ET-1039

Nanotechnology Electrochromism

Author:

Pablo Rivera Mariscal Index

Introduction ...... 3 History of electrochromism ...... 4 Principle of electrochromism ...... 5 Tungsten Oxide (WO3) ...... 6 Applications...... 9 Smart windows ...... 9 Active optic filter ...... 11 Electrochromic mirror ...... 11 Electrochromic displays ...... 11 Conclusions ...... 13 Bibliography ...... 14

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Introduction

Electrochromism is a phenomenon in which some materials display reversibly changing colour by using bursts of charge to cause electrochemical redox reaction. There are many types of materials and structures which, depending on the specific applications, can be used to construct electrochromic devices. Transition metal oxides are a large family of materials possessing various interesting properties in the field of electrochromism. Among them, tungsten oxide (WO3) has been the most extensively studied material, whose use goes from the production of electrochromic windows or smart to the more recently electrochromic displays on paper substrate, which works as anti- counterfeiting systems integrated on packaging. Niquel oxide (NiO) materials have been widely studied as counter electrodes for complementary electrochromic devices, in particular, smart windows. The world leading institutions on NiO efforts include National Renewable Energy Laboratory and Uppsala University. Another common used electrochromic material is polyaniline which can be formed either by the electrochemical or chemical oxidation of aniline. If an electrode is immersed in hydrochloric acid with a small concentration of aniline, then a film of polyaniline will grown on the electrode. Depending on the oxidation state, polyaniline can either be pale yellow or dark green/black. Other electrochromic materials that have found technological application include the viologens and polyoxotungstates.

Since the change is persistent and energy need only be applied to effect a change, electrochromic materials are used to create smart windows. These windows control the amount of and heat allowed to pass through them. One popular application is in the automobile industry where it is used in the rear-view mirrors to automatically tint in various lighting conditions. They are also used in the Boeing 787 Dreamliner and in the ICE 3 high speed trains between the passenger compartment and the driver's cabin, mainly to conceal "unwanted sights" from passengers' view.

Another recent use of electrochromic devices is the creation of small digital displays. Viologen is used in conjunction with dioxide (TiO2) to make these displays, which offer better visibility, low power consumption and many other advantages.

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History of electrochromism

Even though these technologies seem to be very recent, and they are nowadays subject of many investigations, they have actually a history which goes back to the first years of the 18th century.

It was in the 1704 when Diesbach discovered (hexacyanoferrate), a material which suffered a change in colour from transparent to blue under the oxidation of iron. But, it was not since 1930s when Kobosew and Nekrassow first noted electrochemical coloration in bulk tungsten oxide.

On 30 July 1953, T. Kraus provided the first detailed description of electrochemical coloration in thin film of (WO3). However, S. K. Deb was the one who demonstrated the electrochromic coloration in WO3 thin films in 1969. Applying an electric field of the order of 10kVcm-1, S. K. Deb observed electrochromic colour on these films.

The real birth of electrochromic technology is commonly attributed to S. K. Deb seminal paper of 1973, where he described the coloration mechanism of WO3.

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Principle of electrochromism

Without knowing the details of the coloration process, the mere fact that we are dealing with an electrochemical reaction allows us to make several predictions on device performance. In its simplest form the electrochromic reaction can be schematically written as:

Where M denotes an ion of the electrochromic material which can exist in different valency states and A a mobile cation such as H+ or an alkali. An example is the formation of tungsten bronze:

An electrochromic display is thus simply a battery with a visible state of charge. Unless side reactions are involved, the open circuited display retains its charge and, hence, its color. Electrochromic devices thus in general have memory. The voltage required to drive the cell is of the order of a typical galvanic cell voltage (~1V) and the optical density D (log10 of induced absorption) is proportional to the accumulated charge.

Once we know the concept, we can explain it with easier words: Electrochromism involves electroactive materials that, when a small DC voltage is applied, show a reversible colour change. The potential causes redox reactions in the electrochromic and ion storage layers, a process which needs to be accompanied by ion migration across the ion- conducting layer to achieve charge compensation.

Illustration 1: Electrochromism scheme

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The materials and physical composition of electrochromic devices can vary strongly, but the most used electrochromic films are made from two different kinds of electrochromic oxides: ‘cathodic’ ones colouring under ion insertion and ‘anodic’ ones colouring under ion extraction. Figure 2 indicates metals capable of forming oxides of these two categories, taking special attention on vanadium oxides, which can be anodic and cathodic.

Figure 2: Periodic system of elements, except lanthanides and actinides. The shaded boxes indicate the elements which can offer cathodic or anodic electrochromism

However, the most used and studied electrochromic material is the first that was discovered, opening the door to the world of electrochromism: the tungsten oxide.

