Morphology and Photocatalytic Property of Hierarchical Polyimide/ZnO Fibers Prepared via a Direct Ion-exchange Process

A simple and efficient method has been developed for preparing hierarchical nanostructures of polyimide (PI)/ZnO fibers by combining electrospinning and direct ion-exchange process. Poly­(amic acid) (PAA) nanofibers are first prepared by electrospinning, and then, the electrospun PAA fibers are immersed into ZnCl₂ solution. After a subsequent thermal treatment, imidization of PAA and formation of ZnO nanoparticles can be simultaneously achieved in one step to obtain PI/ZnO composite fibers. SEM images show that ZnO nanoparticles are densely and uniformly immobilized on the surface of electrospun PI fibers. Furthermore, the morphology of ZnO can be tuned from nanoplatelets to nanorods by changing the initial concentration of ZnCl₂ solution. Photocatalytic degradation tests show an efficient degradation ability of PI/ZnO composite membranes toward organic dyes. Meanwhile, the free-standing membrane is highly flexible, easy to handle, and easy to retrieve, which enables its use in water treatment. This simple and inexpensive approach can also be applied to fabricating other hierarchically nanostructured composites

Electrically Conductive Polyaniline/Polyimide Nanofiber Membranes Prepared via a Combination of Electrospinning and Subsequent In situ Polymerization Growth

Highly aligned polyimide (PI) nanofiber membranes have been prepared by electrospinning equipped with a high speed rotating collector. As the electrospun polyimide nanofiber membranes possess large surface area, they can be used as the template for in situ growth of polyaniline (PANi) by using FeCl₃ as the oxidant. It is found that PANi nanoparticles can be uniformly distributed on the surface of highly aligned PI nanofibers due to the low oxidization/reduction potential of FeCl₃ and the active nucleation sites of the functionalized PI nanofibers. The as-prepared PANi/PI composite membranes not only possess excellent thermal and mechanical properties but also show good electrical conductivity, pH sensitivity and significantly improved electromagnetic impedance properties. This is a facile method for fabricating high-performance and multifunctional composites that can find potential applications in electrical and aerospace fields

MoSe₂ Nanosheet Array with Layered MoS₂ Heterostructures for Superior Hydrogen Evolution and Lithium Storage Performance

Engineering heterostructures of transition metal disulfides through low-cost and high-yield methods instead of using conventional deposition techniques still have great challenges. Herein, we present a conveniently operated and low-energy-consumption solution-processed strategy for the preparation of heterostructures of MoSe₂ nanosheet array on layered MoS₂, among which the two-dimensional MoS₂ surface is uniformly covered with high-density arrays of vertically aligned MoSe₂. The unique compositional and structural features of the MoS₂–MoSe₂ heterostructures not only provide more exposed active sites for sequent electrochemical process, but also facilitate the ion transfer due to the open porous space within the nanosheet array serving as well-defined ionic reservoirs. As a proof of concept, the MoS₂–MoSe₂ heterostructures serve as promising bifunctional electrodes for both energy conversions and storages, which exhibit an active and acid-stable activity for catalyzing the hydrogen evolution reaction, high specific capacity of 728 F g^–1 at 0.1 A g^–1, and excellent durability with a remained capacity as high as 676 mA h g^–1 after 200 cycles

Self-Templated Growth of Vertically Aligned 2H-1T MoS₂ for Efficient Electrocatalytic Hydrogen Evolution

Semiconductor heterostructures of two-dimensional (2D) transition metal disulfide (TMD) have opened up approaches toward the integration of each function and implementations in novel energy and electronic devices. However, engineering TMD-based homostructures with tailored properties is still challenging. Herein, we demonstrate a solution-processed growth of vertically aligned 1T-MoS₂ using liquid-phase exfoliated 2H-MoS₂ as self-templates. The unique MoS₂-based homostructures not only provide more exposed active sites in the edge and basal plane for the electrocatalytic hydrogen evolution reaction (HER) but also improve the mass transfer due to the introduction of high packing porosity. The resultant all-MoS₂ electrocatalysts with an integration of polymorphous MoS₂ nanostructures exhibit a superior HER activity with a low potential of 203 mV at 10 mA cm^–2, a small Tafel slope of 60 mV dec^–1, and a remarkable cyclic stability. This work thus provides a simple and efficient route for the creation of unprecedented MoS₂-based homostructured materials with exciting properties, especially as an inexpensive alternative to platinum catalysts in electrochemical hydrogen evolution production

