Tian
Li
a,
Yi-Fan
Wang
a,
Zheng
Yin
c,
Jian
Li
d,
Xu
Peng
*ad and
Ming-Hua
Zeng
*ab
aCollaborative Innovation Center for Advanced Organic Chemical Materials Co-constructed by the Province and Ministry, Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, College of Chemistry & Chemical Engineering, Hubei University, Wuhan, 430062, P. R. China. E-mail: pengxu@hubu.edu.cn
bDepartment of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources, Guilin, 541004, P. R. China. E-mail: zmh@mailbox.gxnu.edu.cn
cCollege of Chemistry and Chemical Engineering, Shaanxi University of Science and Technology, Xi'an 710021, P. R. China
dAnHui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei 230601, P. R. China
First published on 19th August 2022
The peripheral N/O chelating of Schiff base ligands, inner bridges, counterions, and metal centers gave rise to a brucite disk cluster [Zn7L6(OCH3)6](NO3)2 (Zn7, (HL = 2-methoxy-6-((methylimino)-methyl)phenolate)) which crystallized into hexagonal prismatic plates. The combination of crystallographic studies, in situ TG-MS, and other characterization techniques showed that with a fixed metal and ligand composition in the precursors, weak correlative interactions (e.g., electrostatic interactions) and shape matching between the cluster core and counterions determine the cluster packing modes in the crystals and affect their phase and morphological changes during pyrolysis. The tracking of the pyrolysis process showed that the peripheral ligands, inner bridge, and counterion decompose first, followed by the Zn7O6 core merging with cubic ZnO, which was then reduced by carbon and eventually evaporated, leaving behind a porous carbon structure. In this process, the solid material composition change was in the sequence {Zn7}-{Zn–O core@C}-{ZnO@C}-{Zn@C}-{C}, which was accompanied by a porosity change from micropores to hierarchical pores, and then to micropores again. The core structure and packing modes of Zn7 evolved into micropores and mesopores, respectively. Micro-mesoporous carbon Zn7-1000 featured a capacitance of 1797 F g−1 at 1 A g−1, where the BET specific surface area was 3119.18 m2 g−1, which, to the best of our knowledge, is the highest value reported for a porous carbon electrode. This work represents an important benchmark for the analysis of dynamic chemical processes involving coordination clusters at high temperatures, and it could lead to important applications in high-performance devices.
Scheme 1 Progressive phase transformation from a heptanuclear zinc cluster (Zn7) to hierarchical porous carbon (Zn7-1000). The microporous carbon samples Zn7-900 and Zn7-1100 are also illustrated. |
X-ray single crystal diffraction analysis showed that Zn7 crystallized in the trigonal system Pc1 space group. It is composed of a [Zn7L6(OCH3)6]2+ cation cluster and two counter anions of NO3− but without any lattice solvents, indicating highly condensed packing (Fig. S2a and Table S1a†). The peripheral Schiff base ligands adopting a η1:η2:η1:μ2 coordination and six μ3-OCH3 as inner bridges grasp seven Zn2+ ions in to a cluster with a disk core. All ZnII atoms are in a six-coordinated octahedral configuration, with the central ZnO6 octahedron surrounded by six outer ZnNO5 octahedrons. The core of the disk is similar to brucite-like fragments and all seven Zn2+ ions are located in a coplanar fashion, with a Zn–OCH3 bond length of 2.036 to 2.127 Å, and a Zn–O–Zn angle of 94.6–101.3°. The benzene ring based peripheral ligands endow the cluster duple open hemispheres at two sides of the disk. Three inner –OCH3– motifs coordinate vertically to the Zn7 plane, which sit inside the bowl and occupy the void space that could potentially host solvent molecules. As a result, weak intermolecular interactions induce a cluster packing mode, where eight Zn7 disks are located at the vertex of a quadrilateral prism with four additional disks at the middle position of the pillar. Two 1D channels with triangular windows are generated in each cell to host the NO3−. The regular vertical arrangement of the disk with duple open hemispheres and the hydrophobic –OCH3– sitting inside the bowl facilitates the close packing of the heptanuclear cations and the facile removal of NO3− trapped in the 1D inter-cluster channel during pyrolysis. The nearest intercluster distance is 11.64 and 14.12 Å along the plane and vertical direction, respectively, considering the center Zn⋯Zn distance for adjacent disks (Fig. S2c†).
