Vehicular

Hydrogen Storage with

ad(ab)sorbent Materials

Channing Ahn,Caltech

Hydrogen and high end useShuttle launches with100,000 kg of LH2

alkaline fuel cell 65%efficiency for electricity.

Aircraft - highflame propagationspeed high energyto wt ratio but lowenergy to vol.

X-43A (Mach 7-10)hydrogen-fueled.

Boeing A2 ondrawing board.Mach 5.5(Concorde wasMach 2).

Planck Mission cryo-cooler

LaNiSn metal hydrides for use in sorbent beds ofclosed cycle Joule-Thomson device

R. C. Bowman, Jr.

Terrestrial mobile storage challenges

Vehicles are beingdesigned by OEMs thatcan achieve > 300 miles(500 km)

• 350 or 700 bar

• 1 to 4 tanks

• Specified range from~200 to > 350 miles

But performance, spaceon-board and cost are stillchallenges for massmarket penetration…

Is there a low pressurealternative?

Courtesy G. Sandrock

Hydrogen Storage basics

Chemical hydrides also being studiedbut dehydrogenation exothermic sono onboard refueling with presentsystems.

http://www.eere.energy.gov/

What about the real (vs ideal) gas law? H2 behavior at 77, 298 and 573K

Gaspak equations here updated fromhydrogen properties from NBS TechnicalMonograph 168, February 1981, (R. D.McCarty, J. Hord and H. M. Roder). Equationof state valid from triple point to 5000 K,pressures to 1200 bar.

At pressures to 100 bar, real and ideal gasbehavior similar for both 77K and RT

35

30

25

20

15

10

5

0

density (

mole

s/liter)

10008006004002000

Pressure (bar)

70

60

50

40

30

20

10

0

density (

gm

/liter)

Real Gas 77K(Gaspak)

Real Gas 298.15 K(Gaspak)

Ideal 298.15 K

Ideal 77K

14

12

10

8

6

4

2

0

density (

mole

s/liter)

100806040200

Pressure (bar)

30

25

20

15

10

5

0

density (

gm

/liter)

Real gas 77K Ideal gas 77K Real gas 298K Ideal gas 298K

Comparison of Gaspak and Refprop at top.

80

60

40

20

0

H2 d

ensity (

kg/m

3)

120100806040200

Pressure (MPa)

NIST refprop 77 K NIST refprop 298 K NIST refprop 573 K Gaspak 77 K Gaspak 298 K Gaspak 573 K

Heat generated for several reaction enthalpies

600

500

400

300

200

100

0

genera

ted h

eat

during (

re)h

ydro

genation (

kW

)

109876543

Rehydrogenation time (minutes)

40 kJ/mole

10 kJ/mole

4 kJ/mole

heat required to bring 80 gallons

water from RT to 100°C

Its all about enthalpy (and a bit of entropy)!

What is the equilibrium pressure of H2 in anadsorbent?

Functional form is the Langmuir isotherm.

What is the equilibrium pressure of H2 inan absorbent?

Functional form is the van’t Hoff plot.

where, p = gaspressure and

0.1

2

4

6

81

2

4

6

810

2

4

6

8100

Dis

socia

tion P

ress

ure

(bar)

4.03.53.02.52.01.5

1000/T (K-1

)

-20050100200300400 20

NaAlH4

Na3AlH6

TiCr1.8H1.7

Mg2FeH6

TiFeH

MmNi5H6

LaNi5H6

CaNi5H4

LaNi4Al

Pd-PdH0.6

LaNi3.5Al1.5

MgH2

Mg2NiH4

Temperature (°C)

Porous solids and adsorption isotherms

Langmuir (1916) recognized problemswith describing sorption in charcoal

‘it is impossible to know definitely thearea on which the adsorption takesplace,’

equations for planer sorption notapplicable to 3d case.

IUPAC isotherm definitions above.

Micropore region of most relevance tohydrogen.

No multilayer phenomena expected.

Electron correlations weak.

