Polymer-free gel electrolyte and its application in TiO2-based electrochromic devices

Electrochromic devices based on polymer-free gel electrolytes (PFGEs) offer several advantages over polymer electrolytes. The preparation and characterization of a novel fumed silica-based PFGE and its applications in TiO2 electrochromic devices (ECD) were the main aims of the present study. First, a series of liquid electrolytes were prepared by mixing lithium chloride (LiCl) and ethylene Glycol (EG) with different molar ratios and their ionic conductivities were measured to get an idea about the highest ionic conductivity composition. The total oxygen atoms of EG to lithium ions of LiCl molar ratio (O:Li+) was altered from 5:1 to 80:1. The highest ionic conductivity was observed for 15: 1 molar ratio with the value being the 1.28 × 10− 2 S cm− 1. This optimized composition was selected for preparing PFGE. In order to prepare PFGE, 10 wt% of fumed silica from the total weight of EG and LiCl were added to the optimized liquid electrolyte EG/LiCl as the polymer-free gelling agent. The maximum ionic conductivity was found in O:Li = 10: 1, with the value being 8.94 × 10− 3 S cm− 1. ECDs were prepared by sandwiching this PFGE between TiO2 electrochromic electrode and fluorine-doped tin oxide (FTO) counter-electrode with the configuration of FTO/TiO2/PFGE/FTO. Notable electrochromic properties of TiO2-coated FTO with higher optical modulation of 64% at 700 nm and 33% at 550 nm by applying 4.2 V and a switching speed of Tbleaching= 42.5 s and Tcoloring= 16.7 s were observed.


Introduction
The expanding crisis of energy and environmental challenges feels a necessity for the evolution of sustainable, economical, and renewable resources.Electrochromism is referred to as unique characteristic of materials in which optical properties being vary their two different color states or transparent to color state reversibly when subjected to a small electric field.In recent two decades, electronics has taken much more attention regarding lowpower consumption in daily life routine.For the technology of smart windows, electrochromism, thermochromism, and photochromism are effective energy conversion technologies which control lighting and temperature [1][2][3].ECDs are widely used in promising applications such as buildings, displays, windows, sunroofs, and agricultural lighting controllers to control and maintain incoming light and heat.
Electrochromism is one of the best experiences for the color changes in a redox reaction of electrochromic materials (ECMs) and active ions.ECD mainly consists of three main components; an active (working) electrode which coated with EC layer, an electrolyte or ion transport medium, and a counter-electrode or ion storage film.The electrolyte should be placed between working electrode and counter-electrode by connecting them ionically but separating physically.ECMs are mostly organic or inorganic compounds [4].The optical properties such as transmittance, absorption, and reflectance can be changed due to changes in the electronic state of complex molecules.This is a reversible process that is associated with an electrochemically induced oxidation-reduction reaction by applied electric potential.It results in the absorption of a range of electromagnetic radiation of different visible colors when switching between redox states.[5,6].When a voltage is applied across the electrochromic material, it is able to change its optical properties.The optical properties should be reversible that means the original state should be recoverable if the polarity of the voltage is changed.
Cation (M + ) insertion to the ECM is the common type and electronic stability is balanced by the absorption of a balancing electron.This can be illustrated in the following stoichiometric Equation [7].
Although lots of materials show electrochromic properties, quite all of them are not commercialized and out of them, tungsten oxide (WO 3 ) has been the most deeply studied ECM used in ECDs [2,14,15].However, the electrochromic properties and coloration cycle of WO 3 strongly depend on its preparation techniques and structure.Vacuum evaporation and sputtering technique are given high coloring performances in WO 3 thin films.
However, these methods are difficult to apply in largearea commercial applications since they are expensive and time-consuming.Among other ECMs, TiO 2 might be a possible substitution for tungsten trioxide owing to its inherent advantages such as low cost, abundance, and low environmental impact.In early studies, transparent films were also fabricated using processed TiO 2 applying simple techniques such as doctor blading [16], screen printing [17], spraying [18], etc.
Apart from the active electrode, another critical component is the electrolyte.It is crucial for the improvement of the efficiency and stability of ECDs due to the ionic movements through them.Liquid-type electrolytes are more common in efficient ECDs and their pragmatic applications are bounded due to some technical issues, such as durability, leakage, and relatively low long-term stability related to other chemical constituents and the presence of water and volatile solvents.Therefore, considerable interest is raised in solid-state or quasi-solid-state electrolytes for ECDs.
In spite of the advantages of gel electrolytes, there are a few drawbacks such as low ionic conductivity and relatively small cycle life.Generally, crystalline polymers are not suitable for batteries and ECDs due to their low conductivity [19].In solvent-doped polymer electrolytes, systems have overcome promising properties such as volatilization, electrode corrosion, and the absence of leakage for ECDs.Although organic polymer electrolytes are widely used in ECDs, several limitations such as cycle life and flammability have been monitored [20,21].
Fumed silica has been used as a substitution for the gelling medium.It is concerned as a high-surface molecule in surface chemistry.In an aprotic solvent, hydroxyl groups can interact with silica via hydrogen bonding of the surface, which results in the formation of a three-dimensional structure.In composite polymer electrolytes (CPEs), fumed silica isn't used as a catalyst like ethylene carbonate, while it is used as a catalyst supporter instead.Silicon is positioned next to carbon in the same group and hence the properties of carbon and silicon are almost same.However, Si-O linkages are more important than C-O linkages in polymers due to high thermal stability, chemical stability, and oxidative stability.There are a lot of studies have been done incorporating SiO 2 and TiO 2 nanoparticles with polymer matrix as a filler to improve the amorphous nature of the gel polymer [22,23] electrolyte.Almost all early research and experiments have been done on SiO 2 composite polymer gel electrolytes, but they did not apply merely fumed silica to the electrolyte of ECD.Apart from this, the availability, cost, and complications in preparing thin films of EC electrodes and counter-electrodes should be taken into account for large-scale production of ECDs using low-cost materials.Due to these reasons, tin oxide (SnO 2 ), which has been an intensively studied semiconducting material for numerous applications and possesses unique properties such as high optical transmittance together with high conductivity, could be a good candidate to be used as the counter-electrode in ECDs.In this study, we have successfully used F-doped tin oxide (FTO) glass substrate as the counter-electrode instead of SnO 2 .Fabrication of low-cost TiO 2 on FTO was done by the doctor-bladed method and it is a very good approach for large-scale production.
By considering all the aforementioned reasons, the fabrication of a novel polymer-free gel electrolyte-based electrochromic device using TiO 2 as the electrochromic material with FTO as the counter-electrode and a mixture of EG/ LiCl/Fumed silica as the ion-conducting electrolyte was focused.

