Xing
Meng
*a,
Hai-Ning
Wang
a,
Xiao-Kun
Wang
b,
Long-Zhang
Dong
b and
Yan-Hong
Zou
a
aSchool of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo, Shandong, 255049, P. R. China. E-mail: mengxing@sdut.edu.cn
bJiangsu Collaborative Innovation Centre of Biomedical Functional Materials, Jiangsu, Key Laboratory of New Power Batteries, College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China
First published on 16th November 2018
Through cation substitution in a zirconium based coordination polymer, an isostructural framework 1@NH4+ has been prepared without any apparent structural change. The proton conductivity of 1@NH4+ is successfully improved. It exhibits high proton conductivity (1.39 × 10−2 S cm−1) at 98% relative humidity and 60 °C.
Ca2Zr(C2O4)4·5.5H2O (1) was synthesized according to the reported procedure.24 Then about 0.500 g of sample 1 was added to 5 mol L−1 of NH4Cl in a solution containing 20 mL of water. The contents were placed in a 20 mL screw-capped vial, which was heated to 45 °C in a water bath for 12 h. A white precipitate was obtained by filtration and washed with water several times to remove excess NH4Cl. In the present study, this is a new approach to mediate the proton conductivity through generating effective proton conduction pathways by cation substitution without changing the main framework. We employ an oxalate based CP Ca2Zr(C2O4)4·5.5H2O (1) owing to the two following considerations: (i) it is an anionic framework, as well as having Ca2+ ions inside its framework; and (ii) the channels are full of water molecules, which may facilitate the generation of hydrogen-bonding networks or act as proton carriers (Fig. 1). Here we mainly describe the influence of cation substitution on the proton conductivity of 1.
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Fig. 1 The three-dimensional framework of 1 (the 1D channel existing along the c axis, and the Ca2+ ions, Zr4+ ions and free water molecules marked in dark teal, dark green and red, respectively). |
Inspired by the previous work, the sample exchanged with NH4+ ions has been synthesized and labeled as 1@NH4+. The structure of 1@NH4+ has been determined to be unchanged with the help of powder X-ray diffraction (PXRD) analyses (Fig. 2a). The different reflection intensities between the simulated and experimental patterns might be attributed to small sizes of as-synthesized powder samples as well as a certain degree of preferred orientation of the crystalline samples during data collection. Additionally, the compositions of the prepared 1 and 1@NH4+ have also been characterized by Fourier transform infrared (FT-IR) and X-ray photoelectron spectra (XPS). The FTIR spectra of 1 and 1@NH4+ are shown in Fig. 2b. The characteristic bands at approximately 1400 cm−1 in 1@NH4+ are ascribed to the stretching vibrations of NH4+.25 Meanwhile, XPS spectra of 1@NH4+ have been used to evaluate its composition, and demonstrate the coexistence of C, N, O, Ca and Zr elements (Fig. 2c). The N 1s XPS spectrum successfully confirms the existence of NH4+, and could be divided into two peaks, which can be ascribed to –NH– segments (399.7 eV) and the protonation states (–NH+–) at 402 eV (Fig. 2d), confirming that NH4+ ions are immobilized into the channels.26 The amounts of Ca and Zr in 1@NH4+ have been determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Table S4, ESI†). Combined with thermogravimetric analysis, we calculate that 1 mol compound 1 loads 0.3 mol NH4+ ions.
To determine the proton conductivity of both samples, the proton conductivities of the two solid samples have been measured by AC impedance spectroscopy using pelletized samples. For 1, due to disordered data points, it is impossible to find the corresponding impedance value meaning the proton conductivity is very low. While, for 1@NH4+, the proton conductivities are 1.07 × 10−6 S cm−1, 3.05 × 10−5 S cm−1, 2.93 × 10−4 S cm−1, 3.93 × 10−4 S cm−1, 4.57 × 10−4 S cm−1, 5.73 × 10−4 S cm−1, and 1.42 × 10−3 S cm−1 at 30 °C, under different relative humidity conditions (50% RH, 60% RH, 70% RH, 80% RH, 90% RH, 95% RH, and 98% RH) (Fig. 3 and Fig. S3, ESI†). Additionally, in order to evaluate the temperature influence, the temperature-dependent proton conductivities have also been measured. These measurements have been carried out in the temperature range of 30–60 °C and 98% RH (Fig. 4). As the temperature increases, the proton conductivity for 1@NH4+ reaches 1.39 × 10−2 S cm−1 under 60 °C and 98% RH, which is comparable to the reported MOFs (Table S3, ESI†). In addition, the PXRD pattern of the tested sample matches well with the original sample, indicating that the sample exhibits good stability before and after the impedance tests (Fig. 2a). The FTIR spectrum of 1@NH4+ after AC impedance measurements has been investigated. As shown in Fig. S5 (ESI†), a characteristic band at approximately 1400 cm−1 in 1@NH4+ exists, which is ascribed to the stretching vibrations of NH4+. Considering that 1 and 1@NH4+ are isostructural, they exhibit different proton conductivities. The difference in proton conductivity may result from the introduced NH4+ ions. The NH4+ ions play an important role in improving proton conductivity. We speculate that NH4+ ions have two roles in the process of proton conduction: increasing proton carriers in the channels, and promoting the formation of a hydrogen-bonding network.19 Additionally, the motion of ammonium ions may also contribute to the high proton conductivity.27 It is worth mentioning that this is the first example of successful control of proton conductivity using cation substitution in zirconium based CPs.
The conductivity of 1@NH4+ increases with increasing RH. This phenomenon suggests that proton conductive behaviors are related to the concentration of water. Inferred from this, the framework could adsorb water molecules into the channel slowly, facilitating the establishment of hydrogen-bond networks and proton-transport pathways. For 1@NH4+, water vapor adsorption and desorption isotherms are collected at room temperature. As shown in Fig. S2 (ESI†), with the increase of vapor pressure, the adsorbed water molecules increase, and the maximum water vapor uptake of 141.1 cm3 g−1 is calculated from the adsorption isotherms, demonstrating that water molecules are able to enter into its channels.
The Grotthuss and vehicle proton conduction mechanisms have been generally used for explaining the proton transfer process in solids.28 The reported activation energy (Ea) is in the range 0.1–0.4 eV for the Grotthuss mechanism and in the range 0.5–0.9 eV for the vehicle mechanism. The results of Ea and the structural analysis of the compound provide an opportunity to further get an insight into the possible proton-conduction pathway and mechanism. A linear correlation (ln(σ) versus 1000/T) is utilized to calculate the Ea of proton transport. In compound 1, the proton conductivity is very low. There exist water molecules in the channel, and the distances between free water molecules along the c-axis direction are about 4.22 Å. This means that a successive hydrogen-bonding network could not be formed solely in compound 1, which results in low conductivity. While in 1@NH4+, the proton conductivity reaches up to 1.39 × 10−2 S cm−1 at 60 °C under 98% RH, and the activation energy is 0.61 eV (Fig. S4, ESI†). This indicates that the proton transport of 1@NH4+ is mainly governed by the vehicle mechanism. The introduced NH4+ and adsorbed H2O molecules as vehicles accept and transport H+ protons under AC impedance measurements, resulting in the vehicle mechanism.
Footnote |
† Electronic supplementary information (ESI) available: Synthetic details, water vapor adsorption and desorption isotherms, the TGA plot, and experimental details of the proton conductivities. See DOI: 10.1039/c8nj04763d |
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