Green hydrogen produced by solar pixels

Researchers from the University of Cambridge and Imperial College London have used a photoactive material as a non-toxic semiconductor alternative to produce green hydrogen. With hydrogen fuel being key in the transition to full decarbonisation and achieving the ambition of net zero emissions by 2050, devices made from readily available oxide and carbon-based materials were shown to make long-lasting clean hydrogen from water.

With most hydrogen coming from fossil fuels, identifying alternatives is crucial, with one method being to use devices that harvest sunlight and split water. As reported in Nature Materials [Andrei et al. Nat. Mater. (2022) DOI: 10.1038/s41563-022-01262-w], here the team looked to improve sustainable solar fuel production as current earth-abundant light-absorbing materials, such as perovskites, are limited due to their performance or stability.

Oxide-based materials are great candidates for energy applications, as they are stable when exposed to air or moisture. Here, bismuth oxyiodide (BiOI), not previously tried for solar fuel applications, was used an effective light harvester. BiOI has energy levels at the right positions for water splitting, and also offers ease of fabrication, low toxicity and useful stability.

The team made artificial leaf devices that copy the natural photosynthesis process in plant leaves, but here make fuels such as hydrogen rather than sugars. The devices were made from BiOI and other sustainable materials, harvesting sunlight to produce O₂, H₂ and CO. The performance and stability of the light absorber was significantly improved by sandwiching it between robust oxide and carbon layers, with the structure being further coated with a water-repellent graphite paste to stop moisture infiltration.

The oxide layers improve the ability to produce hydrogen when compared to stand-alone BiOI, while the artificial leaf devices made of multiple light harvesting areas called pixels offered better performance than standard devices with a single larger pixel of same total size. The approach extended the stability of the BiOI light-absorbing pixels from minutes to a couple of months.

Any faulty pixels can be disconnected and don’t affect the remainder, which meant they could sustain the performance of the small pixels on a larger area. This improved performance also allows such devices to not only make hydrogen but also reduce CO₂ to synthesis gas, an key intermediate in the industrial synthesis of chemicals and pharmaceuticals.

As co-lead author Robert Hoye told Materials Today, “These devices offer a more sustainable way of producing these valuable fuels over the polluting methods that are currently used industrially. As such, they can help us in our ambitions to reach net zero greenhouse gas emissions.” The team could now examine how these concepts translate to different light absorbers and device structures, and whether the multiple pixel design could be applied on a larger scale.

“These devices offer a more sustainable way of producing these valuable fuels over the polluting methods that are currently used industrially. As such, they can help us in our ambitions to reach net zero greenhouse gas emissions.”Robert Hoye