Preparation of electrolyte
First, a series of liquid electrolytes were prepared with a different molar ratio of O:Li + in the electrolyte, and their conductivity values were measured.To prepare the liquid electrolytes, an appropriate amount of vacuum-dried LiCl was added to the EG solvent and a set of liquid electrolytes was prepared by varying the molar ratio of total oxygen atoms from EG to a Li + ions from LiCl (O:Li + = n:1).Here, n was varied from 1 to 80 and their conductivities were measured and highest ion-conducting liquid electrolyte samples were selected and used to prepare the gel electrolytes.In order to prepare the PFGE, different weight percentages of dehydrated fumed silica (10, 12, and 15 wt%) were introduced to the high ion-conducting liquid electrolytes samples, which were optimized previously, and mixtures were stirred until a clear PFGE was obtained.The 10 wt% fumed silica was selected to prepare the PFGE and further characterization was done to this electrolyte.

Electrolyte characterization
The ionic conductivities of the liquid and the gel electrolyte were calculated applying AC complex impedance spectroscopy using Solatron SI-1260 impedance analyzer and Autolab 3 instruments in the frequency range 20 Hz-10 MHz.The liquid electrolyte was filled into a special holder by blocking two stainless steel electrodes and the impedance was measured at room temperature.The PFGE was trapped between two stainless steel electrodes and put it into an oven to measure the AC impedance measurement with respect to different temperatures.Concerning the electrochemical performances and the stability of the liquid and the gel electrolytes, the highest conducting liquid and PFGEs were selected and characterized.They were characterized by testing cycle life and performance by changing the scan rate by using Metrohm Auto lab 2 and Bio Logic SP-300.FTIR spectra of liquid and polymer-free gels were obtained in the range of 4000-550 cm − 1 using Bruker tensor 27 IR spectrophotometer and the attenuated total reflectance attachment was used to obtain the spectrum for the gels and liquids.The morphology and microscopic information of the PFGEs was investigated using Scanning electron microscopy (SEM).

