Thin film of CeO2-SiO2: a new ion storage layer for smart windows

Thin solid films of CeO₂-SiO₂, used as a counter-electrode layer in electrochromic devices, were prepared by the sol–gel dip coating, using an aqueous-based process. The influence of the SiO₂ addition on electrochemistry of the CeO₂ oxide coatings was determined by chronoamperometric measurements. The films exhibit a larger charge storage capacity, which was determined as a function of the coatings thickness. The peak occurrence in the chronoamperometric curve during the deintercalation of lithium ions in the cerium/silicon films is analyzed in terms of trapping energy levels for Li⁺ ions into the film.

In situ UV–Vis spectroelectrochemical measurements of the CeO₂-SiO₂ coatings indicated that the films remained transparent in the visible spectral range during the intercalation process. Powders were characterized by X-ray diffraction after the same thermal treatment of the films, indicating a decrease of crystallinity with the doping. The feasibility for use of these electrodes as ion storage for electrochromic devices was investigated.     

Optical and electrochemical properties of CeO2 thin film prepared by an alkoxide route

Transparent CeO₂ thin solid films, used as ion storage layer in electrochromic devices were prepared by the sol–gel method using an alkoxide route combined with the dip-coating technique. The precursor sol was prepared from a mixture of cerium (IV) methoxyethoxide in anhydrous 2-butanol. Electrochemical Li⁺ intercalation/deintercalation was performed by cyclic voltammetry and the results indicate that the CeO₂/LiClO₄ system is electrochemically reversible. The total inserted/extracted charge of the CeO₂ film was determined by chronoamperometric measurements, which showed an ion storage capacity of 14 mC/cm². The solid-state diffusion of lithium ion into the CeO₂ thin films was investigated by electrochemical impedance spectroscopy.     

Polymer-based symmetric electrochromic devices

The fact that conjugated polymers repeatedly undergo electrochemical doping/undoping processes, which are accompained by color changes, makes these materials very attractive, and much effort has been devoted to their use in advanced devices. There is renewed interest in electroactive polymers that reversibly undergo both p- and n-doping because of their potential application in symmetric electrochemical devices. We employed fused molecules, dithienothiophenes, as monomers to obtain polymers with a narrow band gap suitable for n- and p-doping. The performance results of two symmetric electrochromic devices having as electrodes both poly(dithieno[3,4-b:3',4'-d]thiophene) (pDTT1) and poly(dithieno[3,4-b:2',3'-d]thiophene) (pDTT3) are reported and discussed.     

Brown coloring electrochromic devices based on NiO–TiO2 layers

Brown coloring electrochromic 5×10 cm² windows with the configuration K-glass/NiO–TiO₂/electrolyte/CeO₂–TiO₂/K-glass have been prepared and characterized by optoelectrochemical techniques (cyclic voltammetry, chronoamperometry and galvanostatic measurements). The electrochromic layers have been prepared by the sol–gel technique. As electrolyte either a 1 M aqueous KOH solution or a newly developed starch-based gel impregnated with KOH have been used. The CeO₂–TiO₂ sol–gel layers sintered at 550 °C have been previously characterized in 1 M aqueous KOH electrolyte as a function of the thickness up to 2000 cycles and showed a highly reversible behavior without any corrosion effect. The NiO–TiO₂ sol–gel layers sintered at 300 °C have been extensively characterized in the same electrolyte up to about 7000 cycles. All windows present a deep brown color characteristic of the presence of Ni³⁺ (NiOOH) species, that is fully reversible for several thousands of cycles with a rather-fast kinetics (<30 s). The transmittance of the bleached state however slowly decreases with cycling (permanent coloration). The full-bleached condition can be nevertheless recovered by applying a negative potential for a long duration. Deeper coloration is usually obtained by cycling the windows galvanostatically with a current density of 20 μA/cm². The lifetime of the windows is however limited because of the degradation of the NiO-based layers due to the not fully reversible exchange of OH⁻ that turns the layers mechanically fragile and leads eventually to their complete removal from the substrate. Windows working satisfactorily up to 7000 and 17 000 cycles have been obtained using aqueous KOH electrolyte and starch KOH gel electrolyte, respectively. Memory tests showed that the devices bleach at the open circuit potential from T=39% (colored state) to about T=50% in 60 min.     

