All That Matters

This morning I read an article by the Scientific American editor David Biello on an important topic: the importance of rare earth elements for our economy, and the power of those few countries that export them on a larger scale. (disclaimer: Scientific American is part of Nature Publishing Group, my employer)

David hits an important point there. But to my mind, the problem is far more critical and fundamental than this single, focussed example suggests, and we need to act on it soon.

Salt production at Salar de Uyuni. This salt flat harbours 50% of the world's lithium reserves. Image by Ricampelo via Wikimedia Commons.

The issue is that rare earth elements such as neodymium are essential to green energy and our economy. Neodymium is part of Nd₂Fe₁₄B, a powerful permanent magnet that is used for electromotors, read heads of hard disk drives, etc. Each wind turbine apparently uses 300 kg of neodymium, each Toyota Prius about 1 kg. At present, China produces 97% of all neodymium.

And this is the problem. China has implemented export controls for its rare earth elements resources. In a recent diplomatic spat with Japan, they temporarily restricted the export of rare earth elements to Japan. But the Chinese should not take all the blame for a little realpolitik. Heard of the 1973 oil crisis?

[…]

Solar energy is obviously one of the key renewable energy resources available to us. At the same time researchers are hitting against a glass ceiling. A famous 1961 paper by William Shockley (who co-invented the transistor) and Hans Queisser comes to the conclusion that for a semiconductor such as silicon the maximum conversion efficiency of solar energy into electricity will never be more than about 30%.

Dye-sensitized solar cells. A design similar to these solar cells is now used to demonstrate the creation and extraction of multiple charge carriers per photons.

One reason for this limit is that each light particle only excites one electron. Even if the electron has enough energy to excite two electrons, all this energy is lost and only one electron is excited. And this is the case for pretty much all present commercial solar cell technology. Fortunately, however, there are possible exceptions. Bruce Parkinson and colleagues from the University of Wyoming in the USA have now built a photovoltaic cell that at certain wavelengths of light can generate more than one electron per photon of light. Their approach promises to beat the Shockley-Queisser limit and could lead to solar cells with considerably enhanced efficiency.

In silicon and other semiconductors, if a photon excites an electron all excess energy is predominantly lost as heat. Of course, there are attempts to harvest the heat generated in solar cells, and such approaches could beat the Shockley-Queisser limit. And so could nanostructured materials that use for example plasmonic effects. But a more direct solution would be if the excess energy could be used to excite more than one electron in the first place.

[…]

This week my colleagues at Nature Chemistry landed an impressive scoop, the publication of a paper by Michael Strano and colleagues from MIT on self-regenerating solar cells.

The performance of any kind of solar cell tends to degrade over time. This is particularly the case for organic solar cells, where sunlight can easily destroy the structure of the molecules used. Natural light-harvesting processes have a similar problem, for example during photosynthesis. The way plants solve this problem is through a self-repair mechanism.

Schematic of the regenerating solar cell consisting of light-absorbing proteins, lipid disks and carbon nanotubes. Reprinted by permission from Macmillan Publishers Ltd. Nature Chemistry, advance online publication (2010)

Taking cues from such self-regeneration strategies, Strano and colleagues use a concept that is surprisingly simple. They prepare a solution containing carbon nanotubes, bacterial light-harvesting proteins and discs made from lipid molecules — the structural components that form the membrane of cells. Once the surfactant that keeps all these molecules separate is removed the molecules assemble themselves: the proteins bind to the lipids, which then attach to the carbon nanotubes. No assembly required.

During solar cell operation sunlight is absorbed by the proteins and creates electronic charges that are transported along the carbon nanotubes to the electrical contacts of the solar cell. To regenerate the proteins damaged by the sunlight, surfactant is added again, along with a small quantity of new proteins to replace damaged molecules. This dissolves the structure. But once the surfactant is removed the molecules reassemble, fully repaired.

In the study, the solar cells ran on a 40 hour cycle: 32 hours of operation, followed by 8 hours regeneration. Despite so many hours of regeneration overall cell performance was up by a remarkable 300% in comparison to cells that are not regenerated. And this could be just the beginning. At the moment, as soon as the cells are turned on they lose about 60% efficiency within the first hour or so of operation. Delaying the onset of degradation or finding a more efficient way of regeneration should lead to further enhancements. But who knows, nature may have a solution to this problem, too.

Reference:
Ham, M., Choi, J., Boghossian, A., Jeng, E., Graff, R., Heller, D., Chang, A., Mattis, A., Bayburt, T., Grinkova, Y., Zeiger, A., Van Vliet, K., Hobbie, E., Sligar, S., Wraight, C., & Strano, M. (2010). Photoelectrochemical complexes for solar energy conversion that chemically and autonomously regenerate Nature Chemistry DOI: 10.1038/NCHEM.822