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