Preventing the Collapse Behavior of Polyurethane Foams with the Addition of Cellulose Nanofiber.

Polyurethane foam manufacturing depends on its materials and processes. A polyol that contains primary alcohol is very reactive with isocyanate. Sometimes, this may cause unexpected problems. In this study, a semi-rigid polyurethane foam was fabricated; however, its collapse occurred. The cellulose nanofiber was fabricated to solve this problem, and a weight ratio of 0.25, 0.5, 1, and 3% (based on total parts per weight of polyols) of the nanofiber was added to the polyurethane foams. The effect of the cellulose nanofiber on the polyurethane foams' rheological, chemical, morphological, thermal, and anti-collapse performances was analyzed. The rheological analysis showed that 3 wt% of the cellulose nanofiber was unsuitable because of the aggregation of the filler. It was observed that the addition of the cellulose nanofiber showed the improved hydrogen bonding of the urethane linkage, even if it was not chemically reacted with the isocyanate groups. Moreover, due to the nucleating effect of the cellulose nanofiber, the average cell area of the produced foams decreased according to the amount of the cellulose nanofiber present, and the average cell area especially was reduced about five times when it contained 1 wt% more of the cellulose nanofiber than the neat foam. Although the thermal stability declined slightly, the glass transition temperature shifted from 25.8 °C to 37.6, 38.2, and 40.1 °C by when the cellulose nanofiber increased. Furthermore, the shrinkage ratio after 14 days from the foaming (%shrinkage) of the polyurethane foams decreased 15.4 times for the 1 wt% cellulose nanofiber polyurethane composite.

Solvent-Free Fabrication of Thick Electrodes in Thermoplastic Binders for High Energy Density Lithium-Ion Batteries.

The rapid development of electric vehicles has generated a recent demand for high energy density lithium-ion batteries (LIBs). One simple, effective way to enhance energy density of LIBs is to increase the thickness of electrodes. However, the conventional wet process used to fabricate thick electrodes involves the evaporation of large amounts of organic solvents, which causes an inhomogeneous distribution of conductive additives and binders. This weakens the mechanical and electrochemical network between active materials, resulting in poor electrochemical performance and structural degradation. Herein, we introduce a new strategy to produce homogeneous thick electrodes by using a dry, solvent-free process. Instead of using a conventional PVDF (polyvinylidene fluoride) binder, we employed a phenoxy resin as the binder in dry process for the first time. This thermoplastic binder exhibits better ductile properties than PVDF in the way that it generates a uniform network structure that connects the active materials during the hot press process. This enables the production electrochemically stable electrodes without using organic solvents, which record capacity retention rates of 73.5% over 50 cycles at a 40 mg/cm2 of thick electrodes. By contrast, thick electrodes produced with a PVDF binder via wet processing only have a capacity retention rate of 21.8% due to rapid structural degradation.

KOH-Activated Hollow ZIF-8 Derived Porous Carbon: Nanoarchitectured Control for Upgraded Capacitive Deionization and Supercapacitor.

Herein, the synergistic effects of hollow nanoarchitecture and high specific surface area of hollow activated carbons (HACs) are reported with the superior supercapacitor (SC) and capacitive deionization (CDI) performance. The center of zeolite imidazolate framework-8 (ZIF-8) is selectively etched to create a hollow cavity as a macropore, and the resulting hollow ZIF-8 (HZIF-8) is carbonized to obtain hollow carbon (HC). The distribution of nanopores is, subsequently, optimized by KOH activation to create more nanopores and significantly increase specific surface area. Indeed, as-prepared hollow activated carbons (HACs) show significant improvement not only in the maximum specific capacitance and desalination capacity but also capacitance retention and mean desalination rates in SC and CDI, respectively. As a result, it is confirmed that well-designed nanoarchitecture and porosity are required to allow efficient diffusion and maximum electrosorption of electrolyte ions.

Multifunctional Additives for High-Energy-Density Lithium-Ion Batteries: Improved Conductive Additive/Binder Networks and Enhanced Electrochemical Properties.

