Using nothing but sunlight and photocatalysts, a 100 square meter (1,076 square foot) reactor produced carbon-free hydrogen for three years, demonstrating the potential of the concept. The approach is still considerably less efficient than the more common method, where photovoltaic panels turn the sunlight to electricity first, but in theory the direct approach could bring production costs down further.
Hydrogen is the ultimate clean fuel – when burned or reacted in a fuel cell it produces nothing but water. We already use vast amounts of hydrogen, for example, to make fertilizer and methanol. However, the majority of this is made using fossil fuels (so-called “grey hydrogen”), releasing carbon dioxide. A better way is essential for the current uses, let alone the widespread (although highly contested) dreams of using hydrogen for non-polluting transport, heating, or steel production.
Green hydrogen has no such pollution, relying instead on solar or wind energy to split water molecules into their constituent elements. Although small, this is growing fast, but overwhelmingly relies on conversion to electricity as an intermediary step. Professor Takashi Hisatomi and Professor Kazunari Domen of Shinshu University think we can do better by skipping that stage, and have demonstrated the possibility, although not yet the practicality.
“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,” Domen said in a statement.
As the name suggests, photocatalysts stimulate chemical reactions in the presence of light. Although there are many reactions where this could be useful, the breakdown of water into hydrogen and oxygen is where the world-changing potential lies.
A team led by Hisatomi and Domen built a 100 m2 prototype reactor using sheets of the photocatalyst SrTiO3:Al. Several cocatalysts were placed in solution over these sheets and the water evaporated. Water flowed past the catalysts and gasses were drawn off in tubes.
No energy transformation is 100 percent efficient, so each extra stage lowers the ceiling for maximum total efficiency. For example, the most efficient solar cells in the world struggle to capture 30 percent of the energy in sunlight into electricity, and those in mass production are barely over 20 percent. When the electricity is applied to water, inefficiency occurs again, particularly if cheap nickel-based catalysts are used instead of those made from precious metals. Extensive work is being done to improve this, but even if each stage is 30 percent efficient, the combination means hydrogen fuel ends up with just 9 percent of the Sun’s energy.
If suitable photocatalysts had an efficiency of 10 percent, it would mean more hydrogen for the same amount of sunlight. That would probably enable green hydrogen to finally compete with the grey product in terms of price.
Unfortunately, that is not currently possible. Lab-based studies of direct conversion using simulated sunlight have produced pitifully low efficiencies. It’s normal for any innovation to suffer even greater losses when taken into the real world, but in this case, the builders were in for a pleasant surprise.
“In our system, using an ultraviolet-responsive photocatalyst, the solar energy conversion efficiency was about one and a half times higher under natural sunlight,” said Hisatomi. That’s a consequence of the global standard for simulated sunlight being based on conditions at higher latitudes than Tokyo, the test reactor’s location – the historical northern bias of science has left a legacy even in solar research. Reactors in the tropics, where sunlight has an even higher ultraviolet component, should do better still.
Nevertheless, the work is still nowhere near where it needs to be. “Currently the efficiency under simulated standard sunlight is 1% at best, and it will not reach 5% efficiency under natural sunlight,” Hisatomi said.
Low efficiency doesn’t just drive up costs: inefficient reactors also take up impractical amounts of room, since they need so much sunlight falling on them.
Larger reactors would improve the efficiency somewhat, but real progress depends on finding more efficient photocatalysts. Work in this area started with titanium dioxide, which is common but inefficient, and now focuses on much more complex catalysts such as RhCrOx/SrTiO3:Al.
Other issues that need to be dealt with apply to all forms of water splitting, such as preventing the hydrogen and oxygen from recombining, sometimes explosively, before being safely stored separately.
The four main requirements of direct splitting of water with sunlight is to become commercially viable.
Image Credit: Hisatomi et al/Frontiers Usage Restrictions
To get there, Hisatomi and Domen argue we need a global accreditation process, such as that for claims of solar cell efficiency, with consistent safety regulations and efficiency standards.
There is currently a very large cost difference between hydrogen produced this way and the polluting version. However, Domen is not disheartened. If better photocatalysts emerge, he said; “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.”
Some hope that mining natural hydrogen, recently found to be much more abundant than previously thought, will save us, but the practicality of widespread harvesting has barely been studied.
A review of the state of direct hydrogen production using sunlight is published open access in the journal Frontiers in Science. The team’s demonstration of a 100m2 reactor is published open access in the journal Nature.
