On the Supercapacitive Behaviour of Anodic Porous WO3-Based Negative Electrodes

Recent Progress of Self-Supported Metal Oxide Nano-Porous Arrays in Energy Storage Applications

The demand for high-performance and cost-effective energy storage solutions for mobile electronic devices and electric vehicles has been a driving force for technological advancements. Among the various options available, transitional metal oxides (TMOs) have emerged as a promising candidates due to their exceptional energy storage capabilities and affordability. In particular, TMO nanoporous arrays fabricated by electrochemical anodization technique demonstrate unrivaled advantages including large specific surface area, short ion transport paths, hollow structures that reduce bulk expansion of materials, and so on, which have garnered significant research attention in recent decades. However, there is a lack of comprehensive reviews that discuss the progress of anodized TMO nanoporous arrays and their applications in energy storage. Therefore, this review aims to provide a systematic detailed overview of recent advancements in understanding the ion storage mechanisms and behavior of self-organized anodic TMO nanoporous arrays in various energy storage devices, including alkali metal ion batteries, Mg/Al-ion batteries, Li/Na metal batteries, and supercapacitors. This review also explores modification strategies, redox mechanisms, and outlines future prospects for TMO nanoporous arrays in energy storage.

Synthesis and Capacitive Properties of Mesoporous Tungsten Oxide Films Prepared by Ultrasonic Spray Deposition

Mesoporous tungsten trioxide (WO3) films are prepared by the combination of the template-assisted sol-gel method and ultrasonic spraying deposition (USD) for supercapacitors, and then the surface morphology and electrochemical performance of the films are studied. Compared to WO3 prepared by the traditional hydrothermal synthesis and spin coating method, the films obtained by USD exhibit advantages such as low cost, minimal material usage, and suitability for large-area in-line manufacturing. Additionally, the mesoporous structure of USD-produced films is also supportive of ion transportation. Due to the high specific surface area of WO3 films deposited by USD, it is a material capable of use in a high-performance energy storage device. Through the control of spray coats, the film thickness and specific capacitance can be effectively controlled. Electrochemical measurements show that the mesoporous WO3 films possess excellent electrochemical performance with a maximum specific capacitance of 109.15 F/g at 0.5 A/g. The cycling performance up to 5000 cycles of mesoporous WO3 films is due to the stable nature of nanocrystalline produced by the combination of USD and sol-gel chemistry.

Enhanced Electrochromic Properties of Nanostructured WO3 Film by Combination of Chemical and Physical Methods

WO3 films are the most widely used electrochromic functional layers. It is known that WO3 films prepared by pure chemical method generally possess novel nanostructures, but the adhesion between WO3 films and substrates is weak. However, WO3 films prepared by pure physical method usually show relatively dense morphology, which limits their electrochromic properties. In order to break through these bottlenecks and further improve their electrochromic properties, this work first prepared nanostructured WO3 powder by chemical method, and then using this powder as the evaporation source, nanostructured WO3 films were fabricated by vacuum thermal evaporation method. Properties of nanostructured WO3 films were systematically compared with those of ordinary WO3 films. It turned out that the nanostructured WO3 film exhibited better cyclic stability and memory effect, and also the optical modulation rate was 14% higher than that of the ordinary WO3 film. More importantly, the nanostructured WO3 film showed better adhesion with the ITO substrates. These results demonstrate that a combination of chemical and physical methods is an effective preparation method to improve the electrochromic properties of WO3 films.

Porous Tungsten Oxide: Recent Advances in Design, Synthesis, and Applications

Tungsten oxide (WO₃ ) has received ever more attention and has been highly researched over the last decade due to its being a low-cost transition metal semiconductor with tunable, yet widely stable, band gaps. This minireview briefly highlights the challenges in the design and synthesis of porous WO₃ including methods, precursors, solvent effects, crystal phases, and surface activities of the porous WO₃ base material. These topics are explored while also drawing a connection of how the morphology and crystal phase affect the band gap. The shifts in band gap not only impact the optical properties of tungsten but also allow tuning to operate on different energy levels, which makes WO₃ highly desirable in many applications such as supercapacitors, batteries, solar cells, catalysts, sensors, smart windows, and bioapplications.

