Non-Nitrogen Doped Defective Multi-Walled Carbon Nanotubes (MWCNT) As the Cathode Catalyst for PEMFC
Authors: Chen Jiuting , Eguchi Kouya , Waki Keiko 
Published: February 2020

Recently, non-noble metal catalysts has been intensively studied for proton exchange membrane fuel cell to achieve low cost and high performance. Among them, carbon nanotubes have attracted much attention because of their lower cost and recyclable property. It is suggested that defective structures on the carbon surface with nitrogen doping and small amount of transient metal to form Fe-C-N moiety are important to have high activity.[1][2] However, the role of carbon defects for the oxygen reduction reaction (ORR) is still not clarified. We have been interested in the ORR activity of the defective pure carbon nanotube and found that with defective structures even without nitrogen doping or annealing, the onset potential can also reach to 0.73V vs RHE. [3]

So far high ORR activity always achieved by high temperature annealing process in Argon or ammonia atmosphere. In this study, aiming to further investigate the characteristics of the defective carbon nanotube, optimization of the chemical drilling processes conduct to control the defect structure on MWCNT. By this method high ORR activity MWCNT material could be obtained without annealing or heating treatment.

The MWCNT were provided by Showa Denko KK Japan, acid and heat treatment has been done before experiment. The procedure of making defect on MWCNT is prepared by cobalt deposition and oxidation. Here various conditions have been applied which will lead to different defect structure. These differences cause essential influences on the ORR property of MWCNT. To evaluate the performance of defected MWCNT, both cyclic voltammetry and power generation of cathode catalyst in PEMFC have been measured.
Linear sweep voltammetry test shows that high defect density and shallow defect structure could bring benefits to the ORR property, and 0.65V vs. RHE onset potential has already been achieved by pure MWCNTs in Fig 1, without N doping, metal impurities or annealing process. Temperature program desorption results also proved that different defect making methods would lead to different characteristics of functional group. The details of different defect structures and principles will be furtherly studied by XPS, Raman, ICP analysis and Power Generation.
Acknowledge:
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] Michel Lefèvre, Eric Proietti, Frédéric Jaouen, Jean-Pol Dodelet. "Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells" Science, 324, 71-74, 2009.
[2] Kuanping Gong, Feng Du, Zhenhai Xia, Michael Durstock, Liming Dai. "Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction" Science, 323, 760-764, 2009.
[3] K. Waki, R. A. Wong, H. S. Oktaviano, T. Fujio, T. Nagai, K. Kimoto and K. Yamada. "Non-nitrogen doped and non-metal oxygen reduction electrocatalysts based on carbon nanotubes: mechanism and origin of ORR activity" Energy Environ. Sci., 7, 1950-1958, 2014.

Figure 1