16. ionic conductors for electrochromic devices

Keywords fast ionic conductors electrochromic materials liquid and solid electrolytes

16. IONIC CONDUCTORS FOR ELECTROCHROMIC DEVICES Agnieszka STAŃCO(1), Konstanty MARSZAŁEK, Barbara SWATOWSKA, Zbigniew SOBKÓW (1)

AGH-UST, Department of Electronics, al. Mickiewicza 30, 30-059 Krakow, Poland

Electrochromic devices become often investigated due to the large number of potential applications. Two most important components of such systems are electrochromic electrode and ionic conductor. This chapter presents the impact of various ionic conductors for performance of electrochromic device (ECD). In the past, there was a number of studies on electrolytes used in ECD. Electrochromic materials undergo a colour change when ions and electrons are inserted into them, under the influence of an applied potential. The properties of ionic conductors are very important. In order to achieve proper dynamics of ECD, the ionic radius should be as small as possible (e.g. for H+, Li+ or Na+ ions) and the ionic conductivity should be larger than 10-4 S/cm. On the other hand, the electronic conductivity should be very low – even below 10-12 S/cm. Moreover, the electrolyte must be durable and be well adhesive to the layers. This chapter covers a short review of liquid and solid electrolytes. Experimental part of this report presents application of liquid electrolyte in ECD. Electrochromic cell has been fabricated using electrolyte of sulphuric acid (H2SO4) in ethylene glycol (1:10) and lithium perchlorate (LiClO4) in ethylene glycol (1:10). A six-layer device was assembled according to the following formula: glass/ITO/WO3/electrolyte/ITO/glass, where ITO was In2O3:Sn with sheet resistance about 50 Ω/□, WO3 was the electrochromic film, which have been prepared using the rf reactive sputtering method. The plates with ITO/ WO3 and ITO were joined by a spacer giving 1mm thickness of electrolyte between them. There was achieved sufficient ionic conductivity (σ = 0,4 mS/cm for H+ ion and σ = 4 mS/cm for Li+). The colouring charge density was measured for H+ electrolyte as 370 µC/cm2 and 130 µC/cm2 for Li+ electrolyte.

1.INTRODUCTION Electrochromism is a subject of interest from a lot of researchers and companies because of its possible application in many areas like electronics, optics and of course architecture. The most important application of electrochromism phenomena is in the concept of the „smart window” with using solar energy [1, 2]. This conception is

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considered as the combination of two good features: energy efficiency and comfort of users due to control the flow of sun light and heat passing through the building glazing. Materials that are able to change their optical properties because of application of an electrical potential are named electrochromic [3]. These materials show visible colour changes between a transparent and coloured state. Electrochromic properties can be found in large number of materials. The most important are transition metal oxides, Prussian blue, conducting polymers, viologens (1,1’ – disubstited – 4,4’ – bipyridum salts), and metallopolymers [3, 4]. The materials from metal oxides can be coloured anodically (Ni, Ir) or cathodically (W, Mo) [2,5]. The electrical properties of the layers are changed depending on its states – colourless or coloured, respectively. In the process of electrochemical colouring, the conductivity of the film varies from very small, typical for insulators, to relatively high, almost metallic. For example, this parameter for WO3 coloured by Na+ ions varies from 2·10-6 S/cm to 5·10-2 S/cm [6]. The resistivity of the films also changes from high values (106÷1010 Ωcm) to the values of metals resistances [7]. Tungsten oxide is the most widely studied material in respect of its electrochromic properties [5]. 2.ELECTROCHROMIC DEVICE The typical electrochromic device is a seven – layer “sandwich” as GS / TCO / EF / IC / ISF / TCO / GS, where GS constitutes a glass substrate, TCO – transparent conductor, EF – electrochromic film, IC – ion conductor as liquid or solid electrolyte and ISF – as ion storage film [8]. This structure can be seen in Fig. 1.

Fig. 1. The electrochromic device with liquid – a) and solid electrolyte – b)

The whole system should be permeable in the bleaching state. The first layer coated onto glass substrate constitutes TCO – transparent electronic conductor. Generally it is In2O3:Sn (referred to as Indium Thin Oxide, ITO) or SnO2:F – Fluorine doped Thin Oxide (FTO) [9, 10]. Both should show sheet resistance from 10 Ω/sq to 50 Ω/sq [2]. On the transparent conductive layers are coated electrochromic films or ion storage films – fig.1 [11]. The films (EF and ISF) can be concomitantly coloured and bleached when sufficient potential (about 3-5 V) is applied to the system.

