Properties And Mechanism Study Of Ruddlesden-Popper Type Oxides For Oxygen Reduction Reaction

Energy storage and conversion devices have been stepping into the limelight in an era with problematic issues of energy security and climate change,quick development of the renewable energy market,the popularity of personal consumer electronics and the rapid advancement of electric vehicles.As two major types of energy storage and conversion devices,metal air batteries and electrochemical capacitors?ECs?have been attracting growing worldwide attention.The sluggish oxygen reduction reaction?ORR?and limited service life of the cathode remain two main obstacles for the large-scale applications of metal air batteries.So it is of great theoretical value to carry out the research on the structure-activity relationship for the design and preparation of novel high-performance electrocatalysts.The low energy density hurdles the applications of ECs in the fields of transportation,defense and consumer electronics,making the research and development of new active electrode materials with high spectific capacity a hot field.Due to the structural,electronic configurational flexibility,Ruddlesden?Popper?R?P?type layered perovskite oxides have manifested various physical properties such as paramagnetic?ferromagnetic,ferromagnetic?antiferromagneti,insulator-metal transition and giant magnetoresistance effects.Unfortunately,the study of R?P oxides as electrocatalysts of air elctrode for ORR and active electrode materials for ECS were limited.In the present paper,some pioneering studies on the catalytic activity and mechanism of several R?P oxides have been carried out along with an attempt to improve the ORR activity by mixing with carbonaceous materials to form hybrid catalysts.Besides,the charge storage properties and mechanism of the selected R?P oxides were also investigated.The research results may be helpful to broaden the understanding of the ORR and charge storage mechanism of the R?P type transition metal oxides.The main contents of the study are as follows:Firstly,the ORR activity and mechanism of triple layered R?P type LaSr₃Fe₃O₁₀were studied.La Sr₃Fe₃O₁₀/Graphene oxide?GO?hybrid catalyst was prepared in an attempt to improve the ORR activity of LaSr3Fe3O10.Electrochemistry measurements results showed that oxygen is reduced on LaSr3Fe3O10 and LaSr3Fe3O10-GO via a two electron process.LaSr₃Fe₃O₁₀-GO outperforms LaSr₃Fe₃O₁₀0 in terms of ORR activity,but is inferior to Pt/C.Within the measurement potential windows,both La Sr₃Fe₃O₁₀0 and LaSr₃Fe₃O₁₀-GO exhibit two potential-dependant rate determining steps towards ORR.Secondly,The composition effects of Mn and Co in triple layered R?P type La Sr3Mn3-xCoxO10?x=0,1.0,1.5?on ORR activity and mechanism were investigated.The electrochemistry measurements revealed their four electron path nature for ORR with an activity declining order of La Sr₃Mn_1.5Co_1.5O₁₀?LaSr₃Mn₃O₁₀>La Sr₃Mn₂CoO₁₀.LaSr₃Mn_1.5Co_1.5O₁₀0 was used to mix with conductive acetylene black.The as prepared LaSr3Mn3-xCoxO10/C hybrid electrocatalyst demonstrated a four electron reduction path of oxygen.The conductive agent effects of acetylene black on ORR performance was discussed.As active electrode materials for ECs,the specific capacitance decreased in the order of LaSr₃Mn₃O₁₀>La Sr₃Mn_1.5Co_1.5O₁₀>La Sr₃Mn₂CoO₁₀0 with 225 F g^-1,219 F g^-1and 191 F g^-1for LaSr₃Mn₃O₁₀,La Sr₃Mn_1.5Co_1.5O₁₀0 and LaSr₃Mn₂CoO₁₀,respectively,at a charge-discharge current density of 1 A g^-1.Thirdly,the powder sintering time effects on ORR performance of double layered R?P type LaSr₂Mn₂O₇.The rate-determining step for ORR was discussed.La Sr2Mn2O7/acetylene black hybrid catalyst showed improvement on the ORR activity.As active electrode materials for ECs,the specific capacitance is about 100 F g^-1at a charge-discharge current density of 1 A g^-1.Finaly,the ORR mechanism study revealed a two electron reduction process for La Sr₂Fe₂O₇ and LaSr₂Fe₂O₇/acetylene black hybrid while a four electron process for La Sr₂MnFeO₇/acetylene black hybrid electrocatalyst.The addition of conductive agent acetylene black into LaSr₂Fe₂O₇ improved the limting current density and durability of ORR.LaSr2MnFeO7 proceeds a mixed mechanism of two electron and four electron path towards ORR,while La Sr₂MnFeO₇/C proceeds predominantly four electron path towards ORR.The observed rate-determing step change of La Sr2MnFeO7/C in the measurement potential range was ascribed to the variation of the amount of four electron path for ORR.Based on the aforementioned results,the outlook of R?P type transition metal oxides as electrocatalysts for ORR and active materials for ECs were put forward.