Continuous-Flow Microfluidic Synthesis Enhances C₂₊ Selectivity for Cu₂O Catalysts

The electrochemical reduction of CO₂ to multicarbon (C₂₊) products offers a promising pathway to replace fossil fuels in the chemical and transportation sectors. However, achieving high C₂₊ selectivity requires precisely engineered structures, which, in turn, necessitate advanced synthetic strategies and in situ characterization. Herein, we leverage microfluidic technologies to rationally design and synthesize Cu₂O nanoparticles with tunable features under laminar flow conditions, thereby providing a previously inaccessible level of control over catalyst structure. By tuning flow parameters within the microfluidic platform, we precisely regulate the reaction-diffusion interface, enabling fine control over nanoparticle size, morphology, and defect density. The resulting Cu₂O nanoparticles exhibit a high defect density and intrinsic nanoporosity, two properties known to enhance C₂₊ selectivity during CO₂ electroreduction. In contrast, Cu₂O nanoparticles synthesized via conventional batch methods under identical stoichiometric conditions exhibit larger pore sizes, lower defect densities, and lower C₂₊ selectivity. Using in situ liquid-phase transmission electron microscopy and operando X-ray absorption spectroscopy, we further elucidate the evolution of both catalyst systems. Finally, we demonstrate that surface modification with polyaromatic films further promotes C₂₊ formation. This work highlights microfluidic synthesis as a powerful platform for designing advanced electrocatalysts with tunable structural features and enhanced CO₂ conversion performance to C₂₊ products.