Adding Utility to Carbon Materials: Introducing Dopants Using Highly Soluble Metal Salts and Functionalizing Surfaces via Bromomethylation

Sammanfattning: Carbon-based materials have received intense research interest over the past few decades due to their unique  combination of properties including porosity, non-toxicity, chemical inertness, low density, and electrical conductivity, which has allowed them to find a wide array of applications including supercapacitors, batteries, CO2 capture, fuel cells, and catalysis. To expand their utility, a variety of techniques have been developed to enhance their reactivity and functionality. One such method is doping, wherein heteroatoms (i.e. non-carbon elements) are purposefully incorporated into the carbon structure with the goal of introducing new reactivity to the material. The first paper in this thesis focuses on using soluble Fe salts as dopants for iron/nitrogen-doped ordered mesoporous carbons (Fe-OMC). The anion was found to have a strong effect on the structure, Fe loading, and oxygen reduction reaction (ORR) activity of the Fe-OMC. High Fe loadings of above 3 wt% were obtained for one of the soluble salts, but their activity in polymer electrolyte membrane fuel cells (PEMFCs) did not increase appreciably compared to the standard chloride salt. Electron paramagnetic resonance (EPR) was used to gain insight into the structure and ORR activity of the various Fe species within each Fe-OMC. Another method for increasing the utility of carbon materials is grafting or surface functionalization, which consists of covalently attaching small, organic molecules to the carbon surface. In three papers of this thesis, we report a novel two-step method for the surface functionalization of high surface area carbon materials. The carbons are first subjected to the bromomethylation reaction then, in the second step, many nucleophiles can substitute bromide resulting in monolayer-functionalized surfaces that can be tailored for a specific application. Example nucleophiles include azide, amines, iodide, sulfite, and amide enolates. Several carbon materials efficiently and reproducibly undergo these reactions and the surface-bound groups are stable for months under ambient conditions. This two-step scheme has numerous advantages over other surface modification techniques for carbon including use of solution-phase reagents, minimal harm to the carbon framework, monolayer functionalization, and no carbon pretreatment steps. A total of 12 surface groups were synthesized, which demonstrates the synthetic flexibility of this two-step technique. Four of the twelve modified carbons were used as cathodes in lithium-sulfur (Li-S) batteries. When used with an electrolyte containing lithium nitrate (LiNO3), the functionalized cathodes show increased capacities by virtue of utilizing more S. When used with electrolytes lacking LiNO3, the surface groups attenuate the lithium polysulfide (LiPS) shuttle as measured by the much higher initial Coulombic efficiencies (ICEs) recorded for the functionalized cathodes relative to the unfunctionalized control. The observations with both electrolytes evidence strong interactions between the electroactive S and the surface groups. The higher binding energies (BEs) computed by density functional theory (DFT) support strong interactions between the surface groups and various sulfur species while cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) lend evidence for a significantly reduced LiPS shuttle on the functionalized carbon surfaces. Based on these results with Li-S batteries, we hope that this two-step method of introducing organic groups to carbon surfaces will find wide-spread use in many applications.

  KLICKA HÄR FÖR ATT SE AVHANDLINGEN I FULLTEXT. (PDF-format)