Artificial photosynthesis, clean energy imitating nature

 

 

By Science Columnist Ho-Gwan Ko

 

 

We have an energy source that will probably not be depleted until the destruction of human civilization. Not only us but almost all living beings on the Earth depend on this energy source: the sun. Without it, no life would have been born on this planet. Therefore, the enormous energy that the sun sends to the Earth in the form of light and heat can be called the source of life.

Plants use solar energy directly. They make carbohydrates through photosynthesis using sunlight, carbon dioxide, and water. During the process, the light energy is converted into and stored as chemical energy. Accordingly, plants can make their own energy without eating other life forms. In the process, they also produce oxygen, helping other animals breathe. Animals eat plants, indirectly using solar energy.

Isn’t it amazing that they can make energy only using sunlight? Besides, the photosynthetic process uses carbon dioxide, the culprit of the greenhouse effect. It is like killing two birds with one stone, since it prevents global warming as well as produces energy. In fact, we have been using electricity by converting sunlight into energy using solar cells. However, there is an inevitable loss in energy during the process of storing electricity or converting it to chemical energy. Compared to this, photosynthesis enables plants to convert sunlight directly into chemical energy, store it, and use the energy when necessary.

 

 

▲ Plants make energy with sunlight through photosynthesis and store it, and other animals use it.

 

The idea of artificially replicating photosynthesis was first introduced in 1912 by Italian chemist Giacomo Ciamician. He believed that artificial photosynthesis would reduce fossil fuel use by utilizing sunlight and narrow the economic gap between rich European and poor southern hemisphere countries. “If in a distant future the supply of coal becomes completely exhausted, civilization will not be checked by that, for life and civilization will continue as long as the sun shines!” he explained.

Since then, scientists have started research on how to mimic photosynthesis. It consists of two main processes: First, chlorophyll in plants absorbs light energy to produce chemical energy. This process is called a light reaction. Second, carbohydrates are made using the outputs of light reactions and carbon dioxide, which is called a dark reaction.

Scientists are mainly focused on mimicking the light reaction process. Plants split water with sunlight, creating electrons that will be used for chemical reactions. Artificial photosynthesis reproduces this process using materials that absorb light in replacement of chlorophyll. Being able to make carbohydrates artificially can also help solve global food problems.

Unlike plants, such processes can be used for making high-value-added materials such as methanol and formic acid instead of carbohydrates. Methanol can be used as a fuel in motor engines, replacing petroleum, and formic acid, a raw material for various chemical products, is used to make rubber products, cleaning agents, and pesticides. It can be also used for raw materials for plastics.

In addition, hydrogen is generated during the process of breaking down water, which has gained attention as environmentally friendly energy. Since it can be used for automobiles as well, research on hydrogen vehicles has been actively conducted. Burning hydrogen produces water, emitting no greenhouse gases, such as carbon dioxide and other pollutants. Today, hydrogen is extracted from natural gas or produced through the electrolysis of water; during the process, energy is put in and carbon dioxide is emitted. On the other hand, artificial photosynthesis can solve this problem.

However, we still have a number of problems to solve until the commercialization of artificial photosynthesis. The biggest problem is low efficiency. The photosynthetic efficiency of plants is 3-6%. However, that of artificial photosynthesis has not yet reached 1%. Having been studied for a long time, it cannot beat fossil fuels due to its high cost. Its efficiency should improve a great deal in order to be used for commercial purposes.

There have been a great many efforts around the world to improve the efficiency of artificial photosynthesis. As research results are steadily coming out, we will see the day when energy and various useful substances are created through direct use of sunlight just as plants do.