Title : Synthesis of nitrogen-doped nanocarbons by solution plasma process using cyanoaromatic molecules for catalyst of oxygen reduction reaction
Oxygen reduction reaction (ORR) is the most important process occurring at the cathode in fuel cells. Pt/carbon catalysts have long been known as the most efficient materials for cathode due to its excellent ORR activity. However, their high cost, scarcity, poor long-term durability, and methanol crossover effect have hindered the performance and large-scale production of fuel cells in the coming decade. Nitrogen-doped carbons have recently emerged as a promising material for next-generation fuel-cell cathode instead of Pt/carbon owing to highly efficient ORR activity and excellent durability without methanol crossover. Solution plasma process (SPP) has been gaining a considerable interest in the various synthesis and applications of nanomaterials owing to its several advantages, such as simple equipment, short-time synthesis, ambient condition, and ability to scale up. Very recently, SPP has shown a great potential for the synthesis of carbon nanomaterials by using small organic molecules as precursors such as benzene, toluene, and ethanol. It can also be potentially extended for the synthesis of heteroatom-doped carbons simply by changing the organic precursors. Our previous report has shown that the nitrogen-doped carbons have been successfully synthesized by SPP using the mixture of benzene and pyrazine. However, the ORR performance of synthesized nitrogen-doped carbons was far from that of Pt/carbon. Therefore, further development of nitrogen-doped carbons by SPP is still challenging and highly desirable. In this study, cyano-aromatic molecules, including cyanobenzene (CNBZ), cyanopyridine (CNPD), and cyanopyrazine (CNPZ), were used as the precursors for synthesis. For comparison, undoped nanocarbons were also synthesized from benzene (BZ). The results demonstrated that the nitrogen doping content directly related to the amount of nitrogen atom in the precursors, which varied in the range of 0.6-1.5 at%. The electrocatlytic activity toward ORR in an alkaline medium in terms of onset potential and limiting current density were found to be improved in the order of CNPZ > CNPD > CNBZ > BZ. The ORR tended to proceed via four-electron pathways at −0.4 V for the nitrogen-doped carbons synthesized CNPZ. Moreover, they possess long-term durability and excellent tolerance to methanol crossover effects. Acknowledgement: This research was supported by Japan Science and Technology Agency (JST) under Program on Open Innovation Platform with Enterprises, Research Institute and Academia (OPERA) (No. 18072116).