Wei Cai

Microwave-assisted gas/liquid interfacial synthesis of flowerlike NiO hollow nanosphere precursors and their application as supercapacitor electrodes

A rapid method based on an efficient gas/liquid interfacial microwave-assisted process has been developed to synthesize flowerlike NiO hollow nanosphere precursors, which were then transformed to NiO by simple calcinations. The wall of the sphere is composed of twisted NiO nanosheets that intercalated with each other. Such hollow structure is different from widely reported flowerlike nanostructures with solid cores. An Ostwald ripening mechanism was proposed for the formation of the hollow nanostructures. The products were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution TEM, energy-dispersive X-ray analysis, and N2adsorption-desorption methods. These flowerlike NiO hollow nanospheres have high surface area of 176 m2 g−1. Electrochemical properties show a high specific capacitance of 585 F g−1 at a discharge current of 5 A g−1 and excellent cycling stability, suggesting its promising potentials in supercapacitors.

Nanoporous Nickel Spheres as Highly Active Catalyst for Hydrogen Generation from Ammonia Borane

Hydrogen is one of the best alternative energy carriers because of its abundance, high energy density, and environmentally benign nature. However, difficulties with its controlled storage and release limit the application of hydrogen-based technologies. Many direct hydrogen storage materials, such as metal hydrides, metal–organic frameworks (MOFs), and organic materials, have been studied; however, these materials still do not perform to satisfactory levels with respect to their storage capacity, operating pressure, and operating temperature. Ammonia borane (NH3BH3; AB) is a promising indirect hydrogen storage material owing to its high hydrogen content (19.6 wt%), thermal stability, excellent water solubility at room temperature, and nontoxicity. The maximum release of hydrogen from AB (at room temperature) can be achieved by its hydrolysis in the presence of a suitable catalyst, producing 3 mol H2 per mol AB [Reaction (1)] . This reaction may provide a reliable and easy route for hydrogen supply. Platinum is a very active catalyst for Reaction (1) at room temperature. Besides this high-cost metal, low-cost transition-metal-based catalysts have also been found to show various levels of activity towards the hydrolysis of AB. Several studies have reported the use of nickelor cobalt-based catalysts, with increasing activities. By using nickel nanoparticles and a carbon composite, Sun et al. were able to enhance the activity to a very high level. Finding transition-metal-based catalysts with activities comparable to that of platinum is an important goal.

Design of coherent anode materials with 0D Ni3S2 nanoparticles self-assembled on 3D interconnected carbon networks for fast and reversible sodium storage

There has been tremendous progress in development of nanomaterials for energy conversion and storage, with sodium-ion batteries (SIBs) attracting attention because of the high abundance of raw materials and low cost. However, inferior cycling stability, sluggish reaction kinetics, and poor reversibility hinder their practical applications. In the present study, Ni3S2/carbon nanocomposites with coherent nanostructures were successfully used as anodes in half- and full-cells. Outstanding cycling and rate performances are attributed to a synergistic effect between the Ni3S2 nanoparticles and interconnected carbon networks. The coherent porous framework effectively alleviated volume changes of Ni3S2, shortened the Na+ diffusion path, and accelerated electron transport and ionic diffusion during the electrochemical reaction. More importantly, conversion reaction products can be confined by the entangled carbon networks, leading to reversible redox reactions as demonstrated in ex situ XRD studies. The coherent Ni3S2/C nanocomposites demonstrated a highly reversible charge capacity of 453 and 430 mA h g−1 at a current density of 0.1 and 0.4 A g−1 over 100 cycles, respectively. At a current density of 2.0 A g−1, high rate capacities of 408 mA h g−1 can be attained over 200 cycles. The high performance of Na3V2(PO4)3/Ni3S2 full-cells enrich prospects for future practical applications.

Understanding and manipulating the intrinsic point defect in α-MgAgSb for higher thermoelectric performance

Nanostructured α-MgAgSb has been demonstrated as a good p-type thermoelectric material candidate for low temperature power generation. Nevertheless, the intrinsic defect physics that impedes further enhancement of its thermoelectric performance is still unknown. Here we first unveil that an Ag vacancy is a dominant intrinsic point defect in α-MgAgSb and has a low defect formation energy, shown by first-principles calculations. In addition, the formation of an Ag vacancy could increase the crystal stability. More importantly, intrinsic point defects, namely an Ag vacancy, can be rationally engineered via simply controlling the hot press temperature, due to the recovery effect. Collectively, a high peak ZT of ∼1.3 and average ZT of ∼1.1 are achieved when hot pressed at 533 K. These results elucidate the pivotal role of intrinsic point defects in α-MgAgSb and further highlight that point defect engineering is an effective approach to optimize the thermoelectric properties.

