a Operando differential electrochemical mass spectrometry (DEMS) sweep data obtained during CO2RR on CuO NS catalysts (supported on a flat 0.785 cm2 glassy carbon electrode, catalyst loading: 100 μg cm−2.) using CO2-saturated 0.1 M KHCO3 by continuous cyclic voltammetric scan at 5 mV s−1. b DEMS-derived mass charges for various products formed during the cathodic and anodic voltammetric sweep. The error bars are given as standard error of mean. c Spider plot shows the variations in the onset electrode potential of key products during CO2RR by continuous cyclic voltammetric scan at 5 mV s−1 (see Supplementary Table S2). Product molecules considered are: m/z = 28 CO, m/z = 15 corresponding to methane, m/z = 26 corresponding to ethylene, m/z = 31 corresponding to ethanol. d Faradaic efficiencies of CH4, EtOH and C2H4 on Cu(100) terraces of (from left to right) shrinking width and increasing density of Cu(110) steps. While Cu(100) shows little EtOH, EtOH increases with larger (110) step density and narrower (100) terrace width, peaking at Cu(310). The narrow (100) terraces on Cu(210) prevent EtOH formation in favor of CH4 (data from Ref. 56). e–g Illustration of *CH3 as common intermediate of CH4 and EtOH: Side views of the Cu(210), Cu(310) and Cu(510) single crystal facets. Cu(210) features exclusive *CH3 and *H adsorption toward CH4, while Cu(310) allows for C-C coupling between *CH3 and carbonaceous adsorbates on the (100) terraces toward EtOH. The wider (100) terraces of Cu(510) also enables C-C coupling among carbonaceous adsorbates to C2H4. Grey: C, white: H, red:O.