One-Step Electrodeposited Nickel Cobalt Sulfide Nanosheet Arrays for High-Performance Asymmetric Supercapacitors

A facile one-step electrodeposition method is developed to prepare ternary nickel cobalt sulfide interconnected nanosheet arrays on conductive carbon substrates as electrodes for supercapacitors, resulting in exceptional energy storage performance. Taking advantages of the highly conductive, mesoporous nature of the nanosheets and open framework of the three-dimensional nanoarchitectures, the ternary sulfide electrodes exhibit high specific capacitance (1418 F g^–1 at 5 A g^–1 and 1285 F g^–1 at 100 A g^–1) with excellent rate capability. An asymmetric supercapacitor fabricated by the ternary sulfide nanosheet arrays as positive electrode and porous graphene film as negative electrode demonstrates outstanding electrochemical performance for practical energy storage applications. Our asymmetric supercapacitors show a high energy density of 60 Wh kg^–1 at a power density of 1.8 kW kg^–1. Even when charging the cell within 4.5 s, the energy density is still as high as 33 Wh kg^–1 at an outstanding power density of 28.8 kW kg^–1 with robust long-term cycling stability up to 50 000 cycles

Thermoelectric Properties of Two-Dimensional Molybdenum-Based MXenes

MXenes are an interesting class of 2D materials consisting of transition metal carbides and nitrides, which are currently a subject of extensive studies. Although there have been theoretical calculations estimating the thermoelectric properties of MXenes, no experimental measurements have been reported so far. In this report, three compositions of Mo-based MXenes (Mo₂CT_<i>x</i>, Mo₂Ti­C₂T_<i>x</i>, and Mo₂Ti₂­C₃T_<i>x</i>) have been synthesized and processed into free-standing binder-free papers by vacuum-assisted filtration, and their electrical and thermoelectric properties are measured. Upon heating to 800 K, these MXene papers exhibit high conductivity and n-type Seebeck coefficient. The thermoelectric power reaches 3.09 × 10^–4 W m^–1 K^–2 at 803 K for the Mo₂Ti­C₂T_<i>x</i> MXene. While the thermoelectric properties of MXenes do not reach that of the best materials, they exceed their parent ternary and quaternary layered carbides. Mo₂Ti­C₂T_<i>x</i> shows the highest electrical conductivity in combination with the largest Seebeck coefficient of the three 2D materials studied

Direct Chemical Synthesis of MnO₂ Nanowhiskers on Transition-Metal Carbide Surfaces for Supercapacitor Applications

Transition-metal carbides (MXenes) are an emerging class of two-dimensional materials with promising electrochemical energy storage performance. Herein, for the first time, by direct chemical synthesis, nanocrystalline ε-MnO₂ whiskers were formed on MXene nanosheet surfaces (ε-MnO₂/Ti₂CT_<i>x</i> and ε-MnO₂/Ti₃C₂T_<i>x</i>) to make nanocomposite electrodes for aqueous pseudocapacitors. The ε-MnO₂ nanowhiskers increase the surface area of the composite electrode and enhance the specific capacitance by nearly 3 orders of magnitude compared to that of pure MXene-based symmetric supercapacitors. Combined with enhanced pseudocapacitance, the fabricated ε-MnO₂/MXene supercapacitors exhibited excellent cycling stability with ∼88% of the initial specific capacitance retained after 10000 cycles which is much higher than pure ε-MnO₂-based supercapacitors (∼74%). The proposed electrode structure capitalizes on the high specific capacitance of MnO₂ and the ability of MXenes to improve conductivity and cycling stability

Enhanced ZnO Thin-Film Transistor Performance Using Bilayer Gate Dielectrics

We report ZnO TFTs using Al₂O₃/Ta₂O₅ bilayer gate dielectrics grown by atomic layer deposition. The saturation mobility of single layer Ta₂O₅ dielectric TFT was 0.1 cm² V^–1 s^–1, but increased to 13.3 cm² V^–1 s^–1 using Al₂O₃/Ta₂O₅ bilayer dielectric with significantly lower leakage current and hysteresis. We show that point defects present in ZnO film, particularly V_Zn, are the main reason for the poor TFT performance with single layer dielectric, although interfacial roughness scattering effects cannot be ruled out. Our approach combines the high dielectric constant of Ta₂O₅ and the excellent Al₂O₃/ZnO interface quality, resulting in improved device performance

Oxidant-Dependent Thermoelectric Properties of Undoped ZnO Films by Atomic Layer Deposition

