Department of Chemical Engineering

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Author: search.filters.author.Ghosh, SarbaniSubject: search.filters.subject.Chemical Engineering

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Now showing 1 - 10 of 22
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    Adsorption of hydrogen on single-walled carbon nanotubes with defects
    (Elsevier, 2015-10) Ghosh, Sarbani
    We present molecular dynamics (MD) simulations and density functional theory (DFT) calculations of hydrogen adsorption on single-walled carbon nanotubes (SWCNT) with various kinds of defects. The nature of defects, which is characterized here by the number of carbon atoms present in a ring on the surface of nanotube, plays a significant role in determining the hydrogen adsorption capacity of the SWCNT. Nanotubes containing the Stone–Wales defect with 5 and 8-member rings were found to have the largest hydrogen adsorption ability that increases further with the number of rings with such defects. Whereas, the presence of defects with 5, 3-5-8-member rings and the Stone–Wales defect with 5 and 7-member rings decreases the adsorption ability of the defective SWCNT significantly with respect to defect-free nanotubes. Our results indicate that the huge discrepancies in hydrogen storage capacities of SWCNT reported in the literature could be attributed to the nature of defects present in nanotubes. DFT calculations also reveal that the adsorption energy depends not only on the nature and number of defects present on the surface of nanotube but also on the equilibrium structure of rings.
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    Hydrogen storage capacity of bundles of single-walled carbon nanotubes with defects
    (Wiley, 2016) Ghosh, Sarbani
    We study the hydrogen storage capacity of bundles of single-walled carbon nanotubes (SWCNT) at 80 and 298 K using molecular dynamics simulations. The effect of packing on the storage capacity of the bundles is studied using triangular and square arrays of nanotubes with various separation distance between adjacent nanotubes. The gravimetric storage capacity of the bundles increases with the separation distance between individual nanotubes. At low temperature, the storage capacity of bundles is significantly lower than for isolated SWCNT as the intertube distance is smaller than the adsorbed layer thickness of hydrogen. At high temperature, the adsorbed layer thickness corresponds to only a monolayer of hydrogen around SWCNTs, and hence, hydrogen is captured in the interstitial spaces within the bundle. As the groove volume in the square array is higher than that in the triangular array, the storage capacity of the bundle with square array is higher. An introduction of the Stone–Wales defects on the surface of nanotubes further increases the storage capacity of the bundle due to the higher binding energy of the 8-member rings of the defective SWCNTs. We also observe that more hydrogen molecules are packed in the interstitial spaces due to the deformation of the nanotubes caused by the presence of defective sites
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    Hydrogen storage in Titanium-doped single-walled carbon nanotubes with Stone-Wales defects
    (Elsevier, 2017) Ghosh, Sarbani
    The hydrogen storage capacity of Titanium-doped single-walled carbon nanotubes (SWCNT) containing the Stone-Wales 5,8 defects was studied using molecular dynamics simulations. The equilibrium doping sites and their stability were estimated using density functional theory. Although introduction of structural defects and dopant atom decreases the formation and cohesive energies, the drop in these energies is not large enough to hinder the thermodynamic feasibility of formation of these structures. Moreover, we observed that the stability of SWCNTs, where Ti is doped by replacing two carbon atoms is similar to that of the defect-free nanotube. This particular novel configuration (D5) was also obtained by rearranging the bonds in the 5 and 8-member rings of the Stone-Wales defect. Doping Ti on the defective rings has a more significant effect on the adsorption of hydrogen than doping on the regular 6-member rings. The D5 SWCNT showed the highest gravimetric and volumetric storage capacities at a temperature of 298K and a moderate pressure of 140atm. We also compared the performance of the D5 SWCNT with a recently reported Ti-doped porphyrin SWCNT and observed that the storage capacity of the D5 SWCNT was significantly higher at similar conditions. Our results suggest that Ti-doped SWCNTs with the Stone-Wales (5,8) defects show a promising potential to meet the ultimate goal set by the US Department of Energy for hydrogen storage.
