rd INTERNATIONAL SYMPOSIUM HYDROGEN & ENERGY...Hydrogen Production, Hydrogen Storage, Hydrogen Applications, Theory and Modelling, Fuel Cells, Metal Hydride Batteries, Functional Materials

3nd

Hydrogen & Energy Symposium Braunwald, Switzerland 2009

PROGRAM 1

3rd INTERNATIONAL SYMPOSIUM

HYDROGEN & ENERGY

Hydrogen Production, Hydrogen Storage, Hydrogen Applications, Theory and Modelling, Fuel Cells, Metal Hydride Batteries, Functional Materials

The 3rd

symposium “Hydrogen & Energy” follows the inaugural symposium on 16. February 2007 at Empa and the 2

nd symposium “Hydrogen & Energy” in Braunwald 21. – 25. January 2008. It serves as an

information platform of the fundamental science and the frontiers of research in Sciences and Technology of Hydrogen & Energy.

The second symposium consists of invited keynote lectures reviewing the key elements of the hydrogen cycle. The world leading experts present the current research challenges and newest results in invited and contributing talks. Early stage and experienced researchers present there results and the open questions on posters as well as in a one slide presentation.

The conference takes place in the hotel Alpenblick in the beautiful small village Braunwald on 1'256 m (Eggstöcke 2'449 m) above see level in the Swiss mountains. The village is free of traffic, quiet and offers, beside the scientific program, relaxing moments as well as plenty of sport activities.

The number of participants is limited to 90.

25. - 30. January 2009

Hotel Alpenblick CH-8784 Braunwald Tel (+41) (55) 643 1544 URL: http://www.klausen-resort.ch/en/alpenblick.html

3nd


PROGRAM 2

3nd


PROGRAM 3

CONTENTS

Timetable

Abstracts

SUNDAY 25. 1. 2009

Prof. Dr. Heinz Berke I01: METAL-INDUCED AND METAL-FREE REACTIONS OF DIHYDROGEN AS A BASE FOR CHEMICAL HYDROGEN STORAGE

MONDAY 26. 1. 2009

Dr. Gary Sandrock I02: OVERVIEW OF HYDROGEN STORAGE: GAS, LIQUID, SOLID

Dr. Robert Bowman I03: SOLID STATE NMR STUDIES OF THE BOROHYDRIDES

Dr. Andreas Borgschulte O01: THE TANK IS EMPTY

Dr. Adem Tekin O02: DIFFUSION PATHWAYS OF NH3 IN Mg(NH3)xCl2 FROM DFT CALCULATIONS

Dr. Robin Gremaud O03: HYDROGEN VIBRATIONS AND DIFFUSION MECHANISM IN H/D EXCHANGED LiBH4

Prof. Dr. Joachim Schoenes O04: POLARIZATION DEPENDENT RAMAN SPECTROSCOPY OF LiBH4 SINGLE CRYSTALS

Dr. Zbigniew Lodziana O05: STUCTURE AND STABILITY OF BOROHYDRIDES

Dr. Oliver Friedrichs O06: SYNTHESIS OF LIBH4 BY BORANE ABSORPTION OF LIH

Dr. Elisa Gil Bardaji O07: MECHANOCHEMICAL AND DIRECT WET CHEMICAL SYNTHESIS OF METAL BOROHYDRIDES, BORODEUTERIDES AND PARTIALLY DEUTERATED BOROHYDRIDES

Dr. Riccarda Caputo O08: FIRST-PRINCIPLES STUDY OF METAL BORIDES: ARE THEY INTERMEDIATES OR PRECURSOR COMPOUNDS FOR HYDROGEN STORAGE?

Mr. Claudio Pistidda O09: A DETAILED STUDY ON THE DEHYDROGENATION OF 2NaBH4+MgH2

Dr. Gemma Garcia O10: COMBINATORIAL APPROCH FOR THE DISCOVERY OF NEW MATERIALS FOR HYDROGEN STORAGE

Mr. Sebastiano Garroni O11: DEHYDROGENATION MECHANISM OF THE 2 NaBH4 + MgH2 PREPARED BY BALL MILLING

3nd


PROGRAM 4

TUESDAY 27. 1. 2009

Prof Dr. Ronald Griessen I04: CAPPED METAL-HYDROGEN SYSTEMS

Prof. Dr. Bengt Kasemo I05: LSPR AND QCM-D MEASUREMENTS OF HYDROGEN STORAGE IN NANO-MATERIALS

Mr. Suleyman Er O12: HIGH CAPACITY MOLECULAR HYDROGEN STORAGE NANOMATERIALS

Dr. Nicola Naujoks O13: QUARTZ CRYSTAL MICROBALANCE STUDIES OF HYDROGEN STORAGE IN NANOSTRUCTURED MATERIALS

Prof. Dr. Mathias Getzlaff O14: HYDROGEN-INDUCED PLASTIC DEFORMATION OF RARE EARTH METAL THIN FILMS AND COMPARISON WITH CORRESPONDING NANOPARTICLES

Dr. Kevin Sivula O15: NANOSTRUCTURED PHOTOANODES FOR HYDROGEN PRODUCTION BY SOLAR WATER SPLITTING

Dr. Mauro Palumbo O16: A THERMODYNAMIC DATABASE FOR HYDROGEN STORAGE SYSTEMS

WEDNESDAY 28. 1. 2009

Dr. Geert Brocks I06: FIRST PRINCIPLES MODELING OF MAGNESIUM TRANSITION METAL HYDRIDES

Dr. Martin Johansson I07: HYDROGEN SPLITTING STUDIED BY H-D EXCHANGE

Mr. Florian Buchter O34: EXPERIMENTAL ELECTRONIC CHARGE DENSITY OF LiBH4 FROM MAXIMUM ENTROPY METHOD

Prof Wieslawa Sikora O18: ANALYSIS OF POSSIBLE MAGNETIC STRUCTURES IN ERMN2D2 DEUTERIDE

Prof Dr. Asuncion Fernandez O19: A COMPARATIVE STUDY OF THE ADDITIVE ROLE IN THE MgH2-Nb2O5 SYSTEM vs. THE LiBH4+MgH2-Ti-iso SYSTEM

Dr. Fabrice Leardini O20: LOW TEMPERATURE HYDROGEN DESORPTION FROM MgH2/Mg(OH)2 SYSTEM

Mr. Hidayet Argun O21: PHOTOBIOLOGICAL HYDROGEN PRODUCTION FROM DARK FERMENTATION EFFLUENT

THURSDAY 29. 1. 2009

Prof. Dr. Jens K. Nørskov I08: ATTEMPTS TO UNDERSTAND ELECTROCHEMICAL, PHOTOCHEMICAL AND BIOLOGICAL WATER SPLITTING WITHIN THE SAME CONCEPTUAL FRAMEWORK

3nd


PROGRAM 5

Prof. Dr. Bill David I09: POWDER DIFFRACTION STUDIES OF LIGHT WEIGHT HYDROGEN STORAGE MATERIALS

Prof Dr. Andreas Züttel O22: MECHANISM OF HYDROGEN SORPTION IN COMPLEX HYDRIDES

Dr. Valentina Zakaznova O23: HYDROGEN PRODUCTION BY ALKALINE ELECTROLYSIS: IS THERE A WAY TO INCREASE ITS EFFICIENCY?

Mr. Daniel Marinha O24: La0.6Sr0.4Co0.2Fe0.8O3-x MICROSTRUCTURES OBTAINED BY ELECTROSTATIC SPRAY DEPOSITION FOR IT-SOFCs

Dr. Ulrich Vogt O25: HIGH TEMPERATURE ELECTROLYSIS BY SOEC TECHNOLOGY FOR COST EFFICIENT AND ECONOMICAL HYDROGEN PRODUCTION

Mr. Daniel Wiedenmann O26: FIB/TEM AND EPMA AS COMPLEMENTARY TECHNIQUES FOR THE INVESTIGATION OF SOEC AND SOFC

Mr. Shunsuke Kato O27: IMPACT OF THE SURFACE OXIDATION OF LiBH4 ON THE H2 DESORTPITION PROCESSES

Mr. Jian-Cheng Chen O28: H2 DISSOCIATION ON Ti/Al(100) SURFACES

Mrs. Emilie Deprez O29: SURFACE ANALYSIS AND OXIDATION STATE IN THE REACTIVE HYDRIDE COMPOSITE 2LiBH4+MgH2+Ti-iso

Dr. Jean-Claude Crivello O30: ELECTRONIC PROPERTIES OF SOME INTERMETALLIC COMPOUNDS AND THEIR HYDRIDES

Dr. Maciej Krystian O31: KINETICS OF HYDROGEN SORPTION AND DESORPTION IN ECAP-PROCESSED Mg ALLOY ZK60

Dr. Stefan Schlag O32: THE GLOBAL HYDROGEN MARKET

FRIDAY 30. 1. 2009

Prof. Dr. Bjørn C. Hauback I10: DIFFRACTION STUDIES OF COMPLEX HYDRIDES

Dr. Jens Oluf Jensen I11: PEM FUEL CELLS AT ELEVATED TEMPERATURE

Dr. Philippe Mauron O33: HIGH-PRESSURE AND -TEMPERATURE DSC (DIFFERENTIAL SCANNING CALORIMETER) FOR COMBINED PCT (PRESSURE, CONCENTRATION, TEMPERATURE) MEASUREMENTS OF HYDRIDES

Dr. Emmanuel Wirth O17: NEW TOOLS FOR THE CHARACTERISATION OF HYDROGEN STORAGE MATERIALS

3nd


PROGRAM 6

Prof. Dr. Hans Hagemann O35: VIBRATIONAL SPECTROSCOPY AS A PROBE FOR STRUCTURE AND DYNAMICS OF INORGANIC BOROHYDRIDES

Dr. Anibal Ramirez-Cuesta O36: INELASTIC NEUTRON SCATTERING SPECTROSCOPY OF MOLECULAR HYDROGEN IN POROUS MATERIALS AND SURFACES