Comparative studies of “all sol–gel” electrochromic windows employing various counter-electrodes

Electrochromic (EC) “smart” windows for buildings represent an effective way to modulate the intensity of incoming solar radiation. While it is accepted that WO₃ films represent the best option for the working electrode, the choice of the best counter-electrode is still debatable. Optical properties of counter-electrodes such as Ce, Fe, V and Sn oxides are presented. Electrochromic windows were made with a sol–gel WO₃ active colouring film (150°C), Ce, Fe, V oxide counter-electrodes and a sol–gel organic–inorganic hybrid (Li⁺ormolyte) ion conductor. The electrochromic responses of these devices predicted from the charge capacities, photopic transmittances and coloration efficiencies of individual films are compared with measured values.     

Optimisation of the electrochromic properties of Nb2O5 thin films produced by sol–gel route using factorial design

Electrochemical and electrochromical properties of oxide films are dependent on their microstructure and morphological properties. Thus, the effects of three preparation variables on the electrochemical and electrochromical properties of Nb₂O₅ thin films prepared by the Pechini method were investigated. In order to minimise the number of experiments, a factorial design 2³ was used. The effects of the following variables: CA/EG molar ratio, CA/[Nb] molar ratio and calcination temperature were evaluated. Films prepared with the resin composition CA/EG=1 : 4, CA/[Nb]=10 : 1 and calcined at 500°C, showed the highest values of intercalation charge, transmittance variation and coloration efficiency, 22 mC/cm², 84% and 23 cm² C⁻¹, respectively.     

WO3−x nanowires based electrochromic devices

A simple method was developed to fabricate tungsten oxide (WO_3−x) nanowires based electrochromic devices. The WO_3−x nanowires are grown directly from tungsten oxide powders in a tube furnace. The WO_3−x nanowires have diameters ranging from 30 to 70 nm and lengths up to several micrometers. The WO_3−x nanowires based device has short bleach-coloration transition time and can be grown on a large scale directly onto an ITO-coated glass that makes it potential in many electrochromic applications. The structure, morphology, and composition of the WO_3−x nanowires were characterized using the scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and energy-dispersive spectrometer. The optical and electrochromic performance of the nanowires layer under lithium intercalation was studied in detail by UV–VIS–NIR spectroscope and cyclic voltameter.     

Sprayed CeO2 thin films for electrochromic applications

Cerium dioxide (CeO₂) thin films were prepared by spray pyrolysis using hydrated cerium chloride (CeCl₃·7H₂O) as source compound. The films prepared at substrate temperatures below 300°C were amorphous, while those prepared at optimal conditions (T_s=500°C,s=5 ml/min) were polycrystalline, cubic in structure, preferentially oriented along the (2 0 0) direction and exhibited a transmittance value greater than 80% in the visible range. The cyclic voltammetry study showed that films of CeO₂ deposited on ITO pre-coated glass substrates were capable of charge insertion/extraction when immersed in an electrolyte of propylene carbonate with 1 M LiClO₄.These films also remained fully transparent after Li+ intercalation/deintercalation.     

Using room temperature ionic liquid to fabricate PEDOT/TiO2 nanocomposite electrode-based electrochromic devices with enhanced long-term stability

In this work, poly(3,4-ethylenedioxythiophene)(PEDOT) was electrochemically incorporated with nano- and mesoporous TiO₂ films to form PEDOT/TiO₂ nanocomposite electrochromic electrodes. TiO₂ films were introduced to enhance the interfacial adhesion of the polymers to the substrates and thus increase the long-term stability of electrodes of electrochromic devices (ECDs). Room temperature ionic liquid (RTIL)- 1-butyl-3-methyl-imidazolium tetrafluoroborate ([BMIM]BF₄) was employed to serve as electrolyte during the entire fabrication processes. With these efforts, the ECDs were found to retain up to 95% of their optical response and electroactivity after 10,000 deep, and double potential steps, exhibiting enhanced long-term stability.     

