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News Article | May 4, 2017
Site: www.eurekalert.org

IMAGE:  In computer simulations at Pitt, graphane provides a water-free "bucket brigade " to rapidly conduct protons across the membrane and electrons across the circuit. view more PITTSBURGH (May 4, 2017) ... Hydrogen powered fuel cell cars, developed by almost every major car manufacturer, are ideal zero-emissions vehicles because they produce only water as exhaust. However, their reliability is limited because the fuel cell relies upon a membrane that only functions in when enough water is present, limiting the vehicle's operating conditions. Researchers at the University of Pittsburgh's Swanson School of Engineering have found that the unusual properties of graphane - a two-dimensional polymer of carbon and hydrogen - could form a type of anhydrous "bucket brigade" that transports protons without the need for water, potentially leading to the development of more efficient hydrogen fuel cells for vehicles and other energy systems. The principal investigator is Karl Johnson, the William Kepler Whiteford Professor in the Swanson School's Department of Chemical & Petroleum Engineering, and graduate research assistant Abhishek Bagusetty is the lead author. Their work, "Facile Anhydrous Proton Transport on Hydroxyl Functionalized Graphane" (DOI: 10.1103/PhysRevLett.118.186101), was published this week in Physical Review Letters. Computational modeling techniques coupled with the high performance computational infrastructure at the University's Center for Research Computing enabled them to design this potentially groundbreaking material. Hydrogen fuels cells are like a battery that can be recharged with hydrogen and oxygen. The hydrogen enters one side of the fuel cell, where it is broken down into protons (hydrogen ions) and electrons, while oxygen enters the other side and is ultimately chemically combined with the protons and electrons to produce water, releasing a great deal of energy. At the heart of the fuel cell is a proton exchange membrane (PEM). These membranes mostly rely on water to aid in the conduction of protons across the membranes. Everything works well unless the temperature gets too high or the humidity drops, which depletes the membrane of water and stops the protons from migrating across the membrane. Dr. Johnson explains that for this reason, there is keen interest in developing new membrane materials that can operate at very low water levels-or even in the complete absence of water (anhydrously). "PEMs in today's hydrogen fuel cells are made of a polymer called Nafion, which only conducts protons when it has the right amount of water on it," says Dr. Johnson. "Too little water, the membrane dries out and protons stop moving. Too much and the membrane "floods" and stops operating, similar to how you could flood a carbureted engine with too much gasoline," he added. Dr. Johnson and his team focused on graphane because when functionalized with hydroxyl groups it creates a more stable, insulating membrane to conduct protons. "Our computational modeling showed that because of graphane's unique structure, it is well suited to rapidly conduct protons across the membrane and electrons across the circuit under anhydrous conditions," Dr. Johnson said. "This would enable hydrogen fuel cell cars to be a more practical alternative vehicle." The Johnson Research Group at the University of Pittsburgh uses atomistic modeling to tackle fundamental problems over a wide range of subject areas in chemical engineering, including the molecular design of nanoporous sorbents for the capture of carbon dioxide, the development of catalysts for conversion of carbon dioxide into fuels, the transport of gases and liquids through carbon nanotube membranes, the study of chemical reaction mechanisms, the development of CO2-soluble polymers and CO2 thickeners, and the study of hydrogen storage with complex hydrides. Karl Johnson is a member of the Pittsburgh Quantum Institute. He received his bachelor and master of science degrees in chemical engineering from Brigham Young University, and PhD in chemical engineering with a minor in computer science from Cornell University.

Ewing C.S.,University of Pittsburgh | Ewing C.S.,Pittsburgh Quantum Institute | Veser G.,University of Pittsburgh | Lambrecht D.S.,Pittsburgh Quantum Institute | And 2 more authors.
Surface Science | Year: 2016

Metal-support interactions significantly affect the stability and activity of supported catalytic nanoparticles (NPs), yet there is no simple and reliable method for estimating NP-support interactions, especially for amorphous supports. We present an approach for rapid prediction of catalyst-support interactions between Pt NPs and amorphous silica supports for NPs of various sizes and shapes. We use density functional theory calculations of 13. atom Pt clusters on model amorphous silica supports to determine linear correlations relating catalyst properties to NP-support interactions. We show that these correlations can be combined with fast discrete element method simulations to predict adhesion energy and NP net charge for NPs of larger sizes and different shapes. Furthermore, we demonstrate that this approach can be successfully transferred to Pd, Au, Ni, and Fe NPs. This approach can be used to quickly screen stability and net charge transfer and leads to a better fundamental understanding of catalyst-support interactions. © 2016 Elsevier B.V.

