Scientists from HZB and TU Delft, The Netherlands, were able to store five percent of solar energy in the form of hydrogen using a solar cell and a photo anode made from metal oxide. The new solar fuel device is cost effective, highly stable and highly efficient.
The system is a combination of cheap and chemically stable bismuth vanadate metal oxide and a silicon-based thin film solar cell. The scientists were able to initiate the so-called artificial photosynthesis, where sunlight splits water into hydrogen and oxygen, allowing chemical energy storage. The hydrogen product can be used as fuel directly, or it could be stored in a fuel cell to produce electricity.
The team of scientists, led by Prof. Dr. Roel van de Krol, head of the HZB Institute for Solar Fuels, added a thin layer of the metal oxide to a solar cell, which not only acted as a photo anode for oxygen formation, but also prevented the cell from corrosion. They then optimized the processes of light absorption, separation of charges and splitting of the water molecules. In order to accelerate oxygen formation using the photo anode, the team also added cobalt phosphate catalyst.
The efficiency of separation of electrical charges within the metal oxide film was increased by spraying a solution of wolfram atoms onto the a heated glass substrate to form an internal electric field. This prevented the charge carriers within the metal oxides from recombining. Repeating the process of spraying resulted in evaporation of the solution and creation of 300 nanometers thin photo-active metal oxide film.
The bismuth vanadate was found to be much more efficient than any other metal oxide with more than 80% of the incident photons contributing to the electrical current. The team is now busy with scaling up the process in order to produce sufficient amounts of hydrogen.