Synthesis And Application Of Perovskite Oxide-based Semiconductor Ionic Electrolyte Materials For Low-temperature Solid Oxide Fuel Cells

Solid oxide fuel cells（SOFC）are gaining prominence in the technology industry due to their low emissions and fuel flexibility.Low-temperature（LT）operation is a global SOFC research and development challenge.Exploring new ion-conducting electrolytes at low temperatures is an effective strategy for developing low-temperature solid oxide fuel cells（LT-SOFCs<600°C）.The traditional method of developing efficient ionic conductors is structural doping,but a major barrier remains the unavailability of materials for the low-temperature range.The appealing features of semiconducting oxides and their heterostructures make them an important research topic to fabricate LT-SOFCs with significant output.In this regard,the great versatility of the perovskite oxide structure allows different doping to obtain a variety of properties,which is a key feature in the development of materials for SOFCs.This dissertation includes experimental proof of high ionic conductivities and lower operational temperatures with exceptional fuel cell performance.A few promising perovskite semiconductor oxide materials with electric and mixed ionic electric conductivities,such as single-phase Ba Sn O₃（BSO）and heterostructure composites Nd Ba_0.5Sr_0.5Co_1.5Fe_0.5O_5-δ-Ba Zr_0.1Ce_0.7Y_0.2O_3-δ（BZCY-NBSCF）,SCF-Fe₃O_4,and Ba Ti O₃-Ce O₂.were introduced as an electrolyte membrane with various types of charge conduction exhibited attractive features for the SOFCs operating at low temperature.The developed perovskite structured single-phase semiconductor materials and heterostructures displayed electrochemical performance with low ohmic resistances and high protonic or conduction at a possible low working temperature（550°C）.The results are presented in four sections in this dissertation work:Part Ⅰ:The single-phase perovskite Ba Sn O₃（BSO）was fabricated as an electrolyte membrane for LT-SOFC.The different characterizations verified the single-phase cubic crystalline structure,morphology,chemical states,and surface analysis of the prepared nanoparticles.The prepared electrolyte displayed a proton conductivity of 0.23 S cm^-1and the fuel cell delivered a high output of 843 m W cm^-2 at a low temperature of 550℃.Part Ⅱ:A composite electrolyte was fabricated via tunning a double perovskite structured mixed conductor Nd Ba_0.5Sr_0.5Co_1.5Fe_0.5O_5-δutilizing a proton conductor Ba Zr_0.1Ce_0.7Y_0.2O_3-δ（BZCY）,which results into a fuel cell power density of 470 m W cm^-2and an ionic conductivity of 0.16 S cm^-1 at 550 ^oC.The findings show that interfacial conduction helps in the BZCY-NBSCF composite’s ion transporting process.These interfaces produced among two phases possess a high potential for fabricating high output fuel cell devices.Part Ⅲ:The Sr Co_0.8Fe_0.2O_3-δ-Fe₃O₄ perovskite heterostructure is used as a new functional electrolyte having proton（H⁺）ion conduction.The optimum composition of SCF-Fe₃O₄ has reached an OCV of 1.01 V and a power density of 583 m W cm^-2 at 550^oC,exhibiting an extraordinary ionic conductivity of 0.2 S cm^-1.An energy band alignment mechanism based on a p-n heterojunction may explain the suppression of electronic conductivity and promotion of ionic conductivity in the heterostructure due to the built-in electric field effect.The results reveal that semiconductor SCF can be developed as a promising protonic electrolyte by the SCF-Fe₃O₄ heterostructure approach.Part Ⅳ:A composite of perovskite structure dielectric Ba Ti O₃ and ion-conducting Ce O₂(Ba Ti O_3-x Ce O₂)as an electrolyte material for LT-SOFC is discussed.In comparison with Ba Ti O₃（BTO）,solely 10 percent of Ce O₂ composited with BTO leads to high open-circuit voltage（OCV）and power density of 1.05 V and 609 m W cm^-2respectively,at 550 ^oC.The electrochemical impedance spectrum（EIS）fitted results showed that BTO-10Ce O₂ composites possessed higher ionic conductivity of 0.18 S cm^-1.This is because the heterointerfaces between BTO and Ce O₂ provide a fast ion-conducting path.Conductivity and fuel cell performance measurements indicate that these perovskite semiconductors exhibit a mixture of ionic and electronic conductivities that are balanced in optimum composites,although proton conduction dominates in all of the above-mentioned electrolytes.