CO2 electroreduction into useful chemicals and fuels is a promising technology that might be used to minimize the impact that the increasing industrial CO2 emissions are having on the environment. Although plasma-oxidized silver surfaces were found to display a considerably decreased overpotential for the production of CO, the hydrogen evolution reaction (HER), a competing reaction against CO2 reduction, was found to increase over time. More stable and C1-product-selective SnO x/AgO x catalysts were obtained by electrodepositing Sn on O2-plasma-pretreated Ag surfaces. In particular, a strong suppression of HER (below 5% Faradaic efficiency (FE) at -0.8 V vs the reversible hydrogen electrode, RHE) during 20 h was observed. Ex situ scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), quasi in situ X-ray photoelectron spectroscopy (XPS), and operando X-ray absorption near-edge structure spectroscopy (XANES) measurements showed that our synthesis led to a highly roughened surface containing stable Snδ+/Sn species that were found to be key in the enhanced activity and stable CO/formate (HCOO-) selectivity. Our study highlights the importance of roughness, composition, and chemical state effects in CO2 electrocatalysis.