Study On Oxygen Reduction Performance Of Nitrogen Modified Carbon Supported Platinum Alloy Catalysts

Proton exchange membrane fuel cell（PEMFC）has received widespread attention due to their advantages of high efficiency,cleanliness,and sustainability.PEMFC has been applied in fields such as transportation,fixed,and portable batteries.However,due to the high energy barrier,the oxygen reduction reaction（ORR）is difficult to occur.Therefore,a large amount of platinum has been applied to PEMFC,which hinders its commercialization process.However,commercial Pt/C catalyst still has the drawbacks of poor performance and insufficient lifespan.Therefore,many researchers have regulated the structure and morphology of the active components of catalysts.However,the role of carbon carriers in catalysts cannot be ignored.The physical and chemical structural properties of carriers can directly affect the ORR performance and life of the active components,thereby affecting the effectiveness of catalyst performance.From the perspective of carriers and metal interaction,strategies of functionalization of nitrogen containing groups and nitrogen doped carbon materials were employed.Particle size,and distribution of the active components can be further controlled by regulating the morphology,scale,pore structure,nitrogen content,and nitrogen species of carbon carriers,thereby improving the activity and stability of catalysts.This thesis has laid the theoretical foundation and practical experience for promoting the development of ORR catalysts,and the main thesis contents are as follows:（1）In order to fully utilize the ORR performance of catalysts,both the active components and carbon carriers were explored to regulate the physicochemical structure and electrochemical performance of catalysts.The ethylene glycol reduction method was improved by adding sodium acetate,PtCu nanoparticles with smaller particle sizes were prepared under the influence of electrostatic repulsion.Subsequently,the carbon black was modified with nitrogen containing functional groups to further enhance its loading capacity.H₂O₂and melem modified carbon black（HMC）was obtained by co modifying the carbon black with hydrogen peroxide and nitrogen containing melem groups.Under the combined effect of the two modifications,the defect degree,hydrophilicity and pore structure of carbon black have been optimized.The excellent characteristics of carbon carriers can enhance the interaction between metal nanoparticles and carriers,adjust the electronic properties and crystal structure of Pt,thus improving the oxygen adsorption energy and the number of active sites of the catalyst（PtCu/HMC）.The mass activity of PtCu/HMC is 0.77 A mg _Pt^-1,displaying good ORR performance.Meanwhile,PtCu/HMC demonstrates excellent stability due to the strong interaction between metal nanoparticles and carriers,the mass activity of PtCu/HMC decreases by only 21%during 10,000 potential cycles of accelerated stability testing.（2）In order to avoid excessive oxygen-containing groups from reducing the corrosion resistance of carbon carriers,and to further enhance the mass transfer efficiency and stability of the catalysts,nitrogen doped porous carbon supported PtCu catalyst（PtCu/NHPC）was prepared.Nitrogen doped porous carbon was synthesized through post doping method,using cellulose as the carbon source,sodium bicarbonate as the pore forming agent,and melamine as the nitrogen source.In the process of post doping nitrogen,partial carbon atoms are ablated,making some micropores turn into mesopores,which is conducive to the full contact of metal active sites with the electrolyte,and further accelerates the mass transfer in the catalytic process.The electronic structure and particle size effect of the active components are optimized by NHPC.At the same time,PtCu/NHPC shows excellent catalytic activity due to the spillover effect.During the process of nitrogen doping with carbon,the oxygen content in carbon carriers decreases,which can slow down carbon oxidation and further enhance the corrosion resistance of the carbon carriers.Therefore,PtCu/NHPC exhibits excellent stability after 10000 potential cycles of stability testing（only 6.6%mass activity loss）.（3）In order to further optimize the pore structure,and adjust the nitrogen content and species in carbon carriers,a dual template method was used to prepare nitrogen doped porous carbon（NPC-Co）by in-situ nitrogen doping.Then,Pt Co metal nanoparticles were loaded onto NPC-Co to obtain the catalyst（Pt Co/NPC-Co）.Due to its advantages of easy encapsulation and reduction of metals,polydopamine had been selected as a carbon source and nitrogen source precursor,which could fully utilize its advantages of in-situ doping and obtain uniformly doped carbon carriers with high nitrogen content.Cobalt nanoparticles and zinc oxide served as dual templates,increasing the mesoporous and specific surface area of carbon materials.In addition,cobalt can further increase the content of total nitrogen and graphite nitrogen,thereby enhancing the dispersibility of metal nanoparticles and electronic regulatory effect on Pt,effectively improving the ORR performance of Pt Co/NPC-Co.For Pt Co/NPC-Co,the electrochemical active area（ECSA）is 96.1 m²g _Pt^-1,and its mass activity is 0.53 A mg _Pt^-1.Due to the high proportion of nitrogen content in NPC-Co,the interaction between metal nanoparticles and carriers is enhanced,further improving the stability of the catalyst.After 20,000 potential cycles,the mass activity of Pt Co/NPC-Co decreases by only 23%,while the commercial Pt/C catalyst loses 49%.（4）In order to further improve the activity and stability of ORR catalysts,this research combined in-situ and post doping nitrogen methods.Polystyrene microspheres were used as templates to obtain nitrogen doped porous carbon spheres（NHCS2-DCD）.After adding Pt Co metal nanoparticles,the catalyst（Pt Co/NHCS2-DCD）was obtained.By adjusting the particle size of the polystyrene microsphere（PS）templates,the appropriate PS size was selected.By adjusting the number of coating layers of PS microspheres and optimizing the method of template removal,the carbon carrier with large specific surface area and suitable pore structure was prepared.Polydopamine was selected as an in situ doped nitrogen and carbon source.In order to regulate the nitrogen content and nitrogen species in carbon carriers,dicyandiamide（DCD）was used as a post doping agent,which could further enhance the interaction between metal nanoparticles and carriers The proportion of graphite nitrogen and pyridine nitrogen in the carbon support is adjusted during the nitrogen doping process of DCD,which could help to optimize the oxygen containing binding energy of Pt,and further accelerate the ORR rate of the catalyst.The mass activity of Pt Co/NHCS2-DCD is 0.57 A mg _Pt^-1.After stability testing,the mass activity of Pt Co/NHCS2-DCD only decreases by 21%.In summary,several strategies of functionalization of nitrogen containing functional groups,post doping of nitrogen,in situ doping of nitrogen,and a combination of two doping method were applied.The structural characteristics of carbon materials have been adjusted,which further improve the loading performance and corrosion resistance of carriers.Catalysts with excellent electrochemical performance and stability have been prepared.The mechanism of the interaction between active components and carbon carriers on the electronic properties,crystal structure,and morphology of catalysts have been deeply studied.The thesis has been laied a theoretical and practical foundation for the preparation of efficient and low platinum catalysts.