Electrochemical Properties Regulation And Stability Study Of SrFeO₃-based Hydrogen Electrode For Solid Oxide Cells

Given the limited storage of fossil fuels and the increasingly severe environmental pollution,it is significantly important to promote the commercial application of solid oxide fuel cell（SOFC）for clean and efficient use of fossil fuels,and develop the technology of solid oxide electrolysis cell（SOEC）to obtain high-quality energy by utilizing"abandoned energy"such as abandoned wind,abandoned light,and abandoned water.As an environmentally friendly energy conversion device,solid oxide cells（SOCs）can convert the fuel chemical energy into electrical energy efficiently and vice versa.Strontium ferrite（Sr Fe O₃）perovskite exhibits good ion-electron mixed conductivity,and thus the doped Sr Fe O₃-based solid oxides have been widely used in SOCs oxygen electrodes.At present,the application of doped Sr Fe O₃-based perovskites in SOCs hydrogen electrodes have attracted much attention.This thesis mainly focuses on the structural control and electrochemical performance optimization of the new Sr Fe O₃-based hydrogen electrode with in-situ precipitation of nano-metal particles for solid oxide cell,as well as the operation characteristics under high water content and various complex carbon-containing fuels.It is hoped to clarify the charge conduction process and regulation mechanism of the in-situ precipitation electrode system,reveal the electrochemical performance and stability rules of the electrode under actual working conditions,and construct a heterostructure hydrogen electrode and operating strategy with high-performance and stable operating.The equal proportions of La/Sr（A-site）and Co/Fe（B-site）co-substituted La0.5Sr0.5Co0.45Fe0.45Nb0.1O3-δ（LSCFN）was successfully synthesized by a conventional solid-state reaction.The precipitation process of Co-Fe alloy particles on LSCFN was observed under different reduction conditions,and the corresponding electrochemical performance of LSCFN symmetric cell was investigated,and the effect of electrode structure tailoring on the electrochemical properties as well as stability was discussed.Results show that the precipitated Co_1-xFe_x particle size and Fe content inside under high temperature and3%H₂O-97%H₂ atmosphere increase with the increase of reduction temperature,accompanied with formation of A₂B₂O₅ brownmillerite structure.The increases of ohmic and polarization impedances decrease the electrochemical performance of LSFN symmetric cell.The strategy to pre-reduce at elevated temperature and then operate at lower temperature is demonstrated to effectively mitigate the Co-Fe nanoparticles coarsening and presumably to slow down the anode polarization resistance degradation.SrTi0.3Fe0.7O3-δ（STF）mixed with 10 wt%,20 wt%and 40 wt%of Gd0.1Ce0.9O1.95（GDC）composite anodes were prepared,and the properties and reaction mechanism at low discharge voltage of STF-GDC composite anode single cell was investigated systematically for the first time.Results show that the STF-GDC composite anodes single cell exhibit higher performance and smaller polarization resistance than that of the STF anode single cell.The discharge j-V curve of STF-40GDC composite anode single cell maintains a negative curvature in the entire voltage range,and there exists a concentration polarization under low hydrogen partial pressure.The positive curvature phenomenon for the STF anode single cell disappears.It is concluded that the GDC can be used as an oxygen ion carrier together with STF to accelerate the oxygen ion transport process and expand the triple phase boundary（TPB）,improving the output performance of STF-GDC composite anodes single cell.The regulation mechanism that GDC affects the surface oxygen contentration of STF anodes under different hydrogen partial pressures and discharge voltages was proposed.LSGM（La0.8Sr0.2Ga0.8Mg0.2O3-δ）electrolyte-supported single cells were prepared by using Sr0.95Ti0.3Fe0.63Ni0.07O3-δ（STFN）as electrodes which can precipitate Ni-Fe alloy particles,and the operating characteristics under high water content fuel,as well as electricity production performance and stability using various carbon-containing complex fuel gases were investigated.The influence mechanism of the precipitation anode experienced with different water content fuels on the output performance of STFN single cells is studied and discussed by simulating the actual operating conditions（high fuel utilization,water content～15%or more）.Results show that the Ni-Fe nanoparticles on the nano-heterogeneous anode significantly improves the electrochemical performance of STFN single cell at the relatively low temperature and high H₂O content fuel,i.e.,the maximum power density of STFN single cell under the condition of 700℃,50%H2-50%H2O can still reach～60%of that under pure hydrogen condition.However,the P_max of the STF single cell without Ni-Fe particles on the anode surface is only～40%of that under pure hydrogen condition.STFN anode single cell experienced 20 load cycles at 750℃without obvious performance degradation,and operating stable under high water content fuel（50%H₂-50%H₂O）for more than 360 h indicated that the STFN anode has good operational stability under practical conditions.In the study of electrolytic performance,the electrolytic performance and stability of STFN as hydrogen electrode under different water vapor concentrations were studied and analyzed,as well as the stability of the single cell in the reversible operation mode.Results shows that the electrolytic current of STFN hydrogen electrode single cell under 1.3 V electrolytic voltage at 800°C is-0.66,-0.74,-0.76 A·cm^-2 for 10%,30%,50%H₂O concentration,respectively.The STFN single cell can operate stably for more than 80 h under constant current electrolysis with 10%H₂-N₂-50%H₂O at 750°C,and the performance decay rate is only 0.75%per 100 h.When the cell was carried out in RSOC at 750°C,it can operate more than 240 h with the decay rate of 5.7%per 100 h.Obviously,the STFN hydrogen electrode shows a broad application prospects in renewable energy conversion and storage,distributed off-grid energy supply systems.Finally,using the process gas and by-product gas（which can be used as carbon-based fuel）in actual industrial production as fuel,the feasibility of STFN single cell in practical application is investigated.The STFN single cell using methane steam reformate feed with the reforming degree of～31%can achieves output performance comparable to 57%H₂ feed,i.e.,the P_max at 750℃for both fuels are～0.58 W·cm^-2,and operate stably for more than 100h at 750℃without obvious carbon deposition and performance degradation.The complex by-product gas of a petrochemical enterprise can be used as fuel for STFN anode single cell to operate and shows high output performance.The effect of the composition change of the by-product gas caused by differences in actual operating conditions on the performance of the single cell is studied,and the stable operation mechanism of the STFN anode single cell under the complex by-product gas fuel is clarified.The STFN anode single cell has high output performance under the above two carbon-containing complex fuels,and shows good industrial application prospects.