GENESIS AND EVOLUTION OF POROSITY AND MICROSTRUCTURE IN NANOPOROUS CARBON DERIVED FROM POLYFURFURYL ALCOHOL
Open Access
- Author:
- Burket, Christopher L
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 11, 2007
- Committee Members:
- Henry C Foley, Committee Chair/Co-Chair
Janna Kay Maranas, Committee Member
Andrew Zydney, Committee Member
Angela Lueking, Committee Member - Keywords:
- molecular sieves
graphitization
nanoporous carbon
activated carbon
non-graphitizing carbon
carbon
polymerization
pyrolysis
activation
capacitors - Abstract:
- Rosalind Franklin first termed carbons derived from the pyrolysis of polymer precursors as either graphitizing or non-graphitizing. The classification was determined by the tendency of the amorphous carbon to convert to crystalline graphite when annealed at temperatures above 1000 °C. A unique property of non-graphitizing carbon is native nanoporosity, pores with widths less than 2 nm. The pores act as molecular sieves and are able to separate molecules based on size and shape. Nanoporous carbons (NPC) are utilized in gas separation membranes, catalysis, and electrical double layer capacitors. The disordered structure of the carbon is attributed to the presence of extensive cross-linking in its precursor. Polyfurfuryl alcohol (PFA) is an example of a non-graphitizing carbon precursor. Numerous researchers have studied this polymer-carbon system and acquired extensive knowledge in the areas of polymerization, pyrolysis, activation, and high temperature treatment. Investigating the genesis and evolution of porosity in all aspects of carbon synthesis is critical to engineering the carbon to meet required design criteria, which include pore widths and volumes, surface area, and microstructure. Four topics are examined herein: 1) Tuning of PFA polymerization and cross-linking in order to direct changes in carbon structure and porosity; 2) the autogenesis of porosity during pyrolysis; 3) the evolution of porosity and microstructure during annealing of non-activated and activated carbon; 4) the effectiveness of activated NPC as an electrode in an electrical double layer capacitor. Efforts to modify the molecular weight and degree of cross-linking in PFA were successful; however the variations were not propagated to the carbon. The polymers were assimilated during pyrolysis due to the continued polymerization which occured up to the point of decomposition. Thus bulk production of NPC could proceed from PFA synthesized by any method with the assurance that the properties of the product are consistent. The exception was carbon derived from mixtures of PFA and Triton X-100, a surfactant. With sufficient amount of the surfactant present, mesopores were generated in the nanoporous carbon. Pyrolysis, rather than polymerization, was the key step in the synthesis of purely nanoporous carbon. The pyrolysis of polyfurfuryl alcohol was studied up to 600 °C. Nanopores appeared in the carbon as early as 300 °C along with significant amount of mesopores. As the pyrolysis temperature was increased, nanoporosity was retained, but the mesoporosity disappeared. At 600 °C the average pore width was 0.4-0.5 nm. Between 300 and 400 °C, both polyaromatic domains decorated with hydrogen and oxygen (hetero) atoms and partially decomposed polymer chains coexisted. The unreacted polymer and heteroatoms induced mesoporosity by buffering the nanopores created by polyaromatic domains. Raising the pyrolysis above 400 °C released the buffering material, thereby collapsing the mesopores. A new pathway to synthesize a carbon with both nanoporosity and pre-graphitic structures was discovered by annealing a nanoporous activated carbon at 2000 °C. The activation process permitted the carbon to overcome its intrinsic barrier to graphitization. Prior to activation the nanopore walls were comprised of disordered graphene layers. Activation eliminated the barrier to graphitization by reducing the number of layers and removing carbon material highly susceptible to oxidation. High temperature annealing at 2000 °C of a carbon activated to 84% burnoff induced the formation of pre-graphitic domains amongst the nanoporous carbon. The structures were termed pre-graphitic as order extended in two dimensions. (002) bands were identified and assigned to amorphous, turbostratic, and graphitic carbon. A nanopore volume of 0.50 cm3 gm-1 was preserved after annealing. The product was heterogeneous, exhibiting both nanoporosity, a property of non-graphitizing carbon, and pre-graphite, a property of crystalline graphite. Two electrode capacitors were fabricated utilizing activated nanoporous carbons. The performance of carbons derived from polymers synthesized in the presence of either the solvent tetrahydrofuran or surfactant Triton X-100 was compared. Triton X-100 acted as a mesopore former during pyrolysis. The initial mesoporosity influenced activation by allowing generation of additional mesoporosity at all burnoffs. Whereas, the carbon derived from polymer with tetrahydrofuran did not exhibit significant mesoporosity until a burnoff of 80%. Nanopore volumes as high as 0.98 cm3 gm-1 at an average diameter of 0.82 nm were achieved. Constant current capacitances of 106 F gm-1 and 109 F cm-3 were measured for carbon activated to just 15% burnoff with a nanopore volume of 0.28 cm3 gm-1. The highest capacitance measured was 164 F gm-1. The volumetric capacitance was 88 F cm-3 due to extensive activation and a lower bulk density. Polyfurfuryl alcohol derived nanoporous carbon had tunable porosity and was an excellent material for capacitor electrodes.