Method for improving production of volatile fatty acid through electrocatalytic conversion of carbon dioxide by microorganisms

Abstract

The invention discloses a method for producing volatile fatty acid by improving microbial electro-catalytic conversion of CO2. The core of the method is to construct an efficient and stable heterozygote biological cathode. The method comprises the following steps: inoculating flora mainly comprising Acetobacter burgii and Desulfurization Vibrio in a cathode chamber of an H-type microbial electrosynthesis reactor, and adding a zinc source and a sulfur source to synthesize ZnS nanoparticles in situ on the surfaces of cells, so as to construct the bacterium and ZnS nanoparticle heterozygote biological cathode. The acetic acid yield of the heterozygote biological cathode can reach 3.38 mmol / L / d, and the accumulative yield of one batch is 20.25 mmol / L, which is 2.7 times that of an unmodified carbon cloth biological cathode; the coulombic efficiency of the total volatile fatty acid product reaches 96.47% and is 1.25 times that of a carbon cloth biological cathode; and the coulombic efficiency of acetic acid and the selectivity of acetic acid are respectively improved to 83.27% and 86.32%, which are obviously higher than 40.63% and 52.64% of those of a carbon cloth biological cathode. In addition, the charge transfer resistance of the heterozygote biological cathode is as low as 3.75 ohm and is obviously superior to that of a carbon cloth biological cathode (25.28 ohm), so that long-term stable and high-selectivity acetic acid production is realized.

Description

Technical Field

[0001] This invention relates to the field of microbial electrochemistry, specifically to a method for promoting the conversion of carbon dioxide into volatile fatty acids using a microbial in-situ self-synthesized bacterial@ZnS nanoparticle hybrid biocathode. Background Technology

[0002] Climate change has become one of the greatest environmental challenges facing the world, with greenhouse gas emissions being the primary cause of global warming since the mid-20th century. Carbon dioxide (CO2), the most significant greenhouse gas, accounts for 68% of total anthropogenic greenhouse gas emissions. According to the International Energy Agency (IEA), global CO2 emissions are projected to increase by 63% by 2030 compared to 2004. Against this backdrop, CO2 emission reduction has become a critical global issue. While CO2 capture and storage (CFS) technology is considered an effective way to reduce CO2 emissions, the storage process can lead to resource waste. Therefore, converting captured CO2 into valuable chemicals has become an important research direction for improving resource utilization efficiency and addressing climate change.

[0003] Microbial electrosynthesis (MES) technology, as a green and sustainable method for CO2 resource recovery, has attracted widespread attention in recent years. In the MES system, chemoautotrophic bacteria acquire electrons through the cathode, reducing CO2 into high-value-added chemicals. Compared with traditional chemical conversion methods, MES has advantages such as mild reaction conditions, low energy consumption, and high selectivity. Furthermore, microorganisms can generate a large amount of useful biomass resources while utilizing CO2 for their own growth. However, MES technology still faces challenges such as low electron transfer efficiency and poor product selectivity, which limit its practical application.

[0004] Currently, research on improving electron transfer efficiency mainly focuses on two aspects: strain modification and electrode modification, aiming to enhance the electron transfer rate and adhesion of microorganisms at the microbial-electrode interface. Electron carriers such as cytochromes, ferrugins, quinones, and flavins play crucial roles in electron transfer. While engineering strains using synthetic biology techniques can improve electron transmembrane transfer efficiency to some extent, this method suffers from issues such as exogenous gene expression and adaptability, leading to complex and time-consuming implementation processes. Furthermore, due to the difficulty in establishing the binding of microbial biofilms to electrodes in MES systems, research primarily focuses on increasing electrode surface area and improving interfacial interactions to enhance electron transfer efficiency. To promote microbial attachment and biofilm formation, composite materials such as carbon nanotubes, metallic materials, and graphene / polyaniline have been applied to the electrode surface. However, further increasing electrode microbial attachment requires longer reaction times and the need for mid-process replenishment, increasing the reaction burden and economic costs. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of low electron transfer efficiency at the electrode-microorganism interface and poor selectivity of CO2 conversion products in microbial electrosynthesis systems, and to provide a method for promoting the electrocatalytic conversion of carbon dioxide to volatile fatty acids by microorganisms. This method uses in-situ microbial growth to synthesize ZnS nanoparticles on the cell surface, constructing a bacterial@ZnS nanoparticle hybrid biocathode, which significantly improves transmembrane electron transfer, thereby increasing the yield and coulombic efficiency of CO2 reduction to volatile fatty acids.

