Reviewed by Lexie CornerDec 4 2024
Researchers are improving hydrogen production by developing photocatalytic systems that use sunlight to split water into hydrogen and oxygen, providing a clean alternative to fossil fuel-derived hydrogen. The study was published in Frontiers in Science.
Critical next steps for large-scale solar splitting of water using photocatalysts. Image Credit: Hisatomi et al. /Frontiers
Sunlight and water could be used to produce hydrogen fuel, offering an alternative energy source. Currently, most hydrogen is derived from natural gas, which does not support a transition away from fossil fuels.
Japanese researchers have developed photocatalytic sheets and a panel reactor prototype that demonstrate the potential for large-scale hydrogen production from water.
Sunlight-driven water splitting using photocatalysts is an ideal technology for solar-to-chemical energy conversion and storage, and recent developments in photocatalytic materials and systems raise hopes for its realization. However, many challenges remain.
Kazunari Domen, Professor and Study Senior Author, Shinshu University
Steam Power for the 21st Century
Photocatalysts play a key role in splitting water into hydrogen and oxygen using sunlight by facilitating chemical reactions triggered by light exposure. In one-step excitation systems, a single photocatalyst splits water into hydrogen and oxygen, though these systems have low solar-to-hydrogen conversion efficiency, making them simple but inefficient.
Two-step excitation systems, where one photocatalyst generates hydrogen and another produces oxygen, are currently more efficient.
Obviously, solar energy conversion technology cannot operate at night or in bad weather. But by storing the energy of sunlight as the chemical energy of fuel materials, it is possible to use the energy anytime and anywhere.
Dr. Takashi Hisatomi, Study First Author, Shinshu University
Although the solar-to-hydrogen conversion rate in these systems is higher, they are not yet fully operational. More durable and efficient photocatalysts are needed to withstand daily cycles of operation as the sun rises and sets.
Improving conversion efficiency is also essential to reduce reactor size and operating costs. Currently, refining hydrogen from natural gas remains less expensive.
Many water-splitting processes produce highly explosive oxyhydrogen gas. This risk can be reduced using design principles identified by Domen and Hisatomi's team or avoided by separating hydrogen and oxygen production. Their experiments showed that oxyhydrogen does not explode when ignited in small, narrow compartments. Additionally, materials like soft PVC plastic minimize destructive effects if ignition occurs.
The Future of Fuel
Domen and Hisatomi's team successfully demonstrated a 100 m2 reactor operating for three years. This reactor performed better under real-world sunlight conditions than in a lab setting.
In our system, using an ultraviolet-responsive photocatalyst, the solar energy conversion efficiency was about one and a half times higher under natural sunlight. Simulated standard sunlight uses a spectrum from a slightly high latitude region. In an area where natural sunlight has more short-wavelength components than simulated reference sunlight, the solar energy conversion efficiency could be higher. However, currently, the efficiency under simulated standard sunlight is 1 % at best, and it will not reach 5 % efficiency under natural sunlight.
Dr. Takashi Hisatomi, Study First Author, Shinshu University
The team emphasizes the need for further development of efficient photocatalysts and larger experimental reactors to improve the technology and exceed the 5 % efficiency threshold. Practical testing is essential to make hydrogen a viable fuel option.
They also stress the importance of establishing efficiency standards and safety regulations. Standardized methods for evaluating efficiency will help identify the most effective systems, while accreditation and licensing can ensure safe development of the technology.
Domen explained, “The most important aspect to develop is the efficiency of solar-to-chemical energy conversion by photocatalysts. If it is improved to a practical level, many researchers will work seriously on the development of mass production technology and gas separation processes, as well as large-scale plant construction. This will also change the way many people, including policymakers, think about solar energy conversion, and accelerate the development of infrastructure, laws, and regulations related to solar fuels.”
Journal Reference:
Hisatomi, T., et al. (2024) Photocatalytic water splitting for large-scale solar-to-chemical energy conversion and storage. Frontiers in Science. doi.org/10.3389/fsci.2024.1411644.