Special Issue: Advanced Electrochromic Materials and Devices

Electrochromism is the phenomenon where the optical properties (transmittance, absorbance or reflectance) of a material are dynamically and reversibly changed by the application of a small external electric field [1,2,3,4]. This preeminent technology promises a variety of potential applications, including energy-efficient smart windows [5,6,7,8,9,10,11,12,13,14,15,16], multicolour displays [17,18,19,20,21], military camouflage [22,23] and spacecraft thermal management [24,25]. There has been substantial progress in the electrochromic field over the last serval years. Lots of advanced electrochromic materials and high-performance devices have been developed, significantly contributing to the development and application of electrochromic devices.

As one of the most promising applications of electrochromics, electrochromic smart window can vary the solar irradiation transmittance into buildings based on weather conditions and personal preferences. It has become an attractive technology to reduce a building’s energy need for heating, ventilation, and air conditioning (HVAC), while providing on-demand glare reduction, unobstructed views, and natural daylighting at the same time [5,6,7,8,9,10,11,12,13,14,15,16]. Present inadequate electrochromic performance and high cost are the current technical challenges. Nanostructuring has demonstrated that it is an effective strategy to improve the electrochromic performance of the materials. Recently, Gao et al. reported a high-performance electrochromic device using W₁₇O₄₇ nanowires [5], which show a high optical modulation and a long cycling life. Cai et al. [6] prepared Nb₁₈W₁₆O₉₃ nanomaterial with fast ion diffusion kinetics and demonstrated it to be a promising electrochromic material in smart window applications. Zhang [7,8,9,10,11] and Cao [12,13,14] et al. recently reported dual-band electrochromic smart windows with independent control of visible light and near-infrared transmittance using some semiconductor nanocrystals (such as W₁₈O₄₉ [7], TiO_2-x [11], Nb-TiO₂ [13]), which also show expressive electrochromic performance by notably to contributing the reduction of the building energy consumption. For the practical applications, all-solid-state electrochromic device without liquid leakage is the trend of the development. Diao et al. [15] reported an all-solid-state electrochromic windows with battery-type cathode, which displayed a high electrochromic performance and wide operating temperature range. The slow ion diffusion speed is the key challenge hindering the development of all-solid-state electrochromic devices. Recently, Cao et al. [16] creatively used protons as the diffusing species and designed an all-solid-state electrochromic device with proton-based tandem structure, which show fast response (<1 s), and high stability.

Multicolour display is another promising application of electrochromics due to its colourfulness, nonemissive characteristics and low power consumption [17]. Zhao et al. [18] first reported full-color inorganic electrochromic devices utilizing an ultracompact asymmetric Fabry–Perot nanocacvity, providing a new direction for full-colour tunability of inorganic electrochromic materials. A multicolour bistable electrochromic device based on intramolecular proton-coupled electron transfer was reported by Zhang et al. [19]. This strategy addressed the common issue of organic materials’ bistability and exhibited potential applications in electronic shelf labels. Li et al. [20,21] designed a transparent inorganic multicolour display using an emerging zinc anode-based electrochromic device, which enabled independent operation of top and bottom electrochromic electrodes, thus providing additional configuration flexibility of the devices.

Electrochromic also shows potential applications in military camouflage and spacecraft thermal management through the optical colour or thermal radiation regulation [22,23,24,25]. Wang et al. [22] recently presented smart electrochromic fibers with multicolor, long-range controllability and multi-environmental stability, demonstrating the potential application of electrochromic fibers in adaptive camouflage. A high-performance and robust dual-function electrochromic device for dynamic thermal regulation and electromagenetic interference (EMI) shielding was fabricated by Li et al. [25]. The device exhibited a superior EMI shielding performance, as well as an excellent dynamic thermal regulation performance with a high emittance.

Overall, electrochromics has emerged as an effective technology in many application areas. The issues of electrochromic devices, such as inadequate performance and high cost, are being alleviated through the efforts of the researchers. I believe that electrochromic devices will be widely used in our lifetimes and significantly improve our quality of life in the near future.