Pd nanoparticles supported on WO3/C hybrid material as catalyst for oxygen reduction reaction

Pd nanoparticles supported on WO₃/C hybrid material have been developed as the catalyst for the oxygen reduction reaction (ORR) in direct methanol fuel cells. The resultant Pd–WO₃/C catalyst has an ORR activity comparable to the commercial Pt/C catalyst and a higher activity than the Pd/C catalyst prepared with the same method. Based on the physical and electrochemical characterizations, the improvement in the catalytic performance may be attributed to the small particle sizes and uniform dispersion of Pd on the WO₃/C, the strong interaction between Pd and WO₃ and the formation of hydrogen tungsten bronze which effectively promote the direct 4-electron pathway of the ORR at Pd.     

Investigation of WO3/ZnO thin-film heterojunction-based Schottky diodes for H2 gas sensing

A comparative study of Schottky diode hydrogen gas sensors based on Pd/WO₃/Si and Pd/WO₃/ZnO/Si structure is presented in this work. Atomic force microscopy and X-ray photoelectron spectroscopy reveal that the WO₃ sensing layer grown on ZnO has a rougher surface and better stoichiometric composition than the one grown on the Si substrate. Analysis of the I–V characteristics and dynamic response of the two sensors when exposed to different hydrogen concentrations and various temperatures indicate that with the addition of the ZnO layer, the diode can exhibit a larger voltage shift of 4.0 V, 10 times higher sensitivity, and shorter response and recovery times (105 s and 25 s, respectively) towards 10,000-ppm H₂/air at 423 K. Study on the energy band diagram of the diode suggests that the barrier height is modulated by the WO₃/ZnO heterojunction, which could be verified by the symmetrical sensing properties of the Pd/WO₃/ZnO/Si gas sensor with respect to applied voltage.     

The decoration of TiO2/reduced graphene oxide by Pd and Pt nanoparticles for hydrogen gas sensing

Reduced graphene oxide (RGO) was used to improve the hydrogen sensing properties of Pd and Pt-decorated TiO₂ nanoparticles by facile production routes. The TiO₂ nanoparticles were synthesized by sol–gel method and coupled on GO sheets via a photoreduction process. The Pd or Pt nanoparticles were decorated on the TiO₂/RGO hybrid structures by chemical reduction. X-ray photoelectron spectroscopy demonstrated that GO reduction is done by the TiO₂ nanoparticles and Ti–C bonds are formed between the TiO₂ and the RGO sheets as well. Gas sensing was studied with different concentrations of hydrogen ranging from 100 to 10,000 ppm at various temperatures. High sensitivity (92%) and fast response time (less than 20 s) at 500 ppm of hydrogen were observed for the sample with low concentration of Pd (2 wt.%) decorated on the TiO₂/RGO sample at a relatively low temperature (180 °C). The RGO sheets, by playing scaffold role in these hybrid structures, provide new pathways for gas diffusion and preferential channels for electrical current. Based on the proposed mechanisms, Pd/TiO₂/RGO sample indicated better sensing performance compared to the Pt/TiO₂/RGO. Greater rate of spill-over effect and dissociation of hydrogen molecules on Pd are considered as possible causes of the enhanced sensitivity in Pd/TiO₂/RGO.     

Feasibility of H2 sensors composed of tungsten oxide nanocluster films

The hydrogen (H₂) sensing properties, including the sensor response, response time and recovery time, of different sensor architectures based on tungsten oxide (WO₃) were investigated to assess the feasibility of using WO₃ in producing practical H₂ sensors. Each of the different sensor architectures consists of 3 layers. The first layer is a 2.5-nm palladium (Pd) layer, which is always deposited onto a highly porous WO₃ nanocluster layer. The third layer is an Au/Ti electrode layer, which may be constructed in the form of interdigitated electrodes or 5 × 5 mm² pad electrodes, which is located either on the top surface of the Pd layer or at the bottom of the WO₃ film. Furthermore, the WO₃ layer was also constructed to be either 11.2 nm or 153 nm thick. The sensor design consisting of a 2.5-nm Pd layer on an 11.2-nm WO₃ layer with interdigitated electrodes at the bottom of the layer was found to exhibit the best overall H₂ sensing properties, with excellent cyclic stability over 600 cycles of operation.     

