Carbon Felt Composite Electrode Plates Promote Methanogenesis through Microbial Electrolytic Cells

: Bioelectrochemical systems are widely used in waste utilization processes. Among them, anaerobic digestion (AD) and microbial electrolytic cell coupling (MEC) are cost-effective and efﬁcient waste-to-energy technologies. In this study, the proposal was made that a carbon felt composite electrode plate be applied to an AD-MEC reactor. The control experiment was conducted using an AD reactor (without the external power supply). The result shows that the carbon felt composite electrode plate increased the biogas production of the AD-MEC reactor by 15.4%, and the average methane content increased by 9.49% compared to the control AD reactor. The total methane production of the AD-MEC reactor and control reactor was 302.51 and 407.79 mL, respectively. The total methane production of the AD-MEC reactor was 34.8% higher than the control group. In addition, the authors found that Methanosarcina and Methanosaeta activities in the AD-MEC reactor were signiﬁcantly increased. The carbon felt composite electrode plate applied in AD-MEC may have promoted the methanogenic microorganisms’ interspeciﬁc acetic acid transport process and increased biogas production and methane content.


Introduction
Extensive fossil energy consumption has forced humans to confront energy shortage problems and environmental damage. In order to achieve sustainable development, more countries have turned their attention to waste-to-energy research directions [1,2]. Anaerobic digestion (AD), which converts agricultural and municipal solid waste into biogas, has been widely used as an effective waste-to-energy means [3,4].
Microbial electrolytic cell (MEC) is a technology that uses microorganisms as catalysts for external power sources to input electrical energy and degrade waste or reduce CO 2 while generating clean energy (CH 4 , H 2 ) [5]. In recent years, AD-MEC methane production technology, which couples anaerobic digestion and microbial electrolytic cells, has become a research hotspot [6,7]. Its main advantages include the low cost of microorganisms, abundant sources of electricity and organic matter (carbon source), and mature methane storage and transportation technology [8]. AD-MEC comprises a cathode, anode, electrolyzer, and external circuit. Whether or not the ion exchange membrane is added to the MEC, it can also be divided into single-and double-chamber MEC because the single-chamber MEC has a simple structure, low cost, easy installation and debugging, and low internal resistance. Therefore, the single-chamber MEC is the preferred choice for AD-MEC [9][10][11].
Considering AD-MEC's characteristics, researchers can improve methane production efficiency by optimizing its electrode structure, debugging operating parameters (voltage, pH, temperature, etc. [12,13]), and changing the electrode material [14][15][16][17]. As part of continued research on AD-MEC coupled methane production technologies, the mild and near-neutral pH value under anaerobic digestion operating conditions is an important parameter to improve AD-MEC methane production efficiency [18]. Redox reactions of microorganisms realize the AD-MEC methane production principle. Anode-attached microorganisms catalyze oxidation using organic matter in water to produce electrons and protons. Cathode microorganisms obtain electrons through direct or indirect extracellular electron transfer (EET) to reduce anodized organic matter or CO 2 to produce CH 4 [19]. Increasing the EET and metabolic rates of methanogenic microorganisms has become a vital issue in AD-MEC research [20][21][22]. Several EET pathways of methanogenic microorganisms have been identified, including the direct acquisition of electrons from the surface to reduce CO 2 and produce methane (DET) [23], interspecific hydrogen transport methane production (IHT) [24], interspecific acetic acid transfer methane production (IAT) [25], interspecific formic acid transfer methane production (IFT) [26], and interspecific direct electron transfer methane production (DIET), etc. [27].
Methane production efficiency is affected by the choice of AD-MEC electrode material through the EET process, microbial attachment, proton transport resistance, etc. [28]. Therefore, carbon-based materials such as carbon rods, cloths, felts, brushes, etc., are widely used as electrode materials because they effectively increase microorganisms' redox reaction rate [29][30][31][32][33]. Among them, carbon felt materials provide a good adhesion environment for microorganisms. The graphite felt fiber on its surface promotes microbial EET through the conductive role as archaeal surface fimbriae. Therefore, compared with other conductive materials, researchers prefer carbon felt as an electrode material [34][35][36].
In addition, optimizing the electrode structure also improves the methanogenic efficiency of AD-MEC. Common electrode structures are cylindrical (carbon brush, carbon rod), flat (carbon cloth, carbon felt), and granular filling (granular activated carbon). New electrode structures have also been developed. For example, parallel electrode structures have been used to increase organic matter removal [37].
In up-flow anaerobic reactors, a vertical electrode structure is used; the cathodesanodes are arranged sequentially from bottom to top [34]. Concentric shaft electrodes have also been studied, which achieve lower internal resistance [38]. Other studies have focused on the effect of electrode area on methane production efficiency. They promoted methane production by changing the area size of the cathode-anode. Studies have found that a large area of cathodes can improve methane production efficiency [35].
However, the above studies only optimize AD-MEC regarding electrode material, operating parameters, and electrode structure. Few studies have yet to be conducted on electrode area and electrode distance. In order to solve these problems, the application was proposed that a carbon felt composite electrode plate to an AD-MEC reactor. The carbon felt is used as the cathode and anode of the AD-MEC, connected by a porous insulation cloth. Applying carbon felt composite electrode plates dramatically shortens the distance between the cathode and the anode. It reduces the internal resistance of AD-MEC, increases the microbial attachment area, promotes hydrogen ions transport and the microbial EET process, and improves methanogenic efficiency.
In order to verify the above hypothesis, the constant temperature AD-MEC experimental system was built to compare the effects of carbon felt composite electrode plates applied in AD-MEC on methane production efficiency with traditional AD reactors. Based on anaerobic experiments, the 16S rRNA archaea full-length sequencing analysis of attached microorganisms to the cathode-anode of the carbon felt composite electrode plates was conducted. The effect on methanogenic microorganisms caused by adding a carbon felt composite electrode plate to the AD-MEC reactor was also studied.

