Plants are truly amazing. To produce energy they basically ingest sunlight, water and carbon dioxide. As a result energy is chemically stored in sugars like glucose. (Scientists call them carbohydrates.) We all have heard about this process in school, photosynthesis. The carbohydrates can be further modified by the plants into fats or proteins. Animals like humans rely on all three as food, but also on the oxygen produced as a byproduct. In fact, there was no oxygen in the Earth atmosphere before the first cyanobacteria invented photosynthesis. These cyanobacteria later evolved into the chloroplasts inside plant cells where photosynthesis takes place.
Photosynthesis is very effective in transforming the energy of sunlight into chemical energy in sugars without creating any toxic or polluting waste. For this reason scientists today are trying to artificially create photosynthesis. The goal of these systems is to produce hydrogen or other fuels for engines and electricity. Another advantage would be that carbon dioxide released by the use of fossil fuels could be ”mopped up” from the atmosphere by artificial photosynthesis.
The main difference between artificial and natural photosynthesis is that plants produce carbohydrates, fats and proteins while humans are looking for suitable fuels that can power airplanes or cars. These fuels should ideally resemble fossil fuels and thus enable the use of already existing combustion motors. For this reason, chemists are trying to create different end-products than plants while using the same energy source (sun) and building blocks (carbon dioxide and water).
Plants use their chlorophyll to capture the sunlight while a collection of enzymes and proteins uses this energy to split water molecules into hydrogen, electrons and oxygen. Hydrogen and electrons then form carbohydrates (sugars) with the carbon dioxide and oxygen is released.
For artificial photosynthesis, scientists are mainly interested in the first two steps above. Capturing sunlight is the easy part, as there are plenty of solar-power systems available. Splitting water, however, is trickier and the main challenge. Water is a very stable compound and catalysts are required to initiate the splitting reaction. Catalysts are materials that can accelerate chemical reactions, without being depleted in the process. A great amount of research is being carried out in order to find suitable catalysts for artificial photosynthesis. Among recently published very succesful catalysts were cobalt-based materials. Nevertheless, these systems still require more work and research in order to be optimized for commercialization.
Theoretically, the produced hydrogen could be directly used as a fuel. However, right now it is still more practical to transform hydrogen and carbon dioxide to fuels that closely resemble fossil fuels. This last step can be carried out either with the help of bacteria or other inorganic catalysts like copper. The conversion makes it possible to use the products of artificial photosynthesis in already existing car and airplane engines.
Maybe it will not take too long before we are able to drive cars with fuels directly produced with sunlight and carbon dioxide.