Construction,Optimization And Stability Research Of Ni-Fe Metal Supported SOFC

Compared with the traditional coal-based power generation,the generating efficiency of Solid Oxide Fuel Cell（SOFC）is not restricted by the carnot cycle,it can convert the chemical energy in fuels directly into electricity which is an efficient,safe and environmental friendly energy conversion technology,it owned promising application prospects.Cost and lifetime problem are still main obstacles hindering the commercialization of SOFC technology.Traditional 1G and 2G SOFCs are based on cermet supports for either the electrolyte or anode.Both have drawbacks of low mechanical strength,long start-up time,poor redox cycle capability and poor thermal stability.This hinders SOFCs from replacing widely used low-efficiency and high-polluting heat engines.Recently,many efforts have been devoted to developing metal-supported SOFC（MS-SOFC）,which are sometimes called third-generation SOFC,to replace anode-supported configurations.Using metal as a support and the metal support-anode-electrolyte-cathode structure,the costly electrode and electrolyte layers can be made as thin as possible while ensuring their functionality,which results in an obvious cost reduction.The MS-SOFC can work at 600 ^oC⁸00 ^oC.The difficulty of MS-SOFC preparation is that the metal support is easily separated from the ceramic/metal-ceramic anode.In this paper,the multilayer co-firing structure was used,and the base structure of NiO-Fe₂O₃/NiO-YSZ/YSZ/YSZ was prepared by using the method of tape casting,isostatic pressing,and high temperature calcining,and the nano-structure cathode was obtained by impregnation method.Though MS-SOFC have attracted wide interest,very few studies have been devoted to the oxidation behavior of SOFC metal supports under various anode atmospheres,such as H₂/H₂O or reformed gases.Sufficient oxidation resistance and electrical conductivity research of the porous metal support is thus essential for application.The SOFC cathode plays the most crucial rule in the whole cell and determines the overall cell output performance.Poisoning and corrosion effects by external contaminants should be taken into account for the SOFC cathodes because ambient air is continuously supplied directly to the cathodes during the long term operations.For example,the cathode has always been suffered corrosions from CO₂ and humidity in air during the cell storage and working condition.The main research content includes:First,we investigated the long-term oxidation behavior under anode atmosphere of a Ni-Fe（1:1 weight ratio）support for solid oxide fuel cell（SOFC）applications.Ni-Fe supports were obtained through tape casting,high temperature sintering and pre-reducing in H₂/N₂-3%H₂O at 750 and 1000°C,respectively.The simulated anode atmospheres are H₂/N₂-3%H₂O and H₂/N₂-25%H₂O,and the temperature is 750°C.Calculating the equilibrium phase diagram of Ni-Fe-O₂ using the FACTSAGE Thermochemical Software and Databases.Theoretically,Fe should not be oxidized under H₂/N₂-3%H₂O atmosphere,but will partially oxidized to ASpinel using H₂/N₂-25%H₂O,while Ni should not be oxidized under both anode atmosphere.After theoretical calculation,we began the oxidation behavior test of the Ni-Fe support in SOFC anode atmosphere.The Ni-Fe supports were exposed in simulated anode atmospheres of H₂/N₂-3%H₂O and H₂/N₂-25%H₂O for periods of up to 1000 h at 750°C.The samples were examined for mass change,phase and chemical composition,and microstructure evolution during the annealing process.The Ni-Fe supports exposed to H₂/N₂-3%H₂O showed negligible oxidation,while those exposed to H₂/N₂-25%H₂O showed a 4-6%mass increase,due to the fact that a Fe-rich oxide scale was found on the surface of the Ni-Fe alloy particles.Conductivity measurements showed that the supports annealed in the two atmospheres maintained sufficiently high conductivity.The results from the current work demonstrate that the porous Ni-Fe support can be well employed in MS-SOFC.Second,we investigated the long-term stability test of 3（La,Sr）（Co,Fe）O_3-δgroup electrodes under cathode atmosphere and research on the attenuation mechanism.The selectedcathodewasLa_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ（LSCF-6428）and La_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3-δ（LSCFN-46271）,we also added La_0.4Sr_0.6Co_0.2Fe_0.8O_3-δ（LSCF-4628）cathode as comparison.3 kinds of cathodes was screen printed on both sides of GDC electrolyte.First,to confirm the frequency range corresponding to the three electrochemical processes of cathode,we tested the impedance spectrum of symmetric electrodes under different oxygen partial pressure,the DRT characteristic peaks were