Electrochromic devices (ECDs) are a type of smart material technology that change color or opacity when an electrical voltage is applied to them. They consist of a thin film of electrochromic material, which undergoes a reversible change in its optical properties when electrically charged. They have many important industrial applications for example electrochromic windows can change from transparent to opaque, offering control over heat and light entering a building. These smart windows are particularly beneficial for energy savings in buildings, as it can reduce the need for air conditioning and artificial lighting. Another important application is in the automotive industry where electrochromic materials are used in rearview mirrors and sunroofs in cars. The mirrors can automatically darken in response to bright headlights from trailing vehicles, reducing glare and improving safety. Sunroofs can be adjusted for light transmission, enhancing passenger comfort. They also can be used in small-scale display applications where the display might change color or appearance based on an electrical input. Moreover, electrochromic lenses in sunglasses can change their tint when exposed to different light intensities. This feature allows for optimal vision comfort in varying light conditions. Additionally, privacy Glass: Used in office spaces or even in some home settings, electrochromic glass can switch from transparent to frosted with the flick of a switch, providing instant privacy. Overall, the versatility of electrochromic devices in controlling light and heat transmission, along with their energy efficiency and potential for aesthetic applications, makes them a valuable technology in modern material science and engineering. However, current ECDs have several limitations that affect their usage and widespread adoption, firstly, the relatively slow response time. The transition from one state (transparent) to another (opaque) can take several seconds to a few minutes, which is less desirable in applications requiring rapid changes.
Addressing these limitations is a key area of ongoing research in the field of electrochromic materials and devices, with the goal of making them more practical, convenient, and cost-effective for a wider range of applications. In a new study published in the peer-reviewed Journal Accounts of Materials Research by Professor Xun Cao’ group from the State Key Lab of High-performance Ceramics and Superfine Microstructure at Shanghai Institute of Ceramics, CAS, embarked on a journey to enhance the response speed of ECDs, focusing on WO3 (tungsten trioxide) as their material of choice due to its high coloring efficiency, fast response, and good cycle stability. The enhancement strategies revolved around optimizing the electrolyte, electrode, and electrochromic materials. The team introduced innovative modulation strategies, including the use of multivalent ions and hybrid ion synergy, to expedite ion migration. Foremost among these, their innovative use of tandem proton transport has proven to be effective in reducing response times.
Traditional ECDs are composed of five layers: a transparent conductive layer, ion storage layer, ion conductive layer, electrochromic layer, and another transparent conductive layer. The team’s innovations include the creation of a new electrolyte structure that uses PEDOT: PSS as the proton source, introduces protons as one of the transport ions, and acts as the primary insertion ion in the WO3 layer when the solid-state NaClO4-containing polymer electrolyte layer supplies sodium ions to the PEDOT: PSS layer, which pumps the protons out of the PEDOT: PSS via ion exchange. This increases the switching speed of the device. Electrolyte plays a crucial role in ECD as it facilitates ion transport which is essential for color change. The authors experimented with different types of electrolytes to find one that could provide faster ion migration. They also innovated the structure of the ECD. Traditional ECDs have a five-layer structure, but the team experimented with different configurations to improve performance, with a sandwich-structured electrolyte that delivers lithium ions to both the WO3 and para-electrode materials, with a special focus on the arrangement and materials of the ion-storage and conductive layers.
The implications of the study by Prof. Xun Cao from Shanghai Institute of Ceramics scientists are profound. By dramatically increasing the response speed of ECDs, the research paves the way for their broader adoption in practical applications. Fast-switching ECDs hold immense potential in areas such as smart window applications, where they can contribute to energy efficiency in buildings. In the realm of displays, these devices can lead to the development of more energy-efficient and versatile screens. The research also extends to potential military applications, such as stealth technology, where rapid changes in optical properties can be advantageous.
Jiankang Guo, Hanxiang Jia, Zewei Shao, Ping Jin, and Xun Cao*. Fast-Switching WO3-Based Electrochromic Devices: Design, Fabrication, and Applications. Acc. Mater. Res. 2023, 4, 5, 438–447