Synthesis And Application Of Metal Organic Framework And Their Derivatives In Metal-air Batteries

With the rapidly development of human society,the obviously demand for energy is increasing.Although,fossil fuels provide about 85.7%of the energy to support the entire human society.The fossil fuels are not renewable,and will cause serious environment pollution.Rechargeable lithium-ion batteries are considered as the effective energy conversion system,which has been used in portable electronics and electric transports.However,due to the limited supply of metallic lithium and the unsatisfactory energy density,the further development of the lithium-ion batteries is hindered.Therefore,an effective and environmentally friendly energy storage and conversion device are widely studied.Rechargeable metal-air batteries have attracted intensive interest due to their higher energy density compared with lithium-ion batteries.However,the complex reaction on the catalytic electrode of the metal-air battery would cause a high overpotential and poor reversibility,which severely limited its application.Recently,Because precious metals have good catalytic properties for oxygen reduction(ORR)and oxygen evolution reaction(OER),there are many reports on the application of precious metals(e.g.Pt,Ru,Ir,RuO2,IrO2,etc.)in metal-air batteries,Nevertheless,the high-cost,poor stability and scarcity of the noble metal-based catalysts seriously discount their practical application in the metal-air batteries.Therefore,developing the excellent bifunctional catalysts with low-cost and good durability is very urgent for the metal-air batteries.The derivatives of metal organic framework(MOFs,specially for zeolitic-imidazole frameworks(ZIFs))have excellent ORR/OER catalytic activity because of high N content and transition metal doped.Therefore,derivatives of MOFs have been proved to have potential application prospects in metal-air batteries In this paper,a series of MOFs were synthesized by using 2-methylimidazole as the organic ligand and Co(NO3)2 and Zn(NO3)2 as metal salts.Then,the derivatives were modified to obtain a composite material with good ORR/OER catalytic performance.And these composite materials have been applied to the metal-air battery and showed good performance.(1)In order to solve the problem of poor conductivity of MOFs derivatives,graphene with good conductivity was used for modification.In the experiment,Zn-Co bimetallic ZIFs(Co/Zn-ZIFs)was combined with graphene to obtain graphene loaded with Co/Zn-ZIFs(Co/Zn-ZIFs@rGO).And it was pyrolyzed in an Ar atmosphere to obtain nitrogen-doped graphene(S-Co/N-rGO)loaded with small cobalt particles.Due to the sublimation of Zn at high temperature,the specific surface area of the material would be increased,and the aggregation of Co element was suppressed.The nitrogen-containing organic ligands also make the material with high N content.Therefore,the lithium-air battery based on S-Co/N-rGO can operate stably for 125 cycles at a current density of 200 mA g-1.While the current density was increased to 500 mAh g-1,this battery still can provide a high specific capacity of?4000 mAh g-1.Finally,according to the IR tests for the charge and discharge products,the lithium-air battery based on S-Co/N-rGO has good reversibility In short,S-Co/N-rGO exhibits good electrochemical performance on lithium-air batteries.(2)Although MOFs derivatives show better electrical performance in lithium-air batteries,the lithium anode is easily oxidized,and it would produce dendrites.The electrolyte is also susceptible to decomposition,so it is currently difficult to practically apply to this battery system.Therefore,this research would focus on a safer and more stable water-based zinc-air battery.In this paper,we have modified the graphene oxide(GO)with the ZIF-67 coating ZIF-8(ZIF-67@ZIF-8),and further used the resulting ZIF-67@ZIF-8 modified GO(ZIF-67@ZIF-8/GO)as the precursor to synthesize in-situ encapsulation of the ultra-small C0/C04N nanoparticles into the N-doped carbon nanotubes(N-CNTs)modified reduced GO(Co/Co4N@N-CNTs/rGO).Because the reducing gas released by melamine at high temperature will cause Co to be reduced to metallic Co,which would catalyze graphene to grow a large number of carbon nanotubes Cobalt metal particles would be encapsulated on the top of the carbon nanotubes.This typical core-shell structure of Co/Co4N@N-CNTs not only improves the conductivity of the material,but also suppresses the aggregation of Co particles during the catalytic process,which can improve the catalytic stability.In addition,nitrogen-containing organic ligands can dope the material with N to improve the ORR catalytic performance of the material.Moreover,the evaporation of Zn after pyrolysis and the existence of the rGO matrix can obviously enhance the specific surface area of Co/Co4N@N-CNTs/rGO to ensure enough active space and superior ion/electron transfer for ORR/OER.Therefore,Co/Co4N@N-CNTs/rGO can exhibit half-wave potential and limiting current density of ORR comparable to Pt/C,and can also exhibit better stability and methanol tolerance than Pt/C.On the other hand,Co/Co4N@N-CNTs/rGO also exhibited OER catalytic performance similar to commercial RuO2.Hence,the Co/Co4N@N-CNTs/rGO-based conventional Zn-air battery exhibits the fantastic specific capacity of 783 mAh gzn-1,consciously discharge platform over 6 days,high-power density of?198 mW cm-2 and ultra-long cycling life of 440 h with small overpotential of 0.81 V.Moreover,the flexible Co/Co4N@N-CNTs/rGO based Zn-air cell is also designed.It shows the outstanding mechanical flexibility and good electrochemical performances,which suggests its potential application prospect in wearable electronic devices.(3)Although Co/Co4N@N-CNTs/rGO shows good performance on zinc-air batteries,the OER performance of Co/Co4N@N-CNTs/rGO is still not ideal.Therefore,in this work,Ru is introduced to further improve the OER catalytic activity.Direct pyrolysis of zinc-cobalt bimetallic ZIFs(Zn/Co-ZIFs)can obtain nitrogen-doped carbon cubes loaded with cobalt particles(Co/HP-NC).Then,Ru is loaded by oil bath to obtain Ru/Co bimetal-supported nitrogen-doped carbon cubes(Ru@Co/HP-NC).Because of the sublimation of Zn at high temperature,not only the specific surface area of the material is improved,but also the aggregation of Co at high temperature is suppressed.Moreover,the loading of metal Ru also greatly improves the OER catalytic performance of the material.Therefore,this material can provide an ORR half-wave potential of about 0.81 V and a limiting current density of 6.44 mAcm-2,which is similar to the Pt/C catalyst.And this material exhibits a better catalytic stability and methanol tolerance than Pt/C.Moreover,Ru@Co/HP-NC also exhibit a better OER activity than precious metal RuO2 catalyst.Based Ru@Co/HP-NC cathodes of Zn-air battery show a peak power density of 240 MW cm-2,which higher than Pt/C.What's more,the flexible Ru@Co/HP-NC based Zn-air battery is also designed.it reveals the outstanding mechanical flexibility and good electrochemical performances,which suggests its potential application prospect in wearable electronic devices.