H2 is odourless, colourless, highly flammable gas with low ignition energy [1]. Minor leaks from H2
storage tanks in enclosed environments can quickly reach potentially explosive concentration. Highly
reliable H2 sensors are of great demand. Previous studies identified that WO3 film coated with Pd changes
its colour reversibly from transparent to dark blue when exposed to H2 [2]. Pd is a well-known catalyst
because of its efficiency to dissociate H2 molecules, forming hydrogen ions and electron. Various
gasochromic mechanisms have been proposed to explain the coloration of the Pd/WO3 when exposed to
H2 [3]. The widely accepted mechanism is the spillover effect. Based on this effect, the H electrons dissociated by the Pd were spilled into the WO3 lattice resulting in the reduction of W6+ (transparent)
centre to blue-colored W5+ [3, 4].