Zaoxue Yan

Nanosized tungsten carbide synthesized by a novel route at low temperature for high performance electrocatalysis.

Tungsten carbide (WC) is a widely used engineering material which is usually prepared at high temperature. A new mechanism for synthesizing nanoscaled WC at ultralow temperature has been discovered. This discovery opens a novel route to synthesize valuable WC and other carbides at a cost-efficient way. The novel formation mechanism is based on an ion-exchange resin as carbon source to locally anchor the W and Fe species. As an intermediate, FeWO4 can be formed at lower temperature, which can be directly converted into WC along with the carbonization of resin. The size of WC can be less than 2 nm. The catalyst made with Pt nanoparticles supported on nanosized WC-GC (WC-graphitized carbon) shows enhanced electrocatalytic activity for oxygen reduction reaction. The result also indicates that the Pt nanoparticles deposited on WC-GC are dominated by Pt (111) plane and shows a mass activity of 257.7 mA mg−1Pt@0.9 V.

Low temperature formation of porous graphitized carbon for electrocatalysis

The ion-exchange resin is used as a carbon source to prepare porous structured graphitized carbon. The X-ray diffraction, laser micro-Raman spectroscopy, transmission electron microscopy, Brunauer–Emmett–Teller (BET) surface area, scanning electron microscopy and thermogravimetry are adopted to characterize the properties of the porous graphitized carbon. The factors affecting the performance of the products, including the concentration of K4[Fe(CN)6] and the heating temperature, are evaluated. The results prove that the porous structured carbon with a suitable degree of graphitization results in improved catalytic activities as an electrocatalyst support, owing to the better electronic conductivity and improved mass transfer behavior. The Pt nanoparticles supported on porous graphitized carbon show improved performance for the oxygen reduction reaction and the methanol oxidation reaction compared with that of a commercial Pt/C electrocatalyst.

Synthetic core–shell Ni@Pd nanoparticles supported on graphene and used as an advanced nanoelectrocatalyst for methanol oxidation

In this study, the uniform dispersion of new highly active Ni@Pd core–shell nanoparticle catalysts supported on graphene (Ni@Pd/graphene) was prepared via a two-step procedure involving a microwave synthesis method and a replacement method. Several characterization tools, such as X-ray powder diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) were employed to study the phase structures, morphologies and properties of the Ni@Pd/graphene composite. The results indicated that a uniform dispersion of Ni@Pd core–shell structure nanoparticles on graphene have an average particle size of 4 nm. The Ni@Pd/graphene composite was used as an electrocatalyst for alcohol oxidation in alkaline media for fuel cells. The electrocatalytic activity of Ni@Pd/graphene for ethanol oxidation is 3 times higher than that of the Pd/graphene electrocatalyst at the same Pd loading. The enhanced electrocatalytic properties could be attributed not only to the electric synergistic effect between Pd, Ni and graphene, but also the high use ratio of Pd due to its shell structure.

MoC–graphite composite as a Pt electrocatalyst support for highly active methanol oxidation and oxygen reduction reaction

Molybdenum carbide (MoC or Mo2C) nanoparticles down to 2 nm in size on carbon (C–MoxC) are synthesized through an ion exchange process, and used as a Pt electrocatalyst support for both the methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). XRD, Raman spectrometer, TEM and XPS measurements are used to characterize the structure and properties of the synthesized materials. The results show that the typical Pt/C–MoC exhibits 160 mV negative shift in onset potential for the MOR, and 2.3 times kinetic mass current (276.7 mA mgPt−1) for the ORR compared to commercial Pt/C-TKK. The large improvement in the catalytic activity resulted from the synergistic effect and binding effect between Pt and MoC. The results also show that MoC promotes both the activity and stability of the Pt electrocatalyst to a greater extent than Mo2C, owing to the different synergistic effect and binding energy. In addition, the Mo compound was found to have an obvious catalytic effect on the graphitization of the carbon source, and the graphited carbon was supposed to give more stability as an electrocatalyst support.

A facile route to carbide-based electrocatalytic nanocomposites

Tungsten carbide nanoparticles with diameters less than 10 nm on graphitic carbon (WC@GC) produced from green foxtail grass under catalysis of iron salts have been successfully synthesized by an efficient method for the first time. The materials were characterized by physical and electrochemical techniques. The results showed that the Pt particles and WC on GC have excellent properties as an electrocatalyst for methanol oxidation. The Pt/WC@GC electrocatalyst is over 5 times higher in peak current density at 0.4 V, and 100 mV more negative in onset potential for methanol oxidation reaction than that on the commercial Pt/C electrocatalyst. Since Pt/WC@GC carries higher catalytic activity compared with Pt/C due to its synergistic effect, less Pt will be required for the same performance and it will in turn reduce the cost of fuel cell electrocatalyst. This work demonstrated that the natural plants could be used to uptake targeting precursors for preparing functional materials. The present method is simple, rapid, and scalable to mass production of the nanomaterials. WC@GC is an applicable support material since the composite carbide and graphite particles are electrically conductive and consist of stable components.