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Between the layers is placed an electrolyte, which serves as an ion conductor. When the potential is applied between the transparent conductor, ions are transported between the ion storage film and the electrochromic film. The flow of electrons between the layers changes the optical properties of the device. The electrochemical colouration mechanism for metallic oxides is shown in equation [12]: ABn  xC m  mxe  Cx ABn

(1)

where: A – transition metal, B – oxygen, C – colouring ion (H+, Li+ or Na+), e – electron, ABn – bleached electrochromic material, CxABn – coloured electrochromic material, m – small number of total which may be positive (for materials coloured cathodically) or negative (for materials coloured anodically), n – the number of oxygen atoms, x – number between 0 and 1 [12]. When electrons and ions are incorporated to the electrochromic film, it can be transformed into a material which is heavily absorbing in a visible and infrared range of light. The reverse of potential polarity brings back the original properties of the layer [2]. The electrochromic device shows the memory effect – when there is no move of charge, the system keeps its state (coloured or bleached respectively) [13]. The fundamental element in the electrochromic system is an electrolyte. In order to ensure proper operation of the device, the ion conductivity of electrolyte should be larger than 10-4 S/cm [5]. Other requirement is high thermal and UV stability [14]. Electrolyte also should not cause any degradations in the electrochromic layer. Generally we distinguish liquid and solid (proton and mainly lithium ions) electrolytes which were reviewed several times in the past [5,15-18]. 3.LIQUID ELECTROLYTES The liquid electrolytes were the first used ionic conductive substances in electrochromic devices [15]. They contain a solute in a solvent. The solution can be aqueous or no aqueous. The aqueous H2SO4 electrolytes were first natural choices for the initial investigations on the first electrochromic devices [19]. The ion conductivity σ is various for different electrolytes. The conductivities of exemplary aqueous electrolytes are shown in Table 1 [15]: Table 1. Conductivities of aqueous solutions with 1M electrolytes at room temperature (σ in mS/cm) [15] Cation/ anion H+ Li+

OH– 156

Univalent ClClO4339 – 77,3 74,4

NO3332 73,7

Divalent SO4390 74,4

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K+

213

111,3

13,3

101,5

121

The highest value of σ is 390 mS/cm for sulphuric acid. In addition to conductivity, the most important parameters, which can present the quality of liquid electrolytes are: temperature range in which the electrolytes does not change their properties (freezing and boiling point), limit of potential, and toxicity [15, 20]. The electrolyte must be liquid in wide range of temperatures. The aqueous electrolyte, despite their high boiling point (100oC), have low freezing temperature (0oC) and therefore, in application to large areas are not suitable. On the other hand the water solutions do not exhibit any toxicity, and can be used in other applications, that do not requires wide ranges of temperature [20]. Non-aqueous solvents can be more effective than aqueous [5, 15]. The nonaqueous mediums typically constitute: propylene carbonate (PC), ethylene carbonate (EC), tetrahydrofuran (THF), γ-butyrolactone (γ-BL), 1,2-dimethoxyethane (DME), acetonitrile (AN), methanol, ethanol and others [5, 20]. They, generally, offer a wider range of potential window than aqueous, where the electrochromic device can function safely and without any electrochemical instabilities – fig.2.

Fig. 2. Potential windows for the chosen solvents: a) water, b) acetonitrile, c) 1,2-dimethoxyethane, d) propylene carbonate, e) tetrahydrofuran [5,15]

The range of potential windows depends on many factors: such combination of the solute and solvent or reactions between TCO and the electrolyte. The commonly used PC with LiClO4 can be usable in many electrochromic applications in wide range of used potentials, although it was observed, that PC decomposes into carbon ions and propylene gas on graphite surfaces [21]. Then, carbonate ions react with Li to insoluble Li2CO3 forms. Non-aqueous solutions have larger range of boiling and freezing points (B.P. and F.P., respectively). The freezing points lie between -114,1oC (ethanol) and -43,5oC (ethylene carbonate), and the boiling points lie between 56,3oC (acetone) and 241,7oC

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(propylene carbonate) [20]. On the other hand, the high level of toxicity of some of them does not allow for their practical uses – Table 2. Table 2. Basic chemical properties of non-aqueous solvents [15, 20] Solvent tetrahydrofuran acetonitrile propylene carbonate ethylene carbonate ethanol acetone hexamethylformamide

