Origin of Electrochemical Capacitance of Ni/YSZ Anode and Its Application to Evaluation of Active Reaction Site
Authors: Yashiro Keiji , Takeda Mirai , Hashimoto Shin-ichi , Kawada Tatsuya 
Published: February 2020

1. INTRODUCTION
Electrochemical performance and stability of a Ni-YSZ anode depend on its microstructure. It is indispensable to understand how microstructure changes during operation to ensure the long-term durability of solid oxide fuel cells (SOFCs). It has been reported to observe microstructure change before and after the operation by FIB-SEM [1]. FIB-SEM observation gives us the detain information about anode microstructure, but can’t be used for real time monitoring. Thus, it is better to be combined with a real-time microstructure analysis. As for gadolinium-doped ceria (GDC), which is a non-stoichiometric oxide, Nakamura et al. [2] have reported that chemical capacitance could be used to estimate effective reaction length because chemical capacitance was related to oxygen non-stoichiometry. While YSZ is not nonstoichiometric oxide, we believe that an electrode capacitance can be used to analyze the microstructure change of Ni-YSZ anode during operation. The purpose of this study is to clarify the relationship between microstructure and electrode capacitance of Ni-YSZ anode and to evaluate microstructure degradation by the electrode capacitance.
2. EXPERIMENTAL
Here we assumed relationship between electrode capacitance and double-phase boundary (DPB) between Ni and YSZ because capacitance. Then, electrode capacitance, c
pwas represented by the following equation,
cp =A (LNi-YSZ/2)2(1)
where A is constant; L
Ni-YSZ is the diameter of osculating circle between Ni and YSZ particles, where both particles is assumed to be a sphere shape. This relationship is verified by a model electrode experiment. After this experiment, the same relationship is confirmed with porous Ni-YSZ electrode by C
Mcalculated by model electrode experiment.
The microstructure information was obtained from ternarized cross-section images by FE-SEM.
Firstly, to test the hypothesis, the model nickel electrode was used. Model nickel electrodes, which had diameters of 1000, 500, 100, 50, and 20 μm, were prepared on the substrate of 8YSZ by a pulsed laser deposition. The electrochemical measurement was conducted in an infrared-furnace in 2.3% H2O-H2atmosphere at 600 - 765 °C. A three-probe method was employed. Pt foil was used as a current collector of CE. As to the WE, a Pt-coated tungsten probe was applied to touch onto the electrode surface. Electrode resistance and electrode capacitance were obtained by AC impedance measurement. Secondly, the above method was applied to a practical porous Ni-YSZ cerment anode. The cermet anode was 50vol% Ni-YSZ prepared from 8YSZ and NiO commercial powder. AC impedance measurement was in frequency range of1 M-0.01 Hz at 800 °C.
3. RESULT AND DISCUSSION
3.1. Model electrode experiment
Fig. 1 shows the result of capacitance shows clear dependence on the DPB area. Electrodde capacitance was proportional to the square of diameter. This result supported our hypothesis.
Since verification of the assumption in porous electrode, area specific capacitance, C
M, was calculated in this experiment. In this study, C
M are 1.3-2.1 × 10-4 Fcm-2 at 750 °C. Bieberle et al.[3] reported C
M of 2.6 – 3.9 × 10-4at 700 °C with the model strip electrode. The result agrees with the reported data. The measured electrode capacitance reflect a certain physical phenomenon. It was confirmed that the electrode capacitance correlated to DPB.
3.2. Porous electrode experiment
The experimental value of the capacitance need be compared with DPB area to verify the same relationship for the practical electrode. The experimental value, volume specific capacitance, c
p, and resistance, r
p, were measured by the equivalent model, Gerischer model. The measured capacitance and estimated value by C
Mmeasured by the model electrode were compared. Although absolute values of measured- and estimated electrode capacitances are different order of magnitude, these capacitances are proportional. Therefore, there was relationship between electrode capacitance and anode microstructure.
REFERENCES
[1] Z. Jiao ,N. Shikazono, J. Electrochem Soc., 161(2014) F577 - F582
[2] T. Nakamura, et al, J. Electrochem. Soc., 155 (2008) B1244-B1250
[3] A. Bieberle, L.J. Gaukler, J. Electrochem. Soc., 148 (2001) A646-A656
Fig. 1 Diameter (DPB area) dependence of capacitance in 2.3% H2O-H2at 765 °C

Figure 1