In the effort to move towards carbon-neutral technology, hydrogen generation utilizing sunshine energy (solar-water splitting) has gotten a lot of interest. If chemical products with applications in the health and food industries could be created at the same time as hydrogen, solar-water splitting costs would be reduced, and the technology’s range of applications would be expanded.
Associate Professor Tachikawa et al. of Kobe University discovered that by altering the surface of their previously created hematite photocatalyst, they could make hydrogen peroxide as well as hydrogen safely, economically, and reliably.
Disinfection, bleaching, and soil enhancement are just a few of the uses for hydrogen peroxide. A collaborative research team has succeeded in creating both hydrogen gas and hydrogen peroxide from sunlight and water using a hematite photocatalyst.
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.
CO2-free hydrogen generation using solar energy has gained traction in the effort to achieve a carbon-neutral society. It would be conceivable to construct a solar water-splitting utilization system with even more added value if chemical compounds with applications in the health and food industries could be produced at the same time as hydrogen through photocatalyst-mediated solar water-splitting.
Mesocrystals of hematite can absorb a broad range of visible light. Associate Professor Tachikawa et al. discovered that by creating electrodes containing mesocrystals doped with two distinct metal ions, hydrogen peroxide and hydrogen could be produced safely, economically, and reliably. Disinfection, bleaching, and soil enhancement are just a few of the uses for hydrogen peroxide.
The research group’s next goal is to put this technology into practice. They will aim to integrate the cells into a compact module as a step toward societal deployment while continuing to improve the high efficiency of the produced photocatalyst electrode. They also intend to experiment with different materials and reaction systems to further enhance this mesocrystal technology.
Professor MUTO Shunsuke of Nagoya University’s Institute of Materials and Systems for Sustainability and the Japan Synchrotron Radiation Research Institute (JASRI) (Chief Researcher OHARA Koji and Researcher INA Toshiaki) collaborated on this project.
On March 23, 2022, the findings were published in Nature Communications (Nature Publishing Group) as an advance online publication.
Main Points
- Hematite is not suited for the production of hydrogen peroxide on its own. The researchers created a highly active composite oxide co-catalyst (*6) by doping hematite with various metal ions (tin and titanium) and sintering it.
- The capacity to make hydrogen peroxide in addition to hydrogen on-site will help to lower the cost of solar water splitting while also broadening the technology’s uses. Disinfection, bleaching, and soil enhancement are just a few of the uses for hydrogen peroxide.
Research Background
With the world’s environmental and energy problems worsening, hydrogen has gained traction as a potential next-generation energy source. Photocatalysts might theoretically use sunlight and water to create hydrogen, but such a system would need to achieve a conversion rate of 10% to be commercially viable.
Even if this efficiency is attained, it has been pointed out that the cost of hydrogen will not reach the necessary level. To address these concerns, there is great demand for the development of a cost-effective next-generation solar water-splitting system with high added value that can also produce other important chemicals while producing hydrogen.
Tachikawa et al. previously developed’mesocrystal technology,’ which entails precisely aligning nanoparticles in photocatalysts to control the flow of electrons and holes. They recently applied this approach to hematite and were able to greatly increase the light energy conversion efficiency.
Hematite has never been used in the manufacturing of hydrogen peroxide before. The researchers found that by coating the surface of hematite with a composite oxide of tin and titanium ions, they could create both hydrogen and hydrogen peroxide in a highly efficient and selective manner.
Research Methodology
Mesocrystal technology: The fundamental reason for the decrease in conversion rates in photocatalytic reactions is that the light-produced electrons and holes recombine before they can react with the molecules (in this case, water).
Using solvothermal synthesis, Tachikawa et al. constructed 3D structures of hematite mesocrystals with strongly aligned nanoparticles. By coating and sintering mesocrystals on a conductive glass substrate, they were also able to produce mesocrystal photoelectrodes for water splitting.
Formation of a co-catalyst for producing hydrogen oxide via dopant segregation: Photocatalytic water splitting with hematite usually results in the production of oxygen from the oxidation of the water.
After doping this hematite with tin ions (Sn2+) and titanium ions (Ti4+) and sintering it at 700°C, the tin and titanium dopants segregate, resulting in the development of a composite oxide (SnTiOx) co-catalyst with high selectivity for hydrogen peroxide production.
This structural alteration was discovered utilizing synchrotron-based X-ray total scattering studies on the SPring-8 facility’s beamlines BL01B1 and BLO4B2, as well as a high-resolution electron microscope with electron energy loss spectroscopy.
