The electrochemical conversion of CO 2 into acetaldehyde offers a sustainable and green alternative to the Wacker process. However, current electrocatalysts cannot effectively compete with heterogeneous processes owing to their limited selectivity towards acetaldehyde, resulting in low energy efficiencies. Here we report a theory-guided synthesis of a series of Cu-cluster catalysts (~1.6 nm) immobilized on various heteroatom-doped carbonaceous supports, produced via spark ablation of Cu electrodes (2.6 μg h −1 production rate, 6 Wh energy consumption). These catalysts achieve acetaldehyde selectivity of up to 92% at only 600 mV from the equilibrium potential. In addition, the catalysts exhibit exceptional catalytic stability during a rigorous 30 h stress test involving three repeated startstop cycles. In situ X-ray absorption spectroscopy reveals that the initial oxide clusters were completely reduced under cathodic potential and maintained their metallic nature even after exposure to air, explaining the stable performance of the catalyst. First-principles simulations further elucidate a possible mechanism of CO 2 conversion to acetaldehyde.