Dual-doping in the bulk and the surface to ameliorate the hematite anode for photoelectrochemical water oxidation

Hematite Hollow-Sphere-Array Photoanodes for Efficient Photoelectrochemical Water Splitting

Constructing 3D nanophotonic structures is regarded as an effective method to realize efficient solar-to-hydrogen conversion. These photonic structures can enhance the absorbance of photoelectrodes by the light trapping effect, promote the charge separation by designable charge transport pathway and provide a high specific surface area for catalytic reaction. However, most 3D structures reported so far mainly focused on the influence of light absorption and lacked a systematic investigation of the overall water splitting process. Herein, hematite hollow-sphere-array photoanodes are fabricated through a facile hydrothermal method with polystyrene templates. Validating by simulations and experiments, the hollow sphere array is proved to enhance the efficiency of light harvesting, charge separation and surface reaction at the same time. With an additional annealing treatment in oxygen, a photocurrent density of 2.26 mA cm-2 at 1.23 V versus reversible hydrogen electrode can be obtained, which is 3.70 times larger than that with a planar structure in otherwise the same system. This work gains an insight into the photoelectrochemical water splitting process, which is valuable for the further design of advancing solar driven water splitting devices.

Corrosion resistance and earth-abundance FeS-based heterojunction catalyst for seawater splitting at industrial grade density

The strategic progression toward highly efficient transition metal electrocatalytic electrodes is crucial to achieving efficiency and long-term stability in hydrogen production from authentic seawater sources. This work reports the development of a self-supporting, heterogeneous and corrosion-resistant iron sulfur-based catalytic electrode via a streamlined, one-step process involving sulfide etching and electroless plating on an iron foam substrate (IF). This new electrode, named NiS-FeS@IF, involves a nanostructured NiS-FeS catalytic material that combines in situ, resulting in a thin, ultrathin nanospherical layer on the IF. This construction has low overpotentials of merely 322 mV for the hydrogen evolution reaction (HER) and 563 mV for the oxygen evolution reaction (OER) with a current density of 500 mA cm-2 in alkaline simulated seawater electrolytes. Importantly, the NiS-FeS@IF electrode enduring more than 500 h at an industrial grade high current density of 1 A cm-2 without noteworthy performance deterioration. The unique and uniformly dispersed morphology of NiS-FeS facilitates intensified interfacial electron transfer, optimizes active site exposure and provides efficient channels for the rapid release and mass transfer of gas bubbles. This work introduces a novel approach for the facile preparation of efficient electrode materials.

Rational Design of CoOOH/α-Fe2O3/SnO2 for Boosted Photoelectrochemical Water Oxidation: The Roles of Underneath SnO2 and Surface CoOOH

Hematite (α-Fe2O3) photoanode is a promising candidate for efficient PEC solar energy conversion. However, the serious charge recombination together with the sluggish water oxidation kinetics of α-Fe2O3 still restricts its practical application in renewable energy systems. In this work, a CoOOH/α-Fe2O3/SnO2 photoanode was fabricated, in which the ultrathin SnO2 underlayer is deposited on the fluorine-doped tin oxide (FTO) substrate, α-Fe2O3 nanorod array is the absorber layer, and CoOOH nanosheet is the surface modifier, respectively. The resulting CoOOH/α-Fe2O3/SnO2 exhibited excellent PEC water splitting with a high photocurrent density of 2.05 mA cm-2 at 1.23 V vs RHE in the alkaline electrolyte, which is ca. 3.25 times that of bare α-Fe2O3. PEC characterizations demonstrated that SnO2 not only could block hole transport from α-Fe2O3 to FTO substrate but also could efficiently enhance the light-harvesting property and reduce the surface states by controlling the growth process of α-Fe2O3, while the CoOOH overlayer as cocatalysts could rapidly extract the photogenerated holes and provide catalytic active sites for water oxidation. Benefiting from the synergistic effects of SnO2 and CoOOH, the efficiency of the charge recombination and the overpotential for water oxidation of α-Fe2O3 are obviously decreased, resulting in the boosted PEC efficiency for water oxidation. The rational design and simple fabrication strategy display great potentials to be used for other PEC systems with excellent efficiency.

Unveiling the Influence of Sulfur Doping on Photoelectrochemical Performance in BiVO4/FeOOH Heterostructures

BiVO4 is a promising photoanode for photoelectrochemical (PEC) water splitting but suffers from high charge carrier recombination and sluggish surface water oxidation kinetics that limit its efficiency. In this work, a model of sulfur-incorporated FeOOH cocatalyst-loaded BiVO4 was constructed. The composite photoanode (BiVO4/S-FeOOH) demonstrates an enhanced photocurrent density of 3.58 mA cm-2, which is 3.7 times higher than that of the pristine BiVO4 photoanode. However, the current explanations for the generation of enhanced photocurrent signals through the incorporation of elements and cocatalyst loading remain unclear and require further in-depth research. In this work, the hole transfer kinetics were investigated by using a scanning photoelectrochemical microscope (SPECM). The results suggest that the incorporation of sulfur can effectively improve the charge transfer capacity of FeOOH. Moreover, the oxygen evolution reaction model provides evidence that S-doping can induce a "fast" surface catalytic reaction at the cocatalyst/solution interface. The work not only presents a promising approach for designing a highly efficient photoanode but also offers valuable insights into the role of element doping in the PEC water-splitting system.

