A complete transformation of the different detected surface copper oxide species (Cu 2+ and Cu + ) into metallic copper was observed for all investigated catalyst materials (i.e., Cu, Cu-In and Cu-Sn foams) f ollowing CO2ER in the glovebox and the subsequent transfer to the XPS under an inert atmosphere. This was true regardless of the applied cathodic potential (-0.5 V to -0.9 V) and the different bimetallic compositions studied. Additionally, brief exposure of any of these catalysts to air resulted in significant re-oxidation of their surface into various copper oxide species (Cu + and Cu 2+ ) with small contribution of metallic copper, as shown in Figure and Figures . The Cu LMM Auger analysis revealed the surface transformation to metallic Cu 0 without any detectable residual oxides f ollowing electrocatalysis, and f urthermore shows how the as -prepared materials as well as the postelectrolysis samples exposed to air undergo significant surface oxidation. Taken together, these observations help validate the usef ulness of the quasi in situ XPS approach and highlight the limitations of using ex situ approaches to study electrocatalyst surf aces. The obtained Cu results emphases the existence of mixed heterogeneous surface of separate metals, rather than alloying since we did not detect any meaningful shift of the Cu 2p spectra of the bimetallic systems compared to the Cu 2p of pure Cu. Unlike the Cu surf ace species, the analysis of the In 3d and Sn 3d spectra of bimetallic foams showed that the identified In and Sn surface species are strongly dependent on the initial composition and the applied potentials (Figure ). CO selective bimetallic f oams (Cu-rich, Cu85In15 and Cu85Sn15) showed different surface speciation compared to HCOO - selective catalysts (Cu-poor, Cu25In75 and Cu40Sn60). Bef ore CO2 electrolysis, CO selective bimetallic foams, as well as their respective single elements (In and Sn) exhibited nearly f ully oxidized In and Sn surf ace species with a tiny contribution (<4%) of metallic species (In 0 and Sn 0 ). Looking first at the Cu-In catalysts (Figure ), we see that af ter CO2 electrolysis at -0.5 V (or more negative potentials), the oxidized In surface of the pure In and Cu85In15 catalysts is severely reduced, where the quantification of their surf aces revealed the predominance of metallic In (77-86% In 0 ) with a small contribution of oxidized In species (23-14% In n+ ). On the other hand, Cu25In75 (HCOO -selective catalysts) exhibited a mixture of metallic (58% In 0 ) and oxide (42% In n+ ) In surf ace species before CO2ER. Interestingly, Cu25In75 exhibited exclusively metallic In af ter CO2 electrolysis at all the studied potentials (-0.5 V to -0.9 V RHE). In summary, the obtained XPS results of Cu-In bimetallic f oams reveal that the active surf ace of Cu85In15 f oam is composed of metallic Cu 0 and mixture of metallic In 0 (predominant species) and oxidized In (In n+ ), while HCOO -selective Cu25In75 surf ace is exclusively composed of Cu 0 and In 0 .