Electrocatalytic activity for the oxygen reduction reaction of oxygen-containing nanocarbon synthesized by solution plasma

Oxygen-containing nanocarbon materials were successfully synthesized from a mixed solvent of benzene (BZ) and 1,4-dioxane (DO) by solution plasma. TEM observation demonstrated that the synthesized carbons had a diameter in the nanoscale range, their size increased with an increase in the DO content. XRD and Raman spectroscopy measurements revealed that the nanocarbon samples had the (002) characteristic plane of the turbostratic carbon phase and structural defect sites. XPS spectra showed that the nanocarbon sample synthesized from BZ100 consisted of hydroxyl/epoxide groups as the main surface functional groups on the sample surface, whereas the main surface functional groups of the nanocarbon samples synthesized from mixed solvents containing BZ and DO changed from hydroxyl/epoxide groups to carboxyl groups. The O content in the samples increased with an increase in the DO volume ratio in the mixed solvent. The electrocatalytic activity of the synthesized nanocarbon samples for the oxygen reduction reaction (ORR) was investigated electrochemically using cyclic voltammograms (CVs) and linear sweep voltammograms (LSVs) in an O₂-saturated 0.1 M KOH aqueous solution. It was found that the addition of DO to the mixed solvent could only influence the current density, whereas no significant change in the onset potential was observed. The order of the current density of the nanocarbon samples for the ORR was as follows: BZ90 + DO10 > BZ100 > BZ70 + DO30 > BZ50 + DO50. The number of electron transfers for the ORR of the nanocarbon samples estimated by Koutechy–Levich (K–L) plots was about three at low potentials, indicating the coexistence of two- and four-electron reduction of O₂. The oxygen-containing functional groups formed on the carbon surface and structural defects seemed to play the key roles in enhancing the electrocatalytic activity for the ORR rather than the total oxygen content in the nanocarbon samples.