Sensitive and rapid hydrogen sensors based on Pd–WO3 thick films with different morphologies

WO₃ loaded with noble metals is well-known to be sensitive to reducing gases and can be used as hydrogen sensor. This paper presents a simple and attractive method concerning the preparation of hydrogen sensors based on Pd-loaded WO₃ nanocomposites with different morphologies. The influences of the morphology of WO₃ and the palladium growth on its surface on the hydrogen sensing performances are studied. WO₃ nanospheres, nanowires and nanolamellae were synthesized by different methods starting from the same precursor (H₂WO₄·nH₂O) which has been obtained by acidification of sodium tungstate (Na₂WO₄). The prepared WO₃ nanostructures were modified with the Pd by dispersing them in a PdCl₂ containing solvent using sonication (giving Pd-WO₃ inks). The sensors were prepared by screen-printing thick films (∼10 μm) of these inks on alumina substrates fitted with gold electrodes. The response of Pd-loaded WO₃ sensors to hydrogen was checked for the different morphologies at working temperatures ranging from 180 to 240 °C. The sensors prepared from nanolamellae showed the highest response while the nanowires presented the shortest response time to hydrogen.     

Layered WO3/TiO2 nanostructures with enhanced photocurrent densities

Layered WO₃/TiO₂ nanostructures, fabricated by magnetron sputtering, demonstrate significantly enhanced photocurrent densities compared to individual TiO₂ and WO₃ layers. First, a large quantity of compositions having different microstructures and thicknesses were fabricated by a combinatorial approach: diverse WO₃ microstructures were obtained by adjusting sputtering pressures and depositing the films in form of wedges; later layers of TiO₂ nanocolumns were fabricated thereon by the oblique angle deposition. The obtained photocurrent densities of individual WO₃ and TiO₂ films show thickness and microstructure dependence. Among individual WO₃ layers, porous films exhibit increased photocurrent densities as compared to the dense layer. TiO₂ nanocolumns show length-dependent characteristics, where the photocurrent increases with increasing film thickness. However, by combining a WO₃-wedge type layer with a layer of TiO₂ nanocolumns, PEC properties strikingly improve, by about two orders of magnitude as compared to individual WO₃ layers. The highest photocurrent that is measured in the combinatorial library of porous WO₃/TiO₂ films is as high as 0.11 mA/cm². Efficient charge-separation and charge carrier transfer processes increase the photoconversion efficiency for such films.     

WO3/C supported Pd catalysts for formic acid electro-oxidation activity

Pd-WO₃/C ternary hybrid was designed as a high-efficient catalyst towards formic acid electrooxidation. WO₃/C hybrids were first prepared with two different synthesis order, and then used as the supports to synthesize two kinds of Pd-WO₃/C catalysts by a quick and facile microwave-assisted ethylene glycol method. Compared with Pd/C, the catalytic performances of two Pd-WO₃/C catalysts towards formic acid electrooxidation are significantly enhanced. We elected the better synthesis order and optimized the best proportion of WO₃ and C in the hybrid catalyst. When the mass content of WO₃ is 20% of the mass of the support, Pd nanoparticles with narrower particle size distribution are more uniformly dispersed on the surface of WO₃/C support than the other counterparts, resulting in the highest performance in terms of activity and stability towards formic acid oxidation among all the samples. The reasons for the performance improvement may be: first, Pd nanoparticles in Pd-WO₃/C catalysts are of small size and evenly distributed; second, there may be the catalyst-support interaction between Pd and WO₃, substantially improving the catalytic capability of Pd-WO₃/C catalysts; finally, the hydrogen spillover effect produced by WO₃ significantly expedites the dehydrogenation of formic acid on the surface of Pd-WO₃/C.     

Study on photocurrent of bilayers photoanodes using different combination of WO3 and Fe2O3

