Splitting water into hydrogen and oxygen using solar energy holds the promise of truly renewable fuel – but until now, it hasn’t been feasible outside the laboratory. Writing for Frontiers in Science, renowned experts in the field outline crucial new advances that could make solar production of hydrogen fuel a reality, and call for more research and the development of standards and regulations to take green fuel from proof of concept to practical solution.

Splitting water into hydrogen and oxygen using solar energy holds the promise of truly renewable fuel – but until now, it hasn’t been feasible outside the laboratory. Writing for Frontiers in Science, renowned experts in the field outline crucial new advances that could make solar production of hydrogen fuel a reality, and call for more research and the development of standards and regulations to take green fuel from proof of concept to practical solution.

Steam power for the 21st century

To use sunlight to split water into oxygen and hydrogen, we need photocatalysts. Under light, these catalysts promote chemical reactions which split the water. In one-step excitation systems, the photocatalyst breaks water down to hydrogen and oxygen. These systems are simple but inefficient, with a very low solar-to-hydrogen energy conversion rate. More efficient at present are two-step excitation systems, where one photocatalyst evolves hydrogen from water and the other evolves oxygen from water.

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

These systems have a higher solar-to-hydrogen energy conversion rate – but they’re not yet ready for use. We need to identify the most effective and sustainable photocatalysts, which must be robust enough to tolerate daily start-up and shut-down operations as the sun rises and sets. We also need to increase the efficiency of the conversion as much as possible, to minimize the amount of physical space needed for a reactor and make the reactor cost-effective to run - right now, using natural gas to refine hydrogen fuel is still cheaper.

An additional problem is that many methods of splitting oxygen and hydrogen produce oxyhydrogen, which is highly explosive. This risk can be eliminated by producing oxygen and the hydrogen separately, or managed by using design criteria identified by Domen and Hisatomi’s team. By experimenting with oxyhydrogen, they determined that if the gas is ignited in a small, narrow compartment, it does not explode. Materials are also important: soft PVC plastic does not explode destructively if oxyhydrogen ignites.

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The future of fuel

Domen and Hisatomi’s team have already produced a successful proof of concept, running a 100m2 reactor for three years. This reactor even performed better faced with real-world sunlight than it did in under laboratory 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,” said Hisatomi. “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.”

To move the technology forward and break that 5% barrier, the team says that more researchers need to develop more efficient photocatalysts and build larger experimental reactors. More real-world experiments are needed to develop the technology to the point where hydrogen is a feasible fuel option.

As part of this, the team emphasize the need to institute safety regulations and efficiency standards. An accreditation body and licensing would help to ensure the safe development of the technology, while standardized methods for determining efficiency will help identify the most effective systems.

“The most important aspect to develop is the efficiency of solar-to-chemical energy conversion by photocatalysts,” explained Domen. “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.”

This article is part of the Frontiers in Science multimedia article hub ‘Solar water splitting for green hydrogen’, which also features an editorial, viewpoint, and a policy outlook from other eminent experts: Dr Johan Martens (KU Leuven, Belgium), Dr Markus Antonietti and Dr Christian Mark Pelicano (Max Planck Institute of Colloids and Interfaces, Germany), and Dr Leticia M. Torres-Martínez (Universidad Autónoma de Nuevo León, Mexico) —as well as an explainer with infographics. 

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