Dr. Arndt Remhof O37: HYDROGEN DYNAMICS IN LiBH4 STUDIED BY QUASIELASTIC NEUTRON SCATTERING

POSTER

MONDAY 26. 1. 2009

Mrs. Carine Rongeat P01: SYNTHESIS OF Ca(BH4)2 BY REACTIVE BALL MILLING

Mr. Christian Bonatto Minella P02: HYDROGEN SORPTION PROPERTIES OF MODIFIED CALCIUM BOROHYDRIDE

Dr. Daphiny Pottmaier P03: HYDROGENATION PROCESSES AT LOW PRESSURES IN THE Na-Mg-B SYSTEM PREPARED BY REACTIVE BALL MILLING

Mr. Flavio Pendolino P04: TRUE ACTIVATION ENERGY OF DECOMPOSITION OF LITHIUM BOROHYDRIDE LIBH4

Dr. Chiara Milanese P05: IMPROVEMENT IN THE H2 ABSORPTION KINETICS OF THE Mg – Ni SYSTEM BY C (GRAPHITE) ADDITION

Mr. Filippo Agresti P06: EVIDENCE OF FORMATION OF LiBH4 BY HIGH ENERGY BALL MILLING OF LiH AND B IN HYDROGEN ATMOSPHERE

Dr. Ashley Stowe P07: CATALYTIC ROLE OF NANOSTRUCTURED CARBON ON NaAlH4 HYDROGEN SORPTION

Dr. Mitsuru Matsumoto P08: LIQUID PHASE SYNTHESIS OF MAGNESIUM AMIDE AND AMIDE BASED HYDROGEN STORAGE MATERIALS

Mr. Pascal Martelli P09: IDENTIFICATION OF STRUCTURAL PHASES OF Ca[BH4]2 BY NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

Dr. Francesco Dolci P10: INTERMETIATES IN THE MIXED 2LiNH2/MgH2 HYDROGEN STORAGE SYSTEM

Mr. Deniz Cakir P11: STRUCTURE AND STABILITY OF Li2xMgy(NH)x+y

Mr. Florian Gebert P12: LOW TEMPERATURE RAMAN SPECTROSCOPY OF Mg(BH_4)_2 AND Mg(BD_4)_2

3nd


PROGRAM 7

Dr. Andreas Borgschulte P13: THE TELLER-REDLICH RULE IN COMPLEX HYDRIDES

Dr. Piotr Szymak P14: CONTROL OF SUPPLY SUBSYSTEMS OF FUEL CELL STACK

Dr. Józef Malecki P15: VALIDATION OF A MATHEMATICAL MODEL OF 5kW PEMFC STACK SUPPLIED BY PURE OXYGEN AND HYDROGEN

Dr. Grzegorz Grzeczka P16: CONCEPTION OF POWER SYSTEM OF AN UNDERWATER VEHICLE BASED ON FUEL CELL TECHNOLOGY

TUESDAY 27. 1. 2009

Mr. Jérémie Brillet P17: TANDEM CELL FOR HYDROGEN PRODUCTION: EXAMINING HEMATITE-DSC ARCHITECTURE STRATEGIES

Mr. Michal Gorbar P18: DEVELOPMENT OF NEW MEMBRANES FOR ALKALINE ELECTROLYSER CELLS BASED ON INORGANIC MATERIAL

Dr. Agnieszka Kuna P19: SYMMERTY ANALYSIS OF HYDROGEN RELATED STRUCTURAL TRAMSFORMATIONS

Dr. Ali Marashdeh P20: ALLOYING EFFECT ON H OCTAHEDRAL VS TETRAHEDRAL SITE OCCUPATION IN Pd-Au: FIRST PRINCIPLES CALCULATIONS AND NEUTRON DIFFRACTION MEASUREMENTS

Mr. Abdel El Kharbachi P21: STRUCTURAL CHARACTERIZATION AND THERMODYNAMIC PROPERTIES OF THE LIBH4-MGH2 SYSTEM

Dr. Chiara Milanese P22: KINETIC FEATURES IN THE H2 SORPTION BY NaBH4 + MgH2 COMPOSITES UNDER MECHANICAL ACTIVATION CONDITIONS

Dr. Jonathan Morrell P23: ALKALINE SALT MATERIALS PROPERTY AND AGING STUDY

Dr. Irina Konstanchuk P24: PECULIARITIES OF HYDROGEN INTERACTION WITH NANOCRYSTALLINE MAGNESIUM

Dr. Marco Vittori Antisari P25: METALLOGRAPHIC STUDY ON THE ROLE OF CATALYST PARTICLES IN THE KINETICS OF REACTION BETWEEN MAGNESIUM AND HYDROGEN

Dr. Ashish Khandelwal P26: INVESTIGATION OF MECHANICAL AND KINETIC STABILITY IN COMPACTED MgH2 BASED SAMPLES

Mr. Siarhei Kalinichenka P27: STRUCTURAL AND HYDROGEN SORPTION PROPERTIES OF Mg–Ni–Y ALLOYS PREPARED BY MELT SPINNING

3nd


PROGRAM 8

Mrs. Elsa Callini P28: MAGNESIUM BY GAS PHASE CONDENSATION:MORPHOLOGY MICROSTRUCTURE AND HYDROGEN SORPTION

Mr. Katarzyna Morawa P29: HYDROGEN STORAGE IN LIQUID ORGANIC HYDRIDES

Mr. Jon Bergmann Maronsson P30: COMPUTATIONAL METHODS FOR DESCRIBING REACTION RATES AT INTERFACES OF ENERGY MATERIALS

Mr. Thomas Kollin P31: To be announced

Mr. Ryoji Sahara P32: STRUCTURAL OPTIMIZATION BY TOMBO: CASE STUDY OF HYDRIDES AND MOLECULES

SCIENCE OF HYDROGEN & ENERGY AWARD

List of Participants

Information

Notes

3nd


PROGRAM 9

Timetable

3nd


PROGRAM 10

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3nd Hydrogen & Energy Symposium Braunwald, Switzerland 2009

11

Abstracts

3rd Hydrogen & Energy Symposium Braunwald, Switzerland 2009

12

Metal-Induced and Metal-Free Reactions of Dihydrogen as a Base for Chemical Hydrogen Storage

H. Berke*, Z. Chen, F. Zou, N. Avramovic, C. Jiang, X. Yang, A. Dybov, X.-Y. Liu, Y. Zhao, O. Blacque, T. Fox, H.W. Schmalle, K. Venkatesan Institute of Inorganic Chemistry, University of Zurich, Winterthurer Str. 190, CH-8057 Zurich

Introduction Chemical reactions of dihydrogen comprise the classes of hydrogenation/dehydrogen-ation and hydrodecoupling/dehydrocoupling and the related process of transfer hydrogenation. These transformations may get involved in chemical hydrogen storage as “fuelling” and “refuelling” processes of the storage compounds. Most of these reactions types require splitting of the H2 molecule as the “activating” step, which can be achieved in two formally distinct ways: Homolytic splitting: H2→ 2H•, Heterolytic splitting: H2 → H- + H+. Over decades the homolytic splitting pathways proceeding with oxidative addition of H2 to transition metal centers were elaborated to a quite high level of performance. Respective catalyses became routine type applications.

As a generic descriptor the term “Ionic Hydrogenation” (related are “Hydridic-Protonic Reactions” or “Bifunctional Catalysis”) describes the class of stoichiometric and catalytic hydrogenations /dehydrogenations proceeding with heterolytic splitting of H2 and with subsequent hydride and proton transfers from Lewis acidic centers (normally transition metal centers [1] or since recently also main group element based [2]) and from (Lewis) basic centers to unsaturated mostly organic XY substrates.

X = CH2, Y = CH2, O, NR; E = LnM = transition metal fragment or main group element; B = base As it appears now there are three distinct routes for “Ionic Hydrogenations”, which are

mechanistically distinguished by the sequence of H transfers: (a) hydride before proton transfer, (b) simultaneous hydride and proton transfer, (c) proton before hydride transfer.

Experimental and Results Within the reaction type (a) we investigated stoichiometric reactions using hydridic hydrides, such as WH(CO)n(PMe3)4-n(NO) (n = 0,1) and MoH(dippe)2(NO) complexes (dippe = bis(diisopropylphosphino)ethane), in combination with alcohols as proton sources to hydrogenate organic carbonyl compounds. We recently also studied a catalytic CO2 “Ionic Hydrogenation” using a WH(CPh)(dippe)2 complex to afford a mixture of formic acid and formates in the presence of a base.

Noyori's and Shvo/Casey hydrogenations or transfer hydrogenations are good examples for metal mediated double H transfers of category (b) [3]. In their essential parts these polar processes follow the principal course of the simultaneous double H transfer described in eq 2.

X

Y

H

H

X'

Y'+

X

Y

H

H

X'

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X,Y and X',Y' = atoms or molecular fragments including main group and transition elements

Sat. I Unsat. II Unsat. I Sat. II

Unpolar concerted double H transfers, like the degenerate reaction of ethane with ethylene, have high kinetic barriers and are therefore rarely observed [4], despite the fact that they are symmetry allowed and thermally accessible [4 + 2] processes. It became apparent from our work that the transferred H’s need to be sufficiently different in

X Y

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13

polarities to enable still concerted and low energy transition states. Route (c) occurring with primary H+ transfer was demonstrated to be operative in olefin, imine and ketone hydrogenations [5], where less hydridic transition metal hydrides are combined with relatively strong proton donors as H− and H+ sources. Mechanistically related rhenium and molybdenum catalyzed “Ionic Hydrogenations” of imines, where H2 complexes act as proton source, were studied by us [6]. Related in their reaction mechanisms are metal-free “Frustrated Lewis Pair” catalyzed hydrogenations of imines [2], for which a primary proton transfer was established.

References [1] H. Jacobsen, H. Berke, in Recent Advances

in Hydride Chemistry, p. 89, Eds. M.

Perruzzini, R. Poli, Elsevier, Amsterdam, Netherlands, 2001.

[2] P.A. Chase, G.C. Welch, T. Jurca, D.W. Stephan, Angew. Chem. Int. Ed. 42, 2007, 8050.

