Grain Boundary Engineering in 3D Porous Silver Electrocatalysts for Enhanced CO2-to-CO Conversion.

Silver is a promising electrocatalyst for electrochemical CO2 reduction reaction owing to its high selectivity and efficiency for CO production. However, it still faces a fundamental trade-off between reaction activity and stability. Here, we developed a three-dimensional coral-like porous silver (CP-Ag) catalyst through seed-assisted nanoparticle attachment synthesis, which creates a unique architecture featuring interconnected pores and stable grain boundaries (GBs) between constituent Ag nanoparticles (Ag NPs). Compared to normal Ag NPs, CP-Ag demonstrates superior catalytic performance, maintaining >90% Faradaic efficiency (FE) for CO across a wide potential range (-0.6 to -1.0 V vs. RHE) while achieving 2-times higher current density. Importantly, CP-Ag demonstrated an impressive long-term stability by sustaining nearly 90% FE for CO approximately 40 h at a current density of -50 mA cm-2 in a flow cell. The enhanced catalytic performance arises from three factors: (1) the three-dimensional coral-like morphology increases accessible active sites and promotes charge transfer efficiency; (2) stable GBs between interconnected nanoparticles increase reaction activity; (3) more moderate binding on Ag (100) preferentially promotes *CO intermediate formation. Our findings highlight the importance of simultaneously engineering both morphological and crystallographic features to optimize silver catalysts for CO2 conversion.