Tungsten Oxide Matrix Research Articles

Sodium‐Substituted Tungsten Oxide Nanoflowers: An Efficient Electrode Enhancing the Pseudocapacitive Storage in Aqueous Asymmetric Supercapacitors

AbstractElectrode materials engineering at the nanoscale is essential to improve the electrochemical performance for next‐generation energy storage devices. Herein, a novel approach of sodium substitution in the tungsten oxide matrix is highligting the enhancement of the pseudocapacitor performance of the electrode in an aqueous asymmetric supercapacitor. The sodium‐substituted tungsten trioxide (NWO) and pristine tungsten oxide (PWO) nanoflowers prepared by a single‐step hydrothermal process has presented. The tetragonal crystal structure and nanoflower morphology has maintained even after Na substitution. The electrochemical properties of PWO and NWO have investigated with three‐electrode setups in 1 M H2SO4 aqueous electrolyte. The specific capacitance of PWO and NWO exhibits 104 F g−1 and 476 F g−1 at 1 A g−1, respectively. Furthermore, an aqueous asymmetric supercapacitor (AAS) of NWO demonstrates a specific capacitance of 41 F g−1, an energy density of 13 W h kg−1, and a power density of 3750 W kg−1. An excellent stability of AAS has achieved with 100 % capacitance retention from 1000 to 5000 cycles. The sodium substitution significantly enhanced the pseudocapacitive behavior, attributed to enhanced electroactive sites, conductivity, surface area, and chemical stability. It shows the potential of NWO nanoflowers as a promising candidate for next‐generation supercapacitor devices.

Palladium single atoms from melting nanoparticles on WO3−x for boosted hydrogen evolution reaction

Surface-supported isolated atoms in single-atom catalysts (SACs) grant maximum utilization of metals in heterogeneous catalysis. Herein, we report a feasible pyrolysis strategy to synthesize Pd single atoms by thermally melting Pd nanoparticles on an oxygen-vacancy-rich tungsten-oxide matrix at reduction atmosphere. Near ambient pressure X-ray photoelectron spectroscopy was used to monitor the formation of zero-valence Pd single atoms and the increased metallic feature of WO3−x substrate. Accordingly, the as-obtained zero-valence Pd single-atom catalyst exhibits a markedly boosted HER activity with a low overpotential (η10 = 70 mV) at the current density of 10 mA/cm2 and a small Tafel slope (b = 68 mV/dec), nearly 150 mV and a 3.0-fold enhancement than those of Pd nanoparticles (η10 = 220 mV, b = 133 mV/dec) under the same conditions. In addition, quasi in situ XPS results suggest the hydrogen spillover effect is more likely to occur on Pd single atoms during the electrochemical process. Our work may pave an interesting route for the rational design of highly-efficient single-atom catalysts, and the elucidation of corresponding enhanced reaction mechanisms by the utilization of advanced characterization techniques.

Enhancing the Catalytic Kinetics and Stability of Ru Sites for Acidic Water Oxidation by Forming Brønsted Acid Sites in Tungsten Oxide Matrix

AbstractThe oxygen evolution reaction (OER) suffers from sluggish kinetics even on the benchmark RuO2 catalyst, due to the complex four sequential proton‐coupled electron transfer steps. Severe electrochemical oxidation and dissolution issues also make RuO2 fail as an alternative to highly expensive iridium‐based OER catalysts applied in proton exchange membrane water electrolysis. Herein, an acid‐stable W18O49‐δ matrix‐confined Ru solid solution oxide is developed with considerably reduced Ru loadings beyond commercial RuO2, to enhance the acidic OER kinetics and extend the long‐term durability simultaneously by incorporating Brønsted acid sites. The representative Ru0.6W17.4O49‐δ with 3D urchin‐like morphology achieves an excellent catalytic stability with ultra‐slow degradation rate and a high mass activity of 27 110 A g−1Ru @ 1.53 V versus RHE in 0.1 m HClO4 electrolyte, which is ≈10.8 times higher than that of commercial RuO2. The enhanced electron transfer from W to Ru during the OER process prevents the over‐oxidation of surface Ru sites extending the long‐term stability, while the incorporated Ru‐Obri‐W Brønsted acid sites accelerate the deprotonation step by promoting the mobility of proton from the oxo‐intermediate to the neighboring Obri sites, thus boosting the acidic OER kinetics.

