On the role of hydroxide ions as the mediators of cathodic Cu corrosion.

Understanding changes and degradation of the copper cathode during the electrochemical carbon dioxide reduction reaction (CO2RR) is a fundamental challenge. Advances in operando experimental methods and computational tools have shed light on the degradation mechanisms. However, the role of the hydroxide ions formed from the alkaline Volmer reaction remains under-studied. Here we report a computational investigation of *Cu(OH)n adatom complexes that can form on a roughened surface as possible intermediates that can lead to cathodic copper corrosion. Constant-potential density functional theory calculations are utilized to compute the voltage-dependent free energy surfaces of *Cu(OH)n electrodissolution from adatom defects on a Cu(100) surface under cathodic conditions. The results show that the dissolution of a Cu(OH) adatom complex is endergonic by ∼1 eV at an electrode bias of -1.59 V vs. the standard hydrogen electrode (SHE) and thus quite unfavorable. In contrast, the dissolution of *[Cu(OH)2] at the same bias only requires ∼0.2 eV. The Cu oxidation states of both complexes are +1 regardless of the applied potential. The atomic partial charges and their changes show that two hydroxides of surface-bound *Cu(OH)2 induce an image quadrupole which allows facile dissociation. Pathways to form these two complexes are analyzed using transition state calculations evaluated at constant potential. The presence of oxygen adatoms, for example from subsurface oxygen, is identified to facilitate the formation of *Cu(OH)nvia dissociative chemisorption of water. The disproportionation of surface *OH to form new *O is also found to be favorable under strongly reducing conditions. Including both pathways to form Cu(OH)n and *OH disproportionation, the stochastic simulation model predicts approximately 3.2 monolayers per hour of Cu loss through Cu(OH)n formation. The model also highlights how *OH disproportionation sustains active Cu sites and generates *O, a key intermediate facilitating dissolution to [Cu(OH)2]-(aq). These results show that hydroxide-ion mediated cathodic Cu dissolution without external species is viable. Remaining questions and further work that could validate the proposed mechanism are discussed.