Spatiotemporal colorimetry to reveal electrochemical kinetics of poly(o-toluidine) films along ITO surface

Sheet resistance of transparent conducting electrodes can affect dramatically the electrochromic response of surface confined active species in any electrochromic device. The generated gradient of effective potential on the electrode surface has a direct impact on the electrochemical kinetics of these species. In this work, the electrochromic poly (o-toluidine) (POT) film, a derivative polyaniline, was deposited along a narrow, long transparent indium tin oxide (ITO) supporting electrode with a sheet resistivity of 30 Omega cm(-1). This configuration was used as a model to simulate a surface electrical conduction from the external metallic contact to the opposite extreme of ITO with an important and increasing ohmic drop from 30 Omega to 170 Omega. The color changes on the electrode surface were investigated and parametrized by RGB digital video. The spatiotemporal analysis of color intensity in the red, green and blue channels allowed three electrochromic transitions of POT to be separated. For the first time, the pattern of POT electrochemical kinetics along the electrode surface was revealed. For that, apparent electromonochromatic coefficients were calculated to convert the adimensional RGB intensities to electrochemical currents. The results were explained using a numerical finite difference model varying the ohmic drop and film thickness. The methodology here exposed can be used to characterize electrochemical response of electrochromic devices. (c) 2018 Elsevier Ltd. All rights reserved.