Vanadium substituted Keggin-type POM-based electrochromic films showing high performance in a Li⁺-based neutral non-aqueous electrolyte

Abstract

In this research, three vanadium substituted polyoxometalates (POMs) K_3+n[PW_12−nV_nO₄₀] (abb. PW_12−nV_n) (n = 1, 2, 3)-based electrochromic (EC) films were prepared using the electrodeposition method. The films not only show a good performance in terms of EC properties in an H⁺-based aqueous electrolyte, but also show good EC performance in a Li⁺-based non-aqueous electrolyte. Although the EC activity of POMs is lower in the neutral media than in acid media, the use of the neutral non-aqueous electrolyte does not result in the suppression of the EC performance. The large diffusion coefficient of the Li⁺ in the porous films guarantees the high performance. For the PW₁₁V, PW₁₀V₂ and PW₉V₃-based films, the coefficients of Li⁺ are 4.05 × 10⁻¹¹, 4.11 × 10⁻¹¹, and 1.04 × 10⁻¹¹ cm² s⁻¹, respectively. The PW₁₀V₂-based film shows the shortest response time for coloration which is even shorter than that shown in the H⁺-based aqueous electrolyte. The optical contrast of the films is also nearly the same as which achieved in the aqueous electrolyte media. The application of the non-aqueous neutral electrolyte would be beneficial for the fabrication of the POM-based EC device.

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Introduction

Electrochromic (EC) devices can be electronically darkened or lightened using small applied potentials.¹ The EC devices (ECDs) have been reported to be applied in anti-glare rearview mirrors, portholes and displays.² The high performance of an ECD depends on the synergistic cooperation of the EC material and the electrolytes used. EC materials are the core part of the ECDs, which can be classified into three types: inorganic oxide, organic molecules and polymeric materials.³ The inorganic EC materials have high thermal stability, are ultraviolet (UV)-resistant, give a high performance with a large optical contrast and have long durability. Some examples of tungsten trioxide (WO₃)-based EC devices have already been applied commercially.⁴

Polyoxometalates (POMs) represent a well-known class of metal oxide nano-clusters, which can be decorated or designed rationally to easily obtain certain functions.⁵ Their good water solubility makes it possible to fabricate the POM-based films using a wet chemical process, which is an efficient way to cut the cost of preparing an EC film. The EC properties of the POMs were confirmed using the layer-by-layer (LBL) method.⁶ The recently developed electrodeposition method makes it easy and cheap to prepare high performance EC films. The POM-based near-infrared (IR) EC film and multi-color EC films were successfully prepared using this method.⁷ Unlike the metal oxide, the structure of the POMs molecules is facile to devise during preparation process. Therefore, the EC properties could be adjusted easily by changing the POMs' structure. In this regard, POMs-based EC materials are like the organic EC materials whose EC properties could be easily adjusted by changing their structures. As far as is known, the POMs usually show high electrochemical activity in acid media, and the major research on the POM-based EC films was carried out in acid buffer media. It is reported that POMs could usually express their unique electrochemical properties in acid aqueous electrolyte media.⁸ For example the P₂W₁₅V₃-based film even could exhibit EC multi-color changes in acid media. However, the acidic aqueous electrolyte is not a wise choice to fabricate a device, because the H⁺ has a strong tendency to corrode the surface of the metal oxide electrode and additionally H⁺ is easily polarized to form hydrogen gas bubbles because of its low electrode potential.⁹ The Li⁺-based electrolyte which is commonly used in the transmission metal oxide based EC films is easy to incorporate into or to release from the EC material. A lithium perchlorate (LiClO₄)-based non-aqueous electrolyte features a larger electrochemical window than the H⁺-based electrolyte. However, there are few reports on whether the POM-based films could exhibit EC behavior in the commonly used electrolyte LiClO₄–propylene carbonate (PC) solution. One reason is that most POMs' electrochemical activity cannot be fully expressed in non-aqueous media. Water and H⁺ play important roles in the expression of EC behavior of POMs. Some POMs did not exhibit EC behaviors in the LiClO₄–PC non-aqueous electrolyte. The larger sized Li⁺ diffuses with more difficulty than the H⁺ cation in the POM-based EC films, especially when compared to the condensed POMs-based films prepared by the LBL method. Therefore, the POM itself and the structure of the films would affect the performance of the EC films greatly. The application of the porous substrate would make the Li⁺ easily diffuse in the films. Because of the restriction of the diffusion of the electrolyte, it should be remembered that not all the POM molecules in the film can be reduced in a short time. Therefore, using the smaller sized POM molecules may enhance the ratio of POM molecules to contact with the intercalated Li⁺ cations. The Keggin type POMs with a small size and good electrochemical activity were chosen in the research reported in this paper. The different Keggin type POMs were obtained through the transition metal substitution of some of the tungsten (W) atoms in the basic molecule.¹⁰ It would be a sensible strategy to tune the performance of the POM-based EC films. It is also reported that the vanadium(V) doped WO₃ has a more efficient EC performance.¹¹ In our previous work, the multi-color EC film was obtained using a tri-vanadium substituted Dawson type POM molecule. Therefore, vanadium substituted Keggin type POMs are the clear choice.

