Electrochromic coatings (EC) are switchable thin-film coatings applied to glass or plastic that can change appearance reversibly from a clear to a dark Prussian Blue tint when a small DC voltage is applied. EC windows preserve the outward view while modulating transmitted light, glare and solar heat gains.

An EC coating is a nanometer-thick (1x10^-9 m, 4x10^-8 inch), multi-layer film or stack deposited on a glass or plastic (Figure below). Transparent conductors form the outer layers of the stack, an active electrochromic and passive counter-electrode layer form the middle layers, and an ion-conducting electrolyte layer forms the center portion of the stack. The system works like a battery. A bipolar potential is applied to the outer transparent conductors, which causes lithium ions to migrate across the ion-conducting layer from the counter-electrode layer to the electrochromic layer. A reversible electrochemical reaction takes place causing a tinted Prussian Blue appearance. Reversing the potential causes the ions to migrate back, causing a bleached clear appearance.

The material and physical composition of the EC window can vary and these dictate the unique properties of the EC window: its switching range, speed versus temperature characteristics, power consumption when being switched, durability, and color.

Diagram of a typical tungsten-oxide electrochromic coating.

The reaction that takes place can be grossly simplified [Grandqvist 2000] as follows:

WO₃ + xM⁺ + xe^- ↔ M_xWO₃,
with M⁺ = H⁺, Li⁺, Na⁺ or K⁺, and e^- denoting electrons

For more about the material science of EC window coatings, see: http://windows.lbl.gov/materials/chromogenics/default.htm