Smart windows: Understanding how hydration affects color-changing windows can boost efficiency

Nickel oxide (NiO) is a low-cost, inorganic compound widely used as an anode material inside smart windows because it is a reversible color-changer. Unlike other electrochromic substances, however, researchers have struggled to comprehend how coloration 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 colored states, depending on how much water becomes incorporated into the material.

Chiam and his 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 (see image). 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 color-change cycles.

The team's experiments revealed a complex coloration 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 coloration 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 colored 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," explains Chiam.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering