Viologen-based flexible electrochromic devices

Electrochromic technology has gained significant attention in various fields such as displays,smart windows,biomedical monitoring,military camouflage,human-machine interaction,and electronic skin due to its ability to provide reversible and fast color changes under applied voltage.With the rapid development and increasing demand for flexible electronics,flexible electrochromic devices(FECDs)that offer smarter and more controllable light modulation hold great promise for practical applications.The electrochromic material(ECM)undergoing color changes during the electrochemical reactions is one of the key components in electrochromic devices.Among the ECMs,viologens,a family of organic small molecules with 1,1'-disubstituted-4,4'-dipyridinium salts,have garnered extensive research interest,due to their well-reversible redox reactions,excellent electron acceptance ability,and the ability to produce multiple colors.Notably,viologen-based FECDs demonstrate color changes in the liquid or semisolid electrolyte layer,eliminating the need for two solid electrodes and thus simplifying the device structure.Consequently,viologens offer significant potential for the development of FECDs with high optical contrast,fast response speed,and excellent stability.This review aims to provide a comprehensive overview of the progress and perspectives of viologen-based FECDs.It begins by summarizing the typical structure and recent exciting developments in viologen-based FECDs,along with their advantages and disadvantages.Furthermore,the review discusses recent advancements in FECDs with additional functionalities such as sensing,photochromism,and energy storage.Finally,the remaining challenges and potential research directions for the future of viologen-based FECDs are addressed.

Interface-reinforced solid-state electrochromic Li-ion batteries enabled by in-situ liquid-solid transitional plastic glues

The electrochromic Li-ion batteries(ELIBs) combine the functions of electrochromism and energy storage,realizing the display of energy-storage levels by visual signals. However, the accompanying interfacial issues including physical contact and(electro)chemical stability should be taken into account when the conventional liquid/gel electrolytes are replaced with solid-state counterparts. Herein, the in-situ liquid-solid transitional succinonitrile(SCN) plastic glues are constructed between electrodes and poly(ethylene oxide)(PEO) polymer electrolytes, enabling an interface-reinforced solid-state ELIB.Specifically, the liquid SCN precursor can adequately wet electrode/PEO interfaces at high temperature,while it returns back to solid state at room temperature, leading to seamless interfacial contact and smooth ionic transfer without changing the solid state of the device. Moreover, the SCN interlayer suppresses the direct contact of PEO with electrodes containing high-valence metal ions, evoking the improved interfacial stability by inhibiting the oxidation of PEO. Therefore, the resultant solid-state ELIB with configuration of LiMn_(2)O_(4)/SCN-PEO-SCN/WO_(3) delivers an initial discharge capacity of 111 m A h g^(-1) along with a capacity retention of 88.3% after 200 cycles at 30 ℃. Meanwhile, the electrochromic function is integrated into the device by distinguishing its energy-storage levels through distinct color changes. This work proposes a promising solid-state ELIB with greatly reinforced interfacial compatibility by introducing in-situ solidified plastic glues.

Exploring innovative synthetic solutions for advanced polymer-based electrochromic energy storage devices:Phenoxazine as a promising chromophore

The current investigation offers an innovative synthetic solution regarding electrochromic(EC)and energy storage applications by exploring phenoxazine(POZ)moiety.Subsequently,three POZ-based polymers(polyimide,polyazomethine,and polyamide)were synthesized to ascertain the superior performer.The polyamide exhibited remarkable attributes,including high redox stability during 500 repetitive CVs,optical contrast of 61.98%,rapid response times of 1.02 and 1.38 s for coloring and bleaching,EC efficiency of 280 cm^(2)C^(-1).and decays of the optical density and EC efficiency of only 12.18%and 6.23%after 1000 cycles.Then,the energy storage performance of polyamide PA was tested,for which the following parameters were obtained:74.7 F g^(-1)(CV,scan rate of 10 mV s^(-1))and 118 F g^(-1)(GCD,charging current of 0.1 A g^(-1)).Then,the polyamide was tested in EES devices,which yielded the following EC parameters:an optical contrast of 62.15%,response times of 9.24 and 5.01 s for coloring and bleaching,EC efficiency of 178 cm^(2)C^(-1),and moderate decays of 20.25%and 23.24%for the optical density and EC efficiency after 500 cycles.The energy storage performance included a capacitance of 106 F g^(-1)(CV,scan rate of 0.1 mV s^(-1))and 9.23 F g^(-1)(GCD,charging current of 0.1 A g^(-1)),capacitance decay of 11.9%after500 cycles,and 1.7 V retention after 2 h.Also,two EES devices connected in series powered a 3 V LED for almost 30 s.