[0006] The technical solution adopted in this invention is: A method for improving the electrocatalytic conversion of carbon dioxide by microorganisms to produce volatile fatty acids includes the following steps: S1) A dual-chamber H-type reactor is adopted. The reactor is divided into a cathode chamber and an anode chamber by a proton exchange membrane. The cathode is equipped with a carbon cloth electrode and cathodic electrolyte is added. The anode is equipped with a platinum sheet electrode and anodic electrolyte is added. A reference electrode is also provided. The cathode, anode, and reference electrode are connected to the electrochemical workstation; S2) Inoculate the cathode electrolyte with microbial flora, introduce CO2 into the cathode electrolyte in the cathode chamber as an inorganic carbon source, and add a methane inhibitor to the cathode electrolyte; S3) In the first batch of operation after the start-up of the dual-chamber H-type reactor, zinc and sulfur sources are added to the cathode electrolyte in the cathode chamber to enable ZnS nanoparticles to be biosynthesized in situ on the surface of microbial cells, forming a bacterial@ZnS nanoparticle hybrid biocathode. S4) An electrochemical workstation is used to control the cathode potential to carry out a bioelectrochemical reduction reaction, converting CO2 into volatile fatty acids; S5) When the acetic acid concentration in the cathode electrolyte shows a decreasing trend, replace the cathode electrolyte and the anode electrolyte, and add CO2 and methane inhibitor to the replaced cathode electrolyte, and repeat the operation.

[0007] This invention involves inoculating the cathode chamber of an H-type microbial electrosynthesis reactor with a bacterial community primarily composed of *Acetobacter velutipes* and *Desulfovibrio*. By adding zinc and sulfur sources, ZnS nanoparticles are synthesized in situ on the cell surface, thus constructing a bacterial@ZnS nanoparticle hybrid biocathode. This hybrid biocathode reduces CO2 to produce volatile fatty acids, with an acetic acid yield reaching 3.38 mmol / L / d and a cumulative yield of 20.25 mmol / L in a single batch, 2.7 times that of the unmodified carbon cloth biocathode. The coulombic efficiency of the total volatile fatty acid products reaches 96.47%, 1.25 times that of the carbon cloth biocathode. The coulombic efficiency and selectivity of acetic acid are improved to 83.27% and 86.32%, respectively, significantly higher than the 40.63% and 52.64% of the carbon cloth biocathode. Furthermore, the charge transfer resistance of this hybrid biocathode is as low as 3.75 Ω, significantly better than that of the carbon cloth biocathode (25.28 Ω), thus achieving long-term stable and highly selective acetic acid production.

[0008] In step S2), the microbial flora is *Acetobacter wrasse* (…). Acetobacterium woodii ) and desulfurization vibrio ( Desulfovibrio vulgaris The bacterial flora of *Acetobacter wulei*. Acetobacterium woodii (Item No.: HZB495830) can be purchased from the Hangzhou branch of Wuhan Gray Algae Biotechnology Co., Ltd., as desulfurization Vibrio ( Desulfovibrio vulgaris It can be obtained from the Wuhan University Collection Center (CCTCC AB 2021083).

[0009] In step S2), the volume ratio of inoculated microbial community to cathode electrolyte is 1-10:100.

[0010] In step S2), the methane inhibitor is sodium 2-bromoethanesulfonate, and the concentration of the methane inhibitor after being added to the cathode electrolyte is 0.5-2 g / L.

[0011] In step S3), the zinc source is ZnCl2 and the sulfur source is Na2S2O3; After adding zinc and sulfur sources to the cathode electrolyte in the cathode chamber, the concentration of the zinc source is 1-10 mmol / L and the concentration of the sulfur source is 5-20 mmol / L.

[0012] In step S3), the bacterial@ZnS nanoparticle hybrid biocathode comprises a carbon cloth substrate and a biofilm layer attached to the surface of the carbon cloth substrate. The biofilm layer contains microbial cells and ZnS nanoparticles distributed on and inside the microbial cells. The ZnS nanoparticles are formed in situ by biosynthesis within the biofilm layer using zinc and sulfur sources, creating a conductive interface between the biofilm layer and the carbon cloth substrate. The ZnS nanoparticles contain a cubic zincblende structure. The ZnS nanoparticles are distributed on the extracellular surface and at least partially located in the periplasmic space and / or intracellular space.

