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CARBON-BASED AEROGELS:AMORPHOUS CARBON, NANOTUBE, GRAPHENE, DIAMOND, AND FULLERENE

COPYRIGHT © 2019, AEROGEL TECHNOLOGIES, LLC. ALL RIGHTS RESERVED.

DR. STEPHEN A. STEINER IIIPRESIDENT & CEOAEROGEL TECHNOLOGIES, LLCBOSTON, MA

ISGS 2019 WORKSHOPAUGUST 25, 2019

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CARBON-BASED AEROGELS

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RF Sol RF Gel RFAerogel

CarbonAerogel

80°C3 days

SolventExchanges

SupercriticalCO2 Drying

PyrolysisN2 or Ar

1050°C 10 hrs

600-1050°C

10.5 hoursAr

RF AEROGEL IS PYROLYZED UNDER INERT ATMOSPHERE LEAVING BEHIND CARBON IN SAME MORPHOLOGY

TRADITIONAL AMORPHOUS CARBON AEROGELSMADE BY PYROLYZING AROMATIC POLYMER AEROGELS

Resorcinol-formaldehyde, an aromatic phenolic polymer

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TRADITIONAL AMORPHOUS CARBON AEROGELSMADE BY PYROLYZING AROMATIC POLYMER AEROGELS

§ Many well-described starting material chemistries available including resorcinol-formaldehyde and other related phenolic systems, polyureas, polyimides, and polybenzoxazines

§ Isomorphic with polymer precursor, with 40-60% residue retention and approximately no change in density

§ 2-nm crystallite sizes and typically around 7-nm mean pore size§ Electrical conductivity desirable for electrodes and electrochemistry§ High surface areas, typically around 700 m2/g§ Can be activated with CO2 to make even higher surface areas, up to 3000 m2/g

§ CRITICAL TRADEOFF IN ELECTRICAL CONDUCTIVITY AND SURFACE AREA—electrical conductivity of 2.5 s/m at 0.1 g/cc that drops with decreasing density

§ MECHANICALLY STIFF AND BRITTLE, consistency of charcoal

See for example Baumann, T., et al., J. Non-Crystalline Solids 354, (2008) 3513–3515

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OH

OH

H H

O

H H

OOH

OH

CH2OH

OH

OH

O

OH

OH

O

OH

OHOH

OH OH

OH

O

OH

OH

+

Resorcinol

Formaldehyde

Resorcinol-formaldehyde (RF)Polymer Chains

≈

Polymer ChainsForm Nanoparticles/

Nanofibrils

Nanoparticles/Nanofibrils Form

Gel Network

RESORCINOL-FORMALDEHYDE POLYMER GELS

Na2CO3

in water

80-90°C24-72 h

§ Size and number of clusters controlled by [resorcinol]/[catalyst] (R/C) ratio

§ Values of 50-300 provide acceptable range in which transparent gels can be made

§ Solutions with <7% reactants cured for 7 d at 80-95°C; >7% cured for 1 d at 50°C then by 3 d at 80-95°C

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RF POLYMER SOL-GEL MECHANISM§ Resorcinol is trifunctional, formaldehyde can add to 2, 4, and 6 positions§ Hydroxyl groups are electron donating and ortho/para directing§ Substituted rings condense with each other to form nanoparticles via several reactions

R-CH2OH

R-CH2OH HO-H2C-R’+ R-CH2-O-H2C-R’

R-CH2OH HO-R’+ R-CH2-R’

+ HOH

+ HOH

R-CH2-O-H2C-R’ CH2OR-CH2-R’ +

1

2a

2b

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R-H + CH2OFormaldehyde Addition

Hydroxymethylene Condensation

Hydroxymethylene-Hydroxyl Condensation

Methylene Ether Disproportionation

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PROCESS PARAMETER-PROPERTY RELATIONSHIPSDENSITY VS. R/C RATIO SURFACE AREA VS. R/C RATIO

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MECHANICAL PROPERTIES

COMPRESSIVE STRENGTH VS. DENSITY COMPRESSIVE STIFFNESS VS. DENSITY

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DEGREE OF GRAPHITIZATIONRAMAN SPECTRUM AT 785 nm

D BAND G BAND

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ALTERNATE ROUTE TO RF GELS§ Resorcinol-formaldehyde gels can easily be made in one step at room temperature§ Solvent is acetonitrile instead of water§ Uses HCl as catalyst instead of base§ Eliminates need for high temperature step simplifying molding and eliminating bubbling§ Reduces processing time from 3-7 days to a few hours at room temperature§ Acidic pH and non-aqueous solvent compatibilizes RF chemistry with chemistry used for

epoxide-asssisted gelation of oxides, enabling production of organic-inorganic interpenetrating networks used as precursors for metal aerogels

