Alternating Current‐Driven Electrochromic Devices Based on Benzyl Viologen: AC Frequency Indicator

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.

Effect of trifluoromethyl substituents in benzyl-based viologen on the electrochromic performance: Optical contrast and stability

Colorless-to-black electrochromic materials and solid-state devices with high optical contrast based on cross-linked Poly(4-vinyltriphenylamine)

Colorless-to-black electrochromic devices based on ambipolar electrochromic system consisting of cross-linked poly(4-vinyltriphenylamine) and tungsten trioxide with high optical contrast in visible and near-infrared regions

As a continuing effort to understand the mechanisms of alternating current (AC) transdermal iontophoresis and the iontophoretic transport pathways in the stratum corneum (SC), the objectives of the present study were to determine the interplay of AC frequency, AC voltage, and iontophoretic transport of ionic and neutral permeants across human epidermal membrane (HEM) and use AC as a means to characterize the transport pathways. Constant AC voltage iontophoresis experiments were conducted with HEM in 0.10 M tetraethyl ammonium pivalate (TEAP). AC frequencies ranging from 0.0001 to 25 Hz and AC applied voltages of 0.5 and 2.5 V were investigated. Tetraethyl ammonium (TEA) and arabinose (ARA) were the ionic and neutral model permeants, respectively. In data analysis, the logarithm of the permeability coefficients of HEM for the model permeants was plotted against the logarithm of the HEM electrical resistance for each AC condition. As expected, linear correlations between the logarithms of permeability coefficients and the logarithms of resistances of HEM were observed, and the permeability data were first normalized and then compared at the same HEM electrical resistance using these correlations. Transport enhancement of the ionic permeant was significantly larger than that of the neutral permeant during AC iontophoresis. The fluxes of the ionic permeant during AC iontophoresis of 2.5 V in the frequency range from 5 to 1,000 Hz were relatively constant and were approximately 4 times over those of passive transport. When the AC frequency decreased from 5 to 0.001 Hz at 2.5 V, flux enhancement increased to around 50 times over passive transport. While the AC frequency for achieving the full effect of iontophoretic enhancement at low AC frequency was lower than anticipated, the frequency for approaching passive diffusion transport at high frequency was higher than expected from the HEM morphology. These observations are consistent with a transport model of multiple barriers in series and the previous hypothesis that the iontophoresis pathways across HEM under AC behave like a series of reservoirs interconnected by short pore pathways.

Viologens and extended viologen derivatives with mono- and di-hexyl substituents for highly stable all-in-one ECDs and solar cell powered large-area ECDs

Lab-on-a-chip (LOC) and microfluidic devices have gained more and more importance in biological and chemical fields. A homogeneous mix of multiple reagents and chemicals is often essential to assist the chemical and biological reactions. Electroosmotic flow is an attractive approach for enhancing the homogeneous mix of species in a micro-scale mixer. In this work, a two-dimensional microfluidic mixture with a diamond-shaped split-and-recombine structure, using commercial software package COMSOL Multiphysics is analyzed. The choice of suitable electrode length is made by performing a series of simulations. The influence of AC (alternating current) frequency on the mixing of two fluids is also studied. The mixing efficiency of the micromixer initially increases with an increase in AC frequency and after reaching a maximum value, it starts decreasing. The best suited AC potential frequency for different electrode lengths is different. It is found from the results that the increase in electrode length does not always increase the mixing efficiency of the micromixer. The electrode length of such mixers critically affects the mixing efficiency and the suitable electrode length results in improved mixing of fluids. KeywordsMicromixerElectroosmosisMixing efficiencyElectrode length

Electrochromic devices (ECDs) show reversible color changes on applying external voltages by electrochemical redox reactions. Most ECDs are fabricated using ITO-coated electrodes on glass substrates. With a growing interest in wearable devices, many attempts have been made concerning flexible ECDs. Compared to the conventional devices, the ECDs fabricated on the textile substrate feature highly conformal and curvilinear properties similar to inherent textile aspects. In line with the demand, this study presents the freely deformable and highly durable electrochromic fabric devices (ECFDs) prepared by the spray-coating process. The ECFDs are constructed using a layer-by-layer structure on polyester fabric, consisting of electrodes, electrochromic, electrolyte, and protective layers. Conducting electrodes are designed with the mixture of silver nanowire and PEDOT:PSS, and, on top of it, viologen functionalized polyhedral oligomeric silsesquioxane is subsequently stacked. The additional electrolyte layer is placed for surface hardening and ion transporting purposes. The protection is formed in the outer layer, providing remarkable waterproofness and laundry fastness. The prepared ECFDs reveal reversible and repeated electrochromic performances, along with bending and twisting stability, waterproofness, and washing fastness. The current approach demonstrates the feasibility of color-changing textiles for real-life applications.

