Metal mirror makes window switch from light to dark in seconds

Scientists have made smart glass that switches from transparent to black in one minute thanks to a reversible metal coating process. The material is so cheap and easy to make that it might replace other smart window technologies.

Smart windows can be switched from transparent to tinted and back again. As they block visible radiation, as well as infrared, they can help save on heating or air conditioning costs. In buildings, smart windows can reduce energy consumption by around 10% compared with regular windows.

Most dynamic windows contain electrochromic compounds. They change their colour depending on their redox state, which is controlled by applying a voltage. However, they often fail to combine fast switching, durability and low cost, so they remain limited to niche applications.

A team of US chemists has now made smart glass based on electroplating – reducing ions to their metals when an electric potential is applied. Switching a 10cm×10cm pane from 75% transparency – roughly the same as double glazed windows – to 10% takes only one minute. The researchers found the material survives 4000 light-to-dark-to-light again cycles without a drop in performance.

The device is made out of an indium oxide and nickel oxide electrode sandwiching a 3mm thick electrolyte layer containing bismuth(III) and copper(II) ions. While dissolved, the ions are colourless, but turn a uniform black colour when they deposit on the electrode at 2.5V. Since the electrodes are transparent, the researchers didn’t need to hide them in the window frame, which previously made fast switching and uniform tinting impossible.

The researchers note that their smart windows have cheaper ingredients and cost less to make than devices based on electrochromic compounds. Moreover, the electrodes can be placed between plastic sheets to make flexible windows. This, combined with the devices’ fast switching time and durability, could give them the edge in commercial applications.