Investigation Of Cu-bipy-BTC-derived Carbon-based Catalyst Modified Cathode For Improved Performance Of Microbial Fuel Cell

Microbial fuel cell?MFC?could convert the chemical energy of organic contaminants into electricity directly,which is an energy conversion device combining energy production and environmental protection.Air cathode with oxygen as electron acceptor has high redox potential.However,oxygen reduction reaction?ORR?in cathode is quite sluggish,so highly efficient catalysts are essential to speed up the rate and reduce the cathode overpotential.Platinum?Pt?-based catalysts possess excellent catalytic activity,but the high cost,scarcity,and sensitivity to poisoning greatly hinder their practical application.Therefore,it is urgent to develop efficient and low-cost ORR catalysts to replace Pt.Metal organic framework?MOF?Cu-bipy-BTC has excellent ORR catalytic performance in pH-neutral electrolyte.Thus,it is feasible to modify MFC cathode with Cu-bipy-BTC.The major problem is to improve its electrical conductivity.In this regard,we intended to enhance the electrical conductivity of Cu-bipy-BTC by direct pyrolysis,and simultaneously keep its ORR catalytic activity to improve the cathode performance and consequently boost MFC electricity generation performance.In this work,the precursor Cu-bipy-BTC was prepared using hydrothermal method,and a series of pyrolysis products MOF-T?T=600?700?800?900 C?were prepared by direct pyrolyzing Cu-bipy-BTC in N₂ atmosphere.The results showed that MOF-T had a similar crystal structure with Cu,N-incorporated.Cu-bipy-BTC had only one nitrogen type of pyrrolic nitrogen,but MOF-T had three nitrogen types of pyridinic,pyrrolic and graphitic nitrogen which provided abundant active sites?C–N,Cu–N_x?and thus facilitated the ORR process.Compared to Cu-bipy-BTC,MOF-800 and MOF-900 had lager specific area of 58.4m²·g^-1and 35.7 m²·g^-1respectively.Furthermore,MOF-800 and MOF-900 originated amount of micropores ranging from 0.5¹.3 nm.Electrochemical tests showed that MOF-T had ORR catalytic activities.Considering both the performances of the catalysts and the energy consumption,an optimum pyrolysis temperature of 800°C was proposed in this study.The ORR catalyzed by MOF-800 was the typical hybrid control step and the first-order reaction kinetics.The onset potential of MOF-800 was ca.-40 mV?vs.Ag/AgCl?,and its limiting current density was-3.06mA·cm^-2.Rotating ring–disk electrode test showed that the ORR electron transfer number of MOF-800 was 3.95³.99,and the yield of H₂O₂ was lower than 2.5%,indicating that MOF-800 was highly selective and the ORR followed a 4e^-pathway.The pronounced ORR activities of MOF-800 attributed to its richer porous structure,and larger specific surface area,and abundant nitrogen species compared to Cu-bipy-BTC.Single chamber air-cathode MFCs were constructed with Cu-bipy-BTC and MOF-T as cathode catalysts,and the performances of these MFCs were discussed compared with plain cathode?no catalyst modified?MFC and classic Pt/C modified MFC.The results demonstrated that MOF-800 modified cathode had excellent ORR catalytic performance with a reduction current of ca.-2.2 mA·cm^-2,which was 76%of that of Pt/C cathode.The electrochemical impedance spectroscopy test showed that Cu-bipy-BTC cathode had the largest charge transfer resistance(R_ct)of 176.8?because of its insulating character.When pyrolyzed at high temperature,the R_ctt of the pyrolysis products decreased enormously.MOF-800 cathode had an R_ctt of 1.38?.A maximum power densityof 326±11 mW·m^-2was achieved by MOF-800-MFC,which was 2.6 times of that of Cu-bipy-BTC-MFC and 81.07%of that of Pt/C-MFC.MOF-800 had excellent ORR catalytic performance and good electrical conductivity,and it was expected to replace Pt-based catalyst for practical MFC applications.