Synthesis and Characterization of Metal-Organic Framework as Battery Electrodes

Abstract


INTRODUCTION
A metal-organic framework (MOF) is a porous solid formed by coordination bonds between organic ligands and metal ions or clusters (Xie et al., 2020;Bennett & Cheetham, 2014).MOFs are conductive to electric charges and have many potential applications such as gas adsorption (Chen et al., 2019), electrocatalysis, sensor, energy storage material (Xie et al., 2020), gas separation or storage, membrane, magnetic remembrance (Zheng et al., 2021), three-dimensional patterning, non-linear optics, and white lighting source (Cui et al., 2016).MOFs are mainly used as self-sacrificing templates or porous carbon sources (Zhou et al., 2020).Metal-organic frameworks (MOFs) have been reported to improve the performance of Si-based anode materials due to their three-dimensional (3D) framework and organic/inorganic ligands.
In recent years, various MOFs with large internal spaces and open channels for ion pathways through crystal structures have been of great interest to researchers as alternatives for energy storage and conversion (Boutaybi et al., 2020;Choi et al., 2021).MOFs have diverse and manageable chemical compositions and structures, and many catalytic sites (Zhang et al., 2021;Barthel et al., 2018).Through high-temperature pyrolysis or controlled chemical reactions, MOFs can be easily synthesized into metal compounds with a porous threedimensional composition and crystal structure (An et al., 2021) and adjustable pore size and shape (Zhang et al., 2021), because the MOF has compatibility with aqueous solutions, and long cycle life (1000 cycles).Generally, the sodium ion battery anode material obtained by using MOF as a precursor displays a rich pore structure, which is conducive to fully exposing the active site and thereby improving the electrochemical reaction kinetics of the battery system (Choi et al., 2021).In addition, MOF-derived heteroatom-rich carbon can generate external defects to further enhance the conductivity of the carbon matrix (Li et al., 2021).
Problems encountered in the charging/discharging capability of metal-air batteries are still limited by the slow reaction kinetics of oxygen reduction reactions or oxygen evolution reactions (Yang et al., 2016;Kumar et al., 2021).Materials such as inorganic metal oxides and silicon are also used as electrodes but have the disadvantage of slow diffusion of metal oxide ions, which causes a slow charging or discharging rate of the battery.Likewise, the presence of variations in the electrode volume induces mechanical instability (Boutaybi et al., 2020).The amount of charge storage is also influenced by the morphology of the active ingredient ( Zhang et al., 2021).This research was aimed to determine the physical and electrochemical properties of MOF5 at different synthesis temperatures.By optimizing the synthesis temperature, it is hoped that MOF5 with superior physical and electrochemical properties can be obtained.
To prepare MOF5, 1 g of zinc nitrate tetrahydrate and 0.1 g of acetic acid were dissolved in 40 ml of DMF.The solution was stirred using a magnetic stirrer for about 20 minutes and transferred to a crucible without a lid to be heated using a furnace (Thermo Fisher Scientific, Thermolyne FB1410M-33) at temperatures of 200C-600C for 4 hours.The remaining precipitate (MOF) was immersed three times in DMF and three times in chloroform for 24 hours for each immersion.Then, the MOF was drained and burned without a lid in a furnace at 150C for 2 hours.
The microstructures of MOF5 were observed using a scanning electron microscope (Thermo Fisher Scientific, Phenom Pro X) and their crystal structures were determined using an X-ray diffractometer (Malvern Panalytical, Netherlands).To determine the optical properties of MOF5, Fourier Transform Infra-Red (FTIR PerkinElmer, L182105) and UV-vis spectrophotometer (Shimadzu, UV-2600i) were used.