Cylindrical-type cells have been widely adopted by major battery and electric vehicle manufacturers owing to their price competitiveness, safety, and easy expandability. However, placement of electrodes at the core of cylindrical cells is currently restricted because of high electrode curvature and the lack of specialized electrodes and electrode materials. Here, we report the realization of highly flexible high-energy-density electrodes (active material loading of >98.4%) that can be used at the cores of cylindrical cells. The improved properties result from the introduction of a multifunctional poly(melamine-co-formaldehyde) (MF copolymer) additive, which yields a relatively more fluidic and well-dispersed slurry using only 0.08 wt %. MF copolymer-mediated formation of completely wrapped CNT/PVDF networks on LiCoO2 (LCO) and accompanying contact enhancement between LCO and carbon nanotubes (CNTs) resulted in an increase of electrical and mechanical properties of the two types (composites with or without collectors) of electrodes compared with those of additive-free electrodes. Flexibility tests were carried out by rolling electrodes onto cylinder substrates (diameters of ca. 1 and 10 mm); this process resulted in relatively lower resistance changes of the MF copolymer-containing electrodes than for the reference electrodes. In addition, capacity retention after 100 cycles for cells with and without MF copolymers was approximately 10-20% better for the samples with the MF copolymer than for those without. CNT/PVDF networks with MF copolymers were proven to induce a relatively thin and stable cathode electrolyte interface layer accompanying the chemical bond formation during cycling.

Tailored Nanoarchitecturing of Microporous ZIF-8 to Hierarchically Porous Double-Shell Carbons and Their Intrinsic Electrochemical Property.

Mesostructured polydopamine (PDA) coating has been successfully achieved on the surface of zeolitic imidazolate framework-8 (ZIF-8) particles by incorporating Pluronic F127 (with a pore-expanding agent, 1,3,5-trimethylbenzene) as a pore-directing agent during dopamine polymerization. Upon pyrolysis at high temperatures, mesostructured PDA-coated ZIF-8 particles become hierarchically porous double-shell carbons (HPDCs) with a wide pore size distribution ranging from micro- and meso- to macropores. The formation of a hollow inner shell progresses initially with the shrinkage of ZIF-8 at the periphery where the interface interactions with mesostructured PDA exist, and then the subsequent disintegration of the ZIF-8 core at higher temperatures occurs. Our HPDCs prepared in this study feature physical and electrochemical advantages of hierarchically porous carbons such as high electrochemically accessible surface area, short diffusion distance, and high mass-transfer rate, thus demonstrating significantly improved ion diffusion and surface-enhanced high specific capacitance at high charge-discharge rates. HPDC5.0 therefore exhibits the capacitance retention of up to 76.7% from 1 to 10 A g-1 and maximum specific capacitance of 344.7 F g-1 at 1 mV s-1. It also possesses superior electrochemical stability with about 108% capacitance retention even after 10,000 consecutive cycles of galvanostatic charge-discharge at 10 A g-1.

Layered transition metal dichalcogenide/carbon nanocomposites for electrochemical energy storage and conversion applications.

Layered transition metal dichalcogenide (LTMD)/carbon nanocomposites obtained by incorporating conductive carbons such as graphene, carbon nanotubes (CNT), carbon nanofibers (CF), hybrid carbons, hollow carbons, and porous carbons exhibit superior electrochemical properties for energy storage and conversion. Due to the incorporation of carbon into composites, the LTMD/carbon nanocomposites have the following advantages: (1) highly efficient ion/electron transport properties that promote electrochemical performance; (2) suppressed agglomeration and restacking of active materials that improve the cycling performance and electrocatalytic stability; and (3) unique structures such as network, hollow, porous, and vertically aligned nanocomposites that facilitate the shortening of the ion and electrolyte diffusion pathway. In this context, this review introduces and summarizes the recent advances in LTMD/carbon nanocomposites for electrochemical energy-related applications. First, we briefly summarize the reported synthesis strategies for the preparation of LTMD/carbon nanocomposites with various carbon materials. Following this, previous studies using rationally synthesized nanocomposites are discussed based on a variety of applications related to electrochemical energy storage and conversion including Li/Na-ion batteries (LIBs/SIBs), Li-S batteries, supercapacitors, and the hydrogen evolution reaction (HER). In particular, the sections on LIBs and the HER as representative applications of LTMD/carbon nanocomposites are described in detail by classifying them with different carbon materials containing graphene, carbon nanotubes, carbon nanofibers, hybrid carbons, hollow carbons, and porous carbons. In addition, we suggest a new material design of LTMD/carbon nanocomposites based on theoretical calculations. At the end of this review, we provide an outlook on the challenges and future developments in LTMD/carbon nanocomposite research.

