Reminiscences on the discovery of electrochromic phenomena in transition metal oxides

It has long been known that transition metal oxides of the types WO₃, MoO₃, TiO₂, V₂O₅, etc., show considerable variation in stoichiometry, and that very often the non-stoichiometric compounds are characterized by strong coloration. This observation led to the fabrication of highly disordered thin films of some of these oxides—ones that showed strong coloration upon optical excitation. Back in the 1960s, there was considerable interest in optically bistable phenomena for high-density information recording, and these oxide films were investigated for that purpose. However, the difficulty encountered in the bleaching process led to the proposition that the optically induced color centers could be bleached by an electric field. Subsequent experiments clearly demonstrated that it was not only possible to form color centers by optical excitation and to bleach them by an electric field, but that it was possible to color and bleach by an electric field alone by simply reversing the polarity. This constituted the first successful demonstration of eletrochromic phenomena in thin films of transition metal oxides. This was quickly followed by the development of various innovative device structures for practical application. In this paper, an attempt will be made to reminisce the discovery of the electrochromic phenomena in this type of material, ultimately leading to the current state of this technology.     

Electrochromic oxides: a bandstructure approach

This paper attempts to lay down a foundation for a comprehensive theory of electrochromism among oxides. It is pointed out that crystals of all the well known electrochromic metal (Me) oxides are composed of MeO₆ octahedra in various corner-sharing and edge-sharing arrangements. In electrochromic thin films there are cluster-type and columnar microstructures based on the MeO₆ units. The coordination of the ions leads to electronic bands that are able to explain the presence or absence of cathodic and anodic electrochromism in numerous defect perovskites, rutiles, and layer structures. Optical absorption can be due to intraband effects, interband transitions, and polaron hopping.     

The effect of glass coating emittance and frame rebate on heat transfer through vacuum and electrochromic vacuum glazed windows

The thermal performance of an electrochromic vacuum glazing and a vacuum glazing with a range of low-emittance coatings and frame rebate depths were simulated for insolations between 0 and 1000 W m⁻² using a three-dimensional finite volume model. The vacuum glazing simulated comprised two 0.4 m×0.4 m glass panes separated by a 0.12 mm wide evacuated space supported by a 0.32 mm diameter pillar array spaced at 25 mm. The two glass sheets were sealed contiguously by a 6 mm wide metal edge seal and had either one or two low-emittance coatings. For the electrochromic vacuum glazing, a third glass pane on which an electrochromic layer was deposited was assumed to be sealed to an evacuated glass unit, to enable control of visible light transmittance and solar gain and thus improve occupant thermal comfort. It is shown that for both vacuum glazing and electrochromic vacuum glazings, when the coating emittance value is very low (close to 0.02), the use of two low-emittance coatings only gives limited improvement in glazing performance. The use of a single currently expensive low-emittance coating in both systems provided acceptable performance. Deeper frame rebate depths gave significant improvements in thermal performance for both glazing systems.     

High electrochromic performance of co-sputtered vanadium–titanium oxide as a counter electrode

This study examined the material and electrochromic properties of vanadium–titanium oxides (V–Ti oxides) as a counter electrode material in electrochromic devices. These oxides were deposited on an ITO substrate using a co-sputtering method at different levels of RF power. Electrochemical experiments of these oxides were carried out using half-cell and semi full-cell tests which are good methods for measuring the potential applied to each electrode. The change in electrochromic properties after 1000 cycles of a semi full-cell test was examined. Reversibility and durability of an electrochromic device were improved by increasing the titanium content in V–Ti oxides.     

Solar optical materials for innovative window design

New and innovative optical materials and coatings can greatly improve the efficiency of window energy systems. These potential materials and coatings increase energy efficiency by reducing radiative losses in the infrared, or reducing visible reflection losses or controlling overheating due to solar gain. Current progress in heat mirror coatings for glass a polymeric substrates is presented. Highly doped semiconducting oxides and metal/dielectric interference coatings are reviewed. Physical and optical properties are outlined for antireflection films and transparent aerogel insulation media. The potential for optical switching films as window elements includes discussions of electrochromic, photochromic and other physical switching processes.     

Bifunctional oxygen/air electrodes

A selective review on the materials and construction principles used for bifunctional oxygen/air electrodes is given. The discussion emphasizes the catalytically active materials used for the construction of these electrodes, which are a key component in electrically rechargeable air breathing electrochemical systems. Whereas, in acid electrolytes normally noble metal catalysts must be used, there is a possibility to use less expensive transition metal oxides in alkaline electrolytes. Typical transition metal oxides have the perovskite, pyrochlore and spinel structure.     