Real isotherms and dependencies

4

3

2

1

0

Exc

ess

grav

imet

ric d

ensi

ty (

wt%

)

403020100

Pressure (bar)

2000 m2/gm (TFB170CRF_2HA_b)1460 m2/gm (CRF_4HA) 330 m2

/gm (RF_200)

Surface area dependent behavior synthesizedby T. Baumann and J. Satcher, LLNL

Temperature dependent behaviorfrom IRMOF-1

Why do we get maxima in the adsorption isotherm?Surface excess adsorption; the gas/solid interface

Concentr

ati

on

adsorbed

layer, na

gas phaseremainder

solid

cg

cs=0

0 x1 x

Volume of adsorbed layer:

Amount adsorbed inadsorbed layer:

Total amount of adsorb-ablegas n in system:

Gibbs dividing plane

quantity definedas gas phase

Surface excessamount

GibbsDividingSurface

solid

0 x

For Gibbs dividing surface atsolid surface:

=>

Assume cg small and Va negligible

Excess adsorption

30

25

20

15

10

5

0

Exc

ess

Ads

orpt

ion

6543210

Pressure

Surface area and micropore/slit pore (< 2 nm) geometrydependencies

60

50

40

30

20

10

0

H2 m

ass r

atio

(m

g/g

m)

3000200010000

Specific surface area (m2/gm)

1.00.80.60.40.20.0

micropore volume (ml/gm)

surface area micropore volume

T = 77KP = 35 bar

Understand gravimetric surface areadependence in aerogels* and other carbons

*“Toward New Candidates for Hydrogen Storage: High-Surface-AreaCarbon Aerogels,” H. Kabbour, T. F. Baumann, J. H. Satcher, Jr., A.Saulnier and C. C. Ahn, Chem Mater, 18, (2006).

“Characterization andoptimization ofadsorbents for hydrogenstorage,“ R. Chahine, T.K. Bose, In 11th WHEC,1996; Pergamon Press:Oxford, U.K., 1996.

Details of micropore/slit pore geometry criticalfor volumetric optimization

“Graphene nanostructuresas tunable storage mediafor molecular hydrogen”S. Patchkovskii, J. S. Tse, S.N. Yurchenko, L. Zhechkov,T. Heine, and G. Seifert,PNAS, 102(30), 10439,2005.

“Molecular simulation of hydrogen adsorption in single-walled carbonnanotubes and idealized carbon slit pores,”Q. Wang and J. K. Johnson, J. Chem Phys. 110,(1) (1999).

7

6

5

4

3

2

1

0

H2 G

ravim

etr

ic D

ensity (

wt%

)

500040003000200010000

Surface Area (m2/gm)

CIT_MOF177

LLNL aerogels

ORNL_Baker

GM/CIT_MIL-53(Al)

CIT_MIL's

TRAC-1

MOF74

acf1603_10

Maxsorb (3350)

ORNL_SWNH

LLNL as-prepared aerogel (330 m2/gm)

Liq H2 on surface

fit to frameworks

32 gm/l

28 gm/l

How big is molecular hydrogen?

Landolt-Bornstein (P63/mmc)solid H2 at 4.2 K, a=3.76, c=6.14

Koresh and Sofferliq H2 (3.62Å, 3.44 x 3.85)

Nielsen, McTague and Ellensenadsorbed H2, 3.51Å

Commensurate 3 structure (LiC6)or HC3 => 2.7 wt% (5.4 wt%)

Incommensurate solid H2 on graphite=> 3.85 wt% (7.7 wt%).

Range of graphitic structures

Theoretical surface area of agraphene sheet is 2630 m2/gm.

Activated carbons can have highersurface areas of > 3200 m2/gm, edgecomponents important.

4.35 Å

1.42 Å

8.45 Å

3 structure as x-z cross-section

Incommensurate packing as x-z cross-section

Double packing as x-z cross-section

5.15 Å

1.42 Å

Calculating enthalpies

Gas adsorption calorimetry

combines difficulty of calorimetry andgravimetric analysis

Henry’s law (easy to do butapplicable at zero coveragepressure):

A = nRT

spreading pressure

n = kHp the specific surface excessamount

ln(n/p) = K1 + K2n + K3n2

Advantage is that linearity of semi-logplot extends above Henry’s lawregion.

