The catalytic activity of room-temperature ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM]­[OTf]) for carbon dioxide (CO2) reduction is reported on a polycrystalline silver electrode as a function of the water content and solution pH. The optimal aqueous dilution was determined to be ca. 10% (v/v), resulting in a reduction of the overpotential required to reduce CO2 in [EMIM]­[OTf] by 400 mV. The reduction process and products are largely insensitive to pH. CO2 reduction in [EMIM]­[OTf] with 10% 0.1 M NaHCO3 is achieved with high selectivity for CO with 93.0 ± 4.6% Faradaic efficiency. Hydrogen evolution reaction (HER) is suppressed in the range of neat [EMIM]­[OTf] (500 ppm water content) to a [EMIM]­[OTf]/H2O mixture of 10% water content. Conductivity and viscosity of the [EMIM]­[OTf]/H2O mixture suggest that the ionic liquid ion pair fully dissociates, analogous to dilute KCl solution, where each ion is completely hydrated by water molecules.

Developing affordable electrocatalysts to facilitate the reduction of carbon dioxide (CO2) to high-value products with high selectivity, efficiency, and large current densities is a critical step for the production of liquid carbon-based fuels. In this work, we show that inexpensive post-transition metals [tin (Sn) and lead (Pb)] and their alloys (PbSn) are excellent cathode materials to reduce CO2 in an ionic liquid/acetonitrile/water electrolyte media. Electrochemical impedance spectroscopy measurements show that the PbSn alloys exhibit lower charge-transfer resistance when compared to the pure metal electrodes, as supported by electronic structure calculations. Current densities as high as 60 mA/cm2 are observed with optimal mixtures of ionic liquid, acetonitrile, and water. Reduction product analysis identifies carbon monoxide (CO) and formate (HCOO−) as primary reduced products, with higher selectivity toward formate. Faradaic efficiency for formate on pure Pb and pure Sn was determined to be 80 ± 4 and 86 ± 3%, respectively. FE % improves as either Pb is incorporated into Sn or vice versa, and there is a maximum FE of 91 ± 3% for both 50 and 40% Pb composition.

This paper describes the construction of a microring electrode concentric to an inner injection capillary for voltammetric determination of trace analytes in nanoliter- to picoliter-sized droplets. The gold microring is sandwiched between a pulled fused-silica capillary and borosilicate glass tubing. Compared to polymer-coated microring electrodes, the glass-encapsulated electrode is more robust and does not swell in organic solvents. Consequently, the microring electrode is suitable for voltammetric studies of redox-active species and their accompanying ion transfers between two immiscible solvents. Droplets of variable sizes can be conveniently dispensed from front-loaded sample plugs into an immiscible liquid, greatly simplifying the experimental procedure and facilitating analysis of samples of limited availability. The size of the microring and the volume of the droplet deduced from well-defined voltammograms correlate well with those estimated from their geometric dimensions. The thin-layer cell behavior can be attained with well-defined voltammetric peaks and small capacitive current. Exhaustive electrolysis in single droplets can be accomplished in short times (e.g., ∼85 s in a droplet of 1.42 nL at a microring of 11.4 μm in radius). Anodic stripping voltammetry of Ag deposited onto the microring electrode resulted in a detection limit of 0.13 fmol (14 fg) of Ag(+). The microring electrode/injector assembly can be polished repeatedly and is versatile for various applications (e.g., sample plugs can also be back-loaded via a rotary injection valve and an HPLC pump for flow injection analysis).

Ionic liquids are known to be excellent solvents but their relatively high viscosities can inhibit electrochemical reactions. We describe mixtures of two common ionic liquids, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM OTf) and 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIM OTf), with two common molecular solvents, water and acetonitrile. We analyze the solution viscosity and correlate this to the dynamic response of the electrochemical double layer by recording current vs. time and voltage vs. time transients upon (de)polarization of the electrode. We also test the effects of the added co-solvents in the context of the electrochemical reduction of carbon dioxide. We find that adding water to the ionic liquid significantly reduces the overpotential for the reduction, and that adding acetonitrile significantly increases the current density, matching some of the best performance data currently reported for this reaction. Our results suggest that a three-part mixture of ionic liquid, water, and acetonitrile may lead to further improvements in reaction efficiency.

The electrochemical reduction of atmospheric carbon dioxide to value added chemicals and fuels using renewable energy is a promising route to a sustainable economy. In this work we present results from the electrochemical reduction of carbon dioxide on copper/silver alloy electrodes alongside pure copper and pure silver in acetonitrile-water electrolyte mixtures. EIS measurements show that the copper/silver alloys exhibit lower charge-transfer resistance when compared to the pure metal electrodes. Reduced product analysis identifies carbon monoxide (CO) as the major product on pure Ag, Ag with alloyed with 10% Cu, and Cu alloyed with 10% Ag with Faradaic efficiencies over 60%. Formate is the major reduced product close to the 50:50 (Cu:Ag) ratio with Faradaic efficiencies exceeding 30%. Our results suggest that changes in the metallic lattice and structural strain of the bimetallic electrodes influences reduced product selectivity.