Study On Phase Formation Process And Doping Modification Of Perovskite Type Composite Oxide Materials

Perovskite type composite oxides,as a class of strongly correlated electronic system with multiple degrees of freedom such as charge,spin,orbit,and lattice,possess diverse physical and chemical properties and functional characteristics.Among them,lanthanum perovskite oxides show promising application prospects in the field of electrocatalytic water decomposition due to its rich element reserves,stable crystal structure and variable element composition,and are considered as potential alternative materials for precious metal catalysts.However,at present,the intrinsic activity of perovskite oxides still cannot fully surpass noble metals,and it is difficult to completely replace noble metal catalysts in water splitting reactions.In order to solve this problem,it is necessary to focus on the regulation engineer of intrinsic structure of perovskite oxides and the enhancement of catalytic performance,and conduct a study on the growth and catalytic mechanism of perovskite oxide at the synthesis stage and catalytic stage,respectively,hoping to establish the structure-activity relationships of crystal structure,elemental composition and catalytic performance,and therefore,provide reasonable design ideas for the construction of perovskite-based high-efficiency hydrolysis catalysts.Therefore,this paper first analyzed the phase formation process of perovskite oxides and the regulation of the oxygen evolution performance by doping heterogeneous elements.Then,combined with a variety of intrinsic structure characterization tools and reaction mechanism detection means,a study was conducted on the evolution of crystal structure and electronic structure in the synthesis stage as well as the adsorption and desorption process of intermediates in the electrocatalytic oxygen evolution reactions.Finally,a close connection between the crystal structure and chemical composition of catalysts and their synthesis and catalytic reaction mechanism was established.The main findings of this paper are described below:（1）Study on the Phase Transformation Mechanism of Pure Phase Perovskite LaNiO₃ during Sintering ProcessPVP/PAN/Ni（NO₃）₂/La（NO₃）₃ precursor fibers were prepared by electrospinning method combined with sol-gel method,and after sintering LaNiO₃ perovskite phase was formed.The characterization results of XRD and C_s-corrected TEM confirmed the perovskite phase structure and atomic occupancy of LaNiO₃.By using XRD,XPS,EDS,EELS,and transmission electron microscopy,the effects of different sintering temperatures on the composition and morphology of nanofibers were studied.In-situ gas phase TEM experiments revealed real-time dynamic observation and studied the structural changes and crystal growth process of nanofibers under 21%O₂ atmosphere.The study indicated that at high temperature,the carbon polymer matrix was continuously decomposed during the crystal growth process.The metal ions immersed in the polymer matrix were heated and combined with oxygen in the atmosphere to form oxide crystal embryos,which showed a typical Ostwald ripening growth process.At about 600℃,the crystal phase of LaNiO₃ was generated.At higher temperature,the grains get close to each other,collided,turned,joined,and formed one-dimensional LaNiO₃ nanofibers stacked with nanoparticles.The experiment revealed the growth mechanism of LaNiO₃ nanofibers,providing instructions for the growth regulation and performance modification of materials.（2）Study on the intrinsic structure and electrocatalytic oxygen evolution performance of Nb doped perovskite LaNiO₃ nanofibersNb doped LaNiO₃ nanofibers La Ni_1-xNb_xO₃ were prepared by electrospinning method.XRD and TEM analysis showed that with the doping of Nb at B site,many defects and the Ni O second phase were generated in the La Ni_1-xNb_xO₃ crystal structure.Combined with electrochemical testing,in-situ Raman spectroscopy,in-situ infrared spectroscopy,and in-situ mass spectrometry,the effect of doping on the electrochemical activity of the material was tested.It was shown that doping enhanced the alkaline OER activity of the LaNiO₃,and the adsorbate evolution mechanism（AEM）of La Ni_1-xNb_xO₃dominated in the electrocatalytic process.Based on the analysis of the macroscopic and microscopic electronic valence states,it is proved that doping caused the increase of defect oxygen content and complex energy level splitting near the nickel rich phase.This intermediate energy level promoted the combination of the material and the adsorbed material,and improves the electrocatalytic performance.（3）Study on the synergistic regulation of intrinsic structure and electrocatalytic oxygen evolution performance of LaNiO₃ by Nb and Ir co-dopingIr doped LaNiO₃ nanofibers and Nb,Ir co-doped LaNiO₃ nanofibers were prepared by electrospinning method.The XRD diffraction results of Nb and Ir co-doped LaNiO₃nanofibers showed significant differences compared to single element doping,which peak splittings near diffraction peaks were observed.The atomic structure,element and valence distribution of the materials were further characterized by C_s-corrected TEM,and the structure and performance differences of the co-doped materials were analyzed.Studies indicated that the splitting of diffraction peaks in co-doped LaNiO₃ materials originated from the electronic structure changes introduced by the co-doping of Nb and Ir elements,resulting in the generation of a large number of layered Ruddlesden-Popper defects.At the same time,co-doping caused Nb and Ir elements to occupy the B and A positions in the perovskite structure,respectively.The electrochemical test results showed that co-doped LaNiO₃ nanofibers had an OER overpotential of 360 m V,and the electrocatalytic performance was significantly improved compared to single doping.The results indicated that co-doping led to different structural characteristics compared to single doping.Co-doping introduced different valence metal cations at the A and B sites,leading to the generation of structural and ion defects,causing more oxygen vacancies,providing more active reaction sites,affecting the bonding of B-O bonds,and promoting the migration of intermediate products during the catalytic process.Co-doping improved the electrocatalytic activity of the materials,providing guides for the design and improvement of perovskite type composite oxide materials.