Recent progress and future research directions for electrochromic zinc-ion batteries

In recent times,future energy storage systems demand a multitude of functionalities beyond their traditional energy storage capabilities.In line with this technological shift,there is active research and development of electrochromic-energy storage systems designed to visualize electrochemical charging and discharging processes.The conventional electrochromic-energy storage devices primarily integrated supercapacitors,known for their high power density,to enable rapid color contrast.However,the low energy density of supercapacitors restricts overall energy storage capacity,acting as a significant barrier to expanding the application range of such systems.In this review,we introduce electrochromic zinc(Zn)-ion battery systems,which effectively overcome the limitation of low energy density,and provide illustrative examples of their applicability across diverse fields.Although many recent research works are present for electrochromic Zn-ion batteries,little review has so far taken place.Our objective is to discuss on the current progress and future directions for electrochromic Zn-ion batteries,which are applicable for wearable electronics applications and energy storage systems.This review provides an initial milestone for future researchers in electrochromic energy storage and zinc-ion batteries,which will lead to a stream of future works related to them.

Inhibiting the phase transition of WO_(3)for highly stable aqueous electrochromic battery

Aqueous electrochromic battery(ECB)has shown intense potential for achieving energy storage and saving simultaneously.While tungsten oxide(WO_(3))is the most promising EC material for commercialization,the cycling stability of WO_(3)-based aqueous ECBs is currently unsatisfactory due to the repeated phase transition during the redox process and the corrosion by acidic electrolytes.Herein,we present a titanium-tungsten oxide alloy(Ti-WO_(3))with controllable morphology and crystal phase synthesized by a facile hot injection method to overcome the challenges.In contrast to conventional monoclinic WO_(3),the Ti-WO_(3)nanorods can stably maintain their cubic crystal phase during the redox reaction in an acidic electrolyte,thus leading to dramatically enhanced response speed and cycling stability,Specifically,when working in a well-matched hybrid Al^(3+)/Zn^(2+)aqueous electrolyte,our phasetransition-free cubic Ti-WO_(3)exhibits an ultra-high cycling stability(>20000 cycles),fast response speed(3,95 s/4,65 s for bleaching/coloring),as well as excellent discharge areal capacity of 214.5 mA h m^(-2),We further fabricate a fully complementa ry aqueous electrochromic device,for the first time,using a Ti-WO_(3)/Prussian blue device architecture.Remarkably,the complementary ECB shows>10000 stable operation cycles,attesting to the feasibility of our Ti-WO_(3)for practical applications.Our work validates the significance of inhibiting the phase transitions of WO_(3)during the electrochromic process for realizing highly cyclable aqueous ECB,which can possibly provide a generalized design guidance for other high-quality metallic oxides for electrochemical applications.

