New Insights into the Relationship between Micropore Properties, Ionic Sizes, and Electric Double-Layer Capacitance in Monolithic Carbon Electrodes George Hasegawa,* ,†,‡ Kazuyoshi Kanamori, † Kazuki Nakanishi, † and Takeshi Abe ‡ † Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan ‡ Department of Energy & Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ABSTRACT: The effects of the pore properties and the ionic sizes on the electric double-layer capacitances have been investigated by using the monolithic carbon electrodes with different pore properties. The carbon monoliths with high surface areas which possess homogeneous pore properties in the whole monoliths were prepared from the bridged- polysilsesquioxane gels via the nanophase extraction technique. The detailed investigations of the pore properties of the carbon monoliths were conducted by the nitrogen physisorption measurement as well as the mercury porosimetry. The electrochemical property of each monolithic carbon electrode was examined by the cyclic voltammetry in the different aqueous electrolytes in order to investigate the effects of the ionic sizes. These fundamental analyses have provided new insights into the efficient micropore sizes in each electrolyte for the superior electric double-layer capacitors. 1. INTRODUCTION On growing demands for energy storage, porous electrode materials are attracting increasing attention, and the pore properties are regarded as one of the most important factors for the electrode capability. 1-8 In the case of Li-ion batteries, the pores in the electrode materials help the rapid diffusion of Li + ions especially in the materials with poor ionic conductivities, such as olivine-type materials. 9-11 The pores in the electrodes play a different role in the case of electric double-layer (EDL) capacitors, 12-14 in which the electric charge is stored by a non- faradaic mechanism in the double layer at the electrode/ electrolyte interface. Hence, the capacities of EDL capacitors are dependent on the accessibility of the electrolyte ions to the pores in the electrodes and therefore on the effective surface area. It indicates that the EDL capacitances depend not on the surface areas of the electrodes estimated by the gas adsorption techniques but on the relative difference between pore sizes and the ionic sizes in the electrolytes. In general, it is true that the electrodes with higher surface areas tend to exhibit the higher EDL capacitances, and this is why the activated carbons are mostly used as the electrodes for EDL capacitors. 12-14 In addition, many researchers have discussed the EDL capaci- tances in relation to the estimated surface area, i.e., BET surface area, of the activated carbon electrodes. However, in some cases, the increase in the estimated surface area does not lead to the increase of the EDL capacitances because of the pore size effects. 15-17 The design of the desired pore size in the electrodes is therefore highly important in order to develop the superior EDL capacitors. From this motivation, the relation between pore size and ionic size and the effects on the EDL capacitances have been energetically studied in the past decade. 15-25 In most of the studies, the electrodes of EDL capacitors were prepared as the composite electrodes of the activated carbons which are in powder form, and the relationships between the pore properties of the activated carbon powders and the EDL capacitances were discussed. However, the composite electrodes are prepared from the mixture (slurry) of activated carbons, conductive agent such as acetylene black, and binders which are typically polymers such as poly(vinylidene difluoride) (PVdF) and polytetrafluoro- ethylene (PTFE). This means that the pores of the activated carbons are possibly covered and filled with the conductive agent and the polymer binders, which prevents the precise investigation of the effects of the pore properties on the EDL capacitance. Monolithic electrodes 26-29 are promising candidates as the substitute for the conventional composite electrodes. Since a monolithic material is binder-free and can be used for the electrode without any additional processes, it is possible to exploit the completely clear surface for the formation of EDL. It is therefore expected that the direct information on the effects of the pore properties of the electrode on the EDL capacitance Received: September 11, 2012 Revised: December 3, 2012 Published: December 4, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 26197 dx.doi.org/10.1021/jp309010p | J. Phys. Chem. C 2012, 116, 26197-26203