Analysis of Interfacial Capacitance of Ni-YSZ Anode By Transient Simulation

Abstract

An electrochemical impedance and equivalent circuit analysis are frequently used for electrochemical evaluation of SOFC cells. The impedance analysis enables to obtain the resistance and the capacitance components of the electrode reaction. The electrode resistance is directly related to the cell performance. The electrode capacitance can be sometimes give information about the active electrode reaction site. As to Ni-YSZ electrode, we've tried to elucidate the physical meaning of the capacitance [1]. However, it is not easy to clarify the physical origin of the electrode capacitance. Thus, the capacitance has not been used to evaluate the active reaction zone so far. In this study, the capacitance of Ni-YSZ interface will be simulated by calculating the chemical capacitance derived from YSZ. Then, the origin of the capacitance will be discussed through comparison with the previous studies [1].The structural model to simulate the Ni-YSZ model cell [1] are shown in Fig. 1., which shows simplified model according to symmetry of the cell. In order to limit the electrochemical reaction zone (triple phase boundary), a virtual reaction zone was introduced. Cell conditions for the calculation are following: temperature: 765ºC; oxygen partial pressure of the air electrode: 0.21 bar;hydrogen partial pressure: 1.00 bar, and water vapor partial pressure: 0.03 bar for fuel electrode. Transient simulation was performed to obtain the chemical capacitance by an in-house code, SIMUDEL, where mixed ionic electronic conduction and oxygen nonstoichiometry were taken into consideration [2,3] using the reported conductivity data of YSZ [4].The measured capacitance of the model cell [1] was 5×10-4 Fcm-2. On the other hand, the calculated chemical capacitance of YSZ was 2×10-3 Fcm-2. Even though the oxygen nonstoichiometry of YSZ was small, calculated chemical capacitance was much larger than the observed interfacial capacitance. These difference implies there was unknown factors to promote the response of electron in the actual Ni-YSZ interface. If a thin resistive layer was inserted between Ni and YSZ interface, the calculated chemical capacitance became smaller than that of observed. Sasaki et al. observed an incoherent interface between Ni and YSZ by TEM [5]. This kind of interface structure may be the potential cause of the complicated capacitance.This study was partially supported by NEDO, Japan.[1] M. Takeda, et. al., submitted to J. Electrochem. Soc.[2] K. Terada, et. al., ECS Trans. 35(1), 923 (2011).[3] M. Sato, et. al., Trans. Jpn. Soc. Comp. Eng. Sci. 2017, 14 (2017).[4] J.H. Park, et. al., J. Am. Ceram.Soc.,72[8],1485-87(1989).[5] T. Sasaki, et. al., Mater. Trans. 45(7), 2137 (2004).Fig. 1 Schematic diagrams of the structural model to simulate the chemical capacitance of Ni-YSZ. Figure 1