Taking charge of splitting water

Hydrogen produced using renewable energy, especially solar energy, is expected to become the dominant sustainable fuel of the future. It is known as green hydrogen because it could offer an ideal energy cycle, using sunlight to split water into hydrogen and oxygen, then recombining these products into water when the energy is required. The basic chemistry can already be achieved, but producing green hydrogen on a feasible economic scale remains challenging.

Researchers at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, Imperial College London, and the University of Oxford, UK, have collaborated to build catalytic nanoparticles that significantly advance the economic viability of using sunlight to split water to make hydrogen.

The key step in the activity of any such photocatalyst is to absorb light in a way that kicks electrons into high energy states of separated negative and positive charge. Traditional inorganic semiconductors can achieve this, but they only use a small fraction of the available solar energy; generally less than 5%.

Jan Kosco of the KAUST Solar Center explains that the team developed organic semiconductors that can be tuned by chemical modifications to harvest a significantly larger fraction of light, greatly improving hydrogen production efficiency.  

The key to this success was the fabrication of catalysts that maintain charge separation for longer than previously achieved, increasing the opportunity to exploit it for powering water-splitting before the separated charges recombine. The critical breakthrough was to gain control of the electrochemical activity at the junction between a charge-donating and a different charge-accepting polymer. 

“To the best of our knowledge, the ability to generate such long-lived reactive charges in an organic semiconductor photocatalyst is unique to organic semiconductor heterojunction nanoparticles,” says Kosco.

Chemist, Garry Rumbles,  at the National Renewables Energy Center in Boulder, Colorado, USA, who was not involved in the research, says “This is an excellent ground-breaking paper.” He feels the work is leading the field of organic-based photocatalysts in new and very promising directions.

The researchers are now continuing to investigate different semiconductor blends to explore and refine their full potential. Kosco says they also hope to design nanoparticle photocatalysts to drive other reactions, including oxygen generation and converting carbon dioxide into useful chemical products.

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