In Situ Derived Ni_<i>x</i>Fe_1–<i>x</i>OOH/NiFe/Ni_<i>x</i>Fe_1–<i>x</i>OOH Nanotube Arrays from NiFe Alloys as Efficient Electrocatalysts for Oxygen Evolution

Herein, Ni_<i>x</i>Fe_1–<i>x</i>OOH/NiFe/Ni_<i>x</i>Fe_1–<i>x</i>OOH sandwich-structured nanotube arrays (SNTAs) supported on carbon fiber cloth (CFC) (Ni_<i>x</i>Fe_1–<i>x</i>OOH/NiFe/Ni_<i>x</i>Fe_1–<i>x</i>OOH SNTAs–CFC) have been developed as flexible high-performance oxygen evolution reaction (OER) catalysts by a facile in situ electrochemical oxidation of NiFe metallic alloy nanotube arrays during oxygen evolution process. Benefiting from the advantages of high conductivity, hollow nanotube array, and porous structure, Ni_<i>x</i>Fe_1–<i>x</i>OOH/NiFe/Ni_<i>x</i>Fe_1–<i>x</i>OOH SNTAs–CFC exhibited a low overpotential of ∼220 mV at the current density of 10 mA cm^–2 and a small Tafel slope of 57 mV dec^–1 in alkaline solution, both of which are smaller than those of most OER electrocatalysts. Furthermore, Ni_<i>x</i>Fe_1–<i>x</i>OOH/NiFe/Ni_<i>x</i>Fe_1–<i>x</i>OOH SNTAs–CFC exhibits excellent stability at 100 mA cm^–2 for more than 30 h. It is believed that the present work can provide a valuable route for the design and synthesis of inexpensive and efficient OER electrocatalysts

Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba_{0.5–<i>x</i>}TaO_3–<i>x</i>

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0.25–0.325) compositions are stabilized via the interpolation of Ba²⁺ cations and (TaO)³⁺ groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the <i>c</i> axis. The slightly nonstoichiometric Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0–0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< <i>x</i> < 0.25 nonstoichiometric range, resulting in two closely related (TaO)³⁺-containing and (TaO)³⁺-free TTB phases. The highly nonstoichiometric (TaO)³⁺-containing phase exhibits Ba²⁺ cationic migration. The incorporation of (TaO)³⁺ units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)³⁺ units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions

Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba_{0.5–<i>x</i>}TaO_3–<i>x</i>

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0.25–0.325) compositions are stabilized via the interpolation of Ba²⁺ cations and (TaO)³⁺ groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the <i>c</i> axis. The slightly nonstoichiometric Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0–0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< <i>x</i> < 0.25 nonstoichiometric range, resulting in two closely related (TaO)³⁺-containing and (TaO)³⁺-free TTB phases. The highly nonstoichiometric (TaO)³⁺-containing phase exhibits Ba²⁺ cationic migration. The incorporation of (TaO)³⁺ units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)³⁺ units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions

Chemically Lithiated TiO₂ Heterostructured Nanosheet Anode with Excellent Rate Capability and Long Cycle Life for High-Performance Lithium-Ion Batteries

A new form of dual-phase heterostructured nanosheet comprised of oxygen-deficient TiO₂/Li₄Ti₅O₁₂ has been successfully synthesized and used as anode material for lithium ion batteries. With the three-dimensional (3D) Ti mesh as both the conducting substrate and the Ti³⁺/Ti⁴⁺ source, blue anatase Ti³⁺/TiO₂nanosheets were grown by a hydrothermal reaction. By controlling the chemical lithiation period of TiO₂ nanosheets, a phase boundary was created between the TiO₂ and the newly formed Li₄Ti₅O₁₂, which contribute additional capacity benefiting from favorable charge separation between the two phase interfaces. Through further hydrogenation of the 3D TiO₂/Li₄Ti₅O₁₂ heterostructured nanosheets (denoted as H-TiO₂/LTO HNS), an extraordinary rate performance with capacity of 174 mAh g^–1 at 200 C and outstanding long-term cycling stability with only an ∼6% decrease of its initial specific capacity after 6000 cycles were delivered. The heterostructured nanosheet morphology provides a short length of lithium diffusion and high electrode/electrolyte contact area, which could also explain the remarkable lithium storage performance. In addition, the full battery assembled based on the H-TiO₂/LTO anode achieves high energy and power densities

High Energy Density Asymmetric Quasi-Solid-State Supercapacitor Based on Porous Vanadium Nitride Nanowire Anode

To push the energy density limit of asymmetric supercapacitors (ASCs), a new class of anode materials is needed. Vanadium nitride (VN) holds great promise as anode material for ASCs due to its large specific capacitance, high electrical conductivity, and wide operation windows in negative potential. However, its poor electrochemical stability severely limits its application in SCs. In this work, we demonstrated high energy density, stable, quasi-solid-state ASC device based on porous VN nanowire anode and VO_<i>x</i> nanowire cathode for the first time. The VO_<i>x</i>//VN-ASC device exhibited a stable electrochemical window of 1.8 V and excellent cycling stability with only 12.5% decrease of capacitance after 10 000 cycles. More importantly, the VO_<i>x</i>//VN-ASC device achieved a high energy density of 0.61 mWh cm^–3 at current density of 0.5 mA cm^–2 and a high power density of 0.85 W cm^–3 at current density of 5 mA cm^–2. These values are substantially enhanced compared to most of the reported quasi/all-solid-state SC devices. This work constitutes the first demonstration of using VN nanowires as high energy anode, which could potentially improve the performance of energy storage devices

