The roles of GBs and oxidation states in CuAg systems are not well understood, and the real active sites remain ambiguous. To address these uncertainties, we developed an electro-shock synthesis strategy that enables precise control over the catalyst’s GBs and oxidation structures. In the CuAg system, GBO significantly enhances ECR conversion to methane, doubling its efficiency compared to the GBs alone. In situ XAS spectra reveal the mechanism behind GBO, highlighting stable Cuδ+ species and a unique electron transfer mechanism, distinct from the GBs itself. In situ ATR FT-IR and theoretical calculations confirm the ECR pathway, showing that GBO weakens CO adsorption during ECR. This study fills a critical knowledge gap regarding the relationship between GBO and ECR activity, offering valuable insights for the rational design of electrocatalysts.