H and CO Co-induced Adatom Formation on Cu in CO2 Electroreduction Conditions

Dynamic restructuring of Cu electrode has been observed under electrochemical conditions, and it has been hypothesized to underly the unique reactivity of Cu towards CO2 electroreduction. Surface roughening is one of the key surface phenomena to Cu activation, in which the surface forms numerous atomic vacancies and adatoms. However, the atomic structure of such surface motifs, in the presence of relevant adsorbates has remained elusive. Here, we explore the chemical space of Cu surface restructuring under coverage of CO and H in realistic electroreduction conditions, by a combination of grand canonical density functional theory and global optimization techniques, from which we construct a potential-dependent grand canonical ensemble representation of the surface. Significant vertical displacement of surface Cu atoms is observed in the regime of intermediate and mixed CO and H coverage. This regime, while predicted to be thermodynamically inaccessible, is kinetically controlled, presenting a tough challenge for theory. To investigate the kinetic trapping effects, we develop a quasi-kinetic Monte Carlo simulation to track the evolution of the system during a simulated cathodic scan. The simulation reveals the path that the system takes across the coverage space and the metastable structures that the system evolves into along the way. Chemical bonding analysis is performed on the metastable structures with elevated Cu*CO species to understand its formation mechanism. By molecular dynamics simulations and free energy calculations, the surface chemistry of the Cu*CO species is explored, and we identifies potential mechanisms via which the Cu*CO species may diffuse or dimerize. This work provides rich atomistic insights into the surface roughening phenomenon and the structure of the involved species. It also features generalizable methods to explore the chemical space of restructuring surfaces with mixed adsorbates, and its evolution in non-equilibrium.