Electrochromism is the phenomenon where the color or opacity of a material changes when a voltage is applied.
By doing so, an electrochromic smart window can block ultraviolet , visible or near infrared light instantaneously and on demand. The ability to control transmittance of near infrared light can increase the energy efficiency of a building, reducing the amount of energy needed to cool during summer and heat during winter.
As the color change is persistent and energy need only be applied to effect a change, electrochromic materials are used to control the amount of light and heat allowed to pass through a surface, most commonly smart windows.
One popular application is in the automobile industry where it is used to automatically tint rear-view mirrors in various lighting conditions.
The phenomenon of electrochromism occurs in some transition metal oxides which conduct both electricity and ions , such as tungsten trioxide WO 3. This arrangement results in a three-dimensional nanoporous structure with "tunnels" between individual octahedral segments. The electric field is typically induced by two flat, transparent electrodes which sandwich the ion-containing layers.
As a voltage is applied across these electrodes, the difference in charge between the two sides causes the ions to penetrate the oxide as the charge-balancing electrons flow between the electrodes. These electrons change the valency of the metal atoms in the oxide, reducing their charge, as in the following example of tungsten trioxide: . This is a redox reaction, since the electroactive metal is accepting electrons from the electrodes, forming a half-cell. However, the term electrode often refers to only the flat plate s , more specifically called the electrode substrate.
Photons which reach the oxide layer can cause an electron to move between two nearby metal ions. The energy provided by the photon causes movement of an electron which in turn causes optical absorption of the photon. For example, the following process occurs in tungsten oxide for two tungsten ions a and b :. Electrochromic materials, also known as chromophores , affect the optical color or opacity of a surface when a voltage is applied.
Others include molybdenum , titanium and niobium oxides, although these are less effective optically.
For organic materials , viologens have been commercialized on small scale. A variety of conducting polymers are also of interest, including polypyrrole , PEDOT , and polyaniline. Viologen is used in conjunction with titanium dioxide TiO 2 , also known as titania in the creation of small digital displays. It is hoped [ by whom?
Many methods have been used to synthesize tungsten oxide, including chemical vapor deposition CVD , sputtering , thermal evaporation , spray pyrolysis from a vapor or sol-gel , and hydrothermal synthesis from a liquid. For material synthesis, sol-gel process is widely used due to its advantages of simple process, low cost and easy control.
In the sol-gel process of tungsten trioxide, WCl 6 is dissolved in alcohol and then oxidized by purging O 2 into its solution:. The formation of H 2 is performed by the reaction of alcohol and chlorine that used for the reduction of WO 3 to obtain a blue solution of HWO 3 :. Seven layers are needed for a functional smart window with electrochromic characteristics.
The window is fully transparent now. By applying a lower voltage 2.
Graphene based flexible electrochromic devices
This activation causes reflection of infrared light, thus lowering the greenhouse effect , which in turn reduces the amount of energy needed for air conditioning. Depending on the electrochromic material used, different parts of the spectrum can be blocked, this way UV, VIS and IR can be independently reflected at the will of a customer.
Several electrochromic devices have been developed. Electrochromism is commonly used in the production of electrochromic windows or " smart glass ", and more recently electrochromic displays on paper substrate as anti-counterfeiting systems integrated into packaging. NiO materials have been widely studied as counter electrodes for complementary electrochromic devices, particularly for smart windows.
ICE 3 high speed trains use electrochromic glass panels between the passenger compartment and the driver's cabin.
The standard mode is clear, and can be switched by the driver to frosted translucent , mainly to conceal unsightly collisions from the view of passengers.
Electrochromic windows are used in the Boeing Dreamliner. From Wikipedia, the free encyclopedia. This article includes a list of references , but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations. September Learn how and when to remove this template message.
Bibcode : AnRMS.. Materials Chemistry and Physics. Retrieved 22 August Electrochromism and Electrochromic Devices.
Cambridge University Press. Advanced Functional Materials.
Materials Transactions. Ceramics International. Categories : Chromism Optics.
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