Ion Dynamics at Carbon-Grafted-Polypyrrole Electrode–Electrolyte Interfaces: Study on Charge Carrier Mobility and Ion Co-Adsorption in Liquid and Hydrogel Electrolytes by Electrochemical, Gravimetric, and Computational Methods

We analyzed ionic processes within polypyrrole (PPy) covalently attached to graphene, multiwalled carbon nanotubes (MWCNTs), and single-walled carbon nanohorns (SWCNHs), operating in aqueous and hydrogel electrolytes that contain KCl, NaCl, LiCl, KBr, NaBr, and LiBr salts. Computational studies were carried out for a five-membered PPy chain with a single charged pyrrole using molecular mechanics (MMFFaq) and density functional theory (DFT B3LYP/6-31G). The analysis revealed that cation coadsorption (and corresponding doping of anions) occurs during oxidation of the polymer for potassium and sodium halides, but not for lithium salts. Electro-gravimetric analysis carried out by an electrochemical quartz crystal microbalance confirmed the cation coadsorption on the oxidized polymer in KCl. The highest ion mobility measured for several carbon-grafted PPy composites was found for MWCNT-grafted-PPy in liquid electrolytes. The ionic mobilities for the same materials embedded in a hydrogel electrolyte were comparable, except for graphene-based electrodes (1 order of magnitude lower than that of MWCNT- and SWCNH-grafted-PPy). Ionic mobility is a potential dependent entity. The highest values were observed for KCl in the range of redox activity of PPy, i.e., from 0.522 to 0.922 V vs standard hydrogen electrode (SHE). It significantly dropped at higher potentials due to saturation of PPy (maximum doping). All bromides showed considerably higher mobility as compared to chlorides at polarizations above 0.922 V vs SHE. This phenomenon can be related to additional electrochemical activity of bromides, leading to the formation of Br3 – under positive potentials. Cyclic voltammetry revealed the highest gravimetric capacitance for the MWCNT-grafted-PPy incorporated in hydrogels and synthesized via oxidative radical (516.86 F g–1) or electrochemically aided atom transfer radical polymerization, e-ATRP (456.31 F g–1). The capacitances of hydrogel-embedded composites were significantly higher than those of the same electrodes measured in liquid electrolytes (350.49 F g–1 after oxidative radical polymerization, and 338.43 F g–1 for e-ATRP).