Supplementary Information

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1 Supplementary Information Shape-dependent Electrocatalytic Reduction of CO 2 to CO on Triangular Silver Nanoplates Subiao Liu, Hongbiao Tao, Li Zeng, Qi Liu, Zhenghe Xu, Qingxia Liu, Jing-Li Luo Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada Corresponding authors: Jing-Li Luo, Qingxia Liu Telephone: , luoj@ualberta.ca; qingxia2@ualberta.ca S1

2 Supporting Information contains: S1. The specifications of chemicals and gases S2. Synthesis of triangular Ag nanoplates (Tri-Ag-NPs) and Similarly sized Ag nanoparticles (SS-Ag-NPs). S3. Sample characterizations S3.1 Transmission electron microscopy (TEM) S3.2 UV/vis spectrophotometer S3.3 Fourier transform infrared spectroscopy (FTIR) (Figure S1) S4. Electrochemical measurements S4.1 Linear Sweep Voltammetry (LSV) S4.2 Product analysis of the three-electrode setup for CO 2 reduction S4.3 Electrochemical surface area measurements (Figure S2) S5. Reasons credited for the current density increase of Bulk Ag in Figure 3(d) (Figure S3) S6. Faraday efficiency calculation S7. DFT Calculations S8. The calculations of active adsorption site density S8.1 Data for SS-Ag-NP clusters S8.2 Data for Tri-Ag-NPs S2

3 S9. Supplementary Tables and Figures Table S1. DFT calculated electronic energies and relevant thermodynamic data with suggested fugacities for gaseous species. Table S2: Data for SS-Ag-NP clusters Table S3: Data for Tri-Ag-NP clusters Figure S4. Three-electrode setup for CO 2 reduction. (a) Top view; (b) cathode chamber for the working electrode and reference electrode; (c) anode chamber for the counter electrode. Figure S5. SEM images of SS-Ag-NPs with different scale bars. Figure S6. Electrochemical impedance spectra of the working electrode recorded at open circuit voltage condition under the influence of an ac voltage of 10 mv in CO 2 saturated solution. Figure S7. Faraday efficiencies (FEs) of different working electrodes (Bulk Ag, SS- Ag-NPs, Tri-Ag-NPs, CB and bare GCE) derived from CO 2 reduction. Figure S8. Plots of current densities over time of different catalysts (Bulk Ag, SS-Ag NPs, Tri-Ag-NPs, CB and bare GCE) for CO 2 reduction. Figure S9. TEM images of Tri-Ag-NPs after electrolysis for CO 2 RR. Figure S10. Models for Tri-Ag-NPs with different size. Figure S11. Free energy diagrams for H + to H 2 on different Ag sites. S3

4 S1. The specifications of chemicals and gases Silver nitrate (AgNO 3 ), sodium borohydride (NaBH 4 ), hydrogen peroxide (H 2 O 2, 30%), potassium bicarbonate (> 99.99% trace metal basis) and isopropanol were all purchased from Fisher Scientific Company. Bulk Ag were purchased from Sigma Aldrich. Nafion perfluorinated ion-exchange resin solution (5 wt.% in mixture of lower aliphatic alcohol & H 2 O) was purchased from Sigma-Aldrich. Nafion N-117 membrane (0.18 mm thick) was purchased from Alfa Aesar; The glassy carbon electrode (GCE 10 mm) was purchased from AIDA Science Technology Company, China. Deionized water was taken from a Millipore Autopure system. All chemicals are of analytical grade and used without further purification. Hydrogen (H 2, %), argon (Ar, %), compressed air (extra dry) and carbon dioxide (CO 2, %) were purchased from Prexair; S2. Synthesis of triangular Ag nanoplates (Tri-Ag-NPs) and Similarly sized Ag nanoparticles (SS-Ag-NPs). In a standard synthetic approach, the total volume of the reaction solution is fixed at ml. Typically, a ml aqueous solution combining silver nitrate (0.05 M, 50 µl), trisodium citrate (75 mm, 0.5 ml), and H 2 O 2 (30 wt %, 60 µl) was vigorously stirred at room temperature in air. Sodium borohydride (NaBH 4, 100 mm, 250 µl) was rapidly injected into this mixture to initiate the reduction, immediately leading to a light-yellow solution. After 3 min, the colloidal solution turned to a deep yellow due to the formation of small silver nanoparticles. Within the next several seconds, the morphology started to change from particles to nanoplates accompanied by the solution color changing from deep yellow to red, green, and blue. The entire transition S4

