Li ion exchange in CeO2-TiO2 sol-gel layers studied by electrochemical quartz crystal microbalance

The paper reports first on the electrochemical behavior in liquid Li⁺ electrolytes of 200 nm thick single sol-gel (CeO₂)_0.81-TiO₂ electrochromic (EC) layers deposited by the dip-coating process. The electrolytes were solutions of 1 M LiClO₄ dissolved in dry propylene carbonate (PC) (containing 0.03 wt% of water) and wet PC containing up to 10 wt% of water, respectively. Then an electrochemical quartz crystal microbalance was used as a sensitive detector to analyze the mass changes occurring during the Li⁺ ion exchange processes. These electrochemical processes were studied for 370 nm thick double layers, deposited on gold-coated quartz crystal electrodes and sintered at 450 °C in air. The electrolytes were the same solutions with water content varying from 0.03 up to 3 wt% of water. The processes have been studied in the potential range from −2.0 to +1.0 V vs. Ag/AgClO₄ during 100 voltammetry cycles. The composition of the (CeO₂)_0.81-TiO₂ layers was found to change during the early cycles, mainly because of an irreversible Li⁺ intercalation. It was found, however, that the mass change observed during cycling is not due only to a pure Li⁺ ion exchange process but also involves the adsorption/desorption or exchange of other cations and anions contained in the electrolyte. These ions are Li⁺ and ClO₄⁻ in dry electrolyte and Li⁺, hydrated Li(H₂O)_n⁺ and ClO₄⁻ in wet electrolyte. The improvement of the reversibility of the intercalation and deintercalation processes as well as the faster kinetics observed in wet electrolytes are finally discussed in terms of a model in which the formation of hydrated Li⁺ ions takes an important role.     

Electrosynthesis and characterization of a conductive polythiophene deriving from a terthiophene monomer

The electrochemical polymerization of the 3,3″-di[(S)-(+)-2-methylbutyl]-2,2′:5′,2″-terthiophene (DMBTT) monomer was carried out potentiodynamically on indium tin oxide (ITO), Pt or glassy carbon (GC) electrodes in anhydrous CH₃CN with TBAPF₆ as supporting electrolyte. Films with good adhesion were obtained for all the tested substrates and they were characterized by various analytical techniques including UV–Vis–NIR absorption spectroscopy, cyclic voltammetry (CV), scanning electron microscopy (SEM), and gel permeation chromatography (GPC). The most outstanding properties of the electrochemically synthesized polymer were compared with those of the same polymer obtained by a chemical method. The film obtained on ITO displayed a reversible electrochromic behavior under potential switching between −0.50 and +1.00 V vs. SCE.     

Electrochromic properties of porous NiO thin film as a counter electrode for NiO/WO3 complementary electrochromic window

Highly porous nickel oxide (NiO) thin films were prepared on ITO glass by chemical bath deposition (CBD) method. SEM results show that the as-deposited NiO film is constructed by many interconnected nanoflakes with a thickness of about 20 nm. The electrochromic properties of the NiO film were investigated in a nonaqueous LiClO₄–PC electrolyte by means of optical transmittance, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The NiO film exhibits a noticeable electrochromic performance with a variation of transmittance up to 38.6% at 550 nm. The CV and EIS measurements reveal that the NiO film has high electrochemical reaction activity and reversibility due to its highly porous structure. The electrochromic (EC) window based on complementary WO₃/NiO structure shows an optical modulation of 83.7% at 550 nm, much higher than that of single WO₃ film (65.5% at 550 nm). The response time of the EC widow is found to be about 1.76 s for coloration and 1.54 s for bleaching, respectively. These advantages such as large optical modulation, fast switch speed and excellent cycle durability make it attractive for a practical application.     

