Pyridine-Bridged Axial Coordination Creates Electron-Deficient Co–N ₅ Porphyrin Sites for Selective Photoelectrochemical CO ₂ Reduction on Si Nanowires

The selectivity and efficiency of photoelectrochemical (PEC) CO 2 reduction on Si photocathodes are frequently impeded by ill-defined catalytic interfaces and sluggish interfacial electron transfer kinetics. Here, we report a pyridine-bridged axial-coordination strategy that transforms planar Co–N 4 porphyrins into electron-deficient Co–N 5 motifs and simultaneously strengthens electronic coupling at the semiconductor-molecule interface. Pyridine-functionalized carbon nanotubes serve as a conductive scaffold to disperse 5,10,15,20-tetrakis (4-aminophenyl) porphyrin-cobalt (CoTAPP) and enforce axial ligation, breaking the symmetry of the cobalt center and enhancing electronic coupling to the interface. The resulting CoTAPP-py-CNT/Si photocathode achieves a CO Faradaic efficiency of 95.0% at −0.7 V vs RHE and a solar-to-CO energy conversion efficiency of 1.86%. Combined spectroscopic characterization, kinetic analysis, in-situ infrared spectroscopy, and density functional theory (DFT) calculations demonstrate that axial coordination and the pyridine bridge accelerate interfacial charge transfer, enhance CO 2 adsorption, and promote the *COOH/*CO reaction pathway, thereby enabling highly selective PEC CO 2 reduction.