Ion size dependence in electrochemical polymerization for organic 3D porous networks

Abstract

In the present thesis. we discuss the electrochemical polymerization of conjugated microp orous polymers (CMP) and the effect of electrolyte on polymer morphology and properties. In linear conjugated polymers like PANI, polypyrrole etc., considerable work has been done related to their thin film electrosynthesis under varying conditions like electrode material, electrode preparation procedures, activation post synthesis, electrolyte etc and their effects on optical and electrochemical properties. However, trends observed in such mixed ionic electronic conducting systems may not be assumed in other 3D conjugated porous networks like CMPs. CMPs have gained traction in the last decade due to their intrinsic porous nature and applications in photocatalysis, hole transport, energy storage, biosensing etc. Due to low solution processability, CMPs face a bottleneck in device fabrication. Electropolymerization is an effective method to synthesize thin films of CMPs. To the best of our knowledge, while many reports have studied the effect of variation in parameters like solution pH, electrolyte concentration, potential window onto the morphology of electropolymerized films, systematic reports on electrolyte variation, especially during electropolymerization of CMPs, are nearly nonexistent. Given the amorphous nature of CMPs and the active role of the supporting elec trolyte during electrochemical polymerization, our goal in this thesis was to probe the effect of counterion variation during electropolymerization on various electrochemical properties like electrochromism, impedance etc as well as a primary feature of CMPs- porosity, and ex plore our material’s application in energy storage. We varied the supporting electrolyte used during electropolymerization and analyzed the resultant film’s electrochemical / optical prop erties using various techniques like UV-Vis spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy, spectro-electrochemistry, scanning electron microscopy and atomic force microscopy. We demonstrate the remarkable effect the choice of electrolyte can bring about in various properties of CMPs based on the resultant film’s porosity and fabricate a simple supercapacitor. Compared to trends in linear conjugated polymers, ion size depen dence observed for CMPs is nearly the inverse. The trend of counterion size dependence of porosity in CMPs suggests that the electrolyte choice must be treated as an important parameter to be optimized during device fabrication through electrochemical polymerization in non-controlled environments. Our study reveals direct influence of ion size on porosity in CMPs and hints towards possible ion geometry dependence in electropolymerization of CMPs.