Ling Miao

NiCo2S4 porous nanotubes synthesis via sacrificial templates: high-performance electrode materials of supercapacitors

NiCo2S4 porous nanotubes are synthesised by a sacrificial template method based on the Kirkendall effect. The as-prepared NiCo2S4 nanotube electrode shows a specific capacitance of 1093 F g−1 at a current density of 0.2 A g−1 (933 F g−1 at 1 A g−1).

Hierarchical Configuration of NiCo2S4 Nanotube@Ni–Mn Layered Double Hydroxide Arrays/Three-Dimensional Graphene Sponge as Electrode Materials for High-Capacitance Supercapacitors

Three dimensional (3D) hierarchical network configurations are composed of NiCo2S4 nanotube @Ni-Mn layered double hydroxide (LDH) arrays in situ grown on graphene sponge. The 3D graphene sponge with robust hierarchical porosity suitable for as a basal growth has been obtained from a colloidal dispersion of graphene oxide using a simple directional freeze-drying technique. The high conductive NiCo2S4 nanotube arrays grown on 3D graphene shows excellent pseudocapacity and good conductive support for high-performance Ni-Mn LDH. The 3D NiCo2S4@Ni-Mn LDH/GS shows a high specific capacitance (Csp) 1740 mF cm(-2) at 1 mA cm(-2), even at 10 mA cm(-2), 1267.9 mF cm(-2) maintained. This high-performance composite electrode proposes a new and feasible general pathway as 3D electrode configuration for energy storage devices.

Hierarchical NiCo2S4@NiFe LDH Heterostructures Supported on Nickel Foam for Enhanced Overall-Water-Splitting Activity

Low-cost and highly efficient bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are intensively investigated for overall water splitting. Herein, we combined experimental research with first-principles calculations based on density functional theory (DFT) to engineer the NiCo2S4@NiFe LDH heterostructure interface for enhancing overall water-splitting activity. The DFT calculations exhibit strong interaction and charge transfer between NiCo2S4 and NiFe LDH, which change the interfacial electronic structure and surface reactivity. The calculated chemisorption free energy of hydroxide (ΔEOH) is reduced from 1.56 eV for pure NiFe LDH to 1.03 eV for the heterostructures, indicating a dramatic improvement in OER performance, while the chemisorption free energy of hydrogen (ΔEH) maintains almost invariable. By the use of the facile hydrothermal method, NiCo2S4 nanotubes, NiFe LDH nanosheets, and NiCo2S4@NiFe LDH heterostructures are prepared on nickel foam, of which the corresponding experimental OER overpotentials are 306, 260, and 201 mV at 60 mA cm-2, respectively. These results are good agreement with the theoretical predictions. Meanwhile, the HER performance has little improvement, with an overpotential of about 200 mV at 10 mA cm-2. Due to the dramatic improvement in OER performance, there was an enhancement in the overall water-splitting activity of the NiCo2S4@NiFe LDH heterostructures, with a low voltage of 1.6 V.

Different charge-storage mechanisms in disulfide vanadium and vanadium carbide monolayer

Two-dimensional (2D) transition-metal (TM) compound nanomaterials, due to their high-surface-area and large potential charge capability of TM atoms, have been widely investigated as electrochemical capacitors. However, the understanding of charge-storage mechanisms of 2D transition-metal compounds as electrode materials is still limited. In this study, using density functional theory computations, we systematically investigate the electrochemical properties of monolayer VS2 and V2C. Their electronic structures show a significant electron storage capability of around 0.25 V, referenced to the standard hydrogen electrode, and indicate redox pseudocapacitance characteristics as cathodes. The different charge densities visually confirm that excess electrons tend to localize in the vanadium atoms nearby contact-adsorbed Li ions, corresponding to the redox of vanadium atoms. In contrast, only the electric double layer acts as a charge-storage mechanism in the V2C monolayer. However, the O saturation would induce redox pseudocapacitance in the V2C monolayer. Furthermore, the calculated metallic behavior and low Li ion diffusion barriers substantiate that V2C and VS2 monolayers would manifest low resistance in the charging process. Our findings provide insights for the different charge-storage mechanism of VS2 and V2C monolayers.

