Tungsten Oxide Based Self-Powering Smart Window System

The researchers have designed a self-powering smart window system in which a bifunctional tungsten oxide based device serves as the glass and fibrous dye-sensitized solar cells serve as the frame of the window so that space can be used efficiently. The shining point of this new system is that the number of solar cells can be changed and when converted, it can make this integrated system independent of the angle of incidence as the frame is designed as a cylindrical configuration.

In addition to just powering the dual-function device, the entire self-powered smart window system can automatically adjust its light transmission rate according to changes in weather conditions. That is, when the weather is clear, the solar cells can generate enough power to turn the window into a dark state so that most of the sunlight is blocked, thus keeping the room a comfortable atmosphere; when it is cloudy, the power generated is not enough to turn the window into a dark state and light can partially penetrate so that the room remains bright. The researchers used the solar cell as an automatic intelligent controller to adjust the light transmission rate of the tungsten-based dual-function device according to the weather conditions.

(Picture source: Nanowerk Spotlight)

The other is a device with three functions at the same time, namely solar energy conversion, EC, and energy storage and power supply, without external wires. the EC part and PV part share the same electrolyte, and usually, this integration method sacrifices a certain area of the window. For example, Zhang et al. designed a tri-functional device, a photoelectric color-changing device (PECD), in which both WO₃ and TiO₂ are used as anodes and Pt is used as a counter electrode.

When the device is open-circuited and exposed to sunlight, the dye molecules in the electrolyte are excited and generate electron-hole pairs. Electrons are then injected into the conduction band of TiO₂, which subsequently diffuses into the ITO-PET substrate and then into WO₃, so the WO₃ film turns blue and the sunlight is thus blocked by the device.

When connected to a power application, this colored device can be discharged. Based on tungsten oxide and PANI as the cathode and Al as the anode, the researchers developed a versatile device that can be powered by two methods, namely by sunlight and by exposure to air. Compared to charging methods without solar radiation, the device takes six times less time to fully charge.

Tungsten oxide is also an outstanding material in the field of photochromism. When light is shone on tungsten oxides, electron-hole pairs are created, resulting in a change in their optical absorption. Intuitively manifested as a change in their color, they can be used in displays, smart windows, and light signal processing.

(Picture source: Jin-Long Wang/Nano Lett.)

The band gap of WO₃ is about 2.6 eV, so it can absorb light at a wavelength of 475 nm, which indicates that it is a superior photocatalyst in the visible field compared to TiO₂, which has a band gap of 3.2 eV. Currently, tungsten oxide-based materials are widely used as photocatalysts for carbon dioxide photoreduction, air purification, and other fields. Specifically, tungsten oxide is also widely used in the field of photoelectrochemical cell (PEC) water splitting. For example, researchers have produced 2D/2D WO₃/g-C₃N₄, which shows good H₂ production activity. In addition, due to its relatively low band gap, tungsten oxides are often applied in composites with TiO₂ to extend the photoresponse of the composite to the visible range.

Reference: Han W, Shi Q, Hu R. Advances in electrochemical energy devices constructed with tungsten oxide-based nanomaterials[J]. Nanomaterials, 2021, 11(3): 692.

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