A solar fuel is a synthetic chemical fuel produced directly/indirectly from solar energy sunlight/solar heat through photochemical/photobiological (i.e., artificial photosynthesis, experimental as of 2013), thermochemical, and electrochemical reaction. Light is used as an energy source, with solar energy being transduced to chemical energy, typically by reducing protons to hydrogen, or carbon dioxide to organic compounds. A solar fuel can be produced and stored for later usage, when sunlight is not available, making it an alternative to fossil fuels. Diverse photocatalysts are being developed to carry these reactions in a sustainable, environmentally friendly way.