Electrochromic Systems Research Articles

Recent Advances in NIR-Switchable Multi-Redox Systems Based on Organic Molecules.

In recent years, near-infrared (NIR) dyes, exhibiting absorption in the NIR region (750-2500 nm), have been applied to various optical applications such as security marking, photovoltaic cells and chemotherapy of deep tissues in vivo. Electrochromic systems capable of switching NIR absorption are attractive from the viewpoint of applications for material and life science, and thus several examples have been reported to date. The development of organic-based materials is needed to reduce the environmental impact and improve biocompatibility, however, the switching of NIR absorption based on redox interconversion is still a challenging issue regarding reversibility and durability during interconversion. To overcome this potential instability, several studies on organic electrochromic systems that allow ON/OFF switching of NIR absorption have been developed in recent years. In this review, we focus on redox-active well-defined small molecules that enable ON/OFF switching of NIR absorption, and present recent studies on their intrinsic electrochemical and spectroscopic properties and/or structural characterization of their charged states. We also address more sophisticated electrochromic systems that can modulate their properties in response to external stimuli such as light, heat, and electric potential.

A high-capacity and self-healable electrochromic system based on I0/CuI conversion reaction

Constructing electrochromic devices (ECDs) requires high-performance electrochromic electrodes with not only high color contrasts, but also large charge storage capacities. In this paper, a I0/CuI conversion-type electrochromic system is developed, achieving reversible and profound color switching between colorless transparent and dark brown, while delivering a large charge storage capacity of 246.7 mA h g−1. To improve switching durability, a multifunctional P10 (Polyquaternium-10) binder is introduced into the electrode, which can effectively confine the polyiodide intermediates and suppress the shuttle effect. At the same time, the CuI/Cu redox couple is further activated by controlling the applied voltage. The generated Cu from this couple can spontaneously react with residual I0 species and facilitate the conversion from I0 to CuI, and therefore in-situ rejuvenate the degraded electrochromic performance. Thanks to these rational designs, this electrochromic electrode achieves rapid response speeds (tcoloring: 14 s, tbleaching: 5 s), high optical modulation amplitude (73.4 % at 500 nm), decant coloration efficiency (57.0 cm2 C−1) and impressive cycle stability (without performance degradation after 150 cycles). This work provides a novel electrochromic system for the construction of high-performance electrochromic devices.

Dependence of nanowire length, diameter, and concentration on the electrochromic properties of hybrid silver nanowire network/PEDOT:PSS-based devices

Mixing silver nanowires is a relevant tool to increase the overall conductivity of hybrid electrochromic systems. Herein, the addition of silver nanowires with an average diameter of 30 nm, length of 88 um, and concentration of 3.0 mg/ml lowered the sheet resistance of PEDOT:PSS films from 280 Ω/sq to 34 Ω/sq. Further characterized for their electrochromic behavior, mixing silver nanowires with PEDOT:PSS films allowed lowering the turn-on voltage from -1.5 V to -1.1 V and reducing the switching time from 7 to 3.6 seconds. Furthermore, with the addition of silver nanowires into PEDOT:PSS, an expensive ITO transparent and environmentally impactful electrode material is no longer required. Electrochromic devices based on hybrid AgNW/PEDOT:PSS films showed higher colouration efficiency, lower sheet resistance, lower turn on voltage, and faster switching times as compared to electrochromic devices made with PEDOT:PSS films only.

The Effects of Polymerization on the Performance of Viologen-Based Electrochromic Devices

