Artificial photosynthesis is a chemical process that mimics the natural process of photosynthesis to convert sunlight, water and carbon dioxide into carbohydrates and oxygen. The term artificial photosynthesis is used to refer to any scheme for capturing and storing the energy of sunlight in the chemical bonds of a fuel (a solar fuel). The photocatalytic splitting of water converts water into hydrogen and oxygen and is an important research topic in artificial photosynthesis. Light-driven carbon dioxide reduction is another studied process that replicates natural carbon fixation.
Research in this topic includes the design and assembly of devices for the direct production of solar fuels, photoelectrochemistry and its application in fuel cells, and the engineering of enzymes and photoautotrophic microorganisms for the production of microbial biofuel and biohydrogen from sunlight.
Overview
The photosynthetic reaction can be divided into two oxidation and reduction half-reactions, both of which are essential for fuel production. In plant photosynthesis, water molecules are photo-oxidized to release oxygen and protons. The second phase of plant photosynthesis (also known as the Calvin-Benson cycle) is a light-independent reaction that converts carbon dioxide into glucose (fuel). Artificial photosynthesis researchers are developing photocatalysts capable of carrying out these two reactions. In addition, the protons resulting from the separation of water can be used for the production of hydrogen. These catalysts must be able to react quickly and absorb a large percentage of incident solar photons.
Considering that photovoltaics can provide energy directly from sunlight, the inefficiency of producing fuel from photovoltaic electricity (indirect process) and the fact that sunlight is not constant throughout the day sets a limit for its use. One way to use natural photosynthesis is for the production of a biofuel, which is an indirect process that suffers from low energy conversion efficiency (due to photosynthesis' own low efficiency in converting sunlight into biomass), the cost of harvesting and transportation of fuel, and conflicts due to the growing need for land mass for food production. The purpose of artificial photosynthesis is to produce a fuel from sunlight that can be conveniently stored and used when sunlight is not available, using direct processes, ie to produce a solar fuel. With the development of catalysts capable of reproducing the main parts of photosynthesis, water and sunlight would be the only sources needed to produce clean energy. The only by-product would be oxygen, and producing a solar fuel has the potential to be cheaper than gasoline.
One process for creating a clean and affordable energy source is the development of photocatalytic water separation under sunlight. This method of sustainable hydrogen production is an important objective for the development of alternative energy systems. It is also predicted to be one of the most, if not the most efficient, ways to get hydrogen from water. The conversion of solar energy into hydrogen through a water splitting process assisted by photosemiconductor catalysts is one of the most promising technologies under development. This process has the potential to generate large amounts of hydrogen in an ecologically sound manner. The conversion of solar energy into a clean fuel (H2) under ambient conditions is one of the biggest challenges facing scientists in the twenty-first century.
Two methods are generally recognized for building solar fuel cells for hydrogen production:
A homogeneous system is one in which the catalysts are not compartmentalized, that is, the components are present in the same compartment. This means that hydrogen and oxygen are produced in the same place. This can be a disadvantage, as they form an explosive mixture, requiring separation from the gaseous product. In addition, all components must be active under approximately the same conditions (eg pH).
A heterogeneous system has two separate electrodes, an anode and a cathode, making it possible to separate the production of oxygen and hydrogen. Also, different components do not necessarily have to work under the same conditions. However, the increasing complexity of these systems makes them more difficult to develop and more expensive.
Another area of investigation within artificial photosynthesis is the selection and manipulation of photosynthetic microorganisms, namely green microalgae and cyanobacteria, for the production of solar fuels. Many strains are capable of naturally producing hydrogen, and scientists are working to improve them. Algae biofuels such as butanol and methanol are produced on a laboratory and commercial scale. This method has benefited from the development of synthetic biology, which is also being explored by the J. Craig Venter Institute to produce a synthetic organism capable of producing biofuels. In 2017, an efficient process was developed to produce acetic acid from carbon dioxide using "cyborg bacteria".