Efficient hydrogen conversion achieved through solar water splitting using hematite

A research group led by Associate Professor Takashi Tachikawa of Kobe University’s Molecular Photoscience Research Center has succeeded in developing photocatalysts that can convert an efficient level of hydrogen from water using solar light. It is hoped that methods like this one, which uses titanium-modified hematite mesocrystal-based photoanodes, could form the foundation for a commercial solar water splitting system. This would allow the clean fuel hydrogen to be produced more cheaply and easily than before, making it a viable source of renewable energy.

This was a joint research project with Nagoya University’s Institute of Materials and Systems for Sustainability (Professor Shunsuke Muto) and the Japan Synchrotron Radiation Research Institute (JASRI) (Chief Researchers Koji Ohara and Kunihisa Sugimoto).

The results of this study were first published in the online journal Nature Communications on October 23 2019.

Background

As environmental and energy problems increase, hydrogen has been receiving more attention as a possible clean energy source of the future. Photoelectrochemical (PEC) water splitting (also known as solar water splitting) has been proposed as renewable way to produce hydrogen. In theory, it is a simple method which requires a photocatalyst and sunlight to obtain hydrogen from water. Industrial-scale PEC water splitting systems would lower the commercial price of hydrogen, making it a practical energy source.

However, in order to make PEC water splitting a viable method of producing hydrogen on a large scale, the light-to-energy conversion efficiency needs to be improved. When the photocatalyst is exposed to light, electrons and holes (made by the electrons) are formed on the surface of the photocatalyst. These charges then dissociate to produce hydrogen and oxygen from water molecules. Although experiments with many different photocatalysts have been carried out, a reoccurring problem is that the electrons and holes recombine on the catalyst surface, lowering the conversion efficiency. Other issues include catalyst durability and cost.

In order to control the dynamics of electrons and holes via precise alignment of nanoparticles, Associate Professor Tachikawa et al developed a method using ‘hematite mesocrystal-based photoanodes’ as a photocatalyst. They succeeded in producing a highly efficient light to energy conversion. Mesocrystals are superstructures of nanoparticles with highly ordered structures. This makes them efficient for charge separation and transport. Furthermore, hematite is an abundant natural mineral- making this a potentially low cost method.

Research methodology

Mesocrystal-based photoanodes:

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