Underwater solar cells turn greenhouse gases into 'solar fuel'

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Engineers at Stanford University have discovered how to use the sun’s energy to combine water and carbon dioxide to create chemical products, a process known as artificial photosynthesis, using underwater solar cells.

Functioning under water, the solar cells spur chemical reactions to convert captured greenhouse gases into fuel rather than feeding electricity into the grid.

Stanford materials scientist Paul McIntyre, a pioneer in the emerging field of artificial photosynthesis, led the work, which was published in Nature Materials.

In plants, photosynthesis uses the sun’s energy to combine water and carbon dioxide to create sugar, the fuel on which they live. Artificial photosynthesis would use the energy from specialized solar cells to combine water with captured carbon dioxide to produce industrial fuels, such as natural gas.

Artificial photosynthesis has faced two challenges: ordinary silicon solar cells corrode under water and corrosion-proof solar cells have been unable to capture enough sunlight under water to drive the chemical reactions.

In 2011, McIntyre’s lab managed to make solar cells resistant to corrosion in water. In the new paper, McIntyre and doctoral student Andrew Scheuermann show how to increase the power of corrosion-resistant solar cells, setting a record for solar energy output under water.

"The results reported in this paper are significant because they represent not only an advance in performance of silicon artificial photosynthesis cells, but also establish the design rules needed to achieve high performance for a wide array of different semiconductors, corrosion protection layers and catalysts," McIntyre said in a Stanford news report.

The process could one day play a key role in fighting climate change. The idea is to funnel greenhouse gases from smokestacks or the atmosphere into giant, transparent chemical tanks. Solar cells inside the tanks would spur chemical reactions to turn the greenhouse gases and water into what are sometimes called "solar fuels."

"We have now achieved the corrosion resistance and the energy output required for viable systems," Scheuermann said. "Within five years, we will have complete artificial photosynthesis systems that convert greenhouse gases into fuel."

McIntyre’s lab solved the corrosion problem by coating the electrodes in special cells with a protective layer of transparent titanium dioxide. The first-generation corrosion-proof cells still couldn’t extract enough voltage from the sunlight as it filtered through the water. The researchers made the corrosion-resistant solar cells more powerful by adding a layer of charged silicon between the titanium oxide and the basic silicon cell.

McIntyre and Scheuermann worked on the positive electrode component of solar cells, called anodes. Other researchers have been working on the complementary cathodes. The record performance of the new anode, combined with current cathode technology, makes the entire system feasible, according to the researchers.

Working with Paul Hurley, co-author on the paper and senior research scientist at the Tyndall National Institute in Cork, Ireland, the engineers also provided design principles to help the photovoltaic industry and scientific community build energy-efficient, corrosion-protected solar cells for other purposes.