Crystalline/amorphous tungsten oxide core/shell hierarchical structures and their synergistic effect for optical modulation

Room-Temperature Self-Standing Cellulose-Based Hydrogel Electrolytes for Electrochemical Devices

The trend of research towards more sustainable materials is pushing the application of biopolymers in a variety of unexplored fields. In this regard, hydrogels are attracting significant attention as electrolytes for flexible electrochemical devices thanks to their combination of ionic conductivity and mechanical properties. In this context, we present the use of cellulose-based hydrogels as aqueous electrolytes for electrochemical devices. These materials were obtained by crosslinking of hydroxyethyl cellulose (HEC) with divinyl sulfone (DVS) in the presence of carboxymethyl cellulose (CMC), creating a semi-IPN structure. The reaction was confirmed by NMR and FTIR. The small-amplitude oscillatory shear (SAOS) technique revealed that the rheological properties could be conveniently varied by simply changing the gel composition. Additionally, the hydrogels presented high ionic conductivity in the range of mS cm⁻¹. The ease of synthesis and processing of the hydrogels allowed the assembly of an all-in-one electrochromic device (ECD) with high transmittance variation, improved switching time and good color efficiency. On the other hand, the swelling ability of the hydrogels permits the tuning of the electrolyte to improve the performance of a printed Zinc/MnO₂ primary battery. The results prove the potential of cellulose-based hydrogels as electrolytes for more sustainable electrochemical devices.

Remarkable Near-Infrared Electrochromism in Tungsten Oxide Driven by Interlayer Water-Induced Battery-to-Pseudocapacitor Transition

Near-infrared (NIR) electrochromism is of academic and technological interest for a variety of applications in advanced solar heat regulation, photodynamic therapy, optical telecommunications, and military camouflage. However, inorganic materials with outstanding NIR modulation capability are quite few. Herein, we propose a promising strategy for achieving strong NIR electrochromism in tungsten oxide that is closely related to its electrochemical transformation from battery-type behavior to pseudocapacitance, induced by introducing an interlayer space with water molecules within tungsten oxide. Further evidence demonstrates that the interlayer water molecules significantly reduced the energy barrier to ion diffusion and increased the ion flux in tungsten oxide. As a result, compared with anhydrous WO₃, the as-synthesized WO₃·2H₂O nanoplates exhibited remarkably improved NIR electrochromic properties, including a large transmittance modulation (90.4%), high coloration efficiency (322.6 cm² C^-1), and high cyclic stability (maintaining 93.7% after 500 cycles), which were comparable to those of the best reported NIR electrochromic materials. Moreover, the application of the WO₃·2H₂O nanoplate-based electrochromic device resulted in a temperature difference of 11.9 °C, indicating good solar thermal regulation ability.

Selective photochromism in a self-coated WO3/WO3-x homojunction: enhanced solar modulation efficiency, high luminous transmittance and fast self-bleaching rate

Pursuing excellent photochromic (PC) properties is still a longstanding challenge for energy-efficient smart coatings. Herein, we have prepared a novel series of self-coated crystalline WO₃-amorphous WO_3-x homojunctions by a one-step hydrothermal process, in which oxalic acid was used as a capping agent to prevent polycondensation and the crystallization of [WO₆]. The as-prepared nanopowders with tight interfaces have a large specific surface area and rich surface electrons, which leads to a dramatic PC property reaching up to ΔT _sol = 64.74% at the near-infrared (NIR) range of 1000-2600 nm with high luminous transmittance (T _lum = 94.6% in the virgin state and T _lum = 32.47% in the colored state). Meanwhile, the as-prepared crystalline WO₃-amorphous WO_3-x homojunction reveals fast reversible PC circulation; most particularly, the self-bleaching rate increases at least three times-the self-bleaching time is less than 8 h. Moreover, as the microstructure of the homojunction is tuned, the solar modulation range can be selective and tuned, so that the solar modulation efficiency is up to the best energy-saving state by controlling the main absorption according to the solar energy distribution.