Substrates and Inoculum
The inoculum used in the anaerobic digestion experiment was the anaerobic sludge obtained from a mesophilic anaerobic digester applying pig manure as feedstock in Jiading District, Shanghai, China. The anaerobic sludge was stored in a biochemical incubator at 35 • C before the anaerobic digestion experiment. The inoculated sludge's total solids content (TS) was 10.06 ± 0.15%, and the volatile solids content (VS) was 26.18 ± 0.17%. The effluents used in the anaerobic experiment were diluted digested effluent with a chemical oxygen demand (COD) concentration of 150.5 ± 5.9 mg/L, a pH of 7.42 ± 0.09, and an ammonia nitrogen concentration of 169.2 ± 7.2 mg/L. The mixtures of cellulose, hemicellulose, lignin, and soluble starch from Sinopharm (Sinopharm, Beijing, China) were used as manually simulating rice straw substrates for the AD and AD-MEC reactor. Each reactor received 2 g of the substrate. The rice straw composition measured by the reference study was cellulose 39.9 ± 1.35%, hemicellulose 23.18 ± 0.12%, and lignin 5.70 ± 0.11% [39]. Soluble starch was added to the rest. Figure 1 shows a constant temperature MEC anaerobic digestion experiment station. The anaerobic experimental system consisted of anaerobic digestion, gas collection, and current detection modules. The anaerobic digestion module consisted of a thermostatic incubator and three anaerobic digestion reactors. The reactor is an acrylic box with a length of 15 cm, a width of 10 cm, and a height of 15 cm. The reactor's total and working volumes were 2.25 L and 2 L, respectively. A gas collection valve was set above the reactor and connected to the gas flow meter through a silicone hose. The reactor's top cover was fixed with several fixing screws and sealed with sealant before the experiment began. There was a sampling port on the side of the reactor, and water samples were obtained through the valve daily. The inner side of the reactor had an electrode plate slot so that the electrode plate could be inserted inside and fixed. Copper wires connect the electrode plate to the positive and negative terminal holes of the external power supply above the reactor. The contact position between the copper wire, the electrode plate, and the underwater part was sealed with a sealant. A magnetic stirrer (Shi Bo, ZGCJ-3A, Changzhou, China) was placed under each reactor, controlled by a timer switch. The gas collection module consisted of a gas sampling bag and a flow meter produced by Anaero Technology (Anaero Technology, Cambridge, UK). Gas flow meters calculate the gas collection volume by turning the drum over time. Multiple calibrations are performed before the experiment to record the gas volume when the average drum is flipped. The average flipping volume of the drum calibrated by the gas flow meter before this experiment was 6.41 mL. The gas generated by the reactor is connected to the reactor bottom through a silicone tube. A certain amount of gas enters the reactor to flip the internal drum of the reactor. The number of turns is recorded in a single-chip microcomputer, read by the computer terminal, and multiplied by the drum flipping volume to obtain the gas volume. The current sensing module consists of a direct current (DC) power supply, an ammeter, and a host data logger (Jing Mei, JM342 Modbus, Shenzhen, China). DC power connects both ends of the electrode plate to provide a regulated voltage. The ammeter detects the current flowing through the electrode plate during the anaerobic reaction in real-time. The host recorder records the current data in real-time. Figure 2 shows the AD-MEC reactor principle and the carbon felt composite electrode plate. The electrode plate consists of two pieces of carbon felt 9 cm long, 9 cm wide, and 2 mm thick with a slightly larger porous insulation cloth. The edges of the carbon felt and insulation cloth are bonded with insulating glue to prevent the carbon felt from contacting both sides. Prior to the experiment, the electrode plate is inserted into the reactor groove. One side of the electrode plate is connected to the positive terminal of the power supply through a copper wire. The other side is connected to the negative terminal of the power supply. Upon starting the AD-MEC reactor, the power supply causes potential differences between the anode chamber and the cathode chamber, forcing electrons to flow through the external circuit. Hydrogen ions are transferred through the electrode plate, which promotes the microorganisms' EET process.