[3] R. Noyori, S. Hashiguchi, Acc. Chem. Res. 30, 1997, 97; S. E. Clapman, A. Hazdovic, R. H. Morris, Coord. Chem. Rev. 248, 2004, 2201; Y. Shvo, D. Szarkie, Y. Rahamim, D. F. Chodosh, J. Am. Chem. Soc. 108, 1986, 7400; C. P. Casey, N. A Strotman, S. E. Beetner, J. B. Johnson, D. C. Priebe, T. E. Vos, B. Khodavandi, I. A. Guzei, Organometallics 25, 2006, 1230.

[4] I. Fernandez, M. A. Sierra, F. P. Cossio, J. Org. Chem. 72, 2007, 1488.

[5] R.M. Bullock, Chem. Eur. J. 10, 2004, 2366. [6] X.Y. Liu, K. Venkatesan, H. W. Schmalle, H.

Berke, Organometallics, 23, 2004, 3153; Y. Zhao, H. W. Schmalle, T. Fox, O. Blacque, H. Berke, Dalton Trans. 2006, 73.

Corresponding author: Heinz Berke, email: [email protected], Tel. (+41) (44) 635 4681


14

Monday


15

Overview of Hydrogen Storage: Gas, Liquid, Solid Gary Sandrock Metal Hydride Technology, 810 Hacienda Ave., Davis, CA 95616, USA

A hydrogen-based energy system will require stationary, portable and vehicular H-storage systems. The most technically difficult application is vehicular storage because there are important weight, volume, charge/discharge rate, safety and cost requirements, among others. This presentation will give a broad review of the three basic approaches being considered today (gas, liquid and solid), along with the many subsets of those main categories. Advantages and disadvantages of each technique will be listed, along with the present systems status of each and suggestions for future R&D. This talk is aimed at setting the basic stage for more detailed and focused presentations expected to follow. Gaseous Hydrogen Compressed H2 gas is typically stored in advanced carbon-fiber-wrapped epoxy vessels at 35-70 MPa. This well-established approach suffers from volume and cost problems, among others. The volume problem can be significantly reduced by the cryogas (cold storage) approach.

Liquid Hydrogen There are at least four liquid hydrogen approaches: cryogenic elemental H2, NaBH4 solutions, rechargeable organic liquids and liquid anhydrous NH3. Each approach has advantages and disadvantages. For all four of these sub-approaches, the efficiencies and costs of production and/or regeneration are of concern, along with possible safety considerations.

Solid Hydrogen Solid hydrogen can be classified into three categories. High surface area adsorbents rely on physisorption, with relatively weak binding enthalpies (4-10 kJ/mol H2). Examples include various carbons, MOFs, zeolites, clathrate hydrates and certain polymers. Because of their generally low values of ΔHads, adsorbants generally require low temperatures for effective H-storage. Metal

incorporation offers some hope for increasing ΔHads. Non-reversible metal hydrides fall into two categories: (a) those decomposed by hydrolysis in H2O, and (b) those decomposed thermally. Although they often have attractive volumetric and gravimetric H-capacities, as well as controllable decomposition kinetics, both categories must be chemically regenerated with significant energy penalties. Reversible metal hydrides offer the best opportunities for in situ recharging. This family of solid storage media consists mainly of metallic and complex (mixed ionic-covalent) hydrides. Although all have good volumetric H-capacities, those that also have good gravimetric capacities usually require excessive decomposition temperatures and “catalysis” for acceptable reversibility. It is this particular category of materials where most solid-state storage R&D is presently being performed. There is a focus on metal borohydrides, nanostructure and phase mixtures.

Systems Status A number of prototype vehicular storage systems have been built and/or analyzed for most of the storage categories outlined above. Each has one or more practical shortcoming. More innovative materials and engineering R&D are clearly needed.

Corresponding author: Gary Sandrock, email: [email protected], tel. (+1) (530) 753 0451


16

Solid State NMR Studies of the Borohydrides R. C. Bowman, Jr.1, S.-J. Hwang2, C. Kim2, J. Ku2, M. S. Conradi3, D. T. Shane3, R. L. Corey3,4, J. W. Reiter1, and J. A. Zan1 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA; 2Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena CA 91125 USA; 3Washington University, Dept. of Physics, Saint Louis MO 63130 USA; 4South Dakota School of Mines and Technology, Rapid City SD 57701 USA

High-resolution solid state 11B spectra from LiBH4, Mg(BH4)2, Ca(BH4)2, and LiSc(BH4)4 confirmed there is similar bonding for BH4- ions in these hydrides. Following hydrogen desorption at temperatures ≥ 400 oC, all of these borohydrides formed variable amounts of the B12H12-2 species as well as the elemental amorphous boron (B) or boride (e.g., MgB2, CaB6, etc.) phases. Partial reversibility was noted in LiBH4 + MgH2 mixture and Ca(BH4)2. 1H and 11B relaxation times were also used to assess motions in the low (LT)- and high-temperature (HT) phases of LiBH4, Mg(BH4)2, and Ca(BH4)2,

Introduction Multinuclear nuclear magnetic resonance (NMR) spectroscopy provides novel insights on the compositions, chemical bonding, structures, as well as rotational and translational motions of complex metal hydrides. Because short-range interactions dominate NMR, highly disordered as well as amorphous materials can often be more thoroughly evaluated than is possible with X-ray or neutron diffraction studies that usually require good crystallinity. Taking examples from our recent investigations of several borohydrides, we illustrate how these NMR techniques resolve diverse issues on phase formation and decomposition processes.

Experimental The solid-state NMR techniques Magic Angle Spinning (MAS), cross-polarization (CP) MAS, and multi-quantum (MQ) MAS that were used to assess these borohydrides and their decomposition products were described previously1,2. The nuclear relaxation times were measured from 300 K to over 525 K using the methods reported by Corey, et al.3

Results The presence of the B12H12-2 species following hydrogen desorption from all the borohydrides was established from the 11B CPMAS and MQMAS spectra. While the borides MgB2, CaB6, and ScB2 were formed at sufficiently high temperatures, partial reversibility with H2 gas was only observed for the Ca system and the mixture of LiH and MgB2. Formation of B12H12-2 anions inhibits reformation of the borohydrides. Rotation of the BH4- ions controls the relaxation times of LT-LiBH4, Mg(BH4)2, and Ca(BH4)2 while Li ion diffusion dominates spin relaxation in HT-LiBH4 with an activation energy = 0.7 eV3.

Acknowledgments These studies were partially supported by DOE grants DE-FG02-05ER46256 and DE-AI-01-06EE11105 and were partially performed at the JPL, Caltech, under a contract with NASA.

References [1] S.-J. Hwang, et al., J. Phys. Chem. C 112

(2008) 3164 [2] J. Purewal et al; J. Phys. Chem. C 112 (2008)

8481 [3] R. L. Corey, et al., J. Phys. Chem. C 112

(2008) On-line.

Corresponding author: Robert Bowman, email: [email protected] Tel. (001) 818-354-7941


17

The Tank is empty A. Borgschulte, C. Rongeat, S. Kato, M. Bielmann, R. Gremaud, A. Züttel Empa Materials Science and Technology, Dept. Energy, Environment & Mobility, Sec. Hydrogen & Energy, CH-8600 Dübendorf, Switzerland

What might become a problem when using hydrogen as an energy carrier in car, is a scientific challenge in current research. A future hydrogen fuel gauge is just an indicator of the amount of hydrogen remaining in the used metal hydride. In materials research, the determination of pressure-composition isotherms (pcT) is mandatory to obtain the thermodynamic properties of metal hydrides. Various experimental techniques to measure the plateau pressures of metal hydrides are reviewed, that is gravimetric and volumetric pcT measurements, scanning differential calorimetry, and hydrogenography and resistance measurements on thin films. The agreement of the data for the archetypical example MgH2 is very good. Differences are explained by kinetic effects of the sorption process. The impact of kinetics on the accuracy of the determination of the equilibrium pressure is demonstrated by a comparison of dynamic (flux method) with static pcT measurements of LaNi5Hx. The use of optical probes as hydrogen fuel gauge is demonstrated. Measurement of equilibrium states The thermodynamic parameters of hydride formation can be extracted from the temperature dependence of the equilibrium pressure, by means of the Van ‘t Hoff relation. The technically challenging problem is the measurement of such equilibrium isotherms. In static experiments, the pressure is stepwise enhanced and the system equilibrates. Most frequently used techniques are scanning methods, in which one thermodynamic parameter (either pressure or temperature) is varied and the response of the sample is recorded. As these measurements are by definition dynamic measurements, the equilibrium values have to be extrapolated. The quality of the extrapolation relies on the modelling of the kinetics. We will give an insight in the underlying elementary steps of H-sorption and apply a quantitative model [1] on various experimental examples measured by different systems. Indirect Methods Indirect methods such as hydrogenography [2] and scanning differential calorimetry [3] make use of the impact of the change of the

hydrogen content on other physical properties of the material.

Fig. 1. Equilibrium pressures and kinetics of LaNi5Hx measured gravimetrically.

Optical methods have several advantages, which make them potential candidates as methods to be used in hydrogen fuel gauges.

References [1] Borgschulte et al; Phys. Rev. B 78, 094106

(2008). [2] Gremaud et al; Adv. Mater. 19, 2813-2817

(2007) [3] Rongeat et al., J. Phys. Chem. B 2007, 111,

13301

Corresponding author: Andreas Borgschulte, email: [email protected], Tel. (+41) (44) 823 4639


18

Diffusion Pathways of NH3 in Mg[NH3]xCl2 from DFT calculations Adem Tekina,b, Tejs Veggeb, Jens K. Nørskova a) Center for Atomic-scale Materials Design (CAMD), Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark b) National Laboratory for Sustainable Energy, Technical University of Denmark, DK-4000 Roskilde, Denmark

Ammonia containing metal salts is one of the promising materials to store hydrogen indirectly. They show a fast and reversible ammonia ab- and desorption kinetics. Mg(NH3)xCl2 with x=6,2,1 is selected as a prototype system to enlighten this phenomenon. Bulk diffusion pathways of ammonia in Mg(NH3)xCl2 is investigated by employing a planewave density functional theory approach. Results indicated that diffusion is much faster in the hexa(x=6) ammine phase. Introduction Indirect storage of hydrogen in the form of ammonia stored in metal salts, so-called metal ammines, has many interesting properties as an energy storage medium. Whereas many metal hydrides suffer from low density of hydrogen, slow release or a poor reversibility of ab- and desorption of hydrogen, metal ammines have recently been shown to display superior properties [1]. In contrast to metal hydrides, metal ammines decompose thermally by releasing ammonia which can later be cracked into hydrogen with an ammonia decomposition catalyst. One of the most promising metal ammines for hydrogen storage is Mg(NH3)6Cl2 (9.1 % wt of hydrogen), which has been investigated both theoretically and experimentally [2,3,4]. It has been found that this metal ammine develops a system of nano-sized pores during ammonia decomposition enabling a fast ab- and desorption of ammonia.

