Carbon Containing Manganese Oxide Composites And Their Electrocatalytic Reduction Of Oxygen

In recent ten years,the new power technology has been accelerated to meet the requirement of portable electrical appliance and electric vehicle.Fuel cells convert chemical energy directly into electrical energy with high eficiency and low emission of pollutants,may help to reduce the dependence from fossil fuels,and operating without combustion,can contribute to reduce environmental impact.Therefore fuel cells are a practical answer to the world's pressing need for clean and efficient power due to its high power density,non-polluting,high conversion efficiency and compact design.Fuel cells and metal-air battery that use alkaline electrolyte solutions have been studied in the last decades using non-noble metal catalysts for a long time.As the catalyst,noble metals such as platinum for molecular oxygen reduction are well known.They offer the advantages of high catalytic activity,electronic conductivity and stability,while their high costs and susceptibility to catalyst poisoning are a serious concern for commercial application.As a promising route to replace platinum, manganese oxides MnO_x(such as MnO₂,Mn₂O₃,Mn₃O₄,and Mn₅O₈) as catalyst for oxygen reduction has been long investigated,and a number of research works have focused on the evaluating and optimizing the manganese based catalyst for air cathode. In fuel cells,Gas diffusion oxygen electrodes,where molecular oxygen is electrocatalytically reduced at three-phase interfacial area(solid/liquid/gas),are vital to the performance of fuel cells and metal-air battery.The electrocatalytic oxygen reduction reaction in the gas diffusion electrode is a typical "three-phase interface electrocatalytic reaction".Different from the original catalytic reaction,not only the particle-size of catalyst is important to catalytic efficiency,but also the sufficient and effective three-phase interface catalytical area and direct gas diffusion path are vital to the performance of the gas diffusion electrode.However how to design and synthesize functional material for this "three-phase interface electrocatalytic reaction" is rarely reported.In our work,we mainly synthesize different carbon materials with special structure,in which Mn₃O₄ particles are loaded by wet-penetration method to prepare composite materials,and these composite materials are extensively studied as an air diffusion electrode to compare their catalytic performance.We also introduce a part of research work about alloy and metal oxide metarials used in anode material of Li-ion battery except abovementioned content.1.Synthesis of manganese oxide/carbon composite catalyst and its application for oxygen reductionMn₃O₄ nano-particles were loaded on the surface of different carbon materials by the wet-penetration method,and the catalytic performance of these composite materials were evaluated as air-electrode in oxygen reduction.These composite materials exhibit different catalytic performance by electrochemical measurement.It reveals that the mesoporous carbon(CMK-3) with three-dimensional ordered structure exhibits the better catalytic performance than other materials.The image of TEM indicated that Mn₃O₄ nano-particles were all loaded on the outer-surface of CMK-3.The interconnected mesoporous pores in CMK-3 can provide sufficient gas diffusion and storage channels,meanwhile the Mn₃O₄ particle loading on the out-surface of CMK-3 can contact with both of the electrolyte and oxygen,hence obtain sufficient and effective three-phase interface area and result in its high catalytic activity for oxygen reduction.However,the mesoporous carbon(OMC) with two-dimensional ordered structure exhibits the worst ctatlytic performance due to the lack of effective three-phase interface area on the catalyst particles surface.In order to further improve the catalytic performance of composite material,we design and synthesize mesoporous hollow carbon spheres(HMCS)/manganese oxide composite catalytic material.Different from CMK-3/Mn₃O₄,nano-sized Mn₃O₄ particles embeded in the mesoporous pores of HMCS can further decrease catalyst particle size,and the shell of HMCS is about 60 nm thickness,which can enhance utilizable rate of mesoporous pores.For these reasons,this composite material exhibits excellent catalytic performance.2.Synthesis of metal oxide and alloy/carbon composite materials and its application for anode material of Li-ion batteryIn this part,we synthesized a type of graphitable hollow carbon spheres(GHCS) with high surface area by chemical vapor deposition(CVD) technique,in which CoO nanoparticles were encapsulated to prepare core-shell structure CoO/GHCS composite material by wet-penetration method.The as-synthesized CoO/GHCS composite material exhibits better electrochemical performance as an anode for Li-ion batteries, and the improvement could be attributed to that the graphitable hollow carbon sphere with a good electronic conductivity and high surface severs as depressing medium to prevent CoO from aggregating with other CoO nanoparticles,and provide the enough space to buffer the volume change during the Li-ion insertion and extraction reactions of CoO nano-particles.In the field of alloy anode material,the most important problem is how to limit the volume expanse and pulverization during Li-ion insertion/emersion progress,and recently the common method is preparing the active/inactive composite alloy material or synthesizing nano-sized alloy particles.Even preparing the composite alloy material,there still is a particle second-aggregation phenomenon during Li-ion insertion/emersion progress,which will also lead to significant capacity fading due to the pulverization of alloy material.Heren we synthesize nano-sized Cu₆Sn₅ particles encapsulated by carbon composite material,which is prepared as this route:firstly we synthesize nano-sized Cu₆Sn₅ particles by liquid-phase reduction method,and then coat alloy particles with phenolic-resin;finally the carbon/Cu₆Sn₅ composite material is obtained after carbonization under high temperature.In this structure,carbon layer can enwrap alloy nano-particles integrally and homogeneously by our synthesis method,which can reduce absolute volume change of nano-sized alloy particles during charge/discharge progress,and ensure the existence of core/shell structure. Meanwhile carbon layer can prevent prevent Cu₆Sn₅ nanoparticles from aggregating with each other during the expansion/shrinkage progress.Therefore this composite material exhibits excellent electrochemical performance.