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)