Protection from the Elements

On the top, the dye is shown as an orange circle, attached to the surface of a metal oxide and exposed to the solution. On the bottom, the green coating represents the PMMA dip, which protects the dye and keeps it from detaching, even when exposed to damaging liquids. Adapted with permission from J. Am. Chem. Soc., 2014, 136 (39), pp 13514–13517. Copyright 2014 American Chemical Society.

The sun shines all day, but most of residential power is used after the sun goes down. This is a hurdle to using solar power, because we need a way to store the energy collected from the sun, and current battery systems are often too expensive. Instead of making electricity and then finding a way to store it until it is needed, the Center for Solar Fuels (UNC) is making cells that mimic photosynthesis, using the energy from sunlight to generate energy-dense compounds. These devices often suffer from stability issues, but by using a thin layer of plastic, researchers have figured out a way to protect the devices from degradation.

The solar fuel cells in this work are made by attaching a highly absorptive dye to the surface of a metal oxide (titanium oxide or indium tin oxide in this case). The dye absorbs sunlight, causing an electron to gain energy and jump to the oxide. The dye, having lost an electron, is positively charged and pulls an electron from a nearby molecule. This molecule is specially chosen to drive a chemical reaction. In this case, researchers use a molecule that drives part of the reaction to produce hydrogen.

A major difficulty in making solar fuel cells is long-term stability. Often, the dyes detach when exposed to light in water, especially in acidic or basic environments. If the dye is no longer attached to the oxide, it does not transfer an electron after absorbing light, rendering the dye useless and eventually the cell ineffective. By simply dipping the dye-covered surface in a solution containing a dissolved plastic, the UNC researchers apply a thin, protective layer to keep the dye on the surface.

The plastic, poly(methyl methacrylate) or PMMA, is a common plastic used to make the clear walls of aquariums. The transparent property of the plastic is important in this case because sunlight needs to penetrate the coating and be absorbed by the dye. In addition to being transparent, the plastic coating acts as a sort of glue that prevents the dye from detaching from the oxide surface and protects the dye from acidic and basic conditions. The thickness of the plastic layer must be tuned so as not to impede electron transfer, but still provide the necessary protection. UNC’s Kyung-Ryang Wee, in a research team led by Thomas J. Meyer, easily tuned the thickness by simply changing the concentration of the plastic solution.

This easily fabricated protective coating enhances stability by ~100-fold under long-term light exposure. This breakthrough gives researchers a simple tool to make robust devices and experiment with a large range of solutions and chemicals to improve the efficiency of solar fuel production.

Acknowledgments:

This research was primarily supported by the UNC EFRC Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, supporting M.K.B., L.A., B.H.F., and B.S. In addition, K.R.W. acknowledges a fellowship from the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education for financial support. A.M.L. acknowledges support from the U.S. Government and awarded by the Department of Defense, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate Fellowship, 32 CFR 168a.