Their purpose is not to feed electricity into the grid, but to spur chemical reactions in order to convert captured greenhouse gases into fuel. The cells combine water (H₂O) with captured carbon dioxide (CO₂) to produce pure oxygen (O₂) and methane (CH₄) in a process known as artificial photosynthesis.

Two Challenges: Corrosion and Voltage

Artificial photosynthesis faces two main difficulties:

1.       Ordinary silicon cells corrode underwater.

2.       Corrosion-proof cells do not capture enough sunlight to be effective.

The first problem was solved by coating the cells’ electrodes with a protective layer of transparent titanium dioxide. The coating is so thin that it would require 25,000 layers to equal the thickness of a single sheet of paper.

Even so, the corrosion-proof cells were incapable of extracting enough voltage from sunlight filtered through the water. However, this second problem has recently been solved.

A Silicon Dioxide Voltage Booster

Researchers at Stanford University made the corrosion-resistant cells more powerful by adding a layer of charged silicon between the titanium oxide layer and the basic silicon cell. The result is a device with several layers, each with a different electronic function.

The active silicon layers at the bottom absorb sunlight and excite the electrons. The silicon dioxide booster which forms the next layer increases the voltage. On top of the booster, the transparent titanium dioxide seals the system and prevents corrosion. It also serves as a conductor.

All three layers are coated with iridium, which serves as a catalyst that allows the H₂O and CO₂ molecules to meet. Electricity conducted from the bottom layer breaks the chemical bonds on these molecules and recombines the elements to produce O₂ and CH₄.

Artificial Photosynthesis in Five Years?

“We have now achieved the corrosion resistance and the energy output required for viable systems,” said Andrew Scheuermann, co-author of the published research. “Within five years, we will have complete artificial photosynthesis systems that convert greenhouse gases into fuel.”

The system for artificial photosynthesis Scheuermann worked with Dr. Paul McIntyre on the anode, the positive electrode component of solar cells.

Other researchers are working on complementary cathodes but McIntyre and Scheuermann contend that their anode makes artificial photosynthesis feasible even with current cathode technology.

Even if Scheuermann’s estimate turns out to be overly optimistic, the design principles derived from his research can help other engineers build energy-efficient solar cells that are corrosion-protected. In other words, research on underwater solar cells is going swimmingly.

For more information, read the upcoming article in Nature Materials.