Structure Design And Methanol Electrocatalytic Properties Based On Nitrogen-enriched Porous Carbon Supports

With the limited availability of global fossil fuel and increasing protrude for environmental pollution,the conversion and storage of new energy sources have become a hot issue in the current scientific research.Direct methanol fuel cells?DMFCs?have become one of the most prospect power cells due to their abundant fuel source,high power density and specific energy,and portable device,which have a huge market and broad application in civil,military and traffic devices.At present,the Pt-based catalyst is always the first choice for DMFCs.However,the commercialization of DMFCs has been seriously hindered by the scarcity and prohibitive cost,susceptible CO poisoning and slow reaction kinetics.To design electrocatalysts with exceptional electrocatalytic properties,the structure and composition are two important guiding principles.At this juncture,fully utilizing the synergistic effect of bifunctional effect,electronic effect and strain effect,constructing a specific structure,composition and morphology,developing advanced preparation technology for preparing high-performance electrocatalysts is the main direction of anode catalyst research.In this thesis,we have carried out the following researches on the basis of the current situation and shortcoming of anode catalyst development:Firstly,in order to take the advantage of nitrogen doping on the nucleation and distribution,as well as the hollow structure on the dispersion of noble metal and the transfer rate of electron and mass,we have successfully prepared a nitrogen-enriched porous carbon hollow spheres with an ultra-thin mesoporous?HNPHCS?supported Pt catalyst using silica as a hard template,pyrolysis and microwave-assisted polyol process.SEM and TEM images show that the HNPHCS has an ultra-thin mesoporous?1 nm?and hollow structure?140 nm?with a specific surface area of 805.23 m2 g-1.Thus,large number of Pt nanoparticles with a smaller particle size are evenly distributed on the surface of the HNPHCS support.In addition,the special hierarchical porous structure helps to ensure that the active sites are fully exposed to the three-phase interface,and shortening the charge and mass transfer channels in the catalytic oxidation process to promote the electron and mass transfer efficiently.Therefore,the resulting Pt/HNPHCS catalyst has higher Pt utilization,superior methanol oxidation current density and stronger stability than the commercial Pt/C catalyst and the catalyst prepared without the template.Secondly,in view of the problem that TiN is susceptible to passivation in strong oxidizing solution such as perchloric acid,which leads to the decrease of the conductivity,the surface self-assembly of polyaniline on the TiN surface was successfully prepared by using TiN as core and the surface modification of coupling agent.A novel strategy to generate TiN@nitrogen-riched porous carbon?NDC?supported Pt catalyst?Pt/TiN@NDC?using high-temperature pyrolysis and microwave-assisted polyol process was developed.The results of physical characterization showed that the coupling agent provided sufficient sites for in situ self-assembly of polyaniline,which played an important role in the construction of core-shell structure.The thickness of the porous carbon layer with mesoporous structure is about 7 nm,which will guarantee the smooth transmission channel of electrolyte.The addition of nitrogen-riched porous carbon not only enhances the specific surface area,noble metal distribution and conductivity of TiN nanoparticles,but also significantly improves the passivation of TiN in perchloric acid solution.The results of electrochemical measurements show that Pt/TiN@NDC catalyst exhibit better electrochemical specific surface area,methanol electrooxidation activity and stability than Pt/TiN,Pt/NDC and commercial Pt/C catalyst.Benefited from the electronic effect and bifunctional effect between Pt and supports,the absorbed carbon intermediate species on the surface of the Pt will be oxidized by OH-containing groups.Therefore,the Pt/TiN@NDC catalyst has high anti-CO poisoning ability.Thirdly,based on the unique advantages of the ordered porous structure and the synergistic effect between the components,the hollow TiO₂?H-TiO₂?nanothorns serve as a template to provide sufficient nucleation sites for growing highly ordered polyaniline nanowhisker.Meanwhile,the silica nanoparticles are fully embedded into the polyaniline matrix to produce a mesoporous structure.After high-temperature pyrolysis,etching and microwave-assisted polyol process,the nitrogen-enriched porous carbon nanowhisker coated H-TiO₂ supported Pt catalyst was successfully constructed?Pt/H-TiO₂@N-HPCN-T?.By controlling the reaction parameters systematically,the shape and size of the carbon material can be controlled.It was found that the H-TiO₂ nanothorns could promote the orientation growth of one-dimensional polyaniline nanowhiskers.In addition,the pyrolysis temperature has a significant impact on the microstructure,pore structure and content of supports as well as catalytic properties.The results of electrochemical tests show that the Pt/H-TiO₂@N-HPCN-800 catalyst exhibits the highest electrochemical specific surface area,methanol electro-oxidation activity,CO tolerance and long-term stability at 800 ?.The retention of the forward peak current density of Pt/H-TiO₂@N-HPCN-800 catalyst was 92.9% after 1000 scan cycles.The reasons for the significant enhancement of electrocatalytic performance are attributed to the structural advantages of highly developed three-dimensional porous architectures framework?hollow micro/dual-meso-porous andone-dimensional nanowhisker?,the optimum nitrogen doping level and conductivity,as well as the synergistic effect of electronic and bifunctional effects betweenPt,H-TiO₂ and N-HPCN.