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  • This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 16955–16972 16955

    Cite this: Phys. Chem. Chem. Phys., 2011, 13, 16955–16972

    A multifaceted approach to hydrogen storagew

    Andrew J. Churchard,*a Ewa Banach,b Andreas Borgschulte,c Riccarda Caputo,cd

    Jian-Cheng Chen,e David Clary,f Karol J. Fijalkowski,g Hans Geerlings,bh

    Radostina V. Genova,a Wojciech Grochala,ag Tomasz Jaroń,g

    Juan Carlos Juanes-Marcos,e Bengt Kasemo,i Geert-Jan Kroes,e Ivan Ljubić,f j

    Nicola Naujoks,i Jens K. Nørskov,kl Roar A. Olsen,e Flavio Pendolino,c

    Arndt Remhof,c Loránd Románszki,i Adem Tekin,mn Tejs Vegge,m Michael Zächi

    and Andreas Züttelc

    Received 15th July 2011, Accepted 10th August 2011

    DOI: 10.1039/c1cp22312g

    The widespread adoption of hydrogen as an energy carrier could bring significant benefits, but

    only if a number of currently intractable problems can be overcome. Not the least of these is the

    problem of storage, particularly when aimed at use onboard light-vehicles. The aim of this

    overview is to look in depth at a number of areas linked by the recently concluded HYDROGEN

    research network, representing an intentionally multi-faceted selection with the goal of advancing

    the field on a number of fronts simultaneously. For the general reader we provide a concise

    outline of the main approaches to storing hydrogen before moving on to detailed reviews of

    recent research in the solid chemical storage of hydrogen, and so provide an entry point for the

    interested reader on these diverse topics. The subjects covered include: the mechanisms of Ti

    catalysis in alanates; the kinetics of the borohydrides and the resulting limitations; novel

    transition metal catalysts for use with complex hydrides; less common borohydrides;

    protic-hydridic stores; metal ammines and novel approaches to nano-confined metal hydrides.

    Introduction

    The realisation of the hydrogen economy requires solutions to

    a number of problems involving production, transportation

    and fuel cells, but despite the significant progress made in the

    last decade, storage remains the key barrier to the implemen-

    tation of the hydrogen economy1 in light vehicles.

    The source of this barrier is the prevailing idea that

    consumers will not accept diminished performance compared

    to their fossil fuel powered cars, and that any replacement

    must therefore at least match the latter’s driving range,

    re-fuelling time, durability, price and safety. By working back

    from the current performance levels, the U.S. Department of

    Energy (DOE) developed targets that a hydrogen storage

    system would have to meet to be considered a replacement.

    The targets, which are widely worked to, were released by the

    DOE in 2003 and then revised in 20092 (see Table 1) to reflect

    the more accurate data gathered in the meantime from proto-

    type hydrogen powered vehicles. Though some parameters

    tend to get more attention than others in the literature

    a Interdisciplinary Centre for Mathematical and Computational Modelling, The University of Warsaw, Pawińskiego 5a, 02106 Warsaw, Poland. E-mail: achurch@icm.edu.pl

    b Shell Global Solutions International B.V., Grasweg 31, 1031 HW Amsterdam, The Netherlands c Empa, Materials Science and Technology, Hydrogen and Energy, Überlandstrasse 129, 8600 Dübendorf, Switzerland d ETH Swiss Federal Institute of Technology Zurich, Department of Chemistry and Applied Biosciences, CH-8093 Zurich, Switzerland e Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands f Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Rd., Oxford OX1 3QZ, UK gFaculty of Chemistry, The University of Warsaw, Pasteura 1, 02093 Warsaw, Poland hDelft University of Technology, Faculty of Applied Sciences, Department of Chemical Engineering, P.O. Box 5045, 2600 GA Delft, The Netherlands

    i Chalmers University of Technology, Department of Applied Physics, 41296 Göteborg, Sweden j Department of Physical Chemistry, Ru:er Bošković Institute, Bijenička cesta 54, P.O. Box 180, HR-10002, Zagreb, Republic of Croatia k SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, MenloPark, CA 94025, USA lDepartment of Chemical Engineering, Stanford University, Stanford, CA 94305, USA mRisø National Laboratory for Sustainable Energy and Center for Atomic-scale Materials Design (CAMD), Technical University of Denmark, Denmark

    n Informatics Institute, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey w Electronic supplementary information (ESI) available. See DOI: 10.1039/c1cp22312g

