Structural Design And Performance Studies Of Transition Metal Oxide Composite Nanoarray Oxygen Electrodes

Lithium-oxygen batteries have recently been extensively studied due to their high energy density,but there is still a certain distance from practical applications.At present,they mainly face problems such as large polarization,poor rate performance,and short cycle life.In the research on the positive electrode of the battery,it was found that the root cause of these problems lies in the fact that when an aprotic electrolyte is used,the oxygen reduction reaction（ORR）and oxygen precipitation reaction（OER）on the oxygen electrode are very slow,and the discharge product Li₂O₂ Excessive deposition of pores can also lead to blockage of oxygen,ion and electron transport channels and rapid passivation of the electrodes.Therefore,the efficient catalysts of ORR and OER and the porous structure of oxygen electrodes are very important to optimize the performance of lithium-oxygen batteries.In considering ORR,OER,removal of by-products,and electrical conductivity,multiple functional catalysts or functional materials are usually used to achieve the target function.Among many candidate materials,transition metal oxides have a variety of catalytic activities,are relatively inexpensive,and more importantly,they are very stable in the electrochemical environment of oxygen.The rich variety of transition metal oxides provides a large number of possibilities for the combined design of catalysts to achieve the integration of different functions.In the electrode structure,the binder and conductive agent in the traditional porous composite electrode usually make the utilization rate of the transition metal oxide catalyst low.The array of self-supporting electrodes can effectively display the multiple functions of the composite catalyst.So,in this thesis,a series of nickel,iron oxide-based array oxygen electrodes were perpared by combining the transition metal oxide composite with the electrode configuration design.The composition,composite mode and array structure of nickel-iron compounds and their effects on electrode performance were studied.The design of the composite array structure in this paper not only greatly improves the utilization of catalytic materials,but also provides an ideal research platform for studying the intrinsic catalytic properties of materials.The main research contents and conclusions of this thesis are as follows.（1）Fe₂O₃-NiO nanowire arrays（Fe₂O₃-NiO NAs）on carbon cloth were prepared through a hydrothermal route and electrochemical deposition method followed by annealing in air.Then,SEM and XRD studies confirm that NiO ultrathin nanosheets grow on Fe₂O₃ to form a stable core-shell structure.The three-dimensional growth distribution of NiO catalyst on Fe₂O₃ fully exposed the catalytic active sites,which greatly improved the catalyst utilization rate.At the same time,the array structure provides ample space for matter transport and discharge product accumulation.Used it as an oxygen electrode for lithium-oxygen batteries,at a current density of 0.1 m A cm^-2,Fe₂O₃-NiO NAs exhibits a discharge capacity as high as 4.25 m A h cm^-2 and overall overpotential as low as 1.01 V.with a current density of 0.05 m A cm^–2 and a curtailed capacity of 0.25 m A h cm^–2,it showed a stable cycle of 40 cycles.This work provides a new ideas for constructing efficient oxygen catalytic electrodes through three-dimensional composite structure design.（2）NiO-NiFe₂O₄ heterostructure nanowire arrays（NiO-NiFe₂O₄ NAs）were grown on clean carbon cloth by one-step hydrothermal method and subsequent annealing treatment.XRD,XPS and HRTEM analyses indicate that the successful synthesis of NiO-NiFe₂O₄ NAs.The NiO-NiFe₂O₄ heterostructure integrated the ORR catalytic performance of NiO and the OER catalytic performance of NiFe₂O₄ on the scale of several nanometers,achieving rich ORR/OER bifunctional catalytic active site.At a current density of 0.1 m A cm^-2,NiO-NiFe₂O₄ NAs exhibits a discharge capacity of 3.45 m A h cm^-2,it also shows the lowest overall overpotential of 1.17 V.With a current density of 0.05 m A cm^-2 and a limit capacity of 0.25 m A h cm^–2,it showed a stable cycle of over 45 cycles.This work provides a new idea for the preparation of highly efficient bifunctional oxygen electrodes,that is,integrating ORR and OER catalytic materials on heterostructure nanoarrays.