A Facile Strategy To Construct Au@VxO2x+1 Nanoflowers as a Multicolor Electrochromic Material for Adaptive Camouflage

Recent Advance in Electrochromic Materials and Devices for Display Applications

Electrochromic devices (ECDs) possess the performance advantages in terms of color adjustability, low power consumption, and visual friendliness, emerging as one of the ideal candidates for energy-efficient smart windows, next-generation displays, and wearable electronics. The optical and electrical characteristics of ECDs can be adjusted by modifying the materials or structure of devices. This review summarizes the recent developments of innovative technologies and key materials of ECDs for display applications, highlighting the key issues and development trends in this area.

Porous NiMoO4 Nanosheet Films and a Device with Ultralarge Optical Modulation for Electrochromic Energy-Storage Applications

Reducing building energy consumption, improving aesthetics, and improving occupant privacy as well as comfort by dynamically adjusting solar radiation are important application areas for electrochromic (EC) smart windows. However, the current transition metal oxides still cannot meet the requirements of neutral coloration and large optical modulation. We report NiMoO4 nanosheet films directly grown on fluorine-doped tin oxide glasses. The as-grown NiMoO4 film not only achieves neutral coloration from transparent to dark brown but also shows an ultralarge optical modulation (86.8% at 480 nm) and excellent cycling stability (99.4% retention of maximum optical modulation after 1500 cycles). Meanwhile, an EC device demonstrating good EC performance was constructed. These results will greatly promote the research and development of binary transition metal oxides for both EC and energy-storage applications, and NiMoO4 films may be an excellent candidate to replace NiO films as ion-storage layers in complementary EC devices with WO3 films as EC layers.

New electric field responsive photonic crystals with remarkable yellow-to-green switch for adaptive camouflage

Electric field responsive photonic crystals take on critical significance in developing adaptive camouflage technology, which are promising materials for adaptive camouflage devices with better fabrication processes and color saturation. However, electric field responsive photonic crystals are primarily susceptible to poor fusion with typical background colors and necessitate the continuous implementation of electric fields to attain specific colors, thus limiting their practical applications. Monodisperse polyvinylpyrrolidone modified cadmium sulfide (PVP/CdS) microspheres with large refractive index are well prepared in this study. Liquid photonic crystals, exhibiting earth-yellow and light green under specific electric field, are obtained with PVP/CdS particles dispersed in propylene carbonate in accordance with the subtractive color mixing principle of structural color and initial color. The resulting electric field responsive photonic crystals are characterized by a simple preparation process, fast electrical response, long-time holding of the earth-yellow state, and reversible color changes between earth-yellow and light green in -3.5 V and + 3.5 V electric field switching. This study can contribute to the development of color-changing devices designed for adaptive camouflage applications.

Resonant-Cavity-Enhanced Electrochromic Materials and Devices

With rapid advances in optoelectronics, electrochromic materials and devices have received tremendous attentions from both industry and academia for their strong potentials in wearable and portable electronics, displays/billboards, adaptive camouflage, tunable optics, and intelligent devices, etc. However, conventional electrochromic materials and devices typically present some serious limitations such as undesirable dull colors, and long switching time, hindering their deeper development. Optical resonators have been proven to be the most powerful platform for providing strong optical confinement and controllable lightmatter interactions. They generate locally enhanced electromagnetic near-fields that can convert small refractive index changes in electrochromic materials into high-contrast color variations, enabling multicolor or even panchromatic tuning of electrochromic materials. Here, resonant-cavity-enhanced electrochromic materials and devices, an advanced and emerging trend in electrochromics, are reviewed. In this review, w e will focus on the progress in multicolor electrochromic materials and devices based on different types of optical resonators and their advanced and emerging applications, including multichromatic displays, adaptive visible camouflage, visualized energy storage, and applications of multispectral tunability. Among these topics, principles of optical resonators, related materials/devices and multicolor electrochromic properties are comprehensively discussed and summarized. Finally, the challenges and prospects for resonant-cavity-enhanced electrochromic materials and devices are presented.

Fast and Stable Zinc Anode-Based Electrochromic Displays Enabled by Bimetallically Doped Vanadate and Aqueous Zn2+/Na+ Hybrid Electrolytes

Vanadates are a class of the most promising electrochromic materials for displays as their multicolor characteristics. However, the slow switching times and vanadate dissolution issues of recently reported vanadates significantly hinder their diverse practical applications. Herein, novel strategies are developed to design electrochemically stable vanadates having rapid switching times. We show that the interlayer spacing is greatly broadened by introducing sodium and lanthanum ions into V3O8 interlayers, which facilitates the transportation of cations and enhances the electrochemical kinetics. In addition, a hybrid Zn2+/Na+ electrolyte is designed to inhibit vanadate dissolution while significantly accelerating electrochemical kinetics. As a result, our electrochromic displays yield the most rapid switching times in comparison with any reported Zn-vanadate electrochromic displays. It is envisioned that stable vanadate-based electrochromic displays having video speed switching are appearing on the near horizon.

