Novel Prussian White@MnO2-Based Inorganic Electrochromic Energy Storage Devices with Integrated Flexibility, Multicolor, and Long Life

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.

Homogeneous and Nanogranular Prussian Blue to Enable Long-Term-Stable Electrochromic Devices

The increasing demand for the state-of-the-art electrochromic devices has received great interest in synthesizing Prussian blue (PB) nanoparticles with a uniform diameter that exhibit excellent electrochromism, electrochemistry, and cyclability. Herein, we report the controllable synthesis of sub-100 nm PB nanoparticles via the coprecipitation method. The diameter of PB nanoparticles can be modulated by adjusting the reactant concentration, the selection of a chelator, and their purification. The self-assembled nanogranular thin films, homogeneously fabricated by using optimized PB nanoparticles with an average diameter of 50 nm as building blocks via the blade coating technique enable excellent performance with a large optical modulation of 80% and a high coloration efficiency of 417.79 cm2 C-1. It is also demonstrated by in situ and ex situ observations that the nanogranular PB thin films possess outstanding structural and electrochemical reversibility. Furthermore, such nanogranular PB thin films can enjoy the enhanced long-term cycling stability of the PB-WO3 complementary electrochromic devices having a 91.4% optical contrast retention after 16,000 consecutive cycles. This work provides a newly and industrially compatible approach to producing a complementary electrochromic device with extraordinary durability for various practical applications.

Advanced inorganic nanomaterials for high-performance electrochromic applications

Electrochromic materials and devices with the capability of dynamic optical regulation have attracted considerable attention recently and have shown a variety of potential applications including energy-efficient smart windows, multicolor displays, atuto-diming mirrors, military camouflage, and adaptive thermal management due to the advantages of active control, wide wavelength modulation, and low energy consumption. However, its development still experiences a number of issues such as long response time and inadequate durability. Nanostructuring has demonstrated that it is an effective strategy to improve the electrochromic performance of the materials due to the increased reaction active sites and the reduced ion diffusion distance. Various advanced inorganic nanomaterials with high electrochromic performance have been developed recently, significantly contributing to the development of electrochromic applications. In this review, we systematically introduce and discuss the recent advances in advanced inorganic nanomaterials including zero-, one-, and two-dimensional materials for high-performance electrochromic applications. Finally, we outline the current major challenges and our perspectives for the future development of nanostructured electrochromic materials and applications.

Sodium Dodecyl Sulfate-Adjusted Phase Composition of Hydrated Tungsten Oxides as Stable Self-Supporting Electrodes for Supercapacitors with High Volumetric Specific Capacitance

High pseudocapacitive activity of hydrated tungsten oxides (WO3·xH2O, x = 1 or 2) makes them promising materials for supercapacitors (SCs). During their synthesis, additives such as complexing agents and surfactants generally can only affect the morphology and/or size of the products. Here, we demonstrate that not only morphology and size of WO3·xH2O were affected, its phase composition could also change from WO3·2H2O to WO3·H2O simply by increasing the amount of sodium dodecyl sulfate (SDS) during its anodization synthesis. To the best of our knowledge, such a phenomenon has not been reported before. In addition, SDS introduced a special structure to the products, i.e., WO3·xH2O nanoplatelets constructed from nanoparticle multilayers with abundant nanogaps between the multilayers, which further arranged into nanoflowers with increased amounts of SDS. Benefiting from such a structure, low internal resistance, enhanced stability, and fast redox kinetics, the as-obtained WO3·xH2O/W-3 self-supporting electrode showed a high volumetric specific capacitance of 1402.92 F cm-3 and good cycling stability (a capacity retention of 106% after 10 000 cycles). In addition, an all-solid-state asymmetric SC device based on WO3·xH2O/W-3 delivered high a volumetric energy density of 44.0 mW h cm-3 at 0.5 W cm-3. Our method demonstrates a potential way to fabricate excellent self-supporting electrodes for SCs.

