Tandem System of Dyes Sensitized Solar Cell and Photo Electro-Chemical (DSSC-PEC): Investigation of TiO₂-Nanotube/BiOI as Dark Cathode for Conversion of Nitrogen to Ammonia

Ammonia is an important chemical and is widely used in various chemical industrial processes. Today, the Haber-Bosch process is the industry's primary choice in the production of ammonia, which this process will largely produce CO2 gas emissions and is carried out at extreme temperatures and pressures. In order to provide a more environmentally friendly process, it is necessary to look for alternative technologies in synthesizing ammonia that uses renewable energy and a carbon-free emission process. The objective of this study was to create a tandem Dyes-Sensitized Solar Cells-Photo Electrochemical (DSSC-PEC) system that is capable of converting nitrogen into ammonia. In the DSSC cells, the N719 dye was used to sensitize TiO2-nanotubes (TiO2NT/N719), while photoelectrochemical cell as the catalysis zone used a pair of TiO2-nanotubes/BiOI electrodes, where the conversion of nitrogen to ammonia took place and Ti³⁺/TiO2-nanotubes, where water oxidation took place to provide a source of protons. The preparation of TiO2-nanotubes/BiOI was carried out by using the S-CBD (Sequential Chemical Bath Deposition) method with various depositions of bismuth and iodide ions as the precursors. The results showed that the TiO2-nanotubes/BiOI with repeat deposition (five times) had bandgap energy of 2.26 eV and better photocatalytic activity than bare TiO2-nanotubes, where TiO2-nanotubes/BiOI were active in the visible region and gave a higher current density response. The efficiency of the DSSC system is 1.293%, and the efficiency DSSC-PEC solar to ammonia system is 0.026%. The best ammonia produced under irradiation for 24 hours was 0.274 µmol. Though the efficiency of nitrogen conversion is still low at the moment, however, since the process merely uses input energy of visible light and water as a proton source, the proposed device opens the possibility to provide an artificial photosynthesis tool in nitrogen fixation, which has energy and environmentally friendly feature.