Dynamic Evolution of Copper Nanowires during CO₂ Reduction Probed by <i>Operando</i> Electrochemical 4D-STEM and X-ray Spectroscopy

Nanowires have emerged as an important family of one-dimensional (1D) nanomaterials owing to their exceptional optical, electrical, and chemical properties. In particular, Cu nanowires (NWs) show promising applications in catalyzing the challenging electrochemical CO₂ reduction reaction (CO₂RR) to valuable chemical fuels. Despite early reports showing morphological changes of Cu NWs after CO₂RR processes, their structural evolution and the resulting exact nature of active Cu sites remain largely elusive, which calls for the development of multimodal <i>operando</i> time-resolved nm-scale methods. Here, we report that well-defined 1D copper nanowires, with a diameter of around 30 nm, have a metallic 5-fold twinned Cu core and around 4 nm Cu₂O shell. <i>Operando</i> electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) showed that as-synthesized Cu@Cu₂O NWs experienced electroreduction of surface Cu₂O to disordered (spongy) metallic Cu shell (Cu@Cu^S NWs) under CO₂RR relevant conditions. Cu@Cu^S NWs further underwent a CO-driven Cu migration leading to a complete evolution to polycrystalline metallic Cu nanograins. <i>Operando</i> electrochemical four-dimensional (4D) STEM in liquid, assisted by machine learning, interrogates the complex structures of Cu nanograin boundaries. Correlative <i>operando</i> synchrotron-based high-energy-resolution X-ray absorption spectroscopy unambiguously probes the electroreduction of Cu@Cu₂O to fully metallic Cu nanograins followed by partial reoxidation of surface Cu during postelectrolysis air exposure. This study shows that Cu nanowires evolve into completely different metallic Cu nanograin structures supporting the <i>operando</i> (operating) active sites for the CO₂RR.