Effects on the electrochemical and electrochromic properties of 3,6 linked polycarbazole derivative by the introduction of different acceptor groups and copolymerization

Two novel donor–acceptor type monomers, ethyl 4-(3,6-di(thiophen-2-yl)-9H-carbazole-9-yl)-benzoate (ETCB) and 9-(4-nitrophenyl)-3,6-di(thiophen-2-yl)-9H-carbazole (NDTC), were synthesized and characterized. Both the monomers show good electrochemical activity. UV–vis absorption studies reveal that the spectra of them are obviously different due to the introduction of the acceptor groups with different polarity, and the compound with –NO₂ group has lower band gap. Fluorescent spectral studies indicate that the solution of ETCB in DCM exhibits sky-blue emission, while the NDTC hardly displays the fluorescence because of stronger intramolecular charge transfer. Their polymers can be synthesized by electropolymerization, and both the films show well-defined oxidation and reduction process. Spectroelectrochemical analysis reveals that PETCB film displays the color change from yellow–green (neutral) to blue–purple (oxidized), while the color change of PNDTC film is from yellow (neutral) to gray (oxidized). Both the polymer films exhibit reasonable optical contrast and switching time. Moreover, the copolymer based on ETCB and 3,4-ethylenedioxythiophene (EDOT) is also investigated. The copolymer could show five colors change under different applied potentials and higher optical contrast (50% of 1100 nm) and coloration efficiency (356.88 cm² C^?1).     

A simplified all-polymer flexible electrochromic device

New all-polymeric simplified electrochromic devices have been prepared based in an intrinsically conductive polymer, poly(ethylene dioxythiophene) (PEDOT). In these devices PEDOT acts simultaneously as electrochromic layer and current collector layer simplifying the construction of the classic devices from seven to five layers. The device presents a chromatic contrast in all the visible range with a maximum at 650 nm (ΔT=0.15) between 0 and 3 V. Representative bleaching and coloring times are 20 and 16 s, respectively, for  cm devices. The originality of this work is that advanced electrochromic devices can be constructed using commercially available materials and using simple experimental methods.     

Fabrication of TiO2/Au nanorod arrays employing a positive sacrificial ZnO template and their electrochromic property

A novel method to fabricate large scale TiO₂/Au nanorod array using a positive sacrificial ZnO template has been developed. This method includes a two-step process, (1) preparation of ZnO/Au nanorod array by a simple low-temperature hydrothermal process, and (2) preparation of TiO₂/Au nanorod array by electrochemically induced sol-gel process. The TiO₂/Au nanorod array has showed a reversible electrochromism in lithium-ion-containing organic electrolyte. The coloration and bleaching throughout a visible range can be switched on and off within a few seconds.     

Influencing factors on electrochromic hysteresis performance of ruthenium purple produced by a WO3/Tris(2,2′-bipyridine)ruthenium(II)/polymer hybrid film

A [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine)/WO₃ hybrid (denoted as Ru-WO₃) film was prepared as a base layer on an indium tin oxide electrode by electrodeposition from a colloidal solution containing peroxotungstic acid, [Ru(bpy)₃]²⁺ and poly(sodium 4-styrenesulfonate). A ruthenium purple (RP, Fe^III₄[Ru^II(CN)₆]₃, denoted as Fe^III-Ru^II) layer was electrodeposited on a neat WO₃ film or a Ru-WO₃ film from an aqueous RP colloid solution to yield a WO₃/RP bilayer film or a Ru-WO₃/RP bilayer film, respectively. The spectrocyclic voltammetry measurement reveals that Fe^II-Ru^II is oxidized to Fe^III-Ru^II by a geared reaction of [Ru(bpy)₃]^2+/3+ and Fe^III-Ru^II is reduced by a geared reaction of H_xWO₃/WO₃ in the Ru-WO₃/RP film. These geared reactions produced electrochromic hysteresis of the RP layer. However, the absorbance change in the hysteresis was smaller than that for the Ru-WO₃/Prussian blue bilayer film reported previously, resulting from the lower electroactivities of any redox component for the Ru-WO₃/RP film. The lower electroactivities could be explained by the specific interface between the Ru-WO₃ and RP layers. It might contribute to either an increase of the interfacial resistance between the Ru-WO₃ and RP layers, or formation of the physically precise interface between the layers to make it difficult for counter ions to be transported in the interfacial liquid phase involved in the redox reactions in the film. The specific interface at the Ru-WO₃ and RP layers could be formed possibly by the electrostatic interaction between [Ru(bpy)₃]²⁺ and terminal [Ru(CN)₆]⁴⁻ moieties of RP. It could be suggested by the decreased redox potential of [Ru(bpy)₃]²⁺ in the Ru-WO₃ layer from 1.03 to 0.61 V by formation of the RP layer.     

