An artificial leaf is a solar cell that uses water and an electrical current generated from sunlight to produce hydrogen (for fuel) and oxygen. It is currently in development at laboratories worldwide. If successful, the artificial leaf could serve as an abundant and carbon-free source of hydrogen for use in fuel cells, in which hydrogen reacts with oxygen to produce water and electricity. In combination, artificial leaf and fuel cell technologies would yield very clean energy. See also: Electrolysis; Energy; Energy conversion; Fuel cell; Hydrogen; Oxygen; Solar cell
In nature, photosynthesis converts the energy in sunlight to stored chemical energy in the form of carbohydrates. In the photosynthesis process of green plants, algae, and cyanobacteria, water is split into hydrogen and oxygen—the hydrogen is the energetic component and reacts with carbon dioxide (in a process of proton/CO2 reduction) to form carbohydrates and the oxygen is released as a by-product. See also: Carbohydrate; Energy sources; Photosynthesis
The concept of creating energy-rich fuel in the form of hydrogen gas or a liquid (such as an alcohol) by means of artificial photosynthesis is attractive because it represents a very renewable technology for energy production. In contrast, most of the hydrogen used today is produced through steam reforming of natural gas (methane) [through the reaction CH4 + H2O ↔ CO + 3H2] followed by the water-gas shift reaction (CO + H2O ↔ CO2 + H2). Environmental disadvantages of this process are that it produces the greenhouse gas carbon dioxide (CO2) as a by-product and that it requires a significant input of energy, which itself may come from a polluting source. See also: Alternative fuels for vehicles; Artificial photosynthesis; Energy storage; Greenhouse effect; Methane; Natural gas
The first artificial leaf was developed in 1998 by John Turner of the U.S. National Renewable Energy Laboratory in Golden, Colorado. It had a short lifetime and was very costly to produce, which made it impractical. Probably the best-known researcher currently working on the artificial leaf is Daniel Nocera of Harvard University in Cambridge, Massachusetts, who in 2011 developed a more cost-effective artificial leaf that had a somewhat longer lifetime.
The Nocera leaf used a semiconducting silicon solar cell to absorb light and generate an electrical current to split water into hydrogen and oxygen. Two electrocatalysts—a cobalt phosphate catalyst for oxidizing water into oxygen and hydrogen ions (that is, protons or H+) and a nickel-molybdenum-zinc catalyst for reducing H+ to H2 molecules —were coated on opposite sides of the solar cell to drive the reactions. When the artificial leaf was immersed in water and exposed to light, the reactions proceeded. Water soon corroded the semiconducting material, however, causing the artificial leaf to fail. See also: Catalysis; Corrosion; Electrolysis; Oxidation-reduction
In May 2014, researchers at the California Institute of Technology in Pasadena reported that a thin protective layer of titanium dioxide (TiO2) deposited on the electrocatalysts prevented corrosion yet was transparent enough to allow light to pass through to the semiconductors. Although this represents a breakthrough for the artificial leaf, the technology is still at the laboratory benchtop stage of development. See also: Titanium oxides