Solar Power for Fuel Production

The energy generated by the sun offers a clean alternative just waiting to be harnessed-it's estimated that the total energy output of the sun for one hour could supply the earth with sufficient energy for one year of global energy consumption. Solar energy systems have made great strides over the last decade, but they still suffer from one major limitation-the lack of 24-hour sunlight.

A research team from the EFRC (Energy Frontier Research Center) at the University of North Carolina has offered a novel approach to solving this problem. Mimicking the photosynthesis process, they have constructed a solar energy system that converts solar power into hydrogen fuel instead of electricity, allowing fuel storage for use during nighttime hours.

Compared to a more traditional dye-sensitization solar cell (DSSC), which produces a photocurrent, the new process produces a dye-sensitized photoelectrosynthesis cell (DSPEC). DSPECs use the incoming energy to split water into its hydrogen and oxygen components, storing the hydrogen for use as fuel while releasing the byproduct oxygen into the atmosphere.

The DSPEC's design contains two basic pieces: a chromophore-catalyst assembly and a nanoparticle. The chromophore absorbs light in the visible wavelength, initiating the action of the catalyst to remove the electrons from the water supply. Meanwhile, the conductive nanoparticles move the electrons away to create the hydrogen fuel.

Earlier versions were problematic because of the inability to stabilize the system for balanced electron removal-either the chromophore and catalyst combination would physically break away from the nanoparticles, or the nanoparticles weren't conductive enough to maintain a sufficient flow of electrons to make hydrogen. The system needed to be more balanced and robust to be practical.

Along with a research group from North Carolina State University, the EFRC team solved the conductivity problem by using a simple additive in a novel way. Titanium dioxide (TiO2) is a commonly used industrial additive for things such as providing hiding power in paints, and occurs in several forms. The research team managed to coat individual nanoparticles with extremely thin layers of one form of TiO2, resulting in greatly increased conductivity throughout the entire nanoparticle/TiO2 network.

The team also developed a protective coating that keeps the chromophore and catalyst combination in contact with the nanoparticle/TiO2 surface without degrading the light-absorbing capability of the system. The result is a more rugged system that converts sunlight into hydrogen fuel and oxygen-made from existing materials and known technologies, with little external power, and releasing no greenhouse gases.

The research team is investigating similar mechanisms to use carbon dioxide (CO2) in the atmosphere to produce a carbon-based fuel (for example, methanol). If they succeed on both counts, the researchers will have pulled off a neat trick-using sunlight to produce two different types of fuel while consuming CO2 and producing oxygen. The analogy to photosynthesis becomes even clearer when you think of it that way-turning light energy into chemical energy.

As with any new technology, there are going to be scaling and economic challenges-to date, one of the reasons solar cells have not been more widely used is the high cost required to produce them compared to the output. Even so, this technology bears great promise and is worth watching.