Textured and Hierarchically Porous Hematite Photoanode for Efficient Hydrogen Production via Photoelectrochemical Hydrazine Oxidation

The performance of hematite (α-Fe₂O₃) photoanodes for photoelectrochemical (PEC) water splitting has been limited to around 2-5 mA cm^-2 under standard conditions due to their short hole diffusion length and sluggish oxygen evolution reaction kinetics. This work overcomes those challenges through a synergistic strategy that co-designs the hematite architecture and the surface reaction pathway. We introduce a textured and hierarchically porous Ti-doped Fe₂O₃ (tp-Fe₂O₃) photoanode, synthesized via multi-cycle growth and flame annealing method. This unique architecture features a high texture (110), enlarged surface area, and hierarchically porous structure, which enable significantly enhanced bulk charge transport and interfacial charge transfer compared to typical nanorod Ti-doped Fe₂O₃ (nr-Fe₂O₃). As a result, the tp-Fe₂O₃ photoanode achieves a photocurrent density of 3.1 mA cm^-2 at 1.23 V vs. RHE with exceptional stability over 105 h, notably without any co-catalyst. By replacing the OER with the hydrazine oxidation reaction, the photocurrent further reaches a record-high level of 7.1 mA cm^-2 at 1.23 V_RHE. Finally, when we integrate the tp-Fe₂O₃ with a commercial Si solar cell, it achieves a solar-to-hydrogen efficiency of 8.7%-the highest reported value for any Fe₂O₃-based PV-tandem system. This work provides critical insights into rational Fe₂O₃ photoanode design and highlights the potential of hydrazine as an efficient alternative anodic reaction, enabling waste valorization.