A team of researchers from the German Helmholtz Center for Materials and Energy Berlin (HZB) and the Dutch Delft University (TU Delft) successfully developed an inexpensive solar energy solution for hydrogen production from electrolyzed water. Methods, related results were published in the recently published "Nature & Communications" magazine.
The system developed by the scientists can decompose water into hydrogen and oxygen through sunlight, which allows solar energy to be converted into hydrogen energy and stored. Professor Roal van De Kroll, director of the Solar Fuel Research Institute at the Helmholtz Berlin Center for Materials and Energy, said: "We have combined the best of both worlds. We have used chemical stability and metal oxides. For a low price, combine it with a good but fairly simple thin-film silicon solar cell to get a cheap, very stable and efficient (hydrogen-hydrogenated) unit.â€
When light enters this relatively simple silicon thin-film battery with a metal oxide layer, the system generates a voltage. The metal oxide layer acts as a photoanode and becomes a place where oxygen is formed. It is connected to the solar cells via a graphite conductive bridge. Since only the metal oxide layer is in contact with the electrolyte, other parts of the solar cell will not be corroded. The platinum coil is used as a cathode, which is where hydrogen is formed. A rough calculation can indicate the potential of this technology: With a German solar energy of about 600 watts per square meter, a system of 100 square meters can separate and generate 3 kilowatt-hours of energy stored as hydrogen in an hour of sunlight.
Scientists systematically studied the role of different metal oxides in the process of separation of light from incident charge up to water decomposition in order to further optimize this process. De Kroll said that, theoretically, the anodic efficiency of yttrium vanadate can reach up to 9%. By using an inexpensive cobalt phosphate catalyst, scientists have significantly accelerated the formation of oxygen at photoanodes. The biggest challenge in the study is the highly efficient separation of bismuth vanadate layers. Although metal oxides are stable and inexpensive, charged particles tend to recombine quickly, rendering the process of decomposing water useless. De Kroll and his colleagues found through research that it is helpful to add extra tungsten atoms in the yttrium vanadate layer. The internal electric field generated by these tungsten atoms can well prevent the occurrence of recombination.
The most important photoanode in the system is made of BiVO4, a metal oxide with tungsten atoms added, and sprayed and coated with an inexpensive cobalt phosphate catalyst. To achieve this goal, scientists sprayed a solution containing germanium, vanadium, and tungsten onto a hot glass substrate, and then evaporated the solvent. By spraying multiple solutions at different concentrations, a highly efficient photoactive metal oxide layer with a thickness of about 300 nanometers was obtained. De Kroll said: "We still don't quite understand why vanadate works very well. But we found that more than 80% of the absorbed photons are used. This is really a record metal oxide and it's also physics. The miracle of learning. The next challenge is to scale such systems to square meters so that they can generate more hydrogen.†(Reporter Li Shan)
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