The growing demand of energy and exhaustion of conventional energy resources make the requirement to find a renewable, cost efficient, infinite, and ubiquitous source of energy. Electrochemical cells such as fuel cells, metal-air-batteries, and water splitting cells are one of the most appealing candidates to address the energy crisis. There is a growing interest in oxygen electrode catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of electrochemical cells, as they play a key role in a wide range of renewable energy technologies. Nevertheless, the development of highly-active bifunctional catalysts for both ORR and OER with high efficiency and low-cost features still remains a challenge ^1,2.

Noble metal-based catalysts based on carbon material have used for bifunctional catalysts to overcome the slow reaction kinetics. However, it is not appropriate to use precious metal-based catalysts at commercial scale for the development of electrochemical cells ¹.

For the development of cost-effective catalysts, the researchers performed significant efforts in the field of sulfides-based catalyst. In particular, sulfides - based catalyst materials exhibit the great potential as the bifunctional electrocatalyst due to its unique structure and high concentration of active defect sites ^2–6.

In this study, the transition metals sulfides - doped catalyst are selected and carbon nano-fiber (CNF) is used as a base material to enhance their electrocatalytic activity and durability ^7,8. Hydrothermal method is used to synthesize sulfides-based catalysts ^4,9. X-ray diffraction, Brunauer–Emmett–Teller, scanning electron and transmission electron microscope with energy-dispersive X-ray spectroscopy are used to analyze the physicochemical properties of synthesized catalyst materials ^10,11.

The electrochemical behaviors are analyzed by using rotating disk electrode (RDE) system in the electrolyte of 0.1 M KOH solution. Pt wire used as a counter and Hg/HgO used as a reference electrode. Electrochemical activities are studied by using the linear sweep voltammetry (LSV) at a scan rate of 5 mV s-1 for 1.2-1.7 V for OER and 0.05-1.2 V for ORR. Potential cycling between 1.25 and 1.65 V for 1,500 cycles at a scan rate of 200 mV s-1 are used for the analysis of long-term stability for OER.

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3. B. Chen et al., Nanoscale, 7, 20674–20684 (2015).

4. Q. Liu, J. Jin, and J. Zhang, ACS Appl. Mater. Interfaces, 5, 5002–5008 (2013).

5. M. Shen et al., ACS Appl. Mater. Interfaces, 7, 1207–1218 (2015).

6. C. N. R. Rao and K. P. R. Pisharody, Prog. Solid State Chem., 10, 207–270 (1976).

7. B. Li et al., Nanoscale, 7, 1830–1838 (2015).

8. G.-H. An, E.-H. Lee, and H.-J. Ahn, Phys. Chem. Chem. Phys., 18, 14859–14866 (2016).

9. W. Dong et al., ResearchGate, 40, 243–8 (2010).

10. R. R. Chianelli, T. A. Pecoraro, T. R. Halbert, W.-H. Pan, and E. I. Stiefel, J. Catal., 86, 226–230 (1984).

11. A. Ivanovskaya et al., Langmuir, 29, 480–492 (2013).

Key words: Bifunctional, Transition Metals, Sulfides, Hydrothermal, OER, ORR, Electrochemical, Rotating disk electrode.

* Corresponding authors: jyoung@sejong.ac.kr (J.-Y. Park)