SEOULTECH researchers develop sodium-based next-generation smart electrochromic windows

Thermal management is essential for reducing future heating and cooling energy consumption. Notably, the near-infrared (NIR) component of sunlight is closely associated with heat absorption. Hexagonal tungsten oxide nanorods are promising NIR-blocking electrochromic materials that change their color, transparency, and opacity upon the application of a small electric voltage. Their hexagonal tunnels, known as optically active sites, can effectively accommodate electrolyte ions and enable dynamic NIR modulation. However, maintaining the hexagonal structure requires dopants inside these tunnels for structural integrity.

Although small lithium ions can easily access the hexagonal tunnels, larger sodium ions, despite being more abundant and cost-effective, are considered as limitations due to steric hindrance from dopants inside the tunnels.

In an innovative breakthrough, a team of researchers led by Assistant Professor Sungyeon Heo, along with Mr. Janghan Na, both from Seoul National University of Science and Technology, Republic of Korea, has overcome this dopant-blocking limitation by introducing thermally removable dopants that can be easily eliminated through simple heat treatment. The study was made available online on 20 November 2025, and published in Volume 25, Issue 50 of the journal Nano Letters on 17 December 2025.

“Our strategy allows effective utilization of the hexagonal tunnels for sodium-ion insertion. Consequently, we demonstrate that low-cost sodium electrolytes can achieve large NIR modulation, comparable to that of lithium-based system. This leads to efficient heat-shielding performance without relying on expensive lithium-based systems even with an ultrathin film thickness of 150 nm,” says Dr. Heo.

The nanomaterials developed in this research have strong potential for large-scale production. All synthesis procedures are conducted within a single reactor batch, with precise control over reaction pressure and temperature. This simplicity enables scale-up production, particularly when equipped with integrated systems that automatically monitor and control reaction temperature and pressure. Moreover, since the materials are synthesized in a colloidal form, they are not limited to electrochromic applications. Colloidal nanomaterials can be readily processed into coatings, or composite systems, allowing their use in a wide range of fields beyond electrochromism.

Furthermore, this research offers a practical solution for thermal regulation under diverse climate conditions. In extremely hot regions, such as parts of Africa and the Middle East, electrochromic materials can be maintained in a continuous heat-blocking state to effectively suppress NIR transmission, which is the primary contributor to solar heat gain. In contrast, in regions with distinct seasonal variations, such as Republic of Korea, the optical state can be dynamically adjusted according to user demand and climate conditions. By selectively controlling NIR transmission, the system enables efficient thermal management throughout the year. As a result, both cooling and heating energy consumption can be significantly reduced, improving overall building energy efficiency.

Within the next 5 to 10 years, this progress could enable broader applicability of smart windows and adaptive buildings that automatically regulate heat and light. Such systems would reduce reliance on air conditioning and heating, leading to lower energy consumption while improving indoor comfort.

“Our study demonstrates material designs and processing strategies that are compatible with low-cost Earth-abundant components, such as sodium electrolyte, and scalable synthesis methods, such as single reactor batch. Ultimately, our research supports a transition toward more sustainable environment and could finally reduce energy demand in everyday life,” concludes Dr. Heo.

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Reference
DOI: 10.1021/acs.nanolett.5c04697

About Seoul National University of Science and Technology (SEOULTECH)
Seoul National University of Science and Technology, commonly known as 'SEOULTECH,' is a national university located in Nowon-gu, Seoul, South Korea. Founded in April 1910, around the time of the establishment of the Republic of Korea, SEOULTECH has grown into a large and comprehensive university with a campus size of 504,922 m2.

It comprises 10 undergraduate schools, 35 departments, 6 graduate schools, and has an enrollment of approximately 14,595 students.

Website: https://en.seoultech.ac.kr/

About Assistant Professor Sungyeon Heo
Dr. Sungyeon Heo is an Assistant Professor of Chemical and Biomolecular Engineering at Seoul National University of Science and Technology and leads the Electrochemical Energy Materials Lab (EEML). His group focuses on the development of colloidal semiconductor nanocrystals with various optical, electrical, and electrochemical properties. Based on this, the group is accelerating the development of next-generation electrochemical devices for applications in electrochromic, reflective displays, and defense technologies.

About Mr. Janghan Na
Mr. Janghan Na is a PhD candidate of Chemical and Biomolecular Engineering at Seoul National University of Science and Technology. His research focuses on the synthesis of colloidal metal oxide nanocrystals and the engineering of electrolytes for next-generation electrochromic devices.