Study On Electrocatalysts Of Oxygen Reduction Reaction Based On Nitrogen-doped Carbon Materials

In fuel cells, polymer electrolyte membrane fuel cells (PEMFC), with theadvantages of low operating temperature (60–100C) and the fast start-up, arepromising candidate for application in portable power source and electric vehicletransportation. The major limitation of PEMFC is the performance of oxygen reductionreaction (ORR) at the cathode. Up to now, the carbon black supporting platinum-basednanoparticles (NPs) are widely used as the catalyst for ORR. However, the largescalecommercialization of PEMFC has been hampered by the low catalytic activity and highcost of the Pt-based electrocatalysts. Recently, the investigation of electrocatalystsfocuses on reducing or replacing Pt-based electrode in fuel cells and developping newORR electrocatalysts. Among the developed catalysts, nitrogen-doped carbons and theircomposites are already known to act as efficient catalysts for ORR.Here, the simple and novel methods for the fabrication of nitrogen-dopedgraphene (N-G) and hollow nitrogen-doped carbon microspheres (HNCMS) have beendeveloped and the obtained nitrogen-doped carbons have been applied for ORR inalkaline media. Additionally, we used HNCMS as the catalyst supports of Pt NPs andthe electrocatalytic performances of obtained Pt/HNCMS nanohybrids for ORR in0.5M H2SO4have been studied in detail. The main points of this thesis are as follows:(1) Nitrogen-doped graphene (N-G) were synthesized via a simple hydrothermalreaction of graphene oxide (GO) and urea. N-doping and reduction of GO wereachieved simultaneously under the hydrothermal reaction. The obtained N-G wascharacterized by atomic force microscopic (AFM), transmission electronmicroscopy (TEM) and elemental analysis. Compared with the reduced graphene(B-G) with NaBH4and GO, the onset potential of ORR on N-G was positivelyshifted and an enhanced steady-state diffusion current for N-G was observed. Also,N-G exhibited higher stability and tolerance to crossover effect of methanol thanthat of E-TEK Pt/C.(2) Dopamine, a sustainable and cheap raw material, was selected as the carbon andnitrogen sources to synthesize hollow nitrogen-doped carbon microspheres(HNCMS). The as-prepared HNCMS were characterized by scanning electronmicroscopy (SEM), TEM, automatic micropore and chemisorption analyzer andelemental analysis. The results showed that HNCMS had the uniform diameter and the mesoporous structure, high surface area as well as6.58wt.%of nitrogencontent.The HNCMS were used as a non-noble-metal electrocatalyst for oxygenreduction in alkaline solution and showed high electrocatalytic activity, excellentlong-term stability and tolerance to crossover effect of methanol.(3) Hollow nitrogen-doped carbon microspheres (HNCMS) were synthesized bycarbonization of polydopamine and Pt NPs were deposited on HNCMS viamicrowave-assisted reduction process without the use of any surface modificationand surfactant. TEM, powder X-ray diffraction (XRD) were used to characterizethe catalyst. It was found that Pt NPs were uniformly deposited on HNCMS andthe micropores and mesopores structure in Pt/HNCMS remains after Pt deposition.The electrocatalytic properties of the Pt/HNCMS electrode for ORR have beeninvestigated through cyclic voltammetry and linear sweep voltammetry. The Pt/HNCMS electrode showed~3times higher electrocatalytic activity and4timeslonger durability than those of commercial Pt/C catalysts towards ORR.