Synthesis of porous flower-like Ni-Co-Mo-S nanostructures on Ni foam for battery-supercapacitor hybrid devices

The synergistic effects of multiple components and unique nanostructures were contributed to prepare the high-performance battery-type electrode materials. In this work, Mo element was introduced to form the ternary transition metal oxides/hydroxides of Ni-Co to improve conductivity, and then charge transfer was accelerated to enhance the capacity storage. After sulfidation, the electrical conductivity was further improved, and a porous flower-like nanostructure was formed. Except for that, the composites of transition metal oxides/hydroxides and sulfides were formed via sulfidation. With the help of the synergistic effects of multiple components and a porous flower-like nanostructure, more Faradic redox reactions occurred. Therefore, the as-prepared porous flower-like Ni-Co-Mo-S nanostructures on Ni foam exhibited an excellent areal capacitance of 7.22 C·cm⁻² at 5 mA·cm⁻² and long-cycle stability (96.9% retention after 5000 cycles). Furthermore, a coin-type battery-supercapacitor hybrid (BSH) device was assembled, which achieved 54.54 Wh·kg⁻¹ at 540 W·kg⁻¹ and displayed 74.8% capacitance retention after 3500 cycles. All mentioned above demonstrated that ternary transition metal oxides/hydroxides precursors via sulfidation can form special structures and the composites of transition metal oxides/hydroxides and sulfides to prepare high-performance battery-type electrodes for energy storage.     

Biomimetic molecular water splitting catalysts for hydrogen generation

During the last few decades, the scientific community has been striving hard to develop new and alternative sources for renewable energy and fuel. Hydrogen or carbon free fuels obtained from catalytic water splitting using sunlight offer an attractive solution for a cleaner and greener future. In this pursuit, to establish effective molecular catalytic systems for efficient water oxidation is considered to be a bottleneck, hampering the design, implementation and exploitation of electrochemical and photo-electrochemical modular devices for light driven energy conversion into hydrogen-based storable fuels. From metal oxides to composite materials, noble metal complexes to transition metal organometallics, multinuclear to mono-site catalysts, various water oxidation complexes (WOCs) have been investigated both in a homogeneous environment and on surfaces in photo- or electrochemical conditions. However, a truly biomimetic catalytic system that matches the performance of photosystem-II for efficient water splitting, operating with four consecutive proton coupled electron transfer (PCET) steps to generate oxygen and hydrogen for hundred thousands of cycles at high rate is yet to be achieved. We here present an overview of biomimetic molecular complexes that have been investigated recently for water oxidation catalysis in homogeneous solution using chemical oxidants, or as heterogeneous species for catalytic electrochemical systems.     

Intercalation studies on electron beam evaporated MoO3 films for electrochemical devices

Now-a-days a large number of extensive research has been focused on electrochromic oxide thin films, owing to their potential applications in smart windows, low cost materials in filters, low cost electrochemical devices and also in solar cell windows. Among the varieties of electrochromic transition metal oxides, the molybdenum oxide (MoO₃) and tungsten oxide (WO₃), form a group of predominant ionic solids that exhibit electrochromic effect. The electrochromic response of these materials are aesthetically superior to many other electrochromic materials, because WO₃ and MoO₃ absorb light more intensely and uniformly. In the present case, we have discussed about the electrochromic behaviour of electron beam evaporated MoO₃ films. Moreover, the MoO₃ film can also be used as a potential electro-active material for high energy density secondary lithium ion batteries; because it exhibits two-dimensional van der Waals bonded layered structure in orthorhombic phase. The films were prepared by evaporating the palletized MoO₃ powder under the vacuum of the order of 1 × 10⁻⁵ mbar. The electrochemical behaviour of the films was studied by intercalating/deintercalating the K⁺ ions from KCl electrolyte solutions using three electrode electrochemical cell by the cyclic-voltammetry technique. The studies were carried out for different scanning rates. The films have changed their colour as dark blue in the colouration process and returns to the original colour while the bleaching process. The diffusion coefficient values (D) of the intercalated/deintercalated films were calculated by Randle's Servcik equation. The optical transparency of the coloured and bleached films was studied by the UV–Vis–NIR spectrophotometer. The change in bonding assignment of the intercalated MoO₃ films was studied by FTIR spectroscopic analysis. A feasible study on the effect of substrate temperatures and annealing temperatures on optical density (OD) and colouration efficiency of the films were discussed and explored their performance for the low cost electrochemical devices.     

