Supporting Information

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1 Supporting Information High Rate, Selective and Stable Electroreduction of CO 2 to CO in Basic and Neutral Media Cao-Thang Dinh,1, F. Pelayo García de Arquer,1, David Sinton 2, Edward H. Sargent *,1 1 Department of Electrical and Computer Engineering, University of Toronto, 10 King s College Road, Toronto, ON, Canada, M5S 3G4. 2 Department of Mechanical and Industrial Engineering, University of Toronto, 5 King s College Road, Toronto, ON, Canada, M5S 3G8. These authors contributed equally to this work. (*) Correspondence and requests for materials should be addressed to Edward H. Sargent (ted.sargent@utoronto.ca) (E.H.S)

2 Materials The polymer-based gas-diffusion layer used an expanded PTFE membrane as a gas-diffusion layer with a pore size 450 nm as a catalyst substrate. Ag (99.99%) was sputtered on the substrate using an Angstrom sputtering system at a deposition rate of 1 Å/sec. Carbon black NPs (Sigma-Aldrich, <100 nm) were dispersed in a mixture of methanol and Nafion solution (100 µl of Nafion solution in 1 ml of methanol). The mixture was then sonicated for 1 hour to produce an ink, which was spray coated onto the Ag/PTFE electrodes. Characterization The Ag/PTFE electrodes were characterized using SEM on a Hitachi S-5200 instrument (The Centre for Nanostructure Imaging, University of Toronto) apparatus. The oxidation state of Ag in the samples were studied by X-ray photoelectron spectroscopy (XPS) (model 5600, Perkin-Elmer). The binding energy data were calibrated with reference to the C 1s signal at ev. X-ray diffraction (XRD) were measured on a Philips X Pert Pro Super X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation. CO 2 electroreduction The electroreduction of CO 2 was performed in a flow cell configuration consisting of a gasdiffusion layer, an anion exchange membrane and a nickel mesh anode. 1 The geometric active surface area of both anode and cathode are 1 cm 2. The catalyst electrode, anion exchange membrane and nickel anode were positioned and clamped together using PTFE spacers such that a liquid electrolyte could be introduced into the chambers between the anode and membrane as well as the membrane and the cathode. Gaseous CO 2 would then be passed behind the gas-diffusion layer to diffuse into the liquid electrolyte present at the catalyst. In the catholyte stream, a port

3 drilled into the PTFE spacer is present for a Ag/AgCl reference electrode to be positioned a specific distance from the working electrode. All CO 2 reduction experiments were performed using an electrochemical workstation (Autolab PGSTAT302N) with a Ag/AgCl reference (with 3 M KCl as the filling solution). Electrode potentials after ir compensation were rescaled to the reversible hydrogen electrode (RHE) reference by E RHE = E Ag/AgCl V ph. The electrolytes (1 M KHCO 3 and 1 M KOH, 100 ml) were circulated through the electrochemical cell using peristaltic pumps. The electrolyte flow was keep at 10 ml min -1. The CO 2 (Linde, 99.99%) flow was kept constant at 50 ml min -1 using a mass flow controller. The reactions were run for at least 500 s before the gas products were collected for analysis. The liquid products were collected after 20 minutes of reaction. The gas products from CO 2 RR were analyzed using a gas chromatograph (PerkinElmer Clarus 680) coupled with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The gas chromatograph was equipped with a Molecular Sieve 5A capillary column and a packed Carboxen-1000 column. Argon (Linde, %) was used as the carrier gas. The liquid products were quantified using Nuclear magnetic resonance spectroscopy (NMR). 1 H NMR spectra of freshly acquired samples were collected on Agilent DD2 500 spectrometer in 10% D 2 O using water suppression mode, with Dimethyl sulfoxide (DMSO) as an internal standard. Sixteen second relaxation time between the pulses was used to allow for complete proton relaxation.

4 Figure S1. Representative SEM images of Ag/PTFE sample after carbon nanoparticle coating.

5 Figure S2. Representative NMR spectrum of liquid product from CO 2 RR in 1 M KHCO 3 electrolyte showing the production of formate. The formate Faradaic efficiency is in the range of 1-2 % for all samples.

6 Figure S3. Representative NMR spectrum of liquid product from CO 2 RR in 1 M KOH electrolyte showing the production of formate. The formate Faradaic efficiency is in the range of 3-4 % for all samples.

7 Figure S4. Low magnification SEM image of pristine Ag/PTFE (left) compared to CD-Ag/PTFE (right).

8 Figure S5. XRD spectra of CD-Ag/PTFE compared to pristine Ag/PTFE.

9 Table S1. Summary of CO 2 reduction performances in basic and neutral media at a current density higher than 100 ma/cm 2. Catalyst Electrolytes Current Faradaic Stability (h) References density efficiency (ma/cm 2 ) (%) C/Ag/PTFE 1 M KHCO > This work C/Ag/PTFE 1 M KOH 150 > This work Ag/GDL 1 M NaHCO (2) Ag/C 0.2 M K 2 SO (3) Ag/GDL 0.4 K 2 SO (4) Au/C 0.5 M KHCO Not reported (5) Ag/TiO 2 1 M KOH Not reported (6) Ag/MWCNT (GDE) 1 M KOH Not reported (7) Au/GDL 2 M KOH (*) (8) Ag/GDL 3 M KOH 400 >95 Not reported (9) (*) The sample was washed every 2 hours. References 1. Dinh, C.-T.; Burdyny, T.; Kibria, M. G.; Seifitokaldani, A.; Gabardo, C. M.; García de Arquer, F. P.; Kiani, A.; Edwards, J. P.; De Luna, P.; Bushuyev, O. S. et al. CO 2

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