Investigation Of Graphene-Based Materials Modilfed Electrodes For Improved Performance Of Microbial Fuel Cells

Microbial fuel cell （MFC）, which exploits microbial activities to harvest energy fromorganic matter, has become one of the emerging technologies in the field of environmentalprotection and energy recovery in recent years. MFC can not only directly convert chemicalenergy stored in organic matter into electrical energy, but also have a great potential forsimultaneous wastewater treatment and power generation. At present, low power output andhigh cost are perceived as the two major bottlenecks in the commercialization of MFC.Moreover, the electrode materials play a crucial role in the electricity generation and cost ofMFC. Early studies have shown that electrodes modified with one-dimensional carbonnanotubes （CNTs） can improve the performance of MFCs. Graphene, emerging as a truetwo-dimensional carbon-based nanomaterial, has attracted wide-ranging attention since itsdiscovery in2004. Owing to intrinsically superior electrical conductivity, high charge carriermobility and high surface area, graphene could rival or even surpass the performance of CNTs.Hence, graphene has great potential for electrode modification. In this regard, we investigatedthe feasibility of using graphene-based material to modify anode or cathode of MFC,providing fundamentals for its practical application.In this work, graphite oxide was successfully synthesized by Hummers method and thenthermal split into graphene. FTIR and XRD analyses demonstrated that most of oxygenfunctionalities were removed and as-obtained graphene was well reduced. Iron-andnitrogen-functionalized graphene （Fe-N-G） was produced via a thermal treatment process.Theelectrocatalytic activity of the prepared catalysts toward oxygen reduction reaction （ORR）evaluated by using linear sweep voltammetry （LSV） tests showed that the Fe-N-G catalystshowed excellent ORR activity in neutral electrolytes. When equipped in an air-cathode MFC,the maximum power density of Fe-N-G-MFC (1149.8mW m^-2) was².1times of thatgenerated with the Pt/C-MFC and much higher than that of the MFC with pristine graphene.The modified anodes were prepared through a simple and scalable process:dipping-drying of3pieces of carbon felt in graphene（G）, CNT, and G/CNT hybrid suspension,respectively. Results showed that MFC with G/CNT bybrid anode exhibited the highest power density of760.7mW m^-2, which outperformed the unmodified MFC up to3.3times and was54.9%and90.7%greater than G and CNT modified MFCs, respectively. In addition, thecharge transfer resistance of anode significantly deceased by modified with G/CNT hybrid（39.8Ω） as ccompared to the unmofied anode. Moreover, the scanning electron microscopy（SEM） analysis demonstrated a remarkable increase in the thickness of biofilm by modifiedwith G or G/CNT hybrid. It is worth noting that the diameter of microorganisms wrappedcarbon fiber of G/CNT modified anode was notably increased to⁶9.3μm, which was over5-fold larger than that of plain carbon felt. The significant performance enhancement of MFCwith G/CNT modified anode could be ascribe to the three-dimensional structure created byG/CNT hybrid, which increased the surface area of electrode and the biomass on the electrode,in addition to the superior conductivity and charge transfer mobility of graphene.Furthermore, we developed a novel facile approach to fabricate/modify anode, ie. bydirectly adding graphene oxide （GO） suspension into the anode chamber. Results indicatedthat GO can be reduced to graphene （microbial reduced graphene oxide, mrGO） coupled withincreased power output of MFC. In detail, MFC-C （the GO suspension was added when MFCstarted up） achieved a maximum power density of1134.7mW m^-2,which was55.8%largerthan that of MFC-A （never added GO）. MFC-B （the GO suspension was added at thebeginning of the5th cycle） delivered a maximum power density of921.6mW m^-2,which was26.5%higher than that of MFC-A. The microbial reduced graphene provided goodbiocompatibility, extraordinary conductivity, enhanced extracellular electron-transfer rate aswell as decreased charge transfer resistance (R_ct), which could contribute to the performanceimprovement of the MFCs.