Interfacial Electron Transfer Mechanism Of Surface Functionalized Carbon Nanotubes Anode In Shewanella Putrefaciens Microbial Fuel Cells

Microbial fuel cell?MFC?is a promising renewable power device that can directly convert chemical energy of organic substrates or wastes into electricity through the catalysis by the microbes,which integrates microbial degradation and electrochemical energy harvest.It has attracted significant interest as a renewable energy source and has been applied in many fields,particularly in wastewater treatment.However,the low power density,slow start-up process and high cost of devices impede their commercial applications.In particular,the sluggish anode kinetics mainly caused by the slow extracellular electron transfer from anode electroactive microbes?exoelectrogens?to electrodes is generally considered as one of the main limiting factors for the poor MFC performance.To overcome this obstacle,the researchers applied all kinds of nanostructured materials to improve the catalytic performance of anode.Apparently,anode structure optimization is critical for facilitating bacterial adhesion and interfacial electron transfer,which is also a key strategy to enhance the performance of microbial fuel cells.Carbon nanotubes?CNTs?,as one-dimensional nanomaterials,have some superior electrical,mechanical and chemical properties.In recent years it has shown a broad application prospect of carbon nanotubes as electrode materials in fuel cells,super capacitors and batteries etc.Considering the superior electronic conductivity and the nanostructure,CNTs could enhance the contact between the bacteria and the anode surface as well as the interfacial electron transfer.However,the plain CNTs are easy to tangle and hard to uniformly distributed on the anode surface and not compatible to bacterial cells.Approprate functionalization of the CNTs is necessary before using in MFC anode.In this case,the CNT based anode materials with different functionalization are designed and fabricated to promote the bioelectrocatalytic performance of MFC inoculated with Shewanella putrefaciens CN32?S.putrefaciens CN32?as biocatalyst in this study.The detail mechanisms for the enhancement of these CNT-based anodes on interfacial electron transfer are discussed.T.The main research contents and results are as follows:?1?Firstly,carboxylated carbon nanotubes?CNTb?were successfully prepared and applied as anode material in MFCs.The results show that the maximum current density of the CNTb/CC anode is 1.41 ± 0.06 A m^-2,which is 29% higher than that of the original CNTa/CC(1.09 ± 0.02 A m^-2),indicating that the CNTb / CC anode has a higher MFC current output capability.The maximum output power density of CNTb based MFC is increased to 472 m W m^-2,which is about 1.7 times of the original CNTa/CC anode(272 m W m^-2).The results also show that this simple method can effectively increase the hydrophilicity and biocompatibility of carbon nanotubes.Itsuggests that the change of the hydrophilicity of the anode material can improve the interfacial electron transfer and increase the adsorption capacity of bacteria on the anode surface.This study has laid a foundation for the preparation of carbon nanotube/ionic liquid composites.?2?Secondly,CNT-PMo/CF composites were prepared by physical adsorption method and applied to S.putrefaciens CN32 MFC.The related physical characterization data and electrochemical data show that when the mass ratio of CNTs and PMo is 1: 2,the CNT-PMo/CF composites have a smaller interfacial charge transfer impedance,which is more favorable for the redox reaction of flavins on the electrode surface.Because of the high biocompatibility of PMo and nano-carbon materials,the maximum output power density of CNT-PMo/CF b composite anode is 1235 m W m^-2,which is 6.5 times higher than that of CNT/CF anode(190 m W m^-2)and 12 times higher than that of CF anode(99 m W m^-2).It indicates that the unique surface properties of the anode materials and its enhanced direct electrons of the electron mediator play an important role in enhancing the bio-current of the MFC anode.?3?Finally,an IL functionalized CNT nanocomposite was used to improve anode performance for S.Putrefaciens CN32 MFC.The positively charged IL binds to negatively charged CNTs,and the CNT-IL nanocomposites are also positively charged,and the S.putrefaciens CN32 surface is negatively charged.At the same time,it has been found that when the ratio of CNT and IL reached 1:90,the content of N in the complex was the highest,which allowed fast redox reaction of the FMN secreted by S.putrefaciens CN32 on the anode surface,and these electron mediators can be mediated short distance and fast indirect electron transfer process in the biofilm surface.In addition,due to the high conductivity of both IL and CNT,it ensures that the flavonoid electron mediator achieves rapid electrochemical oxidation at the interface of the nanocomposite anode material,thereby synergistically enhancing the endogenous electron microscopy direct electrochemical process mediated.The CNT-IL/CC anode delivers an around 3-fold higher maximum power density higher than that of the plain CNT/CC anode due to the increased biofilm loading amount as well as the fast interfacial electron transfer between bacteria cells and the electrode,especially for the flavin mediated electron transfer.Furthermore,the CNT-IL anode is quite stable in an MFC system for long-term discharging and achieves higher coulombic efficiency.This work proposes a novel strategy to functionalize CNT based electrode for bio-electrocatalysis that holding great promise for practical MFC applications.