Preparation Of Activated Carbon-based Oxygen Reduction Catalysts And Their Performance In Microbial Fuel Cells

Microbial fuel cell(MFC)generates electricity whiler purifying sewag,which is a promising technology for sewage resource utilization and energy generation.The development of efficient and low-cost cathode oxygen reduction(ORR)catalysts is currently a vital and difficult subject in the MFC field.Activated carbon is one of the most promising non-precious metal catalysts for MFC.However,the low ORR catalytic activity of activated carbon limits its large-scale practical application.In this study,a high-efficiency and inexpensive hoof shell-based activated carbon and an activated carbon-supported CoNC composite catalyst are developed in order to improve the ORR catalytic activity of the activated carbon catalyst in MFC.Hoof shell-based activated carbon(HSC)is prepared by carbonization of pig hoof shells through phosphoric acid activation method.The characterization results show that the surface of HSC is rough and porous,with four elements C,N,O and P evenly distributed in the catalyst surface.It contains pyridine-N and graphite-N catalytic active sites,and the carbon skeleton structure has a high degree of graphitization,which are beneficial for the ORR activity.A glassy carbon electrode is used to test the ORR catalytic performance of HSC prepared at different activation temperatures.The results show that the HSC prepared at the activation temperature of 850?has the lowest impedance and the highest ORR current density.The current density reaches 9.1 A/m2 at-0.2 V,which is 122.0%,51.7%and 51.7%higher than the HSCs prepared at 700?,800? and 900?.Air cathodes are prepared with HSCs and their electrochemical performance in pH neutral solution is tested.The results show that at the optimal activation temperature of 850?,the ORR current density of the HSC air cathode is 0.2 A/m2 at-0.2V,which is 233.3%,150.0%and 5.3%higher than the HSC air cathodes prepared at 700?,800? and 900?.Activated carbon-supported CoNC composite catalyst(CoNC@AC)is prepared by in-situ growth of CoZn-ZIF(a Metal-Organic Frameworks)on activated carbon followed by high-temperature pyrolysis.Characterization results show that CoNC@AC is rich in pyridine-N,Co-Nx and graphite-N catalytic active sites,which can effectively improve the ORR activity of the catalyst;The carbon skeleton of CoNC@AC has a high degree of graphitization,and large numbers of Co nanoparticles and Carbon nanotubes are evenly distributed in the skeleton,which will improve the conductivity of the catalyst and reduce the electron transfer resistance during the ORR process;CoNC@AC has a balanced micropore-mesopore coexistence structure with a high specific surface area of 1186.4 m2/g,which can effectively promote the mass transfer of oxygen,protons and other substances during the ORR process while providing a large number of catalytic active sites.The test results of the rotating disk electrode show that the limit diffusion current density of CoNC@AC is 4.06 mA/cm2 in a pH neutral solution,and the ORR electron transfer number is 3.67-3.89,which are close to commercial Pt/C.This suggests that CoNC@AC catalyzes ORR through an efficient 4e-pathway.CoNC@ACs prepared at different pyrolysis temperatures are used to fabricate air cathodes in order to investigate the electrochemical performance of the catalysts in the actual situations.The results show that at the optimal pyrolysis temperature of 850?,the exchange current density j0 and ORR current density of the CoNC@AC air cathode reach 3.35×10-4 A/cm2 and 11.7 A/m2(at-0.2V)respectively.These values are 73%and 120%higher than the common activated carbon air cathode.The maximum power densities of MFCs using the CoNC@AC catalysts prepared at 800?,850? and 900? are 1880±23 mW/m2,2045±43 mW/m2 and 1872±43 mW/m2,which are 51%-65%higher than common activated carbon MFC.