Non-precious metal electrocatalysts for ORR have been regarded as one of promising candidates for cutting the cost of fuel cell. It has been early considered that combination of transition metals, nitrogen doping together with carbon as support material is essential to have outstanding performance of this class of electrocatalyst.¹ It is reported that in alkaline condition, combination of cobalt oxides (CoO_x) nanocrystal-carbon nanotubes could also exhibit a high ORR activity.² However, not only utilizing a large amount of transition metal oxides (~12-20 at. % Co) but also the annealing process in NH₃ atmosphere are undesirable in the application. On the other hand, recently, it is reported that annealing in the inert atmosphere could also affect the ORR activity for carbon-based catalysts³. However, the understanding of the annealing effect in the inert atmosphere is still limited. Here, we showed that even by annealing the oxidized carbon black (CB) in Ar atmosphere at 900 ^oC and containing less amount of CoO_x, we could achieve high ORR activity in 0.1 M KOH, suggesting that annealing process is essential to enhance the catalytic activity.

As precursor, we used raw CB from Tokai Carbon Co. Ltd (Tokablack #5500). To oxidize CB, raw CB was purified by using acid treatment as reported elsewhere³ and followed by solid-stated oxidation using CoO_x as carbon oxidation catalyst. At first, purified CB was impregnated with 10 wt. % (~2.2 at. % Co), followed with heat treatment under Ar atmosphere at 300 °C for 3 hours and finalized by annealing in air at 230 °C for 25 minutes. Afterward, CoO_x was removed by acid washing in 2M sulfuric acid, resulting in oxidized CB. To study the effect of annealing and removing the functional groups, samples were later treated at 900 ^oC in Ar atmosphere. Effect of the annealing toward ORR activity is shown in Fig. 1. It can be seen that without annealing in the Ar at 900 ^oC, oxidized CB showed lower reduction current than the annealed one. This suggests the important role of annealing to the enhancement of ORR activity. Annealing effect in the different temperatures together with the role of CoO_x-oxidation catalysts on the ORR activity will be further discussed through the detailed characterization of the samples (e.g. RRDE, XPS, TPD and TEM) in the presentation.

References

[1] H. Tang, H. Yin, J. Wang, N. Yang, D. Wang and Z. Tang, Angew. Chem. Int. Ed., 52, 5585 (2013)

[2] Y. Liang, H. Wang, P. Diao, W. Chang, G. Hong, Y. Li, M. Gong, L. Xie, J. Zhou, J. Wang, T. Z. Regier, F. Wei, and H. Dai, J. Am. Chem. Soc., 134, 15849 (2012)

[3] K. Waki, R. A. Wong, H. S. Oktaviano, T. Fujio, T. Nagai, K. Kimoto, and K. Yamada, Energy Environ. Sci., 7, 1950 (2014)