Junchi Wu

Metallic Co4N Porous Nanowire Arrays Activated by Surface Oxidation as Electrocatalysts for the Oxygen Evolution Reaction

Designing highly efficient electrocatalysts for oxygen evolution reaction (OER) plays a key role in the development of various renewable energy storage and conversion devices. In this work, we developed metallic Co4N porous nanowire arrays directly grown on flexible substrates as highly active OER electrocatalysts for the first time. Benefiting from the collaborative advantages of metallic character, 1D porous nanowire arrays, and unique 3D electrode configuration, surface oxidation activated Co4N porous nanowire arrays/carbon cloth achieved an extremely small overpotential of 257 mV at a current density of 10 mA cm(-2), and a low Tafel slope of 44 mV dec(-1) in an alkaline medium, which is the best OER performance among reported Co-based electrocatalysts to date. Moreover, in-depth mechanistic investigations demonstrate the active phases are the metallic Co4N core inside with a thin cobalt oxides/hydroxides shell during the OER process. Our finding introduces a new concept to explore the design of high-efficiency OER electrocatalysts.

Strong-Coupled Cobalt Borate Nanosheets/Graphene Hybrid as Electrocatalyst for Water Oxidation Under Both Alkaline and Neutral Conditions.

Developing highly active catalysts for the oxygen evolution reaction (OER) is of paramount importance for designing various renewable energy storage and conversion devices. Herein, we report the synthesis of a category of Co-Pi analogue, namely cobalt-based borate (Co-Bi ) ultrathin nanosheets/graphene hybrid by a room-temperature synthesis approach. Benefiting from the high surface active sites exposure yield, enhanced electron transfer capacity, and strong synergetic coupled effect, this Co-Bi NS/G hybrid shows high catalytic activity with current density of 10 mA cm(-2) at overpotential of 290 mV and Tafel slope of 53 mV dec(-1) in alkaline medium. Moreover, Co-Bi NS/G electrocatalysts also exhibit promising performance under neutral conditions, with a low onset potential of 235 mV and high current density of 14.4 mA cm(-2) at 1.8 V, which is the best OER performance among well-developed Co-based OER electrocatalysts to date. Our finding paves a way to develop highly active OER electrocatalysts.

A zwitterionic gel electrolyte for efficient solid-state supercapacitors

Gel electrolytes have attracted increasing attention for solid-state supercapacitors. An ideal gel electrolyte usually requires a combination of advantages of high ion migration rate, reasonable mechanical strength and robust water retention ability at the solid state for ensuring excellent work durability. Here we report a zwitterionic gel electrolyte that successfully brings the synergic advantages of robust water retention ability and ion migration channels, manifesting in superior electrochemical performance. When applying the zwitterionic gel electrolyte, our graphene-based solid-state supercapacitor reaches a volume capacitance of 300.8 F cm−3 at 0.8 A cm−3 with a rate capacity of only 14.9% capacitance loss as the current density increases from 0.8 to 20 A cm−3, representing the best value among the previously reported graphene-based solid-state supercapacitors, to the best of our knowledge. We anticipate that zwitterionic gel electrolyte may be developed as a gel electrolyte in solid-state supercapacitors.

Metallic mesocrystal nanosheets of vanadium nitride for high-performance all-solid-state pseudocapacitors

AbstractTransition metal nitrides (TMNs) are of particular interest by virtue of their synergic advantages of superior electrical conductivity, excellent environmental durability and high reaction selectivity, yet it is difficult to achieve flexible design and operation. Herein, mesocrystal nanosheets (MCNSs) of vanadium nitride (VN) are synthesized via a confined-growth route from thermally stable layered vanadium bronze, representing the first two-dimensional (2D) metallic mesocrystal in inorganic compounds. Benefiting from their single-crystalline-like long-range electronic connectivity, VN MCNSs deliver an electrical conductivity of 1.44 × 105 S/m at room temperature, among the highest values observed for 2D nanosheets. Coupled with their unique pseudocapacitance, VN MCNS-based flexible supercapacitors afford a superior volumetric capacitance of 1,937 mF/cm3. Nitride MCNSs should have wide applications in the energy storage and conversion fields because their intrinsic high conductivity is coupled with the reactivity of inorganic lattices.

Regulating the Electrical Behaviors of 2D Inorganic Nanomaterials for Energy Applications

Recent years have witnessed great developments in inorganic 2D nanomaterials for their unique dimensional confinement and diverse electronic energy bands. Precisely regulating their intrinsic electrical behaviors would bring superior electrical conductivity, rendering 2D nanomaterials ideal candidates for active materials in electrochemical applications when combined with the excellent reaction activity from the inorganic lattice. This Concept focuses on highly conducting inorganic 2D nanomaterials, including intrinsic metallic 2D nanomaterials and artificial highly conductive 2D nanomaterials. The intrinsic metallicity of 2D nanomaterials is derived from their closely packed atomic structures that ensure maximum overlapping of electron orbitals, while artificial highly conductive 2D nanomaterials could be achieved by designed methodologies of surface modification, intralayer ion doping, and lattice strain, in which atomic-scale structural modulation plays a vital role in realizing conducting behaviors. Benefiting from fast electron transfer, high reaction activity, as well as large surface areas arising from the 2D inorganic lattice, highly conducting 2D nanomaterials open up prospects for enhancing performance in electrochemical catalysis and electrochemical capacitors. Conductive 2D inorganic nanomaterials promise higher efficiency for electrochemical applications of energy conversion and storage.