Surface Deposition of Ni(OH)_(2) and Lattice Distortion Induce the Electrochromic Performance Decay of NiO Films in Alkaline Electrolyte

NiO,an anodic electrochromic material,has applications in energy-saving windows,intelligent displays,and military camouflage.However,its electrochromic mechanism and reasons for its performance degradation in alkaline aqueous electrolytes are complex and poorly understood,making it challenging to improve NiO thin films.We studied the phases and electrochemical characteristics of NiO films in different states(initial,colored,bleached and after 8000 cycles)and identified three main reasons for performance degradation.First,Ni(OH)_(2)is generated during electrochromic cycling and deposited on the NiO film surface,gradually yielding a NiO@Ni(OH)_(2)core-shell structure,isolating the internal NiO film from the electrolyte,and preventing ion transfer.Second,the core-shell structure causes the mode of electrical conduction to change from first-to second-order conduction,reducing the efficiency of ion transfer to the surface Ni(OH)_(2)layer.Third,Ni(OH)_(2)and NiOOH,which have similar crystal structures but different b-axis lattice parameters,are formed during electrochromic cycling,and large volume changes in the unit cell reduce the structural stability of the thin film.Finally,we clarified the mechanism of electrochromic performance degradation of NiO films in alkaline aqueous electrolytes and provide a route to activation of NiO films,which will promote the development of electrochromic technology.

Electrochromic-Induced Rechargeable Aqueous Batteries: An Integrated Multifunctional System for Cross-Domain Applications

Multifunctional electrochromic-induced rechargeable aqueous batteries(MERABs) integrate electrochromism and aqueous ion batteries into one platform, which is able to deliver the conversion and storage of photo-thermal-electrochemical sources.Aqueous ion batteries compensate for the drawbacks of slow kinetic reactions and unsatisfied storage capacities of electrochromic devices. On the other hand, electrochromic technology can enable dynamically regulation of solar light and heat radiation. However,MERABs still face several technical issues, including a trade-off between electrochromic and electrochemical performance, low conversion efficiency and poor service life. In this connection, novel device configuration and electrode materials, and an optimized compatibility need to be considered for multidisciplinary applications. In this review,the unique advantages, key challenges and advanced applications are elucidated in a timely and comprehensive manner. Firstly, the prerequisites for effective integration of the working mechanism and device configuration, as well as the choice of electrode materials are examined. Secondly, the latest advances in the applications of MERABs are discussed, including wearable, self-powered, integrated systems and multisystem conversion. Finally, perspectives on the current challenges and future development are outlined, highlighting the giant leap required from laboratory prototypes to large-scale production and eventual commercialization.

Bifunctional flexible electrochromic energy storage devices based on silver nanowire flexible transparent electrodes

Flexible electrochromic energy storage devices(FECESDs)for powering flexible electronics have attracted considerable attention.Silver nanowires(AgNWs)are one kind of the most promising flexible transparent electrodes(FTEs)materials for the emerging flexible devices.Currently,fabricating FECESD based on AgNWs FTEs is still hindered by their intrinsic poor electrochemical stability.To address this issue,a hybrid AgNWs/Co(OH)_(2)/PEDOT:PSS electrode is proposed.The PEDOT:PSS could not only improve the resistance against electrochemical corrosion of AgNWs,but also work as functional layer to realize the color-changing and energy storage properties.Moreover,the Co(OH)_(2)interlayer further improved the color-changing and energy storage performance.Based on the improvement,we assembled the symmetrical FECESDs.Under the same condition,the areal capacitance(0.8 mF cm^(−2))and coloration efficiency(269.80 cm^(2)C−1)of AgNWs/Co(OH)_(2)/PEDOT:PSS FECESDs were obviously higher than AgNWs/PEDOT:PSS FECESDs.Furthermore,the obtained FECESDs exhibited excellent stability against the mechanical deformation.The areal capacitance remained stable during 1000 times cyclic bending with a 25 mm curvature radius.These results demonstrated the broad application potential of the AgNWs/Co(OH)_(2)/PEDOT:PSS FECESD for the emerging portable and multifunctional electronics.

