Liquid electrode suitable for amplification of microbial electrolysis cell and carbon sequestration device of liquid electrode

Abstract

The invention relates to a liquid electrode suitable for amplification of a microbial electrolysis cell and a carbon sequestration device of the liquid electrode, and belongs to the field of microbial electrochemical application. The liquid electrode is composed of a solid electrode and humic acid dissolved in a reaction system. As an electron shuttle and a pseudocapacitance energy storage medium, humic acid can efficiently receive electrons from a solid-state electrode and uniformly transmit the electrons to microorganisms in a dissolved state at a long distance in the space of the reactor; therefore, the scale effect problem that the electron transfer efficiency is reduced due to the fact that the size of the microbial electrolysis cell is increased, the electrode distance is increased and the electrode area per unit volume is reduced is fundamentally solved. The carbon sequestration device constructed on the basis of the liquid electrode integrates an autogenous dynamic membrane component taking a stainless steel mesh as a base material and a reverse osmosis product recovery system. The dynamic membrane can effectively intercept humic acid and microorganisms, selectively discharge small molecular products and relieve product inhibition; and the reverse osmosis system realizes high-efficiency recovery and process closed loop of products, and achieves zero emission.

Description

Technical Field

[0001] This invention relates to a liquid electrode suitable for the scale-up of microbial electrolysis cells and its carbon fixation device, belonging to the field of microbial electrochemical applications. Background Technology

[0002] Microbial electrolysis cells, as an emerging electrochemical enhancement strategy, have shown broad application potential in the fields of biosynthesis and carbon fixation. By introducing an external low voltage field (<1.0 V) into the bioreactor system and constructing an anode-cathode interface, microbial electrolysis cells can effectively regulate the microbial community structure and electron transport pathways, promote efficient extracellular electron transport, and thus accelerate the reduction of one-carbon gases into organic compounds such as acetic acid and ethanol by microorganisms.

[0003] Despite the significant advantages of microbial electrolyzers, their large-scale application still faces serious challenges. The core issue lies in the fact that as the reactor volume increases, the electrode area per unit volume (S / V) decreases significantly, and the electrode spacing increases accordingly. This leads to an increase in the system's ohmic resistance and a lengthening of the electron transport path, thus severely weakening electron transport efficiency—a significant "scale effect." Currently, most related research remains at the milliliter to scalable laboratory scale, making engineering scale-up difficult.

[0004] In existing technologies, the main approaches to addressing the "scale effect" include increasing the electrode area per unit volume or increasing the applied voltage. However, increasing the electrode area leads to an exponential increase in investment costs, and the large electrode surface is susceptible to contamination and hindered mass transfer, resulting in poor long-term operational stability of the system. Increasing the applied voltage, on the other hand, exacerbates side reactions such as water electrolysis, causing severe energy loss and reduced energy efficiency. Furthermore, existing technologies also employ methods to construct "scale-up devices" by physically connecting multiple small-scale devices in series. For example, CN118878059A modifies the electrode surface with specific nanomaterials to enhance the electron transport rate and bacterial load at individual interfaces, and increases the overall treatment scale through series connection. However, this does not address the fundamental bottleneck of a single reactor experiencing a dramatic increase in internal resistance and a decrease in electron transport efficiency due to increased electrode spacing and reduced electrode area per unit volume as the volume (scale) of the reactor increases.

[0005] Therefore, there is an urgent need for a new method or device that can fundamentally solve the bottleneck of electron transfer during the scale-up of microbial electrolysis cells, so as to promote the large-scale application of this technology. Summary of the Invention

[0006] To address the aforementioned problems in existing technologies, this invention innovatively proposes the concept of a liquid electrode for a microbial electrolysis cell. By introducing a biocompatible soluble electron storage element, electrons from the solid electrode can be transferred to microorganisms over long distances, providing conditions for increasing the electrode spacing and expanding the reactor capacity.

[0007] The first aspect of the present invention provides a liquid electrode suitable for the scale-up of microbial electrolysis cells, comprising: a solid electrode; and humic acid dissolved in a reaction system; wherein the humic acid serves as an electron shuttle and a pseudocapacitive energy storage medium, for receiving electrons from the solid electrode and transferring electrons to microorganisms in a dissolved state within the reaction system, thereby overcoming the electron transfer bottleneck caused by the increase in volume of the microbial electrolysis cell.

