Study On The Reaction Processes In （La,Sr）（Co,Fe）O_3–δ-Based Composite Oxygen Electrodes For Solid Oxide Cells

Solid oxide cell（SOC）is a kind of energy conversion device with high efficiency,which can operate in solid oxide fuel cell（SOFC）mode or solid oxide electrolysis cell（SOEC）mode.Mixed ionic-electronic conductors（MIEC）,such as familiar La_0.6Sr_0.4Co_0.2Fe_0.8O₃–δ（LSCF）,are applied as oxygen electrodes for SOCs.However,at low-medium temperature under practical condtion,pure LSCF shows low activity and CO₂ and H₂O degradation under partical operation.In recent years,composite electrode has became a hotspot because its performance and stability can be flexibly midified by combining materials with different properties and adjusting structure and morphology of different constituents.However,the oxygen reaction in composite electrode is more complex than pure electrode.For designing and optimizing composite electrodes and exploiting targeted cell fulfilling specific requirements,it is critical to understand the pathways of reactions and the kinetics of related steps.In this thesis,based on the most common and commercial LSCF,oxygen reactions of LSCF-Ce_0.8Gd_0.2O_1.9（LSCF-GDC）and LSCF-Ba Ce_0.7Zr_0.1Gd_0.2O3-δ（LSCF-BCZG）porous composite electrodes were systematically studied,and effects of CO₂ and H₂O on the performance and oxygen reaction were evaluated.In addtion,LSCF-GDC thin-film composite electrodes with different structure and morphology were fabricated.The function of ionic conductor and the oxygen reaction mechanism in composite electrode are explained by the combining research results of porous and thin-film LSCF-GDC electrodes.Finally,cells respectively with LSCF,LSCF-GDC and LSCF-BCZG as oxygen electrodes were prepared,and performances of these cells under practical working condition are investigated.Studying results are as follow:In Chapter 2,pathways and kinetics of oxygen reaction in LSCF and LSCF-GDC porous electrodes are studied systematically.It is found that LSCF behaves as a MIEC at high temperature with low oxygen partial pressure,so that oxygen exchange takes place at the electrode/air interface,and total oxygen reaction proceeds in the reaction region of the electrode;at low temperature with high oxygen partial pressure,LSCF behaves as a pure electronic conductor,so that oxygen exchange takes place at three-phase boundary（TPB）formed between electrode,electrolyte and air,and total oxygen reaction proceeds around electrode/electrolyte interface.GDC can improve the rate of oxygen reaction by accelerate O^2–transport and O₂ diffusion.In LSCF-GDC,oxygen raction tend to proceed in the pathway with oxygen exchange at TPB because the GDC enlarges TPB length.In chapter 3,by the fabrication LSCF-GDC thin-film electrodes with different structure and morphology,the function of GDC in the composite electrode is further revealed.For solid-phase-mixing LSCF-GDC thin-film electrode,the GDC in the thin-film can accelerate O^2–transfer,but the GDC exposed on the surface hinders oxygen exchange.For surface-coating LSCF-GDC thin-film electrodes,GDC nanoparticles coating on the surface of LSCF,which neither catalyzes nor hinders oxygen exchange.However,but with GDC increasing,nanoparticles transform to discontinuous thin-film,which seriously hinders oxygen exchange.In Chapter 4,effects of CO₂ and H₂O on the oxygen reaction are studied by LSCF and LSCF-GDC thin-film electrodes.CO₂ and H₂O in air mainly affect the oxygen exchange proceeding on electrode surface.For LSCF thin-film electrode,both CO₂ and H₂O can suppress oxygen exchange on LSCF thin-film electrode.The effect of H₂O is more significant than CO₂,and CO₂ degradation can be aggravated by H₂O in wet air.Both solid-phasing-mixing and surface-coating LSCF-GDC thin-film electrodes show favorable CO₂ tolerance.Thereinto,GDC nanoparticles significantly improve CO₂ and H₂O tolerance of composite electrode while ensures their favorable performance.In Chapter 5,the performance and oxygen reaction in LSCF-BCZG porous electrode are studied.By comparing LSCF-GDC and LSCF-BCZG,and the relationships between composite properties,oxygen reaction and tolerance are analyzed.It is found that the improvement by BCZG is more significant than by GDC.Comparing with GDC,BCZG with better ionic conductivity accelerate O^2–transport in electrode more effectively.However,BCZG tend to grow and aggregate,so the promotion on O₂ diffusion by BCZG is less significant than GDC.In air with CO₂ and H₂O,LSCF-GDC shows poor CO₂ tolerance but favourable H₂O tolerance.In chapter 6,performances of cells respectively with LSCF,LSCF-GDC and LSCF-BCZG oxygen electrodes are studied under partical working condition,and effects of CO₂ and H₂O on oxygen reaction under charging are analyzed.Cells’performance can be improved by GDC or BCZG in composite oxygen electrodes,and the improvement on electrolysis cell performance is more significant than fuel cell performance.In addition,cell with LSCF-BCZG oxygen electrode shows higher performance than with LSCF-GDC.Cells’performance is degraded by CO₂ in air.In wet air,H₂O can significantly improve the performance of cell with LSCF-BCZG oxygen electrode and alleviates the effect of CO₂ on its performance.By investigating and comparing the oxygen reaction in composite LSCF-based porous and thin-film electrodes,the linkage between thin-film elecrodes and porous electrodes has been built.On this basis,effects of intrinsic factors including electrode compositions,structure and morphology,and extrinsic factors including temperature and surrounding atmosphere on oxygen reaction are explained in details.This research provides guidance for composite electrodes selection,designing and optimization.