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    Beryllium-doped single-walled carbon nanotubes with Stone-Wales defects: A promising material to store hydrogen at room temperature
    (Elsevier, 2017-09) Ghosh, Sarbani
    Hydrogen storage in single-walled carbon nanotubes containing the Stone-Wales defects and doped with metal atoms (titanium and beryllium) has been studied using molecular dynamics simulations and density functional theory calculations. Although, Be is known to be toxic at high temperatures, Be-doped SWCNT shows a promising potential to exceed the DOE target at moderate temperatures and pressures. One of the major advantages of doping Be is its lower atomic weight, which increases the gravimetric storage capacity compared to SWCNTs doped with heavy-wight Ti atoms. In addition, the binding energy of Be is higher than that of Ti, which enhances the capture of hydrogen molecules. The gravimetric and volumetric storage capacities depend not only on the dopant atom but also on the location of doping. SWCNTs in which Be is doped on the octagonal ring of the Stone-Wales defects exhibits higher storage capacity than Be doped on defect-free SWCNTs. At room temperature (298 K), the storage capacity of Be-doped SWCNT containing the Stone-Wales defect exceeds the DOE target of 5.5 wt% (gravimetric) and 40 g H2/L (volumetric) at a pressure of 267 atm, which is significantly lower than that used in high pressure vessels.
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    Hydrogen adsorption in pyridine bridged porphyrin-covalent organic framework
    (Elsevier, 2019) Ghosh, Sarbani
    Covalent organic frameworks (COFs), a class of carbon-based polymeric materials have the potential to be used as hydrogen adsorbent. Three dimensional (3D) COFs, due to their low density and high surface area, although have higher hydrogen adsorption, they have less stability than two dimensional (2D) COFs. Here we studied porphyrin group containing 2D COF, namely H2,P-COF for hydrogen storage using density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations and the results were compared with the most common 2D COFs, COF-1 and COF-5. Cylindrical shaped 2D COFs where isolated unit blocks are stacked in multiple layers due to van der Waals interactions between individual layers, increase the effective surface area for hydrogen storage. A further modification has been done by bridging the inter-layer gap by pyridine molecules. Insertion of pyridine increases the separation distance of layers of 2D COFs as well as the free volume. Feasibility of the structure formation and stability of all the structures were checked using DFT study. To ensure the structural stability of bridged COFs after hydrogen loading, alternating layers of COF were bridged. Single, bi, tri and tetra -pyridine molecules were chemically bonded with the existing carbon ring present in between two C2O2B rings to form pyridine bridged H2,P-COFs. Our GCMC results show a significant increase in storage capacity which is mainly due to an increase in the free volume of the material. The highest capacity of 5.1 wt% and 20 g H2/L at 298 K and 100 bar, above the gravimetric DOE goal, has been found at room temperature for tetra-pyridine doped porphyrin COF structure.
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    Reversible Electronic Solid–Gel Switching of a Conjugated Polymer
    (Wiley, 2019-10) Ghosh, Sarbani
    Conjugated polymers exhibit electrically driven volume changes when included in electrochemical devices via the exchange of ions and solvent. So far, this volumetric change is limited to 40% and 100% for reversible and irreversible systems, respectively, thus restricting potential applications of this technology. A conjugated polymer that reversibly expands by about 300% upon addressing, relative to its previous contracted state, while the first irreversible actuation can achieve values ranging from 1000–10 000%, depending on the voltage applied is reported. From experimental and theoretical studies, it is found that this large and reversible volumetric switching is due to reorganization of the polymer during swelling as it transforms between a solid-state phase and a gel, while maintaining percolation for conductivity. The polymer is utilized as an electroactive cladding to reduce the void sizes of a porous carbon filter electrode by 85%.
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    Electronic Structures and Optical Absorption of N-Type Conducting Polymers at Different Doping Levels
    (ACS, 2019-06) Ghosh, Sarbani
    Theoretical understanding of the electronic structure and optical transitions in n-doped conducting polymers is still controversial for polaronic and bipolaronic states and is completely missing for the case of a high doping level. In the present paper, the electronic structure and optical properties of the archetypical n-doped conducting polymer, double-stranded benzimidazo-benzophenanthroline ladder (BBL), are studied using the density functional theory (DFT) and the time-dependent DFT method. We find that a polaronic state in the BBL chain is a spin-resolved doublet where the spin degeneracy is lifted. The ground state of two electrons corresponds to a triplet polaron pair, which is in stark contrast to a commonly accepted picture where two electrons are postulated to form a spinless bipolaron. The total spin gradually increases until the reduction level reaches cred = 100% (i.e., one electron per monomer unit). With further increase of the reduction level, the total spin decreases until it becomes 0 for the reduction level cred = 200%. The calculated results reproduce the experimentally observed spin signal without any phenomenological parameters. A detailed analysis of the evolution of the electronic structure of BBL and its absorption spectra with increase in reduction level is presented. The calculated UV–vis–NIR spectra are compared with the available experimental results. The electronic structure and optical absorption for different reduction levels presented here are generic to a wide class of conducting polymers, which is illustrated by the corresponding calculations for another archetypical conducting polymer, poly(3,4-ethylenedioxythiophene) (best known as PEDOT).