The simulation of the crystal morphology of Zn7 using the Bravais–Friedel–Donnay–Harker (BFDH) method showed a hexagonal prism-shaped morphology (Fig. S3†),13 which was consistent with that of the obtained sample (Fig. 1a). The [Zn7] units parallel the hexagonal plane of the crystal, and the NO3− anions sit above and below the individual heptanuclear complexes, which interact with the ligand of the Zn7 cluster through C–H⋯O hydrogen bonding interactions. When the observing direction is along the (002) plane and the (100) plane, the surface where the nitrate is located is parallel to the (002) surface while being perpendicular to the (100) surface of [Zn7L6(OCH3)6]2+ (Fig. S3d–f†). For such hexagonal prism crystals constituting the disks arranged in a compact manner, the horizontal and vertical intercluster interactions contribute to the maintenance of the hexagonal morphology during the high temperature carbonization process. When the pyrolysis temperature is 1000 °C, the crystal still maintains a hexagonal prism shape, but shows a slit when viewed from the side of the crystal; slits can be observed parallel to the ab plane (Fig. 1c). This macroscopic observation is strongly related to the thermal decomposition of microscopic molecules.
In the first stage of decomposition (205–280 °C), CH3+, NO+, NO2+, O2+ fragments are detected by TG-MS (Fig. 2b). Among them, the methyl group is attributed to the cracking of μ3-OCH3 and the methoxy group of L, which with the C–O bond in Zn7 of similar strength (Table S1b†) and the O2+ is ascribed to the decomposition of nitrate and its further reactions (2NO3− = 2NO2 + O2, NO2 + C = 2NO + CO2) (Table S2†). It should be noted that the first stage of decomposition is completed at 280 °C, where Zn7 collapses to form an amorphous structure, yet the Zn7 core still remains independent (Fig. S4a†). At the same time, due to the removal of nitrate “planes”, the force between the “planes” where the Zn7 core is located is significantly reduced, and a tendency to separate would start to develop. A heptanuclear [Zn7O12] core with a diameter of about 6.27 Å transform into hexagonal ZnO with a diameter of about 6.50 Å in the temperature range of 280–480 °C (Fig. S4a†). In this process, the elaborately designed cluster with a precise spatial arrangement of ligands, counterions and the Zn7 core enables the coherent decomposition and departure of the nitrate group with methoxy species, which helps to maintain the regular morphology and ordered mesoscopic structure of the resulting ZnO@carbon structure. In the second stage (480–585 °C), the primary chemical reaction is 7ZnO + 7/2C = 7Zn + 7/2CO2 (Table S3†). The stoichiometric number 7 is due to the fact that there are seven Zn atoms in a Zn7 molecule. In the third stage of decomposition (>585 °C), Zn+ is detected by TG-MS because the liquid zinc element began to evaporate and escape (Fig. 2b), and left micropores with a diameter of about 6 Å at 900 °C (Fig. 1d). Moreover, at 1000 °C, the abovementioned separation between the “planes” where the Zn7 cores are located results in a large number of slit-type mesopores at 1000 °C (Fig. 1e).14 In brief, inorganic metal nodes in molecular clusters can be converted into metal oxides by pyrolysis, and further reduced to elemental metals by a carbothermal reduction process, while Zn molecular clusters can generate pore structures of specific sizes in pyrolyzed carbon-based materials during this process. The phase evolution process can be summarized as Zn7[C6H3(CH3O)(O)(NCH3)]6 (OCH3)6(NO3)2 → [Zn7O12]@N,O-doped carbon → [ZnO]@N,O-doped carbon → [Zn]@N,O-doped carbon → N,O-doped hierarchical carbon. Obviously, choosing clusters with specific morphologies for pyrolysis can further change the pore structure of the pyrolysis products and increase the porosity of the derived carbon materials.
