The quantif ication of Sn surface speciation of CO and HCOO -selective Cu-Sn bimetallic dendrites (Figure ) displayed substantial differences compared to the Cu-In bimetallic dendrites under similar measuring conditions. Before CO2ER, the surface of the as-synthesized pure Sn, Cu85Sn15 and Cu40Sn60 f oams exhibited predominantly oxidized Sn (~95% Sn n+ ) with a residual of metallic Sn (Sn 0 ). Af ter CO2ER at -0.7 V, no major changes were detected in Sn surf ace specification of Cu85Sn15 (CO selective) catalysts, where their surf aces are predominantly composed of oxidized Sn species (~87% Sn 2+ and 7% Sn 4+ ). Interestingly, at -0.9 V RHE the Sn 2+ gave way to signif icant increases of Sn 0 and Sn 4+ species. On the other side, HCOO -selective Cu-Sn sample (Cu40Sn60) showed a great increase in the metallic Sn contents after CO2 electrolysis at -0.7 V RHE and -0.9V RHE, where Sn 0 increased from 4% before CO2ER into 48% and ~100% at -0.7 V and -0.9 V, respectively. Additionally, the brief air exposure of any of these catalysts leads to a near complete re-oxidation of their surf aces, highlighting again the effectiveness of using glovebox assisted XPS for identifying the surface-active species. Figure summarizes surface composition and speciation of the all investigated electrodes including CO selective and HCOO -selective bimetallic Cu-In and Cu-Sn foams.