Solar fuel made from artificial leaves

Researchers at the University of Cambridge, with academics at Imperial College London, have developed a bismuth leaf that may one day be able to produce clean hydrogen from water and sunlight, overcoming some of the basic challenges of producing this emerging green energy fuel.

Hydrogen fuel is expected to play a critical role in the transition to decarbonization and in reaching many countries' goals of net-zero emissions. With most hydrogen currently supplied from fossil fuels, scientists have been working toward finding ways to generate hydrogen more sustainably.

One such method is technologies that can harvest sunlight and split water to produce green hydrogen.

Halide perovskites are a family of materials that have shown potential for high performance and low production costs in solar cells. The name "perovskite" comes from the nickname for their crystal structure, although other types of non-halide perovskites (such as oxides and nitrides) are utilized in other energy technologies, such as fuel cells and catalysts.

Perovskite solar cells have shown remarkable progress in recent years with rapid increases in efficiency, from reports of about 3% in 2009 to over 25% today. While perovskite solar cells have become highly efficient in a very short time, many challenges yet remain before they can become a competitive commercial technology.

While many light-absorbing materials have been tested for green hydrogen production, most degrade relatively quickly when submerged in water. Perovskites are the fastest-growing materials in terms of light-harvesting efficiency but are also unstable in water. They also contain lead, which presents a risk of leakage; therefore, researchers have been working to develop lead-free alternatives.

"Bismuth oxyiodide is a fascinating photoactive material that has energy levels at the right positions for water splitting," said Robert Hoye, lecturer in the department of materials at Imperial College London. "A few years ago, we demonstrated that BiOl solar cells are more stable than those using state-of-the-art perovskite light absorbers. We wanted to see if we can translate that stability to green hydrogen production."

Bismuth oxyiodide (BiOl) is a non-toxic semiconductor alternative previously overlooked for solar fuel applications due to its poor stability in water. However, based on previous findings, researchers decided to revisit the promise of this material for the production of green hydrogen.

"We have been working on this material for some time, due to its wide-ranging potential applications, as well as its simplicity of fabrication, low toxicity and good stability," said Judith Driscoll of the department of materials science and metallurgy at University of Cambridge. "It was great to combine the expertise of the different research groups across Cambridge and with Imperial."

The team of researchers created devices that mimicked the natural photosynthesis process occurring in plant leaves, except they produce hydrogen instead of sugars. These artificial leaf devices were made from BiOl and other sustainable materials, harvesting sunlight to produce oxygen, hydrogen, and carbon monoxide.

The researchers found a way to increase the stability of these artificial leaves by inserting BiOl between two oxide layers. The robust oxide-based device structure was then coated with a water-repellant graphite paste, preventing moisture infiltration.

This prolonged the stability of the bismuth oxyiodide light-absorption from minutes to a couple of months, including the time the devices were left in storage.

This significant finding transforms BiOl into a viable light harvester for stable green hydrogen production.

"These oxide layers improve the ability to produce hydrogen compared to stand-alone BiOl," said Robert Jagt, department of materials science and metallurgy at Cambridge.

The researchers also discovered that artificial leaf devices comprising multiple light-harvesting areas (called pixels) demonstrated a higher performance over conventional devices with a single larger pixel of the same total size. This finding could make the scale-up of novel light harvesters much easier and faster for sustainable fuel production.

"Even if some pixels are faulty, we were able to disconnect them, so they don't affect the rest," said Virgil Andrei from the department of chemistry at Cambridge. "This meant we could sustain the performance of the small pixels on a larger area."

Reportedly, this increased performance enabled the device to not only produce hydrogen but also reduce carbon dioxide to synthesis gas, an important intermediate in the industrial synthesis of chemicals and pharmaceuticals.

The findings by the UK researchers demonstrate the potential for these new devices to challenge the performance of existing light absorbers. Furthermore, the new ways of making these BiOl artificial leaves more stable can also be applied to other novel systems, helping to bring other developments towards commercialization.

"This is an exciting development," said Erwin Reisner, department of chemistry at Cambridge. "At the moment, few solar fuel systems show stabilities which are compatible to real-world applications. With this work, we make a step forward towards establishing a circular fuel economy."

The findings have been published in the journal "Nature Materials."