In 1995, a special issue of the journal Accounts of Chemical Research asked leading chemists to describe "holy grail" projects in the field; one of the essays, by Allen J. Bard and Marye Anne Fox, then at the University of Texas at Austin, described the process of splitting water using sunlight. The sheer simplicity of the process conceals its chemical elegance—it takes energy to break chemical bonds, such as the bonds that hold hydrogen atoms to oxygen in a molecule of water, and plants use the energy of sunlight to break those bonds. The result is hydrogen and oxygen. Plants release oxygen into the air and repurpose the hydrogen to make food, in the form of carbohydrates. But hydrogen on its own, as a gas, is a clean and storable form of energy known as a chemical fuel; it can be stored for later use, and that's what Nocera was after.
Source : https://news.nationalgeographic.com/news/innovators/2014/05/140519-nocera-chemistry-artificial-leaf-solar-renewable-energy/