Abstract:

Nitrogen-doped carbon nanotubes (NCNT) were prepared by introducing nitrogen-containing groups into surface of carbon nanotubes of different wall numbers using the pre-oxidation treatment and the ammonia hydrothermal method. Effect of carbon nanotube wall number on the electro-catalytic activity of NCNT for oxygen reduction reaction (ORR) was investigated. X-ray photoelectronic spectroscopy showed that the nitrogen content and the types of nitrogen-containing groups of each NCNT was similar. On the contrary, the content of different nitrogen-containing groups in each NCNT was quite different. Among them, the NCNT sample with an average wall number of 2.5 exhibited the lowest contents of pyridine nitrogen and graphite nitrogen, whereas it exhibited the highest ORR activity in terms of the most positive half-wave potential and the highest limiting current density. This indicated that the ORR activity of NCNT was decided by the number of walls of NCNT, rather than that of pyridine nitrogen and graphite nitrogen active groups when the samples had the similar nitrogen content. For this sample, the inner layer provided an effective conductive path to transfer electrons from the inner layer to the outer layer through the tunneling effect, and then the active sites of the nitrogen-containing groups on the outer wall converted O₂ to OH^-. Nevertheless, the tunneling effect became weaker with increasing the wall number of NCNT, leading to the decrease of the ORR activity of NCNT.

Key words: nitrogen-doped carbon nanotubes (CNT), wall number, oxygen reduction reaction (ORR), alkaline solution, tunneling effect

摘要：

通过预氧化和氨水水热法在含有不同壁数的碳纳米管表面成功引入含氮基团，从而获得了氮掺杂碳纳米管（NCNT），并研究了纳米管壁数对不同NCNT氧还原反应活性的影响。研究表明，各NCNT中氮元素的含量和含氮基团的种类相似，但不同含氮基团的比例则相差较大，其中平均壁数为2.5的NCNT样品含有最低的吡啶氮和石墨氮比例，而该样品却展现出最高的电子转移数和最大的氧还原反应极限扩散电流。分析表明，NCNT的氧还原反应活性决定于纳米管壁数，而不是吡啶氮和石墨氮活性基团的比例，即NCNT的内壁为反应电荷的转移提供了有效导电途径，并通过隧穿效应将电子转移到外壁，而外壁的含氮基团活性位点得到电子从而将O₂转变为OH^-。随着NCNT壁数的增加，NCNT中电子隧穿效应减弱，NCNT的氧还原反应（ORR）活性也随之降低。

关键词: 氮掺杂碳纳米管, 纳米管壁数, 氧还原反应, 碱性, 隧穿效应