Fabrication of the ECD
FTO glass was coated by commercial TiO 2 paste using the "Doctor Blade" technique and it was sintered in a hightemperature furnace at 450 °C.ECDs were fixed as FTO glass/TiO 2 / PFGE/FTO glass with dimensions 2.0 × 1.5 cm 2 as follows.A 0.2 mm spacer was kept on the TiO 2 electrode to maintain even spacing for the electrical insulation from the FTO.Then the frame was filled with the PFGE and another FTO plate was put on the electrolyte layer as a counter-electrode.

Electrochemical performances
Electrochemical properties of the ECD were analyzed by using cyclic voltammetry, and it was carried out using Metrohm Autolab 2 with three-electrode configurations, where TiO 2 -coated FTO served as the working electrode, silver /silver chloride (Ag/AgCl) electrode served as the reference electrode, and carbon rod served as the counter-electrode.The liquid electrolyte has been tested for 500 cycles under cyclic voltammetry for different scan rates ranging from 4 mVs − 1 to 400 mVs − 1 .
The performance of the ECDs was tested, by applying a small voltage externally between two electrodes.The potential difference between the counter-electrode and the working electrode was increased gradually from 0.0 to 4.2 V.The transmittance of the devices was measured by using Shimadzu UV-1800 under a range between 400 and 1100 nm.Transmittance at 700 nm was recorded while toggling applied voltage 4.2 V and − 0.5 V to obtain the kinetic graph.

Ionic conductivity
Clear transparent solutions were obtained for different compositions of EG and LiCl with O to Li ratio of n:1.(O to Li molar ratio is the number of oxygen atoms from EG to the number of Li-ions from LiCl) LiCl salt is highly dissolved in EG and results in highly viscous electrolytes.
The ionic conductivity was calculated using the relation σ = l / RbA, where l is the thickness of the electrolyte sample, A is the contact area between the electrolyte and the electrode and the Rb is the measured resistance.This Rb value was determined by the intercept of the straight line with real axis of the Z'' Vs Z' Nyquist plot, Fig. 2a.
Conductivity variations of the liquid electrolytes at room temperature are shown in Fig. 1 as a function of n: 1 molar ratio.Results indicate that there is an ionic conductivity enhancement in the liquid electrolytes with the increase of n:1 ratio from 5: 1 to 15:1.The highest ionic conductivity value of 1.28 × 10 −2 S cm −1 was observed at n:1 = 15:1.According to the Fig. 1, further increment in the salt concentration (n:1 beyond 15:1) gradually decreases the ionic conductivity.Increment in the ionic conductivity with the Li + concentration is interconnected with the number of mobile ions.However, the fall in the ionic conductivity of liquid electrolyte from 15:1 to 5:1 molar ratio is due to the agglomeration of ions and the hindrance to the ionic mobility due to agglomerations [24].This reduces the free ions in the solution and the ionic mobility [25].If the concentration goes beyond the solubility limit of the salt in the solvent, it will separate as a precipitate and this may cause to make crystalline phase in the electrolyte.Hence, resistance for ion transfer increases and ionic conductivity decreases accordingly.
LiCl, EG, and Nano-Silica polymer-free gel electrolyte was prepared by adding different percentages of dehydrated fumed silica (8, 10, 12, and 15 wt%) into the liquid electrolyte, and mixtures were stirred until a clear gel was obtained.The best gelling properties were observed for the 10 wt% fumed silica and it was selected to prepare the gel electrolytes for further characterization.When 10 wt% of fumed silica was added to the liquid electrolyte, the ionic conductivities of GEs of all O:Li + ratios were dropped compared to the conductivities of liquid electrolytes.The reduction of the ionic conductivity in the gel form is due to hindrance to the ionic mobility in the gel medium due to increased viscosity.The ionic conductivity of the PFGEs increases with the salt concentration until O:Li + = 10:1 and decreases gradually when the salt concentration is increased further.Although the ionic conductivity of the liquid electrolyte was optimized at O:Li + =15:1, the highest conductivity for gels were obtained at O:Li + =10:1 with 10 wt% fumed silica.These results are well matched with the values of pre-exponential factor and the activation energy calculated by Eq. 2.
where A is the Arrhenius constant (pre-exponential factor), E a is the activation energy, and T is the absolute temperature.k is the Boltzmann constant.The calculated values are given in Table 1.The activation energy (E a ) of a gel electrolyte presents the properties of energy characteristics.
(2) = Aexp − E a kT , Among the studied samples, the lowest activation energy was obtained in sample 2 which has the maximum ionic conductivity.
The amount of charge carriers in the electrolyte is proportional to the pre-exponential factor and values obtained for the electrolyte composition for O:Li + =15:1 and O:Li + =7.5:1 ratio were almost the same.This might be due to the reduced ion aggregations of electrolyte O:Li + = 10:1 with the addition of the fumed silica and it shows low activation energy than the O:Li + =7.5:1 ratio.
The Nyquist plot of the gel electrolyte with O:Li + ratio of 10:1 at 25 °C is given in Fig. 2a.High-frequency semi-circle disappearing in the plot is due to poor electrochemical activity and the charge exchange process [26].Figure 2b shows the ionic conductivity variations of studied gel electrolyte samples according to the temperature.It shows an increase in the ac conductivity with increasing temperature.This may be due to the dissociation of ion aggregates with increasing temperature.The curves obtained for the variation of ionic conductivity (σ) of the gel electrolyte systems at elevated temperatures can be well fitted with the Arrhenius equation given in Eq. 2.

