Studies On Electrocatalysts For Oxygen Reduction Reaction For Magnesium-air Fuel Cell

With the rapid development of the economy,more and more fossil energy is consumed,causing a host of environmental issues,among them acid rain and global climate change.Thus,it is necessary to find renewable clean energy in the near future.Batteries,especially rechargeable Li-ion batteries,because of the high specific energy and steady performance have gained the commercial success.Metal-air fuel cell(MFC)offers a promising alternative due to the high specific energy,low-cost and environmental benignity.However,the cathode catalyst for oxygen reduction reaction(ORR)plays an extremely important role in improving the current density,the power density and cell efficiency for Mg-air fuel cell.The carbon black supporting platinum-based nanoparticles(NPs)are widely used as the catalyst for ORR.However,the widespread use of metal-air fuel cell is still hindered by several factors,including high cost and poor durability.Therefore,it is necessary to search a suitable,controllable and high-performance cathode electrocatalyst for metal-air fuel cell.In this paper,we focused on the non-platinum catalysts for oxygen reduction reaction.The main points of this thesis are as follows:First,the preparation of ?-MnO2 NWs/graphene composites as the cathode catalyst for Mg-air fuel cell and its excellent electrochemistry performance.The composites are synthesized by self-assembly of a-MnO2 nanowires(NWs)on the surface of graphene via a simple hydrothermal method.The a-MnO2 NWs/graphene composites showed a higher electrochemical activity than the commercial MnO2.The oxygen reduction peak of the ?-MnO2 NWs/graphene composites catalyst is tested in a 0.1 M KOH solution at-0.252 V,which is more positive than the commercial MnO2(-0.287 V).Based on the Koutecky-Levich plot,the ORR mechanism of the composite involves a one-step,quasi-4-electron pathway.In addition,magnesium-air fuel cell with ?-MnO2 NWs/graphene as catalyst possesses higher current density(140 mA/cm2)and power density(96 mW/cm2)compared to the commercial MnO2.This study proves that the cost-effective ?-MnO2 NWs/graphene with higher power generation ability make it possible for the substitute of the noble metals catalyst in the Mg-air fuel cell.Second,Ag-Co3O4@N-doped carbon/carbon nanotubes composites are synthesized by dispersing Ag nanoparticles throughout the N-doped carbon derived from ZIF-67/CNT,using metal-organic framework as precursors.The results show that the ORR onset potential on the Ag-Co3O4@NC/CNT(-0.0024 V)is more positive than that on 20%Pt/C(-0.0729 V).The half-wave potential of the ORR on Ag-Co3O4@NC/CNT is about-0.198 V,which is similar to 20%Pt/C(-0.20 V).The ORR Tafel slope measured with Ag-Co3O4@NC/CNT catalyst is 95 mV per decade.This value is similar to the one determined in this work for the Pt/C catalyst(86mV/dec).In oxygen reduction reaction(ORR),the synthesized composites possess a one-step 4 electron pathway,and a synergistic effect between Ag nanoparticles and N-doped carbon is proposed.In the Mg-air fuel cell,the synthesized composites show the highest power density of 88.9 mW/cm2 at current density of 140 mA/cm2,better than that of 20%Pt/C.This excellent performance ensures this composite a promising ORR catalyst for Mg-air fuel cell.Finally,La ion was successfully added into the matrix of ZIF-67 during the process of coordination growth of zeolitic imidazalate framework-67.Then,the LaCoO3-Co3O4@NC composites are synthesized by the thermal pyrolysis of La-ZIF-67 under an air atmosphere and a subsequent calcination process.The results show that the Tafel slope of LaCoO3-Co3O4@NC was 73.1 mV/dec,which is close to that of 20%Pt/C.The RRDE measurements were used to investigate the electron-transfer kinetics of electrocatalysts,which reveal the ORR followed by a four-electron process for LaCoO3-Co3O4@NC.The OER activities of LaCoO3-Co3O4@NC,Co3O4@NC,and commercial 20%Pt/C were also examined as a comparison.The onset potential of LaCoO3-CO3O4@NC is 1.37 V(vs.RHE).which is much smaller than that of Co3O4@NC(1.55V)and close to that of 20%Pt/C(1.45V).The electrocatalytic activity indicates that the LaCoO3-Co3O4@NC composites are promising catalysts with a high catalytic activity toward OER.The synthesized composites show the highest power density of 90mW/cm2 at current density of 150mA/cm2,better than that of 20%Pt/C.This excellent performance ensures this composite a promising ORR catalyst for Mg-air fuel cell.