Artificial Photosynthesis Approaches

To recreate the photosynthesis that plants have perfected, an energy conversion system has to be able to do two crucial things (probably inside of some type of nanotube that acts as the structural "leaf"): harvest sunlight and split water molecules.

Plants accomplish these tasks using chlorophyll, which captures sunlight, and a collection of proteins and enzymes that use that sunlight to break down H2O molecules into hydrogen, electrons and oxygen (protons). The electrons and hydrogen are then used to turn CO2 into carbohydrates, and the oxygen is expelled.

For an artificial system to work for human needs, the output has to change. Instead of releasing only oxygen at the end of the reaction, it would have to release liquid hydrogen (or perhaps methanol) as well. That hydrogen could be used directly as liquid fuel or channeled into a fuel cell. Getting the process to produce hydrogen is not a problem, since it's already there in the water molecules. And capturing sunlight is not a problem -- current solar-power systems do that.

The hard part is splitting the water molecules to get the electrons necessary to facilitate the chemical process that produces the hydrogen. Splitting water requires an energy input of about 2.5 volts [source: Hunter]. This means the process requires a catalyst -- something to get the whole thing moving. The catalyst reacts with the sun's photons to initiate a chemical reaction.

There have been important advances in this area in the last five or 10 years. A few of the more successful catalysts include:

  • Manganese: Manganese is the catalyst found in the photosynthetic core of plants. A single atom of manganese triggers the natural process that uses sunlight to split water. Using manganese in an artificial system is a biomimetric approach -- it directly mimics the biology found in plants.
  • Dye-sensitized titanium dioxide: Titanium dioxide (TiO2) is a stable metal that can act as an efficient catalyst. It's used in a dye-sensitized solar cell, also known as a Graetzel cell, which has been around since the 1990s. In a Graetzel cell, the TiO2 is suspended in a layer of dye particles that capture the sunlight and then expose it to the TiO2 to start the reaction.
  • Cobalt oxide: One of the more recently discovered catalysts, clusters of nano-sized cobalt-oxide molecules (CoO) have been found to be stable and highly efficient triggers in an artificial photosynthesis system. Cobalt oxide is also a very abundant molecule -- it's currently a popular industrial catalyst.

Once perfected, these systems could change the way we power our world.