Preparation And Electrochemical Catalytic Properties Of Nitrogen-Doped Carbon Materials For Oxygen Reduction Reaction

Fuel cells are currently considered to be clean and high efficient energy conversion devices because they can convert chemical energy from fuel into electricity in an environmentally benign manner. However, the high cost and scarcity of Pt-based catalysts together with the sluggish oxygen reduction reaction(ORR) kinetics and poor durability has become the biggest barrier to their commercial application. To address this issue, great efforts have been devoted to the development of nonprecious metal catalysts that facilitate the ORR on electrode. Among these non-Pt cathodic catalysts, nitrogen-doped carbon materials have been widely investigated due to their superior ORR catalytic activity, excellent methanol tolerance and durability. In this thesis, nitrogen-doped carbon materials have been successfully synthesized by direct pyrolysis method using tripyrrolyl[1,3,5]triazine(TPT) or TPT-based polymeric networks as carbon and/or nitrogen precursors. The ORR active sites and catalytic mechanism of these catalysts have been investigated based on their morphologies and microstructures, elemental compositions, and ORR performances. The main research contents were shown as follows:(1) Synthesis and electrocatalytic ORR performance of N-doped carbon nanotubes. The N-doped carbon nanotubes(NCNTs) was synthesized by the direct pyrolysis of oxidized CNTs and TPT mixture in N2. The N bonding configuration for NCNTs was investigated by XPS analysis, which indicated that the graphitic-N was predominant for NCNT-900. The electrochemical measurements including cyclic voltammetry, linear scanning voltammetry, chronoamperometry and alternative current impedance were conducted to systematically investigate the electrocatalytic ORR performance. It was found that the as-prepared NCNT-900(pyrolyzed at 900 °C) exhibited excellent electrochemical performance towards ORR in alkaline medium with an onset potential of-0.038 V(vs. Ag/Ag Cl), a large limiting current density of 6.47 m A cm-2, a dominant 4e transfer mechanism, and excellent methanol tolerance and durability.(2) Preparation and electrocatalytic ORR performance of metal-free NC catalyst based on PTPT network. The metal-free N-doped carbon(NC) catalyst was prepared by directly pyrolyzing TPT-based polymeric network(PTPT PN), in which the PTPT PN was synthesized by oxidative-polymerization of TPT using benzoyl peroxide as the oxidizing agent in the presence of Tf OH. The NC-900 catalyst possessed a partial graphitic structure with a high specific surface area of 779 m2 g-1. The N bonding configuration for NC-900 was investigated by XPS analysis, which demonstrated that graphitic-N and pyridinic-N were predominant for NC-900. Compared to commercial Pt/C reference catalyst, the obtained NC-900 catalyst exhibits superior ORR activity in alkaline electrolytes with a high onset potential of 0.008 V(vs. Ag/Ag Cl), a large limiting current density of 5.05 m A cm-2, and excellent methanol tolerance and durability.(3) Preparation and electrocatalytic ORR performance of Fe-N/C catalysts based on PTPT network. The Fe-N/C catalysts were prepared by directly pyrolyzing PTPT network and iron(II) acetate mixture, in which the PTPT network was synthesized by a one-step Friedel-Crafts reaction of TPT with formaldehyde dimethyl acetal using Fe Cl3 as catalyst. The as-prepared Fe-N/C-900 catalyst possessed a partial graphitic structure with a high specific surface area of 710 m2 g-1. The N and Fe contents of the Fe-N/C-900 were found to be 2.1% and 0.6%. The electrocatalytic activities for the ORR were investigated based on rotating disk and rotating disk-ring electrode techniques. Compared to commercial Pt/C catalyst, the Fe-N/C-900 catalyst exhibits superior ORR activity in alkaline electrolytes with a high onset potential of 0.041 V(vs. Ag/Ag Cl), a large limiting current density of 5.43 m A cm-2, and excellent methanol tolerance and durability.(4) Preparation and electrocatalytic ORR performance of Fe-NTP/C-900 catalysts based on TPT and pyrrole(Py) copolymer network. The novel Fe-NTP/C-900 catalyst was prepared by the direct pyrolysis of TPT-Py copolymer network(CPN) and iron(II) acetate mixture, in which the TPT-Py CPN was fabricated by one-step Friedel-Crafts reaction of TPT and Py comonomer with formaldehyde dimethyl acetal using Fe Cl3 as the catalyst. The Fe-NTP/C-900 catalyst possessed a partial graphitic structure with a high specific surface area of 724 m2 g-1. The N and Fe contents of Fe-NTP/C-900 were found to be 2.59% and 0.875%. The catalytic activities for the ORR were investigated based on rotating disk and rotating disk-ring electrode techniques. Compared to commercial Pt/C reference catalyst, the obtained Fe-NTP/C-900 catalyst exhibits superior ORR activity in alkaline electrolytes with a high onset potential of 0.051 V(vs. Ag/Ag Cl), a large limiting current density of 5.79 m A cm-2, and excellent methanol tolerance and durability.