Distribution of Relaxation Times Analysis of SOFCs with Low Tortuosity Structure Anode
Authors: Nagai Kohei , Nagato Keisuke , Okamura Toshihide , Tomizawa Morio , Nakao Masayuki 
Published: December 2020

Fuel cells are systems that generate electricity from hydrogen and oxygen. Specifically, solid-oxide fuel cells (SOFCs) have gained increasing attention due to their high generating efficiency. In the SOFC anode, an electrochemical reaction, in which the oxide ion conducting through the electrolyte yttria-stabilized zirconia (YSZ) combines with the diffusing hydrogen in the pore, occurs. In the operating condition of the SOFC anode, the conductivity of oxide ion in YSZ is lower than that of the conductivity of the electron in the current collector nickel and diffusion of hydrogen gas. The conductivity of oxide ion should be increased for SOFCs to achieve higher generating efficiency.
The YSZ pillar structure was used to increase ionic conductivity. As shown in Fig. 1, the YSZ pillar structure makes the oxide-ion-conducting path more linear. A straight conducting path indicates a low tortuosity factor, which directly affects the ionic conductivity. In our previous study, positive effects on polarization resistance using structures with low tortuosity were reported [1]. However, the effectiveness of a low tortuosity factor on oxide ion conductivity has not been confirmed.
In this study, the mechanism for increasing the generation efficiency of SOFCs with YSZs that have low tortuosity factor structures was investigated. We fabricated various patterns of SOFC cells that have YSZs with low tortuosity factor structures and evaluated their performance using electrochemical impedance spectroscopy (EIS). The measured data were analyzed based on the distribution of relaxation times (DRT) using DRTtools [2].
We use the line-and-space structures as targets for this study. The line-and-space structures can be thought as a 2D counterpart of the pillar structure. Fig. 2 shows a scanning electron microscopy (SEM) image of the cross-section of a fabricated cell. The cells were anode symmetric cells that were fabricated via centrifugal molding and sintering. As shown in Fig. 2, the YSZ line and space structure, YSZ/Ni layer, and Ni current-collecting layer were obtained. The electrochemical measurements were conducted in different temperatures and nitrogen partial pressures.
Fig. 3 shows the DRTs of the measured impedance of the cells. There was no clear relationship between the DRT and shape of line-and-space structures. However, for the cells with line-and-space structures, the DRT peaks in 2k–10k Hz were significantly lower than that of the cells without line-and-space structures. As shown in Fig. 4, the peaks of all the measured cells lowered with increasing temperature and were not affected by nitrogen partial pressure. Therefore, it was inferred that the peak was due to the oxide ionic conductivity.
According to the above results, it was confirmed that the oxide ion conductivity in YSZ increases due to the low tortuosity factor of the anode microstructures. Hence, the polarization resistance of SOFC decreases. Further studies are needed to determine the detailed process of how the shapes of structures with low-tortuosity-factors affect the performance of SOFCs.
References
[1] K. Nagato, K. Shintani, T. Shimura, N. Shikazono, and M. Nakao, “Magnetic Alignment of anode microstructure in solid oxide fuel cell,” Journal of Electrochemical Society, 166, 2 (2019) F144-F148.
[2] T. H. Wan, M. Saccoccio, C. Chen, F. Ciucci, “Influence of the discretization methods on the distribution of relaxation times deconvolution: implementing radial basis function with DRTtools,” Electrochemical Acta, 184 (2015) 483-499

Figure 1