Electrochromism is a phenomenon in which a material displays changes in

color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...

or opacity in response to an electrical stimulus. In this way, a smart window made of an electrochromic material can block specific wavelengths of

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...

, visible or (near)

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

light. 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 Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...

and

heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...

allowed to pass through a surface, most commonly "smart windows". One popular application is in the

automobile A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people instead of goods. The year 1886 is regarded ...

industry Industry may refer to: Economics * Industry (economics), a generally categorized branch of economic activity * Industry (manufacturing), a specific branch of economic activity, typically in factories with machinery * The wider industrial sector ...

where it is used to automatically

tint In color theory, a tint is a mixture of a color with white, which increases lightness, while a shade is a mixture with black, which increases darkness. Both processes affect the resulting color mixture's relative saturation. A tone is produce ...

rear-view mirror A rear-view mirror (or rearview mirror) is a flat mirror in automobiles and other vehicles, designed to allow the driver to see rearward through the vehicle's rear window (rear windshield). In cars, the rear-view mirror is usually affixed to ...

s in various

lighting Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing dayl ...

conditions.

Principle

The phenomenon of electrochromism occurs in some transition metal oxides which conduct both

electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as describe ...

and

ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

, such as tungsten trioxide (WO₃). These oxides have octahedral structures of oxygen which surround a central metal atom and are joined together at the corners. This arrangement results in a three-dimensional nanoporous structure with "tunnels" between individual octahedral segments. These tunnels allow dissociated ions to pass through the substance when they are motivated by an electric field. Common ions used for this purpose are H⁺ and Li⁺. 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: : + ''n''( + e⁻) → This is a

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...

reaction, since the electroactive metal is accepting electrons from the electrodes, forming a half-cell. Strictly speaking the ''electrode'' as a chemical unit comprises the flat plate as well as the semiconducting substance in contact with it. 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'': : + + photon → +

Electrochromic materials

Electrochromic materials, also known as ''chromophores'', affect the optical color or opacity of a surface when a voltage is applied. Among the metal oxides, tungsten oxide (WO₃) is the most extensively studied and well-known electrochromic material. Others include

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ...

titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion i ...

and

niobium Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has s ...

oxides, although these are less effective optically. Viologens are a class of organic materials that are being intensively investigated for electrochromic applications. These 4,4′-bipyridine compounds display reversible color changes between a colorless and a deep-blue color due to redox reactions. Researchers can "tune" them to a deep blue or intense green. As organic materials, viologens are seen as promising alternatives for electronic applications, compared to metal-based systems which tend to be expensive, toxic and a problem to recycle. Possible advantages of viologens include their optical contrast, coloration efficiency, redox stability, ease of design and potential to scale up for large-area preparation. Viologens have been used with phenlyenediamine by Gentex Corporation, which has commercialized auto-dimming rearview mirrors and smart windows in Boeing 787 aircraft.

Viologen Viologens are organic compounds with the formula (C5H4NR)2n+. In some viologens, the pyridyl groups are further modified. Viologens are called so, because these compounds produce violet color on reduction iolet + Latin ''gen'', generator of T ...

has been used in conjunction with

titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolu ...

(TiO₂, also known as titania) in the creation of small digital displays. A variety of conducting polymers are also of interest for displays, including polypyrrole,

PEDOT Poly(3,4-ethylenedioxythiophene) (PEDOT or PEDT; ''IUPAC'' name poly(2,3-dihydrothieno ,4-''b''1,4]dioxane-5,7-diyl)) is a conducting polymer based on 3,4-ethylenedioxythiophene or EDOT. It was first reported by Bayer AG in 1989. Polymer PEDOT p ...

, and

polyaniline Polyaniline (PANI) is a conducting polymer and organic semiconductor of the semi-flexible rod polymer family. The compound has been of interest since the 1980s because of its electrical conductivity and mechanical properties. Polyaniline is one o ...

Synthesis of tungsten oxide

Many methods have been used to synthesize tungsten oxide, including

chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (subst ...

(CVD),

sputtering In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and ca ...

thermal evaporation A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...

spray pyrolysis Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (substra ...

(from a vapor or sol-gel), and

hydrothermal synthesis Hydrothermal synthesis includes the various techniques of crystallizing substances from high-temperature aqueous solutions at high vapor pressures; also termed "hydrothermal method". The term " hydrothermal" is of geologic origin. Geochemists and ...

(from a liquid). In industry, sputtering is the most common method for the deposition of tungsten oxide. For material synthesis, sol-gel process is widely used due to its advantages of simple process, low cost and easy control.

Sol-gel process

In the sol-gel process of tungsten trioxide, is dissolved in alcohol and then oxidized by purging into its solution: : + → + The formation of is performed by the reaction of alcohol and chlorine that used for the reduction of to obtain a blue solution of : : + → + : + → nanoparticles can also be obtained by precipitation of ammonium tungstate para pentahydrate, , or nitric acid, , under acidic conditions from aqueous solutions.

Working principle of electrochromic windows

Multiple layers are needed for a functional smart window with electrochromic characteristics. The first and last are transparent glass made of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

(), the two electrodes are needed to apply the voltage, which in turn will push (or pull) ions from the ion storage layer, through the electrolyte into the electrochromic material (or vice versa). Applying a high voltage (4 V or more) will push lithium-ions into the electrochromic layer, deactivating the electrochromic material. The window is fully transparent now. By applying a lower voltage (2.5 V for example) the concentration of Li-ions in the electrochromic layer decreases, thus activating (N)IR-active tungsten oxide. This activation causes reflection of infrared light, thus lowering the

greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...

, 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, visible and IR light can be independently reflected at the will of a customer.

Applications

Several

electrochromic devices An electrochromic device (ECD) controls optical properties such as optical transmission, absorption, reflectance and/or emittance in a continual but reversible manner on application of voltage (electrochromism). This property enables an ECD to be u ...

have been developed. Electrochromism is commonly used in the production of electrochromic

window A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent mat ...

s or "

smart glass Smart glass or switchable glass (also called a smart window or switchable window) is a glass or glazing whose light transmission properties dynamically alter to control the passage of solar irradiation into buildings. In general, the glass chan ...

", 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 ICE 3, or Intercity-Express 3, is a family of high-speed electric multiple unit trains operated by Deutsche Bahn. It includes classes 403, 406, 407 and 408, which are known as ICE 3, ICE 3M, New ICE 3 and ICE 3neo respectively. Three multisystem ...

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. Electrochromic windows are used in the

Boeing 787 Dreamliner The Boeing 787 Dreamliner is an American Wide-body aircraft, wide-body jet airliner developed and manufactured by Boeing Commercial Airplanes. After dropping its unconventional Boeing Sonic Cruiser, Sonic Cruiser project, Boeing announced th ...

, allowing crew and passengers to control the transparency of the windows and prevent glare.

References

{{reflist

External links

Tutorial on electrochromatic displays
at Gent University (archived fro
the original
on 6 January 2012)
Article on energy efficiency of electrochromic windows
at National Renewable Energy Laboratory (archived fro
the original
on 21 July 2017)
Video of electrochromic glass changing from translucent to transparent
at YouTube Chromism Scattering, absorption and radiative transfer (optics)