Tungsten Oxide (WO3)

Since the discovery of electrochromism, tungsten trioxide, WO3 has emerged as one of the key materials and remains by far the most studied. Several books and review articles are devoted to this oxide. Upon reduction, transparent WO3 thin films switch to a blue colour associated with the formation of MxWO3, as a result of the double injection of electron and M+ cation, as described by the following equation:

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A reversible switch to the initial transparent state is observed on the following oxidation. However, despite a huge amount of work the mechanism at the origin of the coloration amorphous or crystalline WO3 is still the subject of many controversies. Part of the controversies comes from the numerous deposition conditions used to synthesize WO3 thin films, leading to films exhibiting either non stoichiometry or different morphologies, for instance. As we can see in the conference paper of SPIE (see Bibliography), the deposition of WO3 thin films on paper was achieved by using a synthesis method involving the connection of tungsten oxide nanoparticles by UV-irradiation. Crystallized WO3 thin films, deposited on paper substrate covered with a conductive Ag layer, exhibit various morphologies (Fig. 3) associated with higher electrochemical performances for the films prepared from higher specific surface area homemade WO3 powder. The ex-situ evolution of the reflectance spectra for WO3Syn thin films cycled in Paper/Ag/WO3/(0.3 M) HTFSI- BMITFSI/Pt vs Hg/HgO electrochemical cell and progressively colored at various potentials is depicted in Figure 4.

Figure 3: Comparison of 2nd Cyclic Voltammograms of WO3Ald and WO3Syn thin films cycled in WO3/0.3M HTFSI in BMITFSI/ Pt vs. Hg/HgO cells with a scan rate of 10 mV/s and HRSEM images of WO3Ald and WO3Syn thin films, after UV-treatment (UV). All films are deposited on paper/Ag substrates.

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The coloured-bluish state (-0.1V) and the bleached-yellowish state (+0.5V) are associated with reflectance values of about RC ≈ 39% and RB ≈ 60% at 550 nm, respectively. It should be noted that the film is very dark in the coloured state at -0.1V, whereas the bleached step may appear by time to time not completely homogeneous. Indeed, in oxidation a limit in potential of 0.5 V (vs Hg/HgO) was chosen for preventing any instability of the silver Figure 4: Evolution of the Reflectance Spectra vs. wavelength for layer. WO3Syn colored ex-situ at various applied potentials.

As we could see on the investigation done in this conference paper, the characteristics of the WO3 films differ in a significant way even between two films deposited in the same conditions. Hence, even if this material is by far the most studied, it still has problems at a nanometric scale.

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Applications

Smart windows

Colour-changing windows have been available for more than two decades. While they have attracted widespread interest (the National Renewable Energy Laboratory of the U.S. provided a prototype in 1998 for a solar home exhibit at Walt Disney's Epcot Center) they have not become widely available or commercially successful. This is due mainly by the high cost of the smart windows and their low duration (around 10 years).

However, this technology is energy-saving and, hence, eco-friendly. This fact has made researchers try to improve the properties of the smart windows at the same time of trying to low its price. Compared to insulated windows, the energy consumption for cooling when using smart windows can be reduced by half.

Insulated windows are made from multiple layers of glass. Usually the spaces between the panes are filled with a gas. Electrochromic windows are made with a very thin stack of dynamic materials deposited on the outside pane.

The dynamic portion of materials consists of three layers: active electrode layer, counter electrode layer and an ion conductor layer which is between the two former. Currently researchers are experimenting with electrode layers made of nickel and tungsten oxides, using lithium for the ions.

The window changes from clear to tint when a small electric field is applied and the lithium ions move into the working electrode layers. The change can be triggered by sensors in an automated building management system, or by a flick of a switch. Electrochromic windows can block as much as 98 percent of the direct sunlight. Reversing the polarity of the applied voltage causes the ions to migrate back to their original layer, and the glass returns to clear.

Researchers are using metal oxides because light does not degrade them. While current manufacturer warranties typically extend for 10 years, nowadays researchers are aiming to develop windows that perform for 20 years or more.

Although electrochromic windows add yet another powered device to a modern building, they should save far more energy than they consume, since

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powering 1,500 square feet of colour-changing glass (about 100 windows) would require less power than a 75 watt light bulb.

And because the windows modulate the building's interior climate, the rest of the heating, cooling and illumination systems can be smaller, leading to lower construction costs and lower monthly energy bills.