Three-Dimensional Nanoporous Graphene-Carbon Nanotube Hybrid Frameworks for Confinement of SnS₂ Nanosheets: Flexible and Binder-Free Papers with Highly Reversible Lithium Storage

The practical applications of transition-metal dichalcogenides for lithium-ion batteries are severely inhibited by their inferior structural stability and electrical conductivity, which can be solved by optimizing these materials to nanostructures and confining them within conductive frameworks. Thus, we report a facile approach to prepare flexible papers with SnS₂ nanosheets (SnS₂ NSs) homogeneously dispersed and confined within the conductive graphene-carbon nanotube (CNT) hybrid frameworks. The confinement of SnS₂ NSs in graphene-CNT matrixes not only can effectively prevent their aggregation during the discharge–charge procedure, but also can assist facilitating ion transfer across the interfaces. As a result, the optimized SGC papers give an improved capacity of 1118.2 mA h g^–1 at 0.1 A g^–1 along with outstanding stability. This report demonstrates the significance of employing graphene-CNT matrixes for confinement of various active materials to fabricate flexible electrode materials

Nitrogen-Doped Carbon Nanofiber/Molybdenum Disulfide Nanocomposites Derived from Bacterial Cellulose for High-Efficiency Electrocatalytic Hydrogen Evolution Reaction

To remit energy crisis and environmental deterioration, non-noble metal nanocomposites have attracted extensive attention, acting as a fresh kind of cost-effective electrocatalysts for hydrogen evolution reaction (HER). In this work, hierarchically organized nitrogen-doped carbon nanofiber/molybdenum disulfide (pBC-N/MoS₂) nanocomposites were successfully prepared via the combination of in situ polymerization, high-temperature carbonization process, and hydrothermal reaction. Attributing to the uniform coating of polyaniline on the surface of bacterial cellulose, the nitrogen-doped carbon nanofiber network acts as an excellent three-dimensional template for hydrothermal growth of MoS₂ nanosheets. The obtained hierarchical pBC-N/MoS₂ nanocomposites exhibit excellent electrocatalytic activity for HER with small overpotential of 108 mV, high current density of 8.7 mA cm^–2 at η = 200 mV, low Tafel slope of 61 mV dec^–1, and even excellent stability. The greatly improved performance is benefiting from the highly exposed active edge sites of MoS₂ nanosheets, the intimate connection between MoS₂ nanosheets and the highly conductive nitrogen-doped carbon nanofibers and the three-dimensional networks thus formed. Therefore, this work provides a novel strategy for design and application of bacterial cellulose and MoS₂-based nanocomposites as cost-effective HER eletrocatalysts

Flexible Hybrid Membranes with Ni(OH)₂ Nanoplatelets Vertically Grown on Electrospun Carbon Nanofibers for High-Performance Supercapacitors

The practical applications of transition metal oxides and hydroxides for supercapacitors are restricted by their intrinsic poor conductivity, large volumetric expansion, and rapid capacitance fading upon cycling, which can be solved by optimizing these materials to nanostructures and confining them within conductive carbonaceous frameworks. In this work, flexible hybrid membranes with ultrathin Ni­(OH)₂ nanoplatelets vertically and uniformly anchored on the electrospun carbon nanofibers (CNF) have been facilely prepared as electrode materials for supercapacitors. The Ni­(OH)₂/CNF hybrid membranes with three-dimensional macroporous architectures as well as hierarchical nanostructures can provide open and continuous channels for rapid diffusion of electrolyte to access the electrochemically active Ni­(OH)₂ nanoplatelets. Moreover, the carbon nanofiber can act both as a conductive core to provide efficient transport of electrons for fast Faradaic redox reactions of the Ni­(OH)₂ sheath, and as a buffering matrix to mitigate the local volumetric expansion/contraction upon long-term cycling. As a consequence, the optimized Ni­(OH)₂/CNF hybrid membrane exhibits a high specific capacitance of 2523 F g^–1 (based on the mass of Ni­(OH)₂, that is 701 F g^–1 based on the total mass) at a scan rate of 5 mV s^–1. The Ni­(OH)₂/CNF hybrid membranes with high mechanical flexibility, superior electrical conductivity, and remarkably improved electrochemical capacitance are condsidered as promising flexible electrode materials for high-performance supercapacitors