A series of characterizations, including PXRD, ex situ SAXS, PDF, TEM, Raman and BET were conducted to further illustrate the thermal decomposition process of Zn7 and the pore structure changes during pyrolysis. The PXRD patterns of 280–480 °C pyrolysis samples show a clear hexagonal ZnO peak (JCPDS card 65-3411), indicating that the Zn7 core is crystallized into hexagonal ZnO. Meanwhile, the PXRD patterns of 600–800 °C pyrolysis samples also exhibit a strong diffraction peak of ZnO, which is derived from the remaining elemental zinc in the samples oxidized by oxygen in air after the annealing process (Fig. S4†). It should be noted that the PXRD pattern of the Zn7-900 sample has a broad and diffuse diffraction peak near 2θ = 23° and a weak diffraction peak near 2θ = 44°, which is a distinct characteristic of amorphous carbon.15 Also, among the diffraction peak at 2θ = 23°, the diffraction peak of Zn7-1000 is the weakest, which indicates that the degree of graphitization is the lowest (R = 1.36), while the diffraction peak of Zn7-1100 is narrowed to a certain extent, demonstrating an increased degree of graphitization (R = 2.19).16 Furthermore, experimental atomic pair distribution function (PDF) analysis also proves that the peak is centered at 1.42 Å of Zn7-1000 (slightly smaller than other samples, ∼1.44 Å) corresponding to the sp2 C–C bond in graphitic carbon, which is also consistent with the sequential phase evolution process deduced by TG-MS and other structural characterization17 (Fig. 2c and d). Ex situ SAXS patterns of Zn7-900, Zn7-1000, and Zn7-1100 are shown in Fig. S5.† In addition, high-resolution transmission electron microscopy (HRTEM) mapping displays the disordered carbon layer in Zn7-900 and Zn7-1000 samples, while Zn7-1100 contains lattice fringes corresponding to graphitic carbon (002), indicating that amorphous carbon has crystallized into graphitic carbon (Fig. S6†). Similarly, in Raman spectra, the ID/IG value of the pyrolysis products (T = 900, 1000, 1100 °C) remained approximately the same, with an intensity ratio of 0.94, essentially indicating no apparent changes in the degree of carbon defects in the resulting material (Fig. S7†). ICP data of the Zn residual within the sample is shown in Table S4.† According to the analysis of the XPS spectrum, there are predominantly noticeable peaks of C and O elements in the sample (Fig. S8–S10†). The presence of the element O confirms that the surface of the material has oxygen-containing functional groups, which can enhance the hydrophilicity of the surface of the material to the water-based electrolyte and encourage the transfer of electrolyte ions; it can also introduce certain Faraday pseudo capacitors to enhance the capacitor performance.27 The presence of hydrophilic functional groups can also be verified from the PDF data, where a peak at 0.98 Å is ascribed to the considerable O–H bond distance in the sample. Therefore, we used the polyurethane mixed sample and coated the film to conduct the contact angle test. In comparison with the blank, the contact angles of the three samples after high-temperature pyrolysis are θ < 90°, and they all show hydrophilicity. Among them, the contact angle of Zn7-1000 is the smallest, indicating that better hydrophilic characteristics are more conducive to the contact between the ions in the electrolyte solution and the material (Fig. 2e).
In the gas adsorption measurements, a high gas uptake at a low pressure range corresponds to the presence of micropores and the hysteresis loop suggests mesoporous adsorption (Fig. S12†). The 900 °C pyrolysis product has microporous vacancies left by the evaporation and escape of zinc, with a pore size of about 0.6 nm.12,18 In addition, there are other micropores that provide more adsorption sites during energy storage (Fig. 2f).19–22 After being carbonized at a high temperature of 1000 °C, the pore size distribution shows that the Zn7-1000 sample contains mesopores, making the product show a micro-mesoporous hierarchical structure. These pores can become channels for the diffusion of electrolyte ions and accelerating ionization. The diffusion and transfer are rapid, and the surface area utilization rate has significantly improved. Compared with Zn7-900 and Zn7-1100, the adsorption capacity of Zn7-1000 is significantly increased to 1295.16 cm3 g−1 (Fig. S12b†) with a more significant proportion of mesopores. Calculation results of the density functional theory (DFT) method23–25 show that the pore structure of Zn7-1000 primarily consists of pore sizes of 0.6 nm, 0.8 nm, ∼1.2 nm, 3–8 nm, and 20 nm, of which 20 nm is attributed to the gap between the crystal blocks (Fig. 2f). Zn7-1000 has a maximum BET specific surface area, as high as 3119.18 m2 g−1, and a maximum total pore volume of 1.82 cm3 g−1 (Fig. S12d†). According to the curve of the cumulative specific surface area with the pore diameter calculated by the NLDFT method, it can be seen that the increase of the Zn7-1000 specific surface area is mainly due to the contribution of mesopores (Fig. 2g). The ultra-high specific surface area of Zn7-1000 and the extensive micropore–mesoporous hierarchical structure can facilitate more electrolyte ion adsorption sites.26 Briefly, we have discovered the distinct characteristics of Zn7-1000 and other pyrolyzed samples, and these differences are destined to have a significant impact on its electrochemical performance.
