Commercially Available WO₃ Nanopowders for Photoelectrochemical Water Splitting: Photocurrent versus Oxygen Evolution

WO₃ photoanodes with remarkable photocurrent densities are presented. These photoanodes were prepared from three different commercially available WO₃ nanopowders. Doctor blading of the nanopowders followed by a short annealing in air led to nanostructured films. The best photoanode showed a photocurrent density of 3.5 mA cm^-2 at 1.23 V vs. RHE in 1 m CH₃ SO₃ H under AM 1.5 G illumination (100 mW cm^-2 ), surpassing values reported so far for bare WO₃ photoanodes. The study also showed that the photocurrent was strongly dependent on the electrolyte, indicating oxidation of the electrolyte rather than of water. Oxygen evolution measurements performed in different electrolytes revealed that the amounts of oxygen were highly dependent on the electrolyte. By comparing the photocurrent values in the different electrolytes with the amount of evolving oxygen, it was found that the electrolyte producing the highest photocurrent was the electrolyte with the lowest oxygen evolution. Stability measurements showed that the more oxygen is produced, the less stable is the photoanode. These results clearly underline the difficulty to correlate the photocurrent values with oxygen evolution, drawing the attention to one of the major limitations of photoelectrochemical water splitting.