Results We employed a planewave density functional theory approach using the RPBE generalized gradient approximation to account for exchange and correlation effects to determine diffusion mechanisms and

decomposition pathways for Mg(NH3)xCl2. Results are presented for diffusion and rotation rates in x=6,2,1, using a combination of nudged elastic band calculations and transition state theory, showing that diffusion is much faster in the hexa (x=6) ammine phase.

Fig. 1. Ammonia diffusion in Mg(NH3)6Cl2.

References [1] Christensen et al; J. Mater. Chem. 15 (2005)

4106 [2] Jacobsen et al; Chem. Phys. Lett. 441 (2007)

255 [3] Hummelshøj et al; J. Am. Chem. Soc. 128

(2006) 16

[4] Vegge et al; Indirect hydrogen storage in metal ammines, ed. G. Walker, Woodhead Publishing, 2008.

Corresponding author: Adem Tekin, email: [email protected], Tel. (+45) 4525 3184


19

Hydrogen vibrations and diffusion mechanism in H/D exchanged LiBH4 Robin Gremaud1*, Andreas Borgschulte1*, Z. Łodziana1*, Timmy Ramirez-Cuesta2, Joachim Schoenes3, Paul Hug1°, Andreas Züttel1* 1Empa Materials Science and Technology, Dept. Energy, Environment & Mobility, *Sec. Hydrogen & Energy, °Sec. Solid State Chemistry, CH-8600 Dübendorf, Switzerland 2ISIS facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, United Kingdom 3Institute of Condensed Matter Physics, Technical University Braunschweig, Mendelsohnstrasse 3, D-38106 Braunschweig, Germany

The effects of H/D isotope exchange on bulk LiBH4 are studied with Raman, infrared spectroscopy and inelastic neutron scattering. We show that deuterium is exchanged in the BH4- pseudo-molecule at temperature below melting. We are able to detect each individual B(H4-nDn)- unit (n = 1,..,4) and establish that their relative fraction as a function of deuterium fraction exchanged follows a statistical distribution. This shows that in molecular solids like the complex hydride LiBH4, exchange of single hydrogen atoms and its diffusion through the crystal is possible without breaking the molecular unit. Introduction LiBH4 is an example of molecular solid, in which the pseudo-molecule, BH4-, is ionically bound to the counter ion Li+. As a consequence, phonons in LiBH4 can be well approximated by so-called “internal” and “external” vibrations, where internal refers to the motions within the molecular BH4- unit, and external refers to crystalline vibration of BH4- and Li+[1]. Raman and infrared spectroscopy are ideally suited techniques to follow the effects of H/D exchange, especially in the internal vibrations, as the mass change shifts considerably normal vibrations. Results We report on the effect of hydrogen isotope exchange on bulk LiBD4 with Raman spectroscopy at a temperature of 83 K. The experimental results are compared to density functional calculations of the vibrational spectra of B(H4-nDn)- (n=1…4) both isolated and under the influence of the surrounding lattice.

We focus on the B-D stretching modes, as they enable us to detect the presence and to determine the relative fractions of the B(H4-nDn)- subunits.

Fig. 1. B-D stretching region in the Raman spectra for LiBD4 and LiB(H0.55D0.45)4. The peaks labelled as ν1(Dn) (n=1…4) refer to symmetric stretching modes of each B(H4-nDn)- unit.

Reference [1] A. –M. Racu et al.,: J. Phys. Chem. 112, 9716

(2008)

Corresponding author: Robin Gremaud, email: [email protected], Tel. (+41) (44) 823 4933


20

Polarization dependent Raman spectroscopy of LiBH4 single crystals J. Schoenes, B. Willenberg and F. Gebert Institut für Physik der Kondensierten Materie, Technische Universität Braunschweig, D-38106 Braunschweig, Germany

A Raman scattering study of the phonon modes is reported for LiBH4 single crystals. Using the polarization dependence of the lines the symmetry of the modes is determined, allowing a better comparison and a more reliable assignment to computed phonon frequencies. This leads to the revision of a few former assignments made from Raman measurements on polycrystalline samples. Introduction Vibrational spectroscopy, like IR absorption and Raman scattering are particularly suited to study hydrogen bonds, since the light mass of hydrogen generates high vibrational frequencies and the isotope effect on substitution of deuterium for hydrogen is the largest of any element [1,2]. In a previous paper [3] we have reported an extensive low temperature Raman scattering study on LiBH4 and LiBD4 powders. The 27 observed lines have been assigned to phonon modes within the orthorhombic Pnma structure by comparing the experimental values to density functional theory (DFT) values [3].

Experimental In the present contribution we present for the first time Raman scattering measurements on small LiBH4 single crystals. These have been identified among the grains of the powders (Alpha Aesar) by searching for large polarization dependencies of the Raman lines.

Results For optimum orientation Ag modes will appear in a configuration in which the incoming and scattered light have the same polarization but not in a configuration in which the scattered light has a polarization

perpendicular to the incoming light. In contrast, Bg modes will appear for parallel as well as for perpendicular polarization, the relative intensity giving indications whether one deals with B1g, B2g or B3g modes. On the basis of these new results a few of the former assignments have to be revised. Among the external modes this concerns the mode near 307 cm-1 which is definitely of Ag symmetry and not of B2g symmetry as had been concluded on purely energy arguments.

A second issue is the phase transition to a hexagonal structure at about 380 K. Several grains of our powder samples did not show the anticipated changes of the Raman spectra. We have now succeeded in finding a few grains which, indeed, display the characteristic simplification of the Raman spectra when the phase transition to the hexagonal phase occurs.

References [1] H. Kierey, M. Rode, A. Jacob, A. Borgschulte, and J. Schoenes, Phys. Rev. B 63, 134109 (2001)

[2] J. Schoenes, A.-M. Racu, M. Rode, and S. Weber, J. Alloys Compd. 446-447, 562 (2007)

[3] A.-M. Racu, J. Schoenes, Z. Lodziana, A. Borgschulte, and A. Züttel, J. Phys. Chem. A 112, 9716 (2008)

Corresponding author: Joachim Schoenes, email: [email protected], Tel. (+49) (531) 391 5130


21

Structure and stability of borohydrides Zbigniew Łodziana and Andreas Züttel Empa Materials Science and Technology, Dept. Energy, Environment & Mobility, Sec. Hydrogen & Energy, CH-8600 Dübendorf, Switzerland

Borohydrides of elements of Group 1 and Group 2 of the Periodic Table form ionic structures (eg. LiBH4, Mg(BH4)2), while transition metals usually form covalent borohydride molecules that are weakly bounded in Van der Waals crystals. The structural properties of these materials result from type of bonding between BH4 molecular units and metal atoms. We will discuss the stability various borohydrides with respect to electronic bonding and structural properties of compounds. Introduction Metal borohydrides form variety of crystalline structures. They have much better hydrogen storage capacity than alanates due to low mass of boron. Unfortunately, at present borohydrides are considered as irreversible hydrogen storage media that either posses to high decomposition temperature or are unstable at ambient conditions.

Method

We report calculations of the stability of selected borohydrides based on a periodic density functional (DFT) approach. The Kohn–Sham wavefunctions were expanded in plane-wave basis sets with energy cut-off of up to 900 eV. Brillouin zone sampling was performed on meshes with a k-point spacing of ~0.03 Å−1. The analysis of the valence charge of atoms was performed within Bader formalism.

Results The binary alkali borohydrides, with monovalent or divalent cations form crystalline structures with significant charge transfer between metal cation and BH4 molecular unit. The charge on BH4 group ranges between –0.88e for NaBH4 and –0.80e for Ca(BH4)2. During decomposition of

these compounds neutral species with covalent bonds are formed. Borohydrides of tri- or tetravalent metal atoms form closed molecules where significant overlap of the electronic density between metal and BH4 group is observed. The charge transfer between species is smaller (charge on BH4 group is –0.70e for Y(BH4)3 and –0.46e for Ti(BH4)4) and occupied Metal-BH4 bonding orbitals can be distinguished, as can be seen in the Figure 1.

Fig. 1. The electron density at 0.3 e/Å3 of Y(BH4)3 (left) and Ti(BH4)4 (right). Dark sphere is for metal, light gray for boron and small for hydrogen.

These compounds are bound via weak interaction and crystalline structures are unstable even at modest thermodynamic conditions. Small charge transfer and directional bonds indicate that decomposition of metal borohydrides with metal valency larger than two would produce diborane and higher borohydrides.

Corresponding author: Zbigniew Lodziana, email: [email protected], Tel. (+41) (44) 823 4083


22

Synthesis of LiBH4 by borane absorption of LiH Oliver Friedrichs, Andreas Borgschulte, Shunsuke Kato, Florian Buchter, Robin Gremaud, Arndt Remhof and Andreas Züttel Empa Materials Science and Technology, Dept. Energy, Environment & Mobility, Sec. Hydrogen & Energy, CH-8600 Dübendorf, Switzerland

The solvent free synthesis of LiBH4 from LiH in a borane atmosphere at 120°C and ambient pressures is demonstrated. The source of borane is a milled LiBH4/ZnCl2 mixture, in which Zn(BH4)2 is generated by metathesis reaction. The great yield of the reaction of about 74 % LiBH4 shows, that a bulk reaction is taking place upon borane absorption by LiH. This indicates that the formation of B-H bonds is the limiting step for the formation of LiBH4 from the elements. Therefore, the use of diborane as starting reactant allows to overcome the reaction barrier for the B-B bond dissociation and explains the rather moderate synthesis conditions. Introduction In this work the solvent free synthesis of Li[BH4] from LiH in borane atmosphere is presented [1-2]. The absorption reaction is analyzed by volumetric Sieverts method, Raman measurements, X-ray diffraction (XRD) and thermal mass spectrometry (TDS).

