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.

2

Supplementary Figure 2: Low-magnification SEM image of an as-prepared np-Ag electrode.

Inset: SEM image at the center of the cross-section.

3

Supplementary Figure 3: XPS Ag 3d peaks and Al 2p peaks for Ag20Al80 precursors, as-

prepared np-Ag, and np-Ag after 2 hours and 8 hours electrolysis at -0.5 V vs. RHE. The as-

prepared 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 Al2O3. The associated Ag spectrum shows

higher binding energy peaks that may result from forming Ag-Al-O oxide compounds.

4

Supplementary Figure 4: Comparison of CO2 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 pre-

electrolysis).

5

Supplementary Figure 5: CO partial current density (left axis) and CO Faradaic efficiency

(right axis) vs. overpotential on nanoporous silver.

6

Supplementary Figure 6: SEM images of the electrodes after 2 hours electrolysis under various

potentials vs. RHE (scale bar, 1 µm).

7

Supplementary Figure 7: CO2 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.

8

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 Ag2O 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.

9

Supplementary Figure 9: HRTEM images of (a) free-standing Ag nanoparticles and (b) free-

standing 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.

10

Supplementary Figure 10: The comparison of CO2 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.

11

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.

12

Supplementary Figure 12: CO2 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).

13

Supplementary Figure 13: CO partial current density of nanoporous silver vs. (a) CO2 partial

pressure at constant potential and (b) potassium bicarbonate concentration at constant potential.

14

Supplementary Table 1: Summary of silver electrocatalysts for CO2 reduction.

Material Electrolyte pH

Over-

potential

(mv)

jCO

(mA cm-2

)

jCO

(mA mg-1

)

BET

surface area

(cm2)

[C]

Electrochemic

al surface area

(cm2)

[C]

Cell

Type Ref.

Polycrystalline

Ag

0.1 M

NaHCO3 /

CO2

7.2 840 ~4.1 N/A 2[D]

N/A A [1]

Polycrystalline

Ag

0.5 M

KHCO3 /

CO2

7.2 490 0.005 4.8×10-5

2[D]

~16 A This

work

Polycrystalline

Ag

0.5 M

KHCO3 /

CO2

7.2 390 Negligible Negligible 2[D]

~16 A This

work

Ag nanowire

1 mg cm-2

loading

0.5 M

KHCO3 /

CO2

7.2 390 Negligible Negligible ~33[D]

~37 A This

work

Ag nanoparticle

1 mg cm-2

loading

0.5 M

KHCO3 /

CO2

7.2 390 0.022 0.022 71 ~69 A This

work

Ag nanoparticle

10 mg cm-2

loading

0.5 M

KHCO3 /

CO2

7.2 390 0.215 0.0215 710 ~674 A This

work

Ag nanoparticle

6.7 mg cm-2

loading

EMIM-BF4 N/A 170 ~0.61 0.091 N/A N/A B [2]

Ag nanoparticle

6.7 mg cm-2

loading

EMIM-BF4 N/A 670 ~0.92 0.137 N/A N/A B [2]

Ag Nanoparticle

1 mg cm-2

loading

1 M

KOH / CO2 N/A N/A

~1 (-1.4 V

vs.

Ag/AgCl)[E]

~1 (-1.4 V

vs.

Ag/AgCl)[E]

N/A N/A B [3]

Ag

Pyrazole/Carbon

1 mg cm-2

loading

1 M

KOH / CO2 N/A N/A

~3 (-1.4 V

vs.

Ag/AgCl)[E]

~3 (-1.4 V

vs.

Ag/AgCl)[E]

N/A N/A B [3]

Nanoporous

Silver

0.5 M

KHCO3 /

CO2

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 CO2

saturated 1 M KOH was not provided, it is not possible to calculate the overpotentials exactly for these Ag

nanoparticle catalysts.

15

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

16

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 CO2 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 CO2 to CO. Journal of the American

Chemical Society 134, 19520-19523, doi:10.1021/ja308217w (2012).