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Tailoring mesoporous carbons and related materials for energy applications

Sheng Dai ^1,2

¹Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6201, USA

²Department of ����vlog�ٷ����, University of Tennessee, Knoxville, TN 37996-1600, USA

Carbon materials are ubiquitous in catalysis, separation, and energy storage/conversion.  The creation of well-defined carbon architectures is essential for a number of the aforementioned applications.  Recently, we have developed several methods for the synthesis of carbon materials with controlled mesostructures and compositions. The mesostructures of these carbon materials are highly stable and can be further tailored via graphitization and surface functionalization for catalysis and energy-storage applications.  This presentation will be focused on our recent development in (a) self-assembly approaches to the preparation of carbon composite materials for controlling mesostructures and morphologies and (b) surface modification techniques to control the interfacial chemistry of carbon materials for separation, catalysis, and energy-storage applications.

Abstract

This seminar is directed towards undergraduate and graduate students who want to learn about the many career areas available to them after graduation.  Vince will share his career history, P&G research project examples, and the variety of science-related roles that are required to make them happen.  He will also provide information regarding P&G internship opportunities, and the general steps to prepare and apply for them.

About the Speaker 

Vince graduated in 1991 from the ����vlog�ٷ���� ����vlog�ٷ���� Department with a Bachelor of Science degree in ����vlog�ٷ����.  He worked in the environmental science field for one year before joining The Procter & Gamble Co. in 1992.  In his 25 years at Procter & Gamble, he has continually expanded his knowledge and expertise via additional coursework in mass spectrometry, formulation chemistry, chromatography, headspace analysis, modeling and simulation, material science, and lab management.  During this time, Vince has been an integral part of many R&D projects across several businesses and corporate R&D functions, and is a go-to expert, leader, and mentor in his area.  He is currently a Senior Scientist in the Baby Care business responsible for new material innovation and development.

Conductive polymer electrodes have exceptional promise for next generation electronic and electrochemical devices due to inherent mechanical flexibility, printability, biocompatibility, and low cost. Yet conductive polymers continue to suffer from lower conductivity than conventional semiconductors, which ultimately can limit performance.  Electrical conductivity can be increased by increasing the total number of carriers through a charge transfer reaction – oxidation or reduction.  The first half of this talk will focus on the use of spectroscopic methods to evaluate the effects of chemical, electronic, and physical structure changes of organic semiconductors that accompany charge transfer reactions at interfaces, with consequences on device performance. 

The second half of this talk will focus on the unique hybrid electronic-ionic conduction of conductive polymers, which has enabled novel electrochemical devices including bioelectronics.  Two key functionalities of potential-dependent doping at the polymer/electrolyte interface will be addressed: i.) rates of ion migration within the polymer and ii.) rates of charge transfer between a polymer and a redox active molecule.  The potential-dependent microstructure and relative distribution of electronic states (percent doping) are found to be critical in both mechanisms, although happen at different time scales.  For charge transfer, the presence of an inverted regime is observed for the first time, representing a path forward to redox selectivity at polymer electrodes.

Abstract:

Carbon fiber is increasingly visible in materials science and applications.  For example, Boeing’s 787 airframe is constructed from approximately 50 % (w/w) carbon fiber composites, and BMW’s 700 series, i3 and i8 cars make extensive use of carbon fiber.  This practice of lightweighting vehicles is driven by fuel economy, emissions reductions and performance enhancement.  Carbon fiber enables engineering composite materials with the highest tensile performance per mass-density known.  Other applications include advanced space satellite structures tuned for near-zero CTE, giant wind blade spar caps, transmission belts, and a plethora of sporting goods, to name a few.  Further, carbon fiber composites are more resistant to fatigue and corrosion failure than aluminum.  The chemistry and processing to make carbon fibers is a beautiful, rich science but a murky processing bog.  How can a fiber largely comprised of graphitic carbon, pencil fodder or solid lubricant, be so strong? 

The Materials Technologies Group at UK’s Center for Applied Energy Research (CAER), has been active in carbon fiber research and development for the last two decades.  Our work is applied in that we often seek to answer questions orbiting process scale-up, stability, and how processing “knobs” affect product properties.  In this talk, we will delve into (the remarkably few) basic starting chemistries for continuous carbon fiber, and discuss some of the processing, structure and properties observed.  The initial spinning process of the precursor fiber is central and will be highlighted in detail.  Our group operates one of only a few, solution spinlines (apparatus for spinning precursor fiber) outside industry, and the largest in North America, which allows us to explore precursor structure-property relationships.  The overall complexity of making continuous carbon fiber will be contrasted with the relative simplicity of making carbon nanotubes, which can be thought of as miniature discontinuous fibers of carbon.  

