SUPPORTING INFORMATION. Direct Observation on Reaction Intermediates and the Role of. Cu Surfaces

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1 SUPPORTING INFORMATION Direct Observation on Reaction Intermediates and the Role of Bicarbonate Anions in CO 2 Electrochemical Reduction Reaction on Cu Surfaces Shangqian Zhu, Bei Jiang, Wen-Bin Cai, Minhua Shao,#,* Department of Chemical and Biological Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai, , China # Energy Institute, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China Table of contents 1. ATR-SEIRAS set-up and Cu thin film preparation 2. Electrochemical measurements 3. Supporting figures and tables 4. References S-1

2 1. ATR-SEIRAS set-up and Cu thin film preparation The surface enhanced infrared absorption spectroscopy (SEIRAS) with the attenuated total reflection (ATR) configuration was employed in this study. The preparation procedures of Cu film working electrodes were modified from a two-step wet process reported previously involving the initial chemical deposition of a Au thin film (~60 nm) on a hemispherical Si prism followed by the electrodeposition of a Cu thin film. 1 Before chemical deposition of Au, the hemispherical Si prism (20 mm in diameter, MTI Corporation) surface for IR reflection was polished with Al 2 O 3 powder and cleaned in water with sonication. Then the prism was soaked in a piranha solution (3:1 volumetric ratio of 98% H 2 SO 4 (Aldrich) and 30% H 2 O 2 (Aldrich)) for 2 hours. The reflection plane was then treated by 40% NH 4 F (Aldrich) for 90 s, and immersed in a mixture of 4 ml of Au plating solution 2 and 50 μl of 50% HF solution (Aldrich) at 55 C for 5 min. The Au-coated prism was assembled into a homemade spectroelectrochemical cell as the working electrode. An Ag/AgCl (BASi) calibrated by RHE was used as reference, which was introduced near the working electrode via a Luggin capillary. A Pt mesh (1 cm 1 cm) was serving as the counter electrode, which was separated with the main cell body by a dense glass frit (Ace Glass, Inc.). The Au thin film was then cycled in an Ar-saturated 0.1 M HClO 4 (GFS Chemicals) solution at a scan rate of 50 mv s -1 between 0.3 and 1.7 V until a repeatable CV (Figure S1) was obtained (~4 cycles) and rinsed with sufficient water. The electrodeposition of Cu overlayer was conducted in an Ar-saturated 5 mm CuSO 4 (Aldrich) + 50 mm H 2 SO 4 solution. Cu underpotential deposition (UPD) (Figure S2a) was used to evaluate the monolayer charge of Cu (~1.2 mc) on the Au thin film. Then potentiostatic deposition was applied at V vs RHE (Figure S2b) until the total change reached ~150 times of the monolayer charge (~180 mc) evaluated from Cu UPD. The estimated Cu thickness was ~30 nm according to the deposited charge. The prepared Cu thin film was carefully washed with deaerated water for several times, and used immediately. A Nicolet is50 FT-IR spectrometer equipped with a MCT detector cooled with liquid nitrogen was employed for the electrochemical ATR-SEIRAS measurements. All spectra were shown in absorbance (-log(r/r 0 ), with R and R 0 representing the sample and reference spectra, respectively. The spectral resolution was 4 cm -1 for all the measurements if not otherwise mentioned. In all electrochemical and spectroscopic measurements, a CHI electrochemical workstation (Model 760E, CH Instruments) was used. S-2

