Counter Electrodes In Electrochromic Devices Research Articles

Fast-switching electrochromic smart windows based on WO3 doped NiO thin films

Nickel oxide (NiO) is a very promising material for the counter electrode in electrochromic devices (ECDs). In the current research, WO3 incorporated NiO thin films were fabricated via RF sputtering to enhance the electrochromic performance of NiO thin films. An XRD analysis revealed that the addition of WO3 to the cubic NiO thin film disrupts its long-range crystalline structure, resulting in an amorphous state. FESEM revealed the formation of columnar structure which favours ECDs. XPS analysis revealed that the addition of WO3 into NiO enhanced the hole concentration by creating nickel vacancies (nickel defects). Compared to pristine NiO, WO3 mixed NiO thin films showed enhanced optical modulation (66%) with rapid switching speed (0.6s/1.0s for bleaching/colouring). This is mainly because the defect- induced amorphous structure of the nickel tungsten oxide thin film can provide large number of diffusion channels which facilitates the rapid ion intercalation process, and can accommodate large number of ions, and hence enhances the electrochromic performance. This advancement might have a substantial impact on the development of nickel tungsten oxide as a potential counter electrode in high-performance energy saving smart windows.

Ultra-fast switching of energy efficient electrochromic nickel oxide thin films for smart window applications

In today's modern world, energy consumption continues to escalate. In such cases, smart windows play a crucial role in reducing energy consumption and enhancing the quality of life. Electrochromic devices (ECDs) -based smart windows rely profoundly on nickel oxide (NiO) thin films, which act as a counter electrode in ECDs. This work aims to fabricate NiO thin films, with the intention of achieving ultrafast ECD. Through the sputtering technique, energy-efficient ECD is obtained with the highest optical modulation of 60 % at a rapid switching speed of 0.55s for bleaching and 0.95s for coloration. We have also investigated the structural, morphological, vibrational, and optical properties of NiO thin films. XRD analysis revealed the less crystalline or near amorphous nature of NiO thin film. XPS, PL, and Raman studies confirm the existence of defects in the film. The favourable, less crystalline nature, along with the presence of defects, facilitates ultra-fast ion intercalation and de-intercalation process. We believe that prepared NiO film can be used as a promising anodic colourant in electrochromic smart windows with applications in energy-efficient buildings.

Influence of Hf doping on structural, morphological, optical and electrochemical properties of NiO thin films: Electrochromic application

In this study, the authors investigated the influence of hafnium doping on the properties of NiO thin films using the spray pyrolysis (SP) method. A comprehensive analysis was conducted to evaluate the structural, morphological, optical, and electrochemical characteristics of the deposited films. Techniques such as X-ray diffraction analysis (XRD), Raman spectroscopy, scanning electron microscopy (FESEM), energy dispersive analysis by X-ray (EDS), UV–Vis–NIR spectrophotometry, cyclic voltammetry (CV), and chronoamperometry (CA) were employed. XRD examination revealed that the NiO thin films exhibit a polycrystalline cubic phase, with a preferred orientation along the (111) plane. The successful integration of Hf ions into the NiO matrix was confirmed by the Raman spectrum. SEM images depicted the formation of dense and uniform nanograins on all film surfaces, free from any cracks. EDS analysis and elemental mapping indicated uniform and homogeneous distribution of Ni, O, and Hf elements across the films. The optical analysis suggested a decrease in the optical band gap energy from 3.54 eV to 3.49 eV with an increase in hafnium doping concentration. Electrochemical results revealed a continuous improvement in specific capacitance with Hf doping. The NHf6 sample reached 65 F. g−1 at 5 mV s−1, compared to 46 F. g−1 at 5 mV s−1 for the NHf0 sample. However, the NHf4 film exhibited the highest optical density variation (49%), shortest response times (tb = 3.58 s, tc = 4.67 s), and the largest coloration efficiency (63.39 cm2/C), indicating enhanced electrochromic performance at this Hf doping rate. These findings suggest that Hf-doped NiO holds promise as a candidate for use as a counter-electrode in electrochromic devices.

