Investigation of electrochromism process through ultrafast broadband spectroelectrochemistry

Abstract

Electrochromism is the reversible change in color of an electroactive material due to a redox reaction induced by the application of voltage. Electrochromic materials have gained much research attention since the discovery of organic conducting polymers in the late 1970s, with high prospects for diverse applications. Color change in electrochromic materials involves two processes. First, a redox reaction that leads to the oxidation or the reduction of the EC material. Then the intercalation of counter-ions into the material for charge balancing. For instance, to effect a color change in a neutral conducting polymer through oxidation, first, an oxidizing potential is applied and the polymer loses electrons in a heterogeneous electron transfer leading to a net positive charge creation. To achieve further oxidation, electroneutrality is partially restored by anions that diffuse from the bulk solution into the polymer. Since the diffusion rate of counter-ions is much slower than electron transfer, it is widely assumed that the oxidation process is rate-limited by ion transport. The main aim of this work is to put this assumption to test. Relying on the fact that heterogeneous electron transfer is much faster than ionic charge transfer, experiments at very short time scales down to a few microseconds are performed. Under such experimental conditions, counter-ions do not have enough time to move into the electrochromic material and any sign of electrochromism is due to electron transfer. Through Ultrafast Cyclic Voltammetry with online iR compensation, sweep rates in the order of kV/s to MV/s are reached. We used thin films of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) used as the electrochromic material in this work. In monitoring the color change of the polymer film, we employ in situ UV-visible spectroscopy. To keep up with the rapid conversion of the polymer between its redox forms, we collect spectra in aggregate mode where several spectra are averaged into a single spectrum. By using a principal component analysis algorithm, the compound spectra are decomposed and we calculate the time that the polymer spends in each of its redox forms during the Ultrafast Cyclic Voltammetry experiments.