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    Electronic Structures and Optical Properties of p-Type/n-Type Polymer Blends: Density Functional Theory Study
    (ACS, 2020-04) Ghosh, Sarbani
    A blend made of p-type and n-type polymers can act as bipolar/ambipolar material composites that transport both electrons and holes. Although several experimental efforts are currently devoted to p-/n-type blends of conducting polymers, theoretical studies of these systems are missing to a large extent. In the current paper, using the density functional theory (DFT) and the time-dependent DFT, we calculate electronic and optical properties of a p-type/n-type polymeric blend, where we have chosen the poly(3,4-ethylenedioxythiophene)/benzimidazo-benzophenanthroline ladder (PEDOT/BBL) as a model composite system. We demonstrate that in the blend, PEDOT acts as an electron donor and BBL acts as an electron acceptor under doped conditions. However, no charge transfer between the chains takes place for an undoped composite system. Due to a significant difference in the electron affinities and the ionization energies of PEDOT and BBL, the electronic properties of a negatively (positively) doped PEDOT/BBL blend are primarily governed by the chains where negative (positive) charges are localized, i.e., the BBL chains (the PEDOT chains). However, this is no longer valid for the optical absorption where the electronic transition occurs between the two chains and, therefore, the calculated UV–vis–near-infrared (NIR) absorption spectra of the negatively (positively) doped PEDOT/BBL blend are rather different compared to the corresponding spectra of the single BBL chains (PEDOT chains). The electronic coupling between the photoexcited state and the final charge-transfer state of the blend was calculated to be ∼0.08 eV. The results presented here are generic to a wide class of p-type/n-type combinations, which was further confirmed by calculations performed on the polythiophene (PT)/BBL blend
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    Side Chain Redistribution as a Strategy to Boost Organic Electrochemical Transistor Performance and Stability
    (Wiley, 2020-08) Ghosh, Sarbani
    A series of glycolated polythiophenes for use in organic electrochemical transistors (OECTs) is designed and synthesized, differing in the distribution of their ethylene glycol chains that are tethered to the conjugated backbone. While side chain redistribution does not have a significant impact on the optoelectronic properties of the polymers, this molecular engineering strategy strongly impacts the water uptake achieved in the polymers. By careful optimization of the water uptake in the polymer films, OECTs with unprecedented steady-state performances in terms of [μC*] and current retentions up to 98% over 700 electrochemical switching cycles are developed.
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    Effect of Substrate on Structural Phase Transition in a Conducting Polymer during Ion Injection and Water Intake: A View from a Computational Microscope
    (ACS, 2020-12) Ghosh, Sarbani
    Conducting polymers operating in aqueous electrolyte represent mixed electron-ion conductors, where the ion injection and water intake can lead to structural and morphological changes that can strongly affect the material morphology and device performance. In the present paper, using molecular dynamics simulations, we provide an atomistic understanding of the structural phase transitions during electrochemical oxidation and ion injection in a conjugated polymer with glycolated side chains recently reported by Bischak et al. [J. Am. Chem. Soc., 2020, 142, 7434], where the polymer switched between two structurally distinct phases corresponding to different oxidation levels. To outline the structural changes, we calculated the polymer film morphology and X-ray diffraction patterns at different oxidation levels. We demonstrated that the observed phase transition arises due to interplay between several factors, including the effect of the substrate leading to the preferential edge-on arrangement of the chains and formation of lamellas; unzipping of the interdigitated polymer chains during oxidation and ion intake; and changes in the morphology when π–π stacking is absent at low oxidation level and forms at the high oxidation level facilitating the electron mobility and enabling the oxidation of the polymer film. Our calculations quantitatively reproduce the experimental data, which outlines the predictive power of the molecular modeling of the polymer systems that can be utilized for the design of materials and devices with improved performance.