In a May, 2013 paper in NANO Letters, titled “A Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting.” With co-authors are Chong Liu, Jinyao Tang, Hao Ming Chen and Bin Liu, Scientists with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have reported the first fully integrated nanosystem for artificial photosynthesis.
The article is about as fun to read as the title, but it makes a very clear point – artificial photosynthesis is not only possible, it can potentially be done at even greater efficiency than by the plants it is based on. The research is based on solar cells that split water molecules and combine them with airborne carbon dioxide to produce the simple sugar glucose and oxygen. Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division, explains it like this, “The photo-generated electrons in the silicon nanowires migrate to the surface and reduce protons to generate hydrogen while the photo-generated holes in the titanium oxide nanowires oxidize water to evolve oxygen molecules. The majority charge carriers from both semiconductors recombine at the ohmic contact, completing the relay of the Z-scheme, similar to that of natural photosynthesis.”
In English, the artificial photosynthesis cells are comprised of two sides, one made of titanium oxide and the other of silicon. There are also a host of co-catalysts that help the process get started. Each cell works almost identically to a standard solar voltaic cell, but instead of using the displaced electron to create a current, it uses the electron to chemically adjust the structure of molecules in the cell.
The photosynthesis inside the artificial leaf structures is currently not very efficient; in fact it is a paltry 12% efficient. That’s slightly lower than the efficiency of plants. Up to this point, all such projects have been focused on a single solar leaf. This is the first successful attempt at creating a viable network of integrated leaves that act in a similar manner as trees. The total output of the system was similar to that of a 10 ft. Maple tree.
This could be a hugely important advance in solar technology as it has the potential to remove the need for inverters or batteries in solar applications. The solar leaf will produce a storable energy and remove carbon dioxide from the air. This would allow the installation of solar leaves in areas where traditional battery storage systems are not feasible. At the current efficiency level, it doesn’t make sense to pursue this technology, but there are several carbon reducing catalysts that, in theory, should be able to break the 12% threshold.
The only real sticking point in the entire artificial photosynthesis game is that artificial catalysts are still not able to efficiently utilize carbon dioxide in the concentrations that are currently in the atmosphere. Until researchers are able to overcome this fundamental issue, even the most efficient carbon dioxide transferring solar trees will not be feasible on a large scale.
Would you be comfortable with a solar tree near your home? What if they could be made aesthetically pleasing?