Underwater solar cells turn captured greenhouse gases into fuel

Stanford engineers have developed solar cells that can function under water. Instead of pumping electricity into the grid, though, the power these cells produce would be used to spur chemical reactions to convert captured greenhouse gases into fuel.

This new work, published in Nature Materials, was led by Stanford materials scientist Paul McIntyre, whose lab has been a pioneer in an emerging field known as artificial photosynthesis.

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.

Until now, artificial photosynthesis has faced two challenges - ordinary silicon solar cells corrode under water, and even corrosion-proof solar cells had been unable to capture enough sunlight under water to drive the envisioned chemical reactions.

Four years ago, McIntyre's lab made solar cells resistant to corrosion in water. In the new paper, working with doctoral student Andrew Scheuermann, the researchers have shown 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.

Such solar cells would be part of a larger system to fight climate change. The vision 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."

Years of work have gone into developing solar cells that could operate in water permeated by corrosive greenhouse gases.

McIntyre's lab solved the corrosion problem in 2011, by coating the electrodes in these special cells with a protective layer of transparent titanium dioxide.

This coating is so thin that it would take 25,000 layers to stack up to the thickness of a single sheet of paper. But those first-generation, corrosion-proof cells still couldn't extract enough voltage from the sunlight as it filtered though the water.

Scheuermann has shown how to make the corrosion-resistant solar cells more powerful by adding a layer of charged silicon between the titanium oxide and the basic silicon cell.

The resulting device consists of several layers with different electronic functions. The active silicon layer rests at the bottom, absorbing sunlight and exciting electrons.

Above that active layer sits the new silicon dioxide booster, which increases the voltage. On top of the booster the transparent titanium dioxide seals the system and prevents corrosion, and also serves as a conductor.

These three layers are coated with iridium, which serves as the catalyst that allows CO2 and H2O molecules to meet. The electricity conducted from below breaks the chemical bond on these two molecules and recombines the elements to produce pure oxygen and the natural gas methane (CH4).

This system for artificial photosynthesis works like a battery, but in reverse. In this paper, 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 this new anode, combined with current cathode technology, makes the entire system feasible.

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