Electrified <i>Operando</i>-Freezing of\nElectrocatalytic CO<sub>2</sub> Reduction Cells for Cryogenic Electron\nMicroscopy

The ability to freeze and stabilize reaction intermediates\nin their\nmetastable states and obtain their structural and chemical information\nwith high spatial resolution is critical to advance materials technologies\nsuch as catalysis and batteries. Here, we develop an electrified <i>operando</i>-freezing methodology to preserve these metastable\nstates under electrochemical reaction conditions for cryogenic electron\nmicroscopy (cryo-EM) imaging and spectroscopy. Using Cu catalysts\nfor CO<sub>2</sub> reduction as a model system, we observe restructuring\nof the Cu catalyst in a CO<sub>2</sub> atmosphere while the same catalyst\nremains intact in air at the nanometer scale. Furthermore, we discover\nthe existence of a single valence Cu (1+) state and C–O bonding\nat the electrified liquid–solid interface of the <i>operando</i>-frozen samples, which are key reaction intermediates that traditional <i>ex situ</i> measurements fail to detect. This work highlights\nour novel technique to study the local structure and chemistry of\nelectrified liquid–solid interfaces, with broad impact beyond\ncatalysis.