Deposition Behavior of Polyaniline on Carbon Nanofibers by Oxidative Chemical Vapor Deposition.

Oxidative chemical vapor deposition (oCVD) offers unique advantages as a liquid-free processing technique in synthesizing and integrating conducting polymers, including polyaniline (PANI), by enabling conformal coatings onto nanostructured substrates, like carbon nanofibers. With relatively thick nanofiber mats, the challenge is to ensure uniform coating thickness through the porous substrates. Here, the substrate temperature during oCVD is found to be a primary factor influencing PANI coating uniformity. Coating uniformity is enhanced by operating at a higher substrate temperature, where monomer adsorption is believed to be limiting relative to intrinsic reaction kinetics. Also, a higher substrate temperature leads to significantly less PANI oligomers and more PANI in the emeraldine oxidation state. A systematic study of oCVD kinetics with substrate temperature shows a reaction-limited regime at lower substrate temperatures with an activation energy of 12.0 kJ/mol, which is believed to be controlled by the self-catalyzed PANI polymerization reaction that transitions at higher substrate temperatures above 90 °C to an adsorption-limited regime as indicated by a negative activation energy of -18.8 kJ/mol. Overall, by operating within an adsorption-limited oCVD regime, more uniform oCVD PANI coatings on electrospun carbon nanofiber mats have been achieved.

Cobalt Nanoparticle-Embedded Porous Carbon Nanofibers with Inherent N- and F-Doping as Binder-Free Bifunctional Catalysts for Oxygen Reduction and Evolution Reactions.

Efficient, low-cost, non-precious metal-based, and stable bifunctional electrocatalysts are key to various energy storage and conversion devices such as regenerative fuel cells and metal-air batteries. In this work, we report cobalt nanoparticle-embedded porous carbon nanofibers with inherent N- and F-doping as binder-free bifunctional electrocatalysts with excellent activity for both the oxygen reduction and oxygen evolution reaction (ORR/OER) in an alkaline medium. Single-step electrospinning of a solution of the polymer mixture (carbon precursor) and the cobalt precursor followed by controlled pyrolysis with an intermediate reduction step in H2 (to reduce cobalt oxides to cobalt) was utilized to synthesize an integrated freestanding catalyst. The fabricated catalyst with effective structural and electronic interaction between the cobalt metal nanoparticles and the N- and F-doped carbon defect sites showed enhanced catalytic properties compared to the benchmark catalysts for ORR and OER (Pt, Ir, and Ru). The ORR potential at the current density of -3 mA cm-2 was 0.81 VRHE and the OER potential at a current density of 10 mA cm-2 was 1.595 VRHE , resulting in a ΔE of only 0.785 V.

Molecular dynamics study on effect of elongational flow on morphology of immiscible mixtures.

We studied the effect of elongational flow on structure and kinetics of phase separation in immiscible blends using molecular dynamics simulations. Two different blend systems have been investigated-binary blend of polymers and binary mixture of molecular fluids. The interaction potential parameters in both material systems were chosen to ensure complete phase-separation in equilibrium. We found that elongational flow, beyond a certain rate, significantly alters the steady state morphology in such immiscible mixtures. For the case of polymer blends, perpendicular lamellar morphology was formed under elongation rates (ε̇) from 0.05 to 0.5 MD units possibly due to the interplay of two opposing phenomena-domain deformation/rupture under elongation and aggregation of like-domains due to favorable energetic interactions. The elongation timescale at the critical rate of transition from phase-separated to the lamellar structure (ε̇ = 0.05) was found to be comparable to the estimated polymer relaxation time, suggesting a cross-over to the elongation/rupture-dominant regime. Under strong elongational flow rate, ε̇ > 0.5, the formation of disordered morphology was seen in polymer blend systems. The kinetics of phase separation was monitored by calculating domain size as a function of time for various elongational flow rates. The domain growth along the vorticity-axis was shown to follow a power law, Rz(t) ∼ t( α). A growth exponent, α of 1/3 for the polymer blend and 0.5-0.6 for the fluid molecular mixture was found under elongation rates from 0.005 to 0.1. The higher growth exponent in the fluid mixture is a result of its faster diffusion time scale compared to that of polymer chains. The steady state end-to-end distance of polymer chains and viscosity of the polymer blend were examined and found to depend on the steady state morphology and elongation rate.