Our fiber spinning skillset has opened several new arenas of materials development.  One of which, a natural extension of PAN precursor spinning, combined with carbon nanotubes, has led to interesting findings for hollow fiber polymer membranes for gas separations.  Indeed, new fibers generally offer intriguing glimpses at the emerging area of wearables.  

Short Bio:

Dr. Weisenberger is a central KY local, graduating with a B.S. in ����vlog�ٷ���� from Georgetown College in 1999, and later finishing a PhD in Materials Science and Engineering at the ����vlog�ٷ���� in 2007.  His current research focal areas include fiber processing, carbon nanotube composite materials, thermal management materials, waste heat energy harvesting, and membrane separations.  He serves as the group leader of the Materials Technologies Research Group at UKY’s Center for Applied Energy Research (CAER), and as PI on a multitude of externally funded research projects.  His publications have over 1000 peer citations in the refereed literature, and include a diverse group of co-authors.  Dr. Weisenberger has two issued US Patents, and serves on the Executive Committee of the American Carbon Society. He is also adjunct assistant professor in UKY’s Department of Chemical and Materials Engineering. He and his wife have two daughters and live in Lawrenceburg, KY.

Abstract:

 The number of nanoparticle-containing products has increased rapidly over the last decade and nanoparticles can be released into the environment either intentionally or unintentionally during or after production and use. Understanding of their behavior and effects is necessary for estimating risks to the environment and human health. Our nano team studies toxicity, bioavailability as well as transcriptomic, genomic and epigenetic effects of metal and metal oxide manufactured nanoparticles (MNP) in their pristine (as synthesized) and environmentally modified (aged) forms.  For example, Ag-MNPs after entering wastewater streams are rapidly transformed to Ag₂S.  Here we present results on bioavailability and toxicogenomic responses of a model organism, a nematode Caenorhabditis elegans to pristine Ag-MNPs, sulfidized (sAg-MNPs), and AgNO₃. The toxicity and transcriptomic effects of pristine Ag-MNPs involve uptake and bioaccumulation of Ag and is explained by both dissolution and release of Ag ions as well as by particle-specific effects. In contrast, the toxicity of the transformed sAg-MNPs is largely independent of free ion release and Ag bioaccumulation. Cuticle damage is likely to be one of the primary toxicity mechanisms for the sAg-MNPs.

We have been also investigating mutigenerational effects of MNPs and our recent multigenerational study has shown enhanced C. elegans sensitivity for reproductive toxicity as early as second generation for AgNO₃ and Ag-MNPs, but not sAg-MNPs. This suggested that Ag-MNPs may cause mutations or epi-mutations. Using a next generation sequencing approach and changes in levels of DNA (adenine) methylation we are evaluating germ-line mutations and epigenetic modifications that might be induced and passed to subsequent generations after continuous exposure of C. elegans to Ag-MNPs. 

Research into starch metabolism and the fatal neurodegenerative epilepsy called Lafora disease are surprisingly linked by a family of enzymes that we recently discovered called glucan phosphatases. Plants release the energy in transitory starch via a recently identified three-step process: starch phosphorylation, degradation, and dephosphorylation. Dikinases phosphorylate the outer starch glucose units to make them water-soluble and enzyme accessible so that amylases can release maltose and glucose. Following amylase activity, the phosphate must be removed by glucan phosphatases. In the absence of glucan phosphates, plants cannot access the energy stored in starch and the starch granules grow in size while plant growth is stunted. 

We determined the structure of the Arabidopsis glucan phosphatases Starch Excess 4 (SEX4) and Like Sex Four2 (LSF2) with phospho-glucan product bound at 1.62Å and 2.3Å, respectively. The glucose moiety closest to the SEX4 active site is positioned with the oxygen of the C6 carbon directed into the catalytic cleft and we find that SEX4 preferentially dephosphorylates C6 hydroxyls. Alternatively, the glucose moiety closest to the LSF2 active site is positioned with the C3 carbon towards the catalytic cleft and we find that LSF2 exclusively dephosphorylates C3 hydroxyls. Using structural and biochemical insights, we completely reversed SEX4 specificity, providing a method for engineering glucan phosphatase activity.

