Polaron Delocalization Regulating Oxygen Reduction Selectivity and Directional Charge Transfer for Enhanced Dark Photocatalytic H2O2 Synthesis

Abstract Dark photocatalysis decouples light harvesting from catalysis via photogenerated charge storage, enabling H2O2 production under intermittent illumination. However, increasing electron-storage capacity alone does not ensure efficient dark-state H2O2 production, and how stored electronic states influence oxygen reduction reaction (ORR) remains unclear. Here, we show that polaron delocalization contributes to multi-electron ORR selectivity in poly(heptazine imide) (PHI), allowing dark photocatalytic H2O2 synthesis. Sulfonated PHI (S-HPHI) delivers a 2.5-fold higher H2O2 yield (588.9 μmol g–1) than protonated PHI (HPHI, 237.3 μmol g–1) despite comparable net photo-contributed electron-storage capacities (1441–1752 μmol e– g–1). This enhancement is consistent with the higher electron utilization efficiency toward H2O2 formation (81.7% for S-HPHI versus 27.1% for HPHI), enabled by sulfonation-induced polaron delocalization. Combined experimental and theoretical analyses reveal that sulfonation promotes polaron delocalization and disperses structural relaxation during photocharging. These changes are consistent with reduced electron self-trapping and facilitated directional electron transfer to adsorbed O2, which may contribute to suppressing O–O bond cleavage in proton-coupled electron transfer steps. This favors enhanced selectivity toward H2O2 formation even at high stored-electron densities. These findings highlight polaron delocalization as an important factor contributing to improved reaction selectivity in dark photocatalysis.