Efficient Anodized WO ₃ Photoanode for Photoelectrocatalytic Applications: Hydrogen Production and Reduction of CO ₂

Abstract In this work, a nanostructured WO 3 photoanode has been used for photoelectrochemical H 2 production and CO 2 reduction. In particular, we provide a novel method to synthesize tungsten oxide catalysts by anodization of tungsten using an ionic liquid, [EMIN][BF 4 ], as electrolyte in hydrodynamic conditions. We found that the use of appropriate hydrodynamic conditions (200–400 rpm) provides larger and more homogenous nanostructures with higher surface area. All this leads to better morphological and electrochemical properties. An excessive rotation during the synthesis (600 rpm) breaks the uniformity of the morphology of the nanostructures, thus hindering the photoelectrochemical performance. This way, the use of an optimized WO 3 photoanode has shown excellent potential for photoelectrocatalytic water splitting. Moreover, these nanostructures also present good performance in the photoelectrocatalytic CO 2 reduction, leading to the main formation of acetic acid, formic acid, and methanol. In fact, after only 6 h in continuous mode, a remarkable formic acid concentration of 190 µmol/L at 6 h has been obtained. Moreover, we have illustrated the great importance of the anode during the PEC reaction, significantly influencing the concentration of the obtained products owing to the electrons and protons transferred from the oxygen evolution reaction (OER).