In-situ And Ex-situ Modifications On Perovskite Oxide Electrocatalysts To Boost Electrochemical Properties

With the large-scale exploitation and utilization of fossil fuels,global resources are being depleted gradually and environmental pollution has become increasingly serious.In response to these challenges,researchers have paid great attention to efficient and clean energy conversion and storage units.Among them,the rechargeable metal-air battery and fuel cells have advantages,such as high theoretical capacity and high power density.However,the involved oxygen catalytic reaction kinetics is slow,which causes the large overpotential.Therefore,it is of vital importance to develop high-efficiency oxygen reduction reaction（ORR）/oxygen evolution reaction（OER）electrocatalysts.Nowadays,the most commonly used catalysts include carbides,noble metals,and transition metal oxides,and so on.Among them,noble metal-based catalysts（e.g.,Pt/C and Ru O2）showed high catalytic activity,but their high cost,scarcity,and low stability hamper large-scale applications.Perovskite oxides have received widespread attention because of their adjustable structure and abundant resources.The ORR/OER activity of perovskite oxides can be optimized by rationally tuning the compositions,carefully adjusting the microstructure,and intentionally generating oxygen vacancies,et al.In the first work of this thesis,perovskite-type oxides are designed and optimized as oxygen electrocatalysts by substitution and electrospinning.In the second work,external active ions are added to the alkaline electrolyte solution to promote the dynamic and in-situ formation of a highly active perovskite-based oxygen electrocatalyst.With the aim of designing and developing efficient and stable oxygen electrocatalysts,the main results and conclusions are as follows:In the first work（Chapter 3）,to investigate the bifunctionality of oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）,Ni-substituted La Mn O3 perovskite oxides（La Nix Mn1-x O3,x=0.1,0.3,0.5,0.7,and 0.9）and pristine La Ni O3 and La Mn O3 were synthesized using a sol-gel process.After the compositional optimization and electrochemical characterizations,we identify that La Ni0.3Mn0.7O3（SG LNM-3）within the compositions exhibit balanced intrinsic activity for bifunctional ORR and OER.To further improve the apparent activity of the optimized SG LNM-3,electrospinning was used to prepare one-dimensional nanostructured La Ni0.3Mn0.7O3（ES LNM-3）.ES LNM-3 has a higher specific surface area and continuous electron-transfer pathways,resulting in much improved bifunctional activity in comparison to SG LNM-3.In addition to the morphological effect,we ascribe the high electrochemical performance of ES LNM-3 to the high-spin Mn³⁺and low-spin Ni³⁺with electron configurations of t_2g³e_g¹ and t_2g⁶e_g¹（a well-recognized design descriptor）,as well as the high degree of the Jahn-Teller distortion.After assembling ES LNM-3 within Zn-air batteries,promising electrochemical performance such as high power density and rate capability was obtained.Perovskite oxides readily induce the surface reconstruction under electrochemical conditions such as redox potential and alkaline media,leading to elusive active sites and instability issues.In the second work（Charpter 4）,the OER activity and stability of perovskite oxides are significantly boosted through the introduction of exogenous Fe³⁺(Fe_exo³⁺)in the electrolyte.Pr Ba_0.5Sr_0.5Co₂O5+δ（PBSC）as a model electrocatalyst,after cyclic voltammetry in 0.1 M KOH+0.1 m M Fe³⁺,turns to the formation of PBSC+Fe_exo³⁺.PBSC+Fe_exo³⁺ exhibits a more than 10-fold improvement in activity in comparison to pristine PBSC.In addition,PBSC+Fe_exo³⁺shows an extremely low Tafel slope of～50 m V dec^-1,and outstanding stability at 10.0m A cm^-2 for 10 h.The exceptional activity/stability of OER with perovskite electrocatalysts originates from the formation of a thin amorphous layer of Co hydr（oxy）oxide（e.g.,Co Ox Hy）that dynamically interplays with intentional Fe³⁺from the electrolyte(0.1 M KOH+0.1 m M Fe³⁺).Additionally,theoretical calculations support that the exogenous Fe³⁺is preferable to the bulk Fe dopant for participating in the dissolution/re-deposition,and the calculations also reveal the critical importance of the perovskite oxide substrate and the in-situ formed Co hydr（oxy）oxide/perovskite（e.g.,Co OOH/PBSC）for increased activity and stability.Furthermore,the superior activity and stability of PBSC+Fe_exo³⁺ are demonstrated in Zn-air batteries by presenting high open-circuit voltage,narrow potential gap,high power output,and long-term cycle stability（500 cycles）.