Ye J.,University of Pittsburgh | Johnson J.K.,University of Pittsburgh | Johnson J.K.,Pittsburgh Quantum Institute
ACS Catalysis | Year: 2015

The capture and reuse of CO2 as a liquid fuel could reduce the overall anthropogenic carbon footprint but requires a catalytic pathway for CO2 hydrogenation under mild conditions, coupled with a renewable source of H2 or another reducing agent. We have computationally designed eight functional groups having both Lewis acid and base sites for inclusion inside a porous metal-organic framework (MOF) and have evaluated these functionalized MOFs for their catalytic activity toward CO2 hydrogenation. We have used density functional theory to compute reaction energies, barriers, and geometries for the elementary steps of CO2 reduction. The reaction pathways involve two elementary steps for each of the eight functional groups, consisting of heterolytic dissociation of H2 on the Lewis acid and base sites followed by concerted addition of a hydride and a proton to CO2 in a single step. Our analysis of the reaction energetics reveals that the reaction barrier for hydrogen dissociation can be correlated as a function of the chemical hardness of the Lewis acid site. Furthermore, we have identified a Brønsted-Evans-Polanyi relationship relating the barrier for the second step, CO2 hydrogenation, with the H2 adsorption energy on the Lewis sites. Surprisingly, this linear relationship also holds for correlating the hydrogenation barrier with the hydride attachment energy for the gas-phase Lewis acid site. These correlations provide a computationally efficient method for screening functional groups for their catalytic activity toward CO2 hydrogenation. These relationships are further utilized to carry out a Sabatier analysis on a simplified model of the reaction to generate contour plots of the Sabatier activity that can be used to identify properties of the functional groups for maximizing the reaction rate. © 2015 American Chemical Society.

Ewing C.S.,University of Pittsburgh | Ewing C.S.,Pittsburgh Quantum Institute | Veser G.,University of Pittsburgh | McCarthy J.J.,University of Pittsburgh | And 4 more authors.
Journal of Physical Chemistry C | Year: 2015

The interaction between catalytic nanoparticles (NPs) and their supports, which are often amorphous oxides, has not been well characterized at the atomic level, although it is known that, in some cases, NP-support interactions dominate the catalytic activity of the system. Furthermore, there is a lack of understanding of how support preparation affects both the stability of the NP (resistance to sintering) and the catalytic activity. We present first-principles density functional theory (DFT) calculations on amorphous silica supported Pt NPs of various sizes. Our calculations predict that support preparation methods that lead to higher hydroxyl density when NPs are deposited on the support will lead to higher resistance to sintering. We find that the total charge on supported NPs, which can affect catalyst activity, depends linearly on the number of Pt-silica bonds formed during NP deposition. The number of bonds between an NP of a known geometry and the silica support with a known hydroxyl density can be estimated from very fast discrete element method simulations, enabling the prediction of both the net charge and the adhesion energy of the particle from a linear fit correlation derived from DFT calculations of a series of differently sized Pt clusters. This work quantifies interactions between Pt NPs and amorphous silica supports and demonstrates a new method for rapid estimation of NP-support interactions on amorphous supports. (Figure Presented). © 2015 American Chemical Society.

Bi F.,University of Pittsburgh | Bi F.,Pittsburgh Quantum Institute | Huang M.,University of Pittsburgh | Huang M.,Pittsburgh Quantum Institute | And 6 more authors.
Journal of Applied Physics | Year: 2016

LaAlO3/SrTiO3 heterostructures are known to exhibit a sharp, hysteretic metal-insulator transition (MIT) with large enhanced capacitance near depletion. To understand the physical origin of this behavior, the electromechanical response of top-gated LaAlO3/SrTiO3 heterostructures is probed using two simultaneous measurement techniques: piezoresponse force microscopy (PFM) and capacitance spectroscopy. The observed hysteretic PFM responses show strong correlation with the capacitance signals, suggesting an interfacial carrier-mediated structural distortion associated with the gate-tuned MIT. In addition, the frequency dependence of the capacitance enhancement in LaAlO3/SrTiO3 is found to be well-matched to local PFM measurements. Our experimental results provide a fuller understanding of the top-gate tuned MIT in oxide heterostructure, which could be helpful for the development of future oxide-based nanoelectronics. © 2016 AIP Publishing LLC.