In computer simulations of building performance, the electrochromic windows:

 Reduce electricity consumption for cooling by up to 49 percent.  Lower peak electrical power demand by up to 16 percent.  Decrease lighting costs by up to 51 percent.

But what if we could make the smart windows system automatic, making it sensitive to the intensity of the solar light. Well, we can. Since the structure of advanced solar cells is mostly the same as the electrochromic film that covers the outside pane. Hence, putting solar cells spread around the pane can help us power the electrochromic film, making the smart window sensitive to sunlight.

However, we would still need an automated system to control the windows, since we do not want the system to be working the whole year, making it off in winter when we need the sunlight to warm the building.

The technology of smart windows is not only reduced to buildings or sunglasses. It is also used on cars, trains and planes, as we stated in the introduction.

Figure 5: ANA Boeing 787-8 Dreamliner electrochromic window

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Active optic filter

Using the method above to create sensitive to solar light can also help people with vision problems not to worry about having two pairs of glasses: one normal pair and another pair of sunglasses. With only one pair they would have enough. Plus, it is even helpful to people without vision problems, making people like sportsmen, drivers and such not to worry about being annoyed by the sunlight.

The idea remains the same as the smart windows with solar cells, but in this

case it is configured not to be completely Figure 6: electrochromic device installed on glasses opaque.

Electrochromic mirror

A mirror with an electrochromic device on its surface is what we call an electrochromic mirror. As in the two applications above, the idea is the same: reduce the amount of light that can pass through the glass, and in this case, be reflected. As glasses with an active optic Figure 7: normal mirror (left) vs electrochromic mirror (right) filter, the darkest mode of the mirror still needs to let some light get reflected.

This mirrors can avoid blindness make by others cars flashes, which could help preventing accidents.

Electrochromic displays

These displays aim to surpass nowadays displays (such as LCD, E-INK, OLED…) in terms of energy efficiency, price and versatility. The ethyl viologen is the electrochromic material that is currently being tested for this application. Having a big difference between the transparent state and the dark state, we can also obtain a complete set of tones of the same colour by just varying the applied voltage.

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Figure 8: Transmission measurement of EV at different potentials

The advantages of electrochromic displays:

. Cheap The raw materials used to create EC displays are cheap. . Low power consumption As we said on the smart windows section, 100 windows would cost less energy than a 75W light bulb. Plus, when you switch off the power, the colour remains. Moreover, you can add titania molecules to avoid using a backlight, Figure 9: A flexible electrochromic display making the display work as nowadays electronic books. . Fast switching By using porous electrodes, typical switching times can be reduced to the order of 0,2 seconds, instead of the 5 seconds needed with standard electrodes. . Integration of colours without colour filters It is already researched a molecular dye that can display red, green or blue, depending on the applied voltage. . Easy transformation to produce them Existing LCD manufacturers could transform their assembling machines to assemble EC displays without investing too much money.

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Conclusions

Even though it still has a long way to go, electrochromism is one of the technologies of the future – working nowadays. It is true that it has many disadvantages, in example the smart windows high price and low duration, making some electrochromic devices have a rough time when trying to enter in our competitive market.

But it is also true that it has many advantages, and mostly all of them aim to energy efficiency, which is nowadays most important problem: the lack of energy that we will have in the next years.

In a few years we will see this electrochromic materials used in all kinds of places, probably mainly as sunglasses and smart windows, and of course as displays, even though some people would probably not notice the difference.

I am sure that the researchers will be able to reduce the cost and enhance the duration of electrochromic devices, making them able to be competitive in our market.

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Bibliography https://en.wikipedia.org/wiki/Tungsten_trioxide https://en.wikipedia.org/wiki/Electrochromic_devices#cite_note-2 https://mewtoo.uji.es/www.fisica/priv/web_ET1039/presentacions/C4%20electronics%20opt oelectronics%20and%20sensors.pdf http://onlinelibrary.wiley.com/doi/10.1002/9783527615377.fmatter/pdf http://lcp.elis.ugent.be/tutorials/tut_echrom https://en.wikipedia.org/wiki/Smart_glass http://www.innoshade.de/principle-of-electrochromism.html http://www.phys.ufl.edu/~tanner/PDFS/Argun04cm-edot-review.pdf http://nathan.instras.com/documentDB/paper-134.pdf http://www.wiley-vch.de/books/sample/3527336109_c01.pdf http://link.springer.com/chapter/10.1007/978-1-4613-4289-2_9#page-1

Conference Paper in Proceedings of SPIE: Electrochromism: From oxide thing films to devices, by A. Rougier and Abdelaadim Danine. March 2015. DOI: 10.1117/12.2077577

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