Catalyzing High Potential Redox of Inert Molecules
Abstract
The goal of the proposed work is to explore the physical underpinnings that govern electron transfer to kinetically sluggish (ÔinertÕ) gas molecules with high reduction potentials (E > 3 V vs. Li) in nonaqueous environments. Molecular redox reactions are of growing importance to a range of energy-conversion mechanisms in electrochemical systems, but the lack of fundamental understanding and design criteria for nonaqueous electrocatalysis currently limits exploration to reactions that are kinetically facile. One major challenge is that many simple candidate electrocatalysts (carbon and metals) have potentials at open circuit (E < 3 V vs. Li) which are poorly aligned with high-potential reactions. A second hurdle is the lack of understanding of how the electrified interface Ð the surface charge, double layer capacitance, and field strength within the double layer Ð can influence electron transfer, especially when large polarizations are required. In this work, we will examine these phenomena in detail by focusing on one model reaction, the nonaqueous electrochemical reduction of SF6 by Li. This reaction is interesting owing to its high thermodynamic potential (SF6 + 8e- + 8Li+ 6LiF + Li2S, Eo = 3.70 V vs. Li), as well as its large theoretical energy density (3581 Wh/kgSF6 + 8Li) which rivals that of todayÕs advanced electrochemical systems such as Li-O2 and Li-S batteries. SF6 is a classically inert gas, yet we have recently demonstrated that, when dissolved in a nonaqueous electrolyte, SF6 can be activated heterogeneously at a simple carbon electrode, yielding onset reduction potentials between 1.7 Ð 2.2 V vs. Li. These potentials however represent very large (> 1.2 V) kinetic losses, which are not currently understood. This work will study two potential modes of electrocatalysis of SF6 reduction, with the target of raising the reduction potential closer to the thermodynamic value. The first mode focuses on understanding and tailoring the electrified interface to influence the energetics of outer-sphere SF6 reduction. By varying the electrolyte solvent, cation, and electrode surface, the potential of zero charge (PZC) and capacitance will be determined approaching, and at, conditions at which electron transfer to SF6 occurs. Trends linking double layer structure and electron transfer will be established. Armed with this knowledge, strategies to reduce the reduction overpotentials for the outer-sphere reduction of SF6 through rational tuning of the double layer will be pursued. The second mode of electrocatalysis to be studied involves the promotion of inner-sphere electron transfer to SF6 through specific adsorption on transition metal compounds at potentials well-aligned with the targeted thermodynamic potential. The class of transition metal fluorides is chosen as a framework for systematically probing the effects of active site composition and redox behavior on SF6 activation. The possibility of promoting chemisorption through metal-fluoride interactions within the ligand sphere of SF6 will be investigated. Taken together, these two approaches represent complementary yet synergistic approaches to elucidate governing parameters for interfacial and redox phenomena in nonaqueous systems. More broadly, this study will yield generalizable knowledge about how electrified interfaces can be tailored and utilized rationally to unlock desired reactions of interest, relaxing the constraint that such reactions need be a priori kinetically facile.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 06, 2019
- Source ID
- W911NF1910311
Entities
People
- Betar M. Gallant
Organizations
- Army Contracting Command
- Massachusetts Institute of Technology
- United States Army