5 from nanoparticle to nanoplates typically took 2-3 min. The resulting product was collected by centrifugation and washed with deionized water for three times. For the preparation of SS-Ag-NPs, only poly(vinylpyrrolidone) (weight-average molecular weight Mw ~ 29, 000 g/mol) was added to the solution of Ag(NO 3 ), this leads to the formation of Ag nanoparticles. S3. Sample characterizations S3.1 Transmission electron microscopy (TEM): The high-resolution transmission electron microscopy images of all samples were acquired by using a JEOL JEM-2100 with a 200-kV TEM. The TEM uses an electron beam as light source (Lab6 crystal in this case) and the beam goes through (transmission) the specimen. The TEM was operated at an accelerating voltage of 200 kv. Images were achieved using a Gatan digital imaging system through Digital Micrograph software. S3.2 UV/vis spectrophotometer: The measurement of optical property was conducted by using a Varian Cary 50 UV/vis spectrophotometer (190 nm nm). S3.3 Fourier transform infrared spectroscopy (FTIR) To examine if PVP retains on the as-prepared samples, Fourier transform infrared spectroscopy (FTIR) measurements were conducted and the corresponding spectra of air (as reference), PVP, samples before washing and samples after washing were shown in Figure S1 (Note: the resulting product was collected by centrifugation and washed with deionized water for three times). Obviously, a sharp IR band at ~1650 cm -1 was observed for PVP. Samples after washing exhibit invisible characteristic peak as compared to the samples before washing and PVP, an indication of the negligible content of PVP retaining on the as-prepared samples. S5

6 Figure S1. FTIR measurements of air (as reference), PVP, samples before washing and samples after washing S4. Electrochemical measurements S4.1 Linear Sweep Voltammetry (LSV). 20 mg of powders (10 mg of catalyst and 10 mg of carbon black) were dispersed in 1 ml mixed solvent containing 700 µl isopropanol, 200 µl of 5 wt% Nafion solution and 100 µl deionized water to form a homogeneous ink after at least 3 hours ultrasonic. Then 5.0 µl of the catalyst ink was loaded onto a glassy carbon electrode (GCE) of 5 mm in diameter (loading ~ 0.51 mg cm -2 ), the GCE were mechanically polished with alumina paste before loading the catalyst suspension, then the GCE was dried overnight in air at room temperature before testing. The LSV curves were conducted with a Solartron 1255 frequency response analyzer and a Solartron 1286 electrochemical interface instrument in a home-made electrochemical cell using saturated calomel electrode as the reference electrode, a platinum gauze as the counter electrode and the catalyst coated GCE as the working electrode. Electrolyte was saturated by bubbling CO 2 prior to the start of each experiment at a flow rate of 20 ml min -1 for 2 h until a saturated CO 2 condition was reached to ensure that all the oxygen was removed from the electrolyte. The flow of CO 2 was maintained over the solution during the recording of LSV curves in order to ensure its continued CO 2 saturation. The working electrode was repeated at least 20 S6

