Soft matter electrolytes based on polymethylmetacrylate dispersions in lithium bis(trifluoromethanesulfonyl)imide/1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids

Ion transport in a polymer-ionic liquid (IL) soft matter composite electrolyte is discussed here in detail in the context of polymer-ionic liquid interaction and glass transition temperature. The dispersion of polymethylmetacrylate (PMMA) in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF₆) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI) resulted in transparent composite electrolytes with a “jelly-like” consistency. The composite ionic conductivity measured over the range −30 °C to 60 °C was always lower than that of the neat BMITFSI/BMIPF₆ and LiTFSI-BMITFSI/LiTFSI-BMIPF₆ electrolytes but still very high (>1 mS/cm at 25 °C up to 50 wt% PMMA). While addition of LiTFSI to IL does not influence the glass T_g and T_m melting temperature significantly, dispersion of PMMA (especially at higher contents) resulted in increase in T_g and disappearance of T_m. In general, the profile of temperature-dependent ionic conductivity could be fitted to Vogel-Tamman-Fulcher (VTF) suggesting a solvent assisted ion transport. However, for higher PMMA concentration sharp demarcation of temperature regimes between thermally activated and solvent assisted ion transport were observed with the glass transition temperature acting as the reference point for transformation from one form of transport mechanism to the other. Because of the beneficial physico-chemical properties and interesting ion transport mechanism, we envisage the present soft matter electrolytes to be promising for application in electrochromic devices.     

Influence of side chains on electrochromic properties of green donor–acceptor–donor polymers

Three solution processable cathodically coloring green electrochromic polymers, based on 2,3-diphenyl-5,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine, have been synthesized by oxidative FeCl₃ polymerization. The polymers were designed with solubilizing alkyl and oligoethylene oxide side chains to achieve solubility and processability. All three polymers have a small electrochemical bandgap (1.8–1.9 eV) and low oxidation potentials. Spectroelectrochemical studies of polymer films on ITO reveal that the alkyl side chains in head-to-head position on the polymer backbone promote a defined high-energy absorption peak and suppress tailing of charge-carrier absorption into the visible region. Kinetic studies, based on transmission measurements applying a square-wave potential between reduced and oxidized states, show that the polymer with exclusively oligoethylene oxide side chains (P3) had the fastest response times, monitored at the low-energy absorption maxima. The best performing polymer (P1) showed a good optical contrast in the visible region with a ΔT of 26% at 700 nm. An initial test of the electrochemical stability showed that the oligoethylene oxide containing polymers had superior stability over 500 full switches.     

Electrochromic thin films prepared by sol–gel process

New methods are shown for lower temperature preparation of amorphous tungsten oxide thin film and preparation of crystalline iridium oxide thin film by sol–gel process using metal chloride as the starting materials and ethanol as a solvent. These electrochromic materials were combined with gel solid electrolyte, and preparation of fully solid-state electrochromic display (ECD) was made. The transmittance of the ECD could be made to change by 35% by applying a voltage of 3 V for 0.2 sec.     

Improvement of thermal stability of an organic-aqueous gel electrolyte for bismuth electrodeposition devices

The purpose of this work is to study bismuth deposition/dissolution behavior in an aqueous electrolytic gel media, aiming its application in electrochromism. The gel was made from a polymeric animal protein derivative. This polymer is extremely consistent and it becomes a transparent gel in the 350–850 nm range (visible region). The cross-linking reaction of the gel was made with formaldehyde, targeting to improve the thermal stability of the polymeric matrix without considerable loss of optical properties during electrodeposition. Differential scanning calorimetry and thermo-mechanical analysis were used to verify the transition temperatures of the gel, showing that the cross-linked material can be heated up to 90°C without great changes in their mechanical properties. Voltammetric and chromogenic experiments carried out with the electrochromic device using the gel as electrolyte, were very promising and the electrochemical reversibility of Bi/Bi³⁺ redox couple is not affected by the cross-linking reaction showing a large variation of absorbance values.     

Electrostatic layer-by-layer and electrophoretic depositions as methods for electrochromic nanoparticle immobilization

The present paper describes the immobilization of nanoparticles onto conducting substrates by using both electrostatic layer-by-layer and electrophoretic deposition (EPD) methods. These two techniques were compared in high-performance electrochromic electrodes based on mixed nickel hydroxide nanoparticles. In addition to easy handling, EPD seems to be the most suitable method for the immobilization of nanoparticles, leading to higher electrochromic efficiencies, lower response times and higher stability upon coloration and bleaching cycling.     

