Reversible electrochromic performance of Prussian blue coated with proton conductive Ta₂O₅·nH₂O film

Electrochromism of Prussian blue (PB) involving proton insertion/extraction was investigated in an aqueous HCl solution using a Ta₂O₅·nH₂O/PB laminate, formed by spin-coating a peroxo-polytantalate solution on an electrodeposited PB film. Reversible and stable electrochromic performance was observed with more than 10000 repeated coloring and bleaching cycles, demonstrating that the film of Ta₂O₅·nH₂O serves as a proton-conductive protecting layer to prevent PB from dissolving in a strong acid solution. Insertion behavior of proton into the PB framework is compared with those for K⁺, Rb⁺ and Cs⁺ by means of a cyclic voltammogram and equilibrium potential measurements. The equilibrium potentials measured as a function of the degree of intercalation have shown that the maximum accommodation of proton is 3.0 (per Fe³⁺₄[Fe¹¹(CN)₆]₃ unit), which is in agreement with K⁺ and Rb⁺. However, the potential-composition curve observed with the PB-proton system is quite different from others not only in the potential range but also in shape, featuring by a conspicuous inflection at the middle region of the intercalation level. The intercalation behaviors are also discussed using a lattice statistical model.