7 times before data were recorded at a scan rate of 20 mvs -1. All potentials are reported with respect to the reversible hydrogen electrode (RHE). All measurements are independently conducted where a freshly deposited ink was used. Potential (V vs. RHE) = Applied potential (V vs. SCE) V ph S4.2 Product analysis of the three-electrode setup for CO 2 reduction. 25 µl of the catalyst ink was loaded onto the GCE (0.785 cm 2 ) (loading ~0.637 mg cm -2 ). The gas products from the cathode compartment were analyzed using a Hewlett-Packard model Agilent 6890N gas chromatograph (GC) equipped with a packed bed column (HaySep D) operated at 80 C with a thermal conductivity detector and a flame ionization detector. Argon (Air liquid 5.0) was employed as carrier gas with a flow rate of 30 ml min -1. The gaseous products H 2, CH 4 and CO were separated in a molecular sieve column (Alltech, part no , 1.65 m 1/8 in., molecular sieve 13X, 60/80 mesh) and hydrocarbons and CO 2 in a HaySep column (Alltech, part no , 3.5 m 1/8 in., HaySep D, 80/100 mesh). NMR was used to quantify the concentration of liquid-phase products. S4.3 Electrochemical surface area measurement. The electrochemical surface area was performed without removal of oxygen in a flask containing 5.00 mm Pb(NO 3 ) 2, 10 mm HNO 3 and 10 mm KCl. A cyclic voltammetry curve was recorded at 10mV/s between 0 and -0.5 V vs. a Ag/AgCl electrode. The counter electrode was a 30x30 platinum mesh. The deposited amount of lead from Pb 2+ concentrations can be estimated from the deposited charge or from the stripped charge. The UPD peak obtained under the S7

8 experimental conditions shown in Figure 3, at Pb 2+ concentrations of 5 mm and higher, corresponds to a charge of 600 µc/cm 2 1,2 3,4. Figure S2. Cyclic voltammograms of UPD and bulk deposition of Pb in 5 mm Pb(NO 3 ) 2, 10 mm HNO 3 and 10 mm KCl solution for (a) Tri-Ag-NPs, (b) SS-Ag- NPs, and (c) Bulk Ag. (d) CO current density normalized by ECSA S5. Reasons credited for the current density increase of Bulk Ag in Figure 3(d) ECSA = f S, where S stands for the real surface area of the smooth metal electrode, which is generally equal to the geometric area of glassy carbon electrode. The roughness factor f was estimated from the ratio of double-layer capacitance Cd for the working electrode and the corresponding smooth metal electrode (assuming that the average double-layer capacitance of a smooth metal surface is 20μF cm -2 ), that is, f =Cd /20 μf cm -2. The Cd was determined by measuring the capacitive current associated with double-layer charging from the scan-rate dependence of cyclic voltammetric stripping. For this, the potential window of cyclic voltammetric S8

9 stripping was -0.3 V to -0.2 V versus SCE (0.1 M KHCO 3 solution). The scan rates were 20 mv s -1, 50 mv s -1, 80 mv s -1, 100 mv s -1 and 120 mv s -1. The Cd was estimated by plotting the j = (j -j ) at V (where j and j are the cathodic and anodic current densities, respectively) versus SCE against the scan rate. As we can see from Figure 5(right), the Cd increased from 3.65 mf cm -2 to 5.30 mf cm -2 after the stability test. This in turn confirms the increase of ECSA due to the removal of oxide layer and contaminants on the surface of bulk Ag powders, and consequently resulting in the increase of current density during the stability test. Figure S3. Charging current density differences j plotted against scan rates. S6. Faraday efficiency calculation FE 2FV RT 100% FE 2 96,485 C mol V s % N N m mol K K 100% FE, % %, where the unit of V is ml/min. then S9

10 FE V ml min % 100% Vol% = volume concentration of CO in the exhaust gas from the cell (GC data). V (ml/min) = Gas flow rate measured by a flow meter at the exit of the cell at room temperature and under ambient pressure. A = steady-state cell current. S7. DFT Calculations The Vienna Ab initio Simulation Package (VASP) 5-8 was used to perform density functional theory (DFT) calculations. Electron-ion interactions was treated with the projector-augmented plane-wave (PAW) 9,10 method. Revised Perdew-Burke- Ernzerhof (RPBE) generalized gradient approximation (GGA)functional 11 was employed to model the exchange-correlation interactions. Plane-wave cutoff of 400 ev was set for all the computations in this study and spin-polarization was selected. Monkhorst-Pack (15,15,15) k point sampling was set for optimization of unit cell of Ag. Based on the optimized unit cell, flat surfaces (100), (110), (111) and stepped surface (211) with atom slabs and vacuum of 10 Å were built, and cuboctahedra Ag cluster models with 13, 55, 147, 309 atoms were built to simulate SS-Ag-NPs. The Ag cluster with 47 atoms was put in a box of 20 Å 20 Å 20 Å and was used for calculation. During all the structure optimization calculations, top two layers of the surfaces including adsorbate were relaxed, while the bottom two layers were fixed. For Ag S10

