Electrochromic Smart Materials: Fabrication and Applications / Edition 1 available in Hardcover, eBook
Electrochromic Smart Materials: Fabrication and Applications / Edition 1
- ISBN-10:
- 1788011430
- ISBN-13:
- 9781788011433
- Pub. Date:
- 01/07/2019
- Publisher:
- RSC
Electrochromic Smart Materials: Fabrication and Applications / Edition 1
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CHAPTER 1
Introduction to Electrochromism
MING HUI CHUA, TAO TANG, KOK HAW ONG, WEI TENG NEO AND JIAN WEI XU
1.1 General Introduction
Chromic materials are materials which exhibit a reversible colour change in response to an external stimulus such as temperature (thermochromism) and light (photochromism). The source of the colour changes is the variation in absorption spectra of the materials across the UV–visible–near-infrared (NIR) region. Besides the above-mentioned stimuli, oxidation and reduction of certain substance upon application of an electrical bias can also lead to distinct photo-optical and colour changes. This phenomenon is known as "electrochromism". Electrochromic (EC) materials generally exhibit colour changes between two coloured states or between a coloured state and a bleached state. Materials that reveal coloured hues in their oxidised or reduced states are referred to as anodically colouring or cathodically colouring respectively. Several EC materials that exist in multiple redox states reveal the unique ability to switch between several coloured states. This is known as polyelectrochromism. EC materials are highly applicable in smart windows and optical display technology. Furthermore, as the region of optical changes can be extended beyond the UV–visible region into the NIR, the thermal infrared and even the microwave region, these EC materials are potentially useful in defence related applications.
Many different classes of compounds were reported to exhibit EC properties: (i) transition metal oxides (WO3 and TiO2), metal coordination complexes (CoFe(CN)6 and Prussian Blue), organic molecular dyes (e.g. viologen) and organic conducting polymers (e.g. polythiophenes, polyanilines and poly(3,4-ethylenedioxythiophene (PEDOT))). Amongst these, organic EC materials possess advantages such as intense colouration, ease of structural modification, good processability, low cost and good film-forming ability. On the other hand, their inorganic counterparts were reported to exhibit good chemical and electrochemical stabilities as well as a wide range of working temperatures. In addition, organic/inorganic nanocomposites were also developed to combine the advantages of both organic and inorganic EC materials. Such hybrid materials could be prepared from the use of either only EC-active organic or inorganic materials or both. In the EC nanocomposites, much attention has been paid to modifying the interfacial interactions between organic and inorganic parts because such interactions are vital for structure strength, mass transport, electron conduction and EC performance.
1.2 History of Electrochromism
The first EC device was documented by Deb in 1969, where he demonstrated the controlled and reversible changing of colour with the use of tungsten trioxide (WO3). Since then, many classes of EC materials and corresponding devices have been reported, which include metal oxides, viologens and conjugated polymers. Due to their facile colour changes in the visible region, EC materials were highly sought after and employed for optical display applications. Early research in the US, Soviet Union, Japan and Europe on EC materials were motivated by their potential applications in information displays. There were intense research efforts during the first half of the 1970s at several large international companies such as IBM, Zenith Radio, the American Cyanamid Corporation and RCA in the US as well as Canon in Japan, Brown Boveri in Switzerland and Philips in the Netherlands. Through the years, electrochromism continues to receive wide attention in the area of fundamental research. In the mid-1980s, interest in EC materials was boosted again given the potential application in fenestration technology, which was deemed as a way to achieve better energy-efficiency in buildings. The newly conceived "smart" window technology could vary the transmittance of light and solar energy, leading to energy savings and indoor comfort. Moving on, breakthroughs in device engineering and manufacturing techniques allow for electrochromism to move beyond traditional applications such as smart windows and optical displays into emerging applications such as wearable electronics and defence-related technologies.
1.3 Mechanism of Electrochromism and EC Devices
EC materials undergo colour (and sometimes, fluorescence) changes upon the application of an electric field. Generally, the mechanism of EC activities involves the electrochemical oxidation and/or reduction of EC materials, resulting in changes in the optical band-gap, which is thus reflected in colour changes observed. In most cases, a constant supply of electric current is required to sustain a certain colour associated with an electro-oxidised or -reduced state. There are, however, some materials that require almost zero-current consumption to maintain a certain colour state, which is known as the "memory effect". The detailed mechanism of electrochromism will be discussed in subsequent chapters.
