Catalysts based on Cu nanocrystals (NCs) for electrochemical CO₂ -to-C₂₊ conversion with high activity have been a subject of considerable interest, but poor stability and low selectivity for a single C₂₊ product remain obstacles for realizing sustainable carbon-neutral cycles. Here, we used the facet-selective atomic layer deposition (FS-ALD) technique to selectively cover the (111) surface of Cu NCs with ultrathin Al₂ O₃ to increase the exposed facet ratio of (100)/(111), resulting in a faradaic efficiency ratio of C₂ H₄ /CH₄ for overcoated Cu NCs 22 times higher than that for pure Cu NCs. Peak performance of the overcoated catalyst (Cu NCs/Al₂ O₃ -10C) reaches a C₂ H₄ faradaic efficiency of 60.4 % at a current density of 300 mA cm^-2 in 5 M KOH electrolyte, when using a gas diffusion electrode flow cell. Moreover, the Al₂ O₃ overcoating effectively suppresses the dynamic mobility and the aggregation of Cu NCs, which explains the negligible activity loss and selectivity degradations of Cu NCs/Al₂ O₃ -10C shown in stability tests.