Electrochemical formation of Ir oxide/polyaniline composite films

The primary goal of this work has been to electrochemically form and then characterize a composite polyaniline (PANI)/hydrous Ir oxide (IrOx) film. Efforts to electrochemically form IrOx and PANI simultaneously in acidic aniline-containing solutions failed, likely as aniline adsorption on Ir prevents IrOx formation. Successful composite films were therefore made by first forming an anodic IrOx film on bulk Ir and then depositing PANI into its pores. Based on the characteristics of the PANI redox peaks, it is seen that all of the PANI film that is electrochemically active is in direct electrical contact with the Ir surface at the base of the IrOx film pores. This is consistent with the cross-sectional SEM and EDX analyses, showing the formation of films of uniform thickness and composition. Thin films of Ir nanoparticles, subsequently converted to IrOx, were also used as a template for PANI formation within the porous structure. These hybrid films exhibit an enhanced internal porosity, ease of multiple coating formation (up to 20 μm in thickness), high charge densities, unusual electrochromic behavior, and very rapid charge transfer kinetics. The formation of composite IrOx/PANI films also resulted in a widening (by 0.3-0.4 V) of the potential window over which a pseudocapacitive and electrochromic response is seen.     

Photopatterned electrochromic conjugated polymer films via precursor approach

Herein we report the photolithography of electrochromic conjugated polymer (CP) films on the micron scale without exposing the CP to high energy UV radiation. The synthesis of polynorbornene-based precursor copolymers having units with pendant terthiophenes and photocrosslinkable units allows for photopatterning at an earlier stage with respect to the polymerization (chemically or electrochemically) that yields the conducting polymer. The effect that the composition of the photocrosslinkable unit has on the overall process was studied, showing no effect on the electrochemical and optical performance of the conducting polymer. Electrochromic photopatterned structures down to 1 μm were obtained, together with some basic structures for microelectronics. This technique does not have any specific substrate restrictions, and can be used to pattern conducting polymers on flexible, rigid, conducting, or insulating substrates using the present photolithography facilities available to industry and academia.     

Novel electrochromic devices based on complementary nanocrystalline TiO2 and WO3 thin films

Electrochromic devices were elaborated based on two complementary electrodes made of a nanocrystalline metal oxide thin film deposited on conducting glass. The first electrode holds a 5 μm thick nanocrystalline TiO₂ film derivatized by a monolayer of a phosphonated triarylamine which can be rapidly oxidized by electron transfer to the conducting support followed by charge percolation inside the monolayer. The oxidation in accompanied by a blue coloration due to the absorption band at 730 nm of the stable triarylamminum radical cation. The second electrode bears a 0.2 μm thick nanocrystalline WO₃ film which turns from colorless to blue by reduction and lithium ion insertion. The former electrode reaches an absorbance of at least 3 between 700 and 730 nm after full oxidation (16 mC/cm²) at 1.0 V vs. NHE while for the second, complete reduction at −1.3 V (74 mC/cm²) leads to A=2.4 at 774 nm. An electrochromic device comprising both electrodes separated by an electrolytic solution of 0.1 Li⁺ in 4,7-dioxaoctanitrile reaches an absorbance of 2.2 at 700 nm, 4 s after a voltage step to 1.5 V. The system was shown to sustain at least 14400 coloration-discoloration cycles without degradation.