Enhanced Coloration Efficiency of Electrochromic Tungsten Oxide Nanorods by Site Selective Occupation of Sodium Ions

Visible-Near Infrared Independent Modulation of Hexagonal WO3 Induced by Ionic Insertion Sequence and Cavity Characteristics

Dual-band electrochromic materials have attracted significant attention due to their ability to independently control sunlight and solar heat. However, these materials generally exhibit notable limitations, and the mechanisms for their dual-band independent regulation remain poorly understood. Here, the visible-NIR-independent regulation capabilities of hexagonal WO3 (h-WO3) are introduced for the first time. A structure-activity relationship that perfectly links the microscopic ion insertion sequence and cavity characteristics to the macroscopic dual-band electrochromic properties is established. The progressive ion intercalation process and the distinctive optical activity of the cavities are keys for enabling h-WO3 to independently modulate "bright," "cool," and "dark" modes. Notably, h-WO3 demonstrates superior dual-band electrochromic performance with a broadband full shielding effect from 550 to 2000 nm, achieving the widest full shielding band in dual-band electrochromic studies. Additionally, h-WO3 shows a high discharge capacity of 270.9 mAh m- 2 at 0.25 A m- 2, and requires only 49.1 and 209.7 mAh m- 2 to complete a full round-trip switch between "bright-cool" and "bright-dark" modes, respectively. The constructed device offers a dynamic temperature control range of up to 10.5 °C and supports a maximum voltage of 2.86 V, underscoring its considerable potential for practical applications and energy efficiency.

Visible and near-infrared modulating tungsten suboxide nanorods electrochromic films in acidic aqueous electrolytes

Proton-based aqueous electrolytes can be used to achieve high performance electrochromic nanocrystal thin films due to their small ion size. However, acidic aqueous electrolyte systems have not yet been explored in near-infrared (NIR) absorbing plasmonic tungsten oxide nanocrystal films. Here, we demonstrate tungsten suboxide nanorod films with excellent visible and NIR modulation performance in the H+-based aqueous electrolytes, thanks to their mesoporous structure, nanosized domains, and open tunnel structure. Colloidally synthesized WO2.83 nanorods with an average width of 6 nm and length of 48 nm were converted to WO2.90 nanorod film via annealing in air, while still preserving open tunnels. These films exhibit fast switching speed (tc = 0.9 s, tb = 2.1 s), excellent cycling stability over 2500 cycles, wide optical modulation up to ΔT = 53.8 % in the NIR region, and a high coloration efficiency (CE) of 167 cm2 C⁻1 at 1300 nm. Additionally, introducing a thin spacer (25 μm) reduced intrinsic NIR absorption from water, thereby enhancing the NIR modulation properties. These highly performing aqueous proton-electrolytes-based electrochromic devices open new possibilities for implementing visible and NIR electrochromism.

Capturing ion trapping and detrapping dynamics in electrochromic thin films

Ion trapping has been found to be responsible for the performance degradation in electrochromic oxide thin films, and a detrapping procedure was proved to be effective to rejuvenate the degraded films. Despite of the studies on ion trapping and detrapping, its dynamics remain largely unknown. Moreover, coloration mechanisms of electrochromic oxides are also far from clear, limiting the development of superior devices. Here, we visualize ion trapping and detrapping dynamics in a model electrochromic material, amorphous WO3. Specifically, formation of orthorhombic Li2WO4 during long-term cycling accounts for the origin of shallow traps. Deep traps are multiple-step-determined, composed of mixed W4+-Li2WO4, amorphous Li2WO4 and W4+-Li2O. The non-decomposable W4+-Li2WO4 couple is the origin of the irreversible traps. Furthermore, we demonstrate that, besides the typical small polaron hopping between W5+ ↔ W6+ sites, bipolaron hopping between W4+ ↔ W6+ sites gives rise to optical absorption in the short-wavelength region. Overall, we provide a general picture of electrochromism based on polaron hopping. Ion trapping and detrapping were demonstrated to also prevail in other cathodic electrochromic oxides. This work not only provides the ion trapping and detrapping dynamics of WO3, but also open avenues to study other cathodic electrochromic oxides and develop superior electrochromic devices with great durability.