identified through the analysis and as-segment of the relaxation time.The changes in the DRT peaks were correlated with the different electrochemical process.The 3 catholic electrochemical process are:gas diffusion in oxygen electrode,chemical surface exchange of O₂,charge transfer reactions in electrolyte and TPB.The symmetric electrodes were tested in cathode rig under air,CO₂-air and H₂O-air for the corrosion stability,the temperature was 800°C and 650°C.After 100 hs’stability test under air at 800°C,there’s no perovskite phase change of 3 electrode,while the polarization resistance slowly increased,the polarization resistance of LSCF-4628 cathode grew more rapidly than LSCF-6428 and LSCFN-46271.According to the EDS scanning results of elements on the electrode surface,Sr was migrated to the electrode surface,and the Sr enrichment on the electrode surface will affect the catalytic performance and polarization impedance of electrodes.Compared to be tested at 800°C,cathodes maintain good electrode activity at 650°C,polarization resistance didn’t increase rapidly at lower temperature.Choose 20%CO₂-air and 50%CO₂-air as ambient air while doing the durability research on the corrosion resistant properties of cathodes,the temperature are also 800°C and 650°C.The electrodes were examined for phase and chemical composition,and microstructure evolution before and after corrosion test.XRD showed there’s no secondary phase exits,indicates electrodes did not react with CO₂ during test time.According to DRT analysis,the electrode performance degradation and increase of polarization resistance were observed with CO₂ addition into the O₂ stream,CO₂ can inhibit dissociation of the adsorbed oxygen molecule or diffusion of O-species.Then evaluate the effect of humidity on 3 electrodes.The chosen atmosphere were4%H₂O-air and 10%H₂O-air.No secondary phase exited after stability test,but nano-sized particles appeared in the high-magnification SEM images.LSCF-4628 and LSCFN-46271 generated more impurities than LSCF-6428,the EDS point analysis speculated the particles were SrO,after a long time air purging,the nano-particles disappeared.As a result,LSCF-6428 and LSCFN-46271 have good corrosion stability and can be used as MS-SOFC cathode.Finally,4 kinds of MS-SOFC were fabricated,the base structure of the cells were all Ni O-Fe₂O₃/NiO-YSZ/YSZ/YSZ.To made the cathode,the final step was to infiltrated LSCF and LSCFN on the cathode side or steep the whole cell in the impregnating solution.First,test the performance of MS-SOFC with the structure of Ni-Fe/Ni-YSZ/YSZ/YSZ-LSCF.The maximum power density reached 0.72 mW cm^-2 at750 ^oC,and electrolysis voltage of 1.3V at 0.8 Acm^-2 under SOEC mode.The cell had a degradation rate of 10.4%under fuel cell mode and that of 10.2%under water electrolysis mode.Post-test DRT and SEM analysis of the electrodes indicated that the agglomeration of infiltrated LSCF particles,the grown up of Ni particles and the oxidation of Ni-Fe alloy support were possibly responsible for the performance degradation of the cell.Steep the whole cell in the LSCF impregnating solution,which will simplify the cell preparation steps and optimize the anode.LSCF-Ni-Fe/Ni-YSZ/YSZ/YSZ-LSCF structured cell possess better electrochemical performance under same test conditions.At 750 ^oC,the cell had a peak power density of 0.75 W/cm²,the cell had a current density of 0.82 A/cm² at an electrolysis voltage of 1.3 V.The cell has a voltage degradation rate of 6.8%and 8.4%for fuel cell mode and steam electrolysis mode,respectively.The anode side infiltration improved the performance and stability of the cell.The last two kinds of cells with the structure of Ni-Fe/Ni-YSZ/YSZ/YSZ-LSCFN andLSCFN-Ni-Fe/Ni-YSZ/YSZ/YSZ-LSCFNshowedhighelectrochemical performance under SOFC mode,at 750 ^oC,the two cell had the maximum power density of 0.73 W/cm² and 0.82 W/cm².The stability of the cell using LSCFN as cathode is better,which is consistent with the test results of cathode performance and stability in chapter 4.These four kind of cells all have excellent performance and stability.In summary,the Ni-Fe alloy support used in this paper offered sufficient conductivity for application as a MS-SOFC support after long term oxidation test under anode atmosphere.LSCF-6428 and LSCFN-46271 cathode own good corrosion stability under cathode atmosphere and can be used as MS-SOFC cathode.Finally four kinds of Ni-Fe metal supported SOFC were manufactured and tested,and all of the cells had high power density and electrolytic properties.In addition,infiltrate mixed-conducting perovskite to the anode side of the cell is a new optimization method to improve the performance and stability of the cell.