| Microbial fuel cell(MFC)uses microorganisms as catalyst to directly convert chemical energy in organic matter into electrical energy during wastewater treatment.It is a new energy system with sustainable development.Compared with conventional chemical fuel cells,it has unique advantages such as mild operating conditions,low cost,environmental friendliness and so on.The slow extracellular electron transfer between microorganism and electrode leads to the low power density of MFC,which limits its practical application.The output power mainly depends on the degradation rate of substrate,the electron transfer rate of bacteria to anode,circuit resistance,mass transfer rate of protons in liquid,electrode performance and external working conditions.The type and composition of electrode materials,especially anode materials,will affect the adhesion of bacteria and the speed of electron transfer,which plays an important role in the performance of MFC.Therefore,anode materials play an important role in improving the efficiency of extracellular electron transfer of MFC.High efficiency anode biocatalysis requires high loading capacity of biocatalyst and fast electrochemical reaction kinetics.The electrochemical reaction of MFC mainly occurs at the electrode interface.The surface chemical properties of the anode materials(including wettability,surface charge,interface electron transfer ability and surface redox activity)directly affect the heterogeneous electrocatalysis process between the electrode and microorganism,and between the electrode and the electrolyte,which is a major factor affecting the anode performance.In this paper,bacterial cellulose was used as the precursor to prepare anodematerials with different surface properties and structures through different surface modification.The typical electricity producing strain,Shewanella putrefaciens CN32,has been used as biocatalyst to study the influence of different anode materials on the mode of electron transfer at the interface of Shewanella.The main research contents and results are as follows:(1)In order to study the effect of N-doped carbon nanofibers on the extracellular electron transfer of Shewanella putrefaciens CN32,different N-doped carbon nanofibers were prepared by hydrothermal reaction of bacterial cellulose with urea and ammonia solution.The effect of N-doped carbon nanofibers on the extracellular electron transfer was studied by electrochemical behavior analysis in S.putrefaciens CN32 MFC.Through physical characterization,it was found that N element was successfully modified on carbon nanofibers,and the introduction of N element increased the nitrogen-containing functional groups on the surface of CNF.Compared with the unmodified CNF,N-CNF has better adsorption capacity of FMN and higher catalytic current,and thus achieves higher power output in S.putrefaciens CN32 dual-chamber MFCs,in which N-CNF-? has the largest power density.Finally,it is concluded that the N-doped carbon nanofibers mainly promote the FMN mediated indirect electron transfer process.(2)Boric acid is used as B source to prepare carbon nanofibers with different B contents by solution impregnation.A series of physical characterization of different materials are carried out.It is proved that B was successfully modified on carbon nanofibers,which not only increased the boron-containing functional groups on the surface of carbon materials,but also increased the O element content on the surface of carbon nanofibers,and more redox active sites were found on the surface.The specific surface area of carbon nanofiber network decreased significantly due to the crosslinking effect of boric acid.The electrochemical behavior test of the material shows that the overall increase of capacitance current of carbon nanofibers modified by a small amount of B enhances the promotion of FMN oxidation-reduction reaction.However,when the boric acid concentration in the BC precursor is too high,the promotion is weakened.The electrochemical behavior study in the bacterial half-cell also shows that with the increase of boric acid concentration,the release of the material is enhanced Both the point current and the catalytic current are reduced,which shows that the concentration of boric acid is too high,which will destroy the 3D network macropore structure ofbacterial cellulose,at the same time,it will also destroy the mesoporous structure in the material,which is not conducive to the diffusion and transmission of FMN electronic mediator,and is not conducive to the growth and adhesion of bacteria.The appropriate B content can not only promote the indirect extracellular electron transfer,but also enhance the direct electron transfer process mediated by c-type cytochrome.(3)On the basis of the first two studies,CNF@NiO and NDCNF@NiO nanocomposites and CNF@Ni nanocomposites were prepared by hydrothermal reaction bofore and after carbonization,using NiCl2 as the source of Ni.It is found that compared with CNF@Ni nanocomposites prepared before carbonization by the FESEM,the distribution and size of NiO on CNF@NiO and NDCNF@Ni O composites synthesized after carbonization are very uneven.Therefore,in the subsequent experiments,CNF@Ni nanocomposites prepared before carbonization are selected,and their XRD and XPS analysis showed that Ni is successfully grown in CNF networks.The chemical behavior analysis show that CNF@Ni composites greatly improve the redox reaction of FMN,among which CNF@Ni-? has the largest redox peak current of FMN.In the electrochemical behavior analysis in S.putrefaciens CN32 bacterial suspension,the CNF@Ni-? has a large catalytic current and discharging current.As a result,CNF@Ni-? achieves significant improvement on flavin mediated indirect electron transfer.By comparing the XPS analysis of the materials before discharge,it is found that after 48 h of constant potential discharge,the shape of the characteristic peak and the position of binding energy of Ni 2p in CNF@Ni composites have obvious changes,which suggests that the Ni may play a key role in interfacial direct electron transfer. |
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