Transparent dynamic infrared emissivity regulators

Dynamic infrared emissivity regulators, which can efficiently modulate infrared radiation beyond vision, have emerged as an attractive technology in the energy and information fields. The realization of the independent modulation of visible and infrared spectra is a challenging and important task for the application of dynamic infrared emissivity regulators in the fields of smart thermal management and multispectral camouflage. Here, we demonstrate an electrically controlled infrared emissivity regulator that can achieve independent modulation of the infrared emissivity while maintaining a high visible transparency (84.7% at 400-760 nm). The regulators show high degree of emissivity regulation (0.51 at 3-5 μm, 0.41 at 7.5-13 μm), fast response ( < 600 ms), and long cycle life ( > 104 cycles). The infrared emissivity regulation is attributed to the modification of the carrier concentration in the surface depletion layer of aluminum-doped zinc oxide nanocrystals. This transparent infrared emissivity regulator provides opportunities for applications such as on-demand smart thermal management, multispectral displays, and adaptive camouflage.

Selective Electrochromic Regulation for Near-Infrared and Visible Light via Porous Tungsten Oxide Films with Core/Shell Architecture

Dual-band electrochromic smart windows have become a research hotspot owing to their unique ability to selectively control near-infrared (NIR) and visible (VIS) light. However, the design and exploitation of dual-band electrochromic films are still an extreme challenge due to the scarcity of relevant high-performance materials. To solve this issue, we here proposed a type of porous WO₃ film with nanowires/nanoparticles core/shell architecture as a promising candidate, endowing smart windows with a dual-band electrochromic feature. Moreover, the mechanism of the dual-band electrochromism is illustrated by the response of the transmittance spectra in Li⁺-based or TBA⁺-based electrolytes to distinguish the electrochemical behavior and the cyclic voltammetry to determine the degree of diffusion-limited kinetics. Our results indicate that the dual-band electrochromic performance is credited to the progressive electrochemical reduction procedure, in which the capacitive charging process gives rise to NIR regulation and the following ion intercalation contributes to VIS light modulation. Furthermore, we develop a dual-band electrochromic energy storage prototype device utilizing the porous WO₃ film. This work describes a judicious strategy for designing dual-band electrochromic films, promoting the evolution of dual-band electrochromic technology.

Infrared Plasmonic Metamaterials Based on Transparent Nanoparticle Films of In2O3:Sn for Solar-Thermal Shielding Applications

Three-dimensional nanoparticle (NP) assemblies show interesting optical responses that differ from naturally occurring materials, such as metals, oxides, and semiconductors. In this study, we investigate the optical response of thin films comprising Sn:In₂O₃ NPs (ITO NP films) based on the correlation between complex permittivity and infrared (IR) reflectance for solar-thermal shielding applications. IR ellipsometry measurements are conducted to clarify the presence of Lorentz resonances in plasmonic metamaterials. The Lorentz resonances are correlated to the electric field strength at interparticle gaps by varying the Sn dopant concentration, as confirmed using finite-difference time-domain (FDTD) simulations. High solar-thermal shielding performance was obtained owing to selective near-IR reflection based on strong Lorentz resonances as the ITO NP films were electrically polarizable but magnetically inactive. Thermal shielding efficiency was demonstrated via a comparison of the air temperature change in a simulated box used as a model house. Additionally, we demonstrate the significance of NP packing density on the enhancement of the near-IR reflectance. The role of interparticle spacing for high near-IR reflectance was revealed by comparing effective medium approximation analyses and FDTD simulations. This relationship was also demonstrated by the reduction of solar-thermal shielding performance when using aggregated ITO NPs. Our work confirmed that the control of complex permittivity in plasmonic metamaterials must be considered in the structural design of transparent and reflective materials for solar-thermal shielding applications.