[0008] Humic acid is an ideal electron storage element for liquid electrodes in microbial electrolysis cells. It is a natural organic compound with excellent biocompatibility; after treatment, humic acid can dissolve in water; and its molecules contain numerous electroactive groups such as quinones, enabling it to store electrons through pseudocapacitance. Humic acid can rapidly acquire electrons (protons) from the surface or attachments of solid electrodes, and then, driven by the electrode potential, travel back and forth between the electrodes, thus achieving efficient electron transfer between microorganisms and the electrodes.

[0009] A second aspect of the present invention is to address the problems of difficulty in electrode expansion, electron transfer obstruction, and difficulty in electrochemical microbial enrichment during the large-scale application of microbial electrolysis cells, by providing a carbon fixation device based on the liquid electrode.

[0010] A carbon fixation device for scaling up a microbial electrolysis cell includes: a reactor, a liquid electrode, a self-generating dynamic membrane module, and an air inlet device. The reactor is a sealed structure. The liquid electrode, disposed within the reactor, is used to maintain efficient electron transfer during reactor scaling-up. The liquid electrode comprises a solid electrode and humic acid dissolved in the reaction system. The humic acid acts as an electron shuttle and a pseudocapacitive energy storage medium, receiving electrons from the solid electrode and transferring electrons to microorganisms in a dissolved state within the reaction system. The air inlet device is used to introduce carbon-containing gas into the reactor. The self-generating dynamic membrane module, disposed within the reactor, is used to retain the humic acid and discharge liquid containing organic products. The carbon fixation device overcomes the decrease in electron transfer efficiency caused by the increase in reactor volume through the liquid electrode, achieving large-scale scaling-up and efficient carbon fixation of the microbial electrolysis cell.

[0011] In some embodiments, the carbon source in the carbon-containing gas is selected from one or a combination of carbon dioxide and carbon monoxide.

[0012] Furthermore, the solid electrode includes an anode and a cathode; the anode is carbon cloth and is disposed on the inner wall of the reactor; the cathode is made of stainless steel mesh and serves as the substrate of the self-generating dynamic membrane assembly.

[0013] Furthermore, the self-generating dynamic membrane assembly is fixedly connected to a hollow fixed rod, and the stainless steel mesh serves as the membrane substrate. It forms a self-generating dynamic membrane by filtering suspended solids and microbial adhesion. The membrane effluent is discharged from the reactor through the hollow stirring rod.

[0014] Furthermore, the self-generating dynamic membrane module has a hollow cylindrical double-layer structure with a cavity, which is connected to the water outlet pipe; the inner and outer walls of the cavity are both made of stainless steel mesh, which serves as the membrane substrate; through the filtration of suspended solids to form a sludge cake layer and the attachment and growth of microorganisms, a self-generating dynamic membrane is formed on the stainless steel mesh.

[0015] Furthermore, it also includes a product recovery system connected to the outlet of the self-generating dynamic membrane module for separating and concentrating acetic acid and / or ethanol from the effluent and returning the separated liquid to the reactor.

[0016] Furthermore, the product recovery system includes a reverse osmosis unit and a storage tank.

[0017] Furthermore, the electrode area per unit volume (S / V) in the reactor is ≥0.5, and the distance between the cathode and the anode is ≤500 mm; Furthermore, the membrane substrate of the self-generating dynamic membrane module has a pore size of 100-200 mesh, and the effluent flux is controlled at 0.5-50 m³ / s. 3 / (m 2 ·h).

[0018] A third aspect of the present invention is to provide a carbon fixation method based on the aforementioned carbon fixation apparatus, comprising the following steps: (1) Add a solution containing microorganisms and humic acid to the reactor; (2) Carbon-containing gas is introduced into the reactor through the air inlet device; (3) Apply an external voltage of 0.2-2.0 V to the liquid electrode to initiate the reaction; (4) The product liquid containing acetic acid and / or ethanol is discharged through the self-generating dynamic membrane module, and humic acid is retained and recycled in the reactor.

[0019] Furthermore, the carbon fixation method of the present invention further includes: performing membrane separation on the discharged product liquid to recover concentrated acetic acid and / or ethanol, and returning the separated liquid to the reactor.

[0020] Furthermore, the concentration of humic acid in the reactor is 0.5-2.0 g / L.

[0021] Furthermore, the microorganisms in the reactor are strains capable of efficiently converting carbon-containing gases into organic compounds such as acetic acid and ethanol, with their concentration controlled between 1.0 and 20.0 g / L. In some embodiments, the microorganisms are selected from *Clostridium aceticum* and *Acetobacterium woodii*.

