Study On Microwave Constructed And Performance Of Carbon Nanotube Based Oxygen Reduction Catalysts

As an efficient and non-polluting energy conversion device used to alleviate energy depletion and environmental pollution problems,hydrogen-oxygen fuel cells were limited by the slow reaction kinetics of the cathode for large-scale commercial application.Conventional platinum metal-based catalysts were expensive in cost,so a cathode catalyst with high catalytic activity,high durability and low price was needed to solve the problem.Carbon nanotubes were suitable as a support material for cathodic metal catalysts due to their high electrical conductivity,high structural stability and relatively low price.Based on the synergy between metal and support,a series of carbon nanotube-based oxygen reduction catalysts that could be used in alkaline environment were constructed in this thesis by high-energy microwave technology using carbon nanotubes as the original support,which combined with physical and chemical characterization to elucidate the influence of catalyst structure on the oxygen reduction performance.The main contents of this work are as following:（1）By using thermally decomposable ruthenium molybdenum metal carbonyl salt as precursors and carbon nanotubes as support,Ru-Mo₂C@CNT catalyst with low metal loading was prepared by a simple microwave pyrolysis method.The results showed that with the increase of carbon nanotube mass,the size of metal particles gradually decreased and the dispersion was significantly improved,which enhanced the utilization of active sites and thus the oxygen reduction performance.Further increase of the support mass will lead to a relative decrease of the active sites,resulting in a decrease of the oxygen reduction performance.The Ru-Mo₂C@CNT prepared at a 1:4 ratio of ruthenium molybdenum carbonyl salt precursor to carbon nanotube support addition exhibited the best catalytic activity for oxygen reduction with a half-wave potential of0.87 V and an onset potential of 0.94 V.The methanol resistance and stability were better than those of commercial Pt/C catalysts and catalysts using other carbon support.（2）Doping with iodine can regulate the electronic structure of adjacent carbon atoms and increase the electrical conductivity of materials.A series of iodine-doped carbon nanotube oxygen reduction catalysts were prepared using iodine monomers as precursors with the strategy of microwave reconstitution.The results of material characterization showed that the iodine-doped carbon nanotube catalysts with an iodine-to-carbon nanotube mass ratio of 3:1,prepared by the solvent-water method,had the highest iodine doping,larger carbon layer spacing,and more abundant defects compared to the preparation system using dry powder grinding and ethylene glycol as the solvent.Further electrochemical test results showed that the iodine-doped carbon nanotube catalyst had a half wave potential of 0.83 V and an onset potential of 0.92 V,which was higher than the oxygen reduction performance of pure carbon nanotubes.Only 5%of the current was lost after 40000 s of stable operation of the catalyst;the current was almost unchanged with the addition of 3 M methanol,which showed excellent methanol tolerance and stability.（3）To investigate the role of iodine-doped carbon support for metal-carbon nanotube catalysts,a series of Ru-Mo₂C@ICNT catalysts were synthesized by microwave thermal shock method based on the studies in the above two chapters,using ruthenium molybdenum metal carbonyl salts as precursors and iodine-doped carbon nanotubes as support.The characterization results showed that the catalysts synthesized with a 1:4 mass ratio of metal precursors to support exhibited better oxygen reduction performance than other control samples,as indicated by the uniform dispersion of metal particles,abundant active sites,high half-wave potential（0.89 V）,high onset potential（0.99 V）,low Tafel slope(52.19 m V dec^-1),high catalytically active surface area(13.53m F cm^-2).The result of stability test showed that the catalyst ran for 40000 s with only7%current loss,which indicated its excellent long-term stability.（4）The three-dimensional Ru@RGO/CNT catalysts were constructed by microwave assisted ethylene glycol synergistic co-reduction using graphene oxide and carbon nanotubes as the carbon framework and hydrated ruthenium chloride as the metal source,and their oxygen reduction performance under alkaline environment was investigated.Based on the combined morphology and metal addition regulation strategy,the optimal process parameters of 1:2 mass ratio of graphene oxide to carbon nanotubes and 15 mg addition of ruthenium chloride hydrate were used.Synthesized Ru@RGO/CNT showed a three-dimensional layered structure with a half-wave potential of 0.85 V and an onset potential of 0.92 V,indicating that it had the best oxygen reduction performance.In addition,the catalyst exhibited a Tafel slope of 53.14 m V dec^-1 with reaction kinetics exceeding that of commercial Pt/C catalysts.Ru@RGO/CNT was run continuously for 50000 s with current loss at 8%,which exhibited excellent stability.