Differential enthalpy of adsorptionat ‘zero’ coverage then

2.0

1.5

1.0

0.5

0.0

Excess g

ravim

etr

ic d

ensity (

wt%

)

1.00.80.60.40.20.0

Pressure (bar)

IRMOF-8 desorption U.Mich IRMOF-8 IRMOF-1 desorption U.Mich IRMOF-1

Cole, J. H.; Everett, D. H.; Marshall, C. T.; Paniego, A. R.; Powl, J. C.; RodriguezReinoso, F., J. Chem. Soc., Faraday Trans. 1974, I 70, 2154.

Henry’s law analysis cont’d

Plot ln(kH) vs 1/T

Henry Plots at 77K

y = -5.9315x + 3.0398

R2 = 0.9985

y = -3.4828x + 1.731

R2 = 0.9966

y = -5.4471x + 2.9968

R2 = 0.9997

-1.5

-1

-0.5

0

0.5

1

1.5

0.35 0.45 0.55 0.65 0.75 0.85

n/nsat

ln(n

/P)

Ni-doped CA

Co-doped CA

CRF-100 CA

Plot ln(n/p) vs n

Differential enthalpy of adsorption

y = 593.05x - 5.9533

R2 = 0.9992

y = 859.94x - 8.1454

R2 = 0.9991

y = 863.25x - 8.1635

R2 = 0.9999

-6

-5

-4

-3

-2

-1

0

1

2

3

4

0.002 0.007 0.012 0.017

1/T

ln(n

/P)

Ni-doped CA

Co-doped CA

CFR-100

Isosteric enthalpy of adsorption

y = -4.184x + 6.6503

y = -4.2402x + 7.1545

y = -3.9115x + 5.8252

y = -3.1179x + 4.2227

y = -4.3826x + 7.7685

y = -5.8335x + 10.127

y = -1.7587x + 1.6035

y = -2.4962x + 3.0485

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

1.1 1.15 1.2 1.25 1.3 1.35

To/T with To=100K

ln(p

/po(T

o/T

)1/2)

From Benard and ChahineInt. J. Hydrogen Energy, 26 (2001)

Data from IRMOF-1

Rouquerol, F.; Rouquerol, J.; Sing, K., Adsorption by

Powders and Porous Solids. Academic Press: New York,1999.

Increasing molecular hydrogen density via a micro-porousframework structure, MOF-74

Isotherm at left shows thatMOF 74 with SSA of 870m2/gm has surface excesssorption comparable to atypical 1850 m2/gm sorbent(here compared with MIL100and MIL101). Attributed tohigh surface density ofmolecular hydrogen.Volumetric density of 33kg/m3.

Sharper initial isotherm seenat left indicates highersorption enthalpy in Henry’slaw region.

5

4

3

2

1

0

H2 e

xcess g

ravim

etr

ic d

ensity (

wt%

)403020100

Pressure (bar)

MIL101 (BET SSA 2900 m2/gm)

MIL100 (BET SSA 1850 m2/gm)

MOF74 (BET SSA 870 m2/gm)

1.5

1.0

0.5

0.0

H2 e

xcess g

ravim

etr

ic d

ensity (

wt%

)

1.00.80.60.40.20.0Pressure (bar)

MOF-74

MOF-177

MOF-74 as shown above, one of thefew examples of a microporous metalorganic framework. R-3 space groupa=25.9 Å and c=6.8 Å. Zinc octahedra(ZnO6) share edges and form longchains parallel to c-axis. Organicunits link chains leading to channeledstructure with small hexagonalwindows of ~ 11Å diameter. Chainsprovide high density of exposedmetallic sites. Bulk density of 1.2gm/cc. Synthesis of 2 gms required 2months.

Closest hydrogen near neighbor distances in MOF 74 asdetermined at NIST

Rietveld refinement and neutron powder diffraction data for a loading at 3.0 D2:Zn (model showing loadingpositions at right). Cross, red line, green line, and blue line represent the experimental diffraction pattern,calculated pattern, calculated background, and the difference between experiment and calculatedpatterns. Data were collected using 2.0787 Å wavelength neutrons.

