Kinetic Investigation of Electrode Reaction Processes on Ni-GDC Cermet Fuel Electrodes for Reversible Solid Oxide Cells
Authors: Nagatomo Yohei , Ozaki Ryota , Yoshiga Ko , Yasutake Masahiro , Tachikawa Yuya , Lyth Stephen Matthew , Matsuda Junko , Sasaki Kazunari 
Published: December 2024

Introduction

Reversible solid oxide cells (r-SOCs) are electrochemical energy devices that can reversibly switch between power generation by fuel cell operation (SOFC mode) and hydrogen production by steam electrolysis (SOEC mode). R-SOCs can adjust the demand and supply in renewable energy utilization. In order to design high performance and durable r-SOC electrodes for practical usage, it is essential to systematically understand the electrode reaction processes and electrode degradation factors in both SOFC and SOEC modes. Such fundamental information is also necessary for the analysis of electrode performance and the development of performance simulation models. The dependence of polarization resistance on electrode fabrication conditions and operating conditions has been investigated using steady-state polarization and impedance measurements for electrode materials used in either SOFCs or SOECs, and rate-determining reactions have also been discussed [1-5]. However, it is necessary to evaluate the electrode reaction mechanisms of electrode materials suitable for r-SOC operation in an integrated and detailed manner. Therefore, this study aims to systematically evaluate the similarities and differences in the fuel electrode reaction processes in both SOFC and SOEC modes from the viewpoint of dependence on operating conditions by using the distribution of relaxation times (DRT) analysis.

Experimental

As shown in Figure 1 (a), Ni-GDC cermet, a composite of Ni and mixed-conducting Gd0.9Ce0.1O2 (GDC), was used as the fuel electrode material, ScSZ as the electrolyte, and (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF) as the air electrode. GDC layer was inserted between the air electrode and the electrolyte plate as a buffer layer. Electrochemical impedance measurements were performed by supplying H2-H2O gas mixture to the fuel electrode of the fabricated cell, and varying operating temperature, H2-H2O ratio, and current density. Measurements were carried out in both SOFC and SOEC modes. The obtained impedance data were subjected to the DRT analysis to separate the polarization resistance components of the fuel electrode in the relaxation time domain of the electrode response. The area of each DRT peak corresponds to the resistance of each electrode process from which each peak originates. In this study, the dependence of the resistance of each DRT peak on the operating conditions was quantitatively investigated. As shown in Figure 1 (b), the microstructure of the electrodes was observed using FIB-SEM, confirming a porous electrode structure.

Results and Discussion

Figure 1 (c) shows the operating temperature dependence of the typical DRT peaks of the Ni-GDC cermet fuel electrode, in SOFC mode. In the range of 10-1 to 106 Hz where the frequency response was measured, three main DRT peaks appeared, P1: 10-1 to 100 Hz, P2: 100 to 102 Hz, and P3: 102 to 103 Hz, all DRT peak areas decreased with increasing operating temperature. In the presentation, the dependence of each DRT peak on operating parameters and the related electrode reaction processes will be discussed.

References

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Figure 1