Bilayer photoanodes were prepared onto glass substrates (FTO) in order to improve generated photocurrents using UV-vis light by water splitting process. A comparative study of photocatalytic was performed over the films surface using Fe₂O_3, WO₃ and mixture of bicomponents (Fe₂O₃:WO₃). Different types of films were prepared using Fe₂O_3, WO₃ and bicomponents (mixture) on FTO substrates. The films were grown by sol gel method with the PEG-300 as the structure-directing agent. The photo-generated of the samples were determined by measuring the currents and voltages under illumination of UV-vis light. The morphology, structure and related composition distribution of the films have been characterized by SEM, XRD and EDX respectively. Photocurrent measurements indicated surface roughness as the effective parameter in this study. The deposited surfaces by bicomponents or mixture are flat without any feature on the surface while the deposited surfaces by WO₃ appears rough surface as small round (egg-shaped particles) and cauliflower-like. The surface deposited by Fe₂O₃ show rough no as well as WO₃ surface. The deposited surfaces by WO₃ reveal the higher value of photocurrent measurement due to surface roughness. Indeed, the roughness can be effective in increasing contact surface area between film and electrolyte and diffuse reflection (light scattering effect). The solution (Fe₂O₃:WO₃) shows the low photocurrent value in compare to WO₃ and Fe₂O₃ hat it may be due to decomposition the compound at 450 ± 1 °C to iron-tungstate Fe₂(WO₄)₃.     

Synthesis of Pd/TiO2 nanotubes/Ti for oxygen reduction reaction in acidic solution

Highly ordered and uniformly distributed TiO₂ nanotubes on a pure titanium substrate (TNTs/Ti) are successfully fabricated by a pulse anodic oxidation method as the support for Pd electrocatalyst. Pd is electrochemically deposited onto TNTs/Ti support. The sensitization with SnCl₂ and activation with PdCl₂ are critical for the formation of highly dispersed Pd nanoparticles on the TNTs/Ti support. It has been found that both Pd/TNTs/Ti and Pt electrodes show the similar electrochemical behavior in H₂SO₄, implying the possibility to develop the Pt-free alternative electrocatalyst based on the Pd/TNTs/Ti system in acid medium. The preliminary results in this work show that the Pd/TNTs/Ti catalysts have an acceptable catalytic activity for the oxygen reduction reaction (ORR) in acid medium. The factors influencing the structure of TNTs and the catalytic activity of Pd/TNTs/Ti for the ORR are also studied in detail.     

Hydrogen evolution reaction on single crystal WO3/C nanoparticles supported on carbon in acid and alkaline solution

Single crystal tungsten oxide (WO₃) nanoparticles were prepared via a microwave-assisted method. Electrochemical activity for hydrogen evolution reaction (HER) on WO₃ supported on carbon black (WO₃/C) electrocatalyst was first studied in acid solution (0.5 M H₂SO₄) and alkaline solution (1.0 M KOH) at room temperature. The overall experimental results revealed that the electrocatalytic activity for HER on WO₃/C is one order magnitude higher than those obtained with carbon black in 0.5 M H₂SO₄ and is six times than in the case of carbon black in 1.0 M KOH. These results demonstrated that WO₃ could enhance the electrocatalytic activity for hydrogen evolution reaction in acid solution (0.5 M H₂SO₄) and alkaline solution (1.0 M KOH). On the other hand, the kinetic reaction mechanisms were discussed on WO₃/C electrocatalysts and carbon black in acid solution and alkaline solution for HER. Consequently, the rate-determining step changed from Tafel to Volmer due to the incorporation of WO₃ in acid solution. However, the rate-determining step carries through Tafel reaction on both electrocatalysts in alkaline solution though WO₃ was introduced.     

Ag grid induced photocurrent enhancement in WO3 photoanodes and their scale-up performance toward photoelectrochemical H2 generation

The hydrogen generation from photoelectrochemical (PEC) water splitting under visible light was investigated using large area tungsten oxide (WO₃) photoanodes. The photoanodes for PEC hydrogen generation were prepared by screen printing WO₃ films having typical active areas of 0.36, 4.8 and 130 cm² onto the conducting fluorine-doped tin oxide (FTO) substrates with and without embedded inter-connected Ag grid lines. TiO₂ based dye-sensitized solar cell was also fabricated to provide the required external bias to the photoanodes for water splitting. The structural and morphological properties of the WO₃ films were studied before scaling up the area of photoanodes. The screen printed WO₃ film sintered at 500 °C for 30 min crystallized in a monoclinic crystal structure, which is the most useful phase for water splitting. Such WO₃ film revealed nanocrystalline and porous morphology with grain size of ∼70-90 nm. WO₃ photoanode coated on Ag grid embedded FTO substrate exhibited almost two-fold degree of photocurrent density enhancement than that on bare FTO substrate under 1 SUN illumination in 0.5 M H₂SO₄ electrolyte. With such enhancement, the calculated solar-to-hydrogen conversion efficiencies under 1 SUN were 3.24% and ∼2% at 1.23 V for small (0.36 cm²) and large (4.8 cm²) area WO₃ photoanodes, respectively. The rate of hydrogen generation for large area photoanode (130.56 cm²) was 3 mL/min.     

