| Since the turn of the century, energy and environmental issues have become increasingly serious, and the search on microbial fuel cell (MFC) which could generate electrical energy directly from organic pollutants in wastewaters, has experienced rapid increases. Improved the performance of cathode, whose function is to accept electrons and protons, would increase efficiency and power output of the whole MFC. Due to its high oxidation potential, availability, low cost, sustainability and the lack of a chemical product, oxygen is the most suitable electron acceptor for MFC. Further research on oxygen reduction cathode is of importance for the development and industrial application of MFC. Based on the principle of oxygen reduction reaction (ORR), this article studied MFC with two different electrode materials as oxygen reduction cathode. (1) Adopted spectrographic pure graphite (SPG) as oxygen reduction cathode of MFC to make ORR followed the two-electron pathway yielding H2O2. Then, degraded amaranth by in-situ Fenton reaction incorporated into H2O2-producing MFC. (2) Prepared a new type of prussian blue/polyaniline (PB/PANI)-modified electrode, and ORR one the surface of electrode followed the four-electron pathway. MFC with PB/PANI-modified electrode as oxygen reduction cathode had the same performance with the MFC having liquid-state ferricyanide cathode.By using electrochemical methods, it was proved that two-electron oxygen reduction reaction could take place on the surface of SPG. H2O2-producing MFC was constructed by using SPG as oxygen reduction cathode of MFC. According to polarization curve and power curve, the low power output and apparent activation loss in the cathode suggested an unsuitability of using H2O2-producing MFC to generate energy. Operated H2O2-producing MFC with various external resistances, it was found that smaller external resistances were favorable for H2O2 generation. When external resistance of 20Ωwas selected as load and ran for 12 h,73-80 mg/L H2O2 could be obtained in the cathode of H2O2-producing MFC, with the coulombic efficiency of 12.26%, current efficiency of 69.47% and COD conversion efficiency of 8.51%, respectively.Amaranth as the target pollutant was degraded by in-situ Fenton reaction incorporated into H2O2-producing MFC. H2O2-producing MFC with neutral catholyte was able to generate more H2O2, so confirmed MFC-Fenton system operated with the mode of sequence, that is H2O2 generation first and then Fenton reaction. The presence of amaranth in the catholyte had slight impact on the ORR process occurring on the surface of the SPG rod, and H2O2 was unable to decompose amaranth without ironic catalyst. In MFC-conventional Fenton (CF) system with Fe2+ as Fenton reaction catalyst, remove ratio of amaranth (75 mg/L) reached 82.59% within 1 h when 1 mmol/L Fe2+ was applied. In MFC-electrochemical Fenton (EF) system with Fe3+ as Fenton reaction catalyst, remove ratio of amaranth was close to that in MFC-CF system. The cathode potential of MFC-EF could keep stable and the maximum power density was more than twice that of MFC with K3Fe(CN)6 as electron acceptor.Polyaniline was synthesized by electrochemical oxidation method on the surface of SPG as matrix electrode, and prussian blue/polyaniline (PB/PANI)-modified electrode was prepared by doping method. The function of PANI was to stabilize and protect PB film in this modified electrode. By electrochemical methods, PB/PANI-modified electrode showed electrocatalytic activity for four-electron ORR. The performance of modified electrode would be improved when acid and K2SO4 concentrations increased in the electrolyte. In addition, PB/PANI-modified electrode exhibited a good electrochemical stability.PB/PANI/AC was prepared by chemical oxidation method, with nutshell activated carbon as matrix. Characterized by SEM, BET, IR, XRD, and ICP, it was showed that PB film had formed on the surface of electrode and the surface area was large. PB/PANI/AC formed three-dimensional electrode was used as oxygen reduction cathode in MFC. The influences of catholyte pH and kind of aeration gas on the performance of MFC were studied. Results showed that, low pH of catholyte required for high catalytic activities of PB would have a negative impact on electricity-producing microorganisms, and 1.0 mmol/L H2SO4 in the catholyte was acceptable; Power output of MFC aerated by pure oxygen and air were almost the same, taking into account of the high operational cost for pure oxygen aeration, free air was more reasonable. The maximum power density of MFC was 12.82 W/m3, which was close to that of MFC having liquid-state ferricyanide cathode. Continuous operation suggested a good reproducibility and duration of PB/PANI/AC for the application in MFC.
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