Emerging Electrochromic Materials and Devices for Future Displays

With the rapid development of optoelectronic fields, electrochromic (EC) materials and devices have received remarkable attention and have shown attractive potential for use in emerging wearable and portable electronics, electronic papers/billboards, see-through displays, and other new-generation displays, due to the advantages of low power consumption, easy viewing, flexibility, stretchability, etc. Despite continuous progress in related fields, determining how to make electrochromics truly meet the requirements of mature displays (e.g., ideal overall performance) has been a long-term problem. Therefore, the commercialization of relevant high-quality products is still in its infancy. In this review, we will focus on the progress in emerging EC materials and devices for potential displays, including two mainstream EC display prototypes (segmented displays and pixel displays) and their commercial applications. Among these topics, the related materials/devices, EC performance, construction approaches, and processing techniques are comprehensively disscussed and reviewed. We also outline the current barriers with possible solutions and discuss the future of this field.

Tungsten oxide polymorphs and their multifunctional applications

Owing to the natural abundance, easy availability, high stability, non-stoichiometry, and chemical diversity, considerable interest has been devoted to tungsten oxide (WO_3-x) nanomaterials, and many advances have been achieved ranging from traditional catalysts and electronics to emerging artificial intelligence. This review focuses on recent progress of WO_3-x polymorphs and their multifunctional applications. The structural diversity and crystal phase transitions of WO_3-x and recent advances on the general synthesis of various WO_3-x nanostructures are first summarized, since the crystal structure and morphology adjustment obviously affect the physiochemical merits of WO_3-x materials. Then, their applications and related mechanisms in different fields are demonstrated, such as gas sensing, chromogenic (electro-, photo-, gaso-, and thermochromic), photocatalytic (pollutant degradation and water splitting), and emerging applications (biomedical, antibiotic, and artificial intelligence). With the advances highlighted here and the ongoing research efforts, the continuous breakthrough in functionalized WO_3-x nanostructure and their attractive applications is foreseeable in the future.

Optimal Rule-of-Thumb Design of Nickel-Vanadium Oxides as an Electrochromic Electrode with Ultrahigh Capacity and Ultrafast Color Tunability

The use of electrodes capable of functioning as both electrochromic windows and energy storage devices has been extended from green building development to various electronics and displays to promote more efficient energy consumption. Herein, we report the electrochromic energy storage of bimetallic NiV oxide (NiVO) thin films fabricated using chemical bath deposition. The best optimized NiVO electrode with a Ni/V ratio of 3 exhibits superior electronic conductivity and a large electrochemical surface area, which are beneficial for enhancing electrochemical performance. The color switches between semitransparent (a discharged state) and dark brown (a charged state) with excellent reproducibility because of the intercalation and deintercalation of OH^- ions in an alkaline KOH electrolyte. A specific capacity of 2403 F g^-1, a coloration efficiency of 63.18 cm² C^-1, and an outstanding optical modulation of 68% are achieved. The NiVO electrode also demonstrates ultrafast coloration and bleaching behavior (1.52 and 4.79 s, respectively), which are considerably faster than those demonstrated by the NiO electrode (9.03 and 38.87 s). It retains 91.95% capacity after 2000 charge-discharge cycles, much higher than that of the NiO electrode (83.47%), indicating that it has significant potential for use in smart energy storage applications. The superior electrochemical performance of the best NiVO compound electrode with an optimum Ni/V compositional ratio is due to the synergetic effect between the high electrochemically active surface area induced by V-doping-improved redox kinetics (low charge-transfer resistance) and fast ion diffusion, which provides a facile charge transport pathway at the electrolyte/electrode interface.