11 cluster, all the silver atoms including adsorbates were relaxed. The same density of k points as that for unit cell were set to sample the Brillouin zone for surface models, while only Gamma point was included for Ag cluster. Maximum atomic force of 0.05eV was chosen as the convergence criterion for structure relaxation. Fermi-level smearing of 0.1eV was set for surface and cluster calculations, while that of 0.01eV was set for gas-phase species. For all the adsorption models, different adsorption sites of molecule on surfaces were tested and the most stable adsorption configuration was selected. Since purpose of this calculation was to build Gibbs free energy diagram, computational hydrogen electrode (CHE) model 11,12 was used where each electrochemical reaction step is treated as a simultaneous transfer of the protonelectron pair as a function of the applied potential. Underlying reaction mechanisms for CO 2 reduction are, Where * means the corresponding surface where molecules or transition state species adsorbed on. Using initial state where gaseous CO 2 freely above an empty surface as the reference 12, the Gibbs free energy changes of interest can finally be represented as, S11

12 Associated hydrogen evolution reactions and Gibbs free energy changes are shown as below, or To build the Gibbs free energy diagram with the computational hydrogen electrode model, Gibbs free energies for relevant species were calculated with the expression: + + Where E DFT is the DFT calculated electronic energy in VASP, E ZPE is the zero-point vibrational energy, CpdT is the enthalpic correction and TS is the entropy contribution. Harmonic approximation where all 3N degrees of freedom were treated as frustrated harmonic vibrations was selected to treat the adsorbates with negligible contributions from the Ag surfaces, and PV contributions were neglected. Relevant thermodynamic data was calculated with the standard methods 13 and was transferred into Gibbs free energies at K. However, for non-adsorbed molecules, standard ideal gas 12,13 methods were employed to get the later three items from temperature, pressure, and calculated vibrational energies with VASP. As suggested 12, eu value of -0.11V was chosen to represent the approximate minimum driving force such that net free energy change from CO 2 to CO is negative. Also, to account for the overestimation of DFT calculation, +0.45eV correction was added to the CO 2 S12

13 electronic energy 11,13. All the relevant thermodynamic data used to build Gibbs free energy diagrams is listed in Table 1. Table S1. DFT calculated electronic energies and relevant thermodynamic data with suggested fugacities for gaseous species 12. Species/Adsorbate Fugacity /Pa E DFT /ev E ZPE /ev C p dt /ev -TS /ev G /ev CO (g) CO H H 2 O _CO _COOH _H _CO _COOH _H _CO _COOH _H _CO _COOH _H S13

14 Ag_CO Ag_COOH Ag_H S8. The calculations of active adsorption site density S8.1 Table S2: Data for SS-Ag-NP clusters S8.2 Table S3: Data for Tri-Ag-NPs S14

15 S9. Supplementary Figures Figure S4. Figure S4. Three-electrode setup for CO 2 reduction. (a) Top view; (b) cathode chamber for the working electrode and reference electrode; (c) anode chamber for the counter electrode. S15

16 Figure S5. Figure S5. SEM images of SS-Ag-NPs with different scale bars. S16

17 Figure S6. Figure S6. Electrochemical impedance spectra of the working electrode recorded at open circuit voltage condition under the influence of an ac voltage of 10 mv in CO 2 saturated solution. S17

18 Figure S7. Figure S7. Faraday efficiencies (FEs) of different working electrodes (Bulk Ag, SS- Ag-NPs, Tri-Ag-NPs, CB and bare GCE) derived from CO 2 reduction. S18

19 Figure S8. Figure S8. Plots of current densities over time of different catalysts (Bulk Ag, SS-Ag- NPs, Tri-Ag-NPs, CB and bare GCE) for CO 2 reduction. S19

20 Figure S9. TEM images of Tri-Ag-NPs after electrolysis for CO 2 RR. Figure S10. Figure S10. Models for Tri-Ag-NPs with different size. S20

21 Figure S11. Figure S11. free energy diagrams for H + to H 2 on different Ag sites S21

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