Structural Transformation of Hydrated WO3 into SnWO4 via Sn Incorporation Enables a Superior Pseudocapacitor and Aqueous Zinc-Ion Battery

The strategy of crystal structure transformation tunes the electrochemical properties favoring the energy storage performance of the electrode material. Herein, we prepared SnWO4 nanoflakes through Sn incorporation into the tungsten oxide matrix by a single-step wet chemical method. The crystal structure tuning occurs from the orthorhombic structure of hydrated tungsten oxide (WO3·H2O, i.e., HWO) to the hexagonal (SnWO4, i.e., SWO) structure. Simultaneously, the morphology tailoring from nanodisks of HWO to nanoflakes of SWO is realized as a result of the Sn ion exchange mechanism. Further, the electrochemical supercapacitor of SWO nanoflakes demonstrates almost 2-fold enhancement in the specific capacitance of 622 F g–1 at 0.5 A g–1 over HWO nanodisks (255 F g–1). Moreover, the asymmetric supercapacitor (ASC) of SWO//activated carbon (AC) exhibits the specific capacitance of 50 F g–1 at 1 A g–1, with the maximum energy density and power density of 18 Wh kg–1 and 7000 W kg–1, respectively. Furthermore, superior performance of the aqueous zinc-ion battery (AZIB) demonstrates the specific capacity of 75 mA h g–1 at 0.3 A g–1 along with 106% stability and 101% coulombic efficiency after 70 cycles. The improved performance of SWO nanoflakes in the pseudocapacitor and AZIB is attributed to crystal structure tuning, increasing conductivity, enhanced surface area, and Sn redox sites. Therefore, the SWO nanoflakes are favorable cathode materials for commercial energy storage devices.

Tungsten oxide–cellulose nanocrystal composite films for electrochromic applications

Composite films of tungsten oxide and CNCs are prepared through a sol–gel method and their electrochromic (EC) properties investigated. After mixing CNC gel into a tungsten oxide precursor solution, indium-tin-oxide-coated glass substrates are dipped into the composite solution and subsequently annealed at 170°C. The composite films consisted of CNCs dispersed in the tungsten oxide matrix. The resulting nanocomposite was found to be amorphous, exhibiting a high transmission modulation and very good cycling stability. After having tested a range of compositions, a film of WO3 with 10% CNC was found to be the most uniform and showed good EC performance. These results bode well for further work on CNC-EC composites for specific applications, especially when used on flexible substrates.

Synthesis of Au microwires by selective oxidation of Au–W thin-film composition spreads

We report on the stress-induced growth of Au microwires out of a surrounding Au–W matrix by selective oxidation, in view of a possible application as ‘micro-Velcro’. The Au wires are extruded due to the high compressive stress in the tungsten oxide formed by oxidation of elemental W. The samples were fabricated as a thin-film materials library using combinatorial sputter deposition followed by thermal oxidation. Sizes and shapes of the Au microwires were investigated as a function of the W to Au ratio. The coherence length and stress state of the Au microwires were related to their shape and plastic deformation. Depending on the composition of the Au–W precursor, the oxidized samples showed regions with differently shaped Au microwires. The Au48W52 composition yielded wires with the maximum length to diameter ratio due to the high compressive stress in the tungsten oxide matrix. The values of wire length (35 μm) and diameter (2 μm) achieved at the Au48W52 composition are suitable for micro-Velcro applications.

