Xiu-Cheng Zheng

Facile synthesis of 3D nitrogen-doped graphene aerogel nanomeshes with hierarchical porous structures for applications in high-performance supercapacitors

3D nitrogen-doped graphene aerogel nanomeshes (N-GANMs) with hierarchical porous structures were facilely synthesized from graphene oxide and urea using iron nitrate as the etching agent. The resulting materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and N2 adsorption–desorption. The supercapacitor performance was characterized by cyclic voltammetry, galvanostatic charge discharge, and electrochemical impedance spectroscopy, respectively. Compared with the bare graphene aerogel (GA), N-GANMs showed higher specific surface area, more abundant meso-macroporous pores, and much better electrochemical properties. The specific capacitance of N-GANMs was as high as 345.8 F g−1, which was much higher than that of GA and most of the reported carbon-based materials, and the value remains at about 321.0 F g−1 over 2000 cycles at 1.0 A g−1 in 2.0 M KOH. In addition, the N-GANMs demonstrated a high energy density of 20.82 W h kg−1 at a power density of 449.97 W kg−1 and their cycle performance remained approximately 100% after 10 000 cycles at 5.0 A g−1 in 1.0 M Na2SO4. The excellent behaviors might have originated from their hierarchical porous structures and the incorporation of nitrogen.

The enhanced electrocaloric effect in P(VDF-TrFE) copolymer with barium strontium titanate nano-fillers synthesized via an effective hydrothermal method

An effective hydrothermal method is developed to synthesize barium strontium titanate (BST) nanoparticles with a mean size below 30 nm, which are used for the fabrication of nanocomposite films consisting of ferroelectric co-polymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and ceramic nanoparticles. The interface between the nanoparticle and the polymeric matrix is found to improve the polarization and pyroelectric behaviors of the nanocomposites, resulting in a much enhanced electrocaloric effect as compared with that of the pristine P(VDF-TrFE). The electrocaloric temperature change is 2.5 °C under electric fields of ∼600 kV cm−1. The effects of Ba0.75Sr0.25TiO3 nanoparticles on the ferroelectric-to-paraelectric transition of P(VDF-TrFE) are investigated using dynamical mechanical relaxation. The results suggest that the improved electrocaloric effect in the nanocomposites could be related with the localized polarization field near the surfaces of Ba0.75Sr0.25TiO3 nanoparticles.

Synthesis of biofuel via levulinic acid esterification over porous solid acid consisting of tungstophosphoric acid and reduced graphene oxide

Reduced graphene oxide (rGO) was synthesized by chemical reduction of graphene oxide with hydrazine hydrate and used as supports to prepare a series of H3PW12O40 (HPW)-based porous solid acids for the first time. Esterification of levulinic acid with ethanol was used to investigate the catalytic properties of the resulting HPW/rGO catalysts. The results showed that the heterogeneous catalysts possessed a porous structure and that their textural characteristics and catalytic activities were influenced by the loading of HPW. Remarkably, they were efficient in the synthesis of ethyl levulinate, with the one with an HPW loading of 45 wt% exhibiting the best efficiency. The conversion of levulinic acid was as high as 96.9%. Meanwhile, the resulting HPW/rGO catalysts also have satisfactory durability. The conversion of levulinic acid still remains at about 53.1% after five cycles under the present conditions. Furthermore, the various catalytic reaction parameters, such as reaction time, ethanol-to-LA molar ratio, and the catalyst dosage, are optimized to maximize the conversion of levulinic acid over 45 wt% HPW/rGO catalysts.

Exceptionally High Negative Electro-Caloric Effects of Poly(VDF–co–TrFE) Based Nanocomposites Tuned by the Geometries of Barium Titanate Nanofillers

Exceptionally high electro-caloric effects (ECEs) are observed in nanocomposites consisting of poly(vinylidene fluoride-co-trifluoroethylene) (VDF–co–TrFE) copolymer and barium titanate (BT) nanoparticles and nanowires. The poly(VDF–co–TrFE) matrix nanocomposites containing 5% volume fraction of BT nanowires are found to exhibit a negative ECE temperature change as large as 12 °C or a refrigeration effect of 8.3 J/g, which is much larger than those reported to date. The mechanisms of negative ECE and the enhanced negative ECE in the nanocomposites consisting of poly(VDF–co–TrFE) and BT nanowires are explained by the Kauzmann theory on glassy polar states and the interaction between BT nanofillers and the copolymer matrix. The effects of geometries of BT nanofillers on the negative ECEs are elucidated by P-E loop measurements, and dielectric and dynamical mechanical analyses. The nanocomposites, with their enhanced negative ECE tuned by the geometries of BT nanofillers, provide us with promising ECE refrigerants for practical application to small-sized and environmentally-friendly ECE coolers in the heat management of electronic devices.

Esterification of levulinic acid into hexyl levulinate over dodecatungstophosphoric acid anchored to Al-MCM-41

ABSTRACT Aluminum-substituted MCM-41 (Al-MCM-41) were hydrothermally synthesised and used as supports to fabricate the mesostructured H3PW12O40 (HPW) solid acid catalysts via an impregnation method. The influences of various HPW loadings on the structures of the catalysts were verified by X-ray diffraction (XRD), nitrogen physisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible (UV–Vis) spectroscopy and Fourier-transform infrared spectroscopy (FT-IR) measurements and their acidities were evaluated by infrared of pyridine adsorption (Py-IR) and temperature-programmed desorption of ammonia (NH3-TPD) measurements. The optimisation of the reaction conditions for levulinic acid esterification with n-hexanol such as the reaction temperature and time, the reactant molar ratio, and the catalyst dosages was performed to maximise the conversion of levulinic acid. The effects of various HPW loadings on the conversion of levulinic acid were investigated under the optimised and harsh reaction conditions. The reusability of the catalysts was also investigated. The results showed that these catalysts retained the hexagonal mesoporous structure of Al-MCM-41 and the Keggin characteristic of HPW, although their textural parameters decreased with increasing loading of HPW. In particular, the catalysts were found to be efficient in the esterification of levulinic acid with n-hexanol, resulting in hexyl levulinate which could replace the petroleum-derived chemical feedstocks.