Metal Organic Frameworks Derived Carbon-Based Nanomaterials For Zinc-Air Battery

With the increasing energy crisis and environmental pollution,the conversion from traditional fossil fuels to clean,renewable and sustainable energy has become a huge challenge.Zinc-air batteries have the advantages of high theoretical energy density,abundant resources,environmental friendliness,and rechargeability.They are new generation of energy conversion and storage technologies with great development potential.Oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)occur at the air cathode during discharging and charging of Zinc-air batteries.However,the slow kinetics of ORR and OER make it has lower power density and poor cycle stability.Platinum carbon and ruthenium/iridium compounds are highly efficient ORR and OER catalysts,respectively,but high cost and poor stability limit their large-scale application.Moreover,these precious metals are often single-functional catalysts and need to be mixed to achieve bifunctional activity.The development of efficient and stable bifunctional non-noble metal catalysts and air electrodes is the key to the practical application of rechargeable Zinc-air batteries.MOFs-derived carbon-based nanomaterials have excellent electrical conductivity and high electrochemical activity,showing broad application prospects in the field of energy conversion and storage.Nevertheless,the performance in rechargeable Zinc-air batteries of carbon-based nanomaterials derived from pure MOFs still need to be improved,because the active sites and specific surface area will be significantly reduced after calcination at high temperature,and their particle nature make them prone to collapse and agglomeration during charging and discharging process.In this thesis,the authors combines MOFs with different substrate materials to get one-dimensional,two-dimensional,three-dimensional bifunctional catalysts and air electrode,and then explore their electrochemical performance in rechargeable and flexible solid-state Zinc-air batteries.First of all,Zn,Co-ZIF with different metal ratios are combined with one-dimensional tellurium nanotube(Te NT),after high-temperature calcination and sulfur doping,a series of1D N,S co-doped carbon nanotube composites(NSCNT)with different thicknesses are obtained.The one-dimensional tubular structure is conducive to the transfer of reactants,electrolyte diffusion and more exposure of nitrogen and sulfur co-doped active sites,achieving high power density and excellent cyclic stability in rechargeable Zinc-air batteries.When the ratio of Zn and Co is 4:1,NSCNT-4:1 shows the optimal catalytic activity of ORR and OER.The rechargeable Zinc-air battery assembled by NSCNT-4:1 achieves an outstanding power density of 374 m W cm^-2 and a remarkable cycling stability of over 2000 h at 5 m A cm^-2.Subsequently,ZIF-67 is evenly loaded on the graphene oxide according to the induced effect of triethylamine,and two-dimensional(2D)nitrogen-doped carbon nanotubes/graphene(GNCNTs)hybrid is obtained after high-temperature annealing.The as-obtained hybrid(GNCNTs-4)exhibits excellent catalytic activity and durability for the oxygen electrochemical reactions due to the synergistic effect by the hierarchical structure and heteroatom doping.The assembled rechargeable Zinc-air battery achieves a high power density of 253 m W cm^-2 and an excellent cycle stability of over 3000 h at 5 m A cm^-2.Moreover,the flexible solid-state rechargeable Zinc-air batteries assembled by this hybrid oxygen electrocatalyst exhibits a high discharge power density of 223 m W cm^-2,together with cycling stability of over 24 h at 1 m A cm^-2,which can power 45 light-emitting diodes or charge a cellphone.Zinc-air battery is a kind of promising flexible energy storage device.The combination of MOFs with three-dimensional flexible substrate to construct integrated bifunctional air electrode is particularly important for high-performance flexible solid-state Zinc-air batteries.Therefore,a three-dimensional(3D)flexible free-standing bifunctional air electrode composed of N-doped porous carbon array,N-doped carbon nanotubes and carbon cloth(NCNTs@NCAs/CC)is successfully developed,through one step carbonization of 2D Co-MOF array rooted on carbon cloth.The NCNTs@NCAs/CC electrode has an excellent bifunctional catalytic activity and stability.A liquid rechargeable Zinc-air battery assembled by NCNTs@NCAs/CC electrode can provide a remarkable power density of 410 m W cm^-2and excellent cycling stability of over 1500 h at 5 m A cm^-2.Moreover,a flexible solid-state rechargeable Znc-air battery delivers an open circuit potential of 1.55 V and peak power density of 240 m W cm^-2,together with cycling stability of over 40 h at 1 m A cm^-2,and the advantages of the integrated bifunctional air electrode in the flexible solid Zinc-air battery are realized.