Dipole moment-induced asymmetric charge distribution of Fe atomic sites to boost photocatalytic hydrogen evolution

This study developed a selective phosphorization strategy to precisely construct distorted asymmetric FeP₃C₁-coordinated Fe single-atom catalysts (SACs) on graphitic carbon nitride (g-C₃N₄) to overcome the poor charge separation and unbalanced reaction intermediate adsorption arising from the highly symmetric coordination structures of SACs. Moreover, tunable modulation of the local dipole moments and charge distributions was achieved at the Fe sites by finely adjusting the phosphorization time to control the phosphorus doping level. Integrated experimental characterization and density functional theory calculations revealed that the distorted asymmetric FeP₃C₁ configuration significantly enhanced interfacial dipole moments, accelerating photogenerated charge separation while broadening visible-light absorption. Furthermore, the dipole-induced asymmetric charge distribution optimized the adsorption behavior of hydrogen intermediates; this lowered the Gibbs free energy barrier for the hydrogen evolution reaction (HER), leading to the the thermodynamic optimum. The resulting FeP₃C₁/g-C₃N₄ displayed outstanding performance even without noble-metal co-catalysts, achieving an HER rate of 7045 ± 195 μmol g^-1 h^-1 and a peak apparent quantum efficiency of 12.87% at 380 nm. Thus, this work establishes distortion-driven asymmetric coordination engineering and the associated dipole moment enhancement as a new paradigm for efficient solar-to‑hydrogen conversion catalysts designs.