Study On The3D Microstructure Of SOFC Electrode Based On X-Ray Tomography

A solid-oxide fuel cell (SOFC) is a device that directly converts the chemical energy to electrical energy, which as a new and clear energy can solve the problems of Low energy efficiency and environmental pollution. However, it is found that during the SOFC operation its performance degrades badly to shorten its life. This is an important question to restrict the development of SOFC.Improving the SOFC performance is the key to obtain the electrodes that have high efficiency and long life to resist aging and pollution. Thus the most important thing to develop SOFC is how to optimize the porous nano-electrode. However, Current analysis techniques such as scanning electron microscopy (SEM) can only provide two-dimensional (2D) or limited volume characterization of microstructures.The ability to image the porous structures in3D to study the size, distribution, and connectivity of the pores and channels is critical in enabling SOFC technology. The development of the hard X-ray microscopy based on zone plate which has the strong penetrating power and high space resolution make this technique to be widely used in regions of biology、energy and environment science、materials science and microelectronics.National synchrotron radiation laboratory has built a zone plate based hard X-ray Microscopy beamline and station with50nm spatial resolution. The main contributions of this thesis are summarized as follows:1. Testing of X-ray microscopy and developing of the element absorption edge imaging methodThe principle and the main design parameters of optical devices for hard X-ray Microscopy beamline and workstation were introduced. Three imaging methods had been demonstrated. Based on the absorption imaging the element absorption edge imaging method was developed. Using the images of Ni-YSZ anode the differences between the phase contrast imaging and the element absorption edge imaging were compared. The results had indicated the advantage of element absorption edge imaging.2.Three-dimensional reconstruction of Ni-YSZ anode using x-ray element absorption edge The three dimensional (3D) microstructure of Ni-YSZ anode had been reconstructed based on the element absorption edge imaging. The original reconstructed slices were segmented successfully and the three phases (Ni、YSZ and Pore) had been distinguished. The quantitative analysis methods have been developed and used to characterize and quantify the key structural parameters, such as the volume ratio of the three phases (Ni,YSZ, and pore),connected porosity, surface area of each phase, interface of Ni/YSZ, volume-specific three-phase boundary length (TPB where the Ni, YSZ and fuel gas phases come together), and electrical conductivity of the anode. These parameters cannot be obtained by using traditional analysis methods. Obtaining of the3D microstructure would provide necessary information to understand the inter structure.3. Quantitative analysis of micro structural and conductivity evolution of Ni-YSZ anodes during thermal cyclingUnderstanding the mechanism of degradation in solid oxide fuel cells (SOFCs) using nickel/yttria stabilized zirconia (Ni-YSZ) as the anode material is very important for the optimization of cell performance. The effects of thermal cycling on the microstructure of the Ni-YSZ anode were explored using the three-dimensional element absorption imaging technique. It was found that the average Ni particle size increased with thermal cycling, which resulted in the decreased connectivity of the Ni phase. The conductivities of the anodes associated with connectivity of the Ni phase had been proved by measuring the conductivities of the anode samples, which were reduced with the increase in thermal cycle times. Moreover, the three-phase-boundary (TPB) length decreased with thermal cycles, which may lead to poorer performance due to a decrease in the number of electrochemically active sites.4.X-ray tomography of SOFC cathodePorosity of LSM-YSZ cathode is an important parameter, however, it is difficult to measure this parameter using traditional method because the thickness of cathode is too thin. In this work the high-resolution X-ray nanotomography technique was applied to reconstruct the three-dimensional microstructure of a LSM-YSZ composite cathode. Some key microstructural parameters, such as the porosity, representative elementary volume, connected pore volume and pore phase tortuosity, were obtained based on the three-dimensional reconstruction volume data. These parameters may provide valuable information for optimizing the manufacturing processes and improving the device’s reliability.Based on the previous study of LSM-YSZ cathode microstructure, the nano computed tomography (nano-CT) technique was further applied to image the3D microstructures of LSM-YSZ composite cathodes, which were sintered for different time at1423K. Based on the3D reconstructions some key structural parameters, such as the volume ratio of LSM-YSZ solid and pore, surface area and tortuosity, were calculated. It was found that the porosity and tortuosity of pore phase increased according to increasing sintering time; however, the mean diameter of pore and specific surface decreased. An important application of this information will be to improve the performance of the electrode by optimizing the fabrication conditions.The3D microstructure of LaSrFeO-YSZ cathode was imaged using the Fe absorption edge imaging method. The three phases (LaSrFeO、YSZ and Pore) were successfully segmented and distinguished. Some key parameters, such as volume fraction of each phase、connectivity of each phase and TPB length, were calculated. Moreover, the effect of reconstructed volume of LaSrFeO-YSZ cathode on volume fraction of LaSrFeO、YSZ and Pore was investigated.