What Is Electrochromism?

Introduction

When an electric field is applied to an electrochromic material, it results in a modified ability to transmit, reflect or absorb light. The most common change occurs between a colourless (bleached) state and a tinted state, and back again, when a small voltage is applied (typically 1-5 Vdc). Electrochromic materials can interact with the near-infrared (NIR), visible light or ultraviolet (UV) ranges of the electromagnetic spectrum, which are shown below for illustrative purposes.

Electromagnetic spectrum, showing ultraviolet, visible and infrared regions

Typical electrochromic materials include inorganic, organic or hybrid compositions. The materials can either be deposited on the electrodes, or alternately can be dissolved in an electrolytic solution and sandwiched between the electrodes. One successful use case for electrochromic technology is in the architectural sector. For a primer about smart glass windows and skylights, see our article about electrochromic smart glass. Other applications of electrochromic technologies include glare reduction, thermal regulation and information displays in the following sectors:-

• retail signage in supermarkets
• smart warning labels for cold chain transportation of pharmaceuticals
• self-darkening automotive rear-view mirrors
• cabin windows for avionics
• protective eyewear
• thermal regulation for building roofs using electrochromic coatings or tiles that switch between darker (heat-absorbing) colours in winter to lighter (heat-reflecting) colours in summer

ANA Boeing 787-8 Dreamliner with electrochromic cabin window; Image Attribution: Jun Seita from Palo Alto, CA, U.S., CC BY 2.0, via Wikimedia Commons

In this article, we will look more deeply into the physical process of electrochromism, as well as the materials themselves.

The Electrochromic Cell

An electrochromic cell consists of an inner ion storage layer, an electrolytic layer and the electrochromic (EC) layer, all sandwiched between conducting electrodes, as we see below.

Electrochromic cell; Image attribution: Keyur.tithal, CC BY-SA 3.0, via Wikimedia Commons

The electrolytic layer can be in liquid, gel or solid form, and must be ionically conductive and yet electrically insulative. The electrochromic layer can also be in liquid or solid form, with tungsten oxide being a very common material. When a DC voltage is applied to the electrodes, the ions migrate from the ion storage layer through the electrolyte into the electrochromic layer where they intersperse with the atomic structure, changing its band gap so that visible light is absorbed, which tints the cell dark. Electrochromism is reversible, meaning that the material can be switched from the tinted state back to the clear (bleached) state, by simply removing (or reversing) the voltage. There are also some electrochromic materials that transition between two different coloured states as well as ‘polyelectrochromic’ materials, which are capable of exhibiting several colours.

A Short History of Electrochromism

Research into electrochromism was first recorded in 1815 by the Swedish physician and chemist, Jöns Jacob Berzelius (1779 – 1848), who showed that pure tungsten trioxide (which is normally pale yellow) changes colour when warmed under the flow of dry hydrogen gas.

Jöns Jacob Berzelius (1779 – 1848), Swedish physician and chemist; Image Attribution: Johan Way, Public domain, via Wikimedia Commons

In 1824, the German chemist Friedrich Wöhler (1800 – 1882) reported similar results with the chemical reduction of sodium metal. The term ‘electrochromism’ was coined in 1961 by John Rader Platt, an American physicist, while at Bell Laboratories, although he was referring to colour generated via a molecular ‘Stark effect’, which splits the spectral absorption lines of molecules when a strong electric field is applied. Significant progress was made in 1969 by Satyen K. Deb at American Cyanamid Company, who applied an electric field across a thin film of tungsten oxide deposited on quartz, which he called ‘electrophotography’. Nowadays, Deb’s 1973 paper, is cited as the birth of electrochromic technology, since it describes an electrochromic device based on a film of tungsten oxide, immersed in an ion-containing electrolyte, the closest we have to modern day implementations.

Redox

Electrochromism is the result of an electrochemical process called redox (short for ‘reduction-oxidation’), which involves the exchange of electrons and ions:

• Reduction is when a material gains electrons (the material is ‘cathodically coloured’), becoming more negatively charged
• Oxidation is when a material loses electrons (the material is ‘anodically coloured’), becoming more positively charged

The processes of oxidation and reduction do not occur independently but rather simultaneously, with each considered a ‘half-reaction’, and together forming a whole reaction. We can visualise redox as a vertical number line showing electrical charge (oxidation state), such that ‘reduction’ is shown as reducing the electrical charge, by making it more negative.

Redox number scale showing oxidation state

Redox reactions occur either by electron-transfer or by transfer of atoms. For example, iron converts to an oxide when it rusts, increasing the oxidation state of iron, and simultaneously decreasing the oxidation state of the oxygen as it accepts electrons released by the iron. So you see, the two reactions are simply ‘two sides of the same coin’.

