Performance of Solid oxide fuel cells (SOFCs) is affected by microstructural changes and electrochemical reactions in interfacial regions between electrode and electrolyte during the operation. SOFCs are operated at high temperatures using a fuel gas and air. Therefore, an environmental electron microscope that allows for observation of specimens in the gas atmospheres is powerful analytical tool for nano-scale interfaces in SOFCs. The redox reaction in the cell has been reported by using in-site environmental electron microscopy and spectroscopy [1, 2]. The cell reaction, however, at interfacial regions between electrode and electrolyte has not completely been understood. In order to observe the interface in the cell reactions, we have developed an in-situ specimen holder, which can heat and apply the external voltage to the specimen. In this paper, we report details of the in-situ specimen holder and preliminary results of observation of a SOFC structure using this holder in a high-voltage environmental electron microscope.

Figure 1(a) shows the operation of SOFCs based on an oxide ion conducting in electrolyte. Supplying external voltage (Fig. 1(b)), we can make same situations as Fig. 1(a) in the cell. The developed in-situ specimen holder shown in Fig. 2(a) was optimized for the cell reaction in Fig. 1(b). The electrode terminal A is for the external voltage applying, and B and C are for the heating. Figure 2 (b) shows the heater with the electrode terminal consists of the nickel-chrome (NiCr) alloy. Between the heater and the electrode terminal is isolated electrically by insulators. We connect both electrodes of the specimen to the electrode terminal A and the center of the heater, respectively (see Fig. 2 (c)).

Figure 3(a) shows the scanning ion microscope (SIM) image of the full cell specimen prepared by a focused ion beam (FIB) instrument (Hitachi FB-2100). The bulk full cell was constructed by a pulsed laser deposition method on the platinum (Pt) substrate. The cell structure was composed of a gadolinium doped ceria (GDC) of 1 μm thick as electrolyte and a Pt layer of 100 nm thick as the electrode. The gold wire connects the electrode terminal A to the tungsten layer which was deposited on the specimen to protect the cell structure in the FIB thinning process. In-situ observation of the cell structure was performed using the reaction science high-voltage electron microscope [3] (JEOL JEM-1000K RS) at an acceleration voltage of 1 MV. The pressure in the specimen chamber was kept to 1 Pa of the oxygen gas.

Figures 3(b) and 3(c) show the cross-sectional annular dark-field scanning transmission electron microscope (ADF-STEM) images in the oxygen atmosphere at the room temperature (R.T.) and ~600 ℃, respectively. In the case of ~600℃, we applied a voltage of +1V to the electrode terminal A. Figure 3(b) clearly shows the cell structure that has the electrodes (Pt) and the electrolyte (GDC) with the protective (W) layers. We cannot, however, distinguish between Pt and W layers at the higher temperature, as shown in Fig.3(c). These results show Pt layer with W layer is unstable in high-temperature oxidizing environments and obstructs to observe the fuel cell reaction between the Pt electrode and the GDC electrolyte.

References

[1] A. H. Tavabi et al., J. Electron Microsc., 60 (2011) 307-314.

[2] A. H. Tavabi et al., Mirosc. Microanal., 20 (2014) 1817-1825.

[3] N. Tanaka et al., J. Electron Microsc., 62 (2103) 205-215.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Number 25246001, and also partially supported by the program “Global Research Center for Environment and Energy based on Nanomaterials Science” of MEXT, Japan

Figures:

Fig. 1 The operation of SOFCs based on an oxide ion conducting in electrolyte. (a) Power generation, (b) a fuel cell with an external voltage supply.

Fig. 2 Photograph of (a) the tip of the special holder , (b) the NiCr heater with the electrode terminal and (c) a schematic diagram of (a) with (b).

Fig. 3 (a) Low-resolution SIM image of the thin film cell connected between electrode and the heater. (b) (c) Cross-sectional ADF-STEM images of the cell structure in oxygen atmosphere at R.T. and ~600℃, respectively.

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