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

Abstract

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.