Subsurface hydrogen as a hidden driver of copper surface reconstruction in CO2 electroreduction

ABSTRACT Copper (Cu) undergoes significant surface reconstruction during CO2 electroreduction, which is strongly modulated and accelerated by reaction intermediates, yet the atomic-scale mechanism remains far behind the experimental observations. By integrating machine-learning interatomic potentials with large-scale grand canonical Monte Carlo simulations, we systematically investigated *CO- and *H-induced surface roughening across various Cu facets. Our simulations demonstrate that high surface *H (*Hsur) coverage facilitates subsurface hydrogen (*Hsub) incorporation on (100)-dominated Cu facets under typical working conditions (−1 V vs. reversible hydrogen electrode), while such penetration is negligible on (111)-like surfaces. This facet dependence is primarily attributed to a *H-induced hexagonal surface reconstruction observed on (100)-dominated facets, a process driven primarily by geometric rather than electronic effects. Specifically, high *H coverage triggers a partial transition of Cu atoms from ideal 4-fold hollow sites to more closely packed 3-fold arrangements. The local densification expands the spacing at the remaining 4-fold sites, thereby reducing the energy barrier for *Hsub migration into the subsurface. Further analysis reveals that *Hsub alone is sufficient to induce Cu adatom formation, even in the absence of nearby *CO, uncovering a revised structural evolution paradigm for Cu surface roughening. We propose an alloying strategy using low hydrogen affinity metals (Zn, Al, Ga) to effectively suppress *Hsur incorporation, offering a promising pathway for designing Cu-based catalysts with long-term stability.