Influence of Ta5+ dopant on the proprieties of the WO3 photoanode used for hydrogen production

Full text: Hydrogen production via water dissociation using a photoelectrochemical cell (PECC) represents one of the top research topics in the renewable energy domain. If the solar and wind energy represent a solution for the static objectives, the requirement for replacing the fossil fuels in the mobile objectives (for transport) can be fulfilled by hydrogen only. The PECC contains two electrodes where at least one of them should be an optical active material. Under the light radiation a redox system is formed between the electrodes, what leads to obtaining hydrogen and oxygen. Choosing the electrodes materials is a difficult task if one considers two important factors: - the PECC should work using only the (sun)light radiation; - the electrodes are immersed in different electrolytes and as consequence must have high chemical stability. Our previous study, have shown that a PECC containing tungsten trioxide photoanode and platinum cathode can work only by applying a bias of 0.5 eV. The WO3 has shown a good conductivity but a poor chemical stability. In order to improve the photoanode performance we doped the WO3 with Ta5+. The Ta5+ ions were chosen considering the structural and ionic radius compatibility with the W6+ ions. We have doped the WO3 films with 5%, 2.5%, 2%, 1% and 0.5% tantalum ions. The deposition technique used for the film production is Spray Pyrolysis Deposition (SPD) which allows tailoring the material and is not expensive. The precursors were tungsten ethoxide (W(C2H5)6) and tantalum ethoxide (Ta(C2H5)5), the solvent was ethanol and the complexing agent was acetylacetonate. The conduction analysis in liquid state (doped WO3 films immersed in liquid) shows an enhancement of electrical conductivity and chemical stability of the films. The structural analysis proves high crystalline structure for all doped films even without annealing treatment after the deposition. The efficiency of the photoanode was increased from 0.75% for the WO3 doped with 5% Ta5+ to almost 4% for the WO3 doped with 0.5% Ta5+. In doping the WO3 films with tantalum ions two aspects should be considered: using 5% dopant we increase the chemical stability of the film but the optical response decreases. The structural stabilization is an important factor of the doping process and establishing an equilibrium between this properties and the photoconductivity of the material is a critical requirement. The WO3 film doped with 0.5% Ta5+ gives better results from the optical and electrical point of view. (authors)