Probing Electrolyte Influence on CO₂ Reduction in Aprotic Solvents

Abstract

Selective CO 2 capture and electrochemical conversion are important tools in the fight against climate change. Industrially, CO 2 is captured using a variety of aprotic solvents due to their high CO 2 solubility. However, most research efforts on electrochemical CO 2 conversion use aqueous media and are plagued by competing hydrogen evolution reaction (HER) from water breakdown. Fortunately, aprotic solvents can circumvent HER, making it important to develop strategies that enable integrated CO 2 capture and conversion. However, the influence of ion solvation and solvent selection within nonaqueous electrolytes for efficient and selective CO 2 reduction is unclear. In this work, we show that the bulk solvation behavior within the nonaqueous electrolyte can control the CO 2 reduction reaction and product distribution occurring at the catalyst−electrolyte interface. We study different tetrabutylammonium (TBA) salts in two electrolyte systems with glyme ethers (e.g., 1,2 dimethoxyethane or DME) and dimethyl sulfoxide (DMSO) as a low and high dielectric constant medium, respectively. Using spectroscopic tools, we quantify the fraction of ion pairs that forms within the electrolyte. Also, we show how ion pair formation is prevalent in DME and is dependent on the anion type. More importantly, we show that as ion pair formation decreases within the electrolyte, CO 2 current densities increase, and a higher CO Faradaic efficiency is observed at low overpotentials. Meanwhile, in an electrolyte medium where the ion pair fraction does not change with the anion type (such as in DMSO), a smaller influence of solvation is observed on CO 2 current densities and product distribution. By directly coupling bulk solvation to interfacial reactions and product distribution, we showcase the importance and utility of controlling the reaction microenvironment in tuning the electrocatalytic reaction pathways. Insights gained from this work will enable novel electrolyte designs for efficient and selective CO 2 conversion to desired fuels and chemicals.

Interactions of CO2 Anion Radicals with Electrolyte Environments from First-Principles Simulations

“…Such an observation is consistent with the dominant electrostatic interactions and has experimental implications on salt dissociation or formation of ion pairs. 25,27 The positive charge on the NBu 4 + cation is highly shielded by the bulky and electron donating alkyl chains, which means that changing solvents has a relatively minimal effect on the NBu 4 + . However, the strength of the interaction between CO 2 − and NBu 4 + is affected by how strongly CO 2 − is coordinated to the solvent and cationic species.…”

“…Our simulation and physics-based model provides a general and suitable approach to studying solvation effects particularly in an aprotic electrolyte environment, and it was shown that the electrochemical CO 2 RR activity could be correlated to the bulk solvation properties in DME and DMSO. 27 We note the limitation of our model not yet including the electrode surface as well as the pH effect at the…”

“…In nonaqueous solvents, however, alkali metals are known to inhibit CO 2 reduction due to the formation of a passivation layer containing carbonate species. 27,28 Therefore, electrolyte cations such as NX 4 + (X = methyl, ethyl, and butyl groups) are being used, and their catalytic roles were examined. 24,29 In aprotic Li−CO 2 batteries, it was argued that a LiCO 2 intermediate is crucial to tuning CO 2 RR toward a solution-versus surface-mediated pathway.…”

“…Here, we focus on the stabilization effect of the CO 2 − anion radical, as its formation is likely to be the rate-determining step 31 for CO 2 reduction in some aqueous 32−34 and nonaqueous 15,35 media, although this is still under debate. 1 We compare water and nonaqueous solvents (mainly dimethoxyethane (DME) in this work), as glyme-ether-based solvents have been successfully used for electrochemistry (e.g., metal-air/Mg battery) and gas separation processes capturing CO 2 36,37 but have been rarely studied for CO 2 reduction until recently, 27 to the best of our knowledge. To gain understanding of the specific molecular-level interactions, we used ab initio molecular dynamics (AIMD) simulations.…”

Low-Temperature CO2-Assisted Lithium–Oxygen Batteries for Improved Stability of Peroxodicarbonate and Excellent Cyclability

“…However, weak C−O stretching bands of Li 2 CO 3 were detected at 1470 and 1408 cm −1 , revealing the formation of amorphous Li 2 CO 3 during discharge (Figure 1d). Because CO 2 gas is electrochemically inert in the given potential range, 20,25 we first consider the chemical pathway to form Li 2 CO 3 through the Li 2 O 2 and CO 2 reaction (eq 1).…”