Electrochromism: Changing Colors with Electricity

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Electrochromism describes the process of materials changing appearance in response to electricity. The components of the name, “electro” describes the need for electricity and “chromism” describes the involvement of color. Electrochemical oxidation and reduction reactions lead to changes in a material’s absorption characteristics in the UV-Vis-NIR region. As a result, electrochromic materials experience reversible color changes.

Electrochromism is useful for various optical devices for protection from light radiation via “smart windows” that can help to control the environment of indoor spaces or antiglare mirrors for cars to improve safety whilst driving.

Electrochromic materials include:

• Transition metal oxides
• Metal coordination complexes
• Small organic molecules
• Organic conducting polymers

How does Electrochromism work?

Electrochromism is a reversible change in transparency or color under the influence of an electric field or electric current via electrochemical redox reactions. Reduction or oxidation reactions take place as electrons are transfered too or from a material.

When a voltage is applied, electrons and ions (typically small cations like Li⁺ or H⁺) move through the device. These are inserted into or extracted from the electrochromic layer, resulting in a change in its electronic structure, and consequently, its optical properties such as color or light transmittance. The electrochromic reaction can be described by the electrochemical equation:

Oxidized form, O + Electrons + Cation (eg. Li⁺) ↔ Reduced form, R

Example: WO₃ + xe^- + xM⁺ ↔ M_xW^VI_(1-x)W^V_xO₃

A material will change color in their oxidised state or reduced state:

• Anodically coloring electrochromes are molecules that become colored when they are in their oxidized state (when a positive voltage is applied).
• Cathodically coloring electrochromes are the opposite, they become colored in their reduced state (when a negative voltage is applied).

Electrochromic Devices

An electrochromic device typically contains five layers – counter electrode, ion storage layer, ion transport layer, electrochromic layer and then working electrode. There have been many different versions of electrochromic devices created including, all-solid devices, gel devices and liquid devices depending on the components used in each layer.

Electrochromic Device Layers

The observed color changes in electrochromic devices is mainly affected by H⁺ or Li⁺ ion transport for devices. The concentration of these cations in the device significantly affect electrochromic properties such as switching time, cyclicity and staining efficienct. The layers are also usually supported by a single glass or polyester substrate.

Devices that involve 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. The counter electrodes are polyaniline, Prussian Blue, or a mixture of the two, which lead to a large modulation range of visible light.

Devices that involve Li⁺ transport use polymers including, poly-methyl methacrylate (PMMA) copolymerized with polypyrrole, propylene carbonate, silane, polypropylene and polyvinylidene fluoride (PVDF). The polymers become ion-conducting upon the addition of Li-based salts. The counter electrode in these systems use V₂O₅, SnO₂ doped with Mo and Sb, and TiO₂ with or without additions of ZrO₂ or CeO₂.

Performance Indicators

The performance of electrochromic devices can be determined via specific indicator. Different features should be prioritized depending on the desired application of the device. The key performance indicators are:

Electrochromic Materials

Electrochromic materials can be found across a huge range of material categories. The two main categories are organic materials such as small molecules and polymer and inorganic materials such as metal oxides and metal complexes. The color changing properties of all these materials are consistent but the way in which they can change color varies. The color change process itself can be used to classify electrochromic materials:

Transition Metal Oxides

Transition metal oxides exhibit electrochromism when they possess a suitable band gap, redox-active metal centers, sufficient ionic mobility, and strong electron–lattice interactions. Transition metal oxide electrochromes can be broadly classified into anodically coloring and cathodically coloring materials. Oxides of tungsten, molybdenum, vanadium, niobium, and titanium exhibit color changes upon reduction (cathodic coloring), whereas oxides of iridium, rhodium, nickel, and cobalt undergo color changes upon oxidation (anodic coloring). Electrochromic effects occur due to electron–ion separation. Metal atoms (eg. Li+) are introduced into the TMO, and the valence electrons move to the d-levels of the transition metal ion, reducing it. For this to work the injected ions should possess a high diffusion coefficient and a high solubility in the lattice of TMO.