In this paper, vanadium substituted Keggin type POMs K_3+n[α-PW_12−nV_nO₄₀] (n = 1, 2, 3) (α-PW₁₁V, α-PW₁₀V₂ and α-PW₉V₃) are used as EC materials to prepare the EC films. The performance of the films is tested in H⁺-based aqueous electrolyte and LiClO₄-based non-aqueous electrolyte. All the films showed excellent performance in both of the electrolytes. Furthermore, the films are more stable in the LiClO₄-based non-aqueous electrolyte.

Experimental

Materials

The Keggin type vanadium substituted POMs were prepared according to methods found in the literature.¹² The details of the preparation procedure are presented in the ESI.† The titanium dioxide (TiO₂) paste with a particle size of approximately 18 nm was obtained from Dyesol Ltd. Fluorine doped tin oxide glass (14 Ω sq⁻¹, Nippon Sheet Glass) was purchased from HeptaChroma (Dalian, China). The electrolyte is 0.1 M LiClO₄ PC solution and 0.1 M hydrochloric acid (HCl) aqueous solution. The other reagents were all purchased from Aladdin.

The preparation of the POM-based films

The TiO₂ substrate was prepared using the screen printing method, followed by a sinter process at 450 °C for 30 min. The POM-based composite films were prepared with an electrodeposition process, that had been previously reported, with a slight modification. The electrodeposition process is as follows: the counter electrode is a platinum (Pt) plate (1.0 cm²); the reference electrode is a saturated calomel electrode (SCE) and the TiO₂ substrate acts as the working electrode. The working electrode was immersed in the PW₁₁V, PW₁₀V₂ and PW₉V₃ aqueous solution (pH ∼ 2.0, 1.0 mM), then electrodeposition was carried out using cyclic voltammetry (CV) between −1.3 and 0.1 V at a scan rate of 100 mV s⁻¹ for 30 cycles. After that, the films were rinsed with deionized water, absolute alcohol and then dried with hot air. Finally, the films were placed in the oven at a temperature of 150 °C for 30 min.

Characterization

Electrochemical experiments were performed on a CHI-660D electrochemistry station (CH Instrument Corporation, Shanghai, China). The films act as working electrodes: the counter electrode was Pt wire and the reference electrode was an SCE. The electrolyte was 0.1 M LiClO₄ PC solution and 0.1 M HCl aqueous solution. Scanning electron microscopy (SEM) was carried out using an Hitachi S-4800 scanning electron microscope. Atomic force microscopy (AFM) measurements were performed in air with a Seiko SPI 3800N Probe Station. Visible light absorption spectra and transmittance spectra were obtained using a Varian Cary 500 UV-visible near infrared (UV-vis NIR) spectrometer. IR spectra were recorded in the range of 400–2000 cm⁻¹ on a Bruker Alpha Fourier transform infrared spectrophotometer.

