UCLA

Covalent Triazine Frameworks (CTFs) are highly porous materials with two- dimensional or three-dimensional architectures, which can be synthesized with tunable porosity and a variety of functionalities. Their synthesis and characterization has been studied extensively in many prior works. However, ionic functionalities built into CTFs are new and have not been investigated, especially those containing an imidazolium ring and capable of being deprotonated into an active N-heterocyclic carbene. Chapter 1 introduces different chemical architectures developed by chemists, highlighting one of the most investigated topics being Covalent Organic Frameworks. The theory of absorption of gasses on porous solids is also discussed. Chapter 2 outlines the detailed synthesis and characterization of a unique CTF with Reuleaux Triangle shaped pores, which forms by harnessing the geometry of the N-heterocyclic carbene. Aside from nitrogen uptake, we studied absorption of carbon dioxide on an ionically charged framework. Chapter 3 illustrates a method of exfoliating two-dimensional CTFs. Most CTFs are two dimensionally stacked, which have full conjugation, and π-π stacking similar to that of graphite. Yet, their thin film characteristics have never been studied. Here we developed a method to peel these stacked layers apart, characterize the sheets, and produced thin-film devices along with conductivity measurements. Due to CTFs exceptional stability, and intrinsic nitrogen doping, their electrochemical applications are just beginning to be explored. The electrochemical reduction of oxygen, a very important reaction, is one of the main limiting factors to widespread use of fuel cells. In Chapter 4 we create oxygen reduction catalysts using super-acid derived CTF polymers and cobalt salts as precursors. The CTF polymers readily absorb cobalt from solution and are then annealed at 900 �C. The resultant porous catalyst reduces oxygen to water with performance comparable to the industrial standard platinum. Chapter 5 discusses molecular control of porous carbons. We have developed a new pathway by designing molecular precursors to tune the porosity of CTF-derived porous carbon materials. Using CTFs as precursors, while controlling their monomeric length results in a high degree of pore size control. In this process, by choosing using different ratios of long and short monomers we can be tune the size of the CTFs. The resultant framework is heated to desired temperatures of 700 to 900 �C to yield porous carbon materials. The incremental pore size and volumes from molecular tuning these frameworks remains consistent from both pre annealing to post annealing, though some pore expansions can exist. This is the first demonstration of molecular tuning the pore size of porous carbons, and a powerful way of bringing a certain degree of order to traditionally disordered carbon materials. We apply these characteristics of molecular tuning to investigate capacitive related performance as well as oxygen reduction reaction. Through pore tuning, we create different porous carbons with surface areas ranging from 450 m2 g-1 all the way to 2400 m2 g-1 while controlling pore sizes within a few nanometers of each other. Capacitive values rise over ten fold from 14 F g-1 to 180 F g-1. When different size frameworks are loaded with cobalt and used for oxygen reduction, we observe five-fold increases in kinetic current density, and reach overpotentials of 38 mV less than that of platinum on carbon. These electrocatalysis experiments demonstrate the significance of ample reactant delivery to the catalyst.