We report the synthesis of ultra-low-density three-dimensional macroassemblies of graphene sheets that exhibit high electrical conductivities and large internal surface areas. These materials are ...prepared as monolithic solids from suspensions of single-layer graphene oxide in which organic sol−gel chemistry is used to cross-link the individual sheets. The resulting gels are supercritically dried and then thermally reduced to yield graphene aerogels with densities approaching 10 mg/cm3. In contrast to methods that utilize physical cross-links between GO, this approach provides covalent carbon bonding between the graphene sheets. These graphene aerogels exhibit an improvement in bulk electrical conductivity of more than 2 orders of magnitude (∼1 × 102 S/m) compared to graphene assemblies with physical cross-links alone (∼5 × 10−1 S/m). The graphene aerogels also possess large surface areas (584 m2/g) and pore volumes (2.96 cm3/g), making these materials viable candidates for use in energy storage, catalysis, and sensing applications.
Aerogel materials have myriad scientific and technological applications due to their large intrinsic surface areas and ultralow densities. However, creating a nanodiamond aerogel matrix has remained ...an outstanding and intriguing challenge. Here we report the high-pressure, high-temperature synthesis of a diamond aerogel from an amorphous carbon aerogel precursor using a laser-heated diamond anvil cell. Neon is used as a chemically inert, near-hydrostatic pressure medium that prevents collapse of the aerogel under pressure by conformally filling the aerogel's void volume. Electron and X-ray spectromicroscopy confirm the aerogel morphology and composition of the nanodiamond matrix. Time-resolved photoluminescence measurements of recovered material reveal the formation of both nitrogen- and silicon- vacancy point-defects, suggesting a broad range of applications for this nanocrystalline diamond aerogel.
Realization of macroscale three‐dimensional isotropic carbons that retain the exceptional electrical and mechanical properties of graphene sheets remains a challenge. Here, a method for fabricating ...graphene‐derived carbons (GDCs) with isotropic properties approaching those of individual graphene sheets is reported. This synthesis scheme relies on direct cross‐linking of graphene sheets via the functional groups in graphene oxide to maximize electronic transport and mechanical reinforcement between sheets and the partial restacking of the sheets to increase the material density to about 1 g cm‐3. These GDCs exhibit properties 3–6 orders of magnitude higher than previously reported 3D graphene assemblies.
High‐density, machinable, 3D graphene with properties approaching those observed in individual graphene sheets is presented. Synthesis relies upon sp2 carbon cross‐linking between partially restacked graphene sheets to achieve densities of ≈1 g cm‐3. The resulting material possesses electrical and mechanical properties 3–6 orders of magnitude higher than previously reported 3D graphene assemblies, and exceeds the properties of much denser commercial graphite.
Drawbacks of current carbon dioxide capture methods include corrosivity, evaporative losses and fouling. Separating the capture solvent from infrastructure and effluent gases via microencapsulation ...provides possible solutions to these issues. Here we report carbon capture materials that may enable low-cost and energy-efficient capture of carbon dioxide from flue gas. Polymer microcapsules composed of liquid carbonate cores and highly permeable silicone shells are produced by microfluidic assembly. This motif couples the capacity and selectivity of liquid sorbents with high surface area to facilitate rapid and controlled carbon dioxide uptake and release over repeated cycles. While mass transport across the capsule shell is slightly lower relative to neat liquid sorbents, the surface area enhancement gained via encapsulation provides an order-of-magnitude increase in carbon dioxide absorption rates for a given sorbent mass. The microcapsules are stable under typical industrial operating conditions and may be used in supported packing and fluidized beds for large-scale carbon capture.
Zinc(II) cyclen, a small molecule mimic of the enzyme carbonic anhydrase, was evaluated under rigorous conditions resembling those in an industrial carbon capture process: high pH (>12), nearly ...saturated salt concentrations (45% K2CO3) and elevated temperatures (100–130 °C). We found that the catalytic activity of zinc cyclen increased with increasing temperature and pH and was retained after exposure to a 45% w/w K2CO3 solution at 130 °C for 6 days. However, high bicarbonate concentrations markedly reduced the activity of the catalyst. Our results establish a benchmark level of stability and provide qualitative insights for the design of improved small-molecule carbon capture catalysts.
