Reports: DNI555279-DNI5: Hydrocarbons from CO2: Electrochemical Reduction in Task-Specific Solvents

Scott Shaw, PhD, University of Iowa

The goal of this award is to understand and design ionic liquid systems to carry out electrochemical reduction of CO2 to valuable hydrocarbon commodities. Ionic liquids (IL) are solutions of pure ions, that remain liquid at, and below, room temperatures. The result is a conductive, low-vapor pressure fluid with intriguing bulk phase and interfacial phase properties.

Our work in this area began by investigating two major questions regarding ILs: 1) how do ILs behave near (electrode) surfaces, and 2) what effects does absorbed water have on the performance and behaviour of IL materials. These questions were addressed in initial publications by our group, listed as #1 and #2 in the list below. Briefly, we found long-ranging ordered structures in the ionic liquid that developed over periods ~ 30 minutes to 120 minutes. The ordering extends up to several microns, and appears to be independent of substrate material, cation structure, overlying vapor phase, or the presence of water impurities. We are extending this work to examine different classes of IL anions (earlier work focused exclusively on bis(trifluoromethyl)sulfonylimide)) and are incorporating data from our new Sum-Frequency Generation spectrometer. Two additional manuscripts are in preparation.

These results are important because they describe, for the first time, the ability of ionic liquids to form highly-ordered phases of matter that extend orders of magnitude farther from the solid surface than considered previously for any liquid system. While it is widely accepted that some degree of molecular ordering and phase transition occurs in fluids near surfaces and in fluids under shear, observation of an organized fluid layer extending some microns from a solid surface is, to our knowledge, unprecedented until now.

The second area of our early work investigated in the sorption of water into IL media. The presence of water in ionic liquids is difficult to eliminate. Water is known to have a significant effect on the physical properties of ILs, and a few studies have shown that even very low levels of water (ppm) in IL can ‘condense’ on an electrode surface from an IL solution. In order to better understand and predict the absorption of water into ILs, our second publication examined two model ionic liquids: ethylammonium nitrate (EAN), and butyltrimethylammonium bis(trifluoromethylsulfonyl)imide (N1114 TFSI). EAN represents a ‘protic’ IL, which as a class are more hydroscopic than the ‘aprotic’ ILs, such as N1114 TFSI. We reported water sorption rates for these liquids, the electrochemical response to varying amounts of water, and utilized infrared spectra of the fluids to comment on the physical environment of the water (homogenous mixtures or in micro-droplets).

Ultimately, this study shows that water must be closely monitored and controlled. Without proper experimental control, meaningful comparisons of data from IL experiments from different research groups or institutions will be difficult. This work also leads into an important area of work for the CO2 reduction project in understanding the chemical environment of water in the IL. The distinct OH vibrational profiles of water in the protic vs aprotic ILs will direct our upcoming studies on proton transport in ILs to support proton coupled electron transfer reactions of carbon dioxide.

Our most recent work supported by the ACS-PRF examines the capacitive electrochemical behavior of the 1-butyl-3-methylimidazolium tetrafluoroborate (Bmim BF4) polycrystalline gold electrode interface, using fourier transformed, large-amplitude, alternating current voltammetry, as part of a collaboration with Prof. Alan Bond at Monash Univ in Australia. This was a significant project beyond the resulting publication, as it involved one of my students traveling to Australia for four months, learning this advanced electrochemical technique, and returning to Iowa to instruct other students in my group. We expect several additional papers to result from this work, and have two currently in preparation.

The major result of this publication is direct, experimental, observation of the capacitance behaviour of the electrode/IL system without significant hysteresis. Many prior publications have shown a major hysteresis of the capacitance behaviour, dependent on the direction of the applied electrochemical sweep. The electrochemical technique applied here minimizes this hysteresis, creating data sets that only vary ca. 5% based upon scan direction, with is near the scan-to-scan variability of the base measurement.

Overall, our results add significant new information to the understanding of IL interfaces, molecular orientation in materials, and IL materials’ behaviors. These data encourage additional studies to find if long-range ordering may be observed in a broader range of ILs as a possible avenue for forming regular 3-D molecular networks.