Experimental Commercial LiH is heated in borane atmosphere generated by thermal decomposition of Zn(BH4)2. The borane formation is demonstrated by TDS measurements, while the absorption of diborane is monitored by volumetric Sieverts and Raman measurements. After borane absorption the product is analyzed by XRD, in order to determine the newly generated phases in the material.

Results We have shown for the first time the synthesis of LiBH4 by a solid gas reaction between LiH and borane at 120°C and ambient pressures. The reaction is clearly taking place not only on the surface of LiH but in the bulk, which shows that diffusion is not the main problem in the formation process. It indicates that the formation of the B-H bond is the rate limiting step in the LiBH4

formation [3]. With a suitable catalyst supporting the formation of this bond the formation of borohydrides in general might be facilitated. The new synthesis method may be applied also for other promising tetrahydroborate systems [4]. Thereby LiH is replaced by a different metal hydride as CaH2 or MgH2 for example. This will be analyzed in future investigations.

Fig. 1. Schematic presentation of the mechanism of Li[BH4] formation from borane absorption of LiH

References [1] Friedrichs et al.; Angew. Chem. Int. Edit.

(2008) submitted [2] Schlesinger et al.; J. Am. Chem. Soc. 75

(1953) 186. [3] O. Friedrichs et al.; Acta Mater. 56 (2008) 949. [4] A. Züttel et al.; Scripta Mater. 56 (2007) 823.

Corresponding author: Oliver Friedrichs, email: [email protected], Tel. (+41) (44) 823 4153


23

Mechanochemical and direct wet chemical Synthesis of Metal Borohydrides, Borodeuterides and pertially deuterated Borohydrides M. E. Gil Bardají and M. Fichtner Forschungszentrum Karlsruhe, Institut for Nanotechnology, D-76021 Karlsruhe, Germany

Various procedures based on mechanochemical and direct wet chemical synthesis have been used to prepare the compounds Ca(11BH4)2, Ca(11BD4)2, Mg(11BH4)2, Mg(11BD4)2 and the partially deuterated compounds NaBH3D and NaBD3H. Introduction Metal borohydrides can assume a variety of crystal structures depending on the synthesis and the temperature.1 Elastic and inelastic neutron diffraction experiments can be a useful tool to obtain structural information of these materials. In order to perform these measurements calcium and magnesium borohydride as well as borodeuteride (enriched isotop 11B) have been synthesized. In addition, the vibrational spectra of isotopic analogous of BH4- in a crystalline environment can be investigated by using anharmonic DFT calculations. In order to compare these theoretical results, NaBD3H and NaBH3D have been synthesized as probe molecules.

Results Mg(11BH4)2 and Ca(11BH4)2 were synthesized according to the metathesis reaction 2,3:

2Na11BH4 + MCl2 M(11BH4)2 + 2NaCl M = Mg, Ca

The synthesis of the corresponding metal borodeuterides [M(11BD4)2, M = Mg, Ca] was achieved from the starting compound Na11BD4. A direct wet chemical synthesis4 from Et3N·11BD3 and metal deuteride led to the formation of the corresponding metal borodeuterides, too.

MD2 + Et3N·BD3 M(BD4)2 + Et3N M = Mg, Ca

For the synthesis of partially deuterated metal borohydrides, namely NaBD3H and NaBH3D, the previous reaction has been modified by using metal hydride or triethylamine borane, respectively.5

Fig.1. IR spectra of NaBH4, NaBD3H and NaBD4.

Reaction conditions of each procedure as well as characterization of the products will be presented.

References [1] Buchter, et al ; J. Phys. Chem. B 112 (2008) 8042 [2] James, et al; J Prog. Inorg. Chem. 11 (1970) 99. [3] Siegel at al;Metal Hydrides, ed. W. M. Mueller, J. P. Blackledgee and Libowitz G. G., Academic Press, New York (1968) [4] Fichtner at al; Phys. Chem. C 112 (2008)11575 [5] Davis et al; Indiana Academy of Science, 236

Corresponding author: M. Elisa Gil Bardaji, email: [email protected], Tel. (+49) 7247-82-8909


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First-principles study of Metal Borides: Are they Intermediates or Precursor Compounds for Hydrogen Storage? Riccarda Caputo and Andreas Züttel Empa Materials Science and Technology, Dept. Energy, Environment & Mobility, Sec. Hydrogen & Energy, CH-8600 Dübendorf, Switzerland

The importance of alkali and earth-alkali metal borides either as intermediates of the formation reaction of the corresponding borohydrides either as products of their thermal decomposition to upload hydrogen is discussed via first-principles calculations. The hydrogenation and de-hydrogenation cycle runs into a certain degree of irreversibility due to polymorphism of metal borides. Among lithium borides, LiB and Li2B6 are reported and discussed and compared with dehydrogenation phases of LiBH4. Introduction The hydrogenation of lithium borides, in particular of LiB[1] and Li2B6 [2] is modelled and studied from first-principles calculations using density functional theory based methods with GGA functional and norm-conserving type pseudopotential, to model the 1s state of both lithium and boron. Full geometry optimization is employed as implemented in CASTEP and CPMD codes.

Results The absorption of lithium in the α-phase of boron is studied at different compositions in the boron-rich region up to 1:1 atomic ratio Li:B [3]. As increasing the lithium content, the typical icosahedron-based structure of pure boron phase is destroyed [4]. Among all the possible phases so far investigated, LiB and Li2B6 has the lowest energy with the corresponding heat of formation equal to -0.249 eV and -0.448 eV per formula unit respectively, calculated referring to Li (bcc) and α-boron. The hydrogenation of LiB at low concentration of hydrogen has a slight endothermic heat of reaction equal to +0.160 eV per formula unit LiBH1.5. The hydrogenation of Li2B6, at even lower hydrogen concentration, results a highly endothermic reaction. In fact the heat of reaction is equal to +4.994 eV per formula unit Li2B6H0.8. That high energy of reaction

confirms the inertness to hydrogen of Li2B6 experimentally observed.

Figure 1. (a) LiB optimized structure, (b) LiBH1.5 , one of the corresponding hydrogenated phase. (c) Li2B6 optimize structure, (d) the corresponding hydrogenated phase, Li2B6H0.8. (Li, violet, B, pink, H white).

References [1] Wörle, M. et al., Z.Anorg. Allg. Chem., 632 (2006) 1737. [2] Mair, G., et al., Z. Anorg. Allg. Chem., 625 (1999) 1207. [3] Caputo, R. and A. Züttel, (to be submitted). [4] Caputo, R. and A. Züttel, (submitted) to Phys. Chem. Chem. Phys.

*Corresponding author: Riccarda Caputo, email: [email protected], Tel. (+41) (44) 823 4422


A detailed Study on the Hydrogenation of 2NaH + MgB2

Claudio Pistiddaa, Gagik Barkhordarianb, Sebastiano Garronic, Christian Bonatto Minellab, Torben Jensend, Martin Dornheimb, Wiebke Lohstroha, Rüdiger Bormannb, Maximiliam. Fichtnera. a Institut für Nanotechnologie, Forschungszentrum Karlsruhe GmbH, Postfach 3640, 76021 Karlsruhe, Germany b Institute of Material Research, GKSS Research Centre Geesthacht GmbH, Max Planck strasse 1, D-21502 Geesthacht, Germany c Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain d Interdisciplinary Nanoscience Centre (iNANO) and Department of Chemistry, University of Aarhus, Langelandsgabe 140, DK-8000. Denmark The recent discovery of the unique kinetic property of MgB2 in facilitating the hydrogenation of light metal complex borohydrides at moderate conditions has created new prospects to develop high capacity low enthalpy hydrogen storage materials. These new composite materials which consist of a binary light metal hydride (like LiH, NaH and CaH2) and MgB2 can be hydrogenated in much more moderate conditions compared to the usual route of hydrogenating the mixture of the corresponding binary hydride and pure Boron. This suggests that MgB2 is kinetically superior to pure Boron in these reactions; however the corresponding mechanisms are not yet understood. With the aim of clarifying the function of MgB2 functions, we investigated the hydrogenation of NaH+MgB2 by in-situ Synchrotron X-ray diffraction, high pressure titration, and HP-DSC.

Introduction

In the present work the hydrogen desorption behavior of the RHC system 2NaH+MgB2 prepared by ball milling was investigated

Experimental NaH (95% purity) and MgB2 (99.99% purity) were purchased from Sigma Aldrich and Alfa-Aesar, respectively. The NaH and the MgB2 were charged into a hardened steel vial and milled for 1 hour in a Spex 8000 ball mill, with a ball to powder ratio 10:1.Results An extensive in-situ PXD study during absorption of the RHC system 2NaH + MgB2 prepared by BM was performed. The results revealed that the formation of NaMgH3 is

strongly dependent on the hydrogen pressure.

Fig. 1. Series of SR-PXD patterns of the 2NaH+MgB2 system heated under 50 bar of hydrogen pressure from RT to 400°C.

Corresponding author: Claudio Pistidda, email: [email protected]

25


26

Combinatorial Approach for the Discovery of new Materials for Hydrogen Storage Roger Domènech-Ferrer, Gemma Garcia,

Alfonso Sepulveda and Javier

Rodriguez-Viejo † Nanomaterials and Microsystems Group, Department of Physics, UAB, Campus UAB, Bellaterra, 08193 Spain † MATGAS AIE, Campus UAB, Bellaterra, 08193 Spain

We have developed and tested a thin film membrane-based calorimeter to measure kinetically limited phase transitions such as the de/hydrogenation of metallic hydrides. Those microsystems are used as substrates for singular and combinatorial metal hydrides thin film deposition and tested for hydrogen storage applications. Introduction The combinatorial approach provides a mean to quickly investigate a large number of unexplored material for Hydrogen storage combining high throughput synthesis and screening methodologies [1-3]. Furthermore, thin film techniques offer a unique methodology for studying the nanoscale and interphase effects on the materials properties.