Results and discussion

There are several isomers of Keggin type POMs.¹³ In this research, vanadium substituted α-Keggin isomers were employed. The substitution occurs in the same polar site. The structure of the three vanadium substituted Keggin type POMs are shown in Fig. 1. The chemical properties of the three POMs are similar for they adopt the same basic structure.¹⁴ However, the EC properties would be slightly different from each other which was the result of the different amounts of vanadium in the three molecules. Therefore, it is useful to study the EC properties of the isomers to further understand the relationship between the amount of vanadium in the POMs and the EC properties. Stability is an important feature for assessing an EC film. For the POM-based EC composite films, the stability is determined by the intensity of the interaction between the POMs and the TiO₂ substrate. The intensity of the interaction between the POMs and the TiO₂ substrate was confirmed from the IR spectra and the repeated CV tests of the as-prepared films in electrolyte. The IR spectra of pure TiO₂ substrate, α-PW₁₁V and a composite film are shown in Fig. 2. For the TiO₂ substrate, there are no characteristic peaks in the wavenumber range of 1100–800 cm⁻¹. Whereas for the pure α-PW₁₁V, there are three characteristic peaks at 1092, 1047 and 969 cm⁻¹, which could be attributable to the asymmetric stretching vibration of the W–O–W and W–O–V bonds. For the PW₁₁V-based composite film, the characteristic peaks of the PW₁₁V are still retained at 1096, 1045 and 957 cm⁻¹ in the IR spectrum with little shift. The results indicated that there is an intense interaction between α-PW₁₁V and the TiO₂ substrate. Similar results are also found for the α-PW₁₀V₂, α-PW₉V₃-based composite films; such intense interactions between α-PW₁₀V₂ or α-PW₉V₃ and TiO₂ were also established during the electrodeposition process. As shown in Fig. S1,† the characteristic IR peaks of pure α-PW₁₀V₂ were at 1086, 1052 and 960 cm⁻¹, and the characteristic peaks for the α-PW₁₀V₂-based composite films were located at 1083, 1046 and 956 cm⁻¹. As shown in Fig. S2,† the characteristic IR peaks of pure α-PW₉V₃ are at 1084, 961 and 887 cm⁻¹, and the characteristic peaks for the α-PW₉V₃-based composite films were located at 1079, 966 and 885 cm⁻¹. The shift of the characteristic peaks indicates that there is an intense interaction established between the POMs and the TiO₂ substrate. The repeat CV tests were also employed to confirm the intense interaction between the POMs molecules and the TiO₂ substrate. The tests were carried out by placing the as-prepared composite films in the electrolyte (0.1 M LiClO₄ PC solution or 0.1 M HCl aqueous solution). As shown in Fig. S3 and S4,† the peak current value was related to the amount of POM molecules in the film, and the doubling of each cycle of the CV curves indicated that there is no loss of POM during the test. All the three POM-based films showed good stability in both electrolytes. The firmly combination of POMs and TiO₂ substrate could be because of the following reasons: firstly, hydrogen bonds were formed between oxygen atoms of the [PW_12−nV_nO₄₀]⁽ⁿ⁺³⁾⁻ (n = 1, 2, 3) and the surface hydroxyl groups (Ti–OH) of the TiO₂ network; secondly, chemically active surface Ti–OH groups were protonated under an acidic medium to form Ti–OH₂⁺ groups during the electrodeposition process. The Ti–OH₂⁺ group could act as a counter ion for [PW_12−nV_nO₄₀]⁽ⁿ⁺³⁾⁻ (n = 1, 2, 3) and yielded the acid–base reaction.¹⁵

Fig. 1 The structure of α-PW₁₁V (a), α-PW₁₀V₂ (b) and α-PW₉V₃ (c).

Fig. 2 IR spectra of (a) the TiO₂ substrate, (b) α-PW₁₁V and (c) the composite film.