Pd doped WO3 films prepared by sol–gel process for hydrogen sensing

The sol gel method was employed to prepare peroxopolytungstic acid (P-PTA). Palladium chloride salt was dissolved in the sol with different Pd:W molar ratios and coated on Al₂O₃ substrates by spin coating method. XRD and XPS techniques were used to analyze the crystal structure and chemical composition of the films before and after heat treatment at 500 °C. We observed that Pd can modify the growth kinetic of tungsten trioxide nanoparticles by reducing the crystallite size and as a result can improve hydrogen sensitivity. Resistance-sensing measurements indicated sensitivity of about 2.5 × 10⁴ at room temperature in hydrogen concentration of 0.1% in air. Considering all sensing parameters, an optimum working temperature of 100 °C was obtained.     

Single cell study of electrodeposited cathodic electrodes based on Pt-WO3 for polymer electrolyte fuel cells

Cathodic electrodes based on electrodeposited Pt-WO₃ material for proton exchange membrane fuel cells (PEMFC) were studied in single cell configuration. Preparation of the electrodes was carried out by electrodeposition of Pt and WO₃ on commercial gas diffusion layer substrates (microporous carbon black layer on carbon cloth, ELAT E-TEK). The process of simultaneous electrodeposition of Pt and WO₃ is first analyzed from voltammetric curves. It is observed that the deposition of Pt is enhanced when WO₃ is present. Compositional analysis of the electrodes shows metallic platinum and WO₃ in variable proportions. The electrodeposited electrodes were characterized in single PEMFC. Membrane-electrode assemblies were prepared with Nafion^® 117 electrolyte membrane and a standard Pt/C anode. Pt-WO₃ electrodes showed enhanced stability and good response in single cell up to 1500 h. Performance degradation is attributed to a decrease in Pt electroactive area and increase of the internal resistance of the cell. These effects are possibly a consequence of the production of mobile tungsten species, like soluble WO₂ at high current demands and low cathode potentials.     

Structural and electrochemical studies of WO3 films deposited by pulsed spray pyrolysis

Transparent conductive and WO₃ electrochromic thin films were deposited by spray pyrolysis technique. The films were deposited using solutions of WCl₆ in dimethylformamide on SnO₂:F (FTO) substrates with different sheet resistances. Noticeable effects of substrate on structural, morphological and optical properties of the WO₃ films and on its electrochromic behavior are presented and discussed. Hexagonal and monoclinic WO₃ structures were obtained on amorphous glass substrates; also the monoclinic structure on polycrystalline FTO substrates was obtained. Cyclic structural changes during the colored and blanched states were found from XRD and electron diffraction result analysis: The hydrogen tungsten bronze in the tetragonal phase after the hydrogen extraction change to the original WO₃ monoclinic phase.     

Stability of gasochromic WO3 films

Gasochromic films consist of an electrochromic layer such as WO₃ and a very thin catalyst coating, like Pt. Hydrogen is dissociated on the catalyst into H atoms, which color the electrochromic film. A complete bleaching can be achieved in oxygen, whereas flushing with argon or evacuating the sample leads to a comparatively slow and incomplete bleaching. We discuss two kinds of aging processes, i.e. catalysed poisoning by reactants in air and a change in the water content of the WO₃. Poisoning by air increases the time needed for coloring in H₂ and bleaching in O₂ or in Ar. From results with variable WO₃ film thicknesses, we conclude that poisoning results from adsorption of a blocking species on the interior surfaces of the WO₃ pores and not on the catalyst. The adsorption process is accelerated by the catalyst. After drying the device at 100°C in vacuum, there was a severe decrease in the coloring and bleaching rates due to a reduction of the diffusion in the WO₃. Furthermore, the coloration at steady state was more intense. The variation of the water content of the WO₃ was attempted by exposing it to dry or humid atmospheres and was investigated by IR spectroscopy. No changes in water content could be detected, and no significant change in the coloration velocity could be found. To demonstrate the long-term stability of the film, a 1.1 m×0.6 m large window was switched 20,000 times at 20°C over 2 yr without any significant change in performance.     