RESULTS AND DISCUSSION
MOFs are novel crystalline porous materials with outstanding structure, compositional and morphological features and have been widely used in energy storage and conversion fields.Various pure MOFs and their derivatives have been developed and used as anode materials in lithiumion batteries (Reddy et al., 2020).MOF is used as anode material in the manufacture of organic batteries.The synthesis method developed in this research is slow evaporation or solvothermal (Chuhadiya et al., 2021) which is a direct synthesis method.Heating was carried out without a lid for 8 hours and heating with a lid for 2 hours.The surface microstructures of the MOFs made at (a) 200 C, (b) 300 C, (c) 400 C, (d) 500 C, and (e) 600 C are shown in Figure 1.They seem that the increase in heating temperature from 200 C to 600 C during the synthesis causes the particles to get smaller, the particle density is higher, and the porosity is lower.The particles have a spherical shape (Yan et al., 2021).The smaller the pores, the smaller the more active sites for lithium ions.Therefore, MOF5 synthesized at low heating temperatures (200 C -300 C), many active sites for lithium ions are found (Yan et al., 2021).In general, MOFs have a high surface area and porosity properties (Chuhadiya et al., 2021).MOF5 has high microporosity with significant concentrations of 5 and 10 wide pores (Ingersoll et al., 2019).
Energy Dispersive X-ray Spectroscopy curve to determine the composition of the synthesized MOF at   1. Based on table 1, the higher the heating temperature, the higher the zinc content, while the lower the oxygen content.
The crystal structures of the MOFs are formed from metal ions and organic binders and are characterized using an X-ray diffractometer (XRD) (Chuhadiya et al., 2021).The XRD curves of the synthesized MOF at (a) 200 C, (b) 300 C, (c) 400 C, (d) 500 C, and (e) 600 C as shown in Figure 3.There can be seen that heating from 200 C to 600 C does not change the crystal structure of the MOF.This is consistent with the results of research published by (Jeong et al., 2017).Three peaks of high intensities were found at two theta angles of 31.75,34.42, and 36.24.This is a short-range order (Bennett & Cheetham, 2014).In the shortrange regular chain structure Zn, C, N, and O elements were found.This structure is in line with the crystal structure pattern published by previous researchers (Zhou et al., 2021).The next three peaks are found at two theta angles of 47.55, 56.55, and 62.85.This is a long-range order structure (Bennett & Cheetham, 2014).Based on the comparison of MOF1 and MOF5 (Denisov et al., 2019;Shi et al., 2021), it is certainly that MOF5 was successfully produced in this study as shown according to the diffraction pattern in Figure 3.The resulted MOF of direct synthesis is in the form of crystalline thin films (Chuhadiya et al., 2021).For MOF1, three high-intensity peaks were found at two theta angles of less than 15.Likewise, for the MOF2 diffraction pattern, three high-intensity peaks were found at two theta angles of less than 15 (Getachew et al., 2014).MOF5 has a cubic crystal structure (Zeng et al., 2021).
The results of the optical properties characterization of MOF materials in the form of transmittance data (%) to wavelength (cm -1 ).This data was obtained from the MOF sample test using FTIR.FTIR curves of synthesized MOF samples at    resulting MOF transmittance.MOF synthesized at 600 C has the highest transmittance or lowest absorbance.What is interesting is that there is the sharpest increase in transmittance from a heating temperature of 300 C to a heating temperature of 400 C, because it appears to be the largest.Materials that have high transmittance mean low absorbance values.The highest absorbance peak was found in the synthesized MOF at a temperature of 200 C (Shi et al., 2021;Getachew et al., 2014;Zeng et al., 2021).The peaks that appear at wavenumbers around 3409 cm -1 , 2827 cm -1 , 2503 cm -1 , 1380 cm -1 , and 1550 cm -1 are -OH, -CH, -COOH, and C=C, respectively (Cheng et al., 2021;Ensafi et al., 2021;Fang et al., 2021).
The highest absorbance was found in samples whose synthesis process was through heating at 500 C.It was found that in the heating temperature range of 200 C -500 C, the higher the heating temperature caused the MOF absorbance to increase.However, the MOF heating results at a temperature of 600 C showed the smallest absorbance (Wang et al., 2020a;Chen et al., 2021).The highest absorbance, 2.25, was found in the synthesized sample at a temperature of 500 C, and the lowest absorbance, 1.75, was found in the sample heated to 200 C.In the ultraviolet wavelength region (200 nm -350 nm), the MOF has good absorbance, while in the visible light wavelength, 350 nm -600 nm, the absorbance decreases until it approaches 0.5.As shown in Figure 5, MOF's ability to absorb visible light is decreasing.Functional groups containing oxygen can cause a catalytic effect.The non-polar bonding of carbon materials is not conducive to adsorption and cannot prevent the shuttle effect well (Chen et al., 2021).
The electrical properties of the MOF were studied using an Elkahfi I-V meter.The results of current versus voltage measurements for synthesized MOF materials at (a) 200 C, (b) 300 C, (c) 400 C, (d) 500 C, and (e) 600 C as shown in Figure 6.The synthesized MOF with a heating temperature of 200 C appears to be the most   conductive, the generated current is the highest (Wang et al. 2020b;Muthurasu et al., 2021).At a voltage of 10 V, the electric current generated from the samples through the heating process of 200 C, 300 C, 400 C, 500 C, and 600 C respectively is 1.77.10 -4 mA, 2.58.10 -5 mA, 7.17.10 -5 mA, 9.74.10 -5 mA, and 3.02.10 - mA.From Figure 1, the synthesis process at the lowest temperature, 200 C, resulted in the MOF sample with the largest porosity.These more pores serve as a place to accommodate oxygen so that the oxygen evolution reaction activity is superior (Wang et al., 2020b;Tan et al., 2021).The increase in temperature from 300 C to 500 C produces MOF which can generate an increasing electric current as well.According to EDX data, Zn levels increased, while C levels decreased.At high temperatures, the C bonds are broken, and the ZnO compound increases.ZnO is the main contributor to the increase in MOF conductivity, because the electrical conductivity of ZnO is quite high, around 10 4  -1 cm -1 .Meanwhile, the synthesized MOF at 200 C showed the highest current, while the synthesized MOF at 600 C showed the lowest current.
Figure 7 shows the cyclic voltammetry graph of the synthesized MOF thin film at temperatures of 300 C, 500 C, and 600 C.The generated cyclic current is the scan result at a voltage of -1 V to 1 V.The higher the MOF heating temperature, the generated current appears at a smaller initial and final voltage.When the potential is increased from -0.65 V to 1 V, two valleys appear at -0.5 V and 0.5 V, while when the scan is continued in the opposite direction, from 1 V to -0.65 V, a peak appears at 0.35 V.This peak indicates that a cathodic oxygen reduction reaction has occurred (Muthurasu et al., 2021;Tan et al., 2021;Shang et al., 2021).The shape of the voltammetric curve that is formed shows the sign of a Faradaik type battery (Wang et al., 2021).Different MOF synthesis temperatures have different effects on the generated currents.The tendency, higher synthesis temperature, results in a more conductive MOF, the generated current tends to increase.The higher the temperature, the higher the peak current generated is greater.A porous and uniform material has a shorter diffusion trail length, facilitating electrons and electrolyte ions to reach the redox center quickly (Wang et al., 2021).

