published on 18 May 2011 in energy

Artificial photosynthesis

Science is inspired by nature
The history of science and technology is packed with discoveries and inventions inspired by nature. After all, evolution has had billions of years to find the ideal condition which allows each organism to survive and adapt perfectly to every situation and environment. Even Leonardo da Vinci designed his glider observing the structure of the chiropteran wing. In fact, bats all over the world have a thin membrane of skin (called the patagium) that stretches between the fingers of their huge hand, their hind legs and tail. The great inventor built his wings following the same design: a canvas for the wingspan and a wooden frame to stretch and develop it. Another widespread invention has a really remarkable origin. The Swiss engineer, Georges de Mistral, on returning from his beloved walks with his dog, often had to clean his clothes and the fur of his animal friend from the sticky fruits of burdock, a very common plant in the countryside. Intrigued by this fact, the engineer observed these bothersome small balls under the microscope and noticed that they were covered in hooks. When they come into contact with the cloth of a suit or an animal’s fur, the hooks cling strongly to the fabric or hairs: this is the brilliant mechanism with which the burdock spreads its fruits. From this observation de Mistral invented Velcro, which now opens and closes our clothes, our shoes or our camping tents with the distinctive ripping sound. We owe the English inventor Percy Shaw another important creation inspired by nature. Studying the mechanism which allows cat’s eyes to reflect light, he invented the first reflector (cat’s eye) which he patented in 1935. Today this inborn disposition of man to observe and mimic nature has become a true science: biomimetics. There are waterproof materials, similar to the surface of a lotus leaf, which are self-cleaning or computer screens that work according to the same physical mechanism which gives butterflies’ wings infinite shades of colour. This new course of science  also has applications in technologies regarding energy.

Natural photosynthesis
For billions of years green plants have obtained the energy they require to live directly from the Sun by means of a series of chemical and physical reactions which taken together are called photosynthesis. The general scheme of the photosynthetic process is very simple, while its functioning is slightly more complex. Plants combine six molecules of carbon dioxide captured from the air with six molecules of water to obtain one molecule of glucose (a very simple sugar) and six molecules of oxygen. The whole process takes place thanks to sunlight. Therefore solar energy allows the transformation of low-energy simple compounds (water and carbon dioxide) into complex high-energy compounds (sugar). Thus plants collect and store the Sun’s energy in the form of chemical energy. We, together with all the other animals, breathe the oxygen freed into the air by plants and feed directly or indirectly on the energetic molecules which plants produce thanks to photosynthesis.

How photosynthesis works
Photosynthesis takes place in small organelles called chloroplasts that are present in plant cells. Pigments such as chlorophyll (green) and carotenoids (red) that are contained in the membranes of these organelles capture the sunlight and transform it into small negative charges (electrons). The electrons produced are required to split the water molecule into its two fundamental components: hydrogen and oxygen. The oxygen is released into the atmosphere while the hydrogen is needed to transform carbon dioxide into glucose during the night. The amount of solar energy captured by photosynthesis in the world each year is immense, equal to six times the energy consumed by the whole of mankind. Photosynthesis is also the source of food of every living creature because each year it transforms about 115 billion tonnes of carbon dioxide into organic compounds, which can be assimilated by organisms. Look at the experiment “Photosynthesis in action“.

Copying plants
The idea of learning from plants and copying a mechanism similar to photosynthesis to obtain energy from the Sun is several years old. However, all the solutions tested so far have not been very efficient and use rare, expensive or polluting materials. Therefore the objective to reach is to find an inexpensive, clean and efficient system. One of the most important research centres in the world, the Massachusetts Institute of Technology (MIT), seems to have won this challenge. At the MIT, professor Daniel Nocera, who has been studying how to produce an artificial leaf for years, has invented a device that is able to transform solar energy into electricity anywhere and at a low price. Nocera’s leaf is minute, slightly bigger than a playing card, and has been made with inexpensive materials. The prototype built in the MIT laboratories, contrary to earlier models, is stable, reliable and can work non-stop for 45 hours. Let us see how it works. At the heart of the artificial leaf are two rather common metals, nickel and cobalt, which act as catalysts. When the device, immersed in the water, is illuminated by the Sun, it breaks the water molecules and splits hydrogen from oxygen. The hydrogen and oxygen produced are immediately stored in two separate tanks and recombined in a fuel cell, which generates heat and electricity by transforming the two gases back into water. The most ecological aspect of the system is the total absence of harmful emissions and the continuous use of the same water over and over again. These characteristics, together with the leaf’s reasonable price, will make the device appealing mainly in the poor and dry areas of the planet. In fact Professor Nocera claims that, with just one bottle of water, his leaf will be able to produce the electric energy required to satisfy the needs of a family in developing countries. We met Professor Nocera and asked him to tell us about his wonderful discovery.

Energy even during the night
The main negative aspect of traditional solar energy, obtained by installing photovoltaic panels, is the intermittence of generation. In fact, without the Sun, the panels are not active. To be able to use the electric energy produced during the hours of light even during the night, electricity accumulators (basically large batteries) are needed. Actually there are no accumulators which are economical and efficient enough so that this solution can by applied on a large scale. Plants have solved the problem of energy conservation by converting low-energy substances into high-energy molecules. In other words, the sugar produced by photosynthesis works like a chemical battery which absorbs and stores the Sun’s energy. Professor Nocera’s artificial leaf, therefore, resembles the natural model even in this important aspect. Sunlight is converted into chemical energy by the catalyst and can be stored as hydrogen and oxygen.

Hydrogen from water
Producing hydrogen from water is quite simple. When a continuous electric current passes through water, oxygen and hydrogen are produced. This operation is called electrolysis because it splits water using electricity and it was studied by the chemist Michael Faraday in 1832. Electrolysis is easily achievable (Look at the experiment “Electolysis of water“). As we have seen, even the artificial leaf  breaks the water molecule and releases hydrogen, used to feed a fuel cell. The substantial difference between electrolysis and the artificial leaf, however, is the source of energy: the former uses electricity, the  latter sunlight. Therefore the former is expensive, while the latter is not, because sunlight is free of charge. Thus artificial photosynthesis takes place by means of water photolysis, that is, a separation using light.

Future applications
We asked professor Nocera which the energetic future of humanity would be and what role his invention would play. This was the answer he gave us:
“In the future we will have a different relationship with energy. Today we all obtain electricity from a socket in the wall through a distribution network. This means that somewhere there is a big power station which delivers energy to everyone; in other words, a centralised system. I think that in the future each person will be able to generate the energy which he/she requires, autonomously and where it is needed. My invention moves in this direction. For example, my leaf only needs the Sun’s rays and water. The leaf is put into water and it produces the required energy. In many poor countries there is no electricity distribution network. My discovery will be useful especially to them. Many praise me because I wish to help poor people to have energy in an economical and clean way. I usually answer that actually it is the poor people who help me because, if this new way of conceiving energy should become widespread, then also the rest of the world will understand that there is a possibility of making our lives self-sufficient and sustainable from an energetic point of view”

Written by Andrea Bellati

Proceeding of the National Academy of Sciences of the United States of America
PNAS _ October 24, 2006 _ vol. 103 _ no. 43 _ 15729–15735

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