Effect of Hydration Property of Pore Wall on Performance of Electric Double Layer Capacitors
In the present study, mesoporous silicon is utilized as a model electrode. Mesoporous silicon electrodes were prepared by anodization of an n-type Si (100) wafer with a resistivity of less than 0.02 Ω cm in 22 wt% HF solution. After anodization, the electrode was immersed in n-hexane containing propiolic acid or methyl propiolate to make the surfaces hydrophilic and hydrophobic, respectively . Note that we adopted these organic solutes because they have almost the same molecular structure and show opposite hydration properties, i.e., hydrophilic and hydrophobic. Significant differences in pore size are not expected before and after the modification. An aqueous solution containing 0.1 M tetraethylammonium perchlorate was used for the measurement of EDLCs. Cyclic voltammograms (CVs) were measured using the hydrophilic and hydrophobic porous electrodes at a scan rate of 20 mV s–1. The porous silicon electrodes are oxidized above –0.3 V vs. SHE, so that CVs were measured between –0.5 V and –0.3 V. It should be noted that current density in the present study is calculated based on the projected area (0.785 cm2).
A clear difference in hydration property of pore wall is observed in Figure 1. The hydrophobic porous electrode yields a much greater capacitance, meaning that more ions were adsorbed (stored) on the hydrophobic electrode-electrolyte interface. This result suggests that the surface hydration property has an important role on the capacitance of the EDLCs.
 K. Fukami, R. Koda, T. Sakka, T. Urata, K. Aomoto, H. Takada, M. Nakamura, Y. Ogata, and M. Kinoshita, Chem. Phys. Lett., 542, 99 (2012).
 K. Fukami, R. Koda, T. Sakka, Y. Ogata, and M. Kinoshita, J. Chem. Phys., 138, 094702 (2013).