Cuifeng Zhou

Role of carbon coating in improving electrochemical performance of Li-rich Li(Li0.2Mn0.54Ni0.13Co0.13)O2 cathode

Li-rich Li(Li0.2Mn0.54Ni0.13Co0.13)O2 cathode coated with carbon layer has been prepared by a hydrothermal approach followed by a post annealing process. The cathode after surface modification exhibits both enhanced cyclability and improved rate capability compared with the pristine one without coating. The carbon coating process causes a phase transformation from Li2MnO3-like domain to cubic-spinel domain with Fdm symmetry at surface regions of particles. As a consequence, the valence state of Mn on the surface accordingly varies. Such transformed surface spinels, as well as the wrapped carbon layers as a result of this coating strategy are believed to be responsible for the enhanced electrochemical performance.

Hollow nitrogen-containing core/shell fibrous carbon nanomaterials as support to platinum nanocatalysts and their TEM tomography study.

Core/shell nanostructured carbon materials with carbon nanofiber (CNF) as the core and a nitrogen (N)-doped graphitic layer as the shell were synthesized by pyrolysis of CNF/polyaniline (CNF/PANI) composites prepared by in situ polymerization of aniline on CNFs. High-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared and Raman analyses indicated that the PANI shell was carbonized at 900°C. Platinum (Pt) nanoparticles were reduced by formic acid with catalyst supports. Compared to the untreated CNF/PANI composites, the carbonized composites were proven to be better supporting materials for the Pt nanocatalysts and showed superior performance as catalyst supports for methanol electrochemical oxidation. The current density of methanol oxidation on the catalyst with the core/shell nanostructured carbon materials is approximately seven times of that on the catalyst with CNF/PANI support. TEM tomography revealed that some Pt nanoparticles were embedded in the PANI shells of the CNF/PANI composites, which might decrease the electrocatalyst activity. TEM-energy dispersive spectroscopy mapping confirmed that the Pt nanoparticles in the inner tube of N-doped hollow CNFs could be accessed by the Nafion ionomer electrolyte, contributing to the catalytic oxidation of methanol.

Facet-Controlling Agents Free Synthesis of Hematite Crystals with High-Index Planes: Excellent Photodegradation Performance and Mechanism Insight.

Hematite (α-Fe2O3) crystals with uniform size and structure are synthesized through very facile one-pot hydrothermal methods without any additive. The as-synthesized sub-micrometer-sized α-Fe2O3 crystals with small surface areas perform superb visible light photodegradation activities, even much better than most other α-Fe2O3 nanostructures with large surface areas. Profound mechanism analyses reveal that the microwave-assisted hydrothermal (Mic-H) synthesized α-Fe2O3 is enclosed by 12 high-index {2-15} facets. The structure and the low unoccupied molecular orbital (LUMO) of the high-index planes result in the excellent photocatalytic activity. This is the first report on the formation of {2-15} plane group of hematite, and the synthesis of the hematite particles with the {2-15} planes is very simple and no any facet-controlling agent is used. This study may pave the way to further performance enhancement and practical applications of the cheap hematite materials.

Nano-confined multi-synthesis of a Li–Mg–N–H nanocomposite towards low-temperature hydrogen storage with stable reversibility

A Li–Mg–N–H system is a highly promising source of hydrogen storage materials due to its favorable thermodynamics and potential reversibility. Its application has been greatly hindered, however, by its rather high activation energy barriers. Herein, we report a novel multi-reaction methodology for the synthesis of nanosized Li2Mg(NH)2 space-confined into thin-film hollow carbon spheres (THCSs) with a uniform dispersion. It shows that a completely depressed release of ammonia and reversible hydrogen sorption at a temperature of 105 °C, the lowest temperature reported so far, were achieved for the nano-confined Li2Mg(NH)2. Furthermore, a stable cycling capacity close to the theoretical value was also successfully realized, even through up to 20 cycles of de-/re-hydrogenation.