The human EPM2A gene encodes laforin and recessive mutations in EPM2A result in Lafora disease (LD). In the absence of laforin activity, glycogen transforms into a hyper-phosphorylated, water-insoluble, starch-like Lafora body (LB) that drives neuronal apoptosis, neurodegeneration, and eventual death of LD patients. The physiological function of laforin is to dephosphorylate glycogen, yet the mechanism of glycogen dephosphorylation by laforin was unknown. Additionally, LD missense mutations are dispersed throughout laforin, bringing to question the structural mechanism(s) of disease. We determined the crystal structure of human laforin at 2.4 Å bound to oligosaccharides with a phospho-glucan product at the active site. The structure reveals an integrated tertiary structure of the carbohydrate binding module and dual specificity phosphatase domains as well as an antiparallel dimer mediated by the phosphatase domain that results in a tetramodular architecture, positioning the two active sites ~31 Å from each other. We utilized the crystal structure and three solution-based, biophysical techniques along with biochemical analyses of LD patient mutations and structured guided mutations to probe this unique tertiary and quaternary structure. We define a cooperative mechanism of action for laforin as well as establish the effect of LD disease mutations, thereby providing atomic level insights that connect basic glycogen metabolism to human neurodegenerative disease.

These structures are the first of glucan phosphatases, and they provide new insights into the molecular basis of this medically-, agriculturally-, and industry-relevant enzyme family as well as their unique mechanisms of catalysis, substrate specificity, and interaction with glucans.

Metallo-β-lactamases (MBLs) are Zn(II)-containing enzymes produced by bacteria that inactivate all b-lactam containing antibiotics, including the carbapenems, which are antibiotics of last resort. While the MBLs have been studied for over 50 years, there are no clinical inhibitors for these enzymes. Therefore, there are few antibiotics available to treat bacterial infections caused by bacteria that produce a MBL. To address this problem, a multi-institution team of synthetic organic chemists (UC San Diego), medicinal chemists (UT Austin), microbiologists and MD’s (Case Western and the Cleveland VA), and structural biologists/biochemists (Miami University) teamed together to identify and develop new potential leads. In the seminar, recent results on a new inhibitor scaffold will be presented, along with biophysical studies on several other recently reported compounds in the literature.

Abstract:

����vlog�ٷ���� has always been an experimental science, but its guiding principles have come from the realm of physics since the discovery of the nucleus, electrons, and the quantum mechanics that rules them.  The field of quantum chemistry has matured to the point where computations can assist in understanding results form the laboratory, or even in suggesting future experimental work.  To obtain results of sufficient accuracy, the theoretical chemist must use a large basis set to expand the orbitals and a sufficiently sophisticated many-electron wavefunction.  However, when a heavy element is involved, a third complication arises, namely relativity.  This seminar will present a computational complement to results obtained in the Yang Laboratory at the ����vlog�ٷ���� for a Cerium containing molecule.  The talk is intended to teach students the connection between the Schrodinger equation and the relativistic Dirac equation.  The correct physics of the latter can be approximated in a normal quantum chemistry program solving the former equation by modification of one electron integrals.  The presentation of relativistic quantum mechanics will be qualitative - no prior understanding of Einstein is necessary!

Molecular crystals have applications in nonlinear optics, organic electronics, and particularly in pharmaceuticals, as most drugs are marketed in the form of crystals of the pharmaceutically active ingredient. Molecular crystals are bound by dispersion (van der Waals) interactions, whose weak nature generates potential energy landscapes with many local minima that may be extremely close in energy. This often results in polymorphism, the crystallization of the same molecule in several different structures. Crystal structure may profoundly influence the physical and chemical properties, including the electronic and optical properties relevant for device applications.

We perform large scale quantum mechanical simulations to predict the structure of molecular crystals and investigate the effect of crystal packing on their electronic and optical properties. The massively parallel genetic algorithm (GA) package, GAtor, relies on the evolutionary principle of survival of the fittest to find low-energy crystal structures of a given molecule. Dispersion-inclusive density functional theory (DFT) is used for structural relaxation and accurate energy evaluations. Evolutionary niching is performed by using machine learning to perform clustering on the fly. The structure generation package, Genarris, performs fast screening of randomly generated structures with a Harris approximation, whereby the molecular crystal density is constructed by replicating the single molecule density, which is calculated only once. Many-body perturbation theory, within the GW approximation and the Bethe-Salpeter equation (BSE), is then employed to describe properties derived from charged and neutral excitations.

An emerging application of molecular crystals is singlet fission (SF), the down-conversion of one photogenerated singlet exciton into two triplet excitons. SF has the potential to significantly increase the efficiency of organic photovoltaics beyond the Shockley-Queisser limit by harvesting two charge carriers from one photon. However, the realization of SF-based solar cells is hindered by the dearth of suitable materials. We aim to discover new SF materials and optimize the crystal packing of known materials to enhance SF efficiency. We predict that crystalline quaterrylene and a lesser known monoclinic crystal structure of rubrene may exhibit high singlet fission efficiency, possibly rivaling that of the quintessential SF material, pentacene. Quaterrylene has the additional advantages of high stability, a narrow band gap, and a triplet energy in the optimal range to maximize photoconversion efficiency.