WU Yi-Wei YAN Ji-Jun ZHU Mei-Hu GUO Hong-Xu② YING Sho-Ming

a (College of Chemistry, Chemical Engineering and Environment, Minnan Normal University, Zhangzhou 363000, China)

b (Fujian Provincial Key Laboratory of Featured Materials in Biochemical Industry, College of Chemistry and Materials, Ningde Normal University, Ningde 352100, China)

ABSTRACT A block-like metal-organic framework UiO-66 was prepared by in-situ growth one-pot hydrothermal process. The as-synthesized material was characterized by Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. The electrochemical properties used as a supercapacitor electrode material were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge measurement (GCD) and electrochemical impedance spectroscopy (EIS) in 2 M KOH solution, exhibiting a high specific capacitance (311 F·g-1 at 1 A·g-1), suggesting its promising potential as a supercapacitor electrode material.

Keywords: in-situ growth, UiO-66, supercapacitor; DOI: 10.14102/j.cnki.0254-5861.2011-2516

1 INTRODUCTION

With the fast development of modern economy, non- renewable energy sources are increasingly exhausted. To deal with this urgent environmental problem, developing reneable clean energy is the key[1]. Supercapacitors (SCs) with good cycle stability, high power density and good electrochemical reversibility have become a new area of interest for the research community[2]. As the performance of SCs is mainly determined by their electrode materials, the study of high performance materials has been the focus of attention in this field[3].

Metal-organic frameworks (MOFs) are a new class of porous crystalline materials constructed by the coordination of metal ions/clusters and organic bridging ligands in a three-dimensional (3D) space[4]. MOFs have aroused widespread concern in the past few years and have now become one of the most rapidly developing areas of research. Because of their tailorable structure and functionality, high porosity and large internal surface area, MOFs have great potential in a variety of applications, such as dye absorp- tion[5],sensors[6-8], photocatalysis[9]and so forth. In particular, MOFs, as promising electrode materials in the field of electrochemistry, have been investigated as supercapacitors[10]owing to their ultra-high porosity, tunable pore size and chemical composition, and high surface area[11,12].

UiO-66 is one of the most promising candidates for supercapacitor material, which has been synthesized and reported by many researchers[13-15]. In this work, a novel UiO-66 has been prepared by one-pot hydrothermal synthesis and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Fourier transform infrared (FT-IR) spectroscopy. Furthermore, the UiO-66 was used as an electrode material and exhibited excellent electrochemical performance (high specific capacitance of 311 F·g-1at a current density of 1 A·g-1with 94.3% retention over 2500 cycles and good conductivity).

2 EXPERIMENTAL

2. 1 Synthesis

All the chemicals were obtained from commercial sources and used without further purification.

UiO-66 was prepared by mixing ZrCl4(2 mmol) and 1,4-benzendicarboxylate (H2BDC, 4 mmol) in 60 mL mixture of N,N-dimethylformamide and deionized water (ratio of volume is 1:2). A piece of pretreated foamed nickel was added during the ultrasonication of the as-obtained solution, then sealed and heated at 135 ℃ for 12 h. After cooling to room temperature, the as-synthesized solid grown in situ on the nickel foam was rinsed by DMF, absolute ethanol and deionized water for three times, respectively. After that, the white block powder UiO-66 was dried and activated at 80 ℃ for 12 h.

2. 2 Characterization

The structure phases of the materials were characterized by a Rigaku Ultima IV X-ray diffractometer (CuKαirradiation,λ= 0.15406 nm). FTIR spectra were recorded using a NECLET 360 Fourier transform infrared spectrometer in the range of 500～4000 cm-1. Scanning electron microscopy (SEM) was used to scan the samples with Hitachi S-4800 instruments to obtain the morphology of the materials.

2. 3 Electrochemical measurements

Electrochemical performances of the electrodes were characterized using a three-electrode system (As-prepared UiO-66 loaded on nickel foam as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum wire electrode as the counter electrode, respectively.) by CS350H electrochemical workstation (from Wuhan corrtest instrument Corp., Ltd. Wuhan, China). Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) measurement and electrochemical impedance spectrum (EIS) were all in an aqueous 2 M KOH electrolyte.

Fig. 1. XRD patterns (a) and FT-IR spectra (b) of the as-prepared UiO-66

3 RESULTS AND DISCUSSION

3. 1 Structure of UiO-66

The XRD patterns are shown in Fig. 1a. It can be seen that each diffraction peak has a high intensity, indicating that the samples have high crystallinity. Moreover, the positions of diffraction peaks of the materials are consistent with those in the reported literature[13]. It can be concluded based on XRD that the as-prepared sample is UiO-66.

