The Study Of Designed Transition Metal Catalyst And Oxygen Electrocatalysis Structure-activity Relationship

To alleviate the energy crisis and environmental contamination,developing the clean and highly efficient energy storage and conversion technologies to replace the traditional energy technologies based on fossil fuel is crucial.The environmental-friendly energy technologies,such as metal-air batteries,fuel cells and electrochemical water splitting,depend on the central oxygen electrocatalysis.Take the metal-air batteries as an example,oxygen evolution reaction（OER）occurs on charge process and oxygen reduction reaction（ORR）happens in discharge process.However,the kinetics of OER and ORR is sluggish,which is caused by the complex multi-electron process.To reduce activation energy and increase energy efficiency,the key is the development of electrocatalysts with high efficiency.Presently,noble-metal materials have been identified as the best catalysts for OER（Ir O₂,Ru O₂）and ORR（Pt/C）.Nevertheless,the noble-metal catalysts can’t be able to meet the requirement of industrialization development of the mentioned energy technologies,because of their serious scarcity,unacceptable cost and inferior stability.As a consequence,developing the low-cost and efficient catalysts with good stability is crucial to achieve the commercial application of the sustainable energy storage and conversion technologies.Among all known noble-metal-free catalysts,transition metal（Fe,Co,Ni et al）materials have attached much attention due to the abundant resources,regulable electronic structure and good electrochemical activity.Through the deep analysis of relative reports,it can be found that the performance of transition-metal-based catalysts is closely related to the surface state and electronic structure.In this study,multiple electrocatalysts,which are originated from transition metal materials with coordination polyhedron of octahedron,are designed and synthesized via the strategies of cation/anion engineering and interface engineering.To optimize the the surface structure of materials and electronic structure of transition metal elements can effectively improve the electrochemical performance of catalysts.Moreover,the relationship of structure-activity was discussed.The main research contents are as follows:1.A series of Ni doped Fe-based perovskite oxides(Pr_0.5Ba_0.5Fe_1-xNi_xO_3-δ,x=0,0.01,0.025,0.05)were synthesized via the cation strategy.And the impact mechanism of the amount of Ni doping on OER activity was investigated.According to the results of X-ray photoelectron spectroscopy（XPS）,we find that Ni doping can not only induce the formation of oxygen vacancies,but also tailor the electronic structure of Fe.Moreover,the result of fourier transform infrared（FT-IR）spectra indicates that introducing Ni can promote the hydrophilicity of Pr_0.5Ba_0.5Fe O_3-δ（PBF）,which is in favor of accelerating the adsorption of OH^-.Because of the increased oxygen vacancies,optimized electronic structure of Fe together with strengthened OH^-adsorption,Pr_0.5Ba_0.5Fe_0.975Ni_0.025O_3-δ（PBFN2.5）processes the overpotential of 440 m V at 10 m A cm^-2,which is lower than original PBF and commercial Ir O₂.Besides,the durability of PBFN2.5 is superior to that of PBF.This study proves that Ni doping is an effective method for developing the Fe-based perovskites with highly catalytic activity.2.Developing the anion strategy to prepare the highly active catalysts.The series of F doped LiCoO₂(LiCoO_2-xF_x,x=0,0.1,0.2,0.4)catalysts were synthesized via sol-gel method.And the relationship between OER/ORR activity and the amounts of F dopants was systematacially investigated.It can be seen that proper F doping to partially substitute O can result in the formation of oxygen vacancies,thereby improving catalytic activity of LiCoO₂（LCO）.In additional,partial Co³⁺is converted to Co²⁺,which is caused by F doping.When the cation is reductive,the strength of bond between catalysts and intermediates is weakened,which benefits to improve the desorption and transfer of intermediate,thus promoting the kinetics of oxygen electrocatalysis.Due to these favourable factors,LiCoO_1.8F_0.2（LCOF0.2）catalyst displays a higher bifunctional activity for catalyzing OER and ORR than pristine LCO.In practical application,the peak power density of the ZABs using LCOF0.2 as the air electrode is 1.53-fold higher than that of LCO-based ZABs.3.To response the issue of the catalytic activity of transition metal oxides limited by inferior electronic conductivity and small surface area,the nanofibers,consisting of metal and oxides,were designed and built in this study.Firstly,the LiCoO₂ nanofibers（LCO-NFs）with a large surface area were synthesized via electrospinning method.Compared to LCO bulk,there are more electrochemically active sites exposed on the surface of nanofibers,which is beneficial to enhance catalytic activity.Subsequently,metallic Co nanoparticles modified LiCoO₂ nanofibers（Co@LCO-NFs）were prepared via the strategy of in-situ reduction.The existence of metallic Co can increase the electronic conductivity.The OER/ORR bifunctionality and stability of Co@LCO-NFs are superior to that of LCO bulk and LCO-NFs.As for ZABs measurement,the performance of the home-made ZABs equipped with Co@LCO-NFs catalysts are better than that of the Pt/C-Ir O₂ based ZABs.Combining the results of XPS,electron paramagnetic resonance（EPR）spectra and density functional theory（DFT）calculations,the improved electrochemical performance of Co@LCO-NFs hybrids can be attributed to increased electronic conductivity,abundant oxygen vacancies,optimized electronic structure of Co ion together with strengthened Co-O covalency.4.Oxygen vacancies have been identified as the intrinsically active sites of OER and ORR.In this study,the hybrid catalyst（GDC-PBC）,consisting of Gd_0.1Ce_0.9O_2-δ（GDC）modifier and Pa_0.5Ba_0.5CoO_3-δ（PBC）matrix,were constructed.The effects of GDC content on catalytic activity of GDC-PBC hybrid were investigated.Tunning the contents of GDC with abundant oxygen vacancies can tailor the numbers of oxygen vacancies on the surface/interface of the hybrid catalyst,thereby improving electrochemical performance.Among all the as-synthesized samples,the composite catalyst with GDC mass ratio of 20%（20%GDC-PBC）exhibits the best catalytic activity and durability for OER and ORR.Moreover,the ZABs using 20%GDC-PBC as air electrod catalyst shows a higher peak power density(207 m W cm^-2)the PBC based ZABs(175 m W cm^-2)and Pt/C based ZABs(172 m W cm^-2).Combining the results of XPS,EPR and DFT calculations,the improved electrochemical performance of 20%GDC-PBC is originated from the abundant oxygen vacancies and strengthened adsorption of O₂ and H₂O.5.Combining the advantages of anion engineering and interface engineering,the P doped La Sr₃Co_1.5Fe_1.5O_10-δwith exsolved Co₂P modifier（RP-LSCFP500）was synthesized via the phosphorization.The impacts of P doping and Co₂P modification on catalytic activity of La Sr₃Co_1.5Fe_1.5O_10-δ（RP-LSCF）were studied.It can be found that P doping can not only enlarge the concentration of oxygen vacancies,but also modulate the electronic structure of Co and Fe.Compared to original RP-LSCF,the positive shift of Co 2p and Fe 2p XPS spectra of P doped RP-LSCF（RP-LSCFP300）indicates the strengthening of Co/Fe-O covalency,which can accelerate the charge-transfer and adsorption of oxygenated intermediates,thus enables a higher catalytic activity.Besides,the mass-activity of RP-LSCFP500(365.11 A g^-1)is 5.99-fold higher than that of RP-LSCFP300(60.98 A g^-1),indicating that the intrinsic activity of RP-LSCFP300 can be further increased by introducing Co₂P modifier.The improvement can attribute to strong interaction between Co₂P modifier and RP-LSCF matrix with P dopant.