ISGS 2019 WORKSHOP CARBON-BASED AEROGELS: AUGUST 25, 2019 AMORPHOUS CARBON, NANOTUBE, GRAPHENE, DIAMOND, AND FULLERENE DR. STEPHEN A. STEINER III PRESIDENT & CEO AEROGEL TECHNOLOGIES, LLC BOSTON, MA 1 1 COPYRIGHT © 2019, AEROGEL TECHNOLOGIES, LLC. ALL RIGHTS RESERVED. CARBON-BASED AEROGELS 2 TRADITIONAL AMORPHOUS CARBON AEROGELS MADE BY PYROLYZING AROMATIC POLYMER AEROGELS Resorcinol-formaldehyde, an aromatic phenolic polymer RF Sol RF Gel RF Carbon Aerogel Aerogel 80°C Solvent Supercritical Pyrolysis 3 days Exchanges CO2 Drying N2 or Ar 1050°C 10 hrs RF AEROGEL IS PYROLYZED UNDER 600-1050°C INERT ATMOSPHERE LEAVING BEHIND Ar 10.5 hours CARBON IN SAME MORPHOLOGY 3 TRADITIONAL AMORPHOUS CARBON AEROGELS MADE 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 2 § Can be activated with CO2 to make even higher surface areas, up to 3000 m /g See for example Baumann, T., et al., J. Non-Crystalline Solids 354, (2008) 3513–3515 § 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 4 RESORCINOL-FORMALDEHYDE POLYMER GELS Formaldehyde OH O O OH OH OH O ≈ H H OH + OH O HO HO OH Na2CO3 CH OH H H in water 2 HO OH Resorcinol § Size and number of clusters OH OH OH controlled by [resorcinol]/ O [catalyst] (R/C) ratio OH OH § Values of 50-300 provide 80-90°C acceptable range in which Resorcinol-formaldehyde (RF) 24-72 h transparent gels can be made Polymer Chains § Solutions with <7% reactants Polymer Chains Nanoparticles/ cured for 7 d at 80-95°C; Form Nanoparticles/ Nanofibrils Form >7% cured for 1 d at 50°C Nanofibrils Gel Network then by 3 d at 80-95°C 5 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 Formaldehyde Addition 1 R-H + CH2O R-CH2OH Hydroxymethylene Condensation 2a R-CH2OH + HO-H2C-R’ R-CH2-O-H2C-R’ + HOH Hydroxymethylene-Hydroxyl Condensation 2b R-CH2OH + HO-R’ R-CH2-R’ + HOH Methylene Ether Disproportionation 3 R-CH2-O-H2C-R’ R-CH2-R’ + CH2O 6 PROCESS PARAMETER-PROPERTY RELATIONSHIPS DENSITY VS. R/C RATIO SURFACE AREA VS. R/C RATIO 7 MECHANICAL PROPERTIES COMPRESSIVE STRENGTH VS. DENSITY COMPRESSIVE STIFFNESS VS. DENSITY 8 DEGREE OF GRAPHITIZATION RAMAN SPECTRUM AT 785 nm D BAND G BAND 9 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 10 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 11 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 12 MULTIFUNCTIONAL PHENOLIC MONOMERS OH HO OH N H Phloroglucinol 2 More reactive, higher N N crosslinking § Melamine and formaldehyde are mixed in ratio of 1:3.7 H2 N N N H2 in water O OH Melamine § NaOH (10-100 millimoles) is used as base polymerization HO Hexafunctional, catalyst provides high § Melamine is a crystalline solid with limited water OH crosslinking density solubility, so the above slurry is heated for ~15 min at 70°C to form a clear solution 2,4-dihydroxybenzoic acid § Solution is then cooled to 45°C and acidified with HC1 to Provides an ion exchange site pH=1.5-1.8 at RT for doping with metals, § Affords translucent and clear gels neutralize with K2CO3 then gel Pekala, et al., J. Non-Crystalline Solids 145 (1992) 90-9813 OTHER CARBONIZABLE POLYMER SYSTEMS Polyimides Polybenzoxazines NCO O O R N O O O O + NCO-R → O O O O O O O O O O O R O OCN NCO N O O O O → N R + CO2 ↑ O O O O O O O O O O O O O O O O O O O O O O Polyureas See for example Leventis, et al., Chem. Mater., 2014, 26, 1303−1317 14 and Leventis, et al., Chem. Mater., 2016, 28, 67−78 NETWORK MORPHOLOGIES String of Pearls Morphology Element is a Sphere (Examples: Silica, many metal oxides) 15 NETWORK MORPHOLOGIES Leaflike Morphology Element is a Filamentary Structure (Examples: Alumina, acid-catalyzed silica) 16 NETWORK MORPHOLOGIES Wormlike Morphology Element is a Tubule (Examples: Vanadia, some organic polymers) 17 NETWORK MORPHOLOGIES Fibrous Morphology Element is a high-aspect-ratio fibril (Examples: Some polyureas, CNTs) 18 NETWORK MORPHOLOGIES Sheetlike Morphology Element is a sheet or platelette (Examples: Graphene, boron nitride) Sheetlike Morphology Element is a Sheet (Examples: Graphene, two-dimensional boron nitride) 19 ACTIVATING CARBON AEROGELS CO2 + C 2CO 800°C CO2 + Ar 20 min § 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 20 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. 21 APPLICATIONS OF CARBON AEROGELS High- Desalination and Remediation Supercapacitors and Batteries Surface-Area Electrodes See Cooper-Bussman company and Rolison et al. 50 nm Multiwall Nanotubes Activity- Enhancing Catalyst Supports Carbon Aerogel With Zirconia See Steiner III, et al. Aerospace See Ratke and Milow et 22al. See Pekala et al. NANOCARBON AEROGELS APPROACHES 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 23 CARBON NANOTUBES (CNTs) 1 nm 5-25 nm 24 CARBON NANOTUBES (CNTs) 100 nm 10 1 cm 25 HOW TO GROW CNTs Step 1: Provide a Nanoparticle “Seed” nanodiamond semimetal oxide metal a few nm 26 HOW TO GROW CNTs Step 2: Thermally, Chemically Activate Nanoparticle 27 HOW TO GROW CNTs Step 3: Introduce Carbon-Containing Feedstock Gases 28 HOW TO GROW CNTs Step 4: Allow Reactions to Occur on/in Nanoparticle 29 CARBON NANOTUBE AEROGELS § 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.