Study on Bi-functional Co-based Oxides Catalysts for Zinc-air Batteries

Author:Wang Qin

Supervisor:liu zhao ping


Degree Year:2018





Metal air battery is a device that directly transforms the chemical energy of metallic materials into electrical energy.It usually uses high energy density metals as negative electrode,such as magnesium,aluminum and zinc,etc.The positive electrode usually uses oxygen in the atmosphere as fuel.Recently,metal–air batteries have become one of the most promising technologies,based on the nature of high theoretical energy density,rich reserves and environment-friendly.However,the development of rechargeable zinc–air batteries has been hindered by the intrinsically sluggish kinetics of oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).In this thesis,Co-based oxides with good activity,low price and rich raw materials are selected as the study objects.Various strategies are used for regulation and modification,and the physico-chemical properties of the materials are systematically characterized.The electrocatalytic activities of the catalyst materials are evaluated by the rotating disk electrode and the home-made rechargeable zinc-air battery.The mechanism of the factors affecting the catalytic properties of the catalysts is also discussed.The main works achieved in this thesis are as follows:(1)By dpoing in the B-site of La0.8Sr0.2CoO3 with Mn,a series of La0.8Sr0.2Co1-xMnxO3 perovskites have been synthesized.The valence states and the electronic structure of the transition metal cations on the surface of the catalysts have been regulated.And the adsorption properties of oxygen on the surface of catalysts have also been affected.The results show that the appropriate proportion of Mn-doping can increase the catalytic activity of ORR and OER at the same time,in which the doping amount of 60 at.%(LSCM-60)shows the best ORR/OER bifunctional catalytic activity.The potential difference△E between ORR and OER is 1.032 V.Moreover,the LSCM-60 catalyst also has a good performance in rechargeable zinc-air battery.The charge-discharge voltage gap is 1.05 V at a current density of 50mA cm-2.(2)One-dimensional mesoporous nanorod-like spinel Co-Mn-Ni ternary oxides have been prepared by a mixed solvent co-precipitation method.The chemical composition,morphology and specific surface area of the material are regulated by adjusting the ratio of metal elements,and then the charge-transfer ability and oxygen-adsorption capacity of the material in the reaction of ORR and OER are influenced.The results of electrochemical testing show that the optimal bifunctional catalytic activity can be obtained when the atomic ratio of Co/Mn/Ni is 2:3:1,and the potential difference△E between ORR-OER is 0.88 V.Beisides,the stability of CMN-231 catalyst in alkaline solution is also higher than that of noble metal Pt/C and IrO2 catalysts.The zinc--air battery with CMN-231 as catalyst shows a low charge–discharge voltage gap of 0.9 V under the current density of 50 mA cm-2.In addition,the best ethanol/water volume ratio in solvent is 8.(3)The Co3O4/Ag@N-rGO composite catalyst has been prepared by a one-pot process.Transmission electron microscopy(TEM)observation shows that the in-situ grown spherical Co3O4 and Ag nanoparticles are evenly distributed on graphene lamellae.The electrochemical tests confirm that a small amount of Ag plays a significant role in enhancing the catalytic activity of ORR for Co3O4 oxide.Meanwhile,the doping of N element introduces more active sites in graphene,which further increases the catalytic activity toward OER.The potential difference△E of Co3O4/Ag@N-rGO in catalyzing ORR and OER is 0.93 V.Especially,the over potential for OER is lower than that of IrO2 catalyst,which is 437 mV and 442 mV respectively.The discharge-voltage and maximum power density of zinc-air battery using Co3O4/Ag@N-rGO as catalyst are comparable to those of Ag/C catalyst,and the charging voltage is significantly lower than that of Ag/C.Moreover,the stability of charge-discharge cycling is obviously better than that of Pt/C catalyst under the current density of 10 mA cm-2.(4)PVA and PAA based gel electrolytes have been prepared,and their ionic conductivity have been tested by electrochemical method.It is found that the ionic conductivity of PAA based gel electrolyte(0.282 S cm-1)is about 1.84 times higher than that of PVA based gel electrolyte(0.153 S cm-1).The two kinds of electrolytes both have good mechanical tensile and flexural properties.By assembling a flexible zinc-air battery,it is found that the PAA based electrolyte is more suitable for discharging at large current density,and the maximum power density is about 108mW cm-2,while the PVA base electrolyte can only discharge at small current density,and the maximum power density is only 50 mW cm-2.Meanwhile,the PAA based battery has better cycling stability than that of PVA based batteryIn summary,this thesis uses different strategies to regulate the ORR/OER catalytic activities of perovskite,spinel and simple Co-based oxides,and analyzes the reasons for the performance improvement caused by modification and regulation.Meanwhile,the practical application of modified Co-based oxide catalysts in rechargeable zinc-air battery is verified.The research results of this thesis provide ideas and references for the design of transition-metal oxide based ORR/OER bi-functional catalysts,provide guidance for the test and evaluation of rechargeable zinc-air batteries,and also explore the research direction of the flexible zinc-air batteries.