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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Probing the mechanism of CO 2 capture in diamine-appended metal–organic frameworks using measured and simulated X-ray spectroscopy

Diamine-appended metal–organic frameworks display great promise for carbon capture applications, due to unusual step-shaped adsorption behavior that was recently attributed to a cooperative mechanism in which the adsorbed CO 2 molecules insert into the metal–nitrogen bonds to form ordered ammonium carbamate chains. We present a detailed study of this mechanism by in situ X-ray absorption spectroscopy and density functional theory calculations. Distinct spectral changes at the N and O K-edges are apparent upon CO 2 adsorption in both mmen-Mg 2 (dobpdc) and mmen-Mn 2 (dobpdc), and these are evaluated based upon computed spectra from three potential adsorption structures. The computations reveal that the observed spectral changes arise from specific electronic states that are signatures of a quasi-trigonal planar carbamate species that is hydrogen bonded to an ammonium cation. This eliminates two of the three structures studied, and confirms the insertion mechanism. Here, we note the particular sensitivity of X-ray absorption spectra to the insertion step of this mechanism, underpinning the strength of the technique for examining subtle chemical changes upon gas adsorption.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Understanding Small-Molecule Interactions in Metal-Organic Frameworks: Coupling Experiment with Theory

Metal-organic frameworks (MOFs) have gained much attention as next generation porous media for various applications, especially gas separations/storage and catalysis. New MOFs are regularly reported; however, to develop better materials in a timely manner for specific applications, the interactions between guest molecules and the internal surface of the framework must first be understood. In this review, we present a combined experimental and theoretical approach that proves essential for the elucidation of small-molecule interactions in a model MOF system known as M 2 (dobdc) (dobdc 4– = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, or Zn), a material whose adsorption properties can be readily tuned via chemical substitution. Here, we additionally show that the study of extensive families like this one can provide a platform to test the efficacy and accuracy of developing computational methodologies in slightly varying chemical environments, a task that is necessary for their evolution into viable, robust tools for screening large numbers of materials.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Direct Observation of Xe and Kr Adsorption in a Xe-Selective Microporous Metal–Organic Framework

The cryogenic separation of noble gases is energy-intensive and expensive, especially when low concentrations are involved. Metal organic frameworks containing polarizing groups within their pore spaces are predicted to be efficient Xe/Kr solid state adsorbents but no experimental insights on the nature of the Xe-network interaction is available to date. Here we report a new microporous metal organic framework (designated SBU2) that is selective toward Xe over Kr at ambient conditions, with Xe/Kr selectivity of about 10 and Xe capacity of 37.1 wt% at 298 K. Results of single crystal diffraction show Xe selectivity may be attributed to the specific geometry of the pores, forming cages built with phenyl rings and enriched with polar -OH groups, both serving as strong adsorption sites for polarizable Xe gas. The Xe/Kr separation in SBU2 was investigated with experimental and computational breakthrough methods. These experiments showed that Xe broke through the column after 3 min, followed by Kr at 4.5 min after injection of gases mixture, which confirmed that SBU2 has a real practical potential for separating Xe from Kr. Computational calculations show that SBU2 capacity and adsorption selectivity is comparable to the best-performing unmodified MOFs such as Ni-MOF-74 or Co Formate.

Xenon, Krypton↗
DOE OSTI2012

Reticulated Organic Photovoltaics

This paper shows how the self-assembled interlocking of two nanostructured materials can lead to increased photovoltaic performance. A detailed picture of the reticulated 6-DBTTC/C{sub 60} organic photovoltaic (OPV) heterojunction, which produces devices approaching the theoretical maximum for these materials, is presented from near edge X-ray absorption spectroscopy (NEXAFS), X-ray photoelectron spectroscopy (XPS), Grazing Incidence X-ray diffraction (GIXD) and transmission electron microscopy (TEM). The complementary suite of techniques shows how self-assembly can be exploited to engineer the interface and morphology between the cables of donor (6-DBTTC) material and a polycrystalline acceptor (C{sub 60}) to create an interpenetrating network of pure phases expected to be optimal for OPV device design. Moreover, we find that there is also a structural and electronic interaction between the two materials at the molecular interface. The data show how molecular self-assembly can facilitate 3-D nanostructured photovoltaic cells that are made with the simplicity and control of bilayer device fabrication. The significant improvement in photovoltaic performance of the reticulated heterojunction over the flat analog highlights the potential of these strategies to improve the efficiency of organic solar cells.

14 SOLAR ENERGY↗
DOE OSTI2015

Biomimicry in metal-organic materials

Nature has evolved a great number of biological molecules which serve as excellent constructional or functional units for metal-organic materials (MOMs). Even though the study of biomimetic MOMs is still at its embryonic stage, considerable progress has been made in the past few years. In this critical review, we will highlight the recent advances in the design, development and application of biomimetic MOMs, and illustrate how the incorporation of biological components into MOMs could further enrich their structural and functional diversity. More importantly, this review will provide a systematic overview of different methods for rational design of MOMs with biomimetic features. Published by Elsevier B.V.