Experimental We have grown thin film libraries of the Mg-based system using a sequential deposition of pure elements (for instante Mg and Al) by an electron-beam evaporation technique [6]. Presently, we are also growing compositional graded libraries using a co-sputtering technique. Besides, we have developed thin film membrane-based calorimeters that can be used to characterise (de)hydrogenation reactions of metallic hydrides [5].

Results Those Microsystems are used as substrates for singular and combinatorial metal hydride thin film deposition, mainly magnesium-based alloys. After several hydrogenation treatments at different temperatures to

induce the hydride formation, we observe a significant reduction in the onset of dehydrogenation for Mg80Ti20 compared with pure Mg or Mg/Al layers, which confirms the beneficial effect of Ti on dehydrogenation. By using libraries containing different Mg-Al ratios we have also found indirect evidence of the formation of the alanate phase and that aluminum can act as a catalyzer for the hydrogenation reaction. Actually, to complement the high-throughput screening we are developing the Thermal Imaging Technique not only on standard thin films but also coupled with calorimetric in-situ experiments.

References [1] C.H. Olk, G.G. Tibbetts, D. Simon, J.J. Moleski, J. Appl Phys 94, 720–5 (2003). [2] R. Gremaud, C.P. Broedresz, D. Borsa, A. Borgschulte, P. Mauron, H. Schreuders, Adv Mater 19, 2813–7 (2007) [3] S. Guerin, B. E. Hayden, D. C. A. Smith J. Comb. Chem., 10 (1), 37–43 (2008) [5] A. Sepúlveda, A.F. Lopeandía, R. Domènech-Ferrer, G. Garcia, F. Pi, J. Rodríguez-Viejo, F.J. Muñoz; Inter. J. of Hydrogen Energy 33, 2729 – 2737 (2008) [6] G. Garcia, R. Domènech-Ferrer, F. Pi, J. Santiso, J Rodriguez-Viejo; J Combinatorial Chem 9, 230–6,(2007)

Corresponding author: Gemma Garcia, email: [email protected], Tel. (+34) 935811481


27

Dehydrogenation Mechanism of 2NaBH4 + MgH2 prepared by Ball Milling S. Garroni1, C. Pistidda2, M. Brunelli3, G. Vaughan3, S. Suriñach1, M.D. Baró1 1Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain 2GKSS Research Centre, Geesthacht, Germany 3ESRF European Synchrotron Radiation Facility, BP220, Grenoble, France

Hydrogen storage materials based on a mixture of complex and light metal hydrides constitute a class of compounds promising in the field of hydrogen storage for vehicular applications [1]. Particular attention has been paid recently to Reactive Hydride Composites (RHC), where the combination of an alkaline borohydride with a reactive hydride induces a thermodynamic destabilization and a decrease of hydrogen desorption temperature with respect to the component phases [2,3]. Although numerous studies are reported in literature for these functional hydrogen compounds, their dehydrogenation mechanism is not quite clear and their preparation has to be optimized. Introduction In this work we focus on the properties of the system NaBH4 - MgH2 prepared by high energy ball milling. The aim of the work is to study which phases evolve during the hydrogen desorption in the mixture 2NaBH4 + MgH2 in order to understand the mechanism that is involved in that process.

Experimental The mixture was milled in a mol ratio 2:1 by means of Planetary Fritsch P5 mill. The milling was performed under pure Argon atmosphere with a ball to powder ratio of 1:10 and a rotation speed of 230 rpm. With the aim of characterizing the samples, the morphology, thermodynamic and micro-structural properties were investigated by Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC) and Powder X-ray Diffraction (PXD), respectively. In addition in situ Powder X-ray Diffraction (SR-PXD) was performed in order to evaluate the phases evolved during the desorption process.

Results As shown in fig. 1, after 337ºC the desorption of MgH2 starts and consequently the peaks

of Mg appear. The desorption is complete after 388ºC, in according with the DSC analysis performed previously.

Fig. 1. “in situ” XRD patterns measured at different temperatures (25-500º) of 2NaBH4 + MgH2 intimate mixture.

Further analysis confirmed that the formation of MgB2 starts after the complete desorption of MgH2.

References [1] Materials Go/No-Go Decisions Made Within the

Department of Energy Metal Hydride Center of Excellentce (MHCoE), Sandia National Laboratories, Livermore, CA 94551, 2007.

[2] John J. Vajo and Gregory L. Olson, Scripta Mater, (2007) 56, 829-834.

[3] G. Barkhordarian, T. Klassen, M. Dornheim, R. Bormann, J. Alloys. Comp., (2007), 440, L18-L21.

Corresponding author: Sebastiano Garroni, email: [email protected], Tel. (+34) (93) 5811657


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29

Capped Metal-Hydrogen Systems Ronald Griessen, Andrea Baldi, Yevheniy Pivak, Herman Schreuders and Bernard Dam Physics Department, VU university, Amsterdam, the Netherlands

In thin films and nanoclusters the surface to volume ratio is much larger than in polycrystalline bulk materials. We show that the thermodynamics of a Mg layer sandwiched between transition metal (TM) layers can be drastically modified by the elastic constrain imposed by a surface TM layer. The same applies for Mg nanoclusters with a hard MgO surface layer. Introduction Many attempts have been done to destabilize Mg-hydride. We show here that the plateau pressure of Mg-H can be increased by more than one order of magnitude by restricting the free expansion of Mg during hydrogen absorption. One effective way to do so is by sandwiching Mg layers between transition metal layers. Another way is by coating nanocrystallites of Mg by a thin layer of a hard material.

Experimental Ti/Mg/Pd multilayers are deposited on glass in a UHV system (base pressure = 10-8 mbar) by DC/RF magnetron in argon atmosphere. The thickness of the Ti and Pd layers is 10 and 40 nm, respectively, while the thickness of the Mg layer is varied from 10 to 40 nm. This set of samples is used to demonstrate the effect of elastic clamping. The p-c isotherms at 333 K are measured by means of hydrogenography.

Results The width of the plateaus in Fig.1 is proportional to the thickness of the Mg layer that becomes transparent in the hydrogenated state. Most interesting is the large increase in plateau pressure with decreasing Mg thickness. This is predicted by a model in which the lattice expansion of the Mg layer during H uptake is elastically

hampered by the transition metal layers. This model predicts that

where ps and pf are the plateau pressures of a sandwiched (s) and a free (f) Mg layer, respectively. EMg and EMg are the Young moduli and dMg and dMg the thicknesses of the Mg and TM layers. The parameter a depends on characteristic properties of the Mg-H system. A similar model predicts that Mg nano-clusters covered by a thin MgO surface layer have a plateau pressure at 333K that is approx. 100 times higher than the plateau of a pure Mg film.

Fig. 1. P-c isotherms at 333 K of a Mg layer sandwiched between Ti and Pd as a function of the Mg layer thickness (10 – 40 nm).

Corresponding author: Ronald Griessen, email: [email protected], Tel. (+31) (20) 598 7915


30

LSPR and QCM-D Measurements of Hydrogen Storage in nano-Materials. C. Langhammera, N. Naujoksa, E. Larssona, L. Romanszkia, M. Rodahlb, M. Zächa, V. Zhdanova,c, I. Zorica, B. Kasemoa aDepartment of Applied Physics, Chalmers, SE-412 96 Gothenburg, Sweden, b Q-Sense AB, 426 77 Vaestra Froelunda, Sweden, cBoreskov Institute of Catalysis, Russian Academy of Sciences, Novosibirsk 630090, Russia We report (i) QCM measurements of hydrogen uptake/release in submonolayer arrays of Pd nanoparticles, (ii) LSPR measurements on the same system, (iii) preparation of new nanoparticle model systems for QCM and LSPR measurements, (iv) a new experimental set up for QCM measurements with HSM materials and (v) theoretical modeling of hydrogen uptake kinetics in nanoparticles. Introduction Hydrogen storage materials (HSM) in the form of nanoparticles (NP) and nano-structured materials are interesting from the viewpoints of basic physics and applications, and because the thermodynamics and kinetics of the HSM can be modified. A challenge working with small, well-defined NPs is the often limited amount of material available, and the associated reduced sensitivity. We apply QCM, Localized Surface Plasmon Resonance (LSPR) sensing, associated preparation, and modeling to study nano-particle HSM.

Experimental QCM: The QCM technique is well established since long for HSM studies 1. The sample to be studied is deposited on the surface of a piezoelectric quartz crystal sensor, acting as a resonator, allowing measurement of very small frequency (mass) changes, caused by hydrogen uptake or loss. LSPR: LSPR is the optical response caused by collective oscillations of conduction band electrons (plasmons) in a confined particle < light wavelength. Changes in the dielectric environment of the NP, or a change in the NP’s electron structure, cause a change in the optical response which is frequently used for biosensing. We have recently shown that this can be used to sensitively measure

hydrogen uptake and release in Pd nanoparticles2.

Results Combined QCM and LSPR measurements on relatively large Pd NPs demonstrate (i) that thermodynamic values (enthalpy and entropy) for the alpha and beta phases of Pd bulk can be obtained by both methods. The LSPR, calibrated by QCM, has superior sensitivity and can be used to follow fast kinetics in both phases. For sufficiently small particles, interesting new thermodynamics and kinetics are observed, partly addressed also by modeling3. We further give a progress report on a new measurement system for QCM measurements of HSM, and the associated sample preparation to increase the sensitivity of the QCM measurements (see presentation by Naujocs et al).

References [1] G. A. Frazier, R. Glosser, J. Phys. D : Appl.

Phys., 12, (1979).; J. Rydén et. al., J. Less-Comm. Met., 152, 295-309, (1989).; I. Kulchytskyy et. al., Appl. Phys. Lett., 91, 113507, (2007).

[2] C. Langhammer et. al., Nano Lett., 7, 3122-3127, (2007).