[0013] In step S3), the particle size of the ZnS nanoparticles is 20~100 nm, preferably 40~70 nm.

[0014] In step S4), the electrochemical workstation controls the cathode potential to be -0.80 to -0.50 V vs SHE, preferably -0.65 V vs SHE.

[0015] In step S4), the volatile fatty acids include acetic acid, butyric acid, and hexanoic acid.

[0016] In step S5), the cycle for replacing the cathode electrolyte and the anode electrolyte is 5-8 days, preferably 6 days.

[0017] Specifically, a method for improving the electrocatalytic conversion of carbon dioxide by microorganisms to produce volatile fatty acids is carried out according to the following steps: (1) MES Dual-Chamber Reactor: An "H"-type MES dual-chamber reactor was used, with carbon cloth as the cathode, inorganic salt solution as the cathode electrolyte, platinum sheet as the anode, phosphate buffer solution as the anode electrolyte, and Ag / AgCl as the reference electrode. A bioelectrochemical reduction reaction was conducted to convert carbon dioxide into volatile fatty acids. After inoculating the cathode electrolyte with microorganisms (mainly *Acetobacter wuerii* and *Vibrio desulfurans*), pure CO2 was introduced as the inorganic carbon source, and 1 g / L sodium 2-bromoethanesulfonate (BrCH2CH2SO3Na) was introduced as a methane inhibitor. Using an electrochemical workstation, the cathode potential was controlled at -0.65 V vs. SHE, and the concentration of volatile fatty acids (mainly acetic acid, butyric acid, and hexanoic acid) in the cathode electrolyte was monitored. Once the concentration of acetic acid in the cathode electrolyte began to decrease, the anode and cathode electrolytes were replaced, and the inorganic carbon source and methane inhibitor were added back to the cathode electrolyte.

[0018] (2) Bacterial@ZnS nanoparticle hybrid biocathode: In the first batch of the MES reaction, zinc and sulfur sources were added to the cathode chamber to induce in-situ biosynthesis of ZnS nanoparticles and form a bacterial@ZnS nanoparticle hybrid biocathode.

[0019] The inorganic salt solution consists of a phosphate buffer solution, a trace element solution, and a vitamin solution.

[0020] The phosphate buffer solution has the following composition: NH4Cl 0.31 g / L, NaH2PO4·H2O 2.452 g / L, Na2HPO4 4.576 g / L, KCl 0.13 g / L, and deionized water as the solvent. The trace element solution comprises: MgSO4 3 g / L, MnSO4·H2O 0.5 g / L, NaCl 1 g / L, FeSO4·7H2O 0.1 g / L, CaCl2·2H2O 0.1 g / L, CoCl2·6H2O 0.1 g / L, ZnCl2 0.13 g / L, CuSO4·5H2O 0.01 g / L, AlK(SO4)2·12H2O 0.01 g / L, H3BO3 0.01 g / L, Na2MoO4 0.025 g / L, and Na2WO4·2H2O 0.025 g / L, with deionized water as the solvent. The vitamin solution comprises: biotin 0.002 g / L, folic acid 0.002 g / L, pyridoxine 0.01 g / L, riboflavin 0.005 g / L, thiamine 0.005 g / L, niacin 0.005 g / L, pantothenic acid 0.005 g / L, B-12 0.0001 g / L, para-aminobenzoic acid 0.005 g / L, and lipoic acid 0.005 g / L, with deionized water as the solvent.

[0021] Further, the carbon cloth pretreatment method in step (1) is as follows: first, immerse the carbon cloth in a mixed solution of nitric acid and deionized water in a volume ratio of 1:1, sonicate for 15-20 min, and then wash the carbon cloth several times with deionized water; then immerse it in an ethanol solution, sonicate again for 15-20 min, and wash it several times with deionized water to complete the pretreatment.

[0022] Furthermore, the distance between the cathode and anode in step (1) is 3-8 cm, preferably 5 cm.

[0023] Furthermore, the reactor in step (1) is divided into a cathode chamber and an anode chamber by a proton exchange membrane, preferably a Nafion 117 membrane.

