High-Performance Semiconductor Electrolytes Designed By Energy Band Alignment For Low-Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell（SOFC）is an electrochemical energy conversion technology with the functionality of directly converting chemical energy into electricity with high efficiency,low pollution emission and high fuel flexibility,which can fulfill the need of energy at large scale.However,the current development of SOFC is subject to the technological complexity and high-cost issues caused by its high operating temperature（800-1000°C）,which results in a slow progress of SOFC towards commercialization.To face this challenge,the operational temperature of SOFC should be reduced to 400-600°C.In such low-temperature range,the low ionic conductivity of electrolyte will introduce high polarization resistance,leading to more electrochemical loss to the cell.Therefore,developing new electrolyte materials with high ionic conductivity at low temperatures is the key issue to be addressed to develop low-temperature SOFC（LT-SOFC）.To achieve this goal,a lot of efforts have been adopted to develop high oxide ions conductor for LT electrolyte uses,among which,the semiconductor ionic materials with versatile properties have shown great promise to realize good electrolyte functionality.In this regard,this thesis represents an approach of energy band alignment to develop new electrolytes in terms of heterostructure formation and ion doping.Two electrolytes are prepared via heterostructure based on semiconductor Li Ni_0.8Co_0.15Al_0.05O₂（LNCA）and Sm_0.2Ce_0.8O_2-δ（SDC）,and semiconductor Sr Co_0.3Sn_0.7O_3-δand Ce O₂,where both samples are designed via tuning the energy band alignment to realize the charge separation and fast ionic transport.In addition,a perovskite semiconductor Ba_0.5Sr_0.5Zr_0.9Y_0.1O_3-δis tailored by doping to enable hybrid ions(H⁺/O^2-)conducting property for LT-SOFCs（≤600°C）application while considering the energy band alignment of the device.In the first part of the thesis,the incorporation of ionic conductor into semiconductors LNCA and Sr Co Sn O_3-δto design two heterostructure electrolytes displays promising ionic conductivity>0.1 S cm^-1,comparatively(vs 0.01 S cm^-1 of SDC)at 520°C,demonstrating the considerable electrolytic functionality.The peak power density of the SOFC device based on 3D-structured LNCA-SDC heterostructure is as high as 0.735 W cm^-2 at 520°C,while that of the cell based on bi-layer architecture Sr Co Sn O_3-δ-Ce O₂ also show high power output of 0.672 W cm^-2 at 520°C.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.More importantly,the formation of various types of junctions and energy band alignment has obvious impact on the electronic conduction suppression and ions conduction enhancement,which plays a key role in the successful demonstration of two electrolytes in LT-SOFCs.In the second part of the thesis,the hybrid ions(H⁺/O^2-)conduction of semiconductor Ba_0.5Sr_0.5Zr_0.9Y_0.1O_3-δis tailored by doping Gd(Ba_0.5Sr_0.5Zr_0.9-xGd_xY_0.1O_3-δ,x=0,0.05,0.1)to improve the electrolytic functionality in LT-SOFCs.Initially,the materials were investigated by TEM and XPS analysis;it is found the prepared samples with different compositions show homogenous particle distribution and enriched concentration of surface oxygen vacancies as compared to Ba_0.5Sr_0.5Zr_0.9Y_0.1O_3-δ.Further electrical and electrochemical measurements are performed to show that the Gd doping enhances the ionic conductivity and fuel cell power output significantly of Ba_0.5Sr_0.5Zr_0.9Y_0.1O_3-δ,achieving high ionic conductivity of 0.17 S cm^-1 and SOFC power density of 0.805 W cm^-2 at 520°C.This suggests that appropriate Gd doping have caused the generation of massive oxygen vacancies in the lattice of Ba_0.5Sr_0.5Zr_0.9Y_0.1O_3-δto enable the high ionic conductivity.Moreover,the rectification behaviour of Schottky junction（SJ）is verified in terms of I-V characteristic and energy band study,which is found to be able of suppressing the electron transport from anode to electrolyte and promoting the hybrid ions conduction.Through energy band alignment,the above studies have constructed the built-in electric field that capable of modulating ionic and electronic conduction in semiconductor electrolytes and the corresponding SOFCs,achieving improves ionic conductivity and relatively high fuel cell performance,which provides a completely new idea to develop high-performance semiconductor electrolytes for LT-SOFCs.