Design And Characterization Of PrBaCo₂O_5+δ Based Air Electrodes For Solid Oxide Cells

With the rapid development of industrialization and population growth,the consumption of fossil energy increases exponentially and the environment is deteriorating.The development and utilization of sustainable energy becomes the top priority of energy development.Solid oxide cells(SOCs)is an efficient and clean energy conversion device,which can switch freely between electric energy and chemical energy.According to the diversity for current carriers in electrolytes,SOCs can be divided into oxygen ion-conducting(O-SOCs)and proton-conducting(P-SOCs).Conventional O-SOCs operate at high temperatures(800～1000℃)which are demanding in the choice of stack materials and sealing.P-SOCs are more suitable for intermediate and low temperatures(450-700℃)because of the lower activation energy for proton migration.As for Ni-based P-SOCs,the polarization impedance of cells is mainly derived from the oxygen exchange reaction related to protons in the air electrodes,including proton-related oxygen reduction reaction(p-ORR)and protonrelated oxygen evaluation reaction(p-OER).Therefore,it is of great significance to develop air electrodes materials with high activity and durability to improve the performance of P-SOCs.For purpose of improving the kinetics of proton-related oxygen reduction reactions(p-ORR),proton-oxygen ion-electron(H+/O2-/e-)triple-conducting oxides have been developed,which can extend the reaction region from the restricted triple-phase boundary(TPB)to the entire surface of air electrodes.The composition optimization and structural modification of electrodes by doping,composite electrodes,infiltration and other methods is mature and many achievements have been made.However,the properties of materials developed by these methods are limited.In recent years,the methods of surface exsolution and self-assembly for the construction of composite materials with heterostructure have attracted extensive attention.On this basis,this thesis proposes to make exsolution of proton-conducting oxides(PCOs)on the surface of traditional mixed oxygen ionic-electronic conductors(MIECs),and to construct MIEC-PCO heterogeneous interface via self-assembly technique to expand TPB and improve the proton conductivity,structural stability,catalytic activity and durability of PrBaCo2O5+δ(PBC).This thesis mainly includes the following three chapters:In chapter 1,the characteristics,structural composition,working principle and critical materials of SOCs are introduced firstly.Then,the design and modification strategies of air electrodes materials are summarized emphatically.Finally,the research basis and content of this thesis are presented.In chapter 2,four widely used MIECs were used as the parent materials.Zr was partially substituted in B-site,and the target powders were prepared by combustion method.Characterization and analysis methods were used to explore the feasibility and universality of the construction strategy of MIEC-PCO heterogeneous interface.The crystal structure and microstructure of PrBaCo2-xZrxO5+δ(PBCZx,x=0,0.04,0.08,0.12)powders showed that:(1)the oxide self-assembled into B-site deficient perovskite PrBa1-xCo2-xO5+δ(D-PBC,x=0,0.04,0.08,0.12)substrate and proton-conducting BaZrO3(BZO)based perovskite oxide calcined at high temperature in air atmosphere.The BZO based nanoparticles(NPs)were anchored to the surface of substrate;(2)Zr substitution was beneficial to the exsolution of BZO based NPs from the parent oxide under oxidation atmosphere;(3)the exsolution of BZO based NPs was conducive to protonation of D-PBC substrate surface,which effectively constructed triple conductive regions.In chapter 3,the thermal expansion behavior,phase structural stability,oxygen desorption behavior and electrochemical performance of PBCZx were further studied.Investigation results showed that the heterogeneous structure enhanced stability,catalytic activity of oxygen reduction,electrochemical performance and durability of cells,and reduced the thermal expansion rate.When Zr doping content was 8 mol.%,the peak power densities of single cells with PBCZ08 air electrode were 1453 mW cm2 at 700℃,which was 76%higher than those with PBC air electrode(826 mW cm-2).At 700℃,the polarization impedance(0.04 Ω cm2)of single cells with PBCZ08 air electrode was 69%lower than those with PBC air electrode(0.13 Ω cm2).In addition,single cells with PBCZ08 air electrode could stably operate at 600℃ for 140 h.However,under the same operating conditions,single cells with PBC air electrode showed an apparent degradation starting at 20 h.In chapter 4,research contents in this thesis are summarized,and prospect of further research are proposed.