Electrochemical Performance of Fuel-Electrode-Supported Reversible Solid Oxide Cells with a Ni-GDC Functional Layer
Authors: Ozaki Ryota , Nagatomo Yohei , Yoshiga Ko , Kawabata Tsutomu , Sakamoto Mio , Yasutake Masahiro , Tachikawa Yuya , Matsuda Junko , Sasaki Kazunari 
Published: December 2024

Introduction

Reversible solid oxide cells (r-SOCs) can generate electricity in SOFC mode and produce hydrogen in SOEC mode. R-SOCs are promising electrochemical energy devices that can manage power derived from renewable energy sources by switching operating modes (1).
Fuel-electrode-supported cells, which are widely used in both SOFCs and SOECs, exhibit higher performance even at lower operation temperature due to the use of thin electrolytes. Ni-yttria-stabilized zirconia (YSZ) is widely used as the fuel electrode material in fuel-electrode-supported cells because of its high ionic conductivity, high catalytic activity, and compatibility with electrolyte materials. However, Ni-zirconia based cermet such as Ni-YSZ tends to exhibit high polarization resistance, especially in the SOEC mode (2). Furthermore, degradation in the SOEC mode is also an issue (3), and further improvement of electrochemical performance and durability is necessary for the practical use of r-SOCs.
Gadolinia-doped ceria (GDC) has higher ionic conductivity than YSZ, and, as a mixed ionic electronic conductor, the electrochemical reaction extends not only to the triple phase boundary (TPB) but also to the double phase boundary (DPB). Therefore, the electrochemical performance is expected to be improved by substituting Ni-YSZ with Ni-GDC.
Here, in this study, a fuel-electrode-supported reversible solid oxide cell is prepared with a Ni-YSZ fuel electrode support and a thin Ni-GDC fuel electrode functional layer next to the electrolyte, and the electrochemical performance is evaluated.

Experimental

For experiments, a fuel-electrode-supported reversible solid oxide cell was used, as schematically described in Fig. 1. The cell consisted of a fuel electrode support, a fuel electrode functional layer, an electrolyte, a GDC buffer layer, and an air electrode. The materials of the fuel electrode support, the fuel electrode functional layer, and the electrolyte were Ni-(Y2O3)0.08(ZrO2)0.92 (YSZ), Ni-Gd0.1Ce0.9O2 (GDC), and YSZ, respectively, and the half-cell was fabricated by co-sintering these three layers. GDC has been reported to form a highly resistive solid solution with YSZ at high temperatures (4). Therefore, half-cells were fabricated at different co-sintering temperatures to investigate the influence of such solid solution on the cell performance. La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) was used for the air electrode, and the GDC buffer layer was inserted between the electrolyte and the air electrode to suppress interdiffusion.
Current-voltage (I-V) characteristics were measured at 700–800 °C with a supply of 100 ml min-1 of 50%-humidified hydrogen (50 %H2O-50 %H2) to the fuel electrode, and 150 ml min-1 of air to the air electrode. Electrochemical impedance spectra were measured in the frequency range from 1 MHz to 0.1 Hz.

Results and discussion

Figure 2 (a) shows the I-V characteristics at 800 °C of the cells co-sintered at 1300 °C and 1350 °C. The cell co-sintered at 1300 °C exhibited higher I-V characteristics than the cell co-sintered at 1350 °C in both SOFC and SOEC modes. In the SOEC mode, the electrolysis current densities at the thermo-neutral potential (~1.29 V) of the cells co-sintered at 1350 °C and 1300 °C were 1.05 A cm-2 and 1.15 A cm-2 under these fabrication conditions, respectively. Both ohmic and polarization resistances were lower in the cells co-sintered at 1300 °C than in the cells co-sintered at 1350 °C, suggesting that lowering co-sintering temperatures than 1300 °C may further improve their I-V characteristics. The improvement in electrochemical performance by lowering the co-sintering temperature from 1350 °C to 1300 °C can be attributed to several factors, including an increase in TPB density due to a finer fuel electrode microstructure and a suppression of YSZ-GDC solid solution formation. A detailed analysis of these factors is needed in the future.
In this presentation, the electrochemical performance of cells co-sintered at lower temperatures more than 1300 °C and the comparison of the performance of Ni-GDC functional layers with that of Ni-YSZ functional layers will also be presented and discussed.

References

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Moçoteguy, and A. Brisse, Int. J. Hydrogen Energy, 38, 15887 (2013).

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Figure 1