[0024] Furthermore, the operating temperature range of the reactor in step (1) is 15-40℃, preferably 30℃.

[0025] Furthermore, the pH value of the reactor in (1) is 5.5-8.5, preferably 7.0.

[0026] Furthermore, in step (1), the concentration of microorganisms added to the catholyte is 1-10%, preferably 5%; the concentration of sodium 2-bromoethanesulfonate added is 0.5-2 g / L, preferably 1 g / L.

[0027] Furthermore, the solution in step (1) contains saturated CO2, and the cathode chamber is connected to a 250 mL gas bag filled with CO2 gas to stabilize the pressure inside the reactor.

[0028] Furthermore, in step (1), the cathode electrolyte and anolyte are completely replaced every 6 days to remove dead microorganisms, impurities and organic matter produced by the reaction, so as to reduce the product inhibition effect and improve electron transfer efficiency.

[0029] Furthermore, the zinc source in step (2) is ZnCl2, with a final concentration of 1-10 mmol / L, preferably 5 mmol / L.

[0030] Furthermore, the sulfur source mentioned in step (2) is Na2S2O3, with a final concentration of 5-20 mmol / L, preferably 10 mmol / L.

[0031] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in: (1) This invention synthesizes ZnS nanoparticles in situ during the MES reaction to form a highly efficient bacterial@ZnS nanoparticle hybrid biocathode. The biosynthesis of ZnS nanoparticles significantly improves the electron transfer efficiency at the electrode-microorganism interface and reduces charge transfer resistance. Compared with traditional electrode materials, these in-situ synthesized nanoparticles have a more uniform distribution and more surface active sites, thereby improving the electron utilization efficiency and product selectivity in the CO2 reduction process. The biocathode shows a 2.7-fold increase in acetic acid yield, a 1.64-fold increase in acetic acid selectivity, and a 1.25-fold increase in Faraday efficiency, significantly improving product selectivity and efficiency compared to traditional carbon cloth biocathodes.

[0032] (2) The bacterial@ZnS nanoparticle hybrid biocathode of the present invention does not produce secondary pollution during CO2 reduction, meets environmental protection requirements, and has low energy consumption. Compared with traditional CO2 capture and storage technologies, the present invention not only effectively utilizes the greenhouse gas CO2 through microbial electrosynthesis technology, but also converts it into high-value-added chemicals, such as acetic acid, which has good economic and environmental benefits. In addition, the equipment required for the preparation process is simple, the operation is convenient, the conditions are easy to control, and there is no need to use expensive or complex exogenous genetic engineering methods, making this method more promising for widespread application. Attached Figure Description

[0033] Figure 1SEM images of the unmodified carbon cloth biocathode and the bacterial@ZnS nanoparticle hybrid biocathode in the control example and application example 1. (a) Unmodified carbon cloth biocathode; (b) Bacterial@ZnS nanoparticle hybrid biocathode.

[0034] Figure 2 (a) SEM image of the bacterial@ZnS nanoparticle hybrid in Application Example 1; (b) and (c) TEM images of ultrathin biofilm sections of the bacterial@ZnS nanoparticle hybrid; (d) particle size distribution of ZnS nanoparticles.

[0035] Figure 3 This is a SEM image of the cathode on day 6 in Application Example 1 without microbial inoculation.

[0036] Figure 4 Examples 1 show (a) HAADF-STEM and EDS images of an ultrathin biofilm slice of a bacterial@ZnS nanoparticle hybrid; and (b) magnified HAADF-STEM and EDS images of the cell surface. IM, inner membrane; OM, outer membrane.

[0037] Figure 5 (a) TEM, EDS and lattice fringes characterization of ZnS nanoparticles separated and purified in Example 1; (b) XRD pattern; (c) XPS full spectrum; (d) Zn 2p high-resolution XPS spectrum; (e) S 2p high-resolution XPS spectrum.

[0038] Figure 6 The effect of the bacterial@ZnS nanoparticle hybrid biocathode used in in situ biosynthesis on product distribution and selectivity in Example 2. (a) Cumulative concentrations of acetic acid, butyric acid, and hexanoic acid at the end of 6 cycles (day 36); (b) Faraday efficiency of each product; (c) Acetic acid selectivity of carbon cloth and bacterial@ZnS nanoparticle hybrid biocathode.

