RESEARCH NEWS
November 200430
Advances in Hydrogen Storage Materials
Sensors have become a very active
area for advanced materials
development with just under 150
papers on the subject presented at the
recent American Chemical Society
National Meeting.
Researchers from Northwestern
University are developing fluorescent
sensors based on telechelic attachment
of pyrenyl dyes to polydimethylsiloxane
(PDMS). The resulting material, Py-
PDMS-Py, shows fluorescence behavior
suitable for use as an oxygen sensor.
Bradley Holliday and Timothy M. Swager
of Massachusetts Institute of
Technology described the incorporation
of coordinatively unsaturated transition
metal centers into the backbone of a
conducting polymer. This ‘wiring’ of
transition metal receptors in series
results in sensory signal amplification.
By wiring receptors in series with a
conjugated polymer, activation of a
small fraction of the receptors by a
target analyte causes a substantial
reduction in the overall signal, resulting
in very high sensitivity. In resistivity
sensors, binding of a target molecule
anywhere along these molecular wires
disrupts the relatively high conductivity
of the wire by inducing a large increase
in resistance.
Nanowires of LiMo3Se3 condensation
polymers based on triangular Mo3Se3
units provide another type of electrical
resistivity sensor. These nanowires
have a diameter of only 0.85 nm,
exhibit metallic conductivity, and are
soluble in polar organic solvents and in
water, where they form 4 nm thick
bundles containing seven to ten
individual strands. Frank Osterloh of the
University of California, Davis reported
that nanometer-thick films of the
nanowire bundles undergo a reversible
resistivity increase (up to 240%) upon
exposure to solvent vapors. This occurs
because solvent molecules bind to the
wire surface affecting the number and
volatility of the conduction electrons.
The nanowires could, therefore, make
highly sensitive chemical sensors.
Modifying the nanowires by covalently
attaching different alkyl groups allows
specific chemicals to be analyzed.
John K. Borchardt
New materials for improved sensors
Hydrogen-fueled vehicles are being developed to reduce airpollution. Hydrogen storage systems in these vehicles willrequire high energy density and specific energy to achievecomparable performance to conventional gasoline-fueledvehicles. However, the low density of hydrogen moleculesmakes efficient hydrogen storage a challenge. The USDepartment of Energy has set a goal of a 300 mile drivingrange without refueling in its funding plans for research onmaterials for hydrogen storage. Advanced materials underdevelopment to meet this goal include metal hydrides, alloys,intermetallics, sodium and lithium alanates, nanocubes, andcarbon-based materials. Some recent research on the topicwas reported at the 228th National Meeting of the AmericanChemical Society in Philadelphia. Sodium aluminum hydride (NaAlH4) doped with 2% Ti is apromising hydrogen storage material, according toBrookhaven National Laboratory researchers SantanuChaudhuri, Ping Liu, and James Muckerman. They havestudied the multistep hydrogen absorption-desorption cycle.The energetics indicate that an intermediate perovskite
phase, Na3AlH6, is less reactive compared to the endproducts of the hydrogen desorption cycle, sodium hydride,and aluminum. The NaH surface doped with Ti promotesexothermic dissociative absorption of molecular hydrogen (asshown). Computational results indicate NaH {001} surfacesdoped with Ti play a key role promoting exothermicdissociative absorption of molecular hydrogen. Densityfunctional theory is being used to understand therelationships between electronic structure, oxidation state,defects, and hydrogen storage efficiency.A new class of highly porous materials constructed fromoctahedral Zn4O(CO2)6 clusters linked by benzene ring-containing organic units has been developed by Omar M.Yaghi’s group at the University of Michigan and shows greatpromise. The three-dimensional organic linking units arefunctionalized with various organic groups. Long linking unitssuch tetrahydropyrene, tetramethybenzene, triphenylbenzene,and 1,3,5-benzenetribenzoate provide larger pores. The 1,3,5-benzenetribenzoate linking unit provides exceptionally highsurface area (4500 m2/g), a critical design consideration forhydrogen storage materials. Maximum hydrogen uptakecorrelates with the number of organic rings per formula unit:0.9-1.6% by weight. At 77 K, complete hydrogen uptake andrelease can be achieved in minutes. State University of New York at Binghamton researchers arealso studying the use of organic linkers for the preparation ofmetal-organic framework materials using an anionictemplating synthesis methodology. The most promising newmaterial for hydrogen storage developed to date is a cationic,two-dimensional layered material, Pb3F5NO3 stable to 450°C.This material’s interlayer nitrate groups are exchanged forchromate, dichromate, perrhenate, permanganate, benzoate,and terephthalate under ambient aqueous conditions. John K. Borchardt
ACS CONFERENCE REPORT
Al{001} with two next nearest neighbor Ti atoms dissociates hydrogen: (a) the surface
before reaction and (b) after the reaction showing the Ti-H-Al bridges. (Courtesy of
Santanu Chaudhuri of Brookhaven National Laboratory.)
ACS CONFERENCE REPORT
(a) (b)
