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Polymer electrolyte improves fuel cell performance
RESEARCH NEWS
C60 molecules show atom-like states CHARACTERIZATION
Atom-like molecular electronic orbitals can be found bound to the hollow core of buckminsterfullerene (C60) cages in addition to the molecular orbitals present in the carbon framework, say researchers from the University of Pittsburgh and the Donostia International Physics Center in Spain [Feng et al., Science (2008) 320, 359]. The ‘superatom molecular orbitals’ (SAMOs), as they have been named by the team, bring to mind the spherical s, p, and d symmetries of atomic orbitals. “Such orbitals should be a common feature of hollow molecules such as C60, carbon nanotubes, and others derived by wrapping or rolling of molecular sheets”, says Hrvoje Petek of the University of Pittsburgh. The team used scanning tunneling microscopy (STM) and density functional theory calculations to study the electronic structure of single C60 molecules, onedimensional C60 quantum wires, and two-dimensional C60 assemblies. At low temperatures and using a STM bias greater than 3.5 V, a transition from a π molecular orbital to nearly free-electron (NFE)-like character can be observed in these molecules.
BIOMATERIALS
the assembled multilayer films show low methanol permeability. This makes the films of immediate interest in direct methanol fuel cells (DMFCs). Nafion is the standard membrane material used in DMFCs, but it is highly permeable to methanol, hindering the working of the cell and significantly reducing its performance. “We have shown that incorporating these LBL materials into a DMFC improves power output by over 50%,” Hammond says. While the LBL films were previously grown on a Nafion substrate, the group has now successfully produced standalone films (shown), Hammond tells Materials Today.
The exquisite structures generated out of silica by aquatic creatures such as sponges, diatoms, and radiolaria are nucleated and templated by organic molecules, which guide the biosilification that leads to their unique shapes. Now researchers at Nottingham Trent University, UK, and the Limnological Institute in Russia have developed a bimimetic strategy to fabricate silica particles using polyamines to control the growth [Belton et al., Proc. Natl. Acad. Sci. USA (2008) 105, 5963]. The team studied the catalytic activity of naturally occurring polyamines spermine and spermidine and several synthetic polyamines to speed up the condensation of a monosilicic acid from supersaturated solution. The hydrocarbon content of polyamines and partial charging of the nitrogen groups makes these molecules partially hydrophobic, says team leader Carole C. Perry of Nottigham Trent. Polyamines can form emulsions with positively charged amine groups, enhancing the rate of condensation of monosilicic acid to silica in solution. “This causes the phase separation of the polyamines in the form of microemulsions containing droplets of some hundreds of nanometers in diameter. These then act as nano reaction vessels and as the monosilicic acid molecules and anions diffuse into them, they condense to form the hollow silicate spheres,” Perry explains. The ability of polyamines to form these droplets is dependent on the level of charging and their hydrocarbon content. The team also observed a marked increase in the kinetics when using amines separated from each other by three carbon atoms along the backbone of the polymer.
Jonathan Wood
Mark E. Greene
Calculation of a delocalized atom-like orbital on a one-dimensional wire of C60 molecules. (Courtesy of Jin Zhao.)
“Our research shows that hollow molecules with atom-like orbitals can achieve sufficient wave function overlap for electrons to delocalize over a molecular assembly as would free electrons in metals,” adds Petek. “If this could happen close to the Fermi level, such molecules could conduct like metals.” The team will investigate how specific modifications of C60 molecules lower the energy of superatom orbitals, in order to bring them within the reach of practical applications, explains Petek. Katerina Busuttil
Polymer electrolyte improves fuel cell performance ENERGY The high ionic conductivity of novel polymer electrolyte thin films developed by scientists at Massachusetts Institute of Technology (MIT) promises much for improved fuel cells, batteries, and dye-sensitized solar cells [Argun et al., Adv. Mater. (2008) 20, 1539]. Such electrochemical devices are dependent on electrolytes to facilitate charge transport between electrodes. But processing difficulties and safety concerns with liquid or gel electrolytes has tended to limit widespread use of these devices. In contrast, polymer electrolytes can provide mechanical strength and more flexibility in manufacture, but they must still offer fast ion conduction. Paula T. Hammond and colleagues have used the versatility of layer-by-layer (LBL) assembly to build thin films of two different polymers that when brought together offer both high conductivity and mechanical stability. “We have generated an LBL-assembled solid-state thin film with ionic conductivity values over three orders of magnitude higher than the previous best performing multilayer films,” says Hammond. “These new ionic conductors have proton conductivities close to those of much more expensive specialized polymers, making this technique a competitor for fuel cell and other solid-state electrolyte applications.” The team found that by sulfonating a thermally and mechanically stable aromatic polyether and pairing it with a
8
MT1106p7_15.indd 8
Understanding biosilification
A 5 µm thick standalone film suitable for DMFCs that has been assembled by layer-by-layer deposition. (Courtesy of Paula Hammond.) complementary functionalized polymer, they could achieve ionic conductivity values up to 35.3 mS cm–1. Furthermore,
JUNE 2008 | VOLUME 11 | NUMBER 6
09/05/2008 11:59:41
C60 molecules show atom-like states CHARACTERIZATION
Atom-like molecular electronic orbitals can be found bound to the hollow core of buckminsterfullerene (C60) cages in addition to the molecular orbitals present in the carbon framework, say researchers from the University of Pittsburgh and the Donostia International Physics Center in Spain [Feng et al., Science (2008) 320, 359]. The ‘superatom molecular orbitals’ (SAMOs), as they have been named by the team, bring to mind the spherical s, p, and d symmetries of atomic orbitals. “Such orbitals should be a common feature of hollow molecules such as C60, carbon nanotubes, and others derived by wrapping or rolling of molecular sheets”, says Hrvoje Petek of the University of Pittsburgh. The team used scanning tunneling microscopy (STM) and density functional theory calculations to study the electronic structure of single C60 molecules, onedimensional C60 quantum wires, and two-dimensional C60 assemblies. At low temperatures and using a STM bias greater than 3.5 V, a transition from a π molecular orbital to nearly free-electron (NFE)-like character can be observed in these molecules.
