Preparation of Hollow Porous Carbon Nanofibers and Their Performance and Mechanism of Broadband Microwave Absorption.

Developing microwave absorbing composites with lightweight and wide absorption bands is an essential direction for electromagnetic wave stealth and shielding application. In this article, PAN/PMMA blend fibers and sheath-core blend fibers with PAN/PMMA as the sheath and PMMA as the core were spun by uniaxial and coaxial electrostatic spinning, respectively. Porous carbon nanofiber (PCNF) and hollow porous carbon nanofiber (HPCNF) were obtained after pre-oxidation and carbonization of the corresponding two precursor fibers. The microwave absorption composite samples with PCNF and HPCNF as absorbents and paraffin as matrix were prepared, respectively. Their electromagnetic parameters were investigated by the reflective-transmission network parameter method. The microwave absorption properties of the corresponding composites were calculated based on a model for a single-layer planewave absorber from electromagnetic parameters. The results showed diversity between the microwave absorbing performance of the composites filled with PCNF and HPCNF. HPCNF performs better than PCNF as an absorbent; that is, the lowest reflection loss of composite filled with HPCNF is -20.26 dB and the effective bandwidth (lower than -10 dB) is to 4.56 GHz, while the lowest reflection loss of a composite filled with PCNF is -13.70 dB, and the effective bandwidth (lower than -10 dB) is 2.68 GHz when the absorbent content is 7%, and the thickness is 3 mm. Much lower reflection loss and a wider absorption band could be expected from HPCNF. The presence of a hollow structure in HPCNF, which may increase the degree of polarization and provide more interfaces for the interference phase extinction of reflected electromagnetic waves, might help to improve the attenuation of electromagnetic waves and broaden the absorption band.

The Oxygen Reduction Performance of Pt Supported on the Hybrid of Porous Carbon Nanofibers and Carbon Black.

Proton exchange membrane fuel cells (PEMFCs) represent an outstanding clean energy alternative for next-generation power sources. The PEMFC's performance is mainly determined by the sluggish oxygen reduction reaction (ORR) that occurs in its cathode Therefore, the use of electrocatalysts with high electrocatalytic activity and stability for improving the ORR has been a vital direction for the commercialization of PEMFCs. In this article, porous carbon nanofibers (PCNFs) based on a polyacrylonitrile/polymethyl methacrylate (PAN/PMMA) precursor were fabricated by electrospinning followed by carbonization; then, the PCNFs were mixed together with carbon black (CB) in different mass ratios as a hybrid support for Pt nanoparticles. Pt nanoparticles were deposited on the hybrid support by the ethylene glycol reduction method, and the obtained series of Pt/(PCNF + CB) were used as the oxygen reduction electrocatalyst in the cathode. Their electrocatalytic properties, as well as those of Pt/C as a reference, were investigated by cyclic voltammetry scanning (CV) and linear sweep voltammetry (LSV). The results explained that Pt/(PCNF + CB) showed a higher electrochemical activity area and half-wave potential when the PCNF/CB mass ratio was 3/2 than that of commercial Pt/C. Furthermore, the half-wave potential of Pt/(PCNF + CB) only decreased by 4 mV, which was 86 mV lower than that of commercial Pt/C (90 mV) after 2000 ADT cycles, indicating that the incorporation of PCNFs to form a hybrid support could result in corrosion resistance.

Interconnected Hollow Porous Polyacrylonitrile-Based Electrolyte Membrane for a Quasi-Solid-State Flexible Zinc-Air Battery with Ultralong Lifetime.

Quasi-solid-state flexible zinc-air batteries (FZABs) have received enormous attention due to their low cost and high safety. However, the constraints in lifetime resulting from the lack of stable quasi-solid-state electrolyte membranes and efficient bifunctional electrocatalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) hinder the large-scale manufacture and commercialization of FZABs to power electric devices. Herein, a polyacrylonitrile (PAN)-based membrane (HPPANP) fabricated via facile coaxial electrospinning, water dissolution, lyophilization, and KOH preimmersion method was utilized as the quasi-solid-state electrolyte membrane. The interconnected hollow porous structure based on PAN nanofibers endows HPPANP with outstanding electrolyte-uptake/retention capabilities for high ionic conductivity and nanolevel wetted electrolyte/anode interface for uniform Zn dissolution/deposition, thus prolonging the lifespan of the FZABs. In addition, the in situ alkaline hydrolysis of KOH solution supplies HPPANP with abundant oxygen-containing groups, which also improves its ionic conductivity. Additionally, we synthesized a Co/N-doped hollow carbon sphere (CoN-CS) electrocatalyst that exhibits superior ORR and OER electrocatalytic activities with a low potential difference (ΔE) of 0.73 V. Such favorable ORR and OER performances can be mainly attributed to the hierarchical hollow micro/nanostructures with abundant active sites, long-term stability, and favorable electron/ion diffusion pathway. As a result, the assembled FZAB equipped with the CoN-CS catalyst and HPPANP displays high power density (123.8 mW cm-2) and preferable long-term cycling performance (more than 50 h at 3 mA cm-2).

The Construction of Multi-Incorporated Polylactic Composite Nanofibrous Scaffold for the Potential Applications in Bone Tissue Regeneration.

