Study the Modification of Manganese Oxides as Oxygen Reduction Reaction Catalysts for Metal-air Batteries

Author:Sun Shan Shan

Supervisor:liu zhao ping


Degree Year:2017





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,α-MnO2 andδ-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 ofα-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 ofα-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 ofα-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 withα-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 intercalatedδ-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.