B.P. (°C) 66 81,6 241,7 204 78,3 56,3 233

F.P. (°C) -108,5 -43,8 -49,2 -43,5 -114,1 -94,7 7,2

Toxicity (mg/m3) 590 70 – – 1900 2400 cancer suspect reagent

There is a lot of sorts of liquid electrolytes for electrochromic uses. The proper selection of them bases on physical and chemical parameters of solvents and solutes. 4.SOLID ELECTROLYTES The solid electrolytes also are used as ionic conductors for electrochromic devices [18]. They can be inorganic or polymeric (especially its gel forms) [22-24]. In both groups, the most significant and well known are proton conductors based on H + and alkali ions, especially – Li+. Proton conductors have high conductivities. Their transport is related to the movement of surrounded molecules [5]. The most known electrolytes based on them are phosphotungstic acid (PWA) and hydrated zirconium phosphate (ZP). Despite, their conductivity σ can be larger than 0,1 S/cm, they show high instability [5]. Lithium conductors have not high conductivities like some proton ions, but they have excellent durability and can be prepared in thin film form. They are generally used due to necessary of well working of EC systems in many cycles [14]. Polymeric electrolytes can be alternative to liquid or inorganic solid ionic conductors and due to their excellent properties, are suitable to electrochromic devices, especially for “smart windows” concepts. They can be divided into two group: solid polymer electrolytes (solvent-free) and gel polymer electrolytes [25]. The polymer electrolytes combine mechanical flexibility with sufficient ionic conductivity in the solid state. As it was reported [18], in case of the liquid electrolytes, the surface pressure may cause the deformation of window substrates. The flammable and hazardous solvents often although having great properties may cause environmental pollution and be dangerous to the users. In turn, the polymeric electrolytes have not these disadvantages and also in contrast to the solid electrolytes, they have not any problems with complexities of stoichiometry and crystallinity control. The polymeric electrolytes are transparent in wide range of wavelength and are well adhesive to


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electrochromic films. The negative side is that, some polymers can degrade upon UV radiation [15]. The polymer electrolytes consist of a salt dissolved in a high molecular weight polymer compound [26]. The polymer compound should contain molecules (O or N) working as ligands, which have to coordinate the salt ions and then provide the key solvation enthalpy to help formation of the polymer electrolyte. The another important thing is that, the ion should dissociate to move. Among alkali ion conductors working with the polymers, the most known are LiClO4, LiAsF6, LiCF3SO3, LiI, LiBr, etc, and the most significant polymer complexes is poly(ethylene oxide) – PEO and poly(propylene oxide) – PPO, poly(methyl methacrylate) – PMMA, poly(propylene glycol, methyl methacrylate) – PPG+PMMA, and others. Their conductivities with Li+ ions are in range from 10-4 S/cm (for PEO, PPG+PMMA) to 10-6 S/cm (for PEO+PMMA) [5]. Proton conductors can be also performed in polymers containing sulfonic acid. The most prominent materials are poly-vinyl sulfonic acid (poly-VSA), poly-styrene sulfonic acid (poly-AMPS) or poly-perfluoro sulfonic acid (Nafion) [5]. 4.1 GEL ELECTROLYTES As it was earlier mentioned there is also gel polymer group of electrolytes, which can be obtained by sol-gel method. These complexes contain the following components [14,22,27,28]:    

lithium salt as an ionic conductor, especially LiClO4, LiN(CF3SO2)2 or LiCF3SO3, a proper solvent or mixture of solvent, containing PC, EC or γ-BL, polymer matrix, e.g. PVC, PMMA and PEO or their mixtures, solvent for preparation of matrix solvent, e.g. THF or CH3CN.

The conductivity of these electrolytes depends on content by weight of the polymers and should not exceed 30%. The proper weight of the gel polymer per whole electrolyte mixture also should facilitate the gel process and have not any impact for resistivity [14]. It is reported that a proper weight of polymer do not have any influence on the whole electrolyte conductivity. Usage of polymer matrix allows for having good adhesion of electrolyte to the layer and high UV, mechanical and thermal stability and proper transmission [29, 30]. Very important is also the salt solvent content and should have about max 80% to ensure proper dissociation of ions and then ionic mobility that leads to achieve high conductivity (in range of 10 -3 S/cm). The comparison of conductivity of the several solvents used in gelled electrolytes can be describe as follows: PC < PC+EC < PC+ γ-BL < EC+ γ-BL < γ-BL < EC [14]. The lowest conductivity under conditions of the fixed salt concentration was for PC [27]. It is important to choose an appropriate solvent that affects positively the electrolyte conductivity.

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There was tested a lot of combinations of polymers, salts and solvents mixtures, in the conceptions of gel electrolytes [14, 16, 22, 29, 31]. 5.

