Tomohiro Higashi

La5Ti2Cu1−xAgxS5O7 photocathodes operating at positive potentials during photoelectrochemical hydrogen evolution under irradiation of up to 710 nm

A photoelectrochemical (PEC) cell based on a series-connected photocathode and photoanode made of particulate semiconductors is a potentially scalable and inexpensive device for renewable solar hydrogen production via PEC water splitting without any external power supply. The realisation of such PEC devices hinges on the development of photoelectrodes that operate at a small applied voltage. In this study, solid solutions of La5Ti2CuS5O7 (LTC) and La5Ti2AgS5O7 (LTA) were synthesised, and their physical, optical, and PEC properties in the water splitting reaction were discussed. LTC and LTA formed a La5Ti2Cu1−xAgxS5O7 solid solution (LTC1−xAx) over the whole compositional range. The indirect bandgap energy of LTC1−xAx changed nonlinearly with respect to composition, attaining its minimum value (ca. 1.8 eV) at a composition of x ≈ 0.16. Photoelectrodes of Al-doped LTC1−xAx solid solution powder fabricated using the particle transfer method exhibited a photocathodic response regardless of the Ag content. 1%Al-LTC0.9A0.1 photocathodes exhibited the best PEC properties in the hydrogen evolution reaction and yielded a hypothetical half-cell solar-to-hydrogen energy conversion efficiency of 0.25% at +0.6 V vs. RHE, three times higher than the previously reported 1%Sc-LTC. In addition, 1%Al-LTC0.9A0.1 photocathodes were fairly stable at +0.7 V vs. RHE without any protective modifications. Owing to the positive operational electrode potential of 1%Al-LTC0.9A0.1, unassisted PEC water splitting was accomplished using series-connected photoelectrodes made of 1%Al-LTC0.9A0.1 and BaTaO2N, particulate semiconductors with absorption edge wavelengths of 710 and 660 nm, respectively, at a Faradaic efficiency of unity and a solar-to-hydrogen energy conversion efficiency of approximately 0.1%.

Enhancement of Charge Separation and Hydrogen Evolution on Particulate La5Ti2CuS5O7 Photocathodes by Surface Modification

Particulate La5Ti2CuS5O7 (LTC) photocathodes prepared by particle transfer show a positive onset potential of 0.9 V vs RHE for the photocathodic current in photoelectrochemical (PEC) H2 evolution. However, the low photocathodic current imposes a ceiling on the solar-to-hydrogen energy conversion efficiency of PEC cells based on LTC photocathodes. To improve the photocurrent, in this work, the surface of Mg-doped LTC photocathodes was modified with TiO2, Nb2O5, and Ta2O5 by radio frequency reactive magnetron sputtering. The photocurrent of the modified Mg-doped LTC photocathodes was doubled because these oxides formed type-II heterojunctions and extended the lifetimes of photogenerated charge carriers. The enhanced photocathodic current was attributed to hydrogen evolution at a positive potential of +0.7 V vs RHE. This work opens up possibilities for improving PEC hydrogen evolution on particulate photocathodes based on surface oxide modifications and also highlights the importance of the band gap alignment.

A particulate (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 photocathode modified with CdS and ZnS for sunlight-driven overall water splitting

A photocathode prepared using the photocatalyst (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 in powder form, and modified with CdS/ZnS overlayers and a Pt catalyst, exhibits a photocurrent of 4.3 mA cm−2 at 0 VRHE under AM 1.5G simulated sunlight. The photocathode was capable of utilizing photons up to 900 nm, while the onset potential was evaluated to be as high as 0.8 VRHE. Modification with thin CdS and ZnS layers significantly increases the cathodic photocurrent through the formation of a surface p–n junction. The half-cell solar-to-hydrogen conversion efficiency of 0.83% at 0.33 VRHE obtained from this device is among the highest yet reported for a photocathode fabricated from a particulate photocatalyst. A solar to hydrogen conversion efficiency of 0.60% was observed during bias-free overall water splitting using this photocathode together with a BiVO4 photoanode. The present work clearly demonstrates the possibility of efficient hydrogen generation using durable particulate semiconductor photoelectrodes.

