Study On The Properties Of Nitrogen - Doped Metal / Carbon Composite Microbial Fuel Cell Cathode

Microbial fuel cells(MFCs) are a new type of water treatment technology which can produce electricity and purify sewage. MFCs have many advantages such as wide variety of fuel sources, mild reaction conditions, environment–friendly, substrate chemical energy can be directly transformed into electrical energy, none environmental pollution, and so on. However, its low power output and coulombic efficiency, and high catalytic material(commercial Pt/C) costs will still litmit its large–scale applications. The carbon nano–materials have attracted more and more attentions due to its unique molecular structure and excellent mechanical, electrical and chemical properties. The advanced nature of the carbon–based catalyst for generating electricity in MFCs will have a profound impact.It mainly studied the cathode material of MFCs, and chose the best N–doped metal/carbon composites as the electrodes. The choice of the electrodes preparation conditions, types of metal elements, doping content, and the removal rate of COD were investigated. We had prepared two types of cathode catalytic materials, using melamine as the carbon source. By introducing the iron species and silver, the catalytic activity and antibacterial properties of composite materials were correspondingly changed. A single–chamber microbial fuel cell was constructed by using carbon steel mesh as cathode, fiber brush electrodes as anode, and glucose as the simulated wastewater. The composites were characterized and evaluated by XRD, Raman, XPS, TG, BET, SEM, TEM, and other physical and chemical characterization. These tests were used to study the crystalline phase, morphology, and phase transformation of the composites. By using electrochemical analysis, oxygen reducton reaction(ORR) occured in the three–phase interface and its electrochemical behavior through the detection of oxygen reduction were studied.(1) In the range of 620–660 oC, the effects of change of carbonization temperature(interval of 10 oC) on the crystalline phases transformation of N–doped Fe–species/partly–graphitized carbon(Fe–species/NPGC) were significant. It indicated that Fe/Fe3C/NPGC–650 with SBET of 50.27m2/g was obtained at 650 oC, which could be considered as a stable MFCs catalyst. It could generate the power density of 1323 mW/m2 and only had an internal resistance(Rct) of 9.7 ?, indicating that it had better conductivity and ORR activity than other catalysts. It should be noted that the performance of Fe/Fe3O4/Fe3C/NPGC–640 was slightly lower than Fe/Fe3C/NPGC–650 and both of them were higher than Pt/C, which were attributed to that the Fe3 C nanoparticles with high activity were uniformly distributed in the catalysts. It was verified by XPS that the activity(active–sites) and charge transfer capacity across the triphase interfaces were enhanced by the presence of Fe–N species, pyridinic N and pyrrolic N, which are conducive to “capture–consume” the electrons for catalyzing ORR. The synergistic effect between Fe–species and doped N could enhance the performance of MFCs and shorten the starting time.(2) To improve the antibacterial performance, ORR, and electricity-generation efficiency in single–chamber MFCs, N–doped Ag/Fe partly–graphitized carbon composites(Ag/Fe/NPGC)(carbon: iron: silver of 10: 1: 0.5) are obtained at 620–900oC. By comparing with their physical and chemical properties, the Ag/Fe/NPGC with high conductivity, ORR activity and stability is expected to be prepared. When the calcination temperature was 630 oC, the power density can reach to 1791 mW/m2. Ag/Fe/NPGC–630 with SBET of 13.98m2/g has the best catalytic activity as cathode and its power density can reach 1791mW/m2(Rct=5.46?). The maximum voltage output(0.668 V, Rct=6.17?) and coulombic efficiency(33%) are obtained by Ag/Fe/NPGC–640(24.39m2/g). Because of the introduction of double–effect(antibacterial and ORR active) nano–silver, these catalysts can improve the performance of the entire system of MFCs.Nitrogen–doped carbon–based composites as single–chamber microbial fuel cell cathode catalyst are prepared at low temperature by using the melamine as the relatively safe carbon. The composites(Fe–species/NPGC and Ag/Fe/NPGC) have good stability in the long run. Nitrogen atom is successfully embedded into the partly–graphitized carbon structure, improving the ORR activity of composite material. Proton transfer efficiency, reduction of the internal resistance, enhanced performance of the entire electrical system are obtained.