Bionic Leaf Converts Solar Energy Into Liquid Fuel

Photosynthesis fixes CO2 from the air by using sunlight, but this ‘bionic’ leaf’ convert CO2 directly into fuel with the help of a hydrogen-fixing bacterium. . David Leo/Flickr

Plants are able to feed themselves through the miracle of photosynthesis. Scientists, inspired by this process, have also figured out how to convert the sun's energy into useful power — most particularly, electricity. But what if we could take it one step further and transform sunlight directly into liquid fuel? Such a breakthrough could end our dependence on fossil fuels to power our vehicles, for instance.

That breakthrough may have arrived. A team of researchers from Harvard University has developed a "bionic leaf" that not only uses sunlight to split water into hydrogen and oxygen, but is also capable of using that hydrogen in combination with carbon dioxide to generate isopropanol, a liquid fuel, according to Science Daily.

The system is a step up from a previous invention, the artificial leaf, which can mimic the real leaves of plants by splitting water into hydrogen and oxygen. The artificial leaf was developed by Daniel Nocera, who is also senior author on the new bionic leaf study.

To upgrade the artificial leaf to "bionic" status, researchers engineered a bacterium that can convert carbon dioxide plus hydrogen into the liquid fuel isopropanol.

"This is a proof of concept that you can have a way of harvesting solar energy and storing it in the form of a liquid fuel," said Pamela Silver, one of the study's authors. "Dan's formidable discovery of the catalyst really set this off, and we had a mission of wanting to interface some kinds of organisms with the harvesting of solar energy. It was a perfect match."

The bacterium, Ralstonia eutropha, is what makes the whole thing work. New pathways were engineered by researchers so that the organism can produce isopropanol as a byproduct of its natural metabolic processes.

"The advantage of interfacing the inorganic catalyst with biology is you have an unprecedented platform for chemical synthesis that you don't have with inorganic catalysts alone," explained Brendan Colón, another co-author of the paper. "Solar-to-chemical production is the heart of this paper, and so far we've been using plants for that, but we are using the unprecedented ability of biology to make lots of compounds."

How to build a better leaf

The bionic leaf initially worked so well that it was expected to surpass the natural efficiency of photosynthesis, which operates at 1 percent efficiency. But then the researchers hit a snag. As David Biello writes for Scientific American: "... the nickel-molybdenum-zinc catalyst that made its water-splitting chemistry possible had the unfortunate side effect of poisoning the microbes."

It was time to build a better leaf. In 2016, the researchers solved that problem with a different catalyst — an alloy of cobalt and phosphorus — a change that boosted the leaf's efficiency at creating alcohol fuels. By doing so, the researchers' artificial photosynthesis is able to turn about one-10th of the energy in the captured sunlight into fuel, according to the journal Science.

That's giving natural photosynthesis a real run for the money.