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Solar energy fuelled up
RESEARCH NEWS
Solar energy fuelled up ENERGY
Scientists have built a reactor that could one day lead to the efficient production of fuel from sunlight. With solar energy having long been the potential savior for our dwindling energy resources, the fact that it cannot be transported from place to place has thwarted its widespread use. Now, however, a new process that demonstrates how solar-driven thermochemical approaches can be technologically viable may bring that ideal a little closer. A research team led by Professor Sossina Haile, a professor at Steele Laboratories, California Institute of Technology (Caltech), has developed a prototype reactor with a quartz window and a special cavity able to absorb concentrated sunlight through a type of magnifying glass to focus the sun’s rays. The study, a joint effort between Caltech and ETH Zurich and published in Science [Chueh et al. Science (2010) doi: 10.1126/science.1197834], uses ceria as a key ingredient for concentrating solar energy and using it to efficiently convert carbon dioxide and water into fuels. Ceria, an oxide of the most common of the rare earth metals cerium, is used in ceramics, stone and glass polishing, and in the walls of self-cleaning ovens. As ceria can ‘exhale’ oxygen from its
Solar fuel reactor. Courtesy ETH-Zurich and Caltech.
crystalline framework at very high temperatures and then ‘inhale’ oxygen back in at lower temperatures, it is ideal to drive the solarpowered reactions. Dr Haile points out that “What is special about the material is that it doesn’t release all of the oxygen. That helps to leave the framework of the material intact as oxygen leaves. When we cool it back down, the material’s thermodynamically preferred state is to pull oxygen back into the structure.” The inhaled oxygen is stripped off of carbon dioxide and/or water gas molecules pumped
into the reactor, which produces carbon monoxide and/or hydrogen gas, with the latter able to be used for fuelling hydrogen fuel cells. The carbon monoxide, when combined with the hydrogen gas, can also be used to create synthetic gas, the precursor to liquid hydrocarbon fuels. Once the ceria is oxygenated to full capacity, it can then be re-heated, beginning the cycle again. In order to achieve this effect, the reactor’s temperature must reach almost 3000 °F. The researchers were able to use photons for this at the Paul Scherrer Institute in Switzerland, where the reactor was installed on a large solar simulator capable of delivering the heat required. The research could lead to its adoption for large-scale energy plants, bringing the possibility of solar-derived power being available at any time of day or night. Another potential application is using the carbon dioxide emissions from coal-powered electric plants for conversion into transportation fuels. However, the reactor design and materials will need to undergo further improvement to allow this approach to be commercially viable. Laurie Donaldson
Solid smoke NANOTECHNOLOGY Carbon nanotubes (CNTs) represent one of the most potentially useful materials known to man, possessing incredible strength and hardness as well as distinctive thermal and electrical properties. Producing structures that take advantage of these properties is therefore an appealing prospect, and a challenge that has inspired many researchers. Scientists at the University of Central Florida have recently made a step forward in their approach by synthesizing a new “frozen smoke” aerogel from multiwalled CNTs [Zou et al., ACS Nano (2010) 4, 7293]. A conventional gel is a substance composed of a liquid constrained by a cross-linked solid network. An aerogel is similar, but the liquid component has been replaced with gas. The result is an incredibly light, porous substance that can retain the properties of the parent material. Dr Zai and co-workers succeeded in producing an ultralight free-standing multi-walled CNT aerogel monolith after realizing that increasing the interaction
Honeycomb structure of the CNT aerogel. potential between nanotubes was the key to creating the low-density material. The final result was a material with a density of just 4 mg/cm3; the lowest density recorded for a free-standing CNT aerogel. The monolith was capable of being compressed to just 5 % of its volume, with little change in the uncompressed volume once it was allowed to re-expand. After cycling the compression, the aerogel was found to be incredibly resilient, with only a
slight change in the compressive stress after 1000 cycles. However, Zai does point out that there is room for improvement, “Although, the aerogel is very strong and flexible upon compression, we are working on improving its mechanical properties upon elongation”. The electrical conductivity was not as high as for pure MWCNTs, having reduced by approximately half. This was attributed to the presence of the insulating additive that was used to create the strong bonds between nanotubes. However, the conductivity was found to be incredibly dependent on changes in pressure, indicating that CNT aerogels could be used to construct highly sensitive pressure sensors. Other potential uses include energy conversion and storage, as well as catalyst support. The team is currently working on new aerogels composed of functionalized nanotubes, which will hopefully lead to even more applications.