[3] V. P. Zhdanov, B. Kasemo, Chem. Phys. Lett., 460, 158, (2008), and tentatively new results.

Corresponding author: Bengt Kasemo, email: [email protected] Tel. (+46) (31) 772 33 70


31

High Capacity Hydrogen Storage Nanomaterials Süleyman Er1, Gilles A. de Wijs2 and Geert Brocks1 1 Computational Materials Science, Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands. 2 Electronic Structure of Materials, Institute for Molecules and Materials, Faculty of Science, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands. In the first part, lithiated carbon and oxygen molecular structures are considered for hydrogen storage applications. Using First Principles calculations we predict the interaction of hydrogen molecules with such materials. Within these compounds it is found that the Li atoms connected to a central C or O atom bear partial positive charges. Hydrogen molecules are then clustered around these Li atoms via electrostatic interactions (Fig.1). According to our calculations such molecules can attach hydrogen up to ~40 wt. % with average hydrogen binding energies between 0.1 and 0.2 eV/H2. However, to maintain a reversible system, molecular dimerization need to be prevented. Here, we consider attaching the lithiated molecular structures to graphene. The immobilized molecules have a similar interaction with hydrogen molecules as free molecules. However, hydrogen weight percentages are reduced to 5-8 wt. % due to the additional weight of the graphitic template.

Fig. 1. CLi4 (left side figure) and OLi2 molecules in their fully hydrogenated stages.

In the second part, we discuss the hydrogen storage properties of novel boron sheets[1] and buckyballs[2]. We find that the interaction of molecular hydrogen with these systems is weak as in the case of carbon based graphitic and buckyball structures. However, both of the boron structures bind to lightweight alkali and some of alkaline earth metals strongly. Deposited metals on the surface of boron systems bind several hydrogen molecules (Fig.2). Hydrogen weight percentages and binding energies vary depending on the choice of the metal. Ca is found to be a promising doping element for both of the boron sheet (6wt. %H, 0.15 eV/H2) and buckyball (9 wt %H, 0.11 eV/H2) structures.

Fig. 2. Boron buckyball doped with 12 Ca atoms in its fully hydrogenated stage.

References [1] H. Tang and S. Ismail-Beigi, Physical Review

Letters 99, 115501 (2007). [2] Szwacki et al. , Physical Review Letters 98,

166804 (2007).

Corresponding author: Süleyman Er, email: [email protected] , Tel. (+31) (53) 489 3167


32

Quartz Crystal Microbalance Studies of Hydrogen Storage in nanostructured Materials Nicola Naujoks, Loránd Románszki, Michael Zäch, and Bengt Kasemo

Chalmers University of Technology, Applied Physics, SE-412 96 Göteborg, Sweden

Nano-structured materials (thin films and nanoparticles) are investigated regarding their (de-) /hydrogenation properties with the help of Quartz Crystal Microbalance (QCM). Introduction

Reducing the geometrical dimensions of hydrogen storing materials to the nanoscale is expected (and shown) to result in advantageous kinetics. One approach to study this effect is to observe the hydrogenation properties of thin metal films [1] and nanostructured metal particles [2] with the help of a Quartz Crystal Microbalance (QCM). By detecting shifts in resonance frequency the QCM enables the measurement of minute mass changes that correspond to a fraction of monolayer of hydrogen atoms.

Model experiments / QCM To be studied in a QCM setup, the sample structures have to be fabricated onto the surface of quartz sensor, as shown in Fig.1. The feasibility of the QCM technique to investigate hydrogenation of thin metal films is demonstrated at the example of a Pd covered Mg film. Fig.2 shows thermodynamic data of obtained with a QCM setup. Applying this technique to the investigation of hydrogen storage in nanoparticles (NP) requires a method of attaching a sufficiently large amount of NPs onto the crystal (a 2d disperse layer on the sensor surface suffers from a limited sensitivity of the QCM). First results on embedding hydrogen storing NPs into porous supports that are coated onto the crystal will be presented, thereby allowing a larger number of particles to be accessed by

the H2 gas. We will further present a new QCM gas cell setup that will enable measurements over a wider range of temperatures and pressures.

Fig. 1. The samples – thin metal films - are deposited onto quartz crystal sensors.

Fig. 2. Hydrogenation and dehydrogenation cycle of a sandwich structured sample (Pd and Mg)

References [1] A. Krozer, and B. Kasemo; J. Less Comm.

Met. 160 (1990). 323. [2] Langhammer C., Zoric I. and Kasemo B; Nano

Lett. 7 (2007) 3122.

Corresponding author: Nicola Naujoks, email: [email protected], Tel. (+46) (31) 772 3331


33

Hydrogen-induced plastic Deformation of Rare Earth Metal Thin Films and Comparison with corresponding Nanoparticles Mathias Getzlaff1, Astrid Pundt2 1. Institute of Applied Physics, University of Düsseldorf, D-40225 Düsseldorf, Germany 2. Institute of Materials Physics, University of Göttingen, D-37077 Göttingen, Germany The hydrogen induced plastic deformation processes of Gd metal thin films are determined on the nanometer scale by means of the corresponding surface modifications. These results are compared to high Gd islands representing nanoparticles being clamped to the substrate at the interface.

Experimental Surface modification of thin Gd films and high islands representing nanoparticles during hy-drogen adsorption as well as absorption has been investigated on the nanometer scale by means of scanning tunnelling microscopy.

Results The adsorption occurs in two steps. It is initiated by surface imperfections (see left part of Fig. 1). Starting from these nucleation centers a domain-like spreading is present which is strongly hindered at surface steps (see right part of Fig. 1).

Fig. 1. Gadolinium islands on a W(110) surface. The bar corresponds to 100 nm. Left: The system was exposed to 0.2 L hydrogen. Hydrogen is adsorbed at that areas appearing deeper which is due to an electronic effect. Right: With increasing amount (1 L) a domain-like spreading occurs.

The measurements have shown that during further hydrogen loading two different types of surface pattern develop above a particular concentration: disc-like islands and slope fields (see Fig. 2).

Fig. 2. Gadolinium thin films on a W(110) surface. The system was exposed to 70 L hydrogen. Two different patterns occur: disc-like islands (upper part) and slope fields (middle part).

These surface patterns can be well described by two plastic deformation processes in the films that lead to glide steps on the film sur-face. First, the emission of dislocation loops during hydride precipitation occurs. Second-ly, misfit dislocations near the film-substrate interface are present. Since plastic deforma-tion leads to stress release a lot of thin metal films being clamped to a substrate relax pla-stically after reaching a certain hydrogen-in-duced stress that corresponds to a critical hy-drogen concentration. This conclusion is cor-roborated due to free-standing Gd islands being without structural deformation. Overall, combining the ability of preparing high-quality epitaxial thin films with the detailed analysis of the mechanical proper-ties during hydrogen absorption may lead to a deeper fundamental understanding of hy-drogen switchable thin films. It may also improve their industrial applications.

Corresponding author: Mathias Getzlaff, email: [email protected], Tel. (+49) (211) 81 12291


34

Nanostructured Photoanodes for Hydrogen Production by Solar Water Splitting Kevin Sivula, Scott C. Warren, Florian le Formal, Adriana Paracchino, Jeremie Brillet, and Michael Grätzel Laboratoire de Photonique et Interfaces, Ecole Polytechnique Fédérale de Lausanne, CH-1015, Lausanne, Switzerland

Recently we have reported nanostructed, silicon doped Fe2O3 photoanodes prepared by atmosphere pressure CVD which exhibit a remarkable photocurrent under standard conditions (2.2 mA/cm2 at 1.23 V vs RHE and AM 1.5G 100 mW/cm2 simulated solar illumination). When these transparent photoanodes are combined with a bias-supplying photovoltaic cell (a dye sensitized solar cell), unassisted solar-to-hydrogen conversion efficiencies of 3.3 % can be obtained. Here we will describe the current research efforts of PECHouse, the Swiss photoelectrochemical centre of excellence, in continuing the development of these nanostructured photoanodes for solar hydrogen production. Introduction

The world’s most abundant renewable energy source, the Sun, has the potential to meet current and future energy needs. The direct hydrogen production by water photoelectrolysis from solar illumination is one promising route to directly convert solar energy into a storable and transportable medium, but a scalable and inexpensive photoelectrolysis strategy is essential. Due to its abundance, environmental stability, and favorable bandgap, iron (III) oxide (hematite) is a promising photocatalyst for this application. However, the poor photon absorptivity and charge carrier transport of hematite have remained obstinate limitations preventing its full exploitation.

Experimental Nanostructed, silicon doped Fe2O3 photoanodes were prepared by the atmosphere pressure chemical vapor deposition from Fe(CO)5 and Si(OC2H5)4. The process temperature, dopant concentration, and the growth time were varied in order to determine the major factors which influence performance. High resolution electron microscopy, impedance spectroscopy and micro Raman analysis were subsequently used to reveal how the

process conditions affect the nanostructure and crystalinity.

Results We show film growth during deposition to be linear with an incubation time. No concern with electron transport for films up to 600 nm is found, but a higher recombination rate of photo-generated carriers in the hematite near the interface with the fluorine doped tin oxide, as compared to the bulk section of the film is noted. The observed feature sizes of the film are found to depend strongly on this temperature and the presence of silicon dopant precursor (TEOS). We also find evidence for an unusually high donor density which allows the formation of a space-charge field inside the nano-sized features of the polycrystalline hematite photoanode—a factor which could play a major role in the separation of charges and enhancement of performance, even in the surface features of 5 nm.

