Gasochromic effect of palladium doped peroxopolytungstic acid films prepared by the sol-gel route

Gasochromic palladium doped peroxopolytungstic acid (Pd:P-PTA) films have been prepared using dip-coating deposition from peroxopolytungstic acid (P-PTA) sols into which PdCl₂ was added in molar ratios Pd:W = 1:125, 1:100, 1:53, and 1:40. These films exhibit reversible colouring/bleaching changes when exposed to hydrogen or hydrogen/argon mixture (4%) and air, alternatively. Gasochromically coloured and bleached films were characterised using in-situ Fourier transform infrared (FT-IR) spectroscopy. The vibrational modes of as-deposited, coloured and bleached films were assigned and the polaron absorption, which characterises the IR spectra of coloured films, was detected. Colouring/bleaching kinetics of films exposed to H₂ and H₂/Ar mixture as a function of the concentration of the catalyst and temperature of heat-treatment is reported. Proton (σ_pr) and electronic (σ_el) conductivities determined from impedance spectra revealed an increase in σ_el from 10^-5 S cm^-1 in bleached state, to 10^-3 S cm^-1 in coloured state, while σ_pr remained constant (10^-2 S cm^-1).