Studies on the enhancement of solid electrolyte interphase formation on graphitized anodes in LiX-carbonate based electrolytes using Lewis acid additives for lithium-ion batteries

The new electrolyte systems utilizing one type of Lewis acids, the boron based anion receptors (BBARs) with LiF, Li₂O, or Li₂O₂ in carbonate solutions have been developed and reported by us. These systems open up a new approach in developing non-aqueous electrolytes with higher operating voltage and less moisture sensitivity for lithium-ion batteries. However, the formation of a stable solid electrolyte interphase (SEI) layer on the graphitized anodes is a serious problem needs to be solved for these new electrolyte systems, especially when propylene carbonate (PC) is used as a co-solvent. Using lithium bis(oxalato)borate (LiBOB) as an additives, the SEI layer formation on mesophase carbon microbeads (MCMB) anode is significantly enhanced in these new electrolytes containing boron-based anion receptors, such as tris(pentafluorophenyl) borane, and lithium salt such as LiF, or lithium oxides such as Li₂O or Li₂O₂ in PC and dimethyl carbonate (DMC) solvents. The cells using these electrolytes and MCMB anodes cycled very well and the PC co-intercalation was suppressed. Fourier transform infrared spectroscopy (FTIR) studies show that one of the electrochemical decomposition products of LiBOB, lithium carbonate (Li₂CO₃), plays a quite important role in the stablizing SEI layer formation.     

Investigating the solid electrolyte interphase using binder-free graphite electrodes

Binder-free (BF) electrodes simplify interpretation of solid electrolyte interphase (SEI) data obtained from studies of graphite surfaces. In this work, we prepared BF-graphite electrodes by electrophoretic deposition (EPD), and the SEI layers formed on the electrode in lithium cells containing LiPF₆- and LiF₂BC₂O₄-bearing electrolytes were examined by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results showed that the dominant SEI species were lithium alkyl carbonates (ROCO₂Li) and lithium alkoxides (ROLi); Li₂CO₃ was conspicuously absent. Trigonal borate oligomers are most likely present in the SEI of graphite samples cycled in LiF₂BC₂O₄ electrolyte, while lithium fluorophosphates are present on graphite samples cycled in LiPF₆ electrolyte. The SEI layer coverage was greater on graphite samples cycled in LiF₂BC₂O₄ electrolyte than in the LiPF₆ electrolyte. Our results demonstrate that BF-graphite electrodes prepared by EPD are suitable for the study of SEI layer formed in various electrolyte systems.     

The electrochromic and photocatalytic properties of electron beam evaporated vanadium-doped tungsten oxide thin films

The electrochromic and photocatalytic properties of vanadium-doped tungsten trioxide thin films prepared at room temperature (300 K) by the electron beam evaporation technique are reported in this paper. The vanadium to tungsten ratio (V/W) in these films are 0.003, 0.019, 0.029 and 0.047. The optical band gap of the vanadium-doped tungsten oxide (WO₃) thin film initially increases from 3.16 to 3.28 eV for V/W ratio 0.003 then decreases to 3.15 eV for V/W ratio 0.047. These vanadium-doped films switch between neutral gray and transparent states. The coloration efficiency (CE) decreases from 82 cm² C⁻¹ (pure WO₃) to 27 cm² C⁻¹ for the film containing V/W ratio 0.047. The photocatalytic activity has enhanced with vanadium doping and maximum activity of 15% (percentage change in optical density of methylene blue due to photo degradation) has been observed for the film containing V/W ratio of 0.019. The Kelvin probe measurements show that the work function of pure WO₃ films is 4.07 eV and vanadium doping initially increases the work function to 4.19 eV for V/W ratio 0.019 and then decreases it to 3.97 eV for film with V/W ratio 0.047.     