In this work, electrochromic devices were prepared using the redox couple ethyl viologen diperchlorate and 1,1′-diethyl ferrocene in propylene carbonate as an aprotic solvent to facilitate ions separation and diffusion inside the devices. Electrochromic devices were made using electrochromic gel mixtures at the concentrations of 55%, 60% and 65% with respect to the bisphenol A polymer. In particular, two sets of gels were made: one set contained the bisphenol A not-polymerized while and the second one contained the polymerized polymer. Different techniques, such as cyclic voltammetry, UV-vis-NIR, and Raman spectroscopy, were used to study such systems to understand the differences in terms of performances between the different sets of electrochromic devices. Cyclic voltammetry confirmed that the oxidation process of the 1,1′-diethyl ferrocene and the reduction of the ethyl viologen diperchlorate occurred at about 0.4 V. Interesting variations in the transmittances were found between the two groups of samples. The best values of CE were provided by the electrochromic devices based on the polymerized electrochromic gel mixture at a concentration of 60% (EM60). The EM60 device result was CE = 92.82 C/cm2 in the visible region and CE = 80.38 C/cm2 in the near–infrared region, confirming that these devices can be used for energy-saving applications. A structural characterization of the materials used in the two sets of electrochromic devices was made using Raman spectroscopy, and the analysis supports the electrochemical models used to explain the processes involved during operation of the electrochromic systems.

A Wide Color Gamut and Noniridescent Zinc-Anode Asymmetric Electrochromic Device for Self-Sustaining Color-Adaptive Bio-Camouflage System.

Inspired by camouflage-colored organisms, the development of bio-camouflage systems using electrochromic (EC) technology has gained significant interest. However, existing EC systems struggle with achieving a wide color gamut, noniridescent colors, and self-sustainability. Herein, a self-sustainable color-adaptive bio-camouflage system integrating EC and nanogenerator (NG) technologies, enabling environmental color adaptation, and thermal regulation without an external power source is proposed. The system is based on a zinc-anode EC device (ZECD) with an asymmetric structure, incorporating flexible tungsten oxide and vanadium oxide electrodes. During the EC process, tungsten oxide shifts between blue and transparent, allowing near-infrared thermal modulation, while the vanadium oxide transitions from yellow to transparent. This design enables reversible near-infrared modulation and noniridescent color conversion among black, blue, green, yellow, and transparent. For the self-sustainability of the system, an electromagnetic and triboelectric hybrid NG that collects biomechanical energy is developed. In a typical driven cycle, the integrated system transitions colors and achieves significant near-infrared spectral modulation, demonstrating environmental adaptability and thermal regulation. Experiments on human skin and simulated chameleon color changes further confirm the system's effectiveness. This work highlights the potential of integrating EC and NG technologies to advance color-adaptive camouflage systems, opening new an avenue for bio-camouflage design.

Linker Conformation Controls Oxidation Potentials and Electrochromism in Highly Stable Zr-Based Metal-Organic Frameworks.

The development of tailor-made electrochromic (EC) materials requires a large variety of available substances with properties that precisely match the task. Since the inception of electrochromic metal-organic frameworks (MOFs), the field relies only on a limited set of building blocks, providing the desired electrochromic effect. Herein, we demonstrate for the first time the implementation of a Piccard-type system (N,N,N',N'-benzidinetetrabenzoate) into Zr-MOFs to obtain electrochromic materials. With fast switching rates, high contrast ratio, long-life stability, and exceptional chemical and physical stability, the novel material is on par with inorganic EC material. The new EC system exhibits an ultrahigh contrast from the bleaching state, with transmittance in the visible region >53%, to the colored state with a transmittance of ca. 3%. The 5 μm thick film attained up to 90% of the coloring in 12.5 s and exhibited high electrochemical reversibility. Moreover, the conformational lability of the electrochromic ligand chosen is locked via the topology design of the framework, which is not attainable in the solution. Locked conformations of the redox active linker in distinct polymorphous frameworks (DUT-65 and DUT-66) feature different redox characteristics and opens the door to the overarching control of the oxidation pathway in the Piccard-type systems.

Bistable Electrochromic Ionogels via Supramolecular Interactions for Energy-Efficient Displays.