Experimental Methods
Total solids content (TS) and volatile solids content (VS) were determined according to APHA standard methods [40]. The gas volume was collected using a gas flow meter (Anaero Technology, Cambridge, UK). Biogas composition (CH 4 , CO 2 ) was analyzed using a gas chromatograph (Agilent, GC8860, Santa Clara, CA, USA). Chemical oxygen demand (COD) and ammonia nitrogen were measured using a multi-parameter resolution instrument (Lian Hua, 5B-1, Beijing, China) and an ultraviolet-visible spectrophotometer (Lian Hua, UV-5500, Beijing, China). A pH meter (REX, PHSJ-4F, Shanghai, China) was used to measure pH values. Water samples from the reactor and gas samples from the gas bag were collected daily at fixed times. Measurements were repeated three times according to the above method.

Microbial Community Analysis
In order to analyze the carbon felt composite electrode plate's effect on methaneproducing microorganisms in the AD-MEC reactor, the sludge attached to the cathode and anode was collected and sent to a third-party detection platform for microbial diversity detection. The detection item was full-length 16S rRNA archaea sequencing. The target fragment length was 1500 bp, and the primer was 21F (TTCCGGTTGATCCYGCCGG), 1492R (ACCTTGTTACGACTT). After detecting microbial diversity, the effect of adding carbon felt composite electrodes to methanogenic microorganisms was analyzed according to their community composition. The sequencing results were analyzed on a third-party platform, the Majorbio Cloud Platform (https://cloud.majorbio.com/, accessed on 18 March 2023).

Statistical Analysis
Each result was reported as the mean value of measurements from three replicate samples. The standard deviations and statistical differences were analyzed by Microsoft Excel.