Electrochromic devices based on surface-modified nanocrystalline TiO2 thin-film electrodes

Nanocrystalline TiO₂ thin film electrodes on conductive glass were modified with monolayers of different electrochromic compounds (mono-, di- and trimeric N,N′-dialkyl- or-diphenyl-4,4′-bipyridinium salts) equipped with TiO₂ anchoring groups (An=benzoate, salicylate, phosphonate). The synthesis of these compounds is reported. Different approaches have been studied to increase the surface concentration Γ_CS of electrochemically active coloring centers (CS) on TiO₂. The electrodes were checked coulometrically and spectroelectrochemically under potentiostatic conditions in MeCN/TEAP. Γ_CS of mono- and oligomeric viologens was shown to depend on the ratio (CS/An) of CS to anchoring groups (An). A cone-shaped trimeric arborol-type viologen was prepared with the intention to fill out the space above the convex surface of the nanoparticles particularly well. Preliminary results of a new type of TiO₂ solid-phase supported synthesis of the viologens is reported. Electrochromic devices including filters and displays have been prepared. The filter devices (12–100 cm²) consist generally of OTE/TiO₂-poly-viologen/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue/OTE and exhibit optical density changes up to 2 (transparent to blue or yellowish to green and red-brown (at higher potential)) at switching times in the range of 1–3 s. Even higher optical density changes (at slower switching times) were achieved with systems such as OTE/TiO₂-poly-viologen/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue-TiO₂/OTE. The display devices prepared include reflective displays with two to four separately addressable segments ((OTE/TiO₂ (both structured)-oligo-viologen/microcrystalline rutile (reflective layer)/molten salt+spacer/Zn) or (OTE/TiO₂ (both structured)-oligo-viologen/microcrystalline rutile (reflective layer)/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue/OTE), as well as transparent systems with up to four addressable segments such as: OTE/TiO₂ (both structured)-poly-viologen/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue/OTE.     

Alkaline polymer electrolyte fuel cell with Ni-based anode and Co-based cathode

Alkaline polymer electrolyte fuel cells (APEFCs) are a new class of fuel cell that has been expected to combine the advantages of alkaline fuel cells (AFCs) and polymer electrolyte fuel cells (PEFCs). In recent decade, APEFCs have drawn much attention in the fuel cell world. While great efforts have been devoted to the development of high-performance alkaline polymer electrolytes (APEs), prototypes of APEFC using nonprecious metal catalysts in both the anode and the cathode have not been well implemented, except for our previous report where Ni–Cr was used as the anode catalyst and Ag was employed as the cathode catalyst. In the present work, we report our recent progress in this regard. The self-crosslinked quaternary ammonia polysulfone (xQAPS), a high-performance APE that possesses both good ionic conductivity and extremely high dimensional stability, is applied as both the electrolyte membrane and the ionomer impregnated in the electrodes. Carbon-supported Co-polypyrrole (CoPPY/C) is employed as the cathode catalyst and a new Ni-based catalyst, W-doped Ni, is used as the anode catalyst, which features in high oxidation tolerance. H₂–O₂ and H₂-air APEFCs are thus fabricated and show a decent performance with peak power density being 40 and 27.5 mW/cm² at 60 °C, respectively.     

