Effect of Hydration Property of Pore Wall on Performance of Electric Double Layer Capacitors

Electric Double Layer Capacitors (EDLCs) store charges (ions) on electrode-electrolyte interfaces. When using nanoporous electrodes for EDLCs, specific capacitances decrease as the pore size becomes small due to the difficulty in supply of ions from bulk solution into nanopores. In the course of studies in electrodeposition within nanoporous electrodes, we have revealed that metal ions with a relatively large diameter strongly enhance the electrodeposition within hydrophobic nanopores when the solvent of the deposition bath is water [1]. This result indicates that the ion penetration into the pores is enhanced. We have also reported that by choosing an appropriate combination in hydration property of pore wall and ions, a transition phenomenon occurs within nanopores, which results in a high concentration of metal ions compared with the bulk solution [2]. The results reported in the previous studies lead to an idea that the similar enhancement of ion penetration into nanopores is advantageous for further developments of EDLCs using nanoporous electrodes. In this paper, we focus on the effect of hydration property of pore walls on the capacitances with mesoporous silicon electrodes.  Secondly, the effect of pore size on the capacitance is discussed.

   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 [3]. 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.

Reference

[1] 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).

[2] K. Fukami, R. Koda, T. Sakka, Y. Ogata, and M. Kinoshita, J. Chem. Phys., 138, 094702 (2013).

[3] A. Imanishi, S. Yamane, and Y. Nakato, Langmuir, 24, 10755 (2008).