Influence of Small Defects Produced in Electrolytes during Manufacturing Processes on Operated SOFCs

To progress further social implementation, improvement of the durability and reliability are essential. One of the topics for mechanical durability is the evaluation of the effect of small defects in electrolyte. Small defects such as pinholes and unstructured pores in electrolytes layer could not be prevented in the case of electrode-supported type cells because its thickness is only several mm in these days. Through these defects, gas leakage between cathode and anode and the combustion of leaked fuel and air were considered to occur. In this study, we evaluate various influences caused by gas leakage such as thermal stress, anode re-oxidation using model cell.

To evaluate such local phenomenon on operated SOFCs, the new stress evaluation method using micro Raman scattering spectroscopy has been adopted. The stress conditions evaluation method focuses on Raman spectra of samarium-doped ceria (SDC) which was often used as an interlayer. The stress condition of electrolyte can be calculated, considering the thermal stress between interlayer and electrolyte. There are several advantages in In-situ Raman scattering spectroscopy: 1) nondestructive, 2) contact-less, 3) high spatial resolution. Therefore, the method using micro Raman scattering spectroscopy can obtain the local stress conditions of SDC interlayer in SOFCs at operating conditions.

 In this method, the shift of F_2g vibration mode peak of samarium-doped ceria (SDC) was used as a probe of stress conditions evaluation. Because the shift of Raman peak by stress change is not so drastic, e.g. 0.2 cm^-1 / 100 MPa, the accuracy of Raman peak position is essential. SDC Raman spectra change by not only stress but also Sm concentration and temperature. By measuring temperature and Sm concentration dependence of SDC Raman spectra variations, the information about temperature and Sm concentration can be obtained in addition to stress on the measurement points.

In this study, first, this method was applied to measuring the stress condition in SOFCs in operating conditions. Next, We tried to evaluate the effect of the pinhole in electrolyte under operating conditions by using this method. The local stress and temperature were measured. Additionally, to define the mechanism of the gas leak through small defect in electrolyte, the amount of gas leak through pinhole was measured by using gas chromatography.  

 This method was applied to measuring the stress condition in two types of conventional anode supported type SOFC during operating condition. At R.T., the residual stress in electrolyte was a compression of about 400~500MPa. At elevated temperature, the residual stress decreases with increasing temperature, becoming a compression of about 100MPa. This results suggest that the internal stress in SOFCs is caused by thermal stress. During reduction, the internal stress changes to approximately zero. This relaxation stress could be caused by the reduction of NiO.  These results showed that the stress evaluation method using micro Raman scattering spectroscopy can be applied to measuring the local stress condition in SOFCs under operating conditions.

Before evaluate the stress around the small defect in electrolyte under operating conditions by using this method, We tried to define the mechanism of the gas leak through the pinhole in electrolyte by measuring the amount of gas leak through pinhole

Pinholes (Φ150mm) were etched in electrolyte to simulate small defects, and actual gas leakage was evaluated under difference working pressure (0~100 Pa) between cathode side and anode side at R.T. The gas leak was constant value in difference working pressure. This result suggest that gas leak through electrolyte is caused by not pressure difference but also mutual gas diffusion.

 The change of stress conditions and temperature around the pinhole due to the change of gas leakage was measured during operation. After reduction, anode in pinhole was local oxidation state. This reasons was that air leak to anode by gas diffusion increase p(O₂) in this area. Temperature around pinhole increase about 10K inside 100μm distance from pinhole edge by the combustion. The change of the stress around pinhole by the effect of leak gas was nearly zero. To take about 30MPa measurement error in Raman measurement into consideration, these stress caused by local oxidation and combustion were below 30MPa.