Scientists from Japan have developed new materials and methods to increase the efficiency and safety of a proof-of-concept green hydrogen reactor.

The 100m² reactor operated successfully outdoors, outside of laboratory conditions, for three years. Nevertheless, the authors conclude that many challenges remain to be solved before the technology can be a viable component of the clean energy transition.  

Hydrogen is a lightweight, energy-rich fuel famously used to power some rockets. While most industrial hydrogen is still generated using fossil fuels, so called ‘green hydrogen’ is coupled with renewable energy sources such as wind and solar.

“Obviously, solar energy conversion technology cannot operate at night or in bad weather,” says first author Takashi Hisatomi of Shinshu University. “But by storing the energy of sunlight as the chemical energy of fuel materials, it is possible to use the energy anytime and anywhere.”

Hisatomi and colleagues converted sun and water into green hydrogen using photocatalysts, materials that take in light and promote water-splitting chemical reactions to produce oxygen and hydrogen gasses.  

Some photocatalysts split water into hydrogen and oxygen gasses simultaneously but these tend to be less efficient. Two-step systems that separate the production of the two gasses are more efficient and safer but are also more complex as electrons need to be passed between the two reactions.

The team developed a screen-printing technique using ink containing photocatalyst powder to make scalable photocatalyst sheets. Within the sheet, two-step photocatalysts were connected by a conductive layer made of gold or indium tin oxide.

The authors consider these sheets to be a “game-changer” as they are easier to maintain, operate and control for quality than the liquid slurry used in previous designs. Their large-scale, outdoor prototype showed that the technology is safe and has an accompanying improvement in efficiency under natural light conditions.

“In our system, using an ultraviolet-responsive photocatalyst, the solar energy conversion efficiency was about one and a half times higher under natural sunlight,” says Hisatomi.

However, current photocatalysts only achieve around a 1% solar energy to green hydrogen fuel conversion rate. The authors estimate that this rate will need to be at least 5% for reactors to be cost effective, space efficient and competitive with hydrogen made from natural gas.

“The most important aspect to develop is the efficiency of solar-to-chemical energy conversion by photocatalysts,” says senior author Kazunari Domen of Shinshu University.

“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.”

The authors also highlight the importance of governments in setting safety and environmental standards as green hydrogen technology improves.

“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,” says Domen.

The research is published in the journal Frontiers in Science as part of a multimedia article hub on green hydrogen that includes the primary research as well as editorials and a policy outlook.