Fig. 1. characterization of various Cu films.

a A schematic illustration of the synthesis process for LS/MS/HS-Cu and LRS/MRS/HRS-Cu. b Voltammograms of resulting electrodes collected immediately after pre-reduction. Typical TEM images of c HRS-Cu, d MRS-Cu, and e LRS-Cu. f Typical XRD patterns of resulting electrodes. As visual aids, the red dash line indicates the surfaces of the resulting electrodes. The precatalysts of HRS/MRS/LRS-Cu were pre-reduced under −0.5 V versus the RHE for 1 h, and the other conditions of prereduction are the same as that of the CO₂ reduction. The labeling of Cu(200) is used for easier comparison with XRD, in which only (200), the second-order diffraction of (100) could be detected. The peaks marked with an asterisk in XRD patterns originate from carbon-based GDL substrate.