In summary, the results show that CO2 itself, rather than a product of its electrochemical reduction or its influence on pH through carbonate buffer reactions, plays a key role in the degradation of copper during CO2 ER. Copper degradation is observed as long as CO2 is present, even if the applied electrochemical potential is insufficient to drive CO2 electroreduction. In contrast, if CO2 is not present, copper does not corrode, even at applied potentials that are sufficient to drive CO2 ER.