Engineered Sn- and Mg-doped hematite photoanodes for efficient photoelectrochemical water oxidation

Dual co-catalysts activated hematite nanorods with low turn-on potential and enhanced charge collection for efficient solar water oxidation

Hematite (α-Fe₂O₃) photoanode suffers from significant photocarrier recombination and sluggish water oxidation kinetics for photoelectrochemical water splitting. To address these challenges, this work demonstrates the construction of dual co-catalysts modified Fe₂O₃nanorods photoanode by strategically incorporating CoPi and Co(OH)_xfor photoelectrochemical water oxidation. The Fe₂O₃/CoPi/Co(OH)_xnanorods photoanode exhibits the lowest ever turn-on potential of 0.4V_RHE(versus reversible hydrogen electrode) and a photocurrent density of 0.55 mA cm^-2at 1.23V_RHE, 358% higher than that of pristine Fe₂O₃nanorods. The dual co-catalysts modification enhances the light-harvesting efficiency, surface photovoltage and hole transfer kinetics of the hybrid photoanode. The dual co-catalyst coupling also increases the carrier density and significantly reduces the depletion width (1.9 nm), resulting in improved conductivity and favorable band bending, boosting photogenerated hole transfer efficiency for water oxidation.

Sn-Controlled Co-Doped Hematite for Efficient Solar-Assisted Chargeable Zn-Air Batteries

The photoelectrochemical performance of a co-doped hematite photoanode might be hindered due to the unintentionally diffused Sn from a fluorine-doped tin oxide (FTO) substrate during the high-temperature annealing process by providing an increased number of recombination centers and structural disorder. We employed a two-step annealing process to manipulate the Sn concentration in co-doped hematite. The Sn content [Sn/(Sn + Fe)] of a two-step annealing sample decreased to 1.8 from 6.9% of a one-step annealing sample. Si and Sn co-doped hematite with the reduced Sn content exhibited less structural disorder and improved charge transport ability to achieve a 3.0 mA cm^-2 photocurrent density at 1.23 V_RHE, which was 1.3-fold higher than that of the reference Si and Sn co-doped Fe₂O₃ (2.3 mA cm^-2). By decorating with the efficient co-catalyst NiFe(OH)_x, a maximum photocurrent density of 3.57 mA cm^-2 was achieved. We further confirmed that the high charging potential and poor cyclability of the zinc-air battery could be dramatically improved by assembling the optimized, stable, and low-cost hematite photocatalyst with excellent OER performance as a substitute for expensive Ir/C in the solar-assisted chargeable battery. This study demonstrates the significance of manipulating the unintentionally diffused Sn content diffused from FTO to maximize the OER performance of the co-doped hematite.

Facile Fabrication of a Highly Crystalline and Well-Interconnected Hematite Nanoparticle Photoanode for Efficient Visible-Light-Driven Water Oxidation

Facile and scalable fabrication of α-Fe₂O₃ photoanodes using a precursor solution containing Fe^III ions and 1-ethylimidazole (EIm) in methanol was demonstrated to afford a rigidly adhered α-Fe₂O₃ film with a controllable thickness on a fluorine-doped tin oxide (FTO) substrate. EIm ligation to Fe^III ions in the precursor solution brought about high crystallinity of three-dimensionally well-interconnected nanoparticles of α-Fe₂O₃ upon sintering. This is responsible for the 13.6 times higher photocurrent density (at 1.23 V vs reference hydrogen electrode (RHE)) for photoelectrochemical (PEC) water oxidation on the α-Fe₂O₃ (w-α-Fe₂O₃) photoanode prepared with EIm compared with that (w/o-α-Fe₂O₃) prepared without EIm. The w-α-Fe₂O₃ photoanode provided the highest charge separation efficiency (η_sep) value of 27% among the state-of-the-art pristine α-Fe₂O₃ photoanodes, providing incident photon-to-current conversion efficiency (IPCE) of 13% at 420 nm and 1.23 V vs RHE. The superior η_sep for the w-α-Fe₂O₃ photoanode is attributed to the decreased recombination of the photogenerated charge carriers at the grain boundary between nanoparticles, in addition to the higher number of the catalytically active sites and the efficient bulk charge transport in the film, compared with w/o-α-Fe₂O₃.