An O-Carborane Derivative of Perylene Bisimide-Based Thin Film Displaying both Electrochromic and Electrofluorochromic Properties

The widespread application in displays, information encryption, and sensors has boosted studies of electrochromic (EC) systems combining large contrast, fast response, high robustness, and low-cost properties. Herein, we report a film-type new EC system with a non-planar perylene bisimide-carborane derivative (PBI-CB) as the electroactive materials. It was revealed that the film demonstrated outstanding EC properties with response times of 1.18 and 0.94 s for the coloration and bleaching processes, respectively, large transmittance variation around 630 nm (45.7%), and superior stability for more than 200 coloration-bleaching cycles. Moreover, the film also showed precious electrofluorochromic (EFC) properties. The emission around 650 nm at the "on" state could be more than 24.5 times than that at the "off" state, and the response times of the off and on processes could reach 2.2 s and 4.3 s, respectively. Considering the facts that the film was fabricated via simple drop-coating, the EC/EFC operation was performed via a routine three-electrode system and the voltage applied is only -1.3 V, we believe that the EC/EFC system as developed would find applications in smart windows, information encryption, optoelectrical sensing, etc. In addition, the work could also pave the way for developing combined EC/EFC systems via employing known organic fluorophores as the electrochemical active materials, which are not only abundant in numbers but also solution-processable.

Controlling the Shape Anisotropy of Monoclinic Nb12O29 Nanocrystals Enables Tunable Electrochromic Spectral Range

Electrochromic smart windows that modulate the solar transmittance in a wide and selective spectral range can optimize building energy efficiency. However, for conventional materials such as bulk transition metal oxides, the electrochromic spectral range is constrained by their crystal structure with limited tunability. Herein, we report a method to control the shape anisotropy of monoclinic Nb₁₂O₂₉ nanocrystals and obtain a tunable electrochromic spectral range. We demonstrate the synthesis of monoclinic Nb₁₂O₂₉ nanorods (NRs), extending one-dimensionally along the b direction, and monoclinic Nb₁₂O₂₉ nanoplatelets (NPLs), extending two-dimensionally along the b and c directions. Upon electrochemical reduction accompanied by Li insertion, the NR films show increasing absorbance mostly in the near infrared region. In contrast, the NPL films show increasing absorbance in the near infrared region first followed by increasing absorbance in both visible and near infrared regions. To elucidate the influence of shape anisotropy, we used density functional theory to construct the lithiated structures of monoclinic Nb₁₂O₂₉ and in these structures we identified the presence of square planar sites and crystallographic shear sites for Li insertion. By calculating the theoretical spectra of the lithiated structures, we demonstrate that the Li insertion into the square planar sites results in absorption in the near infrared region in both NRs and NPLs due to their extension in the b direction, while the subsequent insertion of Li into the crystallographic shear sites leads to absorption in both visible and near infrared regions, which only occurs in NPLs due to their extension in the c direction.

Reversible Zn2+ Insertion in Tungsten Ion-Activated Titanium Dioxide Nanocrystals for Electrochromic Windows

Zinc-anode-based electrochromic devices (ZECDs) are emerging as the next-generation energy-efficient transparent electronics. We report anatase W-doped TiO₂ nanocrystals (NCs) as a Zn²⁺ active electrochromic material. It demonstrates that the W doping in TiO₂ highly reduces the Zn²⁺ intercalation energy, thus triggering the electrochromism. The prototype ZECDs based on W-doped TiO₂ NCs deliver a high optical modulation (66% at 550 nm), fast spectral response times (9/2.7 s at 550 nm for coloration/bleaching), and good electrochemical stability (8.2% optical modulation loss after 1000 cycles).

Inorganic all-solid-state electrochromic devices with reversible color change between yellow-green and emerald green

All-solid-state electrochromic devices with the structure of glass/ITO/WO₃/Ta₂O₅/Li/V₂O₅/ITO were prepared by an evaporation method. Because of the use of a V₂O₅ complementary layer, the ECD can achieve obvious color change between yellow-green and emerald green with superior cycling performance up to 5000 cycles, indicating the great potential in smart window applications.