Natural Gas Storage onNanoporous Carbon
Jacob Burress, Mikael Wood, Sarah Barker, John Flavin,Cintia Lapilli, Parag Shah, Galen Suppes, Peter Pfeifer
Alliance for Collaborative Research in Alternative FuelTechnology
University of Missouri, Columbia, MO 65211
OverviewOverview
Powdered and monolithicPowdered and monolithicactivated carbons have beenactivated carbons have beenmade with corn cob as startingmade with corn cob as startingmaterial that have a largematerial that have a largemethane storage capacitymethane storage capacity
Pore Space StructurePore Space StructureAnalyzed:Analyzed:
small angle x-ray scatteringsmall angle x-ray scattering(SAXS)(SAXS)
computer simulations of porecomputer simulations of poreformationformation
nitrogen adsorption isothermsnitrogen adsorption isotherms
scanning and transmissionscanning and transmissionelectron microscopyelectron microscopy(SEM/TEM)(SEM/TEM)
methane adsorption isothermsmethane adsorption isotherms
Why are Nanopores Important?Why are Nanopores Important?
In narrow pores, vander Waals potentialsoverlap; creating adeep energy well
Max. CH4 capacity inpores of width 1.1 nm(simulations)
van der Waalspotential of CH4 in poreof width 1.1 nmEnergy loss more thanenough to compressCH4 into dense fluid;remaining energy heat
Binding energy:Binding energy:1717 kJ/mol kJ/mol
Nicholson (Carbon Vol. 36, 1998)
Why are Nanopores Important?Why are Nanopores Important?
~3.7 Å
WidthWidth~6 Å~6 Å
WidthWidth~11 Å~11 Å
WidthWidth~22 Å~22 Å
Definitions of Uptake ValuesDefinitions of Uptake Values
( ), , , ,1 BulkGas
Stored Chamber Sample Gas Chamber Gas Chamber Sample Chamber
Piece
m m m m m=
Stored GasStored Gas
PorePore
Adsorbed FilmAdsorbed FilmBulk (non-adsorbed) GasBulk (non-adsorbed) Gas
Absolute Adsorbed GasAbsolute Adsorbed Gas
( ), , , ,1
absolute BulkGasadsorbed Chamber Sample Gas Chamber Gas Chamber Sample Chamber AdsorbedFilm BulkGas
Skeletal
m m m m m V= +
GibbGibb’’s Excess Adsorbeds Excess AdsorbedGasGas
( ), , , ,1
Excess BulkGasAdsorbed Chamber Sample Gas Chamber Gas Chamber Sample Chamber
Skeletal
m m m m m=
Methane UptakeMethane Uptake
N/AN/A230-239 g/kg230-239 g/kgM/MM/M
180 L/L180 L/L176-182 L/L176-182 L/LV/VV/V
118 g/L118 g/L115-119 g/L115-119 g/LM/VM/V
ANG DOEANG DOETargetTarget
ALL-CRAFT BestALL-CRAFT BestPerformance S-Performance S-33/k33/k
Methane uptake measuredMethane uptake measuredgravimetrically on powdergravimetrically on powdersamples, monoliths measuredsamples, monoliths measuredvolumetrically as well.volumetrically as well.
Values below reported as amountValues below reported as amountof methane stored using a of methane stored using a ““powderpowderdensitydensity”” of 0.5 g/ml of 0.5 g/ml
Summary of Storage DensitiesSummary of Storage Densities
119 g/L
118 g/L
24.9 g/L
Small Angle X-ray ScatteringSmall Angle X-ray Scattering
2r2r
LL( ) 2 2
oI q r L=
Scattering from a cylinder, with finiteScattering from a cylinder, with finitethickness.thickness.
( )( )( ) ( )( )( ) ( )
( )2 2/ 2
1
4 2 2 2 2
0
sin cos * sinsin
sin cos
qL J qrI q d
q L r=
Scattering in the limit L>>rScattering in the limit L>>r
( )( ) ( )
( ) ( )
0
1 1 1
sin 1 cos1.
. , for
qLu qL
I q const duqL u qL
I q const qL L q r
=
� � �
( )2
I qL
( )1
I qqL
( ) 6
2.3Surface
D
D
I q q
Spanning Cluster Pore Space
Computer Modeling of PoreComputer Modeling of PoreFormationFormation
Two stage probabilisticcellular automata (PCA)rule (two separate PCA’sin succession).
Pore space opened frominside outPore space opened fromoutside in, creating aspanning cluster
This models a two stageactivation process.Qualitatively this modelfits well with observeddata.