For real-life applications, EC materials have to be incorporated into functional EC devices. Typically, EC materials exist as thin films within the EC devices, allowing them to be in close contact with electrodes and electrolytes for electric current to flow through the devices. An EC thin-film device normally adopts a multi-layered structure as shown in Figure 1.1, which can be used to tailor the optical properties of a device on applying a voltage, and revert to the original state when the polarity of the voltage is reversed. Having good electrical contact between layers is required to ensure good stability and EC performance. As shown in Figure 1.1, a typical EC device has at least five layers: transparent-conducting oxide (TCO) layer/ ion-storage layer (IS)/ion-conducting layer (electrolyte)/EC layer/TCO layer, superimposed on one substrate or be positioned between two substrates in the laminate configuration. In this configuration, the EC layer is coated on one side of ion conductor while an ion-storage layer is located on the other side of ion conductor. The use of ion-storage layer is to obscure the galvanic-cell basis of operation. The conducting layer is mainly responsible for carrying the charge from a power source to the corresponding EC layer. The ion-conductor, which is made up of small mobile ionic charge carriers, ensures the completion of the circuit by facilitating the transfer of ions between electrodes. Finally, epoxy and relevant sealants are used to ensure that electrolyte is not leaked during operation.
Optical modulation of EC devices is mainly affected by H+ or Li+ transport for devices backed by a single glass or a polyester substrate. However, laminated devices show some difference compared to their counterparts. Particularly, devices using H+ transport normally use electrolytes containing polyethylene oxide (PEO), a copolymer of sodium vinylsulfonic acid and 1-vinyl-2-pyrrolidinone and poly-2-acrylamido-2-methyl-propane sulfonic acid. Meanwhile, the counter electrodes are polyaniline, Prussian Blue, or a mixture of two, which lead to a large modulation range of visible light. On the other hand, laminated devices with Li+ transport are a bit distinct. In other words, the polymers consist of poly-methyl methacrylate (PMMA) copolymerized with polypyrrole, propylene carbonate, silane, polypropylene, glycidyloxypropyl trimethoxysilane copolymerized with tetraethylene glycol, polyethylene glycol methacrylate copolymerized with PEO, ormolyte or polyvinylidene fluoride. Those polymers are ion-conducting via adding an optimal Li salt. In addition, V2O5, SnO2 doped with Mo and Sb, and TiO2 with or without additions of ZrO2 or CeO2 are used as the counter electrode in the system with Li+ transport.
1.4 Applications of EC Materials
Over the past decades, EC materials and devices have been widely applied in a number of areas, particularly information displays, variable reflectance mirrors, smart windows and variable emittance surfaces. The principles of the four stated applications are shown in Figure 1.2. EC materials and devices can be applied to translucent, transparent or mirror surfaces, and the amount of light absorbed, reflected or passing through can be modulated by controlling electric current passing through the devices. In general, all EC devices can be classified based on their operating mode — transmission or reflection.
Recently, there has been a strong resurgence in the development of EC-based display-oriented devices such as "electronic paper" with a focus on cheap printable EC "labels" with excellent viewing properties, "active" authentication devices and sensor platforms with EC-based readout. These will be discussed in further detail in the following subsection.
1.4.1 Smart Glass/Windows
One of the most prominent applications of EC technology is for smart glass and windows. Such EC windows can switch reversibly between transparent and opaque states and across different degrees of opacity simply by varying the electrical potential applied. As such, the amount of external light, glare and solar radiation (hence heat from outside) entering through the window can be modulated easily. This, in turn, leads to potential energy and cost savings as the reliance of indoor lighting and temperature (e.g. air conditioning) control is reduced. Similarly, indoor privacy can be maintained at the wish of the user, simply by switching the smart window from transparent to opaque, effectively eliminating the need for shades or curtains. Smart windows have been used in buildings, vehicles and even on planes. The market size for EC glass was estimated to be $1.17 billion in 2013 and this is expected to expand to $2.59 billion by 2020, representing a compounded annual growth rate of over 10%.
The key advantage of EC smart glass is that it requires electric power only during switching. In contrast, alternative technologies such as suspended particle devices and polymer-dispersed liquid crystal devices require the application of continuous power in order to maintain the glass in a transparent state. Figure 1.3 demonstrates the configuration and mechanism of an EC window. In the configuration, the window functions as an electrochemical cell in which two conducting glass panes are separated by an electrolyte material. At an open-circuit voltage, both the working and counter electrodes are transparent, allowing both heat and light to pass through. The EC window thus exists in the "bright mode". The EC window can switch to "cool mode", where heat is blocked while allowing the natural light to pass through with the reduction of voltage to an intermediate level. Finally, at lower electric potentials, the EC window converts to "dark mode", effectively blocking the transmittance of both heat and natural light.
