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Smart windows assist heat loss prevention

26 Nov 2013

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Researchers at the A*STAR Institute of Materials Research and Engineering in Singapore are working to develop "smart windows" in buildings using electrochromic materials. Such windows will be able to change from transparent to coloured states at the flick of a switch and help prevent heat loss and maximise the amount of natural light passing through. The researchers said understanding how hydration impacts colour-changing windows can improve efficiency and durability.

Nickel oxide (NiO) is a low-cost, inorganic compound widely used as an anode material inside "smart windows" because it is a reversible colour-changer. Unlike other electrochromic substances, however, researchers have struggled to comprehend how colouration occurs in NiO in the presence of common aqueous electrolytes. Part of the problem is that NiO can form different crystal structures in its bleached and coloured states, depending on how much water becomes incorporated into the material.

The research team set out to unravel this puzzle with a 'chemical bath deposition' technique that allowed rapid fabrication of NiO thin films simply by dipping a conductive glass slide into a nickel precursor solution. The researchers annealed the films at increasingly elevated temperatures to gradually drive water out of NiO, checking its structure with X-ray diffraction and infrared spectroscopy along the way. They also investigated how these structures had changed after multiple electrochromic colour-change cycles.

The team's experiments revealed a complex colouration mechanism involving water and NiO particles. Initially, two intertwined reactions hydrated the thin film by turning NiO into nickel hydroxide. This process enhanced the material's optical response to electrical signals by allowing more of the thin film to contribute to colouration reactions. However, repeated cycling caused 'over-hydration' that trapped water molecules inside the thin film structure—a development that degrades electrochromic activity by generating irreversibly coloured nickel oxide hydroxide grains.

The researchers found that a simple high-temperature annealing process could mitigate the effects of over-hydration in the NiO thin film. This improved mechanistic knowledge—in combination with their simple and scalable chemical dip coating technique—helped them to achieve one of the best optical modulations reported for NiO films.

Currently, the team is investigating how to extend their work to flexible substrates. "Fabricating electrochromic thin films on rolls of plastic could make retrofitting onto existing windows affordable and easy," says researcher Sing Yang Chiam.

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