Investigation Of Structure And Steam Electrolysis Performance Of Solid Oxide Cell Using Focused Ion Beam-Scanning Electron Microscopy Tomography

As the core reactor of high-temperature steam electrolysis hydrogen production technology,solid oxide electrolysis cell（SOEC）provides an important route for energy conversion and storage.However,the commercialization of SOEC is hindered due to performance and durability limitations.Investigating the microstructure of SOEC is significant for its performance optimization design.Currently,SOEC characterization is still facing essential scientific problems such as difficulty in quantitative analysis and unclear relationship in structure-performance.This work focuses on the typical O-SOEC that is composed of Ni-8YSZ/8YSZ/LSCF-GDC.The advanced Focused Ion Beam-Scanning Electron Microscopy（FIB-SEM）tomography has been utilized to enable insight into the structure evolution of cell.In this dissertation,the method of sample pretreatment is optimized and the sample-test parameter database is established.More importantly,this work provides theoretical guidance for the optimization of cell fabrication technology.The main work and achievements of this paper include the following aspects.1.The sample pretreatment,optimal testing parameters,and image pretreatment method of Ni-8YSZ/8YSZ/LSCF-GDC large-area cell were determined and applied to SOEC materials characterization reported in this paper.For resin impregnation,the Buehler Epothin resin was verified to be the superior choice.The best imaging parameters of Ni-YSZ hydrogen electrode,YSZ electrolyte and LSCF-GDC oxygen electrode,including voltage,current and detection mode,were determined respectively.Linear shadow correction and gamma correction were utilized to perform gray correction and brightness correction on the original SEM image to improve the accuracy of image segmentation.2.The three-dimensional pore structure of the LSCF-GDC electrodes fabricated at different annealed temperatures was investigated using FIB-SEM tomography,and their performance was evaluated based on impedance spectroscopy and voltage-current curve.The cell fabricated at 1000°C（C1000-1h）showed the best performance as the initial current density of the monitored cells fabricated at 900°C（C900）,1000°C and1100°C（C1100）were 0.5 A cm^-2,0.88 A cm^-2 and 0.66 A cm^-2,respectively.For the C1000-1h oxygen electrode,the polarization resistance associated with gas diffusion and mass transfer processes was proved to be minimal.After SOEC-operation for 130h,the performance of C1000-1h remained stable with a voltage degradation rate of0.016 V kh^-1.Three-dimensional structural analysis showed that the LSCF-GDC oxygen electrode of C900 cell had the smallest porosity and the largest pore tortuosity,which was unfavorable for gas transport.The LSCF-GDC electrode of C1100 cell exhibited the smallest pore surface area,which resulted in limited catalytically active sites.3.The microstructure of electrolyte,electrode pore structure and electrolyte-electrode interface of 8YSZ/Ni-8YSZ/Ni-3YSZ half-cells manufactured using step sintering and co-sintering technique was quantitatively investigated by FIB-SEM tomography,which was in combination with impedance spectroscopy characteristics to discuss the influence mechanism of sintering technique on its SOEC performance.According to the electrochemical test results,the co-sintering cell performed higher initial current density(0.73 A cm^-2)and better stability.FIB-SEM observation showed that the electrolyte of the step sintering cell was slightly thicker,and its porosity（2.71%）was about twice that of the co-sintering cell,which was the main reason for the increase of ohmic polarization.In addition,the pore connectivity of fuel electrode of the step sintering cell was poorer with higher tortuosity,which results in worse performance by increasing gas mass transfer resistance.The co-sintering cell exhibited finer Ni and YSZ at the electrolyte/fuel electrode interface,which could reduce the activation polarization loss of the cell by generating more catalytic reaction sites.4.The initial current density（50%increase）and the durability of the SOEC were enhanced by infiltrating GDC nanoparticles on the conventional Ni-YSZ hydrogen electrode.Subsequently,FIB-SEM tomography was adopted to compare the three-dimensional structure of Ni-YSZ hydrogen electrodes before and after SOEC operation.The results showed that the Ni content of the conventional Ni-YSZ electrode had been significantly reduced by 1.16%after 100-h SOEC operation,while the GDC-modified one presented a negligible decrease.Microscopy results confirmed that further evaporation of Ni was prevented as the GDC nanoparticles formed a protective layer on the Ni surface during the electrolysis operation.Generally,the infiltration of GDC into naked Ni-YSZ electrode alleviated decrease in the tripe phase boundary density and reduced the resistance by providing additional electrochemical active sites.