§ See S. Mulik, C. Sotiriou-Leventis and N. Leventis, Chem. Mater., 2007, 19, 6138

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MACROPOROUS ACID-CATALYZED RF AEROGELS§ Macroporous i.e. multi-micron pore carbon foams§ Uses acetic acid instead of sodium carbonate§ R/C ratio is much larger, e.g., 1300§ Concentration of monomers in solution is 25-55%§ Gels cured at 20°C 1 day, 50°C 1 day, and 90°C for 3 d§ Bone/coral-like structure§ Gel is hard like wood and is incredibly strong, sticks to

molds, hardly shrinks§ Orange to red color§ Pyrolyzable to isomorphic carbon foam§ Can be activated with CO2 etching to achieve monolithic,

strong parts with >3000 m2/g surface area§ Useful for making hierarchically porous carbons with

improved mass transport§ See Brandt, et al., J. Porous Materials, 10, 171–178, 2003

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CHARACTERIZATION METHODS§ Nitrogen sorptimetry—surface area from BET, pore size statistics from BJH model§ Bulk density—dimensional analysis§ Skeletal density—helium pycnometry§ Electrical conductivity—very tricky, four-point probe method using conductive pastes§ Thermal conductivity—calibrated hot plate for small samples, guarded heat flow

meter for large samples§ Powder X-ray diffraction (XRD)—particle size and crystallinity§ X-ray photoelectron spectroscopy (XPS)—degree of oxygenation, elemental purity,

presence and chemical states of dopants§ Scanning electron microscopy (SEM)—imaging and morphology analysis§ Transmission electron microscopy (TEM)—higher resolution imaging and

morphology analysis§ Raman spectroscopy—relative amounts of graphitic vs. defective carbon

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MULTIFUNCTIONAL PHENOLIC MONOMERSOH

OHOH

OH

OHOH

OH

OH

OH

O OH

OH

OH

O

NH2

NH2NH2

N

N

N

NH2

NH2NH2

N

N

N

§ Melamine and formaldehyde are mixed in ratio of 1:3.7 in water

§ NaOH (10-100 millimoles) is used as base polymerization catalyst

§ Melamine is a crystalline solid with limited water solubility, so the above slurry is heated for ~15 min at 70°C to form a clear solution

§ Solution is then cooled to 45°C and acidified with HC1 to pH=1.5-1.8 at RT

§ Affords translucent and clear gels

PhloroglucinolMore reactive, higher

crosslinking

2,4-dihydroxybenzoic acidProvides an ion exchange site

for doping with metals,neutralize with K2CO3 then gel

MelamineHexafunctional, provides high

crosslinking density

Pekala, et al., J. Non-Crystalline Solids 145 (1992) 90-98

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OTHER CARBONIZABLE POLYMER SYSTEMS

OCN

NCO

NCO

+ NCO-R →

→ + CO2 ↑

OO

OO

OO

R

O

NOO

OO

RN

OO

OO

OO

R

O

NOO

OO

O + NCO-R →

→ + CO2 ↑

OO

OO

OO

R

O

NOO

OO

RN

OO

OO

OO

R

O

NOO

OO

O

OOOOOO

OOOO

OO

OOOOOO

OOOO

OO

Polyimides

Polyureas

Polybenzoxazines

See for example Leventis, et al., Chem. Mater., 2014, 26, 1303−1317and Leventis, et al., Chem. Mater., 2016, 28, 67−78

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String of Pearls MorphologyElement is a Sphere

(Examples: Silica, many metal oxides)

NETWORK MORPHOLOGIES

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Leaflike MorphologyElement is a Filamentary Structure

(Examples: Alumina, acid-catalyzed silica)

NETWORK MORPHOLOGIES

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Wormlike MorphologyElement is a Tubule

(Examples: Vanadia, some organic polymers)

NETWORK MORPHOLOGIES

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Fibrous MorphologyElement is a high-aspect-ratio fibril(Examples: Some polyureas, CNTs)

NETWORK MORPHOLOGIES

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Sheetlike MorphologyElement is a sheet or platelette

(Examples: Graphene, boron nitride)

NETWORK MORPHOLOGIES

Sheetlike MorphologyElement is a Sheet

(Examples: Graphene, two-dimensional boron nitride)

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ACTIVATING CARBON AEROGELS

§ Activating means introducing micropores, i.e., <2 nm pores, into carbon structure§ Greatly increases surface area, up to 3200 m2/g!§ Can be performed with a standard electric clamshell furnace§ Example process: in a 2.5-cm quartz tube, flow 20 sccm CO2 with 100 sccm Ar at