Luminescent Au11 nanocluster superlattices with high thermal stability

The poor thermal stability of nanoparticle superlattices heavily inhibits their practical applications. In present research, using stable thiolate-capped Au11(SCH2CH2COO−)7([CH3(CH2)7]4N+)7 nanoclusters as the building blocks, novel luminescent Au11 nanocluster superlattices with high thermal stability have been fabricated by self-assembly. The nanocluster superlattices have a blade-like morphology and extend on a micrometre length scale with the largest over 50 μm. Under an excitation at 400 nm, the fabricated Au11 nanocluster superlattices emit a blue luminescence with the emission peak of 473 nm. The native stability of thiolate-capped gold nanoclusters and the steric repulsion induced by the high-density ligands (SCH2CH2COO−)7([CH3(CH2)7]4N+)7 endows the fabricated superlattices with high thermal stability. The differential scanning calorimetry and thermogravimetric analysis indicates that the superlattices undergo irreversible endothermic transitions in the range of room temperature to 200 °C, which starts at 124 °C and reaches a peak at 160 °C. When processed with heat treatment below the transition temperature or stored for six months at room temperature, there is no obvious difference detected in the emission intensity of the fabricated Au11 nanocluster superlattices. Such thermostability gives the fabricated nanocluster superlattices great potential for many applications, especially for optical devices.

Enhanced thermoelectric and mechanical properties of p-type skutterudites with in situ formed Fe3Si nanoprecipitate

In the present work, p-type skutterudites La0.8Ti0.1Ga0.1Fe3CoSb12 composite with n-type Fe3Si nanoprecipitate was fabricated via an in situ method. Both thermoelectric and mechanical properties of La0.8Ti0.1Ga0.1Fe3CoSb12/xFe3Si composites were thoroughly investigated. With the introduction of homogeneously dispersed nanosized Fe3Si in the matrix, the power factor is almost unchanged due to the counteraction between the decreased electrical conductivity and the significantly enhanced Seebeck coefficient. Simultaneously, the total thermal conductivity was decreased for samples with Fe3Si nanoprecipitate because of the reduced electronic thermal conductivity. As a result, a ZT value of about 1.2 at 700 K has been achieved for La0.8Ti0.1Ga0.1Fe3CoSb12/0.1Fe3Si sample, whose ZT value was slightly enhanced in comparison with the Fe3Si-freeLa0.8Ti0.1Ga0.1Fe3CoSb12 sample. Furthermore, the indentation fracture toughness of La0.8Ti0.1Ga0.1Fe3CoSb12/0.1Fe3Si was improved by nearly 30% compared to the Fe3Si-free skutterudites.

Thermoelectric SnTe with Band Convergence, Dense Dislocations, and Interstitials through Sn Self-Compensation and Mn Alloying

SnTe is known as an eco-friendly analogue of PbTe without toxic elements. However, the application potentials of pure SnTe are limited because of its high hole carrier concentration derived from intrinsic Sn vacancies, which lead to a high electrical thermal conductivity and low Seebeck coefficient. In this study, Sn self-compensation and Mn alloying could significantly improve the Seebeck coefficients in the whole temperature range through simultaneous carrier concentration optimization and band engineering, thereby leading to a large improvement of the power factors. Combining precipitates and atomic-scale interstitials due to Mn alloying with dense dislocations induced by long time annealing, the lattice thermal conductivity is drastically reduced. As a result, an enhanced figure of merit (ZT) of 1.35 is achieved for the composition of Sn0.94 Mn0.09 Te at 873 K and the ZTave from 300 to 873 K is boosted to 0.78, which is of great significance for practical application. Hitherto, the ZTmax and ZTave of this work are the highest values among all single-element-doped SnTe systems.

Damping Capacity of Ni–Mn–Ga–Gd High-Temperature Shape Memory Thin Film

The damping capacity of Ni–Mn–Ga–Gd films annealed for different durations has been investigated by dynamic mechanical analysis (DMA) measurement. The DMA results show that the internal friction peaks are absent in the IF curves, due to very low transformation rate and low transformation strain in the films. With the increase of annealing time, the value of tan delta in martensite first increases then decreases due to the combined effect of grain size and martensite structure change. Owing to the occurrence of high-temperature creep behavior, an abrupt increase of tan delta takes place over 400xa0°C in the film annealed for 10xa0h. The film annealed for 5xa0h shows good thermal stability of phase transformation and keeps a high damping capacity in martensite after ten times of repeated DMA measurement. In terms of the high damping capacity (tan δu2009=u20090.046) in martensite, high transformation temperature (Asu2009=u2009292xa0°C), and good thermal stability, the film annealed for 5xa0h can be used as a damping material for MEMS devices in elevated temperature.

Fresh MoO2 as a better electrode for pseudocapacitive sodium-ion storage

MoO2 shows attractive electronic conductivity beyond other metal oxides as an anode material for sodium-ion storage. A one-pot method has been exploited for the rational design and synthesis of fresh MoO2 nanoclusters, which show improved sodium storage performances in terms of good cycling stability and superior rate capability compared to their annealed counterparts due to the dominant pseudocapacitance.

High thermoelectric performance of p-type Cerium single-filled skutterudites by dislocation engineering

Filled skutterudites, possessing a high power factor and good mechanical properties, have attracted intensive attention for the intermediate-temperature power generation. Since n-type filled skutterudites can achieve a high peak ZT of over 1.5, it is imperative to develop their high-performance p-type counterparts for real applications. In this work, single phase p-type cerium (Ce)-filled skutterudites with dense dislocation arrays were first fabricated by the liquid phase compaction method. It was demonstrated that the dense dislocation arrays significantly reduced the lattice thermal conductivity via strain scattering. Simultaneously, there was almost no influence on the normal carrier transport process. As a result, a high peak ZT value of over 1.1 at 723 K has been obtained for the liquid phase compacted Ce0.8Fe3CoSb12 alloy, which is the record-high value among the single element-filled p-type skutterudites.