Nitrogen Doped Carbon Supported Transition Metal Composites And Their Electrocatalysis

Low temperature fuel cells have been recognized one of sustainable clean energy technologies,due to their advantages including low/zero emission,high energy conversion efficiency,high power density,mild operation conditions and fast response.Nowadays,energy crisis and environmental pollution is becoming more and more serious,it is considered an ideal power sources for new energy automobile.Hydrogen-oxygen fuel cell is a typical low-temperature fuel cell,using the hydrogen as an anode fuel,and in the cathode oxygen is reduced to generate water,at the same time output energy.Sustainable cleaner production of hydrogen is mainly electrolysis of water.At present,no matter for the sluggish oxygen reduction reaction?ORR?or the hydrogen evolution reaction?HER?,platinum-based catalysts are the best catalysts in activity.However,platinum,as a kind of noble metal,is very rare and expensive,and its poor durability is also the major drawback hindering the large-scale application of them.In order to overcome the sluggish ORR and reduce the overpotential of hydrogen evolution to further reduce the costs of fuel cells and electrolyzing water,researchers pay attention to development of the cheap and high efficient non-precious catalysts.So far,the non-precious metal electrocatalysts have poor catalytic activity and stability,which do not meet the requirement of practical applications.This thesis begins from new microstructures and so on to prepare the carbon-based non precious metal ORR and HER catalysts.The main research results are as follows:The graphite flake was oxidized to graphene oxide by improved Hummers method,and mixed with other raw material at low temperature,and then high temperature pyrolysis to prepare the graphene deposited carbon coated Co₃O₄ nanoparticles?Co₃O₄@C/graphene?composite catalyst for ORR.The crystal structure,morphology characteristics,the element valence and specific surface area of composite catalyst was analyzed by making use of XRD?SEM?TEM?Raman?XPS and BET.The results showed that the ORR catalyst was well-crystallized,and its specific surface area reached to 410.6 m² g^-1.The results of electrochemical test in 0.1 M KOH showed that the catalyst had a better ORR activity.The result of the constant voltage scanning test showed that the catalyst remained highly efficient for the ORR with only a loss of < 10 % of its original activity after 10 hours,but the attenuation ratio of Pt/C catalyst was greater than 17%.And Co₃O₄@C/graphene composite catalyst had hardly affected by alcohol toxicity experiment.We think that the high electrocatalytic activity and stability for Co₃O₄@C/graphene composite catalyst could be attributed to the synergistic coupling between Co₃O₄ and then N-doped carbon structure,which make more active for the ORR,and the carbon encapsulated structure,which prevents the Co₃O₄ nanoparticles from the dissociation and/or detachment during the complicated electrocatalytic processes.Co-based N-doped CNT network?Co@NCNT?composite catalyst for ORR and OER was prepared by the pyrolysis of the hybrid precursor of P123,melamine and Co?NO₃?₂ in N₂ atmosphere,and subsequent acid treatment.The morphology characteristics,crystal structure and specific surface area of composite catalyst was analyzed by XRD?SEM?TEM?Raman?XPS and BET.The results showed that the main diffraction peaks corresponded to the lattice of elemental cobalt,mutual crisscross of N-doped CNTs formed a conductive network,which was helpful to improve the electronic conduction,and the existence of porous structure was conducive to infiltrate the electrolyte and further enhance the ORR reaction rate.The results of electrochemical test in 0.1 M KOH showed that the ORR activity of catalyst had a similar to that of Pt/C,and also had a better OER performance.The N-doped carbon coating MoSe₂ nanosheet?MoSe₂/NC?was prepared for HER by the simple two-step process,the first was hydrothermal treatment,and the second was high temperature pyrolysis.This chapter used the ammonium citrate as N and C source,coated on the surface of MoSe₂ nanosheets,and the release of ammonia could further reduce the stack of MoSe₂ nano-layer in the pyrolysis to expose more HER active sites.The analysis results were obtained that the microstructure of the catalyst was 3D flower-like self-assembled MoSe₂ nanosheets,and there had a strong electron interaction between N-doped carbon and ultrathin MoSe₂ nanosheets by XRD ? SEM ? TEM ? Raman ? XPS and BET.The results of electrochemical test in 0.5 M H₂SO₄ showed that,compared with carbon coated MoSe₂ nanosheets?MoSe₂/C?and pure MoSe₂,the 3D Flower-like MoSe₂/NC delivers a high HER performance with small onset potential of 40 mV,and had a smaller Tafel slope and a larger special surface area.