Adding utility to carbon materials: introducing dopants using highly soluble metal salts and functionalizing surfaces via bromomethylation
Licentiate thesis, 2018
Carbon-based materials have received intense research interest over the past few decades due to their outstanding 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. In this thesis, several systematic studies were carried out on copper and iron salts as dopants for ordered mesoporous carbons (OMC). It was found that the selection of the counter anion to the metal cation has a profound influence on the resultant OMC’s structure, chemical composition, metal loadings, and the type of metal obtained (i.e. chelated ions or nanoparticles). We applied a host of characterization methods to elucidate the effect that the anion has on the transition metal-doped OMC. Many copper salts were used to create copper-doped OMCs (Cu-OMCs). High copper loadings of about 5-8 wt% were obtained from using Cu(BF4)2-nH2O as the dopant salt, compared to previous loadings of < 1 wt% using iron salts. The copper species was determined to be metallic copper (Cu0) nanoparticles with diameters of about 40-50 nm. The high copper loadings, however, were found to be deleterious for use as sulfur hosts in lithium-sulfur (Li-S) batteries, with reversible capacities about 50% lower than undoped OMCs. The Cu-OMCs were also tested for O2 reduction on rotating disc electrodes (RDEs), but their catalytic performance was found to be quite poor. The same approach of using different anions was applied to iron salts in the context of polymer electrolyte membrane fuel cells (PEMFCs). The anion was found to have a strong effect on the OMCs structure, iron loading, and O2 reduction activity. High iron loadings of above 3 wt% were obtained for some of the soluble salts, but their activity in PEMFCs did not increase appreciably compared to the standard chloride salt.
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 the last part of this thesis, we report a novel grafting method – the bromomethylation reaction. Several carbon materials efficiently and reproducibly undergo this reaction and surface-bound bromomethyl groups are stable for months in ambient conditions. Subsequently, many nucleophiles can substitute bromide resulting in monolayer-functionalized surfaces tailored for a specific application. We employ diallylamine and ethylenediamine as nucleophiles to produce amine-functionalized carbons for use as the conductive additive for sulfur in Li-S batteries. Such carbons exhibit improved performance over their unmodified precursors demonstrating the utility of this two-step scheme for functionalizing carbon surfaces. We hope that this two-step method of introducing organic groups to carbon surfaces will find wide-spread use in many applications.