Segregation‐Engineered Polarization Synchronizes CO ₂ and Nitrate Reduction for Bias‐Free Urea Synthesis

Overcoming the kinetic mismatch between CO 2 and NO 3 − reduction presents a central challenge for urea photoelectrochemical synthesis. Here, we develop segregation‐engineered Si/Pd–Cu photocathodes where nanoscale phase segregations induce dual‐level interfacial polarization. Cu‐rich segregations favor Schottky‐type band modulation, facilitating photogenerated electron extraction. Simultaneously, Pd‐rich domains expose Pd δ+ –Cu δ− ‐like polarized sites that co‐stabilize CO 2 /NO 3 − ‐derived intermediates, synchronizing their reduction kinetics for efficient C–N coupling. Under AM 1.5 G illumination, the optimized Si/1Pd–3Cu photocathode delivers urea with a remarkable faradaic efficiency up to ≈100% at 0 V vs. RHE, achieving an initial urea partial current density of 1.06 mA·cm −2 . Operando spectroscopies combined with theoretical calculations identify a Pd‐rich governed, low‐barrier C–N coupling pathway operating near the thermodynamic potential. Further integration into photovoltaic photoelectrochemical devices enables light‐driven spontaneous urea synthesis without external bias. This work establishes segregation‐programmed polarization in semiconductor/metal junctions as a powerful, general materials‐design principle for mild and selective multielectron synthesis.