Modern society continues to require more energy due to consumer electronics, transportation, and industrial processes. However increasing the burning of fossil fuels to accommodate the increased energy demand will enviably increase the atmospheric CO₂ content. Because of this, governments have sponsored technologies that utilize renewable resources for energy generation such as solar cell and wind-powered generators. An alternative route would be to reduce CO₂to CO, which can then be used to synthesize other materials via the Fischer−Tropsch process [1].

Imidazolium-based ionic liquids (IL) have sheen shown to sponsor the selective electroreduction of CO₂ on various metals [1]. In acetonitrile, non-noble metals such as Bi and Sn have been shown to not only selectively reduce CO₂ to CO in the presence of imidazolium IL but also obtain similar rates as Ag and Au [2]. Here we follow the electroreduction of CO₂on Cu-Sn alloys fabricated by co-electrodeposition and magnetron sputtering using dynamic electrochemical impedance spectroscopy (dEIS). The comparison of the alloyed materials centers on the morphological differences, with the electrodeposition providing a vast range of nanostructures and sputtering providing a more uniform (structureless) polycrystalline surface.

dEIS consists of applying a series of small-amplitude ac sine waves on top of the slow-sweep “dc” signal used for classical cyclic voltammetry experiments [3]. The technique therefore allows for impedance spectra with up to 50 different frequencies spanning 4 decades in frequency to be collected simultaneously with cyclic voltammetry. This provides further elucidation of the mechanism by deconvoluting the processes that make up the seemingly featureless voltammogram profile.

We demonstrate how the dynamic impedance can be used to not only reconstruct the voltamogram but it can be used to deconvolute the processes, faradaic and non-faradaic, that contribute to the overall rate expression as inferred from Tafel plots. Specifically, we connect condensing of the positively charged imidazolium at the electrode surface to the reduction kinetics. That is, the acidic proton of the imidazolium cation becomes activated at the surface, which facilitates adsorption CO₂. Once adsorbed, the proton-coupled electron transfer becomes active. Lastly, we briefly discuss the used of dEIS to obtain and track the changes in the measured capacitances. Currents can then be normalized to this value to correct for the changes in the catalyst's surface area. We use this to show whether or not the nanostructures obtained during electrodeposition of the catalysts are in fact more active due to an increased surface density of reactive zones, or if the nanostructures just provide an increase in surface area.

Research supported by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Department of Energy’s Office of Basic Energy Sciences Division.

[1] Rosenthal, J. (2014). Progress Toward the Electrocatalytic Production of Liquid Fuels from Carbon Dioxide. Progress in Inorganic Chemistry: Volume 59(pp. 299–338). John Wiley & Sons, Inc. 

[2] Medina-Ramos, J., DiMeglio, J. L., & Rosenthal, J. (2014). Efficient Reduction of CO 2to CO with High Current Density Using in Situ or ex Situ Prepared Bi-Based Materials. Journal of the American Chemical Society, 136(23), 8361–8367. http://doi.org/10.1021/ja501923g

[3] R.L. Sacci, F. Seland and D.A. Harrington, Dynamic Electrochemical Impedance Spectroscopy for Electrocatalytic Reactions, Electrochim. Acta., 131 (2014) 13-19.