RESULTS AND DISCUSSION

The authors carried out the investigations on electrochromic device with liquid electrolytes based on sulphuric acid (H2SO4) in ethylene glycol (1:10) and lithium perchlo-rate (LiClO4) in ethylene glycol (1:10). A six-layer device consisted by: glass – ITO – WO3 – electrolyte – ITO – glass, where ITO as transparent conductor was In2O3:Sn with sheet resistance about 50 Ω/sq. The electrochromic film constituted WO3 (thickness – 400 nm) which have been prepared by using the RF 13.56 MHz magnetron sputtering method in the atmosphere of Ar+O2 and then sealed for about one hour in air atmosphere at 400°C. Detailed description of sputtering system was presented in [32]. The plates with ITO/WO3 and ITO were joined by a spacer giving 1mm thickness of electrolyte between them. The electrochromic device was driven by a coloring potential at 3 V. The measured ionic conductivity was 0,4 mS/cm for electrolyte based on H+ and 4 mS/cm for Li+, respectively. The colouring charge density resulted for H+ electrolyte as 370 µC/cm2 and 130 µC/cm2 for Li+ electrolyte. On the fig. 3 is presented exemplary plate with coated WO3, which was coloured by the electrolyte based on Li+ ions.

Fig. 3. The coloured WO3 layer.

The figure 4 shows the VIS-NIR transmittance spectra of the used electrochromic film. The insertion of Li+ ions changed the transmission from VIS up to the NIR range and the reversible colour of the film from transparent to blue.


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Fig. 4. The transmissions of the WO3 film for transparent and coloured state.

The average difference in transmission for transparent and bleached state in range of wavelength from 600 nm to 900 nm was 70%. The calculated coloration efficiency, as the other important parameter, is presented on the fig. 5, and can be determined by the following formula [33,34]:

 T   CE    ln  b  / Q  Tc  

(3)

where Tb and Tc are transmissions for bleached and coloured state, respectively, Q – is charge density.

Fig. 5. Coloration efficiency at whole spectrum.

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As it can be seen at Fig. 5, the coloration efficiency is the lowest in visible range of light. It would result by the strong absorption of the glass with coated ITO [32]. The values of CE increase for larger wavelengths and achieve maximum for 992 nm. As it is shown also at Fig. 3 this film contributing with proper electrolyte in whole electrochromic device can have real application in “smart window” conception, due to functioning as optical filter. 6.CONCLUSIONS The basic sorts of the electrolytes, which can be potentially used in the investigated electrochromic systems, were reviewed in this work. There are many conceptions of ionic conductors that can be further developed. This review is the basic information for the authors research. There were tested exemplary electrochromic devices with liquid electrolytes (based on sulphuric acid (H2SO4) in ethylene glycol (1:10) and lithium perchlo-rate (LiClO4) in ethylene glycol (1:10) ) and the achieved results were mostly in good agreement with the presented literature. The level of conductivity of obtained liquid electrolytes is proper for electrochromic applications. The authors plan to investigate the gel electrolytes and carry out the experiments leading to characterizations of their thermal, UV and electrochemical stability. ACKNOWLEDGMENTS The authors acknowledge financial support from the budget sources for science 2012, project no 15.11.120.187. REFERENCES [1] Avendaño E., Berggren L., Niklasson G.A., Granqvist C.G., Azens A., Electrochromic materials and devices: Brief survey and new data on optical absorption in tungsten oxide and nickel oxide films, Thin Solid Films No. 496, 2006, 30–36. [2] Granqvist C.G., Azens A., Hjelm A., Kullman L., Niklasson G.A., Rönnow D. Mattsson M.S., Veszelei M., Vaivars G., Recent Advances in Electrochromics for Smart Windows Applications, Solar Energy, Vol. 63, No. 4, 1998, 199–216. [3] Rowley N.M., Mortimer R.J., New electrochromic materials, Science Progress, Vol. 85, No. 3, 2002, 243–262. [4] Carpi F., Rossi D.D., Colours from electroactive polymers: Electrochromic, electroluminescent and laser devices based on organic materials, Optics & Laser Technology No. 38, 2006, 292305. [5] Granqvist C.G., Handbook of inorganic electrochromic materials, Elsvier, Amsterdam – Lausanne – New York – Oxford – Shannon – Tokyo, 1995, 165–166 and 441–451. [6] Dautremont–Smith W.C., Green M., Kang K.S., Optical and electrical properties of thin films of WO3, electrochemically coloured, Electrochim. Acta, No. 22, 1977, 751. [7] Kaneko H., Miyake K., Teramoto Y., Preparation and properties of reactively sputtered tungsten oxide films, Journal of Applied Physics, No. 53, 1982, 3070. [8] Monk P.M.S., Mortimer R.J., Rosseinsky D.R, Electrochromism: Fundamentals and applications, Wiencheim – New York – Basel – Cambridge –Tokyo, VCH, 1995, 14–15.


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