In situ generation of CU- and Ag-Sn alloys from metal sulfides for CO2 reduction

Ag, Cu and Sn based electrocatalysts promise high CO2 reduction kinetics and efficiencies on gas diffusion electrodes. Ag, Cu, Sn sulfide catalysts in particular may offer altered electronic properties and product selectivity, while still being easy to manufacture in scaleable synthesis routes. Comparing the CO2 reduction (CO2RR) performance of Cu3SnS4, Ag3SnS4, Cu2S, SnS and Ag8SnS6 at 100 mA cm-2, formate is found to be the primary CO2RR product with a faradaic efficiency of 57% for Cu3SnS4 and 81% for Ag3SnS4. Characterization by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction revealed the formation of Ag3Sn and Cu3Sn alloys from the corresponding sulfide species during CO2RR. But while the Cu3Sn based electrode surface decomposed into CuO and SnO after 2 h at -100 mA cm-2, metallic Ag3Sn sites on the corresponding electrode surface could be detected by XPS after removing the surface layer. Using density functional theory, the binding energies of *H, *CO and *OCHO on Cu3Sn and Ag3Sn were computed to identify possible catalytic sites. Thereby, Sn was found to render both Cu and Ag highly oxophilic resulting in strong adsorption of carboxylic functionalities, enabling formate production with a partial current density of up to 162 mA cm-2.