Controlling the dispersion and orientation of nanorods in polymer melt under shear: coarse-grained molecular dynamics simulation study.

Incorporation of nanorods (NRs) into a polymer matrix can greatly enhance the material properties, but the aggregation of NRs prevents the full realization of their potential. Using coarse-grained molecular dynamics simulation with the dissipative particle dynamics thermostat, we have systematically examined how key material and processing parameters, such as aspect ratio, particle diameter, rigidity and concentration of NR, polymer chain length, and shear rate can influence the placement and orientation of the self-aggregating NRs in a model polymer melt under shear. When compared with nanoparticles (NPs), the NRs tend to aggregate more severely even under strong shear flow. To improve the dispersion of NRs within the polymer matrix under a given flow condition, we incorporated additional NPs with selective interactions into polymer/NR composites, demonstrating that the current mesoscale simulation study offers insights on how to control the dispersion and orientation of NRs in polymer under shear flow.

Fabrication of transition metal oxide-carbon nanofibers with novel hierarchical architectures.

We report a facile two-step methodology; electrospinning followed by high temperature treatment, to produce manganese oxide-based nanofibers with well-controlled nanoscale architectures. Electrospinning of manganese acetate-based solution (MnOx precursor) has been utilized to fabricate meso-porous manganese oxide nanofibers. These fibers have diameters of about 200-300 nm and fiber mats have been shown to have specific surface area of over 12 m2/g. Scanning and transmission electron microscopy results show that electrospinning has been successfully utilized to create nanofibers with deep inter-connected internal meso-pores for high surface area. In addition, fibers have been spun in a co-axial arrangement to fabricate hollow meso-porous nanofibers, or to develop core-shell nanofibers with nanoparticles of manganese oxides decorated over current conducting carbon core. X-ray diffraction analysis of the oxide fibers confirms the presence of manganese oxides (MnO2, Mn3O4) after calcination at 700 degrees C. These architectures, we believe, are potentially favorable for use in Li-ion batteries, Li-air batteries and supercapacitors.

One-Dimensional, Titania Lepidocrocite-Based Nanofilaments and Their Polysulfide Anchoring Capabilities in Lithium-Sulfur Batteries.

The high theoretical energy density of metal-sulfur batteries compared to their lithium-ion counter parts renders sulfur-based electrode chemistries attractive. Additionally, sulfur is relatively abundant and environmentally benign. Yet, issues like the low conductivity of sulfur, polysulfide (PS) formation, and shuttling have hindered the development of sulfur chemistries. Here, we react titanium carbide powders with tetramethylammonium hydroxide ammonium salts at 50 °C for 5 days and convert them into one dimensional, titania-based lepidocrocite (1DL) nanofilaments (NFs) using our facile bottom-up approach. This simple and scalable approach led to better electrode functionalization, facile tunability, and a higher density of active sites. The 1DL NFs self-assembled into a variety of microstructures─from individual 1DL NFs with minimal cross sections ≈5 × 7 Å2 to 2D flakes to mesoscopic particles. A composite was made with a 1:1 weight ratio of sulfur and 1DL NFs, which were hand-ground, mixed with carbon black and binder in a weight ratio of 70:20:10, respectively. We obtained a specific capacity of 750 mA h g-1 at 0.5C for 300 cycles. The 1DL NFs that, in this case assembled into 2D layers, trapped the polysulfides, PSs, by forming thiosulfate species and Lewis acid-base interactions with the Ti, as confirmed by post-mortem X-ray photoelectron spectroscopy. These interactions were also confirmed by PS adsorption via UV-vis spectroscopy and shuttle current measurements that showed lower PS shuttling in the 1DL NFs cells.

Stabilization of gamma sulfur at room temperature to enable the use of carbonate electrolyte in Li-S batteries.

