Enhancement of Bifunctional Activity of Modified Carbon Nanotube Supported Cobalt Oxide Catalysts

In renewable energy technology, bifunctional catalysts which are related to oxygen electrode play important roles in electrochemical energy storage and conversion devices. However, it is hard to find appropriate bifunctional catalysts for the oxygen evolution (OER) and reduction reaction (ORR) due to their sluggish reaction kinetics [1-4]. Up to now, the best ORR electrocatalysts are the Pt-based catalysts. In addition, iridium and ruthenium oxides are excellent OER catalysts. However, these noble metals have serious drawbacks for using bifunctional electrocatalysts of the water-splitting devices due to the high cost and limited durability. Hence, in this work, various methodologies with transition-metal oxides are investigated for the bifunctional OER & ORR catalysts through the doping, mesoporous nanostructure and surface functionalization. In particular, carbon nanotubes (CNTs) are chosen as a supporting material of transition-metal oxides [5-9]. Further, in order to achieve high efficient electrocatalyst for OERs, CNTs are oxidized by using the potassium permanganate [10]. The physicochemical properties of bifunctional OER & ORR catalysts are investigated by using various analytic techniques such as SEM, TEM, BET, and XRD. For the electrochemical characterization of catalysts, the Faradic efficiency of each material is determined using a rotating ring-disk electrode (RRDE) device. A rotating disk electrode is adopted with an Ag/AgCl reference electrode (in KCl-saturated solution) and Pt counter electrode in 0.1 M KOH electrolyte. Before performance measurements of the catalysts, nitrogen gas is purged in the potential range of 0.05 – 1.2 V versus reversible hydrogen electrode (RHE) at a scan rate of 100 mV s-1 for 50 cycles. The OER activity is measured by linear sweep voltammetry (LSV) from 1.2 to 1.7 V at a scan rate of 5 mV s-1 and is compared the oxygen evolution activity at specific current density. And the durability test for OER is performed by cycling the electrode potential between 1.25 and 1.65 V versus reversible hydrogen electrode (RHE) at 200 mV s-1 for 1500 cycles. The ORR activity is measured by the LSV from 0.2 – 1.1 V at a scan rate of 5 mV s-1 at 400, 900, 1600 and 2500 rpm. High purity oxygen gas (99.999%) is purged for 30 min before each RDE experiment to make the electrolyte saturated with oxygen and is compared the oxygen reduction activity at specific current density. 1. M. Zhang, M. d. Respinis, H. Frei, Nature Chemistry, 6, 362–367 (2014). 2. Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H. Dai, Nature Materials, 10, 780–786 (2011). 3. J. I. Jung, H. Y. Jeong, J. S. Lee, M. G. Kim, J. P. Cho Angewandte Chemie, 53, 4582-4586 (2014). 4. Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Nature Materials, 10, 780-786 (2011). 5. J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough, Y. Shao-Horn, Science, 334, 1383-1385 (2011). 6. J. Suntivich, H. A. Gasteiger, N. Yabuuchi, Y. Shao-Horn, Nature Chemistry, 3, 546-550 (2011). 7. T. Reier, M. Oezaslan, P.Strasser, ACS Catal., 2, 1765-1772 (2012). 8. Y. J. Sa, K. j. Kwon, J. Y. Cheon, F. K., S, H, Joo, J. Mater. Chem. A, 1, 9992-10001 (2013). 9. I. C. Man, H. Y. Su, F. C. Vallejo, H. A. Hansen, J. I. Martínez, N. G. Inoglu, J. Kitchin, T. F. Jaramillo, J. K. Nørskov, J. Kehlet; R. Jan, ChemCatChem, 3,1159–1165, (2011). 10. C. C. L. cCrory, S. Jung, J. C. Peters, T. F. Jaramillo, J. Am. Chem. Soc., 135, 16977–1698 (2013). * Corresponding authors: jyoung@sejong.ac.kr (J.-Y. Park).