(Invited) Edlc Characterization of Carbon Electrodes with Micro to Narrow Mesopores
Authors: Urita Koki , Takahama Sota , Burgos Fernando Vallejos , Urita Chiharu , Notohara Hiroo , Moriguchi Isamu 
Published: December 2024

The capacitance of carbon electrodes on electric double-layer capacitors (EDLCs) is improved by controlling the pore structure to increase the surface area where the electric double-layer can form. Porous carbon electrodes with micropores and narrow mesopores, through which bare electrolyte ions can invade, exhibit a high capacitance [1-3]. Because the electrolyte ions exist as a solvated state with solvent molecules in the electrolyte solution, a desolvation process in pores is required to improve EDLC performance. The desolvation of electrolyte ions spontaneously occurs in certain pore structures. The unique phenomena in nanopores, which are different from those in the bulk phase, were reported in the 1990s [4-6]. One of the representative phenomena is called as "quasi-pressure effect," in which molecules introduced into the pore behave as if they were in a high-pressure field [7-9]. The abnormality in the carbon pores is caused by the strong interaction between the molecules and ions introduced into the pores and the carbon atoms forming the pore walls. Such a strong interaction may be a trigger for desolvation. Accordingly, the pore structure of carbon electrodes must be carefully evaluated to design an ideal porous structure that maximizes EDLC capacitance. Gas adsorption isotherm analysis is one of the techniques used to clarify the porous structure of carbon electrodes. Many analytical methods to determine the specific surface area and pore size distribution (PSD) of adsorption isotherms have been reported. However, the calculated values differed based on analytical methods. The PSD depends on the calculation theories and pore structure models to determine the theoretical adsorption isotherms, whose fitting with the experimental adsorption isotherm provides the PSD of the target porous carbons. Simple pore structures such as slit and cylinder structures have been widely used as pore models. Recently, a set of realistic three-dimensional porous structures was proposed as a model pore structure for porous carbons [10]. Such a realistic model is expected to be effective for evaluating the pore structures of carbon electrodes with micropores and narrow mesopores. On the other hand, it is also considered effective to select Mg ions, which are multivalent ions, as carriers to enhance capacitance. However, Mg ions have a higher solvation energy than Li ions: it is necessary to determine a suitable pore structure for Mg ions. In this study, we designed optimal porous structures of carbon electrodes by clarifying the electric double-layer formation of Li and Mg ions on micro-mesoporous carbons using electrochemical quartz crystal microbalance (EQCM) measurements, while comparing various pore structure determination methods for adsorption isotherms.

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