Glass that quickly becomes opaque could reduce the need for air conditioning in buildings and vehicles.

A ‘smart’ window can turn quickly from clear to dark thanks to layers of nickel oxide and electrolytes.

Transparent windows that become opaque on command can be used to control the amount of light and heat that enter buildings and vehicles. The technology saves energy, but current smart windows darken slowly and often have a blueish tint when partially opaque.

A team led by Christopher Barile at the University of Nevada in Reno developed a window that has layers of nickel oxide, lithium ions and electrolyte gel coating the inside of the pane. When a current is applied to these layers, the nickel oxide, acting as an electrode, soaks up the lithium ions and turns opaque. The current also prompts the electrolyte ions to form deposits on another window coating that serves as a second electrode. This blocks more light. These two effects have no strong colour bias, eliminating the coloured tint of existing smart windows.

Windows made according to this formula can switch states quickly — blocking 94% of light within 60 seconds — and in tests cycled between opaque and transparent at least 4,000 times without significant degradation.