Carbon
Non-Spanning Cluster Pore Space
SEM/TEMSEM/TEM
Due to the small size of theDue to the small size of thepores, ultra high resolutionpores, ultra high resolutionmode was used on themode was used on theHitachi S-4700 FESEM.Hitachi S-4700 FESEM.Beam energy was set to 5 kVBeam energy was set to 5 kVwith a small working distancewith a small working distance(3-4mm)(3-4mm)The beam energy was set toThe beam energy was set to100 and 120 kV for the JEOL100 and 120 kV for the JEOL1200EX TEM1200EX TEM
Top image SEM on sampleTop image SEM on sampleS-33/k, showing entrance toS-33/k, showing entrance topore networkpore network
Bottom image TEM onBottom image TEM onsample S-56 showing ~1.5sample S-56 showing ~1.5nm wide porenm wide pore
200 nm
200 nm
5.00 μm
S-33/k
Nitrogen Adsorption IsothermsNitrogen Adsorption Isotherms
Nitrogen isotherms showNitrogen isotherms showevidence of strongevidence of strongmicroporositymicroporosity
Plateau on linear isothermPlateau on linear isotherm
BET surface area of ~2,200BET surface area of ~2,200mm22/g for sample S-33/k/g for sample S-33/kSurface area of most recentSurface area of most recentsamples found to be ~3,000-samples found to be ~3,000-3,500 m3,500 m22/g/gGives total pore volume ofGives total pore volume of1.22 cc/g, porosity of 0.711.22 cc/g, porosity of 0.71Note: Surface area forNote: Surface area forgraphenegraphene sheet (both sides) sheet (both sides)is 2,965 mis 2,965 m22/g (/g (ChaeChae et. al. et. al.Nature Nature VolVol 427 2004) 427 2004)
Pore Size Distribution fromPore Size Distribution fromNitrogen IsothermNitrogen Isotherm
Done using non-local density functional theory (NLDFT) assumingDone using non-local density functional theory (NLDFT) assumingslit-shaped poresslit-shaped pores
Shows dominance of nanopores, especially pores width ~1.1nmShows dominance of nanopores, especially pores width ~1.1nm
S-33/k
Peak at ~1.13 nm
Methane Adsorption IsothermsMethane Adsorption IsothermsLangmuir givesLangmuir givesgood fit of datagood fit of datawhich is consistentwhich is consistentwith the hypothesiswith the hypothesisthat surface isthat surface iscovered primarilycovered primarilywith a singlewith a singlemonolayer ofmonolayer ofmethanemethaneLangmuirLangmuirparameter ofparameter ofb=0.814 MPab=0.814 MPa-1-1
Asymptotic value of 288.5 grams of adsorbed methane perAsymptotic value of 288.5 grams of adsorbed methane perkilogram carbonkilogram carbonLangmuir fit gives a binding energy of ~22.7 kJ/mol, whichLangmuir fit gives a binding energy of ~22.7 kJ/mol, whichis consistent with the high uptake valuesis consistent with the high uptake values
0.000
0.100
0.200
0.300
0.400
0.500
0.4
- 0
.6
0.6
- 1
.0
1.0
- 1
.5
1.5
- 2
.0
2.0
- 4
.0
4.0
- 6
.06.0
- 1
0.0
10 - 1
5
15 - 2
0
20 - 5
0
>50
Pore Width Range [nm]
Po
re V
olu
me [
cc/g
]
Pore Size Distribution fromPore Size Distribution fromMethane IsothermMethane Isotherm
Shows dominanceShows dominanceof nanopores,of nanopores,especially in poresespecially in poresof width 6-15 Åof width 6-15 Å
Gives total poreGives total porevolume of 1.513volume of 1.513cc/g, porosity ofcc/g, porosity of0.7520.752
Determined viaDetermined viamethod frommethod fromSosinSosin and Quinn, and Quinn,Carbon 34 1335Carbon 34 1335(1996)(1996)
S-33/k
Comparison of MethodsComparison of Methods
0.000
0.100
0.200
0.300
0.400
0.500
0.4
- 0.6
0.6
- 1.0
1.0
- 1.5
1.5
- 2.0
2.0
- 4.0
4.0
- 6.0
6.0
- 10.
0
10 -
15
15 -
20
20 -
50
>50
Pore Width Range [nm]
Po
re V
olu
me
[c
c/g
]
Methane
Nitrogen
~15~15~4~4N/AN/AN/AN/AN/AN/AN/AN/AN/AN/ASAXSSAXS
N/AN/A~11~110.0210.0210.0940.0941.1071.1071.2221.2220.7100.710NitrogenNitrogen
N/AN/A~11~110.0620.0620.2540.2541.1971.1971.5131.5130.7520.752MethaneMethane
AverageAverageNanoporeNanoporeLength [Å]Length [Å]
AverageAverageNanoporeNanoporeWidth [Å]Width [Å]
MacroporeMacropore(>50 nm)(>50 nm)VolumeVolume[cc/g][cc/g]
MesoporeMesopore(2(2––50 nm)50 nm)VolumeVolume[cc/g][cc/g]
MicroporeMicropore(pore diameter(pore diameter0.50.5––2 nm)2 nm)Volume [cc/g]Volume [cc/g]
Total PoreTotal PoreVolumeVolume[cc/g][cc/g]
PorosityPorosity
TEMTEM
Conclusions and FutureConclusions and Future
Activated Carbon made from Missouri corn cobActivated Carbon made from Missouri corn cobhas proven to be an excellent candidate forhas proven to be an excellent candidate foralternative fuel storage at a low cost.alternative fuel storage at a low cost.Department of energy target of 118 g/L has beenDepartment of energy target of 118 g/L has beenmet.met.Carbon has been analyzed with a variety ofCarbon has been analyzed with a variety ofmethods which give consistent results.methods which give consistent results.Carbon is still being optimized.Carbon is still being optimized.Still getting results from the test fixture on theStill getting results from the test fixture on thepickup.pickup.Shows promise for hydrogen storage as wellShows promise for hydrogen storage as well(Mikael Wood et. al. 1:51 PM today in room(Mikael Wood et. al. 1:51 PM today in room502).502).