Stabilized TiN Nanowire Arrays for High-Performance and Flexible Supercapacitors

Metal nitrides have received increasing attention as electrode materials for high-performance supercapacitors (SCs). However, most of them are suffered from poor cycling stability. Here we use TiN as an example to elucidate the mechanism causing the capacitance loss. X-ray photoelectron spectroscopy analyses revealed that the instability is due to the irreversible electrochemical oxidation of TiN during the charging/discharging process. Significantly, we demonstrate for the first time that TiN can be stabilized without sacrificing its electrochemical performance by using poly­(vinyl alcohol) (PVA)/KOH gel as the electrolyte. The polymer electrolyte suppresses the oxidation reaction on electrode surface. Electrochemical studies showed that the TiN solid-state SCs exhibit extraordinary stability up to 15 000 cycles and achieved a high volumetric energy density of 0.05 mWh/cm³. The capability of effectively stabilizing nitride materials could open up new opportunities in developing high-performance and flexible SCs

Localization of Oxygen Interstitials in CeSrGa₃O_7+δ Melilite

The solubility of Ce in the La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ and La_{1.54–<i>x</i>}Ce_<i>x</i>Sr_0.46Ga₃O_7.27+δ melilites was investigated, along with the thermal redox stability in air of these melilites and the conductivity variation associated with oxidization of Ce³⁺ into Ce⁴⁺. Under CO reducing atmosphere, the La in LaSrGa₃O₇ may be completely substituted by Ce to form the La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ solid solution, which is stable in air to ∼600 °C when <i>x</i> ≥ 0.6. On the other side, the La_{1.54–<i>x</i>}Ce_<i>x</i>Sr_0.46Ga₃O_7.27+δ compositions displayed much lower Ce solubility (<i>x</i> ≤ 0.1), irrespective of the synthesis atmosphere. In the as-made La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ, the conductivity increased with the cerium content, due to the enhanced electronic conduction arising from the 4f electrons in Ce³⁺ cations. At 600 °C, CeSrGa₃O_7+δ showed a conductivity of ∼10^–4 S/cm in air, nearly 4 orders of magnitude higher than that of LaSrGa₃O₇. The oxidation of Ce³⁺ into Ce⁴⁺ in CeSrGa₃O_7+δ slightly reduced the conductivity, and the oxygen excess did not result in apparent increase of oxide ion conduction in CeSrGa₃O_7+δ. The Ce doping in air also reduced the interstitial oxide ion conductivity of La_1.54Sr_0.46Ga₃O_7.27. Neutron powder diffraction study on CeSrGa₃O_7.39 composition revealed that the extra oxygen is incorporated in the four-linked GaO₄ polyhedral environment, leading to distorted GaO₅ trigonal bipyramid. The stabilization and low mobility of interstitial oxygen atoms in CeSrGa₃O_7+δ, in contrast with those in La_1+<i>x</i>Sr_1–<i>x</i>Ga₃O_{7+0.5<i>x</i>}, may be correlated with the cationic size contraction from the oxidation of Ce³⁺ to Ce⁴⁺. These results provide a new comprehensive understanding of the accommodation and conduction mechanism of the oxygen interstitials in the melilite structure

Multiple Nucleation and Crystal Growth of Barium Titanate

Crystal growth of cubic BaTiO₃ in the presence of polyethylene glycol-200 (PEG-200) is investigated step by step using powder X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Titanium precursor Ti­(OC₄H₉)₄ aggregates with PEG to form spherical colloidal particles at the very beginning. Multiple nucleation of BaTiO₃ takes place on the surface of these colloidal particles. The nanocrystallites then self-adjust their orientations likely under dipole–dipole interaction and/or intercrystallite interactions enhanced by surface adsorbed polymers, followed by an orientated connection and crystal extension via an Ostwald ripening process. The final BaTiO₃ crystals have a novel dodecahedral morphology. The formation mechanism is proposed to be attributed to the selective adsorption of PEG molecules on the {110} crystal planes, significantly reducing the crystal growth rate on these surfaces. A kinetic model is proposed based on the calculated crystallite sizes using the Scherrer equation. The physical meaning of the model and a significant fake reduction of the crystallite size is discussed

Carbon Quantum Dot Surface-Engineered VO₂ Interwoven Nanowires: A Flexible Cathode Material for Lithium and Sodium Ion Batteries

The use of electrode materials in their powdery form requires binders and conductive additives for the fabrication of the cells, which leads to unsatisfactory energy storage performance. Recently, a new strategy to design flexible, binder-, and additive-free three-dimensional electrodes with nanoscale surface engineering has been exploited in boosting the storage performance of electrode materials. In this paper, we design a new type of free-standing carbon quantum dot coated VO₂ interwoven nanowires through a simple fabrication process and demonstrate its potential to be used as cathode material for lithium and sodium ion batteries. The versatile carbon quantum dots that are vastly flexible for surface engineering serve the function of protecting the nanowire surface and play an important role in the diffusion of electrons. Also, the three-dimensional carbon cloth coated with VO₂ interwoven nanowires assisted in the diffusion of ions through the inner and the outer surface. With this unique architecture, the carbon quantum dot nanosurface engineered VO₂ electrode exhibited capacities of 420 and 328 mAh g^–1 at current density rate of 0.3 C for lithium and sodium storage, respectively. This work serves as a milestone for the potential replacement of lithium ion batteries and next generation postbatteries

Green Light-Excitable Ce-Doped Nitridomagnesoaluminate Sr[Mg₂Al₂N₄] Phosphor for White Light-Emitting Diodes

Green Light-Excitable Ce-Doped Nitridomagnesoaluminate Sr[Mg₂Al₂N₄] Phosphor for White Light-Emitting Diode