Solar Energy Storage in Photochromic Molecules

Transcription

1 Center for Exploitation of Solar Energy & Department of Chemistry, University of Copenhagen Solar Energy Storage in otochromic Molecules Mogens Brøndsted Nielsen Compact long term heat storage workshop, DTU, Jan. 28, 2016

2 Energy Storage using otochromic Molecules Closed Energy Cycle with No CO 2 Emissions or Other Pollutants Light absorption Storage Energy release on demand otoinduced isomerization Energy barrier for back-reaction Energy stored determines storage time light low-energy molecule "trigger" high-energy molecule

3 Material with energy density of 1 MJ / kg Heat release of 1 MJ can be used to bring 3 L of water from room temperature to the boiling point Harvesting light during the day and releasing heat during the night: - maintaining 1 m 3 at 19 o C with outside temperature of -6 o C requires ca. 3 kg of solar battery (when using foam insulation) T. R. Kucharski, Y. Tian, S. Akbulatov, R. Boulatov, Energy Environ. Sci. 2011, 4,

4 Challenges How do we design photochromic molecules with sufficiently high energy densities? How is the energy-releasing back-reaction put on stand-by? energy storage How do we reach high quantum yields How do we avoid photodegradation of molecules over many cycles?

5 Norbornadiene Quadricyclane Couple light heat Energy storage: 1 MJ/kg Quantum yield: 9% Only absorbs UV light Prone to polymerization Z.-I. Yoshida, J. otochem. 1985, 29,

6 trans-azobenzene cis-azobenzene Couple N N light heat N N Energy storage: ca. 0.3 MJ/kg otostationary states, usually with <75% cis

7 Dihydroazulene (DHA) / Vinylheptafulvene (VHF) Couple Quantum yield = 55%

8 Dihydroazulene (DHA) / Vinylheptafulvene (VHF) Couple DHA l max 353 nm VHF l max 470 nm

9 DHA Synthesis Easy to do S. L. Broman, S. L. Brand, C. R. Parker, M. Å. Petersen, C. G. Tortzen, A. Kadziola, K.Kilså, M. B. Nielsen, ARKIVOC 2011, ix,

10 Energy storage: 0.11 MJ / kg We need to modify the molecule to increase this value! S.T. Olsen, J. Elm, F.E. Storm, A.N. Gejl, A.S. Hansen, M.H. Hansen, J.R. Nikolajsen, M.B. Nielsen, H.G. Kjaergaard, K.V. Mikkelsen, J. ys. Chem. A 2015, 119,

11 Energy Storage NC CN Substitute one CN for a H NC H 0.11 MJ / kg 0.25 MJ / kg NC CN NC 0.15 MJ / kg 0.23 MJ / kg H stored energy By minor structural variations we can make a doubling of the energy storage capacity! S.T. Olsen, J. Elm, F.E. Storm, A.N. Gejl, A.S. Hansen, M.H. Hansen, J.R. Nikolajsen, M.B. Nielsen, H.G. Kjaergaard, K.V. Mikkelsen, J. ys. Chem. A 2015, 119, M. Cacciarini, A.B. Skov, M. Jevric, A.S. Hansen, J. Elm, H.G. Kjaergaard, K.V. Mikkelsen, M.B. Nielsen, Chem. Eur. J. 2015, 21,

12 Synthesis of New Targets by Reductive Decyanations NC CN DIBAL-H THF 23% H CN hn H CN NC CN DIBAL-H THF 16% H CN hn very slowly H CN VHF of this DHA: Half-life of 14 s TS calculations: DG = kj/mol Half-life >10 years at rt M. Cacciarini, A.B. Skov, M. Jevric, A.S. Hansen, J. Elm, H.G. Kjaergaard, K.V. Mikkelsen, M.B. Nielsen, Chem. Eur. J. 2015, 21,

13 Triggering the Ring-Closure NC CN Ag + NC CN Effect of Ag + on the thermal ring-closure reaction of VHF: min Solvent: 1,2-dichloroethane C. R. Parker, C. G. Tortzen, S. L. Broman, M. Schau-Magnussen, K. Kilså, M. B. Nielsen, Chem. Commun. 2011, 47,

14 Ring-Closure Reactions in MeCN at Room Temperature Me 2 N fast Me 2 N NC CN NC CN Half-life = 50 min VHF DHA O 2 N slow O 2 N NC CN NC CN Half-life = 490 min VHF DHA S. L. Broman, M. Jevric, M. B. Nielsen, Chem. Eur. J. 2013, 19,

15 Hammett Substituent Constants - Measure of electron-withdrawing / donating effect O OH O + H 2 O + H 3 O K X O X s X = log K X K H X = pk a,h - pk a,x Electron-withdrawing groups have positive s-values (e.g. NO 2 ) Electron-donating groups have negative s-values (e.g. NMe 2 )

16 ln(k) VHF Ring-Closure Hammett Correlation - Effect of changing the aryl group at the vinylic position fast -8.8 NC CN X NC CN X p-cn p-cn -9.0 p-co 2 Me p-no 2 p-no p-i p-cch p-cho m-sac m-cn -9.6 p-f p-br m-i -9.8 p-ome H m-cch slow p-me p-nhac s s m/p (meta m / p or para) Electron-withdrawing group at vinylic position ring enhances the ring-closure S. L. Broman, M. Jevric, M. B. Nielsen, Chem. Eur. J. 2013, 19,

17 VHF Ring-Closure Hammett Correlation - Effect of changing the aryl group at the seven-membered ring X X fast p-nme 2 NC CN NC CN through-conjugation values p-ome p-no 2 slow s p or s p + Electron-donating group at the seven-membered ring enhances the ring-closure S. L. Broman, M. Jevric, M. B. Nielsen, Chem. Eur. J. 2013, 19,

18 Abs Ultrafast Ring-closure of VHF in Locked s-cis Conformation NC CN NC CN 1.0 DHA Half-life: < 2 s (cyclohexane) VHF Wavelength (nm) S. L. Broman, O. Kushnir, M. Rosenberg, A. Kadziola, J. Daub, M. B. Nielsen, Eur. J. Org. Chem. 2015,

19 Dihydroazulene / Vinylheptafulvene Switch Low energy High energy Halting the back-reaction: <1 s Half-life for VHF to DHA back-reaction Infinitely long

20 Another Approach Building in Strain Macrocyclic Structures N N N N Calculations predict increased energy storage and step-wise back-reactions N N E. Durgun, J. C. Grossman, J. ys. Chem. Lett. 2013, 4,

21 Current Status Energy storage capacities of 0.5 MJ/kg may soon be within reach Storage times of days have been reached longer storage times have been reached for various molecules Up to years for the DHA-VHF couple, but triggering the energy release is still a challenge

22 Center for Exploitation of Solar Energy & Department of Chemistry, University of Copenhagen Acknowledgements Dr. Søren L. Broman Dr. Martina Cacciarini Dr. Martyn Jevric Anders B. Skov Alexandru Vlasceanu Prof. Kurt V. Mikkelsen Prof. Henrik G. Kjaergaard Dr. Jonas Elm Stine T. Olsen Anders Gertsen Anne Schou University of Copenhagen & the Carlsberg Foundation