Cation Exchange-Driven Grain Boundary-Rich Nanorings as Efficient CO Reduction Electrocatalysts.

Electrochemical CO reduction reaction (CORR) to formate offers a sustainable pathway for carbon-neutral fuel production, yet achieving high selectivity and activity remains challenging due to competing hydrogen evolution. While grain boundaries (GBs) enhance catalytic performance, the impact of GB density, uniformity, and twisting angles remains unclear. Here, we engineer SnS nanoplates with high-density GBs via cation exchange (CE), preserving sulfur frameworks while inducing strain-driven domain segmentation. The GB-SnS catalyst achieves a formate Faradaic efficiency (FE) of 98.9% at -1.0 V, with a partial current density of 204.6 mA cm at -1.2 V, surpassing single-crystalline (SC) SnS by 3.5-fold. In situ spectroscopy and density functional theory (DFT) reveal high-angle GBs lower *OCHO stabilization barriers while suppressing *H adsorption. Counterintuitively, small-angle GBs raise the *OCHO barrier, impairing CORR-a finding that challenges the assumption that all GBs benefit catalysis. DFT further predicts out-of-plane rotational GBs similarly enhance stabilization, guiding future 3D defect engineering. The catalyst demonstrates industrial viability in a membrane electrode assembly (MEA), sustaining >80% FE at 200 mA cm for 150 h. Statistical analysis of >200 GBs correlates twisting angles with adsorption strength, establishing an angle-dependent design principle. This work decouples geometric/electronic GB effects and pioneers solution-phase CE for scalable defect-rich catalysts, advancing sustainable CO utilization.