Hydrogen production using sunlight energy (solar-water splitting) has gained much attention in the quest to move towards carbon-neutral technologies. If chemical products with applications in the health and food industries could be produced at the same time as hydrogen, this would help to reduce the cost of solar-water splitting, as well as increasing the technology's range of applications. In this study, Kobe University's Associate Professor Tachikawa et al. found that by modifying the surface of their previously-developed hematite photocatalyst, they could safely, cheaply and stably produce hydrogen peroxide as well as hydrogen. Hydrogen peroxide is used for many purposes including disinfecting, bleaching and soil improvement.

Using a hematite (*1) photocatalyst (*2), a joint research team has succeeded in producing both hydrogen gas and hydrogen peroxide (*3) at the same time from sunlight and water. The team included the following members from Kobe University: Associate Professor TACHIKAWA Takashi (of the Molecular Photoscience Research Center) Professor TENNO  Seiichiro (Graduate School of System Informatics/ Graduate School of Science, Technology, and Innovation), Associate Professor TSUCHIMOCHI Takashi (Graduate School of System Informatics) et al.

In the quest to make a carbon neutral society a reality, CO2-free hydrogen production using sunlight energy has gained attention. If chemical products with applications in the health and food industries could be produced at the same time as hydrogen through photocatalyst-mediated solar water-splitting, it would be possible to develop a solar water-splitting utilization system with even greater added value.

Hematite mesocrystals (*4) can absorb a wide range of visible light. In this study, Associate Professor Tachikawa et al. found that by preparing electrodes with mesocrystals doped (*5) with two different metal ions, it was possible to safely, cheaply and stably produce hydrogen peroxide as well as hydrogen. Hydrogen peroxide is used for many purposes including disinfecting, bleaching and soil improvement.

The research group's next aim is to implement this technology. While continuing to improve the high efficiency of the developed photocatalyst electrode, they will try to assemble the cells into a compact module as a step towards societal implementation. They also plan to develop this mesocrystal technology with various materials and reaction systems.

This was a joint research project with Nagoya University's Institute of Materials and Systems for Sustainability (Professor MUTO Shunsuke) and the Japan Synchrotron Radiation Research Institute (JASRI) (Chief Researcher OHARA Koji and Researcher INA Toshiaki).

Main Points

Hematite on its own is not suitable for producing hydrogen peroxide. By doping the hematite with different metal ions (tin and titanium) and sintering it, the researchers developed a highly active composite oxide co-catalyst (*6).

The ability to produce hydrogen peroxide on-site in addition to hydrogen will contribute towards reducing the cost of solar water-splitting, as well as increasing the technology's range of applications. Hydrogen peroxide is used for many purposes including disinfecting, bleaching and soil improvement.

Research Background With the world facing increasing environmental and energy issues, hydrogen has gained attention as one of the possible next generation energy sources. Ideally, photocatalysts could use sunlight and water to produce hydrogen, however it is necessary to achieve a conversion rate of 10% to enable such a system to be adopted industrially. It has been pointed out that even if this efficiency is achieved, the cost of hydrogen will not reach the desired value. To overcome these issues, there is strong demand for the development of a competitive next generation solar water-splitting system with high added value that can produce other useful chemicals at the same time as hydrogen.

In their previous research, Tachikawa et al. developed 'mesocrystal technology', which involves precisely aligning nanoparticles in photocatalysts to control the flow of electrons and their holes. Recently, they have succeeded in dramatically increasing the light energy conversion efficiency by applying this technology to hematite.

Up until now, hematite has not been applied to the production of hydrogen peroxide. In this study, the researchers discovered that by modifying the surface of the hematite with a composite oxide of tin and titanium ions it was possible to produce both hydrogen and hydrogen peroxide in a highly efficient and selective manner.

Research Methodology

Mesocrystal technology: The main problem that causes a conversion rate decline in photocatalytic reactions is that the electrons and holes produced by light recombine before they can react with the molecules (in this case, water). Tachikawa et al. created 3D structures of hematite mesocrystals with highly oriented nanoparticles via solvothermal synthesis (*8). Furthermore, they were able to develop mesocrystal photoelectrodes for water splitting by coating and sintering the mesocrystals on the conductive glass substrate.

Formation of a co-catalyst for producing hydrogen oxide via dopant segregation: Normally, photocatalytic water-splitting using hematite results in oxygen being produced from the oxidation of the water. Doping this hematite with tin ions (Sn2+) and titanium ions (Ti4+) and then sintering it at 700°C causes segregation of the tin and titanium dopants, leading to the formation of a composite oxide (SnTiOx) co-catalyst with high selectivity for hydrogen peroxide production). This structural change was revealed by performing synchrotron-based X-ray total scattering measurements using beamlines BL01B1 and BLO4B2 at the SPring-8 (*9) facility, and by using a high-resolution electron microscope incorporating electron energy loss spectroscopy (*10).

Photocatalyst  formation and performance: The water-splitting reaction was promoted when voltage was applied to the photocatalyst electrode illuminated by artificial sunlight. The researchers investigated the photoelectric current density and the Faradiac efficiency (*11) which indicate the hydrogen production efficiency and the hydrogen peroxide selectivity, respectively. It was revealed that there were positive and negative effects on hydrogen and hydrogen peroxide production if the photocatalyst was doped with only one of the metal ions. On the other hand, hematite doped with both Sn2+ and Ti4+ could produce hydrogen and hydrogen peroxide at the same time in a highly efficient and highly selective manner. In addition, first principle calculations (*12) suggested that the SnTiOx co-catalyst on the hematite consisted of SnO2/SnTiO3 layers of a few nanometers in thickness.

Further Developments

By modifying the surface of the hematite used for the photocatalyst, the research group succeeded in producing hydrogen peroxide, which hasn't been produced in this manner before, in a highly efficient and selective way. Next, the researchers plan to further optimize the photocatalytic electrode and collaborate with industry to develop an onsite system for the production of hydrogen and hydrogen peroxide utilizing sunlight.