Developing three-dimensional (3D) graphene assemblies with properties similar to those individual graphene sheets is a promising strategy for graphene-based electrodes. Typically, the synthesis of 3D ...graphene assemblies relies on van der Waals forces for holding the graphene sheets together, resulting in bulk properties that do not reflect those reported for individual graphene sheets. Here, we report the use of sol-gel chemistry to introduce chemical bonding between the graphene sheets and control the bulk properties of graphene-based aerogels. Adjusting synthetic parameters allows a wide range of control over surface area, pore volume, and pore size, as well as the nature of the chemical cross-links (sp(2) vs sp(3)). The bulk properties of the graphene-based aerogels represent a significant step toward realizing the properties of individual graphene sheets in a 3D assembly with surface areas approaching the theoretical value of an individual sheet.
Alumina aerogels were prepared through the addition of propylene oxide to aqueous or ethanolic solutions of hydrated aluminum salts, AlCl3·6H2O or Al(NO3)3·9H2O, followed by drying with supercritical ...CO2. This technique affords low-density (60−130 kg/m3), high-surface-area (600−700 m2/g) alumina aerogel monoliths without the use of alkoxide precursors. The dried alumina aerogels were characterized using elemental analysis, high-resolution transmission electron microscopy, powder X-ray diffraction, solid-state NMR, acoustic measurements, and nitrogen adsorption/desorption analysis. Powder X-ray diffraction and TEM analysis indicated that the aerogel prepared from hydrated AlCl3 in water or ethanol possessed microstructures containing highly reticulated networks of pseudoboehmite fibers, 2−5 nm in diameter and of varying lengths, whereas the aerogels prepared from hydrated Al(NO3)3 in ethanol were amorphous with microstructures comprised of interconnected spherical particles with diameters in the 5−15 nm range. The difference in microstructure resulted in each type of aerogel displaying distinct physical and mechanical properties. In particular, the alumina aerogels with the weblike microstructure were far more mechanically robust than those with the colloidal network, based on acoustic measurements. Both types of alumina aerogels can be transformed to γ-Al2O3 through calcination at 800 °C without a significant loss in surface area or monolithicity.
We report the synthesis and characterization for the first examples of monolithic low-density carbon aerogel (CA) nanocomposites containing double-walled carbon nanotubes. The CA nancomposites were ...prepared by the sol−gel polymerization of resorcinol and formaldehyde in an aqueous surfactant-stabilized suspension of double-walled carbon nanotubes (DWNTs). The composite hydrogels were then dried with supercritical CO2 and subsequently carbonized under an inert atmosphere to yield monolithic CA structures containing uniform dispersions of DWNTs. The microstructures and electrical conductivities of these CA nanocomposites were evaluated for different DWNT loadings. These materials exhibited high BET surface areas (>500 m2/g) and enhanced electrical conductivities relative to pristine CAs. The details of these results are discussed in comparison with theory and literature.
A panel of five zinc-chelated aza-macrocycle ligands and their ability to catalyze the hydration of carbon dioxide to bicarbonate, H2O + CO2 → H+ + HCO3 –, was investigated using quantum-mechanical ...methods and stopped-flow experiments. The key intermediates in the reaction coordinate were optimized using the M06-2X density functional with aug-cc-pVTZ basis set. Activation energies for the first step in the catalytic cycle, nucleophilic CO2 addition, were calculated from gas-phase optimized transition-state geometries. The computationally derived trend in activation energies was found to not correspond with the experimentally observed rates. However, activation energies for the second, bicarbonate release step, which were estimated using calculated bond dissociation energies, provided good agreement with the observed trend in rate constants. Thus, the joint theoretical and experimental results provide evidence that bicarbonate release, not CO2 addition, may be the rate-limiting step in CO2 hydration by zinc complexes of aza-macrocyclic ligands. pH-independent rate constants were found to increase with decreasing Lewis acidity of the ligand-Zn complex, and the trend in rate constants was correlated with molecular properties of the ligands. It is suggested that tuning catalytic efficiency through the first coordination shell of Zn2+ ligands is predominantly a balance between increasing charge-donating character of the ligand and maintaining the catalytically relevant pK a below the operating pH.
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