Electroreduction of CO2 at Cu and Sn Foams

Previous studies on the electrochemical reduction of CO2 at metallic electrodes suggest that the adsorption of hydrogen species is structure-sensitive.[1] Surface roughening is likely to introduce defects favorable for the reaction of adsorbed hydrogen atoms, an important intermediate step in the electro-reduction of CO2 in protic media.[1, 2] Furthermore, roughened surfaces have been shown to be more selective toward hydrocarbon products, which was attributed to a higher number of uncoordinated metal sites.[2] These finding were confirmed by DFT calculations, which suggest that CO2 activation and reduction occurs at these sites. [3] To further gauge the effect of morphology on the faradaic efficiency and distribution of products obtained during the electroreduction of CO2, Cu and Sn foams were electrosynthesized on Cu substrates using a recently reported process.[4] The metal foams were found to be mechanically stable during their preparation, handling, and use in the electrocatalytic reduction of CO2. Both Cu and Sn foams are attractive metals for the electrocatalytic reduction of CO2because of their low cost and non-toxic nature. Electroreduction of CO2 was performed in a typical H-cell under potentiostatic conditions. The faradaic efficiency of producing formate from CO2 at the metal foams and the equivalent planar metal electrode are compared in Figure 1a and 1b. The faradaic efficiencies obtained with the Cu foam electrode were found to be higher at all potentials with a maximum efficiency of 37% at -1.5V for HCOOH. Previously reported data is included for comparison.[5] Likewise, the faradaic efficiencies for HCOOH generation at the Sn foam electrode were found to be higher at all potentials with a maximum efficiency of 89.5% at -1.7V. Previously reported data with slightly different conditions (Sn gas diffusion cell in 0.5 M NaHCO3) is included for comparison.[6] XRD analysis of the Cu and Sn foams did not reveal major differences in the relative ratio of dominant crystal facets when compared to the equivalent planar samples of high purity metal. The hierarchical nature of the pore architecture along with high surface area of the metal foams could affect the reaction kinetics and contribute to the observed increase in faradaic efficiency. For example, the porous nature of the metal foams may promote interactions between incoming CO2, adsorbed surface species, electrolyte, and reducing equivalents that are novel relative to the same reaction at a planar electrode. The effect of varying the electrodeposition time and resulting foam architecture on the electroreduction of CO2will be the focus of this presentation. References Hori, Y., A. Murata, and R. Takahashi, Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper electrode in aqueous solution. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 1989. 85(8): p. 2309-2326. Tang, W., A.A. Peterson, A.S. Varela, Z.P. Jovanov, L. Bech, W.J. Durand, S. Dahl, J.K. Norskov, and I. Chorkendorff, The importance of surface morphology in controlling the selectivity of polycrystalline copper for CO2 electroreduction. Physical Chemistry Chemical Physics, 2012. 14(1): p. 76-81. Durand, W.J., A.A. Peterson, F. Studt, F. Abild-Pedersen, and J.K. Nørskov, Structure effects on the energetics of the electrochemical reduction of CO2 by copper surfaces. Surface Science, 2011. 605(15–16): p. 1354-1359. Shin, H.C., J. Dong, and M. Liu, Nanoporous Structures Prepared by an Electrochemical Deposition Process. Advanced Materials, 2003. 15(19): p. 1610-1614. Kuhl, K.P., E.R. Cave, D.N. Abram, and T.F. Jaramillo, New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy & Environmental Science, 2012. 5(5): p. 7050-7059. G.K.S. Prakash, F.A. Viva, G.A. Olah, Electrochemical reduction of CO2 over Sn-Nafion® coated electrode for a fuel-cell-like device, Journal of Power Sources, 223 (2013) 68-73.