Electrochemical quartz crystal admittance studies of ion adsorption on nanoporous composite carbon electrodes in aprotic solutions

This paper describes the application of Electrochemical Quartz Crystal Admittance (EQCA) methodology to the tracking of ion adsorption on composite electrode coatings consisting of highly porous activated carbon particles and polyvinylidene difluoride (PVdF) binder rigidly attached to quartz crystal surfaces. Solutions of LiBF₄ and (C₂H₅)₄NBF₄ in propylene carbonate (PC) were used in this study. At small charge densities, the effect of frequency change is nearly of gravimetric nature. We propose a new method to determine the mass contribution to the resonance frequency shift due to adsorption of ions and accompanying solvent molecules, revealing different ion/solvent population ratios for Li⁺, (C₂H₅)₄?N⁺ and BF₄ ^? ions correlated to the ion solvation ability. The EQCA model applied describes the change in the frequency and in resonance peak width in terms of dimensional changes of large carbon particles (bumps) and of pseudo-uniform layers of smaller particles mixed with PVdF. The type of oscillation energy dissipation in composite carbon electrodes with PVdF binder strongly depends on non-uniform potential-induced deformations of electrode particles, and this suggests a strong effect of solvent nature on the mechanical properties of polymeric binders. EQCA may provide important information on the role of polymeric binders during cycling of composite electrodes both for supercapacitors and for Li-ion batteries electrodes.