Electrophoretic Deposition of Binder-Free MOF-Derived Carbon Films for High-Performance Microsupercapacitors.

Recently, miniaturized power supplies have become essential components of micro-electromechanical systems (MEMS) and portable microdevices due to their high-power density, moderate specific energy, and superior long-term cyclability. In this study, microsupercapacitors with ZIF-8-derived carbons as active materials were successfully fabricate by electrophoretic deposition method. The carbon materials on microsupercapacitors, which are directly deposited or obtained by pyrolyzing predeposited ZIF-8 particles, play a crucial role in achieving outstanding electrochemical performances. The microsupercapacitor of 16 interdigital finger electrodes, prepared by electrophoretic deposition of ZIF-8 particles and subsequent pyrolysis, shows maximum specific power 687.6 mW cm-3 , specific energy 2.87 mWh cm-3 , and 97.8 % capacitance retention rate after 10 000 cycles. The simple and facile process of electrophoretic deposition and subsequent pyrolysis of ZIF-8 particles generates a film of densely populated microporous carbon particles on microsupercapacitor, leading to superior capacitive performances.

Gram-Scale Synthesis of Bimetallic ZIFs and Their Thermal Conversion to Nanoporous Carbon Materials.

The hybrid metal-organic frameworks (MOFs) with different Zn2+/Co2+ ratios are synthesized at room temperature with deionized water as the solvent. This use of deionized water can increase the yield of hybrid MOFs (up to 65-70%). After the pyrolysis, the obtained nanoporous carbons (NPCs) show a decrease in the surface area, in which the highest surface area is 655 m2 g-1. The as-prepared NPCs are subjected to activation with KOH in order to increase their surface area and convert cobalt nanoparticles (Co NPs) to Co oxides. These activated carbons are applied to electrical double-layer capacitors (EDLCs) and pseudocapacitors due to the presence of CoO and Co3O4 nanoparticles in the carbon framework, leading to significantly enhanced specific capacitance as compared to that of pristine NPCs. This synthetic method can be utilized in future research to enhance pseudocapacitance further while maintaining the maximum surface area of the carbon materials.

Three-Dimensional Monolithic Organic Battery Electrodes.

Design of freestanding electrodes incorporated with redox-active organic materials has been limited by the poor intrinsic electrical conductivity and lack of methodology driving the feasible integration of conductive substrate and the organic molecules. Single-walled carbon nanotube (SWCNT) aerogels, which possess continuous network structure and high surface area, offer a three-dimensional electrically conducting scaffold. Here, we fabricate monolithic organic electrodes by coating a nanometer-scale imide-based network (IBN) that possesses abundant redox-active sites on the 3D SWCNT scaffold. The substantially integrated 3D monolithic organic electrodes sustain high electrical conductance through a 3D electronic pathway in their compressed form (∼21 µm). A thin and controllable layer (<8 nm) of IBN organic materials has a strong adhesion onto the ultra-lightweight and conductive substrate and facilitates multielectron redox reactions to deliver a specific capacity of up to 1550 mA h g-1 (corresponding to the areal capacity of ∼2.8 mA h cm-2). The redox-active IBN in synergy with the 3D SWCNT scaffold can enable superior electrochemical performances compared to the previously reported organic-based electrode architectures and inorganic-based electrodes.

Nanoarchitectonics for Transition-Metal-Sulfide-Based Electrocatalysts for Water Splitting.

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, it is described how researchers are working to improve the performance of TMS-based materials by manipulating their internal and external nanoarchitectures. A general introduction to the water-splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS-based materials are explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in the HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water-splitting electrocatalysts for both the HER and the OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The aim herein is to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.

