Study Of The Performance Of Lanthanum-strontium-Iron Electrode For Reversible Solid Oxide Cells

Reversible Solid Oxide Cell is an environment-friendly and efficient electrochemical energy conversion device with all-solid-state structure.It can be used not only as solid oxide fuel cell?SOFC?to convert chemical energy in fuels and oxidants to electrical energy without burning,but also as solid oxide electrolysis cell?SOEC?to transform excess power into chemical energy for storage.The oxygen electrode of solid oxide fuel cell?SOFC?and solid oxide electrolysis cell?SOEC?is the place where the oxygen reduction reaction or the oxygen evolution reaction?OER?occurs.The problems of electrochemical activity and stability of the oxygen electrode has been identified as core issues that limit the development and application of SOFC/SOEC devices.Therefore,the synthesis,characterization,properties and degradation mechanism of oxygen electrode materials have been the hot issues in the history of RSOC researches for many years.For example,it's difficult to use conventional synthesis methods such as solid-state reaction and sol-gel to meet the requirements of low-cost,large-scale synthesis of nano-scale perovskite materials.Besides,Co-based perovskite electrode materials exhibit much higher coefficient of thermal expansion?CTE?that do not match that of the electrolytes.In the high temperature processes of preparation and testing of oxygen electrodes,Sr-doped perovskite electrodes will be confronted with the Sr surface segregation,which is associated to the stability of the materials and the cells.To develop stable perovskite oxygen electrode materials with excellent performance and stability,the dissertation focus on the simple cobalt-free material La_0.6Sr_0.4FeO_3-??LSF?system.A series of experimental researches has been carried out in including optimization of synthesis method and process,material design and element doping,characterization of materials,preparation and electrochemical performance testing of RSOC devices.The main research contents and results are listed as follows:?1?A new molten salt method,combining the advantages of solid-state method and liquid-phase method,was introduced to prepare LSF oxygen electrode materials in the dissertation.Nanoscale LSF powders with novel nano-bowl microstructure,single perovskite phase and super high performance were obtained.The processes of synthesis of LSF via molten salt method and sol-gel method were compared.It is concluded that the molten salt corrosion and water washing procedures are the main factors to generate the holes on LSF particles.In addition,the X-ray photoelectron spectroscopy?XPS?characterization technology proves that the average valence of Fe element in LSF synthesized by molten salt method is lowered,with increased oxygen vacancies,which will improve the electrochemical performance of LSF electrode.The possible reason for this phenomenon may be attribute to that the molten salt has poor dissolved oxygen.The X-ray absorption fine structure?XAFS?characterization also shows that the Fe-O coordination number of LSF synthesized by molten salt method is relatively low.At last,the LSF material synthesized by molten salt method was used as the oxygen electrodes of RSOC and tested.The results show that the maximum power density at800^oC reaches 1.73Wcm^-2 in SOFC mode,⁷0%higher than that of LSF electrode prepared by sol-gel method.Besides,when used for high temperature CO₂ electrolysis,the current density achieves 1.5Acm^-2 at 800 ^oC and 1.3V.?2?Since Nb₂O₅ is difficult to be dissolved in common solvents,such as water,acid,alkali and alcohols,it is hard to be used as raw material for sol-gel process.When solid phase method is chosen to doped Nb in the perovskite oxides,higher sintering temperature and longer sintering time are often needed.Therefore,we also use the molten salt to form a“solution”for Nb₂O₅ and other reactants and synthesize La0.6Sr0.4Fe0.9Nb0.1O3-??LSFN?and La0.6Sr0.4Co0.2Fe0.75Nb0.05O3-??LSCFN?at a temperature as low as 850°C.And LSFN and LSCFN are used as electrodes of hydrogen electrode-supported button RSOC,symmetric SOFC,and full-size RSOC,all of which have achieved excellent electrochemical performance.?3?In order to avoid the sintering of LSF and LSFN materials synthesized by molten salt method during the preparation of oxygen electrodes,porous LSF and LSFN powders were used to directly assemble the RSOC oxygen electrodes.The oxygen electrode/electrolyte interface was generated by the discharge polarization process,and the electrochemical performance of the LSF and LSFN butoon cells were tested.The maximum output power density of the direct assembly porous LSF cell is 1.36Wcm^-2at 800°C in SOFC mode,and the electrolysis current density reaches 1.5Acm^-2 at 1.3V and 800°C.As for the directly assembled LSFN cell,the maximum power density is0.68 Wcm^-2 at 800°C.By comparing the polarization curves and electrochemical impedance spectra,we find that the high temperature sintering reduces the ohmic polarization by improving the contact impedance between oxygen electrode and electrolyte.The output performance of directly assembled LSF batteries at 650°C is nearly equal to that of pre-sintered LSF cell,so the directly assembled porous LSF electrode is more suitable to operate at low temperature.?4?To evaluate the stability of Sr-doped perovskite oxygen electrode materials,it is generally required to manufacture the electrodes and treat them under high-temperature with a long time,following by expensive surface analysis.In the last part of this paper,we propose an effective method to study the local atomic structure information of the Sr atom in the pristine materials by using the synchrotron-based XAFS technique.And the relationship between the local structure and the chemical stability of the material is established.Then it's easy to predict the long-term stability of the material before manufacturing the electrode of the cell.The local structure environments of Sr atoms in La_0.6Sr_0.4MnO_3-??LSM?and La_0.6Sr_0.4FeO_3-??LSF?materials synthesized by sol-gel method are studied by XAFS,while the stability of LSM and LSF is characterized under high temperature polarization process via in-situ electrochemical impedance spectroscopy and Cr poisoning experiments.The results show that the stability of Sr atoms in LSM is stronger than that in LSF,and the resistance increase of LSM is lower than LSF under polarization conditions.And the Cr deposition on the surface of LSM is also less severe than that of LSF,indicating that the stability of LSM is higher than that of LSF.