Study On Bio-electrocatalysis Mechanism On The Interface Between Shewanella Biofilm And Mesoporous Carbon Anode

Microbial fuel cells(MFCs)can convert the chemical energy of organic compounds into electricity through the metabolization within the microorganisms.It is a green energy technology that attracts more attentions in recent years.The promising advantage of MFCs is the ability to produce electricity from the organic matter in the wastewater,and thus has a good application prospect in the wastewater treatment and environmental remediation.At present,the lower power output of MFCs restricts its development and practical application as a power supply device.Various factors like cell configuration,proton exchange rate of the proton exchange membrane,the catalytic ability of the cathode and anode as well as the electro-activity of the microorganism affect the performance of MFCs.The bioelectrocatalytic process of anode is considered to be the key to determine the output performance of MFCs,while the slow interfacial electron transfer between biofilm and the electrode is the limiting step in the process,which also restricts the development of MFCs technology.Shewanella species are widely used exoelectrogens in MFCs,which utilize flavins as free electron shuttles or cofactor in outer membrane cytochromes to facilitate the interfacial electron transfer between the biofilm and the electrode.The electron transfer usually includes the following three processes:the first is the bacteria obtain electrons from the substrate and pass them to the flavins located in the periplasmic space of the bacteria cells;the second is the excretion of flavins to the extracellular surroundings followed by the redox reaction of the flavins on the electrode surface;the third is the reduction of the flavins via the MtrC system in the bacterial outer membrane after oxidation on the electrode surface.The application of nanostructured anode is an effective approach to solve the bottleneck of MFCs development.Mesoporous carbon(MC),as an excellent nanomaterial,has unique properties such as uniform pore size,large pore volume,high specific surface area,good electrical conductivity and biocompatibility.It has been widely applied in supercapacitors,lithium ion batteries,oxygen reduction reactions and other fields.However,few reports focus on MCs as MFCs anode materials.Moreover,in these reports,MCs bioelectro-catalytic performance is not as good as expected.It seems that most of its pore area has not been fully utilized.Based on this issue,the effects of pore structure and surface properties of MCs on the bioelectrocatalysis in Shewanella putrefaciens(S.putrefaciens)CN32 MFCs were investigated in this dissertation.The main contents and results are as follows:(1)Room temperature ionic liquids(ILs),the molten salts with melting points below100oC,have been used in various electrochemical systems like fuel cells,supercapacitors.They possess unique physicochemical properties like wide electrochemical windows,good biocompatibility,high conductivity,good dissolving capability,and satisfactory chemical and thermal stability.IL modified carbon electrodes have been used for the determination of important biological compounds and pharmaceuticals.In this work,MC-H was prepared by concentrated nitric acid mesoporous carbon(CMK-3).Then,the MC-IL was modified by the simple chemical reaction on the acidified mesoporous carbon.The surface functionalized MCs were used in S.putrefaciens CN32 MFCs to explore the effect of the surface properties of mesoporous carbon on the bioelectrocatalysis of electrode interface.The physical and chemical characterization shows that the acidification and IL modification do not affect the pore structure a lot.The pore volume decreases slightly and the specific surface area also decreases after IL modificaion.However,the MC-IL shows better bioelectrocatalytic performance.The reason is that the imidazoles with positive charges neutralize the negative charges on the surface of mesoporous carbon,thereby increasing the adhesion between electrodes and negatively charged S.putrefaciens CN32 cells on the surface,and thus increasing the load of bacterial biofilms on electrodes.In addition,the good conductivity of the ionic liquid promotes the redox reaction of the flavins on the interface of the MC-IL anode.The results show that the maximum power density of the MFC with MC-IL anode is 1388mW/m~2,which is 1.2 times of the MFC with MC anode.The surface modified mesoporous carbon of the ionic liquid enhanced the bioelectrocatalytic performance of the MFCs anode,which opened a new way of anode design of high power out MFCs.(2)Mesoporous carbon has quite high specific surface area so that can provide a large number of active sites for redox reaction of flavin mediators,which should substantially improve the power generation of MFCs at a great level.However,the power output performance of MFCs with MC anode is often not satisfying.It seems that a large part of surface area contributed from the pore wall cannot be fully used for anodic redox reaction,but the reason is still not clear.To understand this phenomenon,we used silica as hard template and sucrose as precursors to prepared mesoporous carbon.In this work,three different mesoporous carbon materials(including CMK-3)were used to explore the effect of mesoporous structure on the bioelectrocatalytic properties in S.putrefaciens CN32 MFCs with detail mechanism.The results show that compared with 3nm channel mesoporous CMK-3(MC-I)and MC-II with 14nm spherical pores,MC-III with 40~60nm spherical pores has better bioelectocatalytic properties,although the specific surface area of MC-III is lower than MC-II and MC-I.The reason may be that at the interface between biofilm and electrode,MC-III's large size mesopores provide good pore structure for redox reaction of flavins.The narrower mesoporous cells could not be effectively utilized due to the influence of electric double layer,so the actual electrochemical active area is much smaller than its specific surface area.The results show that the maximum power density of the MFCs with MC-III anode is1700mW/m~2,which is 1.6 times of the MFCs with CMK-3 anode.It reveals that the large spherical mesopores are more favorable than the channel type narrow mesopores for flavin based interfacial electron transfer between biofilm and the electrode.This work will provide some new insights for the design of porous anode for the MFCs.The conclusions of this dissertation reveal the factors that will affect the bioelectrocatalytic performance of a nano porous anode for MFCs and provide original insights to the design of MFCs anode.