To improve the bone regeneration ability of pure polymer, varieties of bioactive components were incorporated to a biomolecular scaffold with different structures. In this study, polysilsesquioxane (POSS), pearl powder and dexamethasone loaded porous carbon nanofibers (DEX@PCNFs) were incorporated into polylactic (PLA) nanofibrous scaffold via electrospinning for the application of bone tissue regeneration. The morphology observation showed that the nanofibers were well formed through electrospinning process. The mineralization test of incubation in simulated body fluid (SBF) revealed that POSS incorporated scaffold obtained faster hydroxyapatite depositing ability than pristine PLA nanofibers. Importantly, benefitting from the bioactive components of pearl powder like bone morphogenetic protein (BMP), bone mesenchymal stem cells (BMSCs) cultured on the composite scaffold presented higher proliferation rate. In addition, by further incorporating with DEX@PCNFs, the alkaline phosphatase (ALP) level and calcium deposition were a little higher based on pearl powder. Consequently, the novel POSS, pearl powder and DEX@PCNFs multi-incorporated PLA nanofibrous scaffold can provide better ability to enhance the biocompatibility and accelerate osteogenic differentiation of BMSCs, which has potential applications in bone tissue regeneration.

Uniform Deposition and Effective Confinement of Lithium in Three-Dimensional Interconnected Microchannels for Stable Lithium Metal Anodes.

Lithium dendrite formation has hindered the practical implementation of lithium metal batteries with higher energy densities compared with those of conventional lithium-ion batteries. Herein, a nanoconfinement strategy to access dendrite-free lithium metal anodes comprising three-dimensional (3D) hollow porous multi-nanochannel carbon fiber embedded with TiO2 nanocrystals (HTCNF) is reported. The transport of the lithium ions is facilitated by the 3D architecture. Functioning as nanoseeds, the TiO2 nanocrystals guide the lithium ions toward forming uniform deposits, which are further confined inside the hollow carbon fibers and the 3D HTCNF layer. Site-selective deposition coupled with the nanoconfinement of lithium metal modifies the Li plating/stripping behavior and effectively suppresses the dendrite growth. The HTCNF-Li cell delivers a stable cycling performance of 1300 h with a voltage hysteresis as low as 6 mV. The assembled HTCNF-Li//LiFePO4 full cell displays a compelling rate performance and enhanced cycling stability with high capacity retention (90% after 400 cycles at 0.5 C). Our results demonstrate a new and potentially scalable route to resolve the lithium dendrite growth issue for enhanced electrochemical performances, which can be further extended to other metal battery systems.

Influence of Incorporated Polydimethylsiloxane on Properties of PA66 Fiber and Its Fabric Performance.

Poly(hexamethyllene adipamide), PA66 fiber has played an important role in varied industrial applications, and its corresponding product would become more competitive if some extra value was added to PA66 fiber. In this article, polydimethylsiloxane (PDMS) was used as an additive to prepare PA66/PDMS blend fibers through melt blend spinning carried out by a screw extruder spinning machine. When the amount of incorporated PDMS was 0.5-3 wt %, the blend melt demonstrated good spinning ability, and the PA66/PDMS blend fibers exhibited excellent mechanical property and reduced hot shrinkage. Moreover, the crystallization and melting behavior of PA66 in the blend fibers turned out to be not affected by the existence of PDMS. In addition, the contact angle of water on the blend fiber surface became larger, while the value of friction coefficient on the surface of fibers got lower with increasing PDMS content in the blend fibers. After evaluating the fabric woven by PA66/PDMS blend fibers using the KES-F KES-FB-2 fabric measuring system, it was found that as PDMS content increased, the flexural rigidity and bending hysteresis would be lower, yet elasticity rate of compression work would be higher, which explained how the fabric composed of the blend fiber performed better in terms of softness and elasticity.

Synergistic Effect of Nitrogen Doping and MWCNT Intercalation for the Graphene Hybrid Support for Pt Nanoparticles with Exemplary Oxygen Reduction Reaction Performance.

The potential of graphene⁻multi-walled-carbon nanotube (G-M) hybrids prepared by the one-pot modified Hummers method followed by thermal annealing has been demonstrated by employing one as an electrocatalyst support for oxygen reduction reaction (ORR). N doping effectively modified the electronic structure of the G-M hybrid support, which was beneficial for the uniform distribution of Pt nanoparticles, and ORR activities were further improved. The newly prepared Pt/N-G-M catalyst demonstrated higher electrochemical activity than Pt/G-M and Pt/G catalysts. Even compared with commercial 20 wt % Pt/C (JM20), Pt/N-G-M delivered a better half-wave potential and mass activity. In terms of the durability test, Pt/N-G-M maintained 72.7% of its initial electrochemical active surface area (ECSA) after 2000 repeated potential cycles between 0 and 1.2 V in acidic media in relation to the 44.4% retention for JM20. Moreover, the half-wave potential for Pt/N-G-M showed only a minimal change, significantly superior to the 139 mV of loss for JM20. It is expected that Pt/N-G-M can be the potential candidate as a highly efficient and durable catalyst if utilized in proton exchange membrane fuel cells (PEMFCs).

Effect of hydrogen bonding on the compressive strength of dihydroxypoly(p-phenylenebenzobisoxazole) fibers.

By the introduction of binary hydroxyl groups into poly(p-phenylenebenzoxazole) (PBO) macromolecular chains, a series of dihydroxypoly(p-phenylenebenzobisoxazole) (DHPBO) chains were prepared, and the effect of the hydroxyl groups on the axial compression property of DHPBO fibers was investigated. The variable-temperature Fourier transform infrared spectrum proved the existence of hydrogen bonds in DHPBO chains. Furthermore, the axial compression bending test showed that the introduction of hydroxyl groups into macromolecular chains apparently improved the compression resistance property of DHPBO fibers. Finally, a proposed arrangement of the hydrogen bonding in DHPBO fibers is presented.