References [1] Kay et al; J. Amer. Chem. Soc. 128 (2006)

15714 [2] Cesar, et al; J. Amer. Chem. Soc. 128 (2006)

4582 [3] Gratzel, M., Nature, 414 (2001), 338

Corresponding author: Kevin Sivula, email: [email protected], Tel. +41 (0) 21 693 3669


35

A thermodynamic Database for Hydrogen storage Systems M. Palumboa,b, J. Urgnanib, J. F. Torresb, D. Baldissinb, M. Bariccob aParticle Simulation and Thermodynamics Group, Computational Materials Science Center (CMSC), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan bDipartimento di Chimica I.F.M. and NIS/INFM/CNISM, Università di Torino, via Giuria 7/9, 10125 Torino, Italy

The thermodynamic behaviour of hydrogen related systems has been investigated by coupling the CALPHAD approach and ab initio calculations. Several binary and ternary systems have been considered, including La-Ni-H, Al-Mg-H, Na-B-H. The development of a thermodynamic database for hydrogen systems is on going. In the latest years, hydrogen storage in intermetallic compounds has been proposed as a suitable way to accumulate energy for several applications [1]. However, the diffusion of this technology, especially in the automotive industry, has been hindered by some difficulties and limitations. Many researchers are active nowadays in this field trying to overcome present drawbacks. To this purpose, knowledge of thermodynamic stability of these compounds is much helpful to study the hydrogenation/dehydrogenation process. In order to be suitable for applications the thermodynamic stability of a candidate material should not be to high, thus avoiding a large heat absorbed/released during hydrogenation/dehydrogenation. Moreover, thermodynamics and phase diagrams are useful to rationalize synthesis reactions of these compounds and to suggest possible alternative reaction routes. By mixing some of these hydrides with other compounds, it is possible to make easier the hydrogenation/dehydrogenation process. A thermodynamic analysis is very useful in rationalizing this behaviour and the CALPHAD approach is the most suited to this goal.

The purpose of this work is to develop a consistent thermodynamic database for hydrogen storage systems. The La-H and La-Ni-H phase diagrams have been reviewed and thermodynamically assessed using the CALPHAD method, while other binary systems (Ni-H and La-Ni) have been adapted to be incorporated in the present database. Other Al, Mg, Na, B-based compounds/systems of interest for hydrogen storage are also included [2-5]. For example, the Al-Mg-H system has been thermodynamically assessed including the magnesium alanate Mg(AlH4)2 which has a theoretical hydrogen content of 9.3 wt% [4]. A critical assessment of available experimental data has been performed. Key ab initio calculations have been carried out. Calculated and experimental thermodynamic properties have then been compared and a satisfactory agreement has been achieved.

References [1] L. Schlapbach et al; Nature 414 (2001) 353 [2] K. Zeng et al; IJHE 24 (1999) 989 [3] B.-M. Lee et al; J. Alloys Compd. 424 (2006) 370 [4] M. Palumbo et al; CALPHAD 31 (2007) 457 [5] J. Urgnani et al; IJHE 33 (2008) 3111

Corresponding author: Mauro Palumbo, email: [email protected]


36

Wednesday


37

First Principles Modeling of Magnesium Transition Metal Hydides Süleyman Er,1 Michiel J. van Setten,2,3 Dhirendra Tiwari,1 Gilles A. de Wijs,2 Geert Brocks1 1Computational Materials Science, Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, the Netherlands. 2Electronic Structure of Materials, Institute for Molecules and Materials, Faculty of Science, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, the Netherlands. 3Institut für Nanotechnologie, Forschungszentrum Karlsruhe, P.O. Box 3640, D-76021 Karlsruhe, Germany.

Lightweight metal hydrides are strong candidates for future hydrogen storage materials. Alloying Mg with lightweight transition metals (TM = Sc, Ti, V, Cr) could provide a viable route towards materials with fast (de)hydrogenation kinetics and suitable thermodynamic properties. We study the structure and stability of MgxTM(1-x)H2, x = [0,1] by first-principles calculations. The stability of these compounds decreases along the series Sc, Ti, V, Cr. The dehydrogenation enthalpy of .MgxTi(1-x)H2 can be markedly improved by adding Al or Si, which destabilizes the hydride, whereas it stabilizes the alloy. MgxTi(1-x)H2 also has remarkable optical properties, such as a black state. We argue that the black state is an intrinsic property of these compounds, unlike similar optical phenomena observed in other metal hydride films. Introduction MgH2 has been studied intensively since it has a relatively high hydrogen gravimetric density of 7.7 wt.%. Bottlenecks in its application are its thermodynamic stability and slow kinetics. Alloying Mg with early transition metals (TMs) is found experimentally to increase the hydrogenation kinetics. Here we study the structure, the thermodynamics and the optical properties of magnesium transition metal hydrides by first-principles calculations at the level of density functional theory.

Results We find that the experimentally observed sharp decrease in hydrogenation rates of MgxTM(1-x)H2 for x > 0.8 correlates with a phase transition from a fluorite to a rutile phase. Varying the transition metal (TM) and the composition x, the formation enthalpy can be tuned over the substantial range 0-2 eV/f.u. Assuming however that the alloy does

not decompose upon dehydrogenation, the enthalpy associated with reversible hydrogenation of compounds with a high magnesium content (x = 0.75) is close to that of pure Mg [1]. Adding Al and Si to MgxTi(1-x)H2, however, destabilizes the hydrides, whereas it stabilizes the MgxTi(1-x) alloy [2]. Thin films of MgxTi(1-x)H2 show an optical black state upon hydrogenation. We calculate the dielectric function and the optical properties of MgxTi(1-x)H2, x = 0.5, 0.75, 0.875, and argue that the black state is an intrinsic property of these compounds [3]. In particular, the plasma frequency drops below 1 eV in disordered structures, which then show a low reflection and transmission in the range 1-6~eV, i.e. a black state.

References [1] Er et al; Phys. Rev. B, submitted (2008). [2] Er et al; in preparation (2009). [3] van Setten et al; Phys. Rev. B, submitted

(2008).

Corresponding author: Geert Brocks, email: [email protected], Tel. (+31) 534893155


38

Hydrogen splitting studied by H-D Exchange M. Johansson Danish National Research Foundation's Center for Individual Nanoparticle Functionality (CINF), Department of Physics, Technical University of Denmark (DTU), 2800 Lyngby, Denmark

It is shown how the H-D exchange reaction can be utilized to measure the splitting rate of hydrogen at 1 bar. Examples of results are given for transition metals, metal alloys and supported metal nano-particles.

Introduction The H-D exchange reaction offers a convenient way to measure the rate of hydrogen splitting on surfaces at atmospheric pressure. Such measurements can help understanding the anode processes of the Proton Exchange Membrane Fuel Cell (PEMFC), and could potentially be useful when designing systems for storage of hydrogen in solids.

Experimental The sticking probability for hydrogen, S, is calculated from the rate of the H-D exchange reaction, which is measured with laterally resolved mass spectrometry. A simple model is used in order to extract S from the rate data [1]. It is assumed that S is the same for H2, HD and D2, and that the desorption rates are second order in the coverages of H and D. The apparent activation energy for hydrogen desorption, Eapp, can be obtained from data for S as a function of temperature.

Results With the anode reaction of the PEMFC in mind, an investigation was carried out for a number of transition metals, deposited as 50 Å films on Highly Ordered Pyrolytic Graphite (HOPG). The values for S and Eapp are in reasonable agreement with literature values obtained at high hydrogen coverage under vacuum conditions [2]. Metals which bind hydrogen strongly would be expected to have a high coverage of H, and thus to give a low value for S. However, Ru and Rh bind H stronger than Pt, but give larger values for S.

The fact that Pt works better than Ru and Rh in a PEMFC may be due to Ru and Rh being oxidized under typical operating conditions. S was also measured with 10 ppm CO added to the hydrogen. The influence of CO on S is strongest for Ir and Pt. These metals show the largest difference between the heats of adsorption for CO and H [3]. S for Pt is significantly more sensitive to CO than the current of a PEMFC with Pt as anode catalyst. This indicates that reactions which consume CO are important for the resistance to CO poisoning of a real PEMFC. Investigations were also carried out for Ru and Pt nano-particles produced by metal evaporation onto sputtered HOPG. There is a maximum in S for a particle diameter, d, of about 3 nm for Ru, but for Pt, S increases with decreasing d. Eapp increases with decreasing d for both Pt and Ru, but the effect is more pronounced for Ru. Presently, metal alloys such as Pt/Bi and Pt/Ru are investigated in order to study the ligand effect separately from the bi-functional effect. So far, it seems that a surface which contains about equal amounts of Pt and Ru gives an even higher value for S than pure Ru, whereas no improvement in S was obtained by adding Bi to a Pt surface.

References [1] M. Johansson, O. Lytken, I. Chorkendorff,

Topics in Catalysis 46 (2007) 175. [2] M. Johansson, O. Lytken, I. Chorkendorff, J.

Chem. Phys. 128 (2008) 034706. [3] M. Johansson, O. Lytken, I. Chorkendorff,

Surf. Sci. 602 (2008) 1863.

Corresponding author: Martin Johansson, email: [email protected], Tel. (+45) 45253191


39

Experimental electronic charge density of LiBD4/LiBH4 from maximum entropy method F. Buchter1, Z. Lodziana1, A. Remhof1, Ph. Mauron1, A. Züttel1, Y. Filinchuk2, T. Noritake3, S. Orimo4 1Empa Materials Science and Technology, Dept. Energy, Environment & Mobility, Sec. Hydrogen & Energy, CH-8600 Dübendorf, Switzerland 2Swiss-Norwegian Beamline, ESRF, BP220, Grenoble Cedex, France. 3Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan. 4Institute for Materials Research, Tohoku University, 980-8577 Sendai, Japan.

Combining neutron diffraction data and X-ray diffraction data, the experimental charge density distribution using the maximum entropy method has been determined for LiBD4/LiBH4. This method gives a fundamental information about the charge transfer and bonding nature between the different atoms within the compound and can be directly compared to ab-initio calculations of the charge density. Introduction Alkali and alkali-earth complex hydrides are promising hydrogen storage materials, but desorb hydrogen at a high temperature and pressure level, due to either their thermodynamical stability or a kinetic barrier. In order to tailor the temperature/pressure level to suitable conditions, it is important to understand their mechanism of formation/decomposition. An accurate experimental investigation of the electronic charge density distribution of the complex hydrides give important information about the charge transfer and bonding nature between the different atoms within the compounds.

Experimental Combining neutron diffraction data for the refinement of the structural model, and subsequently X-ray diffraction data for the structure factor extraction, allows the calculation of the experimental charge density distribution1 (see Figure 1). Due to the limited number of extracted structure factors, the maximum entr

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