Roles of Cobalt-Coordinated Polymeric Perylene Diimide in Hematite Photoanodes for Improved Water Oxidation

Interfacial charge recombination is a permanent issue that impedes the photon energy utilization in photoelectrochemical (PEC) water splitting. Herein, a conjugated polymer, urea linked perylene diimide polymer (PDI), is introduced to the designation of hematite-based composite photoanodes. On account of its unique molecule structure with abundant electronegative atoms, the O and N atoms with lone electron pairs can bond with Fe atoms at the surface of Zr4+ doped α-Fe2 O3 (Zr:Fe2 O3 ) and thus establish charge transfer channels for expediting hole separation and migration. Meanwhile, PDI molecules can passivate the surface states in Zr:Fe2 O3 , which is in favor of suppressing carrier recombination. Particularly, Co2+ is used to coordinate with PDI (Co-PDI) to accelerate hole extraction as well as utilization, and the as-obtained Co-PDI form type-II heterojunction with Zr:Fe2 O3 . Such a photoanode configuration takes advantage of the unique molecule structure of PDI, and the target Co-PDI/Zr:Fe2 O3 photoanodes eventually attain a photocurrent density of 2.17 mA cm-2 , which is inspirational for unearthing the potential use of conjugative molecules in PEC fields.

Expediting hole transfer via surface states in hematite-based composite photoanodes

Regarding the indirect hole transfer route in hematite-based photoelectrodes, the widely accepted viewpoint is that the Fe^IVO states act as a hole transfer medium, while other types of surface states act as recombination centers. Alternatively, it has rarely been reported that the recombining surface states may contribute to the charge transport in modified photoelectrodes. In this study, we employed CoCr layered double hydroxide (LDH)/Fe₂O₃ and CoCr LDH/Zr:Fe₂O₃ as research models to investigate the distinct charge transfer pathways in composite photoanodes. Different from the adverse role of surface states at ∼0.7 V versus the reversible hydrogen electrode (r-SS) in the bare hematite photoelectrodes (Fe₂O₃ or Zr:Fe₂O₃), the r-SS in the composite photoanodes (CoCr LDH/Fe₂O₃ or CoCr LDH/Zr:Fe₂O₃) served as a hole transfer station to induce high-valent Co cations, and the position of r-SS determined the onset potential of the composite photoelectrodes. Moreover, the Fe^IVO states still acted as active intermediates to transport numerous holes to the cocatalyst, which enhanced the charge utilization efficiency at 1.23 V versus the reversible hydrogen electrode (RHE) to a large extent. Besides, a noteworthy fact is that Zr doping increased the number of active Fe^IVO states, which significantly contributed to the enhancement in current density. However, it led to a delayed onset potential because of the positively shifted surface states (r-SS and Fe^IVO). Evidently, the different surface state distributions between Fe₂O₃ and Zr:Fe₂O₃ gave rise to anisotropic charge transfer and recombination behavior in the composite photoanodes. This study gives extensive insight into the hole transfer route in composite photoanodes and reveals the surface state-tuning effects of dopants and cocatalysts, which are significant for a deep understanding of the surface states and optimal design of composite photoanodes via surface state modulation.

Magnetron Sputtered Al Co-Doped with Zr-Fe2O3 Photoanode with Fortuitous Al2O3 Passivation Layer to Lower the Onset Potential for Photoelectrochemical Solar Water Splitting

In this paper, we investigate the magnetron sputtering deposition of an Al-layer on Zr-doped FeOOH (Zr-FeOOH) samples to fabricate a Zr/Al co-doped Fe2O3 (Al-Zr/HT) photoanode. An Al-layer is deposited onto Zr-FeOOH through magnetron sputtering and the thickness of the Al deposition is regulated by differing the sputtering time. Electrochemical impedance spectroscopy, intensity-modulated photocurrent spectroscopy, Mott-Schottky and time-resolved photoluminescence spectra analyses were used to study, in depth, the correlations between sputtered Al-layer thicknesses and PEC characteristics. High-temperature quenching (800 °C) assists in diffusing the Al3+ in the bulk of the Zr-doped Fe2O3 photoanode, whilst an unintended Al2O3 passivation layer forms on the surface. The optimized Al-Zr/HT photoelectrode achieved 0.945 mA/cm2 at 1.0 VRHE, which is 3-fold higher than that of the bare Zr/HT photoanode. The Al2O3 passivation layer causes a 100 mV cathodic shift in the onset potential. Al co-doping improved the donor density, thus reducing the electron transit time. In addition, the passivation effect of the Al2O3 layer ameliorated the surface charge transfer kinetics. The Al2O3 passivation layer suppressed the surface charge transfer resistance, consequently expediting the hole migration from photoanode to electrolyte. We believe that the thickness-controlled Al-layer sputtering approach could be applicable for various metal oxide photoanodes to lower the onset potential.