New Gradient Strategy for Electrochromic (EC) Smart Windows

In an article recently published in the journal ACS Energy Letters, researchers discussed the development of fast-switching electrochromic devices driven by a potential gradient.

Study: Potential Gradient-Driven Fast-Switching Electrochromic Device. Image Credit: JP WALLET/


The serious consequences of global warming necessitate immediate energy conservation and emission reduction efforts. Electrochromic (EC) devices, which can enhance energy efficiency by dynamically changing spectral characteristics and producing a reduction in energy consumption, have received a lot of interest in regard to smart windows for energy-efficient buildings.

Traditional EC devices require an external bias to deliver the voltage or current, which results in increased energy consumption, negating energy savings and impeding the independent and long-term operation of EC devices. Now, numerous energy-harvesting technologies like photovoltaic devices and triboelectric nanogenerators were coupled with EC devices to create self-powered EC device systems to address this issue.

Overall, it appears that there is no technique that can accomplish both universality and high performance in a self-powered EC device's all-in-one structure design. As a result, a novel technique for building highly reversible, fast, and self-switching EC devices with a simpler configuration is required. The driving force that allows a battery to transmit electrical energy to an external circuit is known as the change in the corresponding standard free energy of a cell reaction, which may be measured by finding the electromotive force.

About the Study

In this study, the authors presented a potential gradient technique for the development of a rapid self-chargeable as well as self-dischargeable EC system. The potential difference between zinc (Zn) and Prussian blue (PB) was used in this technique to convert PB to Prussian white (PW) in 1.0 seconds, whereas etched carbon paper (ECP) was used to oxidize PW to its original state in 2.2 seconds. This strategy was also used for other high-performance EC systems, such as Zn||PEDOT||ECP, Zn||WO3||ECP, Zn||PB||ZnHCF, and Al||PB||ECP. A prototype 25 cm2 EC window was built using poly(vinyl alcohol) (PVA) and KCl gel electrolyte to discuss the potential application of large-scale windows.

The researchers discussed the development of a novel fast and self-switching EC system with rapid response time, great visual contrast, and high reversibility by using the technique of building an efficient potential gradient among PB electrode, ECP positive electrode, and Zn negative electrode. By replacing EC or positive/negative electrodes with various materials, the universality of this promising gradient technique was examined.

The team demonstrated the potential applications of the proposed large-area smart windows corresponding to the fast self-switching EC system by assembling 5 x 5 cm2 and 10 x 10 cm2 prototype devices.


The connection of PB to Zn raised the transmittance of the prepared 25 cm2 window from 2% to 66% in 28 seconds, while reversing the connection of PW to ECP dropped it from 73% to 9% in 30 seconds. The discharging capacity of the Zn/PB battery was 23.3 mAh m-2 as determined by galvanostatic discharging at 0.1 mA cm-2. In PB and PW, the Fe(III)/Fe(II) pair was responsible for the discharge plateau at 1.2 V.

After discharging, the open-circuit voltage of PW could fast approach 1.8 V when connected to ECP during the standing process, which showed the self-charging process. At 700 nm, the difference in transmittance between the colored and bleached states of this system was 68.1%. When the PB electrode of area 2x 4 cm2 was connected to Zn electrodes, the transmittance increased by 61.3% in just 1.0 s, while it decreased from 93% to 31.7% in 2.2 s when switched to ECP electrodes, which demonstrated the rapid coloration and bleaching processes. The ID/IG intensity ratio for ECP was 1.105, which indicated that the ECP had a lot of flaws.


In conclusion, this study elucidated that the proposed EC system eliminates the need for external power and addresses the demand for highly reversible, simple, and fast-self-switching EC devices. A potential gradient strategy was proposed for achieving a fast-self-chargeable and -dischargeable EC system without any external supply, in which the potential difference between PB and Zn was established to reduce PB to PW within 1.0 s, and PW could be oxidized to its original state in 2.2 s due to the strong oxidizability of ECP derived from the carbonyl/carbon hydroxyl conversion.

The authors mentioned that the novel potential gradient technique offers new insight into the development of a fast self-switching EC device with promising applications in energy storage and conservation.

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Luo, Y., Jin, H., Lu, Y., Potential Gradient-Driven Fast-Switching Electrochromic Device. ACS Energy Letters 7 1880-1887 (2022).

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Surbhi Jain

Written by

Surbhi Jain

Surbhi Jain is a freelance Technical writer based in Delhi, India. She holds a Ph.D. in Physics from the University of Delhi and has participated in several scientific, cultural, and sports events. Her academic background is in Material Science research with a specialization in the development of optical devices and sensors. She has extensive experience in content writing, editing, experimental data analysis, and project management and has published 7 research papers in Scopus-indexed journals and filed 2 Indian patents based on her research work. She is passionate about reading, writing, research, and technology, and enjoys cooking, acting, gardening, and sports.


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