Electrocolorimetry of electrochromic materials on flexible ITO electrodes

Electrochromic materials are characterized by their colour changes upon applied voltage. Colour can mean many things: a certain kind of light, its effect on the human eye, or the result of this effect in the mind of the viewer. Since the electrochromic materials are developed towards real life applications it is relevant to characterize them with the usual commercial colour standards. A colorimetric study of electrogenerated Prussian blue and electrogenerated polymers based on salen-type complexes of Cu(II), Ni(II) and Pd(II) deposited over transparent flexible electrodes of polyethylene terephthalate coated with indium tin oxide (PET/ITO electrodes) was carried out using the CIELAB coordinates. A cuvette with a designed adapter to allow potentiostatic control was placed on an integrating sphere installed in the sample compartment of a spectrophotometer to run the colorimetric measurements. The colour evolution in situ was measured through the transmittance of the films by potentiostatic control. Chronocoulometry/chronoabsorptometry was used to evaluate maximum coloration efficiencies for the coloration step: 184 (Pd), 161 (Cu) and 83 cm²/C (Ni) and for bleaching: 199 (Pd), 212 (Cu) and 173 cm²/C (Ni) of the Pd, Cu and Ni polymer films, respectively. The Prussian Blue/Prussian White states over the PET/ITO films were relatively reversible while the reversibility and stability of the polymers based on the metals salen-type complexes depends on the metal, Pd being the most stable.     

Design, production and characterisation of an all solid state electrochromic medium size device

An electrochromic device based on a five layer coating deposited on a glass sheet was produced. The layers were all obtained by physical vapour deposition in the same vacuum environment, using a commercial apparatus capable of producing thin films on large area substrates. Small (50×50 mm²) and medium (300×300 mm²) size specimens were prepared for preliminary study, to establish the electrochemical and optical performances of the device. Electrochemical measurements were performed on each active material, as well as on the whole specimens, in order to relate electrochemical properties to process conditions. Also, luminous and solar parameters were obtained from (near) normal incidence spectrophotometry in both the bleached and coloured states, to confirm the values used in the design phase of the electrochromic device. One of the specimens was also submitted to variable angle photometric and radiometric measurements, using an integrating sphere. This data was then used to evaluate the room luminance distribution and energy loads using a building simulation code.     

Ionic liquids in electrochromic devices

The ionic liquid (PYR₁₄TFSI) has proved to be the key material to make a Li-ion conducting element of a complete electrochromic device, when interposed between transparent film electrodes like WO₃ and Li-charged V₂O₅. The key features of this ionic liquid and its mixtures with LiTFSI are the excellent transparency in the visible and NIR optical regions, the good ionic conductivity and the electrochemical compatibility with inorganic Li-intercalation oxide thin film electrodes used in electrochromic devices. The higher optical contrast found during WO₃ colouration with PYR₁₄TFSI-LiTFSI, compared to that in a conventional non-aqueous electrolyte like PC-LiTFSI, was attributed to the larger inertness of the former one (no decomposition reaction at the lowest electrode potential). This highly conductive ionic liquid has been incorporated into a polymer matrix (P(EO)₁₀LiTFSI), in order to obtain a transparent solid electrolyte with high Li ion conductivity and good mechanical stability. Finally this solid PYR₁₄TFSI-P(EO)₁₀LiTFSI transparent ion conductor was interposed between the same electrodes as above in order to yield a fully solid-state, Li-ion electrochromic device. This new solid electrolyte was able to transfer reversibly a Li ionic charge between 5 mC cm⁻² and 10 mC cm⁻² from the lithium storage electrode Li_xV₂O₅ to the WO₃ electrochromic electrode in less than 100 s at room T, darkening the device from an initial 80% to a final 30% transmittance (at 650 nm). Such a device has been tested first under various constant current conditions, and later under potentiostatic control using ±2 V steps. The latter method allows not only for a faster response of the electrochromic system, but provides also an easier life stability test of the device, which withstood 2000 cycles with little changes in its optical contrast.     

Electrochromic multilayer films based on trilacunary Dawson-type polyoxometalate

A new ultrathin multilayer film consisting of polyoxometalate (POM) cluster Na₁₂[P₂W₁₅O₅₆]·18H₂O (P₂W₁₅) and polyallylamine hydrochloride (PAH) were fabricated on quartz and ITO substrates by layer-by-layer self-assembly method. The multilayer films, PSS/PAH/(P₂W₁₅/PAH)_n, were characterized by UV-vis spectra, cyclic voltammetry (CV) and chronoamperometric (CA) measurements. The electrochromic film exhibits both suitable response time and low power consumption due to the presence of lacunary polyoxometalate, providing valuable information for exploring the potential application of polyoxometalate-based electrochromic materials.     

Electrochemical investigations on spray deposited tin oxide thin films

Tin oxide (SnO₂) thin films were prepared by a simple and inexpensive spray pyrolysis technique from an aqueous solution at various substrate temperatures viz. 300, 400 and 500 °C, and their electrochemical studies have been carried out. The thin films have been optically and electrochemically characterized by means of transmittance, cyclic voltammetry and chronoamperometry. The mechanism of reduction and oxidation reactions that took place during the potential cycling is presented. The samples deposited at 500 °C exhibit better performance in terms of coloration efficiency, reversibility, contrast ratio and response time.