Fig. 1b presents the FT-IR spectra of as-prepared UiO-66. It can be seen that there is a distinct absorption peak at 3700～3200 cm-1corresponding to the stretching vibration of O-H, which is attributable to the physisorbed water. It is also possible that the O-H of carboxylate groups is partly deprotonated, since the carboxylate groups in BDC can be readily destroyed. The stretching vibration band of C=O of carboxylic acid exhibits at 1780 ～1800 cm-1. The two characteristic bands at 1600 and 1400 cm-1are attributable to the in- and out-of-phase stretching modes of the carboxylate group (COO-). The absorption peak at 1500 cm-1is ascribed to the skeletal vibration of aromatic rings of organic ligand. The bands are at lower frequencies on account of the O-H and C-H bending mixed with Zr-O.

As depicted in Fig. 2, the particles were constructed from irregular block shaped structure with the size range from 0.5 to 2 μm. The gap between UiO-66 particles can be observed clearly, which provides more efficient contact between electrolyte ion and active materials for charge storage to build high-performance supercapacitor electrodes.

Fig. 2. SEM image of the as-prepared UiO-66

3. 2 Electrochemical properties

The electrochemical measurement results of as-prepared UiO-66 are shown in Fig. 3. There are intense redox peaks (vs.SCE) in CV curves (Fig. 3a), which suggests that the as-obtained capacitance is derived from pseudocapacitance[16]. These peaks are attributed to the intercalation and deinterca- lation of OH-during electrochemical reaction. Furthermore, the shape of CV curves is not distinctly affected by the increase of scan rates, which may be explained by the fact that the solution and electrode resistance can distort the current response at the switching potential and this distortion is dependent upon the scan rate. The potential difference between anodic and cathodic peaks increased are mainly due to the polarization of electrode under a high scan rate[17]. The asymmetry charge-discharge curves (Fig. 3b) of as-synthe- sized UiO-66 further demonstrate the pseudo-capacitance behaviour, matching well with the cyclic voltammetry curve.

Fig. 3. (a) CV, (b) GCD curves of as-prepared UiO-66; the specific capacitance of UiO-66 (c) under various scan rates and (d) at different density of current

The specific capacitance of as-synthesized UiO-66 was 311, 231, 192, 166, 150 and 136 F·g-1at different currents of 1, 2, 4, 6, 8 and 10 A·g-1, respectively. Meanwhile, the specific capacitance under different scan rates and different current density decreased (shown in Figs. 4a and 4b) with the increase of scan rate, which could be ascribed to the incremental voltage drop and insufficient contact of electrolyte and electrode make the ions of electrolyte not fully get into the inner porous channels at higher current densities, resulting in the relatively insufficient faradic redox reaction.

Fig. 4. (a) Nyquist plots of all UiO-66 electrodes and (b) retention of specific capacitance after 2500 cycles

To evaluate the conductivity and charge transport properties at the electrode/electrolyte interface, EIS spectra of the as-prepared UiO-66 were collected at a frequency range from 0.01 Hz to 100 kHz, as shown in Fig. 4a. A semicircle at high frequency is decided by the electrolyte resistance (Rs), material internal resistance (Ri) and charge-transfer resistance

(Rct), while a straight line at low frequency is caused by ion diffusion, which should be related to the capacitance performance of materials. In particular, the node of the semicircle and real axis is RS; the diameter of the semicircle is Rct, also called Faradic resistance. The high-frequency intercept on the real impedance axis is 1.3 Ω in the Nyquist plot, indicating a small series resistance. In low frequency region, the relatively larger angle indicates a good capacitive behavior.

The stability of the supercapacitor was tested by GCD measurements at 4 A?g-1and the obtained results are presented in Fig. 4b. The supercapacitor exhibits good electrochemical stability and maintains a specific capacitance of about 94.3% after 500 GCD cycles. All the results prove that the materials are promising candidates for the application of supercapacitors.

4 CONCLUSION

In this work, we have successfully fabricated UiO-66 using an in situ MOF growth strategy. Here, UiO-66 was grown on nickel foam and characterized by XRD, SEM and FT-IR. The electrochemical performance for UiO-66 was investigated by CV, GCD and EIS. The results show that as-prepared UiO-66 has good reversibility, high specific capacitance (311 F·g-1at 1 A·g-1) and excellent electrochemical stability (94.3% of initial specific capacitance after 2500 cycles). The strategy of in-situ growth on nickel foam UiO-66 has the ability to become as one of the most competitive candidates for the future energy-storage applications.