Recently, carbon based catalysts have attracted much attention as an excellent cathode catalyst for proton exchange membrane fuel cell (PEMFC), because of its low cost and high performance. Multi walled carbon nanotubes (MWCNT) is a special kind of carbon material with higher durability and better cost-efficiency than noble metal catalyst. There are a lot of reports claim that nitrogen doping or adding metal impurity to form M-C-N moiety are important to obtain high activity.[1][2] However, there is another opinion which claimed the non-metal MWCNT itself could also show proper activity and work as the cathode catalyst in PEMFC.[3] Previously, we have reported that only by making defect structure on the surface of MWCNT with catalyst-assistant oxidation, the oxygen reduction reaction (ORR) property could greatly improve.[4] In this study, we are interested in the relationship between oxidation and the ORR activity of the defective carbon nanotube. The further investigation on the defect making indicates that during chemical drilling process, different temperature will lead to an obviously change of the ORR property. A high activity MWCNT with defective structures has been successfully made and its onset potential could reach to 0.76V vs RHE.
2. Experimental
The MWCNT in experiment is VGFX-XA (diameter: 15 nm, approximate length: 1 mm, provided by Showa Denko KK, Japan). Pretreatments including heat and acid treatment have been applied in order to remove the impurities. The defective MWCNT is prepared by chemical drilling process, depositing Cobalt on the surface of MWCNT and followed by oxidation. Here various conditions with different temperature have been applied which will lead to different defective structures. These differences cause essential influences on the ORR property of MWCNT. Cyclic voltammetry, temperature programmed desorption and BET analysis are conducted to investigate those MWCNT and study the ORR mechanism.
3. Result
The onset potential of the defective MWCNT significantly increase with the increasing of the temperature during oxidation from the linear sweep voltammetry measurement as Fig. 1. The oxidation temperature of 600℃ could result in the highest onset potential up to 0.76V vs. RHE, without N doping or metal impurities. Temperature program desorption results also proved that different defect making methods would lead to different functional groups. The details of the defective structures will be furtherly studied by XPS, Raman and ICP analysis.
Acknowledge:
This work partially was supported by the Center of low carbon society strategy (LCS) and Shin-Etsu Chemical Co., Ltd. Japan. Their contribution is greatly appreciated.
Reference:
[1] Z.-Y. Yang, Y.-X. Zhang, L. Jing, Y.-F. Zhao, Y.-M. Yan, and K.-N. Sun, “Beanpod-shaped Fe–C–N composite as promising ORR catalyst for fuel cells operated in neutral media,” J. Mater. Chem. A, vol. 2, no. 8, pp. 2623–2627, 2014.
[2] J. Shui, M. Wang, F. Du, and L. Dai, “N-doped carbon nanomaterials are durable catalysts for oxygen reduction reaction in acidic fuel cells.,” Sci. Adv., vol. 1, no. 1, p. e1400129, 2015.
[3] W. Wei et al., “Unusual high oxygen reduction performance in all-carbon electrocatalysts.,” Sci. Rep., vol. 4, p. 6289, 2014.
[4] K. Waki et al., “Non-nitrogen doped and non-metal oxygen reduction electrocatalysts based on carbon nanotubes: mechanism and origin of ORR activity,” Energy Environ. Sci., vol. 7, no. 6, pp. 1950–1958, 2014.