[0022] In some preferred embodiments, the present invention is applicable to liquid electrode carbon fixation devices for scale-up microbial electrolysis cells, wherein the total concentration of organic matter such as acetic acid in the reactor is ≤100.0 g / L, and the concentration of humic acid is controlled at 0.5-2.0 g / L.

[0023] In some preferred embodiments, the present invention is applicable to liquid electrode carbon fixation devices for scale-up microbial electrolysis cells, where the membrane resistance of the dynamic membrane is controlled to be ≤1.0×10⁻⁶ by adjusting the gas injection rate and stirring speed. 10 m -1 .

[0024] In some preferred embodiments, the present invention is applicable to liquid electrode carbon fixation devices for the scale-up of microbial electrolysis cells, wherein the electrode area (S / V) is ≥0.5 and the distance between the cathode and the anode is ≤500 mm.

[0025] In some preferred embodiments, the present invention is applicable to liquid electrode carbon fixation devices for scale-up microbial electrolysis cells, wherein the recovery rate of organic matter such as acetic acid is ≥80%, and the concentration of organic matter such as acetic acid in the concentrate is ≥20%.

[0026] Beneficial effects: (1) This invention effectively alleviates the scale-up bottleneck of microbial electrolysis cells by introducing a humic acid-based liquid electrode. Utilizing the dissolved state distribution and pseudocapacitive energy storage characteristics of humic acid, it achieves long-distance, uniform electron transfer within the reactor space. This solves the problems of increased resistance and decreased electron transfer efficiency caused by the increased electrode spacing and decreased electrode area per unit volume (S / V) due to the increase in reactor volume. This liquid electrode reduces the system's internal resistance by 2.15 times and increases the scale-up limit of microbial electrolysis cells by 5-10 times. At the same time, the good biocompatibility of humic acid enables it to transfer electrons to non-electroactive microorganisms, reducing the severe dependence on electroactive microorganisms.

[0027] (2) The carbon fixation device constructed in this invention integrates a self-generating dynamic membrane and a reverse osmosis system to achieve efficient biological carbon fixation and product recovery. The device integrates a self-generating dynamic membrane and a reverse osmosis membrane system. The dynamic membrane effectively retains humic acid and microorganisms while selectively discharging small molecule products, thus relieving product inhibition. The reverse osmosis system achieves efficient product recovery and a closed-loop process, achieving zero emissions. Without an external electron donor, this device can achieve efficient bioconversion of carbon dioxide, with a conversion efficiency of 31.9% and an electron utilization rate of 60.9%. (3) The liquid electrode concept and integrated carbon fixation device proposed in this invention provide an innovative solution to the core problems of electrode expansion difficulties, electron transfer obstruction and electrochemical microbial enrichment difficulties in the large-scale application of microbial electrolysis cells, and promote the engineering and industrialization process of this technology. Attached Figure Description

[0028] The accompanying drawings are provided to further illustrate the present application and form part of the specification. They are used together with the embodiments of the present application to explain the present application, but do not constitute a limitation thereof. In the drawings: Figure 1 A schematic diagram of the enlarged liquid electrode and carbon fixation device of the microbial electrolysis cell of the present invention; Figure 2 Example 2: The effect of liquid electrodes on improving the electrochemical performance during the scale-up of microbial electrolysis cells; Figure 3 Example 2: The effect of liquid electrodes on improving the charge spatial distribution and large-scale electrical performance of microbial electrolysis cells; Figure 4 Example 3: Effect diagram of CO2 to acetic acid conversion mediated by liquid electrode in microbial electrolysis cell; Figure 5 Example 4: Effect diagram of CO2 conversion to acetic acid in a microbial electrolysis cell mediated by a liquid electrode. Detailed Implementation

[0029] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification. Many specific details are set forth in the following description to provide a thorough understanding of the present invention; however, the present invention may also be implemented in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0030] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and instruments described are commercially available unless otherwise specified.

[0031] Description of some test methods in the embodiments of this invention: (1) Extraction method of humic acid from sludge Humic acid was extracted from the sludge using the extraction method specified by IHSS. The dehydrated sludge and the sludge hydrolysate obtained after hot hydrolysis at 180°C were freeze-dried and ground into powder, and the following steps were performed sequentially: First, add an appropriate amount of 6 mol / L HCl solution to the obtained humic acid powder to make the pH of the dissolved humic acid = 1.0. Then, add 0.1 mol / L HCl solution to make the volume ratio of humic acid solution to humic acid powder 10:1. Next, place the humic acid solution in a 35℃ air bath shaker and shake for 1 h, then remove and allow it to stand to collect the precipitate. Then, under N2 aeration, dissolve the precipitate with 6 mol / L NaOH, and shake the mixture in a shaker for 15 hours. Remove and allow it to stand to precipitate, obtaining the supernatant. Then, adjust the pH of the supernatant to 1.0 with 6 mol / L HCl, let it stand for 15 hours, and centrifuge to separate the precipitate into humic acid extract and the supernatant into fulvic acid extract.

