Youqi Zhu

Ultrathin nickel hydroxide and oxide nanosheets: synthesis, characterizations and excellent supercapacitor performances.

High-quality ultrathin two-dimensional nanosheets of α-Ni(OH)2 are synthesized at large scale via microwave-assisted liquid-phase growth under low-temperature atmospheric conditions. After heat treatment, non-layered NiO nanosheets are obtained while maintaining their original frame structure. The well-defined and freestanding nanosheets exhibit a micron-sized planar area and ultrathin thickness (<2 nm), suggesting an ultrahigh surface atom ratio with unique surface and electronic structure. The ultrathin 2D nanostructure can make most atoms exposed outside with high activity thus facilitate the surface-dependent electrochemical reaction processes. The ultrathin α-Ni(OH)2 and NiO nanosheets exhibit enhanced supercapacitor performances. Particularly, the α-Ni(OH)2 nanosheets exhibit a maximum specific capacitance of 4172.5 F g−1 at a current density of 1 A g−1. Even at higher rate of 16 A g−1, the specific capacitance is still maintained at 2680 F g−1 with 98.5% retention after 2000 cycles. Even more important, we develop a facile and scalable method to produce high-quality ultrathin transition metal hydroxide and oxide nanosheets and make a possibility in commercial applications.

Two-dimensional ultrathin ZnCo2O4 nanosheets: general formation and lithium storage application

Two-dimensional (2D) multicomponent transition-metal oxide nanosheets are the most promising candidate in low-cost and eco-friendly energy storage/conversion applications. Their surface-enhanced properties and synergic effects are fascinating, yet still underdeveloped. Here, we first report the high-quality ultrathin 2D nanosheets of ZnCo2O4 synthesized on a large scale via microwave-assisted liquid-phase growth coupled with a post annealing procedure. The well-defined and freestanding nanosheets exhibit a micron-sized planar area and ultrathin thickness, suggesting a high surface atom ratio with an unique surface and electronic structure, thus facilitating the charge transfer to enhance the overall performances in electrochemical reaction. When used as anode materials for lithium ion batteries, the ultrathin ZnCo2O4 nanosheets exhibit a high reversible lithium storage capacity of 930–980 mA h g−1 at 200 mA g−1 current density in 200 cycles with an excellent cycling stability and good high-rate capability. Even more importantly, we have extended the facile method for the formation of other analogue nanosheets including binary and ternary transition metal oxides (NiO, Co3O4, NiCo2O4, and CuCo2O4) and make a possibility in exploring more unique properties and promising commercial applications.

Surface-enabled superior lithium storage of high-quality ultrathin NiO nanosheets

Two-dimensional nanomaterials hold great potential for next-generation energy storage and conversion devices. Here, we report a large-scale synthesis of high-quality ultrathin NiO nanosheets. The well-defined nanosheets show a graphene-like morphology with large planar area, ultrathin thickness (<2 nm), and high percentage of surface atoms. In comparison with the bulk material, the NiO nanosheets exhibit unique surface and electronic structure with considerable under-coordinated surface nickel atoms and crystal lattice volume expansion. The detected local coordination geometry and the electronic states endow the ultrathin NiO nanosheets with great potential in surface-dependent electrochemical reactions and catalytic processes. When used as anode materials for lithium-ion batteries, the ultrathin NiO nanosheets exhibit a high reversible lithium storage capacity of 715.2 mA h g−1 at 200 mA g−1 current density in 130 cycles with an excellent cycling stability and rate capability. In particular, the large-area ultrathin 2D nanostructure can shorten lithium ion diffusion paths and provide a large exposed surface for more lithium-insertion channels. The large-scale and cost-efficient synthesis and the excellent electrochemical performance highlight the high-quality ultrathin 2D NiO nanosheets as a competitive anode material for lithium-ion batteries.

Microwave Assisted Synthesis of Porous NiCo2O4 Microspheres: Application as High Performance Asymmetric and Symmetric Supercapacitors with Large Areal Capacitance.

