Supplementary Information for “The Importance of Surface Morphology in Controlling the Selectivity of Polycrystalline Copper for CO 2 Electroreduction” Wei Tang a , Andrew A. Peterson b , Ana Sofia Varela a , Zarko P. Jovanov a , Lone Bech a , William J. Durand b , Søren Dahl a , Jens K. Nørskov b , Ib Chorkendorff a a) Center for Individual Nanoparticle Functionality, Department of Physics, Building 312, Technical University of Denmark, DK-2800 Lyngby, Denmark b) SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, Stanford, CA, United States 1 Experimental Details 1.1 Electrochemical Measurement The electrochemical reactions were carried out in a classical three-electrode cell, which was made of Pyrex glass, using copper as a working electrode, the Hg/HgSO 4 as a reference electrode (in 0.6 M K 2 SO 4 , 0.66 V vs. NHE), and the counter electrode of Pt. The cells of the working electrode and the counter electrode were separated by a solid membrane of commercial Nafion®117, whereby residual protons were exchanged with K + in 0.1M KClO 4 prior to the experiments, so that the oxygen produced on the counter electrode would not affect the electroreduction of CO 2 on the working electrode. To saturate the electrolyte with CO 2 and improve the diffusion of gases in the liquid, gas circulation was driven by a diaphragm pump (KNF, NMP830). The electrolyte was potassium perchlorate (Alrich, 99.99%) of 0.1 M and purified through pre-electrolysis method, by applying a bias of -1.0 V on the substitute Pt wire. The pH value was measured by Orion 720. Cyclic voltammetry (CV) and chronoamperometry (CA) were performed using a numeric potentiostat (Biologic VMP2) controlled via the EC-Lab software. The overpotentials related to CVs for the formation of nanoparticles as well as for the CVs obtained during electrochemical characterization of the surfaces are corrected for ohmic drop between the working and the reference electrode after a series of experimentally determined value for ohmic resistance of 10±2 Ω. Each electrochemical reaction of CO 2 proceeded for 15 min, by applying a bias of -1.1 V vs. RHE on the working electrode. The working electrode was immersed in 15 ml of electrolyte with 39 ml of gas phase volume. A 250-µl sample of gas after the reaction was analyzed by a gas chromatograph (Agilent 6890), equipped with a thermal conductivity detector (TCD) through HP-Molecular Sieve 5A and flame ionization detector (FID) through HP-PLOT Q. The products in liquid phase were analyzed through high-performance liquid chromatography (Agilent 1200) with a BioRad HPX-87H column. The experiments were repeated three times to determine the uncertainty of the results. 1.2 Physical Characterization The surface morphology of the catalyst samples was characterized using scanning electron microscopy (SEM, FEGSEM 200F digital scanning microscope). X-ray Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics This journal is © The Owner Societies 2011