[0032] Purification of humic acid: The humic acid extract was completely dissolved in 0.1 mol / L KOH, and then potassium chloride was added to achieve a potassium ion concentration of approximately 0.3 mol / L. The mixture was then allowed to stand and precipitate. If impurities were present, the humic acid solution was centrifuged to remove suspended impurities before further purification. If no impurities were present, the humic acid solution was directly adjusted to pH 1.0 with 6 mol / L HCl. After standing for 15 hours, the solid extract was obtained by centrifugation. The humic acid extract was then dissolved in a mixture of 0.1 mol / L HCl and 0.3 mol / L HF (volume ratio 1:1). The dissolved humic acid solution was shaken in a shaker for 12 hours, then removed and centrifuged to obtain the humic acid precipitate. Finally, the precipitate was washed repeatedly with ultrapure water to remove chloride ions. Silver nitrate was used to detect the formation of silver chloride precipitate until no precipitate was formed. Finally, the humic acid extract was freeze-dried into powder using vacuum freeze-drying technology.

[0033] (2) Electrochemical impedance spectroscopy method for humic acid The testing instrument was a Chenhua CHI660E electrochemical workstation. Humic acid was used as the working electrode (WE), carbon cloth on the reactor wall as the counter electrode (CE), and a silver / silver chloride (Ag / AgCl) electrode as the reference electrode (RE). 1 mol / L H₂SO₄ was used as the electrolyte. After the system potential stabilized, the test was conducted at the open circuit potential (OCP) or the constant operating bias voltage of the device. The AC signal perturbation amplitude was set to 10 mV, and the scanning frequency range was 10... 5 Hz to 10 -2The impedance real part (Z') and imaginary part (Z'') were recorded in real time at Hz, and Nyquist spectra were plotted. After the test, the experimental data were fitted using electrochemical analysis software (such as ZView) combined with an equivalent circuit model. Method for fixing the humic acid working electrode: Humic acid, acetylene black, and polytetrafluoroethylene (PTFE) were mixed in a mass ratio of 75:15:10 (wt:wt:wt), and ethanol was added to prepare an electrode slurry. The slurry was then coated onto carbon paper (1 cm × 1 cm) and dried under vacuum at 60°C. The electrode loading was controlled at approximately 2 mg / cm². 2 .

[0034] (3) Test method for electron quantity of humic acid The electron quantity of humic acid described in this invention is measured using a mediator electrochemical oxidation / reduction method. A three-electrode system is constructed within an anaerobic operating chamber using an electrochemical workstation. A glassy carbon electrode is used as the working electrode, a platinum sheet electrode as the counter electrode, and a silver / silver chloride (Ag / AgCl) electrode as the reference electrode. For EAC testing, a 0.1 mol / L phosphate buffer solution (pH=7.0) is used as the supporting electrolyte. Mediator violet (MV) or neutral red is added, and reduction electrolysis is performed at a constant potential. After the background current stabilizes, a known mass of humic acid sample is added. The change curve of the reduction current generated by the oxidation-reduction reaction over time is recorded using chronoamperometry until the current returns to the background level. For EDC testing, the mediator ABTS is used for oxidation electrolysis at a high potential (e.g., +0.5V vs. Ag / AgCl), and the sample is added. The values ​​of EAC and EDC are calculated by integrating the it curve using formulas (1) and (2).

[0035] (1) (2) Where Ired and Iox (representing the baseline-corrected reduction and oxidation currents in MER and MEO, respectively) are respectively, F (96485 s A / mol) is the Faraday constant, and m (g) is the mass of added HA.

[0036] (4) Methods for determining the concentration of volatile fatty acids The composition and concentration of volatile fatty acids (VFAs) were analyzed by gas chromatography. Anaerobic digest samples were centrifuged at 8000 rpm for 15 min at 4 °C. The supernatant was filtered through a 0.22 μm aqueous membrane to remove impurities. The filtrate was diluted appropriately, and an equal volume of 3 mol / L phosphoric acid was added for acidification. After thorough mixing, the mixture was stored at -20 °C for a short period. High-performance gas chromatography was used to determine the concentration and composition of VFAs in the pretreated samples. The results were presented as the concentration of each type of VFA, with the total concentration being the sum of all VFAs.