Effect of silver incorporation in phase formation and band gap tuning of tungsten oxide thin films

Silver incorporated tungsten oxide thin films are prepared by RF magnetron sputtering technique. The effect of silver incorporation in micro structure evolution, phase enhancement, band gap tuning and other optical properties are investigated using techniques such as x-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and UV-Visible spectroscopy. Effect of silver addition in phase formation and band gap tuning of tungsten oxide thin films are investigated. It is found that the texturing and phase formation improves with enhancement in silver content. It is also found that as the silver incorporation enhances the thickness of the films increases at the same time the strain in the film decreases. Even without annealing the desired phase can be achieved by doping with silver. A broad band centered at the wavelength 437 nm is observed in the absorption spectra of tungsten oxide films of higher silver incorporation and this can be attributed to surface plasmon resonance of silver atoms present in the tungsten oxide matrix. The transmittance of the films is decreased with increase in silver content which can be due to increase in film thickness, enhancement of scattering, and absorption of light caused by the increase of grain size, surface roughness and porosity of films and enhanced absorption due to surface plasmon resonance of silver. It is found that silver can act as the seed for the growth of tungsten oxide grains and found that the grain size increases with silver content which in turn decreases the band gap of tungsten oxide from 3.14 eV to 2.70 eV.

Activation of dispersed PtSn/C nanoparticles by tungsten oxide matrix towards more efficient oxidation of ethanol

A novel electrocatalytic system for oxidation of ethanol, in which carbon-supported PtSn nanoparticles were modified with ultra-thin films/deposits of tungsten oxide, was proposed, fabricated and characterized here using electrochemical as well as spectroscopic (X-ray diffraction) and microscopic (transmission electron) techniques. The enhancement effect was evident from comparative diagnostic electrochemical experiments utilizing WO3-modified and bare (unmodified) PtSn nanoparticles in acid medium (0.5moldm−3 H2SO4) at room temperature in the presence of ethanol. It is noteworthy that, the chronoamperometric electrocatalytic currents measured at potentials as low as 0.3V (vs. RHE) were significantly larger for WO3-modified PtSn/C relative to bare PtSn/C. In another diagnostic “stripping” experiment, it was found that oxidation of CO-adsorbate occurred in the presence of WO3 at potentials almost 100mV lower in comparison to the unmodified system. The overall activation effect may have origin in interactions of tungsten oxide with tin (from PtSn alloy nanoparticles) leading to stabilization of the catalytic tin oxo species. WO3 may also provide large population of reactive oxo groups at the Pt/Sn-based electrocatalytic interface.

An Efficient Route to Nanostructured Tungsten Oxide Films with Improved Electrochromic Properties

An effective chemical route to nanostructured tungsten oxide films derived from a peroxopolytungstic acid (PTA)/thiourea precursor solution is demonstrated. The conventional procedure of preparing the precursor needs more than 24 h for well-mixing and refluxing the PTA-based solution, while the thiourea-assisted approach takes less than 1 h to prepare the precursor solution because the excess hydrogen peroxide can be efficiently eliminated by oxidation of thiourea. With the precursor solution, tungsten oxide films are deposited by spin coating followed by high temperature annealing. The film annealed at 400 °C possesses a porous nanostructure of nanocrystalline tungsten oxide embedded in an amorphous tungsten oxide matrix, which arises from the gaseous species released through decomposition of thiourea oxides during annealing. The 400 °C-annealed, thiourea-assisted tungsten oxide film exhibits electrochromic (EC) properties superior to those of the film prepared without thiourea, including large transmittance modulation and coloration efficiency, fast response time and adequate reliability. When increasing the annealing temperature to 450 °C, the thiourea-assisted tungsten oxide film is also porous but well-crystallized and shows inferior EC properties. Electrochemical impedance spectroscopy analysis indicates that, in addition to the porous structure, a fast charge-transport rate within the solid portion of the 400 °C-annealed nanostructured film plays a crucial role in enhancing EC performances of the thiourea-assisted tungsten oxide film.

Assembly of Pure Silver‐Tungsten‐Oxide Frameworks from Nanostructured Solution Processable Clusters and Their Evolution into Materials with a Metallic Component

Silver-Tungsten-Oxide Frameworks: Isomerically pure forms of the unique isopolyoxotungstate system are incorporated into highly connected framework materials using Ag(I) as a linking unit. Thermal treatment of the materials leads to the formation of silver microparticles embedded in a tungsten oxide matrix. This novel synthetic approach promises to create a new class of nanostructured tungsten-silver-oxides with exciting physical properties, as well as exemplifies the potential for new frameworks based upon solution processable POM clusters.

