- Email: [email protected]
Putting hydrogen into the framework
RESEARCH NEWS
Putting hydrogen into the framework ENERGY Metallacarboranes can be used as viable natural hydrogen storage materials, say researchers from the US and India [Singh et al., J Am Chem Soc (2010) doi: 10.1021/ja104544s]. Metallacarboranes are clusters consisting of boron and carbon atoms in which one or several BH units are replaced by a transition metal atom to form a cage containing boron, metal and carbon. The remarkable thing is that they are capable of binding hydrogen in a rather unusual way – via the Kubas interaction. While most chemical bonding relies on strong chemisorption involving covalent bonds which are difficult to break or weak physisorption involving van der Waals interactions, the Kubas type of interaction lies somewhere between the two in strength which could potentially change the future of hydrogen storage. Hydrogen storage has long been the focus of many research laboratories but the metal organic frameworks and metal hyrides investigated so far suffer from binding too strongly or too weakly to hydrogen. The Kubas interaction however, provides the right level of binding strength.
The transition metal (green balls) which serves as an anchor for hydrogen storage. Image courtesy of Arta Sadrzadeh, Rice University. The team from Rice University in the US and the Indian Institute of Science in India have used first principle calculations to show that metallacarboranes containing Sc or Ti achieve an average binding energy of around 0.4 eV/H2. This means that they are capable of reversible storage at ambient conditions. Each metal atom can bind up to five H2 molecules and the cages can store around 8 wt % of hydrogen on the metallacarborane cluster, a value which exceeds the
US department of energy goals for set for 2015. Boris Yakobson of Rice University explains to Materials Today why the system is so effective. “The metal organic framework provides a scaffold, and some of its links include transition metal atoms capable of catching and holding H2, yet not too strongly, so it can be released when needed for fuel. Now, to prevent transition metals from aggregating into a blob together, each metal atom must be held, like a gem held in a casing. This is accomplished by placing transition metal atoms in the carborane casings-part of the scaffold of the entire metal organic framework. High porosity of the metal organic framework is important is it allows H2 to enter and leave freely.” In addition to binding up to 5 molecules of H2 via the Kubas interaction, metallacarboranes can also physisorb more hydrogen within the pores of the frameworks. As Yakobson says, “If chemists can synthesize this particular framework with metallacarborane as an element, this (hydrogen storage) may become a reality.”
Katerina Busuttil
Solar cell acts as an artificial leaf ENERGY Scientists have developed a new, water-based, solar device that acts as a type of artificial leaf. The device uses a water-based gel infused with light-sensitive molecules, along with electrodes that are coated with carbon materials, such as carbon nanotubes or graphite. When these molecules are stimulated by the sun they produce electricity, in a similar way to plant molecules synthesising sugars when they are excited. These new, bendable, bio-mimetic solar cells can use both synthetic light-sensitive molecules and also naturally derived products such as chlorophyll, which can be integrated because of their water-gel matrix. The research, which was published in the Journal of Materials Chemistry [Koo et al., J Mater Chem (2011) doi: 10.1039/C0JM01820A] is the first to show that solar cells that closely mimic nature can work effectively. The team aim to fine-tune these water-based photovoltaic devices, making them even more like natural leaves. Lead author of the study, Dr. Orlin Velev, said “The next step is to mimic the self-
Solar cell. regenerating mechanisms found in plants. The other challenge is to change the water-based gel and lightsensitive molecules to improve the efficiency of the solar cells.” Although there is much still to be done before the devices could be sold commercially, the idea of biologically inspired soft devices that generate electricity could one day offer an alternative for current solid-state technologies, especially as they are
more environmentally friendly and easier to dispose of when they reach the end of their natural life. However, it is important to show how they could be made more cheaply and on a larger scale than the current silicon cells. The researchers, from NC State University, the Air Force Research Laboratory and Chung-Ang University in Korea, hope to expand their study to identify how to mimic the materials by which nature harnesses solar energy. To reach that point, as well as improving their efficiency, the team need to replicate the ability of natural leaves to regenerate and replace their organic dye. This would offer a solution to the problems of the long-term stability and performance typical of all organic photovoltaic devices. This is a key advance for the future development of usable bio-mimetic devices – because of the photodegradation of the organic compounds in the devices, it is difficult to mimic the self-regenerating mechanisms found in plants and the decay of the light-sensitive molecules.
