The Structure And Performance Of Doped Barium Cerate Electrolyte For Proton-Conductive Solid Oxide Fuel Cells

Solid oxide fuel cells（SOFCs）are green energy device with high energy conversion efficiency and low pollution emission.Traditional SOFC with oxygen-ion conductor usually works in the temperature range of 800～1000℃.The high working temperature leads to high costs for material preparation and device maintenance.While SOFC with Ba Ce O₃-Ba Zr O₃ based electrolyte are able to work at lower temperature,thereby avoiding a series of problems caused by high temperature,since this electrolyte exhibits relative high proton conductivity at lower temperatures.The Ba Ce O₃ component in Ba Ce O₃-Ba Zr O₃ based electrolyte provides high proton conductivity,but its chemical stability is poor;Ba Zr O₃ component has good chemical stability but its conductivity is low;on the other hand,Ba Zr O₃ makes the electrolyte ask for high sintering temperature due to its awful sinterability.In general,the sinterability and proton conductivity of Ba Ce O₃-Ba Zr O₃-based electrolyte still needs to be further improved.In this work,Ba Ce_0.6Zr_0.2Y_0.2O_3-δ（BCZY）electrolyte was studied.Firstly,the effects of nonstoichiometry of A-site element in this perovskite electrlyte,which is Ba,on the sinterability and conductivity were studied.Secondly,the effects of two types of sintering aids,transition metal oxide Ni O and alkali metal oxide Li₂O,on the above properties of the electrolyte were studied.The thesis focuses on crystal structure,defect chemistry,sintering kinetics,and proton conductivity of BCZY electrolyte.The research consists of the following parts:In the first part of the study,the A-site stoichiometric ratio of BCZY electrolyte with ABO₃ perovskite structure,which means the relative content of Ba ion,was adjusted between 90 mol%and 110 mol%.The following experimental results were found.（1）All of the BCZY electrolyte had cubic perovskite structures.With the increase of Ba content,the lattice parameters increased gradually.（2）The sintering shrinkage of BCZY electrolyte was 2.75%at 1300℃;The sintering shrinkage rate of BCZY electrolyte material is significantly reduced with the absence of Ba;at the same time,the conductivity of the electrolyte decreased obviously;Due to the absence of 5 mol%Ba ion,the conductivity of BCZY at 650℃decreased from 0.00350 S·cm^-1 to 0.00204 S·cm^-1.（3）Excess Ba increased the sinterability of BCZY.When the stoichiometric ratio of Ba was105 mol%and 110 mol%,the sintering shrinkage of the electrolyte at 1300℃increased from 2.75%to 4.9%and 9.8%,respectively;Correspondingly,the relative density of the electrolyte after sintering at 1600℃increased from 91.45%to 94.05%and 92.74%.（4）Appropriate excess of Ba was beneficial to electrical performance.When the stoichiometric ratio of Ba was 105 mol%,the conductivity increased to 0.00427 S·cm-1,while the stoichiometric ratio of Ba was 110 mol%,the conductivity dropped to 0.00203S·cm^-1.In the second part of this work,the effects of Ni O as sintering aid on the phase structure,sinterability and conductivity of BCZY electrolyte materials were studied.（1）0.01～3 mol%Ni O was able to dissolve into the BCZY electrolyte lattice during the sintering process,and there was no evidence showed that the addition of Ni O exceeded the solid solution limit.（2）The sintering temperature for densifying BCZY electrolyte was reduced from 1600℃to 1300℃due to the addition of Ni O sintering aid.The relative density of BCZY electrolyte after sintering at 1600℃for 10 h was 91.45%,while the relative density of BCZY-3Ni sintered at 1300℃for 10 h was 96.82%.（3）The conductivity of BCZY with 3 mol%Ni O increased from 0.00435 S·cm^-1 to 0.00656 S·cm^-1 at 650℃.In the third part of this research,the effects of alkali metal oxide Li₂O as sintering aid on the phase structure,sinterability and conductivity of BCZY electrolyte were studied.It was proved that（1）The addition of Li₂O didn’t change the cubic perovskite structure of BCZY electrolyte.In the experimental process,Li NO₃ was used as the precursor which was added into BCZY electrolyte.Within the doping range of 0～3 mol%of Li NO₃,the content of Li₂O in the sintering system didn’t excess its solid solution limit in the BCZY lattice;（2）Li₂O can effectively improve the sintering kinetic behavior of BCZY electrolyte materials.The sintering shrinkage of BCZY electrolyte at 1300℃increased from 2.75%to 10.37%.Correspondingly,BCZY electrolyte material can be densified at 1300℃,and the relative densities of the BCZY-1Li electrolyte after sintering at 1300℃and 1400℃for 10 h reached 94.25%and 96.12%,respectively.（3）The addition of Li₂O also improved the conductivity of BCZY electrolyte.At 650℃,the conductivity of BCZY-3Li electrolyte sintered at 1300℃increased from 0.00435 S·cm^-1 to 0.00645 S·cm^-1.This thesis showed that in BCZY electrolyt,the absence of A-site element,Ba,could lead to the decline of the sinterability and electrical performance of the electrolyte.The excessive of A-site element,Ba,could effectively improve the sinterability of the BCZY electrolyte.When the excess of Ba ions are 5 mol%,the conductivity of BCZY was improved.The sintering aids of transition metal oxide Ni O and alkali metal oxide Li₂O could effectively improve the sinterability of BCZY electrolyte,lower the sintering temperature needed,and improve the conductivity.In summary,this thesis explored methods to improve the sinterability and conductivity of Ba Ce O₃-Ba Zr O₃-based electrolytes from the views of chemical composition,crystal structure and sintering aids of the electrolyte.It provided overall experimental references for the application of this kind of proton conductive electrolyte in SOFCs.