Vibha Kalra

A free-standing carbon nanofiber interlayer for high-performance lithium–sulfur batteries

Free-standing porous carbon nanofibers with tunable surface area and pore structure have been investigated as an interlayer between the sulfur cathode and the separator to inhibit the shuttling of the intermediate polysulfides in lithium–sulfur (Li–S) batteries. Specifically, the effects of thickness, surface area, and pore size distribution of carbon nanofiber (CNF) interlayers on the performance of Li–S batteries have been studied. The carbon nanofiber interlayer not only reduces the electrochemical resistance but also localizes the migrating polysulfides and traps them, thereby improving the discharge capacity as well as cyclability. It was found that the optimum thickness of the interlayer is a critical factor to achieve good cell performance, in addition to the surface area and pore structure. A high initial discharge capacity of 1549 mA h g−1 at C/5 rate, which is 92% of the theoretical capacity of sulfur, with 98% average coulombic efficiency and 83% capacity retention after 100 cycles was obtained with a meso–microporous carbon nanofiber interlayer.

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.

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.

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.

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.

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.

Co-continuous nanoscale assembly of Nafion–polyacrylonitrile blends within nanofibers: a facile route to fabrication of porous nanofibers

A facile route to fabrication of porous high surface area nanofibers is demonstrated via electrospinning of a polymer blend followed by selective removal of one of the components. First, electrospinnability of Nafion–polyacrylonitrile (PAN) solutions in N,N-dimethylformamide is studied over a wide range of blend compositions. Pure Nafion solutions are not electrospinnable by themselves owing to the presence of aggregates in solution due to ionic interactions, thereby resulting in low solution viscosities. Addition of only 5 wt% PAN in Nafion allows us to successfully electrospin nanofibers, possibly due to the enhancement of entanglements within the solution. An extensional rheology study is conducted to quantify and understand non-electrospinnability. In the second step, the nanoscale internal morphology of Nafion–PAN nanofibers is studied using transmission electron microscopy of microtomed nanofiber sections. The electrospinning process allows us to develop the co-continuous morphology of PAN and Nafion within the nanofibers. To further corroborate our findings and develop high surface area nanofibers, we selectively remove Nafion or PAN to form pure mesoporous carbon (after calcination) and mesoporous Nafion nanofibers respectively. Interestingly, even with an initial composition of 80 : 20 (wt/wt) Nafion : PAN, the nanofibers retain their overall shape after selective Nafion removal, indicating the presence of a continuous PAN phase. The resultant porous carbon nanofibers exhibit a specific surface area of >1500 m2 g−1. While mesoporous carbon nanofibers can have a wide range of applications such as in supercapacitors, high surface area porous Nafion nanofibers will facilitate the development of super-sensitive gas sensors.

Using common salt to impart pseudocapacitive functionalities to carbon nanofibers

A novel and simple method of incorporating pseudocapacitive surface functionalities on free-standing carbon nanofibers using common salt (sodium chloride) is presented. The blend of sodium chloride (NaCl) and polyacrylonitrile is electrospun together, followed by pyrolysis and mild acid treatment to obtain functionalized free-standing (binder-free) carbon nanofibers. The synthesized materials have a low surface area of only 24 m2 g−1, however the electrochemical studies show a five-fold increase in specific capacitance on incorporation of NaCl compared to that without NaCl. The XPS characterization demonstrates that the presence of NaCl leads to enhanced oxygen on the surface of carbon nanofibers, particularly in the form of carboxyl groups. These carboxyl groups then facilitate the adsorption of sulfur functional groups on acid treatment. A high specific capacitance of 204 F g−1, areal capacitance of 1.15 F cm−2, and volumetric capacitance of 63 F cm−3 in 1 M H2SO4 are obtained, which are attributed to the surface functional groups participating in the pseudocapacitive redox reactions. The fabricated nanofibers demonstrate good capacitance retention at high current densities and high cyclability.

Cylindrically confined assembly of asymmetrical block copolymers with and without nanoparticles

Our recent experimental study on electrospinning of block copolymer (BCP)–nanoparticle (NP) nanocomposites has revealed the formation of unique self-assembling structures in submicron scale fibers. In this paper, we use coarse-grained molecular dynamics (MD) simulations to investigate the effect of cylindrical confinement on self-assembly of model asymmetrical BCPs with and without NPs with the aim to understand and control our experimentally found structures. First, the effects of the ratio of the cylindrical confinement diameter to the BCP domain spacing, D/L0, the total polymer chain length, and the polymer–wall interactions on the confined assembly were thoroughly investigated. We examined the core assembled structures along the cylinder axis and constructed a phase diagram for asymmetrical BCP. The structures are categorized by three features: the number of layers of domains, radially interconnected domains, and the number of axially perforated domains. Secondly, NPs with selective attraction towards the (i) minor domain (A) and (ii) major domain (B) were incorporated into asymmetric BCPs. We found that swelling of either domain caused by the inclusion of selective NPs yields different morphologies when compared with a pure BCP with the same effective volume ratio. Interestingly, the effect of confinement on nanoparticle placement was prominently seen if nanoparticles were selectively placed into the minor domain that preferentially wets the confining wall. Finally, the predicted BCP–NP structures are validated by those observed in electrospun BCP–NP nanofibers. The current study demonstrates that coarse-grained MD simulation can offer a useful tool to elucidate, predict and tailor self-assembled structures in electrospun BCP–NP nanofibers.