Microfluidics-Assisted Synthesis of Hierarchical Cu₂O Nanocrystal as C₂-Selective CO₂ Reduction Electrocatalyst

Copper-based catalysts have attracted enormous attention due to their high selectivity for C₂₊ products during the electrochemical reduction of CO₂ (CO₂RR). In particular, grain boundaries on the catalysts contribute to the generation of various Cu coordination environments, which have been found essential for C—C coupling. However, smooth-surfaced Cu₂O nanocrystals generally lack the ability for the surface reorganization to form multiple grain boundaries and desired Cu undercoordination sites. Flow chemistry armed with the unparalleled ability to mix reaction mixture can achieve a very high concentration of unstable reaction intermediates, which in turn are used up rapidly to lead to kinetics-driven nanocrystal growth. Herein, the synthesis of a unique hierarchical structure of Cu₂O with numerous steps (h-Cu₂O ONS) via flow chemistry-assisted modulation of nanocrystal growth kinetics is reported. The surface of h-Cu₂O ONS underwent rapid surface reconstruction under CO₂RR conditions to exhibit multiple heterointerfaces between Cu₂O and Cu phases, setting the preferable condition to facilitate C—C bond formation. Notably, the h-Cu₂O ONS obtained the increased C₂H₄ Faradaic efficiency from 31.9% to 43.5% during electrocatalysis concurrent with the morphological reorganization, showing the role of the stepped surface. Also, the h-Cu₂O ONS demonstrated a 3.8-fold higher ethylene production rate as compared to the Cu₂O nanocube.