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Sustainable Energy

A Practical Artificial Leaf Begins to Unfold

A new prototype that turns the sun’s energy into hydrogen fuel could point the way to commercially viable artificial photosynthesis.

Fuels produced using the sun’s power offer many advantages over conventional solar power.

Fuel made inexpensively through artificial photosynthesis could be the ultimate renewable energy source. Now researchers at Caltech say they have built the first prototype of an artificial leaf that is both efficient and safe. They say the device, which uses light and water to make clean hydrogen fuel, could lead to a commercially viable version in the near future.

The technology harnesses the sun’s energy to split water into hydrogen and oxygen; the hydrogen could be used as fuel for vehicles or be stored for use in generating electricity. A photovoltaic material captures photons and generates electrons, which are passed on to chemical catalysts to do the water splitting. 

To be commercially viable, the system, which is inspired by plant photosynthesis, must be efficient, stable, inexpensive, and safe, says Nate Lewis, a professor of chemistry at Caltech, who led the new research. Some previous demonstrations of artificial leaf technology have shown impressive efficiency but haven’t been stable, he says. “Nothing is close, in terms of efficiency and stability and safety all combined at once, to what we’ve done here,” says Lewis.

A number of groups are pursuing artificial leaf technologies, and are taking a range of different approaches. The prize is a technology that would be better than solar power because its product is an easily stored fuel instead of intermittent electricity. But achieving artificial photosynthesis is a steep technical challenge that requires getting multiple different materials and catalysts to work together.

Key to the enhanced stability is a chemical called titanium dioxide, which the researchers used to protect the photovoltaic materials from the corrosion that occurs due to the chemical reaction that makes oxygen. They applied it as a very thin coating, using a process already used by the semiconductor industry. Crucially, the protective material is compatible with a membrane the group developed to keep the oxygen and hydrogen gas from mixing, which is an explosion hazard. It is also compatible with cheap catalysts that are already used commercially and are made of abundant materials, says Lewis.

The newly demonstrated system represents the accomplishment of a five-year project of the Joint Center for Artificial Photosynthesis, which received $122 million from the federal government in 2010 and will soon be up for new funding. The objective was to demonstrate the direct production of fuel from the sun with efficiency 10 times greater than plants.

The technology still must get a lot cheaper to be competitive with other sources of fuel. Ultimately, Lewis envisions a system that can be “rolled out like a piece of plastic” over a large area. What’s important now, he says, is the proof that achieving high efficiency safely and in a relatively stable manner is even doable. “We will work on the cost after we are walking instead of crawling.”

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