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ET-1039 Electrochromism 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 Página 2 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 glass 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 color 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 light 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 titanium dioxide (TiO2) to make these displays, which offer better visibility, low power consumption and many other advantages. Página 3 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 Prussian blue (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 tungsten trioxide (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. Página 4 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 Página 5 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: Página 6 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. Página 7 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
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