Understand the Degradation Mechanism of Electrochromic WO₃ Films by Double‐step Chronoamperometry and Chronocoulometry Techniques Combined with in situ Spectroelectrochemical Study

Abstract

In this paper, we demonstrate that the double‐step chronoamperometry and chronocoulometry techniques are efficient tools for characterizing the basic electrochromic performance of WO3 films (i. e., coloration/bleaching conversion, charge capacity and coloration efficiency). In combination with in situ spectroelectrochemical study, the variations in the optical modulation and charge capacity of the WO3 film under different potential windows were attributed to different ion diffusion depths and the quantity of WO3 sites participating in the redox reaction. Moreover, the double‐step techniques have distinctive advantages for analyzing the cyclic mechanism of the WO3 film. When the inserted Li+ ions in the coloration process cannot be completely extracted from the film in the next bleaching process, these ions accumulate in the film upon cycling, leading to the degradation of the electrochromic performance. Here the accumulated ions are referred to as “unrecoverable ions”. The abundant formation of unrecoverable ions may be due to incomplete reduction of a portion of LixWO3, which is caused by collapse of the Li+ ion diffusion channels in the deep of film after repeated cycles. All these results support that the double‐step chronoamperometry and chronocoulometry techniques have significant advantages to analyze the cyclic stability and explore the degradation mechanism of electrochromic WO3 films.

Prominent Electrochromism Achieved Using Aluminum Ion Insertion/Extraction in Amorphous WO3 Films

“…This is because the thermal energy is not high enough to produce the octahedral WO 3 crystals at room temperature. 18,19 Compared with the densely packed atomic structure of crystalline, amorphous film could provide higher surface accessibilities, which is more favorable for charge exchange. 5,20 Figure 1b displays the XPS W 4f and W 5p core level spectra of the WO 3 thin film.…”

“…Only broad diffuse halo peaks were found, which was consistent with the uniform surface morphology (insert) indicating the naturally amorphous phase of the film. This is because the thermal energy is not high enough to produce the octahedral WO 3 crystals at room temperature. , Compared with the densely packed atomic structure of crystalline, amorphous film could provide higher surface accessibilities, which is more favorable for charge exchange. , Figure b displays the XPS W 4f and W 5p core level spectra of the WO 3 thin film. According to previous reports, the main peaks at 35.8 eV (W 4f 7/2 ) and 38.0 eV (W 4f 5/2 ) correspond to WO 3 , while the second doublet peaks at 36.4 eV (W 4f 7/2 ) and 35.4 eV (W 4f 5/2 ) originate from W 2 O 5 . , From the fitting peak areas, the relative content of WO 3 and W 2 O 5 was estimated to be 95.5% and 4.5%, respectively.…”

“…Upon extended electrochemical cycling, the constant host–guest interactions would bring about irreversible guest-trapping and host framework damage, leading eventually to degraded electrochromism in optical modulation. ,,− The interaction forms and intensities of guest with host are mainly determined by the host frameworks, types of insertion guests as well as the operation conditions.…”

“…In contrast, severe transmittance degradation was observed in both the bleached and colored states for the film cycled in larger radius dimensions Na + (Figure f). These effects could be assigned to the destruction of ion-diffusion channels induced by excess lattice expansion so that a further Na + insertion is prevented. , Consequently, the increment of trapped ions density for each CA cycle decreased dramatically from initial value of ∼4.3 × 10 19 cm –3 to almost zero around the 100th cycle. In the case of trivalent Al 3+ ions, there was no obvious transmittance degradation for both the bleached and colored states (Figure g), and the optical modulation was almost keep in a steady value of ∼63.0% even after the 300 cycles (Figure h), indicating that the film cycled in Al 3+ was more stable compared to that in Li + and Na + .…”

Dynamic Process of Ions Transport and Cyclic Stability of WO3 Electrochromic Film

“…过程, 包括插入的 Li + 的量, 响应电极的电位和光吸 收; 2) 动力学过程, 包括电极在外部扰动下达到结 构平衡状态的时间行为。通常, Li + 在 WO 3 中传输的 动力学过程主要有两个方面 [7] : 1) 克服 WO 3 /电解 质之间的界面势垒; 2) 引起电致变色反应的离子在 WO 3 薄膜结构内部的扩散行为。采用电化学交流阻 抗(Electrochemical Impedance Spectroscopy, EIS)测 试技术可以建立电流和电压之间的关系, 在阻抗谱 低频区, 拟合的直线斜率与离子在 WO 3 薄膜结构内 部中的扩散动力相对应; 在高频区, 得到的半圆形 曲线的半径对应于 Li + 离子在电解质/WO 3 界面处的 插入阻力大小 [8] 。WO 3 结构中离子的插入行为涉及 到一系列复杂的过程, 包括由结构应变产生的体积 变形 [9] 、电化学反应引起的相变 [10][11] 及电极材料的 电导率变化等 [12][13] [14] , 较大的应力将影响电极结构的稳定性和离子的传 输动力, 缩短电致变色膜的循环使用寿命和降低光 学响应速度。通过制备不同纳米结构的 WO 3 薄膜, 能够得到较快响应速度的电致变色器件 [15][16] , 然而 其稳定性仍然较差, 且其衰减机理不清楚, 需要进 一步研究。 计时电位法是通过施加电流测量电极材料响应 电位的一种电化学表征方法 [17] 。与其它电化学表征 方法(阻抗谱和伏安法)相比, 该技术能够直接探测 溶液-电极系统中不同状态下的电压分布 [18] , 并经 常被用于研究电极系统中的物质传输动力学行为, 例如电极表面附近的质子吸附和传输现象 [19][20][21] 。电 荷量等于电流时间(I-t)曲线的积分 [22] [25] : [29,[34][35] ; 2) 反复的 离子插入/抽出过程中体积应力变化引起的结构破 坏 [13,36] 离子和电子的双注入/抽出模型被认为是 WO 3 薄 膜的电致变色响应的机理 [37] , 其着色反应对应于离 子插入过程, 反应方程为:…”