The morphology of the films was also determined using SEM and AFM. The morphology would affect the transparency of the films and the diffusion of the cations. From the SEM images in Fig. 3, it can be concluded that the diameter of the particles of the TiO₂ substrate is approximately 20 nm, which guarantees the transparency of the device after bleaching is good. There is hardly any change of the particle diameter of films after the PW_12−nV_n (n = 1, 2, 3) was deposited on the films. Fig. 3b–d show no aggregation of POM molecules either on the surface or in the pores after the POMs were deposited. Therefore, the POM molecules would not affect the transparency of the films. The homogeneity of the films was also confirmed by AFM. The AFM image of the as-prepared TiO₂ substrate is shown in Fig. S5,† the root-mean-square (RMS) roughness for the TiO₂ substrate is 15.5 nm. It is regular without aggregation of the POM molecules (as shown in Fig. 4 and S6†). The RMS roughness of the POM composite films was 11.9, 12.4 and 10.5 nm for the α-PW₁₁V, α-PW₁₀V₂ and α-PW₉V₃-based composite, respectively. However, the RMS roughness for the TiO₂ substrate is 15.5 nm which is larger than that of the composite films. This should be attributed to the deposited POM molecules which make the surface of the film more smooth. The thickness of the TiO₂ substrate and the composite films is approximately 4 μm, which was determined using the step profiler.

Fig. 3 SEM images of (a) the TiO₂ substrate, (b) α-PW₁₁V-based composite film, (c) α-PW₁₀V₂-based composite film and (d) α-PW₉V₃-based composite film.

Fig. 4 AFM images of (a) the α-PW₁₁V-based composite film, (b) α-PW₁₀V₂-based composite film and (c) α-PW₉V₃-based composite film.

The EC process of the POMs is also accompanied by the co-intercalation of small cations and an equal quantity of electrons. Usually, for POMs-based EC films, the obvious EC phenomena are detected under acid aqueous solutions. However, the non-aqueous neutral Li⁺-based solutions are preferred more for use in the EC device. Therefore, the EC behaviors of the films were tested in the 0.1 M HCl acid aqueous solution and 0.1 M LiClO₄ PC solution. The diffusion coefficient (D) of the cations is an important parameter for an EC film. In accordance with the Randles–Sevcik equation,¹⁶ the CV at different scan rates was carried out to calculate the D value:

where I_p is the peak current density, n is the number of electrons, C₀ is the concentration of active ions in the solution, ν is the scan rate, and A is the area of the film. D values of the as-prepared films were attained by obtaining the CVs of the films at the scan rates of 25, 50, 75, 100, 125 mV s⁻¹ in 0.1 M LiClO₄ PC solution and in 0.1 M HCl aqueous solution. As shown in the insets of Fig. 5 and 6, the peak current has a good linear relationship with the square root of the scan rate, which indicates that the co-intercalation of H⁺/Li⁺ and the equal quantity of electrons into the POM-based composite films are fast and diffusion confined. When using the 0.1 M HCl aqueous solution as an electrolyte the D values are calculated to be 4.32 × 10⁻¹⁰, 7.83 × 10⁻¹⁰, and 6.15 × 10⁻¹⁰ cm² s⁻¹ for α-PW₁₁V, α-PW₁₀V₂ and α-PW₉V₃-based EC film, respectively. The diffusion of the H⁺ in all the three films is fast. However, for the larger Li⁺ cation the D values decreased by a different extent. When using 0.1 M LiClO₄ PC solution as electrolyte, the D values of the α-PW₁₁V, α-PW₁₀V₂ and α-PW₉V₃-based EC film were 4.05 × 10⁻¹¹ cm² s⁻¹, 4.11 × 10⁻¹¹ cm² s⁻¹, and 1.04 × 10⁻¹¹ cm² s⁻¹, respectively. The Li⁺ and H⁺ ions show the best diffusion in the α-PW₁₀V₂-based film, and diffusion is a little slower in α-PW₁₁V and α-PW₉V₃-based films. The results indicated that the diffusion of Li⁺ is more difficult than that that of H⁺ in the films. The D value for the high performance WO₃-based EC films are also around a 10⁻¹¹ order of magnitude.¹⁷ Therefore, the diffusion of the Li⁺ would not be a barrier to obtain the high performance EC films (Table 1). It is not popular to use a Li⁺-based non-aqueous electrolyte for the POM-based EC film partly because not all the POM show EC properties under the non-aqueous system. It could also be attributed to the structure of the films that the extensively used LBL prepared film is more dense which results in the blocking of the diffusion of the larger Li⁺ in the films. However, the porous films make it possible that the cations could intercalate/deintercalate from the POM molecules swiftly.

