Electrochemical Impedance of Electrode Pattern-Transferred Metal-Organic Flameworks with Pores Filled with Ionic Liquid
Authors: Amano Kentaro , Zheng Yumeng , Kinoshita Kentaro 
Published: December 2024

We have shown that filling the nanopore of MOFs with ionic liquids (ILs) solves drawbacks such as crystal degradation due to moisture absorption and the fragility of MOFs [1]. In addition, the development of crystal growth technique that enables to synthesize mm-size large single crystals of Cu3(BTC)2
[2], which is one of the representative MOFs. By using an IL-loaded large MOF single crystal, IL@MOF, physical property measurements without the influence of grain boundaries are expected to be conducted. As for electric properties, there exists the further difficulty of forming electrodes on MOFs, preventing the elucidation of intrinsic electric properties. In this study, an electrode transfer technique to the surface of a large Cu3(BTC)2 single crystal during the crystal growth was proposed, and impedance measurements were performed with the electrode transferred IL@Cu3(BTC)2.

Electrode transferred IL@Cu3(BTC)2 was prepared by synthesizing large Cu3(BTC)2 single crystals while immersing an SiO2 substrate on which electrode arrays were patterned. With the electrode transferred IL@Cu3(BTC)2, electrochemical impedance (Z) was measured in the frequency (f) range of 4 Hz ~ 5 MHz. To obtain information on an interaction between IL and Cu3(BTC)2 in IL@Cu3(BTC)2, the influence of anion species that compose IL on Z(f) was measured at various temperatures (Ts). Here, the ILs used were [C4mim][BF4], [C4mim][PF6], and [C4mim][Tf2N], that is, three types of ILs that have three different anions of [BF4]-, [PF6]-, and [Tf2N]- and the one common cation of [C4mim]+.

Figure 1(a) shows a photomicrograph image of the electrode pattern-transferred Cu3(BTC)2 single crystal (111) surface. The array of electrodes with the area of 150 µm × 150 µm with a spacing of 20 µm was successfully formed.

Figure 1(b) shows Z(f) of IL@Cu3(BTC)2 at 300 K on the complex impedance plane. The semicircular and linear regions at the high and low frequency range correspond to bulk and Warburg components of Z, respectively. Bulk resistance (R
b) was estimated from the diameter of the semicircles.

Figure 1(c) shows an Arrhenius plot of the T-dependence of R
b for each IL. The linear relationship between 1/k
B
T and log(1/R
b) enables us to estimate the activation energy () based of the formula , where A is a constant. s can be calculated from the slopes in Fig. 1(c) and were estimated to be 0.29 eV for [BF4]-, 0.19 eV for [PF6]-, and 0.15 eV for [Tf2N]-, indicating that the smaller ion size of the anion, the larger the activation energy. Considering the anion size of [BF4]-, [PF6]- and [Tf2N]- are 4.5 Å×4.5 Å, 5.1 Å×5.1 Å and 7.9 Å×2.9 Å respectively[3]. is suggested to become large for smaller anion size. The present result suggests that is a potential barrier height to overcome to migrate from the present pore to the neighboring one. The barrier height is considered to be influenced strongly by the interaction between the anion and the Cu2+ open metal site of Cu3(BTC)2, since the small anion size allows it to reach closer to the Cu2+ open metal site.

Figure 1