200 nm5.00 μm
5.00 μm 100 nm
Density Functional Theory PoreDensity Functional Theory PoreVolume DistributionVolume Distribution
Non-Local Density Functional Theory (NLDFT)Non-Local Density Functional Theory (NLDFT)for slit-shaped pores was used.for slit-shaped pores was used.Relationship between this theory and theRelationship between this theory and theexperimental data is given by the generalizedexperimental data is given by the generalizedadsorption isotherm (GAI)adsorption isotherm (GAI)
where N(P/P0) is the experimental adsorption where N(P/P0) is the experimental adsorptionisotherm data, W is the pore width, N(P/P0,W) isisotherm data, W is the pore width, N(P/P0,W) isthe isotherm on a single pore of width W, andthe isotherm on a single pore of width W, andf(W) is the pore size distribution function.f(W) is the pore size distribution function.
( )0 0
,
MAX
MIN
W
W
P PN N W f W dWP P
=
Brunauer-Emmett-Teller SurfaceBrunauer-Emmett-Teller SurfaceAreaArea
Most widely used method for determination of surfaceMost widely used method for determination of surfacearea of solidsarea of solids
BET Formula given by:BET Formula given by:
where m is the mass of gas adsorbed at a relative pressurewhere m is the mass of gas adsorbed at a relative pressure(P/P(P/P00) (P) (P00 taken as the saturation pressure of adsorptive gas), taken as the saturation pressure of adsorptive gas),mmmonomono is the mass of adsorbate constituting a monolayer ofis the mass of adsorbate constituting a monolayer ofsurface coverage, and C is the BET constant.surface coverage, and C is the BET constant.
( )
0
0 0
1 1 1mono
PCPm
m P PCP P
=
+
BET Theory Cont.BET Theory Cont.
BET equation in linear form:BET equation in linear form:
NNAA is Avogadro is Avogadro’’s number, s number, AACrossSectionCrossSection is the cross-sectional area of is the cross-sectional area ofthe gas molecule, and M is the molecular mass of the gas.the gas molecule, and M is the molecular mass of the gas.
AACrossSectionCrossSection for Nitrogen is 16.2 Å for Nitrogen is 16.2 Å22/molecule/molecule
0
0
0
1 1
1 mono mono
PP C
PPCm CmPm
P
= + �
1
mono
Cslope
Cm=
1
mono
interceptCm
=
1
monomslope intercept
=+
mono A CrossSection
Total
m N ATotal Surface Area S
M= =
Methane Binding EnergyMethane Binding Energy
0
0
1( )
q
RTTb T Langmuir parameter e
p T= =
-1
0
0
293 152.4 ln 0.814
KJq MPa pg T
=
0
0
lnT
q specific heat of adsorption RT bpT
= =
( )0 0,
293
0.52
p T are the reference pressure and temperature
T is the Absolute temperature room temperature K
Universal Gas ConstantR is the Specific Gas Constant
Molar Mass
JR for methane
g K
=
=
Methane Binding EnergyMethane Binding Energy
00 /( )
3 3
1
8
U kT
x y z
kTb e
m=
Localized AdsorptionLocalized Adsorption
0 0( , ) min ( , , )z
U x y U x y z< <
100 2
( )x y zU h+ + +
1
12 1
A
0.814 MPa
3.00 10 s
(0.0160 kg/mol) /
293 K
z
b
m N
T
=
=
=
=
x y z
assume = =
NA = 22.7 kJ/mol
T.L. Hill, An Introduction to Statistical Thermodynamics
(Dover, 1986).G. Bomchil et al., “Structure and dynamics of methane
adsorbed on graphite.” Phil. Trans. Roy.
Soc. Lond. B 290, 537-552 (1980).
Why are Nanopores Important?Why are Nanopores Important?
3.7 Å ~11 Å1.9 Å~4 Å~22 Å
Small Angle X-ray ScatteringSmall Angle X-ray Scattering
Scattering in the limit L>>rScattering in the limit L>>r
( )1
I qqL( )
2I q
L( ) 2 2
oI q r L=
2r2r
LL
( )( ) ( )
( ) ( )
0
1 1 1
sin 1 cos1.
. , for
qLu qL
I q const duqL u qL
I q const qL L q r
=
� � �Scattering from a cylinder, with finiteScattering from a cylinder, with finitethickness.thickness.
( )( )( ) ( )( )( ) ( )
( )2 2/ 2
1
4 2 2 2 2
0
sin cos * sinsin
sin cos
qL J qrI q d
q L r=
2.3SurfaceD
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