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A New Material for Smart Windows and Smart Energy Storage

A New Material for Smart Windows and Smart Energy Storage

Humanity is on the verge of an energy crisis; we need to phase out non-renewable energy sources, such as coal, oil, and natural gas, and minimize the amount of energy used in everyday life. While renewable energy sources such as sunlight and wind power are promising, there are inadequate technologies to store the excess energy generated for later consumption. Thus, scientists have been focusing on various types of batteries, supercapacitors, and other energy-storage devices to address this problem. But what if the storage device itself served other functions as well?

In a recent study, a team of researchers, led by Dr. Chanchal Chakraborty and including Mr. Sayan Halder and Mr. Siddhant Garg from the Department of Chemistry at the Hyderabad Campus of Birla Institute of Technology & Science (BITS) Pilani in India, has developed a new type of material that could single-handedly make buildings much more energy efficient, thanks to its multi-functional nature. Their findings are published in Chemical Engineering Journal.

The researchers sought to address the ever-growing energy requirements of modern buildings by developing an electrochromic supercapacitor device (ESD). The term ‘electrochromic’ refers to a phenomenon in which a material changes its optical properties, such as colour and transparency, in response to an applied electric voltage or current. ESDs change colour depending on their stored energy, so one can visually tell whether they are charged or not.


To create material suitable for high-performance ESDs, the team employed a novel non-conjugated pyridinium cation-containing bisterpyridine ligand (TpyNTpy). By combining it with iron ions, they obtained a low-cost metallo-supramolecular polymer in the shape of nanofibers, which they called Fe-TpyNTpy. This compound can store large amounts of energy and switches between white and purple colours upon charging and discharging.

An electrochromic supercapacitor device based on Fe-TpyNTpy polymer shows a fast colour change from purple to transparent at a low voltage of 1.5 V, making it promising for smart windows and smart energy storage. Picture credits: Dr. Chanchal Chakraborty from BITS Pilani Hyderabad Campus

Using Fe-TpyNTpy, the team developed a small proof-of-concept ESD with remarkable properties. The key to their approach was striking an optimal trade-off between optical memory, which indicates how well the material can retain its optical state, and fast switching. “We successfully balanced two opposite effects of electrochromism using Fe-TpyNTpy, resulting in a material with excellent qualities, including high optical contrast, shorter switching times, high coloration efficiency, and high optical memory that makes the device power efficient,” highlights Dr. Chakraborty.

Additionally, the proposed device exhibited great thermal stability and cyclability, showing no degradation in performance after ten thousand charge–discharge cycles. It could also hold its colour for over 30 minutes when receiving no power. Furthermore, the miniature device showed fast colour change in 2 seconds at a low voltage of 1.5 V, outperforming commercially available costlier alternatives such as tungsten oxide, which shows colour change in 15 to 20 seconds.

One of the most promising applications of the proposed ESD is smart windows. By charging or discharging the Fe-TpyNTpy polymer, one can alter its transparency and thereby control how much sunlight passes through the window and enters the building. In this way, one can easily reduce the amount of energy used for cooling, which is one of the main energy sinks in industrialised countries with hot climates. “Besides industry and transport, buildings use about 35% of the primary energy worldwide. In India, which has a very hot climate, the electrical peak power may be entirely dominated by air conditioning,” explains Dr. Chakraborty. “The proposed material, when applied to smart windows, could reduce the electricity consumption and cost of room cooling, thereby lowering the use of fossil fuels and helping mitigate global warming.

Moreover, since the proposed ESD can efficiently store and deliver energy, smart windows equipped with it could function as power sources for various kinds of electronics. This could be particularly useful during a power outage, since smart windows could help keep the building’s electronic systems online. Worth noting, people in the building would be able to tell easily whether the windows are charged or not by observing their colour. “This innovative technology has the potential to revolutionise smart architectures in smart cities and could transform the way we think about windows and sustainable energy at the same time,” remarks Dr. Chakraborty.

This work was conducted in the Materials Center for Sustainable Energy & Environment (McSEE) established by the research team last year at BITS Pilani Hyderabad Campus. “McSEE serves as a platform for multidisciplinary research, specifically on materials for energy and the environment,” comments Dr. Chakraborty. “With wholehearted support from BITS Pilani, especially from our campus director, Prof. G Sundar, we have created a state-of-art infrastructure for energy research on our campus. We are thankful to BITS Pilani authority for providing us with the opportunity represented by the McSEE center to carry out this cutting-edge research.”

Let us hope that further research at BITS Pilani and its various campuses leads to a future with smart, exciting, and above all, sustainable technologies.