3 2. Electrochemical measurements 2.1 ATR-SEIRAS study on 12 CO 2 -saturated KH 12 CO 3 solution Around 45 ml of 0.1 M KH 12 CO 3 solution was transferred into the spectro-electrochemical cell, which was subsequently assembled into the FTIR optical pathway. The FTIR chamber was continuous purged with N 2 (High purity grade, Hong Kong Specialty Gases Co. Ltd.) at around 5 L min -1. The solution was purged with Ar (ultrahigh purity grade, Hong Kong Specialty Gases Co. Ltd.) for at least one hour until two successive interferograms (subsequently collected with a time interval of ~2 min) did not show any adsorption difference (-log(r 2 /R 1 ) in the CO 2 wavenumber region, which indicated the residual CO 2 in both the FTIR chamber and KH 12 CO 3 solution was removed. Then the working electrode was kept at 0.3 V for 10 min until the current diminished to zero in order to reduce the surface Cu oxide formed in the WE pre-treatment and transfer process. 12 CO 2 (Ultrahigh purity grade, Hong Kong Specialty Gases Co. Ltd.) was then continuously purged into the cell at a flow rate of ~30 ml min -1 for one hour (ph = 6.8). Then potential was swept between 0.3 V and -1.3 V at a scan rate of 2 mv s -1. In the meanwhile, real-time spectra were collect at a resolution of 4 cm interferograms were co-added for each spectrum with a collection period of 50 s. Hence each spectrum was the average result of 100 mv interval in CV. 2.2 ATR-SEIRAS study on Ar-saturated KH 12 CO 3 /KH 13 CO 3 solution The procedure was similar to the corresponding part in Section 2.1. The solution was continuously purged with Ar before and during the spectra collection. The CV potential window was set to 0.3 to -1.0 V. 2.3 ATR-SEIRAS study on 12 CO 2 purged KH 12 CO 3 /KH 13 CO 3 solution The pre-treatment procedure was similar to Section 2.1. Then 12 CO 2 was purged into the cell at 15 ml min -1 for 15 min with keeping the WE potential at 0.2 V (final solution ph = 6.9). Then the potential was stepped to -0.6 V. In the meanwhile, spectra collection was started. A fast IR collection was applied with a resolution of 4 cm -1 and 6 interferograms were co-added for each spectrum. S-3

4 3. Supporting figures and tables Figure S1. Cyclic voltammogram for the Au thin film electrode in an Ar-saturated 0.1 M HClO 4 solution at a scan rate of 50 mv s -1 S-4

5 Figure S2. a) Cyclic voltammogram and b) potentiostatic deposition curve at V for the Au thin film electrode in an Ar-saturated 5 mm CuSO mm H 2 SO 4 solution at a scan rate = 5 mv s -1. S-5

6 Table S1. Band Assignments in Figure 1 Wavenumber (cm -1 ) Assignment 2343 CO ν(c-o) of CO ad ν(c=o) stretching δ(h-o-h) of H 2 O Adsorbed CO 3 2-4a 1402 ν s (O-C-O) of COO - ad Solution CO ν(c-o) of COOH 7a S-6

7 Figure S3. Peak fitting result between 1415 and 1330 cm -1 on spectra collected at a) -0.7 and b) -0.8 V in Figure 1a. Figure S4. ATR-IR spectra of a 0.1 M K 2 CO M KOH solution recorded on a bare ZnSe prism. IR reference was taken in a 0.1 M KOH solution. S-7

8 Figure S5. a) Real-time ATR-SEIRAS spectra and b) CV and integrated band intensities recorded during the anodic scan of the Cu thin film electrode in a CO 2 -saturated 0.1 M KHCO 3 solution. Reference spectrum was taken at 0.3 V vs RHE. S-8

9 Figure S6. Real-time ATR-SEIRAS spectra recorded during the cathodic scan of the Au thin film electrode in a) CO 2 - and b) Ar-saturated 0.1 M KHCO 3 solution. Reference spectrum was taken at 0.3 V vs RHE. References 1. Yan, Y.-G.; Li, Q.-X.; Huo, S.-J.; Ma, M.; Cai, W.-B.; Osawa, M., J. Phys. Chem. B 2005, 109, Miyake, H.; Ye, S.; Osawa, M., Electrochem. Commun. 2002, 4, Nikolic, B.; Huang, H.; Gervasio, D.; Lin, A.; Fierro, C.; Adzic, R.; Yeager, E., J. Electroanal. Chem. Interfacial Electrochem. 1990, 295, (a) Heyes, J.; Dunwell, M.; Xu, B., J. Phys. Chem. C 2016, 120, ; (b) Wuttig, A.; Liu, C.; Peng, Q.; Yaguchi, M.; Hendon, C. H.; Motobayashi, K.; Ye, S.; Osawa, M.; Surendranath, Y., ACS Cent. Sci. 2016, 2, Tammer, M., Colloid Polym. Sci. 2004, 283, (a) Ataka, K.-i.; Yotsuyanagi, T.; Osawa, M., J. Phys. Chem. 1996, 100, ; (b) Dunwell, M.; Yan, Y.; Xu, B., Surf. Sci. 2016, 650, (a) Firet, N. J.; Smith, W. A., ACS Catal. 2016, 7, ; (b) Kakumoto, T., Energy Convers. Manage. 1995, 36, (a) Daiguji, H.; Matsuoka, E.; Muto, S., Soft Matter 2010, 6, ; (b) Zhou, Z.-Y.; Wang, Q.; Lin, J.-L.; Tian, N.; Sun, S.-G., Electrochim. Acta 2010, 55, S-9