pH-dependent mechanism of hydrazine oxidation on hematite-based photoanode

The hydrazine oxidation reaction (HzOR) offers a thermodynamically favorable alternative to the anodic oxygen evolution reaction for photoelectrochemical hydrogen production. Despite its low theoretical potential (-0.33 V_RHE (reversible hydrogen electrode, RHE)), the catalytic efficiency of HzOR is fundamentally limited by an unresolved pH-dependent mechanism. This work deciphers the intrinsic mechanistic shift of HzOR across the full pH range using a Fe₂O₃-based photoanode, stemming from the hydrazine variant between N₂H₄ in alkaline and N₂H₅⁺ in acidic conditions. Photoelectrochemical analysis reveals a non-monotonic, "Z"-shaped correlation between photocurrent density and pH value, with optimal performance in alkaline media. A suite of in-situ diagnostics, including electron paramagnetic resonance, Raman spectroscopy and differential electrochemical mass spectrometry, provides direct evidence for a pH-governed pathway transition: a NN bond cleavage mechanism dominates in acid, generating •NH₂ intermediates, while a sequential deprotonation process prevails in alkali. Femtosecond transient absorption spectroscopy underscores accelerated hole-transfer kinetics in the deprotonation mechanism. Molecular dynamic simulations and density functional theory calculations rationalize the variants and mechanism transition, verify that the sequential deprotonation lowers the energy barrier for HzOR. This study correlates hydrazine variants, reaction mechanism and photoelectrochemical activity of HzOR at different pH values, offering critical guidance on designing an efficient electrolyte appropriate to the pH environment.