Designing an efficient electrocatalysis system for CO₂ conversion to desired value-added products with high selectivity and activity holds promise to enable carbon neutrality and industrial application. Herein, we construct and test two hypotheses: (1) metastable tin oxides play a critical role in enhancing CO₂-to-formate conversion rather than metallic tin, and (2) a catholyte-free electrolyzer, where no electrolyte is present on the cathode side, enables highly concentrated formate generation. We prepare tin oxide thin films on a gas diffusion layer by thermal evaporation and confirm the co-existence of SnO and SnO₂ on the as-prepared tin oxide films. Annealing in air is used to increase the oxidation of tin prior to starting CO₂ electrolysis. In subsequent CO₂ electrolysis, we demonstrate that the selectivity for formate is substantially enhanced while parasitic hydrogen evolution reaction is suppressed on the annealed tin oxide electrodes as compared to the as-prepared. The increased selectivity for formate would suggest the presence of metastable tin oxide species during electrolysis favors formation of *OCHO, a key intermediate of formate generation, while suppressing *H and *COOH formation. We also achieve a concentration of formate at the end of 3 hrs electrolysis is > 1 mol L^-1. We will describe our latest understanding of the change in surface chemistry modulated by post-heat treatment, providing insights into the role of metastable tin oxide on the highly selective CO₂-to-formate conversion.