) reference electrode and working electrode. ECO 2 RR catalytic activities were measured by controlling potentials using chronoamperometry with Biologic VMP3 multichannel potentiostat (Biologic, France). The CO 2 gas flow rate was controlled at 5 sccm using a mass flow controller. At each fixed potential, the generated gas product was diverted and detected in a gas chromatograph online equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID), and the accumulated liquid product in electrolyte was quantitatively determined using nuclear magnetic resonance (NMR) spectroscopy a Bruker AVANCE AV-300 by taking dimethyl sulfoxide (DMSO) as internal standard. The FEs can be calculated as follows: FE = e × F/Q, where e is the number of electrons needed to produce specific product from CO 2 , F is the Faraday constant, n is the total moles of product, Q is total amount of charge during electrolysis. Electrochemical impedance spectroscopy (EIS) analysis was conducted with a frequency range of 10 5 −10 −1 Hz and an amplitude of 5 mV. The ECSA was estimated by measuring the double-layer capacitance (C dl ) of the electrodes by using cyclic voltammetry. The scan rate was varied from 50 to 300 mV s −1 with an interval of 50 mV s −1 in the potential window of the non-Faradaic potential region. The recorded current was plotted as a function of scan rate, and the slop gives the C dl values.

MES experiments
The multichannel MES apparatuses were set up to carried out MES experiments. The measurements were conducted in 100 mL two-chambered H-type reactors separated by proton exchange membranes (Nafion 117, Dupont Co., United States), in accordance with our previous work. [59][60] Three-electrode configurations were employed in all the MES reactors, in which modified CF working electrodes and saturated Ag/AgCl reference electrodes were placed in the cathodic chambers, and Ti meshes coated with Ir and Ru (area of 12.5 cm 2 ) were inserted into the anodic chambers. Three different types of electrodes, including Bi 2 O 3 @CF, Bi 2 O 3 /CF and CF, were employed as working electrodes, and the geometrical areas were controlled to be 2 cm × 2 cm × 0.5 cm for comparing their activities. High purity CO 2 gas was continuously flowed into the cathode side with a flow rate of 3 mL min −1 during the whole processes. Planktonic cells from a long-running MES reactors in our laboratory were trained as inoculum for the MES experiments. The composition of the growth medium was described in our previous work. 60 The pH value of all MES cathodic environment was kept to be around 6.8. All the MES reactors were run for 14 days and the temperature of all MES reactors was kept at 25 ± 2 • C during the operation. Setting potentials ranged from −1.0 to −1.3 V on these three experimental groups with an electrochemical workstation (CHI 1000C, CH Instruments, China). The concentration of produced acetate in the catholyte was detected by HPLC. The biofilms on the cathodes were stained with the LIVE/DEAD BacLight Viability Kit andimaged via Confocal Scanning Laser Microscopy (CSLM, Zeiss LSM710).

Microbial community analysis
The genomic DNA of the biofilm on the Bi 2 O 3 @CF, Bi 2 O 3 /CF and CF cathodes was extracted by using a MO BIO PowerSoil DNA Isolation Kit. The sequencing service was conducted at Magigen Biotechnology Co., Ltd.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.