RESEARCH NEWS

April 200412

Prashant V. Kamat and colleagues

from the University of Notre Dame and

Indiana University Northwest have

shown that fullerene clusters can

improve the performance of electrodes

for direct methanol fuel cells (DMFCs)

[Vinodgopal et al., Nano Lett. (2004)

doi: 10.1021/nl035028y].

“Most DMFC anodes, where the

oxidation of methanol is carried out,

are based on electrocatalysts,”

explains Kamat, “in which metal

nanoparticles such as a 1:1 mixture of

Pt and Ru are deposited on electrically

conducting, high-surface area carbon

films.”

Making smaller fuel cells for portable

electronic devices will require

submicron-sized carbon electrode

supports that minimize the use of

noble metals but retain suitable

catalytic activity. “Carbon nanotubes

and fullerene nanoclusters are the

most suitable candidates for designing

such miniaturized electrodes,” says

Kamat. Films of C60 nanoclusters are

stable to oxidative potentials, highly

porous, exhibit electrocatalytic

properties, and have a high surface

area, all of which could make them

useful for fuel cell applications.

The researchers deposited C60

clusters onto optically transparent

electrodes using electrophoresis to

form a nanostructured film. Pt

nanoparticles were then loaded onto

the C60 film by electrodeposition. This

electrode was tested for methanol

oxidation in a half-cell reaction. The

C60 clusters promote methanol

oxidation at the Pt crystallites. Kamat

attributes the enhancement to the

high-surface area provided by the C60

support. “We are currently working

towards optimizing the performance of

fullerene and carbon nanotube-based

electrodes,” says Kamat.

Jonathan Wood

C60 boostsperformanceNANOTECHNOLOGY

Fuel cells, which react hydrogen and oxygen toproduce energy, are being considered for use inmany applications including to power automobiles.Fuel cell production is projected to grow at morethan 40% annually over the next decade. Theproton-conducting polymer electrolyte membrane(PEM) is one of the main components in fuel cells.Improved membrane performance, particularly athigh temperatures, is needed and intensiveresearch efforts are under way worldwide.Polybenzimidazole’s (PBI) excellent high temperatureproperties have led to its use in demandingapplications. Now researchers from the Centre forElectrochemical Technologies and CEGASA in Spainreport the preparation of proton-conducting polymerelectrolytes based on new porous films of PBI dopedwith phosphoric acid [Mecerreyes et al., Chem.Mater. (2004) 16, 604]. The porous PBI films are prepared by leaching out alow-molecular-weight nonpolymeric compound, calleda porogen, from the film using a selective solventthat dissolves the porogen but not the PBI.

(A porogen is a space-filling material that resistspolymerization, remains suspended in thepolymerization reaction mixture, is dispersed in filmsformed after the polymerization, and can be leachedfrom the polymeric film after formation.) Thismethod allows film porosity levels as great as 75%to be obtained. Scanning electron microscopyindicates the film pore size and morphology stronglydepend on the porogen:PBI ratio. Spherical poresless than 100 nm in size can be obtained at lowporosity, while interconnected void spaces as longas 10-15 µm can be obtained at the highest levels. The researchers evaluated different porogens andfound that pore size decreases in the order dibutylphthalate > dimethyl phthalate > diphenyl phthalate> triphenyl phosphate. Acid uptake of the PBImembranes and, therefore, the ionic conductivity ofthe films increases with porosity. Mechanicallystable membranes with ionic conductivity as high as5 × 10-2 S/cm can be obtained by soaking thehighly porous films in phosphoric acid solutions.John K. Borchardt

Improved polymer membranes for fuel cellsPOLYMERS

Record-breaker on the surfacePOROUS MATERIALS

A new metal-organic framework (MOF) with arecord-breaking surface area has beensynthesized by researchers at The Universityof Michigan, Ann Arbor and Arizona StateUniversity [Chae et al., Nature (2004) 442277,523].Porous materials that have structures withvery high surface areas are important tomany industrial applications, includingcatalysis, separation, and gas storage. “So far, zeolites have been the mostimportant class of materials for theseapplications,” says Adam J. Matzger of TheUniversity of Michigan. The largest recordedsurface area of a zeolite is 904 m2g-1. MOFshave surpassed this value, with the previousbest reaching 3000 m2g-1 (Rosi et al.,Science (2003) 330000, 1127; Materials Today(2003) 66 (7/8) 12). The new material developed by the Michigan-Arizona team, named MOF-177, has asurface area of 4500 m2g-1. The orderedstructure of MOF-177 has extremely largepores and shows stability in the absence ofadsorbed guest species.Block-shaped crystals of MOF-177 were

prepared by combining the carboxylatederivative, 1,3,5-benzenetribenzoate (BTB),which is a triangular unit, with zinc (II) oxidecarboxylate clusters, an octahedral unit. X-ray diffraction confirms the open structureof the Zn4O(BTB)2 crystals in which each zincacetate cluster is linked to six BTB units.The largest pores are calculated to havediameters of 10.8 Å and 11.8 Å. Gassorption studies using N2 show that thisspace is accessible to guest species. “Having large and regular pores is importantin large molecule separations and forcarrying out chemical transformations onthese molecules,” explains Matzger. Theresearchers show that the size of the poresis spacious enough to allow large polycyclicorganic dyes to be adsorbed.The group now intends to concentrate onapplications for MOF-177. “More and moreattention, both by the team at Michigan andin industry,” says Matzger, “will be focusedon moving applications forward in areas suchas hydrogen storage and large moleculeseparations.”Jonathan Wood

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