Development Of Iron-Bearing Multi-Dimensional Electrode For Enhancing CO₂ Electromethanogenesis In Microbial Electrolysis Cell And The Possible Molecular Mechanisms Of Electron Transport

In recent years,increasing fuel consumption has elevated the level of CO₂ in the atmosphere,aggravating the greenhouse effect.For solving the energy crisis and mitigating CO₂emission,microbial electrolysis cell(MEC)for CO₂electromethanogenes is a cutting-edge technology,which enables efficient production of renewable energy and approaches the"carbon neutral".In the MEC system,enriched electroactive biofilm(EAB)can utilize electrons and H₂ from the cathode to reduce CO₂ to CH₄ at appropriate cathode potentials.However,the applied cathode potential to drive the reaction of CO₂ electromethanogenesis is always much more negative than the theoretical value,resulting in energy waste.Therefore,to improve CH₄ production rate in MEC biological electro-activity of EAB and electron transfer needed promoting and regulating,and the development of new electrode meterials is required to reduce cathode potentials.The main research conclusions of this dissertation are as follows:(1)A two-chamber bioelectrochemical cell equipped with a hybrid skirt-shaped cathode was constructed and the electrocatalytic performance of EAB and the electron shuttling mechanisms involved in extracellular electron transfer were systematically studied.The EAB colonizing on biocathode showed an excellent cathodic electrocatalytic activity and the minimum charge transfer resistance.The CH₄production rate of 298.0±46.7 m L/L/d was obtained at the cathodic potential of-1.0V vs.Ag/Ag Cl with the highest coulombic efficiency of 75.8±9.9%.The gel-like extracellular polymeric substances secreted by EAB facilitated the adhesion/aggregation of microbes and EAB development.The microbial results showed CO₂ electromethanogenesis exhibited a positive association with Methanobacterium(54.4%)in EAB.Moreover,metagenome analysis confirmed the presence of direct EET-related genes(i.e.,hdr A,eha A,and ehb C),which accelerated the formation of corresponding functional protein complexes(particularly heterodisulfide reductase A,Hdr A)and electron exchange.(2)In order to shorten the start-up time of MEC,Fe(OH)_x nanosheets were electrodeposited on the graphite felt to prepare Fe(OH)_x@GF cathode for EAB development.A rapid start-up period of 10 days for CO₂ electromethaogenesis was achieved.The CH₄ production rate of Fe(OH)_x@GF biocathode was up to 204.6±28.0m L/L/d,which increased by 2.3 folds than raw GF biocathode(90.5±1.9 m L/L/d)at-1.0 V vs.Ag/Ag Cl.The maximum?_CH4 of Fe(OH)_x@GF biocathode was 88.4±13.3%at-1.0 V vs.Ag/Ag Cl,which was 1.3-fold higher than that of raw GF(67.9±1.9%).At the initial stage of start-up,it is found that Fe(OH)_x nanosheet could benefit attachment behavior and enrichment of EAB by promoting the secretion of protein-dominating extracellular polymeric substances,reducing R_ct and therefore,accelerating the transfer of electrons from the cathode to microorganisms.Methanothrix and Methanobacterium belonging to methanogenic archaea were identified in the mature EAB community.(3)To improve the performance of CO₂ electromethanation and enhance the activity of EAB,a novel FeNi-LDH/NF cathode was prepared by hydrothermal method.At-0.8 V vs.Ag/Ag Cl,the CH₄ production rate was 234.3±7.3 m L/L/d in each cycle,which was 58%higher than that of unmodified NF(148.7±11.6 m L/L/d).The results showed that FeNi-LDH/NF could provide sufficient active sites for cells to adhere and aggregate,and regulate the biofilm thickness of live cells(200?m).As a result,by facilitating micro-channels for nutrient diffusion.the activity metabolism of inner microorganisms could be stimulated and cell apoptosis was also reduced.Statistical methods were used to analyze microbial community and functional genes.The abundance of Methanobacterium in FeNi-LDH/NF biocathode accounted for 75.6%of the total abundance of microorganisms.Moreover,metagenome analysis showed that the CO₂ methanogenesis pathway(M00567)was the highest abundant metabolic pathway(5.7%)of FeNi-LDH/NF.And some related functional genes expression including genes fwd A-H?mtr A-H and hdr ABC were upregulated in FeNi-LDH/NF.(4)The effects of metal elements modified electrodes(Fe/NF,Ni/NF and Ni Fe/NF)on the molecular mechanism of electron transport of EAB were explored.The results showed that the performance of Ni Fe/NF biocathode was the best.The CH₄ production rate of Ni Fe/NF could reach as high as 318.6±27.5 m L/L/d at-0.8 V vs.Ag/Ag Cl.Its excellent performance was benefited from the good biocompatibility of iron-based catalyst and the superior electrocatalytic activity of nickel-based catalyst,confirming that the good synergistic effect of bimetallic catalysts was on the development of EAB.In terms of the 3D spatial structure of EAB,the Ni Fe/NF biofilms reached their peak coverage area ratio of live cells(>7.5%)at a thickness range from 45?m to 110?m.The results of icrobial community showed the Fe largely promoted the proliferation of Methanobacterium?sp.?Pta U1.Bin097 while Ni favored the enrichment of Methanobacterium?congolense.Transcriptomic analysis also revealed that Ni Fe/NF actively stimulated fwd A,ftr,mtr A and hdr A genes expression in the M00567 pathway on EAB.This study provided a promising strategy to modify electrode interface in MEC for CO₂ electromethanogenesis to hasten the process of MEC industrialization.