Bistable electrochromic (EC) materials and systems offer significant potential for building decarbonization through their optical modulation and energy efficiency. However, challenges such as limited design strategies and bottlenecks in cost, fabrication, and color have hindered the full commercialization of energy-saving EC windows and displays, with few materials achieving true bistability. Herein, a novel strategy for designing bistable electrochromic materials is proposed by leveraging supramolecular interactions. These interactions facilitate reversible color transitions, stabilize the colored structure, and enable spatial confinement to inhibit diffusion, thereby achieving bistable electrochromism. The mechanisms and materials underlying these unconventional electrochromic systems are substantiated through detailed characterization. This strategy enables the preparation of low-cost and sustainable transparent electrochromic displays with high performance. Notably, the display information remains clearly visible for more than 2 h without consuming energy. Involving biomass materials and removable device structures also enhances the sustainability and scalability of EC technology applications and development. These results demonstrate the crucial role of supramolecular chemistry in the development of cutting-edge materials for applications such as energy-saving smart windows.

Potential Gradient-Driven Dual-Functional Electrochromic and Electrochemical Device Based on a Shared Electrode Design.

The integration of electrochromic devices and energy storage systems in wearable electronics is highly desirable yet challenging, because self-powered electrochromic devices often require an open system design for continuous replenishment of the strong oxidants to enable the coloring/bleaching processes. A self-powered electrochromic device has been developed with a close configuration by integrating a Zn/MnO2 ionic battery into the Prussian blue (PB)-based electrochromic system. Zn and MnO2 electrodes, as dual shared electrodes, the former one can reduce the PB electrode to the Prussian white (PW) electrode and serves as the anode in the battery; the latter electrode can oxidize the PW electrode to its initial state and acts as the cathode in the battery. The bleaching/coloring processes are driven by the gradient potential between Zn/PB and PW/MnO2 electrodes. The as-prepared Zn||PB||MnO2 system demonstrates superior electrochromic performance, including excellent optical contrast (80.6%), fast self-bleaching/coloring speed (2.0/3.2 s for bleaching/coloring), and long-term self-powered electrochromic cycles. An air-working Zn||PB||MnO2 device is also developed with a 70.3% optical contrast, fast switching speed (2.2/4.8 s for bleaching/coloring), and over 80 self-bleaching/coloring cycles. Furthermore, the closed nature enables the fabrication of various flexible electrochromic devices, exhibiting great potentials for the next-generation wearable electrochromic devices.

Sustainable, low-cost, high-contrast electrochromic displays via host–guest interactions

Electrochromic (EC) displays with electronically regulating the transmittance of solar radiation offer the opportunity to increase the energy efficiency of the building and electronic products and improve the comfort and lifestyle of people. Despite the unique merit and vast application potential of EC technologies, long-awaited EC windows and related visual content displays have not been fully commercialized due to unsatisfactory production cost, durability, color, and complex fabrication processes. Here we develop a unique EC strategy and system based on the natural host and guest interactions to address the above issues. A completely reusable and sustainable EC device has been fabricated with potential advantages of extremely low cost, ideal user-/environment friendly property, and excellent optical modulation, which is benefited from the extracted biomass EC materials and reusable transparent electrodes involved in the system. The as-prepared EC window and nonemissive transparent display also show comprehensively excellent properties: high transmittance change (>85%), broad spectra modulation covering Ultraviolet (UV), Visible (Vis) to Infrared (IR) ranges, high durability (no attenuation under UV radiation for more than 1.5 mo), low open voltage (0.9 V), excellent reusability (>1,200 cycles) of the device's key components and reversibility (>4,000 cycles) with a large transmittance change, and pleasant multicolor. It is anticipated that unconventional exploration and design principles of dynamic host-guest interactions can provide unique insight into different energy-saving and sustainable optoelectronic applications.

Silver Nanowire-Based Transparent Electrodes for V2O5 Thin Films with Electrochromic Properties.

