Make way for the artificial leaf. Researchers have created an electrically conductive and water-resistant film that simplifies the construction of man-made devices that would mimic the ability of plants to convert sunlight and water into fuel.
Leaves use a chemical process called photosynthesis to convert sunlight, water, and carbon dioxide into oxygen and fuel in the form of carbohydrates, or sugars.
For years now, scientists have been trying to mimic this process. Although solar panels already efficiently convert sunlight into electricity, artificial photosynthesis promises a lot more. It aims to use sunlight to split water molecules, releasing oxygen and non-polluting hydrogen fuel that could “be stored for generating electricity or for heating, transported and consumed at will,” said Dimitrios Pantazis, biochemist at the Max Planck Institute for Chemical Energy Conversion in Germany.
In the longer term, such ‘solar fuels’ could also be used in the most energy-intensive sector, transportation.
And artificial leaves could also help address the impact of global warming. “When we burn biomass, food, or fossil fuels, we release CO2 with serious implications for climate change,” said James Barber, a biochemist at Imperial College London. “Artificial photosynthesis would be a way to overcome this. But new technology is required.”
Current prototypes that mimic photosynthesis use two electrodes (a photoanode and a photocathode) and a plastic membrane. The first electrode oxidises water molecules with sunlight and generates oxygen gas, protons, and electrons. The photocathode then recombines the protons and electrons to form hydrogen.
But there is a problem. The electrodes are usually made of semiconductors such as silicon or gallium arsenide, also used in solar panels – which rust when exposed to water. One way to deal with the issue is to cover electrodes with a protective coating. "You want the coating to be many things: chemically compatible with the semiconductor it's trying to protect, impermeable to water, electrically conductive, highly transparent to incoming light, and highly catalytic for the reaction to make oxygen and fuels," said chemist Nate Lewis of the California Institute of Technology in Pasadena, California, a co-author of the latest study.
Previous attempts to develop such a coating involved a complex film that consisted of two layers; the main ingredient typically used was titanium dioxide. But Lewis’s team has instead opted for nickel oxide, and managed to reduce the required protective layers of the photoanode from two to one – enabling the use of a wider range of materials for the construction of an artificial leaf “without sacrificing stability and efficiency,” said Pantazis.
In the journal Proceedings of the National Academy of Sciences (PNAS), Lewis and colleagues write that to create the film, they smashed atoms of argon into a pellet of nickel atoms at high speeds in an oxygen-rich environment. The nickel fragments that popped off the pellet reacted with the oxygen atoms and produced an oxidised form of nickel.
Once applied onto a semiconductor, the film could lead to safe, efficient solar-fuel generators that could be rolled out on a roof as flexible and thin solar panels.
But appropriate working technology is still a long way off. For instance, at the moment the film can only be applied on a specific type of cell, which limits its potential future uses. “A more robust construction of the water-oxidising photoanode, especially one that allows the use of existing high-efficiency semiconductors, could mean lower manufacture and maintenance costs,” said Pantazis – all important if artificial leaves are ever to become a commercial reality.