A long-term stable aqueous aluminum battery electrode based on one-dimensional molybdenum-tantalum oxide nanotube arrays

Aluminum ion aqueous batteries (AIBs) are among the most promising candidates for high energy density devices due to the multivalent redox processes associated with Al³⁺ ion intercalation. However, only a few stable AIB electrode materials have been reported so far. MoO₃ is a very promising electrode material due to its octahedral layered crystal structure which can accommodate multivalent cation by intercalation. However, the poor electrochemical stability of MoO₃ and the sluggish intercalation kinetics of Al³⁺ ion in Mo oxides electrodes limit its practical application. In this work, we propose a strategy to overcome such shortcomings of MoO₃ by fabricating electrodes composed of self-ordered one-dimensional (1D) MoTaO_x nanotubes synthesized via electrochemical anodization of Mo-Ta alloy substrates. We show that this approach allows for direct incorporation of Ta in the Mo oxide nanotubes. The resulting MoTaO_x nanotubes, composed of octahedral MoO₃ and rhombohedral Mo₂Ta₂O₁₁ phases, exhibit remarkable electrochemical stability and Al-ion storage properties in aqueous electrolytes that are superior to that of pristine Mo oxide or other most efficient electrode materials reported to date. Such MoTaO_x nanotube-based electrodes can achieve a specific capacity of 1180 mA h cm^-3 (337 mA h g^-1, 141 μA h cm^-2) at 1.25 A cm^-3 (∼0.35 A g^-1, 0.15 mA cm^-2). More importantly, the capacity retention of such nanotube array electrodes remains above 83% of the initial capacity after 3000 cycles.

Advances in Electrochemical Energy Devices Constructed with Tungsten Oxide-Based Nanomaterials

Tungsten oxide-based materials have drawn huge attention for their versatile uses to construct various energy storage devices. Particularly, their electrochromic devices and optically-changing devices are intensively studied in terms of energy-saving. Furthermore, based on close connections in the forms of device structure and working mechanisms between these two main applications, bifunctional devices of tungsten oxide-based materials with energy storage and optical change came into our view, and when solar cells are integrated, multifunctional devices are accessible. In this article, we have reviewed the latest developments of tungsten oxide-based nanostructured materials in various kinds of applications, and our focus falls on their energy-related uses, especially supercapacitors, lithium ion batteries, electrochromic devices, and their bifunctional and multifunctional devices. Additionally, other applications such as photochromic devices, sensors, and photocatalysts of tungsten oxide-based materials have also been mentioned. We hope this article can shed light on the related applications of tungsten oxide-based materials and inspire new possibilities for further uses.

Two-Dimensional Tungsten Oxide/Selenium Nanocomposite Fabricated for Flexible Supercapacitors with Higher Operational Voltage and Their Charge Storage Mechanism

The present work elaborates the high-energy-density, stable, and flexible supercapacitor devices (full-cell configuration with asymmetric setup) based on a two-dimensional tungsten oxide/selenium (2D WO₃/Se) nanocomposite. For this, the 2D WO₃/Se nanocomposite synthesized by a hydrothermal method followed by air annealing was coated on a flexible carbon cloth current collector and combined separately with both 0.1 M H₂SO₄ and 1-butyl-3-methyl imidazolium tetrafluoroborate room temperature ionic liquid (BmimBF₄ RTIL) as electrolyte. Different physicochemical characterization techniques, viz., transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, are utilized for phase confirmation and morphology identification of the obtained samples. The electrochemical analysis was used to evaluate charge storage mechanism. The half-cell configuration (three electrode system) in 0.1 M H₂SO₄ shows a specific capacitance of 564 F g^-1 at 6 A g^-1 current density, whereas with ionic liquid as electrolyte, a higher specific capacitance of 1650 F g^-1 was obtained at a higher current of 40 mA and working potential of 4 V. Importantly, the asymmetric flexible supercapacitor device with PVA-H₂SO₄ electrolyte shows a working voltage of 1.7 V. A specific capacitance of 858 mF g^-1 is obtained for the asymmetric electrode system with an energy density of 47 mWh kg^-1 and a power density of 345 mW kg^-1 at a current density of 0.2 A g^-1.