Synthesis of Pd on porous hollow carbon spheres as an electrocatalyst for alcohol electrooxidation

Porous hollow carbon spheres (PHCSs) are prepared with glucose as the carbon source and a solid core microporous shell silica (SCMSS) as the template. The PHCSs are composed of broken and complete hollow carbon spheres cemented to each other. There are big gaps between the PHCSs which make it possible to utilize the inner wall of the hollow carbon spheres as an electrocatalyst support. The PHCSs have a high Brauner–Emett–Teller (BET) surface area of 998.1 m2 g−1, and a pore volume of 1.88 cm3 g−1. The Pd nanoparticles supported on PHCS electrocatalysts are highly active for methanol, ethanol and isopropanol electrooxidation. Pd/PHCS has a 3.1 times higher in peak current density and a 80 mV negatively shifted onset potential compared with that of a Pd/C electrocatalyst at the same Pd loadings for ethanol electrooxidation. The porous structure of Pd/PHCS is favorable for mass transfer and remains high activity in higher concentration of ethanol. The Pd/PHCS electrocatalyst is a potential candidate for application in direct liquid alcohol fuel cells.

Ion‐Exchange‐Assisted Synthesis of Pt‐VC Nanoparticles Loaded on Graphitized Carbon: A High‐Performance Nanocomposite Electrocatalyst for Oxygen‐Reduction Reactions

Carbide-based electrocatalysts are superior to traditional carbon-based electrocatalysts, such as the commercial Pt/C electrocatalysts, in terms of their mass activity and stability. Herein, we report a general approach for the preparation of a nanocomposite electrocatalyst of platinum and vanadium carbide nanoparticles that are loaded onto graphitized carbon. The nanocomposite, which was prepared in a localized and controlled fashion by using an ion-exchange process, was an effective electrocatalyst for the oxygen-reduction reaction (ORR). Both the stability and the durability of the Pt-VC/GC nanocomposite catalyst could be enhanced compared with the state-of-the-art Pt/C. This approach can be extended to the synthesis of other metal-carbide-based nanocatalysts. Moreover, this straightforward synthesis of high-performance composite nanocatalysts can be scaled up to meet the requirements for mass production.

Characterization and comparison of uniform hydrophilic/hydrophobic transparent silica aerogel beads: skeleton strength and surface modification

Silica aerogel is a good adsorbent due to the large surface area and chemical stability. However, its inherent poor mechanical properties and hydrophobicity limit applications in water treatment. To improve the adsorption performance of silica aerogel, uniform transparent hydrophilic mesoporous silica aerogel beads with highly degree of sphericity were synthesized through a facile aging immersion and ambient pressure drying route. The designed hydrophilic silica aerogel beads possessed the mesopores, mean diameter of approximately with the average pore size of 12.51 nm, the specific surface area of 582.52 m2 g−1, and the pore volume of 1.46 cm3 g−1. For comparison, hydrophilic silica aerogel beads without skeleton improvement and hydrophobic silica aerogel beads were also obtained, respectively. The synthesized novel hydrophilic silica aerogel beads displayed a significantly improved adsorption performance with a favorable dye absorption capacity, superior to that of previously synthesised silica-based aerogel beads materials. This study demonstrates a facile and low-cost route toward transparent silica aerogel beads with macroscopic beads structures, which can be used as adsorbents, catalytic supports or the template to synthesize novel functional materials requiring high transmittance, high surface area and effective mass transport.

Hollow tungsten carbide/carbon sphere promoted Pt electrocatalyst for efficient methanol oxidation

The surface carbon thickness and particle size of tungsten carbide (WC) are critical to its synergistic effect on noble metal based electrocatalysts. We report the synthesis of hollow tungsten carbide/carbon sphere composites (HWCSs) with a thinner surface carbon layer and smaller particle size. The polystyrene sphere (PS) as a template and P123 as a surfactant favor the dispersion of carbon and WC precursors, leading to HWCSs with a smaller-sized WC and thinner surface carbon. The above factors in turn favor a stable loading of smaller Pt particles on HWCSs, resulting in excellent electrocatalytic activity (due to the synergistic effect) and stability (due to stronger interaction force between WC and Pt) of Pt/HWCS in electrocatalyzing methanol oxidation in acidic media. The present method is imagined to easily prepare other small-sized carbide particles with a thinner surface carbon layer.

CTAB-assisted synthesis and characterization of Bi2WO6 photocatalysts grown from WO3·0.33H2O nanoplate precursors

Using WO3·0.33H2O nanoplates as precursors in cetyltrimethylammonium bromide (CTAB)-assisted processes, bismuth tungstate (Bi2WO6) nanonests and nanosheets were successfully prepared by a facile hydrothermal method, followed by characterization by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectra, nitrogen adsorption–desorption [Brunauer–Emmett–Teller (BET)] analysis, and UV–Vis diffuse reflectance spectra. The results indicated that the concentration of cetyltrimethylammonium bromide played a major role in the formation of Bi2WO6 nanonests. A possible formation mechanism is presented on the basis of the experimental results. The photocatalytic activities of the resulting Bi2WO6 nanostructures were also evaluated by photodegradation of methylene blue under visible-light irradiation. Bi2WO6 nanonests showed higher photocatalytic activity than Bi2WO6 nanosheets, because of their higher surface area and special structure.Graphical Abstract