“Increasing the density of adsorbed hydrogen with coordinatively unsaturated metal centers in metal-organicframeworks,” Yun Liu, Houria Kabbour, Craig M. Brown, Dan A. Neumann, Channing C. Ahn, Langmuir publishedonline March 27, 2008, 10.1021/la703864a

Henry’s law heat for MOF-74

Henry’s law fit indicates -9.4 kJ/moledifferential enthalpy of adsorption at zerocoverage.

3

2

1

0

-1

-2

ln(kH)

0.60.40.20.0

n/nsat

-8

-6

-4

-2

0

2

4

ln(kH)

12840

1000/K

Henry’s law region (differential enthalpy ofadsorption at zero coverage) the heat ofinitial H2 molecule adsorbed onto surface.Non-linearity of semi-log fit an indication ofheterogeneity in sorption site.

Seen at both 77 and 87K.

H2 site occupancy and isosteric heat in MOF 74

Isosteric enthalpy of adsorption tracksenthalpy vs coverage. Initially loadedsite above coordinatively unsaturated Znas determined by neutron diffraction(NIST).

Near neighbor 2.85 Å D2-D2 spacing (site1 to site 3) helps explain high surfacepacking density (range from 2.85 to 4.2Å). 2.85 Å the closest packing of D2 seenunder these conditions! Normally 3.4 Åon graphitic carbon (~3.6 Å in liq H2).

Isosteric enthalpy of adsorption showsdrop by nearly a factor of 2 as a functionof loading. Site heterogeneity an issue.

8

6

4

2

0

H (

kJ/m

ol)

1.61.20.80.40.0

H2 excess gravimetric density (wt%)

Concept tank based on cryo adsorption

R. AhluwaliaArgonne Natl Lab

Assume 4kJ/mole sorption heat, and 0.5kg/min refuelingrate, then total power dissipation needs to be, for 5 kg of H2

is ~16kW, considering only sorption heat

Volumetric activated carbon (AC) analysis

Say high surface area AC ~0.3 gm/cc packing density andGiven 2.1 gm/cc skeletal density, let’s start with 1 liter volume

At 0.3 gm/cc 1 liter => 300 gm ofAC and 300gm/2.1gm/cc or ~0.143liter volume occupied by graphiticcarbon.

For a 5.5wt% hydrogen storagevalue, ~17.5gm H2 in 1 liter. Ifdensity of adsorbed layer similarto LH2 density, then ~250 cc isoccupied by adsorbed H2 gas and~600cc is left as “free” volume.

Let’s fill remainder of void(~600cc) with H2 using gas law.

Gas law contribution to remaining void

For an activated carbon then, where:

R=83.14 cc bar/mol K

At 77K, for pv/RT=n,

40 bar * 600cc(83.14 cc bar/mol K) *77K

= 3.75 mol or 7.5 gm H2

Add this to 17.5 gms H2 adsorbed so get~25 gm/liter total

Our “1 liter tank” has a total adsorbedand gas volume density of 25gH2/300gm AC/liter or 7.8wt%

A tank with 5kg capacity should thenhave an interior volume of ~200 liters.

Let’s apply this analysis to IRMOF-1

Total density is 0.6gm/cc and skeletaldensity is 2.9gm/cc.

In 1 liter, 600gm/2.9gm/cc -> 0.2 liters ofskeletal density of IRMOF.

Be generous and assume 5wt% => 32 gmshydrogen stored and assume this 32 gmsoccupies ~0.46 liters of “adsorbed” space.

Add this 0.46 liters of ‘adsorbed’ H2 to 0.2liters of IRMOF skeletal density and have0.34 liters “free” volume to apply gas lawto get an additional 2.1 gms H2 so ~34 gmstotal in 1 liter (~5.3wt%) or

147 liter interior volume for 5 kg storage.

Let’s optimize the carbon structure

Want graphitic structure with 2 out of 3 planes removed.

>1nm

Now we have instead of 2.1gm/cc as ingraphite, ~0.7 gm/cc packing density use allinterlayer space to pack H2 at 7.7wt%.