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    http://dx.doi.org/10.1039/c1cp22312g http://dx.doi.org/10.1039/c1cp22312g http://dx.doi.org/10.1039/c1cp22312g

  • 16956 Phys. Chem. Chem. Phys., 2011, 13, 16955–16972 This journal is c the Owner Societies 2011

    (especially gravimetric and volumetric capacity), the DOE

    states that all requirements should be met if the implementa-

    tion of the material as a hydrogen carrier is to be successful.

    The field of hydrogen storage is vast. Indicative of the effort

    that has gone into solving the hydrogen storage problem are

    the many, many systems that have been tested, but broadly,

    each can be placed into one of three categories:

    � physical containment (e.g. compression and liquefaction) � physisorption (e.g. adsorption of H2 onto the surface of

    highly porous materials)

    � chemical bonding (e.g. metal hydrides, ammonia) This paper will provide a brief outline of each of these

    groupings before focusing on the last of them. However, it is

    not the intention to cover every aspect, but rather to delve

    deeper into a number of areas that were covered by an

    international research network, fittingly titled HYDROGEN,3

    and we direct the reader to a number of excellent reviews for a

    wider introduction to the topic.4–8

    The first category, physical containment, is perhaps the

    most obvious. Simply storing hydrogen as a compressed

    gas has two important factors to recommend it: the ready

    availability of the hydrogen to the fuel cell and the relative

    simplicity and maturity of the technology involved. For these

    reasons, many of the prototype and demonstration vehicles

    have used compressed gas cylinders, and much work has gone

    into optimising design and reducing costs. Modern cylinders

    store H2 at 350 or 700 bar using light carbon fibre–resin

    composites to provide the required tensile strength with

    either polymer or metal internal liners to act as diffusion

    barriers and a third, protective material on the outer surface.

    Unfortunately the non-ideal behaviour of H2 as a gas

    (progressively less volume reduction is gained per unit increase

    in pressure) means that (at ambient temperature) the ultimate

    target (70 g L�1) is physically not achievable, even ignoring the

    mass of the containment system. To compound the problem,

    higher pressure tanks are significantly heavier, requiring a

    compromise between the gravimetric and the volumetric

    capacity. The conclusions from a comprehensive study were

    that neither 350 bar nor 700 bar tanks would meet the DOE’s

    2015 or ultimate targets.9

    Liquefaction of H2 by cooling to about 20 K is just about

    able to meet the volumetric targets that compressed gas fails,

    if the volume of the tank itself is ignored. As these systems rely

    on open tanks to prevent a build-up of dangerous pressures,

    prevention of boil-off of H2 must be carefully managed.

    However, liquid H2 does not present a realistic solution for

    on-board storage due to the excessive energy cost of producing

    liquid hydrogen (an equivalent of 30% of the fuel’s energy is

    used in the process).

    Cryo-compression, however, marries these two inadequate

    technologies to offer a distinctly promising alternative.

    Fuelling with either liquid hydrogen or cooled, compressed

    H2, (likely a supercritical fluid) increases the volumetric

    capacity, whilst the ability to withstand high pressures reduces

    losses from boil-off. With a minimum of one short journey

    every two days, losses can be all but eliminated and if left for

    several days, there will still be sufficient compressed gas in the

    tank to allow the car to be driven a considerable distance.10

    Perhaps even better, however, the system leaves the choice

    with the consumer: it may be filled by either cheaper ambient

    temperature compressed gas, which provides a lower range

    but may be sufficient for the driver’s immediate needs, or

    more expensive liquid hydrogen if a longer trip is planned with

    a wish to avoid