Mechanistic Insights into Anion-Induced Electrochromism of Ru(II)-Based Metallo-Supramolecular Polymer

The metallo-supramolecular polymer (MSP) is considered one of the most promising electrodes for electrochromic devices due to its intrinsically stable redox properties. Nevertheless, despite extensive work focusing on improving the electrochromic and electrochemical properties of MSPs, little experimental evidence exists from in-depth investigations on the anion-induced electrochromism of MSPs. Herein, Ru-based metallo-supramolecular polymer (polyRu) films with excellent electrochromic performance were fabricated through a novel electrochemical deposition method, and the electrochromic mechanism was further understood. The polyRu films possess fast reaction kinetics with a short switching time of 4.0 s (colored) and 2.8 s (bleached) and highly reversible redox properties due to the resulting impacts on the capacitive behaviors (containing surface, near-surface and intercalation pseudo-capacitance) of the perchlorate ions in the electrochromic process. Moreover, the electrochromic degradation of the polyRu films is considered to stem from the numerous nanopores in the film induced by ClO4- transport and the exchange of counter anions from Cl- to ClO4-. In addition, a physical model, revealing the transport of conduction ions and the evolution of the structure and properties of the polyRu film during the electrochromic process, is presented. It is observed that the charge balance of Ru3+ and Ru2+, achieved through the adsorption/desorption of ClO4- on the film, provides electrochromic and electrochemical reversibility to the polyRu film under positive/negative bias. Correspondingly, a transformation from polyRu·(Cl-)2n to polyRu·(ClO4-)x(Cl-)2n-x in the polyRu film is induced by a counter anion exchange from Cl- to ClO4-. Revealing the detailed perchlorate ion transfer kinetics and electrochromic mechanism in this film can offer new insights into the application of metallo-supramolecular polymers in electrochromic devices.

Flexible Inorganic All-Solid-State Electrochromic Devices toward Visual Energy Storage and Two-Dimensional Color Tunability

Multicolor display has gradually become a sought-after trend for electrochromic devices due to its broadened application scope. Meanwhile, the advantages of inorganic electrochromic devices such as stable electrochemical performance and good energy storage ability also have great attraction in practical production applications. However, there are still huge challenges for inorganic electrochromic materials to achieve multicolor transformation due to their single-color hue change. Herein, we design an inorganic and multicolor electrochromic energy storage device (MEESD) exhibiting flexibility and all-solid-state merits. Prussian blue (PB) and MnO₂, as the asymmetrical electrodes of this MEESD, show good pseudocapacitance property, matching charge capacity, and obvious color change. As a typical electrochromic device, the MEESD shows a fast response of 0.5 s and good coloration efficiency of 144.2 cm²/C. As an energy storage device, the MEESD presents excellent rate capability and volumetric energy/power density (84.2 mWh cm^-3/23.3 W cm^-3). Its energy level can be visually monitored by color contrast and optical modulation. In the charging/discharging process, its color can obviously change to various degrees of yellow, green, and blue along with 40% wide optical modulation at 710 nm. Meanwhile, the stability of the MEESD in a common and humidity environment was analyzed in detail from electrochemical, optical, and energy storage aspects. This work provides feasible thoughts to design multifunctional electrochromic devices integrated with inorganic, flexible, all-solid-state, multicolor, and energy storage properties.

High Electrochromic Performance of Perylene Bisimide/ZnO Hybrid Films: An Efficient, Energy-Saving, and Green Route

The composite or hybrid of organic and inorganic materials is one of the common ways to improve the properties of photoelectric functional materials. Perylene bisimide (PBI) derivatives, as large π-conjugated organic small molecules, are a class of photoelectric functional materials with excellent performance. However, there were few reports on PBIs in the electrochromic field due to the difficulty of film-forming caused by their generally poor solubility. Here, water-soluble PBI derivatives (PDI-COOH and PCl-COOH) were synthesized. The hybrid films (ZnO@PDI-COOH/PCl-COOH) formed by the coordination bond and π-π stacking were prepared via a simple solution immersion method. Fourier transform infrared spectrometry and X-ray diffraction as well as scanning electron microscopy, and energy-dispersive spectrometry results further confirmed the formation of hybrid films. At the same time, electrochemical and spectroelectrochemical analyses revealed that the films have reversible redox activity and cathodic electrochromic properties, which can change from orange-red to purple. The ZnO@PDI-COOH hybrid film formed by coordination bonds exhibits fast switching times (1.7 s colored time and 2.6 s bleached time), good stability (retain 92.41% contrast after 2400 cycles), a low driving voltage (-0.6-0 V), and a high coloration efficiency (276.14 cm²/C). The corresponding electrochromic devices also have good electrochromic properties. On this basis, a large-area (100 mm × 100 mm) electrochromic display device with fine patterning was fabricated by using the hybrid film, and the device shows excellent reversible electrochromic performance. This idea of constructing organic-inorganic hybrid materials with coordination bonds provides an effective, energy-saving, and green method, which is expected to promote the large-scale and fine production of electrochromic materials.