Preparation Of Multicomponent Platinum-based Catalysts And Their Performance For Oxygen Reduction Reaction

The development of hydrogen energy technology can not only meet the needs of economic and social development for clean and renewable energy,but also meet the dual-carbon goals of carbon neutrality and carbon peaking.Fuel cell is the terminal device of hydrogen energy technology and the core component of hydrogen energy technology application.Proton exchange membrane fuel cell has a relatively mature membrane preparation technology,along with enabling quickly start up and operation at low temperature,which has been researched extensively and deeply.A large amount of precious platinum-based catalyst is used in fuel cells,resulting in high costs,which suppresses the commercial application of PEMFC.However,there are no suitable alternatives to platinum catalysts in the short term.Therefore,it is necessary to improve the utilization of platinum catalysts in order to promote the further development of fuel cells.From the perspective of catalysts,it is vital to improve the intrinsic activity and stability of platinum.Multicomponent system provides more possibility to synthesize high-performance catalysts.According to the structural relationship between non-platinum components and platinum component,it can be divided into alloy structure and heterostructure.This thesis designs and prepares a series of multicomponent catalytic systems with the platinum-based catalyst as the research object,aiming at improving the intrinsic activity and stability of catalysts.To relief the issues of strong oxygen adsorption on small-particle-size platinum,a new catalytic system containing atomic-scale metal-nitrogen-carbon supported nanoscale platinum was designed and prepared.Subsequently,this system is introduced into a gradient electrode structure to further promote the utilization of platinum.Focusing on the problem involving stability of platinum under start up/shut down events,a highly stable nitrogen-doped carbon supported highly coupled platinum-tin dioxide（Pt-Sn O₂/NC）catalyst was designed and prepared.The enhanced three-phase interface of Pt-Sn O₂/NC suppresses the Oswald ripening and migration of platinum.In order to realize the large-scale preparation of high-performance catalysts,the high-loaded carbon-supported platinum-cobalt alloy catalyst is designed and realized high-quantity pre-preparation.The specific innovative achievements in this thesis are as follows:（1）The metal-nitrogen-carbon supported platinum complex is structured,promoting the oxygen reduction reaction performance of Pt catalysts.The composite structure of metal-nitrogen-carbon supported metal nanoparticles can be formed after carbonization,deriving from the metal-nitrogen-carbon structure in the metal polyphthalocyanine.Through the galvanic replacement reaction between metal nanoparticles and platinum,platinum nanoparticles can support onto metal-nitrogen-carbon sites by replacing the original metal nanoparticles.The structure and morphology of the catalyst can be controlled by adjusting the solvent,metal content and metal species:solvent has great effect on the size of resultant Pt.The number of metal-nitrogen-carbon sites,content of platinum,particle size and dispersion of platinum can be adjusted by changing the metal ratio in metal-nitrogen-carbon supported platinum.Cobalt/copper can alloy with platinum,along with large particle size/poor dispersion of particles.Nickel does not form an alloy structure with platinum along with a uniform dispersion of nanoparticles.Nanoscale Platinum is adjacent to atomic-scale metal-nitrogen-carbon sites with synergistic effects between the two sites:metal-nitrogen-carbon sites promote the dissociation and desorption of reactive oxygen species on the adjacent platinum sites.The electron transfer between the two sites enhances the interaction between metal and support,improving the stability of platinum.The nickel-nitrogen-carbon supported platinum nanoparticles catalyst shows the best performance for oxygen reduction reaction with mass activiy of 28.0 A mg_Pt^-1 and specific activity of 0.272 A m^-2,which are 6 and 4.5 times that of commercial Pt/C.（2）Based on nickel-nitrogen-carbon catalysts,conducting the ultraviolet light which promotes the displacement reaction and generates gradient light intensity distribution across porous electrode,the gradient catalytic layer with platinum concentration is prepared.The platinum concentration is volcanic distribution with maximum content located in the distance of 0.8μm to 2.4μm from the proton membrane side.Combined with the gradient gas diffusion layer,the gradient electrode promotes the performance of fuel cell.Finite element simulation demonstrates that the gradient structure strengthens the electrochemical process:the gradient catalytic layer promotes the balance of mass transfer of reactant（oxygen and proton）.The gradient gas diffusion layer promotes the discharge of generated water from the inner layer to the outer layer and the entry of air into the inner layer,thus easing the water flooding issues.（3）Nitrogen-doped carbon supported highly coupled platinum tin dioxide electrocatalyst is prepared by low-temperature air pyrolysis of metal polyphthalocyanine platinum tin along with adjusting the pyrolysis temperature and the ratio of platinum/tin.The enhanced interface interaction derives from the Sn-O-Pt interface structure between Pt（100）and Sn O₂（101）crystal plane surrounded by N-doped carbons,forming Pt-N and Sn-N interface structure between nitrogen doped carbon and metal.Electrons flow from highly coupled platinum tin dioxide to nitrogen doped carbon through the interface and transfer from tin dioxide to platinum.After harsh start up/down events,Pt-Sn O₂/NC shows a decrease of 44.6%in mass activity and a decline of 17.1%in maximum power density of membrane electrode assembly,with a much higher stability than Pt/C（94.8%and 62.1%,respectively）.Heat treatment temperature and platinum tin ratio have important effects on the structure and properties of Pt-Sn O₂/NC:it is difficult to decompose the polyphthalocyanine skeleton under low temperature and it is easy to promote the growth of platinum grains under high temperature,resulting in the optimal synthesis temperature of 400°C.Low tin content has insufficient inhibition on platinum grain growth and high tin content will reduce the effective area of platinum,leading to the optimum platinum/tin ratio of 2:1.Thermogravimetric-infrared measurements demonstrates that the formation process of Pt-Sn O₂/NC involves the primary decomposition of polyphthalocyanine platinum,the formation of tin dioxide and growth of Pt,which is suppressed by the fist-generated Sn O₂.（4）Using carbon black as support,high-loaded carbon-supported platinum-cobalt alloy catalysts are successfully synthesized by the ethylene glycol reduction method and high-quantity pre-preparation is realized with an output of 25 g per reactor.Ethylene glycol can form a complex structure with platinum and cobalt,which can strengthen the uniform nucleation and promote the co-deposition of platinum and cobalt to form an alloy structure.The maximum loading of Vulcan 72 carbon black for 2 nm platinum-cobalt alloy nanoparticles is 50 wt.%.The temperature had a significant effect on the structure of the as-prepared catalyst.Too high or too low temperature can lead to inhomogeneous platinum alloys nucleation and low alloying.The chemical state of the carbon support has an obvious influence on the particle size,dispersion of the nanoparticles and the alloy structure.The extremely weak interaction between carbon black and the metal particles is not conducive to the termination of the reaction and the deposition of cobalt while promotes the migration and growth of the particles.The amplification effect of the ethylene glycol reduction system is not obvious.