The development of electrochromic systems, known for the modulation of their optical properties under an applied voltage, depends on the replacement of the state-of-the-art ITO (In2O3:Sn) transparent electrode (TE) as well as the improvement of electrochromic films. This study presents an innovative ITO-free electrochromic film architecture utilizing oxide-coated silver nanowire (AgNW) networks as a TE and V2O5 as an electrochromic oxide layer. The TE was prepared by simple spray deposition of AgNWs that allowed for tuning different densities of the network and hence the resistance and transparency of the film. The conformal oxide coating (SnO2 or ZnO) on AgNWs was deposited by atmospheric-pressure spatial atomic layer deposition, an open-air fast and scalable process yielding a highly stable electrode. V2O5 thin films were then deposited by radio frequency magnetron sputtering on the AgNW-based TE. Independent of the oxide's nature, a 20 nm protective layer thickness was insufficient to prevent the deterioration of the AgNW network during V2O5 deposition. On the contrary, crystalline V2O5 films were grown on 30 nm thick ZnO or SnO2-coated AgNWs, exhibiting a typical orange color. Electrochromic characterization demonstrated that only V2O5 films deposited on 30 nm thick SnO2-coated AgNW showed characteristic oxidation-reduction peaks in the Li+-based liquid electrolyte associated with a reversible orange-to-blue color switch for at least 500 cycles. The electrochromic key properties of AgNW/SnO2 (30 nm)/V2O5 films are discussed in terms of structural and morphological changes due to the AgNW network and the nature and thickness of the two protective oxide coatings.

Poly(neutral red), poly(crystal violet), and poly(congo red) films as color filters for producing new color states in polyaniline electrochromic system

There is a pressing need for efficient strategies to produce new colors in electrochromic (EC) systems. The majority of studies to date have been conducted on the development of advanced synthesis routes for obtaining desired colors in EC materials, but they suffer from complicated synthesis and high costs. Herein, we developed a facile, viable, and efficient approach for tuning EC behavior of polyaniline (PANI) using three polymeric dyes: poly(neutral red) (PNR), poly(crystal violet) (PCV), and poly(Congo red) (PCR) films as color filters. PNR, PCV, and PCR films were electrodeposited on a fluorine-doped tin oxide (FTO) electrode modified with Schiff base network-1 (SNW1) to fabricate FTO/SNW1/PNR, FTO/SNW1/PCV, and FTO/SNW1/PCR, respectively. To inquire about the effects of studied polymeric dyes on the EC performance of PANI, aniline was electropolymerized onto the modified electrodes. The fabrication of the PANI-based EC electrodes, namely FTO/SNW1/PNR/PANI, FTO/SNW1/PCV/PANI, and FTO/SNW1/PCR/PANI, was characterized using cyclic voltammetry, EIS, FT–IR, XRD, FESEM, TEM and spectroelectrochemical approaches. The EC studies confirmed that FTO/SNW1/PNR/PANI, FTO/SNW1/PCV/PANI, and FTO/SNW1/PCR/PANI electrodes change their color in the oxidized states to pale red, yellowish-green, and blue, respectively, if reduced at − 0.6 V. Combining PANI film with three different polymeric dyes can produce new colors in the reduced state of PANI film. FTO/SNW1/PANI, FTO/SNW1/PNR/PANI, FTO/SNW1/PCV/PANI, and FTO/SNW1/PCR/PANI showed high optical contrasts of 66%, 54%, 64%, and 78%, respectively, at 700 nm. This effort provides a new approach for tuning the EC behavior of PANI and puts forward an idea to produce new colors via organic color filters.

Electrolyte design for reversible metal electrodeposition-based electrochromic energy-saving devices

Reversible metal electrodeposition (RME)-based electrochromic devices have been attracting significant research interest due to their merits of low cost, simple configuration, and high extinction coefficients. As the key component in the electrochromic system, RME electrolytes with various metal ions and additives have endowed the RME device with flexible functionalities in energy-saving applications such as energy-efficient displays, smart windows, and camouflages. However, it is still challenging to research a widespread commercial application before some critical issues can be solved such as poor reversibility, low optical memory of the mirror state, and slow switching speed. Here, we offer a critical review of the recent progress of RME electrochromic devices based on aqueous, organic, ionic liquid, and eutectic electrolytes. Furthermore, the main challenges and perspectives for RME electrolytes are highlighted and discussed.

