Study On Preparation And Properties Of Alloy Interconnects For Intermediate Temperature SOFC

Solid Oxide Fuel Cell (SOFC) has been considered as one of major green energy in the 21st century because SOFC has ultra-high energy conversion efficiency and ultra-low pollutant emission. Over the past few years, research and development of both new electrodes and electrolytes and the advent of the anode-supported planar SOFC design have led to a steady reduction in the SOFC operating temperature. Consequently, it becomes more realistic now to use low cost heat-resistant alloys for interconnect components in the SOFC stack. However, there are some serous problems for using alloys as interconnect materials at the high operating temperature of SOFC including the materials and the structure design. The main work of this thesis focuses on the systematic study on the alloy materials and the structure of interconnects in order to make an intensive comprehension on the properties of alloy interconnects and lay the foundation for the fabrication of the SOFC stack.This thesis has evaluated the fundamental properties of SUS430 alloys as SOFC interconnect materials. Taking the LSM20 cathode as an example, the mechanism of chromium poisoning to the cathode was studied in details; and the determinants of the poisoning degree of chromium to the different cathodes were clarified. The compatibility of SUS430 alloy with the Ni-YSZ anode was also studied. Results showed that the thermal expansion coefficient (TEC) of the SUS430 alloy was very close to that of the electrodes under the SOFC operating atmosphere, especially the anode substrate. Although the oxidation growth rate of the alloy was relatively low, the increase of area specific resistance (ASR) was much higher than the acceptable level for SOFC interconnects. The poisoning of chromium to the LSM20 cathode would inhibit the oxygen reduction reaction (ORR) on the surface of LSM20 cathode. At the initial stage of polarization, the dissociative adsorption and the diffusion of oxygen on the LSM20 electrode surface were inhibited by the gaseous Cr species. With the time prolonging, the migration processes of oxygen ions into YSZ electrolyte were inhibited by the solid Cr species, such as Cr2O3 and (Cr,Mn)3O4, deposited on the electrolyte surface. The degree of chromium poisoning to the cathode was determined by the cathode atmosphere, the electrode structure and the cathode materials. Therefore, it is reasonable to inhibit the chromium poisoning to the cathode by lowering the partial pressure of the oxygen and vapor in the cathode atmosphere, selecting the cathode materials with more oxygen vacancies and rationally optimizing the cathode structures. SUS430 alloy was compatible with the Ni-YSZ anode.La0.8Sr0.2FeO3-δ(LSF20) was proposed as the interconnect protective coatings in the cathode side and the metallic nickel was proposed as the interconnect protective coatings in the anode side by the improved APS method. Many kinds of ceramic materials were investigated on their conductivity and TEC, LSM20 and LSF20 were finally selected as the protective coating materials for the interconnects made of SUS430 alloys in this thesis. The protective coating in the interconnect cathode side was prepared by the APS method. The densification technique of APS was improved to adapt to the SOFC high operating temperature. A method of nitrate solution impregnation and sintering in situ was proposed to realize the further densification of the as-prepared APS coating. As a result, this made a perfect effect and the open porosity was below 1%. The properties of the LSM20 and LSF20 coating were compared. Results showed that LSF20 coating could more effectively reduce the high temperature oxidation rate of SUS430 alloys and inhibit chromium evaporation and diffusion through the coating. The ASR of the LSF20 coated SUS430 alloy was only 1m??cm2 after being oxidized in air at 800℃for 1000h, which is only half of the lowest ASR value (2m??cm2) of the interconnects cathode protective coating after the same treatment according to the recent international reports. A metallic nickel coating was prepared by the APS and the electrochemical deposition method, respectively. The nickel coating prepared by the APS method adhered more tightly to the alloy substrate. Therefore, a lower ASR was obtained by SUS430 alloys with the APS nickel coating under the SOFC anodic atmosphere. The current collecting structure was designed in this thesis. The current collector's function and its structural requirements were confirmed by analyzing the results of the electrochemical polarization curves test, electrochemical impedance test and ASR test. Results showed that the electrode contactor can not only reduce the ohmic resistance of the fuel cell, but also improve the catalytic activity of the ORR on the electrode surface. The electrode supporter only played a role in electrical conductivity and did not influence the ORR catalytic activity of the electrodes. A noble contactor of La0.6Sr0.4CoO3-δ(LSC) prepared by solid state reaction method and silver paste was proposed as the cathode contactor. The anode contactor proposed in this thesis was composed of NiO (particles diameter: 0.68μm) and silver paste. A reduction greater than 50% was obtained for the contact resistance between the interconnect and the electrode by fastening the supporters with the silver lead embedded in the silver paste on the contactor surface, and the result was close to the best result presently known.A finite volume quantification analysis model was constructed based on the theory of electrochemical reaction kinetics combined with phenomenology. The idea"ratio factor"was adopted to convert the structural parameters optimization into the selection of the ratio factors. Optimization of the interconnect structural parameters was realized by calculating the simulated unit cells polarization with the changes of the ratio factors in a small step. At the same time, it provided a qualification and theory guidance for the SOFC interconnect structural design. The effects of SOFC operating conditions on the fuel cell performance were also evaluated in this thesis. A set of interconnects were designed and sprayed with the LSF20 protective coating in the cathode side and metallic nickel protective coating in the anode side by the APS method, respectively. The optimized current collectors were applied in the fabrication of the fuel cells. Discharging results were close to those of the simulation.