Extraordinary oxidant-dependent changes in the thermoelectric properties of undoped ZnO thin films deposited by atomic layer deposition (ALD) have been observed. Specifically, deionized water and ozone oxidants are used in the growth of ZnO by ALD using diethylzinc as a zinc precursor. No substitutional atoms have been added to the ZnO films. By using ozone as an oxidant instead of water, a thermoelectric power factor (σS²) of 5.76 × 10^–4 W m^–1 K^–2 is obtained at 705 K for undoped ZnO films. In contrast, the maximum power factor for the water-based ZnO film is only 2.89 × 10^–4 W m^–1 K^–2 at 746 K. Materials analysis results indicate that the oxygen vacancy levels in the water- and ozone-grown ZnO films are essentially the same, but the difference comes from Zn-related defects present in the ZnO films. The data suggest that the strong oxidant effect on thermoelectric performance can be explained by a mechanism involving point defect-induced differences in carrier concentration between these two oxides and a self-compensation effect in water-based ZnO due to the competitive formations of both oxygen and zinc vacancies. This strong oxidant effect on the thermoelectric properties of undoped ZnO films provides a pathway to improve the thermoelectric performance of this important material

Two-Dimensional SnO Anodes with a Tunable Number of Atomic Layers for Sodium Ion Batteries

We have systematically changed the number of atomic layers stacked in 2D SnO nanosheet anodes and studied their sodium ion battery (SIB) performance. The results indicate that as the number of atomic SnO layers in a sheet decreases, both the capacity and cycling stability of the Na ion battery improve. The thinnest SnO nanosheet anodes (two to six SnO monolayers) exhibited the best performance. Specifically, an initial discharge and charge capacity of 1072 and 848 mAh g^–1 were observed, respectively, at 0.1 A g^–1. In addition, an impressive reversible capacity of 665 mAh g^–1 after 100 cycles at 0.1 A g^–1 and 452 mAh g^–1 after 1000 cycles at a high current density of 1.0 A g^–1 was observed, with excellent rate performance. As the average number of atomic layers in the anode sheets increased, the battery performance degraded significantly. For example, for the anode sheets with 10–20 atomic layers, only a reversible capacity of 389 mAh g^–1 could be obtained after 100 cycles at 0.1 A g^–1. Density functional theory calculations coupled with experimental results were used to elucidate the sodiation mechanism of the SnO nanosheets. This systematic study of monolayer-dependent physical and electrochemical properties of 2D anodes shows a promising pathway to engineering and mitigating volume changes in 2D anode materials for sodium ion batteries. It also demonstrates that ultrathin SnO nanosheets are promising SIB anode materials with high specific capacity, stable cyclability, and excellent rate performance

Is NiCo₂S₄ Really a Semiconductor?

NiCo₂S₄ is a technologically important electrode material that has recently achieved remarkable performance in pseudocapacitor, catalysis, and dye-synthesized solar cell applications.− Essentially, all reports on this material have presumed it to be semiconducting, like many of the chalcogenides, with a reported band gap in the range of 1.2–1.7 eV., In this report, we have conducted detailed experimental and theoretical studies, most of which done for the first time, which overwhelmingly show that NiCo₂S₄ is in fact a metal. We have also calculated the Raman spectrum of this material and experimentally verified it for the first time, hence clarifying inconsistent Raman spectra reports. Some of the key results that support our conclusions include: (1) the measured carrier density in NiCo₂S₄ is 3.18 × 10²² cm^–3, (2) NiCo₂S₄ has a room temperature resistivity of around 10³ μΩ cm which increases with temperature, (3) NiCo₂S₄ exhibits a quadratic dependence of the magnetoresistance on magnetic field, (4) thermopower measurements show an extremely low Seebeck coefficient of 5 μV K^–1, (5) first-principles calculations confirm that NiCo₂S₄ is a metal. These results suggest that it is time to rethink the presumed semiconducting nature of this promising material. They also suggest that the metallic conductivity is another reason (besides the known significant redox activity) behind the excellent performance reported for this material

Electropolymerized Star-Shaped Benzotrithiophenes Yield π‑Conjugated Hierarchical Networks with High Areal Capacitance