On the processes of photo-induced hydrogen generation from methanol solution by Ta3N5 and WO3

The hydrogen production rates from deionized water and 20% methanol solution, with or without the presence of Ta₃N₅, WO₃, and the indirect Z-scheme Ta₃N₅/WO₃, were investigated. Under irradiation of a 300 W Xe lamp, all of these three catalysts assisted hydrogen generation in deionized water. In the methanol solution, Ta₃N₅, and WO₃ reduced the hydrogen generation, but Ta₃N₅/WO₃ significantly enhanced the hydrogen production rate by seven times. Under visible light irradiation, the effects of the three catalysts are different from those under full spectrum irradiation. The mechanisms based on the competition of methanol decomposition and water reduction in the presence of catalyst under different irradiation conditions are proposed to explain the different hydrogen generation behaviors.     

High activity of Pd-WO3/C catalyst as anodic catalyst for direct formic acid fuel cell

Pd nanoparticles supported on the WO₃/C hybrid are prepared by a two-step procedure and the catalysts are studied for the electrooxidation of formic acid. For the purpose of comparison, phosphotungstic acid (PWA) and sodium tungstate are used as the precursor of WO₃. Both the Pd-WO₃/C catalysts have much higher catalytic activity for the electrooxidation of formic acid than the Pd/C catalyst. The Pd-WO₃/C catalyst prepared from PWA shows the best catalytic activity and stability for formic acid oxidation; it also shows the maximum power density of approximately 7.6 mW cm⁻² when tested with a small single passive fuel cell. The increase of electrocatalytic activity and stability is ascribed to the interaction between the Pd and WO₃, which promotes the oxidation of formic acid in the direct pathway. The precursors used for the preparation of the WO₃/C hybrid support have a great effect on the performance of the Pd-WO₃/C catalyst. The WO₃/C hybrid support prepared from PWA is beneficial to the dispersion of Pd nanoparticles, and the catalyst has potential application for direct formic acid fuel cell.     

Highly selective hydrogen sensing of Pt-loaded WO3 synthesized by hydrothermal/impregnation methods

Unloaded and 0.25–1.0 wt% Pt-loaded WO₃ nanoparticles were synthesized by hydrothermal method using sodium tungstate dihydrate and sodium chloride as precursors in an acidic condition and impregnated using platinum acetylacetonate. Pt-loaded WO₃ films on an Al₂O₃ substrate with interdigitated Au electrodes were prepared by spin-coating technique. The response of WO₃ sensors with different Pt-loading concentrations was tested towards 0.01–1.0 vol% of H₂ in air as a function of operating temperature (200–350 °C). The 1.0 wt% Pt-loaded WO₃ sensing film showed the highest response of ∼2.16 × 10⁴ to 1.0 vol% H₂ at 250 °C. Therefore, an operating temperature of 250 °C was optimal for H₂ detection. The responses of 1.0 wt% Pt-loaded WO₃ sensing film to other flammable gases, including C₂H₅OH, C₂H₄ and CO, were considerably less, demonstrating Pt-loaded WO₃ sensing film to be highly selective to H₂.     

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.     

An improvement of the hydrogen permeability of C/Al2O3 membranes by palladium deposition into the pores

The development of compact hydrogen separator based on membrane technology is of key importance for hydrogen energy utilization, and the Pd-modified carbon membranes with enhanced hydrogen permeability were investigated in this work. The C/Al₂O₃ membranes were prepared by coating and carbonization of polyfurfuryl alcohol, then the palladium was introduced through impregnation–precipitation and colloid impregnation methods with a PdCl₂/HCl solution and a Pd(OH)₂ colloid as the palladium resources, and the reduction was carried out with a N₂H₄ solution. The resulting Pd/C/Al₂O₃ membranes were characterized by means of SEM, EDX, XRD, XPS and TEM, and their permeation performances were tested with H₂, CO₂, N₂ and CH₄ at 25 °C. Compared with the colloid impregnation method, the impregnation–precipitation is more effective in deposition of palladium clusters inside of the carbon layer, and this kind of Pd/C/Al₂O₃ membranes exhibits excellent hydrogen permeability and permselectivity. Best hydrogen permeance, 1.9 × 10⁻⁷ mol/m² s Pa, is observed at Pd/C = 0.1 wt/wt, and the corresponding H₂/N₂, H₂/CO₂ and H₂/CH₄ permselectivities are 275, 15 and 317, respectively.