Mesoporous ITO Electrodes as Optically Passive Counter Electrodes for Electrochromic Devices

Mesoporous ITO Electrodes as Optically Passive Counter Electrodes for Electrochromic Devices

Advantageous optical characteristics of tantalum vanadium oxide as counter electrode in electrochromic devices

Smart windows are an important technology in terms of energy saving potential in the building sector due to their ability to control visible light and thermal radiation. The essential component of this type of window glazing is an electrochromic thin film. In addition to the widely established tungsten oxide as the optically active material, in particular the counter electrode offers significant potential for improving the overall device performance. In this study, tantalum vanadium oxide films are prepared by reactive radio-frequency sputtering on fluorine-doped tin oxide substrates and optimized in terms of their spectro-electrochemical properties as ion storage layer. We show that an oxide-based tantalum-vanadium alloy is a promising approach to address the open challenges of pure vanadium pentoxide. The coatings exhibit color neutrality in combination with a high transmittance of up to 80% in the as-prepared state and suitable optical transmittance switching. Additionally, we find both a sufficient stability upon cycling and a suitable charge density of about 35 mC cm−2. Thus, the presented oxide-based alloy offers a beneficial performance as an ion storage layer in electrochromic devices.

High-Performance Complementary Electrochromic Device Based on Iridium Oxide as a Counter Electrode.

In complementary electrochromic devices (ECDs), nickel oxide (NiO) is generally used as a counter electrode material for enhancing the coloration efficiency. However, an NiO film as a counter electrode in ECDs is susceptible to degradation upon prolonged electrochemical cycling, which leads to an insufficient device lifetime. In this study, a type of counter electrode iridium oxide (IrO2) layer was fabricated using vacuum cathodic arc plasma (CAP). We focused on the comparison of IrO2 and NiO deposited on a 5 × 5 cm2 indium tin oxide (ITO) glass substrate with various Ar/O2 gas-flow ratios (1/2, 1/2.5, and 1/3) in series. The optical performance of IrO2-ECD (glass/ITO/WO3/liquid electrolyte/IrO2/ITO/glass) was determined by optical transmittance modulation; ∆T = 50% (from Tbleaching (75%) to Tcoloring (25%)) at 633 nm was higher than that of NiO-ECD (ITO/NiO/liquid electrolyte/WO3/ITO) (∆T = 32%). Apart from this, the ECD device demonstrated a fast coloring time of 4.8 s, a bleaching time of 1.5 s, and good cycling durability, which remained at 50% transmittance modulation even after 1000 cycles. The fast time was associated with the IrO2 electrode and provided higher diffusion coefficients and a filamentary shape as an interface that facilitated the transfer of the Li ions into/out of the interface electrodes and the electrolyte. In our result of IrO2-ECD analyses, the higher optical transmittance modulation was useful for promoting electrochromic application to a cycle durability test as an alternative to NiO-ECD.

Films of directionally oriented carbon nanotubes as counter electrodes for electrochromic devices

Connected displays are the salient nature of our e-connected society requiring high-quality electronic conductive materials integrated in smart devices. Herein, we highlight the use of carbon nanotubes (CNTs) not only as substitutes for ITO (In2O3:Sn) but mainly as a counter electrode for electrochromic devices (ECDs). Using a dry spinning process, conductive CNT films with directionally oriented tubes were successfully laid on polyethylene terephthalate (PET) flexible substrates. The CNTs were synthesized as vertically aligned spinnable arrays by chemical vapor deposition. The transparency and optical absorption properties were tuned by adjusting the number of CNT layers (L) within the film from light gray for 1L to dark black above 10L. The CNT films, showing pseudo-capacitive to capacitive behavior in lithium-based electrolytes, were further integrated in ECDs based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) active layers. The fabricated PET/ITO/PEDOT:PSS/electrolyte/CNT/PET device exhibited reproducible and clear color switching from light to dark blue with a coloration efficiency of 88 cm2 C−1. This study highlights novel architecture of ECDs using CNT-based films as conductive counter electrode.

Data on characterization and electrochemical analysis of zinc oxide and tungsten trioxide as counter electrodes for electrochromic devices.

The data presented in this article are related to a research paper entitled “Implementation of High-Performance Electrochromic Device Based on All-Solution-Fabricated Prussian Blue and Tungsten Trioxide Thin Film”[1]. Zinc oxide nanowire (ZnNW) and tungsten trioxide (WO3) were fabricated by different electrodeposition methods and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and used as counter electrodes. The electrochromic (EC) properties of these devices were characterized using optoelectronic analysis during electrochemical applications.

