Synthesis of the CO2 reduction catalyst. The Cu92In8 alloy catalyst was synthesised on 2.36 cm 2 Cu foil substrates via a template-assisted electrodeposition method following previously reported protocols. Prior to electrodeposition, the Cu foil substrates were electropolished in 50% ortho-phosphoric acid by applying 2.0 V for 120 s, rinsed in Milli-Q water and dried under air. The active area for electrodeposition (2.34.3 cm 2 ) was set by masking the substrate with Kapton tape, which is inert and stable under acidic medium. The bimetallic catalyst electrodeposition was conducted in a clean glass beaker containing copper sulfate and indium sulfate precursor salts (total 0.05 M) in 1.5 M sulfuric acid solution (pH ~0.5). A 3-electrode set-up was used with the electropolished Cu foil as working electrode, a Ag/AgCl (double junction, saturated NaCl, Metrohm, Switzerland) electrode as reference, and another 42 cm 2 Cu foil as counter electrode (Supplementary Fig. ). For the galvanostatic deposition process, a current density of j = -0.45 A cm -2 was applied for 100 s to prepare the large-scale catalyst. After electrodeposition, the catalyst was cleaned by dipping into Milli-Q water for 120 s and dried under N2 stream at room temperature.