[0037] In this embodiment of the invention, the distance between the cathode and the anode is set to 70 mm.

[0038] Example 1: Structure and Operation of a Liquid Electrode Carbon Fixation Device This embodiment provides a liquid electrode and its carbon fixation device suitable for the scale-up of microbial electrolysis cells, and the material flow relationship is as follows: Figure 1 As shown.

[0039] The carbon fixation device has a sealed reactor for loading a liquid-phase reaction system containing functional microorganisms. An inlet is located at the top of the reactor for replenishing the nutrient solution containing humic acid; an inlet is located at the bottom for introducing carbon-containing gas. A stirring device is installed at the bottom of the reactor to ensure uniform mixing of the reaction solution and dissolved humic acid.

[0040] Inside the reactor, a self-generating dynamic membrane module is installed. This module is fixedly connected to the center of the reactor top via an independent discharge pipe. The self-generating dynamic membrane module has a hollow cylindrical double-layer structure with a cavity connected to the effluent pipe; both the inner and outer walls of the cavity are made of stainless steel mesh, serving as the membrane substrate. During operation, suspended solids (including microbial cells and humic acid macromolecules) in the reactor are filtered and adhere to the surface of the stainless steel mesh, forming a biological sludge cake layer with filtration function, i.e., the "self-generating dynamic membrane." This dynamic membrane has a dual function: first, it retains large-molecule humic acid and microbial cells, allowing them to be recycled within the reactor; second, it allows small-molecule metabolites (such as acetic acid and ethanol) and water to permeate. The membrane effluent containing the products is discharged from the bottom side wall of the reactor through the discharge pipe and enters the subsequent storage tank.

[0041] Carbon cloth is fixedly installed on the inner wall of the reactor, serving as the anode of the electrochemical system. The stainless steel mesh substrate of the self-generating dynamic membrane module serves as the cathode of the system. A specific concentration of humic acid is added to the reactor. The humic acid dissolves in the reaction solution, and its electroactive groups, such as quinones, can receive and store electrons from the solid cathode in the form of pseudocapacitance. Since the humic acid is uniformly distributed in a dissolved state throughout the reactor space, it can flow and shuttle under the drive of potential, thereby transferring electrons over long distances and efficiently to microorganisms throughout the reactor. The solid cathode (stainless steel mesh), the solid anode (carbon cloth), and the dissolved humic acid together constitute the core of this invention—the liquid electrode system.

[0042] After the dynamic membrane effluent containing products such as acetic acid and ethanol enters the storage tank, it undergoes deep separation through a timed reverse osmosis membrane system to obtain a high-concentration acetic acid / ethanol concentrate. The filtrate produced by reverse osmosis is then returned to the reactor inlet. Simultaneously, unreacted gas discharged from the top of the reactor is circulated back to the bottom inlet via pipeline.

[0043] Working Process: Power is turned on, and an external voltage of 0.2-2.0 V is applied between the anode (carbon cloth) and the cathode (dynamic membrane stainless steel mesh). Humic acid gains electrons at the cathode, becoming reduced, and diffuses throughout the reactor with the agitation of the bottom stirrer, transferring electrons to the microorganisms. The microorganisms use these electrons and carbon-containing gas introduced from the bottom to synthesize organic compounds such as acetic acid. The synthesized small molecule products are discharged through the dynamic membrane with the water flow and concentrated and recovered by the reverse osmosis system. The retained humic acid and microorganisms circulate within the reactor, and unreacted gases are also recycled.

[0044] Example 2: Improvement of electrochemical performance of microbial electrolytic cells (MEC) during scale-up by humic acid liquid electrodes By constructing reaction systems with different effective electrode area ratios (S / V), a pure electrolyte control group (Control) and an experimental group with added humic acid were set up to investigate the effect of the humic acid flowing electrode on the scale-up performance of a microbial electrolyzer (MEC). The experimental group was treated with 1.0 g / L humic acid as the flowing liquid electrode, while the control group used only phosphate buffer electrolyte of the same concentration. The electrochemical impedance spectroscopy (EIS), polarization curves (Tafel), and open-circuit voltage at different spatial locations were monitored under S / V conditions of 0.5, 2, and 5. The entire experiment was conducted under an applied voltage of 2 mA.