Scalable synthesis of holey graphite nanosheets for supercapacitors with high volumetric capacitance.

We prepared a material composed of high-density holey graphite nanosheets (HGNs) that supports a high gravimetric capacitance of 295 F g-1 and a volumetric capacitance of 384 F cm-3 for use as electrodes in supercapacitor devices. This method is a simple and scalable route to obtain large amounts of holey two-dimensional materials with high electrochemical performances.

Nanoarchitectured Graphene-Organic Frameworks (GOFs): Synthetic Strategies, Properties, and Applications.

Graphene-organic frameworks (GOFs) is a new class of graphene-based materials in which structure and properties can be designed by controlling the length and concentration of organic ligands, comparable to their tunable metal-organic frameworks (MOFs) counterpart. The structural properties (e.g., surface area, pore volume) and physico-chemical properties (e.g., electronic, thermal, and mechanical) of GOFs can be tuned based on the synthetic conditions. Such GOFs are promising as the next generation of novel materials for a wide range of potential applications such as H2 storage, electronic devices, sensors, drug carriers, etc. Here we report a review summarizing synthetic strategies, properties, and applications of GOFs.

Prediction of Defect Formation during Resin Impregnation Process through a Multi-Layered Fiber Preform in Resin Transfer Molding by a Proposed Analytical Model.

It is very important to predict any defects occurring by undesired fiber deformations to improve production yields of resin transfer molding, which has been widely used for mass production of carbon fiber reinforced composite parts. In this study, a simple and efficient analytic scheme was proposed to predict deformations of a multi-layered fiber preform by comparing the forces applied to the preform in a mold of resin transfer molding. Friction coefficient of dry and wet states, permeability, and compressive behavior of unidirectional (UD) and plain woven (PW) carbon fabrics were measured, which were used to predict deformations of the multi-layered fiber preforms with changing their constitution ratios. The model predicted the occurrence, type, and position of fiber deformation, which agreed with the experimental results of the multi-layered preforms.

Advanced Functional Carbons and Their Hybrid Nanoarchitectures towards Supercapacitor Applications.

Porous carbons have attracted much attention as electrode materials for supercapacitors due to their enormous surface area, high electrical conductivity, excellent corrosion resistance, high temperature stability, and relatively low cost. The design of porous architectures is considered key for determining electrochemical performance. Pore size distribution, pore size, and pore connectivity strongly affect electrochemical performance. Various carbon materials with pore size ranging from micro- to macropores were extensively studied. Herein, various types of porous carbon-based and hybrid materials from different approaches and their electrochemical applications are summarized. Appropriate tuning of the pore size of carbon materials is essential for ensuring good transport of ions with different sizes throughout the electrolyte, so that the electrode materials can be fully utilized. Many carbon materials were produced from a series of carbonization and activation processes that possess controllable pore structures, including activated carbons, graphite, carbon nanotubes, carbon aerogels, and templated porous carbons. Templated carbon materials were prepared by various approaches, such as direct carbonization from carbon precursors and soft- and hard-template methods. To enhance the electrochemical performance of the electrode materials, heteroatoms, such as nitrogen, sulfur, and boron, were doped into porous carbons. In addition, to optimize the overall capacitance without destroying the stability and morphology of electrode materials, pseudocapacitive materials, such as transition-metal oxides, were introduced into the carbon frameworks. In this review, recent advances in the fabrication of nanoarchitectured porous carbons and metal oxides through various approaches for supercapacitor applications are summarized.

Experimental and Numerical Studies on Fiber Deformation and Formability in Thermoforming Process Using a Fast-Cure Carbon Prepreg: Effect of Stacking Sequence and Mold Geometry.