[0039] Figure 7 Electrochemical performance of the bacterial@ZnS nanoparticle hybrid biocathode synthesized in situ in Example 3. (a) LSV; (b) CV; (c) EIS. Detailed Implementation

[0040] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto: Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0041] Example for comparison: The control example used unmodified carbon cloth as the cathode electrode. The carbon cloth underwent the following pretreatment: The carbon cloth (Suzhou Shengernuo Technology Co., Ltd., WOS1011) was immersed in a 1:1 mixture of nitric acid and deionized water, sonicated for 15-20 min, and then rinsed several times with deionized water. The carbon cloth was then immersed in an ethanol solution, sonicated again for 15-20 min, and rinsed several times with deionized water to complete the pretreatment. Using an electrochemical workstation (VSP, Bio-Logic, France) and an H-type reactor (Tianjin Gaoshi Ruilian Technology Co., Ltd., C007-1), under optimal environmental conditions (culture temperature 30℃, culture medium pH=7), a 5% (v / v) bacterial solution (mainly containing *Acetobacter wulei* and *Desulfovibrio*) was prepared. Acetobacterium woodii Desulfurization Vibrio was used by the Hangzhou branch of Wuhan Gray Algae Biotechnology Co., Ltd. (HZB495830). Desulfovibrio vulgaris The Wuhan University Collection Center (CCTCC AB 2021083) was used to add 80 mL of inorganic salt solution (phosphate buffer solution + trace element + vitamin solution) to the cathode chamber, and 80 mL of 50 mM phosphate buffer solution to the anode chamber. High-purity nitrogen gas was introduced to remove oxygen from the cathode and anode chambers. In addition, pure CO2 was directly introduced into the cathode chamber as an inorganic carbon source (the solution was eventually saturated with CO2, and the cathode chamber was connected to a 250 mL gas bag filled with CO2 gas) and 1 g / L sodium 2-bromoethanesulfonate (methane inhibitor). The cathode potential was controlled by an electrochemical workstation at -0.65 V vs. SHE.

[0042] Example 1: Preparation of a bacterial@ZnS nanoparticle hybrid biocathode In this embodiment, a microbial electrosynthesis system was used to synthesize ZnS nanoparticles in situ, constructing a bacterial@ZnS nanoparticle hybrid biocathode. An electrochemical workstation (same as the control example) and an H-type reactor (Tianjin Gaoshi Ruilian Technology Co., Ltd., C007-1) were used. Under optimal environmental conditions (culture temperature 30℃, culture medium pH=7), a 5% volume concentration bacterial solution (mainly containing *Acetobacter wulei* and *Desulfovibrio*) was prepared. Acetobacterium woodii Desulfurization Vibrio was used by the Hangzhou branch of Wuhan Gray Algae Biotechnology Co., Ltd. (HZB495830). Desulfovibrio vulgarisThe Wuhan University Collection Center (CCTCC AB 2021083) added 80 mL of inorganic salt solution (phosphate buffer solution + trace elements + vitamin solution) to the cathode chamber, and 80 mL of 50 mM phosphate buffer solution to the anode chamber. High-purity nitrogen gas was introduced to purge oxygen from both the cathode and anode chambers. Pure CO2 was directly introduced into the cathode chamber as a carbon source (the solution was ultimately saturated with CO2, and the cathode chamber was connected to a 250 mL gas bag filled with CO2 gas). The cathode potential was controlled at -0.65 V vs. SHE using an electrochemical workstation. The original cathode electrode was carbon cloth as in the control example, and the anode was a platinum sheet. In the first batch, ZnCl2 and Na2S2O3 were added to the catholyte to a final concentration of 5 mmol / L to induce in-situ biosynthesis of ZnS NPs. After this, these reagents were no longer added, and the cathode after this first batch was designated as the bacterial@ZnS nanoparticle hybrid biocathode.

[0043] Each 1 L of inorganic salt solution is composed of 982.5 mL of phosphate buffer solution, 12.5 mL of trace element solution, and 5 mL of vitamin solution.

