Defect-density-dependent spontaneous alloying of PtPd nanoparticles on carbon supports for efficient hydrogen evolution.

Electrocatalytic water splitting is a sustainable route for H2 production, yet the green synthesis of Pt-based alloy catalysts without external reducing agents remains challenging. Herein, we report a defect-rich activated N-doped mesoporous hollow carbon sphere (NMHCS-A) enabling the spontaneous formation of PtPd alloy under ambient conditions without reducing agents or thermal treatment. Structural analyses reveal that KOH activation generates abundant carbon defect-associated species, acting as electron reservoirs to promote the reduction of Pt and Pd precursors and subsequent alloy nucleation. Control experiments show that spontaneous alloy formation occurs only on NMHCS-A with the high defect density, whereas low-defect carbon supports fail to produce detectable PtPd alloy phases under identical conditions. Owing to the strong electronic interaction between Pt and Pd, electron transfer from Pd to Pt optimizes H adsorption and accelerates H2 evolution reaction (HER) kinetics. Consequently, PtPd/NMHCS-A exhibits an overpotential of 28 mV at 10 mA cm-2 in 0.5 M H2SO4 and a Pt mass activity 49 times higher than that of commercial Pt/C. Theoretical calculations reveal that alloy-induced electronic structure reconstruction weakens PtH interactions and brings the H adsorption free energy closer to thermoneutrality. This work provides a defect-density-dependent spontaneous alloying strategy for noble-metal catalysts systhesis.