800°C for 20 min over already pyrolyzed carbon aerogel monolith§ Results in hierarchical microporous/mesoporous morphology§ The more the material is etched, the weaker the monolith§ See Baumann, T., et al., J. Non-Crystalline Solids, 354, (2008) 3513–3515

800°C

20 minCO2 + Ar

CO2 + C 2CO

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DOPING WITH METALS§ RF gel is made with potassium salt of 2,4-dihydroxybenzoic acid instead of resorcinol§ Gels contain K+ -COOH ion exchange sites§ Gels can be soaked in aqueous or solvent-based solution of metal ions, e.g., iron, copper,

cobalt, and nickel§ Ion exchange sites act like diffusion skin that moves ions throughout gel monolith, without

these ions plug up outer surfaces§ Early transition metals like tantalum an tungsten can be introduced by dissolving MClx

compounds in DMF and exchanging into solution§ After exchanging into metal solution several times, exchange into water (or DMF) and then

solvent exchange into acetone or ethanol and supercritically dry from CO2

§ Pyrolysis results in carbon aerogels containing metal and/or metal carbide nanoparticles distributed throughout

§ Particle sizes and phases are a function of pyrolysis time and temperature§ Enables improved electrical conductivity, formation of nanocarbons inside aerogel, and

highly active catalysts§ See publications of Baumann et al., Fu et al., and Steiner III et al.

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APPLICATIONS OF CARBON AEROGELSSupercapacitors and Batteries High-

Surface-Area Electrodes

Desalination and Remediation

MultiwallNanotubes

Carbon AerogelWith Zirconia

50 nm Activity-EnhancingCatalystSupports

AerospaceSee Pekala et al.

See Steiner III, et al.

See Cooper-Bussman company and Rolison et al.

See Ratke and Milow et al.

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NANOCARBON AEROGELSAPPROACHES AND COMPOSITIONS

§ Nanocarbons include carbon nanotubes, graphene, fullerenes, and nanodiamonds§ Aerogels can be made out of these allotropes by

§ Assembling prefabricated nanocarbon structures into an aerogel§ Transforming a precursor into an isomorphic nanocarbon aerogel§ Depositing a nanocarbon aerogel via chemical vapor deposition onto a template§ Elastic smokes formed during chemical vapor deposition synthesis of carbon

nanotubes§ Different nanocarbon structures and amorphous carbon can be combined to make

different aerogels with surprising properties§ Most of the work on nanocarbon aerogels involves assembling prefabricated

nanocarbon structures

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CARBON NANOTUBES (CNTs)

1 nm 5-25 nm

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CARBON NANOTUBES (CNTs)

100 nm 10 1 cm

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HOW TO GROW CNTs

a few nm

Step 1: Provide a Nanoparticle “Seed”

metaloxide

nanodiamondsemimetal

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HOW TO GROW CNTsStep 2: Thermally, Chemically Activate Nanoparticle

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HOW TO GROW CNTsStep 3: Introduce Carbon-Containing Feedstock Gases

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HOW TO GROW CNTsStep 4: Allow Reactions to Occur on/in Nanoparticle

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§ MADE FROM PHYSICAL GELS OF CNTs BOUND WITH POLYVINYL ALCOHOL AND OTHER SURFACTANTS

§ CAN BE REMARKABLY ELASTIC—UP TO 80% ELASTIC DEFLECTION

§ BETTER ELECTRICAL CONDUCTIVITY AT LOW DENSITIES THAN CARBON AEROGELS—0.001-100 S/m AT 7.5 mg/cc VS. 2.5 S/m FOR 0.1 g/cc CARBON AEROGELS

§ CAN BE MADE BY FREEZE DRYING, AVOIDING SUPERCRITICAL DRYING

CARBON NANOTUBE AEROGELS

Mateusz, B., Adv. Mater. 14, 2007, 661.

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MADE FROM DISPERSED DWNTs + RF GLUE AND PYROLYZINGCNT/AMORPHOUS CARBON AEROGEL NANOCOMPOSITES

§ Nanocomposite of double-walled carbon nanotubes glued together with amorphous carbon

§ CNTs are dispersed with surfactant sodium dodecylbenzenesulfonate and mixed with RF sol

§ 585-650 m2/g surface area§ 5-8.1 S/m electrical conductivity—about

100-120% better than carbon aerogelsWorsley, M., Langmuir 24, 17, 2008, 9765.

