Reversible solid oxide cells (r-SOCs, or solid oxide reversible cells) are devices that can efficiently operate in both fuel cell and electrolysis operating modes. It is expected that they can serve as a technology for green, flexible, and efficient energy systems coupled with renewable energy (1). However, a fuel electrode of r-SOCs might be degraded due to redox cycles in switching operation modes. In our research group, we have fabricated redox-resistant anodes with Ni-Co alloy cermet (2). Here in this study, we apply such alloy-based cermet to the fuel electrode, and evaluate electrochemical performance, reverse cycling durability, and the effect of Co addition. Moreover, we compare the cell performance of Ni-Co alloy cermet with conventional Ni-ScSZ fuel electrode.

Experimental

In this study, scandia-stabilized zirconia (ScSZ, 10mol% Sc₂O₃-1mol% CeO₂-89mol% ZrO₂) was used as the solid electrolyte. The powder prepared by mixing Ni-Co-based oxide powder prepared by ammonia co-precipitation with Ce_0.9Gd_0.1O₂ (GDC) in a weight ratio of 48.1:51.9, was used as the fuel electrode material. Ni-Co-GDC fuel electrode with x mol% of Co added to Ni, is denoted as Ni_{(100 – x)}Co_x -GDC fuel electrode. The fuel electrodes of x = 0, 5, 10, and 20 were fabricated. Two types of Ni-GDC cermet were prepared. One was fabricated by using NiO powder by ammonia co-precipitation, and the other was by using commercial NiO powder (Kanto Chemical, Japan). We prepared the Ni-Co alloy cermet by mixing powder and binder, preparing and printing electrode paste on the electrolyte plate, followed by heat treatment. (La_0.6Sr_0.4)(Co_0.2Fe_0.8)O₃ (LSCF) was used as the air electrode material. The electrochemical characteristics and r-SOC reversible cycle durability of the fuel electrodes were evaluated under the condition at an operating temperature of 800℃, while 100 ml/min of 50%-humidified hydrogen fuel was supplied to the fuel electrode, and 150 ml/min of air was supplied to the air electrode. Electrochemical characteristics were evaluated by electrochemical impedance measurements (1255WB, Solartron, UK), which separate ohmic and non-ohmic resistances. The durability in reverse r-SOC operation was evaluated by repeatedly varying current density within ± 0.2 A cm^-2 at a current sweep rate of 1.56 mA cm^-2 s^-1 up to 1,000 cycles. The degradation was evaluated by averaging the percentage change in fuel electrode potential at ± 0.2 A cm^-2.

Results and discussion

Figure 1 shows the initial performance of the cells with each fuel electrode. Figure 2 shows the degradation of each fuel electrode within 1,000 cycles after the r-SOC reverse durability test. The initial performance of Ni-Co-GDC was superior to Ni-ScSZ, and it increased with decreasing Co content. Two types of Ni-GDC exhibited identical performance. Similarly, the r-SOC reverse durability of Ni-Co-GDC was superior to Ni-ScSZ, and it increased with decreasing Co content. Furthermore, two types of Ni-GDC exhibited almost the same durability. These results indicate that GDC contributes to the increased cell performance and durability rather than alloying or the feature of original NiO powder. It appears that Co is not essential for r-SOC fuel electrodes in terms of r-SOC durability. However, fuel electrode materials should be selected by comprehensively considering the performance and the durability at both SOFC and SOEC modes. As Ni-Co alloy shows higher redox cycle durability compared with Ni-GDC (3), Ni-Co alloy is still one of the promising materials for r-SOC fuel electrodes.

Acknowledgements

A part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO).

References

1. N. Q. Minh and M. B. Mogensen, Electrochem. Soc. Interface, 22, 55 (2013).
2. Y. Ishibashi, S. Futamura, Y. Tachikawa, J. Matsuda, Y. Shiratori, S. Taniguchi, and K. Sasaki, J. Electrochem. Soc., 167, 124517 (2020).
3. K. Matsumoto, Y. Tachikawa, J. Matsuda, S. Taniguchi, and K. Sasaki, ECS Trans., 103, 1549 (2021).