Stewart Bland
MARCH 2011 | VOLUME 14 | NUMBER 3
67
Solar energy fuelled up ENERGY
Scientists have built a reactor that could one day lead to the efficient production of fuel from sunlight. With solar energy having long been the potential savior for our dwindling energy resources, the fact that it cannot be transported from place to place has thwarted its widespread use. Now, however, a new process that demonstrates how solar-driven thermochemical approaches can be technologically viable may bring that ideal a little closer. A research team led by Professor Sossina Haile, a professor at Steele Laboratories, California Institute of Technology (Caltech), has developed a prototype reactor with a quartz window and a special cavity able to absorb concentrated sunlight through a type of magnifying glass to focus the sun’s rays. The study, a joint effort between Caltech and ETH Zurich and published in Science [Chueh et al. Science (2010) doi: 10.1126/science.1197834], uses ceria as a key ingredient for concentrating solar energy and using it to efficiently convert carbon dioxide and water into fuels. Ceria, an oxide of the most common of the rare earth metals cerium, is used in ceramics, stone and glass polishing, and in the walls of self-cleaning ovens. As ceria can ‘exhale’ oxygen from its
Solar fuel reactor. Courtesy ETH-Zurich and Caltech.
crystalline framework at very high temperatures and then ‘inhale’ oxygen back in at lower temperatures, it is ideal to drive the solarpowered reactions. Dr Haile points out that “What is special about the material is that it doesn’t release all of the oxygen. That helps to leave the framework of the material intact as oxygen leaves. When we cool it back down, the material’s thermodynamically preferred state is to pull oxygen back into the structure.” The inhaled oxygen is stripped off of carbon dioxide and/or water gas molecules pumped
into the reactor, which produces carbon monoxide and/or hydrogen gas, with the latter able to be used for fuelling hydrogen fuel cells. The carbon monoxide, when combined with the hydrogen gas, can also be used to create synthetic gas, the precursor to liquid hydrocarbon fuels. Once the ceria is oxygenated to full capacity, it can then be re-heated, beginning the cycle again. In order to achieve this effect, the reactor’s temperature must reach almost 3000 °F. The researchers were able to use photons for this at the Paul Scherrer Institute in Switzerland, where the reactor was installed on a large solar simulator capable of delivering the heat required. The research could lead to its adoption for large-scale energy plants, bringing the possibility of solar-derived power being available at any time of day or night. Another potential application is using the carbon dioxide emissions from coal-powered electric plants for conversion into transportation fuels. However, the reactor design and materials will need to undergo further improvement to allow this approach to be commercially viable. Laurie Donaldson
Solid smoke NANOTECHNOLOGY Carbon nanotubes (CNTs) represent one of the most potentially useful materials known to man, possessing incredible strength and hardness as well as distinctive thermal and electrical properties. Producing structures that take advantage of these properties is therefore an appealing prospect, and a challenge that has inspired many researchers. Scientists at the University of Central Florida have recently made a step forward in their approach by synthesizing a new “frozen smoke” aerogel from multiwalled CNTs [Zou et al., ACS Nano (2010) 4, 7293]. A conventional gel is a substance composed of a liquid constrained by a cross-linked solid network. An aerogel is similar, but the liquid component has been replaced with gas. The result is an incredibly light, porous substance that can retain the properties of the parent material. Dr Zai and co-workers succeeded in producing an ultralight free-standing multi-walled CNT aerogel monolith after realizing that increasing the interaction
Honeycomb structure of the CNT aerogel. potential between nanotubes was the key to creating the low-density material. The final result was a material with a density of just 4 mg/cm3; the lowest density recorded for a free-standing CNT aerogel. The monolith was capable of being compressed to just 5 % of its volume, with little change in the uncompressed volume once it was allowed to re-expand. After cycling the compression, the aerogel was found to be incredibly resilient, with only a
slight change in the compressive stress after 1000 cycles. However, Zai does point out that there is room for improvement, “Although, the aerogel is very strong and flexible upon compression, we are working on improving its mechanical properties upon elongation”. The electrical conductivity was not as high as for pure MWCNTs, having reduced by approximately half. This was attributed to the presence of the insulating additive that was used to create the strong bonds between nanotubes. However, the conductivity was found to be incredibly dependent on changes in pressure, indicating that CNT aerogels could be used to construct highly sensitive pressure sensors. Other potential uses include energy conversion and storage, as well as catalyst support. The team is currently working on new aerogels composed of functionalized nanotubes, which will hopefully lead to even more applications.
Stewart Bland
MARCH 2011 | VOLUME 14 | NUMBER 3
67