[0045] like Figure 2As shown in (a / b), the humic acid liquid electrode significantly enhances the electrochemical kinetics of the system. Regarding the Tafel kinetic parameters, as the S / V ratio decreases from 5 to 0, the Tafel slope of the control group increases dramatically from 0.67 V / dec to 1.49 V / dec, indicating that the reduction in electrode area leads to a significant increase in activation overpotential. In contrast, the Tafel slope of the experimental group, with the same electrode area, is only 0.12 V / dec to 0.40 V / dec, far lower than that of the control group, demonstrating the highly efficient electron transfer rate of the humic acid liquid electrode.

[0046] Regarding impedance characteristics, such as Figure 2 As shown in (c / d), the total system resistance of the control group increased dramatically and nonlinearly as the S / V ratio decreased, and the charge transfer resistance (Rct) increased by approximately 1.4 times, exhibiting a severe scale effect. In contrast, the experimental group with the added humic acid liquid electrode showed an average decrease of 2.15 times in Rct across all electrode areas compared to the control group; under the low area ratio condition of S / V=0.5, the internal resistance of the experimental group was reduced by approximately 25.9% compared to the control group. This indicates that the introduction of humic acid as a liquid electrode effectively reduced the internal resistance of the microbial electrolysis cell during scale-up and significantly improved the electrode reaction kinetics.

[0047] Voltage response during constant current operation, such as Figure 3 As shown in (a / b), the real-time voltage variation trends of the experimental and control groups differed significantly under different electrode area ratios. In the control group, the system voltage increased sharply with decreasing S / V, reaching a stable operating voltage of approximately 2.6V at a low area ratio of S / V=0.5, exhibiting extremely strong polarization resistance. In contrast, the experimental group showed significantly optimized operating voltage, with stable voltages of only approximately 1.6V, 1.1V, and 0.9V at S / V ratios of 0.5, 2, and 5, respectively. This means that at the same scale (S / V=0.5), the experimental group reduced operating energy consumption by approximately 1.0V compared to the control group. Simultaneously, the system internal resistance results also indicate that, under the same S / V conditions, the presence of the humic acid liquid electrode can reduce the system internal resistance by 27.5%-38.2% (…). Figure 3 c). Regarding the spatial potential distribution ( Figure 3 d) The control group showed obvious spatial transmission limitation, with a potential decay rate of over 121.6% (relative to the electrode surface potential) at a distance of 2.5 cm from the solid electrode; while the experimental group maintained a relatively uniform open-circuit voltage throughout the entire reactor space.

[0048] The above results indicate that during the scale-up of microbial electrolysis cells (MECs), traditional solid-state electrodes are limited by a point-to-surface electron transfer mode, preventing regions far from the electrode surface from participating in redox reactions, resulting in severe electrode polarization and kinetic losses. The humic acid liquid electrode introduced in this invention utilizes pseudocapacitive energy storage through its abundant quinone and other active groups in its macromolecular structure. These results verify that the humic acid liquid electrode can achieve efficient large-scale scale-up of MECs without increasing the solid-state electrode area, through pseudocapacitive and diffusion effects; it solves the "scale effect" of traditional solid-state electrodes, increasing the effective scale-up limit of MECs by 5-10 times.

[0049] Example 3: Liquid electrode promotes carbon fixation and acid production by electroactive homoacetic bacteria. This embodiment uses the carbon fixation device described in claim 1, inoculating *Clostridium aceticum* as an electroactive homoacetic-producing functional bacterium, with an initial biomass concentration controlled at 10 g / L and humic acid dosage at 1 g / L. To verify the enhanced carbon fixation mechanism of the humic acid liquid electrode, four parallel experiments were set up for comparison: a blank control group (CK) without applied current and no humic acid, a hydrogen-charged group (H2) with continuous external H2 supply, a control group (HAs) with only 1 g / L of undisturbed humic acid added but no current applied, and the experimental group of this invention (HAs+2 mA) with a constant current of 2 mA and 1 g / L of humic acid added. During the experiment, in-situ filtration of the product was performed using a 150-mesh stainless steel mesh dynamic membrane module, and the effluent flux was maintained at 10 m³ / L. 3 / m 2 The mixture then enters the reverse osmosis membrane system for further recovery and concentration of acetic acid, with pure CO2 gas continuously introduced into the bottom of the reactor as the sole carbon source.

