Tailoring Chemistry And Physics Of Nanostructured Anode Materials And Their Bioelectrocatalytically Enhancement Mechanisms In Microbial Fuel Cell

With the ever-worsening environmental pollution and growing shortage of traditional fossil fuels,microbial fuel cell?MFC?as a renewable energy source has received great attention in recent years.This device can simultaneously generate electricity and recycle waste,which provides new opportunities to harvest electricity from biodegradable wastes.Compared with conventional chemical fuel cells that use expensive precious metals or other chemical catalysts,MFC uses a wide variety of microbes as catalysts.Since the microbiological catalysts have rich and flexible metabolic pathways,MFC rendering great potential in applications of portable energy,wastewater treatment,environmental remediation and so on.However,the practical application of MFC is greatly impeded by their lower power density and slow start-up than the conventional fuel cells,which is mainly due to the sluggish electron transfer?ET?between microstrain and electrode as well as low bacterial loadings.To address this challenge,a tremendous amount of research has been carried out to develop various nanostructured anode materials with high surface area and electrical conductivity to promote microbial biofilm growth and expedite electrochemical reaction.Apparently,the underlying effects of anode nanostructure and surface chemistry on microbial metabolism,biofilm growth,electrochemical reaction and electron transfer pathways need more devoted research efforts for their scientific insights and potential practical applications.The sluggish electron transfer pathways between“non-conductive”or“poorly conductive”microbes and electrodes has becoming one of the key hindrances in improvement of the power density,and more studies have shown that the spatial structure and surface properties of anode materials have a great influence on the kinetics of MFC anodes.Nevertheless,the effect of surface physic-chemical and porous structure property of nanostructured anode materials on the bioelectrocatalytic behavior is still need to be further explored.Herein,five carbon based anode nanomaterials with hierarchically porous nanostructures and surface properties are designed and fabricated to promote the bioelectrocatalytic performance of MFC inoculated with Shewanella putrefaciens CN32?S.putrefaciens CN32?as the microstrain.Furthermore,the mechanisms to enhance the bioelectrocatalytic processes such as the biofilm growth ability of S.putrefaciens CN32and it electron transfer process are systematically studied based on synergistic effect of nanomaterials porous structure and surface properties.The main research contents and results are as follows:1.For high performance MFC,it is very important to develop a novel MFC anode with an appropriate porous structure and excellent electronic conductivity to simultaneously boost both electro-and bio-catalysis.In this case,we developed a hierarchical porous graphene aerogel/nickel?G/Ni?electrode without any binders or conducting additives.The electrodes were optimized with different graphene loading for the best electrochemical behaviors.The reduced graphene oxide?rGO?nanosheets have a self-assembled porous aerogel structure distributed pore sizes of 20 nm to 50?m and fill the pores of the nickel foam.Thus,this binder free hierarchical porous G/Ni anode delivers a maximum power density of 3903 mW m^-2 in S.putrefaciens MFCs,which is 2-fold and 13-fold higher than that of the vacuum-dried anode and conventional MFC carbon cloth anode,respectively.The unique hierarchical structure with appropriate macro-/nano-pore distribution and enhanced biocompatibility not only allows bacterial growth on both the outside and inside surfaces of the pores and generates a biofilm to increase the electron transfer/transport between bacteria and graphene,but also the tight electroactive biofilm provides a large amount of S.putrefaciens CN32 cells to generate a high concentration of local electron shuttles around anode along with a short diffusion distance of electron shuttles between bacterial cells and electrodes to greatly promote the direct electrochemistry,thus simultaneously boosting the bio-and electro-catalytic processes for a high power MFC.2.In order to study the effect of the anode material surface properties on bacterial biofilm growth and the biofilm contribution to the bioelectrocatalysis at the interface,a honeycomb-like hierarchical porous carbon-silica?PC/Si?composite with macropores?2-15?m?and mesopores?3-4 nm?has derived from distiller's grains via a simple carbonization procedure and used as an anode in S.putrefaciens CN32 fuel cell for the first time.After optimization with carbonization temperature,the PC/Si anode delivers a much higher maximum power density of 580.7 mW m^-2 in S.putrefaciens CN32 MFC,which is 4.5-fold higher than carbon cloth due to the unique nanostructure and siliceous crusts surface for high bioelectrocatalysis activity.The 3D scaffold with mesopores on the wall provides large surface for both biofilm growth and the flavin redox reaction.At the same time,the siliceous crusts greatly promote the adhesion of bacteria cells on the PC/Si surface so that the large amount of electroactive biofilm can enhance the rate of interfacial electron transfer.After removing siliceous crusts from porous carbon-silica anode,the long distance for the free flavin diffusion will lead to a sluggish electron transfer from exoelectrogen to the electrode and a low power output.Thus,this work demonstrates that the PC/Si composite derived from distiller's grains biomass with hierarchically porous structure and siliceous crusts surface could improve bioelectrocatalysis by a large amount of biofilm as well as the enhanced interfacial electron transfer.3.Considering the contribution from both the surface chemistry and the pore structure,an efficient MFCs anode should to emphasis on regulating the surface chemical structure and porous structure to simultaneously boost bio-catalytic and electro-catalytic process.A hierarchically porous