Yongqi Zhang

Freestanding Co3O4 nanowire array for high performance supercapacitors

We report a single-crystalline Co3O4 nanowire array grown on a nickel foam prepared by a hydrothermal synthesis method for supercapacitor application. The Co3O4 nanowires show sharp tips and have an average diameter of 70 nm, and a length up to 25 μm. Impressively, the as-prepared single-crystalline Co3O4 nanowire array exhibits noticeable pseudocapacitive performance with a high capacitance of 754 F g−1 at 2 A g−1 and 610 F g−1 at 40 A g−1 as well as excellent cycling stability. The enhanced supercapacitor performance is due to the unique one-dimensional (1D) architecture, which provides fast diffusion paths for ions and facilitates the electron and ion transfer on the Co3O4/electrolyte interfaces. Moreover, the 1D nanowire array can accommodate the volume expansion and restrain the pulverization and deterioration of Co3O4 during the repeated cycling process, resulting in enhanced cycling stability.

Solution synthesis of metal oxides for electrochemical energy storage applications

This article provides an overview of solution-based methods for the controllable synthesis of metal oxides and their applications for electrochemical energy storage. Typical solution synthesis strategies are summarized and the detailed chemical reactions are elaborated for several common nanostructured transition metal oxides and their composites. The merits and demerits of these synthesis methods and some important considerations are discussed in association with their electrochemical performance. We also propose the basic guideline for designing advanced nanostructure electrode materials, and the future research trend in the development of high power and energy density electrochemical energy storage devices.

Rapid Synthesis of Cobalt Nitride Nanowires: Highly Efficient and Low‐Cost Catalysts for Oxygen Evolution

Electrochemical splitting of water to produce hydrogen and oxygen is an important process for many energy storage and conversion devices. Developing efficient, durable, low-cost, and earth-abundant electrocatalysts for the oxygen evolution reaction (OER) is of great urgency. To achieve the rapid synthesis of transition-metal nitride nanostructures and improve their electrocatalytic performance, a new strategy has been developed to convert cobalt oxide precursors into cobalt nitride nanowires through N2 radio frequency plasma treatment. This method requires significantly shorter reaction times (about 1 min) at room temperature compared to conventional high-temperature NH3 annealing which requires a few hours. The plasma treatment significantly enhances the OER activity, as evidenced by a low overpotential of 290 mV to reach a current density of 10 mA cm(-2) , a small Tafel slope, and long-term durability in an alkaline electrolyte.

Generic Synthesis of Carbon Nanotube Branches on Metal Oxide Arrays Exhibiting Stable High‐Rate and Long‐Cycle Sodium‐Ion Storage

A new and generic strategy to construct interwoven carbon nanotube (CNT) branches on various metal oxide nanostructure arrays (exemplified by V2 O3 nanoflakes, Co3 O4 nanowires, Co3 O4 -CoTiO3 composite nanotubes, and ZnO microrods), in order to enhance their electrochemical performance, is demonstrated for the first time. In the second part, the V2 O3 /CNTs core/branch composite arrays as the host for Na(+) storage are investigated in detail. This V2 O3 /CNTs hybrid electrode achieves a reversible charge storage capacity of 612 mAh g(-1) at 0.1 A g(-1) and outstanding high-rate cycling stability (a capacity retention of 100% after 6000 cycles at 2 A g(-1) , and 70% after 10 000 cycles at 10 A g(-1) ). Kinetics analysis reveals that the Na(+) storage is a pseudocapacitive dominating process and the CNTs improve the levels of pseudocapacitive energy by providing a conductive network.

Tubular TiC fibre nanostructures as supercapacitor electrode materials with stable cycling life and wide-temperature performance

Highly active electrode materials with judicious design of nanostructure are important for the construction of high-performance electrochemical energy storage devices. In this work, we have fabricated a tubular TiC fibre cloth as an interesting type of stable supercapacitive material. Hollow microfibres of TiC are synthesized by carbothermal treatment of commercial T-shirt cotton fibres. To demonstrate the rationale of nanostructuring in energy storage, the hollow fibres are further covered by interwoven TiC nanotube branches, forming 3D tubular all-TiC hierarchical fibres with high electrical conductivity, high surface area, and high porosity. For energy storage functions, organic symmetric supercapacitors based on the hollow fibre–nanotube (HFNT) TiC cloth electrodes are assembled and thoroughly characterized. The TiC-based electrodes show very stable capacitance in long charge–discharge cycles and at different temperatures. In particular, the integrated TiC HFNT cloth electrodes show a reasonably high capacitance (185 F g−1 at 2 A g−1), better cycling stability at high-rates (e.g., 97% retention at room temperature after 150 000 cycles, and 67% at −15 °C after 50 000 cycles) than other control electrodes (e.g., pure carbon fibre cloths). It is envisaged that this 3D tubular TiC fibre cloth is also useful for solar cells and electrocatalysis.