Laurie Donaldson
NOVEMBER 2010 | VOLUME 13 | NUMBER 11
9
Putting hydrogen into the framework ENERGY Metallacarboranes can be used as viable natural hydrogen storage materials, say researchers from the US and India [Singh et al., J Am Chem Soc (2010) doi: 10.1021/ja104544s]. Metallacarboranes are clusters consisting of boron and carbon atoms in which one or several BH units are replaced by a transition metal atom to form a cage containing boron, metal and carbon. The remarkable thing is that they are capable of binding hydrogen in a rather unusual way – via the Kubas interaction. While most chemical bonding relies on strong chemisorption involving covalent bonds which are difficult to break or weak physisorption involving van der Waals interactions, the Kubas type of interaction lies somewhere between the two in strength which could potentially change the future of hydrogen storage. Hydrogen storage has long been the focus of many research laboratories but the metal organic frameworks and metal hyrides investigated so far suffer from binding too strongly or too weakly to hydrogen. The Kubas interaction however, provides the right level of binding strength.
The transition metal (green balls) which serves as an anchor for hydrogen storage. Image courtesy of Arta Sadrzadeh, Rice University. The team from Rice University in the US and the Indian Institute of Science in India have used first principle calculations to show that metallacarboranes containing Sc or Ti achieve an average binding energy of around 0.4 eV/H2. This means that they are capable of reversible storage at ambient conditions. Each metal atom can bind up to five H2 molecules and the cages can store around 8 wt % of hydrogen on the metallacarborane cluster, a value which exceeds the
US department of energy goals for set for 2015. Boris Yakobson of Rice University explains to Materials Today why the system is so effective. “The metal organic framework provides a scaffold, and some of its links include transition metal atoms capable of catching and holding H2, yet not too strongly, so it can be released when needed for fuel. Now, to prevent transition metals from aggregating into a blob together, each metal atom must be held, like a gem held in a casing. This is accomplished by placing transition metal atoms in the carborane casings-part of the scaffold of the entire metal organic framework. High porosity of the metal organic framework is important is it allows H2 to enter and leave freely.” In addition to binding up to 5 molecules of H2 via the Kubas interaction, metallacarboranes can also physisorb more hydrogen within the pores of the frameworks. As Yakobson says, “If chemists can synthesize this particular framework with metallacarborane as an element, this (hydrogen storage) may become a reality.”
Katerina Busuttil
Solar cell acts as an artificial leaf ENERGY Scientists have developed a new, water-based, solar device that acts as a type of artificial leaf. The device uses a water-based gel infused with light-sensitive molecules, along with electrodes that are coated with carbon materials, such as carbon nanotubes or graphite. When these molecules are stimulated by the sun they produce electricity, in a similar way to plant molecules synthesising sugars when they are excited. These new, bendable, bio-mimetic solar cells can use both synthetic light-sensitive molecules and also naturally derived products such as chlorophyll, which can be integrated because of their water-gel matrix. The research, which was published in the Journal of Materials Chemistry [Koo et al., J Mater Chem (2011) doi: 10.1039/C0JM01820A] is the first to show that solar cells that closely mimic nature can work effectively. The team aim to fine-tune these water-based photovoltaic devices, making them even more like natural leaves. Lead author of the study, Dr. Orlin Velev, said “The next step is to mimic the self-
Solar cell. regenerating mechanisms found in plants. The other challenge is to change the water-based gel and lightsensitive molecules to improve the efficiency of the solar cells.” Although there is much still to be done before the devices could be sold commercially, the idea of biologically inspired soft devices that generate electricity could one day offer an alternative for current solid-state technologies, especially as they are
more environmentally friendly and easier to dispose of when they reach the end of their natural life. However, it is important to show how they could be made more cheaply and on a larger scale than the current silicon cells. The researchers, from NC State University, the Air Force Research Laboratory and Chung-Ang University in Korea, hope to expand their study to identify how to mimic the materials by which nature harnesses solar energy. To reach that point, as well as improving their efficiency, the team need to replicate the ability of natural leaves to regenerate and replace their organic dye. This would offer a solution to the problems of the long-term stability and performance typical of all organic photovoltaic devices. This is a key advance for the future development of usable bio-mimetic devices – because of the photodegradation of the organic compounds in the devices, it is difficult to mimic the self-regenerating mechanisms found in plants and the decay of the light-sensitive molecules.
Laurie Donaldson
NOVEMBER 2010 | VOLUME 13 | NUMBER 11
9