Large areal capacitance is essentially required to integrate the energy storage devices at the microscale electronic appliances. Energy storage devices based on metal oxides are mostly fabricated with low mass loading per unit area which demonstrated low areal capacitance. It is still a challenge to fabricate supercapacitor devices of porous metal oxides with large areal capacitance. Herein we report microwave method followed by a pyrolysis of the as-prepared precursor is used to synthesize porous nickel cobaltite microspheres. Porous NiCo2O4 microspheres are capable to deliver large areal capacitance due to their high specific surface area and small crystallite size. The facile strategy is successfully demonstrated to fabricate aqueous-based asymmetric & symmetric supercapacitor devices of porous NiCo2O4 microspheres with high mass loading of electroactive materials. The asymmetric & symmetric devices exhibit maximum areal capacitance and energy density of 380 mF cm−2 & 19.1 Wh Kg−1 and 194 mF cm−2 & 4.5 Wh Kg−1 (based on total mass loading of 6.25 & 6.0 mg) respectively at current density of 1 mA cm−2. The successful fabrication of symmetric device also indicates that NiCo2O4 can also be used as the negative electrode material for futuristic asymmetric devices.

Microwave-Assisted and Gram-Scale Synthesis of Ultrathin SnO2 Nanosheets with Enhanced Lithium Storage Properties

The rational design and fabrication of SnO2-based anode materials could offer a powerful way of effectively alleviating their large volume variation and guaranteeing excellent reaction kinetics for electrochemical lithium storage. Herein, we present an ultrarapid, low-cost, and simple microwave-assisted synthesis of ultrathin SnO2 nanosheets at the gram-scale. The two-dimensional (2D) anisotropic growth depends on microwave dielectric irradiation coupled with surfactant structural direction, and is conducted under low-temperature atmospheric conditions. The ultrathin 2D nanostructure holds a great surface tin atom percentage with high activity, where the electrochemical reaction processes could be facilitated that highly dependent on the surface. Compared with 1D SnO2 nanorods, the ultrathin SnO2 nanosheets exhibit remarkably improved electrochemical lithium storage properties with a high reversible capacity of 757.6 mAh g(-1) at a current density of 200 mA g(-1) up to 40 cycles as well as excellent rate capability and cycling stability. Specifically, the ultrathin 2D nanosheet could significantly reduce ion diffusion paths, thus allowing faster phase transitions, while the sufficient external surface interspace and interior porous configuration could successfully accommodate the huge volume changes. Even more importantly, we develop a promising strategy to produce ultrathin SnO2 nanosheets to tackle their intrinsic problems for commercial applications.

LiNi1/3Co1/3Mn1/3O2 hollow nano-micro hierarchical microspheres with enhanced performances as cathodes for lithium-ion batteries

LiNi1/3Co1/3Mn1/3O2 hollow nano-micro hierarchical microspheres (NCM-HS) are synthesized using MnCO3 both as a self-template and Mn source. The hollow microspheres with diameters of about 1 μm have walls about 250 nm thick, which are composed of approximately 100 nm primary nanoparticles. NCM-HS cathodes have an initial discharge capacity of 212 mA h g−1 at 0.1 C between 2.5 and 4.5 V. After 40 charge–discharge cycles, the capacity retention at 0.1 C is 85.1%. At higher rates, the reversible capacities of the NCM-HS cathodes are 208.9 (0.5 C), 204.8 (1 C), 180.7 (2 C), 155.7 (5 C) and 135.9 mA h g−1 (10 C). The high performances can be attributed to the distinctive hollow microspherical structures with the 100 nm building blocks, which could effectively reduce the path of Li ion diffusion, increase the contact area between electrodes and electrolyte and buffer the volume changes during the Li ion intercalation/deintercalation processes.

One-step synthesis of zinc–cobalt layered double hydroxide (Zn–Co-LDH) nanosheets for high-efficiency oxygen evolution reaction

Two-dimensional (2D) nanomaterials show great potential for electrocatalysis or other applications that require large surface area. In this work, we developed porous zinc–cobalt layered double hydroxide (Zn–Co-LDH) nanosheets by using a one-step microwave-assisted approach, and examine their oxygen evolution reaction (OER) performance. The Zn–Co-LDH nanosheets with a high specific surface area of 116.4 m2 g−1 exhibit good OER activity, expressed as low onset overpotential, small Tafel slope and large exchange current density. At the overpotential of 0.54 V, the current density of Zn–Co-LDH nanosheets is about 15.06 mA cm−2, which is much higher than that of Zn–Co-LDH nanoparticles. The comparable electrocatalytic performance may be attributed to the porous 2D structure generating more active sites and higher electronic conductivity. Furthermore, the obtained Zn–Co-LDH nanosheets show good stability during long time running at 1.55 V vs. RHE. Accordingly, an effective OER catalyst is exploited by using a simple microwave-assisted synthesis. Such an effective method could be extended to large-scale synthesis of uniform and stable 2D LDH nanomaterials.

