Solar photoelectrocatalytic oxidation of urea in water coupled to green hydrogen production

In past decades, the intensification of human activities has led to an increase in pollution and energy demand. Photoelectrochemical systems have emerged as an alternative for the decentralized management of domestic wastewater with the potential of recovering energy while degrading pollutants such as urea. Tungsten oxide (WO₃) has been traditionally used for water splitting, but the use of this material for the removal of waste from water coupled to hydrogen production is not deeply known until now. This contribution shows an exhaustive and systematic investigation on WO₃ photoanodes for the photoelectrochemical oxidation of urea and the generation of hydrogen, with insights on the reaction mechanism, detailed nitrogen balance investigation of the process, and analysis of the performance compared to well-accepted materials. The WO₃ platelets were successfully synthesized in situ on fluorine doped tin oxide glass by a hydrothermal method. The performance of WO₃ was compared to titanium dioxide (TiO₂) as a benchmark. The photocurrent was enhanced for both electrodes when urea was added to the electrolyte, with WO₃ showing one order of magnitude higher photocurrent than TiO₂. The WO₃ electrode showed a peak incident photon-to-current efficiency of 43% at 360 nm and a much greater rate constant for urea oxidation (1.47 × 10⁻² min⁻¹), compared to the TiO₂ photoanode (16% at 340 nm and 1.1 × 10⁻³ min⁻¹). The influence of different reactor configurations was also evaluated testing one- and two-compartment back-face irradiated photoelectrochemical cells. Hydrogen was generated with a Faradaic efficiency of 87.3% and a solar-to-hydrogen conversion efficiency of 1.1%. These findings aim to contribute to the development of technologies based on the photoelectrochemical production of hydrogen coupled with the oxidation of pollutants in wastewater.