Visualization of Surface Layer Defects in Solid Oxide Fuel Cell Using an Infrared Radiation Based on the Infrared Absorption Properties of CO2
Authors: Nakano Takanori , Sato Kazuhisa 
Published: December 2024

Non-destructive testing methods for quality assurance are essential for the widespread use of all-solid-state electrochemical devices such as Solid Oxide Fuel cell (SOFC) and All-solid-state Lithium ion batteries (AS-LiB). Since these are laminates of ceramics from different materials, they can be subject to mechanical damage during operating and manufacturing. These damages can significantly reduce the electrochemical performance or durability. Therefore, it is important to detect these damages by rapid and non-destructive methods and to determine whether they should be repaired or discarded in order to assure the quality of them. In conventional methods, human visual tests have been used for external inspections and internal inspections have been conducted by operational tests. This process takes a long time and unable to unify inspection standards due to human errors. In addition, these operation tests are subjected to repeated operation and shutdown, thus placing a load on the test specimens. Therefore, attempt is being made to apply various type of wave-based Non-Destructive Testing (NDT) methods to these inspections. However, none of these method is all-purpose, and they all have their strengths and weaknesses. The purpose of this study is to develop a new method to visualize and detect mechanical defects, by utilizing the infrared absorption properties of CO2.
Fig.1(a) shows an overview of detection mechanism. The detection flow is as follow. First, the lower part of the specimen is filled with CO2 and apply a little pressure. Then, CO2 leaks out of the defect. The leaked CO2 is visualized by an infrared absorption property to locate the leakage area and detect the defect. We predict the characteristics of this method as follow. First, because we can get the image data, this method can detect the defect in real-time and can be automated through image processing. However, since CO2 may diffuse from the defect point, the exact shape and size of the defect cannot be determined.
Next, the CO2 visualization method is described. CO2 absorbs infrared radiation which is the same frequency as CO2 vibrational motion’s caused by CO2 molecular structure. Fig. 1(b) shows absorption spectra for CO2. There are three absorption wavelength bands: 2.5~3, 4~5, around 15 µm. To visualize CO2, it is necessary to focus on the infrared wavelength band that CO2 absorbs. In this study, 15 µm is selected as the observation wavelength, which is more easily detectable at room temperature. Fig. 1(c) shows the infrared transmittance at each wavelength for the band pass filter. The infrared reflection properties of the observed surface were also measured to determine whether it reflects infrared rays of the observation wavelength used in this study. In this case, Attenuated Total Reflection (ATR) was measured using a Fourier Transform Infrared Spectroscopy (FT-IR). This time, SOFC was used as the detection target, and the infrared reflectance property of the surface of the cathode (LSCF) was measured. Focusing on the infrared reflection property around the wavelength of 15 µm observed by this method, it was found that the reflectance was more than 98.5, indicating that it was almost completely reflected. Therefore, it can be said that the use of infrared rays at a wavelength of 15 µm is reasonable based on the infrared reflectance properties of the specimen.
The detection device assembled in this experiment will be described. Fig. 1(e) shows the detection device. In order to perform this testing method, a mechanism is needed to hold the cell in place while filling the anode side with CO2. A schematic diagram of the jig fabricated in this study is shown in Fig. 1(d). It has a mechanism for injecting and exhausting gas, and seals are provided to prevent gas leakage from lower part of the cell while holding the cell in place.
We conducted an experiment to see if we could visualize CO2 in a bottle with the device used in this study. An incandescent light bulb was used to wait until the ambient temperature was contact, then CO2 gas was sprayed in front of the lens and the image was taken. It was found that this detection device can capture CO2 gas, and we believe that it can be applied to a detection model in the future.
Based on these results, our future policy is to measure the resolution and time required for evaluation when actual detection is performed. In addition, we would like to compare it with other method and investigate what it is good at detecting.

Figure 1