Researchers at the Solar Energy Institute in Berlin, Germany, at the Helmholtz Center in Berlin and Delft University of Technology in the Netherlands, used a simple solar cell and metal oxide photoanode to achieve a 5% hydrogen transfer rate. This is a breakthrough because the use of solar cells is much simpler than the commonly used triple-point amorphous silicon film or III-V semiconductor high-performance battery.
Researchers said that they combined the advantages of chemical stabilization with the low price of metal oxides, combined with a relatively simple silicon-based thin-film solar cell, and finally achieved a low cost, good stability, and powerful battery. Calculated on the basis of 600 watts of solar energy per square metre in Germany, the 100-square-metre hydrogen production system can store 3 kilowatt-hours of hydrogen energy in sunlight for an hour or night.
Experts combine a simple silicon-based thin-film battery with a layer of metal oxides using yttrium vanadate to act as a photoanode. Because only the metal layer is in contact with water, sensitive batteries can be protected from corrosion. Photoanodes using yttrium vanadate can theoretically make the efficiency of electrochemical cells reach 9%. The experts also significantly accelerated the formation of oxygen in the photoanode by means of a low-cost cobalt phosphate catalyst.
The biggest challenge is the efficient separation of the charge in the yttrium vanadate layer. Although metal oxides have many advantages, charge carriers are easily recombined and lose the function of decomposing water. Researchers found that adding tungsten to the bismuth vanadate layer helps solve the problem. Tungsten atoms are optimally distributed, thereby creating an internal electric field that prevents recombination. This is done by spraying neodymium, vanadium, and tungsten onto a hot glass substrate to vaporize the solvent. After repeated spraying with different tungsten concentrations, a high-efficiency metal active oxide layer of about 300 nm in thickness was finally formed. Although researchers cannot explain why vanadate has such a good effect, it can be determined that more than 80% of the captured photons can be used, which is definitely a new record for metal oxides. The next step is to extend this system to a square meter scale.
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