This past decade has seen extensive research in lithium-sulfur batteries with exemplary works mitigating the notorious polysulfide shuttling. However, these works utilize ether electrolytes that are highly volatile severely hindering their practicality. Here, we stabilize a rare monoclinic γ-sulfur phase within carbon nanofibers that enables successful operation of Lithium-Sulfur (Li-S) batteries in carbonate electrolyte for 4000 cycles. Carbonates are known to adversely react with the intermediate polysulfides and shut down Li-S batteries in first discharge. Through electrochemical characterization and post-mortem spectroscopy/ microscopy studies on cycled cells, we demonstrate an altered redox mechanism in our cells that reversibly converts monoclinic sulfur to Li2S without the formation of intermediate polysulfides for the entire range of 4000 cycles. To the best of our knowledge, this is the first study to report the synthesis of stable γ-sulfur and its application in Li-S batteries. We hope that this striking discovery of solid-to-solid reaction will trigger new fundamental and applied research in carbonate electrolyte Li-S batteries.

Effect of shear on nanoparticle dispersion in polymer melts: A coarse-grained molecular dynamics study.

Coarse-grained, molecular dynamics (MD) simulations have been conducted to study the effect of shear flow on polymer nanocomposite systems. In particular, the interactions between different components have been tuned such that the nanoparticle-nanoparticle attraction is stronger than nanoparticle-polymer interaction, and therefore, the final equilibrium state for such systems is one with clustered nanoparticles. In the current study, we focus on how shear flow affects the kinetics of particle aggregation at the very initial stages in systems with polymers of different chain lengths. The particle volume fraction and size are kept fixed at 0.1 and 1.7 MD units, respectively. Through this work, shear has been shown to significantly slow down nanoparticle aggregation, an effect that was found to be a strong function of both polymer chain length and shear rate. To understand our findings, a systematic study on effect of shear on particle diffusion and an analysis of relative time scales of different mechanisms causing particle aggregation have been conducted. The aggregation rate obtained from the time scale analysis is in good agreement with that determined from the aggregation time derived from the pair correlation function monitored during simulations.

Confined assembly of asymmetric block-copolymer nanofibers via multiaxial jet electrospinning.

Multiaxial (triaxial/coaxial) electrospinning is utilized to fabricate block copolymer (poly(styrene-b-isoprene), PS-b-PI) nanofibers covered with a silica shell. The thermally stable silica shell allows post-fabrication annealing of the fibers to obtain equilibrium self-assembly. For the case of coaxial nanofibers, block copolymers with different isoprene volume fractions are studied to understand the effect of physical confinement and interfacial interaction on self-assembled structures. Various confined assemblies such as co-existing cylinders and concentric lamellar rings are obtained with the styrene domain next to the silica shell. This confined assembly is then utilized as a template to guide the placement of functional nanoparticles such as magnetite selectively into the PI domain in self-assembled nanofibers. To further investigate the effect of interfacial interaction and frustration due to the physically confined environment, triaxial configuration is used where the middle layer of the self-assembling material is sandwiched between the innermost and outermost silica layers. The results reveal that confined block-copolymer assembly is significantly altered by the presence and interaction with both inner and outer silica layers. When nanoparticles are incorporated into PS-b-PI and placed as the middle layer, the PI phase with magnetite nanoparticles migrates next to the silica layers. The migration of the PI phase to the silica layers is also observed for the blend of PS and PS-b-PI as the middle layer. These materials not only provide a platform to further study the effect of confinement and wall interactions on self-assembly but can also help develop an approach to fabricate multilayered, multistructured nanofibers for high-end applications such as drug delivery.

Coarse-grained molecular dynamics study of block copolymer/nanoparticle composites under elongational flow.

Symmetric diblock copolymer/nanoparticle (NP) systems under planar elongational flow have been modeled and simulated using coarse-grained nonequilibrium molecular dynamics. The aim of our present study is to understand how the dispersion of NPs in a block copolymer system is influenced by elongational flow and how the presence of NPs changes the rheology and flow-induced morphology transition in block copolymers. We consider two different kinds of spherical NPs categorized with respect to their interaction potential with the polymeric blocks: (1) selective NPs that show a preference toward one of the blocks of a model diblock copolymer and (2) nonselective NPs that show equal attraction toward both blocks. For unrestricted simulation times during elongational flow, spatially and temporally periodic boundary conditions devised by Kraynik and Reinelt [Int. J. Multiphase Flow 18, 1045 (1992)] have been implemented. Our results show that the concentration peak of both selective NPs at the center of the preferred domain and nonselective NPs at the domain interface becomes broader with increasing elongation rate, suggesting that elongational flow can be used as another parameter to control nanocomposite self-assembly. In addition, our results reveal that the onset of flow-induced transition from lamellar to disordered morphology is greatly influenced by particle-particle and particle-polymer interactions.

