H₂O₂ has a wide range of uses as an oxidant and is also utilized as a high density and environmentally friendly fuel. In this study, crystalline g-C₃N₄ was modified with K⁺, Ni and N-doped carbon by thermal-polymerization of Ni²⁺-anchored and polyvinyl pyrrolidone-encapsulated melamine-cyanuric acid supramolecules, followed by re-calcination in a KCl–LiCl mixture. The obtained strip-like g-C₃N₄ nanocrystals exhibited high charge separation efficiency and good light absorption capability. Density-functional theory calculations confirmed that the orbital hybridization of Ni–N atoms enabled the formation of charge-transfer channels, which can capture electrons, achieve enhanced oxygen adsorption and reduction via a two-electron pathway. Under the synergistic effects of K⁺, Ni and N-doped carbon dopants and crystallinity, the obtained sample exhibited high photocatalytic H₂O₂ yields of 79.6 μM in O₂-saturated pure water, which was 10-fold higher than the yield using pristine g-C₃N₄. This work provides a novel design for production of highly efficient photocatalysts for H₂O₂ production.