Powder-based (CuGa1−yIny)1−xZn2xS2 solid solution photocathodes with a largely positive onset potential for solar water splitting

Photoelectrochemical water splitting has attracted much attention in recent years as an alternative energy source. However, there have been several significant issues such as low efficiency, high cost and less scalability for its practical application. Here, we show that (CuGa1−yIny)1−xZn2xS2 (CGIZS)-based photocathodes fabricated with a particle transfer method exhibited a photocurrent of 4.5 mA cm−2 at 0.6 V vs. RHE along with a largely positive onset potential of 1.0 V vs. RHE under simulated sunlight (AM 1.5G) and an initial solar-to-hydrogen energy conversion efficiency of 1.1% was obtained with photoelectrochemical water splitting using a two-electrode cell composed of the CGIZS-based photocathode and a BiVO4-based photoanode. CGIZS used for the particle transfer method was obtained in a powder state with high crystallinity by a flux method using a molten-salt of LiCl–KCl, and also formed solid solutions with a chalcopyrite single phase structure in a wide range of Ga/In ratios and Zn contents, in which the absorption edges of photocatalysts were tunable up to 880 nm.

Activation of a particulate Ta3N5 water-oxidation photoanode with a GaN hole-blocking layer

Particulate Ta3N5, a material that responds to visible light for photoelectrochemical O2 evolution, was glued to a metallic GaN conducting layer. The electrode was able to oxidize water with 1.8-fold higher efficiency than that without GaN. The GaN layer blocked the hole current from Ta3N5 to the back-contact metal layer and prevented hole–electron recombination.

CdTe-Based Photoanode for Oxygen Evolution from Water under Simulated Sunlight

This study investigated the properties of a photoanode fabricated by depositing a p-type CdTe thin film on a CdS-coated FTO substrate (CdTe/CdS/FTO) via close-space sublimation. This CdTe/CdS/FTO electrode was found to work as a photoanode with a long absorption edge wavelength of 830 nm. In a CdTe-based photoanode such as this, the p-n junction formed at the CdTe/CdS interface promotes charge separation of photoexcited carriers and forces photogenerated holes to move toward the photoanode surface to promote oxidation reactions on the electrode surface. A MoOx buffer layer was also found to play a crucial role in facilitating the transfer of photogenerated holes to surface reaction sites through decreasing the energy barrier at the interface between the CdTe and a surface protective layer. A biphotoelectrode photoelectrochemical cell composed of a CdTe-based photoanode and a CdTe-based photocathode exhibited a solar-to-hydrogen conversion efficiency of 0.22% without an external voltage in response to illumination by AM 1.5G light.

Particulate photocatalyst sheets based on non-oxide semiconductor materials for water splitting under visible light irradiation

Particulate photocatalyst sheets based on a hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP) embedded in a conductive material can realize efficient and scalable water splitting. However, existing sheet systems involve photocatalytic materials with absorption edge wavelengths (λabs) of approximately 500 nm. Thus, the development of sheet systems based solely on photocatalysts with longer absorption edge wavelengths is essential for obtaining solar-to-hydrogen energy conversion efficiency required for practical applications. In this study, we present a photocatalyst sheet system based on Ga-doped La5Ti2Cu0.9Ag0.1S5O7 (Ga-LTCA, λabs = 710 nm) as the HEP and LaTiO2N (λabs = 600 nm) loaded with cobalt oxide (CoOx) as the OEP, both embedded in a Au layer by particle transfer (Ga-LTCA/Au/LaTiO2N). Ga-LTCA/Au/LaTiO2N sheets modified with Rh species evolved hydrogen and oxygen simultaneously under visible light irradiation (λ > 420 nm). The loading of CoOx onto LaTiO2N was found to be essential for successful water splitting, as was the choice of pH values during water splitting. The roles of CoOx and other factors in determining the water splitting activity of the Ga-LTCA/Au/LaTiO2N sheets are described herein, based on the photoelectrochemical properties of LaTiO2N photoanodes.