Controlling nanoparticle location via confined assembly in electrospun block copolymer nanofibers.

Coaxial nanofibers with poly(styrene-block-isoprene) (PS-b-PI)/magnetite nanoparticles as core and silica as shell are fabricated using electrospinning.1-4 Thermally stable silica helps to anneal the fibers above the glass transition temperature of PS-b-PI and form ordered nanocomposite morphologies. Monodisperse magnetite nanoparticles (NPs; 4 nm) are synthesized and surface coated with oleic acid to provide marginal selectivity towards an isoprene domain. When 4 wt% nanoparticles are added to symmetric PS-b-PI, transmission electron microscopy (TEM) images of microtomed electrospun fibers reveal that NPs are uniformly dispersed only in the PI domain, and that the confined lamellar assembly in the form of alternate concentric rings of PS and PI is preserved. For 10 wt% NPs, a morphology transition is seen from concentric rings to a co-continuous phase with NPs again uniformly dispersed in the PI domains. No aggregates or loss of PI selectivity is found in spite of interparticle attraction. Magnetic properties are measured using a superconducting quantum interference device (SQUID) magnetometer and all nanocomposite fiber samples exhibit superparamagnetic behavior.

TiO Phase Stabilized into Freestanding Nanofibers as Strong Polysulfide Immobilizer in Li-S Batteries: Evidence for Lewis Acid-Base Interactions.

We report the stabilization of titanium monoxide (TiO) nanoparticles in nanofibers through electrospinning and carbothermal processes and their unique bifunctionality-high conductivity and ability to bind polysulfides-in Li-S batteries. The developed three-dimensional TiO/carbon nanofiber (CNF) architecture with the inherent interfiber macropores of nanofiber mats provides a much higher surface area (∼427 m2 g-1) and overcomes the challenges associated with the use of highly dense powdered Ti-based suboxides/monoxide materials, thereby allowing for high active sulfur loading among other benefits. The developed TiO/CNF-S cathodes exhibit high initial discharge capacities of ∼1080, ∼975, and ∼791 mAh g-1 at 0.1, 0.2, and 0.5 C rates, respectively, with long-term cycling. Furthermore, freestanding TiO/CNF-S cathodes developed with rapid sulfur melt infiltration (∼5 s) eradicate the need of inactive elements, viz., binders, additional current collectors (Al-foil), and additives. Using postmortem X-ray photoelectron spectroscopy and Raman analysis, this study is the first to reveal the presence of strong Lewis acid-base interaction between TiO (3d2) and S x2- through the coordinate covalent Ti-S bond formation. Our results highlight the importance of developing Ti-suboxides/monoxide-based nanofibrous conducting polar host materials for next-generation Li-S batteries.

Highly Durable, Self-Standing Solid-State Supercapacitor Based on an Ionic Liquid-Rich Ionogel and Porous Carbon Nanofiber Electrodes.

A high-performance, self-standing solid-state supercapacitor is prepared by incorporating an ionic liquid (IL)-rich ionogel made with 95 wt % IL (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and 5 wt % methyl cellulose, a polymer matrix, into highly interconnected 3-D activated carbon nanofiber (CNF) electrodes. The ionogel exhibits strong mechanical properties with a storage modulus of 5 MPa and a high ionic conductivity of 5.7 mS cm-1 at 25 °C. The high-surface-area CNF-based electrode (2282 m2 g-1), obtained via an electrospinning technique, exhibits hierarchical porosity generated both in situ during pyrolysis and ex situ via KOH activation. The porous architecture of the CNF electrodes facilitates the facile percolation of the soft but mechanically durable ionogel film, thereby enabling intimate contact between porous nanofibers and the gel electrolyte interface. The supercapacitor demonstrates promising capacitive characteristics, including a gravimetric capacitance of 153 F g-1, a high specific energy density of 65 W h kg-1, and high cycling stability, with a capacitance fade of only 4% after 20 000 charge-discharge cycles at 1 A g-1. Moreover, device-level areal capacitances for the gel IL cell of 122 and 151 mF cm-2 are observed at electrode mass loadings of 3.20 and 5.10 mg cm-2, respectively.