FTIR analysis
FTIR spectroscopy was conducted to get a better understanding of binding ability among compounds, solubility, and structural behavior of EG/LiCl blends [27].The best ionic conductive solutions of the EG/LiCl series and pure EG were systematically studied in order to determine the interaction of the Li-ions with the solution.In the FTIR spectrum, particular attention on the peaks analogous to functional groups in the range of 4000 − 550 cm − 1 [28] was mainly focused.
Peaks inherent in 3100-3600 and 900-1200 cm − 1 are identical to the (O-H), (C-O-H) and (C-O) stretching modes.The peak in 3100-3600 cm − 1 range was corresponded to the O-H stretching mode of EG [29].The cation, Li + can be expected to coordinate with the O atom of the ethylene glycol as seen by previous studies [30] O-H stretching vibrational frequency decreases with the increasing Li-ion concentration as in Fig. 3a and also given in Table 1.O-H stretching vibrational frequency decreases with the increasing Li-ion concentration as in Fig. 3a and also given in Table 1.The hydration of Li + causes the sudden decrease of the ionic conductivity while increasing the Li + ion of liquids from n:1 = 15:1 to 5:1 molar ratio.This agrees with the diminution of ionic conductivity with the increment of LiCl concentration beyond 15:1 molar ratio.The ion transportation characteristics are mainly affected by the ionic clusters in electrolyte systems.[31] Li-ion agglomeration could be mainly due to the arrangement of ions and molecules as ion pairs and aggregates as discussed elsewhere with similar molecular systems [30].The cation interaction with the polar atom is evident in the solvent molecular modes as follows [32,33].
The C-O-H bending vibration mode has a peak at 1405 cm − 1 , CH 2 wagging vibrations at 1330 cm −1 ,and the C-O stretching vibration modes at 1083 cm − 1 for pure EG is shifted to lower wavenumbers by about 4 cm − 1 wavenumbers as the increase of the salt concentration due to the bathochromic shift (Table 2).The rearrangement of π-electrons in alternating bonds is shorten the distance and condensed each other while increasing the salt concentration [34,35].However, the position of the peak due to C-C stretching mode at 862 cm − 1 does not change significantly with the increasing Li + concentration.It clarifies the minimum effect of Li + ions to the C-C bonds because the attraction of Li-ions to EG molecules is at the O atoms in the ends of EG.This indicates the symmetric bonding of two Li to the two OH groups on the two sides of the EG molecules.
The FTIR spectra of liquid electrolyte and PFGE have almost identical peaks except mainly the Si-O bending vibrations at 801 cm − 1 in the spectrum of PFGE and C-H stretching vibrations at 1260 cm − 1 in the spectrum of liquid electrolyte.
According to Fig. 4, the C-O stretching vibration mode of gel electrolytes has shifted toward lower wavenumbers with the increasing Li + ion concentration as observed in the liquid electrolytes.C-O stretching mode of the pure EG at 1083 cm − 1 is within the range of 1077-1079 cm − 1 for the salt added samples showing the interactions of Li with O atoms of EG molecules.

