Supporting Information. Selective CO2 Reduction on Zinc Electrocatalyst: The Effect of Zinc Oxidation State Induced by Pretreatment Environment

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1 Supporting Information Selective CO2 Reduction on Zinc Electrocatalyst: The Effect of Zinc Oxidation State Induced by Pretreatment Environment Dang Le Tri Nguyen,, Michael Shincheon Jee,, Da Hye Won, Hyejin Jung,, Hyung-Suk Oh, Byoung Koun Min,, and Yun Jeong Hwang *,, Clean Energy Research Center, Korea Institute of Science and Technology (KIST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Division of Energy and Environmental Technology, KIST School, Korea University of Science and Technology (UST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Anamdong, Seongbuk-gu, Seoul , Republic of Korea Green School, Korea University, 145 Anam-ro, Anam-dong, Seongbuk-gu, Seoul , Republic of Korea * Author to whom all correspondence can be addressed. Corresponding author: yjhwang@kist.re.kr, Phone: Number of Pages: 12 Number of Supporting Figures: 11 Number of Supporting Tables: 2 S1

2 Figure S1. Low-resolution of SEM images of (a) Zn foil, (b) E-ZnO, (c) RE-Zn-Ar, (d) RE- Zn-CO2, and (e) RE-Zn-CO2/KCl. S2

3 Figure S2. TEM images of (a, b, c) E-ZnO, (d, e, f) RE-Zn-Ar, (g, h, i) RE-Zn-CO2, and (j, k, l) RE-Zn-CO2/KCl. S3

4 Figure S3. Cyclic voltammetry plots for electrochemical surface area measurements of (a) Zn foil, (b) RE-Zn-Ar, (c) RE-Zn-CO2, and (d) RE-Zn-CO2/KCl. Electrochemical Surface Area Measurement The roughness measurement of the electrodes were conducted using cyclic voltammetry for selected electrodes in 1 M Na2SO4 electrolyte at various scan rates. The scanning potential range was from 0.69 to 0.55 V where no active redox peak existed. The double-layer width of the CV is thereby directly related to the capacitance and proportional to roughness of the electrodes. Specific capacitance values and surface roughness factors were calculated and shown in Table S1. (The current densities were obtained from the double layer charge/discharge curves at the potential of 0.62 V). S4

5 Table S1. The roughness factors of electrochemical surface area (ESCA). Figure S4. CO partial current density-potential curved of the Zn-electrocatalysts. The current density (jco/ecsa) is normalized by the ECSA roughness factors from the Table S1 values. S5

6 Figure S5. Comparison of pre-co2rr morphology of (a) RE-Zn-CO2 and (b) RE-Zn- CO2/KCl by higher-resolution of SEM images showing the formation of the smaller sized nanoparticles when E-ZnO is reduced in the KCl(aq)/CO2(g) condition. Figure S6. Stability test of the RE-Zn-CO2/KCl. Current density (line) and FE (column) of the RE-Zn-CO2/KCl catalyst during 20 h of CO2RR measurement at -1.1 V vs RHE. S6

7 Figure S7. XPS Zn 2p spectra of bulk Zn foil, E-ZnO, RE-Zn-Ar, RE-Zn-CO2, and RE-Zn- CO2/KCl electrodes. S7

8 Figure S8. XPS Cl 2p spectra of Zn-foil, RE-Zn-Ar, RE-Zn-CO2, RE-Zn-CO2/KCl electrodes showing adhesion of Cl when KCl electrolyte was used. S8

9 Figure S9. SEM images after 5 h of CO2RR: (a) Zn foil, (b) RE-Zn-Ar, (c) RE-Zn-CO2, and (d) RE-Zn-CO2/KCl. S9

10 Figure S10. XRD patterns of electrocatalysts after 5 h of CO2RR: Zn foil, RE-Zn-Ar, RE-Zn- CO2, and RE-Zn-CO2/KCl. S10

11 Figure S11. XPS C 1s spectra of electrocatalysts measured (a) pre- and (b) post- 5 h of CO2RR: Zn foil, RE-Zn-Ar, RE-Zn-CO2, and RE-Zn-CO2/KCl. S11

12 Table S2. XPS Zn spectra analysis of Zn electrocatalysts. S12