Tracking Oxygen Evolution Activities of Perovskite Oxide Catalysts By 3D Electrochemical Impedance Spectroscopy

Abstract

IntroductionPromoting the oxygen evolution reaction (OER) is important for more efficient electrochemical water splitting because its sluggish reaction kinetics degenerate the overall efficiency. Perovskite oxides (ABO3) are known as promising electrocatalysts for the OER, which have a flexibility of composition and structure. Among these electrocatalysts, Ba0.5Sr0.5Co0.8Fe0.2O3–δ (BSCF) exhibits good OER activities in an alkaline solution1. One of its specific properties is the change of the surface structure and composition during the OER in alkaline solutions, leading the improvement of the OER activity2. In general, the OER activities of metal oxides significantly depend on electrochemical surface area (ECSA) and activity per surface area (specific activity). However, it is not easy to decouple these two factors when the surface structure and composition change continuously during the OER.Electrochemical impedance spectroscopy (EIS) has been used as a useful tool, which provides information related to electrode/electrolyte interface3. Here, in order to reveal the origin of the OER activities behavior, we used 3D EIS including time axis and tracked the ECSA and specific activity of perovskite oxides during the OER in alkaline solutions.Experimental methodsPerovskite oxides used in this study (BSCF, La0.5Sr0.5Co0.8Fe0.2O3–δ (LSCF) and LaCoO3–δ (LCO)) were synthesized by the sol–gel method.The OER activities of obtained perovskite oxides were evaluated using a three-electrode electrochemical cell with a rotating disk electrode (RDE) in 1.0 mol dm–3 KOH solution. A working electrode was prepared by drop-casting an ink suspension with perovskite oxides (0.25 mg cm–2), acetylene black (0.05 mg cm–2), and Nafion (0.05 mg cm–2) on glassy carbon. Pt wire and a reversible hydrogen electrode (RHE) were used as the counter and reference electrodes, respectively. 3D EIS was obtained in the following steps: First, cyclic voltammetry (CV) was performed at certain cycles (1, 5, 10 ,30, and 50 cycles). Second, the electrode was left at open circuit potential for 5 min. Finally, potentiostatic EIS (PEIS) was recorded 5 times (approximately 10 min), and then Nyquist plots at 5 min from the beginning of the first PEIS were compared.Results and discussionFrom CV measurements of perovskite oxides, while the OER activities of BSCF increased, those of LSCF and LCO slightly decreased with increasing CV cycles. Figure shows Nyquist plots of perovskite oxides after certain CV cycles. The impedance spectra show a semicircle in the high- and mid-frequency regions, respectively. While a semicircle in the high-frequency region was independent on the direct current (DC) potential, the mid-frequency region semicircle was dependent on DC potential. The contrasting behavior indicated that the semicircle in the mid-frequency region corresponds to the charge transfer resistance, which contains the information of double-layer capacitance for the OER. Therefore, we tracked the semicircle in the mid-frequency region during the OER.A semicircle in the mid-frequency region of BSCF gradually decreased with CV cycles, indicating that the BSCF surface changed to a preferred structure during the OER cycles. As a result of the equivalent circuit fitting, the charge transfer resistance decreased and the double-layer capacitance increased with CV cycles. On the other hand, a semicircle in the mid-frequency region of LSCF and LCO remained unchanged or slightly increased.More detailed evaluation of their ECSA and specific activities during OER will be discussed at the conference.Reference[1] J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough and Y. Shao−Horn, Science, 334, 1383 (2011).[2] K. J. May, C. E. Carlton, K. A. Stoerzinger, M. Risch, J. Suntivich, Y.-L. Lee, A. Grimaud and Y. Shao-Horn, J. Phys. Chem. Lett., 3, 3264 (2012)[3] V. A. Alves, L. A. da Silva, and J. F. C. Boodts, Electrochim. Acta, 44, 1525 (1998). Figure 1