Defect engineering-driven enhancement of C₂₊ products over FeCN-modified cu for acidic CO₂ electroreduction

Acidic electrochemical CO₂ reduction (CO₂RR) to high-value C₂₊ products offers a promising route for CO₂ utilization. However, it still faces severe challenges: competitive hydrogen evolution reaction (HER) and slow CC coupling. To address these issues, we propose a defect engineering strategy by Fe-doped carbon nitride (FeCN) modification to reduce Cu defect formation energy, thereby inducing abundant and stable surface defects. We synthesized a magnetron-sputtered Cu nanoparticle (MSCu) catalyst modified with FeCN (MSCu-FeCN) and evaluated its performance in a membrane electrode assembly (MEA) under acidic conditions. At 500 mA·cm^-2, MSCu-FeCN achieves a remarkable C₂₊ Faradaic efficiency (FE) of 73.07 % and a single-pass CO₂ conversion efficiency (SPCE) of 57.30 % with a full-cell voltage of 3.88 V, significantly superior to the original MSCu. Density functional theory (DFT) calculations, cyclic voltammetry curves, and in-situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy confirm that FeCN lowers Cu defect formation energy, increases surface defect density, enhances *CO intermediate coverage, thereby promoting CC coupling. This work presents a defect engineering approach enabling high-efficiency acidic CO₂RR, while providing pivotal insights for industrial-grade electrocatalytic systems.