Fig. 5 Cyclic voltammograms (CVs) at different scan rates (25, 50, 75, 100, and 125 mV s⁻¹) of (a) the α-PW₁₁V-based EC film, (b) α-PW₁₀V₂-based EC film and (c) α-PW₉V₃-based EC film in the H⁺-based aqueous electrolyte. Insets: plots of peak current versus the square root of scan rate.

Fig. 6 Cyclic voltammograms (CVs) at different scan rates (25, 50, 75, 100, and 125 mV s⁻¹) of (a) the α-PW₁₁V-based EC film, (b) α-PW₁₀V₂-based EC film and (c) α-PW₉V₃-based EC film in Li⁺-based non-aqueous electrolyte. Insets: plots of peak current versus the square root of scan rate.

Table 1 The comparison of the D value^a of the POM-based films in H⁺- and Li⁺-based electrolytes

EC film	α-PW11V	α-PW10V2	α-PW9V3
a The unit of the D value is cm^2 s^−1.
H^+-Based electrolyte	4.23 × 10^−10	7.83 × 10^−10	6.15 × 10^−10
Li^+-Based electrolyte	4.05 × 10^−11	4.11 × 10^−11	1.04 × 10^−11

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The color of the films were also tested in H⁺ and Li⁺-based electrolytes. The films all exhibited a gray color in the H⁺-based electrolyte. As shown in Fig. 7, the absorbance of the α-PW₁₁V-based film is 1.28 at a wavelength of 400 nm and the absorbance is 0.85 at a wavelength of 640 nm under an applied potential of −1.6 V. Therefore, the color of the films is deep gray. The α-PW₁₀V₂-based film has two characteristic peaks at wavelengths of 690 nm and 400 nm (Fig. S7a†). The absorbance value of the peak at 690 nm is 1.03 and the absorbance of the peak at 400 nm is 0.78 which results in the color of the film being bluish grey. The absorbance value for a α-PW₉V₃-based film is 2.12 at a wavelength of 380 nm in the UV region, which is the largest of the three POM-based EC films. The α-PW₉V₃-based film also shows a large absorbance in the visible region. Its absorbance is 1.19 at the wavelength of 560 nm (Fig. S7b†). The intense absorbance at 380 nm means that the characteristic peaks in the visible light region were not very distinct. However, the α-PW₉V₃-based EC film shows intense absorbance at 400 nm at a relatively low applied potential, which could be attributed to the fact that the EC property of the α-PW₉V₃-based film is easy to achieve in the acid media. The α-PW₉V₃-based film is most sensitive to the changes of the applied potentials. A slight change of the applied potential results in a large change of the modulation. When the Li⁺-based electrolyte is employed, the absorbance peak at 400 nm becomes weaker and the absorbance peak around 650 nm becomes more intense for the three POM-based EC films. As a result, the extrinsic hue of the films is blue. The EC activity of the POMs becomes low in the non-aqueous media, so the applied potential was increased to achieve a similar modulation scale as the one which was obtained in the H⁺-based electrolyte. For the α-PW₁₁V-based film, the absorbance peak is located at 650 nm (Fig. 7b). Compared to the film in the H⁺-based electrolyte, the absorbance peak at 400 nm becomes less intense and the intensity of the peak at 650 nm still remains similar to that which was obtained in the H⁺-based electrolyte. The slight decrease of the absorbance value could be attributed to the fact that the larger Li⁺ cation has difficulty in diffusing in the film. However, the modulation is also sufficient to apply in a real EC device. The modulation scale of a α-PW₁₀V₂-based film is smaller than that of the α-PW₉V₃-based film at the same applied potential. For the α-PW₁₀V₂-based film, the largest absorbance of the film is 1.2 at a wavelength of 650 nm under the applied potential of −2.0 V (Fig. S8a†). For the α-PW₉V₃-based film, the largest absorbance is 1.3, which is achieved at 650 nm under the applied potential of −2.0 V (Fig. S8b†). For all the three different POM-based EC films, the optical contrast becomes slightly decreased in the Li⁺-based non-aqueous electrolyte compared to that in the H⁺-based aqueous electrolyte.