Brown K.A.,Northwestern University | Brown K.A.,Boston University | He S.,Northwestern University | Eichelsdoerfer D.J.,Northwestern University | And 13 more authors.
Nature Communications | Year: 2016

Complex-oxide interfaces host a diversity of phenomena not present in traditional semiconductor heterostructures. Despite intense interest, many basic questions remain about the mechanisms that give rise to interfacial conductivity and the role of surface chemistry in dictating these properties. Here we demonstrate a fully reversible >4 order of magnitude conductance change at LaAlO 3/SrTiO 3 (LAO/STO) interfaces, regulated by LAO surface protonation. Nominally conductive interfaces are rendered insulating by solvent immersion, which deprotonates the hydroxylated LAO surface; interface conductivity is restored by exposure to light, which induces reprotonation via photocatalytic oxidation of adsorbed water. The proposed mechanisms are supported by a coordinated series of electrical measurements, optical/solvent exposures, and X-ray photoelectron spectroscopy. This intimate connection between LAO surface chemistry and LAO/STO interface physics bears far-reaching implications for reconfigurable oxide nanoelectronics and raises the possibility of novel applications in which electronic properties of these materials can be locally tuned using synthetic chemistry.

Jnawali G.,University of Pittsburgh | Chen L.,University of Pittsburgh | Huang M.,University of Pittsburgh | Lee H.,University of Wisconsin - Madison | And 7 more authors.
Applied Physics Letters | Year: 2015

Terahertz (THz) spectroscopy is an important tool that provides resonant access to free carrier motion, molecular rotation, lattice vibrations, excitonic, spin, and other degrees of freedom. Current methods using THz radiation suffer from limits due to diffraction or low-sensitivity, preventing application at the scale of single nanoscale objects. Here, we present coupling between plasmonic degrees of freedom in a single gold nanorod and broadband THz emission generated from a proximal LaAlO3/SrTiO3 nanostructure. A strong enhancement of THz emission is measured for incident radiation that is linearly polarized along the long axis of the nanorod. This demonstration paves the way for the investigation of near-field plasmonic coupling in a variety of molecular-scale systems. © 2015 AIP Publishing LLC.

Bi F.,University of Pittsburgh | Bi F.,Pittsburgh Quantum Institute | Huang M.,University of Pittsburgh | Huang M.,Pittsburgh Quantum Institute | And 6 more authors.
Applied Physics Letters | Year: 2015

Complex-oxide heterostructures exhibit rich physical behavior such as emergent conductivity, superconductivity, and magnetism that are intriguing for scientific reasons as well as for potential technological applications. It was recently discovered that in-plane magnetism at the LaAlO3/SrTiO3 (LAO/STO) interface can be electronically controlled at room temperature. Here, we employ magnetic force microscopy to investigate electronically controlled ferromagnetism at the LAO/STO interface with LAO thickness t varied from 4 unit cell (u.c.) to 40 u.c. Magnetic signatures are observed only within a thickness window 8 u.c. ≤ t ≤ 25 u.c. Within this window, the device capacitance corresponds well to the expected geometric value, while for thicknesses outside this window, the capacitance is strongly suppressed. The ability to modulate electronic and magnetic properties of LAO/STO devices depends on the ability to control carrier density, which is in turn constrained by intrinsic tunneling mechanisms. © 2015 AIP Publishing LLC.

Bi F.,University of Pittsburgh | Bi F.,Pittsburgh Quantum Institute | Huang M.,University of Pittsburgh | Huang M.,Pittsburgh Quantum Institute | And 8 more authors.
Nature Communications | Year: 2014

Reports of emergent conductivity, superconductivity and magnetism have helped to fuel intense interest in the rich physics and technological potential of complex-oxide interfaces. Here we employ magnetic force microscopy to search for room-temperature magnetism in the well-studied LaAlO3 /SrTiO3 system. Using electrical top gating to control the electron density at the oxide interface, we directly observe the emergence of an in-plane ferromagnetic phase as electrons are depleted from the interface. Itinerant electrons that are reintroduced into the interface align antiferromagnetically with the magnetization at first screening and then destabilizing it as the conductive regime is approached. Repeated cycling of the gate voltage results in new, uncorrelated magnetic patterns. This newfound control over emergent magnetism at the interface between two non-magnetic oxides portends a number of important technological applications. © 2014 Macmillan Publishers Limited. All rights reserved.

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