Structural Design And Study Of Co-based Spinel Nanofibers For Zinc Air Batteries

With the increacing globle energy crysis,it is urgent to explore new clean energy sources and to expand the strategies of energy conversion and storage.Zinc air battery(ZAB)is a promising energy conversion and storage systems owning to its high theoretical energy density,large natural abundance,and environmental friendliness.Generally,increacing the rates of oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)are the key factors to determine the performance of the ZABs.Traditional precious metal Pt-based catalysts are only efficient for ORR,while Ru,Irbased catalysts are only efficient for OER.Therefore,exploring effiencient ORR/OER bifunctional electrocatalysts are urgent,which can improve the specific capacity and power density of ZABs.In this thesis,based on the diversity of active sites for spinel materials,the Co-based spinel nanofibers are modulated by heterostructure construction,dopants,and modulation of electron spin.The activity and stability of ORR and OER are studied,and the performances of these Co-based spinel nanofibers based ZABs are optimized.The main conclutions are as follows?1.It is found that heterojunction interface can improve the performances of the Ni Co2O4 cathod materials.Mesporous nanofibers Ni O/Ni Co2O4 heterojunction were synthesized with nickel and cobalt salts in nonstoichiometric ratio,and abundant Ni3+and Co3+ species were induced by the interfacial engineering.The Ni O/Ni Co2O4 porous nanofibers exhibit highly efficient and durable performances of OER/ORR,including a low overpotential of 357 m V at 10 m A cm-2(OER,Ej=10)and a high half-wave potential of 0.73 V(ORR,E1/2).Ni O/Ni Co2O4 nanofibers based ZAB displays specific capacities of 814.4 m A h g-1 and cycling stability of 175 h.2.It is found that dopants can improve the performances of the Co3O4 cathod materials.Co3-x Fex O4(x=0,1,1.5 and 2)nanofibers were synthesized using electrospinning strategy by replacing partial Co3+ in the octahedral positions of Co3O4 nanofibers with Fe3+.The Co2 Fe O4 nanofibers show the most excellent OER and ORR properties,including the minimum Ej=10 of 350 m V(OER)and the highest E1/2 of 0.78V(ORR).The Co2 Fe O4 nanofibers based ZAB displays the best specific capacity of772.4 m A h g-1,which is higher than 749.0 m A h g-1 of Co3O4 and 756.5 m A h g-1 of Co Fe2O4 nanofibers based ZABs.3.It is found that induction of magnetic field can improve the performances of the Fe Co2O4 cathod materials.Compared with the samples without magnetic field annealing(Fe Co2O4),the secondary annealing with magnetic field(Fe Co2O4-M)can induce more occupying rate of Fe3+ ions in the octahedrons.Fe Co2O4-M shows a rightshifted E1/2 of 20 m V and a left-shifted Ej=10 of 60 m V compared with Fe Co2O4.The maximum power density of Fe Co2O4-M based ZAB is 127.3 m W cm-2,and the gravimetric specific energy is 996.2 W h kg-1,which is about 91.7 % of the theoretical value.Additionally,the flexible ZAB shows excellent cycling stability and good flexibility.4.It is found that magnetic field enviornment can improve the performances of ferromagnetic spinel based ZABs.Magnetic field can change the polarized state of electrons,thus enhance the performances of ferromagnetic materials based ZABs.The Ej=10 of Fe Co2O4 nanofibers shows a left-shifted of 40 m V for the OER by applying an external magnetic field of 1 T.The maximum power density of the Fe Co2O4 nanofibers based ZAB increase from 97.3 m W cm-2 to 108.2 m W cm-2.This method provides a new idea for the application of magnetic cathode electrocatalysts in ZABs.