Outperformed electrochromic behavior of poly(ethylene glycol)-template nanostructured tungsten oxide films with enhanced charge transfer/transport characteristics

Porous tungsten oxide films of nanocrystalline tungsten oxide embedded in an amorphous tungsten oxide matrix have been synthesized via poly(ethylene glycol) (PEG)-template sol-gel technique with peroxopolytungstic acid precursor. The effects of PEG addition on the microstructure and electrochromic performance of the tungsten oxide films are investigated. Charge transfer/transport properties in the tungsten oxide films are studied by electrochemical impedance spectroscopy (EIS) as well. Triclinic tungsten oxide film is formed in the absence of PEG. The PEG-template tungsten oxide film demonstrates an electrochromic performance superior to that of the crystalline tungsten oxide film, including larger transmittance modulation and coloration/bleaching efficiency as well as faster response times. EIS measurements indicate that faster charge-transfer rates at the tungsten oxide/electrolyte interface and larger Li(+) diffusion coefficients in tungsten oxide are achieved in the PEG-template film. We suggest that the PEG-template tungsten oxide film with a porous crystalline/amorphous nanostructure provides an effective means for charge transfer/transport to encourage its superior electrochromic performance.

Nanogravimetric studies of tungsten oxide thin films obtained by the polymeric precursor method

In this work, the intercalation/de-intercalation process of Li + ions in the tungsten oxide matrix was investigated. The reaction mechanism involved was also investigated. The WO 3 films, prepared by the polymeric precursor method, were deposited on a Pt covered quartz crystal using the dip-coating technique. The electrolyte was 0.1 mol L − 1 LiClO 4 in acetonitrile. The morphology and structure of the deposit was accomplished by scanning electron microscopy and X-ray diffraction, respectively. In the electrochemical quartz crystal nanobalance results, it was observed that the mass transport as well as the kinetic processes involved are facilitated in the films cycled at lower potential sweep-rates. The mass variation data as a function of the charge variations in the anodic and cathodic regions indicate the participation of solvent molecules (acetonitrile) during the Li + ion intercalation/de-intercalation process. This was confirmed by the development of a model of the species flux as a function of the potential.

Enhancement of the Electrocatalytic Reduction of Oxygen at Tungsten Oxide Modified Carbon-Supported RuSex Nanoparticles and Co-Porphyrin Centers

Introduction of carbon-supported RuSex nanoparticles or cobalt protoporphyrin into ultra-thin WO3 matrices results in the enhancement of their electroctalytic properties towards electroreduction of dioxygen in acidic medium (0.5 mol dm-3 H2SO4). The enhancement effects are evidenced from the positive shifts of the O2 reduction potentials (ca 150 mV and 200mV, respectively). An important issue is that the mixed-valent tungsten (VI, V) oxide support is characterized by good electronic and protonic conductivities. Further, not only RuSex and/or cobalt protoporphyrin are catalytic but also the tungsten oxide matrix is reactive: it provides protons and is capable of promoting the reduction of hydrogen peroxide intermediate.

Crystallization of chemically vapor deposited molybdenum and mixed tungsten/molybdenum oxide films for electrochromic application

The paper deals with investigation of Mo oxide and mixed W/Mo oxide films showing high electrochromic performance. Films are deposited on Si and conductive glass substrates using pyrolytical decomposition at 200 °C of Mo and W hexacarbonyls in Ar/O 2 atmosphere. The study is focused on structural transformation of the films in dependence on deposition and annealing process parameters. In case of conductive glass substrate (typical for electrochromic devices), the crystallization process in Mo oxide films is almost completed at 400 °C forming triclinic MoO 2.89 and orthorhombic MoO 3 crystalline phases. The structure of mixed W/Mo oxide films is triclinic crystalline phase of tungsten oxide matrix with Mo atoms as substitutes. Discussed are, as well, differences in the crystallization process for the same films, when the substrate is Si. All the films show electrochromic effect, the mixed W/Mo oxide films expressing stronger electrochromic effect with superior color efficiency and optical modulation.