These color changes arise from redox reactions that alter the oxidation states of the metal centers. The associated changes in the electronic structure and band gap modify light absorption properties, leading to the observed differences in color. The visible color change in electrochromic oxides results from small-polaron absorption and intervalence charge transfer between different metal oxidation states (W⁶⁺ → W⁵⁺). A polaron is an electron (or hole) that becomes localized on a metal site and is accompanied by a local lattice distortion, effectively forming a "trapped" charge carrier. As more electrons are inserted during reduction, these polarons can pair up to form bipolarons (two electrons localized in close proximity on adjacent or even the same metal sites).

Polaron hopping involves single electrons moving between adjacent metal sites (W⁵⁺ ↔ W⁶⁺). This is a thermally activated process that directs charge transport during electrochromic switching. At higher levels of electron insertion, bipolaron hopping can occur, where paired electrons migrate between sites. This leads to additional absorption bands (often in the near-infrared) and modifies the optical spectrum, explaining the dual-band absorption sometimes observed in highly reduced oxides. Together, these processes alter the electronic structure and narrow the effective band gap, producing the characteristic color shifts seen in cathodic and anodic electrochromic oxides.

Tungsten Trioxide

Tungsten trioxide (WO₃) was the first electrochromic material to be used in a controlled and reversible color changing device in 1969 and is still widely investigated. It has high functionality, high staining efficiency, high contrast, high chemical stability and a long life cycle. A WO₃ film can be different colors depending on the amount of ions that have intercalated into the film. This is due to the partial reduction tungsten to oxidation state 5⁺ and the changes in the band gap observed from the intercalation of Li⁺. The performance of electrochromic devices based on WO₃ films is dependent on the atomic structure, nanoparticle size, pore size and absorption properties.

Metal Coordination complexes

Many metal coordination complexes are also electrochromic materials because of their intense coloration and redox reactivity. Redox activity can occur at either the metal cations or bonded organic ligands. Color changing properties are linked to low-energy metal-to-ligand charge-transfer (MLCT), ligand-to-metal charge transfer (LMCT) intervalence charge-transfer (IVCT), intra-ligand excitation. These electronic transitions, involving valence electrons, fall in the visible-region and lead to different optical characteristics upon oxidation or reduction of the complex.

Transition metals, such as copper, iron and iridium, are often used as the coordinating metal due to their redox activity. Like with transition metal oxides, new absorption bands at different parts of the visible or near infrared regions are generated by switching between different redox state. In cases where the central cation undergoes redox change, then its initial and final oxidation states are shown in superscript roman numerals. It is less clear for ligands and extra charge lost or gained is usually indicated by a superscripted + or −. Typical electrochromic organic ligands include oligopyridines and phthalocyanines, but there are many options out there for further investigation.

Small Organic Molecules

Small organic molecules are also capable of electrochromism as they contain redox active cores with various substituents that can control the band gap. The band gap in this case is between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. Small molecule materials offer superior features, such as high contrast, low toxicity, rich colors, simple synthetic routes, and low cost. The advantage of small organic molecules is that they can be easily tuned by simple changes to their structure. They exist as salts as they are ionic materials that can undergo redox reactions.

Viologens

Small organic molecules are also capable of being electrochromic. Small molecule materials offer superior features, such as high contrast, low toxicity, rich colors, simple synthetic routes, and low cost. The advantage of small organic molecules is that they can be easily tuned by simple changes to their structure. They exist as salts as they are ionic materials that can undergo redox reactions:

The electrochromic properties of these materials can be modulated by varying the nitrogen substituents on the pyridyl ‘N’. The counter ion used within the salt can also be modified with specific functionalities to enhance electrochromic behaviour. Electrochromic devices based on viologens have advantages of low operational voltages. Efforts have been made to increase their low cycle life and poor device efficiency long term.

