Supporting Information. Christina W. Li and Matthew W. Kanan* *To whom correspondence should be addressed.

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1 Supporting Information CO 2 Reduction at Low Overpotential on Cu Electrodes Resulting from the Reduction of Thick Cu 2 O Films Christina W. Li and Matthew W. Kanan* *To whom correspondence should be addressed. mkanan@stanford.edu Index Experimental methods Fig. S1. SEM, XRD, and XPS characterization of annealed Cu electrodes. Fig. S2. Activity and morphology comparison for annealed electrodes with high CO 2 reduction activity. Fig. S3. High-resolution SEM images of an electrode annealed at 500 C for 12 h and reduced at 0.5 V vs. RHE. Fig. S4. Survey XPS spectra for an annealed electrode before and after electrolysis. Table S1. Total geometric current densities and Faradaic efficiencies for all products for an annealed electrode at selected potentials. Fig. S5. Measurement of double-layer capacitance using cyclic voltammetry. Table S2. Representative capacitances and surface roughness factors for annealed Cu electrodes. page S2 S5 S6-S7 S8-S9 S10 S11 S12 S13 S14

2 Experimental Methods Materials. Na 2 CO 3 (> %) and HClO 4 (67-72% in water) were purchased from Sigma Aldrich; o-phosphoric Acid (85% in water) was purchased from Fisher Scientific; carbon dioxide (99.99%) was purchased from Praxair; copper foil ( %, 0.1 mm thick) and copper wire ( %, 0.5 mm diameter) were purchased from Alfa Aesar. All chemicals were used without further purification. Electrolyte solutions were prepared with DI water (Ricca Chemical, ASTM Type I) M NaHCO 3 solution was prepared by vigorously bubbling CO 2 gas through 0.25 M Na 2 CO 3 solution for at least 6 hours. Annealing procedure. Copper foils were first electropolished in 85% o-phosphoric Acid at 4 V vs. a titanium counter electrode for 5 min. After rinsing with DI water and drying under a stream of N 2, foils were placed in a muffle or tube furnace (Thermo Scientific) under an air atmosphere at the specified temperature and time period. Electrodes were then allowed to cool slowly to room temperature over several hours. Electrochemical measurements. A CH Instruments 760D or 660D potentiostat was used for all experiments. A piece of platinum gauze (Sigma, 99.9%) was used as the counter electrode. The electrolyte used for all CO 2 reduction experiments was 0.5 M NaHCO 3 saturated with CO 2 with ph of 7.2. All potentials were measured against an Ag/AgCl reference electrode (3.0 M KCl, World Precision Instruments or BASi) and converted to the RHE reference scale using E (vs RHE) = E (vs Ag/AgCl) V V*pH. CO 2 reduction electrolyses and product analysis. Electrolyses were performed in a gastight two-compartment electrochemical cell with a piece of SELEMION anion exchange membrane as a separator. Each compartment contained 20 ml of 0.5 M NaHCO 3 electrolyte.

3 The solution in the cathodic compartment was purged with CO 2 for 20 min prior to the start of electrolysis. The headspace of the cathodic compartment was approximately 5 ml. CO 2 gas was delivered into the cathodic compartment at a rate of 5.00 sccm and vented directly into the gas-sampling loop of a gas chromatograph (GC) (SRI Instruments). A GC run was initiated every 15 minutes. The GC was equipped with a packed MolSieve 5A column and a packed HaySep D column. Argon (Praxair, %) was used as the carrier gas. A flame ionization detector (FID) with methanizer was used to quantify CO, C 2 H 4, and C 2 H 6 concentration and a thermal conductivity detector (TCD) was used to quantify hydrogen concentration. The partial current density of CO production was calculated from the GC peak area as follows: where is a conversion factor based on calibration of the GC with a standard sample, p 0 = bar and T = K. HCO 2 H concentration was analyzed on a Varian Inova 600 MHz NMR spectrometer. A 0.5 ml sample of the electrolyte was mixed with 0.1 ml D 2 O and 1.67ppm (m/m) dimethyl sulfoxide (DMSO, Sigma, 99.99%) was added as an internal standard. The 1D 1 H spectrum was measured with water suppression using a presaturation method. Tafel slope. The current density vs. potential data were obtained by stepped-potential electrolyses. Partial current densities for CO production were calculated from the GC spectra every 15 minutes and averaged over 1-2 hours. A 0.5 ml aliquot of the electrolyte was extracted at the end of each potential step. Average partial current densities for HCO 2 H production at each step were calculated from NMR quantification of the HCO 2 H in these aliquots.

