Abundant solar pixels on Earth produce hydrogen for weeks

Devices made from readily available carbon-based materials and oxides can produce clean hydrogen from water for weeks, according to new research.

The findings, co-led by Dr Virgil Andrei, a research fellow at St John’s College, University of Cambridge, with academics from Imperial College London, could help overcome one of the key problems in solar fuel production, where today’s abundant terrestrial light-absorbing materials are limited either by their performance or by their stability.

Underexplored materials for light harvesting

Hydrogen fuel will play a vital role in the transition to full decarbonisation and reaching the UK’s target of net zero emissions by 2050. With most hydrogen currently supplied from fossil fuels, researchers are currently working to find ways to generate hydrogen more sustainably. One way to do this is to make devices that can harvest sunlight and split water to produce green hydrogen.

While many light-absorbing materials have been tested for green hydrogen production, most degrade quickly when immersed in water. For example, perovskites are the fastest growing materials in terms of light gathering efficiency, but are unstable in water and contain lead. This presents a risk of leakage; therefore, researchers have worked to develop lead-free alternatives.

Bismuth oxyiodide (BiOI) is a non-toxic semiconductor alternative that has been overlooked for solar fuel applications due to its poor stability in water. But based on previous findings about BiOI’s potential, the researchers decided to revisit the material’s promise for green hydrogen production.

Dr Robert Hoye, Senior Lecturer in the Department of Materials at Imperial College London, explained: “Bismuth oxyiodide is a fascinating photoactive material that has energy levels at the right positions for the separation of water. A few years ago, we demonstrated that BiOI solar cells are more stable than those using state-of-the-art perovskite light absorbers. We wanted to see if we could translate this stability into green hydrogen production.

Professor Judith Driscoll, Department of Materials Science and Metallurgy, University of Cambridge, said: “We have been working on this material for some time, due to its wide potential applications, as well as its simplicity of fabrication, its low toxicity and its good It was great to combine the expertise of the different Cambridge and Imperial research groups.”

Breakthrough in solar fuel production

The research team created devices that mimicked the natural photosynthesis process occurring in plant leaves, except they produced fuels like hydrogen instead of sugars. These artificial leaf devices were made from BiOI and other sustainable materials, harvesting sunlight to produce O2H2 and co.

Researchers have found a way to increase the stability of these artificial leaf devices by inserting BiOI between two oxide layers. The robust structure of the oxide-based device was additionally coated with a water-repellent graphite paste, which prevented the ingress of moisture. This extended the stability of the bismuth oxyiodide light-absorbing pixels from minutes to months, including the time the devices were stored.

This is an important discovery that turns BiOI into a viable light harvester for stable production of green hydrogen.

“These oxide layers improve the ability to produce hydrogen compared to stand-alone BiOI,” said Dr Robert Jagt (Department of Materials Science and Metallurgy, University of Cambridge), one of the co – main authors.

Researchers further found that artificial leaf devices with multiple light-gathering areas (called “pixels”) performed better than conventional devices with a single, larger pixel of the same total size. This discovery could make scaling up new light collectors much easier and faster for sustainable fuel production.

Dr Virgil Andrei, co-lead author from Cambridge’s Department of Chemistry, explains: “Even if some pixels were faulty, we were able to disconnect them, so they wouldn’t affect the rest. This meant we could maintain the performance of small pixels on a larger area.” This increased performance allowed the device not only to produce hydrogen, but also to reduce CO2 syngas, an important intermediate in the industrial synthesis of chemicals and pharmaceuticals.

To look forward

The results demonstrate the potential of these new devices to challenge the performance of existing light absorbers. New ways to make BiOI artificial leaf devices more stable can now be transferred to other novel systems, helping to bring them to commercialization.

“This is an exciting development! Few solar fuel systems today exhibit stabilities compatible with real-world applications. circular fuel,” said Professor Erwin Reisner (Department of Chemistry, Cambridge), one of the corresponding authors.

The results were published in the journal Natural materials.

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