[0050] Experimental results are as follows Figure 4 As shown, during continuous operation, the experimental group of this invention (HAs + electricity) exhibited significantly better carbon fixation capacity and operational stability than other groups. Regarding product accumulation, the acetic acid concentration in this group increased linearly and rapidly over time, with an acid production rate 2-3 times higher than the blank control group. Furthermore, its acid production intensity increased by 1.35 times without the use of hazardous hydrogen, and it even showed better metabolic sustainability in the later stages of operation. In contrast, the group (HAs) that only added virgin humic acid without electricity produced extremely limited amounts of acid. Figure 4 a). Under conditions without any external organic carbon source or hydrogen input, the experimental group of this invention relied entirely on the current driven by the humic acid liquid electrode to promote the metabolism of Clostridium aceticum. The final carbon conversion efficiency of the system reached 31.9%, and the electron utilization rate reached 60.9%. Figure 4(b) Meanwhile, the coupled application of dynamic membrane and reverse osmosis membrane ensures the efficient recovery and concentration of the product acetic acid. The dynamic membrane has an extremely high rejection rate for bacteria and humic acid, and the concentration of acetic acid in the reverse osmosis concentrate is significantly increased, achieving stable product output.

[0051] The above results demonstrate that the humic acid liquid electrode not only reduces internal resistance in the carbon fixation device, but also enhances the precise delivery of electrons to microorganisms at the level of microbial metabolism. The core mechanism lies in the fact that humic acid macromolecules, with their strong pseudocapacitive storage capacity, are distributed throughout the reactor space after being charged at the cathode, allowing even non-attached planktonic microorganisms to obtain reducing power in real time, thereby significantly increasing the volumetric acid production intensity.

[0052] Example 4: Liquid electrode promotes carbon fixation and acid production by non-electroactive homoacetic bacteria. This embodiment uses the exact same carbon fixation device and process operating conditions as Example 2, except that the functional microorganism used for inoculation is replaced with *Acetobacterium woodii*. This strain is a typical non-electroactive homoacetic bacterium, which lacks the ability to directly take up electrons from solid electrodes under natural conditions. Apart from the strain difference, the parameters in this embodiment, such as the four parallel experimental groups, humic acid dosage, constant current setting, dynamic membrane effluent flux, and the downstream reverse osmosis recovery system, are consistent with those in Example 3.

[0053] Experimental results are as follows Figure 5 As shown, although *Acetobacterium woodlii* itself is not electroactive, its carbon fixation and acid production performance is significantly enhanced under the mediation of a humic acid liquid electrode. In the experimental group of this invention (HAs+electrode), the acetic acid concentration steadily increased over time, and the final acetic acid concentration produced was 1.68 times higher than that of the blank control group. Figure 5 a). Under conditions without any external hydrogen input, the system maintained metabolism entirely through an electrically driven humic acid liquid electrode, achieving a maximum carbon conversion rate of 32.6% and an electron utilization rate of 60.1%. Figure 5 (b) Meanwhile, the stainless steel mesh dynamic membrane exhibits good retention effect on the non-electroactive bacteria and macromolecular humic acid. Combined with the concentration effect of the reverse osmosis membrane, it achieves continuous and efficient recovery of the product acetic acid, and its separation performance indicators are basically consistent with those of Example 2.

[0054] The above results demonstrate that the humic acid liquid electrode has significant universality in enhancing the carbon fixation process in microbial electrolysis cells, effectively driving the electrosynthetic metabolism of non-electroactive microorganisms. The core mechanism lies in the fact that humic acid molecules, acting as excellent electron transfer mediators, acquire electrons on the cathode surface through their redox-active groups and precisely transport the reducing force to the vicinity of *Acetobacterium woodii* cells via spatial shuttle effects, compensating for the physiological deficiency of non-electroactive bacterial species lacking direct electron transport channels.

[0055] In summary, this invention first proposes a humic acid-based liquid electrode to address the "scale effect" bottleneck in the scale-up process of microbial electrolysis cells (MECs). This liquid electrode uses carbon cloth adhered to the inner wall of the device as the anode, a stainless steel mesh substrate of a dynamic membrane as the cathode, and dissolved humic acid as the liquid-phase diffusion electrode, collectively constructing a liquid electrode that solves the problem of MEC scale-up. Humic acid, with its biocompatibility, pseudocapacitive energy storage capacity, and electron shuttle function, can efficiently receive and store electrons from the solid electrode and uniformly transfer electrons to microorganisms over long distances in a dissolved state within the reactor space. Experimental results show that this liquid electrode reduces the internal resistance of the MEC system by 2.15 times, reduces the operating voltage by approximately 1.0V under low electrode area ratio (S / V=0.5), significantly improves electron transfer efficiency, and increases the effective scale-up limit of MECs by 5-10 times. Furthermore, this invention constructs an integrated carbon fixation device based on the aforementioned liquid electrode; this device, with a microbial electrolysis cell as its main body, integrates a self-generating dynamic membrane module and a reverse osmosis product recovery system. The self-generating dynamic membrane, using a cathode stainless steel mesh as its substrate, effectively retains large humic acid molecules and microorganisms while selectively discharging small molecule products (such as acetic acid and ethanol), thus relieving product inhibition. Through a separation-type reverse osmosis membrane filtration process, acetic acid, ethanol, and other products in the effluent are recovered, and the membrane filtrate is recycled back to the reactor, forming a closed loop with no waste generation. Examples demonstrate that, without an external electron donor (such as hydrogen), this device drives electroactive bacteria (Clostridium aceticum) and non-electroactive bacteria (Acetobacterium woodii) to fix carbon, achieving carbon conversion efficiencies of 31.9% and 32.6%, respectively, and electron utilization rates of 60.9% and 60.1%, respectively, realizing highly efficient bioconversion and resource utilization of carbon dioxide.