4 Oxide thickness analysis. For each electrode, a bulk electrolysis was run at 0.5 V vs. RHE for at least 3 h. The amount of charge passed to reduce the Cu 2 O layer per electrode area (Q) was determined by integrating the current densities in the initial plateau region of the electrolysis (~10 ma/cm 2 ) until the total current density fell below 5 ma/cm 2. The CO efficiency was averaged over the first 3 h after reduction of the oxide was complete. Figure 1f consists of electrolyses discussed in Figure 1b 1e, Figure S2a and S2b, and two additional electrodes annealed at 300 C. Estimates of the thickness of the Cu 2 O layer were calculated using the following equation: where n = 2, M = g/mol, and = 6.0 g/cm 3 for the reaction below. Cu 2 O + H 2 O + 2e - 2 Cu OH - Ex situ analyses. Scanning electron microscopy (SEM) images were acquired with an FEI XL30 Sirion Scanning Electron Microscope. X-ray photoelectron spectra were obtained with a PHI VersaProbe II Scanning XPS Microprobe. Powder X-ray diffraction (XRD) patterns were obtained with a PANalytical X'Pert PRO Materials Research Diffractometer with Programmable Divergence Slit (PDS) and PIXcel 3D detector. Surface Area Determination. Surface roughness factors for annealed electrodes relative to unannealed copper foil were determined by measuring double layer capacitances. Cyclic voltammetry (CV) was performed in the same electrochemical cell as in bulk electrolyses with a Nafion proton exchange membrane and 0.1 M HClO 4 electrolyte. CVs were obtained for a potential range in which no faradaic processes were occurring, and the geometric cur-

5 rent density was plotted against the scan rate of the CV. The slope of the linear regression gives the capacitance. A representative set of CVs for an electrode annealed at 500 C for 12h and reduced in 0.5 M NaHCO 3 at 0.5 vs. RHE is shown (Figure S5).

6 Figure S1. SEM, XRD, and Cu 2p XPS characterization of annealed Cu electrodes before and after bulk electrolysis at 0.5 V vs. RHE. In all cases, XPS and XRD indicate the reduction of CuO and Cu 2 O to Cu 0, and SEM shows that the overall morphology before and after oxide reduction remains essentially unchanged. a) Cu annealed at 130 C for 12 h. b) Cu annealed at 300 C for 30 min. c) Cu annealed at 500 C for 15 min.

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8 Figure S2. Comparison of bulk electrolysis data at 0.5 V vs. RHE and SEM images after electrolysis for annealed Cu electrodes with high CO2 reduction activity. The amount of charge passed to reduce the oxide per electrode area (Q) and partial current density for CO production (jco) are also indicated for each electrode. All of these electrodes have similar efficiencies and activities for CO2 reduction to CO despite a range of annealing conditions, but the surface morphologies are very different. a) Cu annealed at 500 C for 15 min, b) Cu annealed at 500 C for 1 h, c) Cu annealed 500 C for 12 h, d) Cu annealed at 700 C for 1 h.

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10 Figure S3. High resolution SEM images of an electrode annealed at 500 C for 12 h and reduced at 0.5 V vs. RHE. The nanowires on the reduced electrode appear to be composed of small particles <50 nm in size.

11 Figure S4. Survey XPS of a Cu electrode a) after annealing at 500 C for 12 h and b) after electrolysis at 0.5 V vs. RHE. Cu 21% O 51% C 28% Cu 45% O 39% C 16%

12 Table S1. Summary of total geometric current densities and Faradaic efficiencies for all products at various potentials on copper foil annealed at 500 C for 12 hours. E vs. RHE V j Faradaic Efficiency / % tot ma cm 2 HCOO MeCHO EtOH PrOH CO C 2 H 4 C 2 H

13 Figure S5. Determination of double-layer capacitance for an electrode annealed at 500 C for 12 h and subsequently reduced in 0.5 M NaHCO 3 at 0.5 vs. RHE. a) CVs taken over a range of scan rates in a potential window where only double-layer charging and discharging is relevant. b) Current due to double-layer charge/discharge plotted against CV scan rate.

14 Table S2. Capacitance values and surface roughness factors measured using CV for selected electrodes discussed in this report. The surface roughness factor for polycrystalline Cu is defined to be 1. Electrode preparation Capacitance Surface Roughness Factor Polycrystalline Cu 29 μf 1 Annealed at 300 C for 1 h 0.96 mf 33 Annealed at 300 C for 5 h 2.0 mf 69 Annealed at 500 C for 15 min 5.8 mf 198 Annealed at 500 C for 12 h 13.9 mf 475 Cu nanoparticle film 1.9 mf 65