Research On Zn-air Battery Based On Carbon-based Materials

In recent years,researchers pay more and more attention to the development of sustainable clean energy to replace the traditional energy such as coal fossil.Fuel batteries and metal air batteries（such as Zn-air batteries,Al-air batteries,etc.）have attracted wide attention due to their advantages such as good safety performance,high energy density,environmentally friendly,rich material sources and simple assembly process.Among them,Zn-air batteries（ZABs）have been widely studied due to their large specific capacity,rich Zn resources and high energy density.However,the two most important reactions of ZABs are oxygen reduction reaction（ORR）and oxygen precipitation reaction（OER）at the air electrode.The slow kinetic process of this reaction has become the key factor restricting the development of ZABs.At present,precious metal catalysts such as Pt,Ru O₂ and Ir have excellent electrocatalytic performance,but they are expensive and have poor cycling stability.Therefore,it is urgent to find the catalyst materials with excellent bifunctional catalytic activity,low cost and good stability.Carbon materials have been widely studied for their large specific surface area,strong electrical conductivity,many catalytic sites,and simple methods doping（such as N,S,etc.）to increase the active sites to improve the catalytic performance.In addition,combining carbon materials with transition metal compounds to design and synthesize excellent bifunctional electrocatalysts is an important way to improve the performance of secondary ZABs.Based on this,we have done the following work:1.A composite material consisting of binary metal oxide-NiCo₂O₄ hollow nanospheres and N-doped carbon nanowebs were designed（NiCo₂O₄@N-CNWs）.First,N-doped carbon nanowebs（N-CNWs）were prepared by liquid phase method.The high specific surface area,hollow tube structure and N-doped structure can significantly increase the catalytic active sites,thus improving the performance of OER.N-CNWs as support material and composited with NiCo₂O₄ hollow spheres,this structure not only can improve the ORR activity of the materials due to the presence of Ni and Co transition metals,but also improve the electrical conductivity of the catalysts and promote the electron mass transfer rate.In addition,N-doped in carbon matrix can form new active sites and reduce ORR overpotential.When used as a bifunctional catalyst for rechargeable ZABs,NiCo₂O₄@N-CNWs showed excellent catalytic activity for ORR（initial potential 0.97 V,half-wave potential 0.81 V）and OER(E_j=10 1.57 V).The ZABs assembled with NiCo₂O₄@N-CNWs as the active material was tested at current density of 5 m A cm^-1.The charge-discharge voltage gap was only 0.98 V,the power density was 93.02 m W cm^-2,and the cycle life could reach233 h.In addition,we also use NiCo₂O₄@N-CNWs to make flexible ZABs,which showed excellent performance.The results showed that NiCo₂O₄@N-CNWs composite material has an important potential application prospect in new energy storage devices.2.In this work,a composite material（Co@NC）of Co nanoparticles dispersed in N-doped polyhedral carbon shells was prepared by using zeolite imidazole framework material（ZIF-67）as the precursor.Then Co@NC was calcined in NH₃ atmosphere to form a Co₄N@NC composite.The results show that N doping in the lattice of Co can further improve the ORR electrocatalytic performance.The half-wave potential is0.842 V,and the Tafel slope is lower than Pt/C and Ir/C.The relationship between the electrocatalytic performance and N doping is further explained by density functional theory（DFT）.DFT calculations show that N doping in Co crystals greatly reduces the reaction energy barrier of the catalytic process RDS(ΔG_*OOH=2.81 e V).When used as a catalyst for ZABs,Co₄N@NC showed excellent stability over 750 h with charge/discharge overpotential of 0.85 V at a current density of 5 m A cm^-2.The flexible ZABs assembled based on Co₄N@NC catalyst also shows excellent performance.At the current density of 5 m A cm^-2,it can maintain stable charge and discharge voltage within 42 h.In the process of charge and discharge,the flexible battery can be bent at any angle,which has almost no effect on the battery charge and discharge performance.At the same time,Co₄N@NC shows good HER performance.We also try to use Co₄N@NC as HER catalyst,and use the ZABs assembled with Co₄N@NC as catalyst to drive water splitting,and successfully realize the water splitting electrolysis process.3.On the basis of the second work,it was found that Co/NC calcined with a certain amount of Se powder,the Co particles and Se powder were gradually transformed into Co Se₂ nanoparticles and dispersed in the N-doped carbon shell.A composite structure（Co Se₂/NC）of Co Se₂ nanoparticles coated in N-doped carbon shell is formed,which can further improve the stability of Co Se₂ and inhibit the agglomeration of Co Se₂ particles.In addition,the existence of carbon layer can improve the conductivity of Co Se₂ material,improve the electron transfer rate and beneficial for electrode material to adapt to high current charge and discharge process.At the current density 5 A g^-1.The reversible specific capacity of Co Se₂/NC maintained at 390.2 m A h g^-1 after 4000 cycles.The excellent sodium storage performance of Co Se₂/NC shows that it is a promising anode materials of potential application value.4.Using prussian blue analogs as self-sacrificing templates,we synthesized Ni Fe S/CNCs with large cavities.The synergistic effect between the symbiotic bimetalline sulfide and the good conductivity dopamine carbon layer promoted the charge transfer,which ensured the full exposure of the active site,and thus obtained the excellent ORR/OER performance.When the current density was 10 m A cm^-2,the OER overpotential of Ni Fe S/CNCs was only 150 m V.ZABs based on Ni FES/CNCS still showed excellent long cycle stability with almost no change in voltage gap during400 h.The results show that Ni Fe S/CNCS is a kind of efficient bifunctional catalyst which can be used in ZABs.