Stability of the liquid electrolyte system
Cyclic voltammetry of optimized FTO/ TiO 2, EG, and LiCl liquid electrolyte from 1 to 500th cycle with scan rate 100 mVs − 1 is shown in Fig. 5. Mostly, TiO 2 -based working electrodes change its color between dark blue and colorless due to the intercalation and deintercalation of ions on the film as shown in Eq. 3 [16,36].Specifically, any cathodic peak has not been identified in all samples and a distinct anodic peak was observed at -0.6 V.It has good compliance with the previous studies for anatase structure of titania [37].
Cyclic voltammetry techniques were performed to study the recyclability of TiO 2 electrode in liquid electrolyte.They were run continuously for 500 cycles in the potential range of − 2.0 V and 0.25 V with the standard Ag/AgCl reference electrode (Fig. 5).The high stability of the electrode and the liquid electrolyte was confirmed by the result obtained from the cyclic voltammetry as it cannot be seen any changes in the shape of the CV or peaks in the CV.
Cyclability and durability of the fumed silica-based PFGE in the application of ECDs with the setout of FTO glass/TiO 2 / PFGE /FTO were characterized by using CV.   Figure 6 shows the CV performance for the first, 25th, 75th, and 100th cycles.According to Fig. 6, oxidation and reduction peaks can be identified due to the existence of lithium in the PFGE.These peaks are represented chemical reactions that take place at the surface of the FTO.The drop of current up to 100 cycles was less than 1 × 10 −3 A. The feature of the voltammogram changes within the first few cycles, but the shape of the cyclic voltammogram does not change sharply by cycling up to 100 cycles.The current density at the cathodic peak which corresponds to the bleaching process has gradually increased up to the 100th cycle and then decreased while the current density for the coloration process which characterizes the charge intercalation into TiO 2 , increased within the first few cycles.However, there is no significant change in either shape of CV or current densities up to 100 cycles, suggesting that the device with the configuration of FTO glass/TiO 2 / PFGE /FTO glass has moderately high stability.

SEM analysis
SEM images of LiCl, EG, and fumed silica gel electrolyte films are shown in Fig. 7a, b, and c.Spherical shape particles are shown in Fig. 7a. of the PFGE belong to the fumed silica matrix and it means that the interaction of weak hydrogen bonding liquids enables it to bond with the surface of the silanol group.Such bonds between particles guide to form PFGE. The uniform distribution of spherical particles can be seen in the SEM images.A large amount of dark liquid region in the micro level is accountable for the high ionic conductivity and it has further increased by a large number of liquid linkages.According to early studies, the increase in silica content causes a decrease of porosity [38].When referring to the layered structure of the PFGE is shown in Fig. 7b, the improvement of the area due to the sandwich layer structure may have increased conductivity.Figure 7, the PFGE with high concentration of pores might be the cause to improve in the ionic conductivity in PFGE.According to the spreading uniformity depicted in Fig. 7c, wrinkle and smooth morphology of the PFGE could be the result of an evenly spreading nature of LiCl which could have been contributed to efficient solubility and its amorphous properties.Dark areas in microstructure mainly represent the liquid linkages influencing solvent retention potential in the PFGE.SEM images of PFGE confirm the healthier compatibility of LiCl, EG, and fumed silica which are the main ingredients in PFGE preparation.

