Reversible solid oxide cells (r-SOCs) are electrochemical energy conversion technologies that serve as both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) (1). Although it is possible to evaluate macroscopic performance of an operating system by measuring actual current and voltage, it is often difficult to observe spatial distribution of power generation inside the cell stacks. Here in this study, performance tests are conducted using an electrolyte-supported single cell composed of typical materials. In parallel, numerical analysis is also performed under the same conditions as in the experiments, incorporating electrochemical data for each material reported in previous studies (2-4). By comparing the I-V characteristics experimentally and numerically obtained, the validity of the electrochemical reaction model is confirmed. Gas species distribution and current density distribution are then examined, and the temperature distribution is analyzed.

Experimental

Figure 1 shows schematic diagrams of a r-SOC used in this study. An electrolyte-supported 50 mm × 50 mm flat plate cell using ScSZ (ScSZ:10mol% Sc₂O₃–1mol% CeO₂–89mol% ZrO₂) was used in this experiment. The fuel electrode was made of NiO-ScSZ cermet, and the air electrode was made of LSM (LSM: (La_0.8Sr_0.2) _0.98MnO₃) and LSM-ScSZ composite layers, screen-printed and sintered. The electrode area was 40 mm × 40 mm (16 cm²). Platinum meshes were used as current collectors. In this study, I-V measurements were performed with 50%-humidified hydrogen supplied at 200 ml min^-1 to the fuel electrode. The air electrode side was open to the ambient atmosphere. The operating temperature was around 800°C. A three-dimensional model of the single cell was created using the numerical analysis software COMSOL Multiphysics (Ver. 5.2), and calculation results were compared with measured results.

Results and discussion

Figure 2 shows the experimental and numerical results of I-V characteristics. The experimental and simulation results were in close agreement, confirming the validity of the simulation model. Figure 3 shows the spatial distribution of cell temperature when current (i =±0.1 A cm^-2) is applied to the r-SOC. During power generation (positive current), the temperature increased above 800°C, while during steam electrolysis (negative current), the temperature decreased. This is well expected because the electrode reaction is exothermic during power generation and endothermic during steam electrolysis. The cell temperature change reaches the maximum at a location around the center of the cell and becomes lower on the periphery. This result is consistent with our recent study showing that exchange current density reaches the maximum where the concentrations of hydrogen and water vapor are comparable (5), leading to a higher current density. The temperature is higher near the fuel inlet than at near outlet. Such distribution is considered to be due to a higher value of local current density near the fuel inlet, besides the effect of radiation.

Acknowledgments

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) (Contract No.20001460-0). Collaborative support by Prof. H. L. Tuller, Prof. B. Yildiz, and Prof. J. L. M. Rupp at Massachusetts Institute of Technology (MIT) is gratefully acknowledged.

References

1. Natsukoshi, K. Miyara, Y. Tachikawa, J. Matsuda, S. Taniguchi, G. F. Harrington, and K. Sasaki, ECS Transactions,103 (1), 375 (2021).
2. Takino, Y. Tachikawa,K. Mori, S. M. Lyth, Y. Shiratori, S. Taniguchi, and K. Sasaki, Int. J. Hydrogen Energy, 45, 6912 (2020).
3. Fukumoto, N. Endo, K. Mori, Y. Tachikawa, J. Matsuda, S. Taniguchi, and K. Sasaki, ECS Transactions, 103 (1), 2007 (2021).
4. Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, Int. J. Hydrogen Energy, 44 (16), 8502 (2019).
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