Degradation Prediction of Solid Oxide Cell Considering Ni Coarsening Model Based on Electrode Microstructure Measurement
Authors: Yoshiga Ko , Nagatomo Yohei , Ozaki Ryota , Matsuda Junko , Tachikawa Yuya , Sasaki Kazunari 
Published: December 2024

Introduction

To further accelerate the development of solid oxide cells (SOCs), a computer-aided design and simulation method is important to visualize the characteristics of SOCs. There is an essential issue with clarifying various internal degradation phenomena for durability which is important for visualizing the characteristics. Various mechanisms have been reported for the degradation of electrodes, electrolytes, and other components [1]. The coarsening of Ni is attracting attention as a major cause of fuel electrode degradation. A theoretical time-dependent model of Ni grain size and TPB density change through microstructural observation has been suggested to simulate the degradation of fuel electrodes in solid oxide fuel cells [2].
Since the electrode degradation is generally affected by loading current, overvoltage, and distributions of gas species and temperature [3], it is important to understand the spatial distribution of these factors when developing a SOC system. Therefore, a precise analysis of degradation factors, such as the causal relationship with operating conditions, is necessary to quantitatively clarify the degradation mechanism. Therefore, if multi-physics simulation technology can be utilized to predict degradation, the development of more durable SOCs can be accelerated.
Our research group has converted the microscopic electrochemical reaction phenomena into a mathematical equation that captures it as macroscopic phenomena by considering the apparent exchange current density, and has reflected it in the simulations [4-6]. Simulation results are consistent with experimental I-V characteristics and temperature distributions, suggesting that various spatial distributions can be predicted for the performance of the initial, non-degraded cells [6].
Therefore, this study aims to establish a method to predict the amount of change in cell performance due to time-dependent degradation of Ni cermet by quantifying Ni coarsening through microstructural observation and reflecting the time-dependent model in the apparent exchange current density equation. The establishment of the suggested method will make it possible to predict SOC degradation when designing the SOC, and is expected to contribute to improving durability in actual SOC system development.

Experimental

To develop the time-dependent Ni coarsening model, we conducted cell durability tests and observed the electrode microstructure after the tests. An electrolyte-supported cell with scandia-stabilized zirconia (ScSZ, 200 µm thick) as electrolyte, Ni-ScSZ cermet as fuel electrode, and gadolinia doped ceria (GDC) buffer layer and lanthanum strontium cobalt ferrite (LSCF) as air electrode was used in this study. Humidified hydrogen was supplied to the fuel electrode at 100 ml min-1 and air was supplied to the air electrode at 150 ml min-1. The electrode microstructure was observed before and after the tests. The fuel electrode microstructure image before the test is shown in Fig. 1.
A three-dimensional model simulating the cell geometry and operating conditions such as temperature and flow rate used in actual experiments was used in the simulation. In this study, calculated I-V characteristics and temperature distributions were obtained in a relatively low current density range consistent with experimental conditions to evaluate the relationship between cell performance and degradation due to Ni coarsening.
In this presentation, we will report the discussion of the calculated results incorporating the Ni time-dependent model compared with experimental results.

References

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Figure 1