Quasi-Operando Liquid-Phase Electron Imaging of Metallic Copper Nanocubes Reveals Step-by-Step Subtle Dissolution, Redeposition, Reattachment, and Fragmentation Mechanisms during CO 2 Electroreduction

Metallic copper nanocubes used in CO2 electroreduction (CO2ER) are known to exhibit moderate binding energies to intermediates and selectivity toward ethylene. However, the process by which they restructure, ultimately redistributing the active sites and causing deactivation, remains challenging to fully elucidate. Herein, we use electrochemical liquid-phase transmission electron microscopy to observe copper nanocube evolution during CO2ER in real time at nanometer resolution. Our statistical analysis reveals that dissolution/redeposition is the primary evolution mechanism. However, unlike other shapes, the thermodynamically active edges of copper cubes attract the redeposited aggregates that reattach to the cubes, moderating their surface-to-volume ratio. Additionally, the fragmentation mechanism was observed, which may occur due to highly defective sites. Our findings illustrate the synergistic effect of the thermodynamically high-energy sites and the role of the kinetic barrier in the dynamics of Cu nanocubes and their secondary aggregates, which affects their stability and as a consequence their selectivity over time.