Zhang, MW↗
DOE OSTI2014

Singlet fission efficiency in tetracene-based organic solar cells

Singlet exciton fission splits one singlet exciton into two triplet excitons. Using a joint analysis of photocurrent and fluorescence modulation under a magnetic field, we determine that the triplet yield within optimized tetracene organic photovoltaic devices is 153% ± 5% for a tetracene film thickness of 20 nm. The corresponding internal quantum efficiency is 127% ± 18%. These results are used to prove the effectiveness of a simplified triplet yield measurement that relies only on the magnetic field modulation of fluorescence. Despite its relatively slow rate of singlet fission, the measured triplet yields confirm that tetracene is presently the best candidate for use with silicon solar cells.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Adding to the Arsenal of Zirconium‐Based Metal–Organic Frameworks: the Topology as a Platform for Solvent‐Assisted Metal Incorporation

Gaining control over the assembly of mesoporous Zr‐based metal–organic frameworks (MOFs) with modifiable metal nodes remains a challenge. The the topology is intriguing, particularly if considering postsynthetic modification, as it presents large sodalite‐like cages surrounded by 12 interconnected 1D channels that allow for efficient diffusion of guest molecules throughout the structure. Herein, we demonstrate the first Zr MOF with the topology, NU‐1200, which was synthesized by controlling the geometry of the organic building unit. In addition, by using NU‐1200 as a platform, we perform solvent‐assisted metal incorporation to form single‐site metalated nodes, a process that is characterized by single‐crystal X‐ray diffraction.

Liu, Tian‐Fu↗
DOE OSTI2018

Xenon Gas Separation and Storage using Metal-Organic Frameworks

The global demand for Xe, a noble gas with applications in electronics, lighting, and the medical industry, is expected to rise significantly over the coming decades. However, the low abundance of Xe in the earth’s atmosphere and the costly cryogenic distillation process that is used to obtain Xe commercially via air separation have limited the scale of applications of Xe. A physisorption-based separation using porous materials may be a viable and cost-effective alternative to cryogenic distillation. In particular, metal-organic frameworks (MOFs) have shown promise as highly Xe-selective, porous solids. In this review, we detail the recent advances of MOFs as adsorbents for noble gas adsorption/separation and the role of computer simulation in finding optimal materials for Xe adsorption.

MOFs, Xenon, Krypton↗
DOE OSTI2014

Water Adsorption in Porous Metal-Organic Frameworks and Related Materials

Water adsorption in porous materials is important for many applications such as dehumidification, thermal batteries, and delivery of drinking water in remote areas. In this study, we have identified three criteria for achieving high performing porous materials for water adsorption. These criteria deal with condensation pressure of water in the pores, uptake capacity, and recyclability and water stability of the material. In search of an excellently performing porous material, we have studied and compared the water adsorption properties of 23 materials, 20 of which are metal organic frameworks (MOFs). Among the MOFs are 10 zirconium(IV) MOFs with a subset of these, MOF-801-SC (single crystal form), -802, -805, -806, -808, -812, and -841 reported for the first time. MOF-801-P (microcrystalline powder form) was reported earlier and studied here for its water adsorption properties. MOF-812 was only made and structurally characterized but not examined for water adsorption because it is a byproduct of MOF-841 synthesis. All the new zirconium MOFs are made from the Zr6O4(OH)(4)(-CO2)(n) secondary building units (n = 6, 8, 10, or 12) and variously shaped carboxyl organic linkers to make extended porous frameworks. The permanent porosity of all 23 materials was confirmed and their water adsorption measured to reveal that MOF-801-P and MOF-841 are the highest performers based on the three criteria stated above; they are water stable, do not lose capacity after five adsorption/desorption cycles, and are easily regenerated at room temperature. An X-ray single-crystal study and a powder neutron diffraction study reveal the position of the water adsorption sites in MOF-801 and highlight the importance of the intermolecular interaction between adsorbed water molecules within the pores.

Furukawa, H↗
DOE OSTI2014

Integrated optical and electrical modeling of plasmon-enhanced thin film photovoltaics: A case-study on organic devices

The nanoscale light control for absorption enhancement of organic photovoltaic (OPV) devices inevitably produces strongly non-uniform optical fields. These non-uniformities due to the localized optical modes are a primary route toward absorption enhancement in OPV devices. Therefore, a rigorous modeling tool taking into account the spatial distribution of optical field and carrier generation is necessary. Presented here is a comprehensive numerical model to describe the coupled optical and electrical behavior of plasmon-enhanced polymer:fullerene bulk heterojunction (BHJ) solar cells. In this model, a position-dependent electron-hole pair generation rate that could become highly non-uniform due to photonic nanostructures is directly calculated from the optical simulations. By considering the absorption and plasmonic properties of nanophotonic gratings included in two different popular device architectures, and applying the Poisson, current continuity, and drift/diffusion equations, the model predicts quantum efficiency, short-circuit current density, and desired carrier mobility ratios for bulk heterojunction devices incorporating nanostructures for light management. In particular, the model predicts a significant degradation of device performance when the carrier species with lower mobility are generated far from the collecting electrode. Consequently, an inverted device architecture is preferred for materials with low hole mobility. This is especially true for devices that include plasmonic nanostructures. Additionally, due to the incorporation of a plasmonic nanostructure, we use simulations to theoretically predict absorption band broadening of a BHJ into energies below the band gap, resulting in a 4.8% increase in generated photocurrent.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