Electrodeposition of Zinc Oxide Nanowires as a Counter Electrode in Electrochromic Devices

Electro deposited ZnO NW for charge storage layer of the electro chromic device.

Investigation of all-solid-state electrochromic devices with durability enhanced tungsten-doped nickel oxide as a counter electrode

Films of tungsten-doped nickel oxide, denoted Ni1-xWx oxide as counter electrode in electrochromic devices were deposited onto indium tin oxide coated glass substrates by reactive dc magnetron sputtering using tungsten-doped nickel alloy target. Electrochromic properties of NixW1-x oxides were evaluated in lithium perchlorate in propylene carbonate. Enhanced cycle stability with moderate optical modulation was obtained for Ni1-xWx oxide film with x = 0.024. Significant degradation in pure NiO film during extended electrochemical cycling caused by microstructural deep trap sites was minimized by W doping. The EC device fabricated with WO3 thin film and Ni1-xWx oxide thin film with x = 0.024 showed stable optical modulation of about 2% reduction from 100 to 1000 cycles. Flexible EC device was also fabricated with WO3 thin film and Ni1-xWx oxide thin film with x = 0.024 deposited on c-ITO/graphene/PET electrode and showed stable cycling performance with maintaining optical modulation of ΔT ≈ 40%.

High performance all-solid-state electrochromic device based on LixNiOy layer with gradient Li distribution

In the complementary electrochromic devices (ECDs), nickel oxide (NiOx) is the mostly studied material as counter electrode for its neutral brown color and relatively large coloration efficiency. However, there exist some obstacles that limit the utilization of NiOx counter electrode in ECDs due to the poor transparency in the bleached state and inadequate ion storage capacity. Here, a new type of counter electrode LixNiOy layers has been investigated, and LixNiOy-based all-solid-state inorganic ECD has been successfully fabricated by magnetron sputtering. The prepared ECD exhibits excellent electrochromic performance with an optical contrast of 72.8% at 550 nm, and desirable response time with 13 s and 3.5 s for coloring and bleaching process, respectively. Furthermore, the advantages of LixNiOy-based ECDs have been demonstrated due to the gradient distribution of Li element in the LixNiOy layer. The LixNiOy layer in the proposed ECD shows a Li-rich surface, which can provide more Li ions for the coloration process. Meanwhile, the bottom of LixNiOy layer is Li-poor, which can accommodate more ions during the bleaching reaction. This structure shows superiority over the traditional NiOx-based structure, as well as some other structures. Hence, our work guided a promising new structure for the electrochromic practical application and commercial mass production.

Electrochemical properties of thin films of V2O5 doped with TiO2

The paper presents a systematic study of the electrochromic properties of thin films of V2O5:TiO2 for a possible utilization as counter-electrode in electrochromic devices. The V2O5:TiO2 thin films were prepared by the sol-gel process and deposited on a substrate of fluorine-tin oxide transparent electrode (FTO) using the dip coating technique and heat treatment at 350 °C for 30 min. The films were characterized by chronocoulometry, cyclic voltammetry (CV), UV-Vis, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), profilometry, and X-ray diffraction (XRD). The best results were obtained for the film of V2O5 with 7.5 mol% of TiO2, which presented highest ion storage capacity of ∼106 mC cm−2 and redox reversibility of 1. The diffusion of the Li+ ions into the thin films was modeled by solving Fick equations with appropriate boundary conditions for a plane sheet geometry. Besides that, these films showed optical modulation of 35% at 633 nm after coloration and bleaching. The XRD patterns revealed that the films have an orthorhombic crystal structure; the AFM and the profilometry confirmed roughness and thickness of 16.76 and 617 nm, respectively.