Synergistic Effect and Electrochromic Mechanism of Nanoflake Li-doped NiO in LiOH Electrolyte

Inorganic metal oxide electrochromic materials have good application prospects for energy-saving windows in buildings and smart display applications.Therefore,the development of electrochromic films with good cycling stabilities,fast color-change response times,and high coloring efficiencies has attracted considerable attention.In this study,nanoflake Li-doped NiO electrochromic films were prepared using a hydrothermal method,and the films exhibited superior electrochromic performances in the LiOH electrolyte.Li^(+)ions doping increased the ion transmission rates of the NiO films,and effectively promoted the transportation of ions from the electrolyte into NiO films.Meanwhile,the nanoflake microstructure caused the NiO films to have larger specific surface areas,providing more active sites for electrochemical reactions.It was determined that the NiO-Li20%film exhibited an ultra-fast response in the LiOH electrolyte(coloring and bleaching times reached 3 and 1.5 s,respectively).Additionally,the coloration efficiency was 62.1 cm^(2)C^(−1),and good cycling stability was maintained beyond 1500 cycles.Finally,the simulation calculation results showed that Li doping weakened the adsorption strengths of the NiO films to OH^(−),which reduced the generation and decomposition of NiOOH and helped to improve the cycling stabilities of the films.Therefore,the research presented in this article provides a strategy for designing electrochromic materials in the future.

Fast and Stable Zinc Anode‑Based Electrochromic Displays Enabled by Bimetallically Doped Vanadate and Aqueous Zn^(2+)/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 V_(3)O_(8)interlayers,which facilitates the transportation of cations and enhances the electrochemical kinetics.In addition,a hybrid Zn^(2+)/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.

An overview of recent progress in the development of flexible electrochromic devices

Electrochromic materials are capable of reversibly switching their colors or optical properties through redox reactions under applied voltages,which have shown great potential applications including smart windows,nonemissive displays,optical filters,among others.Although the current rigid electrochromic devices have shown emerging interest and developed rapidly,many applications(e.g.,wearable/deformable optoelectronics)are blocked due to their inflexible features.Herein,the adaption of rigid electrochromic devices to flexible ones is of particular interest for the new era of smart optoelectronics.In this review,the current state-of-the-art achievements of flexible electrochromic devices(FECDs)are highlighted,along with their design strategies and the choice of electrochromic materials.The recent research progress of FECDs is reviewed in detail,and the challenges and corresponding solutions for real-world applications of FECDs are discussed.Furthermore,we summarize the basic fabrication strategies of FECDs and their potential applications.In addition,the development trend,the perspectives,and the outlook of FECDs are discussed at the end of this Review,which may provide recommendations and potential directions to advance the practical applications of FECDs.

An Electrochromic Nickel Phosphate Film for Large-Area Smart Window with Ultra-Large Optical Modulation

Exploring materials with high electrochemical activity is of keen interest for electrochemistry-controlled optical and energy storage devices.However,it remains a great challenge for transition metal oxides to meet this feature due to their low electron conductivity and insufficient reaction sites.Here,we propose a type of transition metal phosphate(NiHPO_(4)·3H_(2)O,NHP)by a facile and scalable electrodeposition method,which can achieve the capability of efficient ion accommodation and injection/extraction for electrochromic energy storage applications.Specifically,the NHP film with an ultra-high transmittance(approach to 100%)achieves a large optical modulation(90.8%at 500 nm),high coloration efficiency(75.4 cm^(2)C^(-1)at 500 nm),and a high specific capacity of 47.8 mAh g^(-1)at 0.4 A g^(-1).Furthermore,the transformation mechanism of NHP upon electrochemical reaction is systematically elucidated using in situ and ex situ techniques.Ultimately,a large-area electrochromic smart window with 100 cm^(2)is constructed based on the NHP electrode,displaying superior electrochromic energy storage performance in regulating natural light and storing electrical charges.Our findings may open up new strategies for developing advanced electrochromic energy storage materials and smart windows.

