Electrochromic Properties Of WO₃/NiO System Based On Mg²⁺ Electrolytes

Electrochromism is the optical properties of materials change reversibly with the electric field applied.There are many kinds of electrochromic materials.Among them,inorganic electrochromic materials have been widely studied,due to their advantages of the simple preparation process,good cycling stability and UV light resistance.Inorganic electrochromic device is divided into five layers:transparent conductive layer,cathode electrochromic material,electrolyte layer,anode electrochromic material,transparent conductive layer.Electrolytes,as ion storage and transport layer,play an important role in electrochromic devi ce.At present,the most commonly electrolyte ions are H⁺and Li⁺.However,the safety problems of Li⁺and the erosion of H⁺have restricted the application and development of electrochromic materials.The ion radius of Mg²⁺is closest to Li⁺,and Mg is also the abundant element in the earth’s crust.Therefore,the application of Mg²⁺as electrolyte ion instead of Li⁺has the great prospect in the field of electrochromism.The electrochromic behavior and mechanism of cathode electrochromic material based on Mg²⁺electrolyte were studied by taking WO₃ film as an example,which is the most commonly used and studied film.The diffusion process of Mg²⁺in amorphous WO₃ films is a semi-infinite diffusion,and the ion diffusion rate D is of the order of 10^-11～10^-10 cm²/s,which is equivalent to the diffusion coefficient of Li⁺.With the increase of annealing temperature,the ion diffusion rate,the capacity of storing charge and ions of WO₃ films decreases gradually,the optical modulation of WO₃ films decreases,and the cyclic stability of WO₃ films increases.The electrochromic mechanism was studied by Raman spectroscopy and XRD.The results show that when Mg²⁺and electron are injected into amorphous WO₃ films,W⁶⁺is reduced to W⁵⁺,and the small polaron is formed between the W⁶⁺,W⁴⁺and the W⁵⁺states.The transition of small polaron causes optical absorption in the film.The WO₃ film electrochromic device based on Mg²⁺electrolyte was constructed.The device shows large optical transmittance modulation and fast response speed.It proves that Mg²⁺electrolyte can replace Li⁺electrolyte in the electrochromic process of cathode electrochromic material.In order to avoid the REDOX reaction of ITO electrode in WO₃ film electrochromic device,the electrochromic behavior and mechanism of anodic electrochromic material based on Mg²⁺electrolyte were studied.Here,we take the Ni O film as an example.The results show that the electrochromic properties of Ni O films are determined by the crystallinity,particle size and intergranular space.Without annealing,Ni O films have larger optical modulation and better cyclic stability.When Mg²⁺is inserted,it occupies the gap of crystal structure and does not form a strong bond with Ni or O atoms in the film.Meanwhile,Ni³⁺is reduced to Ni²⁺,which reduces the small polaron transition between Ni³⁺and Ni²⁺states in the film.Therefore,the transmittance of the film increases.The Ni O film electrochromic device based on Mg²⁺electrolyte is constructed.The device can successfully get optical modulation,which proves that Mg²⁺electrolyte can replace Li⁺electrolyte in the electrochromic process of anodic electrochromic material.Based on the above,the electrochromic behavior of the complementary electrochromic device based on Mg²⁺electrolyte was studied.Firstly,complementary device based on the liquid Mg²⁺electrolyte was constructed,and its electrochromic behavior was studied by electrochemical and spectral experiments.The optical transmittance modulation of the device is greater than 40%in the whole spectral range,and can reach 50%at 630 nm.In addition,the device also shows the fast response time,the coloration and bleaching response time are 6.4 s and 5.3 s respectively.But the bubbling and leakage problems of liquid devices limit their use.In order to overcome these problems,Mg²⁺gel electrolyte was prepared by knife coating method,and a complementary electrochromic device based on Mg²⁺gel electrolyte was constructed.The optical transmittance modulation of device is greater than 35%,and the response time of coloration and bleaching are 13.9 s and1.3 s,respectively.However,due to the use of gel electrolyte and two layers of ITO,the optical modulation of the device is limited,and the electrochromi sm is not uniform.Therefore,we prepared solid-state electrochromic device based on Mg F₂by vacuum evaporation method.When the coloring time is 100 s,the device can achieve a maximum optical transmittance modulation of 79.7%,and the transmittance of the colored state is close to zero.In addition,the device also shows good cycle stability and fast response speed.When the coloring time is 25 s,the coloration and bleaching response time of the device are 19.2 s and 8.3 s,respectively.The results show that Mg²⁺electrolyte can replace Li⁺electrolyte in complementary electrochromic devices.