Photoelectrochemical water splitting using WO3 photoanodes: the substrate and temperature roles

The influence of a substrate on the performance of WO3 photoanodes is assessed as a function of temperature. Two samples were studied: WO3 deposited on a FTO glass and anodized on a tungsten foil. Current-voltage curves and electrochemical impedance spectroscopy measurements were used to characterize these samples between 25 °C and 65 °C. The photocurrent density increased with temperature for both samples and the onset potential shifted to lower potentials. However, for WO3/FTO, a negative shift of the dark current onset was also observed. The intrinsic resistivity of this substrate limits the photocurrent plateau potential range. On the other hand, this behavior was not observed for WO3/metal. Therefore, the earlier dark current onset observed for WO3/FTO was assigned to the FTO layer. The optimal operating temperatures observed were 45 °C and 55 °C for WO3/FTO and WO3/metal, respectively. For higher temperatures, the bulk electron-hole recombination phenomenon greatly affects the overall performance of WO3 photoanodes. The stability behavior was then studied at these temperatures over 72 h. For WO3/FTO, a crystalline-to-amorphous phase transformation occurred during the stability test, which may justify the current decrease observed after the aging period. The WO3/metal remained stable, maintaining its morphology and good crystallinity. Interestingly, the preferential orientation of the aged crystals was shifted to the (-222) and (222) planes, suggesting that this was responsible for its better and more stable performance. These findings provide crucial information for allowing further developments on the preparation of WO3 photoanodes, envisaging their commercial application in PEC water splitting cells.