Sn-Decorated Cu/Cu₂O Electrodes Enable Selective CO₂ Reduction to Formic Acid under Realistic Flue Gas Streams

The electrochemical reduction of CO₂ is typically investigated under pure CO₂ feeds, but practical deployment must address more complex and dilute sources such as flue gases. Here, we studied Cu/Cu₂O electrodes decorated with tin (Sn) synthesized using a scalable electrodeposition method and post-treatments under both pure CO₂ and reactive nitrogen oxide-containing simulated flue gas, toward formic acid synthesis. Raman spectroscopy and Atomic Force Microscopy analyses revealed that flue gas exposure induces heterogeneous restructuring of the electrode with surface roughening, surface carbonate formation, and localized redeposition processes. Optimal catalyst performance under pure CO₂ was achieved with intermediate Sn coverage of 3 min electrodeposition, delivering Faradaic efficiencies of 80% and production rates of 370 μmol cm^-2 h^-1. Sn-modified Cu₂O electrodes also exhibited high selectivity toward formic acid under acidic gas containing simulated flue gas, reaching Faradaic efficiencies of 90% albeit at production rates of 113 μmol cm^-2 h^-1, despite a 10-fold reduced CO₂ partial pressure. These results demonstrate that interfacial Sn-Cu structures enabled selective CO₂RR even under challenging feed conditions, pointing out both the opportunities and limitations of translating laboratory-scale catalysts to realistic gas streams.