Influence of Oxygen Impurities on the Electrochromic Response of Viologen-Based Plastic Films

This paper concerns the coloring kinetics of electrochromic films obtained by dispersing viologen molecules in a thermoplastic polyacrylate matrix. The experimental data show that the oxygen molecules, which are originally dissolved as ubiquitous impurities in film chemical precursors, and are not eliminated during the film preparation, play a relevant role with respect to the speed of the electrochromic response in the coloring stage. The presence of oxygen can speed up the electrochromic response of the viologen cations or slow it down, depending on the degree of polymerization of the plastic matrix. In stiffer films oxygen accelerates the electrochromic response, while the contrary occurs in less stiff films. A theoretical model of the viologen kinetic electrochromic response has been developed to justify the experimental findings. This takes into account the possible electron exchange reactions with the oxygen molecules. A coherent interpretation of the experimental data has been obtained on the basis of this model.