The Use of Anodic Oxides in Practical and Sustainable Devices for Energy Conversion and Storage

This review addresses the main contributions of anodic oxide films synthesized and designed to overcome the current limitations of practical applications in energy conversion and storage devices. We present some strategies adopted to improve the efficiency, stability, and overall performance of these sustainable technologies operating via photo, photoelectrochemical, and electrochemical processes. The facile and scalable synthesis with strict control of the properties combined with the low-cost, high surface area, chemical stability, and unidirectional orientation of these nanostructures make the anodized oxides attractive for these applications. Assuming different functionalities, TiO2-NT is the widely explored anodic oxide in dye-sensitized solar cells, PEC water-splitting systems, fuel cells, supercapacitors, and batteries. However, other nanostructured anodic films based on WO3, CuxO, ZnO, NiO, SnO, Fe2O3, ZrO2, Nb2O5, and Ta2O5 are also explored and act as the respective active layers in several devices. The use of AAO as a structural material to guide the synthesis is also reported. Although in the development stage, the proof-of-concept of these devices demonstrates the feasibility of using the anodic oxide as a component and opens up new perspectives for the industrial and commercial utilization of these technologies.

Review on Recent Progress in the Development of Tungsten Oxide Based Electrodes for Electrochemical Energy Storage

Current progress in the advancement of energy-storage devices is the most important factor that will allow the scientific community to develop resources to meet the global energy demands of the 21st century. Nanostructured materials can be used as effective electrodes for energy-storage devices because they offer various promising features, including high surface-to-volume ratios, exceptional charge-transport features, and good physicochemical properties. Until now, the successful research frontrunners have focused on the preparation of positive electrode materials for energy-storage applications; nevertheless, the electrochemical performance of negative electrodes is less frequently reported. This review mainly focuses on the current progress in the development of tungsten oxide-based electrodes for energy-storage applications, primarily supercapacitors (SCs) and batteries. Tungsten is found in various stoichiometric and nonstoichiometric oxides. Among the different tungsten oxide materials, tungsten trioxide (WO₃ ) has been intensively investigated as an electrode material for different applications because of its excellent charge-transport features, unique physicochemical properties, and good resistance to corrosion. Various WO₃ composites, such as WO₃ /carbon, WO₃ /polymers, WO₃ /metal oxides, and tungsten-based binary metal oxides, have been used for application in SCs and batteries. However, pristine WO₃ suffers from a relatively low specific surface area and low energy density. Therefore, it is crucial to thoroughly summarize recent progress in utilizing WO₃ -based materials from various perspectives to enhance their performance. Herein, the potential- and pH-dependent behavior of tungsten in aqueous media is discussed. Recent progress in the advancement of nanostructured WO₃ and tungsten oxide-based composites, along with related charge-storage mechanisms and their electrochemical performances in SCs and batteries, is systematically summarized. Finally, remarks are made on future research challenges and the prospect of using tungsten oxide-based materials to further upgrade energy-storage devices.

