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Templated assembly of photoswitches significantly increases the energy-storage capacity of solar thermal fuels


Large-scale utilization of solar-energy resources will require considerable advances in energy-storage technologies to meet ever-increasing global energy demands. Other than liquid fuels, existing energy-storage materials do not provide the requisite combination of high energy density, high stability, easy handling, transportability and low cost. New hybrid solar thermal fuels, composed of photoswitchable molecules on rigid, low-mass nanostructures, transcend the physical limitations of molecular solar thermal fuels by introducing local sterically constrained environments in which interactions between chromophores can be tuned. We demonstrate this principle of a hybrid solar thermal fuel using azobenzene-functionalized carbon nanotubes. We show that, on composite bundling, the amount of energy stored per azobenzene more than doubles from 58 to 120 kJ mol–1, and the material also maintains robust cyclability and stability. Our results demonstrate that solar thermal fuels composed of molecule–nanostructure hybrids can exhibit significantly enhanced energy-storage capabilities through the generation of template-enforced steric strain.

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Figure 1: Synthesis and characterization of azobenzene-functionalized SWCNTs.
Figure 2: Photochemical charging and thermal energy release.
Figure 3: Photochemical and thermal cycling.
Figure 4: Effect of packing photochemical switches in the solid state.
Figure 5: Energy storage as a function of template-packing parameters.


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The authors acknowledge financial support from BP for a BP-MIT Postdoctoral Research Associateship (T.J.K.) and research funds awarded through the MIT Energy Initiative, which supported the synthesis of functionalized SWCNTs, and the Advanced Research Projects Agency-Energy (ARPA-E), US Department of Energy, under Award Number DE-AR0000180, which supported all other work. Calculations were performed at the National Energy Research Scientific Computing Center, supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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T.J.K. synthesized the materials and performed experiments, N.F. performed solid-state fluorescence and spectroscopy measurements, A.M.K. performed DFT calculations and J.O.Z. performed experiments. All authors contributed to the experimental design, analysed the data and wrote the paper.

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Correspondence to Timothy J. Kucharski, Daniel G. Nocera or Jeffrey C. Grossman.

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The authors declare no competing financial interests.

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Kucharski, T., Ferralis, N., Kolpak, A. et al. Templated assembly of photoswitches significantly increases the energy-storage capacity of solar thermal fuels. Nature Chem 6, 441–447 (2014).

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