Development and characterisation of electrochromic devices on polymeric substrates

The development of full solid-state electrochromic (EC) devices on polymeric substrate is underway within a CEC BRITE-EURAM project (Project “FREDOPS”, BE-4137) carried out by four industries, two universities and two research centers from Belgium, Denmark, France, and Italy. The specific goal of this project is to develop a Fast Response Electrochromic Device On Polymeric Substrate (FREDOPS); in order to satisfy the required range of specifications in terms of fast response, long term performance and high contrast ratio, several systems based on different materials have been tested. The full cells consist of an electrochromic material layer and a counter electrode, inserted between two PET/ITO layers and separated by a polymeric electrolyte. Different types of polymeric electrolytes, counter electrodes and electrochromic layers have been developed, studied and checked. Full devices have been assembled using different combinations. Voltammetric and spectrophotometric measurements have been executed to check the electrochromic behaviour of the developed layers in half and full cells. Comparison of the electrochromic performances of different materials based cells has led to the rejection of several solutions due to poor performance and incompatibilities between layers. Considering that the electrochromic devices are finalised for different uses (window, sunroof,…), some performance specifications for each application are defined. A testing bench for cycling and ageing was developed. The present paper discusses these results in order to indicate the best performance.     

A hybrid composite of rGO/TiO2 as a double layer electrode with improved capacitance performance

Reduced graphene oxide (rGO) has unique properties that can revolutionize the performance of the functional devices. rGO hybrids can be designed with transition metal oxides for improved energy storage applications. Herein, a hybrid composite of conductive rGO with titanium dioxide, designed by a simple hydrothermal method, is reported to demonstrate a high double layer capacitance in aqueous electrolyte systems. The mesoporous structure of the composite provides short ion diffusion pathways and the resultant capacitance of the material is 334 F g⁻¹ with ~77% capacitance retention after 7000 charge-discharge cycles.     

A review of catalytic sulfur (VI) oxide decomposition experiments

Sulfur (VI) oxide, also known as sulfur trioxide or SO₃, decomposition is an oxygen-generating decomposition reaction that proceeds in the gaseous system SO₃/SO₂/O₂/H₂O at temperatures above 500 K. Maximum decomposition yield of SO₃ to SO₂ and O₂ is best achieved at temperatures of over 1000 K with an appropriate catalyst. According to the literature, noble metals and some transition metal oxides are highly effective catalysts in the laboratory environment. Sulfur (VI) oxide decomposition is the energetic and temperature limiting step of several endothermic hydrogen generating chemical process heat plants. In particular, the General Atomics Sulfur Iodine cycle and the Westinghouse Hybrid Sulfur cycle are candidates for thermal coupling to a high temperature nuclear reactor. Therefore the sulfur (VI) oxide decomposition reaction is a potential heat sink for a high temperature nuclear reactor. Thus, optimization of catalyst selection is required, both for operational efficiency and safety. In this paper, reaction mechanisms and catalyst composition for sulfur (VI) oxide decomposition are reviewed. Chemical kinetics data from previous sulfur (VI) oxide decomposition experiments are extracted from archival journal papers or other open literature. The available experimental database suggests that Pt-based catalysts have the highest stable activity among the noble metals and Fe₂O₃-based catalysts have the highest stable activity among the transition metal oxides. The decomposition temperature of the corresponding metal sulfate dictates the catalytic activity of a given transition metal oxide.     

A chromium complex under water oxidation: A conversion mechanism and a comprehensive hypothesis

Water splitting toward hydrogen production is a promising method to store energy, but water oxidation is a bottleneck for water splitting. First-row transition metal complexes have been extensively used for water-oxidation reaction. However, only one chromium complex has been applied for water-oxidation reaction until now. Thus, the reinvestigation of water-oxidation reaction by this chromium complex in detail is interesting. Herein, water-oxidation reaction by the chromium complex with diphenoxy N,N′-ethylenebis(salicylimine) (SALEN) ligand is reinvestigated using scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction studies, X-ray photoelectron spectroscopy, thermal gravimetric analysis, elemental analysis, Fourier-transform infrared spectroscopy, and electrochemical methods. All the experiments showed that the chromium complex is neither a catalyst nor a precatalyst for water-oxidation reaction. During OER, a relationship between first-row transition metal complexes and the related metal oxides has been proposed.     

Metal oxide applications in organic-based photovoltaics

Abstract

Organic-based photovoltaics (PV) have attracted increasing attention in recent years and efficiencies exceeding 8% have recently been confirmed. These low cost, lightweight and mechanically flexible devices offer unique advantages and opportunities currently unavailable with crystalline silicon technology. Progress in the field of organic PV has been achieved in part due to the incorporation of transition metal oxides. These offer a wide range of optical and electronic properties, making them applicable in organic-based PV in many capacities. Transparent electrodes can be made from doped metal oxides. The high intrinsic charge carrier mobility of many undoped metal oxides makes them attractive as active materials and charge collectors. Metal oxides can increase the charge selectivity of the electrodes due to the energetic positioning of their valence and conduction bands. Thin films of these materials can manipulate the light distribution inside of organic devices, allowing for improved light harvesting. Metal oxides are stable and can be processed at low temperatures. Consequently, they have been demonstrated as suitable intermediate layer materials in tandem cells. Finally, oxygen-deficient metal oxides can improve the stability of the oxygen- sensitive organic semiconductors. The present work reviews the various applications of metal oxide layers in organic PV devices and summarises the challenges associated with organic/oxide interfaces.     