nitrogen-doped CNTs/rGO composites synthesized with polyaniline as nitrogen source was developed and further used as an anode material to boost the bio-and electro-catalysis of S.putrefaciens CN32 MFC.Owing to the 3-D hierarchically porous structure with excellent biocompatibility for rich bacterial biofilm and also the nitrogen doped surface for flavin adsorption,the N-CNTs/rGO anode delivers a maximum power density of 1137 mW m^-2 in S.putrefaciens CN32 MFCs.It is 8.9-fold higher than that of conventional carbon cloth anode and also higher than that of N-CNTs,N-rGO and non-doped CNTs/rGO anodes.For the reasonable mechanism of greatly improved performance of N-CNTs/rGO anode,the insertion of CNTs into the rGO sheets could effectively inhibit the aggregation of rGO,and result in hierarchically porous network architecture.This structure would offer more accessible rGO surface area for bacterial adhesion and nutrient transport.In addition,the bridging effect of inserted CNTs and compact edge cross-linking among rGO sheets would provide an interconnected electron transfer network,resulting in enhance bio-catalysis process and a fast electron transfer.Furthermore,compared with non-doped composite,the N-CNT/rGO anode could adsorb flavins on the surface to guarantee a high concentration for fast interfacial electron transfer.The incorporation of nanoporous structure into macroporous architectures along with proper surface functionalization can offer a promising strategy to synergistically improve bio-and electro-catalysis,which may boost a rapid development of bioelectrodes in microbial electrochemical system.4.The advantages of nitrogen atom doping on increasing the reaction rate of electron mediator on the anode surface,accelerating the interfacial electron transfer efficiency and improving the bio-electrocatalytic performance have been confirmed.However,the detail functions of the nitrogen groups for the interfacial electron transfer and effect mechanism of the nitrogen-doped structure on the direct electrochemistry has not been disclosed yet.In this case,a thermal treatment is used to control nitrogen doping to molecularly marching with Flavin?mediator?reaction sites,which results in strong absorption to convert diffusive mediator molecules for anchored redox centers.This eventually realizes a fast two-electron transfer based direct electrochemistry on Flavin mononucleotide?FMN?-nitrogen reaction center,in which the anchored FMN offers extremely short electron pathways while the doped nitrogen significantly promotes the charge transfer rate.The power output of the MFC shows that the open-circuit potential and the maximum power output of the MFC with N-CNWs/CC-900 anode is 712.1 mV and 563.5 mW m^-2with a high plateau current density of 1.8 A m^-22 under an external resistor of 1500?,respectively.This is higher than other N-CNWs/CC MFC and 5.6-fold higher than that of the plain carbon cloth MFC(101.1 mW m^-2).After adsorbed FMN,the maximum power density of the MFC can be further enhanced to 2102.88 mW m^-2,which is around 21-fold higher than that of the plain carbon cloth MFC.The optimized N-CNWs shows superior absorption ability and the unique enhancement mechanism of nitrogen atom doping structure on the direct electrochemistry is proposed for the first time:N-doped structure could molecularly match electroactive sites of flavins mediators and thus greatly enhance their two-electron transfer,further to improve the interfacial charge transfer between bacteria and the anode.In addition,the N-CNWs electrode not only can promote FMN absorption but also enhance bacteria cell density adhered on the surface of the electrode.This work discloses the fundamental insights of convert mediated electron-transfer for fast direct electrochemistry,and demonstrate a universal approach to greatly improve the direct electrochemistry process by tuning more detail surface chemistry structure for electroactive sites allowing the mediator accessing the electrode surface.5.In order to further analyze the effect of biofilm growth on the electrocatalysis of S.putrefaciens CN32 MFC anode,the carbon cloth electrode modified with nitrogen-doped carbon nanowires was further modified with a layer of NiO nanowires.The modified oxide layer could enable the electrode to promote bacterial adhesion and electron mediator adsorption.The obtained NiO@N-CNWs/CC anode delivers a maximum power output density of 1216.1 mW m^-2 in S.putrefaciens CN32 MFC,which is 8.32-fold,3.8-fold and 2.15-fold higher than that of carbon cloth,NiO/CC and plain N-CNWs/CC anode,respectively.The NiO nanowire and nitrogen-doped carbon nanowires produce a unique strong synergistic effect on enhancing bioelectrocatalysis.Firstly,NiO nanowire promotes bacterial growth on the anode surface to boost the biocatalytic process.Secondly,the tight electroactive biofilm provides a large amount of S.putrefaciens CN32cells to generate a high concentration of local electron shuttles around anode for a short diffusion distance of electron shuttles between bacterial cells and electrodes to greatly promote the direct electrochemistry.Finally,the synergistic effect from NiO nanowire and nitrogen-doped carbon nanowires accelerates the electron transfer process mediated by flavins between S.putrefaciens CN32 cells and the NiO@N-CNWs/CC anode.This unique synergistic effect significantly boosts the redox reaction of flavins by reduced the anode interface charge transfer resistance.In brief,the superior anode nanomaterials should not only promote biofilm growth but also accelerate the electrochemistry of electron shuttles,thus achieving both fast bio-and electro-catalytic process.Tailoring the anode porous structure and the modification of surface properties could accelerate the extracellular electron transfer at the anode interface while the molecule match adsorption of the mediators could convert the diffusive mediator-based electrochemistry to fast direct electrochemistry for great improvement of the output power density,which is an effective way to realize efficient biocatalysis and electrocatalysis in microbial fuel cell.