Given 2.1 gm/cc skeletal density, let’s startwith 1 liter volume

1 liter will have 700 gm of this idealizedcarbon, assume at 7.7wt%, this carbon willhold ~58 gms/liter

=> 86 liter tank necessary to store 5 kg of H2.

Summary of physiorption in aerogelsand coordination polymers

• Differential enthalpy of adsorption at ‘zero’ coverage different thanisosteric enthalpy of adsorption (which generally decreases withhydrogen loading).

• Higher surface aerogels behave as most carbons adsorbing~1wt%/500m2/gm so goal should be to produce as high a surface areasorbent as possible.

• Pore size should be optimized to be ~1 to 1.1nm to maximizevolumetric density.

• Coordination polymers interesting but overall gravimetric/volumetricdensity properties not much better than carbons.

Equilibrium Behavior of Metal HydridesDissociation pressure isotherms for Li-LiH1

Equilibrium pressure vscondensed phase comp.

Plateau pressure regionwhere pressure independentof composition.

3 phases in equilibrium, twocondensed phases and H2

gas.

Assumptions:1) phase on left a

saturated solution of hydridein metal.

2) phase on rightsaturated solution of metal inhydride.

3) chemical reaction ofplateau region is:

1C. E. Messer, “A survey report on lithium hydride,” USAEC Report NYO-9470, 1960

van’t Hoff data for the Ca-CaH2 system

From W.D. Treadwell and J. Sticher, “Uber den Wasser-stoffdruck von Calciumhydrid, Helv. Chim. Acta 36 (1953)

Equilibrium Behavior of Metal Hydrides

NaAlH4 5.6% total

Na3AlH6

MgH27.6%

calculatedLiBH413.6 %LiH

12.6 %

Approximatedesiredoperatingregime

LaNi5H6 1.4%

0.001

0.01

0.1

1

10

100

Pre

ssu

re

(bar

)

3.53.02.52.01.51.0

1000/T (K-1)

725 °C 400 °C 150 °C 50°C

From J. Vajo, HRL Laboratories

Destabilizing high H hydrides

Alloy cohesive energy is small (~ 5 kJ/atom), therefore destabilization is small

* Reilly and Wiswall (1967)

PURE MgH2 Mg2CuE

NE

RG

Y (

kJ

/mo

l-H

2)

75.3

0

Mg + H2 (7.6%)

MgH2

75.3

0 MgH2 + 1/3MgCu2

67.7

(72.8)2/3 Mg2Cu + H2 (2.6%)

T(1 bar) = 250°C (239°C)

Mg + 1/3Mg2Cu + H2

T(1 bar) = 275°C

Destabilization in LiH

Cohesive energy of LinSi is large (~30 kJ/Li atom) destabilization is large1703-05-00

PURE LiH LiH/Si

181 2Li + H2 (12.6%)

0 2LiH

T(1 bar) = 900°C

EN

ER

GY

(k

J/m

ol-

H2)

181 2Li + 2/nSi + H2

0 2LiH + 2/nSi

~120 Li2.35Si + H2 (5.0%)

T(1 bar) = 490°C

Li4.4 Si

Li1.71 Si

2

2.35

Vajo, J. J. Mertens, F. Ahn, C. C. Bowman, R. C., Jr. Fultz, B., J. Phys. Chem. B 2004, 108, 13977.

LiH + Si kinetic behavior

LiH/Si System: Isotherms* for 2.5LiH + Si

System is reversible; Peq sensitive measure of H(LinSi)

* R. C. Bowman, Jr., JPL

0.01

0.1

1

10

Pre

ssu

re

(bar

)

543210

Weight Percent

2.52.01.51.00.50.0

x (H/Li2.5Si)

476 °C

absorption

desorption

Li2.35Si - confirmed by x-ray

Li1.71Si

confirmed by x-ray

Si + LiH

confirmed

by x-ray

New LiSiHx phase

LiH/Si System: van't Hoff Plot

NaAlH4 5.6% total

Na3AlH6

MgH27.6%

calculatedpure LiBH413.6 %pure LiH

12.6 %

Li2.35Si 5.0 %

Approximatedesiredoperatingregime

LaNi5H6 1.4%

725 °C 400 °C 150 °C 50°C

0.001

0.01

0.1

1

10

100

Pre

ssu

re

(bar

)

3.53.02.52.01.51.0

1000/T (K-1)

Addition of Si increases Peq by > 104 x, decreases H by 60 kJ/mol-H2

From J. Vajo, HRL Laboratories

Theoretical screening ofdestabilization reactions

Large-Scale Screening of Metal HydrideMixtures for High-Capacity HydrogenStorage from First-Principles Calculations

Alapati, Johnson and Sholl, J. Phys. Chem.C, March 2008.

