The common knowledge of the crystal structures of nylons dates back to the early 1950s with the work of Bunn and Garner. It describes the packing of sheets made of hydrogen-bonded stems in an extended chain conformation. These alpha phase structures have a specific powder X-ray diffraction pattern with reflections at 4.4 Å and 3.7 Å. On heating, reflections transform progressively and merge at ≈ 4.2 Å at the so-called “Brill transition”. Other diffraction patterns have been recorded for different types of nylons and thermal histories. These patterns were interpreted only as indicating the existence of “variants” of the alpha phase. However, neither their structure, nor the origin of the Brill transition were established. Recent structural analyses and molecular modeling approaches have provided new insights in these long-standing structural puzzles. The “variants” feature chain conformations that are “pleated”. The Brill transition does not involve the standard extended chains of the alpha phase but corresponds to a dynamic interconversion (≈ 1010/s) between mirror conformations of these pleated stems.
Electrochemical water splitting is a fascinating technology for sustainable hydrogen production, and electrocatalysts are essential to accelerate the sluggish hydrogen and oxygen evolution reactions (HER and OER). Transition-metal-based electrocatalysts have attracted enormous interests due to the abundant resources, low cost, and comparable catalytic performance to noble metals. Among these studies, fibrous materials possess distinct advantages, such as unique structure, high active surface area, and fast electron transport. Herein, the most recent progress of nanofiber electrocatalysts on synthesis and application in HER and OER is summarized, with emphasis on iron-, cobalt-, and nickel-based materials. Moreover, the challenge and prospects of fibrous-structured electrocatalysts on water splitting is provided.
Green and environmentally friendly electrocatalytic nitrogen (N2) fixation to synthesize ammonia (NH3) is recognized as an effective method to replace the traditional Haber–Bosch process. However, the difficulties in N2 adsorption and fracture of hard N≡N bond still remain major challenges in electrocatalytic N2 reduction reactions (NRR). From the perspectives of enhancing N2 adsorption and providing more catalytic sites, two-dimensional (2D) FeS2 nanosheets and three-dimensional (3D) metal organic framework-derived ZnS embedded within N-doped carbon polyhedras are grown on the carbon cloth (CC) template in this work. Thus, a composite NRR catalyst with multi-dimensional structures, which is signed as FeS2/ZnS-NC@CC, is obtained for using over a wide pH range. The uniform distribution of hollow ZnS-NC frameworks and FeS2 nanosheets on the surface of CC largely increase the N2 enrichment efficiency and offer more active sites, while the CC skeleton acts as an independent conductive substrate and S-doping helps promote the fracture of N≡N bond during the NRR reaction. As a result, the FeS2/ZnS-NC@CC electrode achieves a high Faraday efficiency of 46.84% and NH3 yield of 58.52 μg h−1 mg−1 at -0.5 V vs. Ag/AgCl in 0.1 M KOH. Furthermore, the FeS2/ZnS-NC@CC electrode displays excellent NRR catalytic activity in acidic and neutral electrolytes as well, which outperforms most previously reported electrocatalysts including noble metals. Therefore, this work provides a new way for the design of multi-dimensional electrocatalysts with excellent electrocatalytic efficiency and stability for NRR applications.
Wearable and portable electronic devices based on textile structure have been widely used owing to their wearability and comfortableness. However, yarn fineness and the comfort of the fabric cannot satisfy the requirements of smart wearable devices. This work presents a novel strategy to prepare highly integrated PVDF/conductive nylon core-sheath structure piezoelectric yarns for wearable which is fabricated by combining electrospinning strategy with 2D braiding technology. The fineness of single yarns as well as strength are both improved significantly compared to previous works. The piezoelectric outputs of the yarn are still stable after 800 s fatigue test at a frequency of 4 Hz, and the cycle stability can maintain more than 3200 cycles. Furthermore, the piezoelectric yarns are further woven into piezoelectric plain fabric. According to the electrical performance, the length of the piezoelectric yarn and the thickness of the piezoelectric layer would both affect the output electrical performance. The yarn of the 10 cm in length and 600 μm in fineness can produce an output voltage of 120 mV. Meanwhile, Both the piezoelectric yarn and the fabric could generate piezoelectric output signals through human movement, such as bending, walking. Therefore, the electrical and mechanical performance of the piezoelectric yarns prepared in our work could be improved significantly, and the comfortableness and durability performance of the piezoelectric fabric can satisfy most wearing requirements, which would provide some help in the field of piezoelectric wearable devices based on yarns and fabrics.
For obtaining high-throughput production of nanofibers, the preparation mechanism of a self-made spherical section free surface electrospinning (SSFSE) using solution reservoirs with different depths was studied. The effects of the solution reservoir depth on the SSFSE process as well as the quality and yield of polyacrylonitrile (PAN) nanofibers were investigated experimentally using high-speed camera, precise electronic balance and scanning electron microscopy. Furthermore, the results were analyzed theoretically by response surface methodology (RSM) and numerical simulation. The values predicted by the established RSM model and the electric field results obtained by Maxwell 3D were all consistent with the experimental data, which showed that the different depths of the solution reservoir had little effect on the quality of PAN nanofibers, but had great effects on the yields of them. The PAN nanofibers prepared have the best quality and the highest yields when the maximum depth of the solution reservoir was 4.29 mm.
Electrode material has been cited as one of the most important determining factors in classifying an energy storage system’s charge storage mechanism, i.e., as battery-type or supercapacitive-type. In this paper, we show that along with the electrode material, the electrolyte also plays a role in determining the charge storage behaviour of the system. For the purpose of our research, we chose multi-elemental spinal type CuMn2O4 metal oxide nanofibers to prove the hypothesis. The material is synthesized as nanofibers of diameter ~ 120 to 150 nm in large scales by a pilot scale electrospinning set up. It was then tested in three different electrolytes (1 M KOH, 1 M Na2SO4 and 1 M Li2SO4), two of which are neutral and the third is alkaline (KOH). The cyclic voltammograms and the galvanostatic charge–discharge of the electrode material in a three-electrode system measurement showed that it exhibit different charge storage mechanism in different electrolyte solutions. For the neutral electrolytes, a capacitive behaviour was observed whereas a battery-type behaviour was seen for the alkaline electrolyte. This leads us to conclude that the charge storage mechanism, along with the active material, also depends on the electrolyte used.