Other small molecule have been investigated as electrochromic materials and are based on redox active cores including:

Redox Active Core	Colors
Carbazoles	Dark green
Methyoxyphenyls	Blue
Quinones	Purple
Tatrathiafulvalenes	Green
Phenylene diamines	Blue/green
Pyrazolines	Red
Thiophenes	Dark red to purple to navy blue
Furans	Wine-red to orange to orange-pink

Electrochromic Polymers

Electrochromic polymers are renowned for their low cost, easy processing, easy control over color and other properties via synthetic and processing techniques. Electrochromic devices based on polymers offer long lifetimes, high optical contrast, stable oxidation states, excellent switching reproducibility and flexibility so they can be applied to areas that are uneven or foldable compared to their inorganic and small organic counterparts.

Many electrochromic polymers have multiple redox states and are multichromatic. Classic examples of electrochromic polymers include poly(thiophenes), poly(pyrroles), poly(anilines) and other π-conjugated polymers. Poly(imides), poly(amides) and poly(norbornenes) are high performance polymers that can contain electroactive chromophores. These polymers are based on the same redox active cores found in small organic molecules are mentioned in the table above.

PEDOT:PSS

PEDOT:PSS is a cathodic electrochromic material which possesses thiophene redox active sites throughout the polymer backbone. When a negative voltage is applied, doping of electrolyte ions occurs between the polymer backbone, resulting in a change in the energy band gap of the material and thus achieving electrochromism. While the de-doping process occurs under the condition of applied positive voltage, the appearance of color returns to the original form.

Further electrochromic polymers have been developed based on the poly(thiophene) backbone. Different thiophene units, such as thienothiophene (TT) and 2,2'-bithophene (BT), and different substituents, such as triethylene glycol (TEG) have been used to harness reversible electrochromism. p(g2T-TT) films can uptake a much higher density of ions than PEDOT:PSS while demonstrating high volumetric capacitance at low biases.

Organic-Inorganic Composites

Organic-inorganic nanocomposites were also developed to combine the advantages of both organic and inorganic electrochromic materials. Such hybrid materials could be prepared from the use of either only electrochromic-active organic or inorganic materials or both. Modifying the interfacial interactions between organic and inorganic parts has received a lots of research attention because they are so important for structure strength, mass transport, electron conduction and electrochromic performance.

Examples of organic-inorganic composites include many electrochromic polymer and transition metal oxides composites such as PEDOT–WO₃ and polyaniline-TiO₂. As well as metal-organic frameworks which are 3D versions of metal coordination complexes with electroactive polymers. Graphene oxide is also widely used in electrochromic composites.

Electrochromic Applications

Electrochromic materials are used in a wide range of technologies where dynamic control of optical properties is essential. Their ability to modulate light absorption or reflection under an applied voltage makes them valuable in both commercial and scientific fields.

Optical Display

Electrochromic materials can be used in color display technologies such as advertising boards. For this type of application, fast switching between different color states is essential to enable dynamic and responsive visual content. The color changes occur only within the visible spectrum, as there is no requirement for transparency or operation outside the range of human vision. Additionally, the system must provide high color contrast to ensure that the displayed content is vivid and easily distinguishable. Unlike smart windows or optical shutters, a transparent or bleached mode is not necessary for this application.

Variable Reflectance Mirror

Electrochromic technology is widely used in auto-dimming rear-view mirrors for vehicles. These mirrors typically incorporate light sensors that detect glare from the headlights of vehicles behind. When glare is detected, a voltage is applied to the electrochromic (EC) gel layer within the mirror, causing it to darken automatically. This reduces the intensity of reflected light and minimizes visual discomfort for the driver. A well-known example is the Gentex mirror, which employs this EC technology and is widely used in automotive applications around the world.

Smart Windows

One of the most popular applications of electrochromic materials is in smart windows or glass. The smart glass can switch between transparent and opaque states depending on the electrical potential applied. This means the indoor environment or privacy can be controlled easily through the blocking of the suns rays or visibilty from people outside. In 2024 the market size of smart glass was $7.38 billion and this is forecast to rise to over $13 billion by 2030.

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