The electrochemical CO2 reduction performance of the various as-prepared Cu-In and Cu-Sn bimetallic foams with a wide range of compositions was examined in CO2-saturated 0.1 M KHCO3 using a two compartment, three-electrode H-type electrochemical cell. The obtained CO2ER perf ormance results are summarized in Figure and Figures . Figure displays the faradaic efficiency of CO and HCOO -formation as function of Sn and In contents at various applied cathodic potentials. A similar trend in selectivity towards CO and HCOO - pathways is observed for both the bimetallic foam systems, wherein Cu-rich bimetallic (Cu-In and Cu-Sn) foams showed a high CO-selectivity, while Cu-poor foams exhibited a high tendency towards HCOO -production. Even introducing trace amounts (≤1.5 at.%) of either Sn or In into Cu f oam, shifts its selectivity significantly towards CO production, and Sn or In contents of approximately 15 at.% resulted in peak CO selectivity reaching above 90 %. Further increase in either metal results in an abrupt decrease of CO selectivity. Hereaf ter, Cu85In15 and Cu85Sn15, representing the foams with the highest CO-selectivity, will be used for further detailed investigation. It is worth mentioning here that Cu85In15 f oam exhibited its highest FE f or CO of ~92% at -0.6 V RHE, while Cu85Sn15 achieved a similar FE for CO but at more negative potential (-0.7 V RHE). Further increase of the Sn and In contents lead to steadily increasing HCOO -selectivity, plateauing for contents above 50-60 at. %, accompanied by greatly suppressed CO yields. Cu40Sn60 and Cu25In75 will be used hencef orth for indicating the foams with the highest HCOO -selectivity. Both displayed the best HCOO -selectivity at -0.9 V RHE. Further detailed studies were only performed on the Cu-In and Cu-Sn f oams with the best selectivity towards CO (Cu85In15 and Cu85Sn15) and HCOO -(Cu40Sn60 and Cu25In75). Furthermore, Figure shows the distribution of FE f or H2 as a f unction of Sn and In contents (at. %) at various applied cathodic potentials. As shown in this f igure, the selectivity of the competitive undesired hydrogen evolution reaction is significantly suppressed upon modifying Cu foams with even small amounts of either In or Sn. and S6 display the distribution of faradaic efficiencies (FEs) and partial current densities of the dif ferent major products obtained at Cu-Sn and Cu-In bimetallic f oams optimal f or CO and HCOO - production compared to their respective single elements at various applied cathodic potentials. Cu25In75 and Cu40Sn60 showed a similar steady increase of the HCOO -selectivity with the increase of the applied potential (Figures and), however they exhibited quite different selectivity trends for CO and H2. Cu25In75 showed low H2-selectivity with high CO-selectivity at low potentials, while Cu40Sn60 in contrast exhibited high H2selectivity with very low CO-selectivity under similar conditions. On the other hand, the Cu-rich Cu-Sn and Cu-In bimetallic f oams (Cu85In15 and Cu85Sn15) exhibited a kind of similar volcano-shaped trend f or the CO selectivity with maximum at -0.6 V and -0.7 V vs. RHE (all potentials given herein are relative to the RHE) for Cu85In15 and Cu85Sn15, respectively. Despite their similar CO selectivity behavior, they showed differences in the H2 f ormation behavior. Cu85Sn15 f oam showed a high tendency for H2 production at low potentials (<-0.6 V), while Cu85In15 exhibited a high CO selectivity (>50% CO FE) even at relatively lower overpotential (-0.4 V). The pure Sn and In catalysts followed similar CO, H2 and HCOO -trends, where the HCOO -selectivity increases with the potential accompanied with decrease of both H2 and CO production. However, the pure In still shows higher CO and lower H2 selectivity at less negative potentials compared to the pure Sn. More thorough analysis of the physical properties (e.g., porosity, thickness and roughness) of the Cu-In and Cu-Sn bimetallic foams with the best CO and HCOO -selectivity is essential for better understanding the wide variance in the observed currents and selectivity. Thus, the thickness and porosity of these bimetallic foams were estimated from their respective cross-section FIB-SEM (see Figure ). Their roughness, bulk and surface compositions were estimated using XPS (Figure ), EDX (Figure ) and ICP-OES, obtained results summarized in Table . ) compared to Cu85Sn15 (CO-selective), it exhibited higher CO2ER total current (Figure ). This indicates the higher intrinsic activity and the superiority of Cu85In15 over Cu85Sn15. In contrast, Cu40Sn60 (HCOO-selective, Figure ) showed higher CO2ER current compared to the Cu25In75 (Figure ) which could be attributed to its significantly higher electrochemically active surface area. So f ar, we have succeeded in f abricating and fully characterizing the Cu-In and Cu-Sn bimetallic dendrites with tunable performance towards either CO (Cu-rich dendrites) or HCOO -(Cu-poor dendrites) production. The improved CO and HCOO -selectivity on the bimetallic dendrites cannot be attributed to the alloying effect, since the obtained XRD patterns did not exhibit any signs for the existence of a crystalline alloy. Both bimetallic systems (Cu-In and Cu-Sn) showed a similar CO and HCOO-selectivity-dependence on In and Sn contents, where their HCOO -selectivity increases with the applied potential with a simultaneous decrease in t heir tendency to produce CO. Despite their CO and HCOO -selectivity trends, their enhanced ability to yield CO and HCOO -could be attributed to different enhancing mechanisms. Cu25In75 showed a kind of linear increase of HCOO -selectivity at the expense of CO, while Cu40Sn60 seems to enhance the HCOO -selectivity at expense of H2 production. Although the above ex situ characterization provides insight into the observed selectivity trends, it is known that electrocatalysts can transf orm significantly during electrochemical operation, and transition metals in particular can be susceptible to oxidation in air during post-run sample handling. A more precise understanding of the true catalytic interface requires the use of in situ methods capable of characterizing the operating system. Thus, quasi in-situ XPS and XAS together with operando SERS were next used to examine the induced local environment (local pH) and surf ace (speciation and composition) changes during CO2 electroreduction at both bimetallic systems.