What is Electrochromism?
Jump to: How does Electrochromism work? | Electrochromic Devices | Electrochromic Materials | Electrochromic Applications
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+ ↔ MxWVI(1-x)WVxO3
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
Electrodes
Responsible for carrying the charge from a power source to the corresponding electrochromic layer or ion storage layer. Transparent conductive oxides (TCOs) such as indium tin oxide (ITO) and fluorine-doped tin dioxide (FTO) are popular electrode materials as they do not interfere with visible light. As a result, they do not effect the transmission properties for the electrochromic layer. Silver nanowires, conductive metal grids, graphene materials, carbon nanotubes and their composites have also been applied as electrode materials in electrochromic devices due to their high electrical conductivities.
Ion storage layer
Used to decouple the two electrodes to make sure the device does not behave like a battery. It undergoes reversible electrochemical oxidation (reduction) to match with the reduction (oxidation) of EC materials in the electrochromic layer. As a result, it acts like a capacitor or ion-buffer system which is more stable and controllable for electrochromic switching.
Ion transport layer
Usually made up of small mobile ionic charge carriers and ensures the completion of the circuit by facilitating the transfer of ions between electrodes. The materials used in this layer are considered electrolytes either as doped gels, solutions or films, polymers with ionic conductivity and liquid crystals.
Electrochromic layer
The electrochromic layer or film can reversibly change its optical properties, switching between transparent, semi-transparent and colored states. Once an electronic field is applied, ions are transported through the device to the electrochromic layer where redox reactions take place which cause the layer to change color. Electrochromic materials fall into two main categories, organic and inorganic, and will be discussed in more detail below.
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:
Optical Modulation
Optical modulation demonstrates the color-switching ability of the device. It is defined as the difference in absorbance or transmittance at the characteristic absorption wavelength before and after color switching.
Contrast Ratio
The contrast ratio described the ratio of absorbance or transmittance before and after color switching. A high contrast ratio is made possible under strong ambient light due to the light absorption properties of electrochromic devices.
Response Time
The response time is the time it takes for an electrochromic device to reach 90% from one colored state to the other. It can also be referred to as coloring time, bleaching time or fading time. The quicker the response time the better for most devices. Due to the electrochemical driven process, rapid response times (millisecond-scale) are often difficult to achieve.
Coloration Efficiency
The coloration efficiency describes the color change observed for the characteristic absorption wavelength per injected charge in per unit area. Essentially, how efficiently the inputted electricity is converted to color change. A higher coloration efficiency means less charge is required to observe the same optical modulation.
Lifetime
Another key performance indicator is the ability of the electrochromic device to withstand its external environment, even in poor conditions. Ideally, an electrochromic device should reach at least 104–106 cycles without significant optical degradation.
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:
At least one colored and one bleached state
Two distinct colored states
Multicolored electrochromics
Transition Metal Oxides
Oxides like tungsten trioxide (WO3), molybdenum (MoO3), and nickel oxide (NiO) undergo redox-driven color changes. Tungsten trioxide, the first commercial electrochrome, remains widely studied for its high efficiency, stability, and tunable colors via ion intercalation.
Metal Complexes
Coordination complexes (e.g., Cu, Fe, Ir with organic ligands) show intense, tunable color changes through charge-transfer transitions between metals and ligands.
Small Organic Molecules
Viologens and other redox-active molecules (e.g., quinones, carbazoles, thiophenes) offer low cost, tunable colors, and high contrast but often face cycling stability issues.
Electrochromic Polymers
Electrochromic polymers (e.g., PEDOT:PSS, polythiophenes, polyanilines) are low-cost, flexible, and multichromatic. They offer high optical contrast, reproducibility, and mechanical adaptability.
Hybrids
Organic–inorganic composites (e.g., PEDOT–WO3, polyaniline–TiO2, graphene oxide blends) combine conductivity, structural stability, and enhanced electrochromic performance.
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.
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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.
Metal Oxides
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What are Electrochromic Materials?
The two main categories are organic materials such as small molecules and polymer and inorganic materials such as metal oxides and metal complexes. 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.
Learn more...
An Overview of Metal Oxide Nanoparticles
Metal oxide (MOx) nanoparticles (MONPs) are a class of nanomaterial with interesting and diverse chemical, optical, electrical and magnetic properties. Different metals bond to oxygen forming a variety of crystal structures under specific conditions. The shape and nanostructure of these materials effects their surfaces which in turn impacts the way they interact with light, electricity, magnetic fields and other materials.
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References
- A Brief Overview of Electrochromic Materials and Related..., Shchegolkov, A. V. et al., nanomaterials (2021)
- Expanding Color Control of Anodically Coloring Electrochromes Based..., Hawks, A. M. et al., ACS Applied Optical Materials (2024)
- Emerging Electrochromic Materials and Devices for Future Displays, Gu, C. et al., Chemical Reviews (2022)
- Chapter 1: Introduction to Electrochromism, Chua, M. H. et al., The Royal Society of Chemistry (2019)
- Recent advances in electrochromic polymers, Abidin, T. et al., polymer (2014)
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