Enhancement of the oxidation of methyl formate at multifunctional electrocatalyst composed of Pt/Pd and Pt/Ru nanoparticles

Immobilization of carbon-supported Pt/Pd nanoparticles (that have been modified with ruthenium oxo species) within the tungsten oxide film containing Pt/Ru nanoparticles results in the enhancement of the catalytic properties towards electrooxidation of methyl formate in 0.5 mol dm−3 H2SO4. The over-coating of Pt/Pd nanoparticles with cationic Ru-oxo species facilitates their attachment to tungsten oxide matrix. Because oxidation of the ester is mechanistically complex and it is expected to occur most rapidly on the formate component, the comparative measurements have been done with formic acid. In spite of modification of Pt/Pd with Ru-oxo species, the system retains high electrocatalytic activity during oxidation of formic acid. It is rationalized that not only Pt/Ru is catalytic towards oxidation of methyl formate (or products of its hydrolysis) but also oxoruthenium modified Pt/Pd is capable of promoting effectively the oxidation of formic acid or formate component of the ester. The multifunctional activity of co-deposited Pt/Ru and Pt/Pd centers is supported by good electronic and protonic conductivities of hydrated tungsten oxide matrix.

Aerosol Assisted Chemical Vapor Deposition Using Nanoparticle Precursors: A Route to Nanocomposite Thin Films

Gold nanoparticle and gold/semiconductor nanocomposite thin films have been deposited using aerosol assisted chemical vapor deposition (CVD). A preformed gold colloid in toluene was used as a precursor to deposit gold films onto silica glass. These nanoparticle films showed the characteristic plasmon absorption of Au nanoparticles at 537 nm, and scanning electron microscopic (SEM) imaging confirmed the presence of individual gold particles. Nanocomposite films were deposited from the colloid concurrently with conventional CVD precursors. A film of gold particles in a host tungsten oxide matrix resulted from co-deposition with [W(OPh)(6)], while gold particles in a host titania matrix resulted from co-deposition with [Ti(O(i)Pr)(4)]. The density of Au nanoparticles within the film could be varied by changing the Au colloid concentration in the original precursor solution. Titania/gold composite films were intensely colored and showed dichromism: blue in transmitted light and red in reflected light. They showed metal-like reflection spectra and plasmon absorption. X-ray photoelectron spectroscopy and energy-dispersive X-ray analysis confirmed the presence of metallic gold, and SEM imaging showed individual Au nanoparticles embedded in the films. X-ray diffraction detected crystalline gold in the composite films. This CVD technique can be readily extended to produce other nanocomposite films by varying the colloids and precursors used, and it offers a rapid, convenient route to nanoparticle and nanocomposite thin films.

Mesostructured WO3 as a sensing material for NO2 detection

AbstractNanostructured tungsten oxide synthesized from SiO2 templates (SBA-15 and KIT-6) has been used for NO2 gas sensing. Chromium has been added as catalytic additive to WO3 in order to enhance sensor response. Several techniques have been used for identifying both additive location in the tungsten oxide matrix and its oxidation state. Raman spectroscopy confirmed the presence of terminal chromium-oxygen bonds at the material surface. Besides, X-ray photoelectron spectroscopy showed chromium peaks attributable to Cr(III) species. Electrical behavior of pure WO3 has found to be highly dependent on the nanostructure type, i. e. 2D SBA-15 and 3D KIT-6 replicas. Chromium addition diminishes response time and improves sensor response at low NO2 concentrations. Electrical differences due to WO3 nanostructure disappears as a result of additive introduction in the material.

