Performance Of Silver/Iron-Carbon Based Composites As Cathodes In Microbial Fuel Cell

Currently, to solve the increasingly serious problems of water pollution and to explore the new and renewable energy are the two fundamental issues of sustainable development in human society. Microbial fuel cells(MFCs) are a new technology for obtaining energy from wastewater, which can produce electricity and meanwhile purify sewage. For the MFC cathode with electron acceptor of O2, the oxygen reduction reaction(ORR) on an air cathode is one of the key factors affecting the electrical properties of the cell. Therefore, it is important to search the cathode catalyst with high ORR activity, good stability and low cost. Contents of this paper can be divided into two parts. First, ferrous sulfide/(part) graphite carbon(FeS/PGC) or Ag/FeS/PGC catalysts are prepared by carbothermic reduction from the carbonization of pomelo skins as carbon source, iron salt as a graphite catalyst, and thiourea as a sulfur source,or silver nitrate as the silver source under a nitrogen atmosphere. Second, based on our previous work, silver/iron/nitrogen/graphite carbon(Ag/Fe/N/C) composites are prepared from Fe/N/C catalyst after the introduction of Ag source. Through the morphology and property analyses of the above two kinds of catalysts, ORR catalytic activity was detected by means of electrochemical measurement. Then, these catalysts as the air cathode catalyst are used in the single-chamber MFC to study the electricity-generating properties.(1) The presence of FeS in the structure of PGC can enhance the catalytic activity of FeS/PGC and the ORR reaction. The main crystalline phase in FeS/PGC carbonized at different temperatures(600, 700, 800, 900 and 1000 °C) is FeS with a small amount of Fe3O4 and α-Fe. ORR catalytic activity of FeS/PGC-900 is better than others,because the FeS and other nanoparticles are embedded in the interconnected and parallel channels. This structure provides an unobstructed transmission path for O2,while the ORR catalytic active sites increase, so that the activation and reduction ofeffectively-adsorbed O2 on the active center of the surface become easier. The voltage output and stability of Fe S/PGC-x(x=800, 900 and 1000) cathodes are better than those of Pt/C after 120 d opertion in MFC. Power density of 930 ± 10 mW m–2 is obtained by FeS/PGC-900 cathode. It is implied that FeS as the major active component in the catalyst plays an important role in the cathode ORR. The activity and electron transport of anodic bactria are improved by the presence of FeS, which reduces the loss of the released electrons.(2) Because the surface of MFC cathode is easy to form a thick layer of biofilm,the activity of anodic electricity-generating bacteria, electron transfer and proton migration are inhibitted, and eventually, both the catalytic efficiency of O2 reduction and the catalystic activity are greatly lowered. Thus, Ag/FeS/PGC catalysts are successfully prepared by introducing the silver. Power density(1400±10 mW m–2) of Ag/FeS/PGC(Fe:Ag=1:0.6) cathode is better than those of Pt/C. These results indicate that except for the important role of FeS, the nano Ag can be used as the bifunctional(ORR catalytic and antibacterial activities) catalyst, which can avoid the O2 consumation in the process of proliferation, metabolism and decline of bacteria on the surface of the cathode. The synergitic effect between Ag and FeS can enhance ORR efficiency and the electricity- generating property of MFC. In the rotating disk electrode(RDE) tests, the average number of transferred electrons(n) of Ag/FeS/PGC(Fe:Ag=1: 0.6) is 3.89, indicating that dominant pathways for ORR are the 4e oxygen reduction.(3) The introduction of Ag and Fe can effectively improve the amounts of active sites of catalyst. This study prepares the N-doped Ag/Fe/C(Ag/Fe/N/C), which can enhance the O2 adsorption on the catalyst surface to further improve the ORR efficiency. Using melamine as the carbon source, Ag/Fe/N/C is successfully prepared by introducing the Ag, Fe and N species into the skeleton of partly-graphited carbon.The ORR catalytic and antibacterial properties, power output and longterm stability ofthe catalysts are effectively improved by introduction of Ag nanoparticles. The catalytic activity of Ag/Fe/N/C-0.5 is better than others, which has the power density of 1658±10 mW m–2, coulombic efficiency of 32 %, and COD removal rate of above92 %. The dominant pathways for ORR are the 4e oxygen reduction for both Ag/Fe/N/C-0.5 and Pt/C.This study shows that both iron-carbon based and silver/iron-carbon based catalysts have ORR catalytic activity. These catalysts, which can effectively suppress the microbial growth for a long time and improve the efficiency of proton transfer,have good running stability in MFC, high power density and output voltage. These catalysts have the promising potentials for the future application in MFC and provide a new choice for preparation of cathodic ORR catalyst.