Fig. 7 Visible spectra of the α-PW₁₁V-based film in (a) H⁺-based aqueous electrolyte and (b) Li⁺-based non-aqueous electrolyte under different applied potentials.

The response time is also an important parameter and is used to evaluate an EC film. The response time usually refers to coloration/bleaching time extracted for a 90% transmittance change. The intercalation/deintercalation speed of the cation would affect the response time directly. In the H⁺-based aqueous electrolyte, for the α-PW₁₁V-based EC film, the coloration time (t_c,90%) is 15.6 s and the bleaching time (t_b,90%) is 8.7 s (Fig. 8a). For the PW₁₀V₂-based film, the t_c,90% is 17.2 s and the t_b,90% is 6.4 s. The α-PW₉V₃-based film exhibits the shortest response time that is, the t_c,90% is 7.1 s and the t_b,90% is 2.4 s (Fig. S9†). Although the Li⁺ intercalation is more difficult to achieve in the non-aqueous electrolyte than the H⁺ intercalation in the aqueous solution, nevertheless the response times of the three films do not all become slow. As shown in Fig. 8b, the t_c,90% for α-PW₁₁V-based film is 19.5 s which is longer than the one obtained in the H⁺-based aqueous electrolyte, while the t_b,90% is 4.3 s is shorter. For the α-PW₁₀V₂-based film, the response time is shorter than that which is obtained in the H⁺-based aqueous electrolyte, i.e., the t_c,90% is 6.7 s and the t_b,90% is 5.2 s (Fig. S10a†). The response time for the α-PW₉V₃-based film is longer than the one obtained in the H⁺-based aqueous electrolyte, i.e., the t_c,90% is 13.6 s and the t_b,90% is 6.2 s (Fig. 10b). In the H⁺-based electrolyte and Li⁺-based electrolyte, it had nearly the same response time which could be also ascribed to the porous feature of the films. The pores in the films guarantee the swift diffusion of all the different sized cations. Tuning the degree of substitution of the POMs could also influence the response time. The POM molecules belong to the poly anions, and there are different electrons on different molecules, which would also affect the diffusion of the cations. The POM with one vanadium atom substituted (α-PW₁₁V) has four negative charges, the POM with two vanadium atoms substituted (α-PW₁₀V₂) has five negative charges and the POM with three vanadium atoms substituted (α-PW₉V₃) has six negative charges. The anion could attract the Li⁺ cation which made the intercalation easier. However, the attractive force would make the deintercalation of Li⁺ difficult, which affects the bleaching time of the films. The PW₉V₃-based EC film has the longest bleaching time, whereas the PW₁₁V-based EC film has the shortest bleaching time. This phenomenon is also consistent with the results of the diffusion coefficient. The bleaching time of the films in the Li⁺-based electrolyte increases with the increasing of the electrons on the POM molecules.

Fig. 8 Response time for the PW₁₁V-based film in (a) H⁺-based aqueous electrolyte and in (b) Li⁺-based non-aqueous electrolyte.

Fig. 9 (a) Potential, (b) current and (c) transmittance at 650 nm of the PW₁₁V-based EC film during the subsequent double-potential step chronoamperometric experiments at −1.5 V to +1.5 V in H⁺-based aqueous electrolyte.

Fig. 10 (a) Potential, (b) current and (c) transmittance at 650 nm of the PW₁₁V-based EC film during the subsequent double-potential step chronoamperometric experiments of −2.0 V to +1.8 V in Li⁺-based non-aqueous electrolyte.