Electrochromic Materials

Inorganic electrochromic materials include transition metal oxides, with tungsten oxide being the most common example, as well as molybdenum, titanium, niobium oxides and Prussian Blue, produced from ferrocyanide salts.

Prussian Blue pigment, image attribution: Saalebaer, CC0, via Wikimedia Commons

We also find organic materials such as conjugated polymers (with alternating single and double bonds) and aromatic polymers (with an evenly distributed electron density), as well as viologens, currently the subject of intense research.

Viologen redox couple – the left species is colourless, the right species is deep blue or red, depending on the identity of R; Image attribution: Smokefoot, Public domain, via Wikimedia Commons

Viologens are salts of a carbon-based chemical called 4,4´-bipyridine. The viologen molecule has two linked carbon rings, each with one nitrogen substitution, and tints blue when electrochemically reduced (i.e. it gains an electron). Viologens are seen as promising alternatives to inorganic materials due to their superior optical contrast, coloration efficiency, redox stability and potential for large-area scalability. Organic materials give variety, flexibility and low-cost processability. Hybrid materials combine the advantages of both organic and inorganic materials.

Electrochromic Cell Architectures

Electrochromic materials are normally deposited as thin-films and can take on one of several architectures.

Asymmetric Cell

This arrangement has a single electrochromic (EC) film, located either on the anode or the cathode:

Dual-Film Cell

This arrangement has multiple electrochromic (EC) films, which are located on both the anode and the cathode, and tinted simultaneously for greater contrast.

Modes

Electrochromic devices can operate in absorptive or reflective modes, where both modes have at least one optically transparent electrode.

• Absorptive Mode: examples include eyewear, visors or smart glass that have a second optically transparent rear electrode.
• Reflective Mode: examples include information displays and anti-glare mirrors that have a polished metal behind the rear electrode

Power and Switching

Electrochromic devices only consume power when switching between states. The problem is that electrochromic devices have rather long switching times, which is due to the slow diffusion rate of counter-ions during the redox process. Researchers at the University of Maryland have investigated nanotubes as a way to increase the switching speed, by reducing the thickness of the electrochromic films, and thus reducing the diffusion distance of the ions. However, thin films often don’t produce enough contrast, so hollow tubes of electrochromic polymers have been investigated with walls of tens of nanometers thickness but hundreds of nanometers in length. The ions only have to diffuse through the wall to create the redox reaction, with resulting switching times in the order of milliseconds. They have been able to demonstrate films of nanotubes that work in both reflective and transmissive modes, potentially making the arrangement useful for both displays and windows. Reducing the switching time would thus reduce the power requirements and allow the potential for photovoltaically-driven electrochromic devices, avoiding expensive cabling (and inefficient voltage transformations) in a building, vehicle or smart city.

Memory

Once an electrochromic cell has transitioned from clear to tinted (or vice versa), the state remains static for a certain time interval (and without any additional power), thereby exhibiting a ‘memory effect’. Some applications, however, require electrochromic devices to be available 24/7, which is possible through reflective electrochromic technology. This offers the possibility for continuously operated devices, such as smart shelf indicators in supermarkets and cold-chain warning labels for pharmaceuticals. These reflective electrochromic information displays maintain a longer display life by firing a micropulse of current every minute or so without significantly degrading battery life. One such manufacturer is Ynvisible, who manufacture organic electrochromic e-paper information displays for cold-chain logistics, medical devices and wearables. Ynvisible e-paper displays require refreshing every minute but still claim microWatt power consumption, boasting up to 5 years battery life using a simple coin cell.

Outlook

Electrochromism was first discovered in 1815 and has steadily improved in display capabilities, power consumption and switching speed during the last 10 years. In the near future, we can expect to see this technology achieve further recognition in consumer devices, industrial displays, pharmaceutical logistics tracking and transportation as a low-power, low-cost alternative to liquid crystal displays.

References

1. Smart Optical Materials, Encyclopedia of Modern Optics, P. M. Martin, URL 2. Redox, Wikipedia, URL 3. Electrochromic windows, L. Niklaus, URL 4. Electrochromism, Wikipedia, URL 5. Development of Eco-Efficient Smart Electronics for Anticounterfeiting and Shock Detection Based on Printable Inks, URL 6. A Brief Overview of Electrochromic Materials and Related Devices: A Nanostructured Materials Perspective, National Library of Medicine, URL 7. Switching Colours with Electricity, American Scientist, URL 8. A Brief History of Electrochromism, URL 9. Colour and the Optical Properties of Materials, Richard Tilley, ISBN: 978-1-119-55468-4, URL 10. Electrochromism: from oxide thin films to devices, HAL Open Science, URL