CONCLUSION
It can be concluded that the higher the synthesis temperature, the smaller the MOF5 microporosity size and the higher the MOF5 grain density.The increase in heating temperature in the synthesis of MOF5 causes zinc levels to increase, while oxygen, nitrogen, and carbon levels decrease.XRD MOF5 results produced at a synthesis temperature between 200C-600C shows the same three strong peaks, namely in the angle range 2 between 30 and 40.Based on FTIR data, the transmittance of the synthesized MOF5 in the temperature range of 200C-600C worth between 20%-40%, and the higher the synthesis temperature, the higher the transmittance.The absorbance value of MOF5 has not shown consistent results with increasing the synthesis temperature.At the same voltage, the synthesized MOF5 at a lower temperature indicates a higher electric current generated.The electrochemical properties of MOF5 showed that at the synthesis temperature of 500 C, a dominant peak appeared compared to the synthesized MOF5 at temperatures of 300 C and 600 C.

Figure 2 .
Figure2.The detected elements appear on the EDS curve, namely Zn, O, C, and N with high-intensity peaks(Zhou et al., 2020).Nitrogen content is quite small coincides with carbon.The element Zn (Zinc) appears to have a dominant peak, followed by a carbon peak.Quantitatively, the levels of elements in the resulting MOF are shown in Table1.The increase in heating temperature from 200 C to 600 C causes the zinc content of the resulting MOF to tend to increase from 56.99%w to 72.13%w.Meanwhile, the level of oxygen in the resulting MOF tends to decrease from 29.26 %w to 22.33 %w.The levels of carbon and nitrogen elements also tend to decrease.The levels of elements in the composition of the synthesized MOF at ((a) 200 C, (b) 300 C, (c) 400 C, (d) 500 C, and (e) 600 C are shown in Table1.Based on table 1, the higher the heating temperature, the higher the zinc content, while the lower the oxygen content.The crystal structures of the MOFs are formed from metal ions and organic binders and are characterized using an X-ray diffractometer (XRD)(Chuhadiya et al., 2021).The XRD curves of the synthesized MOF at (a) 200 C, (b) 300 C, (c) 400 C, (d) 500 C, and (e) 600 C as shown in Figure3.There can be seen that heating from 200 C to 600 C does not change the crystal structure of the MOF.This is consistent with the results of research published by(Jeong et al., 2017).Three peaks of (a) 200 C, (b) 300 C, (c) 400 C, (d) 500 C, and (e) 600 C as shown in Figure 4.An increased heating temperature causes an increase in the Figure 2. The synthesized EDS MOF curves at temperatures of (a) 200 C, (b) 300 C, (c) 400 C, (d) 500 C, and (e) 600 C.

Figure 3 .
Figure 3. Diffraction pattern of synthesized MOF in the temperature range of 200 C -600 C.

Figure 4 .
Figure 4. FTIR curves of the synthesized MOF in the temperature range of 200 C -600 C.

Figure 6 .
Figure 6.Currents versus voltage curves of the synthesized MOF at temperatures of 200 C -600 C.