Like their natural counterparts, artificial leaves take in sunlight to generate fuel. In the case of the latter, that solar energy is used to break water down into hydrogen and oxygen, and that in turn has been used to create liquid fuels and electricity. Now, a Harvard team has added a new function to their repertoire, turning artificial leaves into low-cost, portable fertilizer factories.

Daniel Nocera first developed his artificial leaf at Harvard in 2011. It absorbs sunlight to drive a chemical reaction that splits water into hydrogen and oxygen, before collecting those gases to produce a range of end products. In its first incarnation, the system fed a hydrogen-hungry bacteria to produce liquid fuels for use in fuel cells, and later versions let it work with impure water and eventually boosted its output to over 10 times the efficiency of natural photosynthesis.

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A similar system developed at Eindhoven University of Technology could produce a range of chemicals, including drugs and pesticides, and the Harvard researchers believe their bionic leaf can have similarly wide applications beyond electricity generation. To that end, they've focused on fertilizer.

"The fuels were just the first step," says Nocera. "Getting to that point showed that you can have a renewable chemical synthesis platform. Now we are demonstrating the generality of it by having another type of bacteria take nitrogen out of the atmosphere to make fertilizer."

The bacteria in question is Xanthobacter, which consumes the hydrogen produced by the leaf and, by also taking in carbon dioxide from the air around it, creates a bioplastic. That's stored inside the bacteria as fuel, which they eventually use to create ammonia, a nitrogen-hydrogen compound that forms the basis of fertilizer.

"I can then put the bug in the soil because it has already used the sunlight to make the bioplastic," says Nocera. "Then the bug pulls nitrogen from the air and uses the bioplastic, which is basically stored hydrogen, to drive the fixation cycle to make ammonia for fertilizing crops."

The radishes on the right were fed fertilizer created by the bionic leaf, and are noticeably larger than the control group on the left (Credit: Nocera Lab, Harvard University)

The team tested the new system on radishes over five crop cycles, and found that plants that received fertilizer made by the bionic leaf grew to be 1.5 times bigger than a control group. The team says this is an indication of the technology's viability for eventually helping farmers in developing countries improve their crop yields by creating their own fertilizer.

"When you have a large centralized process and a massive infrastructure, you can easily make and deliver fertilizer," says Nocera. "But if I said that now you've got to do it in a village in India onsite with dirty water — forget it. Poorer countries in the emerging world don't always have the resources to do this. We should be thinking of a distributed system because that's where it's really needed."

The research was presented this week at the National Meeting and Exposition of the American Chemical Society.

Source: American Chemical Society

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