Solar fuel

A solar fuel is a synthetic fuel produced using solar energy, through photochemical (i.e. photon activation of certain chemical reactions), photobiological (i.e., artificial photosynthesis), electrochemical (i.e. using solar electricity to drive an endogenic reaction such as hydroelectrolysis), or thermochemical methods (i.e., through the use of solar heat supplied by concentrated solar thermal energy to drive a chemical reaction). Sunlight is the primary energy source, with its radiant energy being transduced to chemical energy stored in bonds, typically by reducing protons to hydrogen, or carbon dioxide to organic compounds.

A solar fuel can be produced and stored for later use, when sunlight is not available, making it an alternative to fossil fuels and batteries. Examples of such fuels are hydrogen, ammonia, and hydrazine. Diverse photocatalysts are being developed to carry these reactions in a sustainable, environmentally friendly way.

The world's dependence on the declining reserves of fossil fuels poses not only environmental problems but also geopolitical ones. Solar fuels, in particular hydrogen, are viewed as an alternative source of energy for replacing fossil fuels especially where storage is essential. Electricity can be produced directly from sunlight through photovoltaics, but this form of energy is rather inefficient to store compared to hydrogen. A solar fuel can be produced when and where sunlight is available, and stored and transported for later usage. This makes it much more convenient, because it can be used in situations where direct sunlight is not available.

The most widely researched solar fuels are hydrogen, because the only product of using this fuel is water, and products of photochemical carbon dioxide reduction, which are more conventional fuels like methane and propane. Upcoming research also involves ammonia and related substances (i.e. hydrazine). These can address the challenges that come with hydrogen, by being a more compact and safer way of storing hydrogen. Direct ammonia fuel cells are also being researched.

Solar fuels can be produced via direct or indirect processes. Direct processes harness the energy in sunlight to produce a fuel without intermediary energy conversions. Solar thermochemistry uses the heat of the sun directly to heat a receiver adjacent to the solar reactor where the thermochemical process is performed. In contrast, indirect processes have solar energy converted to another form of energy first (such as biomass or electricity) that can then be used to produce a fuel. Indirect processes have been easier to implement but have the disadvantage of being less efficient than the direct method. Therefore, direct methods should be considered more interesting than their less efficient counterparts. New research therefore focusses more on this direct conversion, but also in fuels that can be used immediately to balance the power grid.

In a solar photoelectrochemical process, hydrogen can be produced by electrolysis. To use sunlight in this process, a photoelectrochemical cell can be used, where one photosensitized electrode converts light into an electric current that is then used for water splitting. One such type of cell is the dye-sensitized solar cell. This is an indirect process, since it produces electricity that then is used to form hydrogen. Another indirect process using sunlight is conversion of biomass to biofuel using photosynthetic organisms; however, most of the energy harvested by photosynthesis is used in life-sustaining processes and therefore lost for energy use.

A semiconductor can also be used as the photosensitizer. When a semiconductor is hit by a photon with an energy higher than the bandgap, an electron is excited to the conduction band and a hole is created in the valence band. Due to band bending, the electrons and holes move to the surface, where these charges are used to split the water molecules. Many different materials have been tested, but none so far have shown the requirements for practical application.

In a photochemical process, the sunlight is directly used to split water into hydrogen and oxygen. Because the absorption spectrum of water does not overlap with the emission spectrum of the sun, direct dissociation of water cannot take place; a photosensitizer needs to be used. Several such catalysts have been developed as proof of concept, but not yet scaled up for commercial use; nevertheless, their relative simplicity gives the advantage of potential lower cost and increased energy conversion efficiency. One such proof of concept is the "artificial leaf" developed by Nocera and coworkers: a combination of metal oxide-based catalysts and a semiconductor solar cell produces hydrogen upon illumination, with oxygen as the only byproduct.