Solar technology is now high on the list of renewable energy sources. The challenge is figuring out the intermediary process between sunlight and fuel. In nature, the leaf is the intermediary and as we know, the process is called photosynthesis.
When plants photosynthesize, they “eat” sunlight, harvesting its energy to convert carbon dioxide from the air and water from the earth into oxygen and glucose, the main energy molecule in plants. That is basically it, although the detailed chemical process is incredibly complex. “When you try to do it yourself, you can only wonder how amazing it all it”, says Uwe Bergmann, a physicist at the Stanford Linear Accelerator Center in the U.S.
Here’s a tiny part of that miracle. It all starts within the leaf, which has a sprawling structure housing a molecule called chlorophyll, which gives plants their green color. As sunlight hits the leaf, photons collide with the chlorophyll molecules, forming the appropriately named “exciton” – an electron-ion pair that traps the photon’s energy. Excitons then travel deep within the so-called reaction center of the leaf where its energy is converted from water and carbon dioxide molecules into simple sugars (primarily glucose) and oxygen. The most amazing part of this process is the way excitons make their way into the deep recess of the leaf’s reactor. Rather than bump around randomly through a forest of chlorophyll molecules, they spread themselves over all possible paths, in effect funneling down through the most efficient route. Little wonder then, that all efforts in the lab to create “artificial photosynthesis” seems like child’s play compared to what nature has designed. The most impressive achievement to date is an artificial leaf created by Daniel Norcera, a chemistry professor currently at Harvard University and his colleagues. This “leaf” is actually a small solar cell that converts water into hydrogen, which is then used to feed a genetically modified bacterium to produce isopropanol, a potential carbon fuel. Nocera’s leaf is an impressive piece of science, but it is currently far from commercially viable. In fact, no other process yet devised can do what nature has done effortlessly for hundreds of millions of years.
Imitating the Sun is hard, but imitating the simple leaf is far from easy.