Sol-gel-based porous Ti-doped tungsten oxide films for high-performance dual-band electrochromic smart windows

Dual-band electrochromic smart windows(DESWs)with independent control of the transmittance of near-infrared and visible light show great potential in the application of smart and energy-saving buildings.The current strategy for building DESWs is to screen materials for composite or prepare plasmonic nanocrystal films.These rigorous preparation processes seriously limit the further development of DESWs.Herein,we report a facile and effective sol-gel strategy using a foaming agent to achieve porous Ti-doped tungsten oxide film for the high performance of DESWs.The introduction of foaming agent polyvinylpyrrolidone during the film preparation can increase the specific surface area and free carrier concentration of the films and enhance their independent regulation ability of near-infrared electrochromism.As a result,the optimal film shows excellent dual-band electrochromic properties,including high optical modulation(84.9%at 633 nm and 90.3%at 1200 nm),high coloration efficiency(114.9 cm^(2) C^(-1) at 633 nm and 420.3 cm^(2) C^(-1) at 1200 nm),quick switching time,excellent bistability,and good cycle stability(the transmittance modulation losses at 633 and 1200 nm were 11%and 3.5%respectively after 1000 cycles).A demonstrated DESW fabricated by the sol-gel film showed effective management of heat and light of sunlight.This study represents a significant advance in the preparation of dual-band electrochromic films,which will shed new light on advancing electrochromic technology for future energy-saving smart buildings.

Reversible Zn^(2+) Insertion in Tungsten Ion-Activated Titanium Dioxide Nanocrystals for Electrochromic Windows

Zinc-anode-based electrochromic devices(ZECDs) are emerging as the next-generation energy-e cient transparent electronics. We report anatase W-doped TiO_(2) nanocrystals(NCs) as a Zn^(2+) active electrochromic material. It demonstrates that the W doping in TiO_(2) highly reduces the Zn^(2+) intercalation energy,thus triggering the electrochromism. The prototype ZECDs based on W-doped TiO_(2) 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).

The research on the high quality TiO_2, MoO_3-doped WO_3 electrochromic film

The high quality TiO2, MoO3-doped WO3 electrochromic film was prepared by the sol-gel method for the first time.The sol, which has hydrogen peroxide （H2O2） and oxalic acid （H2C2O4）, was very stable at room temperature and quite suitable for the deposition of films. The WO3 electrochromic film prepared from this doped sol had excellent performance, such as short response time, no cracks, good adhesion to the substrate, high coloring efficiency and longevity of service.

The Synthesis of 1-Ethyl-1'-(4-vinylbenzyl)-4,4'-bipyridinium Chloride and Iodide and its Electrochromic Property

Abstract： A new compound 1-ethyl-1＇-（4-vinylbenzyl）-4, 4＇-bipyridinium chloride and iodide has been synthesized. The cyclic voltammogram and impedance spectra indicated that a layer of viologen＇s electrochromic （EC） film could be deposited on conductive ITO glass working electrode With polyelectrolyte as ionic conduction layer, solid EC devices based on this compound have been assembled and their thickness was about 2.35 mm. When different voltages were added, they showed blue or violet red color. After optimization, its response time was less than 50 ms, the number of redox circulation was over 107 and the color of coloration states could be kept for 3 days. This kind of EC device can meet the demand of electronic ink.

Boosting Transport Kinetics of Ions and Electrons Simultaneously by Ti3C2Tx(MXene)Addition for Enhanced Electrochromic Performance

Electrochromic technology plays a significant role in energy conservation,while its performance is greatly limited by the transport behavior of ions and electrons.Hence,an electrochromic system with overall excellent performances still need to be explored.Initially motivated by the high ionic and electronic conductivity of transition metal carbide or nitride(MXene),we design a feasible procedure to synthesize the MXene/WO3−x composite electrochromic film.The consequently boosted electrochromic performances prove that the addition of MXene is an effective strategy for simultaneously enhancing electrons and ions transport behavior in electrochromic layer.The MXene/WO3−x electrochromic device exhibits enhanced transmittance modulation and coloration efficiency(60.4%,69.1 cm^2 C^−1),higher diffusion coefficient of Li+and excellent cycling stability(200 cycles)over the pure WO3−x device.Meanwhile,numerical stimulation theoretically explores the mechanism and kinetics of the lithium ion diffusion,and proves the spatial and time distributions of higher Li+concentration in MXene/WO3−x composite electrochromic layer.Both experiments and theoretical data reveal that the addition of MXene is effective to promote the transport kinetics of ions and electrons simultaneously and thus realizing a high-performance electrochromic device.This work opens new avenues for electrochromic materials design and deepens the study of kinetics mechanism of ion diffusion in electrochromic devices.