[0044] The phosphate buffer solution consists of: 0.31 g / L NH4Cl, 2.452 g / L NaH2PO4·H2O, 4.576 g / L Na2HPO4, and 0.13 g / L KCl, with deionized water as the solvent. The trace element solution consists of: MgSO4 3 g / L, MnSO4·H2O 0.5 g / L, NaCl 1 g / L, FeSO4·7H2O 0.1 g / L, CaCl2·2H2O 0.1 g / L, CoCl2·6H2O 0.1 g / L, ZnCl2 0.13 g / L, CuSO4·5H2O 0.01 g / L, AlK(SO4)2·12H2O 0.01 g / L, H3BO3 0.01 g / L, Na2MoO4 0.025 g / L, and Na2WO4·2H2O 0.025 g / L, with deionized water as the solvent. The vitamin solution consists of: biotin 0.002 g / L, folic acid 0.002 g / L, pyridoxine 0.01 g / L, riboflavin 0.005 g / L, thiamine 0.005 g / L, niacin 0.005 g / L, pantothenic acid 0.005 g / L, B-12 0.0001 g / L, para-aminobenzoic acid 0.005 g / L, and lipoic acid 0.005 g / L, with deionized water as the solvent.

[0045] Depend on Figure 1It can be seen that, compared with the unmodified carbon cloth biocathode, the bacterial@ZnS nanoparticle hybrid biocathode has a richer microstructure, which is conducive to microbial colonization. In the early stage (day 6), the carbon cloth biocathode showed limited microbial attachment and sparse surface coverage, while the bacterial@ZnS nanoparticle hybrid biocathode showed dense microbial colonization accompanied by the accumulation of granular surface features. The nanoparticles were densely fixed on the cell surface and uniformly distributed. Figure 2 a). TEM analysis of ultrathin biofilm slices confirmed the presence of nanoparticles with a regular spatial distribution. Figure 2 b, c). Particle size analysis showed that the ZnS nanoparticles were mainly distributed in the range of 40 ~ 70 nm (b, c). Figure 2 d), with a relatively uniform morphology. In non-biological experiments without inoculation, no particulate nanoparticle deposition was observed on the carbon cloth ( Figure 3 This indicates that the ZnS nanoparticles in this system were synthesized in situ by microorganisms. After removing the nanoparticles from the cell surface, HAADF-STEM analysis showed that the nanoparticles were distributed both outside the microbial cell membrane and within the cell space. Figure 4 a). Notably, some of these nanoparticles were observed to be embedded in the periplasm ( Figure 4 b). Energy dispersive spectroscopy confirms that these nanoparticles are mainly composed of Zn and S ( Figure 4 a, b). Furthermore, after separation and purification, TEM, XRD, and XPS analyses confirmed that the nanoparticles were cubic zincblende ZnS (a, b). Figure 5 ).

[0046] Application Example 2: Volatile Fatty Acid Yield and Coulombic Efficiency of Bacterial@ZnS Nanoparticle Hybrid Biocathode Using an electrochemical workstation (same as the control example) and an H-type reactor (Tianjin Gaoshi Ruilian Technology Co., Ltd., C007-1), under optimal environmental conditions (culture temperature 30℃, culture solution pH=7), a 5% bacterial culture (mainly containing *Acetobacter wuerii* and *Desulfovibrio*) and 80 mL of inorganic salt solution (phosphate buffer solution + trace elements + vitamin solution) were added to the cathode chamber. 80 mL of 50 mM phosphate buffer solution was added to the anode chamber. High-purity nitrogen was introduced to remove oxygen from both the anode and cathode chambers. Additionally, pure CO2 was directly introduced into the cathode chamber as a carbon source (the solution was ultimately saturated with CO2, and the cathode chamber was connected to a 250 mL gas bag filled with CO2 gas). The cathode potential was controlled by the electrochemical workstation at -0.65 V vs. SHE. The cathode electrode was the bacterial@ZnS nanoparticle hybrid biocathode prepared in Example 1. The concentrations of volatile fatty acids (acetic acid, butyric acid, and hexanoic acid) in the cathode solution were detected using gas chromatography. A batch electrolyte replacement method was used during operation. When the acetic acid concentration in each batch showed a decreasing trend, the cathode and anolyte were replaced. The cathode was replaced with 80 mL of fresh inorganic salt solution and saturated CO2 solution, and the anolyte was replaced with 80 mL of phosphate buffer solution. This was repeated for six batches (36 days in total). The results are as follows: Figure 6 As shown, the bacterial@ZnS nanoparticle hybrid biocathode achieved an acetic acid yield of 3.38 mmol / L / d in the 6th experimental cycle (36 days), with a cumulative acetic acid yield of 20.25 mmol / L over 6 days, which is 2.7 times that of the carbon cloth biocathode. Figure 6 a). In contrast, the bacterial@ZnS nanoparticle hybrid biocathode produced only 0.07 mmol and 0.02 mmol of butyric acid and hexanoic acid, respectively, representing only 58% and 33% of the yields from the carbon cloth cathode (0.12 and 0.06 mmol). The volatile fatty acids (C...) of the bacterial@ZnS nanoparticle hybrid biocathode... 2+ The coulombic efficiency of the product was 96.47%, which is 1.25 times that of the carbon cloth biocathode (77.10%). Figure 6 b). The coulombic efficiency and acetic acid selectivity of the bacterial@ZnS nanoparticle hybrid biocathode reached 83.27% and 86.32%, respectively, which were 2.05 times and 1.64 times that of the carbon cloth biocathode. Figure 6 c). It can be seen that the bacterial@ZnS nanoparticle hybrid biocathode prepared in Example 1 significantly improved transmembrane electron transfer, thereby increasing the yield and coulombic efficiency of CO2 reduction to volatile fatty acids, and improving the product selectivity of acetic acid.

