Preparation and characterization of palladium nanoparticle-embedded carbon nanofiber membranes via electrospinning and carbonization strategy.

Carbon nanofiber membranes (CNMs) are expected to be used in many energy devices to improve the reaction rate. In this paper, CNMs embedded with palladium nanoparticles (Pd-CNMs) were prepared by electrospinning and carbonization using polyimide as the raw material. The effects of carbonization temperature, carbonization atmosphere, and heating rate on the physicochemical properties of the as-obtained Pd-CNMs were studied in detail. On this basis, the electrocatalytic performance of Pd-CNMs prepared under optimal conditions was characterized. The results showed that highly active zero-valent palladium nanoparticles with uniform particle size could be distributed on the surface of carbon nanofibers. Under vacuum conditions, at a carbonization temperature of 800 °C and a heating rate of 2 °C min-1, Pd-CNMs have lower H2O2 yield, lower Tafel slope (73.3 mV dec-1), higher electron transfer number (∼4), and superior durability, suggesting that Pd-CNMs exhibit excellent electrocatalytic activity for ORR in alkaline electrolyte. Therefore, polyimide-derived CNMs embedded with Pd nanoparticles are expected to become an excellent cathode catalyst layer for fuel cells.

Integrated exploration of experimentation and molecular simulation in ester-containing polyimide dielectrics.

With the growing development of film capacitors in various applications, the requirements for polymer dielectrics have increased accordingly. In this work, a series of ester-cotaining polyimide (EPI) dielectrics were designed and fabricated. Futhermore, integrated exploration of experimentation and molecular simulation is proposed to achieve polymer dielectrics with advanced comprehensive performance, as well as to analyze the dielectric mechanism in-depth. The EPIs show superior thermal resistance and dielectric properties. A Weibull breakdown strength of 440-540 MV m-1, permittivity of 3.52-3.85, dissipation factor of 0.627-0.880% and theoretical energy density of 3.13-4.90 J cm-3 were obtained for the EPIs. The relationship between microscopic parameters and dielectric behavior was investigated in detail. According to the experimental and calculated results, there is close correlation between dipolar moment density (µ/V vdw) and dielectric permittivity (ε r). It is deduced that the integrated research of experiments and molecular simulation would be an effective strategy to reveal the dielectric mechanism as well as assist in the molecular design of polymer dielectrics.

Angular-Shaped Boron Nitride Nanosheets with a High Aspect Ratio to Improve the Out-of-Plane Thermal Conductivity of Polyimide Composite Films.

Polyimide/boron nitride nanosheet (PI/BNNS) composite films have potential applications in the field of electrical devices due to the superior thermal conductivity and outstanding insulating properties of the boron nitride nanosheet. In this study, the boron nitride nanosheet (BNNS-t) was prepared by the template method using sodium chloride as the template, and B2O3 and flowing ammonia as the boron and nitrogen sources, respectively. Then, the PI/BNNS-t composite films were investigated with different loading of BNNS-t as thermally conductive fillers. The results show that BNNS-t has a high aspect ratio and a uniform lateral dimension, with a large dimension and a thin thickness, and there are a few nanosheets with angular shapes in the as-obtained BNNS-t. The synergistic effect of the above characteristics for BNNS-t is beneficial to constructing the three-dimensional heat conduction network of the PI/BNNS-t composite films, which can significantly improve the out-of-plane thermal conduction properties. And then, the out-of-plane thermal conductivity of the PI/BNNS-t composite film achieves 0.67 W m-1 K-1 at 40% loading, which is nearly 3.5 times that of the PI film.

A Novel Triple Crosslinking Strategy on Carbon Nanofiber Membranes as Flexible Electrodes for Lithium-Ion Batteries.

