Smart windows are finding many uses in architectural and vehicle applications. Their ability to reversibly switch from transparent to opaque provides important functionality, like reducing solar heat gain and glare, without sacrificing the benefits of broad views and natural lighting. However, one practical challenge for using them in buildings is the need for an external bias voltage, which means that electricians must run extra electrical wiring, increasing installation and maintenance costs. Now, X.W. Sun and colleagues at Nanyang Technological University and Shanghai Second Polytechnic University have developed a hybrid device that functions as a spontaneously reversible electrochromic window that does not need an external bias, and also as a self-recharging battery. They reported their results in the September 23 issue of Nature Communications (DOI: 10.1038/ncomms5921).

The widespread use of smart windows in buildings could save large amounts of energy, particularly in heat-dominated climates through reduced air conditioning. While their market penetration today is small, a recent analysis by IDTechEx predicts that the smart glass market will grow to $700 million over the next 10 years. The most mature smart window technology uses the electrochromic properties of thin-film tungsten oxide (WO₃), which is normally transparent. Under an external bias, charge-balancing ions (typically Li⁺ from a nickel oxide counter electrode) are intercalated into the film, increasing the optical density and making the film opaque. In pursuit of lower costs, researchers have looked to other materials that display electrochromism. One notable candidate is Prussian Blue (iron hexacyanoferrate, or PB), whose distinctive color is due to intervalence charge transfer between Fe(II) and Fe(III), which makes thin PB films opaque. By selectively reducing the Fe(III) this mechanism is eliminated, and the thin film is bleached to a transparent state. The opacity can be recovered through oxidation.

The Nanyang team noted that the oxidation process to recover PB from a bleached state can occur spontaneously in an aqueous solution that contains dissolved oxygen. This led them to investigate the possibility of a self-recovering electrochromic device. Sun notes that “Prussian Blue is quite unique: it is one of the artificial pigments, and it can form a battery in its reducing and oxidation processes, so we realized it has a lot of potential.” The key step was to identify a counter electrode that could easily lose electrons to PB. After some experimentation, the researchers selected aluminum, constructing a device based on electrodeposition of a PB film on indium-tin-oxide-coated glass, an aluminum counter electrode (only covering a small fraction of the device to avoid obstructing the light), and 3 mol/l KCl aqueous electrolyte. The device can be bleached to transparency by connecting the electrodes and reducing the Fe(III), which occurs over several tens of seconds and shows a maximum optical transparency change of 52.2% for red light (670 nm wavelength).When the electrodes are disconnected, the device spontaneously returns to opaque as the iron is oxidized by oxygen dissolved in the electrolyte. This occurs much more slowly; transparency is reduced by 38.5% after two hours. By applying a 2 V external bias, the researchers were able to significantly accelerate this transition, demonstrating 10-s cycling through +/–10% changes in transparency.

In addition to the electrochromic behavior, the researchers note that the device also functions as a “self-recoverable” battery, with an open-circuit voltage of 1.26 V. During the electrochromic bleaching process the battery is discharging, with a measured discharge capacity of 63.6 mAh/g at –2 V for 30 s. The battery then spontaneously recovers during the electrochromic recovery process; after 24 hours the battery discharges at 61.9% of the original capacity. The spontaneous cycling behavior of the device—considered as either an electrochromic window or a battery—is accompanied by the formation of Al(OH)₃ precipitate in the electrolyte, gradually consuming the Al electrode. However, the researchers note that the rate at which this occurs is negligible, and is unlikely to limit the device performance.

Sebastien Lounis of Lawrence Berkeley National Laboratory agrees that the need for an external bias is a significant problem for the market deployment of smart windows: “With current electrochromic technology, you’re looking at involving both carpenters and electricians for installation, which creates a major headache for the builder and drives up costs.” A bias-free electrochromic window could therefore help accelerate the technology into much wider use.

Prussian Blue has captivated artists and scientists since it was discovered over 300 years ago, and it is now used in everything from art to medicine to machining. But given these results, it may still have surprises in store.