Preparation Of Co-MOF Derivativesand Performance Study Of Zinc-air Battery

The escalating global energy demands and the formidable risks posed by fossil fuels coupled with their rapid depletion have inspired researchers to embark on a quest for sustainable clean energy.The development of clean renewable energy technology is of great significance to human beings.As a kind of electrochemical energy conversion and storage technology,zinc-air battery has attracted more and more attention in this field due to its characteristics of high energy density,long electrochemical stability,low internal resistance,low operating conditions and abundant resources.However,the reaction kinetics of oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）at oxygen electrode in zinc-air battery is slow,which greatly limits the performance of it.Therefore,it is necessary to develop efficient bifunctional catalysts to accelerate the reaction kinetics.Metal-organic framework（MOF）materials have lower density,larger surface area and higher loading capacity than other nanostructures and are often used in the synthesis of various bifunctional catalysts.In this thesis,Co-MOF was mainly used as the research template,and different atoms were introduced through different methods to form synergistic effect and increase the active site.MOF derived materials were obtained by high-temperature processing,and their performance as dual-function catalyst for zinc-air battery were studied.The main work of this thesis is as follows:（1）Using Prussian blue analogue（PBA）as precursor,partial dissolution of the template surface occurs when Zn₃[Fe（CN）₆]₂（Zn Fe-PBA）is immersed in Co²⁺solution,and ion exchange occurs between the dissolved Zn²⁺and Co²⁺.Co Zn Fe-PBA was obtained by recrystallization of Co²⁺and Fe（CN）₆^3-coating on Zn Fe-PBA surface.After high temperature treatment,the bimetallic core-shell material Co Fe@NC was synthesized.The evaporation and dissipation of zinc at high temperature resulted in more holes and increased the active surface area of the catalyst.Synergistic effect of polymetals can increase the active site of the catalyst,thus improving the catalytic performance and stability.After testing,for Co Fe@NC at 10m A cm^-2,the overpotential of OER was 380 m V,and its ORR initial potential was 0.92 V vs.RHE,which showed better activity than Co Fe compounds synthesized by other templates and comparison samples without Co introduction.The zinc-air battery assembled with this catalyst has a high specific capacity(1104.57 m Ah g^-1)and can stably maintain a charge and discharge cycle of up to 120 h.（2）Based on the strong chelation between phytic acid and metal,MOF precursor was synthesized with phytic acid in the synthesis process of cobalt iron terephthalic acid（Co Fe-PTA）,and Co P/Fe P₄@C transition metal phosphide material was synthesized by high temperature treatment.The synergistic effect between bimetals enhanced the activity of the catalyst.The carbon layer can protect the active center and enhance the stability of the catalyst.When Co:Fe=4:1,phosphorus atoms:metal atoms=8:1,Co P/Fe P₄@C had the best comprehensive performance.At 10 m A cm^-2,OER overpotential was 405 m V,and its ORR initial potential was 0.83 V vs.RHE.The zinc-air battery assembled with this catalyst has a high specific capacity of 817.76 m Ah g^-1and can stably maintain a charge and discharge cycle of about 60h.（3）Organic phosphorus ligands were obtained by acid-base combination of weak alkalinity thiourea and the acidic inorganic sodium dihydrogen phosphate,and then Co-MOF was synthesized by the coordination of the ligands with Co²⁺.Metal phosphide was synthesized by high temperature transformation.High temperature polycondensation of thiourea resulted in the formation of C₃N₄nanosheet substrate,which enhanced its electrical conductivity.Residual sulfur and phosphorus together with cobalt produce Co P₂and Co S loaded on C₃N₄.C₃N₄nanosheets have a tendency to coat Co P₂and Co S,which protects the active center to a certain extent and enhances the stability of the catalyst.The heterojunction formed by Co P₂and Co S further enhanced the catalytic activity of the material.The OER overpotential of Co P₂/Co S@C₃N₄at 10 m A cm^-2was 405 m V,which was better than that of Co S@C₃N₄（430 m V）and Co P₂（440 m V）.The ORR initial potential（0.8 V vs.RHE）was larger than Co P₂initial potential(E_on=0.78 V vs.RHE)and Co S@C₃N₄initial potential(E_on=0.79 V vs.RHE),showing better activity.The zinc-air battery assembled with the catalyst has a high specific capacity of 1190.20 m Ah g^-1and can stably maintain a charge and discharge cycle of 52 h.