|M.Sc Student||Horowitz Adi|
|Subject||Nitrogen- containing, Emulsion- templated, Porous|
Carbons With Hierarchical Porosities
|Department||Department of Energy||Supervisor||Professor Michael Silverstein|
|Full Thesis text|
Porous carbons are being developed for a range of energy-storage applications including super-capacitors, batteries, fuel cells, and hydrogen storage. This research has focused on the generation of nitrogen-containing monolithic carbons with hierarchical porosities through the pyrolysis of emulsion-templated porous polymers termed polyHIPEs. PolyHIPEs are macroporous monoliths that are usually synthesized within surfactant-stabilized water-in-oil (W/O) high internal phase emulsions (HIPEs). It should be possible to introduce microporosity and/or mesoporosity into polyHIPE-based carbons using porogens and/or carbon activation.
The objectives of this research were to generate monolithic porous carbons with macro-, meso-, and microporous architectures, to characterize the resulting polymers and carbons, and to describe the effects of the synthesis and pyrolysis parameters on the resulting materials. Two different polyHIPE systems were investigated. One system was based on polyacrylonitrile (PAN) crosslinked with divinylbenzene synthesized in water‑in‑oil HIPEs. Different porogens were incorporated into the polymer matrix to increase the carbon’s specific surface area (SBET). These porogens included solvents (toluene), degradable comonomers (methyl acrylate and a vinyl-terminated oligomeric polycaprolactone), and degradable oligomers (oligomeric polycaprolactone polyols). The other system was based on the hydrothermal carbonisation (HTC) of a renewable resource material (glucose) in the presence of borax and hydrophilic monomers, such as 2-hydroxyethyl methacrylate (HEMA) crosslinked with N,N'‑methylenebisacrylamide (MBAAm), within oil‑in‑water HIPEs. Additionally, nitrogen-containing monomers, 1‑vinylimidazole (VIm) and acrylamide (AAm), were used to increase the nitrogen content of the carbons. The resulting macroporous structures, atomic compositions, macromolecular structures, SBET, microporosities, and thermal properties were characterized. Chemical and physical activations were performed on polyHIPEs and pyrolyzed polyHIPEs from both systems. Physical activation was performed by heating the materials in CO₂ for different dwelling times and temperatures. Chemical activation involved immersing the materials in an aqueous ZnCl₂ solution, drying and pyrolyzing in either N₂ or CO₂.
Generally, the PAN-based polyHIPEs exhibited relatively closed-cell structures, before and after pyrolysis. The as-synthesized PAN-based polyHIPEs had densities of around 0.07 g/cc, relatively low SBET around 11 m2/g and N/C ratios of 0.22. Microporous carbons with SBET as high as 503 m2/g were produced by introducing porogens into these polyHIPEs, with micropores of around 0.71 nm and micropore volumes of around 0.21 cc/g. While activation did not enhance the SBET in the PAN‑based carbons, it did generate a more open-cell structure.
HTC generated open-cell polyHIPEs with densities of around 0.06 g/cc. Carbons with SBET of 100 m2/g were produced through pyrolysis of these polyHIPEs. The N/C ratios were enhanced, from 0.03 to 0.09 through the use of the nitrogen-containing monomers. Physical activation was only able to increase the SBET to around 400 m2/g. Chemical activation successfully generated a carbon with a porous hierarchy containing macropoes, mesopores, and micropores, with an SBET of 1540 m2/g, from an HTC polyHIPE based on glucose, borax, HEMA and MBAAm. The median pore diameter for the micropores and mesopores was 0.74 nm and 4.3 nm, respectively.