After worrying reports that carbon dioxide has reached the highest level for the last three million years, any news about carbon-neutral renewable energy sources is welcome.
Many believe solar technologies are our best option in the future. After all, there’s enough renewable energy in one hour of worldwide sunlight to cover our needs for a whole year. With this in mind, researchers from the US Department of Energy developed the first ever nanosystem for artificial photosynthesis, as reported in a study recently published in NANO Letters.
In nature, photosynthesis is carried out in chloroplasts, where sunlight is captured and converted into energy. Using a similar mechanism to split water and release oxygen, the artificial chloroplast has two light absorbers and co-catalysts. The scientists developed nanowire structures, which have been likened to a tree with silicon trunks and titanium branches. These structures actually resemble a natural forest. Not so much an “artificial leaf”, but think more an “artificial forest”!
How Does It Work?
As light is absorbed by the chloroplast, it releases an electron that is then transported through a chain, going from molecule to molecule, and resulting in the conversion of carbon dioxide (CO2) into long chain carbohydrates (2(CH2O)n) and the release of oxygen (O2).
2n CO2 + 4n H2O + light → 2(CH2O)n + 2n O2 + 2n H2O
Using this mechanism as inspiration, scientists were able to develop a process using two stable semi-conductors – titanium oxide and silicon – linked to co-catalysts and with an ohmic contact in-between. Silicon was chosen to be the photocathode and titanium oxide the photoanode, organised in a tree-like structure to maximise renewable energy production.
Similarly to the real photosynthesis, photo-excited electrons from the silicon nanowires are able to reduce protons and release hydrogen; while the electrons from the titanium oxide nanowires can oxidise molecules of water to produce oxygen. Authors report a 0.12% efficiency to produce energy, but this needs to be considerably improved for commercial use.
On the other hand, a major advantage of this process is its modular approach, allowing for newly-developed individual components simply to be added to the overall system. At the moment, one of the major limitations is unmatched outputs from the silicon cathode and titanium oxide anode, which means the lower current possible for the anode is reducing the system’s overall efficiency. At this stage, the authors are confident that they can solve this problem by finding a higher performance photoanode, which will significantly increase energy conversion rates.
You never know… this may turn out to be the renewable energy of the future!