Unraveling Energy Storage Behavior of Independent Ions in Carbon Electrode for Supercapacitors by Polymeric Ionic Liquids and Electrochemical Quartz Crystal Microbalance

A fundamental understanding of the charge storage mechanism of electrochemical double-layer capacitors (EDLCs) requires an in-depth storage behavior investigation of independent ions on the electrode surface. The direct in-situ observation for energy storage behavior of individual ions under realistic conditions remains challenge. Herein, in-situ monitoring the energy storage behavior of independent ions is realized with assistance of polymeric ionic liquids skillfully. Triangle cyclic voltammogram (CV) curves indicate that polymeric imidazole cations PVBIm+ with huge size cannot enter the active carbon pores upon charging, whereas only free anions can and store energy. The CV and electrochemical quartz crystal microbalance (EQCM) simultaneously detect ion adsorption/desorption during charging. It is shown that PVBIm+ are adsorbed on the electrode surface due to ion adsorption mechanism during the negative polarization and have a small contribution to the capacity. Meanwhile, the mass change shows an ion exchange mechanism during positive polarization. It’s proposed that ion adsorption/desorption is governed by electrostatic attraction and repulsion of charged particles. PVBIm(0.1)+ with a higher polymerization degree may promote desorption of polycations and prevent further adsorption of anions at higher potential in positive polarization, resulting in a sharp decrease in the overall capacity of the electrode. This work not only observes the charging behavior of independent ions, but also reveals the relevant charging mechanism through EQCM, paving the way for panorama of EDLC.