Microwave assisted synthesis of mesoporous NiCo2O4 nanosheets as electrode material for advanced flexible supercapacitors

Mesoporous nickel cobaltite (NiCo2O4) nanosheets are synthesized using a cost effective, ultra fast and environmentally friendly microwave assisted heating method followed by a post-calcination process of the as-prepared precursors. XRD, XPS, BET, SEM, TEM and HRTEM methods are used to characterize the nanosheets. The as-prepared nanosheets with a thickness of around 2 nm possess many interparticle mesopores. The nanosheets have a mesoporous structure, high specific surface area (111.15 m2 g−1), large pore volume (0.3033 cm3 g−1) and narrow pore size distribution (2.25–10 nm). A flexible supercapacitor working electrode of the mesoporous NiCo2O4 nanosheets is prepared on carbon cloth. Cyclic voltammetry, chronopotentiometry and impedance spectroscopy measurements are used to investigate the electrochemical performance of the as-prepared mesoporous NiCo2O4 nanosheet/carbon cloth electrode. The mesoporous NiCo2O4 nanosheets exhibit specific capacitances of 292.5 and 200 F g−1 in 2 M KOH aqueous electrolyte at current densities of 1 and 8 A g−1 respectively. The cyclic performance indicates excellent capacitance retention of 94.5% after 2000 cycles at a current density of 3 A g−1. The excellent cyclic stability can be attributed to the mesoporous nature, high specific surface area, large pore volume and narrow pore distribution of the nanosheets. The synthesized mesoporous NiCo2O4 nanosheets using a microwave method are proved to be excellent electrode material for advanced flexible supercapacitors.

Strongly coupled mesoporous SnO2–graphene hybrid with enhanced electrochemical and photocatalytic activity

A strongly coupled mesoporous SnO2–graphene hybrid has been prepared via direct nucleation, growth, and anchoring of a SnO2 nanocrystal on graphene substrate under microwave irradiation followed by heat-treatment. Investigations reveal that the well-dispersed SnO2 nanocrystals with a uniform particle size of 3–5 nm are homogeneously distributed on the surface of graphene through strong chemical attachment and electrical interaction. The formed structure exhibits a high specific surface area (280.7 m2 g−1) and an ideal synergistic effect, which can provide improved activity and durability for the electrochemical and photocatalytic reaction. Lithium-ion battery performance and photocatalytic activity of the resultant mesoporous SnO2–graphene hybrid are thoroughly investigated. In comparison to bare SnO2 nanoparticles, the hybrid shows substantial enhancement in electrochemical lithium storage properties and photocatalytic hydrogen evolution. More strikingly, the as-synthesized SnO2–graphene hybrid anode could deliver initial discharge and charge capacities of 2445.7 and 1329.4 mAh g−1 with a high initial Coulombic efficiency (54.4%), as well as an excellent cycling stability.

Chrysanthemum-like TiO2 nanostructures with exceptional reversible capacity and high coulombic efficiency for lithium storage

The rational design of hierarchically structured materials is of great significance for developing energy-storage devices. Herein, a novel and uniform chrysanthemum-like TiO2 nanostructure built by well-defined surface-folded nanorods (CLNR-TiO2) has been fabricated by a facile, effective and template-free synthetic method. By only changing the solvent in the reaction another two different morphologies have been obtained, including flower-like structures built by nanoplates (FLNP-TiO2) and microspheres (Microsphere-TiO2). As chrysanthemum-like nanostructures allow efficient Li+ ion diffusion, as well as have better structural stability, CLNR-TiO2 exhibits the best lithium storage performances among the three samples. A superior capacity up to 309.3 mA h g−1 is achieved at a current rate of 0.5 C (1 C = 170 mA g−1) for the first cycle with a high coulombic efficiency of 93.4% and it is significant for practical applications. At a high current rate of 5 C, a high reversible capacity of 198.3 mA h g−1 is obtained in 100 cycles (92% of capacity retention) with excellent rate capacity, high coulombic efficiency and good cycling stability.