1. Development of photocatalysts producing clean hydrogen energy from sunlight and water

Development of photocatalysts producing clean hydrogen energy from sunlight and water

Towards a breakthrough in the efficiency and scalability of the artificial photosynthesis process

We use a great deal of energy in our daily lives, and more than 80% of our energy resources depend on fossil fuel resources. The continuing development of a society reliant on fossil resources may result in energy and environmental problems on a global scale due to resource depletion, destabilization of supply systems, and increased emissions of gases, such as nitrogen oxides, sulfur oxides, and carbon dioxide. These emissions have a greenhouse effect and cause acid rain.

Hydrogen is attracting attention as a new energy medium alternative to fossil fuels. Hydrogen emits only water as a byproduct when burned under ideal conditions and can be stored for a long time as a fuel with high energy density. It is also an important raw material in the chemical industry and can be used to recycle carbon dioxide and nitrogen to produce useful compounds. At present, however, hydrogen is produced from fossil resources. To solve the energy and environmental issues, it is necessary to develop technology that can produce hydrogen without using fossil resources.

The decomposition of water into hydrogen and oxygen using solar energy and photocatalysts is studied as a means of sustainable hydrogen production. The photocatalyst used here is a semiconductor. By absorbing light, negatively charged electrons and positively charged holes are generated, triggering redox reactions associated with decomposition of water. As the reaction proceeds with renewable solar energy, it is expected to enable the production of hydrogen without consuming fossil resources. There has been a great deal of research regarding such chemical processes that make use of solar energy to convert low-energy substances into high-energy substances, which are referred to as artificial photosynthesis.

To make effective use of solar energy, it is necessary to develop a photocatalyst that absorbs visible light and exhibits activity. In Domen-Hisatomi lab at Shinshu University, we mainly researches particulate oxynitrides, nitrides, and oxysulfide semiconductors as photocatalytic materials for water splitting (Fig. 1). Many of these materials are known to exhibit intense long-wavelength visible light absorption. To activate these photocatalytic materials, our laboratory is investigating the detailed correlation between material synthesis methods, physical properties, activation methods, and photocatalytic activity. To make the knowledge obtained in the laboratory useful in the real world, the production of photocatalytic reactors that can be deployed on a large scale is also under investigation.

   Water splitting reaction with particulate photocatalyst

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Fig. 1. Nitride- and sulfide-based particulate photocatalytic materials exhibiting various colors. (Water splitting reaction using photocatalyst sheet Modified from References 1 and 2 with permission)

Our laboratory study a photocatalyst sheet in which two types of particulate photocatalysts are immobilized on a conductive material as a reaction system that can efficiently decompose water under irradiation by sunlight. The photocatalyst sheet has a simple structure and can decompose water simply by installation in water and irradiation with sunlight. In addition, its activity does not decrease even if expanded to a large area. The highest level of solar water decomposition reaction using particulate photocatalysts has been achieved using a photocatalyst sheet with a solar-to-hydrogen energy conversion efficiency of over 1%.

In addition, we are researching panel reactors suitable for deploying photocatalyst sheets over a large area (Fig. 2). When the photocatalyst sheet is housed in the panel reactor, it can decompose water and release hydrogen and oxygen at a practically sufficient speed even with a water depth of only 0.1 mm. Using a shallow water depth, the weight of the water can be significantly reduced compared to conventional flask-type reactors. Therefore, the panel reactor may be manufactured with lightweight and inexpensive materials and is considered to be suitable for deployment over large areas. It has been confirmed that a prototype photocatalyst panel reactor 1 m2 in size can decompose water into hydrogen and oxygen under sunlight irradiation without impairing the original activity of the photocatalyst sheet.

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Fig. 2. (a) A 625 cm2 scale photocatalyst panel reactor. (b) A 3 m2 scale photocatalyst panel reactor module. (c) An overview of the 100 m2 scale photocatalyst panel reactor. (d) Evolution of oxyhydrogen gas from the 100 m2 scale photocatalyst panel reactor. (Modified from Nishiyama, H., Yamada, T., Nakabayashi, M. et al. Photocatalytic solar hydrogen production from water on a 100 m2-scale.Nature 598, 304-307 (2021). Copyright © 2021, The Authors, under exclusive licence to Springer Nature Limited.)

Our laboratory is currently researching photocatalytic materials that can decompose water under long-wavelength visible irradiation, sheet powder photocatalysts, and expanding panel reactors to a scale of 100 m2, from all perspectives of physical chemistry, catalytic chemistry, and chemical engineering. Very recently, we reported the results of the development and demonstration experiment of a 100 m2scale photocatalyst panel reaction system producing and purifying solar hydrogen. We are also working on the evaluation and optimization of the energy efficiency and economic efficiency of the entire artificial photosynthesis system in collaboration with outside researchers.


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