Towards photoelectrochromic modulation of NIR absorption in plasmonic ITO using pentacene films

Pentacene molecules are deposited onto plasmonic ITO nanocrystals as hybrid organic–inorganic bilayers, holding promise as prospective NIR-modulating electrochromic systems being self-powered by VIS-light-driven photocharging.

Nonconventional Symmetric Double-Side Electrochromic Devices Employing a Nafion Conductive Layer to Unlock Superior Durability.

In this work, we present a methodology to create an effective novel double-sided symmetric architecture of solid-state electrochromic devices. This principally new nonconventional configuration provides access to novel electrochromic systems that could be applicable for the creation of smart double-side signage, smart boards, nonemissive displays, and other smart interactive devices that change their color upon application of a voltage. The proposed configuration is based on the assembly of two identical electrochromic materials facing each other through an opaque optical separator. As a proof of concept, we use an electrochromic material based on bis(4'-(pyridin-4-yl)-2,2':6',2″-terpyridine) iron complex, covalently immobilized on screen-printed surface-extended ITO support. The symmetric configuration allows for a drastic enhancement of the overall stability of the device due to both attenuation of the counter electrode polarization and minimization of electrolyte decomposition. A nontransparent ion-permeable separator, in turn, allows observing the color change of only one of the electrodes by cutting off the optical contribution of the electrode located behind it. Further functionalization of the electrochromic material with a thin layer of Nafion is a beneficial strategy to significantly boost up long-term durability of the devices. Applying a layer of Nafion to the electrochromic material results in an increase in ionic conductivity within the device and ensures better retention of electrochromic molecules on the surface, thus minimizing device decomposition during long-term electrochemical cycling. An electrochromic device that bears Nafion-functionalized electrodes can operate (i) in the dual-side mode, where both sides demonstrate effective electrochromic performance; or (ii) in a one-side manner, where only one side of the device changes color. Notably, when operating in the one-side mode, the device withstands 70,000 cycles, after which the performance of the device can be resumed by simply turning the device to the other side (via switching the polarity of the electrodes).

Stable WO3 electrochromic system based on NH4+ hydrogen bond chemistry

As the best-known and extensively-studied electrochromic material, tungsten trioxide (WO3) has drawn tremendous interest due to its low material cost, environmental benignity, dual band regulation, high optical modulation, and high chemical stability. However, these films always suffer from ion-trapping-induced degradation in electrochromic performance when cycled in alkali cation (such as Li+, Na+, K+, etc.) electrolytes. Herein, we report a stable WO3 electrochromic system enabled by a NH4CF3SO3-TEP (Triethyl phosphate) organic electrolyte. Based on comparative experiments and systematic characterizations, the prolonged cycling stability is mainly attributed to the formation of hydrogen bonds between the NH4+ ions and the WO3 lattice, which are much weaker than metallic coordination bonds. Furthermore, the bulky CF3SO3− anion and large TEP solvent molecule are also favorable to stabilize WO3, since they are more resistive to be co-inserted into the WO3 lattice along with the shuttling NH4+ cations. Thanks to these merits, this NH4CF3SO3-TEP electrolyte simultaneously enables great electrochromic activity (76.1 % transmittance modulating ability at 633 nm) and impressive cyclic color-switching stability (1000 times color change without any detectable electrochromic performance or mechanical degradation), overwhelmingly outperforming the conventional Li+/K+ electrolytes. Furthermore, as a demonstration of its application, a WO3/Zn electrochromic device is fabricated based on a NH4+/Zn2+-CF3SO3 dual-cation TEP electrolyte. These findings may shed light on the in-depth understanding of WO3′s “ion trapping” effect, as well as the electrolyte design of high-performance WO3-based electrochromic devices.

