Study On Oxygen Reduction Reaction Activity Of SrCoO_3-? Based Epitaxial Thin Film Electrodes

Solid oxide fuel cells?SOFCs?represent a clean,efficient,and universal chemical energy-electric energy conversion technology.Reducing its operating temperature to intermediate range?650-850??or even lower temperature range?400-650??has a good application prospect.However,the main obstacle to lowering the operating temperature is the slow kinetics of the oxygen reduction reaction?ORR?on the cathode side at lower temperatures,and therefore it is important to explore new cathode materials with good ORR activity.Recently,mixed ion and electron conductor?MIEC?cathodes are widely studied for their good ionic and electronic conductivities.Among them,SrCoO_3-??SC?based cathode is one of the current research hotspots as a MIEC material.In this thesis,the intermediate and low temperature range SOFC cathode materials with good performance are studied in two ways.The first one is tuning the role of doping in ORR activity.The second one is to understand the complex relationship between crystallographic orientation and ORR activity.Both two investigations utilize epitaxial thin film electrodes as the model system.The specific research contents and results are as follows:1.The La_1-xSr_xCoO_3-??from x=0 to x=0.8?thin films were investigated as the electron conductivity and oxygen reduction activity of a medium and low temperature solid oxide fuel cell?SOFC?cathodes.Thin film materials can avoid external interference caused by microstructure and crystal orientation,and better explain the complex relationship between chemical composition,electronic conductivity and ORR activity.With the change of Sr doping concentration,the electronic conductivity and polarization resistance were observed to change.At the Sr doping concentration of x=0.4,the thin film electrode had the best electronic conductivity and polarization resistance.The change in Co³⁺/Co⁴⁺explained that the electronic conductivity and oxygen reduction activity improved with increasing Sr doping concentration?from x=0 to x=0.4?.With the further increase of Sr doping concentration,the doping condition exceeding 0.4 favors the increase of oxygen vacancies,which can be proved by the existence of Co²⁺which balances the charge of the system.Therefore,Co³⁺/Co⁴⁺is no longer the best choice for surface oxygen reduction.The increased oxygen vacancy concentration provides more oxygen ion exchange sites for the surface oxygen reduction reaction process,but at the same time,the electron conductivity is deteriorated,thereby lowering the charge transfer efficiency of the oxygen reduction process.Although the concentration of oxygen vacancies is enhanced,the highly doped LSCO thin films,the non-optimal Co³⁺/Co⁴⁺valence state and poor electronic conductivity synergistically cause a decrease in oxygen reduction activity.2.Design and preparation of novel doped SrCoO_3-?based materials with different orientation epitaxial films.Novel doped SrCoO_3-?-based materials,the design and preparation of epitaxial thin films with different orientations were studied.The possibility of using Sr₂Sc_0.1Nb_0.1Co_1.5Fe_0.3O6-??SSNCF?epitaxial thin film as SOFC low temperature cathode was studied.The SSNCF thin film has similar performance to the La_0.6Sr_0.4Co_0.8Fe_0.2O_3-?cathode film with good performance at low temperature.For SSNCF thin films with different crystal orientations,SSNCF/YSZ grown in the[110]direction exhibits superior ORR activity compared to SSNCF/SDC/YSZ thin film electrodes grown in the[100]direction.By surface element analysis,Sr surface enrichment and lower Co ion oxidation state are detrimental to ORR performance.The epitaxial thin film materials involved in this research have the characteristics of good crystallinity,high surface flatness and good repeatability,which are beneficial to the elimination of the influence of uncontrollable microstructure during the comparison of material properties.In the first part,the effects of Sr doping on the oxygen reduction activity and the surface structure of the Sr-doped?LSCO?cathode film were studied.The second part of the work explored the preparation method of the novel?SSNCF?cathode epitaxial film and studied its anisotropy of performance.The performance comparison between cathode materials has certain reference value.