Developing Ni single-atom sites in carbon nitride for efficient photocatalytic HO production.

Photocatalytic two-electron oxygen reduction to produce high-value hydrogen peroxide (HO) is gaining popularity as a promising avenue of research. However, structural evolution mechanisms of catalytically active sites in the entire photosynthetic HO system remains unclear and seriously hinders the development of highly-active and stable HO photocatalysts. Herein, we report a high-loading Ni single-atom photocatalyst for efficient HO synthesis in pure water, achieving an apparent quantum yield of 10.9% at 420 nm and a solar-to-chemical conversion efficiency of 0.82%. Importantly, using in situ synchrotron X-ray absorption spectroscopy and Raman spectroscopy we directly observe that initial Ni-N sites dynamically transform into high-valent O-Ni-N sites after O adsorption and further evolve to form a key *OOH intermediate before finally forming HOO-Ni-N. Theoretical calculations and experiments further reveal that the evolution of the active sites structure reduces the formation energy barrier of *OOH and suppresses the O=O bond dissociation, leading to improved HO production activity and selectivity.