Thermomechanical\nand Structural Analysis of WO<sub>3</sub> Array for Optimized Photoelectrochemical\nChloride Oxidation\nPerformance

Understanding the crystal structure of WO<sub>3</sub> is essential\nfor optimizing its photoelectrochemical performance. This study comprehensively\nanalyzes the structural characteristics of WO<sub>3</sub> during synthesis\nand investigates their correlation with photoelectrochemical activity.\nStructural analysis, incorporating annealing procedure and WO<sub>3</sub> thickness, identifies a blend of hexagonal, monoclinic, and\northorhombic phases within WO<sub>3</sub> array. Specifically, detailed\nanalysis reveals a predominance of monoclinic WO<sub>3</sub> phase\nalongside the orthorhombic WO<sub>3</sub> phase, both of which are\ncommonly characterized by their monoclinic structure. Three-dimensional\nthermomechanical simulations using the finite element method reveal\nthat thermal displacement in WO<sub>3</sub> layers increases with\nthickness during the thermally induced synthesis process. These results\nhighlight a direct correlation between WO<sub>3</sub> thickness, thermal\ndisplacement, and phase transition, with thicker layers favoring the\ntransformation from orthorhombic to monoclinic structures due to increased\nthermally induced deformation. The heightened monoclinic structure,\nwhich possesses lower symmetry than the orthorhombic structure, induces\nmore defect sites, suggesting increased donor density. Notably, the\nmonoclinic-dominated WO<sub>3</sub> exhibits superior performance\nunder UV–visible irradiation in 0.5 M NaCl. Furthermore, the\nWO<sub>3</sub> array demonstrates over 85% Faradaic efficiency for\nchloride oxidation, indicating preferential selectivity over oxygen\nevolution reaction in 0.5 M NaCl. This study emphasizes the pivotal\nrole of the crystal structure of WO<sub>3</sub> in achieving efficient\nphotoelectrochemical seawater splitting.