Optimization,preparation And Performance Of Fuel Electrode Materials For Solid Oxide Cells

Solid oxide cells（SOCs）are efficient energy conversion device.Electrode is the core component of SOCs,which directly determines the performance and durability of the cell.On the one hand,the development of high performance and stability SOCs electrode materials is carried out by designing micro nano structure oxides.First,transition metals Ni or Co were doped in the B site of Sr₂（Fe,Mo）O_6-δperovskite,the precipitation process of transition metals,material structure evolution and catalytic properties were studied.In addition,the second phase Gd_0.1Ce_0.9O_2-δ（GDC）was added to form composite electrode and the preparation process was further optimizedto improve electrode performance.The performance and stability of the cells in different application scenarios were evaluated.On the other hand,a systematic study on the electrolysis of H₂O by commercial large-scale cells under thermal neutral voltage was carried out.Reveal the relationship between the cell performance and operating condition,and the failure reasons were deeply analyzed,which laid a foundation for the industrial application of SOCs.The details are as follows:In Chapter 3,a new type of carbon resistant fuel electrode material Sr₂Fe Mo_0.65Ni_0.35O_6-δ（SFMN）was developed by doping Ni.The active nanoparticles of Fe Ni₃ are in situ exsolved from the SFMN perovskite oxides in pure hydrogen.A peak power density of 0.439?W?cm^-2at 850℃ is achieved from the cell with SFMN fuel electrode when fueled by hydrogen.More importantly,addition of GDC which can provide ion conduction path in the fuel electrode material reduces the polarization resistance of the electrode.The cell with SFMN-GDC fuel electrode achieves a maximum power density of 0.551,0.476 and 0.392?W?cm^-2at 850℃ fueled by hydrogen,syngas and methane,respectively.The cell has shown negligible degradation for 210 hs in syngas and 600 hs in methane fuel,no carbon deposition was found,indicating that the SFMN-GDC is a promising fuel electrode for solid oxide fuel cells.In Chapter 4,a high activity perovskite oxide materials Sr₂Fe_1.3Co_0.2Mo_0.5O_6-δ（SFCM）was developed by doping Co.In reducing atmosphere of H₂,active cobalt metal nanoparticles are in situ exsolved from the SFCM perovskite oxides.SFCM show the lowest polarization resistance of 0.15?cm² at 850°C in H₂.The cell with SFCM fuel electrode achieves the maximum power density of 1.09,0.981 and 0.29 W?cm^-2at 850℃ when the fuel electrode is fueled by hydrogen,syngas and methane,respectively.Further,after continuously operated in hydrogen for 115 hs,in syngas for 190 hs and in methane for 300 hs,there are no negligible degradations.Synergistic effect between perovskite oxide and exsolved Co nanoparticles contributes to the improved SOFCs performances.In Chapter 5,we report a one-step synthesis method to fabricate electrode material SFCM-GDC.Compared with the traditional method of synthesizing SFCM and GDC separately and then mixing them mechanically,the one-step synthesis method can produce more uniform nano and micro scale composites,which helps to increase the three-phase interface.As a result,the total polarization resistance of SFCM-GDC electrode is effectively decreased to 0.036Ωcm²and 0.047Ωcm² at 850℃ in air and hydrogen,respectively,compared to that of 0.041Ωcm² and 0.074Ωcm² of the mechanically mixed SFCM+GDC.The peak power density of SFCM-GDC symmetrical cell can reach 0.986 W cm^-2 using wet H₂ as fuel at 800℃,much higher than the cell with physical mixed electrode of 0.894 W cm^-2.It is suggested that one-step synthesis method is an effective method to prepare composite electrode with excellent performance.In Chapter 6,the application of SFCM electrode in SOEC mode was studied.The results show that Co nanoparticles are in situ exsolved from the parental SFCM in 10%H₂-90%N₂,and they can keep stable in the atmosphere of 50%CO₂-50%H₂ or 50%CO₂-50%CO.The polarization resistances of the SFCM symmetrical cell are as low as 0.24Ωcm² in 50%H₂O-50%H₂.In addition,the cells for CO₂/H₂O co-electrolysis at 800℃ shows a reasonable durability at different current densities of 0.3,0.6,0.9,1.0,1.2,1.5and 1.8 A cm^-2.Electrochemical impedance spectroscopy and distributed relaxation time analysis indicates that electrode surface exchange and diffusion are the rate-limiting steps in the electrolysis of CO₂/H₂O,and the resistances of these steps can be significantly decreased by the in situ exsolved Co nanoparticles from SFCM electrode,thereby providing higher activity and better stability.In Chapter 7,we investigate the long-term durability of Ni/YSZ fuel electrode supported SOECs operated for electrolysis of steam at the thermoneutral voltage of 1290m V,by testing cells at 800,750,and 700℃（Cell-800,Cell-750 and Cell-700）.The cells experience a significant degree of initial degradation within the first 300 h,with a degradation rate of 2.80,1.93,and 1.77 A cm^-2 kh^-1(～1.80,1.51 and 1.90%kh^-1)for the cells tested at 800,750,and 700℃,respectively.After 300 h of operation,the Cell-750exhibits performance comparable to the Cell-800 and more than 74%higher than the Cell-700,suggesting that among three operating temperatures 750℃is the optimum one.The results show that the major degradation is from the Ni/YSZ electrode for all the cells.Severe microstructural deteriorations including Ni redistribution/agglomeration and the disconnection between Ni and YSZ were observed in the Ni/YSZ electrode for the cell tested at 800°C.Such different degradation behaviors of the cells were ascribed to the difference in electrode overpotential.Our findings highlight the significance of operating temperature optimization on the long-term durability of SOECs when operated in potentiostatic mode.