Multicolored inorganic electrochromic materials: status, challenge, and prospects

Against the backdrop of advocacy for green and low-carbon development, electrochromism has attracted academic and industrial attention as an intelligent and energy-saving applied technology due to its optical switching behavior and its special principles of operation. Inorganic electrochromic materials, represented by transition metal oxides, are considered candidates for the next generation of large-scale electrochromic applied technologies due to their excellent stability. However, the limited color diversity and low color purity of these materials greatly restrict their development. Starting from the multicolor properties of inorganic electrochromic materials, this review systematically elaborates on recent progress in the aspects of the intrinsic multicolor of electrochromic materials, and structural multicolor based on the interaction between light and microstructure. Finally, the challenges and opportunities of inorganic electrochromic technology in the field of multicolor are discussed.

Adaptive and Adjustable MXene/Reduced Graphene Oxide Hybrid Aerogel Composites Integrated with Phase-Change Material and Thermochromic Coating for Synchronous Visible/Infrared Camouflages

Although single-function camouflage under infrared/visible bands has made great advances, it is still difficult for camouflage materials to cope with the synergy detection spanning both visible and infrared spectra and adapt to complex and variable scenarios. Herein, a trilayer composite integrating thermal insulation, heat absorption, solar/electro-thermal conversions, and thermochromism is fabricated for visible and infrared dual camouflages by combining anisotropic MXene/reduced graphene oxide hybrid aerogel with the n-octadecane phase change material in its bottom and a thermochromic coating on its upper surface. Benefiting from the synergetic heat-transfer suppression derived from the thermal insulation of the porous aerogel layer and the heat absorption of the n-octadecane phase-change layer, the composite can serve as a cloak to hide the target signatures from the infrared images of its ambient surroundings during the day in the jungle and at night in all scenes and can assist the target in escaping visual surveillance by virtue of its green appearance. For desert scenarios, the composite can spontaneously increase its surface temperature via its solar-thermal energy conversion, merging infrared images of the targets into the high-temperature surroundings; meanwhile, it can vary the surface color from the original green to yellow, enabling the target to visually disappear from ambient sands and hills. This work provides a promising strategy for designing adaptive and adjustable integrated camouflage materials to counter multiband surveillance in complicated environments.

Robust and stable dual-band electrochromic smart window with multicolor tunability

Dual-band electrochromic smart windows (DESWs) can selectively control the transmittance of near-infrared (NIR) and visible (VIS) light, which can significantly reduce building energy consumption. However, almost all the reported DESW colors switch between clear colorless and dark blue. The single color combined with the dazzling visual experience of blue will undoubtedly limit the application scene of DESWs. Herein, for the first time, we report a robust and stable DESW with multicolor conversion capabilities based on the single-component organic polymer polyaniline (PANI). The results show that the progressive electrochemical reaction enabled PANI film to deliver not only efficient and independent control of NIR and VIS light transmittance but also impressive electrochromic performance-rich color conversion (yellow-green-black), good optical modulation (65% at 633 nm and 59% at 1600 nm), high coloration efficiency (367.1 cm² C^-1 at 633 nm and 299.6 cm² C^-1 at 1600 nm), and excellent cycling stability (optical modulation losses of 6% at 633 nm, and 4% at 1600 nm after 10 000 cycles). Thereby, we demonstrated a prototype PANI-based DESW device (10 × 5 cm²), which delivered a multicolor electrochromism together with independent control and modulation of the VIS (sunlight) and NIR (solar heat) transmittance.

Electrically Conductive Carbazole and Thienoisoindigo-Based COFs Showing Fast and Stable Electrochromism

Thienothiophene thienoisoindigo (ttTII)-based covalent organic frameworks (COFs) have been shown to offer low band gaps and intriguing optical and electrochromic properties. So far, only one tetragonal thienothiophene thienoisoindigo-based COF has been reported showing stable and fast electrochromism and good coloration efficiencies. We have developed two novel COFs using this versatile and nearly linear ttTII building block in a tetragonal and a hexagonal framework geometry to demonstrate their attractive features for optoelectronic applications of thienoisoindigo-based COFs. Both COFs exhibit good electrical conductivities, show promising optical absorption features, are redox-active, and exhibit a strong electrochromic behavior when applying an external electrical stimulus, shifting the optical absorption even farther into the NIR region of the electromagnetic spectrum and achieving absorbance changes of up to 2.5 OD. Cycle-stable cyclic voltammograms with distinct oxidation and reduction waves reveal excellent reversibility and electrochromic switching over 200 cycles and confirm the high stability of the frameworks. Furthermore, high coloration efficiencies in the NIR region and fast switching speeds for coloration/decoloration as fast as 0.75 s/0.37 s for the Cz-ttTII COF and 0.61 s/0.29 s for the TAPB-ttTII COF at 550 nm excitation were observed, outperforming many known electrochromic materials, and offering options for a great variety of applications, such as stimuli-responsive coatings, optical information processing, or thermal control.