Nitrogen-Doped Graphene Nanoribbons as Efficient Metal-Free Electrocatalysts for Oxygen Reduction

Nitrogen-doped graphene nanoribbon (N-GNR) nanomaterials with different nitrogen contents have been facilely prepared via high temperature pyrolysis of graphene nanoribbons (GNR)/polyaniline (PANI) composites. Here, the GNRs with excellent surface integration were prepared by longitudinally unzipping the multiwalled carbon nanotubes. With a high length-to-width ratio, the GNR sheets are prone to form a conductive network by connecting end-to-end to facilitate the transfer of electrons. Different amounts of PANI acting as a N source were deposited on the surface of GNRs via a layer-by-layer approach, resulting in the formation of N-GNR nanomaterials with different N contents after being pyrolyzed. Electrochemical characterizations reveal that the obtained N_8.3-GNR nanomaterial has excellent catalytic activity toward an oxygen reduction reaction (ORR) in an alkaline electrolyte, including large kinetic-limiting current density and long-term stability as well as a desirable four-electron pathway for the formation of water. These superior properties make the N-GNR nanomaterials a promising kind of cathode catalyst for alkaline fuel cell applications

Immobilization of Co–Al Layered Double Hydroxides on Graphene Oxide Nanosheets: Growth Mechanism and Supercapacitor Studies

Layered double hydroxides (LDHs) are generally expressed as [M²⁺_1–<i>x</i>M³⁺_<i>x</i> (OH)₂] [A^<i>n</i>–_{<i>x</i>/<i>n</i>}·<i>m</i>H₂O], where M²⁺ and M³⁺ are divalent and trivalent metal cations respectively, and A is <i>n</i>-valent interlayer guest anion. Co–Al layered double hydroxides (LDHs) with different sizes have been grown on graphene oxide (GO) via in situ hydrothermal crystallization. In the synthesis procedure, the GO is partially reduced in company with the formation of Co–Al LDHs. The morphology and structure of LDHs/GO hybrids are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The growth mechanism of LDHs on GO nanosheets is discussed. Moreover, both LDHs and LDHs/graphene nanosheets (GNS) hybrids are further used as electrochemical supercapacitor materials and their performance is evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge measurements. It is shown that the specific capacitances of LDHs are significantly enhanced by the hybridization with GNS

Hierarchical Nanostructures of Nitrogen-Doped Porous Carbon Polyhedrons Confined in Carbon Nanosheets for High-Performance Supercapacitors

Interconnected close-packed nitrogen-doped porous carbon polyhedrons (NCPs) confined in two-dimensional carbon nanosheets (CNSs) have been prepared through a sustainable one-pot pyrolysis of a simple solid mixture of zeolitic imidazolate framework-8 (ZIF-8) crystals and with organic potassium as the precursors. The hierarchically organized framework of the NCP–CNS composites enables NCPs and CNSs to act as well-defined electrolyte reservoirs and mechanical buffers accommodating large volume expansions of NCPs, respectively. Among the unique composite nanostructures, the NCPs with vast micropores provide electric double-layer capacitances, while the CNSs bridge the individual NCPs to form a conductive pathway with a hierarchical porosity. As a result, the NCP–CNS composites with high electrical integrity and structural stability are used as electrode materials for high-performance supercapacitors, which exhibit excellent electrochemical capacitive characteristics in terms of an outstanding capacitance of 300 F g^–1 at 1 A g^–1, large energy density of 20.9 W h kg^–1, and great cycling performance of 100% retention after 6000 cycles. This work therefore presents a one-pot and efficient strategy to prepare an ordered arrangement of ZIF-8-derived porous carbons toward new electrode materials in promising energy storage systems