Nitrogen Doped Defective Carbon Nanotube Electrocatalyst for Oxygen Reduction Reaction
Authors: Chen Ti , Oktaviano Haryo Satriya , Waki Keiko 
Published: February 2020

Nitrogen-doped non-precious metal catalysts show high oxygen reduction reaction (ORR) activity, which are considered as possible alternatives for fuel cell catalysts. It has been reported that the transition metals and nitrogen doped carbon complex shows outstanding performance of ORR activity, suggesting that metal impurities in carbon nanotube play a very important role to the ORR activity. [1] On the other hand, recently the ORR active sites in N—doped carbon materials are carbon atoms with Lewis basicity next to pyridinic N was proposed. [2] However, the mechanism and the ORR active site are still in controversy. In our previous work, we reported that annealing multi-walled carbon nanotubes (MWCNT) with nano-drilled defective structure in Argon atmosphere could reach high ORR activity. [3] We suggest that the edge of defects is essential for the formation of active site. In order to clarify the active site and further enhance the ORR activity, here, by making defective edges on the MWCNT structure accompanied with nitrogen doping, we showed that the ORR activity has been improved with onset potential reached up to 1.1V vs RHE in 0.1M KOH and 0.88V vs RHE in 0.1M HClO4.
The MWCNT used were provided by Showa Denko KK Japan (VGFX-XA, diameter: 15nm, approximate length: 1μm; containing Fe impurities <1 wt%). As a precursor, functionalized and purified MWCNT was obtained by heat and acid treatment in the mixture of H2SO4/HNO3. [3] Defective MWCNTs were then prepared following our previous report by nano-drilling purified MWCNT using CoOx as oxidation catalyst. [3,4] Then defective MWCNTs were nitrogen doped in 10% NH3/ Argon at 900℃.
The ORR activity of nitrogen doped defective MWCNT in acid was shown in Fig.1 and in alkaline was shown in Fig.2. We found that nitrogen doped defective MWCNTs show very high ORR activity and efficiently doping nitrogen to the defective edge is essential for the enhancement of ORR activity. The optimization of nitrogen doping process and the nitrogen contain along with the role of defect structure on the ORR activity will be discussed further through detailed characterizations with X-ray photoelectron spectroscopy, Temperature Programmed Desorption and Raman Spectroscopy.
Acknowledgement
This work partially was supported by COI STREAM from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Shin-Etsu Chemical Co. ,Ltd. Japan. Their contribution is greatly appreciated.
Reference
[1] Li, Yanguang, et al. "An oxygen reduction electrocatalyst based on carbon nanotube-graphene complexes." Nature nanotechnology 7.6 (2012): 394-400.
[2] Guo, Donghui, et al. "Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts." Science 351.6271 (2016): 361-365.
[3] K. Waki, R. A. Wong, H. S. Oktaviano, T. Fujio, T. Nagai, K. Kimoto and K. Yamada, Energy Environ. Sci., 2014, 7, 1950-1958.
[4] Oktaviano, Haryo S., Koichi Yamada, and Keiko Waki. "Nano-drilled multiwalled carbon nanotubes: characterizations and application for LIB anode materials." Journal of Materials Chemistry 22.48 (2012): 25167-25173.

Figure 1