Preparation And Electrochemical Performance Of Perovskite Oxides With Tuned Microstructures

The worldwide energy crisis and environmental issues have greatly driven researchers to further explore and efficiently utilize renewable energy sources as alternatives to traditional fossil fuels.Meanwhile,integrating intermittent renewable energy sources with advanced energy conversion and storage devices,such as water electrolyzers,metal-air batteries and fuel cells,could make them more dispatchable and efficient.Since the associated oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）largely determine the electrochemical performance of promising rechargeable metal-air batteries and regenerative fuel cells,it is urgent to exploit highly efficient and durable oxygen electrocatalysts for promoting the sluggish kinetics of ORR and OER.Precious metal-based catalysts（such as Pt/C and Ir O₂）are highly active but their prohibitive cost,scarcity and low stability significantly restrict the large-scale application.As a promising alternative,perovskite oxides with the adoption of low-cost and earth-abundant elements show excellent electrocatalytic performance and relatively high durability,thanks to the unique and stable crystal structure.However,perovskite electrocatalysts always suffer from the limited specific surface area and some of them exhibit intrinsically poor electrical conductivity at room temperature,which limit their catalytic activity.In this thesis,morphology engineering and in-situ hybridization with conductive carbon have been applied to optimize the perovskite oxides as highly efficient oxygen electrocatalysts.LaMnO_3+δhas been reported to have the superior intrinsic ORR activity thanks to the e_g filling of～1.However,traditionally prepared LaMnO_3+δis characterized to exhibit a low specific surface area and a limited pore volume,which result in low mass activity.Herein,by applying polymethyl methacrylate（PMMA）as a template,we synthesize a three-dimensionally ordered macroporous LaMnO_3+δthat features ordered and interconnected porous structure,in order to increase catalytic sites.The obtained three-dimensional ordered macroporous LaMnO_3+δexhibits an increased specific surface area of 20.328 m² g^-1 and pore volume of 0.126 cm³ g^-1.Rotating-ring-disk electrode measurement reveals more positive onset potential（0.827 V）and half-wave potential（0.686 V）,and a much higher limited-current density(5.90 m A cm^-2)of the three-dimensionally ordered macroporous LaMnO_3+δcompared to counterparts（Bulk-LM and PEG-LM）,as well as a high electron transfer number（～4）and a better stability.Furthermore,a Li-O₂ battery employing the three-dimensionally ordered macroporous LaMnO_3+δas air electrode exhibits excellent electrochemical performance with a higher initial discharge capacity,a smaller discharge-charge voltage gap,and a higher charge efficiency in comparison with the carbon electrode.Our results suggest that traditional perovskite oxides could be effectively optimized for efficient electrocatalytic reactions.Additionally,we applied a novel solid-liquid co-electrospinning approach to synthesize a hybrid of LaCoO₃ perovskite nanoparticles@nitrogen-doped carbon nanofibers（LCNP@NCNF）as an efficient bifunctional electrocatalyst for ORR and OER.LCNP@NCNF features an integrated structure with well-crystallized LaCoO₃perovskite nanoparticles uniformly distributed in the mesoporous NCNF.In addition,LCNP@NCNF exhibited a large pore volume of～0.164 m³ g^-1 and a high specific surface area of～183.3 m² g^-1.The rotating-electrode measurement revealed the better intrinsic activity and more favorable stability of LCNP@NCNF in comparison with counterparts.Moreover,Zn-air batteries employing LCNP@NCNF as air electrode exhibited excellent electrochemical performance with a smaller discharge-charge voltage gap of～0.95 V and prolonged cycling stability compared with the battery adopting the physically blended LCNP/NCNF.We ascribed the improved electrochemical activity to the enhanced synergistic interaction,originated from the homogeneous coupling of LCNP and NCNF.