In a piece of good news to start off the year, researchers from Ulsan National Institute of Science and Technology in South Korea and Helmholtz-Zentrum Berlin in Germany have announced an efficiency breakthrough for their ‘artificial leaf’ project, which is really more like artificial seaweed. The research, published in Nature Communications, looks at mimicking aquatic plant photosynthesis to split water into its constituent elements.
The theory of solar water splitting is well understood, but finding the right photoelectrode materials has proved difficult. The Korean/German team favoured metal oxide semiconductors for their robustness and low cost, but needed to enhance their poor solar-to-hydrogen conversion rates. The team’s approach was to combine two established metal oxide photoanodes (BiVO4 and Fe2O3), which complement each other’s receptive properties on the solar spectrum. Unfortunately, when the materials were combined directly in a bilayer formation, their performance dropped markedly. Instead, the team came up with a ‘hetero-type dual photoelectrode (HDP)’, in which the materials were placed in parallel to form a tandem cell. BiVO4 as the front photoanode can utilise smaller wavelength photons, while longer wavelengths pass through to be collected by the Fe2O3 layer.
Interestingly, the idea mimics the behaviour of seaweed colonies and marine algae, which have evolved to layer themselves in different coloured bands at different depths in the sea. Green seaweed floats in the shallows and is receptive to red light, while the deeper-penetrating blue light is picked up by red weed. A single seaweed colony can vary its colours selectively as it descends in depth, optimising its photosynthesis for each layer.
The tandem cell set a new performance benchmark for metal oxide photoanodes, and the team believe that the HDP concept could prove a route to practical solar hydrogen production. “We aim to achieve 10% enhanced light harvesting efficiency within three years,” said Professor Jae Sung Lee. “This technology will greatly contribute to the establishment of the renewable-energy-type hydrogen refueling station by supplying cheap fuel for hydrogen fuel cell vehicles.