Preparation And Properties Investigation Of Novel Catalysts For Highly Efficient Oxygen Reduction Reaction

It is well known that platinum is the best catalyst for oxygen reduction reaction?ORR?for proton exchange membrane fuel cells?PEMFCs?,while its slow oxygen reduction kinetics,limited abundance in the earth and high price which are seriously hinder the commercialization of PEMFCs.Accordingly,low platinum and non-platinum ORR catalysts are significant for the development of PEMFCs.Recent studies show that core-shell structures catalyst not only reduce the platinum loading on the catalyst,but also increase ORR activity.Nitrogen-doped carbon-supported catalysts also exhibit excellent activity and stability in terms of oxygen reduction.In this thesis,ORR core-shell catalysts employed transition metal?Fe,W,Mo,V?nitrides as core and platinum as shell were prepared using the principle of surface segregation effect in H₂ or NH₃ atmosphere at relatively high temperature.The corresponding physic-chemical characterizations show that the shell thickness and core composition has close relationships with the ORR catalytic performance,and the corresponding catalytic mechanisms were also discussed.Therefore,the aim of this thesis is to prepare core-shell catalysts that are with controllable structures and with high efficient for oxygen reduction.The main contexts are as follows:?1?The N-Fe M@Pt/C?M=Mo,V and W?core-shell catalysts were successfully synthesized by impregnation method and followed by annealing in ammonia atmosphere at high temperature,and finally de-alloyed by electrochemical method in acid solution.?2?Catalysts with different shell thicknesses were prepared by tuning the molar ratio of elements,and the corresponding catalytic activity for oxygen reduction were investigated by cyclic voltammetry and linear sweep voltammetry.The results display that the electrochemical specific surface area and mass activity?0.9V vs.RHE?for oxygen reduction gradually increased with the reduction of shell thickness.All prepared catalysts in this thesis had significantly enhanced mass activity compared to the standard commercial Pt/C catalyst.Particularly,a remarkable 18.9-fold enhancement in mass activity for ORR was observed on N-Fe₅Mo₁@Pt₃/C.?3?The ORR activities and stabilities were carried out on all the investigated catalysts with different cores before and after 5,000 cycles durability tests.The initial catalytic activities for ORR are in the order of N-Fe₅Mo₁@Pt₃/C>N-Fe₅V₁@Pt₃/C>N-Fe₅W₁@Pt₃/C.After 5,000 cycles,all the catalysts displayed higher ORR activities than the pristine Pt/C catalyst,and only degraded slightly after stability tests.The possible reasons for degradation might be due to the agglomeration and/or detachment of metal particles.Meanwhile,the space vacancies that generated by electrochemical de-alloying process might be filled by the nearby platinum atoms during the accelerated durability tests,which could reduce the electrochemical surface area.