UV-Vis. Spectrum analysis
The transmittance properties of the ECD from 0 to 4.2 V are shown in Fig. 8a.The coloring effect with the voltage of the ECD is illustrated in Fig. 8b.
The optical contrast (ΔT %) and the optical density (ΔOD) of the device refer to the transmittance variation and they are depended on the light absorbency of the ECD.(ΔOD) and (ΔT %) can be calculated using the relation, where T bleached is the transmittance % in the de colored state and T colored is the transmittance of in the colored state.
Values of (ΔT %) and (ΔOD) at 700 nm and 550 nm with respect to the applied voltage have been calculated and tabulated as shown in Table 3.The device shows its minimum transmittance at the colored state at around 700 nm and this implies that the intercalated Li x TiO 2 is blue in color as evident from Fig. 8b.The different colored states of the ECD were obtained with (i) decolored state by applying 0 V, (ii) an intermediate state by applying 2.0 V, and (iii) a fully colored state by applying 4.2 V.
This spectro-electrochemical study further confirms the color variation of ECDs follows Eq. 3. The optical modulations of 64% at 700 nm and 62% at 550 nm were observed , when applying 4.2 V. Though there are no reported data for fumed silica-based PFGE in TiO 2 ECDs, the comparable optical modulation of polymer gel electrolyte-based ECDs is reported by many groups [39,40].The optical contrast and optical density values obtained by our group for polyethylene oxide (PEO)-based TiO 2 electrochromic devices are 61.8% and 1.8, respectively.These are approximately equal to the values we report in this PFGE system [41].This relatively high optical contrast and optical density are due to the high ionic conductivity of the PFGE.Application on chronological amperometric double-step potential characteristics was performed (Fig. 9) to determine the stability during the sequential coloring and bleaching cycles and to calculate the response time of the devices.A double potential step between + 4.2 V and − 0.5 V was applied across the counter-electrode and working electrode for cycling the prepared ECD between colored and decolored states.At the end of every coloring process, − 0.5 V was applied, and then the coloration time t c (the time taken to reduce the light transmission through the ECD from 90 to Fig. 8 a Optical transmittance as a function of wavelength for the electrochromic devices FTO/TiO 2 /non-polymer electrolyte/FTO in various potential stages b Different electrochemical statues of the device FTO/ TiO 2 /non-polymer electrolyte/ FTO (i) as prepared (ii) colored (partially reduced) (iii) highly reduced 10% in the coloration process) and devouring time t b (the time taken to intense the light transmittance through the ECD from 10 to 90% in the decoloring process) [41].Under this definition, coloration and decoloration time for the ECDs of configuration FTO/TiO 2 /PFGE/FTO were around 42 and 17 s, respectively.As compared with the liquid-type electrolytes, our device prepared by PFGEs showed high coloration and bleaching time with relatively low switching performance.But this is very common for most of the gel electrolytes due to its low conductivity.However, the device fabricated by our group in this paper has sufficiently high switching speed compared with the other reported values for polymer electrolytes [42,43].The fast-switching time may benefit from the high ionic mobility in the liquid electrolyte while low switching time may be due to the low transportation of ions in gel electrolytes.The cyclic stability of the ECDs between coloring and decoloring process was performed up to 100 cycles and it has been proven that the reversibility of the ECDs sustained even after 100 cycles.

Conclusion
A novel Li + -conducting PFGE based on fumed silica has been synthesized and characterized.Li + -conducting liquid electrolyte has also been prepared for comparison.Relatively high ionic conductivity of 0.894 × 10 −2 Scm −1 was observed for PFGE.The amorphous nature, numerous pores, and layered structure of the PFGE were confirmed by the SEM and they were liable for the high ionic conductivity in the PFGE.Studies based on cyclic voltammetry results showed the electrochemical characteristics of the reversible process and compatibility of the Li-ion-based electrolyte with the TiO 2 electrode.The possibility of using the PFGE in TiO 2 -based ECDs has been studied.A transformable color switching between colored and bleached states were observed when applied an appropriate voltage between two electrodes of ECD of configuration, FTO/TiO 2 / PFGE /FTO.The reversibility of the ECDs prepared with PFGE has also been confirmed.

Fig. 1
Fig. 1 Room temperature Ionic conductivity of the liquid electrolytes at different (O:Li + ) ratio

( 3 )Fig. 2 aFig. 3 a
Fig. 2 a Nyquist plot for the PFGE of O:Li + =10:1 at room temperature.b variation of the ionic conductivities of PFGE with temperature

1 Fig. 4 Fig. 5
Fig.4 The FTIR Peak shift at 1081 cm -1 with the Li-ion concentration of PFGE

Table 2
Identified vibration modes in liquid electrolyte and PFGE

Table 3
Optical contrast and optical density of the ECDs with configuration FTO glass/ TiO 2 /polymer-free electrolyte / FTO at 700 nm and 550 nm