Supercapacitor Electrodes Based on High-Purity Electrospun Polyaniline and Polyaniline-Carbon Nanotube Nanofibers.

Freestanding, binder-free supercapacitor electrodes based on high-purity polyaniline (PANI) nanofibers were fabricated via a single step electrospinning process. The successful electrospinning of nanofibers with an unprecedentedly high composition of PANI (93 wt %) was made possible due to blending ultrahigh molecular weight poly(ethylene oxide) (PEO) with PANI in solution to impart adequate chain entanglements, a critical requirement for electrospinning. To further enhance the conductivity and stability of the electrodes, a small concentration of carbon nanotubes (CNTs) was added to the PANI/PEO solution prior to electrospinning to generate PANI/CNT/PEO nanofibers (12 wt % CNTs). Scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) porosimetry were conducted to characterize the external morphology of the nanofibers. The electrospun nanofibers were further probed by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). The electroactivity of the freestanding PANI and PANI/CNT nanofiber electrodes was examined using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. Competitive specific capacitances of 308 and 385 F g(-1) were achieved for PANI and PANI-CNT based electrodes, respectively, at a current density of 0.5 A g(-1). Moreover, specific capacitance retentions of 70 and 81.4% were observed for PANI and PANI-CNT based electrodes, respectively, after 1000 cycles. The promising electrochemical performance of the fabricated electrodes, we believe, stems from the porous 3-D electrode structure characteristic of the nonwoven interconnected nanostructures. The interconnected nanofiber network facilitates efficient electron conduction while the inter- and intrafiber porosity enable excellent electrolyte penetration within the polymer matrix, allowing fast ion transport to the active sites.

Porous Carbon Mat as an Electrochemical Testing Platform for Investigating the Polysulfide Retention of Various Cathode Configurations in Li-S Cells.

Two optimized cathode configurations (a porous current collector and an interlayer) are utilized to determine the better architecture for improving the cycle stability and reversibility of lithium-sulfur (Li-S) cells. The electrochemical analysis on the upper-plateau discharge capacity (QH) and the lower-plateau discharge capacity (QL) is introduced for assessing, respectively, the polysulfide retention and the electrochemical reactivity of the cell. The analysis results in line with the expected materials chemistry principles suggest that the interlayer configuration offers stable cell performance for sulfur cathodes. The significance of the interlayer is to block the free migration of the dissolved polysulfides, which is a key factor for immobilizing and continuously utilizing the active material in sulfur cathodes. Accordingly, the carbon mat interlayers provide sulfur cathodes with a high discharge capacity of 864 mA h g(-1) at 1 C rate with a high capacity retention rate of 61% after 400 cycles.

Binder-free three-dimensional high energy density electrodes for ionic-liquid supercapacitors.

We demonstrate a facile methodology to fabricate binder-free porous carbon nanofiber electrodes for room temperature ionic-liquid supercapacitors. The device provides an energy density of 80 W h kg(-1) based on the mass of two electrodes while retaining the high rate capability of supercapacitors with near-ideal CV curves at a high scan rate of 200 mV s(-1).

Coarse-grained molecular dynamics simulation on the placement of nanoparticles within symmetric diblock copolymers under shear flow.

We present molecular dynamics simulations coupled with a dissipative particle dynamics thermostat to model and simulate the behavior of symmetric diblock copolymer/nanoparticle systems under simple shear flow. We consider two categories of nanoparticles, one with selective interactions toward one of the blocks of a model diblock copolymer and the other with nonselective interactions with both blocks. For the selective nanoparticles, we consider additional variants by changing the particle diameter and the particle-polymer interaction potential. The aim of our present study is to understand how the nanoparticles disperse in a block copolymer system under shear flow and how the presence of nanoparticles affects the rheology, structure, and flow behavior of block copolymer systems. We keep the volume fraction of nanoparticles low (0.1) to preserve lamellar morphology in the nanocomposite. Our results show that shear can have a pronounced effect on the location of nanoparticles in block copolymers and can therefore be used as another parameter to control nanocomposite self-assembly. In addition, we investigate the effect of nanoparticles on shear-induced lamellar transition from parallel to perpendicular orientation to further elucidate nanocomposite behavior under shear, which is an important tool to induce long-range order in self-assembling materials such as block copolymers.