A fast-cure carbon fiber/epoxy prepreg was thermoformed against a replicated automotive roof panel mold (square-cup) to investigate the effect of the stacking sequence of prepreg layers with unidirectional and plane woven fabrics and mold geometry with different drawing angles and depths on the fiber deformation and formability of the prepreg. The optimum forming condition was determined via analysis of the material properties of epoxy resin. The non-linear mechanical properties of prepreg at the deformation modes of inter- and intra-ply shear, tensile and bending were measured to be used as input data for the commercial virtual forming simulation software. The prepreg with a stacking sequence containing the plain-woven carbon prepreg on the outer layer of the laminate was successfully thermoformed against a mold with a depth of 20 mm and a tilting angle of 110°. Experimental results for the shear deformations at each corner of the thermoformed square-cup product were compared with the simulation and a similarity in the overall tendency of the shear angle in the path at each corner was observed. The results are expected to contribute to the optimization of parameters on materials, mold design and processing in the thermoforming mass-production process for manufacturing high quality automotive parts with a square-cup geometry.

High-Strength Single-Walled Carbon Nanotube/Permalloy Nanoparticle/Poly(vinyl alcohol) Multifunctional Nanocomposite Fiber.

Magnetic nanocomposite fibers are a topic of intense research due to their potential breakthrough applications such as smart magnetic-field-response devices and electromagnetic interference (EMI) shielding. However, clustering of nanoparticles in a polymer matrix is a recognized challenge for obtaining a property-controllable nanocomposite fiber. Another challenge is that the strength and ductility of the nanocomposite fiber decrease significantly with increased weight loading of magnetic nanoparticles in the fiber. Here, we report high-strength single-walled carbon nanotube (SWNT)/permalloy nanoparticle (PNP)/poly(vinyl alcohol) multifunctional nanocomposite fibers fabricated by wet spinning. The weight loadings of SWNTs and PNPs in the fiber were as high as 12.0 and 38.0%, respectively. The tensile strength of the fiber was as high as 700 MPa, and electrical conductivity reached 96.7 S m(-1). The saturation magnetization (Ms) was as high as 24.8 emu g(-1). The EMI attenuation of a fabric woven from the prepared fiber approached 100% when tested with electromagnetic waves with a frequency higher than 6 GHz. The present study demonstrates that a magnetic-field-response device can be designed using the fabricated multifunctional nanocomposite fiber.

Long-lasting hydrophilicity on nanostructured Si-incorporated diamond-like carbon films.

We investigated the long-lasting hydrophilic behavior of a Si-incorporated diamond-like carbon (Si-DLC) film by varying the Si fraction in DLC matrix through oxygen and nitrogen plasma surface treatments. The wetting behavior of the water droplets on the pure DLC and Si-DLC with the nitrogen or oxygen plasma treatment revealed that the Si element in the oxygen-plasma-treated Si-DLC films played a major role in maintaining a hydrophilic wetting angle of <10° for 20 days in ambient air. The nanostructured patterns with a roughness of ∼10 nm evolved because of the selective etching of the carbon matrix by the oxygen plasma in the Si-DLC film, where the chemical component of the Si-Ox bond was enriched on the top of the nanopatterns and remained for over 20 days.

Nanoscale patterning of microtextured surfaces to control superhydrophobic robustness.

Most naturally existing superhydrophobic surfaces have a dual roughness structure where the entire microtextured area is covered with nanoscale roughness. Despite numerous studies aiming to mimic the biological surfaces, there is a lack of understanding of the role of the nanostructure covering the entire surface. Here we measure and compare the nonwetting behavior of microscopically rough surfaces by changing the coverage of nanoroughness imposed on them. We test the surfaces covered with micropillars, with nanopillars, with partially dual roughness (where micropillar tops are decorated with nanopillars), and with entirely dual roughness and a real lotus leaf surface. It is found that the superhydrophobic robustness of the surface with entirely dual roughness, with respect to the increased liquid pressure caused by the drop evaporation and with respect to the sagging of the liquid meniscus due to increased micropillar spacing, is greatly enhanced compared to that of other surfaces. This is attributed to the nanoroughness on the pillar bases that keeps the bottom surface highly water-repellent. In particular, when a drop sits on the entirely dual surface with a very low micropillar density, the dramatic loss of hydrophobicity is prevented because a novel wetting state is achieved where the drop wets the micropillars while supported by the tips of the basal nanopillars.