Photocatalyst formation and performance: When voltage was given to the photocatalyst electrode irradiated by artificial sunshine, the water-splitting reaction was accelerated. The photoelectric current density and Faradiac efficiency, which represent hydrogen generation efficiency and hydrogen peroxide selectivity, were evaluated by the researchers. When the photocatalyst was doped with only one of the metal ions, it was discovered that there were both positive and negative impacts on hydrogen and hydrogen peroxide production.
Hematite doped with both Sn2+ and Ti4+, on the other hand, may create hydrogen and hydrogen peroxide simultaneously in a very efficient and selective manner. Furthermore, first-principles calculations revealed that the SnTiOx co-catalyst on the hematite was made up of SnO2/SnTiO3 layers with a thickness of a few nanometers.
Further Developments
The study team was able to manufacture hydrogen peroxide in a highly efficient and selective manner by altering the surface of the hematite used for the photocatalyst.
Following that, the researchers want to improve the photocatalytic electrode and work with the industry to develop an onsite system for producing hydrogen and hydrogen peroxide from sunlight. They also intend to expand its use to additional metal oxides and reaction systems.
- Glossary Hematite (α-Fe2O3): A type of iron oxide ore. In addition to being safe, inexpensive, and stable (pH > 3), Hematite can absorb a wide range of visible light (approx. under 600nm).
- Photocatalyst: A substance that can act as a catalyst in reactions involving light illumination. The photocatalyst is placed to a light-absorbing conductive glass substrate (FTO glass). It’s also known as a photocatalyst anode or a photoanode when used as an electrode. The reaction to create hydrogen by splitting water molecules was carried out using a photocatalyst in this work.
- Hydrogen Peroxide: Disinfectants, detergents, cosmetics, bleach, and water purification are just a few of the applications for hydrogen peroxide (H2O2). The antraquinone process, which must be carried out in a large-scale chemical facility and produces organic waste and CO2, produces the majority of hydrogen peroxide. Furthermore, because hydrogen peroxide is unstable, transporting it is costly, and there are questions regarding its safety. This research team, on the other hand, devised a method for producing liquid H2O2 that is safe, low-cost, and environmentally friendly. Because H2O2 has a higher market value than O2, manufacturing hydrogen peroxide at the same time as hydrogen might save money.
- Mesocrystal: Porous crystal formations made up of three-dimensionally aligned nanoparticles. They are hundreds of nanometers or micrometers in size, with pores ranging from 2 to 50 nanometers between the nanoparticles.
- Doping: Changing the physical properties of the crystals by adding a little amount of another substance. Dopant diffusion occurs within the crystal structure, while dopant segregation is the process by which it is deposited on the surface.
- Co-catalyst: To facilitate the process, a material is added with the photocatalyst. To increase hydrogen peroxide production, a tin and titanium composite oxide was used in this work.
- Light energy conversion efficiency: The number of light particles utilized in the reaction (output) divided by the number of light particles fed into the reaction (input).
- Solvothermal method: A method of synthesizing solids using solvents at high temperatures and high pressures.
- SPring-8: SPring-8 is a massive synchrotron radiation facility in Harima Science Park in Hyogo Prefecture, Japan, that now offers the world’s most intense synchrotron radiation. Synchrotron radiation is created when electron beams that have been accelerated to nearly the speed of light are forced to travel in a curved route by a magnetic field, resulting in highly focused strong electromagnetic radiation. Spring-8 conducts a wide spectrum of synchrotron-based research, including nanotechnology, biology, and industrial applications. The Japan Synchrotron Radiation Research Institute (JASRI) is in charge of promoting the usage of SPring-8, which is controlled by RIKEN.
- Electron energy loss spectroscopy: By measuring the energy lost when the incident electron beam excites the electrons in the sample, spectroscopy can be used to determine the composition of a sample and the bonding state of its elements. It is possible to investigate minute regions at high resolutions by combining this approach with scanning transmission electron microscopy.
- Faradaic efficiency: The proportion of total electric current transferred into a system for the purpose of aiding an electrochemical reaction (in this case the production of hydrogen and hydrogen peroxide).
- First principle calculation: Based on Density Functional Theory, a way of determining the movement of electrons within a substance. It allows for the calculation of surface energy absorption properties as well as the optimal structure of a solid or particles.
- Anode: In electro-chemistry, the electrode where the oxidation reaction occurs.
- Cathode: In electro-chemistry, the electrode where the reduction reaction occurs.
