Microfluidics-Assisted Synthesis of Hierarchical Cu 2 O Nanocrystal as C 2 -Selective CO 2 Reduction Electrocatalyst.

Copper-based catalysts have attracted enormous attention due to their high selectivity for C 2+ products during the electrochemical reduction of CO 2 (CO 2 RR). 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 Cu 2 O 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 Cu 2 O with numerous steps (h-Cu 2 O ONS) via flow chemistry-assisted modulation of nanocrystal growth kinetics is reported. The surface of h-Cu 2 O ONS underwent rapid surface reconstruction under CO 2 RR conditions to exhibit multiple heterointerfaces between Cu 2 O and Cu phases, setting the preferable condition to facilitate C-C bond formation. Notably, the h-Cu 2 O ONS obtained the increased C 2 H 4 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-Cu 2 O ONS demonstrated a 3.8-fold higher ethylene production rate as compared to the Cu 2 O nanocube.