Supplementary Figure 1: PXRD patterns of Ag-Al precursors, as-prepared np-ag electrodes and np-ag electrodes after 2 hours electrolysis under -0.5 V vs. RHE.
Supplementary Figure 2: Low-magnification SEM image of an as-prepared np-ag electrode. Inset: SEM image at the center of the cross-section. 2
Supplementary Figure 3: XPS Ag 3d peaks and Al 2p peaks for Ag 20 Al 80 precursors, asprepared np-ag, and np-ag after 2 hours and 8 hours electrolysis at -0.5 V vs. RHE. The asprepared sample and post reacted sample show typical Ag metal spectra with peak separation of 6 ev and no Al residuals. The precursor sample shows a peak at 72.24 which corresponds to Al and a peak at 75.6 ev which usually corresponds to Al 2 O 3. The associated Ag spectrum shows higher binding energy peaks that may result from forming Ag-Al-O oxide compounds. 3
Supplementary Figure 4: Comparison of CO 2 reduction activity of polycrystalline silver with and without a pre-electrolysis process (current density: black line without pre-electrolysis, red line with pre-electrolysis. CO Faradaic efficiency: without pre-electrolysis, with preelectrolysis). 4
Supplementary Figure 5: CO partial current density (left axis) and CO Faradaic efficiency (right axis) vs. overpotential on nanoporous silver. 5
Supplementary Figure 6: SEM images of the electrodes after 2 hours electrolysis under various potentials vs. RHE (scale bar, 1 µm). 6
Supplementary Figure 7: CO 2 reduction activity of nanoporous silver at -0.50 V vs. RHE for 8 hours. Inset: The corresponding SEM image of the electrode after 8 hours electrolysis. 7
Supplementary Figure 8: (a) A typical cyclic voltammogram of Ag within the potential widow of 0 to 1.60 V vs. RHE. The current peak observed at about 1.15 V corresponds to a monolayer formation of Ag 2 O or AgOH. Current transient at constant potential (1.15 V vs. RHE) for nanoporous Ag (b) and polycrystalline Ag (c). The charge required to oxidize one monolayer of np-ag is approximately 160 times as large as that of polycrystalline Ag. 8
Supplementary Figure 9: HRTEM images of (a) free-standing Ag nanoparticles and (b) freestanding Ag nanowires. TEM images of (c) free-standing Ag nanoparticles and (d) free-standing Ag nanowires. SEM images of (e) Ag nanoparticles and (f) Ag nanowires deposited on the Sigracet 25 BC Gas Diffusion Layer. 9
Supplementary Figure 10: The comparison of CO 2 reduction activity of various Ag electrodes at a moderate potential of -0.50 V vs. RHE. Although polycrystalline Ag and Ag nanowires show negligible CO production, they show significant hydrogen production. 10
Supplementary Figure 11: The CO production partial current densities of various Ag electrodes scaled to mass and electrochemical surface area at a moderate potential of -0.50 V vs. RHE. 11
Supplementary Figure 12: CO 2 reduction activity of nanoporous silver at (a) -0.30 V, (b) -0.70 V, and (c) -0.80 V vs. RHE. Total current density versus time on (left axis) and CO Faradaic efficiency vs. time (right axis). 12
Supplementary Figure 13: CO partial current density of nanoporous silver vs. (a) CO 2 partial pressure at constant potential and (b) potassium bicarbonate concentration at constant potential. 13
Supplementary Table 1: Summary of silver electrocatalysts for CO 2 reduction. Material Electrolyte ph Polycrystalline Ag Polycrystalline Ag Polycrystalline Ag Ag nanowire 1 mg cm -2 loading Ag nanoparticle 1 mg cm -2 loading Ag nanoparticle 10 mg cm -2 loading Ag nanoparticle 6.7 mg cm -2 loading Ag nanoparticle 6.7 mg cm -2 loading Ag Nanoparticle 1 mg cm -2 loading Ag Pyrazole/Carbon 1 mg cm -2 loading Nanoporous Silver 0.1 M NaHCO 3 / CO 2 0.5 M KHCO 3 / CO 2 0.5 M KHCO 3 / CO 2 0.5 M KHCO 3 / CO 2 0.5 M KHCO 3 / CO 2 0.5 M KHCO 3 / CO 2 Overpotential (mv) j CO (ma cm -2 ) j CO (ma mg -1 ) BET surface area (cm 2 ) [C] Electrochemic al surface area (cm 2 ) [C] Cell Type 7.2 840 ~4.1 N/A 2 [D] N/A A [1] 7.2 490 0.005 4.8 10-5 2 [D] ~16 A 7.2 390 Negligible Negligible 2 [D] ~16 A 7.2 390 Negligible Negligible ~33 [D] ~37 A 7.2 390 0.022 0.022 71 ~69 A 7.2 390 0.215 0.0215 710 ~674 A EMIM-BF 4 N/A 170 ~0.61 0.091 N/A N/A B [2] EMIM-BF 4 N/A 670 ~0.92 0.137 N/A N/A B [2] 1 M KOH / CO 2 1 M KOH / CO 2 0.5 M KHCO 3 / CO 2 N/A N/A N/A N/A ~1 (-1.4 V vs. Ag/AgCl) [E] ~3 (-1.4 V vs. Ag/AgCl) [E] Ref. This work This work This work This work This work ~1 (-1.4 V vs. N/A N/A B [3] Ag/AgCl) [E] ~3 (-1.4 V vs. N/A N/A B [3] Ag/AgCl) [E] 7.2 390 8 0.1989 2852 ~2650 A This work Note that A stands for gas-tight two-compartment electrochemical cell separated with ion exchange membrane; B stands for flow cell type electrolysis cell; [C] surface area is based on a 1 cm 1 cm apparent electrode size; [D] surface area is estimated based on geometry and mass density; [E] Since the ph of CO 2 saturated 1 M KOH was not provided, it is not possible to calculate the overpotentials exactly for these Ag nanoparticle catalysts. 14
Supplementary Table 2: Summary of geometric current density, CO efficiency, and CO partial current density as a function of potential for nanoporous silver. Potential (V vs. RHE) Geometric current density (ma cm -2 ) CO Faradaic efficiency (%) CO partial current density (ma cm -2 ) -0.20 0.286 0.7 0.002-0.25 0.343 3.5 0.012-0.30 0.603 17.8 0.107-0.35 1.06 65.5 0.692-0.40 3.34 81.0 2.71-0.50 8.97 89.2 8.00-0.60 17.6 92.1 16.3-0.70 29.8 92.3 27.5-0.80 37.3 93.1 34.7 15
Supplementary References: 1. Hori, Y. Modern Aspects of Electrochemistry. Vol. 42 (Springer, 2008). 2. Rosen, B. A. et al. Ionic Liquid-Mediated Selective Conversion of CO 2 to CO at Low Overpotentials. Science 334, 643-644, doi:10.1126/science.1209786 (2011). 3. Tornow, C. E., Thorson, M. R., Ma, S., Gewirth, A. A. & Kenis, P. J. A. Nitrogen-Based Catalysts for the Electrochemical Reduction of CO 2 to CO. Journal of the American Chemical Society 134, 19520-19523, doi:10.1021/ja308217w (2012). 16