[0047] Gas chromatography detection conditions: 7890N GC (Agilent Technologies, USA), using an HP Innowax capillary column (30 m × 320 μm × 0.5 μm) and a flame ionization detector (FID). N2 carrier gas was supplied at a flow rate of 40 mL / min, air at 400 mL / min, and hydrogen at 30 mL / min. The injector and detector temperatures were set to 250 °C and 200 °C, respectively. Column temperature settings were as follows: initial hold at 40 °C for 2 minutes, then ramp up to 180 °C at 10 °C / min, and finally hold for 4 minutes.

[0048] Application Example 3: Electrochemical Performance of Bacterial@ZnS Nanoparticle Hybrid Biocathode Electrochemical performance was tested in a three-electrode electrolytic cell using a cathode, anode, and reference electrode. The cathode was a bacterial@ZnS nanoparticle hybrid biocathode or a carbon cloth biocathode, the anode was a platinum sheet, and the electrolyte was a phosphate buffer solution. The bacterial@ZnS nanoparticle hybrid biocathode and the carbon cloth biocathode were subjected to three-electrode electrochemical tests in a 50 mM phosphate buffer solution to investigate the electrochemical performance of the carbon cloth electrode of the control example and the bacterial@ZnS nanoparticle hybrid biocathode prepared in Example 1. Using an electrochemical workstation (same as the control example), the bacterial@ZnS nanoparticle hybrid biocathode prepared in Example 1 or the carbon cloth biocathode of the control example was used as the working electrode, the platinum sheet (2×2.5×0.1 cm) as the counter electrode, and Ag / AgCl (+0.197 V vs. SHE) as the reference electrode. The LSV curve (…) is shown in the figure. Figure 7 As shown in a), the onset potential of the bacterial@ZnS nanoparticle hybrid biocathode (-0.57 V vs. SHE) is greater than that of the carbon cloth biocathode (-0.65 V vs. SHE), indicating that cathode reactions such as CO2 reduction and hydrogen evolution are more easily initiated. At an operating potential of -0.65 V vs. SHE, the current density of the bacterial@ZnS nanoparticle hybrid biocathode reaches -2.69 mA / cm². 2 This is 2.64 times that of a carbon cloth biocathode. Under strong polarization conditions (-0.8V vs. SHE), the maximum current density of the bacterial@ZnS nanoparticle hybrid biocathode reaches -8.44 mA / cm². 2 This indicates that it exhibits strong catalytic activity under different conditions. CV curve ( Figure 7 (b) shows that there are obvious reduction peaks at -0.40V vs. SHE and -0.20V vs. SHE, and the reduction peak of the bacterial@ZnS nanoparticle hybrid biocathode is significantly larger than that of the carbon cloth biocathode. The results indicate that ZnS nanoparticles improve conductivity and provide additional redox active sites, promoting CO2 reduction and hydrogen evolution. EIS analysis ( Figure 7c) This inference is further supported by the fact that the charge transfer resistance of the bacterial@ZnS nanoparticle hybrid biocathode is only 3.75 Ω, while that of the carbon cloth biocathode is 25.28 Ω.