First-principles calculations have been used tosystematically screen >16 million mixtures of metalhydrides and related compounds to find materials withhigh capacity and favorable thermodynamics forreversible storage of hydrogen. These calculations arebased on a library of crystal structures of >200 solidcompounds comprised of Al, B, C, Ca, K, Li, Mg, N, Na,Sc, Si, Ti, V, and H. Thermodynamic calculations areused that rigorously describe the reactions availablewithin this large library of compounds. Our calculationsextend previous efforts to screen mixtures of this kind byorders of magnitude and, more importantly, identifymultiple reactions that are predicted to have favorableproperties for reversible hydrogen storage.

The ScH2 + 2LiBH4 ScB2 + 2LiH + 5/2H2 System: Experimental Assessment ofChemical Destabilizationa

Destabilization reaction with idealthermodynamic properties (DFT)

H300K = 34.1kJ/molb

8.91 wt% capacityIsothermal kinetic desorptionmeasurments (450ºC)

4.5 wt% desorption

Powder XRD detects only LiH andScH2 crystalline phases in desorptionproduct, i.e. no reaction under theseconditions between ScH2 and LiBH4.Stability of ScH2, ScB2 plays criticalrole in determining overall kinetics.

aHydrogen Sorption Behavior of the ScH2-LiBH4 System:Experimental Assesment of Chemical Destabilization Effects,Justin Purewal, Son-Jong Hwang, Robert C. Bowman, Jr., Ewa

Rönnebro, Brent Fultz and Channing Ahn, accepted for

publication in J. Phys. Chem. C (2008).

bCalculated by Center partner David Sholl and published as Alapati, S. V.; Johnson, J. K.; Sholl, D. S., J. Alloys Compd. 2007,446–447, 23.

Te

mp

era

ture

°C

Gra

vim

etr

ic d

en

sity

(w

t%)

Inte

nsi

ty

The ScH2+2LiBH4 ScB2 + 2LiH + 5/2H2 System (continued): MAS-NMR Spectroscopy

(a) Bloch decay 11B MAS NMR spectra with neat LiBH4

(Sigma-Aldrich) added as a reference. No change ofLiBH4 peak observed after ball milling. 11B MAS andCPMAS spectra (a and b) show formation of elementalboron in the amorphous phase (broad shoulder at ~ 5ppm) and formation of intermediate phase (with peak at-12 ppm), recently identified* as [B12H12]

2- species. Notethat a 11B MAS NMR spectrum of ScB2+2LiH systemafter absorption treatment at high H2 pressure (896 bar,and 460°C for 48 hr) also included where the broaderspinning sidebands in this spectrum are due to un-reacted ScB2. The small peak at -41 ppm, indicatesvery limited LiBH4 formation (~ 3%) in the reactionproduct. So reverse reaction is seen but undertechnologically challenging conditions.

(b) 11B MAS and CPMAS NMR spectra (contact time=0.1ms) of desorbed sample.

*NMR Confirmation for Formation of [B12H12]2- Complexes during Hydrogen

Desorption from Metal Borohydrides, Son-Jong Hwang, Robert C. Bowman,Jr., Joseph W. Reiter, Job Rijssenbeek, Grigorii L. Soloveichik, Ji-ChengZhao, Houria Kabbour, and Channing C. Ahn, J. Phys Chem. C, 112, 3164-3169, 2008.