[0056] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the present invention.

Claims

1. A carbon fixation device for scaling up microbial electrolysis cells, characterized in that, include: Reactor, liquid electrode, self-generating dynamic membrane module, air intake device; The reactor is a sealed structure; The liquid electrode is disposed in the reactor and is used to maintain efficient electron transfer during reactor scale-up. The liquid electrode includes a solid electrode and humic acid dissolved in the reaction system. The humic acid acts as an electron shuttle and a pseudocapacitive energy storage medium, used to receive electrons from the solid electrode and transfer electrons to microorganisms in the reaction system in a dissolved state. The air intake device is used to introduce carbon-containing gas into the reactor; The self-generating dynamic membrane module is disposed in the reactor and is used to retain the humic acid and discharge the liquid containing organic products. The carbon fixation device overcomes the decrease in electron transfer efficiency caused by the increase in reactor volume through the liquid electrode, thereby realizing the large-scale amplification and efficient carbon fixation of the microbial electrolysis cell.

2. The carbon fixation device according to claim 1, characterized in that, The solid electrode includes an anode and a cathode; the anode is carbon cloth and is disposed on the inner wall of the reactor; the cathode is made of stainless steel mesh and serves as the substrate of the self-generating dynamic membrane assembly.

3. The carbon fixation device according to claim 1, characterized in that, The self-generating dynamic membrane module is fixedly connected to a hollow fixed rod. The stainless steel mesh serves as the membrane substrate. It forms a self-generating dynamic membrane by filtering suspended solids and allowing microorganisms to adhere. The membrane effluent is discharged from the reactor through the hollow stirring rod.

4. The carbon fixation device according to claim 1, characterized in that, The self-generating dynamic membrane module has a hollow cylindrical double-layer structure with a cavity, which is connected to the water outlet pipe. The inner and outer walls of the cavity are made of stainless steel mesh, which serves as the membrane substrate. By filtering suspended solids to form a sludge cake layer and allowing microorganisms to attach and grow, a self-generating dynamic membrane is formed on the stainless steel mesh.

5. The carbon fixation device according to claim 1, characterized in that, It also includes a product recovery system connected to the outlet of the self-generating dynamic membrane module for separating and concentrating acetic acid and / or ethanol from the effluent and returning the separated liquid to the reactor.

6. The carbon fixation device according to claim 1, characterized in that, The product recovery system includes a reverse osmosis unit and a storage tank.

7. The carbon fixation device according to claim 1, characterized in that, The electrode area per unit volume (S / V) within the reactor is ≥0.5, and the distance between the cathode and anode is ≤500 mm; and / or, The membrane substrate of the self-generating dynamic membrane module has a pore size of 100-200 mesh, and the effluent flux is controlled at 0.5-50 m³ / h. 3 / (m 2 ·h).

8. A carbon fixation method based on the carbon fixation apparatus according to any one of claims 1-7, characterized in that, Includes the following steps: (1) Add a solution containing microorganisms and humic acid to the reactor; (2) Carbon-containing gas is introduced into the reactor through the air inlet device; (3) Apply an external voltage of 0.2-2.0 V to the liquid electrode to initiate the reaction; (4) The product liquid containing acetic acid and / or ethanol is discharged through the self-generating dynamic membrane module, and humic acid is retained and recycled in the reactor.

9. The method according to claim 8, characterized in that, Also includes: The discharged product liquid is subjected to membrane separation to recover concentrated acetic acid and / or ethanol, and the separated liquid is returned to the reactor.

10. The method according to claim 9, characterized in that, The concentration of humic acid in the reactor is 0.5-2.0 g / L.

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