[0049] Although the present invention has been disclosed above with reference to embodiments, it is not intended to limit the scope of protection of the present invention. Any modifications and refinements made by those skilled in the art without departing from the concept and scope of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A method for improving the microbial electrocatalytic conversion of carbon dioxide to volatile fatty acids, characterized in that, Includes the following steps: S1) A dual-chamber H-type reactor is adopted. The reactor is divided into a cathode chamber and an anode chamber by a proton exchange membrane. The cathode is equipped with a carbon cloth electrode and cathodic electrolyte is added. The anode is equipped with a platinum sheet electrode and anodic electrolyte is added. A reference electrode is also provided. The cathode, anode, and reference electrode are connected to the electrochemical workstation; S2) Inoculate the cathode electrolyte with microbial flora, introduce CO2 into the cathode electrolyte in the cathode chamber as an inorganic carbon source, and add a methane inhibitor to the cathode electrolyte; S3) In the first batch of operation after the start-up of the dual-chamber H-type reactor, zinc and sulfur sources are added to the cathode electrolyte in the cathode chamber to enable ZnS nanoparticles to be biosynthesized in situ on the surface of microbial cells, forming a bacterial@ZnS nanoparticle hybrid biocathode. S4) An electrochemical workstation is used to control the cathode potential to carry out a bioelectrochemical reduction reaction, converting CO2 into volatile fatty acids; S5) When the acetic acid concentration in the cathode electrolyte shows a decreasing trend, replace the cathode electrolyte and the anode electrolyte, and add CO2 and methane inhibitor to the replaced cathode electrolyte, and repeat the operation.

2. The method for improving the microbial electrocatalytic conversion of carbon dioxide to volatile fatty acids according to claim 1, characterized in that, In step S2), the microbial community uses *Acetobacter wrasse*. Acetobacteriumwoodii and desulfuric vibrio Desulfovibrio vulgaris .

3. The method for improving the microbial electrocatalytic conversion of carbon dioxide to volatile fatty acids according to claim 1, characterized in that, In step S2), the volume ratio of inoculated microbial community to cathode electrolyte is 1-10:

100.

4. The method for improving the microbial electrocatalytic conversion of carbon dioxide to volatile fatty acids according to claim 1, characterized in that, In step S2), the methane inhibitor is sodium 2-bromoethanesulfonate, and the concentration of the methane inhibitor after being added to the cathode electrolyte is 0.5-2 g / L.

5. The method for improving the microbial electrocatalytic conversion of carbon dioxide to volatile fatty acids according to claim 1, characterized in that, In step S3), the zinc source is ZnCl2 and the sulfur source is Na2S2O3; After adding zinc and sulfur sources to the cathode electrolyte in the cathode chamber, the concentration of the zinc source is 1-10 mmol / L and the concentration of the sulfur source is 5-20 mmol / L.

6. The method for improving the microbial electrocatalytic conversion of carbon dioxide to volatile fatty acids according to claim 1, characterized in that, In step S3), the bacterial@ZnS nanoparticle hybrid biocathode includes a carbon cloth substrate and a biofilm layer attached to the surface of the carbon cloth substrate. The biofilm layer contains microbial cells and ZnS nanoparticles distributed on the surface and inside of the microbial cells. The ZnS nanoparticles are formed in situ biosynthesized within the biofilm layer by zinc and sulfur sources, thereby forming a conductive connection interface between the biofilm layer and the carbon cloth substrate.

7. The method for improving the microbial electrocatalytic conversion of carbon dioxide to volatile fatty acids according to claim 6, characterized in that, In step S3), the particle size of the ZnS nanoparticles is 20~100 nm.

8. The method for improving the microbial electrocatalytic conversion of carbon dioxide to volatile fatty acids according to claim 1, characterized in that, In step S4), the electrochemical workstation controls the cathode potential to be -0.80 to -0.50 V vs SHE.

9. The method for improving the microbial electrocatalytic conversion of carbon dioxide to volatile fatty acids according to claim 1, characterized in that, In step S4), the volatile fatty acids include acetic acid, butyric acid, and hexanoic acid.

10. The method for improving the microbial electrocatalytic conversion of carbon dioxide to volatile fatty acids according to claim 1, characterized in that, In step S5), the cycle for replacing the cathode electrolyte and the anolyte is 5-8 days.

Images