Preparation And Electrochemical Performance Of Cobalt-vanadium Bimetallic Catalysts

The development of electrochemical energy conversion and storage technologies is critical to cope with the consumption of fossil fuel.Overall water splitting and zinc-air batteries are regarded as promising devices for energy conversion and storage.The key electrocatalytic pathways in the operation of these devices include hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）.However,due to the large overpotential,high price and relatively short service life of their respective electrocatalysts,electric energy is seriously consumed.The shortcomings of these electrocatalysts hinder a wide range of applications of overall water splitting and zinc-air batteries.Therefore,the search for efficient and inexpensive catalysts with HER/OER/ORR properties is the focus of current research.In this thesis,catalysts with HER/OER/ORR performance were prepared by a simple and energy-saving method based on bimetallic cobalt-vanadium,and the effect of metal addition ratio and heat treatment temperature on the electrocatalytic performance were studied,and then they were applied in overall water splitting and zinc air batteries.The relevant research contents are as follows:（1）Preparation and electrochemical performance of V-doped Co-MOF electrocatalysts.Two-dimensional Co-MOF was prepared with metal Co salt and dimethylimidazole ligand as raw materials,and NH₄VO₃ was introduced into Co-MOF by ion exchange method,Simultaneously,VCl₃ was used to replace NH₄VO₃,then pyrolysis was carried out at low temperature to prepare Co V bimetallic MOF with incomplete decomposition.and the effects of different vanadium source on the properties were investigated.The results showed that the different vanadium source introduced changed the crystal form and morphology of the catalyst,but the impact on the performance was not large.Therefore,NH₄VO₃ was selected as the main vanadium source,and the OER/HER/ORR performances of the product was carried out in alkaline environment.The results indicated that:When the Co/V ratio is 8:1 and the pyrolysis temperature is 350℃（Co₈V-MOF-350）,the catalyst showed relatively good OER/HER/ORR catalytic performance,which was due to the synergistic effect between metal Co and V,and the incomplete decomposition of MOF exposing more catalytic metal active sites.For OER and HER reactions,the overpotential at the current density of 10 m A cm^-2(η₁₀)of the samples decreased by 100 m V and 142 m V,respectively,compared with Co-MOF-350.At the same time,it has good ORR initial potential(E_onset).Then,Co₈V-MOF-350 is used to test its performance on overall water splitting,and only 1.76 V voltage is required to drive current density j=10 m A cm^-2.Also,a liquid rechargeable zinc-air batteries was assembled as an air cathode,and it has a high open-circuit voltage of 1.43 V,a peak power density of 21.33 m W cm^-2.Its specific capacity is 428.4m A h g _Zn^-1 at j=10 m A cm^-2.（2）Preparation and electrochemical performance of V-Co₂N/Co N electrocatalyst.Precursor V-doped cobalt hydroxide carbonate（V-Co CH）was prepared by one-step hydrothermal method with metal Co salt,NH₄VO₃,CO（NH₂）₂ and NH₄F as raw materials.,V-doped Co₂N/Co N was prepared by low-temperature nitridation.The sample was composed of two different crystal forms Co₂N and Co N.A small amount of V was doped in Co₂N and Co N,which did not affect the formation of cobalt nitride.By optimizing the doping amount of V and different nitriding temperatures,the catalyst with a Co/V ratio of19:1 and nitriding temperature of 400℃（V-Co₂N/Co N-1-400）exhibited excellent OER/HER/ORR activity.In alkaline solution,theη₁₀ of HER is only 47 m V;η₁₀ of OER is 230 m V;The initial potential of ORR was0.83 V versus reversible hydrogen electrode（vs.RHE）.In view of its excellent catalytic activity,only 1.69V voltage is required to drive the overall water splitting.At the same time,a liquid rechargeable zinc-air battery is assembled as an air cathode,which has a high open-circuit voltage of 1.41 V,good peak power density of 19.59 m W cm^-2 and specific capacity of 352 m A h g_Zn^-1,and it also shows the cyclic stability without voltage attenuation for a long time.