电解质浓度对氧化钨薄膜电致变色性能的影响规律及作用机理

Influence Law and Mechanism of Electrolyte Concentration on Electrochromic Performance of Tungsten Oxide Films

电解质为电致变色器件的变色提供离子,是器件中不可缺少的一部分。然而,目前对电解质层的研究主要集中在复合电解质和极限浓度,少有探究电解质浓度对电致变色性能的影响规律和机理,尤其是循环稳定性。因此,本文系统研究了高氯酸锂(LiClO_(4))电解质浓度(0.1、0.5、1.0、2.0 mol/L)对氧化钨(WO_(3))薄膜循环前后电致变色性能的影响,及对其循环稳定性的作用机理。结果表明,当LiClO_(4)浓度为1.0 mol/L时,WO_(3)薄膜表现出最短的着色/褪色时间,初始电荷储存量高达25.2 m C·cm^(-2),6000圈伏安(CV)循环后,衰退率仅为25.4%,表现出最佳的循环稳定性。该研究详细介绍了电解质浓度对WO_(3)薄膜电致变色性能及其循环稳定性的影响规律及作用机理,对WO_(3)基电致变色器件的设计和制备具有重要的指导意义。

Objective As the main ion supplier of the electrochromic process,the electrolyte layer is one of the most important components for electrochromic devices.Currently,researchers mainly focus on composite electrolyte and limit concentration.There are few studies about the relationship between electrolyte concentration and electrochromic performance,especially cycle stability.Therefore,we aim to investigate the electrochromic performance of tungsten oxide films before and after the cyclic voltammetry-based cycling test in electrolytes with varied lithium perchlorate(LiClO_(4))concentrations(0.1,0.5,1.0,and 2.0 mol/L).The results show that the film using the 1.0 mol/L electrolyte possesses the shortest coloring and fading time.The charge capacity is as high as 25.2 mC∙cm^(-2),and the decay rate of film at 1.0 mol/L electrolyte is only 25.4%after 6000 cycles.We reveal the influence of electrolyte concentration on electrochromic performance,which is of significance for the development and design of tungsten oxide-based electrochromic devices.Methods Tungsten oxide films are prepared on ITO bases by the radio frequency(RF)magnetron sputtering method.The thicknesses of the films are measured by a step meter(Bruker-DektakXT),and the scanning electron microscopy(SEM)images are conducted on Zeiss-Sigma 500 at a voltage of 15 kV.The crystalline structure of tungsten oxide films is examined on an X-ray diffractometer(Philips-X'Pert)by Cu Ka radiation.The LiClO_(4) is dissolved in propylene carbonate(PC)solution to prepare LiClO_(4)-PC electrolytes with varied LiClO_(4) concentrations(0.1,0.5,1.0,and 2.0 mol/L).The response time,cycle performance,and diffusion coefficient of the tungsten oxide film are evaluated by chronoamperometry and cyclic voltammetry tests at a CHI760E electrochemical workstation.The modulation rate is characterized by ultraviolet-visible spectrophotometry(Shimadzu-UV3150).Results and Discussions The prepared amorphous tungsten oxide films exhibit a nano-size peak-like surface structure at a constant thickness of about 500 nm(Fig.1).The electrochromic properties and cyclic stability of these tungsten oxide films at different concentrations of LiClO_(4)-PC electrolyte(0.1,0.5,1.0,and 2.0 mol/L)are evaluated.After 6000 CV cycles,the films in 0.1 mol/L and 1.0 mol/L electrolytes demonstrate a higher optical modulation rate(Figs.2 and 5).In terms of response time,the film in 1.0 mol/L electrolyte shows the shortest coloring and bleaching time both before and after the cyclic voltammetry test(Figs.3 and 6).Additionally,the film in the 1.0 mol/L electrolyte exhibits an initial charge capacity of 25.2 mC∙cm^(-2)(Fig.4).After 6000 CV cycles,its charge capacity is still as high as 18.8 mC∙cm^(-2) with the lowest decay rate of 25.4%,which is superior to the films in the electrolytes at other concentrations(Fig.7).Meanwhile,the film in the 1.0 mol/L electrolyte shows the weakest ion stacking effect and the highest ion diffusion coefficients(Figs.7 and 8).The SEM results also demonstrate that it has the best integrity after 6000 CV cycles(Fig.9).Conclusions We explore the influence of electrolyte concentration on the electrochromic performance of tungsten oxide films.The results reveal that there is no significant difference in the optical modulation rate at various LiClO_(4) electrolyte concentrations.However,the film using the 1.0 mol/L electrolyte possesses the shortest response time and the best cyclic stability.Its charge capacity decreases from 25.2 to 18.8 mC∙cm^(-2) after 6000 CV cycles,with a lower decay rate of 25.4%.The best film integrity of the tungsten oxide after 6000 CV cycles further proves the improved cyclic stability of the 1.0 mol/L electrolyte.In conclusion,in 1.0 mol/L LiClO_(4)-PC electrolyte,the tungsten oxide film shows optimal electrochromic performance,especially long-term cyclic stability.This could be attributed to the fact that the diffusion coefficients of lithium ions of the tungsten oxide film in the 1.0 mol/L electrolyte are significantly higher than those of other concentrations,weakening the lithium ion stacking effect.The proposed concentration-performance relationship of tungsten oxide films is significant for the mechanistic study and future development of electrochromic devices.