Research On Power Generation And Nitrate Removal Of Microbial Fuel Cells Based On Metal-nitrogen Co-doped Carbon Catalysts

Microbial fuel cell（MFC）is an emerging water treatment technology that combines pollutant removal and production capacity.According to the cathode electron acceptor,MFCs can be roughly divided into two categories:one is the air microbial fuel cell（ACMFC）with oxygen in the air as the cathode electron acceptor.Due to the high theoretical reduction potential of oxygen,increasing the productivity of MFCs has attracted widespread attention.However,in the absence of catalyst action,the oxygen reduction reaction of the ACMFC cathode is almost impossible to occur,resulting in low production capacity of ACMFC.The other type of MFC uses oxidizing water pollutants（such as NO₃^-）as cathode electron acceptor,which is reduced to N₂under the catalysis of cathode microorganisms.Currently,the cathode oxygen reduction catalysts for ACMFC are mostly Pt/C,but due to the high cost of Pt/C,this greatly increases the cost of ACMFC.Therefore,finding low-cost and efficient catalysts to replace Pt/C is an urgent problem to be solved.In addition,most of the current research on MFC removal of NO3-is conducted under anaerobic conditions.Although excellent nitrate removal efficiency can be achieved,its production capacity is very low.The discovery of aerobic denitrification bacteria enables O₂and NO₃^-to act as cathode electron acceptor of MFC at the same time,which means that MFC may have the function of removing NO₃^-in sewage while obtaining high productivity.This article conducts research from two aspects:on the one hand,a low-cost,highly active,and stable carbon catalyst（N/Fe/Co-C/CNT）co doped with multiple metals and nitrogen is prepared to replace Pt/C as an oxygen reduction catalyst;On the other hand,N/Fe/Co-C/CNT was used as cathode catalyst,while denitrification bacteria were enriched on the catalyst surface,and O₂and NO₃^-were used as electron acceptor at the same time to study the productivity of MFC and the removal effect of nitrate.The main conclusions are as follows:Firstly,based on the N/Fe-C catalyst,the oxygen reduction catalytic activity and stability of the catalyst can be improved through three aspects:metal content,type and content of multi-metal doping,and carbon matrix.Four types of cataly sts were prepared using calcination method,including N/Fe-C,N/Fe/Co-C,N/Fe/Zn-C,and N/Fe/Co-C/CNT.By removing residual metal ions from the catalyst throug h acid washing,the stability of the catalyst is improved.The results indicate that the catalyst after acid washing has better stability while maintaining catalytic ac tivity.The N,Fe,and Co co doped catalyst（N/Fe/Co-C/CNT（sh））with carbon black and carbon nanotubes as the matrix after acid washing has a corrected pea k potential（-0.38V）,and its oxygen reduction electron transfer number is 3.4,in dicating that ORR is mainly characterized by four electron transfer.In addition,t he microstructure and elemental composition characterization of N/Fe/Co-C/CNT s howed that the formation of functional groups such as graphite nitrogen,pyridine nitrogen,Fe-N,and Co-N was the main reason for the high activity of the cata lyst.Subsequently,the N/Fe/Co-C/CNT-1（sh）catalyst was used for the ACMFC cathode to test the production capacity and stability of ACMFC,and compared with ACMFC using N/Fe/Co-C（sh）,Pt/C,and carbon black as catalysts.Through three months of production capacity,TOC removal,and stability testing,it was found that the maximum power density of MFC-Fe/Co/C/CNT was 15%to 2.9 times higher than other catalysts.The maximum voltage of MFC-Fe/Co/C/CNT could reach 490 m V,and all showed high stability and oxygen reduction activity during the three month experimental period.This indicates that N/Fe/Co-C/CNT（sh）is a catalyst with potential applications.Finally,this paper studied the productivity and nitrate removal capacity of MFC with N/Fe/Co-C/CNT（sh）as cathode catalyst and denitrification bacteria domesticated at the MFC cathode,and O₂and NO₃^-as cathode electron acceptor,and discussed the impact of environmental factors such as C/N ratio,carbon source,external resistance and dissolved oxygen.The results show that the voltage of MFC（DMFC Fe/Co/C/CNT/B）is about 400 m V when N/Fe/Co-C/CNT（sh）catalyst and cathode denitrification bacteria coexist,and the voltage of MFC is about 250 m V when carbon black and cathode denitrification bacteria（DMFC-C/B）coexist.Under low C/N ratio conditions,both DMFC-Fe/Co/C/CNT/B and DMFC-C/B have efficient removal ability for nitrate.Within 2 hours,the nitrate removal rate can reach 80%,and it can be completely removed within 3 hours.These research results indicate that DMFC-Fe/Co/C/CNT/B can effectively remove NO₃^-while significantly increasing production capacity.