A team of engineers from the Berlin center of materials and energy Helmholtz (HZB) and the Delft University of Technology (Technische Universiteit Delft) have developed a new technique for the production of so-called solar hydrogen.
Engineers have created a simple and elegant device that can split water molecules into hydrogen and oxygen, using only the energy of our sun. This technology is called artificial photosynthesis, and it can be used to store the sun's energy in the form of hydrogen. Then, hydrogen can be used either directly or in the form of methane, or it can be used to generate electricity in a fuel cell.
They used a conventional solar cell and metal oxide anode, and in the course of the experiment were about five percent of the solar energy converted into chemical — in the form of hydrogen.
Dutch established solar cell is much easier than ternary compound based on amorphous silicon semiconductors or costly group III-V, which are generally used in this field.
The anode was made from bismuth vanadate (BiVO4) with a small amount of tungsten atoms. Its sprayed on a piece of conductive glass and topped with cobalt phosphate, which acts as a catalyst.
The scheme of the new invention of Dutch engineers (Figure TU Delft).
"We've combined the best of both worlds: they took an inexpensive metal oxide chemically stable and added a thin film of silicon-based solar cell. The result is a budget device for the production of highly efficient solar fuel "- enthusiastically talks about the result of their labors head of the Institute of solar fuel at the center of the Berlin material and energy Roel van de Krol (Roel van de Krol).
The metal oxide layer — only part of the photodetector is in contact with the water and acts as an anode in the production of hydrogen. It is also used to prevent corrosion (rust) on the sensitive silicon element. In the study, researchers optimized the processes of light absorption, charge separation and disintegration of water molecules.
Besides, during manufacture of the designs they could solve another important problem: due to the fact that they used an inexpensive catalyst — cobalt phosphate — the formation of oxygen able to greatly accelerate the photoanode.
The most difficult task was to separate the electric charges of the vanadium-bismuth film. Metal oxides can be inexpensive and stable, but the charge carriers tend to rapid reorganization. In this case, water molecules are split would be impossible. This problem was solved by adding a small amount of tungsten atoms in the film of the bismuth vanadate.
"It was important to distribute these atoms in a special way, so that they create an electric field that prevents the reorganization of the charge carriers," — says van de Krol in a press release.
Lead author van de Krol at work (photo TU Delft).
To this solution researchers took compounds of tungsten and bismuth vanadate, and spray it onto the heated plate of conductive glass, and then part of the solution evaporates. The scientists tested the spray solutions with different concentrations of tungsten as long as a highly efficient metal oxide film with a thickness of 300 nanometers.
"In theory, the efficiency of conversion of solar energy into chemical can be up to 9%, assuming the use of an anode made of bismuth vanadate," — says van de Krol. Dutch scientists have also obtained a result of 5%, and due to its improvements they hope to bring the converter efficiency to 10%.
"Frankly, we do not yet understand exactly why bismuth vanadate works better than other metal oxides. We found that about 80% of the photons incident on the surface contribute to the resulting electric charge. This unexpectedly high result that makes our invention record among metal oxides in this area ", — says van de Krol.
In the next phase of work engineers plan to create a solar cell, the anode area of a few square meters, able to stock enough hydrogen for a demonstration.
Scientists have calculated that the "performance" of the sun in Germany is approximately 600 watts per square meter. This means that 100 square kilometers of the system is theoretically able to store about three kilowatt-hours of energy in the form of hydrogen for one hour at sunny weather. Energy reserves can be used at night or on cloudy days.