The susceptibility of electroplated copper to degradation during CO2 ER was analyzed through in situ morphological analysis after 15 minutes (Figure ). Copper subjected to EWE = -0.8 V showed degrada on pits (red arrows) in 0.01 M KHCO3 saturated with CO2, which is below (in aliis verbis, more reducing than) the onset potential for CO2 ER and HER. These results show that copper is prone to degradation at -0.8 V, even though at standard conditions, this electrochemical potential would strongly favor copper plating. This is shown clearly by the pits and crevices that are visible in copper particles that were originally plated onto the electrode surface (Figure ). Within these confined spaces, the hydrodynamic and chemical conditions may diverge from those in the bulk, potentially leading to chemical reactions occurring outside the anticipated electrochemical potential range for a given redox pair. On the contrary, copper did not exhibit signs of degradation at the same potential if 0.01 M K2SO4 or 0.01 M KHCO3 were saturated with N2 instead of CO2 (Figure and). Morphological analyses of copper particles subjected to CO2 ER conditions in 0.01 M KHCO3 saturated with CO2 show surface roughening, pitting and crevice degradation akin to corrosion observed in typical copper applications such as plumbing . Preferential degradation was observed along grain boundaries, which formed crevices on the particles (Figure ). This result is corroborated by the fact that larger particles, which contain more surface defects / grain boundaries, seem to be more susceptible to corrosion. Figure shows a copper particle that sustained enhanced degradation in the course of the experiment. That copper particle exhibits structural defects that already were present, at least in a small extent, in the as-plated particle (Figure ). This observation corroborates the relation between enhanced degradation and the presence of structural defects that facilitate crevice corrosion. At EWE = -0.8 V, the degradation of the large particles was accompanied by the formation of the smaller re-plated copper nanoparticles. The re-plating of smaller nanoparticles was observed for potentials more reducing than EWE = 0 V vs. SHE'. The bulk of both the degraded and the re-plated particles is composed of metallic copper as revealed by EELS, which is identical to the EELS signature for the as-plated copper particles before they were subjected to CO2 ER conditions (Figure ). The results are different when analyzing the surface of the particles.