Effects Of Metal Ion Doping On Catalytic Performance Of Co₃O₄ Towards Oxygen Reduction/Evolution Reactions And Its Application In Zinc-Air Batteries

Zinc-air batteries（ZABs）with high energy density,low cost and good safety have shown broad applications in the fields of electric vehicles and mobile devices.The air electrode is the key component of ZABs.It occurs the oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）in the respective processes of charge and discharge.Due to the involvement of multi-electron transfer processes,the kinetics of ORR and OER on the air electrode are extremely slow,largely limiting the performance of the ZABs.The development of efficient catalysts to improve the kinetics of the ORR and OER is,therefore,highly desirable.The conventional ZABs use noble metal materials,such as Pt/C and Ru O₂,as ORR/OER catalysts due to their high catalytic activities.These catalysts,however,suffer from drawbacks such as high cost,scarcity in resources,singularity of catalytic activity and poor stability,greatly hindering their commercial utilizations in the ZABs.Exploitation of non-noble metal bifunctional catalysts with low cost,high activity and good stability is,therefore,of great significance to improve the performance of ZABs.Spinel transition metal oxides are potential catalysts for ORR/OER due to their merits of good stability in alkaline media,abundance in resources,low cost and environmentally friendliness.Most spinel transition metal oxides are,however,confronted with the problems of poor electrical conductivity and inferior catalytic activity.So,to make practically usable in the ZABs,the catalytic activity and stability of spinel transition metal oxides have to be improved.Aiming at tackling these problems,several methods such as metal ion doping,carbon material supporting and carbon coating have been adopted to modify the spinel Co₃O₄ in this thesis.Specifically,the N-doped carbon nanotubes supported La doped Co₃O₄ nanoparticles（La-Co₃O₄/NCNTs）,the N-doped carbon nanotubes supported La/Fe co-doped Co₃O₄ nanoparticles（La Fe-Co₃O₄/NCNTs）and the carbon coated La Fe-Co₃O₄/NCNTs（C@La Fe-Co₃O₄/NCNTs）have been prepared and the ORR/OER catalytic activities of Co₃O₄ have been effectively improved.The main contents of this thesis can be described as follows:（1）Preparation of La-Co₃O₄/NCNTs and their electrocatalytic performance for ORR/OER.The La-Co₃O₄/NCNTs catalysts were prepared by a one-step solvothermal method.The results show that the La doping can effectively improve the ORR catalytic activity of Co₃O₄.With the La doping level of 21%,the La-Co₃O₄/NCNTs shows optimum catalytic performance.Its ORR half-wave potential(E_1/2,ORR)and limiting current density（j _L）in 0.1 M KOH solution are 0.803V and 5.61 m A cm^-2,respectively.The high catalytic performance of La-Co₃O₄/NCNTs mainly originates from the controlled regulation of the surface electronic structure of Co₃O₄ by the La doping,which optimizes the adsorption and transformation of reaction intermediates（*OOH,*O,and*OH）on the catalysts surface.Meanwhile,the NCNTs supporting is also important for the good catalytic activity of La-Co₃O₄/NCNTs.The ZABs assembled with La-Co₃O₄/NCNTs show an open-circuit voltage of 1.45V and good stability.Additionally,our results show that the La doping has negligible effect on the OER catalytic activity of Co₃O₄/NCNTs.（2）Preparation of La Fe-Co₃O₄/NCNTs and their electrocatalytic performance for ORR/OER.In order to further improve the catalytic performance of the catalyst,especially for the OER activity,the La Fe-Co₃O₄/NCNTs catalysts were prepared by the solvothermal method.The results show that the La Fe-Co₃O₄/NCNTs catalysts exhibit good ORR/OER catalytic activity.With the La and Fe doping levels of 23%and 15%,respectively,the La Fe-Co₃O₄/NCNTs shows relatively better catalytic activities.Its E_1/2,ORR and OER overpotential at and 10 m A cm^-2 current density(E_η10,OER)in 0.1 M KOH solution are 0.806 V and 386 m V,respectively.The DFT calculations show that the La/Fe co-doping can effectively decrease the ORR and OER overpotential of La Fe-Co₃O₄ and improve its stability.Additionally,our results show that the presence of carbon coating could further improve the ORR/OER catalytic performance of La Fe-Co₃O₄/NCNTs.The carbon coated La Fe-Co₃O₄/NCNTs（C@La Fe-Co₃O₄/NCNTs）shows the higher ORR catalytic performance(E_1/2,ORR=0.813 V)and OER catalytic performance(E_η10,OER=374 m V).Its OER/ORR bifunctional catalytic activity is higher than those of most recently reported catalysts.When the C@La Fe-Co₃O₄/NCNTs is applied in ZABs,the maximum power density can reach up to 122.2 m W cm^-2.Remarkably,the ZABs show excellent cycle stability.In conclusion,both the La doping and La/Fe co-doping are promising methods to enhance the ORR/OER catalytic activities of Co₃O₄-based catalysts.Specifically,the ORR activities of Co₃O₄ can be effectively improved through the respective La doping.In the case of the co-doping,the catalytic activities of Co₃O₄ can be improved to a higher extent.The carbon supporting and carbon coating are also the effective ways to improve the catalytic activity and stability of catalysts.The carbon supporting and carbon coating can not only increase the dispersion of active nanoparticles and prevent them from agglomeration,but also improve the electrical conductivity of the catalysts.It can be expected that the methods reported in the thesis can be extended to the design and preparation of other transition metal-based catalysts with high catalytic activities.It is therefore of great significance for assembling the ZABs with high performance and promoting their commercial applications.