Metal-free Heteroatom-doped Ordered Mesoporous Carbon As Cathode Catalysts Used In Microbial Fuel Cells

Nowadays,it is highly necessary to develop Microbial fuel cell(MFC),a newly green technology armed with two excellent functions of electricity generation and wastewater treatment.MFC can becalled as ?engine? for future energy that could satisfy the crisis of energy and environment in the 21 st century.In an MFC,exoelectrogenic bacteria oxidize organic substrates,transfer electrons to the anode,and produce current flow to cathode,where electrons and protons combine with oxygen to produce water.The oxygen reduction reaction(ORR)occurring on the cathode involves a series of electron transfer processes.Normally,the ORR kinetics is very slow and needs cathode catalysts to speed up the reaction.At the current stage in technology,Platinum(Pt)has long-term been taken as an ultimate ORR catalyst benchmark for its high ORR catalytic activity.However,its limited natural reserves,high cost and low tolerance to toxic pose severe obstscles for real world MFC application.Therefore,it is critical to develop cost-benefit non-Pt-based electrocatalysts to make MFC scale-up practical.This work intends to set metal-free heteroatom-doped carbon materials as the starting point to develop cost-effictive and stable cathode catalysts.We designed and synthesized nitrogen-doped carbon black catalyst(N-CB),nitrogen-doped ordered mesoporous carbon catalysts(N-OMC)and dual metal-free heteroatom N and F-doped OMC(NF-OMC).Diversified catalyst character methods were adopted to analyze structure,morphology,element content,surface chemistry,ORR activity and MFC performance.On the base of the results,the possible correlation between the catalysts structure and ORR reactivity/power generation was established.In addition,the industrial feasibility of using prepared materials as cathodic catalysts in MFC was analysized.Our work is the innovative research work to take OMC into the MFC application.Several important and intriguingresearch conclusions were drawn from our work.The main work can be delineated as follows:(1)Nx-CB derived from the pyrolysis of the low-cost industrial melamine and carbon black were studied as economical and practical cathode catalysts in SCMFCs.The initial mass ratios x(melamine : carbon black)of 10,50 and 70 were used.The results of electrochemical activity and MFC performance manifested that nitrogen doping made great improvement in catalytic character.N50-CB exhibited better ORR activity,higher voltage output and power density generation when compared with N10-CB and N70-CB.SCMFC equipped with N50-CB catalyst yielded the maximum power density of 381 mW/m~2.Combing with XPS,it was the effective contents of nitrogen and higer contents of active nitrogen group that promoted electrocatalytic activity and boosted performance of MFC.(2)On the base of Nx-CBs,Nx-OMCs derived from melamine and OMC were synthesized and studied for the first time in SCMFCs,achieving high voltage output and power generation.The changes in the chemical composition,morphology and structural properties after nitroden doping were systematically characterized.The SCMFC catalyzed by N50-OMC generated the maximum power density of 553± 11 mW/m~2,which was comparable to that of Pt-based carbon cloth cathodecontaining 0.5mg Pt/cm~2(530 ± 7 mW/m~2).The enhanced power density of N50-OMC was attributed to its high amount of graphitic nitrogen functionality,high structural defect degree,high surface area and low charge transfer resistance.In addition,the performance of the SCMFC equipped with N50-OMC presented excellent stability over time.After approximately 2 months of SCMFC tests,thevoltage dropped only by 7% compared with 10% for the Pt/C.This study reveals the great potential of N50-OMC catalysts asstable cost-benefit catalystsfor MFC scale-up.(3)With the view of decreasing the initial mass of doping materials,the dual heteroatom(nitrogen and fluorine)-doped OMC(NF-OMC).The electrochemical activity and MFCs' performance were tentatively explored.The catalyst characterization suggested F atom was not easy to enter the carbon lattice but could promote N loading.Compared with the mono-doped catalyst(N-OMC and F-OMC),SCMFC with NF-OMC catalyst exhibited better ORR activity and higher power generation.However,the inferior electrochemical performance of NF-OMC to N50-OMC probably resulted from the low concentration of F in the prepared material.