The search for practical end-uses for post-combustion CO₂ has generated significant research interest in CO₂ utilization in non-aqueous storage and conversion devices, such as electrochemical reactors, as well as alkali metal-based O₂/CO₂ and -CO₂ batteries. In nonaqueous systems, CO₂ is typically converted to environmentally benign solids such as mineralized carbonates and/or carbon. Employing aprotic solvents for realizing room temperature electrochemical CO₂-to-solids conversion reactions is particularly advantageous because it enables the use of highly reducing alkali/alkaline earth metals (typically Li) that yield large thermodynamic driving forces for the activation and conversion of CO₂. Furthermore, the lack of protons suppresses parasitic side reactions such as hydrogen evolution, and their wide stability window enables access to a wide energy range over which to probe activity, which may be useful for designing new reaction pathways. Due to severe kinetic limitations, however, direct CO₂ reduction, which entails electron-coupled formation of the CO₂ anion radical intermediate,¹ has been reported to have little to no activity in most common nonaqueous solvents.² For example, past literature has shown that nearly all prior reports of Li-CO₂ batteries have almost exclusively relied upon tetraethylene glycol dimethyl ether (TEGDME)-based electrolytes containing either 1 M lithium trifluoromethanesulfonate (LiCF₃SO₃)^3-4 or 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)^5-6 salts. While the choice of such electrolytes in Li-CO₂ batteries appears to be critical, fundamental knowledge regarding why TEGDME-based electrolyte chemistries appear to be most promising for affecting direct electrochemical CO₂ transformations largely remains lacking.

In this work, we study the effect of varying electrolyte compositions on the discharge performance of conventional Li-CO₂ batteries. This entails systematic investigations highlighting the role of key electrolyte parameters such as intrinsic solvent properties, and the concentration and composition of the electrolyte co-salt for facilitating CO₂ discharge. To probe solvent-dependent CO₂ activity, we perform cyclic voltammetry (CVs) and discharge measurements in a diverse set of standard battery-grade solvents (e.g.: TEDGME, DME, PC and DMSO) to investigate the role of physical solvent properties such as donor number (DN), dielectric constant, ionic conductivity, and viscosity for enabling CO₂ reduction. CVs are also conducted in CO₂-rich electrolytes with varying alkali cations to explore the cation-dependence of onset CO₂ reduction potentials and elucidate the discharge reaction mechanism. Furthermore, we also study CO₂ solubility as a function of the identity and concentration of the electrolyte co-salt anion in several TEGDME-based electrolytes to determine how electrolyte compositions modulate transport properties and overall reaction rates. Collectively, these data provide a mechanistic explanation for why certain electrolyte chemistries impart activity for electrochemical CO₂ conversion while others do not, and help establish comprehensive design criteria to guide future electrolyte development for nonaqueous CO₂ conversion.

References:

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