Highly Efficient Photothermal Effect by Atomic-Thickness Confinement in Two-Dimensional ZrNCl Nanosheets

We report a giant photothermal effect arising from quantum confinement in two-dimensional nanomaterials. ZrNCl ultrathin nanosheets with less than four monolayers of graphene-like nanomaterial successfully generated synergetic effects of larger relaxation energy of photon-generated electrons and intensified vibration of surface bonds, offering predominantly an enhancement of the electron-phonon interaction to a maximized extent. As a result, they could generate heat flow reaching an ultrahigh value of 5.25 W/g under UV illumination with conversion efficiency up to 72%. We anticipate that enhanced electron-phonon coupling in a quantum confinement system will be a powerful tool for optimizing photothermal conversion of inorganic semiconductors.

Modulation of Metal and Insulator States in 2D Ferromagnetic VS2 by van der Waals Interaction Engineering

2D transition-metal dichalcogenides (TMDCs) are currently the key to the development of nanoelectronics. However, TMDCs are predominantly nonmagnetic, greatly hindering the advancement of their spintronic applications. Here, an experimental realization of intrinsic magnetic ordering in a pristine TMDC lattice is reported, bringing a new class of ferromagnetic semiconductors among TMDCs. Through van der Waals (vdW) interaction engineering of 2D vanadium disulfide (VS2 ), dual regulation of spin properties and bandgap brings about intrinsic ferromagnetism along with a small bandgap, unravelling the decisive role of vdW gaps in determining the electronic states in 2D VS2 . An overall control of the electronic states of VS2 is also demonstrated: bond-enlarging triggering a metal-to-semiconductor electronic transition and bond-compression inducing metallization in 2D VS2 . The pristine VS2 lattice thus provides a new platform for precise manipulation of both charge and spin degrees of freedom in 2D TMDCs availing spintronic applications.

Very Large-Sized Transition Metal Dichalcogenides Monolayers from Fast Exfoliation by Manual Shaking

For two-dimensional transition metal dichalcogenides (TMD) materials, achieving large size with high quality to provide a basis for the next generation of electronic device geometries has been a long-term need. Here, we demonstrate that, by only manual shaking within several seconds, very large-sized TMD monolayers that cover a wide range of group IVB-VIB transition metal sulfides and selenides can be efficiently harvested from intercalated single-crystal counterparts. Taking TaS2 as examples, monolayers up to unprecedented size (>100 μm) are obtained while maintaining high crystalline quality and the phase structure of the starting materials. Furthermore, benefiting from the gentle manual shaking, we unraveled the atomic-level correlation between the intercalated lattice-strain effects and exfoliated nanosheets, and that strong tensile strain usually led to very large sizes. This work helps to deepen the understanding of exfoliation mechanism and provides a powerful tool for producing large-sized and high-quality TMD nanosheets appealing for further applications.

Magnetic and structural transitions of SrFe2As2 at high pressure and low temperature.

One of key issues in studying iron based superconductors is to understand how the magnetic phase of the parent compounds evolves. Here we report the systematic investigation of paramagnetic to antiferromagnetic and tetragonal to orthorhombic structural transitions of “122” SrFe2As2 parent compound using combined high resolution synchrotron Mössbauer spectroscopy and x-ray diffraction techniques in a cryogenically cooled high pressure diamond anvil cell. It is found that although the two transitions are coupled at 205 K at ambient pressure, they are concurrently suppressed to much lower temperatures near a quantum critical pressure of approximately 4.8 GPa where the antiferromagnetic state transforms into bulk superconducting state. Our results indicate that the lattice distortions and magnetism jointly play a critical role in inducing superconductivity in iron based compounds.

Double-Exchange Effect in Two-Dimensional MnO2 Nanomaterials

Electronic state transitions, especially metal-insulator transitions (MIT), offer physical properties that are useful in intriguing energy applications and smart devices. But to-date, very few simple metal oxides have been shown to undergo electronic state transitions near room temperature. Herein, we demonstrate experimentally that chemical induction of double-exchange in two-dimensional (2D) nanomaterials brings about a MIT near room temperature. In this case, valence-state regulation of a 2D MnO2 nanosheet induces a Mn(III)-O-Mn(IV) structure with the double-exchange effect, successfully triggering a near-room-temperature electronic transition with an ultrahigh negative magneto-resistance (MR). Double-exchange in 2D MnO2 nanomaterials exhibits an ultrahigh MR value of up to -11.3% (0.1 T) at 287 K, representing the highest reported negative MR values in 2D nanomaterials approaching room temperature. Also, the MnO2 nanosheet displays an infrared response of 7.1% transmittance change on going from 270 to 290 K. We anticipate that dimensional confinement of double-exchange structure promises novel magneto-transport properties and sensitive responses for smart devices.