Compared to the significant effort dedicated toward developing efficient electrochromic materials for the working electrodes of electrochromic (EC) devices, the attention paid to developing ion storage counter electrode materials for EC devices has been trivial. Herein, we report that a macroporous crystalline V2O5 film as an ion storage layer paired with a WO3 working electrode results in an EC device with high performance. The macroporous vanadium oxide films are prepared by a simple template-free photodeposition method that allows us to tune the thickness and crystallinity of the film, thus giving access to a full EC device with optimal EC performance: short response time of about 2 s, high electrochromic cycling stability up to 10,000 times, long memory effect over 24 h, and an exceedingly high coloration efficiency of 189 cm2/C that are superior to the state-of-the-art performance of solution-processed vanadium oxide based EC devices. The extraordinary EC performance can be attributed to the macroporous structure, high crystallinity, and optimized thickness of the vanadium oxide films that boost the charge-balancing capability of the films. The easy and controllable preparation and the efficient charge-balancing capability of the macroporous vanadium oxide film make it a promising ion storage material for developing high-performance EC devices.

A novel bis(dihydroxypropyl) viologen-based all-in-one electrochromic device with high cycling stability and coloration efficiency

Electrochromic devices are applied extensively to camouflages, smart windows, heat insulation layers, and automobile rearview mirrors, etc. The amorphous WO3 is a very attractive electrochromic material, whereas it suffers from degradation of optical modulation and reversibility on ion exchange owing to those deep trapped ions with irreversible reaction behavior. Herein, we designed and, by using magnetron sputtering, prepared a composite film with TiO2/WO3/TiO2 double heterojunctions, which is capable of eliminating the deep trapped ions by itself under ultraviolet light (UV) assistance. The electrochromic device based on this composite film, after being recovery by short-time UV irradiation, can maintain a high transmission modulation of 94.72% after 7000 cycles of the voltammetry measurement. This feature allows the device to maintain its initial electrochromic performance after prolonged use. Moreover, the double heterojunction structure can reduce colouring time and enormously improve the colouration efficiency (CE) of electrochromic devices. Experimental research shows that when the thickness of the bottom and upper TiO2 layer of the WO3 film was 145.5 nm and 97.0 nm, respectively, the CE of electrochromic devices reached a perfectly high value (479.3 cm2/C), being much higher than that of WO3 devices (69.5 cm2/C). Functions of the TiO2/WO3/TiO2 double heterojunction in electrochromic device were investigated by combining theoretical analysis and experiment validation, and these results provide a general framework for developing and designing superior electrochromic materials and devices.

Cyanophenol and benzyl phosphonic acid grafted electrodes for poly(butyl viologen) thin film-based electrochromic device

Zinc polyacrylamide hydrogel electrolyte for quasi-solid-state electrochromic devices with low-temperature tolerance

E-skins consisting of soft pressure sensors are enabling technology for soft robots, bio-integrated devices, and deformable touch panels. A well-known bottleneck of capacitive pressure sensors (CPS) is the drastic decay in sensitivity with increasing pressure. To overcome this challenge, we have invented a hybrid-response pressure sensor (HRPS) that exhibits both the piezoresistive and piezocapacitive effects intrinsic to a highly porous nanocomposite (PNC) with carbon nanotube (CNT) dopants. The HRPS is constructed with two conductive electrodes sandwiching a laminated PNC and a stiff dielectric layer. We have simplified the hybrid response into a parallel resistor-capacitor circuit, whose output depends on the AC (alternating current) frequency used for the capacitance measurement. Herein, through theoretical analysis, we discover a dimensionless parameter that governs the frequency responses of the HRPS. The master curve is validated through experiments on the HRPS with various doping ratios, subject to different compressive strains, under diverse AC frequencies. In addition, the relative contribution of piezoresistive and piezocapacitive mechanisms are also found to vary with the three parameters. Based on this experimentally validated theory, we establish a very practical guideline for selecting the optimal AC frequency for the capacitance measurement of HRPSs.

Utilizing the in situ method, we report the fabrication of flexible electrochromic (EC) devices in a one-step lamination procedure. In this study, electrochromic device performance was enhanced via the use of new gel polymer electrolyte (GPE) materials based on poly(ethylene glycol) (PEG) derivatives. PEG serves as the polymer matrix in electrochromic devices (ECDs) that provides not only mechanical stability, but also a wide potential window and compatibility with a variety of salts. Poly(ethylene glycol) dimethacrylate (PEGDMA) in conjunction with poly(ethylene glycol) methyl ether acrylate (PEGMA), containing lithium trifluoromethanesulfonate (LiTRIF) as the salt and propylene carbonate (PC) as a plasticizer; we investigated various electrolyte parameters, including salt loading, the mono/di-functional PEG ratio, and the plasticizer to PEG ratio. Optimized gel systems exceed the mechanical flexibility of indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrates in their sustainable minimum bending radius of curvature, exhibit an ionic conductivity up to 1.36 × 10−3 S cm−1, and yield electrochromic devices (ECDs) with photopic contrasts as high as 53% (without background correction) using poly(2,2-dimethyl-3,4-propylenedioxythiophene) (PProDOT-Me2) as the standard electrochromic material. In addition to ionic conductivity, the crosslink density of the GPEs was found to have an important effect on the photopic contrast of the resultant ECDs. Using these results, 110 cm2 flexible patterned EC displays were assembled as a demonstration of their potential in real world applications.