Abstract
This thesis mainly focuses on the synthesis and fabrication of nitrogen-doped reduced graphene oxide (N-rGO) and its application towards oxygen reduction reaction (ORR). The rotating ring-disk electrode (RRDE) voltammetry was applied to study ORR. In the first part, the electrocatalytic activities of various ORR catalysts including 20 % platinum on Vulcan XC-72 carbon black (Pt/C), manganese dioxide in the α phase (α-MnO2), XC-72 carbon black, α-MnO2/XC-72, and N-rGO were investigated using RRDE technique. During RRDE measurements, the potentials of the Pt ring electrode (Ering) were fixed at 1.03 V, 1.23 V and 1.48 V (vs. RHE) respectively. The results exhibits that the electron transfer number of Pt/C, α-MnO2 and α-MnO2/XC-72 were independent of the Ering between 1.03 V and 1.23 V (vs. RHE). When Ering was varied to 1.48 V (vs. RHE), the electron transfer number decreased. Electron transfer number was also confirmed by Koutecky-Levich plot (K-L plot), which is close to the RRDE results measured at Ering of 1.03 V and 1.23 V (vs. RHE) inferring that Ering between 1.03 V and 1.23 V is more reliable. Moreover, some by-product formed during the reduction reaction of the disk electrode can be only oxidized at the Ering higher than 1.48 V (vs. RHE). To investigate how the ring current (IR) can be changed by the Ering, Ering was scanned from 0.9 V to 1.6 V (vs. RHE). From the linear sweep voltammograms (LSV) of the ring electrode, IR exhibits an small difference between 1 V and 1.3 V (vs. RHE) and large difference between 1.3 V and 1.48 V (vs. RHE). To confirm the species oxidized at the higher Ering, the pH value of the electrolyte was increased and the difference between 1.3 V and 1.48 V became significant indicating the oxidation of OH-; therefore, the choice of the Ering for RRDE should be in the potential range of 1 V to 1.3 V. In the second part, we propose an approach to fabricate N-rGO by microwave-assisted hydrothermal synthesis (MAHS). During the fabrication, melamine and pyrrole were used as nitrogen sources. While applying 25-1 factorial design of experiments and path of steepest ascent/descent (PSA/PSD) experiments, five factors including (A) N-doping temperature, (B) microwave power, (C) holding time of the microwave at the doping temperature, (D) different nitrogen molar ratio of melamine and pyrrole, and (E) total concentration were considered in order to find the optimal values of electron transfer number for ORR. The highest electron transfer number was 3.93 whereas the lowest one was near 2.34. The structures and distributions of nitrogen doped onto r-GO were examined by the x-ray photoelectron spectroscopic (XPS) analysis. The layer-by-layer morphology and the high degree of defects of N-rGO were characterized by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), and Raman spectroscopy. The N-rGO of PSA and PSD was further examined for ORR applications such as zinc-air battery discharge test and hydrogen peroxide (H2O2) electro generation. The N-rGO of PSA was coated on the gas diffusion electrode (GDE) as the cathode for the zinc-air full cell. The results shows that the highest zinc-air battery cell voltage of 1.235V, when the current density maintained at 2 mA cm-2. The N-rGO of PSD was also coated on the GDE and used as a working electrode to generate H2O2. The concentration of 25.27 mg L-1 was reached in 1 hr at constant potential of 0 V (vs. RHE). The highest current efficiency was calculated to be 43 %. A higher concentration was reached at -0.2 V (vs. RHE); however, the current efficiency was only 16.4 %. The H2O2 generation was utilized for electrochemical catalytic decomposition of organics. The degree of degradation of orange G (OG) was analyzed by UV-vis absorption spectra. These results show that the relative absorbance of the azo π-conjugation structure of OG was only 0.38 at a constant potential of -0.2 V (vs. RHE) inferring with a high degree of decoloration.