In situ electrochemical quartz crystal admittance methodology for tracking compositional and mechanical changes in porous carbon electrodes

The state-of-the-art in supercapacitors and capacitive deionization technology utilizes high surface area carbons which store energy in electric double layers (EDLs) at the carbon surface/electrolyte solution interface. To reach high energy density, a fundamental understanding of the nature of ion and solvent adsorption into their porous volume is required. We review herein a novel approach to in situ gravimetric monitoring of the compositional changes in porous carbon electrodes during their charging using the electrochemical quartz crystal admittance (EQCA) method. In addition to gravimetric sensing, mechanical changes in the electrode coating under different charge/discharge/ storage regimes can be monitored with high precision and sensitivity. Adsorption of ions in charged meso- and microporous carbons studied by EQCA was interpreted by linking them to the classical EDL models and capacitive deionization theory. Focus is provided for application of EQCA in characterizing adsorption of different series of ions, including determination of the number of solvent molecules accompanying ions during their adsorption and monitoring the molecular interactions between the adsorbed ions and bulk water.