BIOMATERIALS
the assembled multilayer films show low methanol permeability. This makes the films of immediate interest in direct methanol fuel cells (DMFCs). Nafion is the standard membrane material used in DMFCs, but it is highly permeable to methanol, hindering the working of the cell and significantly reducing its performance. “We have shown that incorporating these LBL materials into a DMFC improves power output by over 50%,” Hammond says. While the LBL films were previously grown on a Nafion substrate, the group has now successfully produced standalone films (shown), Hammond tells Materials Today.
The exquisite structures generated out of silica by aquatic creatures such as sponges, diatoms, and radiolaria are nucleated and templated by organic molecules, which guide the biosilification that leads to their unique shapes. Now researchers at Nottingham Trent University, UK, and the Limnological Institute in Russia have developed a bimimetic strategy to fabricate silica particles using polyamines to control the growth [Belton et al., Proc. Natl. Acad. Sci. USA (2008) 105, 5963]. The team studied the catalytic activity of naturally occurring polyamines spermine and spermidine and several synthetic polyamines to speed up the condensation of a monosilicic acid from supersaturated solution. The hydrocarbon content of polyamines and partial charging of the nitrogen groups makes these molecules partially hydrophobic, says team leader Carole C. Perry of Nottigham Trent. Polyamines can form emulsions with positively charged amine groups, enhancing the rate of condensation of monosilicic acid to silica in solution. “This causes the phase separation of the polyamines in the form of microemulsions containing droplets of some hundreds of nanometers in diameter. These then act as nano reaction vessels and as the monosilicic acid molecules and anions diffuse into them, they condense to form the hollow silicate spheres,” Perry explains. The ability of polyamines to form these droplets is dependent on the level of charging and their hydrocarbon content. The team also observed a marked increase in the kinetics when using amines separated from each other by three carbon atoms along the backbone of the polymer.
Jonathan Wood
Mark E. Greene
Calculation of a delocalized atom-like orbital on a one-dimensional wire of C60 molecules. (Courtesy of Jin Zhao.)
“Our research shows that hollow molecules with atom-like orbitals can achieve sufficient wave function overlap for electrons to delocalize over a molecular assembly as would free electrons in metals,” adds Petek. “If this could happen close to the Fermi level, such molecules could conduct like metals.” The team will investigate how specific modifications of C60 molecules lower the energy of superatom orbitals, in order to bring them within the reach of practical applications, explains Petek. Katerina Busuttil
Polymer electrolyte improves fuel cell performance ENERGY The high ionic conductivity of novel polymer electrolyte thin films developed by scientists at Massachusetts Institute of Technology (MIT) promises much for improved fuel cells, batteries, and dye-sensitized solar cells [Argun et al., Adv. Mater. (2008) 20, 1539]. Such electrochemical devices are dependent on electrolytes to facilitate charge transport between electrodes. But processing difficulties and safety concerns with liquid or gel electrolytes has tended to limit widespread use of these devices. In contrast, polymer electrolytes can provide mechanical strength and more flexibility in manufacture, but they must still offer fast ion conduction. Paula T. Hammond and colleagues have used the versatility of layer-by-layer (LBL) assembly to build thin films of two different polymers that when brought together offer both high conductivity and mechanical stability. “We have generated an LBL-assembled solid-state thin film with ionic conductivity values over three orders of magnitude higher than the previous best performing multilayer films,” says Hammond. “These new ionic conductors have proton conductivities close to those of much more expensive specialized polymers, making this technique a competitor for fuel cell and other solid-state electrolyte applications.” The team found that by sulfonating a thermally and mechanically stable aromatic polyether and pairing it with a
8
MT1106p7_15.indd 8
Understanding biosilification
A 5 µm thick standalone film suitable for DMFCs that has been assembled by layer-by-layer deposition. (Courtesy of Paula Hammond.) complementary functionalized polymer, they could achieve ionic conductivity values up to 35.3 mS cm–1. Furthermore,
JUNE 2008 | VOLUME 11 | NUMBER 6
09/05/2008 11:59:41