Electrochemical synthesis of C2 and C3 hydrocarbons from CO2 on an Ag electrode in DEME-BF4 containing H2O and metal hydroxides

Abstract

In the electrochemical synthesis of hydrocarbon gases from CO2, the improvement of the selectivity of the product by controlling the composition of the electrolyte is attractive as a process that does not require complex electrode structures; however, little is known about the conversion process of CO2 at the ionic liquid-based electrolyte/metal electrode interface. In this study, the electrochemical conversion of CO2 and H2O to C2 and C3 hydrocarbons on a pure Ag electrode in N, N‑diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate (DEME-BF4) containing H2O and metal hydroxides, such as Ca(OH)2, NaOH, and CsOH, was investigated. Quantitative gas analysis of the samples obtained by potentiostatic electrolysis for 30 min in the melt with DEME-BF4:H2O:Ca(OH)2 molar ratio of 2:1:1.8 × 10−4 at room temperature demonstrated that C2H4, C2H6, C3H6, and C3H8 were produced, and the current efficiency of C3H8 was determined to be 11.3 %. The reactant was HCO3− coordinated with DEME+, BF4−, and Ca2+ ions, as identified by Raman spectroscopy combined with density functional theory (DFT) calculations. Moreover, the surface-enhanced Raman spectroscopic data revealed that the intermediate on the Ag electrode during potentiostatic electrolysis was adsorbed 2CO− interacting with Ca2+ ions. DFT simulations also demonstrated that Ca2+ increased the energetic stability of the 2CO−. This study showed that tailoring the electrolyte can lead to molecular-level changes in the phase transformation of CO2 in bulk solution and at the electrode/ionic liquid electrolyte interface and proposed a process that enables the synthesis of unique hydrocarbons such as C3.