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MADE FROM GRAPHENE OXIDE REDUCED BY ETHYLENE DIAMINE AND MICROWAVESHIGHLY ELASTIC GRAPHENE AEROGELS

§ Graphene oxide is dispersed in aqueous solution with ethylene diamine and crosslinked by reduction into a gel by heating to 95°C

§ Gel is freeze dried and functionalized graphene aerogel is further reduced to graphene under microwave irradiation

§ Aerogels are superelastic with up to 90% elastic compression

Hu, H. et al., Adv. Mater. 25, 2013, 2215.

In a typical procedure, graphene oxide dispersion (3 mg mL-1 , 5 mL) was mixed uniformly with ethylenediamine (20 μL) and then sealed in a glass vial and heated for 6 h at 95°C for synthesis of functionalized graphene hydrogel (FGH). After freeze-drying, functionalized graphene aerogel (FGA) was produced. Then, the aerogel was exposed to microwave irradiation in an inert atmosphere for 1 min to give rise to the final graphene aerogel.

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MADE FROM FREEZE CASTING GIANT GRAPHENE OXIDE AND CNT SUSPENSIONSSOL-CRYO METHOD FOR GRAPHENE/CNT AEROGELS

§ Dispersions of giant graphene oxide and carbon nanotubes in water are directly freeze dried

§ Avoids gelation step and uses sol directly§ Resulting aerogel-like material is reduced in hydrazine vapor§ Very scalable§ Densities of 0.16 mg/cc-22.4 mg/cc (on top of air) are

produced

Sun, H. et al., Adv. Mater. 25, 2013, 2554.

To a 100 mL beaker containing GGO aqueous dispersion (1.0 mg/mL-1 , 28 mL), CNTs aqueous dispersion (1.0 mg mL-1, 28 mL) was added. The mixture was stirred with a magnetic bar for 1.5 h, and then poured into the desired mold followed by freeze-drying for 2 days. The as-prepared GGO/CNTs foam (~57 mg) was chemically reduced by hydrazine vapor at 90°C for24 h, followed by vacuum-drying at 160°C for 24 h, affording 42.6 mg of graphene/CNT aerogel (ρ=0.75 mg cm-3 , f = 0.5).

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MADE FROM GRAPHENE OXIDE + RF GLUE AND PYROLYZINGHIGHLY CONDUCTIVE GRAPHENE AEROGELS

§ Amorphous carbon crystallites not seen—pyrolysis reduces graphene oxide and incorporates resorcinol-formaldehyde polymer into graphene matrix

§ 584-1300 m2/g vs 2600 m2/g for graphene sheet§ 87-100 S/m electrical conductivity—two orders of magnitude higher than physical graphene aerogels§ Up to 50 MPa Young’s modulus

Worsley, M. et al., JACS 132, 2010, 14067.Worsley, M. et al., Chem. Commun. 48, 2012,, 8428.

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MADE BY COMPRESSING AMORPHOUS CARBON AEROGEL IN A DIAMOND ANVILNANODIAMOND AEROGELS

AMORPHOUS CARBON AEROGEL

NANODIAMOND AEROGEL

§ Amorphous carbon aerogels are phase transitioned into nanodiamond aerogels in a diamond anvil filled with 22,000 psi (151.6 MPa) of supercritical neon and compressed to 21-25.0 GPa of pressure and heated to about 1580 K by a laser

§ Aerogels convert from black to transparent§ Materials exhibit photoluminescent properties§ Retains morphology of amorphous

carbon aerogel precursor

Pauzauskie, P., et al., PNAS, 2011.

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FULLERENIC CARBON AEROGELS

5 nmZirconia Nanoparticles GraphitizeAmorphous Carbon Aerogels into

Fullerenes Upon Pyrolysis

CVD

AerogelWithZirconia

AerogelWithoutZirconia

Carbon AerogelWith Zirconia

Growth!

S. A. Steiner III, et al., J. American Chemical Society, 2009, 131, 12144.

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CARBON NANOTUBE AEROGELS AND FIBERS BY CVD

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OTHER NANOCARBON AEROGELS§ Aerographite—made by chemical vapor deposition of carbon onto

an evaporating zinc template§ Many, many, many graphene aerogel papers§ Growth of carbon nanotubes by floating catalyst chemical vapor

deposition to form elastic smokes used to make nanotube fibers§ Works of Rolison et al. and Long et al. on energy storage materials

with nanostructured carbon-containing aerogels§ CNT-reinforced polymer-crosslinked silica aerogels, Meador et al.§ Growth CNTs and fullerenes in silica aerogels by chemical vapor

deposition, Hunt et al.

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T H A N K Y O U

Dr. Stephen A. Steiner IIIssteiner@aerogeltechnologies.com

+1 (617) 800-0414 x701www.aerogeltechnologies.com

www.BuyAerogel.comwww.aerogel.orgAEROGEL.ORGAEROGEL.ORG