Enhanced electrochromic performances and cycle stability of NiO-based thin films via Li–Ti co-doping prepared by sol–gel method

Nickel oxide (NiO) has great application prospects as a counter electrode in electrochromic devices, such as smart windows and displays. However, the cycle stability and electrochromic properties of pure NiO films are rather poor in lithium ion based non-aqueous electrolyte. To improve these properties, Li–Ti co-doped NiO films were prepared by sol–gel spin coating technique. Compared with undoped NiO films, Li and Ti doping deteriorate the symmetry of NiO cubic phase, increase lattice disorder and change the valence state of nickel in the films. The investigations of NiO-based films in LiClO4/PC electrolyte reveal enhancement of electrochromic properties and cycle stability with Li–Ti co-doping. Finally, electrochromic devices assembled with poly(3,4-(2,2-dimethylpropylenedioxy)thiophene) and Li–Ti–NiO films exhibit rapid color change within 1s, excellent optical variation for about 56% at 580nm and high stability up to 1,000,000 cycles.

High optical transparency in all redox states: Synthesis and characterization of benzimidazole and thieno[3,2-b]thiophene containing conjugated polymers

The synthesis of a novel monomer based on benzimidazole and thieno[3,2-b]thiophene moieties, 4′-tert-butyl-4,7-di(thieno[3,2-b]thiophen-2-yl)spiro[benzo[d]imidazole-2,1′-cyclohexane] (TTBI), and its electrochemical polymerization are reported. Electrochemical and spectroelectrochemical studies of the polymer were investigated. The high optical transparency and low coloration efficiency during oxidation and reduction processes make the polymer a great candidate for hole-injection layers in light-emitting diodes and photovoltaic devices as well as counter electrodes for electrochromic devices.

Effective utilization of spray pyrolyzed CeO 2 as optically passive counter electrode for enhancing optical modulation of WO 3

Nanocrystalline cerium oxide (CeO 2) thin films were deposited onto the fluorine doped tin oxide coated glass substrates using methanolic solution of cerium nitrate hexahydrate precursor by a simple spray pyrolysis technique. Thermal analysis of the precursor salt showed the onset of crystallization of CeO 2 at 300 °C. Therefore, cerium dioxide thin films were prepared at different deposition temperatures from 300 to 450 °C. Films were transparent ( T ~ 80%), polycrystalline with cubic fluorite crystal structure and having band gap energy (Eg) in the range of 3.04–3.6 eV. The different morphological features of the film obtained at various deposition temperatures had pronounced effect on the ion storage capacity (ISC) and electrochemical stability. The larger film thickness coupled with adequate degree of porosity of CeO 2 films prepared at 400 °C showed higher ion storage capacity of 20.6 mC cm − 2 in 0.5 M LiClO 4 + PC electrolyte. Such films were also electrochemically more stable than the other studied samples. The Ce 4+/Ce 3+ intervalancy charge transfer mechanism during the bleaching–lithiation of CeO 2 film was directly evidenced from X-ray photoelectron spectroscopy. The optically passive behavior of the CeO 2 film (prepared at 400 °C) is affirmed by its negligible transmission modulation upon Li + ion insertion/extraction, irrespective of the extent of Li + ion intercalation. The coloration efficiency of spray deposited tungsten oxide (WO 3) thin film is found to enhance from 47 to 53 cm 2 C − 1 when CeO 2 is coupled with WO 3 as a counter electrode in electrochromic device. Hence, CeO 2 can be a good candidate for optically passive counter electrode as an ion storage layer.

Technology and optoelectronic properties of APCVD Cr 2O 3 and Mo–Cr mixed oxide thin films

Chromium oxide films have been proposed to serve as counter electrode in electrochromic devices. Previously our studies revealed that APCVD Cr 2O 3 films exhibit cathodic electrochromic behaviour, so further technological developments in the structure and composition were carried out for studying their electrochromic (EC) properties. Thin Cr 2O 3 and MoO 3–Cr 2O 3 films were deposited by low-temperature carbonyl CVD process at atmospheric pressure. The deposition temperature of 200 °C was found as optimal since it leads to films with good adhesion to substrates. The chromium oxide films are less investigated, so original studying was necessary to find the correlation between the process parameters, crystal structure and optical quality of the films. The structural characterizations of the films made by FTIR spectroscopy were related to their electrochromic behaviour. Transmittance was measured by UV–VIS spectroscopy and the electrochromic properties were investigated by cyclic voltammetry using different type of electrolytes (type of ions). The results showed that the films have transmittance around 70% in the visible range which is a good initial optical quality for electrochromic applications.

