Preparation And Properties Of Cathode Materials For Intermediate-Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell(SOFC)can directly convert the chemical energy stored in the fuel into electrical energy,and has the advantages of high power generation efficiency,flexible fuel selection,and pollution-free.However,the high operating temperature(800-1000℃)increases the operating cost of the SOFC system,shortens the life of the materials,and greatly reduces the possibility of its large-scale commercialization.The design and development of intermediate-temperature(600-800℃)SOFC system can solve the technical problems caused by high-temperature operation while maintaining the unique advantages of SOFC.However,due to the thermal activation characteristics of the oxygen reduction reaction(ORR),lowering the operating temperature greatly increases the polarization resistance of cathodes,causing the ORR kinetics to become very slow in the intermediate-temperature region.Therefore,the development of efficient and durable cathode materials for intermediate temperature SOFC(IT-SOFC)is the first priority.In this thesis,a surface modification strategy is used to prepare high-performance composite cathode materials and explore the relationship between the microscopic morphology of electrodes and electrochemical performance,and the method of element doping is used to synthesize new perovskite-type cathode materials and the phenomenon of its in-situ activation is studied.The microstructures of A-site deficient(La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ(LSCF)cathode are tailored by PrO2-δ/Pr0.8Ce0.2O2-δ nanoparticles through a facile impregnation process.The LSCF+PrO2-δ cathode shows an area-specific resistance(ASR)of～0.241 Ω cm2 at 600℃,which is about 67%lower than that of pristine LSCF cathode(～0.738 Ω cm2).Furthermore,anode-supported single cells with LSCF+PrO2-δ/Pr0.8Ce0.2O2-δ cathodes show excellent peak power densities(～1457.8 mW cm-2 at 700℃ and～516.7 mW cm-2 at 550 ℃)and enhanced durability(0.9 A cm-2 for～100 h).Analysis shows that PrO2-δ accelerates ORR kinetics by creating an active surface with abundant oxygen vacancies and improving ionic conductivity.The active surface promotes cations migration leading to the formation of active SrxCoyOz,which may further contribute to the improvement of ORR kinetics.It is found that the distinctive LSCF/SrxCoyOz/PrO2-δ hetero-structure plays a vital role in improving the surface stability of LSCF cathode,and the strategy that adjusting the intrinsic activity and thermal expansion behavior of catalytic coatings has also proven to be effective in enhancing the durability of electrode.A self-assembled nanocomposite is developed as oxygen reduction reaction(ORR)catalyst for IT-SOFCs through a co-precipitation method.The nanocomposite is composed of a doped(Mn0.6Mg0.4)0.8Sc0.2Co2O4 spinel oxide(84 wt%),an orthorhombic perovskite phase(11.3 wt%,the spontaneous combination of PrO2 additives and spinel),and a minor Sc2O3 phase(4.7 wt%).The surface of the spinel phase is activated by a self-assembled nano-coating with many heterogeneous interfaces.Thence,the ORR kinetics is obviously accelerated,an area-specific resistance(ASR)of～0.11 Ω cm2 is obtained at 750℃.Moreover,single cell with the cathode shows a peak power density(PPD)of 1144.1 mW cm-2 at 750 ℃,much higher than that of the cell with MnCo2O4 cathode(456.2 mW cm-2).And an enhanced stability of～120 h(0.8 A cm-2,750 ℃)is also achieved,related to the reduced thermal expansion coefficient(10.25 × 10-6 K-1).The improvement in ORR kinetics and stability can be attributed to the refinement of grains,the formation of heterointerfaces,and the enhancement of mechanical compatibility.The perovskite-type Ba(Co0.7Fe0.3)0.8Zr0.2-xScxO3-δ(x=0.08,0.1,0.12)series cathode materials are synthesized by sol-gel method,and the influence of Zr/Sc on the crystal structure and electrochemical properties of BaCo0.7Fe0.3O3-δ(BCF)hexagonal perovskite is studied.The results show that the Zr/Sc co-doped perovskite oxides are all cubic structures,and the space group is Pm-3m.Among them,Ba(Co0.7Fe0.3)0.8Zr0.1Sc0.1O3-δ(BCFZS)has the lowest polarization resistance.For example,its polarization resistances at 700 and 550℃ are 0.0513 and 0.418 Ω cm2,respectively,indicating that BCFZS can show high catalytic activity of oxygen reduction reaction(ORR)in intermediate temperature region.At the same time,the in-situ activation of BCFZS in dry air is observed.After keeping the symmetrical cell at 700℃ for about 14 hours,the polarization resistance reduces from 0.0513 to 0.0315 Ω cm2.Through SEM analysis,it is found that some nanoparticles precipitated in situ on the surface of the electrode.And the in-situ activation process of BCFZS can be regulated by changing the test atmosphere.After keeping the electrode in humid air(～3%H2O)for about 18 hours,the polarization resistance of BCFZS decreases to 0.0224 Ω cm2,and the number of nanoparticles on the surface of the electrode has increased significantly.Therefore,it can be considered that water molecules induce the surface reconstruction of BCFZS.The promoting effect of H2O has been further confirmed by continuous testing in dry/humid air.