Deposition of non-stoichiometric tungsten oxides+MO 2 composites (M=Ru or Ir) and study of their catalytic properties in hydrogen or oxygen evolution reactions

The electrodeposition of composites consisting of a non-stoichiometric tungsten oxide matrix and either RuO 2 or IrO 2 dispersed particles was investigated. These materials were then tested as electrodes for both reduction (hydrogen evolution) and oxidation (oxygen evolution) reactions in acid medium. The composite deposition was carried out by cyclic voltammetry, potentiostatic or galvanostatic electrolysis of suspensions of the RuO 2 or IrO 2 particles in two different media described as appropriate for WO 3 deposition: (i) a colloidal suspension obtained from Na 2WO 4 and H 2SO 4 or (ii) a solution obtained by dissolving W powder in aqueous H 2O 2. All the deposited composites were found to catalyse the hydrogen evolution reaction but none was active in oxygen evolution, presumably due to an inadequate electronic conductivity of the matrix. Such a behaviour differs from that of Pt-containing tungsten oxide electrodes which have been described as suitable anodes for the oxidation of organics.

Oxidation of methanol at an electrocatalytic film containing platinum and polynuclear oxocyanoruthenium microcenters dispersed within tungsten oxide matrix

We propose a composite bifunctional electrocatalytic film that contains platinum microparticles and oxocyanoruthenium microstructures dispersed in the reactive matrix of non-stochiometric tungsten(VI,V) oxides. Polynuclear oxocyanoruthenium can be viewed as a ruthenium analogue of Prussian blue or, more precisely, ruthenium(II,III,IV) hexacyanoruthenate(II,III), that features ruthenium oxo species within the cyanide bridged network. The composite film was fabricated by sequential electrodeposition starting from the preparation of tungsten oxide film into which oxocyanoruthenium and metallic platinum (from chloroplatinate) centers were introduced. To evaluate the reactivity of such a hybrid system toward the oxidation of methanol at fairly low potentials (0.1–0.3 V vs. SCE), cyclic voltammetric and chronoamperometric measurements have been performed at 22 and 60 °C. With approximately the same platinum loading and under analogous conditions, the composite film shows higher catalytic activity than a simple platinized tungsten oxide. It is possible that ruthenium hydroxo species generated within polynuclear oxocyanoruthenium microstructures exhibit an activating effect on neighboring dispersed platinum microparticles. An alternative explanation may involve the possibility of different morphologies of the catalytic films in the presence and absence of oxocyanoruthenium species. At higher potentials, ruthenium(IV)-oxo centers participate directly in the oxidation of methanol. The protonically/electronically conducting and physicochemically stable tungsten oxide/hydrogen bronze matrix is expected to support the activity of platinum and ruthenium centers.

Study of W/HZSM-5-Based Catalysts for Dehydro-aromatization of CH4 in Absence of O2. II. Action of Promoters Zn and Li

By correlating the results of the NH3-TPD characteristic study and the catalyst activity assay of the W/HZSM-5-based catalysts, we confirmed that the intensity and concentration of the surface B-acid sites have pronounced effects on the catalyst performance for dehydro-aromatization of methane (DHAM). It was found experimentally that, by addition of a proper amount of Mg2+, the strong B-acid sites at the catalyst surface could be effectively eliminated, whereas the addition of a proper amount of Zn2+ or Li+ resulted not only in eliminating most of the strong surface B-acid sites but also in generating a kind of new medium-strong acid sites, mostly B-acid sites, simultaneously. The latter could serve as the catalytically active sites for dehydro-aromatization of methane; on such medium-strong surface B-acid sites, the formation of coke would be also alleviated to a greater extent. By simultaneous addition of Mg2+ and Zn2+, optimized adjustment in surface acidity of the catalyst could be realized. On the other hand, the doping of the Zn2+ or Li+ component to the tungsten oxide matrix would facilitate inhibiting aggregation of the W-containing active species and improving dispersion of the W component at the surface of the catalyst, thus leading to a pronounced decrease in the reduction temperature for the hard-to-be-reduced W6+ species and an increase in quantity of the reducible W6+ species at the reaction temperature for DHAM, as has been evidenced by the results of a H2-TPR study on the reducibility of the Zn2+ (or La3+, Li+, Mn2+)-promoted W/HZSM-5 system. The above two roles that Zn2+ and Li+ as promoters played both contributed to the persistence of high methane conversion and benzene selectivity, and the alleviation of coke deposition, as well as the prolongation of the catalyst lifetime.