Pseudocapacitance and excellent cyclability of 2,5-dimethoxy-1,4-benzoquinone on graphene

Electrochemically active organic materials are emerging as low cost, naturally abundant and sustainable alternatives to their metal-based counterparts. However, their usage in energy storage systems is mainly hindered by their poor conductivity, which results in capacitance fade upon cycling. Here, we report a redox-active xerogel composed of 2,5-dimethoxy-1,4-benzoquinone (DMQ) decorated on reduced graphene oxide (rGO) sheets via a hydrothermal method as a high capacitance and long cycle life pseudocapacitive electrode. DMQ not only provided stable redox-active centers but also served as a spacer to avoid rGO sheets aggregation and led to a three-dimensional (3D) hierarchical electrode architecture. When a binder-free 50 μm thick rolled film was tested as a pseudocapacitive electrode, it exhibited an excellent capacitance of 650 F g−1 at 5 mV s−1 (780 F cm−3) in 1 M sulfuric acid, outperforming a large number of reported organic and inorganic electrodes. Most importantly, optimized electrodes showed an excellent capacitance retention of 99% after 25000 cycles at 50 mV s−1. Density functional theory (DFT) calculations are further used to understand the charge storage mechanism, the preferred orientation of the adsorbed molecules, charge density distribution and density of states. Our combined experimental and theoretical findings demonstrate that the careful selection of the conductive substrate, electrode architecture and organic molecules plays a crucial role in achieving high capacitance and long cycling performance.

Architectural design and phase engineering of N/B-codoped TiO2(B)/anatase nanotube assemblies for high-rate and long-life lithium storage

TiO2 polymorphs hold great promise as anode candidates in lithium-ion batteries (LIBs) because of their low cost, enhanced safety and high power capability, but they suffer from poor electrical conductivity and low lithium-ion mobility. Herein, an attractive nanoassembly made of ultrathin TiO2 nanotubes with selected phases and hetero-atom doping was developed through a mild ionothermal reaction. The grain interface of TiO2(B)/anatase, N/B codoping, and nanoassembly were elaborately engineered. They offer effective nanohighways for fast electronic and Li-ion transport, when applied as an anode in LIBs. The carbon-free, N/B-codoped TiO2(B)/anatase nanotube nanoassemblies exhibit exceptionally high rate capability and good durability (160 mA h g−1 at 12 A g−1 and retaining 140 mA h g−1 even after 500 cycles). This work demonstrates that the integrated design of surface states and electronic structures plays a crucial role in exploring new capabilities of TiO2 polymorphs for electrochemical energy storage.

Design of a Tunable Low-Frequency and Broadband Radar Absorber Based on Active Frequency Selective Surface

In this letter, we report on the design, fabrication, measurement, and analysis of a tunable low-frequency and broadband radar absorber applied in the frequency range of 1-12 GHz. Numerical simulations indicate that absorbers based on the resistor-loaded frequency selective surface (FSS) can be resistively tuned to give a broadband absorption of exceeding -10 dB between 1 and 12 GHz. Moreover, superior absorbing performance can be achieved by introducing capacitance paralleled with resistance. Measurement results show that the radar absorber, by way of configuration with active FSS (AFSS) controlled by p-i-n diodes, can be tuned to provide a continuously variable reflectivity level of less than -10 dB from 2 to 11.3 GHz with varied bias voltages. Furthermore, experimental results have good agreement with simulations.

Binding TiO2-B nanosheets with N-doped carbon enables highly durable anodes for lithium-ion batteries

An effective strategy of binding TiO2-B nanosheets with N-doped carbon is developed to construct highly stable TiO2-B (001)|carbon interfaces with enhanced electronic conductivity and Li-uptake capability. The interfaces provide not only additional interfacial lithium-storage capability but also highly conductive pathways for the fast transport of electrons and lithium ions. The hybrid as an anode for lithium-ion batteries exhibits a safe operating potential window of ∼1.5 V, superior rate capability with 180 mA h g−1 at 6 A g−1, and ultra-long cycle life of more than 2000 cycles, holding great promise for safe and high-power energy storage applications.

A novel tunable frequency selective surface absorber with dual-DOF for broadband applications

A novel tunable frequency selective surface (FSS) with dual-degrees of freedom (DOF) is presented, and firstly applied to broadband absorber. Based on a simple prototype unit cell resonator, an approach for achieving multi-resonances is studied. A unit cell pattern with gradient edges is discussed, and variable resistor and variable capacitor are introduced to fully utilize its characteristic of multi-resonances. Bias line is designed to provide bias voltage respectively for two variable devices and provide two operational DOF for FSS. Simulation and measurement results both show that the tunable FSS absorber with dual-DOF has wideband absorption with the reflectivity below -10 dB in 1-5 GHz and with a total thickness of about 10 mm.

Charge transfer induced polymerization of EDOT confined between 2D titanium carbide layers

In situ polymerization of 3,4-ethylenedioxythiophene (EDOT) is achieved on the surface of 2D Ti3C2Tx MXene without using any oxidant, resulting in improved lithium ion storage capability of Ti3C2Tx/poly-EDOT hybrids. A combined experimental and theoretical study revealed the mechanism of charge-transfer-induced polymerization of EDOT, which can be extended to other similar polymers.