Solid state diffusion in ScH2+2LiBH4 =>ScB2+2LiH+4H2

z

yx

y

xz

yx

z

x

y

z

Sc

B

H

Li

ScH2

LiBH4

LiH

ScB2

x

y

z

B

Borohydride analysis: formation of intermediates

NMR Confirmation for Formation of [B12H12]2- Complexes

during Hydrogen Desorption from Metal BorohydridesSon-Jong Hwang,*,† Robert C. Bowman, Jr.‡ Joseph W.Reiter,‡ Job Rijssenbeek,§ Grigorii L. Soloveichik,§ Ji-Cheng Zhao,§ Houria Kabbour,| and Channing C. Ahn,J. Phys Chem. C. (2008).

11B NMR spectroscopy has been employed to identifythe reaction intermediates and products formed in theamorphous phase during the thermal hydrogendesorption of metal tetrahydroborates (borohydrides)LiBH4, Mg(BH4)2, LiSc(BH4) 4, and the mixed Ca(AlH4)2-LiBH4 system. The 11B magic angle spinning (MAS) andcross polarization magic angle spinning (CPMAS)spectral features of the amorphous intermediatespecies closely coincide with those of a modelcompound, closo-borane K2B12H12 that contains the[B12H12]

2- anion. The presence of [B12H12]2- in the

partially decomposed borohydrides was furtherconfirmed by high-resolution solution 11B and 1H NMRspectra after dissolution of the intermediate desorptionpowders in water. The formation of the closo-boranestructure is observed as a major intermediate species inall of the metal borohydride systems we haveexamined.

M(BH4)n -> M + nB + (2n)H2

M(BH4)n -> MHx + nB + [2n-x/2]H2

Assumed dehydrogenation reactions:

But, formation of M2/nB12H12 phases asmajor intermediate species asdetermined by NMR

Summary of hydride and destabilizationreactions

• Equilibrium thermodynamic evaluation a critical starting point indetermining absorbents of engineering value.

• Awareness of constituent formation enthalpies may also be animportant consideration for work where destabilization reactionenthalpies used for materials assessment.

• Reaction pathways not always predictable.

Acknowledgements:

Anne Dailly, now at GMAngelique Saulnier, now at SamsungHouria Kabbour, now at CNRS

John Vajo, HRL LaboratoriesR. C. Bowman Jr., JPL (consulting to DoE)Joe Reiter and Jason Zan, JPLSonjong Hwang, Caltech Solid State NMR facility

Justin PurewalNick StadieDavid Abrecht

Ted Baumann, LLNL

Support through: Energy Efficiency and Renewable Energy at DoE,Ned Stetson and Carole ReadGM, Scott Jorgensen and Jim Spearot

Sieverts apparatus

LN2

VR

VM

VL

AV3

AV4

AV5

AV1 AV0

AV2

3000 psi25000 torr

MV1

H2

He

Ar/N2

Pressurize manifold by pressurizing line before AV5.Open AV5. Wait for equilibration (almost instantaneous).Record pressure (from PR4000 and divide by 4) and manifold temperature (from panel readout).Open AV3. Wait for equilibration (almost instantaneous).Record pressure (from PR4000 and divide by 4) and manifold temperature (from panel readout).Open manual valve to sample. Wait for equilibration (5 min. should be enough).Record pressure (from PR4000 and divide by 4) and manifold temperature (from panel readout).

Isotherms from metal organic frameworks

5

4

3

2

1

050403020100

Pressure (bar)

A.C. 77K A.C. desorption IRMOF-1 77K IRMOF-1 des. IRMOF-8 77K IRMOF-8 des. IRMOF-8 RT IRMOF-1 RT

5

4

3

2

1

014121086420

Pressure (bar)

2.0

1.5

1.0

0.5

0.0

Excess g

ravim

etr

ic d

ensity (

wt%

)

1.00.80.60.40.20.0

Pressure (bar)

IRMOF-8 desorption U.Mich IRMOF-8 IRMOF-1 desorption U.Mich IRMOF-1

Low pressure data to 1 bar not a predictor of saturation behavior, only an indication of sorption heat(high slope => higher heat, IRMOF-1 ~4kJ/mole, IRMOF-8 ~6kJ/mole). Activated carbon samplefrom F. Baker and N. Gallego, ORNL.

A. Dailly, J. J. Vajo and C. C. Ahn, J. Phys. Chem B, v110, p1099, (2006)