Effect of pH value of poly(ethylenimine)–H2SO4 electrolyte on electrochromic response of polyaniline thin films

Electrochromic devices (ECDs) are electrochemical cells with one or two electrochemically active coatings that change color during the reduction–oxidation process. In this work an electrochromic polyaniline (PANI) thin film and a viscous electrolyte, formed by mixing a basic aqueous solution of polyethylenimine (PEI) and concentrated sulfuric acid (H2SO4), have been used as the main components of an ECD. The pH value of the electrolyte is a function of the relative proportion between PEI and H2SO4. Fourier transform infrared spectroscopy (FT-IR) results show that for those PEI–H2SO4 complexes of pH value varied from 2 to 9 sulfate ions of the inorganic acid are associated to the imine groups of PEI. If the pH value of the same system is lower than or equal to 1, HSO4- anions appear in the electrolyte together with a notable reduction of the free water content. Electrochemical impedance spectroscopy study of these electrolytes show that the protonic conductivity of the PEI–H2SO4 complex with pH 1 is almost three orders of magnitude lower than that of the same electrolytes but with a pH value between 2 and 6. Optical switch speed of a PANI/PEI–H2SO4 based ECD at 550 nm under a square potential application of +1.2 V and −1.2 V is proportional to the ionic conductivity of the electrolyte of the device.