Study On Electrocatalysis Of Oxygen Reduction Reaction Based On Nitrogen- Doped Graphene

The proton exchange membrane fuel cells (PEMFCs) have the advantages of high energy conversion efficiency and low pollution. This kind of fuel cell generally uses hydrogen as fuel and oxygen as oxidant, respectively. The work efficiency of the fuel cell largely depends on the reaction rate of oxygen reduction reaction (ORR) on cathode. Usually, the reaction rate of ORR is low, and therefore addition of suitable catalyst is needed for promotion of work efficiency.In recent years, with the further research of carbon materials (such as graphene and carbon nanotube), numerous reports have proved that nitrogen-doped carbon materials have significant catalytic activity toward ORR, which compares favourably with platinum-based catalysts. And owing to the merits of low price, high stability and cycle times, carbon materials have been widely considered as replacement of platinum-based catalysts for ORR. This thesis mainly focused on nitrogen-doped graphene as the catalyst for ORR.(1) Based on the strategy of hydrothermal synthesis of nitrogen-doped graphene and the electrochemical parameters, this work focused on the influencing factors including solvents and species of nitrogen sources, and discussed the optimal synthetic condition of nitrogen-doped graphene in this thesis. Moreover, cobalt and iron has been co-doped with nitrogen to prepare a metallic-nitrogen co-doped graphene. By using XPS and Raman combined with electrochemical test as characterization methods, it suggested nitrogen, oxygen, carbon and metals have great impact to structure and catalytic behavior of nitrogen-doped graphene in this experiment.(2) Considering the potential implications relating to the degree of oxidation of carbon source, a series of graphene oxide with different oxide content have been prepared by using different oxidants, extending reaction time and increasing the ratio of oxidizer. Moreover, a range of nitrogen-doped graphene have been prepared by choosing these graphene oxide and dicyandiamide as precursors. The results suggested that the catalytic behavior of nitrogen-doped graphene prepared by higher degree of oxidation of graphene oxide was mainly dominated by four-electron pathway, showing better catalytic capability for ORR.(3) Aiming at promoting the catalytic capability of nitrogen-doped graphene for ORR, this experiment tried to use hydrazine hydrate and hydroiodic acid to reduce the rest oxygen content of nitrogen-doped graphene. Compared to the raw nitrogen-doped graphene, the electrochemical test showed negative effect including the decrease of current density and negative shift of onset potential after reduction. Assisted by XPS and SEM, it could be attributed to the morphological alteration and composition change of final product:hydroiodic acid seemed to destroy the 3D structure of nitrogen-doped graphene; though hydrazine hydrate could lower the ratio of oxygen, it also caused the decrease of nitrogen content which was very essential to catalytic capability of nitrogen-doped graphene for ORR.