  Identifying and controlling active sites in electrocatalysis remains a grand
challenge due to restructuring of catalysts in the complex chemical
environments during operation. Inactive precatalysts can transform into active
catalysts under reaction conditions, such as oxide-derived Cu (OD-Cu) for
CO$_2$ electroreduction displaying improved production of multicarbon
(C$_{2+}$) chemicals. Revealing the mechanism of active site origin in OD-Cu
catalysts requires in situ/operando characterizations of structure, morphology,
and valence state evolution with high spatial and temporal resolution. Applying
newly developed electrochemical liquid cell transmission electron microscopy
combined with X-ray absorption spectroscopy, our multimodal operando techniques
unveil the formation pathways of OD-Cu active sites from CuO bicrystal nanowire
precatalysts. Rapid reduction of CuO directly to Cu within 60 seconds generates
a nanocrack network throughout the nanowire, via formation of "boundary
nanocracks" along the twin boundary and "transverse nanocracks" propagating
from the surface to the center of the nanowire. The nanocrack network further
reconstructs, leading to a highly porous structure rich in Cu nanograins, with
a boosted specific surface area and density of active sites for C$_{2+}$
products. These findings suggest a means to optimize active OD-Cu
nanostructures through nanocracking by tailoring grain boundaries in CuO
precatalysts. More generally, our advanced operando approach opens new
opportunities for mechanistic insights to enable improved control of catalyst
structure and performance.