In order to solve the problem of low electrical conductivity of carbon nanofiber membranes, a novel triple crosslinking strategy, including pre-rolling, solvent and chemical imidization crosslinking, was proposed to prepare carbon nanofiber membranes with a chemical crosslinking structure (CNMs-CC) derived from electrospinning polyimide nanofiber membranes. The physical-chemical characteristics of CNMs-CC as freestanding anodes for lithium-ion batteries were investigated in detail, along with carbon nanofiber membranes without a crosslinking structure (CNMs) and carbon nanofiber membranes with a physical crosslinking structure (CNMs-PC) as references. Further investigation demonstrates that CNMs-CC exhibits excellent rate performance and long cycle stability, compared with CNMs and CNMs-PC. At 50 mA g-1, CNMs-CC delivers a reversible specific capacity of 495 mAh g-1. In particular, the specific capacity of CNMs-CC is still as high as 290.87 mAh g-1 and maintains 201.38 mAh g-1 after 1000 cycles at a high current density of 1 A g-1. The excellent electrochemical performance of the CNMs-CC is attributed to the unique crosslinking structure derived from the novel triple crosslinking strategy, which imparts fast electron transfer and ion diffusion kinetics, as well as a stable structure that withstands repeated impacts of ions during charging and discharging process. Therefore, CNMs-CC shows great potential to be the freestanding electrodes applied in the field of flexible lithium-ion batteries and supercapacitors owing to the optimized structure strategy and improved properties.

Polyimide-derived carbon nanofiber membranes as free-standing anodes for lithium-ion batteries.

Free-standing and flexible carbon nanofiber membranes (CNMs) with a three-dimensional network structure were fabricated based on PMDA/ODA polyimide by combining electrospinning, imidization, and carbonization strategies. The influence of carbonization temperature on the physical-chemical characteristics of CNMs was investigated in detail. The electrochemical performances of CNMs as free-standing electrodes without any binder or conducting materials for lithium-ion batteries were also discussed. Furthermore, the surface state and internal carbon structure had an important effect on the nitrogen state, electrical conductivity, and wettability of CNMs, and then further affected the electrochemical performances. The CNMs/Li metal half-cells exhibited a satisfying charge-discharge cycle performance and excellent rate performance. They showed that the reversible specific capacity of CNMs carbonized at 700 °C could reach as high as 430 mA h g-1 at 50 mA g-1, and the value of the specific capacity remained at 206 mA h g-1 after 500 cycles at a high current density of 1 A g-1. Overall, the newly developed carbon nanofiber membranes will be a promising candidate for flexible electrodes used in high-power lithium-ion batteries, supercapacitors and sodium-ion batteries.

Application of PVDF Organic Particles Coating on Polyethylene Separator for Lithium Ion Batteries.

Surface coating modification on a polyethylene separator serves as a promising way to meet the high requirements of thermal dimensional stability and excellent electrolyte wettability for lithium ion batteries (LIBs). In this paper, we report a new type of surface modified separator by coating polyvinylidene fluoride (PVDF) organic particles on traditional microporous polyethylene (PE) separators. The PE separator coated by PVDF particles (PE-PVDF separator) has higher porosity (61.4%), better electrolyte wettability (the contact angle to water was 3.28° ± 0.21°) and superior ionic conductivity (1.53 mS/cm) compared with the bare PE separator (51.2%, 111.3° ± 0.12°, 0.55 mS/cm). On one hand, the PVDF organic polymer has excellent organic electrolyte compatibility. On the other hand, the PVDF particles contain sub-micro spheres, of which the separator can possess a large specific surface area to absorb additional electrolyte. As a result, LIBs assembled using the PE-PVDF separator showed better electrochemical performances. For example, the button cell using a PE-PVDF as the separator had a higher capacity retention rate (70.01% capacity retention after 200 cycles at 0.5 C) than the bare PE separator (62.5% capacity retention after 200 cycles at 0.5 C). Moreover, the rate capability of LIBs was greatly improved as well-especially at larger current densities such as 2 C and 5 C.

Direct Nitridation Synthesis of Quasi-Spherical ß-Si3N4 Powders with CaF2 Additive.

In this work, the quasi-spherical ß-Si3N4 powders were synthesized via an efficient direct nitridation strategy with CaF2 as the catalytic material under NH3 atmosphere. The effect of CaF2 on phase composition and crystalline morphology was studied. CaF2 additive can accelerate the nitridation of silicon powders, and the particles of nitridation products tend to have an equiaxed structure with the CaF2 additive increasing. When 4 wt% CaF2 additive or more was added, submicron ß-Si3N4 particles with quasi-spherical morphology and eminent crystal integrity were obtained. In contrast, irregular α-Si3N4 particles appear as the main phase with less than 4 wt% CaF2 additive. The growth mechanism of Si3N4 particles was also discussed. CaxSiyOz liquid phase is crucial in the nitridation of silicon powders with CaF2 additive.