Synthesis Of Doped Nanostructured Carbon Materials And The Electrocatalytic Investigation On Oxygen Reduction Reaction

The usage of the Pt catalyst poses a great challenge to the development of fuel cell technology; therefore, it is urgent to explore novel non-noble metal based electrocatalysts. Nitrogen-doped carbon-based material is considered as the best non-noble catalyst with low cost and high electrocatalytic activity toward oxygen reduction reaction(ORR), which is promising to be used in the application of fuel cells and metal-air batteries. Nitrogen-doped ordered mesoporous carbon(NOMC) features high specific surface area and uniform pore size, which are highly desired to be used as the electrocatalyst in fuel cells.In this work, a novel impregnation method, namely vaporization-capillary condensation impregnation(VCCI) method, is developed to synthesize NOMC. Then, the effect of pyrolysis conditions, SBA-15 templates, nitrogen precursors and further ammonia-activation are extensively investigated on the NOMC. Moreover, the microstructure, elemental composition and chemical state, molecular fraction are extensively investigated by the nitrogen sorption isotherm, electron microscope, X-ray diffraction, thermal gravity analysis, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass. Electrochemical methods, including the cyclic voltammetry, rotating ring-disk electrode and cathodic stripping voltammetry, are applied to investigate the electrochemical behavior in the electrolyte solutions of different pHs(0-2, 7, 12-14).Firstly, the novel VCCI method is developed to synthesize NOMC with high specific surface area successfully. The synthesized C-PY-900 catalyst features an excellent electrocatalytic activity to the ORR, which greatly outperforms the Pt/C catalyst in both halfcell and full-cell tests. The combination of the obtained electrochemical results and the extensive surface analysis suggests that the transition metal might not be directly involved in the active sites. It is further noted that the electrocatalytic activity can be correlated with the fraction of nitrogen-activated carbon atoms(C-N), indicating that the active sites are mostly CN groups in such catalysts.Secondly, the pore structure of SBA-15 yields a significant effect on both the elemental composition(nitrogen content) and the pore structure of the resultant carbon, which thereby changing its electrocatalytic activity for the oxygen reduction reaction. The nitrogen precursor yields a significant effect on both the pore structure and compositions(chemical state of N 1s and C 1s), and thus affecting the electrocatalytic activity of oxygen reduction reaction. The post-treatment in NH3 was found to enhance the nitrogen doping, which facilitates the formation of active sites and thus increasing the ORR performance in alkaline media.Finally, the nitrogen-doped carbon shows a variety of functional groups with enriched reversible redox couples on the surface, acting as the bridge for the charge transfer to the adsorbed oxygen. The mechanism of the ORR depends on the electrode potential and pH of the electrolyte solution. At high potentials, the ORR proceeds by a surface-confined redoxmediated catalysis mechanism, yielding a 4-e reduction reaction, in both acid and alkaline media. At lower potentials, a parallel charge transfer process by the outer-sphere electron transfer(OSET) mechanism gets pronounced in alkaline media, yielding considerable amount of hydrogen peroxide. The nitrogen-doped carbon catalyst shows a superior performance in alkaline media than does in acid media. Such a gap in performance is rationalized by considering the chemical change in the surface at different p H values. The protonation of doped nitrogen atoms hinders the charge delocalization and thus degrades its electrocatalytic activity in acid media.