Electrochromism of Nanocrystal-in-Glass Tungsten Oxide Thin Films under Various Conduction Cations

The nanocrystal-in-glass (nanocrystals embedded amorphous matrix) tungsten oxide (WO₃) thin films with a nanoporous characteristic were prepared via an electron beam evaporation technique. The e-beam evaporated WO₃ thin films present a fast colored/bleached time of 16/11, 16/14, and 12/12 s, a large optical modulation of 92, 91, and 87% at 633 nm, and a high coloration efficiency of 61.78, 62.04, and 67.59 cm² C^-1 in Li⁺, Na⁺, and Al³⁺ electrolytes, respectively. On one hand, the improved electrochromic performance is mainly attributed to the short diffusion distance and buffering effect in the host matrix, which facilitates the ion insertion/extraction and alleviates the structural collapse of the framework. On the other, owing to the strong electrostatic interactions between the trivalent cations and the host, the WO₃ thin films in Al³⁺ possess a shallow diffusion depth and long cycle life. The individual contribution from the capacitance- or diffusion-controlled process is comprehensively demonstrated. Pseudocapacitive behavior in the nanocrystal-in-glass WO₃ thin films is in favor of fast kinetics response and sound cycling stability. Our work offers an in-depth insight of the electrochromic mechanism for nanocrystal-in-glass WO₃ thin films in various electrolytes and sheds light on the fundamental principle in the electrochromic devices.

Self-supported one-dimensional materials for enhanced electrochromism

A reversible, persistent electrochromic change in color or optical parameter controlled by a temporarily applied electrical voltage is attractive because of its enormous display and energy-related applications. Due to the electrochemical and structural advantages, electrodes based on self-supported one-dimensional (1D) nanostructured materials have become increasingly important, and their impacts are particularly significant when considering the ease of assembly of electrochromic devices. This review describes recent advances in the development of self-supported 1D nanostructured materials as electrodes for enhanced electrochromism. Current strategies for the design and morphology control of self-supported electrodes fabricated using templates, anodization, vapor deposition, and solution techniques are outlined along with demonstrating the influences of nanostructures and components on the electrochemical redox kinetics and electrochromic performance. The applications of self-supported 1D nanomaterials in the emerging bifunctional devices are further illustrated.

All-in-One Gel-Based Electrochromic Devices: Strengths and Recent Developments

Electrochromic devices (ECDs) have aroused great interest because of their potential applicability in displays and smart systems, including windows, rearview mirrors, and helmet visors. In the last decades, different device structures and materials have been proposed to meet the requirements of commercial applications to boost market entry. To this end, employing simple device architectures and achieving a competitive electrolyte are crucial to accomplish easily implementable, high-performance ECDs. The present review outlines devices comprising gel electrolytes as a single electroactive layer ("all-in-one") ECD architecture, highlighting some advantages and opportunities they offer over other electrochromic systems. In this context, gel electrolytes not only overcome the drawbacks of liquid and solid electrolytes, such as liquid's low chemical stability and risk of leaking and soil's slow switching and lack of transparency, but also exhibit further strengths. These include easier processability, suitability for flexible substrates, and improved stabilization of the chemical species involved in redox processes, leading to better cyclability and opening wide possibilities to extend the electrochromic color palette, as discussed herein. Finally, conclusions and outlook are provided.

Bi-functional Mo-doped WO3 nanowire array electrochromism-plus electrochemical energy storage

Metal-doping is considered to be an effective way for construction of advanced semiconducting metal oxides with tailored physicochemical properties. Herein, Mo-doped WO3 nanowire arrays are rationally fabricated by a sulfate-assisted hydrothermal method. Compared to the pure WO3, the optimized Mo-doped WO3 nanowire arrays exhibit improved electrochromic properties with fast switching speed (3.2s and 2.6s for coloration and bleaching, respectively), significant optical modulation (56.7% at 750nm, 83.0% at 1600nm and 48.5% at 10μm), high coloration efficiency (123.5cm(2)C(-1)) and excellent cycling stability. In addition, as a proof of concept, the Mo-doped WO3 nanowire arrays are demonstrated with electrochemical energy storage monitored by the electrochromism. This electrode design protocol can provide an alternative way for developing high-performance active materials for bi-functional electrochromic batteries.

Effect of substrate pre-treatment on microstructure and enhanced electrochromic properties of WO3 nanorod arrays

WO₃ nanorod arrays (WNRAs) were successfully synthesized on an FTO substrate pre-coated with a layer of TiO₂ seeds.