The efficiency achieved in catalyst is the key: the average of the incident light that can be used in a system must be analyzed.
Photosynthetic organisms can absorb 50% of incident solar radiation; photochemical cells could use materials absorbing a wider range of solar radiation. Plants have a theoretical threshold of 12% efficiency of glucose formation from photosynthesis, while a carbon reducing catalyst may have a lower threshold value. However, plants are efficient in using CO2 at atmospheric concentrations, something that artificial catalysts cannot perform yet.
The natural photosynthetic reaction consists of two half-reactions: oxidation and reduction. In plant photosynthesis, water molecules are photo-oxidized to release oxygen and protons. The second stage of plant photosynthesis is a light-independent reaction that converts carbon dioxide into glucose. This stage is also known as Calvin-Benson cycle:
Recent studies are focused on photocatalysts to perform both of these reactions independently. Furthermore, the protons can be used for hydrogen production.
Photovoltaic energy is able to provide direct electrical current from sunlight but the main problem of photovoltaic energy is that the sunlight is neither constant nor homogeneous. Artificial photosynthesis aims to produce a fuel from sunlight that could be stored and used when sunlight was not available. In artificial photosynthesis only water and sunlight would be necessary for an environmental friendly energy production.The only by-product would be oxygen. Solar fuel seems to be cheapest than fossil fuels as gasoline.
One process for the creation of energy supply is the development of photocatalytic water splitting under solar radiation. The conversion of solar energy into hydrogen by means of a water-splitting process helped by photosemiconductor catalysts is one of the most promising technologies in development. This process has the potential to generate large quantities of hydrogen in an ecologically method. The conversion of solar energy into hydrogen (a clean fuel) under ambient conditions is one of the greatest challenges facing scientists at present.
Solar fuel cells for hydrogen production can be:
Photosynthetic microorganisms also known as microalgae and cyanobacteria can produce solar fuels. This field research is focusing its efforts to study the suitable selection and manipulation of these microorganisms. Several carbon-based biofuels have also been produced using cyanobacteria, such as 1-butanol.
Filamentous cyanobacteria are nitrogen-fixing microorganisms that possess the enzyme nitrogenase which is responsible for conversion of atmospheric N2 into ammmonia. Molecular hydrogen is a byproduct of this reaction, and is many times not released by the microorganism, but rather taken up by a hydrogen-oxidizing hydrogenase. One way of forcing these organisms to produce hydrogen is then to annihilate uptake hydrogenase activity. Synthetic biology techniques are predicted to be useful in this field.
Artificial photosynthesis has the intention of constructing systems doing the same type of processes that the natural photosynthesis. One of the simplest designs is where the photosensitizer is linked in tandem between a water oxidation catalyst and a hydrogen catalyst:
Photoelectrochemical cells are heterogeneous systems that use light to produce electricity or hydrogen. Dye-sensitized solar cell is a hopeful, recent type of solar cell. This cell depends on a semiconductor for current conduction on one electrode and it is covered by a coating of an organic or inorganic dye that acts as a photosensitizer; the counter electrode is a platinum catalyst for H2 production. These cells have a self-repair mechanism and solar-to-electricity conversion efficiencies rivaling those of solid-state semiconductor ones.
Direct water oxidation by photocatalysts is a more efficient usage of solar energy than photoelectrochimical water splitting because it avoids an intermediate thermal or electrical energy conversion step. Some ruthenium complexes are able to oxidize water under solar light irradiation. They have photostability. Improvement of catalyst stability has been tried resorting to polyoxometalates, in particular ruthenium-based ones.
There are another emerging methodologies as the hydrogen-producing artificial systems and NADP+/NADPH coenzyme-inspired catalyst and photobiological production of fuels.