Study The Modification Of Manganese Oxides As Oxygen Reduction Reaction Catalysts For Metal-air Batteries

With the rapid development of economic,the issues such as shortage of natural resources,environmental pollution and global warming have become worsen and worsen since 21th century.The metal-air batteries as renewable green energy storages have attracted much attentions.However,the sluggish oxygen reduction reaction?ORR?rates in air cathode have restricted the performances and practical applications of metal-air batteries.Developing high electrocatalytic catalysts toward ORR has been an urgent thing.The manganese oxide-based catalysts have drawed much attentions due to their abundant reserves,environmental friendly and considerable catalytic ability.Their catalytic ability can be increased easily by modification,doping/intercalating or combining with other materials.In this thesis,?-MnO₂ and?-MnO₂ which have the best electrocatalytic abilities among all simple manganese oxides are chosen as the research candidates to achieve higher ORR performances.The detailed works of this thesis are as follows:?1?The Ag-MnO₂ composite catalysts with diameter of about 10 nm silver nanoparticles anchored on the surface of?-MnO₂ with high BET surface area were synthesized by a two-step method of sintering and the electroless plating in aqueous.The 50wt%Ag-MnO₂ catalyst is chosen to proceed further study according to the best microstructures.The electrochemical tests on the rotating disk electrode show that the onset potential and half-wave potential are higher of 140 mV and 120 mV than those of?-MnO₂,respectively.The process of ORR happened on its surface undergoes a 4-electron transfer process.The long-term stability of 50wt%Ag-MnO₂ is excellent than that of?-MnO₂ and Ag/C.The aluminum-air battery with 50%Ag-MnO₂ composite catalyst has a peak power density of 204 mW cm^-2,which is much higher than that with?-MnO₂ and Ag/C catalyst.The results indicate that the Ag-MnO₂ composites can be used as an efficient catalyst for aluminum-air batteries.?2?The catalyst of Ag doped manganese oxides dispersed on carbon is developed via efficient and economical strategy liquid phase method.When 17%Ag is doped into the manganese dioxide,the 17%Ag-MnO₂/C had the best ORR catalytic performance with the half-wave potential at 0.8 V?vs.RHE?.The half-wave potential is higher than that of MnO₂/C and Ag/C and close to that of 20%Pt/C.The electron transferred numbers of 17%Ag-MnO₂/C is 3.8³.9 indicating a quasi-4 electron transfer process.The yield of hydrogen peroxide during ORR on the surface of catalyst is below 5%revealing a low production of HO₂^-.The long-term stability of the catalyst is excellent with the ORR current retention as high as 97.1%after 46000 s.The Al-air battery with17%Ag-MnO₂/C as cathode catalyst gives out the peak power density of 315 mW cm^-2 in 4 M KOH aqueous solution with a multicomponent Al alloy.The power density is higher than that of Al-air batteries with MnO₂/C and Ag/C as cathode catalysts,respectively.The production process of Ag-MnO₂/C paves a simple way for developing the Al-air batteries with the cost-effective ORR catalyst.?3?The catalysts of lanthanum and cerium ions intercalated?-MnO₂ dispersed on carbon are further prepared and their microstructures and catalytic performances are investigated in this work,respectively.The aluminum-air batteries using 5%La-MnO₂/C and 4.8%Ce-MnO₂/C as air cathode catalyst were fabricated and tested,respectively.It is confirmed from the O₂-TPD results that the improved electrocatalytic ability of 5%La-MnO₂/C toward ORR was due to the enhanced O₂ adsorptive ability.As for the 4.8%Ce-MnO₂/C,the reasons caused the high ORR performance are different.Combining all the testing results especially the CV behaviors,it is concluded that the ORR performance of 4.8%Ce-MnO₂/C is enhanced first because of the improved high BET value 186 m² g^-1 which is almost three times than that of MnO₂/C;the second reason could be due to the Mn³⁺/Mn⁴⁺redox triggered by the intercalation of Ce ions.The aluminum-air batteries using 5%La-MnO₂/C and 4.8%Ce-MnO₂/C as air cathode catalyst show the higher peak power density of 348.8 mW cm^-2 and 312mW cm^-2 than MnO₂/C in a 4 M KOH solution at room temperature,respectively.The degradation rate of aluminum-air batteries with 4.8%Ce-MnO₂/C is as low as 2%per100 h after more than 300-h discharging at the current density of 100 mA cm^-2 which is much lower than that of MnO₂/C.?4?The Ni ions doped manganese oxides are prepared by a liquid method.The oxygen evolution reaction is improved greatly compared to that of un-doped materials.The rechargeable Al-air batteries was first proposed with a gel-electrolyte and modified Al anode.With a gel-electrolyte in room temperature,the Al-air battery gives a power density of 130 mW cm^-2.To conclude,the manganese oxide based catalysts modified by noble metal silver,doped by silver ions and intercalated by Ce ions have greatly improved their catalytic performance toward ORR.When applied in Al-air batteries,the peak power densities and long-term stabilities are also enhanced apparently.All the preparation processes are easy to scale up which can conduct the commercialization of Al-air batteries.