Photocatalytic two-electron oxygen reduction to produce high-value hydrogen peroxide (H₂O₂) is gaining popularity as a promising avenue of research. However, structural evolution mechanisms of catalytically active sites in the entire photosynthetic H₂O₂ system remains unclear and seriously hinders the development of highly-active and stable H₂O₂ photocatalysts. Herein, we report a high-loading Ni single-atom photocatalyst for efficient H₂O₂ 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-N₃ sites dynamically transform into high-valent O₁-Ni-N₂ sites after O₂ adsorption and further evolve to form a key *OOH intermediate before finally forming HOO-Ni-N₂. 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 H₂O₂ production activity and selectivity.

A paired electrochemical refinery toward the cathodic nitrate reduction reaction (NO₃RR) and anodic glycerol oxidation reaction (GOR) driven by renewable electricity could generate high value-added ammonia and formic acid simultaneously. However, both cathode and anode electrocatalysts often suffer from low efficiency and unclear reconstruction. Herein, we prepare a non-noble NiCu–OH nanocomposite precatalyst composed of crystalline Cu₂(OH)₃(NO₃) and amorphous Ni(OH)₂, which can be reconstructed into high-activity and stable cathodic NO₃RR and anodic GOR electrocatalysts under operating conditions. Various in situ and ex situ experiments demonstrate that NiCu–OH could convert into active Cu nanoparticles coupled with amorphous Ni(OH)₂ under cathodic NO₃RR conditions. Meanwhile, under anodic GOR conditions, NiCu–OH-derived bimetal oxide (NiCuO) changes into active NiOOH species composited with Cu vacancy-rich CuO. Furthermore, an actual paired electrochemical refinery with the reconstructed NiCu-based catalysts toward the NO₃RR and GOR shows a low potential of 1.37 V at a current density of 100 mA cm⁻² to continuously produce ammonium and formate. This study provides a new paradigm for the design of an electrochemical refinery with reconstructed non-noble electrocatalysts toward chemical upgradation.