Spinel manganese-nickel-cobalt ternary oxide nanowire array for high-performance electrochemical capacitor applications.

Aligned spinel Mn-Ni-Co ternary oxide (MNCO) nanowires are synthesized by a facile hydrothermal method. As an electrode of supercapacitors, the MNCO nanowire array on nickel foam shows an outstanding specific capacitance of 638 F g(-1) at 1 A g(-1) and excellent cycling stability. This exceptional performance benefits from its nanowire architecture, which can provide large reaction surface area, fast ion and electron transfer, and good structural stability. Furthermore, an asymmetric supercapacitor (ASC) with high energy density is assembled successfully by employing the MNCO nanowire array as positive electrode and carbon black as negative electrode. The excellent electrochemical performances indicate the promising potential application of the ASC device in the energy storage field.

Fabrication of metal oxide nanobranches on atomic-layer-deposited TiO2 nanotube arrays and their application in energy storage

Due to the chemical stability and easy fabrication by atomic layer deposition (ALD), TiO2 nanotubes are regarded highly useful in constructing branched nanostructured electrodes for solar conversion and electrochemical energy storage devices. Here we present a facile and scalable fabrication of metal oxide nanobranches on ALD pre-formed TiO2 nanotubes. The metal oxide branches can be a wide range (nearly any) of desirable materials, including NiO and Co3O4 demonstrated herein. As an example, the TiO2/NiO nanoarray battery cathode exhibits a relatively high gravimetric capacity value of ~153 mA h g(-1) and a fairly good stability up to 12,000 cycles with a capacitance of 132 mA h g(-1) at 2 A g(-1).

An extended 'quantum confinement' theory: surface-coordination imperfection modifies the entire band structure of a nanosolid

With the miniaturization of a solid, quantum and interface effects become increasingly important. As a result, the band structure of a nanometric semiconductor changes: the band gap expands, the core level shifts, the bandwidth revises, and the sublevel separation within a band increases. Unfortunately, such a thorough change goes beyond the scope of currently available models such as the ‘quantum confinement’ theory. A consistent understanding of the factors dominating the band-structure change is highly desirable. Here we present a new approach for the size-induced unusual change by adding the effect of surface-coordination deficiency-induced bond contraction to the convention of an extended solid of which the Hamiltonian contains the intraatomic trapping interaction and the interatomic binding interaction. Agreement between modelling predictions and the observed size dependency in the photoluminescence of Si oxides and some nanometric III–V and II–VI semiconductors, and in the core-level shift of Cu–O nanosolids has been reached. Results indicate that the spontaneous contraction of chemical bonds at a surface and the rise in the surface-to-volume ratio with reducing particle size are responsible for the unusual change of the band structure of a nanosolid.

Integrated photoelectrochemical energy storage: solar hydrogen generation and supercapacitor

Current solar energy harvest and storage are so far realized by independent technologies (such as solar cell and batteries), by which only a fraction of solar energy is utilized. It is highly desirable to improve the utilization efficiency of solar energy. Here, we construct an integrated photoelectrochemical device with simultaneous supercapacitor and hydrogen evolution functions based on TiO2/transition metal hydroxides/oxides core/shell nanorod arrays. The feasibility of solar-driven pseudocapacitance is clearly demonstrated, and the charge/discharge is indicated by reversible color changes (photochromism). In such an integrated device, the photogenerated electrons are utilized for H2 generation and holes for pseudocapacitive charging, so that both the reductive and oxidative energies are captured and converted. Specific capacitances of 482 F g−1 at 0.5 A g−1 and 287 F g−1 at 1 A g−1 are obtained with TiO2/Ni(OH)2 nanorod arrays. This study provides a new research strategy for integrated pseudocapacitor and solar energy application.

Ultrafast‐Charging Supercapacitors Based on Corn‐Like Titanium Nitride Nanostructures

Ultrahigh rates realized by ALD‐made TiN. The symmetric full‐cell supercapacitors deliver a typical capacitance of 20.7 F cm−3 at a scan rate of 1 V s−1, and retain 4.3 F cm−3 at high rate of 100 V s−1. The devices can be charged and discharged for 20 000 cycles with negligible capacitance loss and with an ultralow self‐discharge current (≈1 μA).