The electrochemical energy devices such as unitized regenerative fuel cells (URFCs) have many advantages in terms of producing hydrogen gas and electricity easily. However, the oxygen electrode of the URFC systems show slow kinetics for oxygen evolution reactions (OERs) and oxygen reduction reactions (ORRs) [1, 2]. As a results, efficiency related with the hydrogen and electricity production are limited in URFCs because of the high polarization resistance (charge- and mass-transfer) at the oxygen electrode. Furthermore, the OERs and ORRs are also restricted by the 4-electrons multi-step electrochemical reactions, then decreasing the overall reaction rate of this system [3].

Up to date, the carbon-supported noble materials, such as Pt/C, Ir/C, PtRu/C, and their alloys with transition metals, have still used to overcome the slow reaction kinetics [4]. However, the utilization of noble metal-based catalysts is not suitable for commercialization of the URFCs. In order to reduce these precious catalysts, many researchers have studied for other type of catalysts such as oxide, carbide, nitride, and carbonaceous materials [1, 5, 6].

In this study, therefore, the perovskite oxide-based catalysts are investigated to improve their electrocatalytic performance and long-term stability for both OERs and ORRs. Several lanthanides (Nd, Sm, and Gd) are doped into A-site of the catalysts to obtain the double perovskite structure [7]. The physicochemical properties of the final products are analyzed by various tools such as X-ray diffraction, scanning electron microscope and transmission electron microscope. For the electrochemical investigations, a rotating disk electrode (RDE) system is used with a 0.1 M KOH solution, a Pt wire and an Hg/HgO for electrolyte, counter electrode and reference electrode, respectively [8, 9]. The computational calculations based on density functional theory (DFT) are also investigated to confirm the relations between oxygen O p-band center and Fermi energy level of the catalysts [6, 10].

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7. T.-H. Lee, K.-Y. Park, N.-I. Kim, S.-J. Song, K.-H. Hong, D. Ahn, A.K. Azad, J. Hwang, S. Bhattacharjee, S.-C. Lee, H.-T. Lim, and J.-Y. Park, J. Power Sources, 331, 495 (2016).
8. N.-I. Kim, Y.J. Sa, S.-H. Cho, I. So, K. Kwon, S.H. Joo, and J.-Y. Park, J. Electrochem. Soc., 163, F3020 (2016).
9. I.-S. So, N.-I. Kim, S.-H. Cho, Y.-R. Kim, J. Yoo, Y. Seo, Y.-S. Seo, B. Park, K. Kwon, and J.-Y. Park, J. Electrochem. Soc., 163, F3041 (2016).
10. J. Kim, X. Yin, K.-C. Tsao, S. Fang and H. Yang, J. Am. Chem. Soc., 136, 14646 (2014).

 Keywords: Unitized regenerative fuel cells; Bi-functional activity; Perovskite; Electrocatalyst; Density functional theory.

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