Cumulative Biogas Production and Daily Biogas Production
The anaerobic experiment was divided into three groups. Blank is the only reactor that adds anaerobic sludge, AD adds anaerobic sludge and substrate, and AD-MEC has applied voltage alongside anaerobic sludge and substrate. Since voltage effects on AD-MEC are not the focus of this study, the applied voltage was set to a constant of 0.7 V for reference [41]. The anaerobic experiment lasted 22 days, after which the reactor produced almost no biogas. A gas flow meter measured the cumulative gas volume produced by each reactor, and the data were entered into a computer terminal. Figure 3 shows each reactor's cumulative biogas yield and daily methane content. Figure 3a shows that the Blank reactor has limited potential because it only adds anaerobic sludge and no substrate. The cumulative biogas production in 22 days of the experimental period was 57.37 mL. Cumulative biogas production in the AD and AD-MEC reactors was the same. The start-up stage accounted for the first five days, the large-scale biogas production stage lasted from the fifth to the tenth day, and the flat stage of biogas production lasted from the tenth to the twenty-second day. The biogas production effect of the AD reactor was significantly improved by adding substrate. The cumulative gas production for 22 days was 539.47 mL. The AD-MEC reactor had the best biogas production effect out of the three groups due to the applied voltage. The cumulative biogas production in 22 days was 622.85 mL, 83.38 mL more than in the AD reactor. Overall, the AD-MEC reactor produced 15.45% more biogas than the AD reactor. Figure 3b shows the daily biogas production of the three reactor groups. The Blank reactor produced less biogas, with 11.52, 11.37, 11.45, 11.55, and 11.48 mL on days 1, 2, 4, 8, and 13, respectively. Since no substrate was added to the reactor, anaerobic microorganisms in the sludge had insufficient carbon sources for anaerobic digestion to produce biogas. In addition, the AD-MEC reactor's start-up speed was significantly faster than the AD reactor. AD-MEC biogas production was stable from days 2 to 12. Its peak biogas production periods were 23.62, 23.55, 47.01, 59, 58.66, 104.68, and 81.57 mL, respectively. During days 4 to 12, the AD reactor was started, and biogas production was relatively stable. The peak biogas production period was from days 4 to 9, 23.55, 23.38, 58, 46.68, 93.49, and 58.43 mL. An AD-MEC reactor has significant advantages over an AD reactor in terms of start-up speed and duration of peak biogas production. The AD-MEC reactor might increase biogas yield by setting an applied voltage, which generates a potential difference in redox reaction among methanogenic microorganisms and promotes their anaerobic digestion reaction. Another possible reason is that the AD-MEC reactor uses carbon felt composite electrode plates, which increase microorganisms' attachment area and improve their reaction efficiency. Carbon felt composite electrode plates have a short cathode-anode difference, thus shortening protons' migration distance between the cathode and anode chambers. Therefore, protons move directionally before the electrode plate. The electrons move in the external circuit to form a complete cycle, improving the cycle efficiency of the whole system's microbial redox reaction. Anaerobic microbial communities and methanogenic archaea were both discussed as reasons for the biogas yield increase. Section 3.4 describes the analysis results.

Daily Methane Content
During the 22-day anaerobic experiment, samples were collected daily from the corresponding gas bags of the AD and AD-MEC reactors at fixed times. The meteorological chromatograph GC8860 (Agilent, GC8860, Santa Clara, CA, USA) detected methane content, as shown in Figure 4.  Figure 4a shows the change in daily methane content in the AD reactor. Three days before the experiment started, the AD reactor was in the start-up stage, the methane content was between 30 and 40%, and methane-producing microorganism activity was low. Methane content increased on the fourth day, and on the ninth day, it peaked at 69.17 ± 1.47%. In addition, the methane content exceeded 60% between days 7 and 13. This phase coincides with the peak of biogas production, as described in Section 3.1. Between days 14 and day 22, the methane content declined to 25.27 ± 0.55%. A possible explanation is that the easily degradable substrate in the reactor was consumed before 13 days, while the remaining refractory substrate inhibited methanogenic microorganism activity. Figure 4b shows changes in the AD-MEC reactor's daily methane content and current intensity. Similar to the AD reactor, the methane content of the AD-MEC reactor was 43.22 ± 0.63, 48.52 ± 0.79, and 42.74 ± 1.14% in the first three days of the experiment, an increase of about 10% over the AD reactor. The average current intensity increased in the first three days and reached 2.05 A on the third day. In the AD-MEC reactor with the applied voltage set, the start-up speed is faster, and the methane-producing microbial activity is higher than in the AD reactor. From days 6 to 18, the methane content in the AD-MEC reactor exceeds 60%, whereas the current intensity slowly decreases from days 3 to 14 at 0.66 A. The current intensity gradually flattens between days 14 and 22 at 0.51 A. The decrease in current intensity is slightly delayed compared to the speed of biogas generation analyzed in Section 3.1, indicating that anaerobic microorganisms' response to current has some lag. This phenomenon can also be derived from the current trend in Figure 4b. The current intensity rises sharply in the first three days, while the daily methane content increases on day 4 and peaks on day 6. It is worth noting that from days 6 to 11, the AD-MEC reactor's methane content rose to more than 70%, reaching 77.98 ± 2.03% on day 7, which was 8.81% higher than the AD reactor's peak. After day 18, the methane content decreased. By the end of the experiment on day 22, the methane content had decreased to 43.37 ± 0.65%. In addition, Figure 4b shows that the AD-MEC reactor is at peak methane content for a longer period. The addition of voltage and setting of carbon felt composite electrodes might explain this increase. These changes not only improved methanogenic microorganism activity but also promoted the degradation of refractory organic matter by microbial communities. Therefore, the carbon felt composite electrode plate applied in the AD-MEC reactor increased biogas production and methane content.