At present, several EC windows are available on the market. Some notable smart window manufacturers include SAGE Electrochromics, Inc., EControlGlas GmbH & Co. KG, Saint Gobain Sekurit, GENTEX Corporation, and Asahi Glass. One well-known application of EC glass is in the windows of the Boeing 787 Dreamliner (Figure 1.3). Used in place of conventional window blinds, EC technology has enabled airline passengers to control the opacity of the windows with the push of a button. Cabin crew can also remotely adjust individual windows or those on the entire plane, freeing them the hassle of checking each individual window before take-off or landing.
Furthermore, energy storage and electrochromism functions can be integrated into a single device, as demonstrated by various groups. By integrating an EC device with a solar cell, photovoltaics, solar cell glazing, or supercapacitor, a self-powered smart device can be obtained (Figure 1.4). For such energy-harvesting smart windows, light energy is converted into electricity when there is strong incident sunlight, which is then stored within the smart window. Concurrently, the colour of the window darkens. When the stored energy is discharged, the window returns to its original colour. The marriage of EC and photovoltaic technology thus effectively eliminates the need for an external electric supply to operate the smart window.
1.4.2 Car Rear-view Mirrors
EC technology has also been applied in anti-glare, auto-dimming rear-view mirrors for automobiles. These mirrors have built-in sensors that can detect glare from the headlights of following vehicles. The built-in sensors of EC auto-dimming rear-view mirrors are usually cameras or photodiode-based photodetectors, which send the signal to a microprocessor. The detection of strong glare will send a charge through an EC gel, which effectively darkens to reduce the glare and discomfort for the driver, thereby improving road safety. No manual adjustment of the rear-view mirror is thus required by the driver, who can focus on driving and road conditions. One such product is the Gentex mirror, millions of which have been sold since 1974.
1.4.3 EC Displays
EC displays produce colour in a subtractive manner, through interaction with transmitted or reflected light from an external light source. This is in contrast to a cathode ray tube or a light emitting diode display which emits light. Beginning in the 1980s, steady development in EC materials has produced materials that can exhibit colour changes from colourless to various colours (such as red, green and blue, or cyan, magenta and yellow). This has thus opened up the possibility to generate full-colour EC displays using the RGB or CMY colour models. Furthermore, they can also be fabricated using printing processes on flexible substrates, making low-cost devices such as e-papers possible. One of the most common forms of EC displays would probably be in digital clocks and watches. A recent example of a fully-printed active-matrix EC display on a flexible substrate which utilizes carbon nanotube thin-film transistors as the backplane was reported. While this display has only 6_6 pixels, it demonstrates the significant potential of EC displays for delivering low-cost, large area devices on flexible substrates.
Figures 1.5 and 1.6 show the structure and performance of an EC display based on ZnO–PEDOT core–shell nanowires. To generate a low power, long term stable and transparent display, the EC material should ideally possess properties such as high contrast, ultrafast switching time, high colouration efficiency, ultrahigh diffusion coefficient and electrochemical stability.
Another example is a stretchable EC display which employs PEDOT and polyurethane (PU) as the major components. The display performance is shown in Figure 1.7. The composite film works as a free-standing EC film in an electrolyte solution, and can be combined with other stretchable materials such as hydrogel as a support. Moreover, the developed EC film and device are useful as a non-emissive display component in a stretchable wearable device to indicate electrochemical signals.
1.4.4 Wearable Apparel and Devices
EC technology has also been applied to wearable apparel such as eyewear. Like transitional lenses, EC lenses for spectacles and sunglasses can be switched between clear and dark states, effectively protecting users from excessive UV radiation and reducing discomfort to the eyes under bright sunlight. The difference between transitional lenses and EC lenses/glasses, however, lies in the former having an auto-dimming function due to photochromic properties of the lenses, whereas the latter operates on a small electric input and is user-controlled. This means that users may switch to a darkened "sunglasses" mode, for example, in a shaded environment, which auto-dimming transitional glasses are unable to do. Nonetheless, the auto-dimming function of EC lenses can also be enabled using photo-sensors and micro-controllers. The low operation voltage and energy consumption of EC devices imply that a single battery can power a device for thousands of switches. In addition, the switching kinetics of EC lenses are comparatively faster than photochromic lenses, and the lenses can switch between more than one colour. For instance, Reynolds et al. reported the use of a colour-mixing method to produce EC lenses that can reveal several shades of brown (Figure 1.8). In addition, the lenses were fabricated using a combination of inkjet printing and blade-coating, clearly demonstrating how these organic EC polymers blends can be easily translatable in a large scale production of EC smart lenses.
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