Study On Ionic Conductivity And SOFC Electrochemical Performance Of Hematite

With the world’s energy and environmental problems becoming more and more serious,clean energy technology has become a research hotspot in the scientific community.Solid oxide fuel cell（SOFC）,as an energy conversion technology that can directly convert chemical energy of hydrogen fuel into electric energy,has the advantages of high efficiency,zero pollution,and noble metal free,and the application prospects in the field of green power generation and low-carbon transportation.However,due to the activation requirement of solid oxide electrolyte and electrode at 800～1000℃,the current operating temperature of SOFC is too high,which hinders the commercialization process of SOFC.Therefore,the development of high-performance electrolytes and electrodes at low temperature has become an urgent need to solve the high-temperature bottleneck of SOFC.In recent years,based on surface,interface,and energy band engineering,researchers have developed a series of semiconductor ionic electrolytes,such as Zn O,Li Al_0.5Co_0.5O₂,Sm Ni O₃and natural hematite（α-Fe₂O₃and Si O₂）,which show considerable ionic conductivity t 400～500℃,and are promising to reduce the operating temperature of SOFCs.Based on the preliminary work of natural hematite electrolytes,the ion conducting and SOFC electrochemical properties of hematiteα-Fe₂O₃are further studied in this thesis.The main contents are as follows:Theα-Fe₂O₃sample is synthesized by a sol-gel method,and the R-α-Fe₂O₃sample is obtained by treatingα-Fe₂O₃in H₂.The phase structure,particle morphology,and surface elemental valence of the two samples are characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）and X-ray photoelectron spectroscopy（XPS）.The DC conductivity ofα-Fe₂O₃and R-α-Fe₂O₃are measured in N₂,Ar and fuel cell atmosphere by DC scanning voltage method.To evaluate their low-temperature electrolyte function,the two samples are applied to SOFC electrolyte layers for I-V characteristics test at 550℃,respectively.The results show that,theα-Fe₂O₃sample is in a form of hexagonal hematite nanoparticles with smooth grain surface and uniform distribution;the surface of R-α-Fe₂O₃samples was partially reduced by H₂,resulting in surface Fe metal particles and oxygen deficientα-Fe₂O_3-δ;compared withα-Fe₂O₃,the R-α-Fe₂O₃has a higher concentration of oxygen vacancy and electronic conduction,so that it has the possibility of O^2-/H⁺/e^-triple conduction in cell operating atmosphere,which greatly improves the ionic conductivity and total conductivity of R-α-Fe₂O₃.However,due to the serious short circuit issue caused by high electronic conductivity,the two samples present much lower SOFC performance than that of natural hematite,which means the preparedα-Fe₂O₃requires further optimization.In order to deal with the short circuit problem ofα-Fe₂O₃electrolyte SOFC,we further imitate the composition of natural hematite to introduce insulator Si O₂intoα-Fe₂O₃to construct two types of"imitative natural hematite"samples via wet chemical method and solid phase mixing method.By comparing and analyzing the phase structure,micro-structure,and SOFC electrochemical performance of the samples,it is found that the imitative natural hematite prepared by solid-phase mixing method shows better electrolyte function and cell performance owing to its micro-structural characteristics.The corresponding SOFC exhibits an open circuit voltage of～1.0 V at 550℃along with an output power density of 250m W/cm²（superior to natural hematite）.On this basis,DC conductivity and XPS tests of the imitative natural hematite by solid-phase mixing method are carried out.It is found the compositing ofα-Fe₂O₃and Si O₂with specific micro-structure and mass proportion can not only block the electron transport of hematiteα-Fe₂O₃,but also improve the concentration of oxygen vacancy and ionic conductivity of the sample.In addition,based on the impedance spectrum analysis of the cell,we further introduce Na₂CO₃into the imitative natural hematite,and find that Na₂CO₃exists in a molten form and contribute fast proton transport channels in the sample,which improves the open-circuit voltage and power outputs of the corresponding SOFC to 1.12 V and 375 m W/cm2（550℃）,respectively,and thus validates the feasibility of optimizing the electrolyte function and cell performance of hematiteα-Fe₂O₃by compositing Si O₂and Na₂CO₃.The above findings manifest the hematiteα-Fe₂O₃electrolyte can gain enhanced ionic conductivity in fuel cell atmosphere,but it also produces a non-negligible electron conduction,which is easy to cause short circuit risk of the cell.By combining Si O₂and Na₂CO₃,the ionic conductivity ofα-Fe₂O₃can be further improved while its electronic conductivity can be reduced,which indicates an effective way to optimize the electrolyte function of hematiteα-Fe₂O₃.The developed imitative natural hematite in this thesis reveals considerable fuel cell performance at low temperature,indicative of a new type of semiconductor ionic material for the development of low-temperature SOFC.