Compared with Zn7-900 and Zn7-1100, which only have a single micropore size, Zn7-1000 combines micropores and mesopores to form a hierarchical porous structure that uses not only mesopores to provide efficient diffusion channels, but also micropores or smaller mesopores that leads to a larger active area, thereby achieving a high capacitance in supercapacitors. In order to verify our above analysis of the pyrolyzed samples from the zinc cluster on the electrochemical performance, we determined their specific capacitance by cyclic voltammetry (CV), galvanostatic charge and discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements, using a three-electrode configuration with Ag/AgCl as the reference electrode. The CV curves of Zn7-900, Zn7-1000, and Zn7-1100 samples range from −0.6 to 1.0 V at a scan rate of 5 to 100 mV s−1. The shape of the CV curves under different scanning rates is not ideally rectangular-like; a prominent redox peak is especially located at ca. 0.3–0.4 V. This phenomenon indicates that a CO group exists in the pyrolyzed samples, which is consistent with XPS analyses.27 In addition, the slight difference in the position of the redox peak is due to the difference in the content of the pyridine nitrogen and the pyrrole nitrogen in the resulting materials (Fig. 4b, S11 and S13†).28
Moreover, GCD curves of Zn7-1000 electrode were tested, as shown in Fig. 3b and S14,† and exhibit a specific capacitance of 1797, 1473, 1297, 1113, and 877 F g−1 at current densities of 1, 2, 5, 10, and 20 A g−1, respectively. This value records the best value among porous carbons as electrodes from MOFs or MOFs-derived materials as precursors (Fig. 4d and Table S5†). Also, the electrochemical impedance spectroscopy test was performed using a three-electrode configuration from 100 kHz to 100 mHz (Fig. 3c). A relatively low equivalent series resistance of only 10.71 Ω is exhibited by Zn7-1000, reflected at the high-frequency region by the intersection of the curve at the real part. Meanwhile, the slope of Zn7-1000 is also larger than that of Zn7-900 and Zn7-1100, revealing a better transport and diffusion of ions in Zn7-1000 samples (Fig. 3d). Zn7-1000 still has a superior specific capacitance retention compared with other samples under a current density of 2 A g−1 to 20 A g−1, which is due to the reduced ion transfer resistance assisted by its microporous–mesoporous hierarchical structure.
Furthermore, we calculated the contribution of fast kinetics from the CV curves of Zn7-900, Zn7-1000 and Zn7-1100 samples, giving 77.1%, 57.4% and 56.6%, respectively (Fig. 4a and S15†).29,30 It should be noted that the value of slow kinetics increases obviously from Zn7-900 to Zn7-1000, which may be due to the introduction of mesopores and the increase of slow kinetics in Zn7-1000 contributing to the higher capacitance. After 10000 charge/discharge cycles at a current density of 10 A g−1, the capacitance retention remained above 96%, indicating that Zn7-1000 has an excellent cycle stability as a supercapacitor material and a good coulombic efficiency which was kept at ∼100% (Fig. 4c).
Footnote |
† Electronic supplementary information (ESI) available. CCDC [2020712]. For ESI and crystallographic data in CIF or other electronic format see https://doi.org/10.1039/d2sc03987g |
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