Adding Utility to Carbon Materials: Introducing Dopants Using Highly Soluble Metal Salts and Functionalizing Surfaces via Bromomethylation
Doctoral thesis, 2019
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.
Samuel Joseph Fretz
Chalmers, Chemistry and Chemical Engineering, Applied Chemistry, Applied Surface Chemistry
S. J. Fretz, C. Janson, W. R. Rosas, and A. E. C. Palmqvist. Influence of Iron Salt Anions on Formation and Oxygen Reduction Activity of Fe/N-Doped Mesoporous Fuel Cell Catalysts
Bromomethylation of high-surface area carbons as a versatile synthon: Adjusting the electrode-electrolyte interface in lithium-sulfur batteries
Journal of Materials Chemistry A,; Vol. 7(2019)p. 20013-20025
S. J. Fretz, M. Agostini, P. Jankowski, P. Johansson, A. Matic, and A. E. C. Palmqvist. Amide- and amine-functionalized ordered mesoporous carbon Li-S battery cathodes
S. J. Fretz, U. Pal, G. M. A. Girard, P. Howlett, and A. E. C. Palmqvist. Sulfonate-functionalization of carbon cathodes as a substitute for lithium nitrate in the electrolyte of lithium-sulfur batteries
Although carbon materials naturally exhibit many desirable traits, a variety of techniques exist to tailor their properties to meet the needs of an application. Among these are doping and surface modification. Doping refers to the purposeful incorporation of non-carbon atoms into the material structure. This is typically accomplished by a judicious selection of an organic precursor used to make the carbon, which contains the desired elements. Since some elements are easy to find in carbon precursors (e.g. O, N, S), these are relatively easy to use as dopants. However, the unique chemistry of transition metals (e.g. Fe, Co, Cu) makes doping with these elements attractive, despite the increased difficulty of incorporating them. In this thesis, Fe-doping of carbon materials is accomplished by using a simple method of dissolving an Fe salt into the organic carbon precursor. The effect of the salt anion on the carbon’s structure and composition is systemically studied and the resultant materials are used as catalysts in polymer electrolyte membrane fuel cells (PEMFCs). Overall, the anion has a profound influence on the properties of the carbon materials.
Another technique used in this thesis is surface modification, wherein small, organic groups are directly attached to the carbon surface. The relatively unreactive carbon surface requires harsh reaction conditions for the surface functionalization to take place to a significant extent, which has led to a dearth of suitable methods. Herein, an alternative, two-step route for the surface modification of carbon materials is proposed and explored. This two-step scheme, referred to as bromomethylation, boasts many advantages over other surface modification methods including mild reaction conditions, high surface coverages, and negligible damage to the carbon structure. Using this method, a total of twelve unique surface-modified carbons are made. Four of these materials were used as hosts for sulfur in lithium-sulfur (Li-S) batteries and all show greatly enhanced performance relative to the unmodified parent carbon. These results demonstrate the potential benefits that bromomethylation can bring to applications requiring the use of carbon materials.
Other Chemistry Topics
Areas of Advance
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4629
Chalmers University of Technology
10:an, Kemigården 4, Chalmers
Opponent: Torbjörn Gustafsson, Department of Chemistry, Ångström Laboratory, Uppsala University, Sweden