Influence of annealing temperature of WO₃ in photoelectrochemical conversion and energy storage for water splitting

The current work demonstrates the importance of WO₃ crystallinity in governing both photoenergy conversion efficiency and storage capacity of the flower structured WO₃ electrode. The degree of crystallinity of the WO₃ electrodes was varied by altering the calcination temperature from 200 to 600 C. For the self-photochargeability phenomenon, the prevailing flexibility of the short-range order structure at low calcination temperature of 200 C favors the intercalation of the positive cations, enabling more photoexcited electrons to be stored within WO₃ framework. This leads to a larger amount of stored charges that can be discharged in an on-demand manner under the absence of irradiation for H ₂ generation. The stability of the electrodes calcined at 200 C, however, is compromised because of the structural instability caused by the abundance insertion of cations. On the other hand, films that were calcined at 400 C displayed the highest stability toward both intercalation of the cations and photoelectrochemical water splitting performance. Although crystallinty of WO₃ was furthered improved at 600 C heat treatment, the worsened contact between the WO₃ platelets and the conducting substrate as induced by the significant sintering has been more detrimental toward the charge transport.