1. Schematic representation of the fabrication of CuO and CuO/ZnO catalyst and CO₂ reduction process

2. (a) Physical picture and (b) simplified model diagram for electrolytic cell of electrochemical CO₂ reduction

3. TEM images of catalysts (a, e) CuO, (b, f) CuO/ZnO-1, (c, g) CuO/ZnO-2 and (d, h) CuO/ZnO-3 (a-d) before and (e-h) after pyrolysis with elemental mapping images of CuO/ZnO-2 ((i) TEM, (j) Cu, (k) O and (l) Zn)

4. (a) XRD patterns of CuO, CuO/ZnO-1, CuO/ZnO-2 and CuO/ZnO-3 electrocatalysts, (b) HRTEM image of CuO/ZnO-2, (c) total XPS survey of CuO/ZnO-2, (d) XPS Cu2p spectra of CuO/ZnO-2 and CuO, (e) XPS Zn2p_3/2 spectrum of CuO/ZnO-2, and (f) XPS high resolution O1s spectra of CuO/ZnO-2 and CuO

5. (a) Linear sweep voltammetry curves (LSVs) in N₂/CO₂-saturated KHCO₃, (b) electrochemical impedance spectra (EIS) of CuO and CuO/ZnO catalyst

6. (a) Reduction activity of CO₂ in CO₂-saturated KOH solution (CO₂-KOH) and N₂-saturated KHCO₃ solution (N₂-KHCO₃); Histograms for FE of the CO₂ reduction products and the ratios of CO/H₂ for (b) CuO, (c) CuO/ZnO-1, (d) CuO/ZnO-2 and (e) CuO/ ZnO-3; (f) Cyclic stability of CuO/ZnO-2 (black square and red dot indicate Faraday efficiencies of H₂ and CO, respectively)

7. Capacitive ΔJ/2 versus scan rate for (a) CuO, (b) CuO/ZnO-1, (c) CuO/ZnO-2, and (d) CuO/ZnO-3

8. (a) Oxidation LSVs of CuO, CuO/ZnO-1, CuO/ZnO-2 and CuO/ZnO-3 in 0.1 mol/L KOH; (b) Infrared spectra of the CuO/ZnO-2 catalyst before and after reaction; (c) Schematic formation process of CO

S1. (a) N₂ adsorption/desorption curves and (b) pore size distributions of CuO, CuO/ZnO-1, CuO/ZnO-2 and CuO/ZnO-3 electrocatalysts

S2. i-t curves of (a) CuO, (b) CuO/ZnO-2, (c) CuO/ZnO-2 and (d) CuO/ZnO-3

S3. TEM image of catalyst CuO/ZnO-2 after cyclic stability test

S4. (a) CV curves for (a) CuO, (b) CuO/ZnO-1, (c) CuO/ZnO-2 and (d) CuO/ZnO-3 at different scan rates

Table 1. Analysis of the surface elemental content