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

Abstract

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 CH3SO3H 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.

Cold sprayed WO3and TiO2electrodes for photoelectrochemical water and methanol oxidation in renewable energy applications

“…The observed photocurrent onsets of around 0.4 V vs. NHE are very consistent with literature values reported for thick and thin film TCO/WO 3 photoanodes in acidic media. 10,11,21,39 The observed photocurrent onset potentials are slightly more positive than the measured flat-band potentials of the cold sprayed electrodes. They were calculated from Mott-Schottky plots to be E FB = −0.35 ± 0.05 V vs. NHE for Ti/ TiO 2 and E FB = 0.24 ± 0.04 V vs. NHE for Ti/WO 3 .…”

“…10 Consequently, the concentration of the electrolyte was reduced to 0.5 M H 2 SO 4 and the cold sprayed TiO 2 on titanium metal and the screen printed TiO 2 on FTO photoanodes were also tested for methanol (model pollutant) oxidation, as displayed in Fig. 7.…”

“…9 However, to the best of our knowledge WO 3 films prepared by cold spray have not been investigated so far. Compared to other coating techniques employed to prepare WO 3 films like doctor blade, 10 magnetron sputtering, 11 atomic layer deposition, 12 spray pyrolysis, 13 and solvothermal deposition, 14 the cold spray technique readily enables an upscaling for the manufacturing of mechanically stable films on metal substrates to an industrial scale with variable electrode sizes. One disadvantage of employing TiO 2 as a photocatalyst is its large band gap energy of 3.0 eV (rutile) which corresponds to an absorption of light with wavelengths ≤413 nm resulting in the utilization of only a very small part of the incoming sunlight (4%-5%).…”

Overestimated solar water splitting performance on oxide semiconductor anodes

“…For TiO2 photoanode, lower O2 evolution amount compared with the amount calculated from photocurrent was also reported with O2 Faradaic efficiencies of 9%-35% in 0.2 mol L −1 H2SO4 under Xe arc lamp illumination [6]. In this consideration, it might be misleading to evaluate the performance of a photocatalytic water oxidation system without measuring the actual O2 evolution rate.Recently, Niederberger and colleagues [7] have clearly pointed out this important yet easily overlooked issue for photoelectrochemical water splitting, i.e., photocurrent alone is insufficient to fully evaluate the performance of a photoanode. In their experiments, four aqueous electrolytes (1 mol L −1 CH3SO3H, 1 mol L −1 H2SO4, 0.1 mol L −1 Na2SO4 at pH 3, and 0.1 mol L −1 Na2SO4 at pH 5.5) were used to explore the influence of the competing reactions over the OER on WO3 photoanodes.…”

“…Recently, Niederberger and colleagues [7] have clearly pointed out this important yet easily overlooked issue for photoelectrochemical water splitting, i.e., photocurrent alone is insufficient to fully evaluate the performance of a photoanode. In their experiments, four aqueous electrolytes (1 mol L −1 CH3SO3H, 1 mol L −1 H2SO4, 0.1 mol L −1 Na2SO4 at pH 3, and 0.1 mol L −1 Na2SO4 at pH 5.5) were used to explore the influence of the competing reactions over the OER on WO3 photoanodes.…”