High-temperature tungsten trioxides obtained by concentrated solar energy. Physicochemical and electrochemical characterization

High-crystalline tungsten trioxides (WO${}_3$) have been synthesized by an environmentally friendly method using concentrated solar energy. The obtained tungsten trioxides (WO${}_3$) at three different temperatures and two oxygen mole fractions used for the highest synthesis temperature were characterized by XRD, SEM, and XPS. Higher crystallinity and concentration of W${}^{5+}$ was observed in tungsten trioxides as the synthesis temperature increased. Nevertheless, despite of the different synthetic conditions used, a mixture of two different crystalline structures was observed in all solar-prepared tungsten trioxides: monoclinic and triclinic. Comparing oxides obtained at 1000 °C, higher concentration of W${}^{5+}$ and more defects were found when using lower oxygen molar fraction (WO${}_3$-1000-2). Their electrochemical performance was evaluated using cyclic voltammetry (CV) in a conventional three-electrode cell in the following three aqueous electrolytes: acidic, alkaline, and neutral media. In the acidic medium, all the tungsten trioxides showed a capacitive behavior, which was enhanced for oxides obtained at 1000 °C due to a mixed valence of W. On the other hand, in the alkaline medium, a catalytic behavior was detected with higher activity towards hydrogen evolution reaction for the oxide with more defects, higher crystallinity, and monoclinic phase, obtained at 1000 °C and a lower oxygen molar fraction in the synthesis. Finally, in the neutral medium, the oxides synthesized at 1000 °C presented a capacitive behavior whereas the oxides prepared at the lowest temperatures (600 and 800 °C) presented electrochemical processes related to a catalytic behavior for water reduction, which must correspond to their minor concentration of defects, as confirmed by XPS.