Cycle test and degradation analysis of WO3/PC+LiClO4/CeO2·TiO2 electrochromic device

We fabricated an electrochromic full cell device adopting WO₃ as a working electrode, and 1 M LiClO₄ in PC with 3% water addition as an electrolyte and CeO₂·TiO₂ with various thicknesses as an ion storage layer. CeO₂·TiO₂ with less than 100 nm shows large charge density but the long-term cyclability is not good due to lithium ion diffusion into ITO thin film. Therefore, the thickness of CeO₂·TiO₂ ion storage layer should be coated at more than 200 mm/min. Long-term cycle test results show that CeO₂·TiO₂ ion storage layer with more than 150 nm thickness and two time coating enhance the long-term stability. SIMS analysis results show that the degradation is due to the remaining lithium ion in the working electrode, WO₃.     

Coloured electrochromic windows based on nanostructured TiO2 films modified by adsorbed redox chromophores

A series of viologens has been synthesised, characterised and tested for their suitability as redox chromophores in electrochromic devices. These viologens contain a phosphonic acid moiety and are irreversibly adsorbed at a transparent nanoporous-nanocrystalline TiO₂ electrode. An electrochromic device consisting of a sandwich of a viologen-modified TiO₂ electrode/electrolyte (γ-butyrolactone, 0.05M LiClO₄, 0.05M ferrocene)/conducting glass shows excellent electrochromic properties: fast switching times (1–2 s), large changes in absorbance, high colouration efficiencies (up to 200 cm²/C) and good long-term stability (>10 000 cycles). Further, the colour changes from transparent or a faint yellow to either a deep blue or a deep green, depending on the nature of the viologen.     

Formation of ultrafast-switching viologen-anchored TiO2 electrochromic device by introducing Sb-doped SnO2 nanoparticles

Ultrafast-switching viologen-anchored TiO₂ electrochromic device (ECD) was developed by introducing Sb-doped SnO₂ (Sb_xSn_1−xO₂, ATO) as counter electrode (CE), and the switching behavior of the fabricated ECD was investigated as a function of Sb-doping concentration. About 9-nm-sized Sb_xSn_1−xO₂ (x=0–0.3) nanoparticles were synthesized by a solvothermal reaction of tin (IV) chloride and antimony (III) chloride at 240 °C, and employed to fabricate 2.4-μm-thick transparent CE. Working electrode (WE) was formed from the 7-nm-sized TiO₂ nanoparticle by a doctor blade method, and the thickness of the nanoporous TiO₂ electrode was 4.5 μm. The phosphonated viologen, bis(2-phosphonylethyl)-4,4′-bipyridinium dibromide, was then adsorbed on the prepared films for the construction of the ECD. The response time was strongly dependent on the doping concentration of Sb in ATO, and the fastest switching response was observed at 3 mol%. At this composition, the coloration time was 5.7 ms, and the bleaching time was 14.4 ms, which is regarded as one of the best results so far reported.     

Synthesis and characterization of terbium doped barium cerates as a proton conducting SOFC electrolyte

A novel proton conductor, BaCe_0.95Tb_0.05O_3−a (BCTb) perovskite was synthesized via the EDTA-citrate acid complexing method, followed by high temperature calcination. The properties of the powders were characterized by thermo gravimetric-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), Scanning electron microscopy (SEM) and the conductivity measurement using the 4-probe method. In order to obtain the pure perovskite structure, the calcination temperature was elevated at 1000 °C or greater. The electrical conductivities of the BCTb perovskite mainly result from protons and are 6.51 × 10⁻³−2.0 × 10⁻² S cm⁻¹ in the temperature range of 600-850 °C with the activation energy of 41.95 kJ mol⁻¹. A Ni-BCTb∣BCTb∣LSCF(La_0.6Sr_0.4Co_0.2Fe_0.8O_3-α)-BCTb fuel cell was subsequently fabricated via a co-pressing/co-sintering method, followed by slurry-coating of the cathode. With 50 mL min⁻¹ pure hydrogen as fuel and the ambient air as oxidant, the maximum output of the fuel cell has reached to 753 mW cm⁻² at 700 °C. The results demonstrate that BCTb perovskite may be a promising electrolyte for the proton-conducting solid oxide fuel cells (PC-SOFCs) after its chemical stability is improved significantly.