Influence of support acidity on the performance of size-confined Pt nanoparticles in the chemoselective hydrogenation of acetophenone

Size-confined Pt nanoparticles of about 1.5 nm have been introduced into [Al]MCM-41 supports with similar acid strength but various population densities of acid sites by means of wet impregnation. The Pt nanoparticles covered preferentially the surface Bronsted acid sites (BAS) of the supports or were located near acid sites rather than on the bigger free space between acid sites even at a very low acid density (6 BAS per 1000 nm2). The free BAS around the Pt particles did not interact with Pt atoms and the electronic properties of the Pt nanoparticles as probed by DRIFTS combined with CO adsorption were similar for Pt/[Al]MCM-41 with and without nearby free BAS. Ionic effects were generated by the Pt-covered acid sites, whereas the population of BAS did not contribute significantly to the ionic effects induced on the Pt nanoparticles. The coverage of BAS of similar strength by platinum nanoparticles led to similar chemoselectivity and product distribution in acetophenone (Aph) hydrogenation, though the density of BAS on the supports increased by more than 11 times. However, increasing the number of BAS on the supports significantly changed the hydrogenation rate. TOFs continuously increased from 125 h−1 up to 534 h−1, when the population of free BAS increased from 18.2 BAS per 1000 nm2 to 39.9 BAS per 1000 nm2. When the free BAS density was further increased to 70.4 BAS per 1000 nm2, the TOF then dropped to 176 h−1. The hydrogenation pathway is similar for both monofunctional (Pt covering all BAS) and bifunctional catalysts (Pt with free BAS), and the reaction was initiated on the Pt surface. This finding indicates that proper tuning of the population density of acid sites on the support can significantly improve the catalytic performance of the supported metal catalysts while keeping similar product selectivities.

Enhancement of the catalytic performance of a CNT supported Pt nanorod cluster catalyst by controlling their microstructure

A novel Pt/CNT catalyst with a hierarchical structure was prepared. The effect of different morphologies of CNT supports on the catalyst microstructure and catalytic performance was studied. TEM tomography was used to analyse the real microstructure of the catalysts, including both the morphology of flower-like Pt clusters themselves and their distribution on the CNTs. The results revealed that Pt flowers were composed of nanorods, which dispersed in both the inner and outer tube surface of CNTs with larger inner tube diameter (TKCNTs). Comparing with the conventional Pt/CNT catalyst where Pt flowers only dispersed on the outer tube surface, the present Pt/TKCNT catalyst with the novel structure exhibited higher activity (enhanced to be ∼1.5 times higher current density) and better catalytic stability for methanol oxidation. Moreover, it displays ∼2 times higher activity for methanol electro-oxidation than that of CB supported Pt nanorod clusters. The results indicate that the catalytic performance of the Pt nanorods supported on different carbon supports depends on both carbon morphology and the distribution of the metal catalyst on the supports. The improved catalytic performance of the Pt/TKCNT catalyst with the novel hierarchical structure could be attributed to the confinement effect of TKCNTs.

Confinement Impact for the Dynamics of Supported Metal Nanocatalyst

Supported metal nanoparticles play key roles in nanoelectronics, sensors, energy storage/conversion, and catalysts for the sustainable production of fuels and chemicals. Direct observation of the dynamic processes of nanocatalysts at high temperatures and the confinement of supports is of great significance to investigate nanoparticle structure and functions for practical utilization. Here, in situ high-resolution transmission electron microscopy photos and videos are combined with dynamics simulations to reveal the real-time dynamic behavior of Pt nanocatalysts at operation temperatures. Amorphous Pt surface on moving and deforming particles is the working structure during the high operation temperature rather than a static crystal surface and immobilization on supports as proposed before. The free rearrangement of the shape of Pt nanoparticles allows them to pass through narrow windows, which is generally considered to immobilize the particles. The Pt particles, no matter what their sizes, prefer to stay inside nanopores even when they are fast moving near an opening at temperatures up to 900 °C. The porous confinement also blocks the sintering of the particles under the confinement size of pores. These contribute to the continuous high activity and stability of Pt nanocatalysts inside nanoporous supports during a long-term evaluation of catalytic reforming reaction.