Mimicking photosynthesis for cheap hydrogen fuel

Production of inexpensive hydrogen for automotive or jet fuel may one day be possible by mimicking photosynthesis, but a number of hurdles first must be overcome.

Scientists at Pennsylvania State University have developed an artificial system that can mimic photosynthesis in the hope of creating a practical, inexpensive way to make jet fuel. Using the energy in blue light, their work has yielded only 2 to 3 percent hydrogen. The blue light is much less efficient than other solar energy conversion technologies, but the investigators have hope.

Although some researchers have used solar cells to make electricity or used concentrated solar heat to split water, both processes are energy-intensive. The key to direct conversion, scientists say, is electrons. As with the dyes that occur naturally in plants, inorganic dyes absorb sunlight, and the energy kicks out an electron. When left on its own, the electron can recombine to create heat, but if channeled – molecule to molecule – far enough away from where it originated, it can reach the catalyst and split the hydrogen from the oxygen in water.

Recombination of electrons is not the only problem the scientists face. They also must address the oxygen-evolving end of the system, which currently limits the lifetime of the system to a few hours. Even though natural photosynthesis has the same problem, it can repair itself by periodically replacing the oxygen-evolving complex and the protein molecules around it. The researchers have not yet been able to provide a fix to the oxidation process.

Currently, they are using only blue light, but they would like to expand into the entire visible spectrum from the sun. In addition, their experiments use only expensive components – titanium oxide and platinum dark electrodes, and an iridium oxide catalyst. Substitutions are necessary, and researchers at other institutions have begun working on an alternative solution. An MIT group is investigating cobalt and nickel catalysts, and manganese is under investigation at Yale and Princeton universities.

The system uses only one photon at a time, but the Penn State researchers anticipate that a two-photon system, albeit more complicated, would be more effective in using the full spectrum of sunlight. Research will continue, and they will focus their efforts to track all the energy pathways in the cell to understand the kinetics, with the hope of modeling the cells and adjusting the portions to decrease energy loss and increase efficiency.