Design Of Catalytic Function And Ionic Transport In Perovskite Oxides For Low Temperature Solid Oxide Fuel Cell

Solid oxide fuel cells(SOFCs)have attracted worldwide attention due to their high conversion efficiency,fuel flexibility and low emissions.The main challenges of this technology for marketing acceptance are associated with cost due to the high temperature operation(700-1000?).Therefore,intensive research efforts have been devoted to lowering the operating temperature from the higher temperature(800-1000?)to low-temperature range(LT,300-600?).To achieve this goal,material selection plays a dominant role,which involves improving the conductivity and electro-catalytic behavior of existing materials or developing new materials.Great efforts have been made to develop high oxygen ion conductors through structural doping,but a big challenge is still remained about availability of materials for the LT range.Various attractive properties of perovskite oxides and their hetero-structure make them a prominent research area for the development of LT-SOFCs.However,this thesis presents both experimental and theoretical investigation on redox in active elements such as Ti and Sb doped Sr Fe O_3-?(SFT)and Ba Sr Fe O_3-?(BSFSb)perovskite and their heterostructures with Sm-doped Ce O_2-?(SDC)for both ion-conducting electrolyte membrane and oxygen reduction electro-catalysts uses in LT-SOFCs(below 600?).These prepared perovskites oxides exhibit promising performance such as low area-specific resistance(ASR)of 0.20(?)cm²and maximum power density of 738 m Wcm^-2at low temperature of 550?.Moreover,two semi-conducting-ionic hetero-structured composites(SFT-SDC&BSFSb-SDC)are designed for electrolyte membrane applications with increased ionic conductivity and low polarization resistance by incorporating Sm doped Ce O_2-?(SDC)into SFT and BSFSb,respectively.The designed heterostructure composites have exhibited a high ionic conductivity of>0.1 Scm^-1(vs 0.01 Scm^-1of SDC)at 550?,to realize feasible LTSOFCs with a remarkable power density of around 1000 m Wcm^-2,showing excellent electrolyte functionality.The crystallographic,morphological and spectroscopic characterizations are carried out to gain deep insight into the material features.Further investigation finds that hetero-phasic interfacial conduction plays a crucial role in the good ionic transport property and the device performance.It has been found interfacial energy-band reconstruction lead to electronic confinement and create a build-in electric field(BIEF)to promote the ionic conduction.The Kelvin probe force microscopy(KPFM)is used to measure the surface potentials at the interface to confirm the BIEF in the heterostructure,which assists the ionic transport effectively.In addition,X-ray photoelectron spectroscopy and density functional theory(DFT)are used to study the charge transfer mechanism and enhanced ionic conductivity in the heterostructure materials,respectively.The junction effects and energy band structure studies verified the electronic compressibility and working principle of the heterostructure materials for reliable electrolyte function.This dissertation presents the work on semiconducting perovskite and their heterostructure with a novel strategy to improve the catalytic and ionic properties of the materials and demonstrate of the optimal performance for LT-SOFCs.