Chemical Oxygen Demand, Ammonia Nitrogen and pH
After microporous membrane filtration, each reactor's daily COD, ammonia nitrogen, and pH changes were measured. Figure 5a shows daily COD changes for each reactor during the 22-day experiment. COD's changing trend in the Blank reactor was generally flat. It always remained between 200 and 230 mg/L without significant changes. This uniformity indicates that the dissolved organic matter content in this anaerobic sludge batch was low, making it suitable as a control. The AD reactor's COD on day 2 increased significantly from 198.3 ± 8.2 to 637.2 ± 7.4 mg/L, indicating that the easily degradable organic matter in the substrate started to hydrolyze. From days 2 to 4, COD had flattened. One possible reason is that the hydrolysis rate of organic matter and the gas production rate of microorganisms using organic matter were balanced. COD in the AD reactor dropped significantly on the sixth day, indicating that a substantial amount of COD was consumed to produce biogas. This result was consistent with the situation analyzed in Section 3.1. The AD reactor entered its peak biogas production period on day 6. After day 13, the AD reactor's COD content flattened off, decreasing to 250.7 ± 12.7 mg/L on day 22. This decline shows that after the easily degradable organic matter in the substrate is consumed, the remaining refractory organic matter microorganisms are difficult to use and reduce biogas production efficiency in the later stages. In the AD-MEC reactor, COD rose on the following day, unlike in AD reactors. From days 3 to 6 in the AD-MEC reactor, COD decreased first and then increased. One possible reason is that the applied voltage setting facilitates the hydrolysis of organic matter in the reactor. Additionally, it increases the rate of biogas production by anaerobic microorganisms and facilitates the start-up time of the anaerobic reaction. Studies have shown that when complex compounds are treated with applied voltage, biogas production increases by promoting COD's conversion to CH 4 [42,43]. On day 9, the AD-MEC reactor's COD content was reduced to 120.4 mg/L. COD levels in the reactor slowly recovered after day 10, reaching 371.3 ± 16.4 mg/L on day 13. COD decreased slowly after days 13 to 22 of the experiment. On day 22, the AD-MEC reactor's COD content was 121.9 ± 7.6 mg/L.
There is the possibility that adding applied voltage and carbon felt composite electrode plates promotes the hydrolysis of refractory organic matter in the substrate and improves the anaerobic microorganisms' efficiency in utilizing organic matter. This may also explain why the AD-MEC reactor's final biogas yield was 15.45% higher than the AD reactor.  Figure 5b shows the ammonia nitrogen trend in three reactor groups. It is almost a flat curve in the blank reactor, with the ammonia nitrogen concentration remaining unchanged from 116.85 ± 4.3 mg/L at the beginning of the reaction to 123.1 ± 4.1 mg/L at the end. The ammonia nitrogen trend in the AD and AD-MEC reactors is almost consistent. The concentration of ammonia nitrogen in the AD reactor increased from 139.9 ± 3.6 mg/L at the beginning of the reaction to 165.7 ± 3.8 mg/L at the end. The concentration of ammonia nitrogen in the AD-MEC reactor increased from 143.8 ± 3.7 mg/L at the beginning of the reaction to 166.7 ± 3.9 mg/L at the end. The slight increase in ammonia nitrogen in the reactors can be attributed to the addition of substrate during anaerobic digestion. The applied voltage did not significantly affect changes in ammonia nitrogen concentration during anaerobic digestion. Figure 5c shows pH changes for the three groups of reactors. Changes in the pH curves of the Blank, AD, and AD-MEC reactors were minor, all remaining between 7.41 ± 0.04 and 7.87 ± 0.07, indicating that either the addition of substrate or setting the applied voltage did not affect the anaerobic digestion process.

