Energy Storage in Clathrates and Related Molecular

Compounds

Wendy L. MaoGeological and Environmental Sciences & Photon Science, SLAC

Stanford University

Clathrates Filled Ices van der Waals Compounds

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

Molecular Compounds

Clathrate structures

cubic

cubic

hexagonal

W. Mao et al, Physics Today 2007

sI

sII

sH

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

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?

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

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

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

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

Clathrates in the Solar system

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

Porco et al, Science 2006Kieffer et al, Science 2006

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

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 IcFilled ice Ih

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

van der Waals compounds

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

He(N2)11

9 GPa

Hydrogen Storage

Requirements:1. High hydrogen content (by mass and volume)2. Moderate P-T synthesis3. Moderate P-T storage4. Easy hydrogen release5. Environmentally friendly by-products6. Cost and availability7. Safety

C2 (filled Ice Ic) H2-H2O

C1 (filled Ice II) H2-6H2O

C2

Vos et al, PRL 1993

H2 + H2O system

H2 + H2O system

W. Mao et al, Science 2002

300 MPa250 K

249 Kt = 0

t = 30 min HHsII (Hydrogen hydrate in sII clathrate structure) H2-2H2O

H2

H2O

H2 + H2O system

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)

Lokshin et al, PRL 2004

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

.. ..

H2 + H2O system

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)

W. Mao et al, Science 2002Lokshin et al, PRL 2004

Somayazulu et al, Science 1996

H2 + CH4 system

CH4(H2)2

H2 + CH4 system

Needs more characterization Structure Hydrogen occupancy Phase diagram Metastable synthesis paths

0.4 GPa, 86 K 1.0 GPa, 120 K

H4Mcrystals

liquid

0.4 GPa, 86 K 1.0 GPa, 120 K

H4Mcrystals

liquid

W. Mao et al, CPL 2005

CH4(H2)4 has 33.4 wt% H2

(not including H in CH4)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 5 10 15 20 25 30 35 40

Liquidhydrogen

100

density: 5 g/cm3

2 g/cm3

1 g/cm3

0.7 g/cm3

CH4(H2)4

Light element hydridesTransition

metal hydridesHydrocarbons

2007

2015

HHsII

C2

2010

Gravimetric H2 density (wt %)

Vol

umet

ric H

2 de

nsity

(kg

H2/

L)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 5 10 15 20 25 30 35 40

Liquidhydrogen

100

density: 5 g/cm3

2 g/cm3

1 g/cm3

0.7 g/cm3

CH4(H2)4

Light element hydridesTransition

metal hydridesHydrocarbons

2007

2015

HHsII

C2

2010

Gravimetric H2 density (wt %)

Vol

umet

ric H

2 de

nsity

(kg

H2/

L)

W. Mao et al, Physics Today 2007

Hydrogen storage capacity

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