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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)