Energy Storage in Clathrates and Related Molecular Compounds

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

2. Molecular Compounds • Clathrates • Filled Ices • van der Waals Compounds

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

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

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 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? Image courtesy of Petrobas

8. 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? Methane ice worm

9. 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? Figure courtesy of G. Dickens

10. 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? Image courtesy of G. Klinkhammer Flare from the Mallik 2002 production test well

11. Ubiquitous presence in the Universe? CO2 and CH4 clathrates on Mars CH4 clathrates on Titan Source of plumes on Saturn’s moon Enceladus Clathrates in Halley’s comet Clathrates in the Solar system Porco et al, Science 2006 Kieffer et al,Science 2006

12. Molecular Compounds • 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

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 Ih Filled ice Ic Filled ice II

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

15. He(N2)11,Vos et al, Nature 1992 Ne(He)2,Loubeyre et al, PRL 1993 Ar(H2)2,Loubeyre et al, PRL 1994 CH4-H2,Somayazulu et al, Science 1996 van der Waals compounds He(N2)11 9 GPa

16. Hydrogen Storage Requirements: • High hydrogen content (by mass and volume) • Moderate P-T synthesis • Moderate P-T storage • Easy hydrogen release • Environmentally friendly by-products • Cost and availability • Safety

17. H2 + H2O system C2 • C2(filled Ice Ic) H2-H2O • C1(filled Ice II) H2-6H2O Vos et al, PRL 1993

18. 300 MPa 250 K 249 K t = 0 t = 30 min H2 + H2O system H2O H2 • HHsII(Hydrogen hydrate in sII clathrate structure) H2-2H2O W. Mao et al, Science 2002

19. Two filled ices and sII H2 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. C2 filled Ice Ic) H2 + H2O system 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 .. .. Chemical stabilization Florusse et al, Science 2004

21. Two filled ices and sII H2 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. C2 filled Ice Ic) H2 + H2O system W. Mao et al, Science 2002 Lokshin et al, PRL 2004

22. H2 + CH4 system Somayazulu et al, Science 1996 CH4(H2)2

23. H2 + CH4 system Needs more characterization Structure Hydrogen occupancy Phase diagram Metastable synthesis paths CH4(H2)4 has 33.4 wt% H2 (not including H in CH4) W. Mao et al, CPL 2005

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

25. Astrophysics Trifid Nebula Spectra from WL5, protostar in the r Ophiuchus cloud complex Sandford et al, Science1993 Experimental Results Telescope Observations • HH-sII in small, icy bodies?