Electrochromic Cu₃（HHTP）₂ Electrodes Based On Different Ion Electrolytes

In recent years,electrochromic technology and materials have developed rapidly and have played an important role in energy saving,emission reduction,smart display,and military camouflage.Metal organic framework（MOFs）materials have excellent designability and multi-dimensional pore structure.The design of ligands and metal molecules can achieve excellent conductivity of MOFs,which is very consistent with the requirements of electrochromic materials.Cu₃（HHTP）₂,as a new type of conductive MOFs,has been widely used in capacitance and sensing fields due to its excellent conductivity and redox activity.However,the current conductive MOFs had complex film formation,poor film uniformity and the uncontrollable thickness.In addition,the relevant research mainly uses traditional organic lithium-ion electrolyte,which has the risk of explosive and high cost.To solve the above problems,Cu₃（HHTP）₂ thin films were prepared by liquid phase epitaxial spraying and layer-by-layer self-assembly.Furthermore,the electrochromic ion adaptation properties of the films in various electrolytes of organic system and water system were studied.Finally,we assembled the Cu₃（HHTP）₂//PEDOT electrochromic devices to show the practical application value.Firstly,Cu₃（HHTP）₂films were prepared on FTO conductive glasses by liquid phase epitaxial spraying.The different thicknesses of the Cu₃（HHTP）₂ thin films were prepared by controlling the number of spraying cycles.We systematically studied the influence of thickness on the electrochromic properties of the films.The results show that the films can be transformed into yellow/blue at-1.5 V/0.2 V.With the increase of the thickness,the optical modulation range of the films increases first and then decreases,the maximum optical contrast reaches 27.5%,and the coloring/bleaching time increases from 7.6 s/5.5 s to 20.3 s/10.3 s.The excellent electrochromic properties of Cu₃（HHTP）₂ thin films are provided by low electron transfer resistance and high ion diffusion coefficient.This work provides a new idea for the study of MOFs materials.In order to improve the problems of excessive influence factors and poor repeatability,Cu₃（HHTP）₂ thin films were prepared on FTO glasses by layer-by-layer self-assembly.The results show that the films prepared by layer-by-layer self-assembly have larger pore structure on the surface,and the electrodes have better electrochromic adaptability to large ion electrolytes.compared with the traditional Li Cl O₄/PC or Na Cl O₄/PC electrolyte,Cu₃（HHTP）₂ films shows the lower contact resistance and higher ion diffusion efficiency in the ionic liquid[EMIm]BF₄electrolyte.The switch speed of the electrochromic electrode is significantly improved.The coloring time is reduced from 10.3 s to 8.0 s,and the bleaching time is reduced from 23.6 s to 5.2 s.In order to show the practical application value of the films,5 cm×5 cm Cu₃（HHTP）₂//PEDOT electrochromic device based on[EMIm]BF₄ electrolyte was assembled.The device showed 33.7%transmittance change at 660 nm.This work demonstrates the potential of MOFs/ionic liquid electrochemical system in the field of electrochromic device.In order to avoid the hidden danger of explosion and the high cost of traditional organic electrolytes,we selected the easily available water-based electrolytes in this chapter.Firstly,the properties of Cu₃（HHTP）₂ thin films in Na Cl aqueous solution were investigated.When the concentration was 100 m M,the films showed the transmission change of 25.8%,and the color/bleaching time was 5.0 s/4.5 s.In addition,the thin films still has certain electrochromic properties under extremely low Na Cl ion concentration（0.1 m M）,which proves that the thin films has the characteristics of high ion sensitivity.Then the properties of Cu₃（HHTP）₂ films in different ionic aqueous solutions（KCl,Mg Cl₂ and Ca Cl₂）were investigated.The transmission change were kept above 20%,and the coloring/bleaching time could be controlled within 5s.we chose seawater as electrolyte,the films showed excellent electrochromic performance（16.9%transmittance change and 3.5 s bleaching speed/2.0 s coloring speed）,which proved that the films had the characteristics of high ion compatibility.Based on the high sensitivity of Cu₃（HHTP）₂ films,according to the color change depth and time of the films,it can be used as a visual detector of ion concentration to judge the ion concentration range of the solution.This work has proved the advantages of Cu₃（HHTP）₂thin films in high sensitivity and high adaptability,and greatly expanded the application scope of conductive MOFs in research and application.