Correlation between lithium storage and diffusion properties and electrochromic characteristics of WO_3 thin films

As essential electrochromic（EC） materials are related to energy savings in fenestration technology,tungsten oxide（WO3） films have been intensively studied recently.In order to achieve better understanding of the mechanism of EC properties,and thus facilitate optimization of device performance,clarification of the correlation between cation storage and transfer properties and the coloration performance is needed.In this study,transparent polycrystalline and amorphous WO3 thin films were deposited on SnO2：F-coated glass substrates by the pulsed laser deposition technique.Investigation into optical transmittance in a wavelength range of 400-800 nm measured at a current density of 130 μA·cm-2 with the applied potential ranging from 3.2 to 2.2 V indicates that polycrystalline films have a larger optical modulation of ～ 30% at 600 nm and a larger coloration switch time of 95 s in the whole wavelength range compared with amorphous films（～ 24% and 50 s）.Meanwhile,under the same conditions,polycrystalline films show a larger lithium storage capacity corresponding to a Li/W ratio of 0.5,a smaller lithium diffusion coefficient（2×10-12cm2·s-1 for Li/W=0.24） compared with the amorphous ones,which have a Li/W ratio of 0.29 and a coefficient of ～2.5×10-11cm2·s-1 as Li/W=0.24.These results demonstrate that the large optical modulation relates to the large lithium storage capacity,and the fast coloration transition is associated with fast lithium diffusion.

Electrochromic Properties of Sputtered Ti-Doped WOa Films

Ti-doped WO3 films were prepared by the mid-frequency dual-target magnetron sputtering method. The structure and electrochromic properties of the Ti-doped WO3 films were analysed by X-Ray diffraction （XRD）, Raman spectroscopy, spectrophotometer, cyclic chronoam- perometry and atomic force microscopy （AFM）. The results indicate that the erystallinity decrease after the doping of titanium, the channels for ion injection and extraction increase, the responding speed with 5.1% titanium doped becomes faster, and its circle life increases more than four times compared with the undoped WO3 film. In the coloured state, the W-O-W bonds decrease, but the W = O bonds increase. Since the W-O-W bonds break down for Li＋ ions＇ injection and more W = O bonds form, it is more convenient to inject Li＋ ions into the Ti-doped film than undoped film because more W-O-W bonds break down in the coloured state.

Influence of Different Substrates on the Electrochromic Property of Polyaniline Films

The polyaniline （PANI） films doped with complex acid （sulfuric acid and sulfosalicylic acid） were prepared using the potentiostatic method on bare nickel flake （NF） and flexible polyethylene terephthalate （PET）/indium tin oxide （ITO） substrates. The contents of the PANI films,surface elements, electrochromic property and electrical conductivity were characterized by energy dispersive X-ray spectrometer （EDS）, cyclic voltammetry （CV） and electrochemical impedance spectroscopy （EIS）. The experimental results show that differences exist among cycle stability, redox reversibility and response time of polyaniline films on these two kinds of substrates, but the electrochromic phenomenon of the PANI films is in substantial agreement. The equilibrium transmittance spectra in the visible region （400-800 nm） for the PANI film on flexible PET/ITO substrate was obtained at different applied potential from -0.4 to 1.5 V. The results show that the transmittance of the PANI film by applying voltage is adjustable in a row and has excellent electrochromic performance.