Economical and sustainable conversion of sunlight into other useful forms of energy that are easily transportable is currently a significant research challenge. One approach that has been explored is the use of solar thermal fuels. Upon absorption of light energy, photoactive molecules switch to a metastable, higher energy conformation. This results in the storage of energy (ΔH), which can be released when an external trigger is applied and thermal barrier E a is overcome—see (a) in figure. The fuel can then be recharged on exposure to light and the cycle is repeated. Although solar thermal fuels have important advantages—they are potentially 100% renewable, produce no emissions, and are easily transportable—they were dismissed as unfeasible decades ago because the fuel degraded after only a few cycles of energy conversion and release. Interest in the concept was renewed, however, with the synthesis of a Ru-fulvalene compound. This constituted a new solar thermal fuel that does not degrade, although its low volumetric energy density (several orders of magnitude lower than Li-ion batteries), and the expense of Ru, makes it impractical. Recently, however, A.M. Kolpak and J.C. Grossman of the Massachusetts Institute of Technology designed a new, stable solar thermal fuel, which has a predicted volumetric energy density equal to that of Li-ion batteries, by covalently bonding photoactive azobenzene derivatives to carbon nanotubes (CNTs).
As reported in the June 10 online edition of Nano Letters (DOI: 10.1021/nl201357n), Kolpak and Grossman used density functional calculations to show that the CNT facilitates a close-packed, crystalline arrangement of azobenzenes—see (b) in figure. The packing interactions and the symmetry-breaking of the azobenzenes (a consequence of their attachment to the CNT) both contribute to a large increase in ΔH. In the packed state, the researchers found that the position at which the azobenzenes were attached to the CNT (para, meta, or ortho with respect to the azobenzene nitrogen double bond) could be used to alter the value of ΔH. By also substituting hydroxyl groups for phenyl hydrogens, and thereby enabling the formation of hydrogen bonds between adjacent azobenzenes, the researchers found that they could simultaneously increase the relative stability of the cis and trans isomers and the energy barrier between them. In one example (Figure b), with a meta linkage and a total of six hydrogen bonds per azobenzene in the trans state, ΔH increased by 260% over gas-phase azobenzene and E a increased by 20%, giving a half-life for the photoexcited state of the azobenzene-CNT fuel of greater than one year, a marked improvement over gas-phase azobenzene.
Kolpak and Grossman said that their azobenzene-CNT fuel will be water soluble due to the hydroxyl groups on the solvent-accessible surfaces. Increased solubility can be achieved with substitution of additional polar groups at positions in contact with the solvent, which should not affect ΔH or E a, so that formation of highly concentrated solutions is possible. The researchers estimated that the temperature of the heat released by their azobenzene-CNT fuel to be 620 K, but said that even higher temperatures could be achieved by lowering the heat capacity of the fuel.
Kolpak and Grossman said, “In addition, our results suggest that one can generalize the concept of hybrid photoisomer-substrate nanostructures to other photoactive molecules, different substrates, and different linker chemistries, potentially leading to a wide range of novel degradation-resistant, high-stability, and high-energy density solar thermal fuels.”
(a) The solar thermal fuel—2,2’,5’-trihydroxy diazobenzene molecules covalently attached to an (8,0) carbon nanotube (CNT)—undergoes a photo-induced trans-to-cis isomerization, which stores 1.55 eV per azobenzene moiety. The back reaction during storage is prevented by the thermal barrier, E a, which is overcome by an external trigger to control the release of the stored energy. C, N, and O atoms are represented by gray, blue, and red spheres, respectively, with nanotube carbons slightly lighter in color. Inset: the corresponding energy storage/release curve for isolated azobenzene.
(b) In order to illustrate packing interactions, azobenzenes are shown here bound only to a single row of carbon atoms of an (8,0) CNT in the direction of the tube axis. Dashed lines are hydrogen bonds. Reproduced with permission from Nano Lett. (DOI: 10.1021/nl201357n). © 2011 American Chemical Society.
