The rapid degradation of copper-based cathodic catalysts under CO2 ER affects their activity, selectivity and overall performance . The selectivity towards CO2-reduced products lowers over time and hydrogen evolution becomes the prevalent reaction 10-30 mins into the reaction . The fundamental drivers of those changes are not well understood. DeWulf and co-workers produced methane and ethylene with 65% selectivity using copper foil but reported a selectivity drop to 0% after 2h, which was correlated to the accumulation of carbon and graphitic species at the catalyst surface . Similar results were also obtained for rotating-disk copper electrodes . Copper deactivation during CO2 ER was initially attributed to accumulation of metal impurities . Later, surface sensitive chemical techniques such as X-ray Photoelectron Spectroscopy (XPS), showed that the deposition of metal impurities on the electrode surface was not the main deactivation phenomenon. Voltammetry on deactivated Cu electrodes showed unaccounted-for peaks at approximately -0.1 or -0.56 V versus SHE. The extent of deac va on correlated with the electric charge of the anodic peak, which was interpreted as a correlation with the concentration of deactivation species . Huang and co-workers observed detachment of small clusters and pitting corrosion in copper nanocubes subjected to CO2 ER using transmission electron microscopy (TEM) techniques . The degradation of a Cu-based catalyst into 2-4 nm densely packed nanoparticles was also observed on an ethylene-selective copper catalyst . The results suggested that morphological changes and corrosion were linked to the reducing electrochemical potential applied and formation of metastable metal anions that spontaneously oxidized to form small Cu nanoparticles. The authors identified in situ electron microscopy as a key tool to gain new insights and address unresolved questions of copper degradation during CO2 ER . Collectively, these studies underscore the fact that significant advances are needed to accurately identify and understand the mechanisms and agents by which copper degradation is promoted under electrochemical conditions. Uncovering the basis of degradation requires tightly coupled experiment-theory integration to obtain mechanistic understanding of the factors contributing to variations in the material performance.