Performance Regulation Of Electroactive Biofilms And Enhanced Mechanism Of Extracellular Electron Transfer Process

Microbial fuel cell（MFC）can realize wastewater treatment and energy recovery simultaneously by utilizing the metabolism of microorganisms,which has shown great application value in environmental protection and energy conversion.However,MFC still faces some limitations such as low energy density and slow electron transfer efficiency in practical application.This is mainly due to the insufficient adhesion of microorganisms on the anode surface and the slow extracellular electron transfer（EET）process between the microorganisms and electrode interface.MFC anode is an important site for electroactive biofilm（EAB）growth and EET process,and its surface properties have a direct and important influence on the performance of anode and MFC.In this thesis,the effective attachment and rapid enrichment of exoelectrogens on the anode surface were achieved by regulating the surface properties of the anode material;the EET process was strengthened by optimizing the bridging between the anode interface and the outer membrane cytochrome c（OM c-Cyts）;the activity of EAB was improved by regulating the distribution of extracellular polymer substance in the anode biofilm through introducing active materials.The main research contents and results are as follows:（1）In order to improve the electrocatalytic activity and microbial adhesion of the anode,polydopamine（PDA）with strong hydrophilicity and reduced graphene oxide（r GO）with good conductivity were used to modify the surface of carbon cloth（CC）substrate.After modification,the PDA microspheres and the r GO wrinkled film were interconnected and uniformly distributed on the CC electrode surface.The surface roughness of the CC-PDA-r GO electrode increased from 58 nm to 180 nm,and both the specific surface area and the electrochemically active area were significantly improved.The surface hydrophobicity of the CC electrode was transformed into hydrophilicity（water contact angle of 51.6°for CC-PDA-r GO）.The increased surface area and improved hydrophilicity are beneficial to the effective attachment and growth of microorganisms,and the biomass of CC-PDA-r GO anode biofilm was 840.6μg/cm²,which was much higher than that of the CC（198.6μg/cm²）.The MFC with the CC-PDA-r GO anode exhibited a maximum power density of 2047 m W/m²,which was 6.1times that with the CC anode.The charge transfer resistance between the biofilm and the anode was reduced from 266.6Ωto 31.3Ω.The electron collection capability of the CC-PDA-r GO anode was improved,and the EET process was facilitated.The EET process mediated by the binding of flavin and OM c-Cyts at the CC-PDA-r GO electrode interface was enhanced.The CC-PDA-r GO anode biofilm had the best biofilm activity,and the green fluorescence intensity of living cells was significantly higher.This study showed that the targeted anode modification with suitable materials can effectively improve the electrocatalytic activity of the anode and promote the bacterial attachment and MFC electricity production.（2）The conductive pigment proteins on the bacterial outer membrane,namely OM c-Cyts,play an important role in the EET process.From the point of view of improving the affinity between the electrode interface and OM c-Cyts,iron porphyrin(Fe Por,C₃₄H₃₂Cl Fe N₄O₄)as the active center of OM c-Cyts was adopted to modify CC.In addition,based on the results from the first part,polyquaternium-7（PQ）with good hydrophilicity and stability was used for surface modification of CC.The Fe Por-PQ composite anode was prepared using solvent evaporation method and solvent soaking method.On the one hand,the co-modification of Fe Por and PQ improved the roughness,hydrophilicity,and electrochemically active area of the electrode surface,which was beneficial to the adhesion of bacteria,the transport of nutrients,and the occurrence of redox reactions.On the other hand,the introduction of active materials formed a structure doped with N and Fe heteroatoms on the surface of the electrode,the pyrrolic N and graphitic N can obtain electrons from OM c-Cyts more easily,and the EET process at the anode interface was promoted.The electron transfer rate constant was increased from 1.26 s^-1 to 2.15 s^-1,and the contribution of fast kinetic processes to the current was increased from 19.54%～27.75%to 45.22%～56.62%.With the improved EET efficiency,the MFC with Fe Por-PQ anode achieved the maximum power density of 2165.7 m W/m²,which was 1.92 times that of CC.The introduction of Fe Por and PQ significantly increased the cell viability of the anode biofilm,and the exoelectrogen Geobacter was efficiently enriched with the abundance increasing from 34.35%to55.84%,realizing the formation of a highly electroactive biofilm.This study showed that the co-modification of Fe Por and PQ was an effective method to simultaneously enhance the EET and enrichment of exoelectrogens.（3）The results from the second part indicated that the introduction of active materials with similar structure to the center of OM c-Cyts into the anode interface can effectively enhance the EET efficiency,but its effect on the composition and structural characteristics of EAB remains to be explored.The spatial structure of EAB is closely related to the electrocatalytic activity of biofilms.In this part of the study,iron phthalocyanine(Fe Pc,C₃₂H₁₆Fe N₈),which is similar in structure to Fe Por and has more abundant N atoms,was introduced into the CC electrode.The effects of Fe Pc on the composition,structural characteristics,and electrochemical performance of EAB were explored,and the electron transfer mechanism between Fe Pc-CC anode biofilm and electrode was revealed.The elemental composition analysis showed that the element ratio of N in the Fe Pc-CC electrode was 16.03%,and that of Fe was 3.05%.The successful introduction of N and Fe atoms can change the distribution of active sites on the electrode surface and form more effective electrochemical active sites.The charge transfer resistance at the Fe Pc-CC interface was reduced to 6.74Ω,the direct electron transfer process on the electrode was enhanced,and the peak current density corresponding to OM c-Cyts Omc A was 3.66 A/m²,which was 2.6 times that of the CC anode.The MFC maximum power density of Fe Pc-CC anode was 2419 m W/m²,which was 4.3 times that of CC anode（560 m W/m²）.The changes of cell viability and EPS distribution in the biofilm longitudinal direction were analyzed by CLSM,and it was found that the proportion of viable cells in Fe Pc-CC（0.53±0.03）was higher than that in CC（0.46±0.01）.The relative proportion ofα-D-glucopyranose polysaccharides in the biofilm was significantly lower than that in CC.The reduction of insulating polysaccharides can enhance long-distance electron transport in EAB,thereby enhancing biofilm activity.The abundance of typical exoelectrogen Geobacter increased from 6.97%to 44.83%.This study showed that the introduction of Fe Pc effectively regulated the activity and selectivity of EAB,thereby enhancing the electrochemical activity of EAB,and realizing the promotion of electron transfer and the improvement of MFC electricity production performance.