Structure Design,Catalytic Mechanism And Application Of Low-/Non-Pt Oxygen Reduction Electrocatalysts

Developing highly efficient and stable low-/non-Pt electrocatalysts is significant for the commercialization of fuel cells and metal-air batteries.In this thesis,for the two systems of low-Pt with intermallic structure and manganese（Mn）-based oxides without Pt,we designed and synthesized low-/non-Pt oxygen reduction electrocatalysts with excellent oxygen reduction reaction（ORR）performance by considering the particle size,crystal structure,element composition and structual defects of the catalysts.We revealed the intrinsic catalytic mechanism by combining in situ synchrotron X-ray absorption spectroscopy and the density functional theory calculation.Furthermore,we studied the practical application of electrocatalysts in fuel cells and metal-air batteries.1.The ultralfine nitrogen（N）-doped tetragonal-ordered intermetallic Pt Ni catalyst supported onto Ketjenblack was synthesized.The mass activity and specific activity of the catalyst are 10.1 and 7.7 times than those of commercial Pt/C catalyst,respectively.And the stability of the catalyst after 30,000 cycles test was improved23.8%compared to that of commercial Pt/C.The investigation of the catalytic mechanism indicated that the introduced tensile strain via the N-doping and ordered structure greatly optimizes the surface adsorption of oxygenated species on the Pt surface,leading to the enhanced catalytic performance.The high durability of the catalyst is attributed to the pinning effect due to the chemically durable Ni-N bonding and the formation of intermetallic phase comprising alternating Ni₄-N and Pt planes.Moreover,the mass activity and the mass activity loss of the catalyst in PEMFC testing are both higher than the US Department of Energy 2020 target.2.We successfully synthesize ultrafine N-doped hexagonal-ordered intermetallic Pt Cu catalyst supported onto Ketjenblack.The mass activity and specific activity of the catalyst are 5.2 and 3.9 times than those of commercial Pt/C catalyst,respectively.And the stability of the catalyst after 20,000 cycles test was improved 17.4%compared to the commercial Pt/C.The investigation of the catalytic mechanism indicated that both the formation of the ordered intermetallic structure and the N-doping can introduce the electronic effects on the Pt surface,regulating the optimum adsorption of oxygenated species on the Pt surface,thus accounting for the improved intrinsic activity and stability of the catalyst towards the ORR.3.We presented a facile synthesis of N-doped carbon encapsulated hexagonal-ordered intermetallic Pt-Co-Fe nanoparticles with different atom ratios of Co and Fe.The catalysts with the optimal Fe concentration（Co:Fe=7:3）and 1～2layers of carbon sheet exhibited the best ORR activity and durability.The investigation of the catalytic mechanism indicated that the excellent ORR performance and extraordinary stability can be attributed to the protective effect of N-doped carbon shell,the strong catalyst-support interaction,and the synergistic interaction between the Pt-Co-Fe core and N-doped carbon shell.4.We systematically studied the relationships between the concentrations of oxygen defects and the ORR intrinsic activities of ultrafine Mn-based mullite nanocrystals(Sm Mn₂O_5-δ).The optimized electrocatalyst with oxygen defect concentration（δ=0.14）exhibits highest ORR intrinsic activity and stability.Theoretical calculations indicated that the introduction of oxygen vacancies reduces the work function,and resulting in stronger catalyst-intermediates interaction,thus facilitating electron transfer processes and enhancing the intrinsic ORR activity.Moreover,the optimized catalysts reveal excellent long-term discharge stability in netural Mg-air battery,outperforming the state-of-the-art Pt/C equipped batteries.