Progress in Artificial Photosynthesis

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Plants perform photosynthesis with sunlight. Photo by Adrain Cable.

With artificial photosynthesis, carbon dioxide (CO2) and water could be converted into carbohydrates and oxygen. A study published September 29, 2011, performed at the University of Illinois, USA, in collaboration with the company Dioxide Materials, showed how it is possible to perform this process at laboratory scale using much less energy than before.

Natural photosynthesis

Natural photosynthesis is a process performed by plants, which transforms carbon dioxide (CO2) and water (H2O) into molecular oxygen (O2) and carbohydrates, generally sugars. This reaction only takes place in the presence of sunlight, which supplies the energy necessary for the reaction.

Photosynthesis is an essential process for the lives of plants, as it provides them with the nutrients necessary for their growth. This process, however, is also very important for the whole environment: in fact, the oxygen produced gets released into the atmosphere; this allows the life of all aerobic species on the planet.

In addition to these benefits, in recent years photosynthesis has been considered also from an environmental point of view. The photosynthesis reaction reduces the CO2 concentration in the atmosphere; therefore it could mitigate the negative consequences of increased CO2 emissions due to human activity.

Artificial Photosynthesis: Principle and Problems

The idea behind artificial photosynthesis is to reproduce artificially the natural process that takes place in the plants. In this way, using the energy from the sun, and natural molecules such as CO2 and water, we could obtain carbohydrates, which could be used in many ways, for instance as fuels.

The main problem of this artificial reaction is that a relatively high amount of energy is necessary to perform it.

The step of the process which requires most energy is the conversion of CO2 into CO; this is because, for the conversion to occur, an ionic intermediate species, “CO2”, has to be formed. For this to happen, a negative charge (electron) has to be added to the carbon dioxide, which requires a high quantity of energy.

Recent progress in artificial photosynthesis

In September 2011, Science published the results of a study performed by Professor Masel and his co-workers. The research was performed in collaboration between the company Dioxide Materials, and the Department of Chemical and Biomolecular Engineering of the University of Illinois (USA).

In this study, professor Masel and his coworkers showed how to convert CO2 into CO using a much smaller amount of energy. To achieve this, they performed the reaction using an ionic liquid solution, based on the salt 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4).

Chemical formula of EMIM-BF4. Photo by Clara Piccirillo

Ionic Liquid

EMIM-BF4 is a salt that, when liquid, keeps its ionic structure; the negative ion is BF4 while the positive one is the carbon- and nitrogen-based ring (see the Figure to the left).

In the presence of this ionic liquid, a complex gets formed between the “CO2” and the liquid itself; this facilitates the formation of the “CO2” intermediate. In this way, a smaller amount of energy is necessary to form the intermediate.

The complex between the “CO2” and the EMIM-BF4, however, is reversible; this means that, subsequently, the reaction can continue and “CO2” can be converted into CO.

Important Results

Commenting on this study, Professor Masel said:

“We observed a substantial decrease in the energy needed to convert CO2 into CO – about 0.3 Volts instead of almost 1 Volt. Furthermore, the reaction efficiency is quite high, about 96 % of CO2 is converted into CO, with only 4 % of hydrogen H2 produced as by-product. More work needs to be done on this process, to make it faster, to scale it up and make it commercially viable. These results, however, are very important, as they are a step in the right direction, to make artificial photosynthesis really feasible.”

This success could lead to further advances in the field, and a more successful artificial photosynthesis process.

Sources

B.A. Rosen et al.: “Ionic Liquid–Mediated Selective Conversion of CO2 to CO at Low Overpotentials.” Science DOI: 10.1126/science.1209786. Accessed November 2011.

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