Fe and Sn Single-Site-Based Electrodes for High-Current CO₂ Reduction in Acid Media and Stable Zn–CO₂ Batteries

Electrocatalytic CO2 conversion is a promising method for reducing the dependence on fossil fuels and lowering CO2 emissions. However, challenges such as suppression of the competing hydrogen evolution reaction (HER) and long-term stability, especially in acidic media, among others, hinder its industrial application. Herein, Fe and Sn single sites supported on an N-doped carbon support (FSNC) was prepared by direct pyrolysis of selected precursors. XANES and EXAFS measurements confirmed the presence of Fe and Sn single atoms coordinated to N or O atoms in the N-doped carbon. An analogous material synthesized by deposition of Fe and Sn precursors on a previously fabricated N-doped carbon matrix (FS/NC), followed by thermal reduction, rendered Fe–O small clusters and Sn single atoms. FSNC was tested for CO2 reduction, obtaining a CO Faradaic efficiency (FE) of 92%, while the CO FE of FS/NC was 63%. We attributed the differences in selectivity to the interaction between the Fe and Sn single sites, while the Fe–O clusters are inactive for this reaction. Double-layer capacitance (CDL) and electrochemical impedance spectroscopy (EIS) measurements confirmed a larger electrochemically active surface area and lower charge-transfer resistance, respectively, in FSNC. In addition, FSNC demonstrated a high CO FE (90%) under acidic conditions (pH = 2.1), demonstrating that this electrocatalyst can effectively suppress the HER under acidic conditions. Moreover, 5 cm2 electrodes containing FSNC were fabricated, and their stability was tested for 20 h of continuous operation in an electrochemical flow cell at different current densities (50–350 mA/cm2), demonstrating improved stability at high current densities and under acidic conditions. Finally, FSCN-based cathodes were also tested in a Zn–CO2 battery, achieving a maximum power density of 2.54 mW/cm2 at 0.48 V with a current density of 5.2 mA/cm2 and demonstrating outstanding rechargeability and stability upon 50 continuous charge–discharge cycles for 50 h.