Artificial photosynthesis involves four stages, making it very similar to natural photosynthesis :
1. Light harvesting: the collection of light particles (photons) by antenna molecules and the concentration of the collected energy in a reaction centre.
2. Charge separation: at the reaction centre, the collected sunlight is used to separate positive (‘holes’) and negative (electrons) charges from each other.
3. Water splitting: positive charges are directly injected into catalytic centres where they are used to split water into hydrogen ions (protons) and oxygen.
4. Fuel production: electrons from step 2 are given more energy from new photons and subsequently combined with the hydrogen ions and possibly CO2 to produce either hydrogen or a carbon-based fuel.
The chemical processes involved in the production of hydrogen (see figure 4) are simpler than those involved in the production of carbon-based fuels. Four protons and four electrons, released by splitting water with four photons, go to create two hydrogen molecules using the energy from a further four photons:
Hydrogen is an attractive carbon-free energy carrier, which may play a lead role in future renewable energy technology. The drawback of hydrogen is that it is a highly explosive gas, while the existing energy infrastructure is based on liquids.
The carbon-based fuels that may be produced by means of artificial photosynthesis are not complex molecules like carbohydrates, but simpler molecules such as methane, methanol and carbon monoxide. The processes by which these fuels are produced are more complex than those involved in the production of hydrogen, because in many cases more than four electrons and protons and more than eight photons play a role in the reaction. The storage of energy in carbon-based fuels represents a major scientific challenge. However, carbon-based fuels have the advantage that many are liquid and could therefore be integrated into the existing energy infrastructure relatively easily.