1. Energy Storage in Clathrates and Related Molecular Compounds Wendy L. Mao Geological and Environmental Sciences & Photon Science, SLAC Stanford University

2. Clathrates Filled Ices van der Waals Compounds Molecular Compounds

3. Clathrates Crystalline structures based on a hydrogen-bonded water framework (‘host’ lattice) with cavities which contain ‘guest’ molecules Filled Ices van der Waals Compounds Molecular Compounds

4. Clathrate structures cubic hexagonal W. Mao et al, Physics Today 2007 sI sII sH

5. Clathrates – an old science In 1778, Joseph Priestley may have been the first to discover clathrates taking advantage of the cold winters in Birmingham to refrigerate his samples of sulfur dioxide + water Credit for the discovery is usually given to Michael Faraday’s boss, Sir Humphry Davy who reported a clathrate in the chlorine + water system in 1811

6. Clathrates on Earth Clathrates represent major flow assurance problem in natural gas and oil pipelines Methane clathrate in sI structure is the most abundant form of hydrocarbon on Earth (> 60%) Found in terrestrial marine sediments and permafrost Source of global climate change? Potential energy resource?

7. Clathrates represent major flow assurance problem in natural gas and oil pipelines Methane clathrate in sI structure is the most abundant form of hydrocarbon on Earth (> 60%) Found in terrestrial marine sediments and permafrost Source of global climate change? Potential energy resource? Image courtesy of Petrobas Clathrates on Earth

8. Clathrates represent major flow assurance problem in natural gas and oil pipelines Methane clathrate in sI structure is the most abundant form of hydrocarbon on Earth (> 60%) Found in terrestrial marine sediments and permafrost Source of global climate change? Potential energy resource? Methane ice worm Clathrates on Earth

9. Clathrates represent major flow assurance problem in natural gas and oil pipelines Methane clathrate in sI structure is the most abundant form of hydrocarbon on Earth (> 60%) Found in terrestrial marine sediments and permafrost Source of global climate change? Potential energy resource? Figure courtesy of G. Dickens Clathrates on Earth

10. Clathrates represent major flow assurance problem in natural gas and oil pipelines Methane clathrate in sI structure is the most abundant form of hydrocarbon on Earth (> 60%) Found in terrestrial marine sediments and permafrost Source of global climate change? Potential energy resource? Image courtesy of G. Klinkhammer Clathrates on Earth Flare from the Mallik 2002 production test well

11. Clathrates in the Solar system Ubiquitous presence in the Universe? CO 2 and CH 4 clathrates on Mars CH 4 clathrates on Titan Source of plumes on Saturn’s moon Enceladus Clathrates in Halley’s comet Porco et al, Science 2006 Kieffer et al, Science 2006

12. Clathrates Filled Ices Hydrates with structures related to known ice phase Guest molecules occupy channels within ice structure rather than cages van der Waals Compounds Molecular Compounds

13. Formed at higher pressures He hydrate (ice II), Londono et al JCP, 1992 Hydrogen hydrates (ice II and ice Ic), Vos et al, PRL 1993 Methane hydrate III (ice Ih), Loveday et al, PRL 2001 Filled Ices Filled ice II Filled ice Ic Filled ice Ih

14. Clathrates Filled Ices van der Waals Compounds Stoichiometric crystals of mixtures of atoms and molecules held together by weak van der Waals forces Molecular Compounds

15. van der Waals compounds He(N 2 ) 11, Vos et al, Nature 1992 Ne(He) 2, Loubeyre et al, PRL 1993 Ar(H 2 ) 2, Loubeyre et al, PRL 1994 CH 4 -H 2, Somayazulu et al, Science 1996 He(N 2 ) 11 9 GPa

16. Hydrogen Storage Requirements: 1.High hydrogen content (by mass and volume) 2.Moderate P-T synthesis 3.Moderate P-T storage 4.Easy hydrogen release 5.Environmentally friendly by-products 6.Cost and availability 7.Safety

17. C 2 (filled Ice I c ) H 2 -H 2 O C 1 (filled Ice II) H 2 -6H 2 O C2C2 Vos et al, PRL 1993 H 2 + H 2 O system

18. W. Mao et al, Science 2002 300 MPa 250 K 249 K t = 0 t = 30 min HHsII (Hydrogen hydrate in sII clathrate structure) H 2 -2H 2 O H2H2 H2OH2O

19. H 2 + H 2 O system Two filled ices and sII H 2 clathrate found at high P Can be quenched to much lower P with low T Studied using XRD, Neutron diffraction, Raman and IR spectroscopy Chemical stabilization of clathrate phase Not enough hydrogen storage, kinetic limitations Different P-T range (e.g. C 2 filled Ice Ic) Lokshin et al, PRL 2004

20. Use THF as a promoter molecule to fill large cage Forms sII clathrate at 277.3 K at ambient P Florusse et al, Science 2004 Chemical stabilization..

21. H 2 + H 2 O system Two filled ices and sII H 2 clathrate found at high P Can be quenched to much lower P with low T Studied using XRD, Neutron diffraction, Raman and IR spectroscopy Chemical stabilization of clathrate phase Not enough hydrogen storage, kinetic limitations Different P-T range (e.g. C 2 filled Ice Ic) W. Mao et al, Science 2002 Lokshin et al, PRL 2004

22. Somayazulu et al, Science 1996 H 2 + CH 4 system CH 4 (H 2 ) 2

23. H 2 + CH 4 system Needs more characterization Structure Hydrogen occupancy Phase diagram Metastable synthesis paths W. Mao et al, CPL 2005 CH 4 (H 2 ) 4 has 33.4 wt% H 2 (not including H in CH 4 )

24. W. Mao et al, Physics Today 2007 Hydrogen storage capacity

25. Astrophysics Experimental Results Spectra from WL5, protostar in the  Ophiuchus cloud complex Sandford et al, Science 1993 Trifid Nebula HH-sII in small, icy bodies? Telescope Observations