Transparent Conductive Dielectric-Metal-Dielectric Structures for Electrochromic Applications Fabricated by High-Power Impulse Magnetron Sputtering

The growing applications of electrochromic (EC) devices have generated great interest in bifunctional materials that can serve as both transparent conductive (TC) and EC coatings. WO₃/Ag/WO₃ (WAW) heterostructures, in principle, facilitate this extension of EC technology without reliance on an indium tin oxide (ITO) substrate. However, these structures synthesized using traditional methods have shown significant performance deficiencies. Thermally evaporated WAW structures show weak adhesion to the substrate with rapid degradation of coloration efficiency. Improved EC durability can be obtained using magnetron sputtering deposition, but this requires the insertion of an extra tungsten (W) sacrificial layer beneath the external WO₃ layer to prevent oxidation and associated loss of conductivity of the silver film. Here, we demonstrate for the first time that a new method, known as high-power impulse magnetron sputtering (HiPIMS), can produce trilayer bifunctional EC and TC devices, eliminating the need for the additional protective layer. X-ray photoelectron spectroscopy and X-ray diffraction data provided evidence that oxidation of the silver layer can be avoided, whilst stoichiometric WO₃ structures are achieved. To achieve optimum WAW structures, we tuned the partial pressure of oxygen in the HiPIMS atmosphere applied for the deposition of WO₃ layers. Our optimized WO₃ (30 nm)/Ag (10 nm)/WO₃ (50 nm) structure had a sheet resistance of 23.0 ± 0.4 Ω/□ and a luminous transmittance of 80.33 ± 0.07%. The HiPIMS coatings exhibited excellent long-term cycling stability for at least 2500 cycles, decent switching times (bleaching: 22.4 s, coloring: 15 s), and luminescence transmittance modulation (Δ T) of 34.5%. The HiPIMS strategy for the fabrication of ITO-free EC coatings for smart windows holds great promise to be extended to producing other metal-dielectric composite coatings for modern applications such as organic light-emitting diodes (OLEDs), liquid crystals, and wearable displays.

Effective Utilization of Waste Red Mud for High Performance Supercapacitor Electrodes

In recent years, metal oxide-based, inexpensive, stable electrodes are being explored as a potent source of high performance, sustainable supercapacitors. Here, the employment of industrial waste red mud as a pseudocapacitive electrode material is reported. Mechanical milling is used to produce uniform red mud nanoparticles, which are rich in hematite (Fe2O3), and lower amounts of other metal oxides. A comprehensive supercapacitive study of the electrode is presented as a function of ball-milling time up to 15 h. Ten-hour ball-milled samples exhibit the highest pseudocapacitive behavior with a specific capacitance value of ≈317 F g-1, at a scan rate of 10 mV s-1 in 6 m aqueous potassium hydroxide electrolyte solution. The modified electrode shows an extraordinary retention of ≈97% after 5000 cycles. A detailed quantitative electrochemical analysis is carried out to understand the charge storage mechanism at the electrode-electrolyte interface. The formation of uniform nanoparticles and increased electrode stability are correlated with the high performance. This work presents two significant benefits for the environment; in energy storage, it shows the production of a stable and efficient supercapacitor electrode, and in waste management with new applications for the treatment of red mud.

Fabrication of Mo-Doped WO₃ Nanorod Arrays on FTO Substrate with Enhanced Electrochromic Properties

Well-oriented and crystalline WO₃ nanorod arrays (WNRAs) decorated with Mo were synthesized on fluorine doped tin oxide (FTO) substrate by the hydrothermal method. The effects of Mo doping, hydrothermal reaction time, and hydrothermal temperature on the morphologies and electrochromic properties of as-prepared WNRAs were studied thoroughly. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and chronoamperometry techniques were used to characterize the structures and properties of obtained WNRAs. The results demonstrate that the average diameter of the as-prepared WNRAs ranged from 30 to 70 nm. During the decoration of Mo on the WNRAs, the growth density of as-prepared WNRAs decreased and the surfaces became rough. However, the decorated Mo on WNRAs synthesized at 180 °C for 5 h with a Mo/W mole ratio of 1:40 exhibited better electrochromic properties than single WNRAs. They exhibited high optical modulation (61.7%), fast bleaching/coloring response times (3 s/9 s), high coloration efficiency values (73.1 cm²/C), and good cycling stability.