High-surface-area π-conjugated polymeric networks have the potential to lend outstanding capacitance to supercapacitors because of the pronounced faradaic processes that take place across the dense intimate interface between active material and electrolytes. In this report, we describe how benzo­[1,2-<i>b</i>:3,4-<i>b</i>′:5,6-<i>b</i>″]­trithiophene (<b>BTT</b>) and tris­(ethylenedioxythiophene)­benzo­[1,2-<i>b</i>:3,4-<i>b</i>′:5,6-<i>b</i>″]­trithiophene (<b>TEBTT</b>) can serve as 2D (trivalent) building blocks in the development of electropolymerized hierarchical π-conjugated frameworks with particularly high areal capacitance. In comparing electropolymerized networks of <b>BTT</b>, <b>TEBTT</b>, and their copolymers with EDOT, we show that <b>TEBTT</b>/EDOT-based copolymers, i.e., P­(<b>TEBTT</b>/EDOT), can achieve higher areal capacitance (e.g., as high as 443.8 mF cm^–2 at 1 mA cm^–2) than those achieved by their respective homopolymers (<b>PTEBTT</b> and PEDOT) in the same experimental conditions of electrodeposition (<b>PTEBTT</b>: 271.1 mF cm^–2 (at 1 mA cm^–2) and PEDOT: 12.1 mF cm^–2 (at 1 mA cm^–2)). For example, P­(<b>TEBTT</b>/EDOT)-based frameworks synthesized in a 1:1 monomer-to-comonomer ratio show a ca. 35× capacitance improvement over PEDOT. The high areal capacitance measured for P­(<b>TEBTT</b>/EDOT)-based frameworks can be explained by the open, highly porous hierarchical morphologies formed during the electropolymerization step. With >70% capacitance retention over 1000 cycles (up to 89% achieved), both <b>PTEBTT</b>- and P­(<b>TEBTT</b>/EDOT)-based frameworks are resilient to repeated electrochemical cycling and can be considered promising systems for high life cycle capacitive electrode applications

Tunable Multipolar Surface Plasmons in 2D Ti₃C₂<i>T</i>_<i>x</i> MXene Flakes

2D Ti₃C₂<i>T</i>_<i>x</i> MXenes were recently shown to exhibit intense surface plasmon (SP) excitations; however, their spatial variation over individual Ti₃C₂<i>T</i>_<i>x</i> flakes remains undiscovered. Here, we use scanning transmission electron microscopy (STEM) combined with ultra-high-resolution electron energy loss spectroscopy (EELS) to investigate the spatial and energy distribution of SPs (both optically active and forbidden modes) in mono- and multilayered Ti₃C₂<i>T</i>_<i>x</i> flakes. With STEM-EELS mapping, the inherent interband transition in addition to a variety of transversal and longitudinal SP modes (ranging from visible down to 0.1 eV in MIR) are directly visualized and correlated with the shape, size, and thickness of Ti₃C₂<i>T</i>_<i>x</i> flakes. The independent polarizability of Ti₃C₂<i>T</i>_<i>x</i> monolayers is unambiguously demonstrated and attributed to their unusual weak interlayer coupling. This characteristic allows for engineering a class of nanoscale systems, where each monolayer in the multilayered structure of Ti₃C₂<i>T</i>_<i>x</i> has its own set of SPs with distinctive multipolar characters. Moreover, the tunability of the SP energies is highlighted by conducting <i>in situ heating</i> STEM to monitor the change of the surface functionalization of Ti₃C₂<i>T</i>_<i>x</i> through annealing at temperatures up to 900 °C. At temperatures above 500 °C, the observed fluorine (F) desorption multiplies the metal-like free electron density of Ti₃C₂<i>T</i>_<i>x</i> flakes, resulting in a monotonic blue-shift in the SP energy of all modes. These results underline the great potential for the development of Ti₃C₂<i>T</i>_<i>x</i>-based applications, spanning the visible–MIR spectrum, relying on the excitation and detection of single SPs

Plasma-Assisted Synthesis of NiCoP for Efficient Overall Water Splitting

Efficient water splitting requires highly active, earth-abundant, and robust catalysts. Monometallic phosphides such as Ni₂P have been shown to be active toward water splitting. Our theoretical analysis has suggested that their performance can be further enhanced by substitution with extrinsic metals, though very little work has been conducted in this area. Here we present for the first time a novel PH₃ plasma-assisted approach to convert NiCo hydroxides into ternary NiCoP. The obtained NiCoP nanostructure supported on Ni foam shows superior catalytic activity toward the hydrogen evolution reaction (HER) with a low overpotential of 32 mV at −10 mA cm^–2 in alkaline media. Moreover, it is also capable of catalyzing the oxygen evolution reaction (OER) with high efficiency though the real active sites are surface oxides in situ formed during the catalysis. Specifically, a current density of 10 mA cm^–2 is achieved at overpotential of 280 mV. These overpotentials are among the best reported values for non-noble metal catalysts. Most importantly, when used as both the cathode and anode for overall water splitting, a current density of 10 mA cm^–2 is achieved at a cell voltage as low as 1.58 V, making NiCoP among the most efficient earth-abundant catalysts for water splitting. Moreover, our new synthetic approach can serve as a versatile route to synthesize various bimetallic or even more complex phosphides for various applications