Spray deposited titanium oxide thin films as passive counter electrodes

Titanium dioxide (TiO 2) thin films were deposited from methanolic solution onto fluorine doped tin oxide coated conducting glass substrates by spray pyrolysis technique. The electrochemical properties of TiO 2 thin films were investigated using cyclic voltammetry, chronoamperometry, chronocoulometry and iono-optical studies, in 0.1N H 2SO 4 electrolyte. Performance of the films deposited at three different substrate temperatures, viz. 350, 400 and 450 °C is discussed in view of their utilization in electrochromic devices, as counter electrode. The magnitude of charge storage capacity, Q/ t (4.75–6.13 × 10 −3 mC/(cm 2 nm)) and colouration efficiency (3.2–4.3 cm 2/mC) of TiO 2 rank these films among the promising counter electrodes in electrochromic devices.

Electrochromic degradation in nickel oxide thin film: A self-discharge and dissolution phenomenon

Nickel oxide thin films known as optical active counter electrode in electrochromic devices were grown by Pulsed Laser Deposition (PLD) at room temperature (RT) under a 10 −1 mbar oxygen pressure. From intense electrochemical characterizations of as-deposited NiO electrodes in alkaline medium, a mechanism taking into account the three typical steps of their cycling life, namely the activation period, the steady state and the degradation period, is proposed. The reversible color change from transparent to brownish, generally ascribed to the Ni(II)/Ni(III) couple in the literature, was clearly identified to the electrochemically active nickel hydroxide/oxy-hydroxide phases. The reason lies in a spontaneous chemical conversion of NiO into Ni(OH) 2 when the film is immersed in KOH medium. Afterwards, once cycling is performed, there is in competition with the electrochemical process, a self-discharge phenomenon associated to a partial dissolution of the thin-film oxidized phases. This second process becomes predominant on cycling leading to a progressive degradation of the electrochromic performances.

Covalent layer-by-layer type modification of electrodes using ferrocene derivatives and crosslinkers

A series of mono-substituted ferrocenes (Fecp 2 R, R = CONHCH 2N(C 2H 4NH 2) 2 ( 4), R = CH 2NHCH 2N(C 2H 4NH 2) 2 ( 5)) and di-substituted ferrocenes (Fe(cp R) 2, R = COF ( 3)) have been prepared, with 4 and 5 representing new compounds. The ferrocenes were grown layer-by-layer on metal oxide electrodes and crosslinked. The underlying principle of synthesis is based on repetitive and fast amide forming reactions between trimesic acid chloride ( 2) or Fe(cp COF) 2 ( 3) and tris(2-aminoethyl)amine ( 6), or one of the amine-substituted ferrocenes 4 or 5 on a 3-aminopropyl triethoxysilane (APS) ( 1) modified ITO, FTO or ATO electrode. Linear growth of the films electroactivity with the number of cascade steps is observed for up to 12 cascade steps yielding 1.1 × 10 −8 ferrocene centers/cm 2 on a flat ITO electrode, whereas 3 cascade steps yield 2.2 × 10 −7 ferrocene centers/cm 2 on a mesoporeous ATO electrode. In case of the ATO electrode, the inner walls of the mesopores exhibiting a very large surface are completely modified in the procedure. Beside this large coulometry fast electron transfer kinetics, high stability and low optical density in both oxidation states is observed. The electrodes were checked for their potential use as optically transparent counter electrodes in electrochromic devices.

Electrochemically Inactive Nickel Oxide as Electrochromic Material

Nickel oxide thin films, known as optically active counter electrodes in electrochromic devices, were grown at room temperature under an oxygen pressure of mbar, using pulsed laser deposition (PLD). The influence of the film thickness, potential window, and extended cycling on the electrochromic properties was studied by means of complementary electrochemical, X-ray diffraction, and electron microscopy techniques. Independently of the substrate nature, transparent as-deposited films were crystallized with a [1 1 1] preferred orientation. The reversible color change from transparent to brownish is ascribed to a faradic process, involving the redox couple, the intensity of which is enhanced with decreasing film thickness. Finally, taking into account a first chemical reaction when NiO thin films are immersed in KOH, a mechanism based on hydroxide electrochemically active phases is proposed for the electrochromic properties of PLD NiO thin films. © 2004 The Electrochemical Society. All rights reserved.