Researchers around the world are working to find clean, efficient, and safe ways to produce and store fuel. Meanwhile, plants are expertly converting natural resources into energy, by way of photosynthesis. This is why the U.S. Department of Energy launched the Joint Center for Artificial Photosynthesis (JCAP) at Caltech in 2010. The research hub has one main goal: to develop artificial solar-fuel generation technology. Five years into the program, researchers have now developed an artificial leaf—a complete, efficient, safe, integrated system that uses the power of the sun to split water into hydrogen and oxygen to make fuel. “This new system shatters all of the combined safety, performance, and stability records for artificial leaf technology by factors of 5 to 10 or more,” says Nate Lewis, the JCAP’s scientific director.
The prototype is just one square centimeter, and converts ten percent of the sun’s energy into stored energy in the form of a chemical fuel. It can operate continuously for more than 40 hours. The system has three main parts: two electrodes and a membrane. The photoanode uses sunlight to oxidize water molecules and generate protons, electrons, and oxygen gas. A photocathode then recombines the protons and electrons to form hydrogen gas. The most critical component is a semi-permeable plastic membrane. It keeps oxygen and hydrogen gases separate (in order to prevent explosion), while still allowing the ions to flow into the electrical circuit in the cell.
The electrodes are made of semiconductors, which absorb light efficiently but rust easily when exposed to water. The researchers solved this problem by coating the electrodes with titanium dioxide (the material found in white paint, toothpaste, and sunscreen) to prevent corrosion, in a layer that is thin enough to still allow light and electrons to pass through. Active, inexpensive catalysts also speed up the water-splitting reaction. The system is described in the August 27 online issue of the journal Energy and Environmental Science.
“Our work shows that it is indeed possible to produce fuels from sunlight safely and efficiently in an integrated system with inexpensive components,” said Lewis. 
“Of course, we still have work to do to extend the lifetime of the system and to develop methods for cost-effectively manufacturing full systems, both of which are in progress.”
