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Record-breaker on the surface
mt0704pg7-23.qxd
09/03/2004
09:47
Page 12
RESEARCH NEWS
Record-breaker on the surface POROUS MATERIALS
A new metal-organic framework (MOF) with a record-breaking surface area has been synthesized by researchers at The University of Michigan, Ann Arbor and Arizona State University [Chae et al., Nature (2004) 427, 523]. Porous materials that have structures with very high surface areas are important to many industrial applications, including catalysis, separation, and gas storage. “So far, zeolites have been the most important class of materials for these applications,” says Adam J. Matzger of The University of Michigan. The largest recorded surface area of a zeolite is 904 m2g-1. MOFs have surpassed this value, with the previous best reaching 3000 m2g-1 (Rosi et al., Science (2003) 300, 1127; Materials Today (2003) 6 (7/8) 12). The new material developed by the MichiganArizona team, named MOF-177, has a surface area of 4500 m2g-1. The ordered structure of MOF-177 has extremely large pores and shows stability in the absence of adsorbed guest species. Block-shaped crystals of MOF-177 were
prepared by combining the carboxylate derivative, 1,3,5-benzenetribenzoate (BTB), which is a triangular unit, with zinc (II) oxide carboxylate clusters, an octahedral unit. X-ray diffraction confirms the open structure of the Zn4O(BTB)2 crystals in which each zinc acetate cluster is linked to six BTB units. The largest pores are calculated to have diameters of 10.8 Å and 11.8 Å. Gas sorption studies using N2 show that this space is accessible to guest species. “Having large and regular pores is important in large molecule separations and for carrying out chemical transformations on these molecules,” explains Matzger. The researchers show that the size of the pores is spacious enough to allow large polycyclic organic dyes to be adsorbed. The group now intends to concentrate on applications for MOF-177. “More and more attention, both by the team at Michigan and in industry,” says Matzger, “will be focused on moving applications forward in areas such as hydrogen storage and large molecule separations.” Jonathan Wood
Improved polymer membranes for fuel cells POLYMERS Fuel cells, which react hydrogen and oxygen to produce energy, are being considered for use in many applications including to power automobiles. Fuel cell production is projected to grow at more than 40% annually over the next decade. The proton-conducting polymer electrolyte membrane (PEM) is one of the main components in fuel cells. Improved membrane performance, particularly at high temperatures, is needed and intensive research efforts are under way worldwide. Polybenzimidazole’s (PBI) excellent high temperature properties have led to its use in demanding applications. Now researchers from the Centre for Electrochemical Technologies and CEGASA in Spain report the preparation of proton-conducting polymer electrolytes based on new porous films of PBI doped with phosphoric acid [Mecerreyes et al., Chem. Mater. (2004) 16, 604]. The porous PBI films are prepared by leaching out a low-molecular-weight nonpolymeric compound, called a porogen, from the film using a selective solvent that dissolves the porogen but not the PBI.
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April 2004
(A porogen is a space-filling material that resists polymerization, remains suspended in the polymerization reaction mixture, is dispersed in films formed after the polymerization, and can be leached from the polymeric film after formation.) This method allows film porosity levels as great as 75% to be obtained. Scanning electron microscopy indicates the film pore size and morphology strongly depend on the porogen:PBI ratio. Spherical pores less than 100 nm in size can be obtained at low porosity, while interconnected void spaces as long as 10-15 µm can be obtained at the highest levels. The researchers evaluated different porogens and found that pore size decreases in the order dibutyl phthalate > dimethyl phthalate > diphenyl phthalate > triphenyl phosphate. Acid uptake of the PBI membranes and, therefore, the ionic conductivity of the films increases with porosity. Mechanically stable membranes with ionic conductivity as high as 5 × 10-2 S/cm can be obtained by soaking the highly porous films in phosphoric acid solutions. John K. Borchardt
C60 boosts performance NANOTECHNOLOGY 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
09/03/2004
09:47
Page 12
RESEARCH NEWS
Record-breaker on the surface POROUS MATERIALS
A new metal-organic framework (MOF) with a record-breaking surface area has been synthesized by researchers at The University of Michigan, Ann Arbor and Arizona State University [Chae et al., Nature (2004) 427, 523]. Porous materials that have structures with very high surface areas are important to many industrial applications, including catalysis, separation, and gas storage. “So far, zeolites have been the most important class of materials for these applications,” says Adam J. Matzger of The University of Michigan. The largest recorded surface area of a zeolite is 904 m2g-1. MOFs have surpassed this value, with the previous best reaching 3000 m2g-1 (Rosi et al., Science (2003) 300, 1127; Materials Today (2003) 6 (7/8) 12). The new material developed by the MichiganArizona team, named MOF-177, has a surface area of 4500 m2g-1. The ordered structure of MOF-177 has extremely large pores and shows stability in the absence of adsorbed guest species. Block-shaped crystals of MOF-177 were
prepared by combining the carboxylate derivative, 1,3,5-benzenetribenzoate (BTB), which is a triangular unit, with zinc (II) oxide carboxylate clusters, an octahedral unit. X-ray diffraction confirms the open structure of the Zn4O(BTB)2 crystals in which each zinc acetate cluster is linked to six BTB units. The largest pores are calculated to have diameters of 10.8 Å and 11.8 Å. Gas sorption studies using N2 show that this space is accessible to guest species. “Having large and regular pores is important in large molecule separations and for carrying out chemical transformations on these molecules,” explains Matzger. The researchers show that the size of the pores is spacious enough to allow large polycyclic organic dyes to be adsorbed. The group now intends to concentrate on applications for MOF-177. “More and more attention, both by the team at Michigan and in industry,” says Matzger, “will be focused on moving applications forward in areas such as hydrogen storage and large molecule separations.” Jonathan Wood
Improved polymer membranes for fuel cells POLYMERS Fuel cells, which react hydrogen and oxygen to produce energy, are being considered for use in many applications including to power automobiles. Fuel cell production is projected to grow at more than 40% annually over the next decade. The proton-conducting polymer electrolyte membrane (PEM) is one of the main components in fuel cells. Improved membrane performance, particularly at high temperatures, is needed and intensive research efforts are under way worldwide. Polybenzimidazole’s (PBI) excellent high temperature properties have led to its use in demanding applications. Now researchers from the Centre for Electrochemical Technologies and CEGASA in Spain report the preparation of proton-conducting polymer electrolytes based on new porous films of PBI doped with phosphoric acid [Mecerreyes et al., Chem. Mater. (2004) 16, 604]. The porous PBI films are prepared by leaching out a low-molecular-weight nonpolymeric compound, called a porogen, from the film using a selective solvent that dissolves the porogen but not the PBI.
12
April 2004
(A porogen is a space-filling material that resists polymerization, remains suspended in the polymerization reaction mixture, is dispersed in films formed after the polymerization, and can be leached from the polymeric film after formation.) This method allows film porosity levels as great as 75% to be obtained. Scanning electron microscopy indicates the film pore size and morphology strongly depend on the porogen:PBI ratio. Spherical pores less than 100 nm in size can be obtained at low porosity, while interconnected void spaces as long as 10-15 µm can be obtained at the highest levels. The researchers evaluated different porogens and found that pore size decreases in the order dibutyl phthalate > dimethyl phthalate > diphenyl phthalate > triphenyl phosphate. Acid uptake of the PBI membranes and, therefore, the ionic conductivity of the films increases with porosity. Mechanically stable membranes with ionic conductivity as high as 5 × 10-2 S/cm can be obtained by soaking the highly porous films in phosphoric acid solutions. John K. Borchardt
C60 boosts performance NANOTECHNOLOGY 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