Neutral color and self-healable electrochromic system based on CuI/Cu and $${{\rm{I}}_3}^ - /{{\rm{I}}^ - }$$ conversions

Neutral color and self-healable electrochromic system based on CuI/Cu and $${{\rm{I}}_3}^ - /{{\rm{I}}^ - }$$ conversions

Deposition and optical properties investigation of components for multilayer electrochromic system

Electrochromic materials are materials that change their optical properties under the influence of an applied electrical potential. They can be based on different types of electrochromes, ranging from metal oxides (e.g., WO3, MO3, TiO, or NiO) to organic materials (such as SmPc2, EuPc2, or YbPc2). The choice of electrochromic material determines the subsequent choice of components, such as the electrolyte, also called the fast ion conductor, and the ion storage layer. In this paper, the authors present methods for the deposition of the individual functional layers of the electrochromic system, together with the parameters of the deposition processes. The construction of the high-vacuum apparatus on which the layers were deposited is presented. The results of structural and optical measurements of the deposited layers are also presented.

Durable multicolor electrochromic fibers based on metal oxidecoated steel tapes

Electrochromic devices (ECDs) have the ability to selectively change color under the influence of electric current or potential and can keep the selected color after disconnection. They are used as smart windows, e-ink displays and even as flexible tapes. Most multicolor ECDs incorporate traditional “sandwich” architecture, which requires at least one electrode to be made of a transparent conductive polymer or thin film ceramics, and can be quite fragile and sensitive. In this paper, a flexible and durable multicolor ECD tape is reported, which uses advanced architecture, steel foil electrodes, and multiple coloration systems, for example intercalation used in tungsten and vanadium oxide based electrochromic systems, as well as a redox dye, for example methyl orange. The presented ECD tapes can utilize different metal oxides and color mechanisms with coloration efficiency around 10 cm2 C−1 for V2O5 and WO3 in both visible and IR range, and more than 30 cm2 C−1 for ZnO with added methyl orange dye. Due to the use of stainless-steel foil as a substrate, the assembled ECD tape has excellent durability properties, withstanding more than 39 MPa of pressure, and could therefore be used even in the most adverse conditions, for applications such as adaptive camouflage, color changing clothes, or other applications

A liquid crystal smart window for energy saving and harvesting

Energy conservation, particularly in buildings, has become a critical focus area. Among the various components of a building, windows play a significant role in the exchange of heat and light. Electrochromic systems, known for their ability to switch between transparent and opaque states with the aid of external energy, have served as the basis for conventional smart windows. This research introduces a novel approach to smart window technology by incorporating a luminescent solar concentrator (LSC) that incorporates thin photovoltaic cells at the window's edges, coupled with a thermally and optically adjustable liquid crystal cell. The device intelligently transitions between an energy-saving, opaque mode (day mode) and a transparent, idle mode (night mode), based on ambient temperature and incident light levels. Through this innovative design, the proposed smart window offers enhanced energy efficiency and harvesting capabilities, promising significant benefits for sustainable building practices.

High-Voltage Pulse-Assisted Operation of Single-Layer Electrochromic Systems for High Performance and Reliability.

A single-layer electrochromic device (SL-ECD) based on ionic conductors containing EC chromophores provides a very simple platform that can be readily fabricated by sandwiching the EC layer between two electrodes. The operation of SL-ECDs is governed by the diffusion of redox species due to their SL structure, which causes a relatively slow dynamic response. In this study, we propose an effective high-voltage pulse injection strategy to improve the performance of SL-ECDs. Applying a programmed voltage wave composed of DC and high-voltage pulses promotes coloration/bleaching switching without degrading device stability, which is more advantageous than applying high DC voltages. We modified the input voltage profile by considering fundamental parameters, such as the amplitude and duty ratio of additional voltage pulses. The coloration and bleaching dynamic responses with the optimized voltage wave are ∼62 and ∼20% faster, respectively, compared with those with the simple DC input. Furthermore, the additionally injected pulse aids in increasing the coloration efficiency from ∼95.3 to ∼168.6 cm2 C-1. Another notable feature of this system is that the device operates stably when a programmed voltage wave is used. These results indicate that the concept of high-voltage pulse-assisted operation of SL-ECDs is a straightforward but effective method for improving device performance without changing the EC chromophore or device structure.