Halogen Regulation of Ag (111) Surfaces for CO2 Electroreduction: A DFT Study of Electronic Structure Modulation and CO2RR/HER Competition

Electrochemical CO2 conversion powered by renewable energy offers a promising route for carbon utilization and carbon neutrality. Ag is highly selective for syngas (CO/H2) production, and halogen modification has proven effective in regulating its CO2 electroreduction performance; however, the effects of halogen species and substitution concentration remain unclear. Herein, density functional theory calculations were used to systematically investigate how Cl, Br, and I substitution at 1.25%, 2.5%, and 5% regulates the structural evolution, electronic properties, intermediate adsorption behavior, and CO2RR/HER competition on Ag(111) surfaces. The results demonstrate that the catalytic behavior of Ag(111) under halogen substitution is strongly dependent on both halogen species and substitution concentration. Low-concentration halogen substitution favors *COOH formation, whereas medium-to-high Br and I substitution induces local electronic reconstruction and weakens key intermediate stabilization. Among the investigated systems, 1.25% Br substitution provides the most favorable balance between *COOH stabilization and *H adsorption, thereby improving CO2RR/HER competition. Electronic structure analyses further show that this behavior arises from the combined effects of Ag–C orbital interactions, local coordination, and charge redistribution rather than a single d-band-center shift. These findings provide atomic-level insight into halogen-regulated Ag catalysts and guidance for selective CO2 electroreduction.