Alpha Diversity Analysis
The carbon felt composite electrode plate is an electrode with a flat shape. In the case of stirring in the reactor, it can be assumed that the microbial community richness on the electrode surface is uniform. Therefore, samples were taken separately from the upper part of each piece of carbon felt. Three sludge samples were collected from the cathode and anode to analyze the carbon felt composite electrode plate's effect on methane-producing microorganisms in the AD-MEC reactor. The samples were sent to a third-party testing platform for microbial diversity detection. This study used alpha diversity analysis to evaluate the community richness and diversity of cathodes and anodes on carbon felt composite electrodes. In order to evaluate the alpha diversity of microbial communities, the Chao index and Observed species index were used to determine community richness, Shannon and Simpson's indices were used to determine community diversity, and the Goods coverage index was used to assess community coverage. Table 1 shows specific values: A1, A2, and A3 represent three anode samples, and B4, B5, and B6 represent three cathode samples. Table 1 shows that the Chao index and Observed species of the three anode samples are higher than the three cathode samples, indicating that the anode community richness is higher than the cathode. The Simpson indices of the anode and cathode samples are similar. A cathode sample with a lower Shannon index than an anode sample has a higher biodiversity.  Figure 6a shows the phylum-level archaeal sequence analysis results. Euryarchaeota has the highest relative abundance in the anode and cathode samples. The relative abundance of Euryarchaeota in the three anode samples, A1, A2, and A3, was 49.35, 77.89, and 57.23%, respectively. The relative abundance of Euryarchaeota in the three cathode samples, B4, B5, and B6, was 60.78, 77.249, and 77.014%, respectively. Euryarchaeota dominates AD-MEC. Most studies have shown that Euryarchaeota is essential to the hydrocarbon conversion process, the most representative of which is the methane flora. In addition, Euryarchaeota may play an essential role in promoting refractory substrate hydrolysis [38]. Figure 6b shows the genus-level archaeal sequence analysis results. The acetic acid methanogenic archaea Methanosaeta and Methanosarcina dominate the three anode and three cathode samples. The relative abundances of Methanosaeta in the three samples, A1, A2, and A3, were 21.43, 49.27, and 21.91%, respectively. The relative abundance of Methanosaeta in the three cathode samples, B4, B5, and B6, was 23.15, 39.07, and 38.18%, respectively. The relative abundance of Methanosaeta in the cathode sample was approximately 2.6% higher than in the anode sample. This suggests that applying carbon felt composite electrode plate to the AD-MEC reactor increases Methanosaeta activity, and the cathode Methanosaeta is higher. Previous studies had similar findings [44]. The relative abundance of Methanosarcina in the three anode samples was 23.53, 21.21, and 30.74%, respectively. The relative abundance of Methanosarcina in the three cathode samples was 32.73, 25.71, and 30.33%, respectively. The proportion of methyl meat in the cathode sample is approximately 4.43% higher than that in the anode sample. Methanosarcina belongs to acetic acid methanogenic archaea. This phenomenon indicates that adding a carbon felt composite electrode plate to the AD-MEC reactor may promote changes in the microbial community structure under applied voltage. Consequently, Methanosarcina microbiota activity increases with the thicker cell wall and promotes the interspecific acetic acid transfer process (IAT). Studies have found that Methanosarcina converts acetic acid, hydrogen protons, and CO 2 into methane, inhibits pH drop, and maintains methanogenesis at a high activity level, thereby improving methanogenic efficiency [45]. In addition, Bathyarchaeia was also found in the cathode and anode of the carbon felt composite electrode plate. The relative abundance of Bathyarchaeia in the three anode samples was 2.14, 2.328, and 1.807%, respectively. The relative abundance of the three cathode samples increased to 5.503, 5.728, and 4.126%, respectively. This finding indicates that carbon felt composite electrode plates improve the Bathyarchaeia activity in the AD-MEC reactor and promote the hydrogen transport process (IHT) between methanogenic archaea. In previous studies, similar findings were found in AD-MEC reactors, where applied voltage enhanced hydrogen trophic methanogenic archaea on the electrode, promoted electromethanogenesis, and improved methanogenic efficiency [46].

Conclusions
Using carbon felt composite electrodes in the AD-MEC reactor significantly improved its biogas yield and methane content. The AD-MEC reactor produced 41.69 mL more biogas yield per gram of substrate than the control group. The total methane production of the AD-MEC reactor and control reactor was 302.51 and 407.79 mL, respectively. The total methane production of the AD-MEC reactor was 34.8% higher than the control group. In addition, the acetic acid methanogenic flora activity on the carbon felt composite electrode plate in the AD-MEC reactor significantly improved. The application of a carbon felt composite electrode plate with small electrode spacing and a large area to the AD-MEC reactor promotes the microbial extracellular electron transfer process and improves methane production. These findings demonstrate its potential application in AD-MEC.