The double-potential step chronoamperometric experiment was carried out to evaluate the performance of the films. The transmittance of the films was recorded simultaneously using UV-vis spectroscopy. The double-potential steps were −1.5 V to +1.5 V for α-PW₁₁V-based EC films. Fig. 9b and c show the corresponding changes of the current and transmittance. The optical contrast of α-PW₁₁V-based EC films is 75.8% (λ = 650 nm) at the applied potential of −1.5 V. A residual leakage current was rapidly achieved during coloration and the current decays rapidly to zero during bleaching for the three EC films. Similar double-potential step chronoamperometric experiments were also carried out to test α-PW₁₀V₂- and α-PW₉V₃-based EC films in H⁺-based electrolyte. For the α-PW₁₀V₂-based film (Fig. S11†), the optical contrast was 74.5% at 650 nm under the applied double-potential steps of +1.0 V to −1.0 V. For the α-PW₉V₃-based EC film (as shown in Fig. S12†), the optical contrast was 79.7% at 550 nm under the applied double-potential steps of +1.0 V to −1.5 V. When the films were placed in the LiClO₄-based electrolyte, the optical contrast became smaller or needed a more negative applied potential to achieve the same optical contrast as that which was obtained in the H⁺-based electrolyte. As shown in Fig. 10, the α-PW₁₁V-based film exhibited the maximum absorbance at a wavelength of 660 nm and the modulation was 44.7% at 650 nm under applied double-potential steps of +1.8 V to −2.0 V. The α-PW₁₀V₂ and α-PW₉V₃-based EC films displayed similar results in the LiClO₄-based electrolyte. As shown in Fig. S13 and S14,† the α-PW₁₀V₂-based EC film achieved an optical contrast of 66.3% at a wavelength of 690 nm under applied double-potential steps of +1.5 V to −1.8 V. The α-PW₉V₃-based EC film achieved an optical contrast of 77.6% at a wavelength of 655 nm under applied double-potential steps of +1.8 V to −1.8 V. The stability and reversibility of the α-PW_12−nV_n (n = 1–3)-based EC films were tested by repetitive double-potential step chronoamperometric both in the H⁺-based electrolyte and in the Li⁺-based electrolyte. As shown in Fig. S15,† the response time for coloration and bleaching, and the optical contrast of the PW_12−nV_n (n = 1–3)-based EC films did not change noticeably after 500 cycles in the Li⁺-based non-aqueous electrolyte. When the test cycles up to 1000, there was a slight decay of the optical contrast of the three EC films. The optical decays were 5.3%, 4.9% and 4.7% for PW₁₁V-, PW₁₀V₂- and PW₉V₃-based films, respectively. Also, when changing the electrolyte to the H⁺-based aqueous electrolyte, the optical contrast of the PW_12−nV_n (n = 1–3)-based EC films did not change noticeably after 500 cycles. However, when the cycles were increased to 1000, the optical contrast decays became obvious. The optical decays were 5.1%, 9.3% and 8.4% for PW₁₁V-, PW₁₀V₂- and PW₉V₃-based film, respectively (Fig. S16†). Therefore, the non-aqueous electrolyte aids the long term stability of the POM-based EC film compared to the H⁺-based electrolyte.

Conclusions

In summary, all the results of this research indicated that the POM-based EC films can not only show good performance in acidic aqueous electrolyte, but they also exhibit high performance in a neutral non-aqueous system. The porous morphology of the films guarantees the cations' swift intercalation and deintercalation. Although the D values of the Li⁺ in the P₂W_18−nV_n (n = 1–3)-based EC films are smaller than that of the H⁺ in the film, they are still large enough to give a relatively short response time. The performances of different POM-based films were also different, which could be attributed to the difference of the POM molecules. Changing the number of substituted vanadium atoms resulted in the change of their EC film performance. The use of the Li⁺-based non-aqueous electrolyte avoided the disadvantages accompanied with the H⁺-based aqueous electrolyte. The application of the neutral and non-aqueous-based electrolyte in the POM-based EC film is an important progression in the development of the POM-based EC device. The work of developing high performance POM-based EC films and devices is in progress in our group.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21501084, 21271096), the Liaoning Province Doctor Startup Fund (20141052), and the Natural Science of Liaoning University (2013LDQN19 and LDGY201412).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03037h

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