Artificial photosynthesis system converts water into hydrogen

The energy available in sunlight is a resource we’ve only begun to really get a handle on. Current photovoltaic-cell technology, typically a semiconductor-based system, is expensive, not very efficient, and only does instant conversions from sunlight to electricity. But an artificial photosynthesis system or a photo-electrochemical cell that mimics what happens in plants could potentially create an endless, relatively inexpensive supply of all the clean “gas” and electricity we need to power our lives in a storable form. A team of four chemists at the University of Rochester are developing a new kind of a system that derives usable hydrogen fuel from water by using only sunlight.

“Everybody talks about using hydrogen as a super-green fuel, but actually generating that fuel without using some other non-green energy in the process is not easy,” says Kara Bren, professor in the Department of Chemistry. “People have used sunlight to derive hydrogen from water before, but the trick is making the whole process efficient enough to be useful.”

Bren and the rest of the University of Rochester team will be investigating artificial photosynthesis, which uses sunlight to carry out chemical processes analogous to the ones the plants do. What makes the Rochester approach different from past attempts to use sunlight in order to produce hydrogen from water is that their device is divided into three “modules” that allow each stage of the process to be manipulated and optimized far more easily compared to other existing methods.

The first module uses visible light to create free electrons. A complex natural molecule called a chromophore that plants use to absorb sunlight will be re-engineered to efficiently generate reducing electrons. The second module will be a membrane covered with carbon nanotubes which act as molecular wires so small that they are only one-hundred-thousandth the thickness of a human hair. To prevent the chromophores from re-absorbing the electrons, the nanotube membrane channels the electrons away from the chromophores and toward the third module. In the third module, catalysts put the electrons to work forming hydrogen from water. The hydrogen can then be used in fuel cells in cars, homes, or power plants of the future.

By separating the first and third module with the nanotube membrane, the chemists hope to isolate the process of gathering sunlight from the process of generating hydrogen. This isolation will allow the team to maximize the system’s light-harvesting abilities without altering its hydrogen-generation abilities, and vice versa. Bren says this is a distinct advantage over other systems that have integrated designs because in those designs a change that enhances one trait may degrade another unpredictably and unacceptably.

Bren says it may be years before the team has a system that clearly works better than other designs, and even then the system would have to work efficiently enough to be commercially viable. “But if we succeed, we may be able to not only help create a fuel that burns cleanly, but the creation of the fuel itself may be clean.”

The project has caught the attention of the U.S. Department of Energy, which has just given the team nearly $1.7 million to pursue the design.