电沉积法制备电致变色材料

应用电沉积法制备WO3、NiO、M0O3,WO3/M0O3等电致变色薄膜。WO3薄膜、NiO薄膜、M0O3薄膜具有较好的电致变色特性,漂白态和着色态的透射率之差平均为30％左右。WO3/M0O3掺杂薄膜漂白态和着色态的透射率之差平均为40％左右,显示掺杂有利于增强薄膜的电致变色特性。WO3 NiO互补型电致变色体系漂白态和着色态的透射率之差平均为40％左右,显示互补型电致变色材料有利于增强电致变色特性。在双注入模型的基础上,根据过渡金属配合物显色机理,提出解释电致变色机理的“配位场模型”,认为在电致变色中可能存在三种电子跃迁方式。     

层状富镍过渡金属氧化物正极材料衰减机理研究进展

层状富镍过渡金属氧化物正极材料因具有比容量高、价格低廉以及对环境友好等特性而受到广泛关注,并已在产品中应用。但其本身固有的一些缺陷,如在循环过程中结构稳定性较差、高温循环衰减过快、导电系数较低及储存性能不佳等,极大地限制了其在各个领域的广泛应用。本文对近年来高镍层状过渡金属氧化物正极材料在循环过程中容量衰减有关机理进行概括总结,并针对不同衰减机理给出简要改进方法。     

Hydrothermal synthesis of electrode materials pyrochlore tungsten trioxide film

Hydrothermal synthesis methods have been successfully used to prepare new transition-metal oxides for cathodes in electrochemical devices such as lithium batteries and electrochromic windows. The tungsten oxides were the first studied, but the method has been extended to the oxides of molybdenum, vanadium and manganese. Sodium tungsten oxide films with the pyrochlore structure have been prepared on gold/alumina and indium-doped tin oxide substrates. These films reversibly and rapidly intercalate lithium and hydrogen ions.     

Effect of calcination temperature on morphology,crystallinity and electrochemical properties of nano-crystalline metal oxides (Co3O4, CuO,and NiO) prepared via ultrasonic spray pyrolysis

Nano-crystalline metal oxides (Co₃O₄, CuO, and NiO) are synthesized as anode materials for lithium-ion batteries by an ultrasonic spray pyrolysis method. The effects of calcination temperature on the morphology, crystallite size and electrochemical properties of the metal oxides are investigated. X-ray diffraction (XRD) studies show that the crystallite size varies with the final calcination temperature. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations reveal that the calcination temperature strongly influences the morphology of the prepared metal oxides and this results in different electrochemical performance. The existence of a nano-scale microstructure for the prepared metal oxides has a strong relationship with irreversible capacity and capacity retention.     

Thermal performance of an electrochromic vacuum glazing

Using a three-dimensional finite volume model, the thermal performance of an electrochromic vacuum glazing was simulated for insolation intensities between 0 and 1200 W m⁻². The electrochromic evacuated glazing simulated consisted of three glass panes 0.5 m by 0.5 m with a 0.12 mm wide evacuated space between two 4 mm thick panes supported by 0.32 mm diameter pillars spaced on a 25 mm square grid contiguously sealed by a 6 mm wide metal edge seal. The third glass pane on which the electrochromic layer was deposited was assumed to be sealed to the evacuated glass unit. The simulations indicate that when facing the indoor environment, the temperature of the glass pane with the electrochromic layer can reach 129.5 °C for an incident insolation of 600 W m⁻². At such temperatures unacceptable occupant comfort would ensue and the durability of the electrochromic glazing would be compromised. The glass pane with the electrochromic layer must therefore face the outdoor environment.     

Low-operating temperature resistive nanostructured hydrogen sensors

With increasing attention on hydrogen (H₂) produced by renewable energy methods for transport and other applications, an updated evaluation of H₂ monitoring techniques is timely. The emergence of nanomaterials with unique properties enables a new class of H₂ sensing materials, exhibiting important attributes such as high surface area to volume ratio, reactivity and number of active sites. The development of these materials allows for improved H₂ sensing methods with increased sensitivity, selectivity, speed and improved economy for both financial cost and power consumption. These advances enable small-scale, high-performance H₂ sensors for use in emerging hybrid renewable energy systems, including Internet of Things (IoT) or mobile systems. While literature reports on H₂ sensors include thermal conductivity, resistive, optical, acoustic, electrochemical and catalytic methods, this review will focus on resistive H₂ sensing nanomaterials. Three classes of these nanomaterials are discussed: metal oxides, transition metal dichalcogenides, and graphene based nanomaterials. As a key aim of this review is improved economy of use, we will elaborate only sensors operating at temperatures lower than 200 °C, with a focus on potential for room temperature operation.