Graduate Thesis Or Dissertation
 

Preparation and Properties of Porous Carbon Materials

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vm40xv81j

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  • Porous carbon is indispensable in modern technology applications. It is used for energy storage, gas separation, water purification, catalyst support, and chromatography. The diversity of its applications stems from its unique properties, including high specific surface area, tunable pore volume, and chemical stability. Specifically, the large surface area provides high capacitance for the electric double layer capacitor, and catalytic sites for the chemical reaction and binding substrate for other catalysts in the metal air battery, fuel cell, and water splitting. Tunable pore size holds the same importance as the high surface area. Micropores are always prerequisite for the high surface area, and dramatically affect the solvated ions in the capacitive behavior. Mesopores and macropores are necessary for the mass transfer, which is the most dominant process in drug delivery, gas separation, and ions diffusion in the capacitor and fuel cell. The carbon crystal structure always determines chemical stability. Always, the more graphitic or the more graphenic is, the more chemically stable the carbon is. On the contrary, the structure of amorphous carbon is apt to be damaged under high potentials or in the harsh chemical environments, such as strong acid or base. Porous carbon can be synthesized by inorganic template filling, polymer carbonization or catalytic activation; however, these methods are constrained by high cost, tedious preparation process and low yield, which further limit their practical application. In this thesis, I will introduce a porous graphene preparation by CO₂ activation and magnesiothermic reduction of CO₂. On one side, porous carbon preparation by physical activation, such as H₂O and CO₂, and chemical activation, such as ZnCl₂, H₃PO₄, and KOH, has been used industrially for decades, and plenty studies have been devoted to revealing the pore size or surface area changes during the activation, such as volume of micropores increased by H₃PO₄, lower activation temperature and higher yield by ZnCl₂ and increased surface area and graphitic feature by KOH. However, detailed study of the carbon structure evolution during the activation remains unknown. The reaction mechanism of carbon activated by CO₂ is the simplest due to the single product formation of CO, and the simplicity of this reaction makes possible the elucidation of structural evolution of carbon during CO₂ activation. After analyzing the structural evolution revealed by neutron total scattering, TEM, and other characterizations, we have come to the conclusion that the defective graphenic domains are removed, and turbostratic domains are thinned after the nanoporosity is generated in the initial activation. Furthermore, the tailor-designed porous carbon is synthesized in a short activation time with a high surface area with the guide of the mechanistic insights into the structural evolution of carbon. The synthesis of porous carbon by magnesiothermic reduction of CO₂ holds a very similar design principle as the widely used inorganic template methods. However, the magnesiothermic reduction of CO₂ has the following advantages: (1) the template MgO forms simultaneously with porous carbon, and thus no template preparation is required; (2) MgO template can be easily removed by HCl without using highly corrosive and dangerous HF; (3) carbon source CO₂ is almost free compared to the expensive and tedious polymer synthesis in the inorganic template process. Moreover, some further study widens this novel synthesis method: Zn is added to increase the surface area from ~800 m²/g to 1900 m²/g, Cu is added to increase the graphitic and graphenic features, N2 is added to realize the N-doping. In this thesis, the application of porous carbon includes electric double layer capacitor, Li-O₂ battery, microbial fuel cell, and potassium ion batteries. For the electric double layer capacitor, porous carbon with surface area high up to 1900 m²/g showed a high capacitance of 190 F/g even at the high current density of 10 A/g or high sweep rate of 2000 mV/s. The N-doped porous carbon not only increased the capacity if Li-O₂ battery from 5300 mAh/g to 9600 mAh/g, but also lowered the overpotential in the charging process which led to a more stable cycle life. The increased degree of local crystallinity in porous carbon enabled an improved electrochemical performance in the microbial fuel cell, which has the guiding significance on the catalyst design for the microbial fuel cell. The local curvature of the porous carbon provided the epitaxial template for the growth of polynanocrystalline graphite, which showed an improved cycling life for potassium ion battery compared with graphite.
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