Fe and Sn Single-Site-Based\nElectrodes for High-Current\nCO<sub>2</sub> Reduction in Acid Media and Stable Zn–CO<sub>2</sub> Batteries

Electrocatalytic CO<sub>2</sub> conversion is a promising\nmethod\nfor reducing the dependence on fossil fuels and lowering CO<sub>2</sub> emissions. However, challenges such as suppression of the competing\nhydrogen evolution reaction (HER) and long-term stability, especially\nin acidic media, among others, hinder its industrial application.\nHerein, Fe and Sn single sites supported on an N-doped carbon support\n(FSNC) was prepared by direct pyrolysis of selected precursors. XANES\nand EXAFS measurements confirmed the presence of Fe and Sn single\natoms coordinated to N or O atoms in the N-doped carbon. An analogous\nmaterial synthesized by deposition of Fe and Sn precursors on a previously\nfabricated N-doped carbon matrix (FS/NC), followed by thermal reduction,\nrendered Fe–O small clusters and Sn single atoms. FSNC was\ntested for CO<sub>2</sub> reduction, obtaining a CO Faradaic efficiency\n(FE) of 92%, while the CO FE of FS/NC was 63%. We attributed the differences\nin selectivity to the interaction between the Fe and Sn single sites,\nwhile the Fe–O clusters are inactive for this reaction. Double-layer\ncapacitance (C<sub>DL</sub>) and electrochemical impedance spectroscopy\n(EIS) measurements confirmed a larger electrochemically active surface\narea and lower charge-transfer resistance, respectively, in FSNC.\nIn addition, FSNC demonstrated a high CO FE (90%) under acidic conditions\n(pH = 2.1), demonstrating that this electrocatalyst can effectively\nsuppress the HER under acidic conditions. Moreover, 5 cm<sup>2</sup> electrodes containing FSNC were fabricated, and their stability\nwas tested for 20 h of continuous operation in an electrochemical\nflow cell at different current densities (50–350 mA/cm<sup>2</sup>), demonstrating improved stability at high current densities\nand under acidic conditions. Finally, FSCN-based cathodes were also\ntested in a Zn–CO<sub>2</sub> battery, achieving a maximum\npower density of 2.54 mW/cm<sup>2</sup> at 0.48 V with a current density\nof 5.2 mA/cm<sup>2</sup> and demonstrating outstanding rechargeability\nand stability upon 50 continuous charge–discharge cycles for\n50 h.